Dual-mode SPR/SERS optical fiber sensor for ultra-trace mercury ions detection

The detection of mercury ions (Hg²⁺) is crucial due to its harmful effects on health and environment. In this article, what we believe to be a novel dual-mode optical fiber sensor incorporating surface plasmon resonance (SPR) and surface-enhanced Raman scattering (SERS) is proposed for ultra-trace Hg²⁺ detection. The sensing probe comprises gold (Au)/graphene oxide (GO) composite membrane structure and Au nanospheres (AuNPs), which are connected via double-stranded DNA. In the presence of Hg²⁺, two single-stranded DNA (ssDNA) modified on the sensing region and AuNPs asymmetrically pair to form a thymine (T) - Hg²⁺ - T structure, facilitating AuNPs attachment to the sensing region. This attachment induces spectral changes, thereby enabling Hg²⁺ detection. In the SPR mode, the limit of detection (LOD) for Hg²⁺ is 1.82 × 10^-12 M. In the SERS mode, AuNPs generate numerous "hot spots" that amplify the Raman signal through electromagnetic enhancement mechanism (EM), the Au/GO composite membrane can undergo charge transfer with Raman molecule to further enhance the Raman signal through chemical enhancement mechanism (CM), thus achieving detection of Hg²⁺ with a LOD of 3.94 × 10^-13 M. The synergy between SPR mode and SERS mode enhances cross-validation of results and improves accuracy and reliability of the assay. Therefore, the sensor proposed in this paper demonstrates strong potential for diverse practical applications.