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

• The oxidase-like activity of BSA-Ag NCs was “switched-on” selectively by Hg2+.
• Hg2+ stimulated Ag NCs had a better affinity than HRP considering TMB as a substrate.
• The triggered enzyme mimicking activity was utilized to colorimetrically detect DNA.
• This colorimetric sensor for DNA was label-free, facile and fast.

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 3,3′,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.

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