Dithiol Self-Assembled Monolayer Based Electrochemical Surface Plasmon Resonance Optical Fiber Sensor for Selective Heavy Metal Ions Detection

Herein, we report the simultaneously application of surface plasmon resonance (SPR) spectroscopy and electrochemistry (EC) technology for the real-time quantitative investigation of heavy metal ions, with high selectivity and sensitivity. A surface of optical fiber coated over with nanometric gold film and then modified with a dithiol self-assembled monolayer of 1,6-hexanedithiol (HDT) was developed as the dual mode electrochemical-surface plasmon resonance (EC-SPR) optical fiber sensor. The limit of detection of the proposed sensor exposed in Hg ²⁺ ions solution without and with the multiple cyclic voltammetry (CV) scans to activate the HDT monolayer can be improved from 0.47 μM to 0.22 μM. More importantly, the CV scans can activate the redox reaction of Hg ²⁺ ions, and then easily strip the Hg ²⁺ ions from the HDT monolayer to restore the active sites, and endow the designed sensor the characteristics of repeated use. Under simultaneous SPR monitoring, the differential pulse voltammetry (DPV) currents of HDT-functionalized optical fiber can also act as a microelectrode to develop electrochemical sensor platform for monitoring Cu ²⁺ , with a limit of detection of 94 nM and a dynamic range up to 90 μM. As a comparison, the EC-SPR sensor still has good analysis performance for both Hg ²⁺ and Cu ²⁺ ions in presence of Ni ²⁺ , Fe ²⁺ , Co ²⁺ , Pb ²⁺ , K ⁺ , among others. We therefore demonstrated that the proposed fiber-based EC-SPR sensor has the advantages of real-time, miniaturized, and cost-effective sensing, paving the way for environmental monitoring applications for on site inspection.