Abstract: An infrared (IR) sensor and a method of detecting molecular species in a liquid. In one embodiment, the IR sensor includes: (1) an IR light source configured to emit IR light, (2) a sensing element configured to receive the IR light, the IR light generating an evanescent field about the sensing element as the IR light propagates therethrough, molecules in a subject liquid interacting with the evanescent field and affecting a characteristic of the IR light and (3) an IR light detector configured to receive the IR light from the sensing element and detect the characteristic.