Highly fluorescent hybrid Au/Ag nanoclusters stabilized with poly(ethylene glycol)- and zwitterion-modified thiolate ligands

Abstract

We report a simple strategy to grow highly fluorescing, near-infrared-emitting nanoclusters (NCs) made of bimetallic Au/Ag cores, surface capped with a mixture of triphenylphosphine and various monothiol ligands. The ligands include short chain aliphatic monothiols, which yields hydrophobic NCs, and poly(ethylene glycol)- or zwitterion-appended monothiols, which yield NCs that are readily dispersible in buffer media. The reaction uses well-defined triphenylphosphine-protected Au₁₁ clusters (as precursors) that are reacted with Ag(I)–thiolate complexes. The prepared materials are small (diameter <2 nm, as characterized by TEM) with emission peak at 730–760 nm and long lifetime (∼8–12 μs). The quantum yield measured for these materials in both hydrophobic and hydrophilic dispersions is ∼40%. High-magnification dark field STEM and X-ray photoelectron spectroscopy measurements show the presence of both metal atoms in the core, with measured binding energies that agree with reported values for nanocluster materials. The NIR emission combined with high quantum yield, small size, colloidal stability in buffer media and ease of surface functionalization afforded by the coating, make these materials suitable for investigating fundamental questions and potentially useful for biological sensing and imaging applications.