Thermal Sensitive Microgels with Stable and Reversible

pubs.acs.org/Langmuir © 2010 American Chemical Society

Thermal Sensitive Microgels with Stable and Reversible Photoluminescence Based on Covalently Bonded Quantum Dots Hongjing Dou, Weihai Yang, Ke Tao, Wanwan Li, and Kang Sun* State Key Lab of Metal Matrix Composites, and School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China Received April 2, 2009. Revised Manuscript Received February 17, 2010 In this study, thermal sensitive microgels functionalized with carboxyl groups were synthesized directly from hydroxypropyl cellulose (HPC) and acrylic acid (AA) without using any organic solvent. Furthermore, covalently bonded hybrid microgels with novel thermosensitivity in terms of size and fluorescence were fabricated from these HPCPAA microgels and cysteamine-capped CdTe quantum dots (QDs). The composition of the hybrid microgels were characterized by thermal thermogravimetric analysis (TGA) and coulometric titration. It was verified that the weight percent of CdTe QDs was ca. 40%, and the percent of poly(acrylic acid) varied between 9.0% and 13.6%. Through a systematic study, it was found that both the size and the fluorescent intensity of the microgels decreased as the temperature increased from below the lower critical solution temperature (LCST) to above the LCST of the HPC. Different from most reported cases, it was found that the thermal sensitive alteration of the current hybrid microgels’ size and fluorescent intensity are reversible. The novel fluorescent properties are deduced to be related to the structural characteristics of the microgels, i.e., the QDs are covalently bonded to the microgels and the dispersion of QDs in the microgels is spatially homogeneous. As a consequence of this special structure, the refractive indexes of the microgels were changed and the surface defects of the QDs were reduced, and therefore affected the fluorescent properties of the resulting hybrid microgels.

1. Introduction Water-soluble microgels that are responsive to environmental temperature stimuli, known as thermal sensitive microgels, have attracted intensive attentions after the pioneering work reported by Pelton et al.1-3 Thermal sensitive Microgels can be used in biomedical fields, such as drug and biomacromolecule delivery,4-6 because the microgels possess an abrupt volume variation at a certain volume phase transition temperature (TVPT), are watersoluble, and have a high porosity and characteristic morphology. Moreover, if the high porosity of the thermal sensitive microgels is utilized to load functional nanocrystals, the microgels could be able to perform several tasks in a parallel or coordinate manner, and their application would be effectively broadened.7 A variety of functional nanocrystals have been loaded in thermal sensitive microgels to form multifunctional hybrid par*To whom correspondence should be addressed. Tel: 86-21-34202743, Fax: 86-21-34202745, Email: [email protected].

(1) Pelton, R. H.; Chibante, P. Colloids Surf. 1986, 20, 247. (2) Hoshino, F.; Fujimoto, T.; Kawaguchi, H.; Ohtsuka, Y. Polym. J. 1987, 19, 241. (3) Meunier, F.; Elaissari, A.; Pichot, C. Polym. Adv. Technol. 1995, 6, 489. (4) Pelton, R. Adv. Colloid Interface Sci. 2000, 85, 1. (5) Freiberg, S.; Zhu, X. X. Int. J. Pharm. 2004, 282, 1. (6) Godovsky, D. Y. Adv. Polym. Sci. 2000, 153, 163. (7) Alvarez-Lorenzo, C.; Concheiro, A. Mini-Rev. Med. Chem. 2008, 8, 1065. (8) Lu, Y.; Mei, Y.; Drechsler, M.; Ballauff, M. Angew. Chem., Int. Ed. 2006, 45, 813. (9) Mei, Y.; Sharma, G.; Lu, Y.; Ballauff, M.; Drechsler, M.; Irrgang, T.; Kempe, R. Langmuir 2005, 21, 12229. (10) Karg, M.; Lu, Y.; Carb
o-Argibay, E.; Pastoriza-Santos, I.; P
erez-Juste, J.; Liz-Marz
an, L. M.; Hellweg, T. Langmuir 2009, 25, 3163. (11) Menager, C.; Sandre, O.; Mangili, J.; Cabuil, V. Polymer 2004, 45, 2475. (12) Pich, A.; Bhattacharya, S.; Lu, Y.; Boyko, V.; Adler, H. P. Langmuir 2004, 20, 10706. (13) Sauzedde, F.; Elaissari, A.; Pichot, C. Colloid Polym. Sci. 1999, 277, 1041. (14) Sauzedde, F.; Elaissari, A.; Pichot, C. Colloid Polym. Sci. 1999, 277, 846. (15) Deng, Y.; Wang, L.; Yang, W.; Fu, S.; Elaissari, A. J. Magn. Magn. Mater. 2003, 257, 69.

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ticles, including noble metals,8-10 metal oxides,11-15 quantum dots,16,17 and biominerals.18,19 Among them, quantum dots (QDs) were of great interest owing to their size-tunable optical properties and high photoluminescence (PL) quantum yields. While preserving the applications of thermal sensitive microgels, the loading of QDs can not only make the delivery, release, and interaction processes traceable, but also potentially lead to other applications in biomedical fields, such as high-throughput screening or bioassays. Furthermore, because the hydrophilic-hydrophobic transition of a polymer network at the volume phase transition affects the physicochemical environment of the loaded QDs, their fluorescent properties discontinuously change at the TVPT, and this makes QD loaded microgels suitable to be used as temperature indicators by acting as a fluorescent switch. For example, poly(N-isopropylacrylamide) (PNIPAM) gels were prepared in the presence of CdTe QDs by entrapping them within the polymer network without covalent bonds.20 When the TVPT was approached, the PL intensity decreased conspicuously. Similar phenomena were also found in other fluorescent thermal sensitive microgel systems, in which the QDs were entrapped or diffused within microgels by hydrophobic forces,21 hydrogen bonding,22 or electrostatic interactions.23 Alternatively, if QDs were physically bound in PNIPAM microgels by covalent bonding, (16) Bai, C.; Fang, Y.; Zhang, Y.; Chen, B. Langmuir 2004, 20, 263. (17) Pich, A.; Hain, J.; Lu, Y.; Boyko, V.; Prots, Y.; Adler, H. P. Macromolecules 2005, 38, 6610. (18) Zhang, G.; Wang, D.; Gu, Z.; Hartmann, J.; M€ohwald, H. Chem. Mater. 2005, 17, 5268. (19) Zhang, G.; Wang, D.; Gu, Z.; M€ohwald, H. Langmuir 2005, 21, 9143. (20) Li, J.; Hong, X.; Liu, Y.; Li, D.; Wang, Y.; Li, J.; Bai, Y.; Li, T. Adv. Mater. 2005, 17, 163. (21) Shen., L.; Pich, A.; Fava, D.; Wang, M.; Kumar, S.; Wu, C.; Scholes, G. D.; Winnik, M. A. J. Mater. Chem. 2008, 18, 763. (22) Gong, Y.; Gao, M.; Wang, D.; M€ohwald, H. Chem. Mater. 2005, 17, 2648. (23) Kuang, M.; Wang, D.; Bao, H.; Gao, M.; M€ohwald, H.; Jiang, M. Adv. Mater. 2005, 17, 267.

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fluorescence could be observed only when the temperature is higher than the TVPT.24 The sensitivity of the fluorescent properties of microgels to the TVPT makes them desirable for applications in chemical and temperature sensors. However, with respect to bioassays, high-throughput screening, drug carrier labeling, and drug release tracing, it is of great importance to keep photoluminescence stable near the TVPT. Many studies have reported that the fluorescent properties of QDs are directly related to their surface bonding and chemical environment.20-24 However, it is currently unknown what will happen to the fluorescent properties of the QDs if the structure of QD loaded microgels changes. The investigation of this issue is significant for understanding the influence of the interaction between QDs and macromolecular networks and, furthermore, the effect of this interaction on the fluorescent properties of the QDs. Herein, we designed a novel thermal sensitive microgel system based on hydroxypropyl cellulose (HPC) and poly(acrylic acid) (PAA), and fabricated a hybrid microgel by binding CdTe nanocrytals into HPC-PAA microgels. The structure and optical properties of the composite microgels were respectively characterized. It was found that their fluorescent intensity is not sensitive to the TVPT and remains stable across a range of temperatures, a feature which is attributed to the microgels’ structure. The results of the current work may be helpful for understanding the relationship between the structure and the optical property of QD-macromolecule hybrid particles.

2. Experimental Section Chemicals. Hydroxypropyl cellulose (HPC, Mw = 80 000, DS = 2.5) and N-(3-dimethylaminopropyl)-N0 -ethylcarbodiimide hydrochloride (EDC) were purchased from Sigma-Aldrich and used directly. Acrylic acid (AA) was dried by MgSO4 and then vacuum distilled before use. N,N0 -Methylene bisacrylamide (MBA) (Fluka) was recrystallized from methanol. Cerium(IV) ammonium nitrate (CAN) (Shanghai Sinopharm Chemical Reagent Co. Ltd.) was recrystallized from dilute nitric acid containing an appropriate amount of ammonium nitrate. Water-soluble CdTe nanocrystals stabilized by cysteamine (Sigma-Aldrich) were prepared according to our previous work.25 The water used in all of the experiments is Milli-Q water. All chemicals, unless otherwise noted, were obtained from Shanghai Sinopharm Chemical Reagent Co. Ltd. Microgels Synthesis. The synthesis of microgels follows our previous report,26-28 which is based on the graft polymerization of AA from HPC at the presence of the bifunctional monomer MBA where the polymerization was initiated by CAN. An aqueous solution of 37 mg CAN in 1.25 mL of 0.1 mol/L nitric acid and 0.4 mL AA were successively added to 50 mL of 1.2 wt % aqueous solution of HPC under gentle stirring and nitrogen bubbling. The reaction proceeded at 30 °C for 30 min, then 8 mL AA and 2.8 g (0.018 mol) MBA were added, and thereafter, the reaction was kept at 30 °C and a pH =1-2 for 4 h, followed by adjusting the pH to 7.0 with 1 mol 3 L-1 NaOH. Excess precursors and side-productions were removed over a 72 h period by dialysis using a membrane bag with a molecular weight cutoff of 14 000. The resultant microgels were freeze-dried and stored for further treatment. To load the quantum dots into the microgel synthesized as above, 15 mg of HPC-PAA microgel powder was dissolved in 1 mL of water, mixed with a certain amount of EDC (the molar (24) Agrawal, M.; Rubio-Retama, J.; Zafeiropoulos, N. E.; Gaponik, N.; Gupta, S.; Cimrova, V.; Lesnyak, V.; L
opez-Cabarcos, E.; Tzavalas, S.; Rojas-Reyna, R.; Eychm€uller, A.; Stamm, M. Langmuir 2008, 24, 9820. (25) Yang, W.; Li, W.; Dou, H.; Sun, K. Mater. Lett. 2008, 62, 2564. (26) Dou, H.; Yang, W.; Sun, K. Chem. Lett. 2006, 35, 1374. (27) Dou, H.; Tang, M.; Sun, K. Macromol. Chem. Phys. 2005, 206, 2177. (28) Tang, M.; Dou, H.; Sun, K. Polymer 2006, 47, 728.

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ratio of EDC and carboxyl groups in HPC microgels is 10/1), and stirred for 5 min. Then, 15 mg of cysteamine-capped CdTe powder was added at a neutral pH, and the reaction was allowed to proceed with stirring for 4 h at 25 °C (the weight content of cysteamine in cysteamine capped CdTe is ca. 20% determined by TGA). After that, the hybrid microgels were collected by centrifugation at 12 000 rpm for 10 min, and dispersed in water for characterization. The synthesis of the fluorescent microgels is schematically illustrated in Scheme 1. Characterization. The size and zeta-potential of the HPC microgel and the hybrid microgels were recorded on a Malvern ZetaSizer Nano ZS900 after being allowed to equilibrate at the designated temperatures for 15 min. To determine the content of the QDs in the hybrid microgels, thermogravimetric analysis (TGA) measurements were performed at a ramp rate of 10 °C/ min under a nitrogen atmosphere using a TGA 2050 (TA Instruments). For transmission electron microscopy (TEM) studies, the negative staining of the samples was realized by mixing the aqueous dispersion of the microgels with 5% phosphotungstic acid aqueous solution before drop-casting them onto a carbon film coated copper grid. TEM images were taken on a JEOL JEM-2100F microscope at an accelerating voltage of 200 kV. Fluorescence data were obtained by using a RF-5301 Shimadzu spectrofluorophotometer at the designated temperatures in the cycling range from 25 to 50 °C. The measurements were performed after allowing the sample to equilibrate to the designated temperature for 15 min, and the concentration of the samples for fluorescence measurements were 0.5 mg/mL. Coulometric titration was performed on a 785 DMP Titrino (Metrohm, Switzerland). About 10 mg of the HPC-PAA microgels (powder) or hybrid microgels was dissolved in 40 mL of water, and the pH value was adjusted to about 4, followed by the titration of 2 mmol/L NaOH. Two equivalence points can be found, one at a pH = 7 and one at a pH = 10. As displayed in eq 1, the mass content of PAA was calculated by the volume of NaOH titrated between the two equivalence points (V/L), where xAA is the weight percentage of PAA chains in microgels, and mgel (g) is the weight of microgels. xAA ¼ ð2  10 -3  V  72Þ=mgel

ð1Þ

3. Results and Discussion 3.1. HPC Microgels. As a water-soluble polysaccharide, HPC has been approved by the Food and Drug Administration (FDA) to be used in biomedical fields owing to its excellent biocompatibility.29 The mechanism of the synthesis of HPC-PAA microgels is similar to that of the Dextran-PAA microgels in our previous publications.26-28 Both HPC and PAA are watersoluble macromolecules, and it has been confirmed that many polysaccharides, including HPC, have complexation with PAA in an acid medium owing to the hydrogen bond interaction between the carboxyl groups and proton-acceptors in the glucose units.27,28 Thus, the driving force for the formation of microgels can be attributed to the complexation between PAA grafts and HPC segments. The mechanism of this one-pot synthesis as proposed is illustrated in Scheme 1. After the initiation of the graft copolymerization of AA from HPC, the ‘‘micelle-like’’ nanoaggregates that form are attributed to the complexation between HPC and PAA. As no macroscopic precipitation takes place in this process, we suggest that the HPC chains with fewer complexed segments may remain solvated and thus stabilize the complex aggregates. Subsequently, participation of the bifunctional monomer, MBA, leads to further fixation of the structure. Thanks to the shielding effect of the HPC segments on the exterior, interparticle crosslinking was prevented. The stability of the microgels is confirmed (29) Nagai, T.; Machida, Y. Adv. Drug Delivery Rev. 1993, 11, 179.

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Scheme 1. Chemical Structure of HPC (A) and the Schematic Presentation for the Fabrication of HPC Microgels and Hybrid Microgels (B)

by the fact that they still hold an integral structure at a pH = 7 where the hydrogen bonds between the HPC and the PAA have been destroyed because of the deprotonation of the carboxy groups in PAA. HPC polymer chains in water have LCST in the range 4045 °C,30 which means that the microgels based on HPC could also possess a thermal sensitivity. In order to verify the thermal sensitivity, the sizes of the microgels dispersed in pure water at different temperatures were determined by DLS. As shown in Figure 1a, a decrease in size is observed at about 43 °C, which confirms the thermal sensitive hydrophilic to hydrophobic transition of HPC at around 43 °C. In addition, it is interesting that the zeta-potential of the microgel in water displayed a continuous decrease from -5 mv to -23 mv as the temperature increased from 36 to 60 °C. The negative value of zeta-potential is reasonable because in neutral Mill-Q water, the PAA chains were mostly ionized, since the pH value was higher than their pKa. With the increase in temperature, the HPC on the exterior of the microgels became hydrophobic and aggregated into the interior of the microgel. As a result, ionized PAA chains were left on the exterior of the microgels, which is the origination of the thermal sensitivity of the zeta-potential. Furthermore, the microgels’ size and zetapotential can almost return to their initial values as the temperature cycles from 25 to 70 °C and back to 25 °C, demonstrating their reversibility. The transformation at the TVPT also affects the light scattering of the microgels, resulting in a distinct opalescence of their aqueous solution (Figure 1b inset). Figure 2 shows the TEM image of HPC-PAA microgels. It is demonstrated (30) Pham, A. T.; Lee, P. I. Pharm. Res. 1994, 11, 1379.

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by TEM that the diameter of the microgels is between 150 and 250 nm. The diameter of the microgels observed by TEM is slightly smaller than that determined by dynamic light scattering (DLS). This decrease of size is caused by the shrinkage due to water evaporation during sample preparation, and a similar phenomenon was also observed in the morphology observations of various micelles or particles composed with hydrophilic components.31,32 It is noteworthy that, unless stained by phosphotungstic acid, the samples could not be observed by TEM because of their low electron density. This approach provides a convenient strategy to fabricate polysaccharide-based microgels with stable structures, thermal responsibility, and functional groups without using any organic solvent or surfactant. According to the results of coulometric titration, the weight content of PAA chains calculated from eq 1 is 24.3%, which is much less than the weight content of AA in the sum of HPC and all monomers used in the synthesis of microgels. We deduced that the reason might be chain transfer and other side reactions which occurred during the free radical polymerization of AA stemming from the HPC backbones. 3.2. Structure of Fluorescent Microgels. EDC is a commonly used activating agent for the connection of -COOH with -NH2 in a biochemical conjugation.33,34 In our case, as the CdTe nanocrystals were stabilized by cysteamine, they can be easily connected to the microgels by covalent bonding between the (31) Dou, H.; Jiang, M.; Peng, H.; Chen, D.; Hong, Y. Angew. Chem., Int. Ed. 2003, 42, 1516. (32) Dou, H.; Sun, K.; Yang, W. Macromol. Chem. Phys. 2006, 207, 1899. (33) Richert, L.; Boulmedais, F.; Lavalle, P.; Mutterer, J.; Ferreux, E.; Decher, G.; Schaaf, P.; Voegel, J. C.; Picart, C. Biomacromolecules 2004, 5, 284. (34) McCarthy, J. R.; Weissleder, R. Adv. Drug Delivery Rev. 2008, 60, 1241.

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Figure 3. Typical TEM image of hybrid fluorescent microgels; insets show magnification of a hybrid microgel (upper) and an entrapped CdTe nanocrystal (bottom).

Figure 1. Variation of (a) the hydrodynamic diameter (Dh) and (b) the zeta-potential of HPC microgels in water as a function of temperature. Inset in (b) shows the images of the HPC microgels dispersed in water at 25 and 45 °C.

Figure 2. Typical TEM image of HPC microgels stained by 5% phosphotungstic acid.

-NH2 in cysteamine and the carboxyl groups in the microgels with the activation of EDC. As a powerful instrument, TEM was used by many research groups to observe the morphology of particles with stable structure. Moreover, based on the distinct contrast of polymeric matrix and QDs in hybrid microgels, the TEM image can display the contrast clearly enough to distinguish (35) Bhattacharya, S.; Eckert, F.; Boyko, V.; Pich, A. Small 2007, 4, 650.

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the location of QDs in dry state.21,24,35,36 For example, RubioRetama et al. presented the TEM image of the hybrid microgels with QDs loaded on the exterior part, which confirmed further the feasibility of identifying the location of QDs in hybrid microgels by TEM observation of microgels.36 A typical TEM image of fluorescent hybrid microgels (Figure 3) shows that the size of the fabricated hybrid microgels is in the range 150-230 nm, which is consistent with that of the microgels before QD loading. Almost every microgel particle is covered by dark dots, which is evident at higher magnification (the upper inset of Figure 3), proving the successful immobilization of the CdTe nanocrystals. When compared with unaltered HPC microgels, microgels containing CdTe QDs contrast better and the samples can be directly observed without staining, indicating the loading of inorganic nanocrystals from the interior to the exterior of the microgels. Furthermore, nanocrystals showing clear lattice fringes were found in fluorescent microgels, and an example is shown in the bottom inset of Figure 3. The nanocrystal displays an interplanar distance of about 0.23 nm, corresponding to the (220) plain of zinc blende structured CdTe nanocrystals (about 0.226 nm).37 The macrographs of fluorescent microgels taken at 25 and 50 °C are shown in Figure 4a,b, respectively. When the microgels are aggregated by the adjustment of temperature, fluorescence can be observed only in the sediment. Additionally, the supernatant was measured by photoluminescence spectroscopy and showed that almost no quantum dots were contained within it. These results demonstrate that the cysteamine-capped CdTe nanocrystals were successfully entrapped within the microgels and would not be pulled out at a temperature higher than the TVPT. To further evaluate the loading of QDs, thermogravimetric analyses (TGA) were performed for fluorescent microgels immobilized with four kinds of quantum dots, which show fluorescence peaks at about 530 (540 nm after being entrapped in microgels), 565, 600, and 640 nm corresponding to their different sizes. As shown in Figure 5, the residuals of the microgels after being heated to 550 °C are ascribed to the CdTe, because the (36) Rubio-Retama, J.; Zafeiropoulos, N. E.; Serafinelli, C.; Rojas-Reyna, R.; Voit, B.; L
opez-Cabarcos, E.; Stamm, M. Langmuir 2007, 23, 10280. (37) Guo, J.; Yang, W.; Wang, C. J. Phys. Chem. B 2005, 109, 17467.

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Dou et al. Table 1. Composition of Four Kinds of Microgels hybrid microgels

percentage of AA (wt%)

molar ratio between residue AA and polysaccharide unit

Microgel-CdTe@530 nm Microgel-CdTe@565 nm Microgel-CdTe@600 nm Microgel-CdTe@640 nm

9.0 11.7 12.9 13.6

0.44 0.59 0.66 0.71

Table 2. Photoluminescence Peak Positions at Different Temperature

Figure 4. Photographs of fluorescent microgels dispersed in water, taken at 25 °C (a) and 50 °C (b) after 5 min under sunlight (upper) and ultraviolet (bottom), respectively. The concentration of hybrid microgels aqueous solutions are all 15 mg/mL.

Figure 5. TGA curves of hybrid microgels with emission peaks of QDs at 540, 565, 600, and 640 nm, respectively, inset shows the TGA curves of HPC microgels.

polysaccharide and other organic molecules decompose at lower temperatures (inset of Figure 5a). It can be observed that the weight of CdTe in the microgels, regardless of the QD size, is about 40 wt %. By using coulometric titration, the amount of 5026 DOI: 10.1021/la903667r

PL wavelength after entrapment (nm)

PL wavelength before entrapment (nm)

25 °C

30 °C

40 °C

50 °C

530 565 600

541 566 600

539 566 600

540 569 606

539 572 615

residual carboxyl groups in the hybrid microgels can be measured, as shown in Table 1. Although the loading weight is the same for all samples, the residual carboxyl groups increased with increasing QD size. Combined with the TGA results, it can be concluded that the consumption of carboxyl groups is ascribed to the different amounts of cysteamine of the same weight, but different-sized, QD surfaces, and this point is also proof for the covalent bonding between the QDs and the microgels. It is difficult to characterize the amide groups formed from the reaction of the carboxyl and the amino groups in the hybrid microgels directly from IR and NMR spectra, because there were also amide groups in the microgels originating from the crosslinker, MBA. Therefore, a control experiment was performed without using EDC. By the electrostatic forces between the ionized carboxyl and the amino groups in water, some QDs can be entrapped into/onto the microgels; however, the resultant hybrid microgels are much different from those produced through the covalent coupling of the carboxyl and the amino groups. First, the mass percentages (about 5% by TGA) are much lower than those of the hybrid microgels subjected to covalent coupling. In addition, the QDs attracted by electrostatic forces are easily released from the microgels once the temperature increases above their TVPT. From the results of the control experiment, it is reasonable to conclude that the QDs are covalently coupled throughout the microgels. 3.3. Optical Properties of Fluorescent Microgels. Optical properties of fluorescent microgels were evaluated in two respects: the emission peak position and the photoluminescence intensity. The change in peak position is shown in Table 2. After being encapsulated into HPC microgels, 530 nm CdTe nanocrystals were red-shifted to about 540 nm, while the fluorescent wavelength of two other kinds of QDs were kept almost the same. In comparison, when the temperature increases, the photoluminescence peaks of the microgels loaded with the 565 and 600 nm CdTe nanocrystals shifted to longer wavelengths, while the microgels loaded with 540 nm QDs showed almost no change. These phenomena can be ascribed to the Forster energy transfer, as has been observed in previous reports.21-23 On the basis of the same entrapping weight from the TGA results and, with respect to the QDs’ sizes, it can be surmised that when compared to the microgels having 565 and 600 nm CdTe QDs, a higher number of 530 nm CdTe QDs can be entrapped in the microgels. This leads to a closer space interval, less than the Forster energy transfer radius, in the fluorescent microgels containing the 530 nm CdTe QDs. However, in the case of the 565 or 600 nm QDs, the space interval did not approach the Forster energy transfer radius. Therefore, the red-shift happened after the shrinkage of the microgels rather than during the entrapping process. Langmuir 2010, 26(7), 5022–5027

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Figure 6. Change of photoluminescence intensity of CdTe QDloaded microgels as a function of temperature.

The most interesting property of resultant hybrid microgels lies in their photoluminescence intensity. As shown in Figure 6, unlike other kinds of thermosensitive fluorescent microgels,21-24 although the intensity decreases with increasing temperature, it does not possess a conspicuous change at the TVPT. Even more interesting is that the intensity can almost revert back to the original value if the temperature returns to 25 °C after one temperature cycle. Contrary to this phenomenon, in other cases where QDs were well-dispersed in thermal sensitive microgels,21-23 a conspicuous decrease of intensity was observed when the temperature approached the TVPT. This phenomenon was explained by two possible reasons. First, the refractive index of the microgel increases with respect to the aqueous media and the microgel scatters more photons than in the swollen state. Therefore, the photoluminescence intensity is considerably reduced because no photons reach the QDs located the inner portion of the microgel and only the QDs in the outer layers would take part in the excitation. Second, because the QDs were dispersed through the whole of the microgels in these cases, the collapse of the polymer chains would result in the QDs becoming closely packed in the gels, and therefore leads to a sharp decrease in fluorescent intensity when compared to the swollen state.38 However, in our case, although the refractive index increases and the QDs are also dispersed throughout the thermosensitive microgels, a discontinuous decrease of photoluminescence intensity was not observed at the TVPT. Therefore, in addition to the two reasons proposed, there should be other factors that affect the fluorescent intensity. In another study that coated QDs mainly on the exterior of the microgels,24 the fluorescence could only be observed when the temperature was higher than the TVPT. In this scenario, fluorescence enhancement above the TVPT was explained by a change in the environmental pH and by the reduction of the number of QDs surface defects. The latter is caused by the relaxation of the covalent bonds between the QDs and microgel. In cases where the (38) Kagan, C. R.; Burray, C. B.; Bawendi, M. Phys. Rev. B 1996, 54, 8633.

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photoluminescence intensity sharply decreased at the TVPT, the QDs were entrapped by hydrogen bonds, by electrostatic forces, or by hydrophobic-hydrophilic forces, and the bond type is the only other factor that could possibly affect the photoluminescence intensity. According to our results, it can be confirmed that the relaxation of the covalent bonds during the microgel collapse reduces the number of QD surface defects, and therefore provides an enhancement of photoluminescence intensity. Thus, the slight decrease of photoluminescence intensity as the temperature increases can be explained by the coordinating influences of these factors: changing refractive index, polymer network collapse, and relaxation of the QD-microgel bonding. The photoluminescence intensity decrease for the pure quantum dots with increasing temperature was generally explained by the oxidation of the QD surfaces, and therefore, this kind of decrease is irreversible.39,40 However, it is observed here that the photoluminescence intensity of the hybrid microgels can almost return to their original value if the temperature goes back to 25 °C after one cycle, which suggests that the QD surfaces would not be oxidized. This indicates that there is a measure of protection afforded to the QDs by the covalently linked microgel networks, which prevents their oxidization, and further confirms the influencing factors of fluorescent intensity. To the best of our knowledge, this reversibility has not been observed in QD-containing hybrid nanostructures.

4. Conclusion Thermal sensitive microgels functionalized with carboxyl groups were synthesized directly from HPC and AA without using any organic solvent. By using the thermal sensitive HPC microgels and cysteamine-capped CdTe nanocrystals, a novel multifunctional hybrid microgel was designed and synthesized by the classical bioconjugation method. In this structure, QDs were covalently bonded to the polymer chains and dispersed throughout the microgel. Like the HPC microgel’s size, the variation of the fluorescent intensity of hybrid microgels was found to be reversible when cycling the temperature from below to above TVPT and back to the initial temperature. Such phenomena are believed to be related to the structure of the hybrid microgels, the presence of the covalent bonds, and, consequently, the cooperative effect of the reduction of QD surface defects and the microgel’s refractive index. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (No. 20904032), the Science and Technology Committee of Shanghai (No. 08ZR1415700), and the Ph.D. Programs Foundation of Ministry of Education of China (No. 200802481131). We thank Instrumental Analysis Center of SJTU for the assistance on measurements. We also thank Shanghai Sunny New Technology Development Co. Ltd. for their support. (39) Wuister, S. F.; Doneg
a, C. M.; Meijerink, A. J. Am. Chem. Soc. 2004, 126, 10397. (40) Kapitonov, A. M.; Stupak, A. P.; Gaponenko, S. V.; Petrov, E. P.; Rogach, A. L.; Eychmuller, A. J. Phys. Chem. B 1999, 103, 10109.

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