A paper based microfluidic device is fabricated that can rapidly detect very low concentrations of uric acid (UA) using 3,5,3',5'-tetramethyl benzidine (TMB), H2O2 and positively charged gold nanoparticles ((+)AuNPs). In the presence of (+)AuNPs, H2O2 reacts with TMB to produce a bluish-green colour which becomes colourless on reaction with UA. This colorimetric method can detect as low as 8.1 ppm of UA within <20 minutes on white filter paper. This technique provides an alternative way for UA detection.
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 HAuCl 4 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 Au 25 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.
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