Applications of Nanomaterials in Plasmonic Sensors

Dear Colleague,

Plasmonic sensors (PS) have been actively used for biochemical analysis, medical diagnostics, and environmental monitoring for more than three decades. The operation principle of plasmonic sensitive elements (SE) rests on the resonant nature of excitation of surface plasma oscillations, with the resonance conditions depending strongly on the refractive index of the ambient medium. The latter, in turn, changes, for example, due to the binding or dissociation of the target analyte, or other chemical or physical processes, which is then detected by the sensor. In other words, in most cases such devices are no more than refractometers, but such that are capable of ultra-sensitive label-free measurements, which explains the popularity and widespread use of such devices. The first designs of plasmonic sensors made use of flat (Otto and Kretschmann schemes) or cylindrical (fiber optic schemes) metal / dielectric interfaces as SEs. Over time, plasmonic nanoparticles began to be applied to these boundaries or the boundaries were coated with ultrathin layers of various materials, including those based on graphene and its derivatives. This enabled tunability of the excitation conditions and parameters of the boundary-guided surface plasmon polaritons (SPP) and, as a result, significantly improved the metrological characteristics of plasmonic refractometers, at the same time making them more compact and expanding their range of application.

It is interesting that graphene-based composites in recent years are increasingly considered not only from the point of view of enhancing classical plasmon refractometer schemes, but also as an independent basis for building ultra-sensitive measuring devices. This is due to the unique properties of this 2D material: high mobility of charge carriers, the ability to guide SPP in a wide spectral range with very low absorption losses, the ability to control optical properties, large surface area, remarkable mechanical strength, chemical inertness, and intrinsic biocompatibility. Many researchers are convinced that further progress of sensors based on SPP will be largely determined by the development of sensitive elements based on 2D plasmonic nanocomposites.

It must be noted that nanoscale inhomogeneities on the surface of noble metals, nanoparticles of these metals, as well as core-shell nanostructures or even carbon-based nanotubes can also serve not only as an enhancement to classical refractometers based on propagating SPPs, but as independent sensitive elements of PS. The sensitivity of the spectral parameters of the localized surface plasmon resonance (LSPR) to variations in the ambient refractive index, which is characteristic of such objects, is normally much lower than that of the devices based on propagating SPP. However, the LSPR spectrum is highly dependent on the size and configuration of the subwavelength nanostructures. Therefore, by choosing a proper SE geometry, it is possible to adjust the spectral position of the resonance peak and its shape so as to best match the optical properties of the analyzed substances. It is for this reason that a great number of publications have appeared in recent years on biochemical or medical sensors based on nanoparticles and other subwavelength plasmonic structures.

It should be especially noted that the local field enhancement near subwavelength structures, caused by LSPR, opens up additional measurement possibilities, since it can significantly affect optical processes in molecules, for example, dramatically enhance photoluminescence or Raman scattering (SERS), providing extremely high measurement sensitivity. In recent years, reports have appeared on the possibility of detecting a signal from a single molecule using Raman scattering-based SEs on a single subwavelength plasmonic nanostructure.

However, it should also be mentioned that at ultra-low concentrations of analyte molecules, the latter will be highly dispersed throughout the volume and the probability of their interaction with the localized surface plasmons is very low. This problem can be solved by surrounding the SE with an additional superhydrophobic surface. A drop of an aqueous solution deposited on such a surface slides over it during evaporation due to low adhesion and, ideally, brings the analyte molecules onto the plasmonic sensitive element. Superhydrophobic properties of the surface are achieved by nanopattering, which constitutes another area of ​​application of nanomaterials in the design of plasmonic sensors.

By arranging specifically tailored subwavelength plasmonic nanostructures in certain order on a plane or in space, one can obtain so-called metasurfaces or metamaterials whose collective response will differ significantly from the response of its individual structural units. Taking advantage of this property, one can design SEs with very unusual, unique properties. Ultimately, this seems to be a promising strategy to achieve unprecedented refractometric sensitivity and resolution, as well as meet various requirements of diverse applications.

This Special Issue is devoted to the current trends in the use of nanomaterials in plasmonic sensors, including but not limited to the range of topics covered in this brief introduction.

Prof. Dr. Oleg Vitrik
Guest Editor

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