Review pubs.acs.org/CR

Upconversion Luminescent Materials: Advances and Applications Jing Zhou,† Qian Liu,† Wei Feng, Yun Sun, and Fuyou Li* Department of Chemistry & State Key Laboratory of Molecular Engineering of Polymers & Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, Shanghai 200433, P. R. China S Supporting Information *

3.4.3. Re(I) Diimine Tricarbonyl Complexes As Sensitizers 3.5. Cyclometalated Complexes As Sensitizers 3.5.1. Cyclometalated Ir(III) Complexes As Sensitizers 3.5.2. Cyclometalated Pt(II) Complexes As Sensitizers 3.5.3. Pt(II) Bis(phosphine) Bis(aryleneethynylene) Complexes As Sensitizers 3.5.4. Pt(II) and Pd(II) Complexes with SchiffBases As Sensitizers 3.6. Organic Dyes As Sensitizers 3.6.1. Heavy Atom-Substituted Fluorophores As Sensitizers 3.6.2. Fullerene−Chromophore Dyads As Sensitizers 3.6.3. Biacetyl Derivatives As Sensitizers 3.7. Single-Molecular TTA-Based Upconversion Systems 3.7.1. [Ru(dmb)2(bpy-An)]2+ As Single-Molecular TTA-Based Upconversion Material 3.7.2. Polymers Containing Heavy Metal Complexes As Single-Molecular TTA-Based Upconversion Material 4. Synthesis of Upconversion Luminescent Nanoparticles (UCNPs) 4.1. Synthesis of Water-Dispersible TTA-Based UCNPs 4.1.1. Direct Loading of Sensitizer and Annihilator into Hydrophilic Dendrimers 4.1.2. Embedding Sensitizer and Annihilator into Micelles 4.1.3. Embedding Sensitizer and Annihilator into Cross-Linked Polystyrene Nanoparticles 4.1.4. Amphiphilic Polymer Loading and SiO2 Coating 4.1.5. Water-Dispersible Upconversion Nanocapsules 4.2. Synthesis of TTA-Based Upconversion Microparticles 4.3. Synthesis of Lanthanide UCNPs 4.3.1. Hydrothermal Synthesis 4.3.2. Synthesis in High-Boiling Solvents

CONTENTS 1. Introduction 2. Upconversion Luminescence Process 2.1. Upconversion Mechanism of Lanthanide Upconversion Nanophosphors (UCNPs) 2.2. Host, Activator, and Sensitizer for Lanthanide UCNPs 2.3. Upconversion Mechanism Based on Triplet− Triplet Annihilation (TTA) 2.4. Upconversion Efficiency 3. Components of TTA-Based Upconversion Systems 3.1. Annihilators of TTA-Based Upconversion Systems 3.1.1. Requirements of the Annihilators 3.1.2. Polycyclic Aromatic Hydrocarbons As Annihilators 3.1.3. Heterocyclic Compounds As Annihilators 3.2. Requirements of Sensitizers 3.3. Porphyrin/Phthalocyanine Complexes As Sensitizers 3.3.1. Porphyrin Complexes As Sensitizers 3.3.2. Benzoporphyrin Complexes As Sensitizers 3.3.3. Other Condensed Porphyrin Complexes As Sensitizers 3.3.4. Metal Phthalocyanines As Sensitizers 3.4. Polyimine Complexes As Sensitizers 3.4.1. Ruthenium(II) Polyimine Complexes As Sensitizers 3.4.2. Pt(II) Polyimine Acetylide Complexes As Sensitizers

© 2014 American Chemical Society

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Chemical Reviews 4.3.3. Other Synthetic Methods 4.4. Surface Engineering of Lanthanide UCNPs 4.4.1. One-Step Hydrothermal Synthesis of Hydrophilic Lanthanide UCNPs 4.4.2. One-Pot/Multistep Synthesis of WaterDispersible Lanthanide UCNPs 4.4.3. Two-Step Conversion Synthesis of Water-Dispersible Lanthanide UCNPs 5. Tuning and Optimization of the Upconversion Properties 5.1. Measurement of Upconversion Emission/ Excitation Spectra 5.2. Tuning Upconversion Emission Bands of Lanthanide UCNPs 5.2.1. Tuning the Ratio of Multiple Upconversion Emission Bands 5.2.2. Single-Band Upconversion Emission of Lanthanide UCNPs 5.3. Tuning the Emission Color of TTA-Based Upconversion 5.4. Optimization of Upconversion Efficiency of Lanthanide UCNPs 5.4.1. Crystalline Phase and Host 5.4.2. Yb3+ Cluster at Sublattice Level 5.4.3. Formation of Core−Shell Structures 5.4.4. Surface-Plasmon-Coupled Emission (SPCE) Effect 5.4.5. High Excitation Power Density 5.5. Optimization of TTA-Based Upconversion Efficiency 5.5.1. Reducing the Degree of Aggregation of Annihilators 5.5.2. Creating an Intraligand Excited State of Heavy-Metal Complexes As Sensitizers 5.5.3. SPCE Effect 5.5.4. Magnetic Field Effects 5.5.5. Sensitizer(I)−Annihilator−Sensitizer(II) System 5.5.6. Single-Sensitizer/Dual-Annihilator System 5.5.7. Optimization of Experimental Conditions 5.6. Upconversion Emission Lifetime 5.7. Tuning Upconversion Excitation Wavelengths 6. UCNPs for Bioimaging 6.1. Bioimaging Methods and Equipment 6.1.1. Laser Scanning Upconversion Luminescence Confocal Microscopy 6.1.2. Small-Animal Upconversion Luminescence Imaging System 6.1.3. Fluorescence Diffuse Optical Tomography (FDOT) 6.2. Features of Upconversion Bioimaging 6.2.1. Lack of Autofluorescence from Biosamples 6.2.2. Improving Penetration Depth in Upconversion Bioimaging 6.2.3. Low Photobleaching in Upconversion Bioimaging 6.2.4. Nonblinking in Lanthanide-Based Upconversion Bioimaging

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6.2.5. Low Detection Limitations in WholeBody Animal Imaging 6.2.6. Multiplex Upconversion in Vivo Bioimaging 6.3. UCNPs for Bioimaging at Different Levels 6.3.1. Lanthanide UCNPs for Cell Imaging 6.3.2. TTA-Based UCNPs for Cell Imaging 6.3.3. Lanthanide UCNPs for the Bioimaging of C. elegans 6.3.4. Lanthanide UCNPs for in Vivo Bioimaging of Different Animal Species 6.4. UCNPs for Functional Bioimaging 6.4.1. Lanthanide UCNPs for Lymphatic Upconversion Imaging 6.4.2. TTA-Based UCNPs for Lymphatic Imaging 6.4.3. Lanthanide UCNPs for Vascular Imaging 6.4.4. Lanthanide UCNPs for Cell Tracking 6.4.5. Lanthanide UCNPs for Tumor Targeting 6.5. Lanthanide UCNPs for Multimodality Bioimaging 6.5.1. Lanthanide UCNPs for Magnetic Resonance Imaging (MRI) 6.5.2. UCNPs for X-ray Computed Tomography (CT) Imaging 6.5.3. UCNPs for Positron Emission Tomography (PET) Imaging 6.5.4. UCNPs for Single-Photon Emission Computed Tomography (SPECT) Imaging 6.5.5. UCNPs for Multimodality Bioimaging 7. Lanthanide UCNPs for Therapies 7.1. Lanthanide UCNPs for Photodynamic Therapy (PDT) 7.1.1. Silica Layer As a Carrier of Photosensitizers 7.1.2. Polymers as Photosensitizer Carriers 7.1.3. Covalent Bonding of Photosensitizers onto Lanthanide UCNPs 7.2. Lanthanide UCNPs-Based Nanocomposites for Chemotherapy 7.2.1. Lanthanide UCNPs Combined Simply with Chemotherapy 7.2.2. Lanthanide UCNPs for PhototriggerInduced Chemotherapy 7.2.3. Lanthanide UCNPs for Chemotherapies Based on Photoinduced Isomerization 7.3. Lanthanide UCNPs Combined with Photothermal Therapy (PTT) 8. Biosafety of UCNPs 8.1. In Vitro Biosafety Assessment of Lanthanide UCNPs 8.1.1. MTT (or MTS) Analysis 8.1.2. IC50 Value 8.1.3. TEM Observations 8.2. Pharmacokinetics of Lanthanide UCNPs 8.2.1. Using the ICP-AES Technique 8.2.2. Using in Vivo Upconversion Luminescence Imaging 8.2.3. Using Radioactive Analytical Techniques 8.3. Toxicity of Lanthanide UCNPs in Small Animals 8.3.1. Toxicity of Lanthanide UCNPs in C. elegans and Zebrafish

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Chemical Reviews 8.3.2. Toxicity Evaluation of Lanthanide UCNPs Injected into a Mouse Model 8.4. Biosafety Assessment of Lanthanide UCNPs on Plants 8.5. Biosafety Assessment of TTA-Based UCNPs 9. Detection Applications of Lanthanide UCNPs 9.1. Lanthanide UCNPs as Nanothermometers 9.2. Upconversion Detection Based on the Inner Filter Effect 9.2.1. Lanthanide UCNPs as pH Sensors 9.2.2. Lanthanide UCNPs as CO2 or Ammonia Probes 9.2.3. Lanthanide UCNPs as a Cr6+ Probe 9.2.4. Lanthanide UCNPs as Probes for Antioxidants 9.3. Design Strategy for Upconversion LRET Detection 9.4. Upconversion LRET Detection by Alteration of the Spectral Overlap between Donor and Acceptor 9.4.1. Lanthanide UCNPs as CN− Probe 9.4.2. Lanthanide UCNPs as a NO2− Probe 9.4.3. Lanthanide UCNPs as a Cu2+ Probe 9.4.4. Lanthanide UCNPs as Hg2+ and MeHg+ Probes 9.4.5. Lanthanide UCNPs as an Oxygen Probe 9.4.6. Lanthanide UCNPs as a pH Probe 9.4.7. Lanthanide UCNPs as a GSH Probe 9.5. UC-LRET Detection by Alteration of the Distance between Donor and Acceptor 9.5.1. Lanthanide UCNPs for DNA/RNA Detection 9.5.2. Lanthanide UCNPs for Immunoassay 9.5.3. Lanthanide UCNPs as Luminescent Probes Based on Ligand−Acceptor Interaction 9.5.4. Lanthanide UCNPs as Enzyme-Activity Assay 9.5.5. Lanthanide UCNPs as an ATP Probe 9.5.6. Lanthanide UCNPs as Hg2+ Probe 9.6. Summary of Upconversion Detection Systems 10. Upconversion Materials as a Lighting Source 10.1. Solid-State TTA-Based Upconversion Film for Lighting 10.1.1. Co-doping Both Sensitizer and Annihilator into a Polymer Matrix 10.1.2. Doping the Sensitizer in an Emissive Polymer Matrix 10.1.3. TTA-Based Upconversion Luminescence in Nanocrystalline ZrO2 Films 10.1.4. TTA-Based Upconversion Luminescence in Nanofibers and Mats 10.2. Lanthanide UCNPs for Lighting 10.3. TTA-Based Upconversion Materials for Color-Display Devices 10.4. Lanthanide UCNPs for Anticounterfeiting Applications 10.5. Lanthanide UCNPs for Fingermark Detection 10.6. Lanthanide UCNPs s for 3D-Displays 11. Upconversion Materials as a Second Excitation Source

Review

11.1. Upconversion Materials for Photocurrent Generation 11.1.1. Lanthanide UCNPs for Photocurrent Generation 11.1.2. TTA-Based Upconversion Materials for Photocurrent Generation 11.1.3. Lanthanide UCNPs for Solar Cells 11.1.4. TTA-Based Upconversion Materials for Solar Cells 11.2. Upconversion Materials for Photocatalysis 11.2.1. Lanthanide UCNPs for Photocatalysis 11.2.2. TTA-Based Upconversion for Photocatalysis 11.3. Upconversion Materials for Solar Fuels 11.4. Upconversion Materials for Photoisomerization 11.4.1. Lanthanide UCNPs for Photoisomerization of Diarylethenes 11.4.2. Upconversion Materials for the Photoisomerization of Azobenzene 12. Summary and Prospects 12.1. Future Directions in the Optimization of Upconversion Materials 12.1.1. Optimization of the Upconversion Efficiency of Lanthanide UCNPs 12.1.2. Optimization of the Photostability of TTA-Based Upconversion Materials 12.1.3. Development of NIR Emissive TTABased Upconversion Materials 12.1.4. Decreasing the Aggregation Quenching in Water-Dispersible TTA-Based UCNPs 12.1.5. Nanotoxicity of Lanthanide UCNPs 12.2. Future Directions in the Application of Upconversion Materials 12.2.1. Lanthanide UCNPs for Therapy Applications 12.2.2. Lanthanide UCNPs for Sensing 12.2.3. Applications of TTA-Based Upconversion Materials in Biology and Medicine 12.2.4. Application of TTA-Based Upconversion Materials in Optical/Electrical Devices Associated Content Supporting Information Author Information Corresponding Author Author Contributions Notes Biographies Acknowledgments Abbreviations References

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1. INTRODUCTION Luminescence-based techniques continue to attract considerable attention due not only to their current range of applications but also to their wide potential in the fields of optical devices and biomedicine. To date, numerous luminescent materials, such as fluorescent proteins,1 organic dyes,2 metal complexes,3−7 semiconductors,8 noble metal nanoparticles,9 as well as lanthanide-doped inorganic phos-

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phors,10,11 have been developed for use in various applications. Most of these conventional materials exhibit luminescent emission with a Stokes shift (Scheme 1). That is they emit

observe the emission from Er3+ by pumping Yb3+ in a glass matrix in the 1960s,13,14 most of the reported upconversion emissive materials have incorporated lanthanide ions as sensitizers and emitters because the f-electron configurations of these ions have abundant energy levels, and many of them possess long lifetimes (∼ms). Lanthanide-doped materials show unique upconversion luminescence properties including large anti-Stokes shifts of several hundred nanometers (even >600 nm, about 2 eV), sharp emission lines, long upconversion luminescence lifetimes (∼ms), and superior photostability. In early studies, these materials were incorporated into inorganic hosts and were mainly used in lasers, anticounterfeiting applications, and optical devices.15−17 In the past 10 years, nanoscale lanthanide upconversion nanophosphors (UCNPs), with controlled size, structure, morphology, and surface ligands, have all been synthesized.18−23 Applications in the bioimaging of living cells and small animals, biosensors, chemosensors, and other optical fields have undergone considerable development.24−29 In this review we will focus on advances in lanthanide UCNPs, rather than in bulk materials since progress made in relation to lanthanide bulk materials can already be found in several published reviews.15−17 Another approach used to achieve upconversion luminescence emission is based on TTA. This process was first reported in the 1960s by Parker and co-workers, who employed organic chromophores with their absorption and emission both in the ultraviolet (UV) region (phenanthrene/naphthalene or proflavin/anthracene).30 Up until a few years ago this remained a mere curiosity for photochemists, because of the relatively low efficiency and significant O2-induced upconversion luminescence quenching coupled with the low photostability of the organic chromophores employed.31−34 Since it was established that TTA-based upconversion luminescence emission can even be observed by the naked eye upon excitation with a commercial green laser at low power (2 h) even under continuous 690 nm irradiation with a high power density of 30 W cm−2). 3.3.3.2. Metal Tetrakisquinoxalinoporphyrin As a Sensitizer. The palladium(II) tetrakisquinoxalinoporphyrin complex (PdPQ4, Chart 2) displays an absorption Q band in the wavelength range 650−700 nm with ε = 1.05 × 106 M−1 dm−1.82 Schmidt and co-workers integrated a model system of PdPQ4&rubrene for TTA-based upconversion emission.82 They demonstrated that more than 60% of the triplet state of the rubrene (annihilator, Chart 1) decayed through a highly effective TTA process. 3.3.3.3. Metal Anthraporphyrin As a Sensitizer. Further extension of the π-system through annelated aromatic rings can result in their Q-bands reaching as far as the NIR region. Baluschev and co-workers demonstrated that the use of the tetraaryl anthraporphyrin palladium complex (PdTAP, Chart 2) as sensitizer allowed the direct photon upconversion of lowintensity NIR light at 785 nm (100 mW cm−2) to yellow emission (570 nm) with rubrene (annihilator) in toluene (Figure 1c).61 The QEUC of this system in toluene was 1.2% (cPdTAP = 5 × 10−5 M, crubrene = 1 × 10−3 M). 3.3.4. Metal Phthalocyanines As Sensitizers. Phthalocyanine (Pc) exhibits intense absorbance in the red to NIR regions and low triplet energy suitable for sensitizing longwavelength emissions for TTA-based upconversion systems. Introducing a heavy-metal atom such as Pd into phthalocyanine greatly enhances its potential for intersystem crossing. Castellano and co-workers reported a yellow upconversion emission from rubrene (λem = 560 nm) upon excitation (λex =

Figure 1. (a) Upconversion photograph of the system of PdOEP&DPA, excited with the green part of the focused sun spectrum, no filter was used. Reprinted with permission from ref 76. Copyright 2006 The American Physical Society. (b) Upconversion photograph of PtTPBP&2CBPEA in deaerated DMF under excitation with 635 nm laser. Reprinted with permission from ref 78. Copyright 2009 American Chemical Society. (c) Upconversion photograph of PdTAP&rubrene under excitation at 785 nm laser, no optical filter was used. Reprinted with permission from ref 61. Copyright 2008 WileyVCH Verlag GmbH & Co. KgaA, Weinheim. (d) Upconversion photograph of PtTPTNP&PDI solution in deaerated toluene under 690 nm excitation. Reprinted with permission from ref 81. Copyright 2013 The Royal Society of Chemistry.

3.3.1.3. ZnTPP As a Sensitizer. Despite without heavy-metal atom, zinc tetraphenylporphyrin (ZnTPP, Chart 2) has also been reported as the sensitizer. Recently, Steer and co-workers have investigated the upconversion mechanism based on TTA process involving ZnTPP (Chart 2) as the sensitizer, using perylene (Chart 1) as the annihilators.53 In the upconversion system of ZnTPP&perylene, bimolecular TTET occurred with high efficiency. 3.3.2. Benzoporphyrin Complexes As Sensitizers. Platinum(II) tetraphenyltetrabenzoporphyrin (PtTPBP, Chart 2) shows strong absorption at 430 nm (the Soret band) and 611 nm (the Q band),77 and a triplet-state lifetime of 41.5 μs. To date, PtTPBP has been successfully used to sensitize 2CBPEA, perylene, and BODIPY dyes (Chart 1) for achieving TTA-based upconversion emission. 3.3.2.1. PtTPBP&2CBPEA (or Perylene). Castellano and coworkers reported PtTPBP&2CBPEA as a red-to-blue upconversion system.78 Upon the excitation of PtTPBP in DMF at 635 nm, the upconversion emission from 2CBPEA located at 490 nm was clearly discernible by the naked eye (Figure 1b). As determined by transient absorption spectroscopy, the rate constant for the TTA process was 5.64 × 109 M−1 s−1. Interestingly, it was possible to visualize the upconversion system in a polyurethane polymer for months. PtTPBP could also sensitize the upconversion emission of perylene in benzene. It is centered at 451 nm, with a maximum QEUC of 0.65% (cPtTPBP = 3.3 × 10−6 M, c2CBPEA = 1.67 × 10−4 M; power density, 125 W cm−2).77 In addition, the palladium(II) complex 405

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Chart 3. Chemical Structures of the Ru(III) Polyimine Complexes As Sensitizers

upconversion enhancement resulted from an increase in the fluorescence QY of DPA in comparison with that of anthracene and a decrease in the extent of dimerization. 3.4.1.2. Ru(II) Polyimine Complexes with Chromophoric Moieties As Sensitizers. By introducing chromophoric moieties (coumarin, BODIPY, or pyrene and their derivatives) into polyimine ligands, Zhao and co-workers synthesized four series of sensitizers based on Ru(II) polyimine complexes (Ru-1−Ru13, Chart 3) for achieving upconversion emission of the annihilator (DPA or perylene).86−89 In these chromophorecontaining Ru(II) complexes, the excited-state lifetime was prolonged by switching the emissive state from metal-to-ligand charge transfer (3MLCT) to the intraligand excited state (3IL) or by balancing the 3MLCT and 3IL excited states. At the same time, the upconversion emission was greatly improved. For example, Ru-4 showed a 3IL excited state with a long lifetime of 58.4 μs, and the QEUC of the Ru-4&DPA system was determined as 9.6% in CH3CN (cRu‑4 = 1.0 × 10−5 M, cDPA = 4.3 × 10−5 M, power density of 0.07 W cm−2). This is approximately ten times higher than that of Ru[(dmb)3]2+&DPA (QEUC = 1%).86 In particular, the QEUC of the Ru-11&DPA in CH3CN was shown to be as high as 15.2% (cRu‑11 = 1.0 × 10−5 M, cDPA = 4.0 × 10−5 M; power density of 0.071 W cm−2).88 3.4.1.3. Supermolecular Complex Pyr1RuPPZn2 As a Sensitizer. The complex Pyr1RuPPZn2 (Chart 3) is a supermolecule consisting of one Ru(II) complex and two porphyrin zinc complexes linked through two ethynyl groups.

725 nm) of a 1,4,8,11,15,18,22,25-octabutyloxyphthalocyanine Pd(II) complex (PdPc(OBu)8, Chart 2) in toluene as well as in solid polymer films.83 However, in aerated toluene, the rubrene annihilator was oxidized upon exposure to NIR light. Its low photostability in aerated media limits the application of the PdPc(OBu)8&rubrene system. 3.4. Polyimine Complexes As Sensitizers

3.4.1. Ruthenium(II) Polyimine Complexes As Sensitizers. Ruthenium(II) polyimine complexes exhibit efficient intersystem crossing. The efficiency of the singlet to triplet transition is near unity which together with a long-lived triplet excited state mean that these advantages of Ru(II) complexes are beneficial for their role as sensitizers (Chart 3). 3.4.1.1. [Ru(dmb)3]2+ Complex As a Sensitizer. Castellano and co-workers reported [Ru(dmb)3]2+ (Chart 1) as a sensitizer for use in three TTA-based upconversion systems using anthracene and its derivatives DMA and DPA (Chart 1) as the annihilators.35 Moreover, they observed that the photodimerization of anthracene is sensitized to visible light by [Ru(dmb)3]2+.84 For the [Ru(dmb)3]2+&DMA system, under excitation at 514.5 nm, both upconversion emission (λem = 420−470 nm) and normal fluorescence of the DMA excimer (λem = 500−700 nm) were produced at a high DMA concentration (90 mM).85 Furthermore, the substitution of anthracene by DPA (which is sterically bulky) provided an approximately 24-fold enhancement of upconversion emission. In addition, photodimerization was not observed.35 The 406

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Chart 4. Chemical Structures of the Pt(II) Polyimine Complexes As Sensitizers

Chart 5. Chemical Structures of the Re(I) Diimine Complexes As Sensitizers

have become attractive as triplet sensitizers because their photophysical properties can be fine-tuned using different acetylide ligands. Eisenberg and co-workers reported the TTAbased upconversion emission of DPA sensitized by [Pt(ttpy)(CCPh)]ClO4 (Pt-1, Chart 4).90 The blue upconversion emission from DPA proved to be very stable for several hours under continuous laser irradiation in a deaerated solution. Recently, Zhao and co-workers synthesized ten Pt(II) diimine bisacetylide complexes (Pt-3 ∼ Pt-12, Chart 4) which proved efficient sensitizers. By introducing some chromophoric moieties such as coumarin, difluoroboron, naphthalenediimide, fluorescein, and fluorine-conjugated naphthyl and pyrenyl units, Pt-3−Pt-12 showed enhanced absorption in the visible region and prolonged triplet-state lifetimes when compared to the

Utilizing a combination of Pyr1RuPPZn2 and PDI (annihilator, Chart 1) in the 2-methyltetrahydrofuran (MTHF) solution, Castellano and co-workers demonstrated an upconversion emission centered at 541 nm with a QEUC of 0.9% using laser irradiation at 780 nm (csensitizer = 3.3 × 10−6 M, cannihilator = 4.2 × 10−4 M).69 Using tetracene (Chart 1) as the annihilator, a green upconversion emission at 505 nm was successfully observed upon excitation of Pyr1RuPPZn2 at 780 nm, and the triplet energy level of Pyr1RuPPZn2 was determined to be higher than 1.27 eV (3tetracene*). Moreover, no significant decrease in upconversion emission intensity was measured even over several hours. 3.4.2. Pt(II) Polyimine Acetylide Complexes As Sensitizers. Platinum(II) polyimine acetylide complexes 407

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Chart 6. Chemical Structures of the Cyclometalated Ir(III) Complexes As Sensitizers

3.5. Cyclometalated Complexes As Sensitizers

model complex Pt-2 (Chart 4). Upon excitation of Pt-3 at 473 nm, the upconversion emission intensity of DPA was found to be five times higher than that with Pt-2, due to the large molar extinction coefficient and long-lived 3IL excited state of Pt-3. A high QEUC of 14.1% was measured for Pt-3&DPA in toluene (cPt‑3 = 1.0 × 10−5 M, cDPA = 1.32 × 10−4 M).91 Similarly, Pt-4 containing ethynylpyrene,92 Pt-5 containing difluoroboron,93 Pt-6 and Pt-7 containing naphthalenediimide,92,94 Pt-8 containing fluorescein,95 Pt-9, Pt-10, and Pt-11 bearing fluorine-containing acetylide ligands appended to naphthyl, pyrenyl, and naphthalenediimide moieties,96,97 respectively, could be successfully used as sensitizers for TTA-based upconversion. Of particular note is the fact that Pt-9 proved to be weakly phosphorescent, but it could still be used for the highly efficient sensitizing upconversion emission of DPA. This result is very important for the design of weakly or nonemissive metal complexes for use as sensitizers. 3.4.3. Re(I) Diimine Tricarbonyl Complexes As Sensitizers. The use of rhenium(I) diimine tricarbonyl complexes as triplet sensitizers is usually limited by their weak absorption in the visible range. By introducing coumarin, naphthyl, and BODIPY moieties into Re(I) diimine tricarbonyl complexes (Chart 5), however, the triplet-state lifetimes of Re2, Re-3, Re-4, and Re-5 are prolonged to 86, 64, 111.8, and 52.8 μs, respectively. This is believed to result from their intraligand triplet excited states.98,99 The Re-2 and Re-3 complexes proved excellent sensitizers in the effort to achieve upconversion emission from DPA. In addition, Re-4 and Re-5 could effectively further sensitize perylene to exhibit blue upconversion emission. Of these the QEUC of Re-2&DPA in toluene was found to be as high as 17% (cRe‑2 = 5.0 × 10−6 M, cDPA = 1.5 × 10−5 M).98

3.5.1. Cyclometalated Ir(III) Complexes As Sensitizers. Cyclometalated iridium(III) complexes possess long-lived excited states and have absorption bands that can tail into the visible region. This allows such complexes to serve as sensitizers for TTA-based upconversion. Castellano and coworkers demonstrated the possibility of upconversion emissions of pyrene and tert-butylpyrene (Chart 1) using [Ir(ppy)3] (Chart 6) as the sensitizer.64 However, in [Ir(ppy)3]&pyrene, the pyrene excimer was generated, which reduced the QEUC. When tert-butylpyrene was employed as the annihilator, this broad emission which is centered at 470 nm disappeared. This indicates that the steric bulk of the two tert-butyl groups effectively suppressed the formation of the excimer. Recently, by incorporating light-harvesting coumarin into the complex Ir(ppy)3, Zhao and co-workers synthesized two cyclometalated Ir(III) complexes Ir-1 and Ir-2 (Chart 6) that showed intense absorption in the visible region.100 Using Ir-1 with a long-lived 3IL state of 75.5 μs as the sensitizer, intense upconversion emission from DPA in CH3CN was collected and a QEUC of up to 23.4% (cIr‑1 = 1.0 × 10−5 M, cDPA = 8.0 × 10−5 M) was found. This was in spite of the fact that the phosphorescence of Ir-1 is quite weak. Furthermore, pyrenylfused imidazole ligands and naphthal or naphthalimidemodified cyclometalated ligands were introduced into Ir(III) complexes (Ir-3−Ir-5) in order to increase the absorption in the visible range and to create a long triplet-state lifetime. These complexes (Ir-3, Ir-4 and Ir-5) were used as sensitizers for highly effective TTA-based upconversion in toluene, with QEUC values of 23.7%, 7.1%, and 14.4%, respectively (csensitizer = 1.0 × 10−5 M, cDPA = 6.0 × 10−5 M).101 In addition, after attaching BODIPY and naphthalenediimide units to the 2,2′-bipyridine 408

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Chart 7. Chemical Structures of the Cyclometalized Pt(II) Complexes As Sensitizers

Chart 8. Chemical Structures of Pt(II) Bis(phosphine) Bis(aryleneethynylene) Complexes As Sensitizers

4.3 × 10−5 M),104 which may be ascribed to the intense absorption in the visible region and the long-lived 3IL excited state. 3.5.3. Pt(II) Bis(phosphine) Bis(aryleneethynylene) Complexes As Sensitizers. After the attachment of visible light-harvesting antennae, such as BODIPY, naphthalize, thiazocoumarin, naphthalimide, and phenyl acetylide, to Pt-23−Pt-31 (Chart 8), they showed enhanced absorption in the visible region and long-lived triplet excited states (16.0−139.9 μs) compared with the reference complexes Pt-21 and Pt-22.107,108 In particular, the binuclear complexes Pt-24 and Pt-25, with two Pt(II) coordination centers connected to the π-cores of the BODIPY ligands, showed red-shifted absorption at 643 nm compared to the mononuclear Pt(II) complexes Pt-23 (λabs = 589 nm) and Pt-26 (λabs = 602 nm). Moreover, these Pt(II) complexes could be used as triplet sensitizers for upconversion

(bpy) ligand through −CC− bonds, three Ir(III) complexes (Ir-6−Ir-8) were obtained which proved suitable for sensitizing the upconversion emission of perylene in CH3CN with QEUC values of 1.2%, 2.8%, and 6.7%, respectively (csensitizer = 1.0 × 10−5 M, cperylene = 5.0 × 10−5 M; power density of 0.070 W cm−2).102,103 3.5.2. Cyclometalated Pt(II) Complexes As Sensitizers. Recently, Zhao and co-workers synthesized eight Pt(II) complexes (Pt-13−Pt-20, Chart 7), in which the thiazocourmarin or naphthalenediimide ligand was cycloplatinated.104−106 Although Pt(II) complexes of this nature exhibit quite weak phosphorescence, they can be used as sensitizers for upconversion emission of DPA. This is a further indication that weakly phosphorescent complexes can also be used as sensitizers. In particular, the QEUC of Pt-13&DPA in CH2Cl2 was found to be as high as 15.4% (cPt‑13 = 1.0 × 10−5 M, cDPA = 409

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Chart 9. Chemical Structures of Pt(II) and Pd(II) Complexes with Schiff-Bases

Chart 10. Chemical Structures of Heavy-Atom Substituted Fluorophores As Metal-Free Organic Sensitizers

1, Pt-33, Pd-2, Pt-34, and Pd-3 showed the strong green upconversion emission from Per-1 (Chart 1) in toluene, with QEUC values of 2.0%, 1.6%, 6.2%, 2.0%, and 2.0%, respectively (csensitizer = 1 × 10−4 M, cPer‑1 = 5 × 10−4 M).

emission with perylene and perylenebisimide (PBI, Chart 1) as the annihilators. Among these upconversion systems, the highest QEUC obtained was 27.2% for Pt-31&DPA in toluene (cPt‑31 = 1.0 × 10−5 M, cDPA = 6.0 × 10−5 M; power density of 0.070 W cm−2).108 3.5.4. Pt(II) and Pd(II) Complexes with Schiff-Bases As Sensitizers. Recently, Borisov and co-workers demonstrated that five complexes (Pt-32, Pt-33, and Pd-1−Pd-3, Chart 9) incorporating donor−acceptor Schiff-bases could be used as red-light-excited triplet sensitizers to achieve the upconversion emission of perylene-based annihilators (Per-1 and Per-2, Chart 1).109 The molar absorption coefficients (ε) of these complexes in the red-light region range up to >105 M−1 cm−1. Upon excitation at 635 nm, deoxygenated systems sensitized with Pd-

3.6. Organic Dyes As Sensitizers

Although Parker and Hatchard have reported that they could sensitize a TTA-base upconversion system in 1962,30 currently, the available sensitizers for TTA-based upconversion are still mainly limited to heavy-metal complexes. Recently, metal-free organic triplet sensitizers have attracted increasing attention. 3.6.1. Heavy Atom-Substituted Fluorophores As Sensitizers. The heavy atom effect enhances the rate of a spin-forbidden process and also favors intersystem crossing 410

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Chart 11. Chemical Structures of C60-BODIPY Dyads As Sensitizers

Chart 12. Chemical Structures of the Diacetyl Derivatives As Sensitizers

(Chart 1) was observed when BD-9 was used as a triplet sensitizer. 3.6.1.2. Bromo-Substituted Naphthalenediimide As Sensitizer. Another strategy for the design of heavy-atom-bearing sensitizers is to introduce a bromo substituent. Recently, 2,6dibromo-3,7-diaminonaphthalenediimide (Br-NDI, Chart 10) has been reported as showing strong absorption at 526 nm (ε = 2.1 × 104 M−1 cm−1) as well as weak fluorescence (QY = 0.2%). It has been utilized as a sensitizer to produce the blue upconversion emission of perylene in toluene with a QEUC of 18.5% (cBr‑NDI = 1.0 × 10−5 M, cperylene = 8.3 × 10−5 M; power density of 0.070 W cm−2).115 The lifetime of the upconversion emission was determined to be 153.2 μs, which is 5 orders of magnitude longer than the fluorescence lifetime of perylene. 3.6.2. Fullerene−Chromophore Dyads As Sensitizers. C60 is typically used as a spin converter, and its triplet energy level can be efficiently generated without the heavy-atom effect. The efficiency of the triplet excited state is close to unity. Steer and co-workers observed that the upconverted emission from anthanthrene mixed with C60 in toluene upon CW excitation at 532 nm.116 However, C60 has a very weak absorption in the

(ISC) from the singlet state to the triplet state. The heavy atom usually quenches fluorescence and enhances phosphorescence emission. As a result, introducing a heavy atom such as iodine or bromine into organic dyes offers an effective strategy for obtaining triplet sensitizers for TTA-based upconversion. 3.6.1.1. Iodo-Substituted Fluorophores As Sensitizers. Sun and co-workers reported a blue upconversion emission (434 nm) from DPA using 2,4,5,7-tetraiodo-6-hydroxy-3-fluorone (TIHF, Chart 10) as a sensitizer.110 Unfortunately, the QEUC of this system was relatively low (0.6%). Recently, Zhao and coworkers proposed a library of organic triplet sensitizers based on iodo-substituted BODIPY (BDP-3−BDP-19, Chart 10).111−113 Compared with the parent model, the iodoBODIPY derivatives showed reduced fluorescence QY and long-lived triplet excited states. In particular, BDP-11 showed a long-lived triplet excited state of 95.2 μs at room temperature, and the QEUC of BDP-11&perylene in toluene was found to be as high as 16.5% (cBDP‑11 = 1.0 × 10−5 M, cperylene = 1.3 × 10−5 M).114 Moreover, the upconversion emission of BDP4&perylene could be observed in PEG-1500 polymer films. In addition, significant upconversion emission of 1CBPEA 411

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Chart 13. Chemical Structures of Single-Molecular TTA-Based Upconversion Materials

3.7.2. Polymers Containing Heavy Metal Complexes As Single-Molecular TTA-Based Upconversion Material. Baluschev and co-workers reported the covalent bonding of 8 wt % dopant Pd-porphyrin to the end of a poly(pentaphenylene) L-5Ph chain to form the polymer PdTTP (Chart 13). This strongly reduced the dopant phase separation and aggregation, and thus improved the TTA-based upconversion emission.67 When PdTTP was excited at 532 nm, the characteristic blue emission of L-5Ph (420−490 nm) was observed. Compared with the dopant system, the upconversion process in PdTTP proved to be more stable and efficient. In addition, Ghiggino and co-workers demonstrated that ruthenium-centered polymers containing Ru(bpy)3 core and pendant DPA arms displayed upconversion emission through efficient intramolecular TTET from [Ru(bpy)3] (sensitizer) to DPA (annihilator).123,124 Unfortunately, the obvious singlet− singlet back-energy-transfer process from DPA to [Ru(bpy)3] still exists in this system.

visible range. To enhance the absorption in the visible range, Zhao and co-workers demonstrated six C60-BODIPY dyads (Chart 11) as sensitizers by introducing the chromophoric BODIPY moiety.117−119 The absorption of the C60-BODIPY dyads could be tuned by changing the radiation-harvesting antenna. The formation of the C60-localized triplet excited state was confirmed by nanosecond time-resolved transient absorption spectroscopy. Upconversion emission from perylene (Chart 1) was observed upon excitation of C60-BODIPY with visible light, with the highest QEUC of 7.0% in toluene (csensitizer = 1.0 × 10−5 M, cperylene = 4.1 × 10−4 M).118 3.6.3. Biacetyl Derivatives As Sensitizers. Castellano and co-workers demonstrated visible-to-UV upconversion using two simple organic chromophores, namely biacetyl (as the sensitizer, Chart 12) and 2,5-diphenyloxazole (as the annihilator, Chart 1) in benzene.120 The excitation of biacetyl&2,5-diphenyloxazole at 442 nm resulted in UV emission centered at 360 nm. The QEUC reached 0.5% under excitation with a lower power density of 0.389 W cm−2. In addition, Guo and co-workers developed five ketocoumarin compounds (KC-1−KC-5, Chart 12) as metal-free triplet sensitizers which showed effective absorption in the visible region (ε = ∼105 cm−1 M−1) and long-lived triplet excited states (up to 199.7 μs). However, only KC-1 (λabs = 449 nm) could effectively sensitize DPA in toluene to show upconversion emission with a QEUC of 11.3% (cKC‑1 = 1.0 × 10−5 M, cDPA = 1.3 × 10−5 M).121

4. SYNTHESIS OF UPCONVERSION LUMINESCENT NANOPARTICLES (UCNPS) Nanoscale upconversion materials have attracted much research interest in recent years because of their unique application in the fields of bioimaging and detection. Efficient control of the synthetic procedures used can produce size-, phase-, and shapetunable nanomaterials. The upconversion luminescence properties can also be tuned and improved by the formation of specific nanostructures. Because of the different nature of TTA-based and lanthanide-based UCNPs, the corresponding synthesis methods are discussed separately.

3.7. Single-Molecular TTA-Based Upconversion Systems

In contrast to the above-mentioned TTA-based upconversion systems comprising separate sensitizer and annihilator components, single-molecular upconversion materials integrating the sensitizer and annihilator into one system (Chart 13) can also be utilized. However, due to the singlet−singlet back energy transfer from the annihilator to the sensitizer, it is difficult to obtain high-efficiency TTA-based upconversion emission based on single-molecular systems. 3.7.1. [Ru(dmb)2(bpy-An)]2+ As Single-Molecular TTABased Upconversion Material. Castellano and co-workers reported that the single molecule [Ru(dmb)2(bpy-An)]2+ (Chart 13), which combines an Ru(II) complex and anthracene unit, yielded a delayed TTA-based upconversion emission of anthracene at 410 nm in CH3CN upon excitation with a 450 nm laser.122 However, the upconversion efficiency of this complex was found to be rather low, only about one-third of that of the dual-dye system ([Ru(dmb)3]2+&anthracene) under identical experimental conditions. This could be attributed to quenching of the anthracene fluorescence by the intramolecular MLCT ground state.

4.1. Synthesis of Water-Dispersible TTA-Based UCNPs

As described in section 2.4, most upconversion systems based on TTA processes are in the organic phase, hence the integration of sensitizer and annihilator into an aqueous environment while maintaining the upconversion efficiency is a significant challenge. In this section, we discuss the advances in synthetic methods used to prepare water-dispersible TTAbased UCNPs. 4.1.1. Direct Loading of Sensitizer and Annihilator into Hydrophilic Dendrimers. A simple synthetic strategy is to directly load the hydrophobic sensitizer and the anthracene into hydrophilic dendrimers. Chujo and co-workers assembled PtOEP (sensitizer, Chart 2) and anthracene (as the annihilator, Chart 1) into a water-dispersible G2 dendrimer with a core of cubic octameric polyhedral oligomeric silsesquioxane (POSS) (Scheme 5a).125 Upon excitation at 537 nm, upconversion emission of anthracene at 380−450 nm occurred in the 412

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hydrophilic and hydrophobic moieties and can form micelles. Miteva and co-workers have demonstrated the possibility of a micellar carrier (30−35 nm), in which the sensitizer and annihilator were embedded, made of amphiphilic polyoxyethanyl-tocopheryl sebacate (PTS) block copolymers for TTAbased upconversion emission in a water environment.127 The micelles could be successfully loaded with PdPh4TBP (sensitizer) and perylene (annihilator) at concentrations ranging from 10 μM to 10 mM. As shown in Figure 2, blue upconversion emission at 480 nm was obtained under excitation at 635 nm, with a QEUC of 2.4% (csensitizer = 2 × 10−5 M, cannihilator = 4 × 10−4 M; solvent, H2O).

Scheme 5. Schematic Illustration in Frabricating WaterDispersible TTA-Based UCNPsa

Figure 2. Upconversion emission spectra for PdTPBP&perylene in PTS/H2O mixture (red line) and in toluene (black line) with a notch filter. Inset: an upconversion photograph of the studied water solution, daylight conditions. Reprinted with permission from ref 127. Copyright 2011 IOP Publishing Ltd. and Deutsche Physikalische Gesellschaft.

4.1.3. Embedding Sensitizer and Annihilator into Cross-Linked Polystyrene Nanoparticles. By embedding both PtOEP and DPA within cross-linked polystyrene nanoparticles with a diameter of 16 nm (Scheme 5c), Monguzzi and co-workers reported the formation of another low-powerexcited upconversion nanomaterial.128 After excitation at 532 nm (90%. Subsequently, 18F-labeled NaYF4:Yb,Tm nanoparticles were used to investigate the biodistribution (1 h) of UCNPs by PET imaging and to image the sentinel lymph node in vivo. Using this method, two other lanthanide UCNPs (citrate-modified NaYF4:Gd,Yb,Er251 and α-CD-modified NaYF4:Yb,Tm265) were labeled with radioactive 18F and further applied in the PET imaging of small animals. Unfortunately, the relatively short half-life of radioactive 18F precludes monitoring of the long-term distribution of 18F-labeled lanthanide UCNPs. 6.5.4. UCNPs for Single-Photon Emission Computed Tomography (SPECT) Imaging. 153Sm (half-life of 46.3 h) is a γ-emitter and it has been incorporated into SPECT imaging probes that are currently both in preclinical and clinical use. Currently, to track the pharmacokinetics of lanthanide UCNPs in vivo, two methods have been developed to label them with the radionuclide 153Sm. The first is a unique cation-exchangebased postlabeling method developed by our group.472 By simply mixing the UCNPs and 153Sm3+ in aqueous solution for 1 min at room temperature and atmospheric pressure, 153Sm was postlabeled onto NaLuF4 nanoparticles in fetal calf serum with high yield (>99%) and excellent stability (>99%, 72 h). This postlabeling method with radioactive 153Sm is also applicable to other kinds of rare earth nanoparticles, such as oxides, fluorides, and phosphates. The second method is to dope radioactive 153Sm into the lanthanide nanophosphor during the synthesis process. Recently, our group prepared Yb3+, Tm3+, and 153Sm3+ codoped NaLuF4 nanoparticles by using a one-step hydrothermal method150,224 and a thermal decomposition method.347,473 The radioactive 153Sm-labeled NaLuF4:Yb,Tm nanoparticles thus obtained were accurately tracked in vivo using SPECT imaging. In addition, our group has developed radioactive/ upconverting NaLuF4:153Sm,Yb,Tm nanoparticles as a blood pool imaging probe for in vivo SPECT imaging.244 6.5.5. UCNPs for Multimodality Bioimaging. Each conventional diagnostic imaging technique, including MRI, CT, nuclear, and optical imaging, has its own advantages and disadvantages. MRI and CT have the advantage of being noninvasive techniques for both in vivo imaging and 3D tomography. Nuclear imaging, including PET and SPECT, exhibit ultrahigh sensitivity in vivo. However, MRI is limited by its low sensitivity. In addition, CT and nuclear imaging expose patients to the hazards of X-ray radiation and radioactivity, respectively. upconversion imaging has a relatively good sensitivity on the subcellular scale but has the drawback of a relatively low tissue penetration depth (∼cm). Multimodality imaging can compensate for the deficiencies of individual imaging modalities and give a more accurate or extensive information. Numerous reports have described the combination of conventional diagnostic imaging techniques with upconversion imaging to achieve such multifunctional imaging (Scheme

15). To date, dual-modality upconversion/MRI, upconversion/ CT, and upconversion/PET and multimodality upconversion/ Scheme 15. Schematic Representation of Multifunctional Lanthanide UCNPs for Multi-Modality Bioimaging Including Upconversion, X-ray CT, MRI, PET, and SPECT Imaging

MRI/PET, upconversion/MRI/CT, upconversion/CT/ SPECT, and upconversion/MRI/CT/SPECT imaging systems have all been reported. For example, Yan and co-workers fabricated core−multishell NaYF4:Yb,[email protected]@[email protected] nanoprobes for upconversion luminescence, CT, and MRI trimodal imaging.474 Our group developed a multifunctional NaLuF4:Yb,[email protected] NaGdF4:153Sm nanocomposite, which was confirmed as being effective and applicable for four-modality bioimaging combining upconversion luminescence, CT, MRI, and SPECT imaging. The resulting probe can be used to investigate tumor angiogenesis.347

7. LANTHANIDE UCNPS FOR THERAPIES Some nanocomposites containing lanthanide UCNPs have been developed for therapeutics, ranging from photodynamic therapy (PDT), photothermal therapy (PTT), controlled drug release, and the targeted delivery of small interference RNA (siRNA) to multifunctional cancer therapies. The reasoning behind introducing lanthanide UCNPs into therapeutic agents is to allow the agent to operate in the NIR range in vivo. The functions of such lanthanide UCNPs in these nanocomposites to date can be divided into two main categories. The first is to use the wavelength-converting ability of lanthanide UCNPs, which extends the operating range of therapeutic agents from the UV or visible to the NIR region. Such upconversion nanocomposites are often designed for PDT or drug carriers with phototrigger and photoisomerization processes. The other option is to use lanthanide UCNPs as photoluminescent probes to monitor the distribution and metabolism of drugs. If there is efficient energy transfer between UCNPs and the drug molecules, the upconversion emission of lanthanide UCNPs can also be used as a signal to monitor the amount of drug that has been released. Nanocomposites of this type have been designed for drug release for both PTT and chemotherapy. Detailed mechanisms and examples of applications of lanthanide UCNPs for therapy are described below. 7.1. Lanthanide UCNPs for Photodynamic Therapy (PDT)

Unlike chemotherapy, radiotherapy, or surgery, PDT is a technique for cancer treatment that involves killing the diseased cells by excitation of a photosensitizer with high-energy light to 435

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Chart 15. Chemical Structures of the Photosensitizers Used in Lanthanide UCNPs for PDT

produce cytotoxic reactive oxygen species (ROS).475 The commonly used photosensitizers include zinc(II) phthalocyanine (ZnPc), merocyanine 540, meso-tetraphenyl porphine (TPP), tris(bipyridine)ruthenium(II), tetrasubstituted carboxy aluminum phthalocyanine (AlC4Pc), porphyrin, hematoporphyrin, silicon phthalocyanine dihydroxide, methylene blue (MB), rose bengal (RB), and Chlorine6 (Ce6) as shown in Chart 15. Considerable effort has been focused on constructing hybrid nanophosphors for PDT based on lanthanide UCNPs.476 7.1.1. Silica Layer As a Carrier of Photosensitizers. Pure lanthanide UCNPs have no ability to carry or deliver drugs. The formation of a silica coating on the surface of lanthanide UCNPs to form heterogeneous core−shell nanostructures is one of the typical methods of choice for producing a carrier layer. Zhang and co-workers reported the fabrication of core−shell NaYF4:Yb,[email protected] nanocomposites loaded with merocyanine 540 in the silica shell.477 After 45 min of 974 nm irradiation, these nanoparticles showed primary PDT effects on carcinoma cells. Other core−shell NaYF4:Yb,[email protected] particles loaded with tris(bipyridine)ruthenium(II)478 and NaYF4:Gd,Yb,[email protected] particles loaded with methylene blue479 have also been reported as capable of generating reactive oxygen species. To enhance the loading efficiency of the photosensitizer, Zhang and co-workers synthesized other core−shell NaYF4:Yb,[email protected] and NaYF4:Yb,[email protected]@mSiO2 nanocomposites413 with ZnPc incorporated into the mesoporous silica (mSiO2) shell. After 5 min of 980 nm irradiation, the oxygen species generated were readily released from the mSiO2 layer. Recently, Zhang and co-workers481 used the multiwavelength-emission capability of NaYF4:Yb,[email protected] under

a single 980 nm excitation for the simultaneous activation of two photosensitizers (MC540 and ZnPc, Chart 15) for the purpose of producing enhanced PDT. 7.1.2. Polymers as Photosensitizer Carriers. The use of polymer coatings is another means of constructing PDT systems. Zhang and co-workers coated NaYF4:Yb,Er nanoparticles with polyethylenimine (PEI) to allow the physical adsorption of the photosensitizer ZnPc (Chart 15) as well as conjugation with folic acid.481,482 The resulting nanoparticles exhibited the targeted binding of cancer cells and induced significant cell destruction upon excitation at 980 nm for 5 min (Scheme 16). Coating an amphiphilic polymer onto the surface of hydrophobic UCNPs can provide a hydrophobic interlayer Scheme 16. Schematic Representation of the Action Mechanism of Lanthanide UCNPs-Based PDTa

a

After NIR irradiation at 980 nm, NaYF4:Yb,Er acts as a nanotransducer by converting NIR light into visible emissions which excites the nearby ZnPc photosensitizers and these in turn convert the surrounding molecular oxygen to singlet oxygen.

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Scheme 17. Schematic Representation of (a) NaYF4:Yb,[email protected]@mSiO2 Nanocomposite, (b) [email protected]@[email protected] NaYF4:Yb,Er Nanocomposite, and (c) [email protected]@NaYF4:Yb,Er Nanorattles

Figure 15. Experimental design for uncaging D-luciferin and subsequent bioluminescence through the use of photocaged silica-coated NaYF4:Yb,[email protected] upconversion nanoparticles. Bioluminescent images of freely luciferase activity in living mice that were treated with Dluciferin. Reprinted with permission from ref 335., Copyright 2012 Wiley-VCH Verlag GmbH & Co. KgaA, Weinheim.

region for loading selected photosensitizers. 483−486 For example, Liu and co-workers reported the fabrication of a nanocarrier by applying a PEGylated amphiphilic polymer onto the surface of OA-coated NaYF4:Yb,Er nanoparticles. This provided a hydrophobic region that could be loaded with Ce6 (Chart 15) for PDT.485 Excellent tumor destruction was achieved in a Balb/c mouse as a result of the intratumoral injection of 40−50 μL of Ce6-loaded polymer-coated upconversion nanocomposite (20 mg mL−1 UCNP, ∼1.5 mg mL−1 Ce6) followed by exposure to 980 nm light (0.5 W cm−2) alternating between on and off at 1 min intervals over 30 min. On the basis of a similar hydrophobic interaction, Gu and coworkers demonstrated that amphiphilic N-succinyl-N′-octyl chitosan-modified OA-capped NaYF4:Yb,Er nanoparticles could be loaded with ZnPc also for PDT.486 The tumor volume was significantly reduced after 14 days, and the tumor inhibitory ratio was calculated to be approximately 76%. 7.1.3. Covalent Bonding of Photosensitizers onto Lanthanide UCNPs. Another strategy is to attach a photosensitizer onto the surface of lanthanide UCNPs through covalent bonding. Zheng and co-workers reported the synthesis of NaGdF 4:Yb,[email protected] @SiO2 nanocomposites with AlC4Pc (Chart 15) covalently incorporated inside the silica shells.406 After 12 h of incubation with these nanocomposites (100 mg mL−1), MEAR cells were exposed to a 980 nm laser (0.5 W cm−2) for 5 min, and almost 40% of the cells were killed. In addition, NaGdF4:Yb,[email protected]@mSiO2 nanocomposites bearing covalently grafted photosensitizers (hematoporphyrin, silicon phthalocyanine dihydroxide)487 as well as NaYF4:Yb,Er covalently assembled with rose bengal488 or ZnPc489 (Chart 15) have all been employed for in vitro PDT.

drug release. Nowadays, lanthanide UCNPs-based nanocomposites, including UCNP coated with the polymer TWEEN or hydrogels and nanocomposites with porous or hollow structure are being as drug and siRNA carriers for chemotherapy. Among them, lanthanide UCNPs modified with SiO2 or mesoporous SiO2 could provide porous or hollow nanostructures as suitable drug delivery systems (Scheme 17).490−493 For example, Lin and co-workers constructed a multifunctional drug carrier system by loading ibuprofen into core−shell [email protected]@[email protected]:Yb,Er nanocomposites.491 The loaded IBU molecules (6.2 wt %) were completely released from the nanocomposites. In addition, the controlled release of the drug in solution could be monitored by the change in upconversion emission intensity. These authors also fabricated NaYF4:Yb,[email protected] nanofibers for drug delivery.494 Su and Yeh and co-workers reported that doxorubicin (DOX) could be thiolated onto the surface of NaYF4:Yb,[email protected], forming a disulfide bond.495 siRNA with double-stranded RNA shows great potential as a new treatment for the sequence-specific silencing of genes and human diseases amenable to manipulation at the gene expression level. Upconversion imaging offers an ideal method for monitoring the delivery of siRNA to specific cells and detecting its intracellular fate.496−499 Zhang and co-workers demonstrated the use of anti-Her2 antibody-conjugated NaYF4:Yb,[email protected] nanoparticles for the targeted imaging of Her2 receptors of SK-BR-3 cells and the targeted delivery of siRNA.496 Exogenous luciferase gene expression assays revealed a luciferase gene silencing effect of 45.5% attributable to the siRNA delivered by the nanoparticles. 7.2.2. Lanthanide UCNPs for Phototrigger-Induced Chemotherapy. A phototrigger can be defined as a photoresponsive functional group that can absorb light of a specific wavelength and subsequently release covalently bonded molecules. Conventional phototriggers are based on UV light. However, these can cause cellular damage and cannot penetrate into deep tissue. Due to the ability of lanthanide UCNPs to accomplish NIR-to-UV upconversion, and the deep tissue

7.2. Lanthanide UCNPs-Based Nanocomposites for Chemotherapy

7.2.1. Lanthanide UCNPs Combined Simply with Chemotherapy. Chemotherapy is still a main strategy of clinic treatment for tumor disease. Combination of lanthanide UCNPs with chemotherapy can provide a possibility of imaging-guided chemotherapy and monitoring the degree of 437

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Scheme 18. Schematic Illustration of the DMNPE Caged Plasmid DNA and siRNA and Their Activation by the UV Upconversion Emission from Lanthanide UCNPsa

a

Inset demonstrates the superior penetration depth of NIR light comparing with UV light in tissue.

Scheme 19. Schematic Illustration of the NIR-Triggerred Upconversion-Based Therapy Process and the Photolysis of the Prodrug under Upconversion Emission from the Nanocomposites502

peptide onto NaYF4:Yb,[email protected] nanoparticles.500 After illumination with NIR light, release of the antitumor platinum prodrug was selectively triggered, and it was possible to simultaneously image the apoptosis induced by the activated cytotoxicity in real time. Qu and co-workers synthesized NaYF4:Yb,[email protected] nanoparticles with UV-photocleavable 4(hydroxymethyl)-3-nitrobenzoic acid (ONA) molecules covalently attached to the surface. NIR light was then locally converted into UV light for the cleavage of the photocaged linker to realize cell release on-demand.501 The remote activation of biomolecules in deep tissues was achieved by Zhang and co-workers using NIR-to-UV emission. They did this by first loading photocaged plasmid DNA/siRNA into the mesopores of NaYF4:Yb,Er/[email protected] nanocomposites and using 4,5-dimethoxy-2-nitroacetophenone (DMNPE) as a phototrigger.499 This strategy is illustrated in Scheme 18.

penetration of NIR radiation, much effort has been devoted to utilizing the upconversion emission of lanthanide UCNPs to regulate the release of biomolecules or drugs. Zhao and co-workers reported the use of micelles of poly(ethylene oxide)-block-poly(4,5-dimethoxy-2-nitrobenzyl methacrylate)-coated NaYF4:Yb,Tm nanoparticles.337 Irradiation at 980 nm caused the micelles to dissociate and the loaded hydrophobic species were then released. These light-responsive polymeric systems were considered suitable for potential biomedical applications. The upconversion phototrigger system for bioluminescence imaging was developed by Xing and coworkers. They reported that D-luciferin-conjugated NaYF4:Yb,[email protected]@SiO2 nanocomposites were capable of the controlled phototriggering and uncaging of D-luciferin by NIR light (Figure 15).335 These authors also incorporated a specific photoactive platinum prodrug and an apoptosis-sensing 438

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incubated with these nanocomposites were effectively destroyed. Recently, organic dyes with significant photothermal effects have also been conjugated with lanthanide UCNPs to achieve upconversion imaging and PTT dual modality.505

Murine melanoma B16−F0 cells were transfected with green fluorescent protein (GFP)-plasmid-loaded UCNPs and embedded in a polydimethylsiloxane (PDMS) device. This was subsequently subcutaneously transplanted beneath the skin of a Balb/c mouse. The PDMS device was explanted after 48 h of NIR irradiation. Ex vivo confocal fluorescence microscopy imaging results indicated that the photocaged nucleic acids had been successfully activated in the deep tissues. An animal cancer therapy model of an upconversion phototrigger system was developed by our group.502 A yolk− shell nanostructure containing 50 nm NaYF4:Yb,[email protected] nanoparticle as the core was designed as shown in Scheme 19. The anticancer drug chlorambucil caged in a hydrophobic 7amino-coumarin derivative could be loaded. Under irradiation at 980 nm, NaYF4:Yb,Tm transformed the NIR light to the UV region in order to break the chemical bond of the aminocoumarin site. The uncaged anticancer drug chlorambucil was then released from the yolk−shell nanostructure. This nanosystem was used to deliver the drug to the tumor site, and was subsequently released by NIR light stimulation. More importantly, the NIR-triggered release of chlorambucil was found to inhibit the growth rate of highly malignant sarcoma 180 (S180) tumors in mice. 7.2.3. Lanthanide UCNPs for Chemotherapies Based on Photoinduced Isomerization. The upconversion emissions of lanthanide UCNPs lead to the trans−cis photoisomerization of azobenzene. Recently, Shi and co-workers described the upconversion-induced release of the anticancer drug DOX.503 NaYF4:Yb,[email protected]@mSiO2 nanocomposites were synthesized as drug vectors whose silica pore walls were modified with azobenzene. The cell-penetrating TAT peptide was attached to the outer surface to enhance cellular uptake. Reproducible photoisomerization of the grafted azobenzene was regulated using UV and visible upconversion emissions of the UCNPs. The azobenzene molecules provide a driving force for propelling the drug release from the silica host. Cell toxicity assays revealed that both the working power and the irradiation time of the NIR laser could be adjusted to efficiently regulate the dose of the anticancer drug.

8. BIOSAFETY OF UCNPS The increasing use of upconversion emissive materials has raised concerns about their potential risks to human health. The toxicity evaluation of lanthanide-based and TTA-based UCNPs both in vitro and in vivo is discussed in this section. 8.1. In Vitro Biosafety Assessment of Lanthanide UCNPs

8.1.1. MTT (or MTS) Analysis. Numerous researchers have reported on the in vitro cytotoxicity of lanthanide UCNPs by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide), MTS (3-(4,5-dimethylthiazol-2-yl)-5-(3-carbo-xymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, sodium salt), and CCK-8 mitochondrial metabolic activity assays. Data concerning the cytotoxicity of lanthanide UCNPs are summarized in Table S2. Various concentrations of lanthanide UCNPs in the range 0.05−20000 μg mL−1 and with different incubation periods ranging from 1 to 336 h were studied. More than 75% of the cells remained viable in the majority of cases. This demonstrates the low cytotoxicity of the lanthanide UCNPs. In the cases listed in Table S2, the viabilities of cells treated with different sized (5−400 nm) lanthanide UCNPs showed no obvious differences. This indicates that particle size is not the key determinant of cytotoxicity. 8.1.2. IC50 Value. Although the IC50 value is another important index of cytotoxicity, only one example, by our group, of IC50 determined for lanthanide UCNPs has been reported. After the incubation with KB cells, for a dopaminemodified NaYF4:Yb,[email protected] nanocomposite with a 20 nm core (NaYF4:Yb,Tm) and a 5 nm shell (FexOy), the IC50 values were approximately 295 and 190 μg mL−1 at 24 and 48 h, respectively,.451 8.1.3. TEM Observations. TEM characterization can also be used for organ cell analysis in vitro. Our group determined the accurate distribution site of NaLuF4:Yb,Tm nanoparticles in HeLa cells using TEM imaging with no obvious morphological alteration observed.463 Using TEM it was found that lanthanide UCNPs were first trapped by macrophages in the blood sinus and then moved into the hepatocytes.

7.3. Lanthanide UCNPs Combined with Photothermal Therapy (PTT)

PTT employs photoabsorbers to generate heat from light absorption which leads to the thermal ablation of cancer cells. Combining upconversion imaging with PTT creates one effective theranostic method. Due to their surface plasmon resonance absorption, Au or Ag nanoparticles can be used as PTT agents with NIR irradiation. Therefore, incorporating Ag or Au nanoparticles with lanthanide UCNPs offers a suitable approach for PTT. For example, Song and co-workers have demonstrated core−shell NaYF4:Yb,[email protected] nanocomposites for upconversion imaging and therapeutic applications.242 When HepG2 cells and Bcap-37 cells incubated with the NaYF4:Yb,[email protected] nanocomposites (600 μg mL−1) were exposed to 980 nm (1.5 W cm−2) irradiation for 20 min, their viabilities decreased to 4.62% and 5.43%, respectively. In addition, Liu and co-workers synthesized PEG-modified NaYF4:Yb,[email protected]@Au nanocomposites (∼195 nm) for the magnetically targeted PTT of tumor-bearing mice.468 Silica is another choice as an intermediate layer for attaching Au nanoparticles to UCNPs. Chow and co-workers reported the fabrication of NaYF4:Yb,[email protected]@[email protected] nanocomposites also for PTT.504 Human neuroblastoma BE(2)-C cells

8.2. Pharmacokinetics of Lanthanide UCNPs

The pharmacokinetics of materials is used to directly determine the toxic mode of action and thus the duration of toxic effects. To date, the biodistribution and excretion of lanthanide UCNPs has been carefully studied using inductively coupled plasma-atomic emission spectroscopy (ICP-AES), upconversion luminescence imaging, and radioactive analysis techniques. 8.2.1. Using the ICP-AES Technique. For biodistribution studies, it is practical and reliable to measure the concentration of lanthanide ions in the different tissues by ICP-AES. Zhang and co-workers investigated the in vivo biodistribution of SiO2coated271 and PEI-modified430 NaYF4:Yb,Er/Tm nanoparticles in animal models. After the intravenous injection of SiO2coated NaYF4:Yb,Er nanoparticles (∼40 nm) into a Wistar rat at a dose of 10 mg (kg wt)−1, the nanoparticles first accumulated in the lung and heart, and were then gradually excreted in the urine or faeces. Finally they were completely cleared from the body by the seventh day postinjection.271 A similar biodistribution was observed for PEI-modified NaYF4:Yb,Er nanoparticles (∼50 nm) after intravenous injection.430 439

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distributed in the liver and spleen. Therefore, the particle size and surface ligands of the lanthanide UCNPs appear to significantly affect their biodistribution in living animals after intravenous injection. Unfortunately, few data are as yet available about the excretion of lanthanide UCNPs, and those obtained so far demonstrate that these particles are mainly excreted by the hepatobilary pathway, and rarely through the kidneys. In addition, the excretion rate is slow in most cases.

However, in Liu’s study, after injection of PAA-coated NaYF4:Yb,Tm (35 nm) and PEG-modified NaYF4:Yb,Tm (30 nm), the Y3+ levels in the organs, except in the lung, did not decrease significantly over 3 months according to ICP measurements.506 8.2.2. Using in Vivo Upconversion Luminescence Imaging. Compared with ICP-AES measurement, in vivo upconversion luminescence imaging is a more powerful tool for directly visualizing the dynamic biodistribution of lanthanide UCNPs. One example is the long-term tracking of the in vivo biodistribution of PAA-coated NaYF4:Yb,Tm (∼11.5 nm), as developed by our group, using 980 nm-to-800 nm upconversion emission as the detection signal.245 Within 24 h postinjection, uptake by the spleen increased, while uptake by the liver increased rapidly in only 5 min and then decreased. Thereafter, the signals from the liver and spleen decreased further and almost no upconversion emission signals were detected from these organs 14 days after injection. The upconversion emission signals were observed from the intestine and faeces, indicating a hepatobiliary excretion pathway. In addition, Liu and co-workers studied the postinjection biodistribution of PAA-coated NaYF4:Yb,Tm (∼35 nm) and PEG-modified NaYF4:Yb,Tm (∼30 nm), using the upconversion luminescence imaging technique.506 8.2.3. Using Radioactive Analytical Techniques. Compared with qualitative optical imaging, radioactive analytical techniques offer higher sensitivity (subpicomolar), and can be used to quantitatively monitor the biodistribution of nanoparticles both in vivo and in vitro. Utilizing PET imaging in vivo and gamma counter detection, our group studied the biodistribution of three kinds of 18Flabeled lanthanide UCNPs with different surfactants and sizes: citrate-coated NaYF4:Gd,Yb,Er (22 nm),251 polymer F127modified NaYF4:Yb,Tm (∼20 nm),471 and α-CD-modified NaYF4:Yb,Er nanocrystals (18 nm).265 These three UCNPs showed similar distribution characteristics. That is, a rapid accumulation in the liver and spleen after intravenous injection with a further increase in the spleen concomitant with a decrease in accumulation in the liver. Meanwhile, uptake by the heart, lung, kidney, and other organs was very low. Compared to 18F used for PET imaging, the radioactive isotope 153Sm, with its longer half-life (46.3 h), is suitable for long-term monitoring of lanthanide nanoparticles by SPECT imaging. Recently, our group studied the biodistribution and excretion of citrate-modified NaLuF4:Gd,Yb,Tm (∼22 nm) after labeling with 153Sm3+ in a nude mouse using SPECT imaging. After intravenous injection, most radioactive signals were found to originate from the liver and spleen, with very few from the lung and kidney. At 3669 min after injection, the radioactive signals from the liver and spleen showed 34% and 49% losses, respectively. These data were further confirmed by an in vitro test using a gamma counter. The results demonstrated a tendency of gradual excretion of the nanoparticles from the body.472 As mentioned above, the lanthanide UCNPs were found to be mainly distributed in the liver and spleen, with very few in bone and the heart, lung, kidney, and other organs.507 Interestingly, PEG-modified NaYF4:Yb,Tm (∼30 nm),506 NaYF4:Yb,Er,153Sm (trace 153Sm, seed > leaf > stem) on the fifth day was quantitatively measured by means of a Geiger counter. Furthermore, when these cit-UCNP-treated bean sprouts were introduced into the stomachs of mice, the UCNPs were excreted in the faeces without adsorption or retention. Hematoxylin-eosin (HE) stain results showed no detectable toxic effects to the main organs.

9.1. Lanthanide UCNPs as Nanothermometers

Some 4f transitions of lanthanide ions are temperaturesensitive. On the basis of the temperature-dependent upconversion emission intensity, various lanthanide UCNPs have been developed as nanothermometers. For example, Zhang and co-workers demonstrated that ZnO:Er3+ nanocrystals (48, 65, and 80 nm) have a thermal sensitivity of 0.0062 K−1 based on the temperature-dependent upconversion emission intensity.215 Similarly, a temperature probe applicable in liquids and cells has been developed by Capobianco and co-workers.510 The ratio between the upconversion emissions centered at 525

8.5. Biosafety Assessment of TTA-Based UCNPs

Study of the biosafety of TTA-based UCNPs has been limited, with only in vitro data published to date. Using the MTT assay, when HeLa cells were incubated with a concentration of 320 μg mL−1 of PdOEP&DPA-loaded SiO2 nanoparticles, around 5% cell death was detected thus indicating low cytoxicity.129 In 441

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nm (2H11/2 → I15/2) and 545 nm (2S3/2 → 4I15/2) from PEIcoated NaYF4:Yb,Er nanoparticles (∼18 nm) provided an optical means for sensing biological samples by measuring the temperature changes of an individual HeLa cell up to its thermally induced death at 45 °C. Moreover, the 3H4 → 3H6 (800 nm) and 1G4 → 3H6 transitions (480 nm) of Tm3+ ions are also sensitive to temperature changes. In addition, NaYF4:Yb,Er,511,512 CaF2:Yb,Tm,232 Yb2Ti2O7:Mo,Er,423 and Y2O3:Yb,Tm513 nanoparticles have also been reported as nanothermometers. Although the current nanothermometer design based on lanthanide UCNPs have proved to show high sensitivity and stability, they have not yet been applied directly for in vivo temperature sensing.

fabricated from a gas-permeable but proton-impermeable polymer to make it insensitive to the external pH. Wolfbeis and co-workers demonstrated the use of a CO2 probe based on the optical interrogation of a polystyrene film (12 μm) with permeation selectivity for CO 2 and which contained NaYF4:Yb,Er nanoparticles as well as BTB/TBA ion pairs (Scheme 20). 517 The upconversion emissions of the Scheme 20. Cross-Section of the Probe Film and Schematic Representation Explaining the Inner Filter Effect of the Lanthanide UCNPs That Act As Nanolampsa

9.2. Upconversion Detection Based on the Inner Filter Effect

Wolfbeis and co-workers made significant advances in this field of upconversion emission detection based on the inner filter effect by introducing functional groups.29 Some requirements must be met: (i) the probe should interact with the additional analyte through supramolecular recognition or chemical reaction; (ii) the probe or the reaction product should have an intense absorption band that overlaps with the upconversion emission of the lanthanide UCNPs; and (iii) the interaction should cause a significant change (either increase/decrease or shift) in the absorption band of the probe. This in turn should tune the upconversion emission of the lanthanide UCNPs. To date, organic dyes and Au nanoparticles have been developed as probes to achieve upconversion emission detection based on the inner filter effect. 9.2.1. Lanthanide UCNPs as pH Sensors. On the basis of the inner filter effect, Wolfbeis and co-workers first designed a pH-sensitive film based on a polyurethane hydrogel that hosted NaYF4:Yb,Er nanorods (∼950 nm in length and ∼50 nm in width) and contained the pH indicator bromothymol blue (BTB, Chart 16).514 BTB displays a large spectral shift in response to changing pH value, and its absorption band partially overlaps with the green and red upconversion emissions of the nanorods. Due to a pH-dependent inner filter effect, the green and red upconversion emission intensities of the membrane excited at 980 nm were affected by pH in the 6− 10 range and the response occurred within 30 s. Similarly, Qin and co-workers515 developed optical thin films for pH sensing which incorporated NaYF4:Yb,Er nanorods (∼1 μm in length and ∼200 nm in width) and the chromophore ETH-5418 (Chart 16) in hydrophobic polymer matrices. Au nanoparticles represent another kind of ideal absorbant material for use as an inner filter. Yan and co-workers attached cysteine to Au nanoparticles which when combined with 120 nm NaYF4:Yb,Er nanocrystals made it possible to construct a pH-sensitive system.516 A change in pH affects the static electric properties of cysteine, which in turn influences the assembly of the Au nanoparticles. In a solution of pH 3, the Au nanoparticles assemble, causing a red-shift to 619 nm in the extinction peak. This suppresses the red upconversion emission from the Er3+. After increasing the pH to 11, the Au nanoparticles disassemble. The extinction peak reverts to 523 nm, and the red upconversion emission of Er3+ recovers. This process can be repeated over more than three cycles. 9.2.2. Lanthanide UCNPs as CO2 or Ammonia Probes. The sensing strategy based on the inner filter effect has been extended to the sensing of acidic gases such as carbon dioxide (CO2), or bases such as ammonia. Usually, the probe is

a

Their light is screened by the CO2 sensitive film.517.

NaYF4:Yb,Er nanoparticles (40−100 nm) at 542 and 657 nm strongly overlap with the absorption band of the BTB (Chart 16). When the concentration of CO2 was increased, the BTB changed from blue to yellow form, the overlap was reduced, and the upconversion emission of the NaYF4:Yb,Er nanoparticles was found to increase. The detection limit of CO2 was 0.11%. Similarly, Wolfbeis and co-workers developed an ammonia probe based on a polystyrene matrix containing NaYF4:Yb,Er nanoparticles (60−90 nm) and phenol red (Chart 16).518 Dissolved ammonia was found to increase the local pH, thus causing a strong increase in the absorption of the phenol red at 560 nm. This, in turn, caused the green upconversion emission to be screened off. The detection limit was determined as 400 μM. 9.2.3. Lanthanide UCNPs as a Cr6+ Probe. On the basis of a similar inner filter effect, Ren and co-workers519 developed a system containing NaYF4:Yb,Er nanoparticles (∼60 nm) and diphenylcarbazide (DPC, Chart 16) to be used for the determination of traces of Cr6+ in water. The detection principle was based on the complementary overlap of the green upconversion emission band of the NaYF4:Yb,Er nanoparticles with the absorption band of the pink complex (Cr(III)-DPC) generated by the quantitative reaction between DPC and the Cr6+. The detection limit was 2.4 × 10−8 mol L−1. 9.2.4. Lanthanide UCNPs as Probes for Antioxidants. Utilizing the inner filter effect, Dong and co-workers520 fabricated polyoxometalate-modified NaYF4:Yb,[email protected] nanoparticles for sensing antioxidants, such as albumin from bovine serum (BSA), glucose, glutathione (GSH), among others, in aqueous solution. The method exhibit excellent detection limits of 0.02 μM and 0.01 mM for GSH and glucose, respectively. 9.3. Design Strategy for Upconversion LRET Detection

While distinct from the inner filter effect, LRET is still a powerful spectroscopic technique for studying nanoscale interactions, such as conformational and distance changes between molecules and nanosystems.11 The unique upconversion emission properties of lanthanide UCNPs have greatly 442

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Scheme 21. Schematic Illustration of the UC-LRET Process Based on the Alternation of the Spectral Overlap between Donor and Acceptora

a

(a) On addition of the analytes, LRET is suppressed and upconversion emission is recovered; (b) On addition of the analytes, LRET occurs and upconversion emission is suppressed; (c) If the receptor is highly emissive and can be excited by the upconversion emission, then fluorescence is observed. On addition of the analytes, the adduct can not be excited by the upconversion emission, and the fluorescence is turned off.

Scheme 22. Schematic Illustration of the Upconversion LRET Process Based on the Distance Change between Donor and Acceptora

a

(a) On addition of the analytes, the distance between the donor and acceptor (quencher) is enough for LRET to occur. This corresponds to upconversion emission quenching; (b) On addition of the analytes, the distance between the donor and acceptor (quencher) is too far away to turn off the LRET and the upconversion emission is recovered; (c) On addition of the analytes, the distance between the donor and acceptor (fluophore) is enough for LRET and fluorescence of the acceptor is observed.

expedited their application in the UC-LRET detection of DNAs, proteins, metal ions, and others. The strategy of upconversion detection is based on the different energy transfer efficiencies of a luminescent system before and after interaction with the target species. The spectral

overlap and the distance between the donor (UCNPs) and the acceptor are the two major factors that determine the UCLRET efficiency. In one potentially suitable type of system, the absorption spectrum of the energy acceptor shows a significant change (increased or decreased absorbance or wavelength shift) 443

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Scheme 23. Proposed Recognition Mechanism and the Upconversion LRET Process of OA-Ir-9-UCNPs Towards CN−

In addition, based on a highly efficient LRET process and using a self-assembly method based on hydrophobic−hydrophobic interactions, our group combined NaYF4:Yb,Ho nanoparticles (20−40 nm), an amphiphilic polymer (poly(maleic anhydride-alt-1-octadecene)-PEG), and a CN−-responsive chromophoric Ir(III) complex Ir-10 (Chart 16) in one hybrid nanostructure with the view to detecting CN− in pure water.263 This upconversion probe was a capable of a detection limit of 37.6 μM in water. 9.4.2. Lanthanide UCNPs as a NO2− Probe. It is known that NO2− at high concentrations has an adverse effect on various biological processes. Wang and co-workers developed an NO2− probe based on the LRET process between NaYF4:Yb,Er nanoparticles (donor) and ANDBS (as the acceptor, Chart 16) (Scheme 21b).523 ANDBS was generated by a quantitative reaction with NO2− under Griess conditions. The detection limit for NO2− was found to be 0.0046 μg mL−1. 9.4.3. Lanthanide UCNPs as a Cu2+ Probe. Both Cu2+ deficiency or high levels of Cu2+ can lead to various diseases. Hence, the detection of Cu2+ has attracted considerable attention. On the basis of a UC-LRET process, two Cu2+sensitive rhodamine derivatives (Chart 16) have been reported as energy acceptors for NaYF 4 :Yb,Er nanoparticles (donor).524,525 In the presence of Cu2+, the rhodamine derivatives changed from a nonfluorescent to a fluorescent form and the green-to-red upconversion emission ratio decreased (Scheme 21c). These two rhodamine-modified lanthanide UCNPs permits selective and sensitive quantification of Cu2+. 9.4.4. Lanthanide UCNPs as Hg2+ and MeHg+ Probes. Hg2+ is one of the most hazardous and ubiquitous pollutants released through natural events or human activities. On the basis of the UC-LRET process, our group reported a chromophoric Ru(II) complex (N719 in Chart 16)-assembled NaYF4:Yb,Er,Tm nanophosphor (N719-UCNPs) for Hg2+ detection in both solution and for monitoring intracellular Hg2+.526 Upon the addition of Hg2+, a significant blue-shift of the absorption maximum of N719 from 541 to 485 nm occurred. This leads to a decrease in the spectral overlap between the absorption band of N719 and the green

in the presence of the target analyte (Scheme 21). Because various chemodosimeters designed to detect ions and other functional species with significant color change have been developed in recent years,521 the combination of these chemodosimeters with lanthanide UCNPs offers the most convenient route for the construction of upconversion LRET probes. Another system is based on a change in the distance between the donor and acceptor. This one utilizes a linker that is responsive to the target species (Scheme 22). On the basis of these two kinds of approaches, various upconversion LRET probes, with excellent detection limits, have been developed for the detection of different target species. 9.4. Upconversion LRET Detection by Alteration of the Spectral Overlap between Donor and Acceptor

To date, most of the reported UC-LRET probes for the detection of ions and small molecules, such as cyanide anion (CN−), nitrite (NO2−), copper ion (Cu2+), mercury ion (Hg2+), oxygen (O2), and glutathione (GSH), as well as pH determination, rely on the alteration of the spectral overlap between donor and acceptor. 9.4.1. Lanthanide UCNPs as CN− Probe. It is well established that CN− is extremely toxic to mammals. Our group reported the use of an Ir(III) complex (Ir-9)-coated NaYF4:Yb,Er,Tm nanoparticles (∼20 nm) for the detection of CN− based on LRET from the donor UCNPs to the energy acceptor Ir-9 (Scheme 23).522 To improve the reaction rate, hydrophobic OA was used as a coligand to coat the surface of the UCNPs. After the addition of CN−, this anion reacted with an α,β-unsaturated carbonyl moiety, which induced a weak absorption in the visible region. This reduced the spectral overlap between the absorption of Ir-9 and the green upconversion emission. Therefore, LRET was suppressed (Scheme 21a), and the upconversion emission ratio (I540/ I800) of the 540 to 800 nm region was increased. On this basis, the nanoprobe provided a very low detection limit of 0.18 μM CN− in the mixed solvent DMF/H2O (9:1, v/v). Moreover, the ratiometric nanoprobe was shown to be successful for the monitoring of intracellular CN−. 444

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upconversion emission of the UCNPs (Scheme 21a), and results in the recovery of the upconversion emission at 541 nm. The Hg2+ detection limits in water for this nanoprobe was found to be as low as 1.95 ppb. Moreover, the N719-UCNPs system has been shown, using upconversion bioimaging, to be capable of monitoring changes in the distribution of Hg2+ in living cells. This method has the potential to be extended to other similar systems.527 Recently, using hCy7 (Chart 16) with an MeHg+-responsive absorption in the red-to-NIR range as the energy acceptor, our group has applied the upconversion system for MeHg+ detection in vivo.528 With the assistance of an amphiphilic polymer (poly(maleic anhydride-alt-1-octadecene)-PEG), NaYF4:Yb,Er,Tm nanoparticles (∼25 nm) and hCy7 molecules were loaded together. The addition of MeHg+, shifted the absorption peak of Cy7 from 670 to 845 nm, which caused a decrease in the upconversion emission at 800 nm from Tm3+ (Scheme 21b) and an increase in the upconversion emission at 670 nm from Er3+ (Scheme 21a). The signal change at 800 nm was successfully used to monitor the presence of MeHg+ in vivo in the liver of a mouse model exposed to this toxin by upconversion luminescence in vivo imaging. 9.4.5. Lanthanide UCNPs as an Oxygen Probe. O2 is a critical component in many physiological and pathological processes in living cells.497 Wolfbeis and co-workers reported an O2 probe made by incorporating NaYF4:Yb,Tm nanoparticles (80−120 nm) and the O2-sensitive phosphorescent complex Ir-11 (Chart 16) into a thin layer of ethyl cellulose.529 After excitation at 980 nm, the blue upconversion emission of the NaYF4:Yb,Tm nanoparticles were found to excite the Ir-11 (Scheme 21c). Moreover, this upconversion-induced green emission of Ir-11 was, in turn, dynamically and fully reversibly quenched by O2. This upconversion system was capable of detecting O2 at concentrations between 0% and 20%, and its response time was 10−12 s. 9.4.6. Lanthanide UCNPs as a pH Probe. Vinogradov and co-workers530 combined NaYF4:Yb,Er nanoparticles with porphyrin derivatives (P-Glu4, Chart 16) to produce a ratiometric pH nanoprobe. By detecting the gradual change in the red/green upconversion emission ratio, the pH could be successfully monitored. 9.4.7. Lanthanide UCNPs as a GSH Probe. GSH is an antioxidant that prevents damage in important cellular components caused by reactive oxygen species (ROS). Liu and co-workers reported that NaYF4:Yb,Tm nanoparticles (∼30 nm) modified with a MnO2-nanosheet could be used for GSH detection (Scheme 24).531 The MnO2-nanosheet formed on the surface of the lanthanide UCNPs served as an efficient quencher for the upconversion emission (Scheme 21b). When GSH was added to the system, the upconversion emission was recovered as a result of the reduction of MnO2 to Mn2+. The detection limit with this system was found to be 0.9 μM and its potential was further demonstrated by monitoring intracellular GSH using upconversion microscopy.

Scheme 24. Schematic Illustration for the Design Strategy of NaYF4:Yb,Tm Nanoparticles Modified with a MnO2Nanosheet for GSH Detection531

(donor) and acceptor, strong interaction between the acceptor and the surface ligand of the lanthanide UCNPs should be induced on addition of the analyte. DNA−DNA, antigen− antibody, and ligand−acceptor interactions have all been successfully utilized for sensing molecules such as DNA, proteins, ATP, glucose, among others. 9.5.1. Lanthanide UCNPs for DNA/RNA Detection. 9.5.1.1. Sandwich-Type Hybridization Format. A typical design strategy for DNA detection is based on a sandwichtype hybridization format involving two DNA-labeled conjugation derivatives and the complementary target DNA. Two DNA-labeled conjugation derivatives representing the two DNAs are labeled with lanthanide UCNPs and the energy acceptor, respectively. On the basis of this sandwich-type hybridization format incorporating two short oligonucleotides with designed sequences and a longer target nucleotide, both Zhang’s group532 and our group266 have reported two UCLRET systems to be used as DNA probes. In these two systems, the same fluorophore N,N,N′,N′-tetramethyl-6carboxyrhodamine (TAMRA) was chosen as the energy acceptor. However, different linkers, either an SiO2 layer or strepavidin/biotin, were adopted to conjugate one short DNA strand with the NaYF4:Yb,Er nanoparticles. Scheme 25 shows a schematic representation of this UCNP sandwich-type hybridization format for DNA detection using a strepavidin/biotin linker. The NaYF4:Yb,Er nanoparticle and TAMRA were selected on the basis of the emission wavelength of the Scheme 25. Schematic Illustration of Lanthanide UCNPs for the UC-LRET Detection of DNA266

9.5. UC-LRET Detection by Alteration of the Distance between Donor and Acceptor

Using this strategy of UC-LRET sensing (Scheme 22), it is necessary that the energy acceptor has a large absorption crosssection. To date, organic dyes, metal nanoparticles, and carbon nanomaterials have all been successfully used as energy acceptors for quenching the upconversion emission. Moreover, to achieve the tunable distance between the lanthanide UCNPs 445

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9.5.2.1. Organic Dyes as Energy Acceptors. Kuningas and co-workers reported a competitive homogeneous immunoassay for 17β-estradiol in serum. In it La2O2S:Yb,Er (210−350 nm) was coated with a 17β-estradiol-specific recombinant antibody Fab fragment and used utilized as a donor. The 17β-estradiolconjugated Oyster-556 dye served as the acceptor.539 Using upconversion emission at 600 nm as the detection signal, detection limits of 0.4 and 0.9 nM were observed in buffer and serum, respectively. Equilibrium in the assay was reached within 30 min. Similarly, Ukonaho and co-workers540 demonstrated that Y2O2S:Yb,Er particles (210−350 nm) could be used in two-site immunoassays for free prostate-specific antigens (PSAs). A detection limit of 0.53 ng L−1 was achieved. 9.5.2.2. Au Nanoparticles as Energy Acceptors. In contrast to using organic dyes as energy acceptors, Xu and co-workers designed a UC-LRET immunoassay for the goat antihuman immunoglobulin G (IgG) antibody using Au nanoparticles as the energy acceptors (Scheme 26).541 The upconversion

NaYF4:Yb,Er nanoparticle (as energy donor) overlapping with the absorption of TAMRA (the energy acceptor). In the presence of the target DNA, TAMRA was brought close to the NaYF4:Yb,Er nanoparticle, and energy transfer could take place from the green upconversion emission of the NaYF4:Yb,Er nanoparticle to TAMRA. This leads to light emission from TAMRA at 575 nm and the detection limits reach the nmol L−1 level. In addition, Zhang and co-workers533 used a similar probe system for the detection of point mutation of the gene associated with sickle cell disease. A methicillin-resistant Staphylococcus aureus (MRSA) DNA sequence probe was also reported by Zhang and co-workers534 They utilized citratecapped NaYF4:Yb,Er nanoparticles as the energy donor to covalently conjugate with the captured oligonucleotides. A very thin layer of citrate capping allowed close contact between TAMRA and the NaYF4:Yb,Er nanoparticles with an efficient energy transfer process as the result. The detection limits were found to reach 0.18 nM. Recently, Liu and co-workers535,536 reported a lanthanide UCNPs-based sensor for DNA and thrombin. It is based on altering the distance between the donor and the acceptor. 9.5.1.2. Dual-Channel Sandwich Hybridization Assays. Distinct from the above-mentioned single-channel upconversion detection, Rantanen and co-workers constructed a dualparameter, homogeneous sandwich hybridization assay.537 They utilized 2.3−6.0 μm upconversion particles (UCPs) labeled with a capture-oligonucleotide as the donor. Alexa Fluor 546 or Alexa Fluor 700-labeled oligonucleotides were used as acceptors. The emissions of the acceptors excited by energy transfer were measured at 600 and 740 nm with no significant mutual interference. The upconversion emission signal was directly proportional to the amount of the analytes (Beta-actin or HLA-B27) and the single-stranded target-oligonucleotide sequences were detected in a concentration range of 0.03 to 0.4 pmol. 9.5.1.3. Multiplexed DNA Assay. Multicolor labeling provides an opportunity to either confirm one target with more information or to detect multiple targets simultaneously. Zhang and co-workers reported the multiplexed biodetection of nucleic acid with upconversion microbarcodes.333 A model DNA hybridization system containing oligoprobes and triplecolor encoded beads (NaYF4:Yb,Tm, NaYF4:Yb,Ho, and NaYF4:Yb,Ho,Tm nanocrystal-tagged beads) was used for these multiplexed assays. 9.5.1.4. Intercalating Energy Acceptors into DoubleStranded dsDNA. In contrast to the above-mentioned DNAlabeling method by the energy acceptor, Zhang and co-workers demonstrated another strategy, for DNA detection. Hence the strategy is to intercalate the energy acceptor into doublestranded DNA.538 This upconversion probe was composed of the capture-DNA-labeled NaYF4:Yb,Tm nanoparticle as the donor and an intercalating dye (SYBR Green I) as the acceptor. When the target-DNA was present, hybridization took place between the target DNA and the capture-DNA; the dye was then intercalated by the double-stranded DNA, giving rise to significantly stonger emission. The detection limit of this probe was calculated to be 20 fmol. 9.5.2. Lanthanide UCNPs for Immunoassay. Immunoassays are highly sensitive and specific. The formation of antibody−antigen complexes is measured through an indicator reaction, which results in amplification of the measured product and enhancement of the detection sensitivity.

Scheme 26. Schematic Illustration for the UC-LRET Immunoassay between a NaYF4:Yb,Er Nanoparticle (Donor) and a Au Nanoparticle (Acceptor)541

emission band at 542 nm of human IgG-conjugated NaYF4:Yb,Er nanoparticles (∼46.8 nm) partially overlapped with the absorption band (λabs = 530 nm) of rabbit antigoat IgG-conjugated Au nanoparticles. When goat antihuman IgG was added, an LRET system was formed and the upconversion emission was quenched (Scheme 22a). The detection limit was quite low and of the order 0.88 μg mL−1. 9.5.3. Lanthanide UCNPs as Luminescent Probes Based on Ligand−Acceptor Interaction. On the basis of strong ligand−acceptor interactions, lanthanide UCNPs coupled with metallic nanoparticles or organic fluorophores as energy acceptors can also be used as luminescent probes for avidin, glucose, and lectin. Due to their high affinity, an avidin−biotin system has been utilized to construct a upconversion LRET system for avidin detection.259,542 For example, Li and co-workers developed a highly sensitive upconversion probe for the detection of avidin using biotin-functionalized Na(Y1.5Na0.5)F6:Yb,Er (50 nm) and biotin-functionalized Au nanoparticles (acceptor).259 After addition of the avidin, Au-biotin nanoparticles were conjugated to the biotin-functionalized UCNPs through the specific avidin−biotin interactions. This led to LRET from the upconversion emission to the absorption at 520 nm of Au nanoparticles (Scheme 22a). The upconversion emission intensity at 540 nm was gradually quenched. 446

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Figure 16. Digital upconversion emission photographs of EO-EPI thin films doped with (a) PdOEP&DPA upon 544 nm excitation and (b) PdPc(OBu)8&rubrene upon 725 nm excitation, (c) Tecoflex EG-80A polymer bars doped with PdOEP&DPA upon 544 nm excitation, (d) PtTBPB&2CBPEA upon 635 nm excitation and (e) PtTBPB&BDP-2 upon 635 nm excitation. upconversion emission images (snapshots) of the running all-organic 2D-displays containing (f) PdTPBP&perylene, (g) PdTPBP&BPEA, and (h) PdTPBP&rubrene. Reprinted with permission from ref 31. Copyright 2010 Elsevier B.V. Reprinted with permission from ref 560. Copyright 2008 IOP Publishing Ltd. and Deutsche Physikalische Gesellschaft.

Based on a similar principle, Zhang and co-workers543 developed an aptasensor which is composed of aptamerfunctionalized NaYF4:Yb,Er (donor) and Au nanoparticles (acceptor) designed for platelet-derived growth factor detection. The detection limit was as low as 10 nM. Liu and co-workers234 reported a glucose probe based on the upconversion LRET between ConA-labeled NaYF4:Yb,Er nanoparticles (∼50 nm, donor) and chitosan-labeled graphene oxide (GO, acceptor). Capobianco and co-workers192 synthesized a lectin probe by employing an LRET process between mannose-coated poly(amidoamine)-NaGdF4:Yb,Er (donor) and fluorescent lectin RITC-ConA (acceptor). The detection limit of glucose was 25 nM. 9.5.4. Lanthanide UCNPs as Enzyme-Activity Assay. Enzymes are vital in almost all cellular processes, such as signaling pathways, metabolism, and gene expression. Pang and co-workers235 reported an upconversion measurement for enzyme activity (thrombin) based on the LRET from NaYF4:Yb,Er nanoparticles (donor, 50 nm) to carbon nanoparticles (acceptor, 40−60 nm). The NaYF4:Yb,Er nanoparticles were covalently tagged with thrombin aptamer, which bound to the surface of the carbon nanoparticles through π−π stacking interactions. Upon the addition of thrombin, a quadruplex structure was formed leading to weakened π−π interactions, and hence the acceptor was separated from the donor, thereby blocking the LRET process (Scheme 22b). The probe showed detection limits of 0.18 nM in an aqueous buffer and 0.25 nM in spiked human serum samples. 9.5.5. Lanthanide UCNPs as an ATP Probe. The function of adenosine triphosphate (ATP) as the cornerstone of metabolic reactions is to store energy within a cell and produce that energy when needed. Li and co-workers reported an ATP probe based on the UC-LRET between single-stranded

DNA (ssDNA)-NaYF4:Yb,Er (donor, 29 nm) and graphene oxide (GO, acceptor).246 When ATP was added, aptamer−ATP complexes were formed with a rigid structural conformation imposed by the aptamer sequences. This weakened the π−π stacking interaction of ssDNA and GO, and the ssDNAUCNPs were separated from the GO surface. In this way, the upconversion emission was enhanced (Scheme 22b). The detection limit for ATP was found to be 80 nM. On the basis of a similar interaction between aptamer and ATP, Zhang and coworkers constructed another LRET-based ATP bioprobe with a detection limit of 20 μM. But in this case they used the organic dye TAMRA as the energy acceptor.146 9.5.6. Lanthanide UCNPs as Hg2+ Probe. Li and coworkers reported an Hg2+ probe based on the LRET between a thymine (T)-rich oligonucleotide (5′-NH2TTCTTCTTTCTTCCCCTTGTTTGTTG-3′)-conjugated NaYF4:Yb,Er (29 nm, donor) and GO (acceptor).246 Upon addition of Hg2+, formation of the dsDNA structure through THg2+-T interactions caused the lanthanide UCNPs to move away from the GO surface (Scheme 22b). This in turn leads to upconversion emission recovery. The detection limit for Hg2+ was 0.5 nM. 9.6. Summary of Upconversion Detection Systems

The detection of target species is realized through a change in emission intensity of one upconversion emission band and/or the intensity ratio of two upconversion emission bands. These changes in upconversion emission can be achieved through two principle routes. The first relies on the intrinsic properties of the lanthanide UCNPs; that is, their upconversion emission properties can be directly tuned through alteration of the chemical or physical surroundings by the presence of the target species. The second route is to introduce a functional additive (such as a chromophore, luminophore, or magnetic particle) as a responsive moiety. Addition of the target species can change 447

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Figure 17. (a) Schematics of the colloid-electrospinning process for embedding upconversion nanocapsules in nanofibers. (b) Photograph of a fiber mat under illumination of a 633 nm laser (light intensity = ∼500 mW cm−2). The green dot in the center is upconversion emission. The notch filter was used in order to suppress the scattered laser light. Reprinted with permission from ref 131. Copyright 2013 American Chemical Society.

based upconversion emission of PtOEP&DPA in a PMMA matrix could be modulated. This was attributed to the effect of the magnetic field on the TTA process in DPA, and it demonstrates that triplet−triplet fusion is a diffusion-limited process in solid films.381 To date, most TTA-based upconversion films have been prepared by dissolving the preformed polymer host in an organic solution containing the TTA-based upconversion couple followed by solvent removal. However, it is difficult to achieve precise doping. Castellano and co-workers developed a facile fabrication strategy for TTA-based upconversion polymers that could be easily molded into a variety of forms under ambient conditions and made with minimal processing using polyurethane precursors (Clear Flex 50, CLRFLX).547 Interestingly, the resulting transparent films containing PdOEP&DPA showed high flexibility and good mechanical properties and exhibited a linear incident power dependence with QEUC exceeding 20% (power density, 0.200 W cm−2). Recently, Weder and co-workers have developed a new method for preparing TTA-based upconversion emissive rigid PMMA films. They used compression-molding of premixed blends (at 240 °C) and subsequently quenched the samples in a molecularly mixed state. By kinetically trapping the dyes through rapid cooling, high concentrations of DPA (up to 25% w/w) and a series of different concentrations (0.005%−0.5% w/w) of PdOEP were seccessfully incorporated into the polymeric materials with minimal phase separation.548 10.1.2. Doping the Sensitizer in an Emissive Polymer Matrix. In the second assembly strategy, the polymer matrix plays the role of annihilator. In 2003, Baluschev and co-workers reported the first example of doping PdOEP into a fluorescent PF2/6 film (Chart 2).549 Under excitation with a 532 nm laser, blue upconversion emission from PF2/6 was detected. Furthermore, the system was extended to other blue-emitting polymers, such as L-5Ph (Chart 2).67 Compared to PF2/6, the QEUC of the L-5Ph system is five times higher, which results from the reduced reabsorption of upconversion emission by the sensitizer (PtOEP). Laquai and co-workers synthesized another blue TTA-based upconversion polyspirobifluorene-anthracene copolymer doped with low concentrations of PtOEP.550 Importantly, the upconversion emission intensity from this

the interaction or distance between the additive and lanthanide UCNPs; thus the upconversion emission properties of the latter can be tuned. It is worth noting that the introduction of an analyte-sensitive chromophore or luminophore to construct the LRET system is the most popular strategy for utilizing lanthanide UCNPs as luminescent probes.

10. UPCONVERSION MATERIALS AS A LIGHTING SOURCE Due to the special photon upconversion process, lanthanide UCNPs and TTA-based upconversion materials have been applied in other fields. By using the upconversion emission visible to the naked eye, such materials can be used as lighting sources for visual lighting, anticounterfeiting, and fingerprint detection. 10.1. Solid-State TTA-Based Upconversion Film for Lighting

To further exploit the upconversion process, solid-state film materials with TTA-based upconversion emission need to be developed. To date, two strategies have been adopted: namely codoping both sensitizer and annihilator into a polymer matrix, or doping the sensitizer into an emitting polymeric annihilator. 10.1.1. Co-doping Both Sensitizer and Annihilator into a Polymer Matrix. To date, polymers such as ethylene oxide/epichlorohydrin copolymer (EO-EPI), polyurethanes, poly(methyl methacrylate) (PMMA), and cellulose acetate have all been successfully used as matrices for achieving TTAbased upconversion emission. Doping both sensitizer and annihilator has been used. In 2007, Castellano and co-workers reported a TTA-based upconversion process in a solid film by codoping PdOEP&DPA into a host polymer.544 After excitation at 544 nm, delayed TTA-based upconversion emission from DPA was observed (Figure 16). Similarly, by codoping PdPc(OBu)8&rubrene into an EO-EPI polymer matrix, yellow upconversion emission was observed under excitation at 730 nm (Figure 16). 83 Dinnocenzo and co-workers demonstrated green-to-blue and blue-to-UV upconversion in rigid PMMA films.545 Monguzzi and co-workers reported red-to-green photon upconversion in a solid polymer cellulose acetate film by doping with PtTPBP&BPEA.546 Interestingly, Mezyk and co-workers discovered that, in the presence of a magnetic field, the TTA448

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treatment. May and co-workers applied thermal polymerization to preparing OA-stabilized β-NaYF4:Yb,Er/PMMA monoliths.558 Lin and co-workers prepared β-NaYF4:Yb,Er/Tm− PMMA bulk nanocomposites by an in situ radical photopolymerization method.559

copolymer system was an order of magnitude higher than that from the anthracene-free system. 10.1.3. TTA-Based Upconversion Luminescence in Nanocrystalline ZrO2 Films. Morandeira and co-workers have reported a TTA-based upconversion system based on nanocrystalline ZrO2 films codoped with PtOEP&DPA under simulated sunlight excitation.551 Time-resolved emission suggests that triplet energy migration is occurring through the Dexter mechanism. The upconversion QEUC was as low as 0.06%, which may be related to the unfavorable adsorption of the DPA annihilator on the ZrO2 surface. Furthermore, a derivative of DPA, methyl 4-(10-p-tolylanthracen-9-yl)benzoate (MTAB, Chart 1) was introduced and chemisorbed onto a mesoporous ZrO2 film. Compared with the DPA-based system, significant enhancement in the absolute magnitude of upconversion emission of the MTAB|ZrO2 sealed cell was achieved.552 10.1.4. TTA-Based Upconversion Luminescence in Nanofibers and Mats. Recently, Turshatov and co-workers reported ultralight red-to-green upconverting mats consisting of rigid polymer nanofibers with average diameters between 250 and 500 nm.131 The nanofibers were prepared by simultaneously electrospinning an aqueous solution of a PVA polymer (with excellent oxygen-barrier properties) and upconversion nanocapsules containing the PdTPB&BDP-2 couple, as shown in Figure 17. The pronounced oxygen-barrier properties of the PVA matrix prevented the penetration of oxygen very efficiently. As a result, bright-green upconversion emission which was detectable by the naked eye was observed for a macroscale mat, even under an ambient atmosphere (Figure 17). The mat displayed a very weak photobleaching of the upconverting dye under excitation below ∼136 mW cm−2. This successful fabrication of upconverting mats is expected to promote the rapid development of TTA-based upconversion materials for practical applications in the future.

10.3. TTA-Based Upconversion Materials for Color-Display Devices

Baluschev and co-workers have demonstrated the fabrication of all-organic, transparent, flexible TTA-based upconversion color displays by dispersing both sensitizer and annihilators into a transparent polymeric matrix.560 Blue, green, and orange displays were generated by using the TTA-based upconversion couples of PdTPBP&perylene (λ e m = 475 nm), PdTPBP&BPEA (λem = 513 nm), and PdTPBP&rubrene (λem = 560 nm) with only 633 nm excitation light (100 h. 10.4. Lanthanide UCNPs for Anticounterfeiting Applications

Through invisible and transparent patterning and detectable NIR-to-NIR or NIR-to-vis upconversion emission, lanthanide UCNPs have great potential for applications in security technology, e.g., forgery, tampering, or counterfeiting prevention. Prasad and co-workers developed photopatterned security marks using t-butyloxycarbonyl (t-BOC)-coated NaYF4:Yb,Er or NaYF4:Yb,Tm nanoparticles (∼23 nm).561 This allowed the lanthanide UCNPs to be dispersed in chloroform and then cured with UV radiation after the printing process. 10.5. Lanthanide UCNPs for Fingermark Detection

Fingerprint detection techniques can provide greater contrast between the developed fingerprint and the background. Latent fingermarks on nonporous and semiporous surfaces, including glass, plastic, aluminum, soft drink labels, and Australian polymer banknotes, have been developed with dry and wet lanthanide UCNPs powders and cyanoacrylate staining techniques.562−564 For example, Roux and co-workers reported fingermark detection using NaYF4:Yb,Er nanoparticles with size distributions ranging from ∼0.2 to 2 μm.563 A fingermark upconversion detection process suppressed strong background interference and showed higher sensitivity and selectivity than that of traditional luminescence techniques. Subsequently, these authors developed another fingermarker detector consisting of YVO4:Yb,Er nanoparticles. These revealed fingermark ridges on different substrates.564 Recently, Yuan and co-workers reported NaYF4:Yb,Er nanoparticles (260 nm) functionalized with a lysozyme-binding aptamer. These were used to target and detect fingerprints by recognizing lysozyme in the fingerprint ridges.565 These results illustrate the potential of lanthanide UCNPs for the detection of latent fingermarks.

10.2. Lanthanide UCNPs for Lighting

There have been some examples of the incorporation of lanthanide UCNPs into matrices, such as transparent glass ceramics, thin films, polymers, and resins, in order to facilitate their use on a bulk scale. Wang and co-workers successfully obtained a series of transparent glass ceramics with embedded lanthanide UCNPs, such as NaYF4:Yb,Er (∼21 nm)553 and YF3:Yb,Er,Tm (∼20 nm).554 These glass ceramics had high transparency and their high chemical and mechanical stabilities were derived from the silica-based glass matrix. Teshima and co-workers reported a flux coating method for the coating of NaYF4:Ln (Ln = Yb, Er, Tm) nanocrystals onto glass substrates at 350 °C. This provided a convenient route for directly constructing upconversion layers in the functional devices.555 Inorganic thin films containing lanthanide UCNPs are often produced by sol−gel techniques. For example, van Veggel and co-workers reported the use of a sol−gel method to produce an SiO2 thin film incorporating LaF3:Yb,Er/Ho/Tm nanoparticles.556 They further demonstrated the generation of white light from SiO2/ZrO2 sol−gel thin films incorporating combinations of LaF3:Yb,Er/Ho/Tm nanoparticles.557 The ability to disperse lanthanide UCNPs in transparent polymer matrices, such as PMMA or polydimethylsiloxane (PDMS), has led to the development of new optical displays. Several groups have reported the synthesis of transparent lanthanide UCNPs-PMMA nanocomposites by an in situ polymerization route without the need for postdeposition heat

10.6. Lanthanide UCNPs s for 3D-Displays

The ability to integrate transparent upconversion emissive polymer nanocomposites can facilitate the manufacture of 3D upconversion displays. Liu and co-workers showed that lanthanide UCNPs could be successfully used in 3D displays.323 The image was created with an NaYF4:Yb,Er,Gd (∼40 nm)PDMS composite and a computer-directed 980 nm laser. Compared to conventional 3D displays which provide different images to the viewer’s left and right eyes, this method can 449

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Scheme 27. Structure of the Integrated Devicea

a

Low-energy photons pass through the active layer of the device and subsequently lead to upconversion emission in the TTA-based upconversion layer. Upconverted photons that are absorbed by the active layer provide extra current to the device.

transform NIR light to the green region avoids the technical challenges associated with the use of NIR quantum dots-based detectors. 11.1.2. TTA-Based Upconversion Materials for Photocurrent Generation. Upconversion-powered photocurrent generation based on the TTA process has been achieved.569 Castellano and co-workers utilized PdOEP as a sensitizer and DPA as an annihilator for producing an upconversion-powered photocurrent.570 Nanostructured WO3 photoanodes (Eg = 2.7 eV) in aqueous solution were sensitized using a tandem benchmark composition of PdOEP and DPA dissolved in degassed toluene and contained in an adjacent optical cell. The generated photocurrent or dark current was found to be dependent on whether or not the optical cell was degassed or aerated. The long-wavelength (>500 nm) sensitization of the WO3 was closely related to the Q-band absorption of PdOEP. 11.1.3. Lanthanide UCNPs for Solar Cells. 11.1.3.1. Silicon Solar Cells. Gibart and co-workers introduced a Yb3+,Er3+ codoped vitro-ceramic upconversion layer on the back surface of a GaAs solar cell.571 Under excitation from a Ti-sapphire NIR laser with a power of 1 W at 891 nm, the efficiency was measured as 2.5%. Shalav and co-workers reported the use of NaYF4:Er3+ as an upconversion material to extend the absorption of a silicon solar cell.572 The external efficiency of the solar cell reached 2.5%, under illumination with a 1523 nm laser with a power of 5.1 mW. When silver was coated on the back of the upconversion materials to reflect all photons back to the solar cell layer, the external efficiency was enhanced to 3.4%.573 de Wild and co-workers used β-NaYF4:Yb,Er powders in a thin-film hydrogenated amorphous silicon (a-Si:H) solar cell.574 Because the band gap of a-Si:H is as high as 1.7 eV (∼730 nm for photons), the absorption of photons in the upconversion layer will have less of a reducing effect on the efficiency of the solar cell. The external efficiency was measured as 0.03% under illumination with a 980 nm laser (28 mW). The same group further explored different upconversion materials further and by using Gd2O2S:Yb,Er instead of NaYF4:Yb,Er developed an amorphous silicon (a-Si) solar cell with an efficiency of up to 0.50%. 11.1.3.2. Dye-Sensitized Solar Cells (DSSC). The use of upconversion nanocomposites containing LaF3:Yb,Er in a dyesensitized solar cell was reported by Demopoulos and coworkers.575 Approximately 2.4% of an increase in the

obtain real 3D imaging by precisely controlling the focused spots of the excitation lasers. The relatively modest cost of CW infrared lasers makes the large-scale use of these displays feasible. However, additional work is necessary to find lanthanide UCNPs with the required low excitation powers. This is in order to reduce the NIR laser power necessary to achieve a stable color output.

11. UPCONVERSION MATERIALS AS A SECOND EXCITATION SOURCE Using upconverted photons with higher energy as a second excitation source, upconversion emissive materials have been developed for use in electronic devices, photocurrent generation, solar cells, data storage, photodeformable films, and reaction catalysts. 11.1. Upconversion Materials for Photocurrent Generation

The energy from sunlight cannot be fully utilized by solar cells because of spectral mismatch. This is especially true for lower energy NIR photons. The introduction of upconversion materials allows the conversion of lower energy (sub-band gap) photons to higher energy ones, which can then be absorbed by the solar cell resulting in improved efficiency. To date, however, there have only been a few reports of proof-ofconcept experiments. This is because of the relatively low upconversion efficiency. de Wild and co-workers reviewed theoretical and practical examples of the application of upconversion materials in solar cells in 2011.566 11.1.1. Lanthanide UCNPs for Photocurrent Generation. The efficient upconversion of photons with sub-band gap energies to create a hole−electron pair in a semiconductor would allow the fundamental limitation of single-junction photovoltaic devices (which utilize only light of a specific wavelength) to be overcome. This is because in this case NIR light can be used. Perepichka and co-workers demonstrated an example of a photoelectronic device composed of CdSe QDs attached to ∼22 nm NaYF4:Yb,Er nanoparticles (CSNY).567 On illumination with a 0.86 W 980 nm laser, a photocurrent as high as 5 nA was measured resulting in a significant on/off ratio of up to 360. The switching between the two states was reversible over a number of cycles. These results imply that CSNY could constitute a promising reversible and stable NIR photoconductivity switch. Chow and co-workers also fabricated an NaYF4:Yb,Er-based model device as a solution-processed NIR detector.568 The unique ability of NaYF4:Yb,Er to 450

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fulvalene diruthenium (FvRu2) derivative (Scheme 28). This solar fuel using noncoherent truncated white light was achieved

photocurrent density was observed. Other parameters, such as the overall efficiency, open-circuit voltage, and fill factor, remained the same. The main challenge in this work lies in overcoming the spectral mismatch between the upconversion emission and the absorption of the dye (N719). In addition, Chen and co-workers designed an upconversion-based solar cell by introducing a film of NaYF4:Yb,Er into a conventional dyesensitized solar cell.576 The maximum output power was 0.47 mW under irradiation with a 1 W 980 nm laser. 11.1.4. TTA-Based Upconversion Materials for Solar Cells. Great advances in TTA-based upconversion materials have been achieved by Schmidt and co-workers in enhancing the performance of solar cells including a-Si and DSSC.577−581 The TTA-based upconversion system was based on PQ4PdNA as the sensitizer and rubrene as the annihilator. One strategy is incorporating TTA-based upconversion system into DSSC in order to fabricate an integrated device (Scheme 27). A benzene solution-based upconversion emissive cell with an optimal thickness of ∼120 μm was contained within an encapsulated chamber. The dye D149 (see Scheme 27) was adopted as the photosensitizer for TiO2 nanocrystals to match the yellow upconversion emission from rubrene as the second excitation source. It is important to note that the relative enhancement of the charge carrier conversion efficiency (IPCE) vs the incident photon wavelength was perfectly matched with the absorption spectrum of PQ4PdNA, thus clearly indicating the source of the enhanced performance to be the TTA-based upconversion process. The relative enhancement of the IPCE was calculated to be about 13%.

Scheme 28. Fulvalene Diruthenium Derivative (R = 1,1dimethyltridecyl) Used As a Recyclable Solar Fuel

based on a microfluidic device having a large illuminated area (∼400 mm2). Importantly, no fouling in the microfluidic channels, even over 50 h of running time, was observed. One limiting factor was the low quantum yield (0.2%) of photochemical conversion for the FvRu2 system used.584 11.4. Upconversion Materials for Photoisomerization

The upconversion emissions of lanthanide UCNPs have been used to induce open/closed-ring photoisomerization of photochromic diarylethenes (DTEs) and the trans−cis photoisomerization of azobenzene. Therefore, combining lanthanide UCNPs with diarylethene or azobenzene moieties represents an attractive strategy for fabricating the next generation of data memory devices or optical switches. 11.4.1. Lanthanide UCNPs for Photoisomerization of Diarylethenes. In 2008, our group explored in this area by employing a diarylethene derivative DTE-1 (Chart 17) in a PMMA film loaded with LaF3:Yb,Ho nanoparticles.585 The green upconversion emission of the lanthanide UCNPs overlaps with the absorption band of the photostationary state of DTE-1 in the closed-ring form and an intermolecular energy transfer process occurs to quench this emission. This quenching route is blocked when the DTE-1 is converted into the open-ring form following UV irradiation. The upconversion emission signals can be read by confocal imaging. Based on this principle, a unique route for highly efficient nondestructive optical memory can be developed in the DTE/UCNPs hybrid system. Yan and co-workers reported the first microdevice incorporating “erasing”, “rewriting”, and “reading” operations based on orderly assembled nanopatterns of monodispersed NaYF4:Yb,Er nanoparticles (∼20 nm) and DTE-2 (Chart 17). DTE-2, with its reversible photoisomerization properties, allows the data bits to be rewritable. UCNPs with nondestructive NIR excitation were employed as a very sensitive readout window.586 Photoswitchable nanocomposites can also be applied in photoswitchable upconversion bioimaging. Branda and coworkers attached another diarylethene derivative, DTE-3 (Chart 17), to the surfaces of NaYF4:Yb,Er nanoparticles (∼25 nm) by means of a copper-catalyzed cycloaddition “click” reaction.587 Using UV and visible light, they demonstrated photoswitching images by means of two-photon fluorescence microscopy of immobilized nematodes, C. elegans N2 hermaphrodites, after incubation for 3 h with DTE-3-modified UCNPs dispersed in buffer. One of the major drawbacks of the systems mentioned above is the need to use UV and visible light to induce the ringclosing and opening reactions, respectively. This clearly will restrict their application. Branda and co-workers illustrated the concept of “remote control” by using two types of lanthanide UCNPs (∼25.3 nm NaYF4:Yb,Tm, and 25.6 nm NaYF4:Yb,Er)209 or core−shell−shell NaYF4-based UCNPs (35.7

11.2. Upconversion Materials for Photocatalysis

Photon upconversion has also been applied to photocatalysis, using lanthanide UCNPs and TTA-based upconversion materials as photoconverters to extend the working wavelength of the catalyst from the UV or visible to the NIR range. 11.2.1. Lanthanide UCNPs for Photocatalysis. Yan and co-workers demonstrated that the NIR-induced photocatalytically driven degradation of rhodamine B can be achieved with the assistance of ordered mesoporous TiO2 codoped with Yb3+ and Tm3+.582 In contrast, no photodegradation of rhodamine B was observed for mesoporous SiO2:Yb,Tm nanoparticles under the same conditions. In this work, the aim of achieving NIR photocatalysis by the upconversion process was successfully realized. Recently, Lu and co-workers synthesized double-shellstructured β-NaYF4:Yb3+,Tm3+/[email protected]@TiO2 upconversion photocatalysts for the degradation of rhodamine B under 980 nm laser irradiation. In these systems, the role of TiO2 is to absorb both UV and visible light in order to make sufficient use of the upconverted light from Yb3+,Tm3+/Er3+ for photocatalysis.583 11.2.2. TTA-Based Upconversion for Photocatalysis. Kim and co-workers demonstrated the TTA-based upconversion-induced •OH production by photons with energy lower than the band gap of a semiconductor photocatalyst.136 Upconversion capsules were dispersed in an aqueous solution containing a Pt/WO3 photocatalyst with a band gap energy of 2.8 eV. After excitation at 532 nm (2.3 eV), •OH generation was observed, and the concentration of •OH increased as the incident light power was increased. 11.3. Upconversion Materials for Solar Fuels

Moth-Poulsen and co-workers demonstrated that the TTAbased upconversion from the PdOEP&DPA couple could be used to facilitate a solar energy harvesting reaction of a 451

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Chart 17. Molecular Structure of Diarylethene Derivative and the Interconverting Reversible Photoreactions

model, in which NIR excitation is used as the tool for a photoswitchable device combining diarylethene and lanthanide UCNPs. In addition, these authors also wrapped NaYF4:Yb,Tm nanoparticles in a UV-blocking PEG shell. In this way, the multiphoton process can be selectively employed and any direct activation by UV light is minimized. Such nanosystems should offer an enhanced level of control over photochemistry in many applications.588 11.4.2. Upconversion Materials for the Photoisomerization of Azobenzene. The upconversion emission of lanthanide UCNPs may be used for the photoisomerization of azobenzene. Cross-linked liquid-crystal polymers (CLCPs) exhibit unique properties, such as elasticity, anisotropy, stimulus-responsiveness, molecular-cooperation effects, and large thermally induced contractions. Yu and our group incorporated NaYF4:Yb,Tm nanoparticles (∼70 nm) into a film of CLCPs containing azotolane (Figure 19).589 The main upconversion emission peaks of the nanoparticles at 450 and 475 nm overlapped with the absorption band of the azotolane CLCP film, so that the upconversion emissions of the nanoparticles led to the trans−cis photoisomerization of the azotolane units and an alignment change of the mesogens. After exposure to a 980 nm laser, the resulting composite film was immediately deformed. The deformed film completely reverted to the initial flat state after removal of the NIR light. This kind of novel photodeformable CLCP system based on lanthanide UCNPs has potential as an artificial muscle-like actuator or as an optical switch. In order to decrease the operation laser power density to achieve more effective control of the photoisomerization process, we applied TTA-based upconversion in a CLCP film in order to trigger the photoisomeriza-

nm) and NaYF4:Yb,[email protected]:Yb,[email protected] (38.8 nm) and NaYF4:Yb,[email protected]:Yb,[email protected] (38.8 nm) samples, respectively.50 They used DTE-4 (Chart 17) as the photoswitch (Figure 18). The ring-closing and ring-opening reactions were promoted by the UV upconversion emission from NaYF4:Yb,Tm and the visible upconversion emission from NaYF4:Yb,Er, respectively. This design represents a new

Figure 18. Bidirectional photoswitching of a THF solution of DTE dispersed with NaYF4:Yb,[email protected]:Yb,[email protected] nanoparticles by varying only the intensity of the NIR light. Reprinted with permission from ref 50. Copyright 2010 American Chemical Society. 452

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remains low. Meanwhile, little or no data concerning the absolute quantum efficiencies of these modified lanthanide UCNPs have been reported. Hence the further improvement of the upconversion efficiency of lanthanide UCNPs is needed. Among the methods already developed, the surface plasmon resonance-enhanced effect has definite potential for improving the upconversion efficiency. However, where the synthetic procedures are concerned, it is difficult to accurately control the thickness and degree of coating to obtain the required nanocomposites. In light of the relatively simple and easy to control growth of a shell layer on lanthanide UCNPs, assembly of core−shell nanostructures is currently the most popular strategy for improving the overall upconversion efficiency of lanthanide UCNPs. 12.1.2. Optimization of the Photostability of TTABased Upconversion Materials. Organic species tend to have low photostabilities. The conjugated planar structures of aromatic hydrocarbon annihilators makes them susceptible to oxidation. Hence, there is a need to develop new annihilators that are resistant to photo-oxidation. 12.1.3. Development of NIR Emissive TTA-Based Upconversion Materials. Another issue that needs to be addressed concerns the excitation and emission wavelengths. NIR excitation and emission are the key determinants of the optical imaging depth. An appropriate upconversion system working with both excitation and emission in the optical window 650−1100 nm needs to be designed. This will minimize absorption in tissues in biological applications. In addition, with a view to obtaining improved upconversion efficiency and lower toxicity, further exploration of new sensitizers and annihilators is necessary. 12.1.4. Decreasing the Aggregation Quenching in Water-Dispersible TTA-Based UCNPs. Annihilators are used at high concentrations in TTA-based upconversion systems and this makes them prone to aggregation and leads to upconversion emission quenching. Such aggregation-induced upconversion quenching becomes serious when the sensitizer and annihilator are integrated into one hydrophilic nanoparticle. There are two strategies for solving this problem: one is the development of new methods to integrate the sensitizer and annihilator, whereas the second is the design of new annihilators with a bulk steric hindrance effect in order to decrease aggregation-induced quenching. 12.1.5. Nanotoxicity of Lanthanide UCNPs. Nanotoxicology studies and safety assessment are absolutely essential for clinical application. Such nanotoxicology studies include nanoparticle distribution and transformation in the body, excretion and any induced changes of the organs or tissues both at macroscopic and microscopic levels. One important problem is the aggregation of lanthanide UCNPs in biological media. The interaction of lanthanide UCNPs with proteins in the blood is still unclear. Of particular importance is the study of what constitutes a safe dose. Nanotoxicity studies and safety assessments of lanthanide UCNPs with different components of tunable sizes and various surface coating species are essential if their full potential is to be realized. In particular, the development of smaller biocompatible lanthanide UCNPs needed to overcome nonspecific organ uptake and monocytemacrophage system scavenging still remains an open challenge for improving blood circulation and targeting efficiency.

Figure 19. Photographs of the azotolane CLCP/NaYF4:Yb,Tm composite film bending toward the light source along the alignment direction of themesogens, remaining bent in response to the CW NIR irradiation at 980 nm (power density = 15 W/cm2), and becoming flat again after the light source was removed. The size of the composite film was 8 mm × 2 mm × 20 μm. Reprinted with permission from ref 589. Copyright 2011 American Chemical Society.

tion.590 PtTPBP and 9,10-di(bisphenylphosphoryl)anthracene (BDPPA) were doped into the film as sensitizer and annihilator, respectively. The upconversion efficiency reached 9.3%, allowing reduction of the operation laser to a relatively low power density of 200 mW cm−2 at 635 nm.

12. SUMMARY AND PROSPECTS This review has covered recent developments concerning lanthanide UCNPs and TTA-based upconversion materials. For lanthanide UCNPs, significant advances have been made both in their controllable synthesis and surface engineering. Currently tuning of their size, phase, composition, and surface chemistry, as well as their upconversion emission colors and efficiency are all possible. The current applications for these lanthanide UCNPs which include bioimaging, sensors, drugs, displays, solar cells, photocatalysis, photoisomerization, among others, have been summarized. For TTA-based upconversion materials, current research is focused on identifying pairs of compounds that are effective sensitizers and acceptors. The mechanism and decay dynamics of the upconversion process, the accomplishment of TTA-based upconversion in aqueous media, and applications in lighting, photocatalysis, and bioimaging are all currently the focus of research. Based on the advantages and the current limitations of upconversion emissive materials, several future research directions aimed at specific applications can be summarized: 12.1. Future Directions in the Optimization of Upconversion Materials

12.1.1. Optimization of the Upconversion Efficiency of Lanthanide UCNPs. The main factors responsible for the low upconversion efficiencies of lanthanide UCNPs are first the low absorption cross sections of lanthanide ions and second the energy losses through nonradiative transitions. Due to nonradiative decay from defects on their surfaces, the absolute QYUC values of the lanthanide UCNPs of sizes 8−100 nm have been measured as less than 0.6%. These are much lower than those of the corresponding bulk materials. Although numerous methods, such as the assembly of core−shell structures, annealing processes, surface modification, surface plasmon resonance-enhanced effects, and so on, have all been used to overcome this drawback, the overall upconversion efficiency 453

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12.2. Future Directions in the Application of Upconversion Materials

lanthanide-based ones, thereby achieving higher efficiency. However, the fabrication techniques of the relevant materials need to be developed in order to convert the current molecular systems into solid-state materials. This is expected to be the main future focus of investigation for applications of TTAbased upconversion materials.

12.2.1. Lanthanide UCNPs for Therapy Applications. Lanthanide UCNPs have been utilized for therapy applications, such as PTT, chemotherapy, PDT, and the photocontrolled release of drugs. However, in the reported PTT and chemotherapy applications, lanthanide UCNPs have served mainly as luminescent indicators to reveal the position of the drug carrier. In some cases they were used to indicate the amount of drug left in the nanocomposite through an energy transfer process. In PDT and the photocontrolled release of drugs, the upconversion process is used as a wavelength transforming function, with the function of the drug carrier controlled by NIR light. However, most of the reported upconversion-driven therapies have focused on in vitro experiments such as cell-level therapy. Further development of upconversion-driven therapies based on lanthanide UCNPs requires assessment of the effectiveness of this therapy in smallanimal tumor models before subsequent assessment in larger animals and deeper tissues. In addition, owing to the high sensitivity of upconversion imaging based on lanthanide UCNPs, upconversion imaging-guided surgery is expected to enable immediate confirmation of correct tissue sampling or clearer margins in cancer nodule resection thus eliminating the need to send specimens to pathology for processing. 12.2.2. Lanthanide UCNPs for Sensing. To date, lanthanide UCNPs have been applied in chemosensors and biosensors based on the inner filter effect and LRET process. A further issue that needs to be addressed concerns clinical detection using upconversion emission as an output signal. The working wavelength needs to be shifted to the near-infrared range to enable greater detection depth. Obtaining uniform nano/submicrosized lanthanide UCNPs is also required for sensor systems to ensure their repeatability and reliability. In addition, flow cytometry analysis using lanthanide UCNPs as probes is expected to offer new detection methods. 12.2.3. Applications of TTA-Based Upconversion Materials in Biology and Medicine. To date, only rare examples of the application of TTA-based upconversion materials in biology and medicine have been reported. This is in spite of the fact that the TTA-based upconversion process has some intrinsic advantages. Some potential directions for TTA-based upconversion materials, include as biosensors, in targeted tumor imaging, lymphatic tracking, in TTA-based upconversion imaging guided surgery, TTA-based upconversion emission triggered drug release, as well as photodynamic therapies among others. Recently, Bonnet and co-workers utilized TTA-based upconversion PEGylated liposomes with blue emission in conjunction with a 630 nm clinical grade laser as the excitation source to trigger the photodissociation of polypyridyl ruthenium complexes from liposomes.591 This result provide a good example to design TTA-based upconversion materials for therapy. The TTA-based upconversion-emissive liposomes were prepared by embedding the sensitizer and annihilator molecules into their hydrophobic layers. 12.2.4. Application of TTA-Based Upconversion Materials in Optical/Electrical Devices. Benefiting from large absorption cross sections and high upconversion efficiencies, TTA-based upconversion materials promise to enhance the performances of current optical and electrical devices. In particular, as demonstrated in solar cells, TTA-based upconversion can utilize more sunlight than the corresponding

ASSOCIATED CONTENT S Supporting Information *

Summaries of detailed physiochemical properties of upconversion materials. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author

*Fax: 86-21-55664621. Tel: 86-21-55664185. E-mail: [email protected] fudan.edu.cn. Author Contributions †

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest. Biographies

Jing Zhou was born in 1984 in Inner Mongolia, China. She received her B.S. degree from Inner Mongolia University (2007). In 2012, she received her Ph.D. degree from Fudan University under the direction of Prof. Fuyou Li. She is currently a lecturer in the Department of Chemistry in Capital Normal University. Her research interest is focused on multifunctional upconversion nanoparticles for imaging and therapy.

Qian Liu was born in 1985 in Shijiazhuang, China. She received her B.S. degree from South West University (China) in 2008. She received 454

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her Ph.D. degree (2013) from Fudan University under the direction of Prof. Fuyou Li. Her current research is focused on upconversion materials for bioimaging.

Fuyou Li was born in Zhejiang, China, in 1973. He received his B.S. degree in 1995 and Ph.D. degree in 2000 from Beijing Normal University. He worked as a postdoctoral researcher at Peking University from 2000−2002. He was as an associate professor at Peking University from 2002−2003 and Fudan University from 2003− 2006. He has been working as a full Professor at Fudan University since 2006. His current research interests involve upconversion luminescent materials and luminescent chemodisimeters for sensing and bioimaging. To date, he has 200 scientific publications which have received more than 10 000 citations.

ACKNOWLEDGMENTS The authors wish to thank State Key Basic Research Program of China (2015CB931800 and 2012CB932403), the National Science Foundation of China (21231004 and 21375024), Shanghai Sci. Tech. Comm. (12JC1401300), and The CAS/ SAFEA International Partnership Program for Creative Research Teams for financial support.

Wei Feng was born in 1982 in Wuhan, China, and received his B.S. degree in 2004 and Ph.D. degree in 2009, both in chemistry from Peking University. He subsequently worked as postdoctoral fellow on the synthesis and applications of upconversion nanomaterials with Prof. Chunhua Yan in Peking University for two years. He is currently an associate professor in the Department of Chemistry at Fudan University. His research interests lie in the design and synthesis of luminescent nanomaterials with a focus on their biological applications.

ABBREVIATIONS 1CBPEA 1-chloro-bis(phenylethynyl)anthracene) 2CBPEA 2-chloro-bis(phenylethynyl)anthracene) 3D three-dimensional α-CD α-cyclodextrin ε molar extinction coefficient AlC4Pc tetrasubstituted carboxy aluminum phthalocyanine BODIPY 4-bora-3a,4a-diaza-s-indacene derivative BPEA 9,10-bis(phenylethynyl)anthracene BPEN 9,10-bis(phenylethynyl)naphthacene BTB bromothymol blue C. elegans Caenorhabditis elegans CT X-ray computed tomography CTAB cetyltrimethylammonium bromide CUCLM confocal upconversion luminescence microscopy CW continuous-wave DLS dynamic light scattering DMA 9,10-dimethylanthracene DOX doxorubicin DPA 9,10-diphenylanthracene EDTA ethylene diaminetetra-acetate EDXS energy dispersed X-ray spectrometer EELS electron energy loss spectroscopy EMCCD electron multiplying charge coupled device EO-EPI ethylene oxide/epichlorohydrin copolymer ET energy-transfer FA folic acid FITC fluorescein isothiocyanate FRET Förster (fluorescence) resonance energy transfer Gd-DTPA gadopentetic acid GSH glutathione HAADF high-angle annular dark-field HE hematoxylin and eosin stain IBU ibuprofen IC50 half-maximal inhibitory concentration ICP-AES inductively coupled plasma atomic emission spectroscopy IgG immunoglobulin G L-5ph polymer poly(ladder-type) pentaphenylene Ln lanthanide LRET luminescence resonance energy transfer

Yun Sun was born in 1982 in MengYin, China. He received his B.S. degree (2004) and MS degree (2008) in Biotechnology at Yantai University and Donghua University, respectively. He received his Ph.D. degree (2012) at Fudan University under the direction of Profs. Fuyou Li and Chunhui Huang. His current research is focused on upconversion nanophosphors for bioimaging.

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LSUCLM

laser scanning upconversion luminescence microscopy MFNPs multifunctional nanoparticles MLCT metal-to-ligand charge-transfer mPEG−OH polyethylene glycol monomethyl ether MR magnetic resonance MRI magnetic resonance imaging MTS (3-(4,5-dimethylthiazol-2-yl)-5-(3-carbo-xymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, sodium salts MTT methyl thiazolyltetrazolium NIR near-infrared OA oleic acid OM oleylamine PAA poly(acrylic acid) PAH polycyclic aromatic hydrocarbon Pc phthalocyanine PDI N,N-bis(ethylpropyl)perylene-3,4,9,10-tetracarboxylicdiimide PDMS polydimethylsiloxane PDT photodynamic therapy PEG poly(ethylene glycol) PEI polyethylenimine PET positron emission tomography PF2/6 poly(9,9-bis(2-ethylhexyl)fluorene) PMAO poly(maleic anhydride-alt-1-octadecene) PMMA poly(methyl methacrylate) POSS polyhedral oligomeric silsesquioxane PSA prostate-specific antigen PSAA poly(styrene/acrylic acid) copolymer PtOEP (2,7,8,12,13,17,18-octaethyl-porphyrinato)Pt(II) PTS polyoxyethanyl-tocopheryl sebacate PTT photothermal therapy PVP polyvinylpyrrolidone QD quantum dot QE quantum efficiency QY quantum yield RE rare earth RGD arginine-glycine-aspartic peptide ROS reactive oxygen species siRNA small interference RNA SNR signal-to-noise ratio SPCE surface-plasmon-coupled emission SPECT single-photon emission computed tomography ssDNA single-stranded DNA STEM scanning transmission electron microscopy TAMRA N,N,N′,N′-tetramethyl-6-carboxyrhodamine TAP tetra-nathraporphyrin TEM transmission electron microscope TIHF 2,4,5,7-tetraiodo-6-hydroxy-3-fluorone TRITC tetramethylrhodamine isothiocyanate TTA triplet−triplet annihilation TTET triplet−triplet energy transfer UCNPs upconversion nanophosphors UV ultraviolet XPS X-ray photoelectron spectroscopy

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Upconversion luminescent materials: advances and applications.

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