LSPR and Interferometric Sensor Modalities Combined Using a Double-Clad Optical Fiber

Abstract

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.

Mapping Nanoparticles in Hydrogels: A Comparison of Preparation Methods for Electron Microscopy

“…This was found to be due to the inhomogeneous distribution of the particles in the hydrogel. The LSPR response for decreasing interparticle distances, where dominated by the side-by-side plasmon coupling between spheroidal GNPs [53]. This was likely due to an ordered tendency of the spheroidal particles in the hydrogel.…”

“…By focusing on detecting mainly SE and BSE intensities, the studies are limited by assuming that the GNRs are embedded in the hydrogel after fixation, dehydration, and drying. This assumption is based on previous results obtained in different hydrogel-GNR sensor configurations in [51][52][53], where the LSPR signal was stable during different swelling experiments, i.e., the GNRs were shown to be contained in the hydrogel. Due to these results, the use of general distribution analytical techniques such as X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDAX), or prompt gamma neutron activation analysis (PGAA) are omitted to direct the focus on morphological characterization and the study of GNRs positioned in hydrogel polymer networks.…”

“…We have in earlier work developed proof-of-concepts of fiber optic (FO) sensors based on using NMNPs immobilized in polyacrylamide hydrogels and combining LSPR and interferometric sensing modalities [25,[50][51][52][53]. The interferometric sensing modality detects the change in volume of a stimuli-responsive hydrogel, whereas the LSPR sensing modality detects receptor-analyte recombinations on the NMNP surface.…”

“…The dipole-dipole coupling and RI sensitivity of the LSPR in NMNPs may as well be utilized for detecting the change in volume of stimuli-responsive hydrogels for significant swelling and deswelling. The LSPR response of spherical or spheroidal gold nanoparticles (GNP) in hydrogels has shown to deviate from the LSPR response of randomly distributed spherical or spheroidal GNPs [51][52][53]. The plasmon coupling between spherical GNPs occurred for an interparticle distance that was larger than the estimated plasmon coupling distance [51].…”

“…Here, the morphology of 10 wt % polyacrylamide (AAM-BIS) and poly(acrylamide-co-acrylic acid) (AAM-AAC-BIS) hydrogels, with and without gold nanorods (GNRs) as composite materials, was studied using conventional SEM. Low GNR densities were used (10/µm 3 ) as for the earlier FO sensor studies in [51][52][53]. The hydrogels were prepared for SEM using CPD or the drying with HMDS and characterized concerning pore structures and GNR positions in the polymer network.…”