Highly sensitive optical fiber SPR sensor based on chitosan for the detection of trace Cu²⁺ ion in aqueous solution

In this paper, a highly sensitive fiber-optic sensor was proposed for the detection of Cu²⁺ traces in aqueous solution which has a nonnegligible impact on human health. Based on the principle of SPR, a uniform chitosan (CS) material was coated on a multimode fiber-single mode fiber-multimode fiber (MSM) structure with silver layer by glutaraldehyde (GLA) and graphene oxide modification. The amino and hydroxyl groups in the material have a recognition function for Cu²⁺, which can chelate with Cu²⁺ and change the ambient layer refractive index of the sensor leading to a shift in the SPR spectrum. Trace Cu²⁺ can be detected by monitoring the shift in the resonance wavelength of the SPR spectrum. The results illustrate that within the range of 0.1 ~ 1 mg/L Cu²⁺ concentration, as the Cu²⁺ concentration increasing, the SPR spectrum undergoes a red shift and the maximum detection sensitivity of Cu²⁺ ions achieves 49.466 nm/(mg/L), with a good linearity of 0.996. The temperature experimental results indicate that the sensor has good stability at room temperature (25 ~ 35 °C). The stability and repeatability characteristics of the system are also investigated. The good performance provides a new method in trace Cu²⁺ in-situ detection.