A chemical sensing device for detecting a fluid, the sensing device comprising: a semiconductor substrate (1, 6) comprising at least one semiconductor substrate region, wherein said at least one substrate region comprises a first substrate and a second substrate wherein the first substrate comprises a first through-etched portion and wherein the second substrate comprises a second through-etched portion; a first dielectric layer formed on the surface of the first substrate and a second dielectric layer formed on the surface of the second substrate, wherein the dielectric layers form a first dielectric membrane on the surface of the first substrate at the location of the first through-etched portion and a second dielectric membrane on the surface of the second substrate at the location of the second through-etched portion; an optical source for emitting an infra-red signal; an optical detector for detecting the IR signal emitted from the optica source; and at least one further substrate formed either on the first dielectric layer and under the second dielectric layer or on the second dielectric layer and under the first dielectric layer; wherein said at least one further substrate defines a portion of an optical path for the IR signal to propagate from the optical source to the optical detector; and wherein the optical source is formed at least partially in the first dielectric membrane at the location of the first through-etched portion and wherein the optical detector is formed at least partially in the second dielectric membrane at the location of the second through-etched portion, and wherein the optical source and the optical detector are vertically spaced relative to one another, and wherein the at least one further substrate forms a vertical spacer between the first and second dielectric membranes. a semiconductor substrate (1, 6) comprising at least one semiconductor substrate region, wherein said at least one substrate region comprises a first substrate and a second substrate wherein the first substrate comprises a first through-etched portion and wherein the second substrate comprises a second through-etched portion; a first dielectric layer formed on the surface of the first substrate and a second dielectric layer formed on the surface of the second substrate, wherein the dielectric layers form a first dielectric membrane on the surface of the first substrate at the location of the first through-etched portion and a second dielectric membrane on the surface of the second substrate at the location of the second through-etched portion; an optical source for emitting an infra-red signal; an optical detector for detecting the IR signal emitted from the optica source; and at least one further substrate formed either on the first dielectric layer and under the second dielectric layer or on the second dielectric layer and under the first dielectric layer; wherein said at least one further substrate defines a portion of an optical path for the IR signal to propagate from the optical source to the optical detector; and wherein the optical source is formed at least partially in the first dielectric membrane at the location of the first through-etched portion and wherein the optical detector is formed at least partially in the second dielectric membrane at the location of the second through-etched portion, and wherein the optical source and the optical detector are vertically spaced relative to one another, and wherein the at least one further substrate forms a vertical spacer between the first and second dielectric membranes. wherein said at least one further substrate defines a portion of an optical path for the IR signal to propagate from the optical source to the optical detector; and wherein the optical source is formed at least partially in the first dielectric membrane at the location of the first through-etched portion and wherein the optical detector is formed at least partially in the second dielectric membrane at the location of the second through-etched portion, and wherein the optical source and the optical detector are vertically spaced relative to one another, and wherein the at least one further substrate forms a vertical spacer between the first and second dielectric membranes.