Discrimination of Bulk and Surface Refractive Index Change in Plasmonic Sensors with Narrow Bandwidth Resonance Combs

Abstract

A method to enable surface plasmon resonance (SPR) sensors to discriminate between bulk and surface-localized refractive index changes is demonstrated with modified gold-coated tilted fiber Bragg grating SPR sensors (TFBG-SPR). Without this capability, all high-resolution SPR sensors should be using reference channels and strict temperature control to prevent the contamination of the desired detection of surface-localized chemical or binding events by drift of the refractive index of the medium, in which the experiment is carried out. The very fine comb of high-quality-factor resonances of a TFBG-SPR device coupled to the large differential sensitivity of some of the resonances to various perturbations is used to measure unambiguously the refractive index changes within a surface layer thinner than 25 nm from those of the bulk surrounding. The enabling modification of the conventional TFBG-SPR is a reduction of the gold coating from its optimum value near 50–30 nm: at this lower thickness, a surface plasmon wave can still be excited by a limited number of cladding mode resonances, but at the same time, the metal is thin enough to allow modes away from the SPR to tunnel across the metal and probe the bulk RI value. Measurements and simulations of the deposition of a self-assembled monolayer of 1-dodecanethiol in ethanol show that the bulk refractive index changes as small as 0.0004 can be distinguished from the formation of a 1 nm thick coating on the surface of the fiber.

Accurate Correction of the “Bulk Response” in Surface Plasmon Resonance Sensing Provides New Insights on Interactions Involving Lysozyme and Poly(ethylene glycol)

“…Future work may implement more precise measurements of the RI of the bulk solution, 45 but it needs to be measured at the sensing spot to avoid artifacts from the liquid exchange process influencing the kinetics. Notably, the metal film thickness may be altered 5 to improve the resolution in the TIR angle, although this will also influence the resolution in the SPR angle.…”

“…Future work may implement more precise measurements of the RI of the bulk solution, 45 but it needs to be measured at the sensing spot to avoid artifacts from the liquid exchange process influencing the kinetics. Notably, the metal film thickness may be altered 5 to improve the resolution in the TIR angle, although this will also influence the resolution in the SPR angle. We analyzed the interaction between poly(ethylene glycol) brushes and the protein lysozyme at fully physiological conditions, i.e., an ordinary PBS buffer, and found an equilibrium affinity of ∼200 μM for these particular PEG brushes.…”

Mode-division and spatial-division optical fiber sensors

“…A similar approach can be taken for plasmonic-assisted sensors, by adding a layer of metal to a calibrated grating in the simulations and calculating wavelength shifts for the given sensing modality [70,[183][184][185][186][187]. Figure 24 shows how the simulated spectrum of a gold-coated TFBG in liquid varies with the refractive index of the liquid.…”

Research on the High Temperature and High Pressure Gold-Plated Fiber Grating Dual-Parameter Sensing Measurement System

“…In 2021, Yanjun Zhang et al [27] conducted research on goldplated FBG; the sensitization characteristics of the sensor were theoretically analyzed, and the response characteristics of the sensor were studied. To date, gold-plated FBG is mostly used in plasmon resonance [28][29][30] and battery biomarkers [31,32], and there have been few studies on the temperature and pressure.…”