We demonstrate a new single point, multi-parameter, fiber optic sensor concept based on a combination of interferometric and plasmonic sensor modalities on an optical fiber end face. The sensor consists of an extrinsic Fabry-Perot interferometer in the form of a hemispherical stimuli-responsive hydrogel with immobilized gold nanoparticles (GNPs). The GNPs exhibit local surface plasmon resonance (LSPR) that is sensitive towards the local refractive index (RI) of the surrounding environment, while the stimuli-responsive hydrogel is sensitive towards specific chemical compounds. We evaluate the quality of the interferometric and LSPR signals as function of GNP concentration and of hydrogel swelling degree stimulated by ethanol solutions. The GNPs have little influence on the visibility of the Fabry-Perot etalon. The swelling degree of the hydrogel, with corresponding bulk RI changes, have little influence on the local surface RI of the GNPs. We expect this novel sensor concept to be of great value for biosensors for medical applications.applications.

We report on characterization of an optical fiber-based multi-parameter sensor concept combining localized surface plasmon resonance (LSPR) signal and interferometric sensing using a double-clad optical fiber. The sensor consists of a micro-Fabry-Perot in the form of a hemispherical stimuli-responsive hydrogel with immobilized gold nanorods on the facet of a cleaved double-clad optical fiber. The swelling degree of the hydrogel is measured interferometrically using the single-mode inner core, while the LSPR signal is measured using the multi-mode inner cladding. The quality of the interferometric signal is comparable to previous work on hydrogel micro-Fabry-Perot sensors despite having gold nanorods immobilized in the hydrogel. We characterize the effect of hydrogel swelling and variation of bulk solution refractive index on the LSPR peak wavelength. The results show that pH-induced hydrogel swelling causes only weak redshifts of the longitudinal LSPR mode, while increased bulk refractive index using glycerol and sucrose causes large blueshifts. The redshifts are likely due to reduced plasmon coupling of the side-by-side configuration as the interparticle distance increases with increasing swelling. The blueshifts with increasing bulk refractive index are likely due to alteration of the surface electronic structure of the gold nanorods donated by the anionic polymer network and glycerol or sucrose solutions. The recombination of biotin-streptavidin on gold nanorods in hydrogel showed a 7.6 nm redshift of the longitudinal LSPR. The LSPR response of biotin-streptavidin recombination is due to the change in local refractive index (RI), which is possible to discriminate from the LSPR response due to changes in bulk RI. In spite of the large LSPR shifts due to bulk refractive index, we show, using biotin-functionalized gold nanorods binding to streptavidin, that LSPR signal from gold nanorods embedded in the anionic hydrogel can be used for label-free biosensing. These results demonstrate the utility of immobilizing gold nanorods in a hydrogel on a double-clad optical fiber-end facet to obtain multi-parameter sensing.

The paper presents results obtained by experimentally testing fiber optic based SPR (Surface Plasmon Resonance) sensors for quality monitoring of aqueous media. The research focuses on the direct impact of the temperature in such measurements for certain applications, being introduced as a parasitic effect. A novel and cost effective configuration for a fiber optic based SPR sensor system, exploiting a compensation method of the thermo-optic effect, is also presented. Its working principle is already proven based on previous results and performances obtained from experimental work, and it is ready to be implemented. The sensor is basically a refracto-meter in the range of 1.33-1.37 RI (Refractive index), thus for aqueous media, intended to be used as a long-term monitoring device for the quality of the swimming pool water (real time chlorine concentration analysis) compensating the parasitic effect of temperature.

The activity hereby presented is part of a project aimed at developing a compact all-fibre setup that exploits the Surface Plasmon Resonance (SPR) phenomenon to detect changes of chemical composition in a flowing liquid. The work has reached a lab prototype level, consisting of a plastic pipe with an embedded multi-mode optical fibre SPR sensor used as a refractometer. A demonstrator has been realised connecting the pipe to a water reservoir through a recirculating pump. Introduction of a contaminant into the flowing water produces a shift of the SPR. The system, tested with isopropyl/water mixtures, shows the capability to measure refractive index variations with a sensitivity of 2000 nm/RIU (RIU stands for Refractive Index Unit). The control unit is developed upon a commercial easy-to-use platform for demonstration and educational purposes.