3. Biochemistry of Targets and Probes

The interaction of biological molecules with the surface is one of the most relevant phenomena when it comes to interfaces between CMOS circuits and biological systems. We may have nonspecific or specific interactions depending on the nature of the interface. If our interface is developed for a specific sensing aim, then the only interacting molecules are expected to be the molecular targets. For example, if we develop an implantable system for measuring human glycemia (the measure of glucose in the blood), then glucose must be the only molecule to interact with our interface. Then we can define specific interactions, with all interactions occurring at the interface that are specifically related to the aim of the Bio/CMOS interface we are dealing with. In biosensing, the specifically related molecules are called target molecules, or simply targets. In the example of a CMOS circuit for measuring glycemia, glucose is the biosensing target. However, all biological systems typically contain many different components that would interact with our interface. For example, 1 μl of blood contains millions of cells and thousands of proteins and metabolites. We can define as nonspecific interactions all interactions occurring at the interface that are not specifically related to the aim of our Bio/CMOS interface. Sometimes, the molecules that have nonspecific interactions at the Bio/CMOS interface are also called nontarget molecules, or simply nontargets. The interaction of these nonspecific molecules generates nonspecific electrical signals, which are nonetheless registered by our CMOS circuit. We can also consider these signals a kind of interference.