Ultra-sensitive optical fiber SPR Cu2+sensor enhanced by a multilayers Schiff base-hydrogel with temperature compensation

Given the dual role of copper ions (Cu 2+ ) as essential micronutrients and harmful pollutants, the demand for developing sensitive and selective sensors capable of trace-level Cu 2+ monitoring in water bodies is urgently needed. In this paper, an ultra-sensitive surface plasmon resonance (SPR) optical fiber Cu 2+ sensor is proposed, which consists of a multimode fiber (MMF) and a 1 cm single-hole dual-core micro-structured fiber (MOF). Femtosecond laser micromachining is employed to fabricate microfluidic channel within the MOF, followed by polydimethylsiloxane (PDMS) infusion to induce an effective refractive index (RI) difference between dual cores to construct a Michelson interferometer (MI) for temperature monitoring. Simultaneously, a gold film is deposited on the MOF surface served as the SPR excitation channel, and multilayers Schiff base-functionalized hydrogel composites are immobilized onto the gold film through electrostatic layer-by-layer (LBL) self-assembly. The specific chelation between Cu 2+ with Schiff base ligands triggers hydrogel swelling, thereby inducing significant resonance wavelength shifts in the SPR spectrum. Experimental results demonstrate that the sensitivity of Cu 2+ has increased by 6 times, with the maximum sensitivity reaching −504.96 nm/(mg/L) and the limit of detection (LOD) as low as 11.0149 nM. The MI channel exclusively responds to PDMS variations due to temperature, exhibiting insensitivity to Cu 2+ concentration fluctuations, thereby eliminating the temperature cross-talk in the SPR channel. The proposed method provides a new thought for the integrated design of sensitive and specific sensors, which has great potential in food safety, environment and biology monitoring.