Microwave-Assisted Formation of Gold Nanoclusters Capped in Bovine Serum Albumin and Exhibiting Red or Blue Emission

In this study, a microwave-assisted method for preparing fluorescent gold nanoclusters capped in bovine serum albumin (AuNCs@BSA) was developed. Through fluorescence spectrometry, transmission electron microscopy, and X-ray photoelectron spectroscopy (XPS), the effect of reaction temperature on the size and oxidation state of AuNCs@BSA samples was determined. Blue-emitting AuNCs@BSA (B-AuNCs@BSA), with weak bonds between AuNCs and BSA, were prepared at 135 °C. Red-emitting AuNCs@BSA (R-AuNCs@BSA) were also formed, and their formation was followed by an aggregation of polydispersed AuNCs@BSA. Through infrared spectrometry and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, it was confirmed that the R-AuNCs@BSA, consisting of 18 Au atoms, was successfully embedded in BSA. In addition, XPS measurements revealed that Au⁺ ions were present only in the R-AuNCs@BSA. Steady-state and lifetime results revealed the importance of the size and oxidation state of Au⁺ in the stabilization of the AuNCs@BSA and in the presence of a long lifetime component.