Phase Transfer of Aqueous Gold Colloidal Particles Capped with

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Langmuir 2001, 17, 3766-3768

Phase Transfer of Aqueous Gold Colloidal Particles Capped with Inclusion Complexes of Cyclodextrin and Alkanethiol Molecules into Chloroform Neeta Lala, Sachin P. Lalbegi, S. D. Adyanthaya, and Murali Sastry* Materials Chemistry Division, National Chemical Laboratory, Pune - 411 008, India Received November 13, 2000. In Final Form: February 13, 2001 Colloidal gold particles synthesized in water have been capped with octadecanethiol (ODT) molecules rendered water-soluble by threading with R-cyclodextrin (R-CD) molecules. Thereafter, the gold nanoparticles could be transferred into an organic solvent such as chloroform by vigorous shaking of a biphasic mixture of the R-CD-threaded ODT-stabilized gold hydrosol and chloroform. The phase transfer of the gold nanoparticles could be observed as a rapid transfer of color from the aqueous phase to the organic phase. During shaking of the biphasic mixture, it is believed that R-CD molecules are dislodged from the octadecanethiol molecules, rendering the gold particles hydrophobic and amenable to phase transfer. Transmission electron microscopy studies of films of the gold nanoparticles in chloroform formed by solvent evaporation indicate a uniform size distribution of the nanoparticles and hexagonal close-packing of the particles in the film.

Cyclodextrins (CDs), which are cyclic oligosaccharides consisting of six, seven, or eight glucopyranose units (R-, β-, and γ-CDs respectively),1 are being increasingly studied as versatile host molecules in a variety of applications.2a Host-guest interactions provide a simple means of stepwise functionalization of surfaces and, consequently, self-assembled monolayers (2D SAMs) of CDs on gold/ silver thin films have been studied in some detail.2 As in the case of alkanethiols on gold where reports on the selfassembly of these molecules on 2D gold surfaces3 were rapidly followed by studies into the self-assembly on gold nanoparticles (3D SAMs),4 self-assembly of thiolated CDs on gold5 and Pt/Pd colloidal particles6 has also been attempted. Surface-derivatization of fullerenes with CDs has recently been shown to lead to a new class of watersoluble bucky balls.7 In a novel twist, R-CD molecules have been used to solubilize thiol molecules in water by formation of inclusion complexes (ICs) and, thereafter, to self-assemble the ICs on gold nanoparticle surfaces8 (here again, in parallel with 2D SAMs of CD-alkanethiol ICs).2d In this note, we advance investigations into surface modification of colloidal gold particles with ICs of R-CD and octadecanethiol (ODT) molecules and demonstrate * To whom correspondence should be addressed. Phone: +91 20-5893044. Fax: +91 5893044/5893952. E-mail: [email protected]. (1) Saenger, W. Angew. Chem., Int. Ed. Engl. 1980, 19, 344. (2) (a) Nelles, G.; Weisser, M.; Back, R.; Wohlfart, P.; Wenz, G.; Mittler-Neher, S. J. Am. Chem. Soc. 1996, 118, 5039 and references therein. (b) Rojas, M. T.; Koninger, R.; Stoddart, J. F.; Kaifer, A. E. J. Am. Chem. Soc. 1995, 117, 336. (c) Maeda, Y.; Kitano, H. J. Phys. Chem. 1995, 99, 487. (d) Yan, J.; Dong, S. Langmuir 1997, 13, 3251. (e) Fukuda, T.; Maeda, Y.; Kitano, H. Langmuir 1999, 15, 1887. (f) Kitano, H.; Taira, Y.; Yamamoto, H. Anal. Chem. 2000, 72, 2976. (3) Ulman, A. Introduction to ultrathin organic films: from Langmuir-Blodgett to self-assembly; Academic Press: San Diego, CA, 1991. (4) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (5) Liu, J.; Mendoza, S.; Roman, E.; Lynn, M. J.; Xu, R.; Kaifer, A. E. J. Am. Chem. Soc. 1999, 121, 4304. (b) Liu, J.; Ong, W.; Roman, E.; Lynn, M. J.; Kaifer, A. E. Langmuir 2000, 16, 3000. (6) Alvarez, J.; Liu, J.; Roman, E.; Kaifer, A. E. J. Chem. Soc., Chem. Commun. 2000, 1151. (7) Samal, S.; Geckeler, K. E. J. Chem. Soc., Chem. Commun. 2000, 1101. (8) (a) Liu, J.; Xu, R.; Kaifer, A. E. Langmuir 1998, 14, 7337. (b) Liu, J.; Alvarez, J.; Kaifer, A. E. Adv. Mater. 2000, 12, 1381.

Figure 1. UV-vis spectra recorded from the as-prepared gold colloidal solution (curve 1), the gold colloidal solution after capping with R-CD-threaded ODT molecules (curve 2), the chloroform solution after phase transfer of the gold nanoparticles (curve 3; see text for details), and the aqueous gold colloidal solution after phase transfer of the gold nanoparticles into chloroform (curve 4). The inset is a picture of test tubes containing solutions of chloroform and R-CD-threaded ODTcapped gold hydrosol before (test tube on the left) and after phase transfer of the gold nanoparticles into chloroform (test tube on the right). The cartoons illustrate the nature of surface modification of the gold particles in the aqueous phase and in the organic phase.

that such gold colloidal particles may be phase-transferred into organic media such as chloroform by simple shaking of the gold hydrosol-chloroform biphasic solutions (Figure 1, inset). During shaking of the biphasic mixture, the R-CD molecules are dislodged from the ICs thereby rendering the alkanethiol-capped gold particles hydrophobic and amenable to phase transfer. This report thus adds to the growing literature on phase transfer of colloidal gold,9 silver,9a,10 and Q-state CdS11 particles synthesized in an aqueous medium into nonpolar organic phases, with the interest in such phase-transfer protocols being the ability

10.1021/la0015765 CCC: $20.00 © 2001 American Chemical Society Published on Web 05/11/2001

Phase Transfer of Aqueous Gold Colloidal Particles

to direct the nanoparticles into environments having different physicochemical properties. In a typical experiment, ODT (Aldrich; used as-received) was added to 50 mL of an aqueous solution of 10-2 M R-CD (Aldrich, used as-received) kept in a water bath at a constant temperature of 40 °C. The amount of ODT added to the aqueous R-CD solution was such that, if completely solubilized, would lead to a 10-3 M concentrated ODT solution. Thus, a 10-fold excess of R-CD was taken in the solution to maximize the dissolution of ODT in water. Under continuous stirring, it was observed that the ODT powder slowly went into solution and completely disappeared after ca. 48 h of stirring. The dissolution of ODT in water is known to occur by the threading of the R-CD molecules by the hydrocarbon chains of ODT, thus, leading to the formation of ICs.8 Colloidal gold particles were prepared by borohydride reduction of a HAuCl4 solution as described in detail elsewhere.12 This procedure yields a clear, ruby-red colored solution (pH ∼ 9) of the gold nanoparticles of dimensions 35 ( 7 Å.12 Equal volumes (20 mL) of the gold hydrosol and the aqueous solution of ICs of R-CD and ODT were mixed together, and the capping of the gold particles by the ICs was effected. Please note that this marks a point of departure from the protocol adopted by Kaifer and coworkers for the synthesis of gold colloidal particles stabilized by R-CD-threaded alkanethiols.8 In that report, both the R-CD and alkanethiol molecules were directly added to the colloidal solution, resulting in the formation of a precipitate under prolonged stirring.8 We have observed that capping of the gold particles after separately solubilizing the ODT molecules resulted in a colloidal solution that was stable for many days before eventually precipitating out of solution after a couple of weeks of storage. After 12 h of mixing of the gold nanoparticle and R-CD-ODT solutions, 20 mL of this solution was taken in a test tube and an equal volume of chloroform was added, resulting in the formation of two separate layers of liquid. A picture of the test tube containing the biphasic mixture is shown in the inset of Figure 1 (test tube on the left). The associated cartoon shows the surface of a colloidal gold particle capped with water soluble, R-CD-threaded ODT molecules from the aqueous phase. Because chloroform is denser than water (density of chloroform ) 1.47 gm/cm3), it settles at the bottom of the test tube as a transparent layer, with the colored gold hydrosol on top (Figure 1, inset; test tube on the left). Vigorous shaking of this biphasic mixture resulted in rapid transfer of color to the organic phase. The test tube to the right in the inset of Figure 1 shows the biphasic mixture after the shaking process, the ruby red color now appearing in the chloroform solution at the bottom of the test tube. This result indicates almost complete transfer of the gold nanoparticles into chloroform, presumably by displacement of the R-CD molecules from the ICs during the shaking process. As a consequence, the gold particles, which were water-soluble by virtue of the threaded R-CD molecules, are rendered hydrophobic and amenable to phase transfer as observed. This is illustrated in the cartoon associated with the chloroform phase (inset of Figure 1; test tube to the right) showing the hydrophobic gold nanoparticles covered by just ODT molecules. (9) (a) Sarathy, V. K.; Kulkarni, G. U.; Rao, C. N. R. J. Chem. Soc., Chem. Commun. 1997, 537. (b) Kumar, A.; Mukherjee, P.; Guha, A.; Adyanthaya, S. D.; Mandale, A. B.; Kumar, R.; Sastry, M. Langmuir 2000, 16, 9775. (c) Sastry, M.; Kumar, A.; Mukherjee, P. Coll. Surf. A 2001, 181, 255. (10) Wang, W.; Efrima, S.; Regev, O. Langmuir 1998, 14, 602. (11) Kumar, A.; Mandale, A. B.; Sastry, M. Langmuir 2000, 16, 9299. (12) Patil, V.; Malvankar, R. B.; Sastry, M. Langmuir 1999, 15, 8197.

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Figure 2. (A) TEM micrograph of a drop-coated film of the ODT-stabilized gold nanoparticles after phase transfer into chloroform. The scale bar corresponds to 100 nm. (B) Particle size distribution histogram of the gold nanoparticles imaged in the TEM picture (Figure 2A). The solid line is a Gaussian fit to the data.

Figure 1 shows the UV-vis spectra13 recorded from the as-prepared gold hydrosol (curve 1), the gold hydrosol after addition of R-CD-ODT IC solution and stabilization for 12 h (curve 2), the chloroform solution after phase transfer of the gold particles as described above (curve 3), and the aqueous phase after phase transfer (curve 4). The resonance at ca. 520 nm in spectra 1-3 arises because of excitation of surface plasmon oscillations in the gold particles and is responsible for the striking color of gold sols.14 It is observed that the surface plasmon resonance is broadened and shifted marginally to the red on addition of the R-CD-ODT IC solution to the gold hydrosol, indicating surface binding of the ODT molecules to the gold surface via thiolate linkages.8 The surface plasmon resonance is broader for the chloroform solution of the gold nanoparticles, indicating some degree of aggregation of the particles. However, the organic solution of the gold nanoparticles was extremely stable in time with no precipitation observed even months after the phase transfer had been effected. The gold nanoparticle-chloroform solution was rotary evacuated and resulted in a brownish solid which could be readily dissolved in different organic solvents such as ethanol, toluene, benzene, etc. It is also seen that almost complete phase transfer of the gold particles had occurred and only a weak plasmon resonance was recorded from the aqueous phase after phase transfer (curve 4). The molar ratio of gold in the organic phase to that of gold remaining in the aqueous phase after phase transfer was roughly calculated to be 30. Films of the phase-transferred gold nanoparticles were deposited by drop-coating the gold nanoparticle-chloroform solution onto carbon-coated grids for transmission electron microscopy (TEM)15 measurements. Figure 2A shows the TEM micrograph obtained from the gold nanoparticle film, and it can clearly be seen that the surface is covered with fairly extended regions of closepacked, hexagonally arranged gold nanoparticles. The facile formation of close-packed structures of nanoparticles over large length scales by simple solvent evaporation is an attractive aspect of the preparation of surfactantstabilized nanoparticles in volatile organic solvents.9a,16 (13) UV-vis spectra of the colloidal solutions were recorded on a Hewlett-Packard 8542A diode array spectrophotometer at a resolution of 4 nm. (14) Underwood, S.; Mulvaney, P. Langmuir 1994, 10, 3427. (15) TEM measurements were carried out on a JEOL model 1200EX instrument operated at an accelerating voltage of 120 kV. (16) Wang, Z. L. Adv. Mater. 1998, 10, 13 and references therein.

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The histogram of the particle diameters is plotted in Figure 2B, where a bimodal particle size distribution is observed. The solid line in the figure is a Gaussian fit to the data and leads to an average particle diameter of 65 ( 5 Å for the more frequently occurring gold nanoparticles and ca. 40 Å for the smaller particles. Although the as-prepared gold particle solution has been measured to consist of particles of 35 Å diameter (corresponding to the smaller particle size peak in Figure 2B), the presence of a large proportion of particles of size 65 Å indicates some aggregation of the particles, evidence of which can be seen in the top and lower left edges of the TEM picture (Figure 2A). The average center-center distance of the nanoparticles in the TEM picture was estimated to be 95 Å. This distance is larger than the close-packing distance of 65 Å for the gold particles and arises because of overlap of hydrocarbon chains of neighboring particles. The chains are partially interdigitated in agreement with the observation of others on self-assembled films of gold nanoparticles stabilized by alkanethiols.9a,17 To get a better insight into the nature of coordination of the alkanethiol molecules on the gold nanoparticle surface before and after phase transfer, FTIR measurements of drop-cast films of the nanoparticles on a Si (111) substrate as well as thermogravimetry/differential thermal analysis (TGA/DTA)18 of the rotovapped powder of the gold nanoparticles was carried out. FTIR spectra (data not shown) showed the presence of two bands at 2920 and 2850 cm-1 for films cast from the aqueous phase before phase transfer and the organic phase after phase transfer. These bands correspond to the methylene antisymmetric and symmetric vibrations respectively from the ODT molecules on the gold surface.19 The important difference in the two films was the disappearance of a strong band at ca. 3400 cm-1 after phase transfer in the chloroform phase. This band arises due to excitation of O-H vibrations and is known to appear as an intense peak in films of CDs.2a The absence of this band in films of the phasetransferred gold particles clearly indicates that the R-CD molecules in the ICs bound to the gold particle surface are dislodged during shaking of the biphasic mixture and supports the contention that this process is responsible for the hydrophobization of the nanoparticles and the subsequent phase transfer. (17) Terrill, R. H. et al. J. Am. Chem. Soc. 1995, 117, 12537. (18) TGA/DTA data of the gold nanoparticle powder were recorded on a Seiko Instruments model 32 TG/DTA system at a heating rate of 10 °C/min. (19) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604.

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The results of the TGA/DTA analysis are shown in Figure 1 of the supplementary information. Two prominent weight losses at 230 and 310 °C are observed and are endothermic and exothermic processes, respectively. The weight loss at 230 °C amounting to ca. 40% of the sample weight is attributed to desorption of surface-bound ODT molecules. This temperature agrees well with reported values for ODT bound to gold17,20 as well as CdS nanoparticle surfaces.11 The exothermic feature at 310 °C is likely to be due to sintering of the gold nanoparticles because of desorption of the stabilizing ODT molecules from the particle surface. This process of sintering of the gold nanoparticles is accompanied by desorption of the gold nanoparticles and an almost complete loss of the nanoparticles by the time the sample temperature reached 500 °C. This behavior is very similar to that observed by us in our earlier studies on ODT-stabilized CdS nanoparticles.11 Two endothermic features at 30 and 50 °C are observed in the TGA/DTA data with no weight loss accompanying these processes. The endothermic feature at 50 °C is due to melting of the ordered regions of the hydrocarbon chains, possibly in the ordered, interdigitated hydrocarbon regions of the film. This feature has been observed earlier in alkylamine/alkanethiol stabilized gold films,17,20 silver nanoparticles stabilized with fatty acids,21 as well as in ODT-stabilized CdS nanoparticle films.11 The feature at 30 °C indicates melting in less ordered regions of the hydrocarbon chains and is an aspect that requires further study. In conclusion, the capping of gold nanoparticles by ODT molecules solubilized in water via formation of ICs with R-CD molecules has been demonstrated. The gold nanoparticles capped with the ICs through thiolate bond formation with the ODT molecules may be phasetransferred into chloroform by dislodging the R-CD molecules. This is easily accomplished during vigorous shaking of biphasic mixtures of the gold hydrosol and chloroform and results in hydrophobic, ODT-capped gold particles amenable to phase transfer. Acknowledgment. The authors thank Dr. Mohan Bhadbade, Physical Chemistry Division, NCL Pune for assistance with the TEM measurements. Supporting Information Available: TGA-DTA measurements of powder of gold nanoparticles capped with ODT after phase transfer into chloroform (Figure 1). This material is available free of charge via the Internet at http://pubs.acs.org. LA0015765 (20) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 4723. (21) Patil, V.; Mayya, K. S.; Pradhan, S. D.; Sastry, M. J. Am. Chem. Soc. 1997, 119, 9281.