Biogenic Quantum Dots for Sensitive, Label-Free Detection of Mercury Ions

Nanoparticle-based fluorescent probes, typically fabricated by a chemical synthesis route, have been widely used for monitoring trace heavy metals in environments. However, the high-cost and complicated, aggressive fabrication processes restrict their widespread application. In this work, we report the first use of biogenic quantum dots (Bio-QDs) as a highly sensitive, low-cost fluorescent probe for label-free detection of mercury ions (Hg²⁺), with comparable performance to conventional chemically synthesized counterparts. Fluorescent Bio-QDs with uniform sizes (1.6 ± 0.3 nm) and unique core-shell structure (CdSSe_1- core and protein- and phosphate-rich capping) were assembled by Escherichia coli cells. The Bio-QDs were extracted and directly used as a Hg²⁺ probe, which exhibited sensitive, linear fluorescent response to Hg²⁺ concentration in the range of 1.5-100 nM. Interestingly, it even enable a naked-eye detection of Hg²⁺ in a higher concentration range of 0.1-10 μM by simply raising the Bio-QD load. The underlying detection mechanisms, involving substitution of the Cd atoms with Hg from water, were revealed by Raman spectra, X-ray absorption fine structure, and density functional theory calculations. Our work implies a high potential of green-synthesized Bio-QDs for environmental monitoring applications, which may not only broaden the application ranges of Bio-QDs, but also advance the development of environmental analytical techniques toward higher sustainability.