Fluorescent gold nanoclusters as pH sensors for the pH 5 to 9 range and for imaging of blood cell pH values

The authors describe the use of gold nanoclusters (AuNCs) with a diameter of similar to 2 nm for fluorescent sensing of pH values in the range from 5 to 9. The AuNCs were synthesized in the presence of bovine serum albumin (BSA) which acts as both a reducing agent and capping agent. The resulting AuNCs were characterized in terms of size and surface chemistry using TEM and FTIR. The BSA-capped AuNCs display red luminescence, with excitation/emission peaks at 470/640 nm, which is strongly modulated by the pH indicator bromothymol blue (BTB). The effect depends mainly on an inner filter effect due to spectral overlap between the absorption BTB and the emission of the AuNCs. The pH nanosensor responds to pH values in the range from 5 to 9 which is the socalled physiological pH range. The method was applied to detect changes in the pH values that occur after the death of red blood cells. Such pH changes are considered as a potential forensic marker for estimating the time passed since death. The results show the BTB-BSA-AuNC system to be capable of detecting respective intracellular pH changes.