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 3,3′,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.