Abstract: Disclosed are an electrode and a sensor-system for capacitively and/or photovoltaic sensing contaminants in a fluid. A micro-reactor including such sensor-system and methods for measuring and producing are also disclosed. In at least one embodiment, the capacitance electrode includes a transducing element; an affinity layer that in use is in contact with a fluid to be measured, wherein the affinity layer comprises a multi-array surface; and an insulating layer separating the transducing element and the affinity layer, wherein the electrode is arranged such that in use contaminants in the fluid may interact with the affinity layer.