A portable smartphone assisted multi-color colorimetric sensor for the detection of mercury ions based on the peroxidase-like activity of platinum nanoparticles

An innovative multi-color colorimetric method integrated with smartphone detection was designed with great sensitivity and specificity for the determination of mercury ions (Hg²⁺) in aqueous samples based on platinum nanoparticles (PtNPs) and gold nanorods (AuNRs). The peroxidase-mimicking activity of PtNPs triggered the conversion of 3,3',5,5'-tetramethylbenzidine (TMB) into its oxidized form (TMB²⁺), and the resulting TMB²⁺ could quantitatively etch AuNRs, inducing a distinct color transition from blue-green to pale red. By forming metallophilic interactions with the PtNPs, Hg²⁺ effectively suppressed the etching process of the AuNRs by inhibiting PtNPs' peroxidase-like catalytic activity. Based on this principle, a correlation allowed for the quantification of Hg²⁺ by measuring the average color intensity (<i>I</i>_c) calculated from red, green, and blue (RGB) values of the reaction solution, exhibiting a linear response ranging from 10 to 400 nM with a detection limit (LOD) of 5.1 nM. The method also exhibited outstanding selectivity and anti-interference capability against competing metal ions. Robust practical applicability was confirmed in complex real water samples with satisfactory spike-and-recovery results. Moreover, the proposed sensor integrated AuNRs served as a multi-color indicator for Hg²⁺-modulated peroxide-mimicking enzyme activity of PtNPs, with smartphone-based RGB analysis, presenting a rapid and cost-effective solution for field-based environmental monitoring of mercury contamination.