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.

Keywords: FP interferometer; LSPR; double-clad optical fiber; gold nanorods; multiparameter sensor; reflection-based OF sensor; single-point sensor; smart hydrogel.

Figure 1

Illustration of the double-clad optical fiber combining interferometric and plasmonic sensor modalities with $n_{\mathrm{core}}=1.46208$, $n_{1\mathrm{st}\mathrm{cladding}}=1.45713$, and $n_{2\mathrm{nd}\mathrm{cladding}}=1.44344$. Light in the range of 1500–1600 nm (${\lambda }_{\mathrm{I}}$) is confined as single transverse mode both in the fiber and in the hydrogel volume, with reflection at the OF–hydrogel interface and hydrogel–solution interface illustrated with red color. Multi-mode with light in the range of 450–850 nm (${\lambda }_{\mathrm{II}}$) is guided in the first cladding with numerical aperture illustrated with green on fiber-end face. The FP interference is measured with ${\lambda }_{\mathrm{I}}$, while the LSPR signal from gold nanorods (GNR) is measured with ${\lambda }_{\mathrm{II}}$.

Figure 2

(a) Resonance permittivities (Equation (11)) of two identical GNRs in the s-s or e-e configuration for decreasing d; (b) ${\lambda }_{\max }$ (Equation (14)) of two identical GNRs in the s-s or e-e configuration for decreasing d. ${\epsilon }_m=n_m^2={1.33}^2$, ${\lambda }_p=183$ nm, GNR width = 19 nm, GNR length = 50 nm, and $\mathrm{AR}=50/19$.

Figure 3

Setup of the fiber-optic instrument based on reflection measurements.

Figure 4

(a) Reflectance in the ${\lambda }_{\mathrm{I}}$ range from the hydrogel with GNRs in pH of 4.5; (b) autocorrelation function of the interferometric spectrum.

Figure 5

(a) Reflectance from the GNR-hydrogel with smoothing function, in pH 4.5 with $R_{\lambda }\left(\mathrm{pH}4.5\right)$; (b) reflectance from the GNR-hydrogel in (1) pH 3.0 with $R_{\lambda }\left(\mathrm{pH}3.0\right)$ and $R_{\lambda }\left(\mathrm{pH}4.5\right)$ and (2) 40 wt % glycerol at pH 4.5 with $R_{\lambda }$ (40 wt % glycerol, pH 4.5) and $R_{\lambda }\left(\mathrm{pH}4.5\right)$.

Figure 6

(a) FSR measured for the hydrogel deswellling from pH 4.5 to 3.0 for two sampled series with mean, minimum and maximum values from 4 sampled FSRs; (b) FSR measured for increasing bulk RI with pH 4.5 and 3.0 for one sampled series.

Figure 7

(a) LSPR peak position measured for the hydrogel deswelling from pH 4.5 to 3.0 for two sampled series with mean, minimum and maximum values from 4 sampled LSPR peak positions; (b) the error of the LSPR peak position by holding the reference spectrum constant at $R_{\lambda }\left(\mathrm{pH}4.5\right)$ for pH 4.5 to 3.0. RI = 1.33 (Milli-Q water).

Figure 8

(a) LSPR peak position as a function of bulk RI with pH 4.5 and 3.0 for one sampled series; (b) the error of the LSPR peak position by holding the reference spectrum constant at $R_{\lambda }\left(\mathrm{pH}4.5\right)$ for increasing bulk RI with pH 4.5 and 3.0.

Figure 9

${\lambda }_{\mathrm{II}}$ reflectance spectra for bare GNRs, biotin-functionalized GNR and biotin–streptavidin recombination on GNRs in hydrogel in Milli-Q water with pH at 4.5.