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Synthesis and characterization of alanine-capped water soluble copper sulphide quantum dots
Materials Letters 75 (2012) 161–164
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis and characterization of alanine-capped water soluble copper sulphide quantum dots S.M.M. Nelwamondo a, M.J. Moloto b,⁎, R.W.M. Krause a, N. Moloto c a b c
Department of Chemical Technology, University of Johannesburg, P.O. Box 17011, Doornfontein, Johannesburg, 2028, South Africa Department of Chemistry, Vaal University of Technology, Private Bag X021 Vanderbijlpark, 1900, South Africa Molecular Science Institute, School of Chemistry, The University of the Witwatersrand, Private Bag 3, Wits, 2050, South Africa
a r t i c l e
i n f o
Article history: Received 15 November 2011 Accepted 19 January 2012 Available online 28 January 2012 Keywords: Covellite Water soluble Quantum dots Alanine Capping agents
a b s t r a c t Copper sulphide has interesting chemistry as it is capable of forming various stoichiometries and couple to that, it has excellent electrical, optical and magnetic properties. Its optical and magnetic attributes renders it a suitable candidate in a nano-regime for biological applications. We therefore report on the synthesis of single-phase, water soluble CuS nanoparticles using a simple colloidal route with alanine as the stabilizing and compatibility ligand. We further report on the optical and morphological properties of CuS nanoparticles as a function of temperature. The employed synthetic method successfully yielded water soluble CuS nanoparticles with a single crystal phase. The binding mode of alanine on the surface of the nanoparticles was shown to be pH dependent. The temperature had an effect on both the optical and morphological properties of the particle. The highest synthetic temperature (100 °C) resulted in particles with properties superior to the ones synthesized at lower temperatures and the surface coverage of the nanoparticles with alanine improved with increasing temperature. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Copper sulphide is known to exist for a long time in a range of stable and unstable phases including copper-poor CuS (covellite) to copper-rich Cu2S (chalcocite). Other phases include Cu1.75S (anilite), Cu1.8S (digenite) and Cu1.94S (djurleite). Copper sulphide Cu2-xS (x b 2), is a p-type semiconductor, whose conductivity is due to copper vacancies and decreases from copper-poor CuS to copper-rich CuxS. CuS and Cu2S exhibits band gaps around 1.7 and 1.2 eV respectively, while CuxS (x = 1.94, 1.8 and 1.75), exhibit band gaps between 1.05 and 1.96 eV [1]. Covellite (CuS) is a representative I–VI semiconductor and due to quantum confinement effects, CuS nanoparticles exhibit novel optical and electrical properties as compared to the bulk material [2], which is known to exist in two forms i.e., the less crystalline (brown) and more crystalline green CuS [3]. Many physical and chemical routes developed include microwave irradiation techniques [4], sonochemistry [5], solid state reaction [6], and hydrothermal/solvothermal synthesis [7], CuxSy has far more potential in biological applications compared to cadmium based II–VI semiconductors as they contain a less toxic metal. Methods used to synthesize water soluble quantum dots include aqueous based preparations of thiol-capped CdS [8], CdSe [9] and CdTe [10–12]. In this method the capping molecules are amphiphilic molecules containing
⁎ Corresponding author. Tel.: + 27 169509207; fax: + 27 867563592. E-mail address: [email protected] (M.J. Moloto). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.01.079
a thiol group strongly coordinated to the QDs surfaces and a polar group (―OH, ―COOH and ―NH2), that confers compatibility with aqueous solutions to the QDs. We report on the synthesis of watersoluble covellite copper sulphide nanoparticles via a simple and relatively low temperature colloidal method using alanine as a stabilizer. 2. Experimental 2.1. Materials Copper acetate monohydrate, β-alanine, sodium hydroxide, thioacetamide (TAA) and methanol were purchased from Sigma Aldrich and used as received. 2.2. Synthesis of the nanoparticles Alanine (2.0 g) was dissolved in a mixture of 30 cm 3/20 cm 3 water and methanol respectively. At 40 °C, 0.5 g of copper acetate was added and after complete dissolution, 0.5 g of thioacetamide in 10 cm 3 of distilled water was added to the reaction mixture. Sodium hydroxide solution (1.25 × 10 − 11 M) was added until the pH of 10. The reaction mixture was then maintained at 40 °C for 30 min while stirring under nitrogen gas, after which a green precipitate formed. The solution was then allowed to cool to room temperature and the precipitate was obtained by centrifugation and dried overnight. Following the same procedure, CuS nanoparticles was synthesized at different temperatures i.e. 70 and 100 °C.
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Scheme 1. Representation of the (i) effect of pH on alanine and (ii) binding motifs of alanine on the surface of CuS nanoparticles.
2.3. Characterization The optical spectra were taken using an Analytikjena SPECORD 50 UV–vis spectrophotometer and an LS 45 Perkin-Elmer spectrometer with a Xenon lamp at room, respectively. The surface interaction of alanine on the nanoparticles was characterized using an FT-IR Perkin Elmer 100 spectrometer. The TEM images were obtained using a JEM2100 JEOL electron microscope using copper grids. X-ray diffraction (XRD) patterns on powdered samples were measured on Phillips X'Pert materials research diffractometer using secondary graphite monochromated Cu Kα radiation (λ = 1.54060 Å) at 40 kV/50 mA. The samples were prepared by dispersing a small amount of CuS in distilled water 3. Results and discussion As copper sulphide is susceptible to forming various phases, careful control and choice of the reaction and its condition is essential. A simple but effective colloidal method adapted from method reported by C. Tan et al. has been used [13]. The colloidal synthesis involves the reaction of a thioacetamide with a copper salt with alanine acting as a stabilizing agent and possesses two possible binding sites as shown in Scheme 1(ii). The choice of alanine is to ensure stabilization of the quantum dots through one functional group while reserving the other for water solubility. The binding of the alanine to nanoparticles is dependent on the pH of the reaction. Because alanine is an amino acid, the pH has an effect on the electronic characteristics of the molecule as depicted in Scheme 1(i). Garcia et Al. in 2008 reported that alanine can be stabilized in different forms depending on the medium of the solution [14]. The pH 10 was chosen with the interest of hydroxyl which can bind on the surface of nanoparticles.
Fig. 1. Infrared spectra of (a) pristine alanine and (b) alanine-capped CuS nanoparticles.
Fig. 2. Absorption and emission spectra of alanine-capped CuS nanoparticles synthesized at 40 (a), 70 (b) and 100 °C (c).
S.M.M. Nelwamondo et al. / Materials Letters 75 (2012) 161–164
FTIR spectra showed characteristics overlapping bands at 2500–3200 cm − 1 assigned to O―H and N―H stretching frequencies of alanine. The oxide moiety which result when alanine is at pH 10 interact strongly with the surface of CuS. The alanine capped-CuS (Fig. 1(b)) has a broad vibration band associated with N―H stretching frequency. Since copper is a p-type semiconductor, it is likely to interact with the negatively charged oxide moiety of alanine. The N―H band of alanine capped-CuS nanoparticles is more pronounced as compared to the free alanine because the bonds are strained since they are now bonded on the surface of the nanoparticles. The other differences were observed on the C O and C―O which are broader that in free alanine. These differences arise from the electron cloud being evenly distributed around the O―C―O− moiety. This confirms that the bonding of alanine to the surface of the nanoparticles occurs through route B of Scheme 1(ii). The optical properties of alanine capped CuS nanoparticles were characterized using UV-Visible and photoluminescence spectroscopy. The absorption spectra of all the samples showed a blue-shift in the absorption band edge compared to the bulk CuS (1022 nm) as a result of quantum confinement effects. The excitonic peaks observed in the spectra were associated with the formation of covellite. As the temperature increased, Fig. 2(a–c) there was a successive red-shifting of the band edge from 504 to 516 nm and 522 nm for particles synthesized at 40, 70 and 100 °C respectively. The increase to higher wavelength was attributed to Ostwald ripening. The emission spectrum depends on the particle surface state, size and surface passivation. The photoluminescence spectra (Fig. 2(a–c)) of CuS nanoparticles (λexc = 250 nm) showed broad emission peaks for particles synthesized at 40 and 70 °C. This indicated that the particles were polydispersed whilst a narrower emission peak for particles synthesized at 100 °C was observed. The FWHM were 250, 125 and 50 nm for particles synthesized at 40, 70 and 100 °C respectively. The sharpening of the emission peak with increased temperature suggested that the size and shape of the nanoparticles was approaching monodispersity.
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The morphology and size of CuS nanoparticles were investigated by XRD and TEM (Fig. 3). The XRD patterns (Fig. 3(II)) obtained at different temperatures namely, 40, 70 and 100 °C could be indexed to a CuS hexagonal phase (JCPDS card no.: 06-0464). No evidence of impurities or other phases could be detected from the diffractograms. The estimated average crystallite sizes were calculated using Scherer's equation to give 5.4, 7.8 and 11.7 nm for particles synthesized at 40, 70 and 100 °C respectively. The TEM images of CuS nanoparticles synthesized at different temperatures are depicted in Fig. 3(I). Fig. 3(I-a) showed that CuS particles prepared at 40 °C had mixed morphologies of triangular, spherical and rod shaped nanoparticles. At 60 °C (Fig. 3(I-b)) rodshaped and deformed nanoparticles formed while at 100 °C (Fig. 3(I-c)), the particles were predominantly rod shaped. As the temperature is increased, the reaction approaches a critical temperature where one type of morphology dominates. With increase in temperature, there is an increase in the average kinetic energy of the system. This causes the non-preferential morphology to destabilize, leaving only the stable particles and as the temperature is further increased the properties of the nanoparticles improves, such as crystallinity and monodispersity. The particles size for different temperature were difficult to be compared and calculated from the TEM images since the particles were of different shapes in 40 °C and some of the particles were deformed in 60 °C unlike in XRD were by the instrument can give the estimated sizes for the whole sample. A solubility test was performed by dissolving equal amount of particles synthesized at various temperatures in an equal amount of water and sonicated at room temperature for a fixed amount of time. The concentration was measured to establish the solubility of the particles. The solubility increased with an increase in synthetic temperature. This could be as results of the degree of monodispersity of the particles. The particles approach uniformity with increase in temperature. The solubility of all the particles confirms the coating of the surface with alanine. At 100 °C, nearly all the particles were
Fig. 3. TEM images (i) and XRD patterns (ii) of alanine-capped CuS nanoparticles synthesized at (a) 40 °C, (b) 70 °C and (c) 100 °C.
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soluble in water (>90%), this suggest that this synthetic temperature is the best condition for alanine-capped synthesis of copper sulphide nanoparticles. Perhaps a small increase beyond 100 °C can result in more uniform sizes and morphologies. 4. Conclusions The method employed, successfully yielded water soluble CuS nanoparticles with a single crystal phase. It was shown that the binding mode of alanine on the surface of the nanoparticles was pH dependent. The highest synthetic temperature (100 °C) resulted in particles with properties superior to the ones synthesized at lower temperatures and the surface coverage of the nanoparticles with alanine improved with increasing temperature evident from the degree of water solubility of the particles. Acknowledgements The authors would like to acknowledge the following institutions for financial, research space and equipment support; the NRF (South Africa) and University of Johannesburg.
References [1] Kim Y-Y, Walsh D. Nanoscale 2010;2:240–7. [2] Anders H, Michael G. Chem Rev 1995;95:49–68. [3] Brelle MC, Torres-Martinez CL, McNul JC, Mehra RK, Zhang JZ. Pure Appl Chem 2000;72:101–17. [4] Mu C-F, Yao QZ, Qu XF, Li ML, Fu SQ. Colloids Surf A Physicochem Eng Aspects 2010;37:114–21. [5] Wang H, Zhang JR, Zha XN, Xu S, Zhu JJ. Mater Lett 2002;55:253–8. [6] Parkin IP. Chem Soc Rev 1996;25:199–207. [7] Liu J, Xue D. Mater Res Bull 2010;45:309–13. [8] Vossmeyer T, Katsikas L, Giersig M, Popovic IG, Chemseddine A, Eychmuller A, Weller H. J Phys Chem 1994;98:7665–73. [9] Rogach AL, Kornowski A, Gao M, Eychmuller A. J Phys Chem B 1999;103:3065–9. [10] Rogach AL, Katsikas L, Kornowski A, Su D, Eychmuller A, Weller H. Ber Bunsen-Ges Phys Chem 1996;100:1772–8. [11] Rogach AL, Katsikas L, Kornowski A, Su D, Eychmuller A, Weller H. Ber Bunsen-Ges Phys Chem 1997;101:1668–70. [12] Gao MY, Kirstein S, Möhwald H, Rogach AL, Kornowski A, Eychmüller A, et al. J Phys Chem B 1998;102:8360–3. [13] Tan C, Zhu Y, Lu R, Xue P, Bao C, Liu X, et al. Mater Chem Phys 2005;91:44–7. [14] Garcia AR, Brito de Barros R, Lourenco JP, Ilharco LM. J Phys Chem A 2008;112: 8280–7.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis and characterization of alanine-capped water soluble copper sulphide quantum dots S.M.M. Nelwamondo a, M.J. Moloto b,⁎, R.W.M. Krause a, N. Moloto c a b c
Department of Chemical Technology, University of Johannesburg, P.O. Box 17011, Doornfontein, Johannesburg, 2028, South Africa Department of Chemistry, Vaal University of Technology, Private Bag X021 Vanderbijlpark, 1900, South Africa Molecular Science Institute, School of Chemistry, The University of the Witwatersrand, Private Bag 3, Wits, 2050, South Africa
a r t i c l e
i n f o
Article history: Received 15 November 2011 Accepted 19 January 2012 Available online 28 January 2012 Keywords: Covellite Water soluble Quantum dots Alanine Capping agents
a b s t r a c t Copper sulphide has interesting chemistry as it is capable of forming various stoichiometries and couple to that, it has excellent electrical, optical and magnetic properties. Its optical and magnetic attributes renders it a suitable candidate in a nano-regime for biological applications. We therefore report on the synthesis of single-phase, water soluble CuS nanoparticles using a simple colloidal route with alanine as the stabilizing and compatibility ligand. We further report on the optical and morphological properties of CuS nanoparticles as a function of temperature. The employed synthetic method successfully yielded water soluble CuS nanoparticles with a single crystal phase. The binding mode of alanine on the surface of the nanoparticles was shown to be pH dependent. The temperature had an effect on both the optical and morphological properties of the particle. The highest synthetic temperature (100 °C) resulted in particles with properties superior to the ones synthesized at lower temperatures and the surface coverage of the nanoparticles with alanine improved with increasing temperature. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Copper sulphide is known to exist for a long time in a range of stable and unstable phases including copper-poor CuS (covellite) to copper-rich Cu2S (chalcocite). Other phases include Cu1.75S (anilite), Cu1.8S (digenite) and Cu1.94S (djurleite). Copper sulphide Cu2-xS (x b 2), is a p-type semiconductor, whose conductivity is due to copper vacancies and decreases from copper-poor CuS to copper-rich CuxS. CuS and Cu2S exhibits band gaps around 1.7 and 1.2 eV respectively, while CuxS (x = 1.94, 1.8 and 1.75), exhibit band gaps between 1.05 and 1.96 eV [1]. Covellite (CuS) is a representative I–VI semiconductor and due to quantum confinement effects, CuS nanoparticles exhibit novel optical and electrical properties as compared to the bulk material [2], which is known to exist in two forms i.e., the less crystalline (brown) and more crystalline green CuS [3]. Many physical and chemical routes developed include microwave irradiation techniques [4], sonochemistry [5], solid state reaction [6], and hydrothermal/solvothermal synthesis [7], CuxSy has far more potential in biological applications compared to cadmium based II–VI semiconductors as they contain a less toxic metal. Methods used to synthesize water soluble quantum dots include aqueous based preparations of thiol-capped CdS [8], CdSe [9] and CdTe [10–12]. In this method the capping molecules are amphiphilic molecules containing
⁎ Corresponding author. Tel.: + 27 169509207; fax: + 27 867563592. E-mail address: [email protected] (M.J. Moloto). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.01.079
a thiol group strongly coordinated to the QDs surfaces and a polar group (―OH, ―COOH and ―NH2), that confers compatibility with aqueous solutions to the QDs. We report on the synthesis of watersoluble covellite copper sulphide nanoparticles via a simple and relatively low temperature colloidal method using alanine as a stabilizer. 2. Experimental 2.1. Materials Copper acetate monohydrate, β-alanine, sodium hydroxide, thioacetamide (TAA) and methanol were purchased from Sigma Aldrich and used as received. 2.2. Synthesis of the nanoparticles Alanine (2.0 g) was dissolved in a mixture of 30 cm 3/20 cm 3 water and methanol respectively. At 40 °C, 0.5 g of copper acetate was added and after complete dissolution, 0.5 g of thioacetamide in 10 cm 3 of distilled water was added to the reaction mixture. Sodium hydroxide solution (1.25 × 10 − 11 M) was added until the pH of 10. The reaction mixture was then maintained at 40 °C for 30 min while stirring under nitrogen gas, after which a green precipitate formed. The solution was then allowed to cool to room temperature and the precipitate was obtained by centrifugation and dried overnight. Following the same procedure, CuS nanoparticles was synthesized at different temperatures i.e. 70 and 100 °C.
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Scheme 1. Representation of the (i) effect of pH on alanine and (ii) binding motifs of alanine on the surface of CuS nanoparticles.
2.3. Characterization The optical spectra were taken using an Analytikjena SPECORD 50 UV–vis spectrophotometer and an LS 45 Perkin-Elmer spectrometer with a Xenon lamp at room, respectively. The surface interaction of alanine on the nanoparticles was characterized using an FT-IR Perkin Elmer 100 spectrometer. The TEM images were obtained using a JEM2100 JEOL electron microscope using copper grids. X-ray diffraction (XRD) patterns on powdered samples were measured on Phillips X'Pert materials research diffractometer using secondary graphite monochromated Cu Kα radiation (λ = 1.54060 Å) at 40 kV/50 mA. The samples were prepared by dispersing a small amount of CuS in distilled water 3. Results and discussion As copper sulphide is susceptible to forming various phases, careful control and choice of the reaction and its condition is essential. A simple but effective colloidal method adapted from method reported by C. Tan et al. has been used [13]. The colloidal synthesis involves the reaction of a thioacetamide with a copper salt with alanine acting as a stabilizing agent and possesses two possible binding sites as shown in Scheme 1(ii). The choice of alanine is to ensure stabilization of the quantum dots through one functional group while reserving the other for water solubility. The binding of the alanine to nanoparticles is dependent on the pH of the reaction. Because alanine is an amino acid, the pH has an effect on the electronic characteristics of the molecule as depicted in Scheme 1(i). Garcia et Al. in 2008 reported that alanine can be stabilized in different forms depending on the medium of the solution [14]. The pH 10 was chosen with the interest of hydroxyl which can bind on the surface of nanoparticles.
Fig. 1. Infrared spectra of (a) pristine alanine and (b) alanine-capped CuS nanoparticles.
Fig. 2. Absorption and emission spectra of alanine-capped CuS nanoparticles synthesized at 40 (a), 70 (b) and 100 °C (c).
S.M.M. Nelwamondo et al. / Materials Letters 75 (2012) 161–164
FTIR spectra showed characteristics overlapping bands at 2500–3200 cm − 1 assigned to O―H and N―H stretching frequencies of alanine. The oxide moiety which result when alanine is at pH 10 interact strongly with the surface of CuS. The alanine capped-CuS (Fig. 1(b)) has a broad vibration band associated with N―H stretching frequency. Since copper is a p-type semiconductor, it is likely to interact with the negatively charged oxide moiety of alanine. The N―H band of alanine capped-CuS nanoparticles is more pronounced as compared to the free alanine because the bonds are strained since they are now bonded on the surface of the nanoparticles. The other differences were observed on the C O and C―O which are broader that in free alanine. These differences arise from the electron cloud being evenly distributed around the O―C―O− moiety. This confirms that the bonding of alanine to the surface of the nanoparticles occurs through route B of Scheme 1(ii). The optical properties of alanine capped CuS nanoparticles were characterized using UV-Visible and photoluminescence spectroscopy. The absorption spectra of all the samples showed a blue-shift in the absorption band edge compared to the bulk CuS (1022 nm) as a result of quantum confinement effects. The excitonic peaks observed in the spectra were associated with the formation of covellite. As the temperature increased, Fig. 2(a–c) there was a successive red-shifting of the band edge from 504 to 516 nm and 522 nm for particles synthesized at 40, 70 and 100 °C respectively. The increase to higher wavelength was attributed to Ostwald ripening. The emission spectrum depends on the particle surface state, size and surface passivation. The photoluminescence spectra (Fig. 2(a–c)) of CuS nanoparticles (λexc = 250 nm) showed broad emission peaks for particles synthesized at 40 and 70 °C. This indicated that the particles were polydispersed whilst a narrower emission peak for particles synthesized at 100 °C was observed. The FWHM were 250, 125 and 50 nm for particles synthesized at 40, 70 and 100 °C respectively. The sharpening of the emission peak with increased temperature suggested that the size and shape of the nanoparticles was approaching monodispersity.
163
The morphology and size of CuS nanoparticles were investigated by XRD and TEM (Fig. 3). The XRD patterns (Fig. 3(II)) obtained at different temperatures namely, 40, 70 and 100 °C could be indexed to a CuS hexagonal phase (JCPDS card no.: 06-0464). No evidence of impurities or other phases could be detected from the diffractograms. The estimated average crystallite sizes were calculated using Scherer's equation to give 5.4, 7.8 and 11.7 nm for particles synthesized at 40, 70 and 100 °C respectively. The TEM images of CuS nanoparticles synthesized at different temperatures are depicted in Fig. 3(I). Fig. 3(I-a) showed that CuS particles prepared at 40 °C had mixed morphologies of triangular, spherical and rod shaped nanoparticles. At 60 °C (Fig. 3(I-b)) rodshaped and deformed nanoparticles formed while at 100 °C (Fig. 3(I-c)), the particles were predominantly rod shaped. As the temperature is increased, the reaction approaches a critical temperature where one type of morphology dominates. With increase in temperature, there is an increase in the average kinetic energy of the system. This causes the non-preferential morphology to destabilize, leaving only the stable particles and as the temperature is further increased the properties of the nanoparticles improves, such as crystallinity and monodispersity. The particles size for different temperature were difficult to be compared and calculated from the TEM images since the particles were of different shapes in 40 °C and some of the particles were deformed in 60 °C unlike in XRD were by the instrument can give the estimated sizes for the whole sample. A solubility test was performed by dissolving equal amount of particles synthesized at various temperatures in an equal amount of water and sonicated at room temperature for a fixed amount of time. The concentration was measured to establish the solubility of the particles. The solubility increased with an increase in synthetic temperature. This could be as results of the degree of monodispersity of the particles. The particles approach uniformity with increase in temperature. The solubility of all the particles confirms the coating of the surface with alanine. At 100 °C, nearly all the particles were
Fig. 3. TEM images (i) and XRD patterns (ii) of alanine-capped CuS nanoparticles synthesized at (a) 40 °C, (b) 70 °C and (c) 100 °C.
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soluble in water (>90%), this suggest that this synthetic temperature is the best condition for alanine-capped synthesis of copper sulphide nanoparticles. Perhaps a small increase beyond 100 °C can result in more uniform sizes and morphologies. 4. Conclusions The method employed, successfully yielded water soluble CuS nanoparticles with a single crystal phase. It was shown that the binding mode of alanine on the surface of the nanoparticles was pH dependent. The highest synthetic temperature (100 °C) resulted in particles with properties superior to the ones synthesized at lower temperatures and the surface coverage of the nanoparticles with alanine improved with increasing temperature evident from the degree of water solubility of the particles. Acknowledgements The authors would like to acknowledge the following institutions for financial, research space and equipment support; the NRF (South Africa) and University of Johannesburg.
References [1] Kim Y-Y, Walsh D. Nanoscale 2010;2:240–7. [2] Anders H, Michael G. Chem Rev 1995;95:49–68. [3] Brelle MC, Torres-Martinez CL, McNul JC, Mehra RK, Zhang JZ. Pure Appl Chem 2000;72:101–17. [4] Mu C-F, Yao QZ, Qu XF, Li ML, Fu SQ. Colloids Surf A Physicochem Eng Aspects 2010;37:114–21. [5] Wang H, Zhang JR, Zha XN, Xu S, Zhu JJ. Mater Lett 2002;55:253–8. [6] Parkin IP. Chem Soc Rev 1996;25:199–207. [7] Liu J, Xue D. Mater Res Bull 2010;45:309–13. [8] Vossmeyer T, Katsikas L, Giersig M, Popovic IG, Chemseddine A, Eychmuller A, Weller H. J Phys Chem 1994;98:7665–73. [9] Rogach AL, Kornowski A, Gao M, Eychmuller A. J Phys Chem B 1999;103:3065–9. [10] Rogach AL, Katsikas L, Kornowski A, Su D, Eychmuller A, Weller H. Ber Bunsen-Ges Phys Chem 1996;100:1772–8. [11] Rogach AL, Katsikas L, Kornowski A, Su D, Eychmuller A, Weller H. Ber Bunsen-Ges Phys Chem 1997;101:1668–70. [12] Gao MY, Kirstein S, Möhwald H, Rogach AL, Kornowski A, Eychmüller A, et al. J Phys Chem B 1998;102:8360–3. [13] Tan C, Zhu Y, Lu R, Xue P, Bao C, Liu X, et al. Mater Chem Phys 2005;91:44–7. [14] Garcia AR, Brito de Barros R, Lourenco JP, Ilharco LM. J Phys Chem A 2008;112: 8280–7.