This invention relates to a system and method for detecting target molecules using an optical sensor element including a photonic crystal constituted by a membrane of a chosen transparent material, being provided with a number of defined openings in a chosen pattern, a chosen number of the openings providing capture molecules having a high affinity to target molecules, the pattern being adapted to provide resonance at a chosen wavelength, or range of wavelengths. The system and method also moving a first fluid flow containing target molecules through the openings and after this conducting a second fluid flow containing a second reactant being different from the capture molecules but having a high affinity for the target molecules but a low affinity for other possible molecules in the first flow. By illuminating the sensor element at a chosen wavelength thus obtaining a resonance and imaging means for providing an image of the sensor element it is possible to detect tight leaking from the resonator. Analyzing means may then detect the captured molecules as well as the reactant having reacted with them based on the amount of light leaking from the resonator.