A fluidic apparatus for detection of a chemical substance, a biosensor, and a method of fabricating the fluidic apparatus. The fluidic apparatus includes a fluidic structure arranged to receive a sample containing a target substance, and a trapping structure, in fluid communication with the fluidic structure and arranged to immobilize the target substance in a detection region, wherein the detection region of the trapping structure is arranged to alter a physical characteristic of an incident light signal which represents a concentration of the target substance contained in the sample.