We disclose a chemical sensing device for detecting a fluid. The sensing device comprises: at least one substrate region comprising at least one etched portion; a dielectric region formed on the at least one substrate region, the dielectric region comprising at least one dielectric membrane region adjacent to the at least one etched portion; an optical source for emitting an infra-red (IR) signal; an optical detector for detecting the IR signal emitted from the optical source; one or more further substrates formed on or under the dielectric region, said one or more further substrates defining an optical path for the IR signal to propagate from the optical source to the optical detector. At least one of the optical source and optical detector is formed in or on the dielectric membrane region.