An integrated optical sensor operating in the frequency domain is disclosed together with a system and method for detecting the presence of a sample to be detected with the optical sensor. The optical sensor comprises a substrate for supporting the optical sensor, at least two substantially identical gratings or grating-based resonators, a waveguide directional coupler coupled to the gratings and arranged to receive light from a light source and propagate the light through to the gratings, and a flow cell for housing each of the gratings, each flow cell having a fluidic input and a fluidic output, one flow cell for receiving a reference fluid and another flow cell for receiving a fluid with the sample to be detected; the sample to be detected alters the effective refractive index of the propagating mode inside the grating by binding or being adsorbed by the grating to alter the optical frequency of the light that propagates through the grating. In an embodiment, the grating is surface functionalized with a sensitive layer to enable sample adherence or adsorption.