Abstract: A method and device for detecting the presence of an unbound portion of therapeutic agents in a bodily fluid sample provided in which the bodily fluid includes an endogenous component of geometrical size larger than the therapeutic agent. A photonic integrated circuit has a porous material enrichment layer, a waveguide formed in the enrichment layer, and a blocking means provided on an outer surface of the enrichment layer. The enrichment layer with blocking means is permeable to the unbound therapeutic agent and prevents penetration of the endogenous component into pores of enrichment layer. The blocking means is configured to cooperate with the enrichment layer to maintain therapeutic agent access to the pores of the enrichment layer and prevent light-matter interaction between the at least one endogenous component and light guided in the waveguide, when contacting the liquid sample with the photonic integrated circuit.