Atomically precise nanoclusters predominantly seed gold nanoparticle syntheses

Abstract

Seed-mediated synthesis strategies, in which small gold nanoparticle precursors are added to a growth solution to initiate heterogeneous nucleation, are among the most prevalent, simple, and productive methodologies for generating well-defined colloidal anisotropic nanostructures. However, the size, structure, and chemical properties of the seeds remain poorly understood, which partially explains the lack of mechanistic understanding of many particle growth reactions. Here, we identify the majority component in the seed solution as an atomically precise gold nanocluster, consisting of a 32-atom Au core with 8 halide ligands and 12 neutral ligands constituting a bound ion pair between a halide and the cationic surfactant: Au₃₂X₈[AQA⁺•X^-]₁₂ (X = Cl, Br; AQA = alkyl quaternary ammonium). Ligand exchange is dynamic and versatile, occurring on the order of minutes and allowing for the formation of 48 distinct Au₃₂ clusters with AQAX (alkyl quaternary ammonium halide) ligands. Anisotropic nanoparticle syntheses seeded with solutions enriched in Au₃₂X₈[AQA⁺•X^-]₁₂ show narrower size distributions and fewer impurity particle shapes, indicating the importance of this cluster as a precursor to the growth of well-defined nanostructures.

Fig. 1. Illustration of a typical anisotropic metal nanoparticle synthesis.

Reactions proceed via the rapid reduction of a gold halide salt a to nucleate small seed particles b, which then act as heterogeneous nucleation sites in a subsequent reaction to facilitate the controlled growth of particles with well-defined shapes c. This work identifies the seed intermediates as an atomically precise cluster with 32 gold atoms (yellow), 8 halides (blue), and 12 alkyl quaternary ammonium (AQA)-halide bound ion pairs (purple) as surface ligands.

Fig. 2. Identification of the chemical formula of gold nanoclusters found in seed solutions.

a Schematic of the chemical features of Au₃₂X₈[AQA⁺•X^-]₁₂ clusters and the diversity of surface ligands that can stabilize them. b Experimental absorption spectra of Au₃₂ clusters synthesized with C₁₆TAB (blue), C₁₄TAB (orange), and C₁₆TAC (green) ligands, molecular structures of which are shown as insets. Simulated absorption spectra generated by a combined MD and DFT approach with NH₄Cl ligands (grey) shows excellent agreement. c MALDI mass spectrometry of clusters synthesized with C₁₆TAB (blue), C₁₄TAB (orange), and C₁₆TAC (green) ligands all show an intense peak arising from the intact Au cluster with smaller peaks assigned to the loss of n cationic surfactants. d Aggregated data showing cluster mass vs. ligand mass for 48 distinct seed syntheses, allowing for determination of the number of surfactant ligands (12), halide anions (20), and gold atoms (32) by linear interpolation/extrapolation. Individual datapoints shown as squares represent clusters with a single type of ligand, circles represent mixed-ligand clusters.

Fig. 3. Characterization of the structure of Au₃₂X₈[AQA⁺•X^-]₁₂.

a Low-resolution cryogenic TEM shows ≈1 nm metal clusters and the absence of larger particles while b high-resolution STEM shows clusters with atomic resolution; images in b have been processed and represented in false color (see Methods) to visually enhance contrast with greyscale pixel intensities represented by the color scale. c Because of beam-induced motion of clusters on a graphene grid, numerous frames of a single cluster can be captured and individual atoms can be identified (red dots), scale is identical to panel b. d Averaging the Au atom counts over many frames yields values that are consistent with the assignment of a 32-atom core. Error bars represent +/- standard deviation from N = 15-18 independent measurements. e Proposed pseudo-icosahedral structure of Au₃₂X₈[AQA⁺•X^-]₁₂ built up as concentric radial shells moving left to right: hollow Au₁₂ icosahedron, Au₂₀ pentagonal dodecahedron, X₂₀ halides, (NH₄)⁺₁₂ ammonium headgroups, [C₁₆TA⁺•X^-]₁₂ bound ion pairs. Note that the Au₃₂X₈[NH₄⁺•X^-]₁₂ and Au₃₂X₈[C₁₆TA⁺•X^-]₁₂ structures are directly taken from DFT and MD simulation models, respectively.

Fig. 4. Mass spectrometry analysis of nanocluster ligand exchange.

a, b Exchange of the bound ion pair and c, d halide ligands on Au₃₂X₈[AQA⁺•X^-]₁₂ clusters can both be resolved in MALDI spectra. b, d Dotted lines indicate the mass of single-ligand clusters and grey bars indicate the location of the distribution of mixed-ligand clusters if binding of each molecule was equally favorable. Peaks are indexed in red to denote the number of exchanged ligands and the ratio of molecules in solution are given as insets. a, b Equilibration to a mixture of ligands with differing alkyl chain lengths results in a preference for [C₁₄TA⁺•Br^-] over [C₁₆TA⁺•Br^-] bound ion pairs. c, d Equilibration to a mixture of ligands with differing halide counterions results in a preference for Br over Cl. (Blue circles for Br^-; green Cl^-; orange [C₁₄TA⁺•Br^-]; magenta [C₁₆TA⁺•Br^-]; red [C₁₆TA⁺•Cl⁻]).

Fig. 5. Morphological analysis of gold nanorods synthesized with traditional seed solutions (red) or solutions enriched in Au₃₂ nanoclusters (blue).

a Representative TEM images. b Percent of total particles constituting rod, sphere, and cube shapes. c Size distribution of gold nanorod major and minor axes normalized to the average with shaded region representing 95% confidence ellipse. Full datasets (see Supplementary Fig. 24b) consist of N = 766 and 2,990 measurements for Au₃₂ and traditional seeds, respectively. 500 points from each dataset have been selected at random to improve visibility.