A New Method to Determine the Response of Kretschmann's Structure-Based Biosensors

Optical sensors have been used to detect refractive index variations in several areas, such as medicine or the environment, due to their excellent characteristics, mainly in reproducibility, sensitivity, and response time. Sensors based on surface plasmon polaritons (SPPs), such as Kretschmann's, are a promising alternative since they allow miniaturization of the devices, useful for "in vivo" detection. In this article, a more accurate method is proposed to obtain the reflectance at the interfaces of the Kretschmann structure. The successive steps of the new method allow a simple physical interpretation of the whole process, unlike previous methods. Experimental results and others obtained from the previous methodologies allow the validation of the proposed method. The method validation from experimental results is performed in BK7-Au-air and BK7-Cr-Au-air structures, revealing an excellent agreement of the reflectance curves. Also, the new method is applied to three simulated Kretschmann sensors already presented in the literature, BK7-Au-analyte, SF10-Au-analyte, and SF10-Au-graphene-analyte structures, for refractive index variation of the analyte between 1.33 and 1.35, leading to a good agreement between both methodologies results. The operating point of SF10-Au-analyte and SF10-Au-graphene-analyte structures is analyzed and optimized. The analyte refractive index changes lead to reflectance variations higher than 92% in the infrared region, meaning the reflectance almost varies from the highest to the lowest possible value.