Three-layer optical sensor “prism–metal–water suspension of inclusions” based on surface plasmon resonance

An optical scheme of the Kretchman type is considered, in which the receptor layer and the analyte layer bordering the metal (gold) nanolayer do not exist separately but represent a single receptor-analyte layer in the form of an aqueous suspension of microinclusions. The presence of microinclusions forms the effective dielectric constant of the suspension, which is different from the dielectric constant of the components (water and inclusions). Surface plasmon resonance (SPR) consists in the possibility of determining the resonance wavelength and resonance angle of incidence from the minimum of the reflection coefficient and significantly depend on the sign, in this case, of the effective dielectric constant of the suspension. Therefore, in this study, the main attention was devoted to the effective permittivity sign of the suspension, since the SPR exists only at its positive values. The influence of two factors on the effective dielectric constant was investigated. These are the concentration of inclusions to the aqueous suspension and their dielectric constants. It is shown that for protein and nucleic inclusions, the effective dielectric constant cannot be negative for any concentrations if the dielectric properties of the inclusions are considered to be exclusively real. However, the real part of the dielectric constant of protein and nucleic molecules varies from −1 to ≈2.5, depending on the length of the molecular chain. Therefore, a study of the effect of the real part of the dielectric constant of such inclusions on the effective dielectric constant of the suspension for several actual concentration values was also conducted. It is confirmed that for protein and nucleic inclusions, in the general case, the effective dielectric constant of the suspension can also be only positive and the SPR is implemented at any concentration in the selected range (concentration > 10%). It is also shown that for inclusions with a purely negative (< –1) real part of the dielectric constant, negative values of the effective dielectric constant of the suspension can also be realized.