A Dual-Readout Magnetic Nanoparticle-Based Enzyme Assay for the Sensitive Detection of Hg(II) Ions in Drinking Water

Development of miniaturized sensitive techniques for the analysis of toxic mercury ions (Hg²⁺) in water is of immense interest due to public health concerns. A miniaturized colorimetric FeMNPs-AlOx probe was developed for rapid and selective detection of Hg²⁺ ions in drinking water with a significantly low limit of detection 0.1 ng L^–1. A linear working range of detection of 0.5–100 ng L^–1 for Hg²⁺ ions was achieved for 200 μL assay volumes with 10 min analysis times. The sensing system comprises alcohol oxidase enzymes (AlOx) coupled to iron oxide magnetic nanoparticles (FeMNPs) (average particle size 40 nm) via silane cross-linkers. The activity of the sensing probe was quantified using methanol, horseradish peroxidase (HRP) enzyme, and O-phenylenediamine (OPD) as the colorimetric substrate. The developed sensing probe was tested for analysis in drinking water samples with recoveries of 93–100%. Selectivity toward Hg²⁺ ions was achieved using citrate buffer as a masking agent. Analysis of Hg²⁺ ions was also carried out in the presence of Certipur certified reference material (CRM) for a multielement solution (As, Be, Cd, Cr­(VI), Hg, Ni, Pb, Se, Tl). The developed FeMNPs-AlOx sensing probe exhibited good storage stability (with 75% retention of original response) over one month when stored at 4 °C. The developed probe also provided clear visual detection of Hg²⁺ ions in drinking water samples via colorimetric sensing.