Optimization of Synthesis of Bovine Serum Albumin-Encapsulated Fluorescent Gold Nanoclusters ^†

Gold nanoclusters (AuNCs) are nanoscale structures consisting of a few to tens of gold atoms. AuNCs exhibit size- and scaffold-dependent photoluminescence, which allows their usage as analytical sensors or as fluorescent probes for biological imaging [1,2]. Typical synthesis involves mixing a metal precursor (HAuCl4) with bovine serum albumin (BSA), adding sodium hydroxide (NaOH) to increase the pH of the reaction mixture (and starting the reduction of Au(III)), and finally exposing the solution to microwave irradiation. In this contribution, different molar ratios of the reactants were used to optimize the fluorescent properties of AuNCs. The change in molar ratios affected the fluorescence intensity and position of fluorescence emission maximum of AuNCs; simultaneously, it altered the period which is necessary for reaching the fluorescence maximum. It was observed that with an increasing concentration of NaOH (in the pH range of 9–13), the fluorescence maximum position manifested itself via a bathochromic shift, and the maximum value of fluorescence intensity decreased and was reached in a shorter period than at lower pHs. Indeed, a lower pH value of the reaction mixture can lead to an efficient increase in the fluorescence quantum yield of AuNCs. This might be caused by several simultaneous factors: (i) conformational changes in BSA are less pronounced at pH 9 than at pH 13, for instance; and (ii) faster reduction of Au(III) proceeds at higher pH values, which may consequently lead to overgrowth of nanoclusters to non-fluorescent particles. Both factors may contribute to the generation of a higher number of Au nanoclusters of smaller sizes possessing excellent fluorescent properties while working at lower pH values. The choice of an appropriate fluorescent standard is tremendously important, but often underestimated by many researchers.