Quantum Dot-Based Sensing Mechanisms for Environmental Pollutant Detection

Abstract

Identifying trace-level environmental pollutants, such as heavy metal ions, organophosphate pesticides, and emerging contaminants, poses a significant analytical challenge due to the constraints of traditional laboratory techniques like atomic absorption spectroscopy and high-performance liquid chromatography. Although these methods are precise, they are expensive, time-intensive, and not suitable for field deployment or real-time monitoring. Semiconductor and carbon-based quantum dots (QDs) have shown potential as optical transducers for pollutant detection because of their size-dependent photoluminescence (PL), high quantum yield, and ability to interact with target analytes through surface-mediated processes. This research systematically explores QD-based fluorescence sensing mechanisms for detecting mercury(II) ions (Hg2+) and the organophosphate pesticide chlorpyrifos, utilizing thioglycolic-acid-capped CdTe QDs and nitrogen-doped carbon dots (N-CDs) as sensing platforms. The study monitored photoluminescence quenching relative to analyte concentration and interpreted the results using Stern-Volmer kinetics, uncovering both static and dynamic quenching mechanisms driven by electron transfer and inner-filter effects. The CdTe QD probe demonstrated a linear detection range of 0-50 micromolar for Hg2+ with a detection limit (LOD) of 0.34 micromolar (68 ppb), while the N-CD probe achieved an LOD of 0.19 micromolar (38 ppb) for chlorpyrifos within a linear range of 0-40 micromolar. Selectivity tests against eleven common interfering ions and two co-occurring pesticides confirmed high specificity, with the N-CD probe showing less than 15% cross-reactivity for all interferents except lead(II) and, as expected, chlorpyrifos-family analogues. Time-resolved photoluminescence measurements and X-ray photoelectron spectroscopy of the QD surface before and after exposure to analytes support a combined static-complexation and photoinduced-electron-transfer quenching mechanism. These findings, validated against real environmental water samples spiked with known pollutant concentrations, indicate that QD-based optical sensors can achieve sensitivity levels relevant to regulatory standards while providing a cost-effective, field-portable alternative to traditional instruments. The paper concludes by discussing the remaining challenges for practical deployment, including QD stability, matrix interference, and multiplexed detection.