Figures of Merit for Refractometric LSPR Biosensing

Abstract

The aim of this chapter is to describe routes to improve the features of plasmonic nanoparticles as refractometric based biosensors. Taking advantage of the tunability of their localized surface plasmon resonance (LSPR), we explore the sensing performance as a function of the LSPR spectral position. Firstly, we show the ambiguities that can arise from the description of the sensitivity in the wavelength and energy scales. However, we will see how such ambiguities can be circumvented with the introduction of the figure of merit (FOM), defined as the quotient between sensitivity and width of the resonance peaks, since this parameter is equivalent in both energy and wavelength scales. The spectral analysis reveals that the sensitivity to local changes of refractive index close to the metal surface can be comparable or even larger than that of conventional SPR sensors when the resonance position of the nanoparticles is properly selected. Indeed, for a fixed nanoparticle volume, we show that the surface sensitivity only depends on the spectral position of the resonance, whereas the shape of the particle only plays a secondary role. In addition, the FOM displays an optimized spectral region, located between 700 and 900 nm in the case of gold, which coincides with the region where the quotient between real and imaginary parts of the dielectric constant of the metal is maximized.

Finally, we study the influence of the supporting substrate and adhesion layers, required to improve the mechanical stability of the nanoparticles, on their refractometric FOMs. Just the presence of the substrate can induce a sensitivity reduction larger than 40%, especially in substrates with high refractive index. On the other hand, the adhesion layer can drastically downgrade the sensing performance of plasmonic nanoparticles due to the large decrease of their scattering or extinction cross-sections and the substantial broadening of their LSPR peaks. In the particular case of Cr adhesion layers, the FOM can show a fourfold decrease, while the scattering cross-section can be reduced close to one order of magnitude, adding drastic constraints to the limit of detection and performance of nanoplasmonic sensors. Therefore, minimization of the substrate refractive index and the thickness of the adhesion layers are prerequisites to ensure excellent limits of detection in refractometric nanoplasmonic sensors.