Biosensors are bioengineered detection systems that can be expressed in cellular systems and are used to monitor specific signaling processes inside a cell. Usually, the concept behind biosensors is that the sensor can bind to a cellular component of interest (e.g. second messenger) which elicits a conformational change leading to a reporter signal that can be either fluorescent or luminescent.

One popular example is the biosensor GECO which increases its fluorescent intensity in the presence of calcium.