β-In2S3 quantum dot-modified graphite for indirect monitoring of chemically speciated mercury in canal water

Abstract

Monitoring mercury (Hg²⁺) in aquatic systems is critical for ecological risk assessment, yet a key challenge lies in assessing its bioavailable fraction, which is governed by chemical speciation. Herein, a novel electrochemical sensor for the indirect monitoring of chemically speciated mercury in environmental waters is reported. The sensor is based on surfactant-tailored β-indium sulfide (In₂S₃) quantum dots (QDs) immobilized using a chitosan ink on a disposable graphite electrode. The sulfide-rich surface of the In₂S₃ QDs provides high-affinity binding sites for Hg²⁺. The sensing mechanism relies on the Hg²⁺-induced enhancement of electron transfer for a ferrocyanide redox probe, rather than direct Hg electrochemistry. This indirect approach proved highly sensitive to the free Hg²⁺ ions, as demonstrated by a pronounced counterion effect: the signal intensity followed the trend Hg(NO₃)₂ > (CH₃COO) ₂Hg > HgCl₂, reflecting the lability and bioavailability of the mercury species. The sensor exhibited a detection limit of 21.2 nM, high selectivity against common interferents (Zn²⁺, Cd²⁺, and Pb²⁺), and excellent operational stability, retaining 86% of its signal after 30 days. Crucially, the sensor performed reliably in the analysis of canal water, achieving accurate quantification with recoveries of 101.50 ± 1.25% without any sample pre-treatment. Hence, this robust and low-cost In₂S₃/GPE sensor provides insights into mercury speciation, supporting more accurate environmental monitoring and ecological risk assessment in contaminated aqueous systems.