Two-dimensional graphene analogues for biomedical applications.

Chem Soc Rev

Published on 18 December 2014. Downloaded by Northwestern University on 19/12/2014 12:08:26.

REVIEW ARTICLE

Cite this: DOI: 10.1039/c4cs00300d

View Article Online View Journal

Two-dimensional graphene analogues for biomedical applications Yu Chen,ab Chaoliang Tan,c Hua Zhang*c and Lianzhou Wang*a The increasing demand of clinical biomedicine and fast development of nanobiotechnology has substantially promoted the generation of a variety of organic/inorganic nanosystems for biomedical applications. Biocompatible two-dimensional (2D) graphene analogues (e.g., nanosheets of transition metal dichalcogenides, transition metal oxides, g-C3N4, Bi2Se3, BN, etc.), which are referred to as 2D-GAs, have emerged as a new unique family of nanomaterials that show unprecedented advantages and superior performances in biomedicine due to their unique compositional, structural and physicochemical features. In this review, we summarize the state-of-the-art progress of this dynamically developed material family with a particular focus on biomedical applications. After the introduction, the second section of the article summarizes a range of synthetic methods for new types of 2D-GAs as well as their surface functionalization. The subsequent section provides a snapshot on the use of these biocompatible 2D-GAs for a broad spectrum of biomedical applications, including therapeutic (photothermal/photodynamic therapy, chemotherapy and synergistic therapy), diagnostic (fluorescent/ magnetic resonance/computed tomography/photoacoustic imaging) and theranostic (concurrent diagnostic imaging and therapy) applications, especially on oncology. In addition, we briefly present the biosensing applications of these 2D-GAs for the detection of biomacromolecules and their in vitro/

Received 10th September 2014

in vivo biosafety evaluations. The last section summarizes some critical unresolved issues, possible

DOI: 10.1039/c4cs00300d

challenges/obstacles and also proposes future perspectives related to the rational design and

www.rsc.org/csr

construction of 2D-GAs for biomedical engineering, which are believed to promote their clinical translations for benefiting the personalized medicine and human health.

1. Introduction The fast development of biomedicine and nanobiotechnology provides broad efficient strategies as promising alternatives towards disease diagnosis and therapy, especially on oncology.1–6 It is believed that these emerging techniques strongly depend on the fabricated biomaterial systems at nanoscale with desirable structures, compositions, morphologies and physicochemical properties. It has been demonstrated that the morphology of nanomaterials did have significant impact on their biological performances such as cellular uptake, biodistribution, excretion and even blood circulation durations.7–10 The most explored morphology of diverse nanosystems for biomedical applications is the spherical nanoparticles (NPs), most probably due to the easy preparation of such type of shape. Moreover, other types of a

Nanomaterials Center, School of Chemical Engineering and AIBN, University of Queensland, Queensland, 4072, Australia. E-mail: [email protected] b State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, P. R. China c School of Materials Science and Engineering, Nanyang Technological University, 639798, Singapore. E-mail: [email protected]

This journal is © The Royal Society of Chemistry 2014

nanostructures with rich topologies, such as tubes,11,12 wires,13 ellipsoidal,14,15 and cages,16–18 have also been successfully constructed as either drug delivery nanosystems or synergistically therapeutic agents for cancer treatment.19 Previous studies on mesoporous NPs have proved that the large surface area of mesopores is highly favorable for loading guest drug molecules (Fig. 1a).2,20–26 Therefore, it is expected that the nanostructures with high surface area would be appealing candidates for biomedical applications. As a newly emerging class of nanomaterials, two-dimensional (2D) nanosheets with planar topography exhibit some unique properties that originate from their ultrathin thickness and 2D morphological feature, such as high surface-area-to-mass ratio and specific physicochemical properties, enabling them very promising nanoplatforms for biomedical applications (Fig. 1a).27–34 Representatively, biocompatible graphene derivatives, such as graphene oxide (GO) and reduced GO (rGO), have been recently demonstrated to be attractive candidates for biomedical applications, including the anticancer drug delivery,35–38 gene transportation,39–41 photothermal therapy (PTT),42–44 photodynamic therapy (PDT),45 biosensing46,47 and even tissue engineering,48–50 which showed superior performances compared to other conventional

Chem. Soc. Rev.

View Article Online


Review Article

Chem Soc Rev

nanostructures such as NPs, tubes, wires, and cages. Graphenebased hybrid nanomaterials by integrating graphene with other functional nanomaterials have been also developed for diagnostic imaging such as fluorescent imaging,51–54 magnetic resonance imaging (MRI),55,56 computed tomography (CT),57–59 and radionuclide imaging.60–62 Their corresponding biological effects/ behaviors have also been revealed, including cytotoxicities, biodistribution, excretion, and hemo/histocompatibility.63–69 However, the biomedical engineering and clinical translation of GO/rGO suffer from many severe problems. For example, the photothermal conversion efficiency of hydrophilic GO during photothermal therapy (PTT) is relatively low due to the existance of abundant structural defects. In addition, the in vivo biodegradation rate of GO/rGO is extremely low; therefore,

the long term accumulation of GO/rGO in the body might cause some severe biosafety issues. In addition to graphene nanosheets, the 2D graphene analogues, which are referred to as 2D-GAs, with an ultrathinlayered feature, such as transition metal dichalcogenides (referred to as TMDs), transition metal oxides, graphitic carbon nitride (designated as g-C3N4), boron nitride (BN), and Bi2Se3, are also receiving considerable research interest due to their unique physical, chemical, and electronic properties in the past few years.70–77 In this review, the 2D-GAs mainly refer to the inorganic materials with planar topology, ultrathin thickness (single or several atomic layers), which have structural similarity with graphene but exhibit distinctive physicochemical/ biological properties. Although having similar structural features

Yu Chen received his bachelor degree in Polymer Material Sciences and Engineering at Nanjing Tech University and PhD degree at Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS). Since then, he has worked as a research associate in SICCAS. He is now a research associate at the University of Queensland. His research focuses on the design, synthesis and biomedical applications of multifunctional inorganic nanodrug-delivery systems for chemotherapy, molecular imaging, synergistic agent for hyperthermia and non-virus gene delivery

Chaoliang Tan received his BE degree from Hunan University of Science and Technology in 2009. After he received his MS degree from South China Normal University, he moved in 2012 to the School of Materials Science and Engineering of Nanyang Technological University in Singapore where he is a PhD candidate under the supervision of Professor Hua Zhang at present. His research interests Chaoliang Tan focus on the synthesis, assembly and applications of two-dimensional nanosheets (e.g. graphene and single- or few-layer transition metal dichalcogenides) and their composites.

Hua Zhang obtained his BS and MS degrees at Nanjing University in 1992 and 1995, respectively, and completed his PhD with Prof. Zhongfan Liu at Peking University in 1998. As a Postdoctoral Fellow, he joined Prof. Frans C. De Schryver’s group at Katholieke Universiteit Leuven (Belgium) in 1999, and then moved to Prof. Chad A. Mirkin’s group at Northwestern University in 2001. After he worked at NanoInk Inc. (USA) Hua Zhang and Institute of Bioengineering and Nanotechnology (Singapore), he joined Nanyang Technological University in July 2006. His current research interests focus on synthesis of two-dimensional nanomaterials and carbon materials (graphene and carbon nanotubes), and their applications in nanoand bio-sensors, clean energy, and water remediation.

Lianzhou Wang is currently a Professor in School of Chemical Engineering and Research Director of Nanomaterials Centre, the University of Queensland (UQ), Australia. He received his PhD degree from Shanghai Institute of Ceramics, Chinese Academy of Sciences in 1999. Before joining UQ in 2004, he has worked at two national institutes (NIMS and AIST) of Japan for five years. Wang’s research interests include Lianzhou Wang the design and application of nanostructured materials, including two-dimensional layered compounds for energy, environment and relevant applications.

Yu Chen materials, including contrast agents for ultrasound/magnetic vehicles.

Chem. Soc. Rev.


View Article Online


Chem Soc Rev

Review Article

Fig. 1 (a) Schematic illustration of the conventional spherical mesoporous NPs and new 2D planar nanostructures for drug delivery. (b) Summary of the biocompatible 2D-GAs for biomedical applications.

with graphene, these 2D-GAs possess much different physicochemical properties, composition and surface status. In this regard, these biocompatible 2D-GAs could be ideal alternatives or complementary candidates of graphene-based nanomaterials for biomedical applications such as drug delivery, phototheram/ photodynamic therapy, diagnostic imaging, and biosensing (Fig. 1b). In this review, the state-of-the-art progress, challenges and perspectives of 2D-GAs in biomedical applications will be highlighted and discussed. In particular, the very recent progresses of 2D planar biomaterial nanosystems, including their synthesis strategies, surface engineering and diagnostic/ therapeutic performance against cancers, as well as the biosensing applications and biosafety evaluations will be summarized with a focus on 2D MoS2, WS2, BN, Bi2Se3, g-C3N4 and MnO2. Of note, in this article, we will exclude the studies on biocompatible 2D nanosheets of layered double hydroxides (LDHs) for biomedical purposes because several excellent reviews concerning this topic have already been published.78,79

2. Synthesis of 2D-GAs Layered materials have been investigated for decades, whereas the recent research interest on graphene in single-layer sheet fashion has triggered the increasing research efforts on this family of materials due to some unique physicochemical properties arising from the thickness reduction to single or few layers. For instance, the indirect bandgap of bulk MoS2 is 1.3 eV, but it can convert to a direct bandgap of 1.8 eV in its single-layer form.80 Various kinds of single- or few-layer TMD nanosheets, such as MoS2, WS2, MoSe2, NbSe2, TiS2, ZrS2, TaS2, and WSe2, have been successfully constructed for a range of applications such as catalysis, electronic/optoelectronic devices, energy harvesting and storage.70,73,80–88 In addition, many other 2D nanomaterials, including transition metal oxides (e.g., TiO2


and MnO2),89–93 g-C3N4,94–96 BN97–99 and Bi2Se3 (ref. 100) are also receiving extensive investigation. To date, a number of synthetic methods, including mechanical exfoliation,101–103 liquid-phase exfoliation,104,105 ion-intercalation and exfoliation,106,107 chemical vapor deposition (CVD),108–112 and hydro-/solvo-thermal synthesis,113 have been developed for the preparation of various classes of 2D-GAs. Generally, these synthetic methodologies can be divided into two distinct categories, i.e. top-down and bottom-up approaches. 2.1

Top-down synthesis

The top-down methods are based on the direct exfoliation of layered bulk crystals by various driving forces. The typical topdown method is the mechanical-exfoliation approach, which was first used to produce graphene sheets (Fig. 2a and b). Similarly, atomically thin sheets of layered 2D-GAs can also be fabricated by this procedure from their initial bulk materials featured with stacks of strongly bonded layers with weak interlayer attractions. By using the micromechanical cleavage, Geim et al. successfully prepared several types of single-layer nanosheets such as graphene, NbSe2, Bi2Sr2CaCu2Ox, BN and MoS2.101 Ultrathin 2D-GAs prepared by this approach are pristine and well-crystallized sheets with large size (up to tens of micrometer) deposited on certain substrates (e.g. SiO2/Si), which are suitable for fundamental understanding of their intrinsic properties. However, this approach is featured with relatively low throughput; therefore, it cannot be used for those applications (e.g. biomedicine) that require a large amount of solution processed 2D nanosheets. Alternatively, low-energy ball milling was further employed to substitute the initial Scotch tape for mechanical exfoliation, which could mechanically peel hexagonal boron nitride (h-BN) particles into BN nanosheets while only little crystallographic damage would be generated within the in-plane crystal structures. Importantly, this ball milling

Chem. Soc. Rev.

View Article Online


Review Article

Chem Soc Rev

Fig. 2 (a) Typical synthetic procedure for surface-modified 2D layered nanosheets by the typical top-down exfoliation approach. (b) Schematic illustration of biomedical applications of 2D-GAs against cancers, including drug delivery, photo/photodynamic therapy, diagnostic imaging and other specific applications such as biosensing.

method could potentially produce BN nanosheets in high yield and large scale.102 Chemical exfoliation of layered bulk crystals in liquid is one of the most developed strategies to synthesize 2D-GAs for biomedical applications, especially suitable for large-scale production and easy control of the crucial structural/compositional parameters of prepared 2D-GAs. The intercalation of bulk TMD crystals by lithium ions (Li+) has been proven to be effective in the high-yield production of single-layer TMD nanosheets. As a typical paradigm, we recently developed an electrochemical Li-intercalation and exfoliation method to precisely control the intercalation process and enhance the product yield (Fig. 3).106 By using this method, single- and few-layer TMD nanosheets such as MoS2, TaS2, TiS2, WS2, ZrS2, NbSe2, WSe2,

Fig. 3 Li-intercalation and exfoliation method for preparation of 2D TMDs. Reprinted with permission from ref. 106. Copyright 2011 Wiley-VCH.

Chem. Soc. Rev.

Sb2Se3, Bi2Te3, and BN can be prepared in high yield and large scale.106,107 Recently, a H2SO4-assisted liquid-exfoliation approach was developed to prepare highly dispersed WS2 nanosheets.114 One of the advantages of this intercalation process is that it can be processed in air and water compared with the Li-intercalation method. As an interesting paradigm, Coleman et al. reported a simple and general dispersion/ exfoliation method to prepare several kinds of 2D nanosheets such as MoS2, WS2, MoSe2, TaSe2, NbSe2, NiTe2, BN, and Bi2Te3.104 This method simply dispersed the commercial layered bulk powders into the solvents with dispersive, polar and H-bonding components of the cohesive energy density within certain well-defined ranges according to the Hansen solubility parameter theory. Solvents such as N-methylpyrrolidone (NMP) and isopropanol (IPA) were demonstrated as the most effective solvents to minimize the energy for exfoliation. Note that the yield of single-layer nanosheets for this method is relatively low compared to the electrochemical Li-intercalation and exfoliation method.106,107 Exfoliation by ultrasonication in liquids containing various solvents, surfactants and polymers can produce the single-layer nanosheets in large scale depending on the mechanical effect of the ultrasound. It should be noted that this exfoliation process can produce 2D-GAs with specific surface modification, which can avoid the re-aggregation of as-formed nanosheets. Importantly, such a surface modification can endow these 2D-GAs with high stability in physiological conditions, which is very essential for further in vivo biomedical applications. However, the liquid exfoliation can potentially cause large amount of


View Article Online

Chem Soc Rev

defects within the nanosheets, the impurities of the product, and single-layer and multilayer 2D-GAs co-exist in the exfoliating solution, and it is rather difficult to separate them.


2.2

Bottom-up approach

Aforementioned top-down methods are only applicable to the layered bulk compounds. Alternatively, the bottom-up approaches, such as CVD growth and wet-chemical synthesis, are also widely used for preparation of 2D-GAs. The CVD method can produce highquality and large-area 2D-GA nanosheets with atomic thickness. For instance, single-layer MoS2 films can be deposited onto amorphous SiO2 substrates by the CVD process with MoO3 and S powers as the reactants at 650 1C.112 The pre-treatment of the substrate with rGO, perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt and perylene-3,4,9,10-tetracarboxylic dianhydride could facilitate the growth of MoS2 nanosheets. Large-area MoS2 atomic layers on a SiO2 substrate could be also prepared by a similar CVD approach.115 By etching the SiO2 substrate, the fabricated MoS2 nanosheets could be transferred to arbitrary substrates for further characterizations and applications. Typically, the CVD method is suitable for largescale device fabrication,111,112,115 but not suitable for biomedical applications, especially for therapeutic purposes, for which small sheet size is highly desirable. Hydro-/solvo-thermal synthesis is expected to produce highquality 2D TMD nanosheets that can be adapted to biomedical applications.113,116–118 Previous results showed that single- or fewlayer MoS2 nanosheets could be synthesized by the hydrothermal treatment of molybdic oxide and KSCN in deionized water at 453 K.113 However, there is still no appropriate synthetic procedure to prepare nano-sized TMD nanosheets with high dispersity and controlled structural parameters by hydro-/solvo-thermal synthesis, which is essential to satisfy the strict requirement of biomedicine. An effective bottom-up hydro-/solvo-thermal synthesis of 2D-GA nanosheets is still rare; however, it is expected that abundant 2D-GA nanosheets with controlled and desirable structural parameters could be synthesized by this method in the near future based on its simple and controllable nature. 2.3

Surface-modification and multifunctionalization

Similar to other nanosystems in biomedicine, the exfoliated thin 2D-GAs are not stable in physiological conditions, and thus surface modification is essential to endow them with the high dispersity and stability in a physiological environment, site-specific targeting capability and improved biocompatibility. Generally, the surface engineering of 2D-GAs is to modify their surface with certain polymers through physical adsorption or weak non-covalent chemical bonding. As a typical paradigm, a simple in situ sonication-assisted exfoliation approach assisted with poly(acrylic acid) (PAA) was developed to fabricate highly water-soluble WS2 nanosheets by modifying PAA onto the surface of WS2 via the relatively strong coordination interaction between carboxyl groups and tungsten atoms.119 The enhanced stability of PAA-modified WS2 sheets is evidenced by the fact that they could be stored for weeks at ambient temperature. Similarly, the adsorption of bovine serum albumin (BSA) onto the surface of WS2 nanosheets could also enhance their dispersity and stability in phosphate buffered


Review Article

saline (PBS).114 Recently, we successfully grafted the surface of MnO2 nanosheets with polyethylene glycol (PEG) molecules to improve their stability in physiological condition for concurrent pH-responsive drug releasing and T1-weighted MRI.120 It should be noted that such a surface modification would decrease the anchoring points of 2D-GAs for therapeutic molecules; thus, the modification degree should be carefully conducted and controlled. The decoration of 2D-GAs with other functional materials can further broaden their applications. For instance, the integration of 2D nanosheets with contrast agents (CAs) can endow them with concurrent diagnostic imaging and therapeutic performance (designated as theranostic).121–123 However, such a multifunctionalization strategy is still in infancy compared to the widely explored GO/rGO because of the synthetic and integrating difficulties.84,124,125 It is believed that the fast development on synthesis and applications of 2D-GAs will lead to diverse 2D nanosheet-based multifunctional nanosystems for biomedical engineering in the near future.

3. Therapeutic applications of 2D-GAs The unique 2D planar structure and diverse chemical compositions lead to unique properties of 2D-GAs for biomedical applications. Till now, these 2D nanosheets have been explored for a variety of biomedical applications such as drug delivery, photothermal/photodynamic therapy, diagnostic imaging, and biosensing. Moreover, the biological effect and behavior of these 2D nanosheets are also now under extensive exploration. It has been demonstrated that these 2D nanosheets present relatively high biocompatibility and biosafety such as low cytotoxicity and high hemo-/histo-compatibility. In this section, we mainly focus on the introduction of these biocompatible 2D nanomaterials for various biomedical purposes (Table 1 and Fig. 2b). 3.1

Photothermal therapy

Photothermal therapy (PTT) typically employs laser to generate heat and induces hyperthermia within tumor tissues, causing the denaturation of proteins, the disruption of cell membrane and corresponding irreversible damage to cancer cells.126 However, the nonselectivity and high power density of laser therapy can damage both the normal and tumor tissues, causing severe side effects.127 The introduction of photothermal agent (PTA) can increase the selectivity of laser, which means that the heat can only be generated within the local microenvironment of tumor tissue at relatively low power density of laser.128–136 GO and rGO have been extensively explored as the PTA for in vitro and in vivo photothermal ablation of cancer cells, due to their strong lightabsorbance in the near-infrared (NIR) window of wavelength in the range of 700–1300 nm.42,55,131,132 Biological tissues are transparent to this NIR window; thus, the light-penetrating depth is high and tissue-induced light-absorption is low, which causes the high photothermal conversion efficiency within tumor tissues. Similar to the high photothermal conversion efficiency of GO or rGO, some 2D-GAs with various chemical compositions have also been reported as PTA for efficient PTT. Recently, amphiphilic

Chem. Soc. Rev.

View Article Online

Review Article Table 1

Chem Soc Rev

Summary of representative in vivo diagnostic imaging and therapeutic applications of 2D-GAs


Nanosheet Surface Sheet size type modifications (nm)

Imaging Therapeutic modality modality

Administration mode Performance

MoS2

PEG

50 (AFM)

—

MoS2

Chitosan

80 (AFM)

CT

WS2

BSA

20–100 (TEM) CT

PTT & PDT

Intratumor

WS2

PEG

50–100 (TEM) CT

PTT

Bi2Se3

PVP

90 (TEM)

PTT

Intravenous & Intratumor Intratumor

MnO2

PEG

80–100 (TEM) MRI

Chemotherapy Intratumor

MnO

PEG

8–70 (TEM)

—

CT

MRI

PTT & Intravenous & Chemotherapy Intratumor PTT & Intratumor Chemotherapy

chemically exfoliated MoS2 (designated as ceMoS2) was synthesized via the Morrison method to break the interlayer van der Waals force in bulk MoS2 through ultrasonication.133 The assynthesized ceMoS2 exhibited the typical 2D planar morphology with an average sheet size of 800 nm and thickness of 1.54 nm (n = 40 sheets, Fig. 4a). The mass extinction coefficient of assynthesized ceMoS2 was calculated to be 29.2 Lg1 cm1 by the NIR absorbance (Fig. 4b), which was about 7.8-fold of GO nanosheets (3.6 Lg1 cm1) and better than that of rGO nanosheets (24.5 Lg1 cm1). The temperature of the aqueous solution could be rapidly increased to above 40 1C at ceMoS2 concentrations in the range of 38 ppm to 300 ppm (Fig. 4c). The in vitro photothermal evaluation against HeLa cells (Fig. 4d) showed that the cells could be completely killed after co-incubation with ceMoS2 and NIR irradiation (l = 800 nm, 20 min) because the solution temperature could be quickly

Intravenous

(a) High PTT efficiency (b) Synergistic PTT and chemotherapy outcome (a) Preliminary excellent CT imaging performance (b) Enhanced synergistic PTT and chemotherapeutic efficiency (a) Strong CT imaging signals of the injected site of tumor (b) Synergistic PTT and PDT outcome (a) Excellent CT imaging performance (b) Complete tumor eradication by PTT (a) Contrast-enhanced tumor CT imaging (b) Complete tumor eradication by PTT (a) pH-responsive intelligent drug releasing (b) pH-responsive MR imaging of tumor tissue Significantly contrast-enhanced MRI in the whole body

Ref. 140 141 114 134 100 120 164

raised to over thermal-ablation thresholds. This report on the photothermal effect of MoS2 nanosheets provides direct evidence that the photothermal conversion efficiency of MoS2 nanosheets was comparable to the hydrophobic rGO, but they exhibited a hydrophilic nature. In addition, this report on in vitro PTT of cancer cells using 2D-GAs as the PTA clearly reveals that graphene can be substituted by other 2D-GA nanosheets for efficient PTT. However, the large sheet size (around 800 nm) and lack of surface modification of ceMoS2 restrict their further in vivo PTT against tumors. WS2 is another representative member of the TMD family. Similar to MoS2, 2D WS2 nanosheets also exhibit promising photothermal conversion capability for PTT. 2D WS2 nanosheets with the sheet size of 50–100 nm were synthesized by the Morrison method to reveal their PTT efficiency (Fig. 5a).134 After further PEGylation through the W–S bond between lipoic

Fig. 4 (a) Atomic force microscopy (AFM) image of ceMoS2 sheets (inset: the diameter and thickness diagrams). (b) Absorbance spectra of ceMoS2 at different concentrations (inset: Beer’s law plot at 800 nm). (c) The temperature increase with the prolonged exposure duration to laser at l = 800 nm (0.8 W cm2) at elevated concentrations. (d) MTT results to determine HeLa cell viabilities after diverse treatments. Reprinted with permission from ref. 133. Copyright 2013 Wiley-VCH.

Chem. Soc. Rev.


View Article Online


Chem Soc Rev

Review Article

Fig. 5 AFM images of (a) WS2 and (b) WS2–PEG. Inset: photographs of WS2 and WS2–PEG dispersed in saline at the concentration of 0.05 mg mL1. (c) IR thermal images of tumor-bearing mice after the intratumor (i.t.) and intravenous (i.v.) administration of WS2–PEG after exposure to the irradiation of the 808 nm laser at the power density of 0.8 W cm2. (d) The tumor volume changes and (e) the corresponding survival curves of mice after different treatments. Reprinted with permission from ref. 134. Copyright 2013 Wiley-VCH.

acid-conjugated PEG (LA–PEG) and WS2 nanosheets, the as-synthesized PEGylated WS2 nanosheets (designated as WS2– PEG) could be easily dispersed into saline with high stability. The average thickness of WS2–PEG was determined to be about 1.6 nm by AFM characterization (Fig. 5b). According to the UV-vis-NIR spectrum of WS2–PEG nanosheets, the extinction coefficient WS2–PEG at 808 nm was calculated to be as high as 23.8 Lg1 cm1. Importantly, the capability of WS2–PEG for PTT was systematically revealed in vivo. It was found that the surface temperature of the tumor could be quickly raised to about 65 1C within a 5 min-irradiation by a 808 nm laser at the power density of 0.8 W cm2 (Fig. 5c), irrespective of the intratumor or intravenous administration of WS2–PEG into mice. In particular, the administration of WS2–PEG combined with laser irradiation could bring with the significantly improved in vivo photothermal-therapeutic efficiency, where the tumors could be completely eradicated without obvious reoccurrences (Fig. 5d). The survival of mice could also be significantly improved because of this high PTT outcome (Fig. 5e). Furthermore, it was found that 2D Bi2Se3 nanoplates are also efficient in absorbing the NIR laser and converting it into heat for PTT.100 Polyvinylpyrrolidone (PVP)-coated 2D Bi2Se3


nanoplates with the sheet size of about 90 nm were synthesized with the outer layer thickness of about 3.9 nm and inner layer thickness of about 21.5 nm. The high photothermal conversion efficiency was shown by a fast temperature increase to 50.2 1C after 5 min-irradiation via the 808 nm laser with a power density of 1 W cm2 at Bi2Se3 concentration of 50 mg mL1. The co-incubation with Bi2Se3 nanoplates combined with laser irradiation could cause 75% H22 cell death at the concentration of 50 mg mL1. After intratumor injection of Bi2Se3 nanoplates assisted by the 808 nm laser irradiation, the tumors could be completely eradicated, further demonstrating their high in vivo photothermal efficiency against tumor. 3.2

Drug delivery

One of the outstanding structural characteristics of 2D nanosheets is their large surface-to-volume ratio, which can provide numerous anchoring points available for loading guest molecules (Fig. 1a).137 2D graphene and its derivatives have the strong absorption ability for anticancer drugs through supramolecular p–p staking and hydrophobic interaction due to the ultrahigh surface area and unique sp2-bonded carbonaceous surface.53,66 In addition, GO and rGO were demonstrated as

Chem. Soc. Rev.

View Article Online


Review Article

efficient carriers for gene macromolecules or even contrast agents (CAs).53,66,67 Similar to the GO or rGO sheets, the 2D-GAs are also expected to have a similar desirable function for drug delivery based on the following considerations. First, inorganic 2D-GAs show relatively high chemical/physiological stability compared to the organic liposomes and micelles. Therefore, the drug molecules can be released from the carrier via a sustained manner to avoid the explosive release of drugs caused by the break-up of traditional organic nanocarriers. Second, the high surface-to-volume ratio of 2D-GAs endows the carrier with high drug-loading capacity. Third, the unique chemical composition of some members in the 2D-GA family can form specific interactions with drug molecules, which can endow the carrier with high drugloading capacity and response to external triggers for on-demand drug releasing. Last but not least, the multifunctionalities of the carriers themselves can bring the synergistically therapeutic outcome such as photothermal-/chemo-therapy and/or theranostics (concurrent diagnostic imaging and therapy). The aforementioned features and functions are difficult to achieve by traditional organic liposomes or micelles. Similar to the structure of graphite, graphitic-phase carbon nitride (g-C3N4) is generally regarded as the N-substituted graphite via a regular manner. As a new 2D semiconductor material, g-C3N4 nanosheets exhibit intriguing performance in photochemical and electrochemical catalysis.94,139 However, the biomedical applications of g-C3N4 require their specific physical/chemical/biological properties, i.e., small sheet size, high dispersity, high hydrophilicity and low toxicity. Recently, highly dispersed g-C3N4 nanosheets were synthesized with the hydrodynamic diameter of only 55 nm and thickness of about 1.1 nm by chemical oxidation of bulk g-C3N4 followed by ultrasonic exfoliation.138 The as-synthesized g-C3N4 could not

Fig. 6 The scheme of 2D g-C3N4 nanosheets for synergistically photodynamic therapy and chemotherapy. Reprinted with permission from ref. 138. Copyright 2014 Royal Society of Chemistry.

Chem. Soc. Rev.

Chem Soc Rev

only encapsulate/deliver anticancer drugs (doxorubicin, Dox) but could also act as the photosensitizer for photodynamic therapy (Fig. 6). Importantly, the loading amount of Dox on g-C3N4 could reach as high as 18 200 mg g1, which is much higher than traditional mesoporous NPs. The release of Dox from g-C3N4 was pH-dependent where the acidity could accelerate the releasing rate of drugs from the carrier. Furthermore, the Dox-loaded g-C3N4 exhibited comparable cytotoxicities compared to free Dox molecules. Biocompatible transition metal oxide nanosheets can also act as drug delivery nanosystems for chemotherapy. In particular, biocompatible MnO2 nanosheets exhibit a unique break-up nature under mild acidic environment. We recently constructed a pHresponsive drug-releasing platform based on MnO2 nanosheets.120 The drug molecules (Dox) could be quickly released from MnO2 nanosheets when Dox-loaded MnO2 nanosheets were impregnated into an acidic environment (Fig. 7a). The successful loading of Dox onto MnO2 nanosheets were demonstrated by the changes of Zeta potential from 40.6 mV to +22.3 mV (Fig. 7b). The Dox-releasing in neutral solution after 5 h was only 24.8% (Fig. 7c). Based on the disintegration of MnO2 nanosheets, the releasing rate showed a significant increase in a mild acidic environment where the 5 h-releasing amounts could reach 58.9% (pH 6.0) and 94.3% (pH 4.6), respectively. Such an ultrasensitive intelligent drugreleasing performance can concurrently reduce the side-effects and improve the therapeutic efficiency of anticancer drugs because

Fig. 7 (a) The scheme of the break-up nature of 2D MnO2 nanosheets for pH-responsive drug releasing. (b) Zeta potentials of as-prepared MnO2 nanosheets and Dox-loaded MnO2 nanosheets. (c) Cumulative-releasing percentage of Dox from Dox-loaded MnO2 nanosheets at different pHs. (d) T1 values of the releasing medium after 300 min drug-releasing (top image, MRI-T1 images from left to right: water, buffer solution at pH 7.4, 6.0 and 4.6). Reprinted with permission from ref. 120. Copyright 2014 Wiley-VCH.


View Article Online


Chem Soc Rev

these drugs are mostly released within the acidic microenvironment of tumor tissues. Importantly, the disintegration of MnO2 nanosheets could substantially improve the T1-weighted MRI performance (Fig. 7d), which provides the potential for monitoring the drug-releasing procedure by T1-weighted MR imaging. Furthermore, in vitro cellular evaluations showed that Dox-loaded MnO2 nanosheets could enhance the anticancer outcome and circumvent the multidrug resistance (MDR) of cancer cells by escaping the pumping effect of molecular pumps such as P-glycoprotein. The 2D nanosheets can be further assembled to form porous 3D materials for drug transportation. For instance, a highly water soluble, porous and biocompatible BN material was constructed for anticancer drug delivery.98 The 3D porous BN nanostructure was formed by self-assembling a few-nanometer sized hyroxylated BN layers along the [0001] direction. It is well known that BN is one of the typical structural analogues of carbon materials, in which C atoms are substituted by alternating B and N atoms. The fabrication of the porous BN materials was based on the thermal substitution reaction of C atoms in graphitic carbon nitrides, which could produce porous BN with unprecedently high hydroxylation degrees, guaranteeing their high hydrophilicity. Importantly, the as-synthesized porous BN nanostructure exhibited extremely high drug-loading capacity of as high as 309 wt% (for Dox), which is rationalized to the inherent light nature of BN components and extra p–p interactions between the BN framework and aromatic Dox molecules. Furthermore, the Dox-loaded

Review Article

BN showed enhanced in vitro therapeutic efficiency against LNCaP cells compared to free Dox molecules. 3.3

Synergistic therapy

2D nanosheets of TMDs, such as MoS2 and WS2, have been demonstrated as the efficient therapeutic agents for photothermal therapy. It is anticipated that anchoring anticancer agents onto the surface of these 2D nanosheets can bring the synergistically therapeutic outcome, i.e., combined chemotherapy and photothermal therapy, which is difficult to be achieved by using Au NPs as the photothermal agent. On this ground, highly dispersed 2D MoS2 nanosheets were synthesized by a chemical-exfoliation approach (Fig. 8a), which was further modified by lipoic acidconjugated PEG (LA–PEG) to improve their stability in PBS.140 The mass extinction coefficient of as-synthesized MoS2 nanosheets was determined to be 28.4 Lg1 cm1 by UV-vis-NIR absorbance spectra, much higher than that of traditional GO (3.6 Lg1 cm1) and better than that of rGO (24.6 Lg1 cm1). Similar to g-C3N4, the high surface area of the atomic-thin planar nanostructure endows MoS2–PEG with extraordinarily high drug-loading capacity, which could reach 239% for Dox, 39% for Ce6 and 118% for SN38, much higher than PEGylated GO with 150%, 25% and 15% loading amount for Dox, Ce6 and SN38, respectively. After intravenous administration of Dox-loaded MoS2–PEG into 4T1 tumorbearing mice, a significantly synergistic therapeutic efficiency could be achieved after further tumor exposure to the 808 nm

Fig. 8 (a) Schematic illustration of the synthetic procedure of the MoS2–PEG and drug-loading process. (b) The scheme of intravenous administration of Dox-loaded MoS2–PEG, and the combined PTT and chemotherapy. (c) IR thermal images of 4T1 tumor-bearing mice after various treatments. (d) The temperature-variations of tumor as a function of irradiation duration after exposure to the 808 nm NIR laser. (e) Tumor volume changes as a function of time after the various treatments. Reprinted with permission from ref. 140. Copyright 2014 Wiley-VCH.


Chem. Soc. Rev.

View Article Online


Review Article

NIR laser (Fig. 8b). The tumor temperature could be quickly increased to 45 1C after laser irradiation (Fig. 8c and d). The enhanced tumor-suppressing effect could be achieved in the group of MoS2–PEG/Dox combined with 808 nm laser irradiation (Fig. 8e) compared to the groups of MoS2–PEG/Dox and MoS2– PEG combined with the NIR exposure, demonstrating the high synergistically therapeutic outcome. Similar to 2D MoS2 nanosheets for synergistically concurrent chemotherapy and PTT, a modified oleum treatment exfoliation process was developed to synthesize single-layer MoS2 nanosheets for efficient NIR-triggered on-demand drug releasing and synergistically chemo-photothermal therapy.141 After further surface modification with chitosan (CS), the MoS2–CS could load anticancer agent Dox for NIR laser-triggered drug releasing. The ON/OFF introduction of 808 NIR laser could quickly raise the temperature of the releasing media and produce the pulsatile drug-releasing patterns where the ‘‘ON status’’ of the laser quickly promoted the releasing of Dox. The releasing percentage could reach 80% at the power density of 1.4 W cm2 while only less than 20% Dox released without the NIR laser triggering. The intratumor administration of Dox-loaded MoS2–CS into pancreatic tumor-bearing mice could cause the high accumulation of the nanosheets into tumors. After the irradiation by 808 NIR irradiation (0.9 W cm2), the temperature of tumors raised rapidly (DT = 22.5 1C) to induce the hyperthermia and trigger the Dox releasing. The maximum tumor growth-inhibition effect could be achieved by such a synergistically chemo-photothermal therapy. In addition to the encapsulation and delivery of anticancer drugs for chemotherapy, 2D layered nanosheets can also be used to deliver other therapeutic agents such as photosensitizers. Methylene blue (MB) was loaded onto the surface of WS2 nanosheets as the photosensitizer to generate the cytotoxic singlet oxygen for photodynamic therapy (PDT).114 Combined with the photothermal effect of WS2 nanosheets, the localgenerated heat could promote the fast release of MB molecules and significantly restore the generating amounts of singlet oxygen. The high synergistically therapeutic effect could be

Chem Soc Rev

realized by combining irradiations of 665 nm and 808 nm lasers. These results give the strong evidence that 2D-GA nanosheets can act as the carrier of photosensitizers for PDT. In addition, chemically exfoliated MoS2 nanosheets exhibited unique antibacterial activity due to their 2D planar structure and high conductivity, which could induce the membrane/ oxidative stress and produce reactive oxygen species.142

4. Diagnostic imaging of 2D-GAs The unique physicochemical property of 2D nanosheets provides an excellent platform for diagnostic imaging, which indicates that they can act as the CAs to improve the imaging performance of diverse imaging modalities. Compared to traditional CAs, these 2D layered nanomaterials exhibit their specific features for contrast-enhanced imaging, mainly based on their chemical composition, layered structure and physicochemical properties. By combining the concurrent therapeutic and diagnostic performances, these 2D-GAs can further act as the theranostic agents for diagnostic imaging and therapy simultaneously. 4.1

Fluorescent imaging

Compared to conventional organic fluorescein, inorganic quantum dots (QDs) exhibited significantly enhanced fluorescent performance for bio-imaging such as tunable wavelength, enhanced photostability and high quantum yields.143–146 However, traditional inorganic QDs typically contain toxic heavy metal atoms (e.g., Cd-based QDs), which severely restrict further clinical translations. Graphene QDs, a newly developed metal-free fluorescent nanomaterial, showed potential utilization for fluorescent imaging.147,148 However, these graphene QDs suffer from a low photo-response and the intrinsic hydrophobicity. To develop more reliable inorganic fluorescent nanomaterials, ultrathin g-C3N4 nanosheets were developed to act as probes for two-photon fluorescence imaging of cellular nucleus based on the p-conjugated electronic structure and rigid C–N

Fig. 9 ((a) inset: size distribution) TEM, (b) AFM and (c) height diagram of g-C3N4 QDs. Confocal fluorescent images (one-photon and two-photon) of HepG2 cells after co-incubation with (d) 1,1-dioctadecyl-3,3,3 0 ,3-tetramethylindocarbocyanine perchlorate (Dil), fluorescein diacetate (FDA), g-C3N4 QDs, and (e) Dil, FDA, 4 0 ,6-diamidino-2-phenylindole (DAPI), respectively (d1 and e1: one-photon fluorescence of Dil; d2 and e2: one-photon fluorescence of FDA; d3: one-photo fluorescence of g-C3N4 QDs; d4: two-photon fluorescence of g-C3N4 QDs; d5: merged image of d1, d2 and d3; d6: merged image of d1, d2 and d4; e3: one-photon fluorescence of DAPI; e4: two-photo fluorescence of DAPI; e5: merged image of e1, e2 and e3; e6: merged image of e1, e2 and e4). Reprinted with permission from ref. 149. Copyright 2014 Wiley-VCH.

Chem. Soc. Rev.


View Article Online


Chem Soc Rev

planes of thin-layered nanostructures.149 Theoretical calculation shows that single-layered g-C3N4 with small sheet size can easily cause two-photon absorption (TPA). g-C3N4 QDs with the diameter of only a few nanometers (B4 nm, Fig. 9a) and the thickness of a single C–N layer (B0.35 nm, Fig. 9b and c) were prepared by three steps, including acid treatment, exfoliation and further hydrothermal treatment. The synthesized g-C3N4 QDs could concurrently absorb two near-infrared photons and emit bright fluorescence in the visible-light region, which is the typical TPA feature. Fig. 9d shows that g-C3N4 QDs could enter the nuclei of HepG2 cells while g-C3N4 with relatively large sheet size (B30 nm and B100 nm) could not pass through the nuclear membrane to penetrate into the nuclei. The twophoton fluorescence-imaging performance (Fig. 9d and e) of g-C3N4 QDs was compared with the commercial nuclear staining regent 4 0 ,6-diamidino-2-phenylindole (DAPI). It was found that the two-photon fluorescence nuclear imaging of g-C3N4 QDs agreed with that of DAPI, but the cost of g-C3N4 QDs was relatively low. Importantly, the nuclei information could be effectively lightened up after the two-photon fluorescence imaging of g-C3N4 QDs, demonstrating their high intracellular and intranuclear fluorescent-imaging capabilities. A one-step degradation of C3N4 nanosheets with the assistance of catalase was developed to produce fluorescent N-doped carbon (N–C) dots with particle size of 5 nm, which can overcome the drawbacks of large sheet sizes of C3N4 while the excellent fluorescent properties could be maintained. The N–C dots exhibited low cytotoxicity and strong blue fluorescence at the excitation wavelength of l = 405 nm, which can also be used for cell imaging.150 Xie et al. also found that the ultrathin liquid-exfoliated g-C3N4 showed an extremely high photoluminescence quantum yield up to 19.6% for intracellular bioimaging.151 Note that bioimaging of g-C3N4 nanosheets for intracellular imaging is highly effective. However, their application for further in vivo fluorescent imaging is severely restricted because of their emission of blue fluorescence, which is not in the range of the widely accepted NIR window of wavelengths (700–1300 nm). 4.2

of high Z are expected to act as CAs for CT imaging. The use of WS2–PEG as the CA for CT imaging was successfully demonstrated due to the higher Z of W (Z = 74) compared to the clinically used I (Z = 53).134 The enhanced in vitro CT performance of WS2–PEG was revealed compared to commercial iodine-based CT CAs (Iopromide). Importantly, WS2–PEG could accumulate within tumor tissues to give the apparent contrastenhanced CT imaging after the intravenous injection of WS2– PEG. Zhao et al. also showed the high CT-imaging performance of BSA-modified WS2 nanosheets for nude mice bearing HeLa tumors after intratumoral administration of the CAs.114 The capability of MoS2 nanosheets as the CAs for CT imaging was preliminarily revealed, showing slightly higher in vitro imaging performance compared to the commercial Iopromide.141 Bismuth (Z = 83) is regarded as the element with almost the highest atomic number (Z) among various metal elements with satisfactory biocompatibility. Different from most explored Bi2S3 NPs for CT imaging,153,154 2D topological insulator bismuth selenide (Bi2Se3) nanoplates possess specific functions of concurrent photothermal therapy and CT imaging.100 In addition to the low cytotoxicity of Bi,155 Se is an essential trace element for the human body. Thus, Bi2Se3 nanoplates are expected to show high biocompatibility. Compared to other metals, Bi shows one of the largest X-ray attenuation coefficients (Bi: 5.74, Au: 5.16, Pt: 4.99,

Photoacoustic tomography

Photoacoustic tomography (PAT) has attracted much recent attention for diagnostic imaging due to its specific features of high imaging depth and spatial resolution.152 As a new diagnosticimaging modality, PAT typically requires the CAs to further improve their imaging performance. Nanoprobes with strong NIRabsorbance are generally regarded as the desirable CAs for PAT. Based on the excellent NIR-absorbance performance, Liu et al. employed PEGylated WS2 nanosheets as the CAs for PAT imaging.134 Under a PAT imaging system using a 700 nm laser as the excitation source, strong photoacoustic signals of the 4T1 tumor could be recorded after either intratumor or intravenous administration of WS2–PEG while only major blood vasculatures could be found in mice without the administration of WS2–PEG CAs. 4.3

Review Article

Computed tomography imaging

An element with high atomic number (Z) can be used as the CA for computed tomography (CT). 2D-GAs composed of elements


Fig. 10 (a) In vitro CT images of Bi2Se3 nanoplates and Iopamidol at varied concentrations. (b) CT HU value of Bi2Se3 nanoplates and Iopamidol as a function of Bi and I concentrations. (c) In vivo CT coronal imaging of tumor-bearing mouse after the intratumor administration of Bi2Se3, and corresponding three-dimensional CT reconstruction images. Reprinted with permission from ref. 100. Copyright 2013 Nature Publishing Group.

Chem. Soc. Rev.

View Article Online


Review Article

Ta: 4.3 and I: 1.94 at 100 keV).154 Thus, the in vitro CT images (Fig. 10a) of Bi2Se3 nanoplates displayed the substantially enhanced HU values compared to the commercial Iopamidol at the same Bi and I concentrations (Fig. 10a and b). The enhanced CT-imaging performance of Bi2Se3 nanoplates indicates that the low doses of CAs can cause the equivalent CT contrasts compared to clinical iodinate-based CAs, which can concurrently increase CT-imaging accuracy and the biosafety of CAs. After the in situ injection of Bi2Se3 nanoplates into tumor tissues, a significantly brighter CT signal could be observed within tumor tissues compared to the surrounding soft tissues (Fig. 10c), demonstrating the high in vivo CT imaging performance of Bi2Se3. Although the intratumor injection is not a preferred manner, this preliminary in vivo assessment indeed showed their high performance for in vivo CT imaging. It is anticipated that the in vivo CT-imaging performances after intravenous administration can be further realized by additional surface modifications such as PEGylation or targeting engineering. 4.4

Magnetic resonance imaging

Due to the high spatial resolution, excellent contrast difference of three-dimensional soft tissues and non-invasive feature, magnetic

Chem Soc Rev

resonance imaging (MRI) has been extensively applied for clinically diagnostic imaging.156,157 Although Gd-based CAs are demonstrated to be effective in improving the accuracy of diagnosis, the US Food and Drug Administration (FDA) has warned about their potential toxicities related to nephrogenic systemic fibrosis with impaired kidney function.158 Comparatively, manganese-based T1-weighted MRI CAs show high potential for substituting Gd-based MRI CAs because of the necessary daily uptake of manganese for physiological metabolism by the human body.159 In addition, the human body can control the homeostasis of manganese.160 The slow development of manganese-based MRI CAs is mainly due to their relatively low imaging performance (typically r1 o 0.5 mM1 s1) compared to commercial Gd-based agents (r1 E 3.4 mM1 s1).161–163 The high in vivo MRI performance of PEGylated 2D MnO nanoplates (r1 = 5.5 mM1 s1) was demonstrated by Hyeon et al.164 They found that the contrast-enhanced T1-weighted MRI could occur almost in the whole body by using the intravenous administration of 8 nm-sized PEGylated MnO nanoplates as CAs. Kidney excretion of MnO CAs was also observed due to the small particulate size of PEGylated MnO nanoplates.

Fig. 11 (a) TEM image of PEG–MnO2 nanosheets. In vitro dynamic measurement of T1-weighted MR imaging of PEG–MnO2 in (pH = 4.6, b) mild acidic environment and (pH = 7.4, c) neutral condition. (d) Axial and (e) coronal T1-weighted MR imaging of 4T1 tumor-bearing nude mice before (d1 and e1) and after (d2–d9 and e2–e9) injection of PEG–MnO2 nanosheets saline solution within tumor and normal subcutaneous tissue. Quantitative T1-wieghted MRI signal intensity before and after the injection of PEG–MnO2 nanosheets saline solution (f: axial tumor region in d, g: coronal tumor region and normal subcutaneous tissue in e). Reprinted with permission from ref. 120. Copyright 2014 Wiley-VCH.

Chem. Soc. Rev.


View Article Online


Chem Soc Rev

Very recently, we solved the critical issue of low-MRI performance of Mn-based CAs based on the unique break-up nature of 2D MnO2 nanosheets.120 The highly dispersed MnO2 nanosheets were fabricated by the typical chemical exfoliation approach. The surface was further under PEGylation to improve their stability in physiological conditions (Fig. 11a). The in vitro dynamic evaluation of T1-weighted MRI showed that the T1 signal gave a substantial increase after the incubation of MnO2 nanosheets within the mild acidic environment (pH 4.6, Fig. 11b) while the impregnation of PEG–MnO2 nanosheets under neutral condition (pH 7.4) did not cause the obvious positive-signal enhancement (Fig. 11c). The relaxation rate (r1 value) was only 0.007 mM1 s1 in the neutral solution due to the high valence (IV) of Mn, but increased to 3.4 mM1 s1 and 4.0 mM1 s1 under the mild acidic environment of pH 6.0 and pH 4.6, respectively. Such an ultrasensitive pH-triggered MRI enhancement was due to the gradual disintegration of MnO2 nanosheets. The released MnII ions from MnO2 nanosheets had the maximum chance to interact with water molecules, thus leading to the enhancement of MRI signals. Importantly, the in vivo evaluation on tumor xenograft further demonstrated that the acidic tumor microenvironment can trigger the break-up of MnO2 nanosheets to enhance the T1-weighted MRI while the normal tissues with neutral pH microenvironment cannot cause such a change (Fig. 11d–g).165 Such a unique break-up nature of 2D MnO2 nanosheets can also facilitate the excretion of nanosheets out of the body due to the extremely small size of leaked MnII ions cleared by kidney. It is well known that traditional CAs, e.g., Fe3O4, Au and Iopamidol, perform well in various diagnostic-imaging modalities. Comparatively, the introduced 2D GA-based CAs can exert some specific or enhanced performances in these imaging modalities such as tumor microenvironment-responsive MR imaging,120 significantly enhanced in vitro CT imaging performance100 and imaging-guided cancer therapy.100,120,134 However, these new 2D GA-based CAs have not been screened by systematic in vivo evaluations regarding their biosafety and performance compared to the mostly explored CAs like Fe3O4 and Au NPs. Nevertheless, the preliminary results of 2D-GAs indicate their potential for clinical translations in diagnostic imaging.

Review Article

5. Biosensors In addition to the high performance of 2D-GAs for therapy and diagnostic imaging of cancer, these 2D nanosystems can also be used as novel biosensing platforms for the detection of biomolecules and bio-effects. Compared to various NP-based biosensors, 2D-GAs have two apparent advantageous features in biosensing applications. On the one hand, the large surface area of 2D planar structures allows the immobilization of large amount of sensing molecules to concurrently cause the short assay duration and low detection limit. On the other hand, the unique physical property of 2D nanosheets can exert some unusual performance such as the fluorescent quenching effect caused by the photoinduced electron transfer (PET) from the excited fluorophore to the conduction band of 2D nanosheets. Recently, we demonstrated that single-layer MoS2 nanosheets could be used as a sensing platform for detection of DNA based on its strong fluorescence-quenching ability.166 As shown in Fig. 12a, MoS2 nanosheet could absorb dye-labeled singlestranded DNA (ssDNA) via van der Waals force between nucleobases and the basal plane of MoS2, by which the fluorescence of the dye was quenched. Afterwards, the fluorescence-quenching effect can be significantly inhibited when the ssDNA was hybridized with its complementary target DNA to form doublestranded DNA (dsDNA). The availability of nucleobases to the basal plane of MoS2 was substantially reduced due to the restriction of nucleobases between the densely negative-charged helical phosphate backbones, causing the weak interaction between dsDNA and MoS2 and subsequent retention of the fluorescence of the dye. This MoS2-based sensor shows a detection limit of 500 pM to DNA (biomacromolecules) and 5 mm to adenosine (small molecules). This homogeneous process is simple and fast (within a few minutes), which shows great promise for molecular diagnostics.166 As another example, based on the 2D g-C3N4 nanosheet, a DNA biosensor was designed by using the affinity changes of g-C3N4 to DNA probes when they targeted the analyte and the corresponding PET-based fluorescence quenching effect.168 In addition, the atomistic and quantum simulations showed that the single-layer MoS2 is highly

Fig. 12 (a) The scheme of the fluorometric DNA assay based on 2D MoS2 nanosheets as the biosensor. Reprinted with permission from ref. 166. Copyright 2013 American Chemical Society. (b) The translocation of DNA through the single-layer MoS2 nanopore. Reprinted with permission from ref. 167. Copyright 2014 American Chemical Society.


Chem. Soc. Rev.

View Article Online


Review Article

suitable for nanopore-based DNA sequencing with the signal-tonoise ratio (SNR) of more than 15 (Fig. 12b).167 Based on the fluorescence quenching effect of 2D WS2 nanosheets, Liu et al. established two biosensors, i.e., the nuclei acid hybridization model and protein–aptamer model, to detect the oligonucleotides and proteins.119 The binding of bioprobes onto WS2 nanosheets is reversible, which indicates that the interactions between bioprobes and WS2 nanosheets can be interrupted by the addition of other biomolecules. Such a reversible process was reflected by the fluorescence quenching and recovery based on a special energy-transfer process. In addition to being used as the fluorescent quencher, Jiang et al. found that WS2 nanosheets showed differential affinity towards short oligonucleotide fragment versus the ssDNA probe. Based on this mechanism, they developed a WS2-based biosensor to detect microRNAs (miRNAs) with a detection limit of 300 fm.169 The intrinsic photoluminescence (PL) effect of 2D MoS2 can be further used for the in situ sensing of biological systems, in which ion intercalation plays an important role.170 It has been demonstrated that the PL effect of 2D MoS2 nanosheets is caused by the hybridization between Pz orbitals of S atoms and d orbitals of Mo atoms, which can be manipulated by intercalating alkaline (Li+, Na+ and K+) and H+ ions. The quasi-2D MoS2 nanoflake was recently designed to monitor the ion transfer during enzymatic activities and ion exchange in both viable and nonviable cells.170 The oxidation of glucose by glucose oxidase can generate H+ and electrons, which can intercalate into the MoS2 nanoflake to quench its PL with the assistance of an external electric field because the intercalated H+ and electrons can transform the semiconducting phase of MoS2 into the metallic HxMoS2 phase. The PL modulation of quasi-2D MoS2 by ion intercalation was further used to investigate the ion exchange in yeast cells. Nonviable yeast cells could not quench the PL of quasi-2D MoS2 nanoflake because the ions (K+) could not intercalate into MoS2 without the assistance of a driving force. Comparatively,

Chem Soc Rev

the viable yeast cells could generate an electric field across the membrane by the transfer of K+ ions to the exterior of cells, which could drive the intercalation of K+ ions into quasi-2D MoS2 to quench the fluorescence. The control experiment using polystyrene particles (B5–10 mm) to substitute the yeast cells could not cause the PL quenching effect of MoS2 nanoflake, and the procedure of cell death caused by introducing methanol could gradually restore the PL of quasi-2D MoS2 nanoflake. Thus, this sensing ability of MoS2 could be used to reveal the cell viabilities. Importantly, this sensing procedure was expected to monitor the ion-related biological and medical procedures.170 Compared to traditional one-dimensional carbon nanotubes and silicon nanowires, 2D nanosheets are more suitable for fieldeffect transistor (FET)-based biosensors because of their unique electronic states and planar morphology. For example, there is no bandgap in pristine graphene while the single-layer MoS2 has a direct band gap of 1.8–1.9 eV. Sarkar et al. recently used MoS2 as the channel material for fabricating a new FET-based biosensor for the detection of pH and biomolecules.171 For the biosensing procedure, the dielectric layer covering the MoS2 channel was initially modified with biotin to capture streptavidin. The charged streptavidin could induce a gating effect after the capture to modulate the device current (Fig. 13a–d). For biosensing applications, the device showed significant current decrease after adding streptavidin in the 0.01 PBS solution while the addition of pure buffer could not cause the current change (Fig. 13e). This current decrease was due to the negative charge of the streptavidin in the 0.01 PBS solution. Comparatively, the addition of streptavidin solution with the pH value lower than the isoelectric point (pI) could cause the current increase compared to that in pure PBS buffer solution (Fig. 13f). This ultrasensitive and specific protein biosensing procedure could achieve the sensitivity of 196 even at 100 fm concentration. This work gives strong evidence that 2D MoS2 can be an excellent candidate for the next-generation low-cost FET-based biosensors for diagnostics.

Fig. 13 (a) The scheme of the principle of the MoS2-based FET biosensor. (b) Optical image of the MoS2 flake on 270 nm SiO2 grown on Si substrate. (c) Optical image of the MoS2 FET biosensor device. (d) Photograph image and schematic illustration of the chip with the biosensor device and macrofluidic channel. (e) The current change of a MoS2-based FET biosensor functionalized with biotin after the addition of streptavidin solution in pure buffer (0.01 PBS). (f) The current change after the addition of streptavidin solution at a pH of 4.75, less than the pI of streptavidin. Reprinted with permission from ref. 171. Copyright 2014 American Chemical Society.

Chem. Soc. Rev.


View Article Online

Chem Soc Rev


6. Biosafety/toxicity evaluations of 2D-GAs The biosafety and toxicity of 2D-GAs determine their further clinical translations. Similar to other organic/inorganic nanosystems, the biological effects such as the in vitro cellular uptake/location/toxicity and in vivo biodistribution/degradation/ excretion of these 2D nanosheets are highly related to their crucial structural/compositional parameters and physicochemical properties such as the sheet size, morphology, dispersion, surface status and hydrophilicity. Because of the use of various 2D-GAs in biomedicine, some crucial parameters of 2D-GAs, such as solubility, biodegradation and biocompatibility, are different from one another. In addition, the biocompatibility/biosafety evaluations of different types of 2D-GAs are still in progress. Therefore, it is still too early to claim the biosafety of 2D-GAs, though there are still some promising progresses indicating their potential in biomedicine. It has been demonstrated that several members of TMDs exhibit low cytotoxicities towards living cells. For instance, the chitosan-modified MoS2 (MoS2–CS) sheets revealed low cytotoxicities against KB (human epithelial carcinoma cell line) and Panc-1 (pancreatic carcinoma, epithelial-like cell line) cell lines, even at the high concentration of 400 mg mL1.141 After the modification of MoS2 with chitosan, the in vitro hemolytic effect of MoS2–CS against red blood cells (RBCs) was extremely low even at the concentration of as high as 800 mg mL1. PEGylated MoS2 nanosheets also showed the low cytotoxicity against HeLa cells at the high concentration of 0.16 mg mL1.140 In addition, BSA-modified WS2 nanosheets exhibited low in vitro cytotoxicities against HeLa cancer cells by using a typical standard Cell Counting Kit-8 (CCK-8) assay.114 Liu et al. found that PEGylated WS2 nanosheets had no obvious in vitro toxicity using several cell lines at the high concentration of up to 0.1 mg mL1 by the typical MTT assay,134 including murine breast cancer cells 4T1, human epithelial carcinoma cells HeLa and human embryo kidney cells 293T. Further cytotoxicity investigation revealed that the release of lactate dehydrogenase (LDH) and reactive oxygen species (ROS) was at normal levels compared to the untreated cells, further indicating the low cytotoxicities of PEGylated WS2 nanosheets. Very recently, exfoliated MoS2 and WS2 nanosheets were found to be much less hazardous than GO or halogenated graphene by MTT and WST-8 assays against human lung carcinoma epithelial cells A549.172 The in vivo potential cytotoxicity of PEGylated WS2 nanosheets against Balb/c mice was systematically assessed by the typical hematoxylin and eosin (H&E) assay, serum biochemistry assay and complete blood panel test (dose: 20 mg kg1).134 It was found that no obvious abnormal behavior of mice was observed during the assay (45 days after PTT). H&E result showed high histocompatibility and all the indexes of the blood test were in the normal range. The biocompatibility of PEGylated MoS2 nanosheets were assessed by the same procedure, which showed the low toxicities to Balb/c mice at the dose of 3.4 mg kg1 and feeding duration of 30 days.140 In addition, the surface status and sheet size of 2D-GAs affected their in vivo biodistribution and


Review Article

excretion. The kidney excretion was observed on the 8 nm-sized PEGylated MnO nanoplates.164 This unique clearance behavior is of high significance for the biosafety of these nanosheets.143 In addition, the circulation time and enhanced permeability and retention (EPR)-mediated tumor targeting of 2D-GAs strongly depend on the surface organic modifications, particle sizes, hydrophilicity and dispersity/stability, which are similar to most explored nanosystems. The long blood-circulation time and enhanced accumulation within tumor tissues can also be achieved as long as aforementioned compositional/structural parameters are optimized. For instance, the PEGylated MoS2 nanosheets exhibited relatively long blood-circulation duration with the blood level of 3.67% of injected dose per gram tissue (%ID/g) retained even after the post-injection for 24 h. The tumor-accumulation amount at 24 h post-injection was determined to be 6.62% ID/g.140 Compared to GO or rGO sheets, the research on the biosafety of 2D-GA nanosheets is still at the very early stage. Only preliminary in vitro and in vivo assessments have been performed on the cytotoxicities or acute toxicities to mice. Although these results have shown promising biosafety of these 2D nanomaterials under the investigated doses, the evaluations on the biodistribution, tolerant threshold, degradation and clearance have not been systematically conducted yet. Especially, some metal elements of 2D-GAs (e.g., W, Mo, and Mn) are very rare in biological systems. Therefore, their retaining, excretion, potential toxicity and longterm influence to animals should also be revealed and determined to guarantee their safe application clinically. In this regard, more detailed research results concerning the evaluations on the biocompatibility and biosafety of these 2D-GAs are urgently required.

7. Conclusion and outlook This review summarized some of the recent important progresses of inorganic 2D layered nanosheets for biomedical applications. Note that many of them only emerged over the past few years. 2D-GAs with specific planar morphology and physicochemical properties have been demonstrated to be highly effective in drug delivery, diagnostic imaging and biosensing. Compared to GO/rGO sheets, these 2D-GAs can bring many more specific characteristics due to their abundant chemical compositions and diverse biological effects in biomedicine. However, the investigation of these 2D-GAs in biomedicine is still at their early stage, and several unresolved critical issues are to be addressed to further facilitate the advances of this field (Fig. 14). (i) From the materials point of view, one of the key challenges of 2D-GAs for biomedical applications is the lack of controllable and standard synthetic methodologies to obtain the nanosheets with desired structural/compositional parameters such as sheet size, dispersity, hydrophilicity and surface functionalities, which are the critical factors to determine the in vivo biological effects and therapeutic/diagnostic performances. Most of the reported nanosized 2D-GA nanosheets were fabricated by exfoliation from

Chem. Soc. Rev.

View Article Online


Review Article

Chem Soc Rev

Fig. 14 Summary of the development on 2D-GAs for biomedical applications, including the design/construction of 2D layered nanosheets based on the practical clinical-use criteria, current status and future developments of these 2D-GAs in biomedicine.

their bulk crystals. However, this method can only produce 2D-GAs with wide sheet-size distribution. The bottom-up synthesis is regarded as the alternative approach to solve this issue, while how to produce the 2D-GAs on a large scale is still a big challenge. (ii) Detailed biological and biosafety assessments of these 2D-GA nanosheets are urgently required to ensure their further clinical translations. Although the preliminary evaluation has demonstrated potentially low toxicities of several sub-families of 2D-GAs (e.g., MoS2, WS2, MnO2, and g-C3N4), their potential risks should be further determined and revealed from current acute toxicity assessment to further chronic toxicity evaluation. The assessments should be further concentrated on the biodistribution, biodegradation, excretion and potential toxicities to specific organs, including neurotoxicity, reproductive toxicity and the influences to embryonic development. In addition, animal models should be further updated to large animals such as pigs or primates. Only after the biosafety of these 2D nanosheets is well proved, it will be possible to further carry out potential clinical trials. (iii) The application fields of these 2D-GA nanosheets are much less than the widely explored 2D GO/rGO, partially due to the short research history of the 2D-GAs. In addition to the reported applications in drug delivery, diagnostic imaging and biosensing, these 2D-GA nanosheets also show great potentials in other biomedical aspects such as gene therapy, tissue engineering, and radiosensitization. In addition, the nonspecific accumulation of these 2D nanosheets in the reticuloendothelial system (RES) is still high due to the lack of targeting specificities. Thus, the surface engineering of 2D-GAs with targeting functions can endow these carriers with the capability

Chem. Soc. Rev.

to accumulate at specific cells or sub-cellular organelles, and reduce the accumulation in RES organs. Based on this targeting strategy, the high therapeutic/diagnostic performance is expected to be enhanced and the side-effects to normal tissues can be mitigated simultaneously. (iv) The 2D-GA nanosheets are expected to be further integrated with many other biocompatible components, by which multiple diagnostic and therapeutic purposes can be realized. Integration of CAs with 2D layered nanosheets can bring with multiple functions of concurrent diagnostic imaging and therapy (theranostics). In addition, integration of different therapeutic modalities can realize the synergistically therapeutic outcome. However, it should be noted that such a multifunctionalization process requires more strict and complex biosafety evaluations of each component and whole integrated nanosystems for further potential clinical translations. Compared to the mostly explored and nearly mature liposomes, micelles, Fe3O4 and Au NPs, the biomedical applications of 2D-GAs are now still at an early stage. We understand that much more research effort is necessary to be conducted before these 2D GA-based materials come into the clinical stage. Encouragingly, these new material-based nanosystems in biomedicine indeed provide new opportunities to promote the generation of new diagnostic-imaging and therapeutic modalities such as enhanced/intelligent MR imaging, on-demand drug releasing, and synergistic therapy, which are difficult to be realized by traditional liposomes, micelles and inorganic nanocrystals. The biomedical applications of 2D-GAs are now the much focused research frontier in materials science, biomedicine and nanobiotechnology. Although there are still some unresolved issues, the relatively high biosafety at appropriate doses and


View Article Online


Chem Soc Rev

demonstrated performance in biomedicines shed light on new pathways towards efficient clinical translations. To realize this goal, much closer collaborations among the researchers and experts from multidisciplinary fields should be further established to promote the clinical translations of these 2D-GAs. Given the very short research history and rapid development trajectory of this field, we are confident that the biosafety and pharmaceutical issues of the elaborately designed and synthesized 2D-GA nanosheets could be further addressed in the near future, which may eventually lead to clinical trials to benefit our human health. In addition to the emerging applications in biomedicine, it should be noted that these new 2D-GAs have shown important roles in energy- and environment-related areas. For instance, they can be used for flexible supercapacitors30,173,174 or electrode materials of lithium-ion batteries175–180 with enhanced electrochemical performance. In light harvesting area, these 2D-GAs can be used as efficient and stable photocatalysts for solar fuel hydrogen production via a water splitting process.94,181–186 In addition, these 2D-GAs also exhibit high performance in gas sensors,187 phototransistors188,189 and various device applications.190–192 The new application opportunities of 2D-GAs in turn also promote the rapid development of new synthetic methodologies of 2D-GAs with desirable structural and compositional parameters. On this ground, previous studies over the past few years are expected to pave a solid base for using these 2D-GAs to benefit our society not only in human healthcare but also more broadly in other important areas such as energy and environment.

List of abbreviations 2D-GAs 2D TMDs g-C3N4 LDH PTT PTA PDT MRI CT PAT QDs GO/rGO PEG PVP MB BSA NPs NIR CAs Dox MDR PET FET

Two dimensional graphene analogues Two dimensional Transition metal dichalcogenides Graphitic-phase carbon nitride Layered double hydroxide Photothermal therapy Photothermal agent Photodynamic therapy Magnetic resonance imaging Computed tomography Photoacoustic tomography Quantum dots Graphene oxide/reduced graphene oxide Polyethylene glycol Polyvinylpyrrolidone Methylene blue Bovine serum albumin Nanoparticles Near-infrared Contrast agents Doxorubicin Multidrug resistance Photoinduced electron transfer Field-effect transistors


Review Article

RBCs ROS RES CVD EPR

Red blood cells Reactive oxygen species Reticuloendothelial system Chemical vapor deposition Enhanced permeability and retention

Acknowledgements L. Z. Wang acknowledges the financial support from Australian Research Council through its Discovery and Future Fellowship schemes. Y. Chen acknowledges the financial support from the National Nature Science Foundation of China (Grant No. 51302293), the Natural Science Foundation of Shanghai (13ZR1463500), the Shanghai Rising-Star Program (14QA1404100) and the Biomedical ECR Grant 2014. This work was also supported by MOE under AcRF Tier 2 (ARC 26/13, No. MOE2013-T2-1-034), AcRF Tier 1 (RG 61/12, RGT18/13, and RG5/13), and Start-Up Grant (M4080865.070.706022) in Singapore. The NTU-HUJ-BGU Nanomaterials for Energy and Water Management Programme under the Campus for Research Excellence and Technological Enterprise (CREATE), which was supported by the National Research Foundation, Prime Minister’s Office, Singapore, is also acknowledged.

References 1 V. Wagner, A. Dullaart, A. K. Bock and A. Zweck, Nat. Biotechnol., 2006, 24, 1211–1217. 2 C. E. Ashley, E. C. Carnes, G. K. Phillips, D. Padilla, P. N. Durfee, P. A. Brown, T. N. Hanna, J. W. Liu, B. Phillips, M. B. Carter, N. J. Carroll, X. M. Jiang, D. R. Dunphy, C. L. Willman, D. N. Petsev, D. G. Evans, A. N. Parikh, B. Chackerian, W. Wharton, D. S. Peabody and C. J. Brinker, Nat. Mater., 2011, 10, 389–397. 3 D. Peer, J. M. Karp, S. Hong, O. C. FaroKhzad, R. Margalit and R. Langer, Nat. Nanotechnol., 2007, 2, 751–760. 4 R. K. Jain and T. Stylianopoulos, Nat. Rev. Clin. Oncol., 2010, 7, 653–664. 5 M. Ferrari, Nat. Rev. Cancer, 2005, 5, 161–171. 6 Y. Chen, H. Chen and J. Shi, Mol. Pharmacol., 2014, 11, 2495–2510. 7 Y. Geng, P. Dalhaimer, S. S. Cai, R. Tsai, M. Tewari, T. Minko and D. E. Discher, Nat. Nanotechnol., 2007, 2, 249–255. 8 T. Yu, D. Hubbard, A. Ray and H. Ghandehari, J. Controlled Release, 2012, 163, 46–54. 9 X. L. Huang, L. L. Li, T. L. Liu, N. J. Hao, H. Y. Liu, D. Chen and F. Q. Tang, ACS Nano, 2011, 5, 5390–5399. 10 T. Yu, K. Greish, L. D. McGill, A. Ray and H. Ghandehari, ACS Nano, 2012, 6, 2289–2301. 11 Z. Liu, X. M. Sun, N. Nakayama-Ratchford and H. J. Dai, ACS Nano, 2007, 1, 50–56. 12 Y. H. Bai, Y. Zhang, J. P. Zhang, Q. X. Mu, W. D. Zhang, E. R. Butch, S. E. Snyder and B. Yan, Nat. Nanotechnol., 2010, 5, 683–689.

Chem. Soc. Rev.

View Article Online


Review Article

13 F. Peng, Y. Su, X. Wei, Y. Lu, Y. Zhou, Y. Zhong, S.-T. Lee and Y. He, Angew. Chem., Int. Ed., 2013, 52, 1457–1461. 14 Y. Piao, J. Kim, H. Bin Na, D. Kim, J. S. Baek, M. K. Ko, J. H. Lee, M. Shokouhimehr and T. Hyeon, Nat. Mater., 2008, 7, 242–247. 15 Y. Chen, H. R. Chen, S. J. Zhang, F. Chen, L. X. Zhang, J. M. Zhang, M. Zhu, H. X. Wu, L. M. Guo, J. W. Feng and J. L. Shi, Adv. Funct. Mater., 2011, 21, 270–278. 16 M. S. Yavuz, Y. Y. Cheng, J. Y. Chen, C. M. Cobley, Q. Zhang, M. Rycenga, J. W. Xie, C. Kim, K. H. Song, A. G. Schwartz, L. H. V. Wang and Y. N. Xia, Nat. Mater., 2009, 8, 935–939. 17 S. E. Skrabalak, J. Y. Chen, Y. G. Sun, X. M. Lu, L. Au, C. M. Cobley and Y. N. Xia, Acc. Chem. Res., 2008, 41, 1587–1595. 18 Y. Chen, P. Xu, M. Wu, Q. Meng, H. Chen, Z. Shu, J. Wang, L. Zhang, Y. Li and J. Shi, Adv. Mater., 2014, 26, 4294–4301. 19 Y. Chen, H. Chen and J. Shi, Adv. Healthcare Mater., 2014, DOI: 10.1002/adhm.201400127. 20 Y. Chen, H. Chen and J. Shi, Adv. Mater., 2013, 25, 3144–3176. 21 Y. Chen, H. R. Chen, L. M. Guo, Q. J. He, F. Chen, J. Zhou, J. W. Feng and J. L. Shi, ACS Nano, 2010, 4, 529–539. 22 Y. Chen, H. R. Chen and J. L. Shi, Acc. Chem. Res., 2014, 47, 125–137. 23 Z. X. Li, J. C. Barnes, A. Bosoy, J. F. Stoddart and J. I. Zink, Chem. Soc. Rev., 2012, 41, 2590–2605. 24 P. P. Yang, S. L. Gai and J. Lin, Chem. Soc. Rev., 2012, 41, 3679–3698. 25 J. E. Lee, N. Lee, T. Kim, J. Kim and T. Hyeon, Acc. Chem. Res., 2011, 44, 893–902. 26 Y. Chen, P. F. Xu, H. R. Chen, Y. S. Li, W. B. Bu, Z. Shu, Y. P. Li, J. M. Zhang, L. X. Zhang, L. M. Pan, X. Z. Cui, Z. L. Hua, J. Wang, L. L. Zhang and J. L. Shi, Adv. Mater., 2013, 25, 3100–3105. 27 H. Li, J. Wu, Z. Yin and H. Zhang, Acc. Chem. Res., 2014, 47, 1067–1075. 28 A. B. Kaul, J. Mater. Res., 2014, 29, 348–361. 29 X. Zhang and Y. Xie, Chem. Soc. Rev., 2013, 42, 8187–8199. 30 X. Peng, L. Peng, C. Wu and Y. Xie, Chem. Soc. Rev., 2014, 43, 3303–3323. 31 V. Sorkin, H. Pan, H. Shi, S. Y. Quek and Y. W. Zhang, Crit. Rev. Solid State Mater. Sci., 2014, 39, 319–367. 32 Q. Tang and Z. Zhou, Prog. Mater. Sci., 2013, 58, 1244–1315. 33 T. F. Jaramillo, K. P. Jorgensen, J. Bonde, J. H. Nielsen, S. Horch and I. Chorkendorff, Science, 2007, 317, 100–102. 34 C. H. Lui, L. Liu, K. F. Mak, G. W. Flynn and T. F. Heinz, Nature, 2009, 462, 339–341. 35 H. Q. Bao, Y. Z. Pan, Y. Ping, N. G. Sahoo, T. F. Wu, L. Li, J. Li and L. H. Gan, Small, 2011, 7, 1569–1578. 36 X. J. Fan, G. Z. Jiao, W. Zhao, P. F. Jin and X. Li, Nanoscale, 2013, 5, 1143–1152. 37 Z. Liu, J. T. Robinson, X. M. Sun and H. J. Dai, J. Am. Chem. Soc., 2008, 130, 10876–10877. 38 L. M. Zhang, J. G. Xia, Q. H. Zhao, L. W. Liu and Z. J. Zhang, Small, 2010, 6, 537–544.

Chem. Soc. Rev.

Chem Soc Rev

39 B. A. Chen, M. Liu, L. M. Zhang, J. Huang, J. L. Yao and Z. J. Zhang, J. Mater. Chem., 2011, 21, 7736–7741. 40 L. Z. F. L. Z. Feng, S. A. Zhang and Z. A. Liu, Nanoscale, 2011, 3, 1252–1257. 41 L. M. Zhang, Z. X. Lu, Q. H. Zhao, J. Huang, H. Shen and Z. J. Zhang, Small, 2011, 7, 460–464. 42 J. T. Robinson, S. M. Tabakman, Y. Y. Liang, H. L. Wang, H. S. Casalongue, D. Vinh and H. J. Dai, J. Am. Chem. Soc., 2011, 133, 6825–6831. 43 H. W. Yang, Y. J. Lu, K. J. Lin, S. C. Hsu, C. Y. Huang, S. H. She, H. L. Liu, C. W. Lin, M. C. Xiao, S. P. Wey, P. Y. Chen, T. C. Yen, K. C. Wei and C. C. M. Ma, Biomaterials, 2013, 34, 7204–7214. 44 K. Yang, S. A. Zhang, G. X. Zhang, X. M. Sun, S. T. Lee and Z. A. Liu, Nano Lett., 2010, 10, 3318–3323. 45 B. Tian, C. Wang, S. Zhang, L. Z. Feng and Z. Liu, ACS Nano, 2011, 5, 7000–7009. 46 M. Pumera, Mater. Today, 2011, 14, 308–315. 47 Y. Wang, Y. Y. Shao, D. W. Matson, J. H. Li and Y. H. Lin, ACS Nano, 2010, 4, 1790–1798. 48 S. Sayyar, E. Murray, B. C. Thompson, S. Gambhir, D. L. Officer and G. G. Wallace, Carbon, 2013, 52, 296–304. 49 S. H. Ku, M. Lee and C. B. Park, Adv. Healthcare Mater., 2013, 2, 244–260. 50 G. Lalwani, A. M. Henslee, B. Farshid, L. J. Lin, F. K. Kasper, Y. X. Qin, A. G. Mikos and B. Sitharaman, Biomacromolecules, 2013, 14, 900–909. 51 G. Gollavelli and Y. C. Ling, Biomaterials, 2012, 33, 2532–2545. 52 C. S. Wang, J. Y. Li, C. Amatore, Y. Chen, H. Jiang and X. M. Wang, Angew. Chem., Int. Ed., 2011, 50, 11644–11648. 53 K. Yang, L. Z. Feng, X. Z. Shi and Z. Liu, Chem. Soc. Rev., 2013, 42, 530–547. 54 K. Yang, H. Gong, X. Z. Shi, J. M. Wan, Y. J. Zhang and Z. Liu, Biomaterials, 2013, 34, 2787–2795. 55 K. Yang, L. L. Hu, X. X. Ma, S. Q. Ye, L. Cheng, X. Z. Shi, C. H. Li, Y. G. Li and Z. Liu, Adv. Mater., 2012, 24, 1868–1872. 56 T. N. Narayanan, B. K. Gupta, S. A. Vithayathil, R. R. Aburto, S. A. Mani, J. Taha-Tijerina, B. Xie, B. A. Kaipparettu, S. V. Torti and P. M. Ajayan, Adv. Mater., 2012, 24, 2992–2998. 57 Y. T. Chen, F. Guo, Y. Qiu, H. R. Hu, I. Kulaots, E. Walsh and R. H. Hurt, ACS Nano, 2013, 7, 3744–3753. 58 Y. S. Jin, J. R. Wang, H. T. Ke, S. M. Wang and Z. F. Dai, Biomaterials, 2013, 34, 4794–4802. 59 G. Lalwani, J. L. Sundararaj, K. Schaefer, T. Button and B. Sitharaman, J. Mater. Chem. B, 2014, 2, 3519–3530. 60 H. Hong, Y. Zhang, J. W. Engle, T. R. Nayak, C. P. Theuer, R. J. Nickles, T. E. Barnhart and W. B. Cai, Biomaterials, 2012, 33, 4147–4156. 61 S. Shi, K. Yang, H. Hong, H. F. Valdovinos, T. R. Nayak, Y. Zhang, C. P. Theuer, T. E. Barnhart, Z. Liu and W. Cai, Biomaterials, 2013, 34, 3002–3009. 62 B. Cornelissen, S. Able, V. Kersemans, P. A. Waghorn, S. Myhra, K. Jurkshat, A. Crossley and K. A. Vallis, Biomaterials, 2013, 34, 1146–1154. 63 D. Bitounis, H. Ali-Boucetta, B. H. Hong, D. H. Min and K. Kostarelos, Adv. Mater., 2013, 25, 2258–2268.


View Article Online


Chem Soc Rev

64 C. Y. Cha, S. R. Shin, N. Annabi, M. R. Dokmeci and A. Khademhosseini, ACS Nano, 2013, 7, 2891–2897. 65 C. Chung, Y. K. Kim, D. Shin, S. R. Ryoo, B. H. Hong and D. H. Min, Acc. Chem. Res., 2013, 46, 2211–2224. 66 L. Y. Feng, L. Wu and X. G. Qu, Adv. Mater., 2013, 25, 168–186. 67 H. Y. Mao, S. Laurent, W. Chen, O. Akhavan, M. Imani, A. A. Ashkarran and M. Mahmoudi, Chem. Rev., 2013, 113, 3407–3424. 68 K. Yang, Y. J. Li, X. F. Tan, R. Peng and Z. Liu, Small, 2013, 9, 1492–1503. 69 K. Y. K. Yang, J. M. Wan, S. A. Zhang, Y. J. Zhang, S. T. Lee and Z. A. Liu, ACS Nano, 2011, 5, 516–522. 70 S. Z. Butler, S. M. Hollen, L. Y. Cao, Y. Cui, J. A. Gupta, H. R. Gutierrez, T. F. Heinz, S. S. Hong, J. X. Huang, A. F. Ismach, E. Johnston-Halperin, M. Kuno, V. V. Plashnitsa, R. D. Robinson, R. S. Ruoff, S. Salahuddin, J. Shan, L. Shi, M. G. Spencer, M. Terrones, W. Windl and J. E. Goldberger, ACS Nano, 2013, 7, 2898–2926. 71 J. D. Lin, C. Han, F. Wang, R. Wang, D. Xiang, S. Qin, X.-A. Zhang, L. Wang, H. Zhang, A. T. S. Wee and W. Chen, ACS Nano, 2014, 8, 5323–5329. 72 Z. Zeng, C. Tan, X. Huang, S. Bao and H. Zhang, Energy Environ. Sci., 2014, 7, 797–803. 73 C. Tan, X. Qi, X. Huang, J. Yang, B. Zheng, Z. An, R. Chen, J. Wei, B. Z. Tang, W. Huang and H. Zhang, Adv. Mater., 2014, 26, 1735–1739. 74 X. Gu, W. Cui, H. Li, Z. Wu, Z. Zeng, S.-T. Lee, H. Zhang and B. Sun, Adv. Energy Mater., 2013, 3, 1262–1268. 75 H. Li, J. Wu, X. Huang, G. Lu, J. Yang, X. Lu, Q. Zhang and H. Zhang, ACS Nano, 2013, 7, 10344–10353. 76 X. Huang, Z. Zeng, S. Bao, M. Wang, X. Qi, Z. Fan and H. Zhang, Nat. Commun., 2013, 4, 1444. 77 S. Wu, Z. Zeng, Q. He, Z. Wang, S. J. Wang, Y. Du, Z. Yin, X. Sun, W. Chen and H. Zhang, Small, 2012, 8, 2264–2270. 78 Q. Wang and D. O’Hare, Chem. Rev., 2012, 112, 4124–4155. 79 K. Ladewig, Z. P. Xu and G. Q. Lu, Expert Opin. Drug Delivery, 2009, 6, 907–922. 80 Q. H. Wang, K. Kalantar-Zadeh, A. Kis, J. N. Coleman and M. S. Strano, Nat. Nanotechnol., 2012, 7, 699–712. 81 M. S. Xu, T. Liang, M. M. Shi and H. Z. Chen, Chem. Rev., 2013, 113, 3766–3798. 82 O. Lopez-Sanchez, D. Lembke, M. Kayci, A. Radenovic and A. Kis, Nat. Nanotechnol., 2013, 8, 497–501. 83 M. Chhowalla, H. S. Shin, G. Eda, L. J. Li, K. P. Loh and H. Zhang, Nat. Chem., 2013, 5, 263–275. 84 X. Huang, B. Zheng, Z. Liu, C. Tan, J. Liu, B. Chen, H. Li, J. Chen, X. Zhang, Z. Fan, W. Zhang, Z. Guo, F. Huo, Y. Yang, L.-H. Xie, W. Huang and H. Zhang, ACS Nano, 2014, 8, 8695–8701. 85 G. Sun, J. Liu, X. Zhang, X. Wang, H. Li, Y. Yu, W. Huang, H. Zhang and P. Chen, Angew. Chem., Int. Ed., 2014, 53, 12576–12580. 86 Z. Yin, X. Zhang, Y. Cai, J. Chen, J. I. Wong, Y.-Y. Tay, J. Chai, J. Wu, Z. Zeng, B. Zheng, H. Y. Yang and H. Zhang, Angew. Chem., Int. Ed., 2014, 53, 12560–12565.


Review Article

87 Z. Yin, B. Chen, M. Bosman, X. Cao, J. Chen, B. Zheng and H. Zhang, Small, 2014, 10, 3537–3543. 88 C. Xu, S. Peng, C. Tan, H. Ang, H. Tan, H. Zhang and Q. Yan, J. Mater. Chem. A, 2014, 2, 5597–5601. 89 L. Wang and T. Sasaki, Chem. Rev., 2014, 114, 9455–9486. 90 Y. Omomo, T. Sasaki, L. Z. Wang and M. Watanabe, J. Am. Chem. Soc., 2003, 125, 3568–3575. 91 R. Z. Ma and T. Sasaki, Adv. Mater., 2010, 22, 5082–5104. 92 L. Z. Wang, N. Sakai, Y. Ebina, K. Takada and T. Sasaki, Chem. Mater., 2005, 17, 1352–1357. 93 R. Deng, X. Xie, M. Vendrell, Y.-T. Chang and X. Liu, J. Am. Chem. Soc., 2011, 133, 20168–20171. 94 X. C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen and M. Antonietti, Nat. Mater., 2009, 8, 76–80. 95 X. H. Li and M. Antonietti, Chem. Soc. Rev., 2013, 42, 6593–6604. 96 G. Liu, P. Niu, C. H. Sun, S. C. Smith, Z. G. Chen, G. Q. Lu and H. M. Cheng, J. Am. Chem. Soc., 2010, 132, 11642–11648. 97 D. Golberg, Y. Bando, Y. Huang, T. Terao, M. Mitome, C. Tang and C. Zhi, ACS Nano, 2010, 4, 2979–2993. 98 Q. Weng, B. Wang, X. Wang, N. Hanagata, X. Li, D. Liu, X. Wang, X. Jiang, Y. Bando and D. Golberg, ACS Nano, 2014, 8, 6123–6130. 99 C. Zhi, Y. Bando, C. Tang, H. Kuwahara and D. Golberg, Adv. Mater., 2009, 21, 2889–2893. 100 J. Li, F. Jiang, B. Yang, X. R. Song, Y. Liu, H. H. Yang, D. R. Cao, W. R. Shi and G. N. Chen, Sci. Rep., 2013, 3, 1–7. 101 K. S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevich, S. V. Morozov and A. K. Geim, Proc. Natl. Acad. Sci. U. S. A., 2005, 102, 10451–10453. 102 L. H. Li, Y. Chen, G. Behan, H. Z. Zhang, M. Petravic and A. M. Glushenkov, J. Mater. Chem., 2011, 21, 11862–11866. 103 H. Li, G. Lu, Y. L. Wang, Z. Y. Yin, C. X. Cong, Q. Y. He, L. Wang, F. Ding, T. Yu and H. Zhang, Small, 2013, 9, 1974–1981. 104 J. N. Coleman, M. Lotya, A. O’Neill, S. D. Bergin, P. J. King, U. Khan, K. Young, A. Gaucher, S. De, R. J. Smith, I. V. Shvets, S. K. Arora, G. Stanton, H. Y. Kim, K. Lee, G. T. Kim, G. S. Duesberg, T. Hallam, J. J. Boland, J. J. Wang, J. F. Donegan, J. C. Grunlan, G. Moriarty, A. Shmeliov, R. J. Nicholls, J. M. Perkins, E. M. Grieveson, K. Theuwissen, D. W. McComb, P. D. Nellist and V. Nicolosi, Science, 2011, 331, 568–571. 105 R. J. Smith, P. J. King, M. Lotya, C. Wirtz, U. Khan, S. De, A. O’Neill, G. S. Duesberg, J. C. Grunlan, G. Moriarty, J. Chen, J. Wang, A. I. Minett, V. Nicolosi and J. N. Coleman, Adv. Mater., 2011, 23, 3944–3948. 106 Z. Y. Zeng, Z. Y. Yin, X. Huang, H. Li, Q. Y. He, G. Lu, F. Boey and H. Zhang, Angew. Chem., Int. Ed., 2011, 50, 11093–11097. 107 Z. Zeng, T. Sun, J. Zhu, X. Huang, Z. Yin, G. Lu, Z. Fan, Q. Yan, H. H. Hng and H. Zhang, Angew. Chem., Int. Ed., 2012, 51, 9052–9056. 108 K. K. Kim, A. Hsu, X. Jia, S. M. Kim, Y. Shi, M. Hofmann, D. Nezich, J. F. Rodriguez-Nieva, M. Dresselhaus, T. Palacios and J. Kong, Nano Lett., 2012, 12, 161–166.

Chem. Soc. Rev.

View Article Online


Review Article

109 S. Chatterjee, Z. Luo, M. Acerce, D. M. Yates, A. T. C. Johnson and L. G. Sneddon, Chem. Mater., 2011, 23, 4414–4416. 110 X. S. Wang, H. B. Feng, Y. M. Wu and L. Y. Jiao, J. Am. Chem. Soc., 2013, 135, 5304–5307. 111 K. K. Liu, W. J. Zhang, Y. H. Lee, Y. C. Lin, M. T. Chang, C. Su, C. S. Chang, H. Li, Y. M. Shi, H. Zhang, C. S. Lai and L. J. Li, Nano Lett., 2012, 12, 1538–1544. 112 Y. H. Lee, X. Q. Zhang, W. J. Zhang, M. T. Chang, C. T. Lin, K. D. Chang, Y. C. Yu, J. T. W. Wang, C. S. Chang, L. J. Li and T. W. Lin, Adv. Mater., 2012, 24, 2320–2325. 113 H. Matte, A. Gomathi, A. K. Manna, D. J. Late, R. Datta, S. K. Pati and C. N. R. Rao, Angew. Chem., Int. Ed., 2010, 49, 4059–4062. 114 Y. Yong, L. Zhou, Z. Gu, L. Yan, G. Tian, X. Zheng, X. Liu, X. Zhang, J. Shi, W. Cong, W. Yin and Y. Zhao, Nanoscale, 2014, 6, 10394–10403. 115 Y. J. Zhan, Z. Liu, S. Najmaei, P. M. Ajayan and J. Lou, Small, 2012, 8, 966–971. 116 Y. Y. Peng, Z. Y. Meng, C. Zhong, J. Lu, W. C. Yu, Z. P. Yang and Y. T. Qian, J. Solid State Chem., 2001, 159, 170–173. 117 Y. Y. Peng, Z. Y. Meng, C. Zhong, J. Lu, W. C. Yu, Y. B. Jia and Y. T. Qian, Chem. Lett., 2001, 772–773. 118 H. Matte, B. Plowman, R. Datta and C. N. R. Rao, Dalton Trans., 2011, 40, 10322–10325. 119 Y. X. Yuan, R. Q. Li and Z. H. Liu, Anal. Chem., 2014, 86, 3610–3615. 120 Y. Chen, D. Ye, M. Wu, H. Chen, L. Zhang, J. Shi and L. Wang, Adv. Mater., 2014, 26, 7019–7026. 121 T. Lammers, S. Aime, W. E. Hennink, G. Storm and F. Kiessling, Acc. Chem. Res., 2011, 44, 1029–1038. 122 S. Mura and P. Couvreur, Adv. Drug Delivery Rev., 2012, 64, 1394–1416. 123 Y. Chen, H. Chen and J. Shi, Expert Opin. Drug Delivery, 2014, 11, 917–930. 124 X. Huang, X. Qi, F. Boey and H. Zhang, Chem. Soc. Rev., 2012, 41, 666–686. 125 X. Huang, C. L. Tan, Z. Y. Yin and H. Zhang, Adv. Mater., 2014, 26, 2185–2204. 126 E. S. Shibu, M. Hamada, N. Murase and V. Biju, J. Photochem. Photobiol., C, 2013, 15, 53–72. 127 X. H. Huang, P. K. Jain, I. H. El-Sayed and M. A. El-Sayed, Lasers Med. Sci., 2008, 23, 217–228. 128 S. Lal, S. E. Clare and N. J. Halas, Acc. Chem. Res., 2008, 41, 1842–1851. 129 M. Ma, H. R. Chen, Y. Chen, X. Wang, F. Chen, X. Z. Cui and J. L. Shi, Biomaterials, 2012, 33, 989–998. 130 Z. Zhang, L. Wang, J. Wang, X. Jiang, X. Li, Z. Hu, Y. Ji, X. Wu and C. Chen, Adv. Mater., 2012, 24, 1418–1423. 131 M. Li, X. J. Yang, J. S. Ren, K. G. Qu and X. G. Qu, Adv. Mater., 2012, 24, 1722–1728. 132 Z. M. Markovic, L. M. Harhaji-Trajkovic, B. M. TodorovicMarkovic, D. P. Kepic, K. M. Arsikin, S. P. Jovanovic, A. C. Pantovic, M. D. Dramicanin and V. S. Trajkovic, Biomaterials, 2011, 32, 1121–1129.

Chem. Soc. Rev.

Chem Soc Rev

133 S. S. Chou, B. Kaehr, J. Kim, B. M. Foley, M. De, P. E. Hopkins, J. Huang, C. J. Brinker and V. P. Dravid, Angew. Chem., Int. Ed., 2013, 52, 4160–4164. 134 L. Cheng, J. Liu, X. Gu, H. Gong, X. Shi, T. Liu, C. Wang, X. Wang, G. Liu, H. Xing, W. Bu, B. Sun and Z. Liu, Adv. Mater., 2014, 26, 1886–1893. 135 R. C. Jin, Y. C. Cao, E. C. Hao, G. S. Metraux, G. C. Schatz and C. A. Mirkin, Nature, 2003, 425, 487–490. 136 P. K. Jain, X. Huang, I. H. El-Sayed and M. A. El-Sayad, Plasmonics, 2007, 2, 107–118. 137 Y. Chen, P. Xu, Z. Shu, M. Wu, L. Wang, S. Zhang, Y. Zheng, H. Chen, J. Wang, Y. Li and J. Shi, Adv. Funct. Mater., 2014, 24, 4386–4396. 138 L. S. Lin, Z. X. Cong, J. Li, K. M. Ke, S. S. Guo, H. H. Yang and G. N. Chen, J. Mater. Chem. B, 2014, 2, 1031–1037. 139 S. B. Yang, X. L. Feng, X. C. Wang and K. Mullen, Angew. Chem., Int. Ed., 2011, 50, 5339–5343. 140 T. Liu, C. Wang, X. Gu, H. Gong, L. Cheng, X. Shi, L. Feng, B. Sun and Z. Liu, Adv. Mater., 2014, 26, 3433–3440. 141 W. Yin, L. Yan, J. Yu, G. Tian, L. Zhou, X. Zheng, X. Zhang, Y. Yong, J. Li, Z. Gu and Y. Zhao, ACS Nano, 2014, 8, 6922–6933. 142 X. Yang, J. Li, T. Liang, C. Ma, Y. Zhang, H. Chen, N. Hanagata, H. Su and M. Xu, Nanoscale, 2014, 6, 10126–10133. 143 H. S. Choi, W. Liu, P. Misra, E. Tanaka, J. P. Zimmer, B. I. Ipe, M. G. Bawendi and J. V. Frangioni, Nat. Biotechnol., 2007, 25, 1165–1170. 144 X. H. Gao, Y. Y. Cui, R. M. Levenson, L. W. K. Chung and S. M. Nie, Nat. Biotechnol., 2004, 22, 969–976. 145 I. L. Medintz, H. T. Uyeda, E. R. Goldman and H. Mattoussi, Nat. Mater., 2005, 4, 435–446. 146 X. Michalet, F. F. Pinaud, L. A. Bentolila, J. M. Tsay, S. Doose, J. J. Li, G. Sundaresan, A. M. Wu, S. S. Gambhir and S. Weiss, Science, 2005, 307, 538–544. 147 Q. Liu, B. D. Guo, Z. Y. Rao, B. H. Zhang and J. R. Gong, Nano Lett., 2013, 13, 2436–2441. 148 H. G. Zhang, H. Hu, Y. Pan, J. H. Mao, M. Gao, H. M. Guo, S. X. Du, T. Greber and H. J. Gao, J. Phys.: Condens. Matter, 2010, 22, 302001. 149 X. Zhang, H. Wang, H. Wang, Q. Zhang, J. Xie, Y. Tian, J. Wang and Y. Xie, Adv. Mater., 2014, 26, 4438–4443. 150 N. Liu, J. Liu, W. Kong, H. Li, H. Huang, Y. Liu and Z. Kang, J. Mater. Chem. B, 2014, 2, 5768–5774. 151 X. Zhang, X. Xie, H. Wang, J. Zhang, B. Pan and Y. Xie, J. Am. Chem. Soc., 2013, 135, 18–21. 152 L. H. V. Wang and S. Hu, Science, 2012, 335, 1458–1462. 153 O. Rabin, J. M. Perez, J. Grimm, G. Wojtkiewicz and R. Weissleder, Nat. Mater., 2006, 5, 118–122. 154 K. L. Ai, Y. L. Liu, J. H. Liu, Q. H. Yuan, Y. Y. He and L. H. Lu, Adv. Mater., 2011, 23, 4886–4891. 155 G. G. Briand and N. Burford, Chem. Rev., 1999, 99, 2601–2657. 156 P. Caravan, J. J. Ellison, T. J. McMurry and R. B. Lauffer, Chem. Rev., 1999, 99, 2293–2352. 157 E. Terreno, D. D. Castelli, A. Viale and S. Aime, Chem. Rev., 2010, 110, 3019–3042.


View Article Online


Chem Soc Rev

158 Y. Chen, H. Chen, S. Zhang, F. Chen, S. Sun, Q. He, M. Ma, X. Wang, H. Wu, L. Zhang, L. Zhang and J. Shi, Biomaterials, 2012, 33, 2388–2398. 159 M. Kueny-Stotz, A. Garofalo and D. Felder-Flesch, Eur. J. Inorg. Chem., 2012, 1987–2005. 160 Y. Chen, H. R. Chen, Y. Sun, Y. Y. Zheng, D. P. Zeng, F. Q. Li, S. J. Zhang, X. Wang, K. Zhang, M. Ma, Q. J. He, L. L. Zhang and J. L. Shi, Angew. Chem., Int. Ed., 2011, 50, 12505–12509. 161 T. Kim, E. J. Cho, Y. Chae, M. Kim, A. Oh, J. Jin, E. S. Lee, H. Baik, S. Haam, J. S. Suh, Y. M. Huh and K. Lee, Angew. Chem., Int. Ed., 2011, 50, 10589–10593. 162 T. Kim, E. Momin, J. Choi, K. Yuan, H. Zaidi, J. Kim, M. Park, N. Lee, M. T. McMahon, A. Quinones-Hinojosa, J. W. M. Bulte, T. Hyeon and A. A. Gilad, J. Am. Chem. Soc., 2011, 133, 2955–2961. 163 H. B. Na, J. H. Lee, K. J. An, Y. I. Park, M. Park, I. S. Lee, D. H. Nam, S. T. Kim, S. H. Kim, S. W. Kim, K. H. Lim, K. S. Kim, S. O. Kim and T. Hyeon, Angew. Chem., Int. Ed., 2007, 46, 5397–5401. 164 M. Park, N. Lee, S. H. Choi, K. An, S.-H. Yu, J. H. Kim, S.-H. Kwon, D. Kim, H. Kim, S.-I. Baek, T.-Y. Ahn, O. K. Park, J. S. Son, Y.-E. Sung, Y.-W. Kim, Z. Wang, N. Pinna and T. Hyeon, Chem. Mater., 2011, 23, 3318–3324. 165 Y. Chen, Q. Yin, X. F. Ji, S. J. Zhang, H. R. Chen, Y. Y. Zheng, Y. Sun, H. Y. Qu, Z. Wang, Y. P. Li, X. Wang, K. Zhang, L. L. Zhang and J. L. Shi, Biomaterials, 2012, 33, 7126–7137. 166 C. F. Zhu, Z. Y. Zeng, H. Li, F. Li, C. H. Fan and H. Zhang, J. Am. Chem. Soc., 2013, 135, 5998–6001. 167 A. B. Farimani, K. Min and N. R. Aluru, ACS Nano, 2014, 8, 7914–7922. 168 Q. B. Wang, W. Wang, J. P. Lei, N. Xu, F. L. Gao and H. X. Ju, Anal. Chem., 2013, 85, 12182–12188. 169 Q. Xi, D. M. Zhou, Y. Y. Kan, J. Ge, Z. K. Wu, R. Q. Yu and J. H. Jiang, Anal. Chem., 2014, 86, 1361–1365. 170 J. Z. Ou, A. F. Chrimes, Y. C. Wang, S. Y. Tang, M. S. Strano and K. Kalantar-zadeh, Nano Lett., 2014, 14, 857–863. 171 D. Sarkar, W. Liu, X. Xie, A. C. Anselmo, S. Mitragotri and K. Banerjee, ACS Nano, 2014, 8, 3992–4003. 172 W. Z. Teo, E. L. K. Chng, Z. Sofer and M. Pumera, Chem. – Eur. J., 2014, 20, 9627–9632.


Review Article

173 M. F. El-Kady, V. Strong, S. Dubin and R. B. Kaner, Science, 2012, 335, 1326–1330. 174 M. R. Lukatskaya, O. Mashtalir, C. E. Ren, Y. Dall’Agnese, P. Rozier, P. L. Taberna, M. Naguib, P. Simon, M. W. Barsoum and Y. Gogotsi, Science, 2013, 341, 1502–1505. 175 T. Stephenson, Z. Li, B. Olsen and D. Mitlin, Energy Environ. Sci., 2014, 7, 209–231. 176 J. Xiao, D. Choi, L. Cosimbescu, P. Koech, J. Liu and J. P. Lemmon, Chem. Mater., 2010, 22, 4522–4524. 177 M. Pumera, Z. Sofer and A. Ambrosi, J. Mater. Chem. A, 2014, 2, 8981–8987. 178 L. David, R. Bhandavat and G. Singh, ACS Nano, 2014, 8, 1759–1770. 179 R. Bhandavat, L. David and G. Singh, J. Phys. Chem. Lett., 2012, 3, 1523–1530. 180 C. L. Tan and H. Zhang, Chem. Soc. Rev., 2015, DOI: 10.1039/c4cs00182f. 181 M. A. Lukowski, A. S. Daniel, F. Meng, A. Forticaux, L. S. Li and S. Jin, J. Am. Chem. Soc., 2013, 135, 10274–10277. 182 U. Maitra, U. Gupta, M. De, R. Datta, A. Govindaraj and C. N. R. Rao, Angew. Chem., Int. Ed., 2013, 52, 13057–13061. 183 M. A. Lukowski, A. S. Daniel, C. R. English, F. Meng, A. Forticaux, R. J. Hamers and S. Jin, Energy Environ. Sci., 2014, 7, 2608–2613. 184 J. H. Lee, W. S. Jang, S. W. Han and H. K. Baik, Langmuir, 2014, 30, 9866–9873. 185 F. H. Saadi, A. I. Carim, J. M. Velazquez, J. H. Baricuatro, C. C. L. McCrory, M. P. Soriaga and N. S. Lewis, ACS Catal., 2014, 4, 2866–2873. 186 M. S. Faber, M. A. Lukowski, Q. Ding, N. S. Kaiser and S. Jin, J. Phys. Chem. C, 2014, 118, 21347–21356. 187 H. Li, Z. Yin, Q. He, H. Li, X. Huang, G. Lu, D. W. H. Fam, A. I. Y. Tok, Q. Zhang and H. Zhang, Small, 2012, 8, 63–67. 188 Z. Yin, H. Li, H. Li, L. Jiang, Y. Shi, Y. Sun, G. Lu, Q. Zhang, X. Chen and H. Zhang, ACS Nano, 2012, 6, 74–80. 189 G. L. Frey, K. J. Reynolds, R. H. Friend, H. Cohen and Y. Feldman, J. Am. Chem. Soc., 2003, 125, 5998–6007. 190 D. Jariwala, V. K. Sangwan, L. J. Lauhon, T. J. Marks and M. C. Hersam, ACS Nano, 2014, 8, 1102–1120. 191 G. Eda and S. A. Maier, ACS Nano, 2013, 7, 5660–5665. 192 S. Ghatak, A. N. Pal and A. Ghosh, ACS Nano, 2011, 5, 7707–7712.

Chem. Soc. Rev.

Polymer-Enriched 3D Graphene Foams for Biomedical Applications.

Preparation and functionalization of graphene nanocomposites for biomedical applications.

Synthesis, toxicity, biocompatibility, and biomedical applications of graphene and graphene-related materials.

Functional nanoparticles for biomedical applications.

Crosslinking biopolymers for biomedical applications.

Titanium nanostructures for biomedical applications.

Three-dimensional printing of high-content graphene scaffolds for electronic and biomedical applications.

graphene oxide composite hydrogels for biomedical applications.

keratin composites for biomedical applications.

Magnetic nanoparticles adapted for specific biomedical applications.

Fluorescent probes for biomedical applications (2009-2014).

Dual-function antibacterial surfaces for biomedical applications.

Engineering Stem Cells for Biomedical Applications.

Stimuli-responsive nanomaterials for biomedical applications.

Dielectrophoresis for Biomedical Sciences Applications: A Review.

Carbon nanotubes reinforced composites for biomedical applications.

Biocompatible electrospun polymer blends for biomedical applications.

Designing fractal nanostructured biointerfaces for biomedical applications.

High-energy proton imaging for biomedical applications.

Functional supramolecular polymers for biomedical applications.

Design of Hydrogels for Biomedical Applications.

Biopolymeric formulations for biocatalysis and biomedical applications.

Photocontrolled nanoparticle delivery systems for biomedical applications.

A thermostatic system for biomedical applications.