Effect of the materials’ properties in the design of high transmittance and low FWHM SiO2/TiO2 thin film optical filters for integration in a malaria diagnostics device

Abstract

Malaria is an infectious disease, highly prevalent in world regions with lacking healthcare conditions. Nowadays, malaria diagnostic methods in these endemic regions are mainly based on microscopy and rapid diagnostic tests by immunochromatographic assays. Here, it is presented an optical diagnostic method, based on reflectance spectrophotometry, through hemozoin (Hz) quantification, towards an innovative non-invasive malaria diagnostic device. Therefore, a set of optical filters, with high transmittance and low full width at half maximum (FWHM) at specific wavelengths, is designed for being integrated in the device. These allow the full reconstruction of the optical reflectance spectrum, able to distinguish between healthy and infected samples, with a detection limit up to 12.5 parasites/μl of red blood cells. This work presents the design, performance simulation, and optimization of 16 highly selective narrow band-pass optical filters, based on multilayer stacks of SiO2/TiO2 thin films. The optical properties of the thin films layer materials, in particular the refractive indexes, are the main focus in this study. Three different reflective indexes were evaluated and the results showed that, for all the simulated conditions, each filter is sensitive to a single wavelength with a FWHM < 25 nm and peak transmittance intensity > 90%, but slight variations were observed for the different refractive indexes. The simulation results proved that these 16 optical filters designs are extremely sensitive to the material properties, although they are the best option regarding the required optical response, assuring feasibility and being adequate for the fabrication process.