High-sensitivity optical chemosensor based on coated long-period gratings for sub-ppm chemical detection in water

Abstract

In this work, the use of fiber long-period gratings (LPGs)—coated with nanoscale overlays of Syndiotactic Polystyrene (sPS) in the nanoporous crystalline $\delta$ form as specific and highly sensitive chemical sensors for in water monitoring—is proposed. The approach presented here, combines the excellent sorption properties of $\delta$ form sPS as a chemosensitive layer with the excellent refractive index sensitivity of LPG-based sensors as ideal transducers. In particular, when overlays with a high refractive index compared with the cladding one are deposited along the grating region, as in this case, the refraction-reflection regime at the cladding-overlay interface occurs. As result of this mechanism, the attenuation bands of coated LPGs would respond to the optical changes induced in the sensitive overlay due to chemical sorption by a significant modification of the peak central wavelength and intensity. The sensitivity depends strongly on the overlay thickness and the grating coupled cladding mode. Here, sensor probes were prepared by using a dip coating technique and a proprietary procedure to obtain the $\delta$ form sPS. An experimental demonstration of the sensor capability to perform sub-ppm detection of chloroform in water at room temperature is reported here. Also, the effects of the overlay thickness and the cladding mode order on sensor sensitivity and response time have been numerically and experimentally investigated.