A chitosan/porous silicon biosensing platform for the detection of carboxylic acid-containing drugs in water is prepared and characterized. Parasitic layer-free films of mesoporous silicon are electrochemically etched and functionalized by covalent attachment of chitosan oligomers. The presence of the chitosan species covering the inner and outer surface of the porous silicon films is confirmed by infrared spectroscopy and nitrogen adsorption analysis. The obtained hybrid platform offers both an important porosity, with a higher surface area than the original porous silicon substrate, and an important surface charge, very suitable for sensing charged molecules. Ibuprofen is chosen as a drug model for the sensing experiments in water. Optical interferometry measurements are performed on the chitosan/porous silicon substrate exposed to ibuprofen solutions of various concentrations, and compared to the same experiments performed on a nonfunctionalized porous silicon surface. Results of the sensing experiments show that the presence of chitosan increases the sensitivity of the sensing porous film by more than one order of magnitude compared to the nonfunctionalized porous film. Detection in water of BHB, a model molecule for illicit rape drug GHB, is also demonstrated at concentrations compatible with forensic analysis, using the chitosan/porous silicon hybrid platform. ; Beniamino Sciacca, Emilie Secret, Stéphanie Pace, Philippe Gonzalez, Francesco Geobaldo, Françoise Quignard and Frédérique Cunin