Patent ID: 7239766

Claim:
A nitrogen oxide sensor comprising: a nitrogen oxide sensing element made by a method comprising: incorporating a diazotizing reagent which reacts with nitrous ions to produce a diazo compound and a coupling reagent which couples with the diazo compound to produce an azo dye into a sol; and allowing the sol to gel; wherein the gel comprises a microporous optically transparent inorganic matrix comprising immobilized diazotizing reagent and immobilized coupling reagent; a light source; and an optical detector; wherein the sensing element is coupled to the light source by one or more transmitting optical fibers and wherein the sensing element is coupled to the detector by one or more receiving optical fibers such that light from the light source is transmitted through the one or more transmitting optical fibers and impinges on the sensing element and light impinging on the sensing element is transmitted through the one or more receiving optical fibers to the detector; wherein the sensing element is in the form of a film; wherein ends of the transmitting and receiving fibers adjacent the sensing element are bundled into an optical fiber cable having a planar cable end adjacent the sensing element, and wherein the planar surface of the film sensing element is oriented at an angle to the planar cable end adjacent the sensing element; and wherein the angle of orientation of the film sensing element to the planar cable end is the Brewster's angle (β B ) as calculated from the refractive index of the sensing element (n 2 ) and the refractive index of the medium through which the light travels before impinging on the sensing element (n 1 ) as set forth below: β B = arc ⁢ ⁢ tan ⁡ ( n 2 n 1 ) .