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Switchable Photoluminescence of CdTe Nanocrystals by Temperature-Responsive Microgels M. Agrawal,† J. Rubio-Retama,*,† N. E. Zafeiropoulos,† N. Gaponik,§ S. Gupta,† V. Cimrova,‡ V. Lesnyak,§ E. Lo´pez-Cabarcos,| S. Tzavalas,† R. Rojas-Reyna,† A. Eychmu¨ller,§ and M. Stamm† Leibniz-Institut fu¨r Polymerforschung Dresden e.V, Hohe Strasse 6, Dresden 01069, Germany, Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, HeyroVsky´ Sq. 2, 162 06 Prague 6, Czech Republic, Physical-Chemistry, TU Dresden, Dresden 01062 Germany, and Physical Chemistry Department, Pharmacy Faculty, Complutense UniVersity, Madrid 28040, Spain ReceiVed May 5, 2008. ReVised Manuscript ReceiVed June 2, 2008 In the present study, we report a method for preparing a fluorescent thermosensitive hybrid material based on monodisperse, thermosensitive poly(N-isopropyl acrylamide) (PNIPAM) microgels covered with CdTe nanocrystals of 3.2 nm diameter. The CdTe nanocrystals were covalently immobilized on the surface of PNIPAM microgels. The chemical environment around the CdTe nanocrystals was modified by changing the temperature and inducing the microgel volume-phase transition. This change provoked a steep variation in the nanocrystal photoluminescence (PL) intensity in such a way that when the temperature was under the low critical solution temperature (LCST) of the polymer (36 °C) the PL of the nanocrystals was strongly quenched, whereas above the LCST the PL intensity was restored.
Introduction The production of smart materials that are able to respond to external stimuli is one of the most active fields in polymer chemistry. Microgels are probably one of the most used responsive systems because their high specific surface allows us to obtain materials with very short response times.1,2 The phenomenon of the gel volume transition in response to external stimuli (e.g., temperature, pH, ionic strength, and electric field)3,4 has prompted increased scientific interest in investigating gels as potential actuators. A thermally induced volume transition appears in hydrogels on the basis of weakly cross-linked polymers that exhibit an LCST. Among others, microgels based on PNIPAM, poly(N-vinylcaprolactam-co-glycidyl-methacrylate), or poly(ethylene glycol methyl methacrylates) are promising materials as actuators because they exhibit an LCST,5 abruptly changing their volume at a given temperature.14 In microgels with an LCST, the change in the polymer network21 at the volume phase transition modifies the physicochemical environment of the components immobilized inside the gel, influencing their behavior. Quantum dots (QDs) are very sensitive to environmental changes because their PL efficiency may be varied as a result of variations in surface properties such as surface defects and capping agents.6,7,10 The QDs’ environ* Corresponding author. E-mail:
[email protected] or
[email protected]. † Leibniz-Institut fu¨r Polymerforschung Dresden e.V. ‡ Academy of Sciences of the Czech Republic. § TU Dresden. | Complutense University. (1) Lo´pez-Cabarcos, E.; Mecerreyes, D.; Sierra-Martin, B.; Romero-Cano, M. S.; Strunz, P.; Fe´rnandez-Barbero, A. Phys. Chem. Chem. Phys. 2004, 6, 1396. (2) Rubio-Retama, J.; Lo´pez-Cabarcos, E.; Lo´pez-Ruiz, B. Biomaterials 2003, 24, 2965. (3) Daly, E.; Saunders, B. R. Langmuir 2000, 16, 5546. (4) Nerapusri, V.; Keddie, J. L.; Vincent, B.; Busnak, I. A. Langmuir 2006, 22, 5036. (5) Meunier, F.; Elaı¨ssari, A.; Pichot, C. Polym. AdV. Technol. 1994, 6, 489. (6) Erskine, L.; Emrick, T.; Alivisatos, A. P.; Fre´chet, J.M. J. Polym. Prepr. (Am. Chem. Soc., DiV. Polym. Chem.) 2000, 12, 1102. (7) Zhang, H.; Ciu, Z. C.; Wang, Y.; Zhang, K.; Ji, X. L.; Lu¨, C. L.; Yang, B.; Gao, M. Y. AdV. Mater. 2003, 15, 777.
mental sensitivity has also been used to create chemical sensors, which respond to the nature of the solvent varying as previously reported by Ionov and co-workers.24 Previously, Li and coworkers8 synthesized smart materials, which combine the thermoresponsive properties of PNIPAM gels with the photoluminescent features of CdTe QDs. To do that, they prepared gels in the presence of QDs with the aim of entrapping them within the polymer network. By this procedure, they obtained gels with PL properties without covalently binding the QD to the polymer matrix. The PL intensity of the resulting material strongly depended on the temperature. Thus, when PNIPAM approaches the LCST, the PL intensity decreases conspicuously, and a red shift of the emission band is observed. A similar approach was followed by Wang and co-workers,9 who entrapped CdTe QDs by infiltration in weakly cross-linked PNIPAM microgels, with results that resemble those reported by Li et al. Both articles related the red shift of the PL with the aggregation of the QDs in the collapsed polymer matrix, whereas the quenching effect of the PL was associated with the increment in the scattering of the collapsed gel microparticles. In this scenario, the refractive index of the microgel increases with respect to the aqueous media and the microgel scatters more photons than in the swollen state. Thus, the PL intensity is considerably reduced because no photons reach the QDs placed in the inner part of the microgel and only the QDs in the outer layers would take part in the excitation and emission phenomena. These studies showed the possibility of modifying the PL of the QDs by changing their environment. However, if the QDs were connected to the microgel by covalent bonds, then their response to the environmental variations could be significantly increased. Owing to this idea, we have covalently attached CdTe nanocrystals to PNIPAM microgels with 10% cross-linking content. The high cross-linking rate prevents the (8) Li, J.; Hong, X.; Liu, Y.; Li, D.; Wang, Y.; Li, J.; Bai, Y.; Li, T. AdV. Mater. 2005, 17, 163. (9) Gong, Y.; Gao, M.; Wang, D.; Mohwald, H. Chem. Mater. 2005, 17, 2648. (10) Gaponik, N.; Talapin, D. V.; Rogach, A. L.; Hoppe, K.; Shevchenko, E. V.; Kornowski, A.; Eychmu¨ller, A.; Weller, H. J. Phys. Chem. B 2002, 106, 7177.
10.1021/la801347d CCC: $40.75 2008 American Chemical Society Published on Web 07/23/2008
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Scheme 1. Reaction Steps Carried Out to Obtain Microgels Covered with CdTe Quantum Dots
infiltration of QDs into the microgels, facilitating their immobilization onto the gel surface by covalent bonding. Thus, by placing the QDs in the outer shell of the microgels we would be able to prevent the PL reduction due to the scattering. The synthesized hybrid material shows reversible, temperaturedependent on-off PL behavior in the region around the LCST. When the microgels are swollen, the PL intensity is quenched whereas when the microgels are collapsed the PL emission is enhanced. Furthermore, the covalent bonds between the PNIPAM chains and the QDs permit the conformational changes of the polymer chains to be transferred more effectively to the QD surface, thus controlling their luminescence.
Experimental Section Materials. The monomers of N-isopropyl-acrylamide and acrylic acid, the cross-linker N,N-methylene-bis-acrylamide, the initiator ammonium persulfate (APS), and cadmium dichlorate hexahydrate and cysteamine were purchased from Sigma. Al2Te3 (lumps) used as a source of H2Te was purchased from CERAC Inc. Synthesis of Amine-Terminated CdTe Quantum Dots. Amineterminated CdTe QDs were produced in aqueous media following
Figure 1. (A) SEM micrograph of P(NIPAM-AA) microgels. (A′) Size distribution of the microgels in an aqueous dispersion. (B) SEM micrograph of P(NIPAM-AA-CdTe) microgels. (B′) Size distribution of the P(NIPAM-AA-CdTe) microgels in an aqueous dispersion.
the procedure described by Gaponik et al.10 Cd(ClO4)2 · 6H2O was dissolved in water, and the thiol stabilizer (cysteamine) was added with stirring. The pH of the solution was maintained at about 6 during the process. Subsequently, the solution was placed in a threenecked flask fitted with a septum and valves and was deaerated by bubbling nitrogen. H2Te gas (generated by the reaction of Al2Te3 lumps with a H2SO4 solution under an N2 atmosphere) was passed through the solution while stirring, together with a slow nitrogen flow. CdTe precursors were formed at this stage, and their appearance was accompanied by a change in the solution color to dark red. The precursors were converted to CdTe nanocrystals by refluxing the reaction mixture at 100 °C under open-air conditions with a condenser attached. The size of the CdTe nanocrystals was controlled by the duration of reflux, and the crystal diameter estimated from the absorption spectrum was 3.2 nm.11 Synthesis of Carboxylic Terminal Microgels. PNIPAM microgels were prepared by surfactant free-radical polymerization12,13 of 50 mL of an aqueous solution formed by N-isopropylacrylamide (0.1 M, 550 mg) and N,N-methylenebisacrylamide (0.007 M, 50 mg) at 70 °C using 250 mg of ammonium persulfate as an initiator. With the aim of creating a carboxylic group-rich shell to which CdTe QDs should be covalently attached, we added 35 mg of sodium acrylate (0.01 M final concentration) 15 min after starting the reaction. This resulted in microgels enriched in carboxylic groups. The introduction of COOH groups into the microgels was studied by FTIR (Supporting Information). In the FTIR spectra of microgels synthesized with sodium acrylate “P(NIPAM-AA)”, a band appears around 1730 cm-1 that was assigned to the stretching vibrational mode of COOH groups. The COO- stretching band was not observed because of the overlapping of the amide III band. Pelton12 and Pichot13 and co-workers demonstrated previously that the copolymerization of NIPAM with charged monomers creates microgels in which the charged molecules are preferentially distributed on the surface.12–14 This effect is due to the special method of polymerization of the PNIPAM microgels. During this process, PNIPAM growing chains create a hydrophobic core, which is not accessible to charged molecules. As a result, the charged molecules polymerize preferentially on the surface; consequently, the superficial negative charge of the microgels increased from 4.7 (11) Rogach, A. L.; Franzl, T.; Klar, T. A.; Feldmann, J.; Gaponik, N.; Lesnyak, V.; Shavel, A.; Eychmu¨ller, A.; Rakovich, Y. P.; Donegan, J. F. J. Phys. Chem. C 2007, 111, 14628. (12) Pelton, R. H.; Chivante, P. Colloids Surf. 1986, 20, 247. (13) Meunier, A.; Elaı¨ssari, A.; Pichot, C. Polym. AdV. Technol. 1994, 6, 489. (14) Rubio-Retama, J.; Zafeiropoulos, N. E.; Serafinelli, C.; Rojas-Reyna, R.; Voit, B.; Lopez-Cabarcos, E.; Stamm, M. Langmuir 2007, 23, 10280. (15) Kagan, C. R.; Burray, C. B.; Bawendi, M. Phys. ReV. B. 1996, 54, 8633.
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Figure 2. TEM micrograph of P(NIPAM) microgels covered with CdTe nanocrystals. The image on the right depicts a detailed micrograph of microgels. The microgels have a PNIPAM core surrounded by a shell enriched with carboxylic groups to which the nanocrystals were bonded. The material that is not included in the microgels must be aggregated from CdTe nanocrytals in the outer shell of the microgels.
Figure 3. Experimental diameter of microgels as a function of temperature (black, P(NIPAM-AA) microgels; green, PNIPAM-AA-CdTe microgels).
Figure 4. PL spectra of P(NIPAM-AA-CdTe) microgels taken at different temperatures between 25 and 41 °C. The excitation wavelength was 480 nm.
C/g for PNIPAM microgels to 27.6 C/g for poly(NIPAM)-acrylate shell microgels (P(NIPAM-AA)). After the addition of the acrylate monomer, the mixture was refluxed for 4 h in a nitrogen atmosphere, and then the microgels were filtered and dialyzed against distilled water for 2 days. Finally, the microgels were collected, freeze dried, and stored at room temperature. Covalent Bonding of Cysteamine-Capped QDs to the Microgels. To covalently attach the quantum dots onto the surface of the microgels, the carboxylic groups were activated, making them suitable for reaction with the amine-terminated CdTe nanocrystlas. To do that, we dispersed 50 mg of microgel in 5 mL of diethyl carbodiimide (EDC, 200 mM) and N-hidroxyl succinamide (NHS, 50 mM) aqueous solution. After the carboxyl groups were activated, the microgel dispersion was added drop by drop into an aqueous dispersion of 1 mg/mL amine-terminated CdTe nanocrystals at neutral pH. The mixture was allowed to react for 30 min. Then, the microgels were collected by centrifugation and stored in the dark. Scheme 1 summarizes the preparation process of the hybrid material. Characterization. The microgel particles were studied using scanning electron microscopy (SEM) in a JEOL (JSM-6400) microscope and transmission electron microscopy (TEM) in a JEOL2000FX microscope operating at 200 kV. Dynamic light scattering experiments (DLS) were carried out to examine the evolution of the
particle size at the volume-phase transition temperatures using a Malvern Nano-ZS system equipped with a He-Ne laser working at 632.8 nm. The suspension of microgels was diluted to a concentration of 0.02% (w/w) to prevent multiple scattering and to diminish colloidal interactions. The time correlation function of the scattering intensity, g(t) ) 〈I(0) I(t)〉, was measured, and the mean hydrodynamic radius was obtained as a function of temperature using cumulative analysis. The charge of the microgels was determined using a model PCD 03 particle charge detector from µMu¨tek. The negative charge of the microgels was inferred from the amount of poly(diallyl-dimethyl-ammonium-chloride) necessary to reach the isoelectric point of a 0.1% (w/w) microgel dispersion. PL spectra were collected using a Perkin-Elmer LS 55, which was coupled to a thermostatic bath. The colloidal stability of the sample was analyzed with a Lumifue from Lumi GmbH.
Results and Discussion Figure 1A shows an SEM micrograph of the neat P(NIPAMAA) microgels. As we can observe in Figure 1A′, monodisperse microgels with a mean diameter of 610 nm were obtained. Figure 1B shows the P(NIPAM-AA) microgels with CdTe nanocrystals immobilized on its surface (P(NIPAM-AA-CdTe)).
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Figure 5. (A) PL intensity of the P(NIPAM-AA-CdTe) (see above) microgels at 620 nm as a function of temperature. (B) PL intensity measured during repeated heating (40 °C) and cooling (25 °C) cycles of the microgels. Scheme 2. Proposed Models for the Quenching and Antiquenching Effects
The incorporation of the CdTe nanocrystals reduces the mean diameter of the microgel to 550 nm (Figure 1B′) but does not modify the shape of the distribution. The smaller diameter of the hybrid microgels could be interpreted as an effect of the reduction of the superficial charge due to the transformation of carboxylic groups into amide groups that would permit the approximation of neighboring chains above and below the LCST, resulting in a smaller microgel size. Such an assumption is supported by the reduction of the COOH stretching band intensity at 1730 cm-1 in P(NIPAMAA-CdTe) microgels when compared with the same band in P(NIPAM-AA) microgels (Supporting Information). Such a transformation would reduce the total negative charge of the microgels, therefore decreasing their swelling and their diameter. Figure 2 shows a TEM micrograph of the microgels covered with the CdTe nanocrystals. In Figure 2, one can observe that after the CdTe immobilization the microgel surface becomes more electro-opaque. This effect is even more evident at higher magnification. As illustrated in the right panel of Figure 2, the coverage of microgels with QDs appear to be rough with aggregates of CdTe nanocrystals instead of a monolayer-like coating. We attribute this structure to the nonuniform distribution of the carboxylic groups in the microgel shell that can be weakly
cross-linked. The longer poly(acrylic acid) chains have more lateral carboxylic groups to which the CdTe nanoparticles can be attached and subsequently aggregated. The lack of uniformity of the shell, as observed from the micrographs, is the result of either the irregular distribution of nanocrystals during the microgel collapse when samples were prepared for TEM or the nonuniform distribution of nanocrystals on the microgel surface. With the aim of studying the thermal response of the hybrid materials, we have measured the diameter of the microgels with the QDs as a function of the temperature in the region of the volume-phase transition (Figure 3). As can be observed in Figure 3, one of the effects of incorporating the nanocrystals is the reduction of the diameter of the microgel in both the swelled and the collapsed states. Nevertheless, the volume-phase transition of the hybrid materials remains at 35 °C, which roughly corresponds to the LCST of the microgels without CdTe nanocrystals. It is worth pointing out the increment in the LCST of the P(NIPAM-AA) microgels with respect to those of pure PNIPAM at 35 and 32 °C. This effect is related to the hydrophilic character of the acrylic acid incorporation. This monomer introducse ionic repulsion, which could hamper the aggregation of the NIPAM segments, shifting
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to higher temperature the volume-phase transition as was previously reported by Kratz and co-workers.18 In a similar way, the immobilization of inorganic nanoparticles can also induce modifications in the LCST of the PNIPAM. In this case, the presence of nanoparticles introduces a steric impediment, which avoids the approaching PNIPAM chain, shifting the LCST to higher temperatures.14,21 The PNIPAM microgels do not show PL in the swollen or in the collapsed state. However, this scenario changes when the CdTe nanocrystals are immobilized on the surface of the microgels. Figure 4 depicts the PL spectra of the P(NIPAMAA-CdTe) microgels carried out in the temperature range from 25 to 41 °C. As can be seen in Figure 4, the collapsed P(NIPAM-AACdTe) microgels show a broad emission peak at around 640 nm that disappears when the microgels are in the swollen state (see above). The PL peak is red-shifted in comparison with the emission peak of the pure CdTe nanocrystal solutions (around 605 nm at 25 °C, Supporting Information). This red shift as well as the broadening, which is also observed in the P(NIPAMAA-CdTe) microgels, can be partially explained by the thermally assisted population of trap states near the valence band or similar effects discussed in previous work.22,23 Additionally, it could also be related to energy-transfer processes that occur when the nanocrystals become close-packed.15 With the aim of visualizing the connection between the volumephase transition of the microgels and the enhancement of the fluorescence in Figure 5A, we have plotted the maximum PL intensity as a function of temperature. We can observe a conspicuous fluorescence increase at the same temperature at which the microgels began to collapse, clearly indicating the connection between the two phenomena. Figure 5B shows that the PL quenching and enhancement processes are fully reproducible after subjecting the sample to five heating and cooling cycles. There are two ways to explain how the collapse of the P(NIPAM-AA-CdTe) microgels could trigger the fluorescence enhancement. One is related to the change in the pH around the CdTe nanocrystals, and the other is related to the reduced number of surface defects of the nanocrystals during microgel collapse. The influence of the PL efficiency as a function of the pH value of the colloidal solution has been reported by Zhang et al.16 and Gao et al.17 Thus, Gao and co-workers observed an increment of the PL efficiency of CdTe nanocrystals when the nanocrystals were covered with poly(acrylic acid) at pH below 3. They interpreted this result as an effect of the coordination between carboxyl groups and cadmium ions on the particle surface. However, Kratz et al.18 explained that the collapse of P(NIPAM-co-acrylate) networks requires the reduction of the negative charge by protonation of the COO- groups, which produces COOH groups. The above-mentioned results could (16) Zhang, Z.; Zhow, Z.; Yang, B.; Gao, M. J. Phys. Chem. B 2003, 107, 8. (17) Gao, M.; Kirstein, S.; Mo¨hwald, H.; Rogach, A. L.; Kornowski, A.; Eychmu¨ller, A.; Weller, H. J. Phys. Chem. B 1998, 102, 8360. (18) Krazt, K.; Hellweg, T.; Eimer, W. Colloids Surf., A 2000, 170, 137. (19) Flory P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1975. (20) Wuister, S. R.; Donega´, C. M.; Meijerink, A. J. Am. Chem. Soc. 2004, 126, 10397. (21) Pich, A. Z.; Adler, H. J. P. Polym. Int. 2007, 56, 291. (22) Kapitonov, A. M.; Stupak, A. P.; Gaponenko, S. V.; Petrov, E. P.; Rogach, A. L.; Eychmuller, A. J. Phys. Chem. B 1999, 103, 10109. (23) Walker, G. W.; Sundar, V. C.; Rudzinski, C. M.; Wun, A. W.; Bawendi, M. G.; Nocera, D. G. Appl. Phys. Lett. 2003, 83, 17. (24) Ionov, L.; Sapra, S.; Synytska, A.; Rogach, A. L.; Stamm, M.; Diez, S. AdV. Mater. 2006, 18, 1453.
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explain the increment of the PL emission in the P(NIPAMAA-CdTe) microgels because above the LCST these microgels could have an external shell rich in COOH groups, which could wrap the CdTe nanocrystals, provoking a modification of the fluorescence properties of P(NIPAM-AA-CdTe). However, the reduction of the number of surface defects during microgel collapse provides the second scenario for PL enhancement. In the swollen state, the polymer chain tends to dissolve as a result of the free energy of mixing (∆G mix) that is given by Flory-Huggings theory19
∆Gmix ) KbT
V[ (1 - φ) ln(1 - φ) + χφ(1 - φ)] a3
where V is the volume of the gel, a is the monomer segment length, φ is the volume fraction of the polymer, and χ is the Flory-Huggins interaction parameter. However, because the microgels are cross-linked there is an elastic component (∆Gelas) that hinders the volume expansion at high swelling, creating tensions localized at the cross-linking points. Because the polymer is covalently bound to the cysteamine molecules anchored on the CdTe nanocrystals, these covalent bonds could act as cross-linking points introducing an elastic tension in the bond that could stretch the interface of the surfactantCdTe nanocrystals, creating surface states that can quench the PL. In contrast, above the LCST the polymer is less soluble, reducing the mixing free energy and diminishing the elastic tension and consequently the number of surface trap states acting as emission quenching centers. This phenomenon is similar to the PL temperature antiquenching effect recently reported by Wuister at al.20 These authors found that the freezing of the solvent (water) induced strain in the capping shell and that the short stabilizer molecules (2-mercaptoethanolamine) propagate the strain to the surface of the nanocrystals, creating surface quenching states. In our case, this effect was shown to be reversible. Scheme 2 depicts the possible mechanisms proposed.
Conclusions A novel hybrid material based on CdTe nanocrystals covalently immobilized on the surface of P(NIPAM-AA) microgels was synthesized. The presence of the CdTe nanocrystals provides fluorescence to the microgels at temperatures above the LCST, whereas at temperatures below the LCST an intensive quenching effect was observed. Such variations in the fluorescence efficiency were related to changes in the environment of the polymer network, which could modify the physical-chemistry properties of the CdTe surface. This could increase or reduce the number of active surface states, strongly affecting the PL of the composite. Acknowledgment. This work was supported by the Ministry of Science and Technology (MAT2006-13646-C03-01) and the CAM-UCM Program for research groups. Partial support of COST Action D43 and the EU-NoE Phoremost is acknowledged. J.R.-R. acknowledges the Areces Foundation for a postdoctoral fellowship to perform this work. We acknowledge the support of the Grant Agency of the Academy of Sciences of the Czech Republic (grant no. IAA4050409) and of the Ministry of Education, Youth and Sports of the Czech Republic (grant no. 1M06031) Supporting Information Available: FTIR spectra of microgels and PL spectra of nanocrystals. This material is available free of charge via the Internet at http://pubs.acs.org. LA801347D
