Detection of Cu²⁺ in an Aqueous Solution Based on CS/PAA-Modified Fiber-Optic SPR Sensor

A surface plasmon resonance (SPR) sensor based on multimode fiber-single-mode fiber-multimode fiber (MMF-SMF-MMF) structure was designed to detect the concentration of Cu<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup>. The sensor uses a magnetron sputtering instrument to coat the surface of the fiber with a 50 nm silver film and modify it with chitosan (CS)/polyacrylic acid (PAA) film to detect Cu<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> concentration. The Cu<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> absorption by CS/PAA results on a greater refractive index change, inducing a larger variation response in the surface plasma wave. The detection limit of the sensor is 0.1127 ppm in the range of 0.01-100 ppm, and the maximum sensitivity is 1185.35 nm/ppm and -130.47 nm/ppm from 0.01-1 ppm and 1-100 ppm, respectively, indicating that the sensor performs well at low concentrations. In addition, the experiment shows the effect of the number of layers of CS/PAA film on the sensitivity of the sensor, which provides new ideas for the field of metal ion detection.