Optical fiber plasmonic sensor for the ultrasensitive detection of copper (II) ion based on trimetallic Au@AgPt core-shell nanospheres

This article presents for the first time an ultrasensitive localized surface plasmon resonance (LSPR) sensor system based on multimode-singlemode-multimode fiber (MMF-SMF-MMF) structure for the accurate measurement of copper (II) Ion (Cu²⁺). Citrate-stabilized Au@AgPt core-shell nanospheres (NSs) with a rough AgPt alloy outer shell were synthesized in house, and then adhered onto the sensing section of MMF-SMF-MMF structure through silane coupling agent. Finally, the Au@AgPt NSs-coated optical fiber probe surface was covered with a layer of PEI as the chelating agent for specific Cu²⁺ trapping. The detection process of Cu²⁺ is conducted by monitoring the resonance intensity variation of the transmission spectrum, which is due to the local refractive index changes result- ing from the specific interactions between Cu²⁺ in solution and PEI immobilized on the sensor surface. The proposed PEI-Au@AgPt NSs modified fiber sensor achieved an ultralow Cu²⁺ detection level down to 10⁻¹⁶ mol/L. The facile fabrication, pollution-free, high sensitivity and cost-effectiveness of the optical fiber sensor system make it rather promising for the precise Cu²⁺ monitoring in a wide range of biological application scenarios.