U-shaped fiber optic SPR sensor for trace Copper ion detection

The accurate detection of heavy metal ions in aqueous environments is of paramount significance for both environmental conservation and the safeguarding of human health. Based on the surface plasmon resonance (SPR) principle, this study presents a U-shaped multimode fiber SPR sensor designed for detecting copper ion (Cu²⁺) concentration. The sensor eliminates traditional splicing processes by employing a single segment of multimode fiber, thereby significantly enhancing structural stability and sensing performance. The U-shaped sensing region was fabricated using an alcohol lamp. It was subsequently functionalized by sputtering an Au film, followed by coating with barium titanate (BaTiO₃), polyacrylic acid (PAA), and chitosan (CS) films. The specific adsorption of Cu²⁺ by the PAA/CS composite film induces a decrease in the sensing region's effective refractive index (RI), leading to a blue shift in the resonance wavelength. Experimental results demonstrate that the sensor exhibits a sensitivity of -686.2 nm/ppm and a limit of detection (LOD) of 0.0146 ppb for Cu²⁺ across a concentration range of 0.01 to 0.30 ppm, highlighting its superior capability for detecting low-concentration analytes. Moreover, it can accurately detect the presence of Cu²⁺ in aqueous solutions. Furthermore, the sensor shows outstanding stability, repeatability, and specificity, with a relative standard deviation (RSD) of 0.22% in stability tests and a maximum error of 0.0075 ppm in Cu²⁺ concentration detection. The sensor is characterized by simple fabrication, low cost, and compact structure, offering a promising approach for the application of fiber optic SPR technology in heavy metal ion detection.