Assay of biothiols by regulating the growth of silver nanoparticles with C-dots as reducing agent.

Recently, the development of optical probes for the assay of thiols, e.g., cysteine (Cys), homocysteine (Hcy), and glutathione (GSH), has been an active research area due to their biological significance. We have found that carbon dots (C-dots) exhibit direct reduction of Ag(+) to elemental silver (Ag(0)) and the resulting Ag(0) formed a silver nanoparticle (Ag-NP) spontaneously. The excessive C-dots consume free Ag(+) in the solution by binding Ag(+) with functional groups on the C-dots surface and thus inhibits the growth of Ag-NPs. Biothiols can coordinate with Ag(+) through thiol groups, and afterward, the Ag(+)-biothiol complex gradually releases free Ag(+) to ensure its reduction by C-dots and thus facilitates the growth of Ag-NPs on C-dots surface. A colorimetric assay procedure is thus developed for fast detection of biothiols based on Ag-NPs plasmon absorption. The linear calibration range can be regulated by controlling the concentration of Ag(+). Two linear ranges were obtained for the biothiols assay at different levels, which offer ultrahigh sensitivity for the assay of an ultratrace amount of biothiols with detection limits of 1.5, 2.6, and 1.2 nM for Cys, Hcy, and GSH, respectively. The precisions for the assay of Cys, Hcy, and GSH at 20 nM are achieved as 3.1%, 3.1%, and 2.4%. In addition, the sensing system exhibits good selectivity toward biothiols in the presence of other amino acids, the major metal cations, and biomolecules in biological fluids. For the assay of 20 nM Cys, 150-fold of coexisting amino acids, 2500-fold of Ca(2+), Mg(2+), glucose, and ascorbic acid, and 38-fold of HSA are tolerated. In the assay of Cys in human plasma, spiking recoveries of 94% to 108% are obtained at 100 μM.