Abstract: A colorimetric sensor for detecting an analyte of interest in a fluid sample includes a photonic structure comprising a first receptor, wherein the photonic structure may be configured such that, when an analyte contacts the first receptor within the photonic structure, a refractive property of the photonic structure changes thereby to cause a detectable color change in the photonic structure. The first receptor may comprise an optical absorber indicator, wherein a second receptor is part of a structure of the optical absorber indicator, such that, when the analyte contacts the second receptor, the analyte causes a photo-induced electron transfer to induce a color change of the optical absorber indicator.