Three kinds of colloids containing gold nanoparticles (AuNP) were obtained by three different methods of synthesis, using castor oil as dispersant agent and ...
Synthesis and optical properties of different colloidal systems of gold nanoparticles in a chiral dispersant agent Mario R. Meneghettia, Monique G. A. da Silvaa, Márcio A. R. C. Alencarb, Jandir M. Hickmannb Instituto de Química e Biotecnologia, Universidade Federal de Alagoas, 57072-970, Maceió, AL, Brazil; b Instituto de Física, Universidade Federal de Alagoas, 57072-970, Maceió, AL, Brazil.
a
ABSTRACT Three kinds of colloids containing gold nanoparticles (AuNP) were obtained by three different methods of synthesis, using castor oil as dispersant agent and tetrachloroauric (III) acid as gold source. The colloidal systems were characterized by Uv-vis spectroscopy and transmission electron microscopy (TEM). Each method gave rise to quasispherical shape and different size distribution of AuNP. The TEM images of the nanostrutured systems show that from each method of synthesis, nanoparticles of different average sizes, equal to 7, 15, and 55 nm, were produced. These characteristics are reflected by the presence of different maximum wavelength absorption, indicating that each colloid presents distinct surface Plasmon resonance bands. Keywords: Nanoparticles, chiral medium, organic materials, surface plasmon
1. INTRODUCTION Today, nanoscience and nanotecnology are promising fields of research and technological development. The reduced size dimensions of nanostructured materials leads to very singular properties that are mostly not observed with their bulk counterparts forms, opening new applications for these materials in several areas. Among the myriad of nanostructured systems, materials that contain metal nanoparticles are one of the most studied, having a major impact in material science, chemistry, physics, and biological research. Areas like microeletronics,1 chemical sensing,2 data storage,3 biodetection,4 optics5 and catalysis6, for example, are having a large improvement due to the manipulation of nanostructured systems, i.e. nanotechnology. In fact, metal nanoparticules can be considered intermediates between molecules and solid state, in other words, metal nanoparticles can combine properties of a bulk system and an atomic character.7 In particular, colloidal metal nanoparticles systems are been intensively exploited, taking advantage of the very small size of the metal particles. Their specific properties, characteristics, and potential applicability depend not only on the nanoparticles properties, like chemical constitution, size and shape, but also on the surrounding medium, e. g. stabilizers, dispersant agents, etc.8 However, rational synthesis of metal nanoparticles remains as one of the most important challenges for chemists to develop new materials that can reach special and different properties. A series of methods have been developed for formation of stable colloidal systems containing metal nanoparticles.9 In general, those methods make use of: i) metal complexes as metal source, e.g. HAuCl4, Pd(OAc)2, etc.; ii) inductive agents, e.g. NaBH4, H2, citrated, NaOH, temperature, etc., which allow the formation of the cluster; and iii) stabilizing agents, e.g. surfactants, functionalized oligomers and polymers, etc, which difficult particle agglomeration.10 The preparation of the metal nanoparticles is facilitated by the right choice of ligands or stabilizers, which prevent the coalescence of the particles. For example, in aqueous solution, polymeric stabilizers are very efficient,11 however, in organic medium, long chain surfactants or specific ligands are normally employed.12 Other options for stabilization are isolation of the nanoparticles in micelles or microemulsions,13 or also by formation of a small pellicle of silica.14 Stability of the colloidal fluid system is an important characteristic in order to acquiring a material relative stable in relation to the time. The thermodynamic favored tendency of agglomeration of the colloidal metal nanoparticles, macroscopically detected by precipitation or flocculation, is avoided by addition of stabilizing agents. These compounds
Plasmonics: Metallic Nanostructures and their Optical Properties IV, edited by Mark I. Stockman, Proc. of SPIE Vol. 6323, 63231S, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.681320
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interact with the colloidal particles, recovering them. The non agglomeration in this case is attributed to steric effect, depletion or electrostatic repulsion.15,16 There are few examples of use of vegetable oils and derivatives as dispersive or stabilizer agents. For instance, oleic and lauric acid are employed as stabilizers for the synthesis of magnetic nanoparticles.17 Nevertheless, we recently have observed that castor oil is a very attractive dispersant agent for AuNP.18 It is a very singular vegetable oil, with particular molecular structures, and physical-chemical properties, for example, high viscosity, optical activity19, etc. Also, we have verified that castor oil presents strong thermal nonlinear optical response.20 A large spatial self-phase modulation effect is observed when a Gaussian laser beam interacts with castor oil, and a huge enhancement of this effect is obtained when AuNP are dispersed in this oil.18 In this work, we report the synthesis of different colloids of gold nanoparticles dispersed in castor oil, produced from three different methods. The nanoparticles properties were investigated using electron transmission microscopy and UVVis spectroscopy. It is observed that each method gives rise to metal nanoparticles of different average size and shape. The linear absorption spectra of these systems corroborate with this result.
2. EXPERIMENTAL SECTION 2.1 Castor oil properties For many years, castor oil was known just as a purgative. However, since few decades ago, castor oil affords now a wide range of reactions leading to the formation of several derivatives. These chemicals are on par with petrochemical products, and are used in a number of industrial applications. Its derivatives are used in food (additive), textile (surfactants, pigment wetting agents), plastics (polyamide resin known as Rilsan used for coating metals, plasticizers, coupling agents), perfumes and cosmetics (emulsifiers, deodorant), electronics (capacitor), pharmaceuticals, paints, inks, adhesives and lubricants. In terms of molecular constitution, castor oil, in fact, is a mixture of triglycerides,19 containing predominantly the ester form of an unsaturated and hydroxylated fatty acid, the ricinoleic acid, (9Z,12R)-12-hydroxy-9-octadecenoic acid. A representative illustration of the molecular structured of the castor oil is depicted in Figure 1, where the most relevant molecular constituent is pointed out.
HO O
O
O O
O
HO
O
HO Figure 1: A representative molecular structure of the triglycerides of the castor oil.
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It worth to remark that castor oil is an anisotropic liquid medium, which have a well known optical activity. This oil presents also low linear absorption, as can be seen in figure 2, and large thermal nonlinearity20.
Absorbance (arb.units)
2,5 2,0 1,5 1,0 0,5 0,0 200
300
400
500
600
700
800
Wavelength (nm) Fig. 2. Absorption spectrum of castor oil.
2.2 Synthesis of AuNP, using NaBH4 as reducing agent The colloidal gold nanoparticles were prepared based on two-phase method, using NaBH4 as reducing agent21. Into a 250 mL round bottom flask a mixture of 20 mL of a 0.1 M aqueous solution of hexadecyltrimethyammonium bromide, 14 mL of a 1.0 % (w/v) aqueous HAuCl4•3H2O solution, 100 mL of toluene, 30 mL of ethanol, and 10 mL of castor oil were vigorously stirred for 15 minutes. The orange color organic phase was separated, and 50 mL of a 20 mM aqueous solution of NaBH4 was drop by drop added under vigorous stirring. With this procedure the mixture becomes gradually a rose suspension. After 24 hours of vigorous stirring, 20 mL of ethanol are added, and the wine color organic phase is again separated, dried under MgSO4, and all volatiles were removed in vacuum.
2.3 Synthesis of AuNP, in the presence of KOH The colloidal gold nanoparticles were prepared based on the standard KOH method.22 Into a 100 mL round bottom flask 10 mL of castor oil, 10 mL of ethanol, 1.0 mL of 2.0% (w/v) aqueous HAuCl4•3H2O solution, and 1.0 mL of 0.1 M KOH aqueous solution were vigorously stirred at room temperature for 15 min, leading to a grayish-black suspension. The mixture was heated at 80°C for 24h. During this period the color changes to deep wine-red. The biphasic mixture was separated and the organic phase was centrifuged to eliminate water residues. The colloid was dried in MgSO4 and all volatiles were removed in vacuum, leading to a deep red colloidal system.
2.4 Synthesis of AuNP, using citrate as reducing agent The colloidal gold nanoparticles were prepared based on the standard citrate method.23 Into a 250 mL round bottom flask 100 mL of 0.01% (w/v) aqueous HAuCl4•3H2O solution were brought to boil, and then 3.5 mL of 1% (w/v) aqueous trisodium citrate were added under vigorous stirring. In few minutes, the mixture becomes wine-red. After 2 hours, the aqueous colloidal system was allowed to cool, and then 10 mL of castor oil are added. The mixture was heated at 80°C for 2 hours. The system is cool down again and the biphasic mixture was separated and the organic phase was centrifuged to eliminate water residues. The colloid was dried in MgSO4 and all the volatiles were removed in vacuum, leading to a deep blue colloidal system.
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2.4 Colloid characterization All colloidal systems were characterized by TEM and UV-vis spectroscopy. The absorption spectra were recorded using a MultiSpec-1500 Shimadzu UV-vis spectrophotometer. The optical path length used was 1.0 cm, employing net castor oil as the reference. Transmission electron micrographs of the nanoparticles were carried out at Centro de Microscopia Eletronica e Microanalise da Universidade Luterana do Brasil – ULBRA, using a EM 208S-Philips. The samples of the AuNP were prepared, depositing a drop of the colloid solution on a carbon grid.
3. RESULTS AND DISCUSSION Three methods to yield metal nanoparticles colloids based on chemical reactions in solution have been adapted to prepare AuNP dispersed in castor oil. As previewed, each method leads to different size and shape metal nanoparticles. All three methods consist in two-phase synthesis. Water and castor oil are always present during the preparation, presenting between them a very small fraction of solubility. The first method of synthesis is adapted from a typical two-phase synthesis, involving the transfer of AuCl4- ions into an organic dispersant, in this case, toluene/castor oil with the use of a tetralkylamonium salt following by reduction with NaBH4. The TEM image of the AuNPs dispersed in castor oil produced by this method is presented in figure 3. As can be seen in this figure, approximately spherical nanoparticles with an average core diameter of 7 ± 2 nm are obtained by this method. The linear absorption spectrum of this colloid is depicted in figure 4. It presents a large absorption band, centered at 542 nm, which is due to the nanoparticles surface plasmon resonance.
50 nm
Figure 3. TEM image of the AuNP prepared using NaBH4 as reducing agent.
The second method, is based on the oldest technique of preparation of colloidal AuNP, using KOH and temperature to disproportionate Au(III) salts to Au(I) and Au(0). In this case, spherical gold particles are produced, presenting an average diameter of 15 ± 5 nm. A typical TEM image of these nanoparticles is presented in figure 5. As expected the linear absorption spectrum of this colloidal system shows a slightly different absorption peak than in the previous case. As can be seen in figure 6, the absorption peak owing to the surface Plasmon resonance of the gold nanoparticles is centered at 558 nm in the present case.
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λmax= 542 nm
Absorbance (arb. units)
1,0 0,8 0,6 0,4 0,2 0,0 400
500
600
700
800
Wavelength (nm) Fig. 4. Absorption spectrum of the colloid containing AuNP in castor oil, prepared using NaBH4 as reducing agent.
w
I
100 nm Fig. 5. TEM image of the gold nanoparticles produced in presence of KOH.
The third method involves the reduction of HAuCl4, employing citrate as reducing agent. In this case, the gold nanoparticles are assembled in a raspberry-like morphology, very different from the previous cases. These structures have an average core diameter of 55 ± 5 nm, see figure 7. The linear absorption spectrum of the colloid containing the berry-shaped nanoparticles centered at 664 nm, as can be seen in figure 8, which indicates the strong dependence of the
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surface plasmon resonance with the size and shape of the nanoparticles clusters. It is important to remark that, the characteristic color of spherical Au nanoparticles colloids is red, while the colloidal solution of the berry-shaped AuNPs is blue.
Absorbance (arb. units)
2,0
λmax = 558 nm
1,6 1,2 0,8 0,4 0,0 400
500
600
700
800
Wavelength (nm) Fig. 6. Absorption spectrum of colloid containing AuNP in castor oil, prepared in presence of KOH.
50 nm Fig. 7. TEM image of the gold nanoparticles produced in presence of citrate.
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λmax= 664 nm
Absorbance (arb. units)
1,75 1,50 1,25 1,00 0,75 0,50 400
500
600
700
800
Wavelength (nm) Fig. 8. Absorption spectrum of colloid containing AuNP in castor oil, prepared in presence of citrate.
The optical activity and the nonlinear optical properties of these colloids are currently being investigated24. Preliminary results suggest that the thermal and electronic contributions to the nonlinear refractive indexes of these colloids are dependent on the nanoparticles size. However, a complete investigation has not been performed yet.
4. CONCLUSION Three distinguished and stable colloids of gold nanoparticles (AuNP), using castor oil as dispersant agent were obtained by three different methods of synthesis, using tetrachloroauric(III) acid as gold source. Each method leads to different size and shape metal nanoparticles. Images obtained by TEM of the nanostrutured systems show that from each method of synthesis, nanoparticles of avarege size equal to 7, 15, and 55 nm are obtained. Different maximum wavelength absorption was also observed, indicating that each colloid presents distinct suface plasmon resonance bands.
Acknowledgments The authors thank the financial support from Instituto do Milênio de Informação Quântica, CAPES, CNPq, FAPEAL, PADCT, Nanofoton network, FINEP and ANP-CTPETRO. M.R.M. thanks to Dr. G. Machado and Prof. Dr. J. Dupont of the Instituto de Química of Universidade Federal do Rio Grande do Sul for the TEM images of the gold nanoparticles.
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