Application of Plasmonic Sensors

Ane Galarneau^*
*Correspondence: Ane Galarneau, Research Director, CNRS Institute Charles Gerhardt of Montpellier, France, Email:

Introduction

Its large enhancement factors and the ability to recognise species fingerprints, has become the most common surface-enhanced spectroscopic sensor. The boost in SERS is attributed to the strong EM contribution caused by SPR excitation and the modest chemical contribution caused by the charge transfer effect. The SERS-active substrate improves both the incident and dispersed electric fields in terms of EM enhancement. As a result, the overall Raman enhancement is the product of the intensity enhancements at the incident and Raman-scattered frequencies, which is roughly equivalent to the fourth power of the EM field enhancement and can approach 109 -1010 under ideal excitation setups. SERS, in addition to the putative chemical action [1].

The nanogaps between metal nanostructures or sharp features of single nanostructures that create substantial EM field amplification during resonant stimulation are the main characteristics of a SERS-active substrate. So far, aggregates and self-assembled colloidal metal nanoparticles, nanofabricated arrays of metal NPs on substrates, metal island films, and roughened electrochemical metal electrodes have been created utilising either the bottomup or top-down approach.

However, during SERS measurements on such surfaces, a variety of issues frequently develop. First, the laser power on a low SERS-activity substrate must be high, potentially causing heat damage to the sample. Second, photoinduced chemical reactions are conceivable, which obscures or eliminates inherent spectrum information and complicates spectral analysis. The reactions might occur as a result of either the incoming laser's photochemical effect or plasmonic phenomena, such as the creation of plasmon-induced hot electrons. Third, because the molecules are generally permanently deposited on the metal surface, the SERS-substrates are not reusable, which raises the cost of SERS sensing devices. Some new SERS strategies have been developed to address, at least in part, the aforementioned shortcomings.

SPR sensors, on the other hand, are based on the resonant peak shift of SPs caused by changes in the refractive index of the surrounding environment. The molecules adsorbing on the surface of metal nanostructures have refractive indices that differ from the surrounding medium. Although the change is minor, the movement in SPR peak locations can nevertheless be noted. The amount of peak shift per refractive index unit change is commonly used to characterise the performance of an SPR sensor. Significant efforts have been made to increase the performance of both localised surface plasmon resonance sensors and surface plasmon polaritons sensors.

To account for the effect of peak width on sensing performance, the sensitivity is divided by the full width at half maximum, yielding a figure of merit that is a more trustworthy measure of sensing performance. As a result, narrow peaks are chosen for sensing. FoM values for LSPR sensors are typically less than 10 and somewhat higher for SPP-based sensors [ A variety of nanostructures and sensing approaches have been investigated in try to enhance the FoM. The multipolar modes of LSPR have a higher FoM than the dipolar mode, and Fano-resonances in plasmonic NP oligomers can also produce high FoM values for sensing. The refractive index sensing method was used to propagate SPPs in metal nanowires.

The incident wavelength and the dielectric properties of both the surroundings and the metal dictate the wavelength of SPP. The near-field distribution of SPP in a metal NW of limited length is governed by the interference of SPP modes in the NW and may be detected visually using the quantum-dots imaging technique [2]. The period of the near-field distribution is determined by the dielectric constant of the environment. Furthermore, the surroundings influence the NW's output spectrum. Such characteristics result in a novel form of SPP sensor. This review article will discuss current breakthroughs in plasmonic sensors, including as novel SERS sensing approaches, LSPR sensing employing multipolar modes and Fano-resonances, and metal NWsbased SPP sensors.

Metallic nanoparticles have attracted a lot of attention because they have a unique optical response that is driven by the stimulation of localised surface plasmon resonances. These resonances are caused by the coherent oscillation of free electrons in a metallic particle, which is induced by an electromagnetic field. Because of their sensitivity to the medium around the nanoparticle, LSPRs are being researched for application in the construction of sensitive biological sensors with ultrasmall detection volumes. The figure of merit of plasmonic sensors is often described as the resonance shift caused by a change in the refractive index of the surrounding dielectric, normalised by the resonance line width. Narrow resonances, as a result, increase the FoM of plasmonic sensors.

A direct experimental comparison of the refractive index sensing capabilities of LSPRs in gold nanoparticles with propagating surface plasmon polaritons on extended gold surfaces, also known as surface plasmon resonances, was recently published. Despite having a low bulk refractive index sensing figure of merit of usually, LSPR-based sensing was shown to be a highly competitive technology to traditional SPR for the detection of changes in the refractive index near to the surface. The considerable confinement of the LSPR electromagnetic field surrounding the nanostructures has been linked to the great sensitivity. Recent advances in the FoM of gold nanoparticle sensors have been made possible by linking plasmonic resonances in systems including two or more nanoparticles, nanoholes, and nanowells.

This connection can result in narrow Fano resonances caused by interference between a discrete state and a continuum of states. The heightened sensitivity of Fano resonances in periodic arrays of metallic particles to minute changes in the refractive index of the surrounding medium is demonstrated in this article [3]. We show that linking these localised resonances with Rayleigh anomalies improves the FoM by more than an order of magnitude when compared to LSPRs. Rayleigh anomalies reflect the start of diffraction in a grating. At the wavelength and angle of incidence corresponding to the Rayleigh anomaly, there is a transition between a diffracted order propagating in the plane of the array and an evanescent diffracted order.

Because of this connection, narrow resonances develop in the array's transmittance and reflectance spectra. These resonances are known as surface lattice resonances, and they were initially hypothesised by Carron et al. in the context of surface-enhanced Raman scattering. Schatz and colleagues expanded on this notion in future research. The earliest experimental evidences of this phenomena were found in 1D arrays [4]. Only lately has there been unambiguous proof of the stimulation of these modes in arrays of microscopic particles and nanoantennas, as well as the first demonstration of the use of these resonances for optical sensing. Surface lattice resonances are defined as the interference of a large localised surface plasmon resonance owing to Fano resonances.

This interference produces the asymmetric line forms that are characteristic of Fano resonances. Furthermore, we discover that the frequency difference between the surface lattice resonance and the Rayleigh anomaly ultimately governs the FoM for arrays of low loss metals, that is, metals with an imaginary component of permittivity significantly less than the modulus of the real component. The frequency difference is determined by the array's lattice constant and particle size. In disordered arrays of nanoparticles, which do not display collective behaviour, such a universal scaling of the FoM does not occur. Because the spectrum location of surface lattice resonances is regulated by geometrical factors, they provide enormous flexibility in the design of plasmonic response [5].

Conflict of Interest

None.

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