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Structure of a Thiol Monolayer-Protected Gold Nanoparticle at 1.1 Å Resolution

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Science  19 Oct 2007:
Vol. 318, Issue 5849, pp. 430-433
DOI: 10.1126/science.1148624

Abstract

Structural information on nanometer-sized gold particles has been limited, due in part to the problem of preparing homogeneous material. Here we report the crystallization and x-ray structure determination of a p-mercaptobenzoic acid (p-MBA)–protected gold nanoparticle, which comprises 102 gold atoms and 44 p-MBAs. The central gold atoms are packed in a Marks decahedron, surrounded by additional layers of gold atoms in unanticipated geometries. The p-MBAs interact not only with the gold but also with one another, forming a rigid surface layer. The particles are chiral, with the two enantiomers alternating in the crystal lattice. The discrete nature of the particle may be explained by the closing of a 58-electron shell.

Nanometer-size metal particles are of fundamental interest for their chemical and quantum electronic properties and of practical interest for many potential applications (1, 2). With the development of facile routes of synthesis (3), gold nanoparticles coated with surface thiol layers have been studied in most detail. The particles are typically heterogeneous as synthesized, and though their size distribution may be narrowed by fractionation or other means (49), no atomically monodisperse preparation has been reported, and no atomic structure has been obtained. Electron microscopy (EM) (10, 11), powder x-ray diffraction (PXRD) (12), and theoretical studies have led to the idea of Marks decahedral (MD) and truncated octahedral geometries of the metal core, with crystalline packing and {111} faces (13). According to this idea, discrete core sizes represent “magic numbers” of gold atoms, arising from closed geometric shells (14). Alternatives of amorphous (15), molten, or quasimolten (16) cores have also been proposed. The structure of the surface thiol layer is similarly obscure. The nature of the gold-sulfur interaction (17), the fate of the sulfhydryl proton (18), and the conformation of the organic moiety all remain to be determined. The thiols are exchangeable and presumed to be mobile, further impeding structural analysis (19, 20).

Through systematic variation of solution conditions for gold nanoparticle synthesis, we have obtained particles sufficiently uniform in size for the growth of large single crystals, opening the way to x-ray structure determination. We report here on x-ray analysis of a nanoparticle whose gold core and surface thiol structures differ markedly from what had been anticipated. The gold particles were coated with p-MBA and crystallized from a solution containing 40% methanol, 300 mM sodium chloride, and 100 mM sodium acetate, at pH 2.5 (21). The crystals were in the centrosymmetric space group C2/c, so diffraction showed no anomalous differences. We obtained initial phases using dispersive differences between data collected at the Au LIII edge and a low-energy remote, giving an electron density map that revealed 102 gold atoms and 44 p-MBAs. All electron density was accounted for by the structure (except for solvent water), so the clusters were entirely homogeneous and the numbers of gold atoms and p-MBAs were precise (Fig. 1A). The structure was refined at a resolution of 1.15 Å to Rwork and Rfree of 8.8 and 9.5%, respectively. The particles proved to be chiral, with half of an enantiomer in the asymmetric unit of the crystal (Fig. 1B and table S1).

Fig. 1.

X-ray crystal structure determination of the Au102(p-MBA)44 nanoparticle. (A) Electron density map(redmesh) and atomic structure (gold atoms depicted as yellow spheres, and p-MBA shown as framework and with small spheres [sulfur in cyan, carbon in gray, and oxygen in red]). (B) View down the cluster axis of the two enantiomeric particles. Color scheme as in (A), except only sulfur atoms of p-MBA are shown.

Most gold-gold distances in the core lie in the range 2.8 to 3.1 Å (figs. S1 and S2). The core may be described as a 49-atom MD (2,1,2) with four atoms on the central axis, two 20-atom caps with C5 symmetry on opposite poles (expanding to 89 the number of gold atoms with fivefold rotational symmetry), and a 13-atom band with no apparent symmetry on the equator (Fig. 2A). Alternatively, the MD may be described as five twinned face-centered cubic (fcc) or hexagonal close-packed (hcp) crystallites (Fig. 2, B to D) (22). All 102 gold atoms are found in environments with 12 nearest neighbors—fcc, hcp, icosahedral, or truncated decahedral—except that atoms near the surface lack from 1 to 10 neighbors. The 13 equatorial atoms occupy two different environments, which deviate slightly from local hcp or truncated decahedral (figs. S3 and S4). It is the number and geometry of the equatorial atoms that impart chirality to the core, and the deviations from local symmetry may reflect the interaction of the equatorial atoms with the p-MBA monolayer.

Fig. 2.

Packing of gold atoms in the nanoparticle. (A) MD (2,1,2) in yellow, two 20-atom “caps” at the poles in green, and the 13-atom equatorial band in blue. (B) View of the MD with the axis horizontal, showing the fcc planes (α, β, γ, and α′). When viewed in the direction of the arrow, the fourth plane (α′) overlaps with the first (α), conforming to the definition of fcc planes. (C) View of the MD with the axis perpendicular to the page, showing how it may be regarded as five twinned crystals. (D) Same view as in (C) but showing hcp crystallites. When viewed in the direction of the arrow, the third plane (δ′) overlaps with the first(δ), conforming to the definition of hcp planes.

Gold atoms up to 5.5 Å from the center of the particle do not contact sulfur, those in a shell of radius 6.0 to 6.3 Å bind one sulfur, and those in a shell of radius 7.5 to 8.0 Å bind two sulfurs (Fig. 3A and fig. S5). All sulfur atoms lie in a shell of radius 8.3 ± 0.4 Å and bind in a bridge conformation (23) to two gold atoms; atleast one of the gold atoms binds two sulfurs, forming a “staple” motif (Fig. 3, B and C). The gold-sulfur distance ranges from 2.2 to 2.6 Å (fig. S6). Gold-sulfur-gold angles are 80° to 115°, and sulfur-gold-sulfur angles are 155° to 175° (fig. S7). If the surface is taken as all gold atoms interacting with sulfur, then the coverage by p-MBA (thiol:gold ratio) is 70%, which is much higher than the values of 31 and 33% for benzenethiol (24) and alkanethiols (17) on Au(111) surfaces, reflecting the curvature of the nanoparticle surface.

Fig. 3.

Sulfur-gold interactions in the surface of the nanoparticle. (A) Successive shells of gold atoms interacting with zero (yellow), one (blue), or two (magenta) sulfur atoms. Sulfur atoms are cyan. (B) Example of two p-MBAs interacting with three gold atoms in a bridge conformation, here termed a staple motif. Gold atoms are yellow, sulfur atoms are cyan, oxygen atoms are red, and carbon atoms are gray. (C) Distribution of staple motifs in the surface of the nanoparticle. Staple motifs are depicted symbolically, with gold in yellow and sulfur in cyan. Only the gold atoms on the axis of the MD are shown (in red).

The thiol monolayer is stabilized not only by gold-sulfur bonding but also by interactions between p-MBA molecules. These interactions are of three types: phenyl rings stacked on one another with the centers offset by the ring radius (Fig. 4A), phenyl rings interacting at right angles (T-stacking) (Fig. 4B), and sulfur interacting with a phenyl ring (Fig. 4C). Eighteen of the sulfur atoms are located over the face of a phenyl ring at a distance of about 3.55 ± 0.25 Å, similar to sulfur atoms engaged in aromatic-thiol π hydrogen bonding in proteins (25). Almost all sulfur atoms are also engaged in lone pair bonding to a phenyl edge (25). Most p-MBAs are linked through chains of such interactions extending from one pole of the nanoparticle to the other (Fig. 4D). This ordering of p-MBAs exemplifies the “self-assembly” of a thiol monolayer on a gold surface (26).

Fig. 4.

p-MBA/p-MBA interactions in the surface of the nanoparticle. Color scheme as in Fig. 3B. (A) Phenyl rings stacked with faces parallel. (B) Phenyl rings stacked edge-to-face. (C) Phenyl ring interacting with sulfur. (D) Chains of interacting p-MBAs, extending across the surface of the nanoparticle, indicated by a different color for each chain.

The chains of p-MBA interactions extending across the nanoparticle establish the chirality that is apparent from the view of the nanoparticle down the MD axis (Fig. 1B). Most sulfur atoms, bonded to gold atoms in two different shells and to a phenyl ring, are also chiral centers. One enantiomer has 22 sulfur centers with R configuration, 18 with S configuration, and 2 with no readily assigned chirality, because they are bonded to two gold atoms in the same shell.

The pairing of enantiomeric particles in the crystal demonstrates a surface complementarity of the particles (fig. S8). Interparticle interactions in the crystal thus reflect the chirality of the surface thiol layer. These interactions are of several types. Hydrogen bonding between carboxylic acids occurs at many crystal contacts (fig. S9), in some cases mediated by water molecules (27). Such interactions are frequent near the equator, where the phenyl rings extend outward from the particle surface. The p-MBAs from different nanoparticles interdigitate through phenyl-phenyl interactions, especially at the MD poles (fig. S10). Such interactions can explain the common finding that distances between neighboring clusters in two-dimensional gold particle arrays are less than twice the length of the fully extended thiol (28, 29).

The very existence of a discrete Au102(p-MBA)44 nanoparticle is a notable finding from this work. Discrete sizes have been explained in the past by geometrical or electronic shell closing. The arrangement of gold atoms, with polar caps and an equatorial band, argues against geometrical shell closing. If, however, each gold atom (5d106s1) contributes one valence electron, and 44 are engaged in bonding to sulfur, then 58 electrons remain, corresponding to a well-known filled shell. Indeed, a naked cluster in the gas phase containing 58 gold atoms shows exceptional stability (3032).

There are several connections of the Au102 nanoparticle structure with previous work. First, structures of small gold, silver, and platinum clusters, and of large platinum-palladium clusters, include fivefold symmetry elements and, in one case, also include thiols bridging between pairs of gold atoms (3336). Second, EM, PXRD, and theoretical studies of large gold clusters have given results that are consistent with a MD (1012). Third, theoretical studies have raised the possibility of distinct gold-sulfur units capping a central gold core (37). Fourth, the fcc packing in the core, with a gold-gold distance of 2.8 to 3.1 Å, corresponds with the fcc packing in bulk metallic gold, with a gold-gold distance of 2.9 Å. Fifth, the staple motif, containing alternating gold and sulfur atoms (Fig. 3C), resembles the gold-thiol polymers believed to represent intermediates in the process of nanoparticle formation (38). Finally, circular dichroism measurements on gold nanoparticle preparations have shown chiro-optical activity (39).

We have screened 15 crystals derived from multiple gold nanoparticle preparations and obtained the same Au102 structure, so the unusual arrangement in the 13-atom equatorial band is a consistent result. Other nanoparticle preparations, however, which have also given rise to large single crystals, will doubtless reveal other core structures, from which rules or principles of core assembly may ultimately be derived. It remains to investigate the chemical and physical properties of the Au102 nanoparticle, as well as to explore the theoretical basis of the gold packing and gold-thiol interactions that we have observed.

Supporting Online Material

www.sciencemag.org/cgi/content/full/318/5849/430/DC1

Materials and Methods

Figs. S1 to S10

Table S1

Structure Parameter Files

References and Notes

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