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Spherical and Planar Gold(0) Nanoparticles with a Rigid Gold(I)-Anion or a Fluid Gold(0)-Acetone Surface† Guangtao Li,‡ Matthias Lauer,‡ Andrea Schulz,‡ Christoph Boettcher,§ Fengting Li,‡ and Ju¨rgen-Hinrich Fuhrhop*,‡ Freie Universita¨ t Berlin, FB Biologie, Chemie, Pharmazie, Institut fu¨ r Chemie/Organische Chemie, Takustrasse 3, D-14195 Berlin, Germany, and Forschungszentrum fu¨ r Elektronenmikroskopie, Fabeckstrasse 36a, D-14195 Berlin, Germany Received January 23, 2003 We propose here a new model for gold nanoparticles, which are thought to be solubilized by either a Au(I)-citrate or Au(I)-sulfide layer. In the case of pure Au(0) particles, stabilization of the colloidal solutions occurs by inverse micelles or by tight adsorption of solvent covers, in particular acetone. The main evidence for this model is as follows: Standard 20 nm citrate gold particles precipitate from the red colloidal solution in water in the form of a black metal powder after addition of sodium borohydride or as blue coagulate after acidification with HCl. Both processes are irreversible. If the same particles are carrying a lipoate coating, they dissolve in water at pH 11, coagulate at pH 3, and redissolve quantitatively at pH 11. This cycle has been repeated several times without any change of the 20 nm particles. No irreversible color change of the colloidal solution or fusion of the particles takes place upon acid-base treatment. Sodium borohydride, on the other hand, precipitates the lipoate-coated particles irreversibly. It is concluded that Au(I) salts make up the surface of the spherical citrate- or sulfide-covered particles and stabilize it by Au(I)-Au(I) binding interactions (aurophilic effect). If a large amount of acetone is added to the citrate gold solution, the electroneutral acetone replaces the citrate and the 20 nm particles dissociate to form 3 nm particles. They tend to fuse quickly to form Au(0) platelets with the same thickness of about 3 nm. The mixture of acetone-covered platelets and spheres remains in solution upon addition of either sodium borohydride or HCl. No reduction or charge neutralization occurs. It is concluded that acetone is adsorbed to Au(0) on the surface of the particles in solution. Dried material tightly adsorbs acetone in a η1-orientation (IR: 1635 cm-1) perpendicular to the gold surface where only the oxygen is bonded to the metal surface. Acetone-covered platelets were then also coated with lipoate, and an Au(I)SR layer was again formed. This was indicated again by irreversible precipitation of the blue colloid in aqueous solution by sodium borohydride and formation of a black metal powder. The Au(I)-lipoate-coated particles produced indefinitely stable blue colloidal solutions at pH 11, coagulated at pH values below 4, and redissolved at pH 11. Analogies between curved or planar gold nanoparticles and molecular assemblies of amphiphilic lipids are briefly discussed.
Introduction Soluble gold nanoparticles are usually made either by reduction of AuCl4- ions with sodium borohydride in biphasic toluene-water systems and in the presence of alkylthiolates (“Brust gold”)1 or by reduction with citrate at pH 9 in water (“citrate gold”).2 Both colloidal particles carry anions, citrate or sulfide, on their surface; both anions are soluble in the solvents surrounding the colloidal particles but do not separate from the gold surface. Brust gold forms brown solutions in organic solvents showing a broad plasmon absorption band over the whole visible range. It is made, for example, by reduction of tetraoctylammonium aurotetrachloride [N(C8H15)4+ AuCl4-] with sodium borohydride in toluene and in the presence of dodecanethiol (C12H25SH). Transmission electron micrographs show no sign of a C18 coating, and the crystalline areas stretch up to the edges of the particles.1 These gold particles have frequently been further functionalized with thiolates containing redox active dyes.3 X-ray photoelec* To whom correspondence should be addressed. † Dedicated to the memory of Professor David O’Brien. ‡ Freie Universita ¨ t Berlin. § Forschungszentrum fu ¨ r Elektronenmikroskopie. (1) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (2) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 55. (3) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27.
tron spectroscopy measurements of gold [Au(4f)] binding energies are always near to those of Au(0), whereas the S(2p) binding energies are close to those of RS-.1,4-6 We assume that Au(I)SR particles are at first formed and then further reduced by borohydride:
[AuISR]n + BH4- f Aux(SR)y Since the particles are generally collected in a toluene solution, one may visualize the final particles as inversed micelles with a nucleus consisting of gold.7 Variations of the Au/S ratio from 1:1 to 6:1 induced the formation of particles with diameters ranging from 1.5 to 20 nm.7 Digestive ripening in toluene-acetone gave uniform 3-6 nm particles.8 Both observations indicate a strong influence of the organic carrier system on the size of the gold nucleus, which may either grow or divide. It was, however, also found that rigid, photoactive thiol domains were (4) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17. (5) Zhong, C.-J.; Brush, R. C.; Anderegg, J.; Porter, M. D. Langmuir 1999, 15, 518. (6) Noh, J.; Ito, E.; Nakajima, K.; Kim, J.; Lee, H.; Hara, M. J. Phys. Chem. B 2002, 106, 7139. (7) Leff, D. V.; Ohara, P. C.; Heath, J. R.; Gelbart, W. M. J. Phys. Chem. 1995, 99, 7036. (8) Stoeva, S.; Klabunde, K. J.; Sorensen, C. M.; Dragieva, I. J. Am. Chem. Soc. 2002, 124, 2305.
10.1021/la0300277 CCC: $25.00 © 2003 American Chemical Society Published on Web 06/27/2003
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tightly bound to crystalline gold particles where the formation of a closed membrane structure was impossible.9,10 Aqueous solutions of gold particles obtained by citrate reduction of AuCl4- in boiling water at pH 9 or with subsequent treatment with lipoate or alkylsulfides11 have a different appearance. They produce a much sharper plasmon band at 520 nm, their typical diameter is 20 nm, their surface is smooth and spherical rather than crystalline, and they are stable in water.2,11-13 The energy associated with Au-S chemisorption is 28 kcal/mol (thiolate on gold)13 and has cautiously been traced back to the formation of Au(I)-thiolate on the surface.9 Protons have been proposed as oxidants for Au(0):
RSH + Aun(0) f RS- + Au+‚Aun(0) + (1/2)H2 Ab initio geometry optimization located the sulfur atom in hollow sites of gold [111] and similar environments.14 The most astonishing fact concerning both types of nanoparticles is the stability of the sulfide-coated gold particles in hot toluene, which should dissolve thiols as well as decompose polymeric Au(I)-thiolates,15 and the strong binding of Au(I) citrate to the surface of Au(0) spheres in water, which should also rapidly remove the surface citrate. Citrate ions stabilize solutions of 20 nm gold particles down to concentrations of about 10-4 M, which is more than 3 orders of magnitude below its saturation concentration. Dodecylthiol remains on the surface of 4 nm gold particles in boiling toluene,11 which is a perfect solvent for this molecule and presumably also for polymeric and unstable Au(I) salts.16 The special stability of the sphere’s surfaces can, however, be rationalized by assuming the well-established d10-d10 binding interaction between Au(I) ions (aurophilic effect) or a related stabilizing effect.14 In this paper, we show that anion-coated gold particles are not stable against sodium borohydride or HCl, which indicates the presence of Au(I) surfaces neutralized by organic anions. Pure Au(0) particles, on the other hand, are stabilized by adsorption of acetone on the Au(0) surface. It will be shown that such particles are stable against the attacks of either borohydride or HCl. Citrate replacement by acetone or removal of citrate by ion exchange then induces the rearrangement of 20 nm gold spheres to soluble platelets with a thickness of 3-5 nm. The properties of the soluble, acetone-covered platelets with some crystalline order are described for the first time. We also repeated an old experiment by Turkevich,12 who observed fusion of citrate gold particles after depletion of citrate by ion exchange. Experimental Section Preparation of Citrate-Gold and Acetone-Gold Particles. Citrate gold particles were prepared following the standard method2 with slight modification. HAuCl4 (40 mg) was dissolved (9) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389. (10) Hu, J.; Zhang, J.; Liu, F.; Kittredge, K.; Whitesell, J. K.; Fox, M. A. J. Am. Chem. Soc. 2001, 123, 1464. (11) Weisbecker, C. S.; Merritt, M. V.; Whitesides, G. M. Langmuir 1996, 12, 6317. (12) Enu¨stu¨n, B. V.; Turkevich, J. J. Am. Chem. Soc. 1963, 85, 331712. (13) Biggs, S.; Mulvaney, P.; Zukoski, C. F.; Grieser, F. J. Am. Chem. Soc. 194, 116, 9150. (14) (a) Jansen, M. Angew. Chem. 1987, 99, 1136. (b) Pyykko¨, P. Chem. Rev. 1997, 97, 597. (15) (a) Al-Sa’ady, A. K. H.; Moss, K.; McAuliffe, C. A.; Parish, R. V. J. Chem. Soc., Dalton Trans. 1984, 1609. (b) Wojnowski, W.; Becker, B. Z. Anorg. Allg. Chem. 1994, 620, 1417. (16) McNeillie, A.; Brown, D. H.; Smith, W. E.; Gibson, M.; Watson, L. J. Chem. Soc., Dalton Trans. 1980, 767.
Li et al. in 100 mL of Milli-Q water and heated to boiling. Water (5 mL) containing 120 mg of trisodium citrate was given, and the solution was refluxed for another 30 min. The resulting red sol was cooled slowly to room temperature. Instead of water, a 1:1 mixture of water-acetone was used as the reaction medium to prepare acetone-gold particles. Following the same procedure and conditions as in the preparation of citrate gold, the blue acetonegold sol was made. Self-Assembling of Lipoic Acid Monolayers on CitrateGold and Acetone-Gold Particles. A 10 mL portion of the above citrate gold sol was diluted with acetone to 20 mL, and 1 mL of acetone containing 20 mg of lipoic acid was quickly added under stirring. The resultant reaction mixture was stirred overnight. The monolayer-coated particles were separated from the excess lipoic disulfide by repeated precipitation with HCl (pH 2), centrifugation, and redispersion in NaOH aqueous solution at pH 11. The same procedure was used to coat acetonegold platelets. Titration of Gold Particles with HCl. UV/vis was employed to monitor the aggregation process of particles by titration using HCl (5 mmol). Citrate gold particles exhibited the typical plasmon absorption at 520 nm. After addition of 10 µL of HCl (5 mmol), the absorption peak continuously shifted to the long-wavelength region. The solution became turbid, and finally a blue-grey precipitate was formed. This could not be redispersed with NaOH in aqueous solution (pH 11). Spherical gold particles coated with a lipoate monolayer showed the same visual changes as citrate gold, but the resulting precipitate was completely redissolved after addition of NaOH in aqueous solution (pH 11). Acetonedissolved gold did not produce any precipitates after addition of HCl, nor did the blue color of the solution change. In the case of lipoate-coated gold platelets, however, the reversible precipitation occurred in the same manner as with the corresponding spheres. Reduction of Gold Particles with NaBH4. To 5 mL of the above citrate gold solution, 20 mg of NaBH4 was added. The color changed immediately from red to blue, and a black powder precipitated. The same procedure was applied to the citrategold membrane and to acetone-gold platelets both coated with a lipoate membrane. The same black precipitate always appeared within a few seconds. In the case of acetone-covered particles, however, the addition of sodium borohydride produced neither a change in color nor any precipitate. Ion-Depleted Gold Colloids on Various Substrates. HAuCL4 (3.38 mg, 0.1 mmol) was dissolved in 50 mL of Milli-Q water, and 3.86 mg (1.3 mmol) of sodium citrate dihydrate was added. The yellow solution was refluxed for 20 min and then treated with 3 g of a mixed ion-exchange resin containing strongly acidic and basic ion exchangers (Serdolit MB-1, Serva; a mixed bed ion exchanger, -CH2-NH(CH3)2 plus -SO3- in a ratio of 1:1.5, both on polystyrene). The resin was removed by decantation and filtration. The substrates were prepared as follows. Mica (Plano, Wetzlar/ Germany) and highly oriented pyrolitic graphite (HOPG; Plano, Wetzlar/Germany) were freshly cleaved prior to use. A commercial gold-coated platelet (Arrandee/Schroer, Germany) was annealed in a hydrogen flame for about 6 min at yellow heat. It was cooled in Milli-Q water and plunged into a solution of mercaptoethylamine (Sigma/Aldrich) in ethanol (5 mM). The mica platelets were rinsed with a mixture of ethanol and Milli-Q water (1:1), dried under a stream of nitrogen, and then treated with a 50 µL droplet of the freshly prepared colloidal solution described above. Excess fluid was blotted off after 45 s. Scanning Force Microscopy (SFM). Images were recorded using a MultiMode scanning probe microscope (Digital Instruments, Inc., Santa Barbara, CA) that was operated in the tapping mode. Nanosensors (NCH-W) etched silicon cantilevers were used with a typical resonance frequency in the range of 280-324 kHz and a spring constant in the range of 29-48 N/m. All samples were measured at room temperature and in air environment. The scanning rate was usually 1.0 Hz. For areas above 25 µm2, the scan rate was decreased to 0.2 Hz. SFM height measurements were based on the cross-sectional profiles. Transmission Electron Microscopy (TEM). Droplets of freshly prepared solutions were placed on hydrophilized (BALTEC MED 020 plasma glow discharge for 30 s at 8 W) carbon grids, blotted, and subsequently frozen in liquid nitrogen. Freeze-
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drying of the samples was performed over 6 h at a sample temperature of -100 °C using a BALTEC MED 020 device. The prior transfer of the frozen sample was accomplished with a Gatan Cryotransfersystem (model 626). Dried samples were directly examined with a Tecnai F20 FEG transmission electron microscope at suitable magnifications. Calibration of focal length and magnification was accomplished using gold calibration standards (Plano, Wetzlar/Germany). The size of individual particles in three dimensions is accessible by goniometer tilting experiments and yields almost perfect spherical globules. Intensities in the images, which are within the linear sensitivity range of the photographic emulsion, can thus be calibrated with respect to the mass thickness. Line scan intensity profiles of digitized micrographs have been calculated in the context of IMAGIC 5 software (Image Science GmbH, Berlin, Germany) and were used to retrieve the thickness of the aggregates in good approximation.
Results Coating of Citrate-Gold Nanoparticles. Citrate gold particles were produced as usual by reduction of 10-3 M solutions of AuCl4- in boiling water at pH 8-9 with a 4-fold excess of trisodium citrate.2 Transmission electron micrographs showed loose clusters of gold spheres with a uniform diameter of 20 ( 2 nm (Figure 1A). Such particles remained dispersed in water over periods of months. If, however, the pH was lowered to pH 2-3 (HCl), rapid coagulation was observed and the precipitate only partly redissolved upon going back to pH 9-11 by adding sodium hydroxide. Addition of sodium borohydride to the red citrate gold solution at pH 9 gave black precipitates of gold, which could not be redissolved. HCl presumably leads to an irreversible dissociation of Au(I)-citrate bonds, and sodium borohydride is thought to reduce Au(I) on the surface of particles. Self-assembly of lipoic acid monolayers17 on the relatively large citrate gold particles was tried in various solvents. An acetone-water mixture (1:4) was found to be most suitable to keep the gold particles as well as lipoic acid disulfide in solution. The short lipoate disulfide with an S-S bridge at one terminal reacted with gold surfaces but did not react with acetone to form thioketals. Separation of the particles from the reagent occurred by repeated precipitation with HCl (pH ) 2), centrifugation, and redispersion in sodium hydroxide solution at pH 11. TEM pictures characterize their typical aggregation behaviors. Citrate gold appears to form loose clusters (Figure 1A); lipoate-coated particles are separated at pH 11 (Figure 1B). At pH 3, tight clusters are formed, which show the expected short distance between neighboring particles of less than 1 nm. This minimal distance is caused by the lipoate coatings in densely packed arrays (Figure 1C). Figure 2 reproduces the spectroscopic changes observed upon titration with HCl (5 mM) and immediate backtitration with sodium hydroxide (5 mM). At pH 11, the lipoate-coated gold particles showed the same 520 nm plasmon absorption band as citrate gold at pH 9. Upon titration with HCl, the color of the sol changed rapidly from red to blue (Figure 2A). Shifts and shapes of the absorption bands were essentially the same as observed for coagulating gold particles coated with ω-thiol fatty acids.11 After several minutes, the blue colloidal solution became turbid and gray precipitates became detectable by the naked eye. Upon addition of sodium hydroxide, however, these precipitates redissolved completely at pH 11. The 520 nm band showed no change (Figure 2B), neither in width nor height after several cycles (Figure 2C). The Au(I)-sulfide bond is thus much more stable to
Figure 1. Transmission electron micrographs of (A) the citrate gold particles prepared at pH 9, (B) the same particles after coating with lipoate at pH 11, and (C) the coagulate after addition of HCl at pH 3.
(17) Berchmans, S.; Thomas, P. J.; Rao, C. N. R. J. Phys. Chem. B 2002, 106, 4647.
acid treatment than that of the citrate. To the best of our knowledge, no other totally reversible coagulation of
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Figure 2. (A) Changes of the visible spectrum of an aqueous solution (NaOH, pH 11) of lipoate-coated gold particles upon titration with HCl. (B) Absorption band of the same solution at pH 11 before (a) and after titration with HCl and backtitration with NaOH (b); spectrum of the supernatant at pH 3, 10 min after the end of the acid titration (c). (C) Completely reversible aggregation by pH.
medium-sized gold colloids is known. Treatment with sodium borohydride again led to a quantitative disruption of the Au(I)-anion bond, and the metal precipitated as black powder. Acetone-Gold Platelets. During the self-assembly experiments, we observed that the simple dilution of aqueous citrate gold solutions with acetone18 caused a color change from red to blue within 20 min without any addition of lipoate or acid. At first, it was supposed that coagulation was caused by the lowering of the dielectric constant of the medium and was presumably responsible for the color change. Transmission electron micrographs, however, did not show coagulates comparable to those in Figure 1C. Instead we observed highly curved, nontransparent platelets (Figure 3A) together with flat and (18) (a) Davies, A. E. J. Phys. Chem. 1929, 33, 274. (b) Sparks, S. C.; Szabo, A.; Szulczewski, G. J.; Junker, K.; White, J. M. J. Phys. Chem. B 1997, 101, 8315.
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crystalline areas together with small spherical particles (Figure 3B,C). Freeze-drying of freshly prepared solutions on hydrophilized carbon grids allowed a smooth deposition of the platelets for TEM. The platelets appear in a networklike distribution and are accompanied by two types of particles differing in size, one in the diameter range of 3-5 nm and the other in the range of 30-50 nm (Figure 3A,B). For the thin platelets, a thickness of 3-5 nm was estimated from intensity line scans which were calculated from digitized images. These platelets originate probably from a breakdown of the 20 nm particles to small particles and subsequent fusion (see Discussion). High-resolution TEM imaging clearly resolved atomic layer lines in platelets as well as in all particles. Gold crystal standards (Plano, Wetzlar/Germany) for the calibration of magnifications were used and allowed evaluation of the layer lines of the different acetone-gold aggregates in digitized and Fourier transformed (FFT) images quantitatively. The determined values could all be attributed to typical Miller indices of gold crystal planes [111, 110, 100, and higher orders] in different spatial orientation. Diffraction patterns of individual areas of platelets as well as of particles reveal the same statistically varying orientation of the specimen. It seems likely that the thin regions of the platelets originate from the fusion of the 3-5 nm particles, whereas the thicker regions originate from fusion and thinningout of the 20 nm particles. The 20 nm gold spheres were still omnipresent and appeared with high dark contrast (not shown). Attempts to perform thickness measurements by SFM failed because of the platelets’ high flexibility which prevented a sound adsorption on graphite (HOPG) or mica substrates. Instead, ill-defined lumps with a thickness of about 100 nm were found. Addition of sodium borohydride to the acetone-covered particles produced neither any change in color nor any precipitate. We conclude that the surface of the gold particles was made exclusively of Au(0). Control experiments with the Au(I) coated particles discussed above showed that addition of acetone had no measurable protective effect for the Au(I) particles. To improve the yield of the thin platelets, we replaced the aqueous citrate solution for the aurate reduction by a 1:1 water/acetone solution keeping sodium citrate and tetrachloro-aurate concentrations constant. Blue colloidal solutions were obtained, which were stable for at least a month. Slow sedimentation then took place and led to black slurries and a faintly blue supernatant. Simple hand shaking, however, reconstituted the blue solution, and the absorption spectra were identical to those of fresh solutions. Transmission electron micrographs of dried samples showed mainly large coagulates with foamy edges together with a large number of 3-5 nm particles (Figure 4A). Micrometer scale thin platelets of irregular shape showing crystalline patches (Figure 4B,C) and isolated 20-50 nm particles occurred in large numbers. Addition of sodium borohydride had again no effect on the acetonecovered Au(0) particles. The thin platelets were then trapped by coating them in acetone-water mixtures with lipoate disulfide. TEM pictures now indicated a large population of large crystalline thin sheets (Figure 5A,B), and characterization of the unfolded platelets by scanning force microscopy, for example, atomic force microscopy (AFM) in the tapping mode, was now possible. A uniform thickness of 3-5 nm was found (Figure 5C). This is similar to the particle diameter of the isolated particles observed on electron micrographs.
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Figure 3. (A) Thick gold platelets as obtained by the fusion of citrate gold particles after addition of acetone. (B) Thin gold platelets as observed in the same preparation. The arrows indicate crystalline areas. The voids on the bottom (V) contain 3 nm particles. (C) Magnification of a typical crystalline area [100]. (D) Electron diffraction of section given in (C).
Reduction with Sodium Borohydride. Addition of sodium borohydride to the lipoate-coated particles in the presence of an excess of acetone led immediately to the precipitation of black gold metal powder. The acetonecovered platelets and coagulates without lipoate are thus essentially electroneutral and kept in solution only by solvation of the carbonyl group. Infrared spectra of centrifuged and air-dried acetone-gold platelets showed a band at 1635 cm-1, which is typical for η1-acetone perpendicular to the gold surface where only the oxygen is bonded to the metal surface.19 Addition of HCl to blue solutions of these platelets caused neither a color change nor any precipitation, verifying the assumption of an essentially electroneutral surface (Figure 6A). The blue solution of the lipoate-coated platelets, on the other hand, decolorized within less than an hour upon addition of HCl, and a black precipitate was formed. At pH 11, this precipitate dissolved again to form a blue solution (Figure 6B). Removal of Counterions. In a final set of experiments, we studied the fusion of electroneutral gold particles in the absence of acetone or any other organic solvent. For (19) Sim, W. S.; King, D. A. J. Phys. Chem. 1996, 100, 14794.
this purpose, the citrate gold particles were mixed in water with Serdolit MB-1, a mixed bed ion-exchange polymer, which removed sodium, Au(I), and citrate ions from the solution. In bulk aqueous solution, ion depletion leads to a limited fusion of citrate gold particles within several days.12 When these ion-depleted solutions were deposited on a thioethanolamine-coated gold surface, no change of the 20 nm spheres took place. They were chemically immobilized by the amino groups and appeared as separated dots (Figure 7A). On a graphite surface (HOPG), the 20 nm particles remained mobile on the wet surface. They flattened spontaneously and fused to form irregular curved structures with a uniform thickness of about 3 nm and typical diameters of 100-200 nm (Figure 7B). On mica, no flattening and fusion was observed, but the 20 nm particles aggregated to form large dendritic assemblies with a typical height of 30 nm, which is about 50% thicker than the diameter of single particles (Figure 7C). Discussion Organic Sulfide Coatings of Gold Nanoparticles. The experiments with the ion-depleted citrate gold particles demonstrate that the coagulation and fusion of naked Au(0) spheres strongly depend on interactions with
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the substrate. An organic cystamine surface does not induce a change of the shape of the spheres. They are fixated in the position where they contact the surface at first. Mica slightly flattens the particles and allows free migration leading to dendritic aggregates with a height of two to three particles and a strong side-on flattening of interacting gold surfaces. Crystalline graphite (HOPG) leads to a drastic flattening of the spherical particles from 20 to 3 nm and fusion. Naked gold thus behaves like a wax, which melts on graphite, is slightly modified on watercovered mica, and retains its shape on waxlike surfaces. This behavior is comparable to that of noncovalent, micellar fibers.20 Evaporated 2 nm gold particles on mica fused upon addition of HCl.21 To summarize: gold nanoparticles without an anion coating are very soft and tend to fuse. Organic coatings, in particular citrate ions at pH 9 and alkylsulfides at pH values between 3 and 10, on the other hand, strongly stabilize the gold particles. Experimental details and possible implications are as follows: (i) Neither graphite nor gold or mica changes the spherical appearance significantly, although most of its material is still Au(0). These findings suggest that Au(I)-Au(I) interactions on the surface prevent the coalescence and/or crystallization of gold(0) within nanoparticles. (ii) Citrate gold is long-lived in aqueous solutions, where citrate anions should diffuse away from neutral gold surfaces. Fusion occurs only if citrate is dialyzed away. One may assume that citrate acts as a water-soluble counterion for Au(I), which is tightly bound to gold colloids in water, because at least two of the carboxyl groups can combine simultaneously to the Au(I) surface. Crystal structures of various citrates usually show chelate-type ligand fields around metal ions with some of the carboxyl groups still protonated.22,23 Such gold(I) complexes allow for hydrogen-bonded networks on the polycationic gold surface. Crystal structures of gold citrates are not known, presumably because they disintegrate to form Au(0) and oxidation products of citric acid. On the surface of the gold colloid, however, the positive charge may be distributed over several gold atoms and the oxidation power may be very low. (iii) The fact that sodium borohydride decomposes citrate gold and its sulfide derivatives immediately indicates that gold(I) is present and necessary to stabilize the spherical nanoparticles. (iv) Sulfide coatings are extremely stable in water as well as in toluene, even if the coating material is wellsoluble in the bulk solvent. If, however, the gold surface is readily accessible in small, highly curved particles, they are also destroyed by sodium borohydride reduction. (v) The aggregation of lipoate-coated gold particles is fully reversible upon acid-base treatment. No fusion is observed, although the particles approach each other in the coagulates to distances of less than a nanometer and although lipoate is water-soluble. HCl also precipitates citrate gold, although citric acid is water-soluble. Decomposition of the Au(I) salts maybe responsible for this effect. Electroneutral acetone-covered gold particles are insensitive to pH changes.
Figure 4. Transmission electron micrographs of typical gold platelets as obtained by citrate reduction of AuCl4- in acetone/ water 1:1. (A) Thick platelets; the inscribed circles correspond to 20 nm, the typical diameter of a citrate gold particle (see Figure 1A). (B) Thin platelets with 20 and 3 nm particles and (C) magnification of the marked area in (B).
(20) Messerschmidt, C.; Draeger, C.; Schulz, A.; Rabe, J. P.; Fuhrhop, J.-H. Langmuir 2001, 17, 3526. (21) Thompson, D. W.; Collins, I. R. J. Colloid Interface Sci. 1994, 163, 347. (22) O’Brien, P.; Salacinski, H.; Motevalli, M. J. Am. Chem. Soc. 1997, 119, 12695. (23) Matzapetakis, M.; Kourgiantakis, M.; Dakanali, M.; Raptopoulou, C. P.; Terzis, A.; Lakatos, A.; Kiss, T.; Banyai, I.; Iordanidis, L.; Mavromoustakos, T.; Salifoglou, A. Inorg. Chem. 2001, 40, 1734.
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Figure 5. (A) Transmission electron micrograph of thin lipoate-coated acetone-gold platelets. Crystalline arrays dominate. (B) Magnification of the inscribed area in (A). (C) AFM image of the same type of lipoate-coated, acetone-gold platelets. The thickness is 3-5 nm.
(vi) NaBH4 only destroys gold particles with anionic coatings, but not those with an electroneutral acetone layer as a solubilizing cover. (vii) Addition of NaCN to sulfide-coated 20 nm particles does not lead to the destruction of the particles. The sulfide prevents corrosion of the large particles; the coating must form a closed, impermeable monolayer, which is tightly bound to the gold surface.11,24
(viii) Physical measurements indicate “chemical binding” of the sulfide, although Au(I) could not be demonstrated conclusively.4-6 (ix) Tetrathiafulvalene is oxidized to the +1 state by citrate gold in the presence of pyridine.25 (24) Li, G.; Fuhrhop, J.-H. Langmuir 2002, 18, 7740. (25) Sandroff, C. J.; Herschbach, D. R. Langmuir 1985, 1, 131.
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Figure 6. Visible spectra of acetone-covered gold platelets (A) at (a) pH 11 and (b) pH 3 and (B) after coating with lipoate at pH 11 (a) before acidification and (b) after precipitation with HCl and redissolution with NaOH. (c) Visible spectrum of the supernatant after (reversible) precipitation with HCl.
These findings clearly demand the action of an “aurophilic effect” for particles with anionic coatings. Without such an effect, the observed stability of surface mono- and multiple layers in bulk solvents for the organic anions is difficult to understand. It is feasible that the positive charge on gold is distributed over two or more gold atoms.9,26 The nature of chemisorption of anions on gold colloidal surfaces has been under considerable debate for decades now, but no agreement on a single surface structure of the large variety of gold nanoparticles has been reached. The surface chemistry observed on the macroscopic level, however, distinguishes perfectly well between charged Au(I) surfaces and noncharged Au(0) surfaces as well as between soluble and precipitating particles. If forces in the order of 5-10 kcal/mol can be assumed for Au(I) counterions of the anionic coatings, and if these binding forces are drastically diminished upon reduction to Au(0), then all but one of the experimental findings can be rationalized. In the case of the 1-3 nm particles introduced by Brust, one must assume that the alkylthiolates form micelles or inverse micelles with S-, SH, or S-S headgroups, possibly stabilized by S--SH and charge-transfer bonds, and entrap the tiny gold clusters. Au(I) surfaces do not survive the attack of borohydride ions. The reduction of surface Au(I) by BH4- ions usually leads to precipitation of large gold particles in the form of black powders. Treatment with acetone, on the other hand, washes away Au(I)sulfide and -citrate coatings and then disintegrates the large particles; 3-5 nm Au(0) nanoparticles are formed, (26) Ehlich, H.; Schier, A.; Schmidbaur, H. Inorg. Chem. 2002, 41, 3721.
Figure 7. AFM pictures of citrate-depleted gold spheres in water as deposited on different substrates: (a) on cystaminecoated gold, showing isolated spheres, (b) on graphite (HOPG), where fusion takes place, and (c) on mica, where dendritic, thick plates are formed by fusion.
which have a strong tendency to fuse to acetone-covered platelets showing some crystallinity (Figures 3-5). Literature reports on the action of acetone on gold include the formation of nonspherical crystallites from AuCl4-,1
Spherical and Planar Gold(0) Nanoparticles
Langmuir, Vol. 19, No. 16, 2003 6491
Figure 8. Model of Au(0) spheres coated with Au(I) or Au(0) surface layers in the presence of organic thiols (∼), citrate (-), and acetone (formula given).
the uptake of vaporized gold,27 and the extraction of gold from charcoal by acetone.28 Acetone-Gold. The dissolution of gold by acetone certainly starts with the formation of the η1-adsorbate, which remains stable for a long time after the centrifugation and air-drying of the colloidal gold. This adsorbate readily redissolves in water/acetone (Figure 8). The basic keto oxygen atom may take up protons from water as well as Au(I) ions from the colloid’s surface, or it may also add to the R-carbon atom as an enolate. Such structures are known from gold-ketone adducts29 and may contain acetonealcohol and other condensation products of acetone, which may be responsible for the formation of viscous films, which we always found on the surface of aged acetone-gold platelet probes by AFM. The reduction of AuCl4- by acetone takes many hours and presumably uses the enolate of acetone as the reductant. Water/acetonesoluble platelets containing large crystalline areas are obtained by the cooperation of citrate reduction and acetone-driven dissolution and crystallization. The fact that the microcrystallites do not precipitate irreversibly in acetone/water points to an enormous stability of the η1-acetone, which is comparable to the stability of Au(I)-citrate in water. Inversion of the dipole by treatment with HCl also does not destroy it. The model given below (Figure 8) summarizes our findings and conclusions. Comparison of Gold Nanoparticles with Lipid Vesicles. Similar rearrangements take place in aggregates of amphiphilic lipids, and a comparison may help to bring the phenomena observed with the gold nanoparticles in a common perspective. Micelles have a diameter of 3-5 nm, release monomers, and rapidly convert to planar bilayer structures, if the headgroup loses its hydration sphere. Sodium myristate micelles, its halfprotonated fibers and fully protonated crystals,30 and the conversion of stearate micelles to stearic acid vesicles provide examples.31 Micelles with negatively charged surfaces may be seen as analogues of Au(I)-citrate- or -sulfide-coated Au(0) particles. They fuse if the surface charge is removed, and they are reformed if solvents (27) Tian, F.; Klabunde, K. J. New J. Chem. 1998, 22, 1275. (28) Espiell, F.; Roca, A.; Cruells, M.; Nunez, C. Hydrometallurgy 1988, 19, 321. (29) Bock, B.; Flatau, K.; Junge, H.; Kuhr, M.; Musso, H. Angew. Chem., Int. Ed. Engl. 1971, 10, 225. (30) Tra¨ger, O.; Sowade, S.; Boettcher, C.; Fuhrhop, J.-H. J. Am. Chem. Soc. 1997, 119, 9120. (31) Hargreaves, W. R.; Deamer, D. W. Biochemistry 1978, 17, 3759.
disintegrate the bilayer. Gold citrate and -sulfide behave like solvated headgroups; the gold atoms in the center play the role of a hydrophobic core. Acetone also converts curved vesicular lecithin double layers to interdigitated planar bilayers,32,33 because the hydrophilic headgroups are partially dehydrated. This leads to less repulsive hydration forces on the curved surface, and the hydrophobic effect within the alkane core also disappears. Acetone not only dehydrates the gold surface and removes the citrate ions, but it also penetrates between gold atoms and diminishes the binding forces between them. It even may form soluble and unstable enolates with Au(I). The gold platelets separated by acetone adlayers are finally comparable to Myelin figures of lipids, where hydration keeps the bilayers apart. A similar comparison between gold(I) compounds and amphiphiles has been drawn recently in connection with the crystallization behavior of Au(I) compounds,34 which implied the aurophilic interaction as an analogue of the hydrogen bond. Hydrogen bonds also stabilize the surface of micelles, if the headgroup is large enough to cover the surface of the sphere.35 The mechanical properties of the liquid gold and alkyl chain cores are thus comparable. The headgroups of the gold particles, however, can not only be modified in the same way as those of micelles and vesicles but can also be totally removed with acetone. The Au(I) surface can thus be changed to Au(0) without precipitation. In a second self-assembly step, the first citrate or sulfide coating may then be replaced by another one. Acknowledgment. Financial support by the Deutsche Forschungsgemeinschaft (SFB 348 “Mesoscopic Systems”), the European TMR research network “Carbohydrate Recognition”, the Fonds der Deutschen Chemischen Industrie, and the FNK of the Free University is gratefully acknowledged. We also thank the DFG for generously providing the Tecnai F20 transmission electron microscope and Professor H. Mo¨hwald, Golm, for helpful discussions. LA0300277 (32) Kinoshita, K.; Asano, T.; Yamazaki, M. Chem. Phys. Lipids 1997, 85, 53. (33) Kinoshita, K.; Yamazaki, M. Biochim. Biophys. Acta 1996, 1284, 233. (34) Codina, A.; Ferna´ndez, E. J.; Jones, P. G.; Laguna, A.; Lo´pezde-Luzuriaga, J. M.; Monge, M.; Olmos, M. E.; Pe´rez, J.; Rodrı´guez, M. A. J. Am. Chem. Soc. 2002, 124, 6781. (35) Gouzy, M.-F.; Lauer, M.; Gonzaga, F.; Schulz, A.; Fuhrhop, J.H. Isolable Micelles with a Fluid Core Made of Kanamycin A 6′Stearoylamide. Langmuir 2002, 18, 10091.
