Impact of Quantum Dot Purification Methods on Sensing Performance for Heavy Metal Detection in Environmental Pollution Monitoring

Abstract

The demand for rapid, low-cost and robust approaches to environmental monitoring has intensified interest in fluorescence-based platforms for heavy-metal detection in water. Carbon quantum dots (CQDs) have emerged as a promising class of eco-friendly nanomaterials with sensing capabilities, photostability, and tunable optical properties. Here, we investigate CQDs as fluorescent probes for heavy-metal ions and examine how purification at coarse and fine levels controls their photophysics and sensing performance. Dialysis (DL) and silica-gel column chromatography (CC) were used to obtain CQDs-DL and CQDs-CC, which were compared in terms of optical properties and surface chemistry. CQDs-DL exhibited a higher relative abundance of amine-containing groups than CQDs-CC and showed stronger fluorescence quenching in the presence of Hg²⁺ in aqueous solution. To improve stability and enable solid-state sensing, CQDs were immobilised in a TEMPO-oxidised cellulose nanofibers (TOCN) matrix to form bacterial nanocellulose-based biocomposites (CQDs/TOCN). After immobilisation, CQDs-CC displayed enhanced chemical stability and a more linear quenching response across the tested heavy-metal ions, whereas CQDs-DL produced a stronger but more heterogeneous response consistent with a broader distribution of emissive species. Finally, real-sample analysis confirmed the sensitivity of the platform, revealing distinct fluorescence changes upon mercury exposure. These findings indicate that, although dialysis is widely used as a “standard” purification method, it is not necessarily the most effective approach for all sensing applications. Column chromatography provided superior purification, leading to improved sensing performance in selected environmental monitoring scenarios. Overall, this study highlights the potential of CQDs-based sensors as a real-time, eco-friendly platform for monitoring heavy metal contamination in water sources, while emphasizing the critical role of purification in tailoring their sensing behaviour.