Limits to optical chemical sensing fluctuations versus ultimate performance

The role of chemical sensors in modern process industry cannot be overestimated, since they are encountered at different stages of process control, helping maintain product quality and at the same time allowing to keep an eye on environmental issues and pollution problems. Numerous device families are currently in use, where optical chemical sensors are probably the fastest, the simplest and the most sensitive. Affinity sensors represent a class of these devices based on adsorption of chemical analytes. The prime example are plasmonic sensors, which are label-free, ultra sensitive and ultra fast. Literature usually defines only the ideal sensitivity of such devices and their performance is taken as granted. In reality there are many parasitic processes that decrease their performance and introduce significant measurement uncertainties. Noise and fluctuations in such sensors can be generated by both extrinsic or intrinsic sources and a significant part of them is a consequence of fundamental processes that at the same time ensure the very function of the devices. The main intrinsic mechanisms include adsorption-desorption, optical flicker and thermal noise, while the level of the extrinsic noise is related with the quality of the interrogating beam source and the photo detector. In this contribution we offer a possible systematization of different sources of noise in refractometric chemical sensors and analyze the influence of such fluctuations to the ultimate device performance.