An aptamer-controlled Mn-Au nanozyme for H2O2-free and smartphone-assisted colorimetric detection of mercury ions.

A novel Hg2+ sensing strategy has been developed based on a Mn-Au hybrid nanozyme with prominent oxidase-like activity and an aptamer-controlled catalytic switching mechanism. First, a Mn-Au hybrid nanozyme (MnAuNPs) with a distinctive "fried-egg-like" core-shell morphology was synthesized, which exhibited remarkable oxidase-like activity and rapidly oxidized 3,3',5,5'-tetramethylbenzidine (TMB) into the characteristic blue product (OxTMB) without H2O2. The Hg2+-specific aptamer can adsorb onto the surface of MnAuNPs through electrostatic and Mn-phosphate coordination interactions, which blocks the catalytic sites and markedly suppresses the oxidase-like activity. In the presence of Hg2+, the specific T-Hg2+-T coordination triggers the detachment of the aptamer from the MnAuNPs surface, thereby restoring oxidase-like activity and inducing distinct color and absorbance change. Accordingly, an effective colorimetric platform for Hg2+ detection was developed, exhibiting a linear response over the range of 1.0-50 μM with a limit of detection (LOD) of 0.32 μM. In parallel, a paper-based smartphone-assisted sensing platform was developed, enabling on-site Hg2+ detection in the range of 5.0-50 μM with LOD of 5.0 μM. Through the colorimetric and smartphone-assisted method, accurate quantification of Hg2+ in the Yellow River water and wastewater samples were achieved with the results in good agreement with those obtained by ICP-OES. Overall, this work establishes a simple and portable H2O2-free colorimetric sensing strategy based on Hg2+-induced catalytic switching for environmental Hg2+ monitoring.