Surface Plasmon Resonance Enhanced Plug-and-Play Copper Ion Sensor Based on a U-Shaped Optical Fiber

For human health and environmental protection, the identification of heavy metal ions in seawater is of great importance. Here, we presented and demonstrated a surface plasmon resonance (SPR) sensor for plug-and-play type copper ion (Cu2+) detection based on a U-shaped optical fiber. The suggested sensor is made of a U-shaped multi-single–multi (MSM) mode optical fiber coating with a gold film, which can produce a stronger evanescent field and greatly enhance the sensing characteristic of the SPR sensor. The maximum refractive index (RI) sensitivity of 7045.61 nm/RIU can be obtained by optimizing the relevant parameters of the sensor through simulation and experiment. The functionalization of the sensor was realized by chitosan and synthetic Schiff base in a simple way. The specific complexation of Cu2+ by Schiff base induces an RI change on the sensor surface, achieving a sensitivity of −35.35 nm/lg(mg/L) in the concentration range of the 0.07–1 mg/L. The developed sensor probe is capable of detecting Cu2+ concentration as low as $4.61\times 10^{-4}$ mg/L. Additionally, the repeatability, stability, specificity, and response time have been assessed to demonstrate that the sensor has significant capacity for future use in metal ions detection.