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Three-Dimensional Self-Assembly of Gold Nanocolloids in Spheroids Due to Dialysis in the Presence of Sodium Mercaptoacetate Eiki Adachi† L’Ore´ al Tsukuba Center, 5-5 Tokodai, Tsukuba, Ibaraki 300-2635, Japan Received February 18, 2000. In Final Form: April 26, 2000 Elemental gold nanocolloids were synthesized by reducing an aurate aqueous solution in the presence of sodium mercaptoacetate. The nanocolloids, 4.6 nm in diameter, self-assembled in highly monodispersed spherical aggregates, 30 nm in diameter, when the nanocolloid suspension was dialyzed in water. Elimination of excess ions, due to dialysis, caused the three-dimensional self-assembly of the nanocolloids. The spheroid and also the elemental nanocolloids before dialysis did not produce the plasmonic absorption peak that is characteristic of conventional gold colloids. Chemisorption of mercaptoacetate onto the nanocolloids caused the peak to disappear. The features of the spheroid structure and assembly mechanism are presented.
Introduction Metallic nanocolloids are characterized by specific heat,1,2 nonlinear optical susceptibility,3-5 and singleelectron transfer6 differing from the bulk because of quantum effects originating in the confinement of conduction electrons in the nanometric volume.7-10 Their close-packed assembly is expected to produce complex electronic and optical functions based on quantum mechanical coupling of conduction electrons localized in each nanocolloid.11-14 Nanocolloids are generally synthesized by reducing a metal salt in the presence of stabilizing organic surfactants. This aqueous colloid is largely chargestabilized, so it is difficult to fabricate a close-packed assembly of charged colloids. It was recently proposed to synthesize stable uncharged nanocolloids in organic solvents due to the chemisorption of alkanethiol having a methylene chain 8-12 long on nanocolloid surfaces,15,16 providing steric repulsion among nanocolloids. Thiolderivatized nanocolloids assemble in a two-dimensional (2D) close-packed array supported on a solid surface17,18 † Phone: +81-298-47-7984. Fax: +81-298-47-7985. E-mail:
[email protected].
(1) Stewart, G. R. Phys. Rev. B 1997, 15, 1143. (2) Goll, G.; Lo¨hneysen, H. v.; Kreibig, U.; Schmid, G. Z. Phys. D: At., Mol. Clusters 1991, 20, 329. (3) Ricard, D.; Roussignol, P.; Flytzanis, C. Opt. Lett. 1985, 10, 511. (4) Hache, F.; Ricard, D.;l Flytzanis, C. J. Opt. Soc. Am. B 1986, 3, 1647. (5) Hache, F.; Ricard, D.; Flytzanis, C.; Kreibig, U. Appl. Phys. A 1988, 47, 347. (6) Devoret, M. H.; Grabert, H. In Single Charge Tunneling; Grabert, H., Devoret, M. H., Eds.; Plenum: New York, 1992; p 1. (7) Schmid, G. Chem. Rev. 1992, 92, 1709. (8) deHeer, W. A. Rev. Mod. Phys. 1993, 65, 611. (9) Scho¨n, G.; Simon, U. Colloid Polym. Sci. 1995, 273, 202. (10) Weller, H. Curr. Opin. Colloid Interface Sci. 1998, 3, 194. (11) Kolagunta, V. R.; et al. Proc.sElectrochem. Soc. 1996, 95-17 (Quantum Confinement), 56-69. (12) Andres, R. P., et al. Science 1996, 273, 1690. (13) Osifchin, R. G.; Andres, R. P.; Henderson, J. I.; Kubiak, C. P.; Dominey, R. N. Nanotechnology 1996, 7, 412. (14) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath, J. R. Science 1997, 277, 1978. (15) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, J. D.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 7, 801. (16) Sarathy, K. V.; Raina, G.; Yadav, R. T.; Kulkarni, G. U.; Rao, C. N. R. J. Phys. Chem. B 1997, 101, 9876. (17) Harfenist, S. A.; Wang, Z. L.; Alvarez, M. M.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. 1996, 100, 13904. (18) Sato, T.; Brown, D.; Johnson, B. F. G. Chem. Commun. 1997, 11, 1007.
even though the nearest nanocolloids are electronically isolated because of the long methylene chain. The assembly subsequently requires displacement of alkanethiol to an electrical conductive linker, e.g. aromatic dithiol, for coupling. This is achieved by immersing the 2D array in an aromatic dithiol solution.11,12 Displacement is required when a long alkanethiol is used as the stabilizing reagent. This restricts the assembly of nanocolloids into layered structures because it takes place at the interface. The size of the gap between the nearest nanocolloids also determines the strength of coupling. Another way of coupling is to keep gaps between individual nanocolloids close enough for electron tunneling.6 This suggests that a short thiol should be used instead of a long thiol. The shorter the thiol length, the better the strength, potentially resulting in uncontrollable aggregates due to the lack of steric repulsion in organic solvents. This should be compensated by another repulsion, e.g. electrostatic repulsion, to avoid aggregates. Compensation requires that aqueous nanocolloids be synthesized in the presence of short thiol having a dissociative group with subsequent control of aggregates. The aggregates are probably a novel close-packed assembly, i.e., three-dimensional (3D), due to control that extends the dimension from 2D to 3D, promising an expanded variety in functions. Gold nanocolloids were synthesized by reducing an aureate solution in the presence of sodium mercaptoacetate. We found that nanocolloids self-assemble in stable spheroids monodispersing in water, due to subsequent dialysis. Materials and Methods Hydrogen tetrachloroaurate(III) tetrahydrate (HAuCl4‚4H2O, MW ) 411.8), sodium borohydride (NaBH4, MW ) 37.8), and sodium mercaptoacetate (HSCH2COONa, MW ) 114.1) were purchased from Wako Chem., Co., Ltd., Osaka, Japan, and dissolved in pure water (MilliQPlus, Millipore Inc., Bedford, MA) to prepare 0.53 mM of HAuCl4‚4H2O, 0.15 M of NaBH4, and 20 mM of HSCH2COONa aqueous solutions. A 1 mL amount of the sodium mercaptoacetate solution was added to 94 mL of the aurate solution in a Pyrex beaker during stirring; 5 mL of NaBH4 solution was then immediately added. The solution quickly turned from transparent light yellow to brown, indicating the synthesis of elemental gold nanocolloids. The solution pH went from pH 3 to 4 at the start of reduction. Concentrations of HAuCl4‚4H2O, NaBH4, and HSCH2COONa eventually became 0.5, 7.5, and 0.2
10.1021/la000244x CCC: $19.00 © 2000 American Chemical Society Published on Web 07/06/2000
Self-Assembly of Gold Nanocolloids in Spheroids
Figure 1. (a) TEM image of gold nanocolloids immediately after reduction. Nucleation appears to start although spheroids do not form without dialysis. The bar corresponds to 200 nm. (b) TEM image of spheroids after overnight dialysis. They are similar in size, although slightly large spheroids can occur. Spheroids do not form perfectly if dialysis is stopped after 4 h. The bar corresponds to 200 nm. (c) Spheroids under high magnification. Each consists of elemental nanocolloids. The bar corresponds to 100 nm. Drawings depict the spheroid synthesis procedure. mM. The total volume was 100 mL. The solution was first filtered using a syringe filter (0.2 µm pore, polyvinylidene fluoride, Whatman Inc., Clifton, NJ) to remove large contaminants. It was subsequently dialyzed overnight in 2 L of pure water to obtain a spheroid suspension (Figure 1), using a dialysis tube (Spectra/Por MWCO ) 6-8000, Spectrum Medical Industries Inc., Gardena, CA). The suspension pH finally became 5 by the end of the dialysis. The volume of suspension was extracted after reduction and overnight dialysis and dried on grids covered with a collodion film for transmission electron microscopy (TEM). The grids were subsequently coated with carbon in a vacuum evaporator to protect the samples from contaminants in the air and avoid damage during observation. The prepared samples were observed by a TEM at 80 kV acceleration voltage (Leo EM902, Leo Electron Microscopy Ltd., Oberkochen Germany). Spheroid diameter was measured by dynamic light scattering (DLS) using 632.8 nm wavelength He-Ne laser (DLS/SLS-5000, ALV, Co., Ltd., Langen Germany). Optical absorption spectra of the spheroid suspension and conventional gold colloid suspension, synthesized in the same way as the spheroid synthesis but without HSCH2COONa, were measured by a spectrophotometer (V-550, Jasco Corp., Hachioji Japan) at 400-700 nm range using a quartz cuvette. The spheroid suspension was concentrated 20 times by removing water using an ultrafiltration membrane (Diafo, Amicon Inc., Beverly, MA). This concentration resulted in the removal of free ions of the additives in the suspension. A volume of the suspension was dried on a gold-coated glass plate in a desiccator for Fourier transform infrared reflection-absorption spectroscopy (FTIRRAS) (Spectrum 2000 FTIR, Perkin-Elmer Analytical Instruments Corp., Norwalk, CT) to evaluate the existence of mer-
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Figure 2. (a) TEM image of a spheroid. Individual elemental gold nanocolloids are identified as black dots 4.6 ( 0.8 nm in diameter. A substance encapsulates the spherical aggregate of nanocolloids, which probably originates in carbon-coating. The acceleration voltage is 80 kV. The bar corresponds to 30 nm. (b) Depiction of a spheroid. The aggregate diameter is 30 ( 7 nm. (c) Electron diffraction pattern of spheroids. The bars in the diffraction pattern and the TEM image correspond to 10 mm and 100 nm. Each black dot corresponds to one spheroid. Diffraction rings corresponding to the (111), (200), (220), and (311) planes of gold crystal indicate that nanocolloids composing spherical aggregates are gold crystal. captoacetate in the spheroids. Transmission FTIR spectra of mercaptoacetic acid and sodium mercaptoacetate were measured for comparison. All experiments were conducted at room temperature (∼23 °C).
Results Elemental nanocolloids did not assemble immediately after reduction, although nucleation appeared to have started (Figure 1a). Spheroids self-assembled after overnight dialysis (Figure 1b). Each black dot in Figure 1b was a spheroid that was a spherical aggregate of elemental nanocolloids (Figure 1c). The suspension pH became 5 by the end of the dialysis without any buffers. Spheroids were not obtained by adjusting pH to 5 only using a buffer (the buffer used was 10 mM GTA composed of 3,3dimethylglutaric acid, Tris (2-amino-2-(hydroxymethyl)1,3-propanediol), and 2-amino-2-methyl-1,3-propanediol at equal mole amounts). Spheroids were not found in the reduced suspension left overnight and not synthesized without HSCH2COONa. The diameters of the nanocolloid and the spheroid were 4.6 ( 0.8 and 30 ( 7 nm, obtained by averaging them respectively in TEM images. The spheroids, which are spherical aggregates of nanocolloids, were covered with a substance (Figure 2a). Aggregates were clearly composed of nanocolloids (Figure 2b). A similar substance covered conventional gold colloids when the TEM sample was prepared in the same way as the spheroid TEM sample. The electron diffraction pattern from spheroids indicated that nanocolloids were composed
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Figure 4. Transmission FTIR spectra of HSCH2COOH on a sodium chloride (NaCl) plate, HSCH2COONa mixed with potassium bromide (KBr) powder, and FTIR-RAS spectrum of spheroids dried on a gold-coated glass plate. The 3 characteristic peaks (1710, 1580, and 1385 cm-1) in the spheroid spectrum indicate the carboxylic acid dimer (1710 and 1385 cm-1) and carboxylate (1580 and 1385 cm-1) because each set of peaks corresponds to the carboxylic acid dimer of HSCH2COOH and carboxylate originating in HSCH2COONa. Figure 3. (a) Size distribution of spheroids in water. The single peak corresponds to 34 nm in diameter. The distribution is the Laplacian transform of a correlation function obtained by DLS, which is a function of decay time proportional to spheroid size. (b) Size distribution of a latex particle in water. The latex particle (38 ( 7 nm in diameter) is a standard sample for magnification calibration in TEM. (c) VIS spectra of spheroid (solid line) and conventional gold colloid (dotted line) suspensions. A conventional gold colloid suspension was synthesized by reducing the Au3+ aqueous solution with NaBH4 but without HSCH2COONa. Although the conventional colloid suspension shows an absorption peak at 518 nm, the spheroid suspension does not show a similar peak. The absorption spectrum of nanocolloid suspension before dialysis is almost the same as that of the spheroid suspension.
of gold crystal because the diameter ratio of each circle and the most inner circle matched with that of gold crystal (Figure 2c). Spheroid size distribution in water showed a single peak corresponding to 34 nm in diameter obtained from DLS experiments (Figure 3a). The decay time is proportional to the averaged diameter of particles dispersed in solvents. Dispersion was almost equal to that of standard latex particles (38 ( 7 nm in diameter) (Figure 3b). Although conventional gold colloid suspension (17 nm in diameter) showed an absorption peak at 518 nm, the spheroid suspension did not (Figure 3c). The absorption spectrum of the nanocolloid suspension before dialysis was almost the same as that of the spheroid suspension after dialysis. Peaks at 1710, 1580, and 1385 cm-1 of the FTIR spectrum of dried spheroids indicated the existence of carboxylic acid and carboxylate because peaks at 1710 cm-1 of HSCH2COOH and at 1580 cm-1 of HSCH2COONa coexisted in the spheroid spectrum (Figure 4). This shows that sodium mercaptoacetate, which is used in the spheroid synthesis, causes the three peaks. The spheroid maintained its assembled form for 1 month at room temperature. Discussion A spheroid was a spherical aggregate of nanocolloids according to TEM observation. The spheroid diameter in water (34 nm) was larger than that in a vacuum (30 ( 7 nm). This indicates shrinkage of spheroids on the TEM grid, due to drying during carbon-coating. A substance covered the spheroid that was sandwiched between a collodion film and the carbon coating. A similar substance also covered conventional gold nanocolloids when the TEM
sample was prepared in the same way as the spheroid TEM sample. This indicates that the substance mainly comes from the carbon coating, not the spheroids, because such clear substances were not found without the carbon coating. The FTIR spectrum of dried spheroids shows carboxylic acid (1710 cm-1) and carboxylate (1580 cm-1). Thiol groups specifically chemisorb on gold colloids, indicating that each nanocolloid in the spherical aggregate chemisorbs mercaptoacetate onto the surface. Almost all free ions of mercaptoacetate were eliminated from the original spheroid suspension because it was concentrated 20 times. This spectrum therefore mainly shows signals of mercaptoacetate in the spheroids. The largest peak at 1385 cm-1 should be small because peaks of pure sodium mercaptoacetate and mercaptoacetic acid near 1400 cm-1 are small compared to peaks at 1580 and 1710 cm-1. The orientation of mercaptoacetate on nanocolloids does not cause the largest peak because the dried spheroids are oriented randomly. Analysis of chemical composition in the spheroids will give us more precise details about the origin of these peaks. However, the composition is currently uncertain. The absorption spectrum of the spheroid suspension with dialysis and also the nanocolloid suspension without dialysis did not produce a peak in the range of 518 nm, which is a plasmonic absorption of conventional gold colloids in water. The absorption peak arises from the plasmon resonance of conduction electrons as a function of the electron number density and dielectric constant of the dispersion medium. Even if counterions condense at surrounding areas of the spheroids in water, this does not cause the absorption peak to disappear. The difference in dielectric constants between the areas and bulk water merely shifts the peak wavelength. The chemisorption of mercaptoacetate onto nanocolloids probably inhibits the plasmon resonance of electrons in visible region, causing the peak to disappear. The peaks of the FTIR spectrum and the disappearing of the absorption peak indicate that mercaptoacetate exists in the spheroid. This ensured the chemisorption of mercaptoacetate onto elemental gold nanocolloids composing the spheroids. The reduction of gold ions initially produces a certain size of nanocolloids in the presence of mercaptoacetate ions. The nanocolloids are subsequently stabilized by chemisorption of mercaptoacetate onto the surfaces.
Self-Assembly of Gold Nanocolloids in Spheroids
Nucleuses composed of nanocolloids are formed by the end of the reduction process. However, nanocolloids do not self-assemble in monodispersed spheroids without subsequent dialysis, indicating that the elimination of excess chemicals is necessary for self-assembly to occur. Nanocolloids chemisorbing sodium mercaptoacetate as like-charged colloids are intuitively repulsive, originating in negative charges of dissociated carboxylates on nanocolloids. Repulsive nanocolloids cannot assemble in a spheroid without attraction. Self-assembly of nanocolloids in a spheroid thus requires attraction among nanocolloids. It was recently inferred in experiments19-22 and calculations23-25 that electrostatic attraction exists between likecharged colloids, arising from correlation among counterions localized near individual colloids.25 Similar attraction among nanocolloids can originate in counterions, i.e., either Na+ or Au3+, or both, to carboxyl terminals of mercaptoacetate on nanocolloids. The FTIR peak at 1580 cm-1 is larger than that at 1710 cm-1, suggesting that the spheroid is carboxylate anion-rich, indicating cations as the counterions. It appears that this attraction causes the nucleation and subsequent assembly of elemental nanocolloids. The elimination of the excess free ions results in an appropriate balance of repulsion and attraction (19) Khan, A.; Fontell, K. Colloids Surf. 1984, 11, 401. (20) Kang, C.; Khan, A. J. Colloid Interface Sci. 1993, 156, 218. (21) Larsen, A. E.; Grier, D. G. Nature 1997, 385, 230. (22) Grier, D. G. Nature 1998, 393, 621. (23) Guldbrand, L.; Jo¨nsson, B.; Wennerstro¨em, H.; Linse, P. J. Chem. Phys. 1984, 80, 2221. (24) Kjellander, R.; Marcelja, S. Chem. Phys. Lett. 1984, 112, 49. (25) Linse, P.; Lobaskin, V. Phys. Rev. Lett. 1999, 83, 4208.
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among nanocolloids in a spheroid. This balance seems to be an important factor in determining how the spheroids monodisperse. Conclusions It was found that elemental gold nanocolloids threedimensionally self-assembled in a spheroid via the process of dialysis. The spheroid was a spherical aggregate of nanocolloids according to TEM observation, and the size distribution was no less monodispersing than the standard latex particles in water. Spheroid suspension did not show the characteristic plasmonic absorption peak. Attraction, which originates in counterions localized near nanocolloids,25 was suggested as a factor causing the nucleation and subsequent assembly of nanocolloids. Elimination of excess chemicals during dialysis, balancing repulsion and attraction among nanocolloids, appears to be the origin of nanocolloid self-assembly in monodispersed spheroids. Details of chemical composition in spheroids are currently uncertain. The self-assembly of nanocolloids produced a large number of spheroids dispersed in water. This is, in fact, a promising feature in applications of the spheroid. Acknowledgment. We thank Dr. Y. Okada and Dr. K. Yase for their assistance in TEM, Dr. R. Azumi for the FTIR spectroscopy (National Institute of Material and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 3058565, Japan), and Dr. Ikkai for assistance in DLS experiments (L′Ore´al Tsukuba Center). LA000244X
