Combination of conventional electrochemical etching and subsequent metal-assisted chemical etching (MACE) was used to obtain double etched porous silicon (DEPSi) structures with good optical properties and high affinity to bacteria. Second etching step increased the roughness of the surface and created more cavities for entrapping of bacteria, while optical scattering on the surface was still insufficient and quality of infrared Fabry-Perot interferograms remained high. Obtained DEPSi structures were used for detection of bacteria (E. Coli). Fast Fourier transform of infrared spectra showed reversible broadening of the main band, what allows to detect bacteria in concentrations down to 10(4) CFU per mL. (C) The Author(s) 2017. Published by ECS. All rights reserved.