Solubility-Mediated Access to a Diverse Series of Atomically Precise Gold Nanoclusters Protected by p -Mercaptobenzoic Acid

Abstract

A series of p-mercaptobenzoic acid (L-H)-protected gold nanoclusters (AuxLyz) was synthesized in methanol/water mixtures with controlled mixing ratios, leveraging size-dependent solubility differences. Isolation via polyacrylamide gel electrophoresis and electrospray ionization mass spectrometry yielded seven fractions containing single species: [Au25L18]− (Au25), Au44L28 (Au44), [Au75L38]− (Au75), Au92L44 (Au92), Au122L48 (Au122), Au146L57 (Au146), Au185L63 (Au185). Three additionally obtained fractions contain distinct but multiple species: [Au60L31]− and [Au65L34]− (Au60/65); Au100L42 and Au102L44 (Au100/102); Au129L52, Au130L52, and Au131L52 (Au129–131). Furthermore, two less pure fractions tentatively assigned to Au∼211L∼70 (Au∼211) and Au∼262L∼80 (Au∼262) were obtained. For the majority of these AuNCs, the number of valence electrons does not align with the predictions of the spherical jellium model, except for Au25, Au60, Au100, and Au102. These discrepancies suggest that the high stability of the majority of isolated AuNCs originates from geometric factors. Powder X-ray diffraction, aberration-corrected transmission electron microscopy, and density functional theory calculations revealed a structural evolution from nonfcc to fcc-structured Au cores between Au122 and Au129–131, with notable exceptions of Au44 and Au92, which possess fcc structures despite their smaller sizes. Cryogenic optical absorption spectroscopy revealed that surface plasmon resonance becomes discernible in Au122 and larger AuNCs. These findings imply that the SPR response of Au129–131 and larger AuNCs is associated with fcc-based Au cores, while the SPR response of nonfcc Au122 is ascribed to the coupling between Au core electrons and the π-electrons in the L ligands. These water-soluble AuNCs will serve as key components for biorelated applications, catalysis, and the assembly of functional materials.