The internal surface of porous silicon (PS) nanostructures was chemically modified by octadecyl and carboxylic acid groups by applying the hydrosilylation reaction as well as by aminopropyl and vinyl groups applying the silanization reaction. Concentrations of the chemically grafted groups and thicknesses of grafted layers were determined by measurements of PS refractive index in the infrared spectral range. The Landau-Lifshitz-Looyenga effective media model was used to relate the measured refractive index values to a volume fraction and then to the concentration of the grafted groups. The described quantitative method was applied to determine the sensitivity limits of PS-based sensing devices.