Sputter Deposition toward Short Cationic Thiolated Fluorescent Gold Nanoclusters: Investigation of Their Unique Structural and Photophysical Characteristics Using High-Performance Liquid Chromatography.

We herein present the preparation of short, bulky cationic thiolate (thiocholine)-protected fluorescent Au nanoclusters via sputter deposition over a liquid polymer matrix. The obtained Au nanoclusters showed near-infrared fluorescence and had an average core diameter of 1.7 ± 0.6 nm, which is too large compared to that of the reported fluorescent Au nanoclusters prepared via chemical means. We revealed the mechanism of formation of this unique material using single-particle electron microscopy, optical measurements, X-ray photoelectron spectroscopy (XPS), and high-performance liquid chromatography fractionations. The noncrystallized image was observed via single-particle high-angle annular dark-field scanning transmission electron microscopy observations and compared with chemically synthesized crystalline Au nanoparticle with the same diameter, which demonstrated the unique structural characteristic speculated via XPS. The size fractionation and size-dependent fluorescence measurement, together with other observations, indicated that the nanoclusters most probably contained a mixture of very small fluorescent species in their aggregated form and were derived from the sputtering process itself and not from the interaction between thiol ligands.