A simple and low cost optical sensor, based on porous silicon nanotechnology, has been used to detect and quantify the presence of atrazine pesticide in water and humic acid solutions. In both cases, a well defined optical signal variation can be registered, even at low concentration as 1 ppm. The phenomenon can be ascribed to the capillary infiltration of liquid into the pores, which changes the average refractive index of the structure. Due to the resonant cavity enhanced operation of the proposed sensors, very low detection limits can be reached.