A silicon nitride coated LSPR based fiber-optic probe for possible continuous monitoring of sucrose content in fruit juices

Abstract

This paper describes a sensor configuration for stable, long term, continuous monitoring of sugar content in the product of fruit juice packaging industry. An improvement in refractive index (RI) sensitivity of a gold nanoparticle (AuNP) coated, U-shaped fiber-optic probe was achieved by a deposition of silicon nitride (SiNx) film on the probe surface. AuNP of two distinct sizes, 18nm and 39nm, were bound to fiber core surfaces to generate LSPR, when excited by light passing through the fiber. Following this, a thin film of SiNx was deposited over the AuNP coated sensing region of the fiber-optic probe, using hot filament chemical vapor deposition. We investigated the effect of SiNx film coating on RI sensitivity for five different thicknesses: 11nm, 16nm, 20nm, 23nm and 40nm. Several concentrations of sucrose solution were used to vary the bulk RI from 1.333 to 1.387 in the sensing region. We have observed 1.24-times RI sensitivity enhancement in case of 39nm AuNP covered with 20nm SiNx film compared to only AuNP coated fiber-optic probe. We also successfully determined the sugar content in three different packaged juices (at various dilution levels). The sensor proved its potential for measurement of sugar in solutions having complex composition with error <0.8Brix, and can be used for monitoring sugar content in production units.