Label-free colorimetric sensor for mercury(II) and DNA on the basis of mercury(II) switched-on the oxidase-mimicking activity of silver nanoclusters

In this paper, a novel colorimetric biosensor for Hg2+ and DNA molecules is presented based on Hg2+ stimulated oxidase-like activity of bovine serum albumin protected silver clusters (BSA-Ag NCs). Under mild conditions, Hg2+ activated BSA-Ag NCs to show high catalytic activity toward the oxidation of 3,30,5, 5'-tetramethylbenzidine (TMB) using ambient dissolved oxygen as an oxidant. The oxidase-like activity of BSA-Ag NCs was switched-on selectively in the presence of Hg2+, which permitted a novel and facile colorimetric sensor for Hg2+. As low as 25 nmol L (1) Hg2+ could be detected with a linear range from 80 nmol L (1) to 50 mmol L (1). In addition, the sensing strategy was also employed to detect DNA molecules. Hg2+ is known to bind very strongly and specifically with two DNA thymine bases (T) to form thymine-Hg2+- thymine (T-Hg2+-T) base pairs. The hairpin-structure was disrupted and Hg2+ ions were released after hybridization with the DNA target. By coupling the Hg2+ switched-on the oxidase-mimicking activity of BSA-Ag NCs, we developed a novel label-free strategy for facile and fast colorimetric detection of DNA molecules. More important, target DNA can be detected as low as 10 nmol L (1) with a linear range from 30 to 225 nmol L (1). Compared with other methods, this method presents several advantages such as the independence of hydrogen peroxide, high sensitivity and good selectivity, avoiding any modification or immobilization of DNA, which holds a great potential of metal NCs for clinical application in biosensing and biotechnology. (C) 2015 Elsevier B.V. All rights reserved.