CdS quantum dots immobilized on calcium alginate microbeads for rapid and selective detection of Hg2+ ions

Abstract

Mercury is extremely toxic to the environment and is detrimental to human health. We describe here the development of an analytical method comprising mercaptoacetic acid capped CdS quantum dots (QDs) immobilized on calcium alginate microbeads (referred here as CA@CdS) for rapid, selective and quantitative detection of Hg²⁺ ions based on a fluorescence quenching phenomenon. The feasibility of detecting Hg²⁺ ions in this method is extended to real time analysis by spiking 0.5 mg L⁻¹ and 1.0 mg L⁻¹ Hg²⁺ ions in batches of municipal tap water. The mechanism of fluorescence quenching has been explained on the basis of adsorption of Hg²⁺ (adsorption capacity was determined to be 88.33 μg g⁻¹). The selectivity of Hg²⁺ ions is attributable to its soft acid–soft base interaction between Hg²⁺ and the sulphydryl group of the mercaptoacetic acid capped CdS QDs. The fluorescence quenching phenomenon satisfied the Stern–Volmer equation and was linearly correlated with Hg²⁺ ions concentrations in the range of 0.015 and 2.0 mg L⁻¹. The limit of detection of Hg²⁺ ion was determined to be 0.008 mg L⁻¹ at an optimized condition, i.e., pH 6 and contact time 30 min; and the detection was not affected by other experimentally relevant cations like Na⁺, Mg²⁺, K⁺, Ca²⁺, Mn²⁺, Fe²⁺, Cu²⁺, Pb²⁺, Cr³⁺, Zn²⁺, As³⁺, Cd²⁺; and anions like Cl⁻, CO₃²⁻, HCO₃⁻, HPO₄⁻, SeO₄⁻, SO₄²⁻. The major advantage of this nano-fluoroprobe is that it can detect as well as remove Hg²⁺ ions from an aqueous medium. The fluoroprobe can be recovered and re-used for subsequent cycles of detection and removal of Hg²⁺ ions.