Fabrication and characterization of a highly sensitive fiber-optic cholesterol sensor utilizing cholesterol oxidase (ChOx) have been carried out. The operation of the sensor is based on combined phenomenon of localized and propagating surface plasmons. Two different immobilization schemes of enzyme ChOx have been used. In one, the enzyme is entrapped in the hydrogel and in second the enzyme is immobilized over the graphene oxide (GO) nanosheets. The synthesis of GO is confirmed by XRD, TEM, and Raman spectroscopy and the binding between ChOx and GO is verified through the Raman spectroscopy and Fourier transform infrared spectroscopy techniques. Following three types of probes having different configurations are fabricated and compared: 1) enzyme entrapped gel layer over silver coated unclad fiber; 2) enzyme immobilized over silver and GO coated unclad fiber; and 3) enzyme immobilized over silver, GO, and silver nanoparticles coated unclad fiber. The performance of all these probes have been compared in terms of operating range, sensitivity and limit of detection. The sensors have many advantages, such as compact size, modest cost, high sensitivity, high selectivity, and capability of remote sensing.

The surface plasmon resonance (SPR) technique is a remarkable tool, with applications in almost every area of science and technology. Sensing is the foremost and majorly explored application of SPR technique. The last few decades have seen a surge in SPR sensor research related to sensitivity enhancement and innovative target materials for specificity. Nanotechnological advances have augmented the SPR sensor research tremendously by employing nanomaterials in the design of SPR-based sensors, owing to their manifold properties. Carbon-based nanomaterials, like graphene and its derivatives (graphene oxide (GO)), (reduced graphene oxide (rGO)), carbon nanotubes (CNTs), and their nanocomposites, have revolutionized the field of sensing due to their extraordinary properties, such as large surface area, easy synthesis, tunable optical properties, and strong compatible adsorption of biomolecules. In SPR based sensors carbon-based nanomaterials have been used to act as a plasmonic layer, as the sensitivity enhancement material, and to provide the large surface area and compatibility for immobilizing various biomolecules, such as enzymes, DNA, antibodies, and antigens, in the design of the sensing layer. In this review, we report the role of carbon-based nanomaterials in SPR-based sensors, their current developments, and challenges.

In this paper, we present numerical and experimental results on Localized Surface Plasmon Resonance (LSPR) refractive index (RI) sensitivity, Figure of Merit (FoM), and penetration depth (d_p) dependence on spherical gold nanoparticles (AuNPs) size, and the effects of AuNP dimer interparticle distance (d_s) studied numerically. These parameters were calculated and observed for d = 20, 40, 60, 80, and 100 nm diameter spherical AuNPs. Our investigation shows d = 60 nm AuNPs give the best FoM. The AuNP dimer interparticle distance can significantly influence the RI sensitivity. Therefore, the effect of distances between pairs of d = 20 nm and 60 nm AuNPs is shown. We discuss the importance of penetration depth information for AuNPs functionalized with aptamers for biosensing in the context of aptamer size.