Nucleation-Controlled Approach to Synthesize Sub-2 nm Water-Soluble Purine-Modified Green-Fluorescent Gold Nanoclusters

Purine-modified gold nanoclusters (Au NCs) of diameter 1.4 ± 0.34 nm have been synthesized through a modified, single-phase Brust-Schiffrin method with the introduction of a controlled nucleation approach. The synthesized Au NCs present high chemical stability in neutral to basic environments and across various salt concentrations (NaCl and PBS buffer (pH:7.4)), which overcomes previous size and water-solubility limitations and highlights opportunity for future bioimaging and biosensor applications. The stability of these purine-modified Au NCs in pure water is attributed to the large electron cloud density of sulfur prompted by a greater prevalence of negatively charged nitrogen groups during thiol adsorption. This facilitates a high density and efficient packing of the functional, self-assembled monolayer that protects the Au core from direct interaction with the liquid environment. The combination of reduced size and improved distribution of 6-mercaptopurine, including the increased contribution of Au(I) complexes, has led to increased fluorescence emission intensity of these purine-modified Au NCs. Overall, the synthesis protocol developed in this work provides the foundation for a tailored approach to create a stable, green-fluorescent, water-soluble dispersion of thiol-stabilized Au NCs for bio-related nanotechnology applications.