No abstract

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.

A silver thin-film-based fiber-optic plasmonic sensor for the detection of a phenolic analyte, catechol, is elicited in the present study. The sensor relies on the interaction of catechol with a cetyltrimethylammonium-bromide (CTAB)-functionalized zinc oxide/carbon nanotube (ZnO/CNT) nanocomposite coated over the silver film. A simple sol−gel method is reported for the synthesis of the nanocomposite. The morphological, elemental, and structural characterizations of the nanocomposite are confirmed by various microscopic and spectroscopic techniques. The sensor's performance is studied for the catechol concentration range of 0−100 μM. The fabricated probe shows efficiency in a very wide pH range depending on the concentration of CTAB utilized in the preparation of the probe. Various operational parameters are adjusted to obtain the optimum sensor performance. Efficiently reproducible and selective performance is obtained after analyzing various possible interferants like dopamine, ascorbic acid, hydrazine, and so on. The approach is found to be feasible for real sample analysis with an efficient recovery percentage. The highest sensitivity of the sensor, 5.46 nm/μM, is obtained at the lowest catechol concentration with a limit of detection of 0.1 μM. The utilization of an optical fiber makes the approach very simple and miniaturized for on-field detection or real-time remote sensing applications. The surface plasmon resonance results of the sensor reveal the excellent performance of the CTAB-functionalized ZnO/CNT nanocomposite in the effective quantification of catechol.

A highly sensitive ammonia gas sensor exploiting the gas sensing characteristics of tin oxide (SnO₂) has been reported. The methodology of the sensor is based on the phenomenon of surface plasmon resonance (SPR) with a fiber-optic probe consisting of coatings of silver as a plasmonic material and SnO₂ as the sensing layer. The sensing principle relies on the change in refractive index of SnO₂ upon its reaction with ammonia gas. The capability of the sensor has been tested for a 10 to 100 ppm concentration range of ammonia gas. To enhance the sensitivity, probes with different thicknesses of SnO₂ have been fabricated and characterized for ammonia sensing. It has been found that at a particular thickness the sensitivity is highest. The reason for the highest sensitivity at a particular thickness has been evinced theoretically. The electromagnetic field distribution for the multilayer structure of the probe reveals the enhancement of the evanescent field at the tin oxide-ammonia gas interface, which in turn manifests the highest shift in resonance wavelength at a particular thickness. The selectivity of the probe has been tested for various gases, and it has been found to be most accurate for the sensing of ammonia. A sensor utilizing optical fiber, the SPR technique, and metal oxide as sensing element combines the advantages of a miniaturized probe, online monitoring, and remote sensing on one hand and stability, high sensitivity and selectivity, ruggedness, and low cost on the other.

Surface-enhanced Raman scattering (SERS) is an effective and widely used technique to study chemical reactions induced or catalyzed by plasmonic substrates, since the experimental setup allows us to trigger and track the reaction simultaneously and identify the products. However, on substrates with plasmonic hotspots, the total signal mainly originates from these nanoscopic volumes with high reactivity and the information about the overall consumption remains obscure in SERS measurements. This has important implications; for example, the apparent reaction order in SERS measurements does not correlate with the real reaction order, whereas the apparent reaction rates are proportional to the real reaction rates as demonstrated by finite-difference time-domain (FDTD) simulations. We determined the electric field enhancement distribution of a gold nanoparticle (AuNP) monolayer and calculated the SERS intensities in light-driven reactions in an adsorbed self-assembled molecular monolayer on the AuNP surface. Accordingly, even if a high conversion is observed in SERS due to the high reactivity in the hotspots, most of the adsorbed molecules on the AuNP surface remain unreacted. The theoretical findings are compared with the hot-electron-induced dehalogenation of 4-bromothiophenol, indicating a time dependency of the hot-carrier concentration in plasmon-mediated reactions. To fit the kinetics of plasmon-mediated reactions in plasmonic hotspots, fractal-like kinetics are well suited to account for the inhomogeneity of reactive sites on the substrates, whereas also modified standard kinetics model allows equally well fits. The outcomes of this study are on the one hand essential to derive a mechanistic understanding of reactions on plasmonic substrates by SERS measurements and on the other hand to drive plasmonic reactions with high local precision and facilitate the engineering of chemistry on a nanoscale.