An electrode comprising a conducting substrate for detecting species such as nitric oxide (NO), carbon monoxide (CO), oxygen (O 2 ) and hydrogen (H 2 ) and a polymer matrix formed from a first layer only or first and second layers with the second layer applied to the first. The matrix forms a permselective barrier. Each layer has a first pre-coat or first and second pre-coats. Each pre-coat may be formed by: depositing a liquid form of at least one halogenated polymer such as fluorinated or chlorinated polymers onto a substrate and allowing the material to dry. A liquid form of at least one halogenated polymer adheres the first and second layers to the substrate. The electrode allows for detection of the target species and allows real time in vivo measurements to be taken. Blood flow can also be monitored. An electrode bundle with electrodes for different target species is also provided.