Abstract:

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 consists in mixing a metal precursor (HAuCl₄) with bovine serum albumin (BSA), then 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 a microwave irradiation. In this contribution, different molar ratios of the reactants were used to optimize AuNCs fluorescent properties. 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), position of fluorescence maximum manifested itself by a bathochromic shift, 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 leads to an efficient increase of the fluorescence quantum yield of AuNCs. It might be caused by several simultaneous factors: (i) conformational changes of BSA are less pronounced for instance at pH 9 than at pH 13; (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.

Acknowledgement:

This research was funded by Grant Agency of the Czech Republic, grant number 19-03207S and Internal Grant Agency of Palacký University, grant number IGA_PrF_2021_003.

References:

[1] Zhang L., Wang E. (2014) Metal nanoclusters: New fluorescent probes for sensors and bioimaging. Nano Today 9, 132-157.

[2] Shang L., Dong S., Nienhaus G.U. (2011) Ultra-small fluorescent metal nanoclusters: Synthesis and biological applications. Nano Today 6, 401-418.