About this Journal Submit a Manuscript Table of Contents
Journal of Nanomaterials
Volume 2012 (2012), Article ID 478153, 10 pages
http://dx.doi.org/10.1155/2012/478153
Research Article

Polycation-Capped CdS Quantum Dots Synthesized in Reverse Microemulsions

Institut für Chemie, Universität Potsdam, Karl-Liebknecht-Straße 24-25, Haus 25 Golm, 14476 Potsdam, Germany

Received 21 February 2012; Accepted 21 June 2012

Academic Editor: Grégory Guisbiers

Copyright © 2012 Karina Lemke and Joachim Koetz. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

This paper is focused on the formation and recovery of cadmium sulfide (CdS) nanoparticles in two different types of polycation-modified reverse microemulsions using low molecular weight poly(diallyldimethylammonium chloride) (PDADMAC) and poly(ethyleneimine) (PEI). Both polymers were incorporated in a quaternary w/o microemulsion consisting of water, toluene-pentanol (1 : 1), and sodium dodecyl sulfate (SDS), as well as in a ternary w/o microemulsion consisting of water, heptanol, and 3-(N,N-dimethyl-dodecylammonio)-propanesulfonate (SB). UV-vis and fluorescence measurements in the microemulsion illustrate the capping effect of the polycations on the formation of the CdS quantum dots. The nanoparticles are redispersed in water and characterized by using UV-vis and fluorescence spectroscopy, in combination with dynamic light scattering. From the quaternary microemulsion, only nanoparticle aggregates of about 100 nm can be redispersed, but, from the ternary microemulsion, well-stabilized polycation-capped CdS quantum dots can be obtained. The results show that the electrostatic interactions between the polycation and the surfactant are of high relevance especially in the solvent evaporation and redispersion process. That means only that in the case of moderate polycation-surfactant interactions a redispersion of the polymer-capped CdS quantum dots without problems of aggregation is possible.

1. Introduction

The importance of small, monodisperse semiconductor particles has greatly increased in the recent years, due to the growing interest in technological applications of the resulting materials in electronic devices like solar cells or LEDs [14], medical diagnostics [57], and photo catalysis [810]. The optical, optoelectronical, and magnetic properties of such materials are strongly depending on the size according to the well-investigated size quantization effect [1114]. Nanoscalic semiconductors like cadmium sulfide show a size-dependent onset of light absorption, and a blue shift of the fluorescence band with decreasing particle size. Therefore, the formation of monodisperse cadmium sulfide (CdS), CdSe, or ZnS nanocrystals, so-called quantum dots, is of importance. For the preparation of such materials, different methods can be used, for example, sol-gel [15, 16] or solvothermal [17, 18] and irradiation processes [19, 20]. An important way to synthesize monodisperse nanoparticles of very small dimensions is to use a template phase. The nanoparticles can be formed in block copolymer micelles or polymer microgels [2124] in dendrimers or microemulsions [2531]. Especially microemulsion template phases are widely used as “nano-reactors” to make ultrafine particles [32, 33]. For this process, two w/o microemulsions are mixed together containing reactant A and reactant B, respectively. Hereby, the size of the droplets, and the bending elasticity as well as the rigidity of the droplet film is of importance, due to the fact that droplet diffusion and fusion is the rate-determining step in the microemulsion and not the reaction between the components A and B [31]. Two different mechanisms for the droplet exchange process are discussed in the literature, that is, the water channel mechanism and the droplet coalescence mechanism [34].

By adding polymers one can influence the droplet size [3537], the droplet-droplet interactions [3840], and the stability [41] and rigidity [4244] of the surfactant film in w/o microemulsion due to polymer-surfactant interactions.

Our own experiments have shown that polyelectrolytes can be incorporated into water-in-oil microemulsions, too [4547]. Molecular dynamic simulations of inverse micelles in presence of a cationic polyelectrolyte have shown that the polymer is located nearby the surfactant film [48]. The polyelectrolyte-modified water-in-oil microemulsions can be used as a template phase for the formation of different types of very small nanoparticles, for example, BaSO4, ZnS, gold, and magnetite [4952].

It has to be mentioned here that it is possible to produce ultrafine CdS nanoparticles inside the microemulsion droplets with a narrow size distribution in absence of an additive [26, 32, 53], but an efficient recovery of the nanoparticles from microemulsion is still an open problem [54]. There are different approaches, for example, water or temperature-induced separation [55, 56], precipitation by antisolvents [57] or surfactants [58], or a so-called cloud point extraction [59], but due to their high surface energy the ultrafine particles tend to coagulate irreversibly during the separation process. Therefore, it is necessary to protect the nanoparticles during the separation and recovery process. To overcome this problem organic molecules can be added to stabilize the particles during the recovery process. Agostiano et al. have used, for example, thiophenol as a capping agent to redisperse CdS nanoparticles in pyridine [26], and Tamborra et al. immobilized octylamine-capped CdS nanocrystals in polymers, that is, poly(methylmethacrylate) or polystyrene [60].

To stabilize nanoparticles in water, polyelectrolytes are of special relevance due to their electrosterical stabilization effect [61, 62]. Therefore, polyelectrolyte-modified microemulsions are of special interest, because of the fact that the polyelectrolytes are already present during the process of particle formation, solvent evaporation as well as during the process of redispersion in water.

This paper describes the influence of two polycations incorporated into microemulsions on the CdS nanoparticle formation process and the possibility to recover the nanoparticles after solvent evaporation and redispersion in water.

The aim of this paper is to learn more about the role of electrostatic interactions in the nanoparticle recovery process.

Therefore, two microemulsions were used: one containing the strong anionic SDS and the other the amphoteric SB.

The formation of CdS in both types of microemulsion is possible by mixing microemulsions containing the corresponding salts, that is, CdCl2 and (NH4)2S [63]. The nanoparticle formation was investigated by means of UV-vis and fluorescence spectroscopy, and the redispersed particles were also characterized by UV-vis and fluorescence measurement, as well as dynamic light scattering (DLS).

2. Experimental Part

2.1. Materials

The poly(diallyldimethylammonium chloride), with a molecular weight of Mw = 21.000 g/mol, was synthesized by radical polymerization. DADMAC (N,N′-Diallyl-N,N′-dimethylammonium-chloride) (65 wt. % in water) was purchased from Sigma-Aldrich and used as received. The branched Poly(ethyleneimine) with a molecular weight of Mw = 25.000 g/mol was obtained from the BASF. Toluene (>99%; Fluka), pentanol (>99%, Fluka), and heptanol (>99%, Merck) were used without further purification. Sodium dodecylsulfate (SDS) (>99% Fluka), cadmium chloride (CdCl2) (>99%, Fluka), and a 20 wt% aqueous solution of (NH4)2S are used as obtained. 3-(N,N-dimethyl-dodecylammonio)-propanesulfonate (SB) (>97%) was obtained from Raschig company. Water is purified with the Milli-Q Reference A+ water purification system (Millipore).

2.2. Phase Diagram

The determination of the isotropic phase range of all systems mentioned above has been carried out by titration of the pseudobinary oil-cosurfactant/surfactant, or oil/surfactant mixture with a 1 wt. % aqueous polymer solution. The mixture was shaken or treated by ultrasonification and optically tested in order to survey a transparent phase region of reverse microemulsions (L2-phase).

2.3. Preparation of CdS Nanoparticles

CdS nanoparticles are prepared by mixing two microemulsions: one containing 40 mmol/L CdCl2 and 2 wt% polymer and the other 40 mmol/L (NH4)2S. The spontaneous induced formation of the nanoparticles in the w/o microemulsion droplets occurs after shaking the received microemulsion mixture of the corresponding precursor salts. Afterwards the mixture was dried under vacuum at 40°C for one week to remove the solvents (water, toluene, pentanol, and heptanol). The received powder was redispersed in water by ultrasonification for further characterization.

2.4. Characterization of CdS Nanoparticles
2.4.1. UV-Vis Spectroscopy

Absorption spectra are obtained by using a Cary 5000 UV-vis NIR spectrophotometer (Varian) in a wave length range between 200 and 800 nm. For this, the microemulsion samples, as well as the redispersed samples, were placed in a quartz cuvette with a path length of 1 cm.

2.4.2. Fluorescence Spectroscopy

Fluorescence measurements are carried out by using a FluoroMax-3 spectrometer (Horiba) in the wave length region between 370 and 700 nm at the excitation wavelength between 340 and 370 nm.

2.4.3. Dynamic Light Scattering and Electrophoretic Light Scattering

The determination of the particle size and the particle size distribution by dynamic light scattering measurements is carried out at a fixed angle of 173° (backscattering) at 25°C using the Zetasizer Nano ZS (Malvern), equipped with a He-Ne laser and a digital autocorrelator. The averaged particle diameters were obtained from five separate measurements by using a peak analysis by number, volume, or intensity.

The zeta potential that means the electrokinetic potential at the effective shear plane between the moveable and nonmovable part of the double layer, was measured by means of the Zetasizer Nano ZS (Malvern) based on the principle of electrophoretic light scattering.

3. Results

3.1. Phase Behaviour

The partial phase diagrams with PEI and PDADMAC, determined at room temperature, show a transparent phase region of reverse microemulsions (L2-phase) in the “oil” corner in all systems (Figures 1(a) and 1(b)). For the quasiternary system toluene-pentanol (1 : 1)/SDS/water, the L2-phase is extended towards the water corner when PDADMAC is incorporated in comparison to the PEI-modified system. The phase range of the L2-phase of the ternary system heptanol/SB/water is decreased by replacing PDADMAC with PEI. In both cases the reduction of the L2-phase in presence of PEI is accompanied by lower interactions between the branched polymer and the surfactant film.

fig1
Figure 1: Partial phase diagram of the L2-phase of the modified quasi ternary system toluene-pentanol (1 : 1)/SDS/water (a) and of the modified ternary system heptanol/SB/water (b).

Furthermore, an extension of the phase range of the L2-phase for both polymers in direction to the water corner can be observed in the quasiternary SDS-modified system. This effect can be explained by an enhancement of the bending elasticity of the surfactant film caused by interactions between the anionic surfactants and the polycations.

3.2. Nanoparticle Formation
3.2.1. Toluene-Pentanol (1 : 1)/SDS/Water Template Phase

The formation of CdS nanoparticles has been carried out at Point A with a composition oil/surfactant/water = 88/6/6, that means at a water to surfactant ratio, 𝑅 = 1 . 0 . For checking the influence of the droplet size on the results, additional investigations were made at point B (composition oil/surfactant/water = 70/20/10), at 𝑅 = 0 . 5 . After elimination of the reference spectra, the absorption spectra are given in Figure 2(a) in absence and in presence of low molecular weight PEI and PDADMAC, respectively.

fig2
Figure 2: Absorption (a) and fluorescence spectra (b) of the microemulsion system toluene-pentanol (1 : 1), SDS, water after CdS nanoparticle formation in presence of PEI or PDADMAC at point A.

For poly(ethyleneimine) two pronounced absorption maxima, the first one at 324 nm and the second one at 348 nm, could be observed. Note, that the presence of two absorption maxima could be a hint for the existence of two different particle fractions. However, the fluorescence spectrum for the PEI modified system shows only one well defined band with a maximum at 508 nm (Figure 2(b)), which should be more asymmetric, if there are different particle fractions. Due to that, one can conclude that a surface modification or cluster formation with PEI influence the absorption behaviour of the particles in that characteristic way. Similar absorption and emission spectra were obtained by us with PEI of significant lower and higher molar masses [63].

In comparison a modification with poly(diallyldimethylammonium chloride) leads to a broad shoulder between 390 and 440 nm, similar to the unmodified water system, in good agreement to the results given in references [63, 64]. Fluorescence measurements (Figure 2(b)) show an expanded emission peak between 450 and 800 nm for PDADMAC with a maximum at 690 nm. In both systems the spectra indicate the formation of nanometer-sized CdS nanoparticles, taken into account that the position and the height of the absorption band are well related to the size of the semiconductor particles [26]. For that reason, one can assume a mean particle size range between 2 are 10 nm for the CdS nanoparticles formed in presence of PEI or PDADMAC. Nevertheless, the blue shift of the two absorption bands in the UV-vis spectrum indicates the formation of PEI-capped small CdS nanoparticles with a narrow size distribution, in good agreement with the shift of the emission peak towards the blue region.

Note that similar results were obtained at point B (compare Figure  8 in the Supplementary part), indicating that the droplet size is of minor relevance.

The blue emission colour of the PEI-modified and the orange colour of the PDADMAC-modified systems excited with a near-UV lamp (shown in Figure  9 in the Supplementary Material available online at http://dx.doi.org/10.1155/2012/478153) underline the statements given before.

Summarizing the results, one can conclude that the PEI-modified nanoparticles are of about 2 nm, and the PDADMAC-modified ones of about 4 nm in size. This is in full agreement with calculated diameter based on the band gap according to reference [65] summarized in Table 1.

tab1
Table 1: Experimental values of the band gap ( 𝐸 𝑔 ) corresponding to the calculated diameter of the CdS nanoparticles.
3.2.2. Heptanol/SB/Water Template Phase

To compare the characteristics of the CdS nanoparticles synthesized in the ternary system with the nanoparticles synthesized in the quaternary system, the formation of the particles has been carried out in all cases at point A and point B, respectively. In Figure 3(a) the absorption spectra (after elimination of the reference spectra) are given for CdS nanoparticles in presence of PEI and PDADMAC in comparison to the spectrum in absence of a polymer.

fig3
Figure 3: Absorption (a) and fluorescence spectra (b) of the microemulsion system heptanol, SB, water after CdS nanoparticle formation in presence of PEI or PDADMAC at point A.

The absorption spectra for PEI show two absorption maxima, the first one, weakly distinctive, at 330 nm and the second one at 360 nm. In comparison to the results in the quaternary microemulsion a marginal bathochrome shift of both maxima can be observed. The shoulder, obtained for CdS nanoparticles modified with PDADMAC, between 350 and 420 nm is shifted to lower wavelengths. The hypsochrome shift indicates the formation of smaller particles in case of the ternary microemulsion system. This can be explained by lower interactions between the amphoteric surfactant and PDADMAC located more in the inner core of the droplets. Due to this the particle growing process inside of the droplets is more restricted. PEI is not going to be affected by this effect, because of the branched structure of the polymer located more in the inner part of the droplet and significant weaker electrostatic interactions. Fluorescence measurements illustrated in Figure 3(b) show a broad emission peak with a maximum at 520 nm. The emission peaks are nearly in the same range already obtained in the quaternary microemulsion. According to these results one can conclude that PEI-capped CdS nanoparticles are formed in the same size range for both systems. This will be confirmed by the blue emission colour of the PEI-modified particles, which is quite similar in both microemulsions (Figures  9 and 10 in the supplementary part).

Fluorescence measurements for PDADMAC modified particles show a broad emission peak between 400 and 700 nm, with an emission maximum shifted to lower wavelengths at 590 nm. Our results show that in comparison to the PEI-modified system these particles are of course larger, but smaller in comparison to the quaternary SDS-based system. This corresponds with the yellow emission colour (Figure  10 in the supplementary part). Taking into account the emission colour and the fluorescence spectra one can conclude that PDADMAC-capped CdS nanoparticles of about 3 nm have been produced.

At point B similar results were obtained for the PEI-modified system, in contrast to a significant smaller absorption maximum in presence of PDADMAC, disappearing in absence of a polymer. These results underline the additional templating effect of the PEI in the SB-based microemulsion.

It is noteworthy that the different absorption and fluorescence behaviour cannot only trace back to different particle sizes. We also have to take into account that two different polycations, on the one hand branched PEI and on the other hand linear PDADMAC, in two different types of microemulsion, are used. Capping exchange at the nanoparticle surface can also influence the optical properties of the system, as already shown by Tamborra et al. [60]. In general one can conclude stronger electrostatic interactions (illustrated in Scheme 1) of the polycations with SDS, especially for the linear PDADMAC. Therefore, PDADMAC is located at the interphase and stabilize the surfactant film. In contrast, branched PEI, located more in the inner part of the droplet, influences the particle-growing process much more (additional templating effect).

478153.sch.001
Scheme 1: Schematic representation of the location and interactions of the polycations PEI and PDADMAC within the w/o microemulsion droplets of a SDS-based system (a) and a SB-based system (b).
3.3. Characterization of the Redispersed Nanoparticles
3.3.1. Nanoparticles Redispersed from the Toluene-Pentanol (1 : 1)/SDS/Water Template Phase

After a complete solvent evaporation, the received powder was redispersed in water by ultrasonification. The obtained turbid solution was filtrated to separate aggregates and bad stabilized individual nanoparticles. To get more information on the size and surface charge of the redispersed particles dynamic and zeta potential measurements were conducted. The results of dynamic light scattering shown in Figure 4 demonstrate that the diameters of the redispersed CdS nanoparticles are significant larger than the particle dimensions estimated in the microemulsion. Two main particle fractions with diameters of about 440 nm (±120 nm) and 120 nm (±20 nm) can be found for PEI, and of about 270 nm (±140 nm) and 65 nm (±20 nm) for PDADMAC, respectively. According to the results of UV-vis and fluorescence measurements in the microemulsion, one can conclude that individual-polymer capped CdS nanoparticles formed in the microemulsion droplets can be not redispersed after solvent evaporation without particle aggregation. Furthermore, zeta potential measurements show for both polymers a negative value of about −50 ± 2 mV. Taking into account that SDS (a strong anionic surfactant) is in excess, one can assume that the interactions between the surfactant and the polycations are strong enough to form polycation-surfactant complexes combined by a destabilization of the individual nanoparticles by stripping the polycation from the surface of the CdS particles.

478153.fig.004
Figure 4: Size distribution of redispersed CdS nanoparticles produced in the microemulsion system toluene-pentanol (1 : 1), SDS, water (PEI) or (PDADMAC) at point A.
3.3.2. Nanoparticles Redispersed from the Heptanol/SB/Water Template Phase

After solvent evaporation, the received powder can be completely redispersed in water by ultrasonification without problems of phase separation. Dynamic light-scattering experiments demonstrate that in the transparent solution only one fraction of nanoparticles can be obtained, with a mean diameter of 10 nm (±3 nm) in presence of PEI and 13 nm (±4 nm) in presence of PDADMAC, respectively (Figure 5).

478153.fig.005
Figure 5: Size distribution of redispersed CdS nanoparticles produced in the microemulsion system heptanol SB, water (PEI), or (PDADMAC) at point A.

The particles show a positive zeta potential of +8 ± 3 mV. From this data, one can conclude a polymer adsorption onto the surface of the CdS nanoparticles.

For further characterization UV-vis and fluorescence measurements were conducted. For PEI two absorption maxima could be observed in Figure 6(a), the first one at 320 nm and the second one at 350 nm. A broad shoulder between 390 and 450 nm can be observed in presence of PDADMAC, a hint for larger nanoparticle diameter.

fig6
Figure 6: Absorption (a) and fluorescence spectra (b) of redispersed CdS nanoparticles produced in the microemulsion system heptanol, SB, water (PEI), and for the system with PDADMAC at point A.

A direct comparison, between the absorption peaks obtained in the microemulsion and the redispersed aqueous system, shows no significant differences in the PEI system, but a marginal bathochrome shift of the absorption band for the PDADMAC modified system. According to this one can conclude that CdS nanoparticles could be redispersed without a significant change in the particle dimension much better in presence of PEI.

The fluorescence spectrum of the redispersed CdS nanoparticles in presence of PEI is shown in Figure 6(b); for PDADMAC the emission peak is very small and correspondingly not significant. One can see that the emission peak for PEI-capped CdS nanoparticles is shifted to lower wave length and becomes more narrow in comparison to the microemulsion. The blue emission colour for PEI supports our finding that PEI-capped CdS quantum dots are redispersed without a change in the particle dimension. According to our assumption that redispersed PDADMAC-capped nanoparticles are larger than in the microemulsion, the emission colour is turned to orange for the redispersed particles, illustrated in Figure  11 in the supplementary part.

Based on the UV-vis spectroscopically obtained band gap the particle size of quantum dots can be determined according to Patidar et al. [65]. The calculated particle size is in the same range as expected according to UV-vis and fluorescence spectroscopy, as well as the observed emission colour (compare Table 1).

In order to assure the size and the shape of the particles and explain the discrepancy between DLS and UV-vis data a supporting technique, like transmission electron microscopy (TEM), should be helpful as already shown by Pons et al. [66]. Unfortunately, it is not possible to get some micrographs of these systems with a TEM due to the excess of surfactants. Noteworthy, that CdS nanoparticles produced in a SB-based hexane-pentanol microemulsion could be successfully analyzed only by high-resolution transmission electron microscopy (HRTEM). The particle dimensions of 2-3 nm (shown in Figure  12 in the supplementary part) are also in that case in disagreement with the diameter of 9 ± 2 nm observed in DLS. Inspired by these results we have synthesized in addition CdS nanoparticles in absence of surfactants, that means in a diluted aqueous PEI-solution. By this procedure we were able to produce PEI stabilized CdS nanoparticles with quite similar properties. However, in that case, that means in absence of surfactants, TEM can be successful applied. The TEM micrographs (Figures  7(a) and  7(b)) in the supplementary part) clearly show that in addition to the individual nanoparticles of about 3 nm (Figure  7(a)) particle aggregates of about 10 nm (Figure  7(b)) exist.

In the heptanol-based microemulsion system investigated here, individual CdS nanoparticles of about 3 nm are redispersed, which aggregated to clusters containing 3-4 nanoparticles. Therefore, one can conclude that in the intensity plot of the DLS experiments predominantly the QD clusters are detected. The existence of two absorption peaks in the UV-vis spectrum can be related to QD’s varying in the surrounding medium in dependence on the state of aggregation.

To underline the effect of the polyelectrolytes in the microemulsion template phase as well as in the recovery process we made two additional experiments.

In a “reference” experiment we produced CdS particles in the SB microemulsion in absence of a polyelectrolyte, followed by the solvent evaporation and redispersion in analogy to the procedure described above. In a second experiment we added the polyelectrolyte PEI during the redispersion procedure. The absorption and fluorescence spectra (given in Figure  13 in the supplementary part) clearly demonstrate that in absence of the polyelectrolyte quantum dots cannot be recovered. When the PEI is added during the redispersion procedure, only a marginal part of the nanoparticles can be stabilized.

4. Conclusion

Our results show that the quaternary template phase consisting of water, toluene-pentanol (1 : 1), and the anionic surfactant SDS in presence of PEI or PDADMAC can be successfully used for the synthesis of polymer-capped CdS nanoparticles. Unfortunately a recovery of the quantum dots without a particle aggregation is not possible due to the strong surfactant polycation interactions.

When the ternary template phase with the amphoteric SB surfactant is used, the polymer-capped nanoparticles produced in the microemulsion template phase can be recovered. That means the individual polymer-capped quantum dots and QD clusters are stable during the process of solvent evaporation and can be redispersed.

The results show on the one hand that the recovery process is only successful when the electrostatic interactions between the polycation and the surfactant headgroups are moderate. On the other hand one can see that the polyelectrolytes should be incorporated already into the microemulsion, because of the additional templating effect of the polymer during the particle formation process, and the protecting effect during the recovery process.

Conflict of Interests

The authors declare no conflict of interests with Rashing company.

Acknowledgements

The authors would like to thank the UP Transfer GmbH for the financial support. The Raschig company is gratefully acknowledged for the supply of the SB surfactant, and the BASF for providing the PEI sample.

References

  1. L. Han, D. Qin, X. Jiang et al., “Synthesis of high quality zinc-blende CdSe nanocrystals and their application in hybrid solar cells,” Nanotechnology, vol. 17, no. 18, pp. 4736–4742, 2006. View at Publisher · View at Google Scholar · View at Scopus
  2. W. U. Huynh, J. J. Dittmer, and A. P. Alivisatos, “Hybrid nanorod-polymer solar cells,” Science, vol. 295, no. 5564, pp. 2425–2427, 2002. View at Publisher · View at Google Scholar · View at Scopus
  3. C. Wei, D. Grouquist, and J. Roark, “Voltage tunable electroluminescence of CdTe nanoparticle light emitting diodes,” Journal of Nanoscience and Nanotechnology, vol. 2, no. 1, pp. 47–53, 2002. View at Publisher · View at Google Scholar · View at Scopus
  4. I. Matsui, “Nanoparticles for electronic device applications: a brief review,” Journal of Chemical Engineering of Japan, vol. 38, no. 8, pp. 535–546, 2005. View at Publisher · View at Google Scholar · View at Scopus
  5. S. Nie, Y. Xing, G. J. Kim, and J. W. Simons, “Nanotechnology applications in cancer,” Annual Review of Biomedical Engineering, vol. 9, pp. 257–288, 2007. View at Publisher · View at Google Scholar · View at Scopus
  6. M. N. Rhyner, A. M. Smith, X. Gao, H. Mao, L. Yang, and S. Nie, “Quantum dots and multifunctional nanoparticles: new contrast agents for tumor imaging,” Nanomedicine, vol. 1, no. 2, pp. 209–217, 2006. View at Publisher · View at Google Scholar · View at Scopus
  7. S. Santra, J. Xu, K. Wang, and W. Tan, “Luminescent nanoparticle probes for bioimaging,” Journal of Nanoscience and Nanotechnology, vol. 4, no. 6, pp. 590–599, 2004. View at Publisher · View at Google Scholar · View at Scopus
  8. N. Serpone and R. Khairutdinov, “Application of nanoparticles in the photocatalytic degradation of water pollutants,” Studies in Surface Science and Catalysis, vol. 103, pp. 417–444, 1997.
  9. J. J. Zou, C. Chen, C. J. Liu, Y. P. Zhang, Y. Han, and L. Cui, “Pt nanoparticles on TiO2 with novel metal-semiconductor interface as highly efficient photocatalyst,” Materials Letters, vol. 59, no. 27, pp. 3437–3440, 2005. View at Publisher · View at Google Scholar · View at Scopus
  10. W. Zhang, Y. Zhong, J. Fan et al., “Preparation, morphology, size quantization effect and photocatalytic properties of CdS Q-nanocrystals,” Science China Chemistry B, vol. 46, no. 2, pp. 196–206, 2003.
  11. L. E. Brus, “A simple model for the ionization potential, electron affinity, and aqueous redox potentials of small semiconductor crystallites,” The Journal of Chemical Physics, vol. 79, no. 11, pp. 5566–5572, 1983. View at Scopus
  12. L. E. Brus, “Electron-electron and electron-hole interactions in small semiconductor crystallites: the size dependence of the lowest excited electronic state,” The Journal of Chemical Physics, vol. 80, no. 9, pp. 4403–4409, 1984. View at Scopus
  13. A. Henglein, “Small-particle research: physicochemical properties of extremely small colloidal metal and semiconductor particles,” Chemical Reviews, vol. 89, no. 8, pp. 1861–1873, 1989. View at Scopus
  14. H. Weller, “Colloidal semiconductor Q-particles: chemistry in the transition region between solid state and molecules,” Angewandte Chemie, vol. 32, no. 1, pp. 41–53, 1993. View at Scopus
  15. E. Lifshitz, I. Dag, I. Litvin et al., “Optical properties of CdSe nanoparticle films prepared by chemical deposition and sol-gel methods,” Chemical Physics Letters, vol. 288, no. 2–4, pp. 188–196, 1998. View at Scopus
  16. B. Bhattacharjee, D. Ganguli, S. Chaudhuri, and A. K. Pal, “Synthesis and optical characterization of sol-gel derived zinc sulphide nanoparticles confined in amorphous silica thin films,” Materials Chemistry and Physics, vol. 78, no. 2, pp. 372–379, 2003. View at Publisher · View at Google Scholar · View at Scopus
  17. Y. Li, F. Huang, Q. Zhang, and Z. Gu, “Solvothermal synthesis of nanocrystalline cadmium sulfide,” Journal of Materials Science, vol. 35, no. 23, pp. 5933–5937, 2000. View at Publisher · View at Google Scholar · View at Scopus
  18. Q. Lu, F. Gao, and D. Zhao, “The assembly of semiconductor sulfide nanocrystallites with organic reagents as templates,” Nanotechnology, vol. 13, no. 6, pp. 741–745, 2002. View at Publisher · View at Google Scholar · View at Scopus
  19. Y. Yin, X. Xu, X. Ge, Y. Lu, and Z. Zhang, “Synthesis and characterization of ZnS colloidal particles via γ-radiation,” Radiation Physics and Chemistry, vol. 55, no. 3, pp. 353–356, 1999. View at Publisher · View at Google Scholar · View at Scopus
  20. M. Shao, Q. Li, B. Xie, J. Wu, and Y. Qian, “The synthesis of CdS/ZnO and CdS/Pb3O4 composite materials via microwave irradiation,” Materials Chemistry and Physics, vol. 78, no. 1, pp. 288–291, 2003. View at Publisher · View at Google Scholar · View at Scopus
  21. I. W. Hamley, “Nanostructure fabrication using block copolymers,” Nanotechnology, vol. 14, no. 10, pp. R39–R54, 2003. View at Publisher · View at Google Scholar · View at Scopus
  22. M. Moffitt, H. Vali, and A. Eisenberg, “Spherical assemblies of semiconductor nanoparticles in water-soluble block copolymer aggregates,” Chemistry of Materials, vol. 10, no. 4, pp. 1021–1028, 1998. View at Scopus
  23. J. Zhang, S. Xu, and E. Kumacheva, “Polymer microgels: reactors for semiconductor, metal, and magnetic nanoparticles,” Journal of the American Chemical Society, vol. 126, no. 25, pp. 7908–7914, 2004. View at Publisher · View at Google Scholar · View at Scopus
  24. D. Mandal and U. Chatterjee, “Synthesis and spectroscopy of CdS nanoparticles in amphiphilic diblock copolymer micelles,” Journal of Chemical Physics, vol. 126, no. 13, Article ID 134507, 8 pages, 2007. View at Publisher · View at Google Scholar · View at Scopus
  25. T. F. Towey, A. Khan-Lodhi, and B. H. Robinson, “Kinetics and mechanism of formation of quantum-sized cadmium sulphide particles in water-Aerosol-OT-oil microemulsions,” Journal of the Chemical Society, Faraday Transactions, vol. 86, no. 22, pp. 3757–3762, 1990. View at Publisher · View at Google Scholar · View at Scopus
  26. A. Agostiano, M. Catalano, M. L. Curri, M. Della Monica, L. Manna, and L. Vasanelli, “Synthesis and structural characterisation of CdS nanoparticles prepared in a four-components “water-in-oil” microemulsion,” Micron, vol. 31, no. 3, pp. 253–258, 2000. View at Publisher · View at Google Scholar · View at Scopus
  27. E. Caponetti, L. Pedone, D. Chillura Martino, V. Pantò, and V. Turco Liveri, “Synthesis, size control, and passivation of CdS nanoparticles in water/AOT/n-heptane microemulsions,” Materials Science and Engineering C, vol. 23, no. 4, pp. 531–539, 2003. View at Publisher · View at Google Scholar · View at Scopus
  28. P. S. Khiew, S. Radiman, N. M. Huang, and M. S. Ahmad, “Studies on the growth and characterization of CdS and PbS nanoparticles using sugar-ester nonionic water-in-oil microemulsion,” Journal of Crystal Growth, vol. 254, no. 1-2, pp. 235–243, 2003. View at Publisher · View at Google Scholar · View at Scopus
  29. P. S. Khiew, N. M. Huang, S. Radiman, and M. S. Ahmad, “Synthesis and characterization of conducting polyaniline-coated cadmium sulphide nanocomposites in reverse microemulsion,” Materials Letters, vol. 58, no. 3-4, pp. 516–521, 2004. View at Publisher · View at Google Scholar · View at Scopus
  30. I. Capek, “Preparation of metal nanoparticles in water-in-oil (w/o) microemulsions,” Advances in Colloid and Interface Science, vol. 110, no. 1-2, pp. 49–74, 2004. View at Publisher · View at Google Scholar · View at Scopus
  31. J. Eastoe, M. J. Hollamby, and L. Hudson, “Recent advances in nanoparticle synthesis with reversed micelles,” Advances in Colloid and Interface Science, vol. 128–130, pp. 5–15, 2006. View at Publisher · View at Google Scholar · View at Scopus
  32. C. Petit, T. K. Jain, F. Billoudet, and M. P. Pileni, “Oil in water micellar solution used to synthesize CdS particles: structural study and photoelectron transfer reaction,” Langmuir, vol. 10, no. 12, pp. 4446–4450, 1994. View at Scopus
  33. M. A. López-Quintela, “Synthesis of nanomaterials in microemulsions: formation mechanisms and growth control,” Current Opinion in Colloid & Interface Science, vol. 8, no. 2, pp. 137–144, 2003. View at Publisher · View at Google Scholar · View at Scopus
  34. P. D. I. Fletcher, A. M. Howe, and B. H. Robinson, “The kinetics of solubilisate exchange between water droplets of a water-in-oil microemulsion,” Journal of the Chemical Society, Faraday Transactions 1, vol. 83, no. 4, pp. 985–1006, 1987. View at Publisher · View at Google Scholar · View at Scopus
  35. M. J. Suarez, H. Lévy, and J. Lang, “Effect of addition of polymer to water-in-oil microemulsions on droplet size and exchange of material between droplets,” Journal of Physical Chemistry, vol. 97, no. 38, pp. 9808–9816, 1993. View at Scopus
  36. D. Papoutsi, P. Lianos, and W. Brown, “Interaction of polyethylene glycol with water-in-oil microemulsions. 3. Effect of polymer size and polymer concentration,” Langmuir, vol. 10, no. 10, pp. 3402–3405, 1994. View at Scopus
  37. P. Lianos, S. Modes, G. Staikos, and W. Brown, “Interaction of poly(oxyethylene glycol) with cyclohexane-pentanol-sodium dodecyl sulfate water-in-oil microemulsions,” Langmuir, vol. 8, no. 4, pp. 1054–1059, 1992. View at Scopus
  38. A. Kabalnov, U. Olsson, K. Thuresson, and H. Wennerström, “Polymer effects on the phase equilibrium of a balanced microemulsion: adsorbing versus nonadsorbing polymers,” Langmuir, vol. 10, no. 13, pp. 4509–4513, 1994. View at Scopus
  39. C. González-Blanco, L. J. Rodríguez, and M. M. Velázquez, “Effect of the addition of water-soluble polymers on the structure of aerosol OT water-in-oil microemulsions: a fourier transform infrared spectroscopy study,” Langmuir, vol. 13, no. 7, pp. 1938–1945, 1997. View at Scopus
  40. M. J. Suarez and J. Lang, “Effect of addition of water-soluble polymers in water-in-oil microemulsions made with anionic and cationic surfactants,” Journal of Physical Chemistry, vol. 99, no. 13, pp. 4626–4631, 1995. View at Scopus
  41. W. Meier, “Poly(oxyethylene) adsorption in water/oil microemulsions: a conductivity study,” Langmuir, vol. 12, no. 5, pp. 1188–1192, 1996. View at Scopus
  42. J. Appell, C. Ligoure, and G. Porte, “Bending elasticity of a curved amphiphilic film decorated with anchored copolymers: a small angle neutron scattering study,” Journal of Statistical Mechanics, vol. 2004, Article ID P08002, 2004. View at Publisher · View at Google Scholar
  43. K. C. Tam and E. Wyn-Jones, “Insights on polymer surfactant complex structures during the binding of surfactants to polymers as measured by equilibrium and structural techniques,” Chemical Society Reviews, vol. 35, no. 8, pp. 693–709, 2006. View at Publisher · View at Google Scholar · View at Scopus
  44. A. Shioi, M. Harada, M. Obika, and M. Adachi, “Structure and properties of fluids composed of polyelectrolyte and ionic surfactant in organic phase: poly(acrylic acid) and didodecyldimethylammonium bromide,” Langmuir, vol. 14, no. 17, pp. 4737–4743, 1998. View at Scopus
  45. M. Fechner, M. Kramer, E. Kleinpeter, and J. Koetz, “Polyampholyte-modified ionic microemulsions,” Colloid and Polymer Science, vol. 287, no. 10, pp. 1145–1153, 2009. View at Publisher · View at Google Scholar · View at Scopus
  46. M. Fechner and J. Koetz, “Polyampholyte-surfactant film tuning in reverse microemulsions,” Langmuir, vol. 27, no. 9, pp. 5316–5323, 2011. View at Publisher · View at Google Scholar · View at Scopus
  47. C. Note, S. Kosmella, and J. Koetz, “Structural changes in poly(ethyleneimine) modified microemulsion,” Journal of Colloid and Interface Science, vol. 302, no. 2, pp. 662–668, 2006. View at Publisher · View at Google Scholar · View at Scopus
  48. A. H. Poghosyan, L. H. Arsenyan, H. H. Gharabekyan, S. Falkenhagen, J. Koetz, and A. A. Shahinyan, “Molecular dynamics simulations of inverse sodium dodecyl sulfate (SDS) micelles in a mixed toluene/pentanol solvent in the absence and presence of poly(diallyldimethylammonium chloride) (PDADMAC),” Journal of Colloid and Interface Science, vol. 358, no. 1, pp. 175–181, 2011. View at Publisher · View at Google Scholar · View at Scopus
  49. J. Koetz, J. Bahnemann, G. Lucas, B. Tiersch, and S. Kosmella, “Polyelectrolyte-modified microemulsions as new templates for the formation of nanoparticles,” Colloids and Surfaces A, vol. 250, no. 1–3, pp. 423–430, 2004. View at Publisher · View at Google Scholar · View at Scopus
  50. J. Koetz, J. Baier, and S. Kosmella, “Formation of zinc sulfide and hydroxylapatite nanoparticles in polyelectrolyte-modified microemulsions,” Colloid and Polymer Science, vol. 285, no. 15, pp. 1719–1726, 2007. View at Publisher · View at Google Scholar · View at Scopus
  51. J. Baier, J. Koetz, S. Kosmella, B. Tiersch, and H. Rehage, “Polyelectrolyte-modified inverse microemulsions and their use as templates for the formation of magnetite nanoparlicles,” Journal of Physical Chemistry B, vol. 111, no. 29, pp. 8612–8618, 2007. View at Publisher · View at Google Scholar · View at Scopus
  52. C. Note, J. Koetz, L. Wattebled, and A. Laschewsky, “Effect of a new hydrophobically modified polyampholyte on the formation of inverse microemulsions and the preparation of gold nanoparticles,” Journal of Colloid and Interface Science, vol. 308, no. 1, pp. 162–169, 2007. View at Publisher · View at Google Scholar · View at Scopus
  53. J. Zhang, L. Sun, C. Liao, and C. Yan, “Size control and photoluminescence enhancement of CdS nanoparticles prepared via reverse micelle method,” Solid State Communications, vol. 124, no. 1-2, pp. 45–48, 2002. View at Publisher · View at Google Scholar · View at Scopus
  54. M. F. Nazar, O. Myakonkaya, S. S. Shah, and J. Eastoe, “Separating nanoparticles from microemulsions,” Journal of Colloid and Interface Science, vol. 354, no. 2, pp. 624–629, 2011. View at Publisher · View at Google Scholar · View at Scopus
  55. M. J. Hollamby, J. Eastoe, A. Chemelli et al., “Separation and purification of nanoparticles in a single step,” Langmuir, vol. 26, no. 10, pp. 6989–6994, 2010. View at Publisher · View at Google Scholar · View at Scopus
  56. B. Abécassis, F. Testard, and T. Zemb, “Gold nanoparticle synthesis in worm-like catanionic micelles: microstructure conservation and temperature induced recovery,” Soft Matter, vol. 5, no. 5, pp. 974–978, 2009. View at Publisher · View at Google Scholar · View at Scopus
  57. R. Zhang, J. Liu, J. He et al., “Organic reactions and nanoparticle preparation in Co2-induced water/P104/p-xylene microemulsions,” Chemistry, vol. 9, no. 10, pp. 2167–2172, 2003. View at Publisher · View at Google Scholar · View at Scopus
  58. A. Salabat, J. Eastoe, A. Vesperinas, R. F. Tabor, and K. J. Muteh, “Photorecovery of nanoparticles from an organic solvent,” Langmuir, vol. 24, no. 5, pp. 1829–1832, 2008. View at Publisher · View at Google Scholar · View at Scopus
  59. J. F. Liu, R. Liu, Y. G. Yin, and G. B. Jiang, “Triton X-114 based cloud point extraction: a thermoreversible approach for separation/concentration and dispersion of nanomaterials in the aqueous phase,” Chemical Communications, no. 12, pp. 1514–1516, 2009. View at Publisher · View at Google Scholar · View at Scopus
  60. M. Tamborra, M. Striccoli, R. Comparelli, M. L. Curri, A. Petrella, and A. Agostiano, “Optical properties of hybrid composites based on highly luminescent CdS nanocrystals in polymer,” Nanotechnology, vol. 15, no. 4, pp. S240–S244, 2004. View at Publisher · View at Google Scholar · View at Scopus
  61. I. R. Collins, “Surface electrical properties of barium sulfate modified by adsorption of poly α, β aspartic acid,” Journal of Colloid and Interface Science, vol. 212, no. 2, pp. 535–544, 1999. View at Publisher · View at Google Scholar · View at Scopus
  62. D. H. Chen and Y. Y. Chen, “Synthesis of barium ferrite ultrafine particles by coprecipitation in the presence of polyacrylic acid,” Journal of Colloid and Interface Science, vol. 235, no. 1, pp. 9–14, 2001. View at Publisher · View at Google Scholar · View at Scopus
  63. J. Koetz, K. Gawlitza, and S. Kosmella, “Formation of organically and inorganically passivated CdS nanoparticles in reverse microemulsions,” Colloid and Polymer Science, vol. 288, no. 3, pp. 257–263, 2010. View at Publisher · View at Google Scholar · View at Scopus
  64. M. L. Curri, A. Agostiano, L. Manna et al., “Synthesis and characterization of CdS nanoclusters in a quaternary microemulsion the role of the cosurfactant,” Journal of Physical Chemistry B, vol. 104, no. 35, pp. 8391–8397, 2000. View at Scopus
  65. D. Patidar, K. Rathore, N. Saxena, K. Sharma, and T. Sharma, “Energy band gap studies of CdS nanomaterials,” Journal of Nano Research, vol. 3, pp. 97–102, 2008.
  66. T. Pons, H. T. Uyeda, I. L. Medintz, and H. Mattoussi, “Hydrodynamic dimensions, electrophoretic mobility, and stability of hydrophilic quantum dots,” Journal of Physical Chemistry B, vol. 110, no. 35, pp. 20308–20316, 2006. View at Publisher · View at Google Scholar · View at Scopus