Effect of Adenine on the Photoluminescence Properties and Stability of

Apr 7, 2009 - Relia Diagnostic Systems, Burlingame, California 94010. ReceiVed: July 09, 2008; ReVised Manuscript ReceiVed: February 18, 2009...
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J. Phys. Chem. C 2009, 113, 6929–6935

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Effect of Adenine on the Photoluminescence Properties and Stability of Water-Soluble CdTe Quantum Dots Xuejiao Feng,† Qingkun Shang,*,† Hongjian Liu,‡ Hongdan Wang,† Wenlan Wang,† and Zhidan Wang† Chemistry Faculty, North-East Normal UniVersity, Changchun, 130024, People’s Republic of China, and Relia Diagnostic Systems, Burlingame, California 94010 ReceiVed: July 09, 2008; ReVised Manuscript ReceiVed: February 18, 2009

In this paper, it has been found that a small biomolecule adenine can greatly affect the photoluminescence properties and stability of water-soluble CdTe quantum dots. The interaction between CdTe QDs and adenine is characterized by TEM, fluorescence microscope, and photoluminescence (PL), IR, and UV-vis spectra. The influences of reflux time, pH value, ionic strength, storage time, the ratio of CdTe QDs to adenine on the PL intensity, and the stability of CdTe-adenine conjugates are investigated in detail. The PL intensity and stability of CdTe-adenine conjugate are largely enhanced compared to that of pure CdTe QDs. At the same time the fluorescence quantum yield of the CdTe-adenine conjugate reaches 23.04% in comparison with 20.27% of the parent CdTe QDs. The existence of coordinated bond and hydrogen bond between CdTe QDs and adenine is illuminated by IR analysis. These bonds together with the passivation of adenine on the surface of CdTe are possibly the main reasons for the stability of the conjugate. As a new stabilization agent, adenine has no effects on the properties of the biological system but can increase the stability and PL intensity of CdTe QDs, which will enlarge the application of QDs in biomedicine and other fields. Introduction Colloidal nanocrystals, often referred to as quantum dots (QDs), have attracted extensive scientific and industrial interests as a consequence of their strong size-dependence and unique optical and electronic properties,1-8 which give rise to their potential application in a variety of fields, including lightemitting devices,9,10 photonic crystals,11,12 nonlinear optical devices,13 and especially in biological labels.14-17 Originally, many groups tried to synthesize QDs mentioned above, and succeeded to some extent, through an organic method. The biological applications of these kinds of QDs have been hampered by their inherently low solubility in water. Then, new chemical strategies were established to solve this problem. One is to replace the surface-capping molecules on QDs prepared in organic solvents with water-soluble thiols.18-20 The other is to directly synthesize CdTe QDs in aqueous solution by using water-thiols as a stabilizing agent.21-24 With the solubility of QDs solved, substantial progress has been achieved by several groups recently, which has led to a burst of activities in the forming of biomolecule-QDs conjugation. Mamedova et al.22 demonstrated the preparation, structure, and interunit energy transfer with the antenna effect of albumin-dTe nanoparticle bioconjugates. Several groups, such as those of Meeker,25 Gerion,26 and Parak,27 have focused on the interaction between DNA and QDs. Most of the work in this area has centered on improving the luminescent property and stability of highly capping QDs with proteins, DNA, or other biological molecules and application to cell label. To obtain a better application of these conjugates, it is very * To whom correspondence should be addressed. Phone: +86 431 85099787. E-mail: [email protected]. † North-East Normal University. ‡ Relia Diagnostic Systems.

important to understand the effects of biomolecules on the PL intensity and stability of QDs. In this paper, thioglycolic acid (TGA)-capped CdTe is selected as one of the most robust and highly luminescent nanparticle materials which are directly synthesized in aqueous medium. The effects of adenine, a part of nucleic acid, on the photoluminescence property and stability of CdTe QDs have been studied. The interaction mechanism between CdTe QDs and adenine are discussed. The results demonstrate that the PL intensity and stability of the CdTe-adenine conjugate are largely enhanced compared to that of pure CdTe QDs. It is believed the application of QDs in biomedicine and other fields will be extended by using adenine as a new stabilization agent. Experimental Proceduces Instrument. The emission spectra were obtained on a CARY eclipse luminescence spectrometer. Absorption spectra were recorded on a CARY 500 UV-vis-NIR spectrometer. IR spectra were recorded with a Nicolet Magna 560 FTIR spectrophotometer. TEM imagines were obtained with a JEOL 100CXII transmission electron microscope. Fluorescence micrographs were obtained with a Nikon ECUPSE TE 2000-U fluorescence microscope. The pH measurements were done with a PHS-3C pH meter. All optical measurements were carried out at room temperature under ambient conditions. Reagents and Materials. The chemicals used in the experiments included Cd (AC)2 · 2H2O (analytical purity), sodium borohydride (96%), tellurium powder (99.999%), adenine (Biology Reagent), guanine (Biology Reagent), and thioglycolic acid (TGA, purity). All chemicals were used without further purification. Preparation of Water-Soluble CdTe QDs. The CdTe QDs were prepared by using the reaction between Cd2+ and NaHTe in the presence of thioglycolic acid (TGA) as the stabilizing

10.1021/jp806116m CCC: $40.75  2009 American Chemical Society Published on Web 04/07/2009

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Figure 1. UV-vis absorption spectrum (a) and PL spectrum (b) of CdTe QDs and CdTe-adenine conjugates.

agent following the method described previously.28,29 Sodium borohydride was reacted with tellurium powder (2:1 molar ratio) in water to produce NaHTe. The molar ratio of Cd2+:Te2-:TGA was 1:0.5:2.5.The pH value of the solution was adjusted to 7.6 with 0.1 M NaOH. The oxygen in the system was removed by flowing nitrogen when the system was kept in a reflux distillation state at 96 °C for 2 h. 2-Propanol was used to precipitate and purify the CdTe QDs. The free TGA was removed by centrifugation and separation. Preparation of CdTe-Adenine Conjugates. Purified CdTe colloidal solution and aqueous adenine were mixed in different ratios and heated to 96 °C and kept under reflux distillation for different times. The effects of different reflux time, the pH value of the reaction system, the ionic strength, the ratio of adenine to CdTe, and the storage time on the photoluminescence of CdTe QDswereinvestigated.ToinvestigatethestabilityofCdTe-adenine, CdTe-guanine conjugates were prepared. Results and Discussion Characterization of CdTe-Adenine. Figure 1 presents both absorption and PL spectra of CdTe QDs and CdTe-adenine. The absorption spectrum (Figure 1a) indicates that both CdTe QDs and CdTe-adenine have a broad range of absorption. The absorbance of the CdTe-adenine conjugate is higher than that of CdTe. Compared to the original CdTe, the absorption wavelength of the conjugate is red shifted. The difference in absorption spectra shows that the interaction between CdTe QDs and adenine exists. Meanwhile, the PL spectra of the CdTe QDs, CdTe-adenine, overlaid with the absorption spectra in Figure 1b, are measured at the excitation wavelength of 335 nm. The PL spectrum of CdTe-adenine is characterized by good symmetry, and is sufficiently narrow with a full width at halfmaximum (fwhm) of 49 nm, which is about 17 nm narrower than the value of CdTe QDs. This reflects that the CdTe-adenine has a relatively narrow size distribution and keeps unique optical properties. The PL intensity of CdTe-adenine is higher than

that of the pure CdTe QDs. A possible reason was the coordinated interactions between CdTe surface and nitrogen atoms of adenine, which can efficiently remove the dangling bonds and surface defects, resulting in higher fluorescence intensity than that of the original TGA-capped CdTe QDs. The maximum emission peak of CdTe-adenine is at 600.0 nm, which shifts to the red 40 nm in comparison with that of CdTe QDs (559 nm). This may be attributed to the formation of the CdTe-adenine conjugate, leading to an increase of particle size. This argument can be supported by the TEM images of CdTe-adenine conjugates and CdTe QDs, as shown in Figure 2. One can see from the figure that the diameter of CdTe-adenine is bigger than that of CdTe QDs. The IR spectra of adenine and CdTe-adenine conjugate are shown in Figure 3. For adenine the absorption bands occur at 3500-3000 cm-1 (ν(OH,H2O), ν(NH2,NH)), 1673 cm-1 (σ(N-H)), 1602 cm-1 (σ(CdN)), and 1350-1250 cm-1 (ν(C-N)). The most pronounced IR absorption bands of CdTe-adenine conjugate occur at 3500-3000 cm-1 (ν(OH,H2O)), 1675 cm-1 (νas(COO-)), 1450 cm-1 (νs(COO-)), and 1400 cm-1 (σ(OH)) for the thioglycolic acid stabilized QDs. The S-H vibration (ca. 2580 cm-1) has not been observed, possibly because of the formation of the covalently bound QDs surface. The existence of a hydrogen bond between -NH2 of adenine and -COOH of CdTe QDs can be illuminated by the broad band around 3440 cm-1, a decreased absorption at 1675 cm-1, a blue-shift from 1673 cm-1 to 1609 cm-1, and a wide region from 1424 to 1675 cm-1. In addition, a weak adsorption at 1602 cm-1 for the CdN bend vibration is possibly derived from the decrease in density of the electron cloud on the heterocycle. This may result from the coordination between -NH2 of adenine and the Cd atom of CdTe QDs. The conjugation of QDs and adenine is also proved by fluorescence micrographs of free CdTe QDs and CdTe-adenine, as shown in Figure 4. The green luminescence of CdTe QDs can be seen as a whole in Figure 4A. But orange luminescence

Stability of Water-Soluble CdTe Quantum Dots

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Figure 2. TEM images of CdTe QDs (a) and CdTe-adenine conjugates (b).

results in the PL intensity of conjugates increasing. In the second equation, the -NH2 group of adenine may combine with the -COOH of thioglycolic acid from the surface of QDs to form the hydrogen bond, which enhances the interaction between QDs and adenine. This interaction influences the quantum yields (QY) of QDs. By using Rohodamine 6G as a PL reference, the PL QY of CdTe-adenine conjugates can be estimated by,30

QYx ) [(ArLxnx2)/(AxLrnr2)]QYr

Figure 3. IR spectra of adenine and CdTe-adenine conjugates.

for CdTe-adenine conjugates is clearly observed in Figure 4B. The formation of CdTe-adenine conjugation causes the increase of the particle size and the variation of the emission color. These fluorescence images are in good agreement with the results shown in Figures 2 and 3. The photographs of the emission colors of CdTe and CdTe-adenine under the radiation of a UV lamp are shown in Figure 4C. The color variation from green (CdTe QDs) to orange (CdTe-adenine conjugate) was another evidence that indicates the interaction between CdTe and adenine. A possible mechanism of the interaction between CdTe QDs and adenine is proposed in Scheme 1. In the first equation, the adenine molecule exists in F-π, π-π conjugation, which activates the -NH2; there are also lots of dangling bonds on the surface of CdTe QDs according to the Cd2+ ions with unoccupied orbitals. These Cd2+ ions can coordinate with the -NH2 group of adenine easily. The coordination sites of Cd with the N atom in adenine are different from that of Cd with thio-sulfur. The surface dangling bonds are passivated, which

Here, QYx and QYr are the absolute quantum yields of QDs sample and Rohodamine 6G, respectively. The room quantum efficiency of Rohodamine 6G in ethanol is 95% from the literature. Ax and Ar are compared to the absorption value at this excitation wavelength, respectively. nx and nr are the refractive index of solvents: nethanol is 1.359, nwater is 1.333 at room temperature. Lx and Lr are PL integrate intensities of CdTe QDs and Rohodamine 6G excited at 470 nm. The PL QY of CdTe-adenine conjugates are thus estimated to be 23.04%, which is higher than that of CdTe QDs (20.27%). Effect of Reflux Time. It is shown in Figure 5 that different reflux times can affect both the peak position and the PL intensities of CdTe-adenine conjugates. The PL emission intensity increases as the reflux time increases from 0 to 1 h, due to the improvement of the crystallization and annealing effect of defects. The maximum PL emission occurs at a reflux time of 1 h. This can be explained in terms of the mechanism of Ostwald ripening and defocusing.31 However, refluxing for more than 1 h results in a decrease of PL emission intensity because of a broad distribution and relatively small surface/ volume ratio of the CdTe QDs. In addition, the values of fwhm increase from 44 up to 53 nm with an increase in refluxing to more than 1 h, which is also ascribed to the effect of Ostwald ripening. Reflux time also hasa great effect on the peak position of CdTe-adenine conjugates. The peaks of CdTe-adenine conjugates shift from 558 nm to 624 nm as the reflux time is prolonged. The shift can be understood by the consequence of quantum confinement. It is known that the wavelength of fluorescence depends on the bandgap and thus on the size of

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Figure 4. Fluorescence micrographs of free CdTe QDs (A) and CdTe-adenine (B) and photographs of the emission colors of CdTe and CdTe-adenine under the radiation of a UV lamp (C).

SCHEME 1: Possible Mechanism of Interaction between CdTe QDs and Adenine

the quantum dot.32,33 The larger the particle size of QDs is the longer the wavelength of the fluorescence emission peak of QDs will be.34,35 When the reflux time is prolonged, the particle size of CdTe-adenine conjugates may increase concomitantly. Therefore the emission peaks of CdTe-adenine have shifted to the red. Effect of pH Value. The PL spectra recorded under different pH values are shown in Figure 6. The intensity of CdTe-adenine first increases gradually as the pH values increase from 5.44 to 7.31, and then decreases when the pH gets higher. The strongest fluorescence intensity of CdTe-adenine is obtained at pH 7.31, while that of CdTe is obtained at pH 7.60, which is the same as our previous observations.36 At a higher pH value (pH >8), desorption of adenine from the QDs surface may occur, resulting in the aggregation of QDs and decrease of PL intensity. Meanwhile, when the pH is less than 5.44 or more than 8.36, the surface defects and dangling bonds cannot be passivated well. Therefore, the CdTe QDs become unstable and agglutinable with a lower PL intensity. In addition, the PL peak shifts to the red with the formation of the conjugate and the particle size increases in the experimental pH condition. The bandwidth broadens when the pH is less than 5.44 or more than 8.36 compared to that of CdTe QDs. All these results indicate that the photoluminescence efficiency of the conjugates strongly

depends on the pH value of the solution. The photoluminescence properties of CdTe-adenine conjugates are best in neutral pH condition. This will provide a wide application in biosystems. Effect of the Molar Ratio of Adenine to CdTe. Figure 7 shows the maximum PL intensities of CdTe-adenine conjugates at different molar ratios of adenine to CdTe. The molar ratio of adenine to CdTe has a small effect on the PL intensity of the CdTe-adenine conjugate in the range between 0.2 and 5. This indicates that the space around CdTe QDs has been saturated with adenine molecules when the molar ratio reaches 0.2. However, when the ratio is bigger than 5, the PL intensity decreases greatly. In general, if the ratio of adenine to CdTe is too high, the excessive adenine molecules generate some additional traps, which is the center of nonradiative recombination. This will result in the decrease of PL intensity. We found a small change of PL intensity when the molar ratio increased from 0.2 to 5, which means the nonradiative recombination cannot be formed until enough adenine molecules exist. Effect of the Ionic Strength. The PL spectra of the CdTe-adenine conjugates with different ionic strength are shown in Figure 8. It can be seen that the intensity of the CdTe-adenine conjugates increases gradually as the concentra-

Stability of Water-Soluble CdTe Quantum Dots

Figure 5. PL spectra of CdTe-adenine at different reflux times: (a) CdTe; (b) 0 h; (c) 0.5 h; (d) 0.83 h; (e) 1.0 h; (f) 1.5 h; (g) 2.0 h; (h) 2.5 h. (Inset: PL intensity vs. reflux time.)

Figure 6. PL spectra of CdTe-adenine under different pH values: (a) CdTe; (b) pH 6.30; (c) pH 7.31; (d) pH 8.36. (Inset: PL intensity vs. pH value.)

Figure 7. The maximum PL intensities of CdTe-adenine conjugates at different molar ratios of adenine to CdTe.

tions of NaCl increase from 0 to 0.05 mol · L-1, then decreases greatly when the concentration of NaCl is higher than 0.05 mol · L-1. There are many electriferous ions in the solution, such as -COO-/Cd2+ ions on the surface of CdTe QDs and N+/N- of

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Figure 8. PL spectra of CdT-adenine conjugates under different ionic strength controlled by sodium chloride solution: (a) CdTe; (b) CNaCl ) 0.00 mol · L-1; (c) CNaCl ) 0.01 mol · L-1; (d) CNaCl ) 0.02 mol · L-1; (e) CNaCl ) 0.05 mol · L-1; (f) CNaCl ) 0.07 mol · L-1; (g) CNaCl ) 0.09 mol · L-1. (Inset: peak vs. CNaCl.)

Figure 9. PL spectra of CdTe-adenine and CdTe-guanine conjugates and CdTe versus storage time.

the adenine. When Na+/Cl- ions are added into the solution, they could passivate the surface of the CdTe QDs and effectively diminish the contribution of the nonradiative channel for electron-hole recombination, and consequently enhance the PL intensity. But if the concentration of NaCl is too high, the counterion, Na+ and Cl-, can reduce the binding affinity of QDs to adenine, thus decreasing the PL intensity. Effect of Storage Time. Before QDs are used for biological and medical applications, it is necessary to know the effect of storage time on stability. We selected CdTe-adenine sample, compared with original CdTe QDs, CdTe-guanine, for tracking the variation of their optical properties on different storage times. Figure 9 shows the PL spectra of samples incubated in a refrigerator at 4 °C for 31 days. The PL intensities of CdTe-adenine conjugates increase in the first 12 days and then become constant, which is higher than the initial value of the parent CdTe QDs aqueous solution. Due to their high surface energy and reactivity, the surfaces of the QDs can be attached preferably and dynamically by adenine. Thus QDs will be passivated more efficiently by adenine, resulting in a more obvious enhancement in PL intensity during the initial storage time. With the increase of storage time, adenine-coated CdTe QDs build a potential barrier, preventing the surface deteriora-

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Figure 10. PL peak positions of CdTe QDs, CdTe-adenine, and CdTe-guanine conjugates versus storage time.

tion and aggregation and protecting the stabilization in aqueous solution. It can further be concluded that the enhancement of PL intensity and photochemical stability is credited to the covalent bonding between adenine and QDs, as illustrated in Scheme 1. Additionally, the PL intensity of pure CdTe increases significantly for the first five days, then deceases to a plateau that is still higher than the initial value of the parent solution. This can be ascribed to the photocatalytic oxidation of the thiol ligands on the surface of QDs to form the disulfides by using CdTe QDs as the photocatalyst. This photocatalytic process prevented the photooxidation of QDs themselves. After nearly all of the thiol ligands on the surface of QDs are converted into disulfides, the system undergoes several different pathways.37 In our work, the disulfides are insoluble in aqueous, they likely formed a micelle-like structure around the QDs core and kept the QDs stable for a long period of time. Herein, the CdTe QDs are stable in solution, but the PL intensity is not strong, compared to that of the CdTe-adenine. The adenine-coated CdTe QDs are not only stable, but also have enough PL intensity. As shown in Figure 9, we also made a further comparison between CdTe-adenine and CdTe-guanine and found that guanine-coated CdTe QDs are less stable and almost quenched in a month. It is observed that fluorescence intensity of the CdTe-guanine loses 73% after 31days. The structure difference between adenine and guanine resulted in guaninecoated CdTe QDs being unstable. Both oxygen and nitrogen atoms on guanine can easily coordinate with the cadmium atom of CdTe QDs, directly leading to decreased concentration of free Cd2+, and resulting in more dangling bonds of Te atoms. Figure 10 shows the PL peak position of CdTe-adenine, CdTe-guanine conjugate, and CdTe QDs stored in the refrigerator at 4 °C as a function of storage time. For PL peak position, a very interesting phenomenon is observed. The PL peak positions of CdTe-adenine, CdTe QDs are almost constant for a month. This can be explained by the dynamic equilibrium between free and bound capping ligands, adenine, in which the particle size is almost constant, and the PL peak position is correlated to particle size. In other words, with the increase of storage time, adenine can reduce the dangling bonds on the CdTe QDs surface, but once these defect sites are saturated, adenine also can substitute the native capping agent TGA through ligand exchange, resulting in a constant PL peak position. The same phenomenon is also detected in guanine-coated CdTe.

The conjugates of CdTe stabilized by thioglycolic acid and adenine have been synthesized in aqueous solution. Adenine can enhance the PL intensity and stability of CdTe QDs effectively. Many factors affect the PL intensity of CdTe-adenine conjugates in our experimental conditions. To obtain higher PL intensity and more stable conjugates the condition should be controlled in a lower ionic strength solution, less than 1 h reflux time, neutral pH, and lower 5/1 molar ratio of adenine to CdTe. The coordination between the Cd of CdTe QDs and the N of adenine, hydrogen bonding between adenine and thioglycolic acid, and the special p-π, π-π structure of adenine are proposed as the main reasons for the increase of PL intensity and stability of conjugate. On the basis of our above results, it is suggested that adenine may be another stabilization agent for CdTe QDs to further prevent the aggregation of CdTe QDs. It is useful to understand the reaction mechanism between QDs and nucleic acid and make a good application in biological, medical, and other fields. Acknowledgment. The authors acknowledge the support from the Science and Technology Foundation of Jilin Province (20060572,20070705)andChangchunCity(05GG56,2007GH26), China. References and Notes (1) Li, Y.; Rizzo, A.; Cingolani, R.; Gigli, G. AdV. Mater. 2006, 18, 2545. (2) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E. C.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (3) Jacobs, K.; Zaziski, D.; Scher, E. C.; Herhold, A. B.; Alivisatos, A. P. Science 2001, 293, 1803. (4) Tang, Z. Y.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237. (5) Coe, S.; Woo, W. K.; Bawendi, M. G.; Bulovic, V. Nature 2002, 420, 800. (6) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (7) Wang, Q.; Seo, D.-K. Chem. Mater. 2006, 18, 5764. (8) Wang, X.; Du, Y.; Ding, S.; Wang, Q.; Xiong, G.; Xie, M.; Shen, X.; Pang, D. J. Phys. Chem. B 2006, 110, 1566. (9) Gao, M.; Lesser, C.; Kirstein, S.; Mo¨wald, H.; Rogach, A. L.; Weller, H. J. Appl. Phys. 2000, 87, 2297. (10) Lin, Y. W.; Tseng, W. L.; Chang, H. T. AdV. Mater. 2006, 18, 1381. (11) Rogach, A. L.; Susha, A.; Caruso, F.; Sukhorukov, G.; Kornowski, A.; Kershaw, S.; Mo¨wald, H.; Eychmuller, A.; Weller, H. AdV. Mater. 2000, 12, 333. (12) Fleischhaker, F.; Zentel, R. Chem. Mater. 2005, 17, 1346. (13) Mamedov, A. A.; Belov, A.; Giersig, M.; Mamedova, N. N.; Kotov, N. A. J. Am.Chem. Soc. 2001, 123, 7738. (14) Jiang, W.; Mardyani, S.; Fischer, H.; Chan, W. C. W. Chem. Mater. 2006, 18, 872. (15) Schroedter, A.; Weller, H.; Eritja, R.; Ford, W. E.; Wessels, J. M. Nano Lett. 2002, 2, 1363. (16) Zheng, Y.; Gao, S.; Ying, J. Y. AdV. Mater. 2007, 19, 376. (17) Jiang, W.; Singhal, A.; Zheng, J.; Wang, C.; Chan, W. C. W. Chem. Mater. 2006, 18, 4845. (18) Gaponik, N.; Talapin, D. V.; Rogach, A. L.; Hoppe, K.; Shevchenko, E. V.; Kornowski, A.; Eychmuller, A.; Weller, H. J. Phys. Chem. B 2002, 106, 7177. (19) Wang, S.; Mamedova, N.; Kotov, N. A.; Chen, W.; Studer, J. Nano Lett. 2002, 2, 817. (20) Wuister, S. F.; Swart, I.; van Driel, F.; Hickey, S. G.; de Mello Donega, C. Nano Lett. 2003, 3, 503. (21) Rogach, A. L.; Susha, A. S.; Caruso, F.; Sukhorwkov, G. B.; Kornowski, A.; Kershaw, S.; Mo¨hovald, H.; Eychmu¨eller, A.; Weller, H. AdV. Mater. 2000, 12, 333. (22) Mamedova, N. N.; Kotov, N. A.; Rogach, A. L.; Studer, J. Nano Lett. 2001, 1, 281. (23) Rogach, A. L.; Kotov, N. A.; Koktysh, D. S.; Ostrander, J. W.; Ragoisha, G. A. Chem. Mater. 2000, 12, 2721. (24) Radtchenko, I. L.; Sukhorukov, G. B.; Gaponik, N.; Kornowski, A.; Rogach, A. L.; Mo¨hovald, H. AdV. Mater. 2001, 13, 1684. (25) Meeker, K.; Ellis, A. B. J. Phys. Chem. B 2000, 104, 2500.

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