In-situ fiber sensor for ultra-trace Hg²⁺ with sandwich-structured SERS substrate

Due to the highly toxic and bioaccumulative nature of mercury ions, even trace or ultra-trace levels of mercury ions in the environment, food, and biological samples pose a critical threat to ecological safety and human health. However, ensuring the detection of ultra-trace mercury ions while simultaneously guaranteeing high practicality, selectivity, and low cost remains an extremely formidable challenge. To achieve this goal, an ultra-trace Hg²⁺ fiber-optic SERS sensor based on an AgNPs/4-ATP/AgNPs sandwich-structured substrate was developed. In this architecture, the outer AgNP layer serves as the Hg²⁺ reaction interface, while the inner AgNP layer enhances the resulting SERS signal variation. Integration of the sandwich structure onto the fiber tip enables rapid measurement using a portable Raman spectrometer. The sensor produced a concentration-dependent response over 10^-16-10^-11 M, with a calculated detection limit of 4.5 × 10^-16 M. Experimental results demonstrated good Hg²⁺ ion specificity, time-stability with an RSD below 1.5% over 20 min, and sensor-to-sensor consistency with an RSD of 2.55%. Tap-water spiking experiments yielded recoveries of 73.99%-118.55%, with relative standard deviations of 3.38%-14.38%, preliminarily demonstrating the feasibility of the method for Hg²⁺ detection in real water samples. This fiber-tip sandwich-structured sensing strategy provides a potential approach for ultra-trace environmental Hg²⁺ monitoring, although further validation under more diverse and complex real-world conditions is still required.