Nanoparticles-enhanced fiber optic plasmonic sensor for trace-level detection of mercury (Hg2+) ions

Continuous release of heavy metal ions (HMIs) from industrial, mining, and domestic wastewater sources poses a serious threat to the environment and living organisms. Among HMIs, mercury (Hg²⁺) is particularly hazardous due to its high toxicity, bioaccumulation potential, and widespread presence in aquatic environments. We report a low-cost and highly sensitive optical fiber-based surface plasmon resonance (SPR) sensing technique to detect Hg2+ using copper nanoparticles (CuNPs) and iron nanoparticles (FeNPs). Fabrication of the sensor was mainly in two distinct phases. In first phase, CuNPs and FeNPs were synthesized via the environmentally sustainable exploding wire (EW) method, which minimizes the use of hazardous chemicals and reduces energy consumption. The second phase included the development of optical fiber-based SPR sensing probes and their application in Hg2+ sensing. The nanoparticles were deposited onto a gold (Au)-coated optical fiber to develop two distinct SPR sensing probes: one functionalized with CuNPs and the other with FeNPs. The structural and compositional characteristics of the sensing probes were thoroughly investigated using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Experimental characterization of sensors revealed that both sensing probes exhibited high sensitivity toward Hg²⁺ in aqueous media, with detection capabilities in sub-micromolar range. The CuNPs-modified probe demonstrated better analytical performance, achieving a sensitivity of 1.29 nm/µM and a limit of detection (LOD) of 0.40 µM, compared to 0.96 nm/µM sensitivity and 0.51 µM LOD for the FeNPs-modified probe. The results underscore the potential of this approach for large-scale deployment in environmental monitoring and water quality assessment.