Fiber-optic metal ion sensor based on thin-core fiber modal interferometer with nanocoating self-assembled via hydrogen bonding

A new fiber-optic metal ion sensor based on a thin-core fiber modal interferometer (TCFMI) is presented. Poly(4-vinylpyridine) (P4VP) and poly(acrylic acid) (PAA) are layer-by-layer deposited on the side surface of the TCFMI via hydrogen bonding for the detection of metal ion in aqueous solutions. Ultraviolet–visible (UV–vis) absorption spectroscopy is employed to monitor the self-assembly process, and the thickness and morphology of the nanocoating are characterized by scanning electron microscopy (SEM). When the sensor is immersed into the aqueous solutions containing metal ions, the refractive index (RI) of the nanocoating is changed because of the association of metal ions with the nanocoating, which was verified by energy dispersive X-ray (EDX). Based on the RI sensing capability of TCFMI, one can detect the concentration of metal ions through the measurement of the dip wavelength of the sensorʹs transmission spectrum. The experiment results exhibit that the sensor is reusable and with a fast response in a wide metal ion concentration range (10 nM–0.1 M). The limit of detection (LOD) of the sensor is around 9.6 nM experimentally.