Copper ion concentration sensing using a CS/PAA-coated SNMNMS fiber interferometer: analysis via adsorption kinetics

A copper ion concentration sensor based on a chitosan/polyacrylic acid (CS/PAA)-functionalized single-mode fiber (SMF)-no-core fiber (NCF)-multimode fiber (MMF)-NCF-MMF-SMF (SNMNMS) interferometer was fabricated and demonstrated. The wavelength shift of the interference dip was used as an optical indicator of copper ion adsorption in the CS/PAA functional coating. The relationship between wavelength shift and adsorption time was well described by a pseudo-first-order (PFO) kinetic model, allowing the adsorption rate constant and equilibrium behavior to be accurately extracted under different copper ion concentrations. A quantitative correlation between wavelength shift and copper ion concentration was thereby established. The proposed sensor exhibited a sensitivity of 0.00132 nm/(mg/L) and a linearity of 0.998 for copper ion concentrations ranging from 100 to 1000 mg/L, with a response time of 16 min. The standard deviation of the wavelength shift in repeated measurements was below 0.021 nm. The wavelength shift induced by copper ion adsorption was significantly larger than that caused by other heavy metal ions, indicating excellent selectivity. Owing to the strong chelating ability of ethylenediaminetetraacetic acid (EDTA), efficient desorption and sensor regeneration were achieved, ensuring stable performance and measurement accuracy in complex environments.