A light source emits light into a first portion of a living body. Functionalized particles within the body are configured to specifically bind to a target analyte, and upon receiving the emitted light, undergo a reaction that separates a detectable label from the functionalized particle. A sensor device is configured to detect a response signal from a second portion of the living body that is indicative of an abundance of the detectable label in the second portion. A control system uses the sensor device to obtain sensor data indicative of the response signal from the second portion of the living body detected by the sensor device during a measurement interval, and determines a presence or absence of the target analyte within the first portion of the living body based in part on the obtained data.