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A Facile Strategy to Fabricate Thermoresponsive Polymer Functionalized CdTe/ZnS Quantum Dots: Assemblies and Optical Properties Bingxin Liu, Cuiyan Tong,* Lijuan Feng, Chunyu Wang, Yao He, Changli Lü* Novel thermoresponsive CdTe/ZnS quantum dots (QDs) decorated with a copolymer ligand (CPL) containing 8-hydroxyquinoline and NIPAM units are prepared through coordinate bonding in aqueous solution. The dependence of the morphology and optical properties of the QDs/CPL assemblies formed via coordinate bonding on the experimental conditions is studied. The coordinate induced self-assemblies are observed by controlling the molar ratio of QDs and CPL. The self-organized structure of QDs/CPL proceeds through a first step of QDs-chains, followed by a necklace-like single annular chain, and subsequently increases its annular chain structure, forming a network. The CPL functionalized QDs can emit multiple colors from the cooperating interaction between the inherent emission (606 nm) of the QDs and the surfacecoordinated emission (517 nm) of the CPL complex formed on the QD surface. For QDs-CPL systems, both Förster resonance energy transfer (FRET) and a high rate of photoinduced electron transfer (PET) are simultaneous, the latter mainly contributing to PL quenching. The thermoresponsive QDs/CPL assemblies also exhibit dual reversible PL properties between the inherent emission of QDs and surface-coordinated emission.

1. Introduction Semiconductor quantum dots (QDs), also known as semiconductor nanocrystals, have special physical dimensions that lay between those of bulk and discrete molecules.[1] Differing from their corresponding individual molecules or bulk materials, QDs exhibit unique size-dependent optical properties due to quantum confinement effects.[2–5] Studies of QDs have therefore generated tremendous interest over the past decade and they are often considered B. X. Liu, Dr. C. Y. Tong, C. Y. Wang, Y. He, Prof. C. L. Lü, College of Chemistry, Northeast Normal University, Changchun, 130024 , P. R. China E-mail: [email protected]; [email protected] Dr. L. J. Feng Centre of Analytical and Test, Beihua University, Jilin, 132013, P. R. China Macromol. Rapid Commun. 2014, 35, 77−83 © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

for different applications, ranging from light-emitting diodes (LEDs),[6,7] solar cells[8] and lasers[9] to biological sensors[10,11] and medicine.[12] Compared with traditional organic dyes, these water-soluble QDs are more attractive due to their strong photostability and size-dependent emission wavelength.[13] Poly(N-isopropylacrylamide) (PNIPAM) is a thermoresponsive polymer with a lower critical solution temperature (LCST) of around 34 °C in water.[14] The polymer undergoes phase transition at its LCST because of the collapse of individual chains from hydrated coils to hydrophobic globules. The attention of researchers was attracted by its thermoresponsive behavior, which is caused by a critical hydrophilic/hydrophobic balance of polymer side groups.[15] Thus the incorporation of QDs into PNIPAM microgels could provide a new generation of multifunctional QDs with practical applications. The QDs can be entrapped in the polymer matrix using

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DOI: 10.1002/marc.201300634

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different approaches. The polymerization of polymeric monomers in the presence of QDs is a simple strategy,[16] but it is difficult to prevent the QDs from aggregating during the polymerization. Wu et al. reported CdS QDs in situ embedded in the interior of p(NIPAM-AAm-PBA) microgels in response to a change in glucose concentration.[17] The PAAm segments designed in the microgels were not only able to combine with the Cd2+ precursors and stabilize the CdS QDs produced, but also greatly improved the stability of the resulting hybrid microgels. Shen and co-workers incorporated the QDs into already preformed polymer microspheres via hydrophobic forces.[18] They employed the carboxyl groups of these microgels to replace oleic acid from the surface of oleic acid-stabilized QDs. Gong[19] and Kuang[20] fabricated polymer microspheres embedded with QDs by hydrogen bonding and electrostatic interactions, respectively. However, these thermally responsive QDs–PNIPAM hybrids only showed PL quenching with each increment of temperature.[21] CdTe–PNIPAM microspheres synthesized by Wang et al. exhibited a red-shift of about 36 nm in the emission wavelength.[22] It is known that 8-hydroxyquinoline (HQ) and its derivatives can coordinate on the surface of nanoparticles such as ZnS and CdS to form stable fluorescent complexes because these particles have a great number of surface metal atoms.[23] The inherent luminescent emission of QDs may have a cooperating interaction with that of the HQ–metal complexes formed on the QDs surface. Thus, HQ and its derivatives can be used as a functional ligand to tailor the optical properties of HQ–QDs hybrids when HQ is anchored to the semiconductor nanoparticles.[24] However, the intrinsic fluorescence of these QDs, such as ZnS and ZnS:Mn, is too weak to clearly investigate the cooperating fluorescence interaction between the different emitting centers. To the best of our knowledge, there are, as yet, no reports on the direct exploration of ligand-functionalized PNIPAM protected highly fluorescent QDs via coordinate bonding in aqueous solutions. In this paper, 5-(2-methacryloylethyloxymethyl)8-quinolinol (MQ) was selected as a functional ligand to copolymerize with NIPAM monomer to fabricate a novel random copolymer p(NIPAM-co-MQ) which could then play a critical role in thermosensitive materials. Then the multifunctional hybrid QDs, which combine the features of photoluminescence and temperature sensitivity, were prepared through coordination bonds between QDs and copolymer ligand (CPL). The PL properties of the copolymer functionalized QDs also showed interesting changes at different temperatures and molar feed ratios of QDs to CPL. The assembly morphology and the PL properties of the copolymer functionalized QDs were systematically investigated.

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2. Results and Discussion The novel copolymer ligand (CPL) of p(NIPAM-co-MQ) was prepared by conventional free radical copolymerization of NIPAM and MQ monomers, and its structure was confirmed by 1H NMR (see Figure S1 in the Supporting Information). The 1H NMR response signals of MQ are very small due to the low percentage of MQ units on the copolymer. To more clearly observe the characteristic peaks of the MQ units in the copolymer, the partially amplified 1H NMR spectrum is shown in the inset of Figure S2 in the Supporting Information. The signal at 9.80 ppm was assigned to the H atomic nucleus of Ph-OH, and the chemical shifts of the pyridine ring protons for the MQ units appeared at 8.75–7.08 ppm. The above results indicate that the MQ monomers have been successfully incorporated into the copolymer. The signals of aromatic protons a and 2 in Figure 1 were used

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to calculate the molar ratio of NIPAM and MQ units in CPL and the result was 58:1. GPC measurements revealed ¯n ) of the that the number average molecular weight (M copolymer was around 26 000. This value is higher than that of the MMA-MQ copolymers.[25] Moreover, the copolymer had a relatively narrow molecular weight distribution with a polydispersity index (PDI) of 1.63. Temperature-dependent optical transmissions at 600 nm for the copolymer in aqueous solution did not vary until about 29 °C (see Figure 1). However, a sharp decrease in transmittance was observed between 30 and 36 °C, which is associated with the “coil-to-globule” transition of the connecting PNIPAM chain segments.[19] As shown in Figure 1b, the aqueous solution of copolymer appeared semi-transparent white after the phase transition with a higher temperature above 30 °C. Therefore the CPL keeps its thermoresponsive property with the lower critical solution temperature (LCST) of around 30 °C in water, which is lower than that of pure PNIPAM. This result indicates that the incorporation of a hydrophobic comonomer will reduce the amount of hydrophilic groups, which will result in a decrease in the hydrophilicity of the copolymer.[19,26,27] In this work, CdTe/ZnS QDs were synthesized through the in situ growth of a ZnS shell on the surface of CdTe cores in alkaline solution with an average diameter of about 3.7 nm (Figure 2a and 2b).[28] In order to study the morphology of the QD/CPL assemblies induced by CPL, TEM studies were carried out for QDs/CPL-2, 5, 7 and 9 with different molar feed ratios (see Figure 2c to 2f). The

QD-chains assembly can be observed in Figure 2c when the [QDs]/[CPL] is 3:100. With increasing CPL dosage, it is noteworthy that single annular QD-chains are formed for QD/CPL-5 with thick chains. QD/CPL-7 shows the double annular chains which may be formed through branching of the single annular chain assembly. Because the average diameter of the single annular chain is almost twice that of the double annular chain, the QD-chains tend to form a network with increasing CPL dosage. For QD/CPL-9, the degree of chain assembly and network formation is enhanced, and branching and interconnectivity between chains is observed. It can be seen that the generation of the network structure crosslinked by QDs results from the stable coordinate bond between the copolymer ligand and the zinc atoms on the QDs surface. There are three reasonable binding schemes that could exist between QDs and CPL: (i) the copolymers could connect with their binding groups to different QDs, forming a network, and they could be wrapped around the QDs; (ii) the copolymers could be linked to the same particle at multiple sites and form loops on the QDs’ surfaces; (iii) the copolymers could bind to the QDs with only a single HQ moiety, leaving the rest of the unreacted HQ moiety. From the TEM images, a possible coordination structure resulting from the bridging of neighboring QDs induced by CPL chains and the copolymers connecting with their binding groups to different QDs is supposed. The formation process for the coordinate induced self-assembly is presented in Scheme 1. The interaction between QDs and CPL is supported by the UV–vis spectra (Figure S2 in the Supporting Information). Here, the pure complex of CPL and zinc ions (ZnCPL) was also prepared with a molar ratio of 1:1 by the addition of Zn(O2CCH3)2 to CPL aqueous solution, and its UV-vis absorption spectrum is also given in the inset of Figure S2 in the Supporting Information. The absorption peak at 259 nm for QD/CPL systems is associated with the π–π* electron transition from the quinoline ring and this peak red-shifts by about 14 nm compared with that of the pure Zn-CPL complex. A new absorption band,

Figure 2. TEM image (a) and HRTEM image (b) of pure QDs, and TEM images of QDs/CPL assemblies: QDs/CPL-2 (c); QDs/CPL-5 (d); QDs/CPL-7 (e); QDs/CPL-9 (f).

Scheme 1. Schematic representation of the assembly formation process and the thermoresponsive mechanism of QDs/CPL systems.

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caused by the metal-quinolate transition on the surface of QDs, was observed at 380 nm, and the intensity of this peak increased with an increasing molar fraction of CPL. The above results indicate that an increasing amount of MQ segments on CPL chains are anchored to the surface of QDs to form the metalloquinolates. In addition, these absorption bands are red-shifted with increasing CPL content when compared with that of the pure Zn-CPL complex. Actually, the absorption band at 380 nm should be assigned to the charge transfer transition from the oxidecontaining phenolato moiety of the quinolate ligand to its nitrogen-containing pyridyl moiety.[29] In addition, the new species at λabs = 380 nm is similar to that of pure Zn-CPL complex, indicating that the new species should be attributed to the single coordination bond of the Zn-CPL complex on the surface of QDs due to the steric hindrance effect provided by the surface of the QDs and the existence of Zn–S bonds, as compared to conventional small coordination compounds such as Zn(MQ)2.[30] The PL properties of the CPL functionalized QDs with different molar feed ratios were studied (Figure 3a). There are two emission centers, including the inherent luminescent emission (606 nm) of QDs and the surface coordination emission (517 nm) of the Zn-CPL complex formed on the surface of the QDs. With an increasing concentration of CPL in the QDs solution, the characteristic emission of the QDs is suppressed (except for QD-CPL-1 and QD-CPL-2) but not absolutely quenched, and the surface coordination emission of the Zn-CPL complex on QDs is increased. It was also noted that the emission peaks almost did not shift when [QDs]/[CPL] changed from 3:10 to 3:400. The λmax values of the QDs/CPL systems had blue-shifts of 3, 5, 12, 65 and 77 nm when the ratios were 3:600, 3:800, 3:1000, 3:2000 and 3:3000, respectively, compared with that of pure QDs. The full width at half maximum (FWHM) of the PL spectra also gradually broaden due to the cooperating interaction between the QDs and CPL. As a result, the luminescence emission of QDs/CPL can be tailor-made, from orange red to yellow, and then green. The corresponding digital images under daylight and PL photographs of QDs/CPL systems in aqueous solutions are shown in Figure S3 in the Supporting Information. It can also be seen from Figure 3a that the PL intensities at about 606 nm for QD/CPL-1 and QD/CPL-2 increase while other samples show PL a quenching phenomenon at room temperature. It was hypothesized that the fluorescence enhancement could be caused by Förster resonance energy transfer (FRET) due to the intrinsic spectral overlap. Figure 3b shows the absorption and emission spectra of pure QDs and the emission spectrum of the pure Zn-CPL complex. Actually, the metalloquinolates formed on the surface of the QDs have an almost identical emission peak to that of the pure Zn-CPL complex. It can be seen that there is an obvious spectral overlap between

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Figure 3. (a) PL emission spectra of QDs/CPL systems and (b) spectral overlap of absorption (solid line) and emission spectrum (dashed line) of pure QDs and emission spectrum (dotted line) of pure Zn-CPL complex.

the luminescence emission of the Zn-CPL complex and the absorption band of pure QDs. This result reveals that the FRET between the inherent emission and the surface-coordination emission in QD/CPL-1 and QD/CPL-2 may contribute to the enhancement of the inherent emission of QDs at about 606 nm. Along with a decrease in the molar feed ratio of QDs and CPL from QDs/CPL-3 to 9, the fluorescence intensity of QDs is gradually quenched and the PL spectra are blueshifted. The fluorescence quenching in QDs/CPL systems is possibly caused by (i) the formation of a non-photoactive ground state complex and (ii) charge separation photoinduced electron transfer (PET) from the photoexcited QDs to the coordinated CPL on the surface of QDs and charge recombination.[31,32] Time-resolved emission experiments gave further insight into the excited-state deactivation pathways of QDs in the presence of CPL. The emission decay traces were acquired at 606 nm (see

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Figure S5 in the Supporting Information), and all emission decay traces were accurately fitted by the biexponential decay function in Equation (1):[33] I = A1 exp(−t/τ1 ) + A2 exp(−t/τ2 )

(1)

where I is the normalized emission intensity, A1 and A2 are the pre-exponential factors, and t is the time after pulsed-laser excitation. The corresponding average lifetimes (τav) and the individual lifetime components (τ1,2) along with their fractional amplitudes are presented in Table S1 in the Supporting Information. The result indicates that an inner ET pathway occurs from CdTe core to ZnS shell. It can be clearly seen from Figure S4 and Table S1 in the Supporting Information that the emission kinetics of QDs/CPL decay much faster from pure QDs to QDs/CPL-9. The dynamic nature of the emission quenching is evident by inspection of the emission decay traces, which approach the baseline more rapidly at higher CPL concentrations.[31,34] Thus it was hypothesized that the dynamic quenching supports a mechanism of electron transfer from the photoexcited QDs to the Zn-CPL complex due to the electron accepting nature of the Zn-CPL complex on the surface of QDs. To further understand the electronic behavior between QDs and CPL, cyclic voltammetry (CV) measurements were carried out for the determination of band structure parameters, viz. the highest occupied molecular orbital (HOMO) or valence band edge (VB) and lowest unoccupied molecular orbital (LUMO) or conduction band edge (CB).[35] The positions of the VB and CB were determined to be −5.8 and −3.9 eV vs a vacuum based on the onset oxidation potential and the wavelength at the bottom tangent of the spectral line in the ultraviolet absorption spectrum in the present work (see Figure S5 in the Supporting Information).[36] As discussed above, the zinc (II) atoms on the surface of QDs should adopt a single coordination geometry due to the unconventional metaloquinolate complexes formed on the QDs’ surfaces with the steric confinement effect. Thus, the LUMO level of the pure Zn-CPL complex (Zn2+/CPL = 1/1) was adopted as that of the metaloquinolate complexes formed on the surface of the QDs. This LUMO level was determined from CV curves to be –4.9 eV vs a vacuum. As discussed above, both CV measurements and time resolved PL studies confirmed that the electron transfer from the conduction band of QDs to the Zn-CPL complex formed on the surface of QDs is thermodynamically feasible because the Fermi level (–4.9 eV) of Zn-CPL complex is much lower than the conduction band (–3.9 eV) of the QDs. Thus the photogenerated electron is trapped by Zn-CPL complex on the surface of the QDs and the interaction between QDs and CPL could obviously affect the optical properties of the QDs. As discussed above, it was assumed that FRET and ET are simultaneousness in the QDs/CPL systems and the

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Scheme 2. Schematic diagram of the FRET from Zn-CPL complex to QDs and ET from photoexcited QDs to Zn-CPL complex.

dominant mechanism was the ET pathway when [QDs]/ [CPL] was less than 3:100 since the QDs and CPL are linked with a coordinate bond. To describe the mechanism for the FRET and/or ET in QDs/CPL systems, a model is proposed in Scheme 2. Here, CPL plays an important role as an energy donor in the FRET pathway, while it is an electron acceptor in the ET pathway. As the CPL has the sensitive thermally triggered response, focus was maintained on the PL properties of QDs capped with copolymer ligand at different temperatures. Figure 4a shows the PL emission spectra of the QDs/CPL-5 sample with the [QDs]/[CPL] ratio of 3:600. With rising temperature, a remarkable decrease in the PL intensity of QDs was observed and the maximum emission wavelength blue-shifted by 73 nm when the temperature was above the LCST (around 30 °C). Both the PL intensity and maximum emission wavelength were almost unchanged when the temperature was above 37 °C. However, it was found that the fluorescence of pure QDs was not quenched even at high temperatures, e.g., 100 °C. The obvious quenching could be found above the LCST when the inclusion complexes formed through coordinate bonds linking QDs and polymer ligand. Thus, the scattering[22] and aggregation of QDs[37] caused by the volume transition of CPL networks upon heating may lead to the reduction in PL intensity at 606 nm. However, the surface-coordination emission originating from the Zn-CPL complex on QDs’ surfaces still retains. As a result, the dominant photoluminescence is changed from the emission of QDs (606 nm) to that of Zn-CPL complex (517 nm). The QDs/CPL system can therefore be considered as a dual reversible photoluminescence switch by environmental temperature control. six cycles of heating– cooling were also performed and it was found that the temperature-responsive behavior of the QDs were highly reversible, as shown in Figure 4b. This progress of dynamic quenching and the reversible temperature-sensitive PL property is described by Scheme 3.

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the fraction of CPL, the fluorescence intensity of the QDs was quenched and an obvious blue-shift was observed in PL spectra. It is presumed that both Förster resonance energy transfer (FRET) and a high rate of photoinduced electron transfer (PET) may contribute to the PL quenching. Moreover, the dual luminescence switching properties of QDs capped with CPL can be regulated by varying the temperature. These novel multifunctional hybrid QDs are expected to find potential applications as optical sensors, devices and nanothermometers in a living cell.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements: The authors appreciate the financial support of the National Natural Science Foundation of China (21074019) and the Natural Science Foundation of Jilin Province (20101539). Received: August 19, 2013; Revised: October 19, 2013; Published online: November 27, 2013; DOI: 10.1002/marc.201300634 Keywords: assemblies; CdTe/ZnS quantum dots; 8hydroxyquinoline; optical properties; thermoresponsive copolymers

Figure 4. (a) PL emission spectra of QDs/CPL-5 at different temperatures (inset: digital PL images at 25 °C and 37 °C) and (b) corresponding cycles of heating–cooling at above and below the LCST.

Scheme 3. Emission pathways above and below the LCST.

3. Conclusion Novel thermoresponsive QDs decorated with CPL containing 8-hydroxyquinoline and NIPAM units by coordination bonding have been successfully fabricated. The interesting QDs’ assembly behaviors were observed by controlling the molar ratio of QDs and CPL. Upon increasing

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ZnS quantum dots: assemblies and optical properties.

Novel thermoresponsive CdTe/ZnS quantum dots (QDs) decorated with a copolymer ligand (CPL) containing 8-hydroxyquinoline and NIPAM units are prepared ...
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