Vernier-Effect-Enhanced Fabry-Pérot Interferometric Optical Fiber Sensor for Highly Selective Copper-Ion Detection

Abstract

A cascaded single–mode–multimode–hollow-core fiber (SMF-MMF-HCF) optical sensor based on Fabry-Pérot interferometer (FPI) is proposed for ultra-sensitive copper ions (Cu(II)) detection. The cascaded structure provides an unfilled-air reference cavity and an air-material hybrid sensing cavity with slightly detuned free spectral ranges, and by precisely tailoring their lengths, the Vernier effect is generated to intrinsically amplify the sensitivity. The terminal HCF section is partially loaded with an ion-imprinted polymer to provide specific Cu(II) recognition. The specific binding of Cu(II) changes the effective optical path of the material section, thereby inducing a distinct spectral shift of the Vernier envelope. Experimental results demonstrate that the sensor exhibits an ultra-high average sensitivity of −66 nm/$\mathrm{\mu }$M and a remarkably low limit of detection (LOD) of 0.98 nM within a broad dynamic range. Furthermore, the sensor shows exceptional selectivity against interfering ions and excellent reusability. The practical feasibility of the sensor was validated by successfully detecting Cu(II) in tap water and biological fluid samples with high recovery rates, confirming its reliability against complex matrix interference and highlighting its great potential for trace heavy metal monitoring in environmental and biomedical applications.