Dithiothreitol-Facilitated Synthesis of Bovine Serum Albumin–Gold Nanoclusters for Pb(II) Ion Detection on Paper Substrates and in Live Cells

In this work, a bright red fluorescent protein-capped gold nanocluster, AuNC@BSA, is developed in a green synthesis approach, and its application as a sensor for detection of Pb(II) ion in water and in live cells is demonstrated. AuNC@BSA is prepared by dithiothreitol (DTT)-mediated activation of bovine serum albumin (BSA) followed by the reduction of HAuCl4 in an aqueous medium. The incorporation of DTT assists in the reduction of disulfide bridges present in cysteine residues, which in turn increases the reducing power of BSA forming a significant number of the Au25 clusters that enhances the bright red fluorescence of AuNC@BSA at 660 nm when excited at 520 nm with a highest quantum yield of ∼20.0% reported so far. AuNC@BSA as nanosensor selectively detect Pb(II) ion in water with a detection limit of 1.0 ppb, lower than the World Health Organization limit (10.0 ppb), and follow quenching-based sensing mechanism through metallophilic interaction between Au(I) and Pb(II) ion. In presence of a strong chelating agent, ethylenediaminetetraacetic acid, AuNC@BSA shows regeneration ability after the treatment with Pb(II) ion. On paper-based substrate AuNC@BSA also exhibits a distinct change in red fluorescent color with metal ion concentration. The cellular study with AuNC@BSA demonstrates excellent biocompatibility that makes them a suitable candidate for biosensing of Pb(II) ion in biological medium. In HeLa cells, AuNC@BSA shows bright red fluorescence under the confocal microscope, which is quenched when incubated with Pb(II) ion enriched cells up to a limit of 1.0 ppm proving their imaging ability for in vitro applications. For the first time, such efficient green synthesis of a gold nanosensor for a sensitive and selective detection of Pb(II) ion on paper and in live cells is reported in this study.