Optical sensing elements for nitrogen dioxide (NO2) gas detection, a sol-gel method for making the sensing elements and fiber optic sensors incorporating nitrogen dioxide gas optical sensing elements

• US 20050089260A1
• Filed: 10/27/2003
• Published: 04/28/2005
• Est. Priority Date: 10/27/2003
• Status: Active Grant

• Current Assignee: Mississippi State University
• Original Assignee: Mississippi State University
• Inventors: Mechery, Shelly John, Singh, Jagdish P.
• Granted Patent: US 7,239,766 B2
• Time in Patent Office: Days
• US Class Current: 385/12
• CPC Class Codes: C03B 19/12 by liquid-phase reaction processes
• CPC Class Codes: C03C 1/006 to produce glass through wet route
• CPC Class Codes: C03C 11/00 Multi-cellular glass ; Porous or hollow glass or glass particles
• CPC Class Codes: G01N 2021/772 Tip coated light guide
• CPC Class Codes: G01N 2021/7773 Reflection
• CPC Class Codes: G01N 21/783 for analysing gases
• CPC Class Codes: G01N 2201/08 Optical fibres; light guides
• CPC Class Codes: G01N 2201/084 Fibres for remote transmission
• CPC Class Codes: G01N 31/22 using chemical indicators G01N31/02 takes precedence
• CPC Class Codes: G01N 33/0037 NOx
• CPC Class Codes: G02B 6/06 the relative position of the fibres being the same at both ends, e.g. for transporting images
• CPC Class Codes: G02B 6/4298 coupling with non-coherent light sources and/or radiation detectors, e.g. lamps, incandescent bulbs, scintillation chambers
• CPC Class Codes: Y02A 50/20 Air quality improvement or preservation, e.g. vehicle emission control or emission reduction by using catalytic converters
• CPC Class Codes: Y10T 436/177692 Oxides of nitrogen

Abstract

A sensing element, a method of making a sensing element, and a fiber optic sensor incorporating the sensing element are described. The sensor can be used for the quantitative detection of NO₂ in a mixture of gases. The sensing element can be made by 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 form an optically transparent gel. The sensing element changes color in the presence of NO₂ gas. The temporal response of the absorption spectrum at various NO₂ concentrations has also been recorded and analyzed. Sensors having different design configurations are described. The sensing element can detect NO₂ gas at levels of parts per billion.

Claims

1. A method of making a nitrogen oxide sensing element 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 inroganic matrix comprising immobilized diazotizing reagent and immobilized coupling reagent.

2. The method of claim 1, wherein the sol comprises a metal alkoxide or a metalorganic compound.

3. The method of claim 1, wherein the sol comprises an alkoxy silane.

4. The method of claim 1, wherein the sol comprises tetramethyl orthosilicate (TMOS).

5. The method of claim 1, wherein the diazotizing reagent is sulfanilamide (SFA).

6. The method of claim 1, wherein the coupling reagent is N,N-dimethyl-1-naphthylamine (DMNA).

7. The method of claim 1, further comprising transferring the sol into the cavity of a mold and allowing the sol to gel in the mold.

8. The method of claim 7, wherein the mold is a tube and wherein the cavity of the mold is the hollow interior of the tube.

9. The method of claim 1, wherein the sol is formed into a film.

10. The method of claim 1, wherein the sensing element comprises equal molar proportions of diazotizing reagent and coupling reagent.

11. The method of claim 7, wherein the tube comprises polytetrafluorethylene (PTFE).

12. A nitrogen oxide sensing element made by the method of claim 1.

13. The sensing element of claim 12, wherein the shape of the sensing element is a cylindrical solid body.

14. The sensing element of claim 12, wherein the sensing element is a film.

15. The sensing element of claim 12, wherein the shape of the sensing element is a disc.

16. A nitrogen oxide sensor comprising:

a sensing element as set forth in claim 12;

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.

17. The sensor of claim 16, wherein the transmitting and receiving optical fibers are combined in the form of a Y-shaped optical cable having a single distal end comprising ends of both transmitting and receiving fibers, a first proximal end comprising the ends of one or more transmitting fibers and a second proximal end comprising the ends of one or more receiving fibers;

wherein the distal end of the Y-shaped optical cable is placed in optical communication with the sensing element;

wherein the first proximal end of the Y-shaped optical cable is placed in optical communication with the light source; and

wherein the second proximal end of the Y-shaped optical cable is placed in optical communication with the detector.

18. The sensor of claim 17, wherein the optical cable comprises a single transmitting fiber and a plurality of receiving fibers.

19. The sensor of claim 18, wherein the single transmitting fiber is surrounded by the plurality of receiving fibers at the distal end of the optical fiber cable.

20. The sensor of claim 18, wherein the optical cable comprises six receiving fibers.

21. The sensor of claim 16, wherein the sensing element is in the form of a film.

22. The sensor of claim 21, further comprising a mirror, wherein the mirror is positioned adjacent the sensing device such that light transmitted from the one or more transmitting fibers through the sensing element is reflected by the mirror, passes through the sensing element a second time and is transmitted into and through the one or more receiving fibers to the detector.

23. The sensor of claim 21, wherein ends of the transmitting and receiving fibers adjacent the sensing element are positioned on the same side of the film sensing element.

24. The sensor of claim 23, wherein the sensing device is in the form of a film and wherein the sensing element is positioned such that light impinging on the sensing element is reflected by the sensing element into one or more receiving fiber ends.

25. The sensor of claim 24, wherein the sensor does not comprise a mirror.

26. The sensor of claim 16, wherein the sensing element is in the form of a three-dimensional elongate solid body having a proximal end and a distal end.

27. The sensor of claim 26, wherein the proximal end of the sensing element is positioned adjacent ends of the one or more transmitting fibers and wherein the distal end of the sensing element is positioned adjacent ends of the one or more receiving fibers such that light transmitted through the transmitting fibers passes through the sensing element and through the receiving fibers to the detector.

28. The sensor of claim 16, wherein the sensing element is in the shape of a cylindrical solid body.

29. The sensor of claim 21, 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.

30. The sensor of claim 29, wherein the angle of orientation of the film sensing element to the planar cable end is the Brewster'"'"'s angle (PB) as calculated from the refractive index of the sensing element (n₂) and the refractive index of the medium through which the light travels before impinging on the sensing element (n₁) as set forth below:

${\mathrm{\beta <br><br>}}_B=\mathrm{arc}<br><br>\text{\hspace{1em}<br><br>}<br><br>\tan <br><br>\left(\frac{n_2}{n_1}\right).$

31. The sensor of claim 16, wherein the light source has a spectral distribution ranging from 250 nm to 870 nm.

32. The sensor of claim 16, wherein the detector is a spectrometer.

33. The sensor of claim 32, wherein the spectrometer comprises a grating and a CCD, wherein light transmitted from the sensing element to the detector passes through the grating and impinges on the CCD.

34. The sensor of claim 16, further comprising a personal computer interfaced to the detector for collecting data.

35. A nitrogen oxide sensing element comprising:

a microporous matrix of an optically transparent inorganic compound;

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;

wherein the diazotizing reagent and the coupling reagent are immobilized in the microporous matrix.

36. The sensing element of claim 35, wherein the microporous matrix comprises a metalorganic compound.

37. The sensing element of claim 35, wherein the microporous matrix comprises SiO₂.

38. The sensing element of claim 35, wherein the diazotizing reagent and the coupling reagent are present in equal molar proportions.

39. The method of claim 35, wherein the diazotizing reagent is sulfanilamide (SFA).

40. The method of claim 35, wherein the coupling reagent is N,N-dimethyl-1-naphthylamine (DMNA).

41. A nitrogen oxide sensor comprising:

a film sensing element comprising a microporous matrix of an optically transparent material, 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;

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; and

wherein ends of the transmitting and receiving fibers adjacent the sensing element are positioned on the same side of the film sensing element.

42. The sensor of claim 41, further comprising a mirror, wherein the mirror is positioned adjacent the sensing element opposite the ends of the transmitting and receiving fibers such that light transmitted from the one or more transmitting fibers through the sensing element is reflected by the mirror, passes through the sensing element a second time and is transmitted into and through the one or more receiving fibers.

43. The sensor of claim 41, wherein the film sensing element is positioned such that light impinging on the sensing element is reflected by the sensing element into one or more receiving fiber ends.

44. The sensor of claim 43, wherein the sensor does not comprise a mirror.

45. The sensor of claim 43, 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 film 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.

46. The sensor of claim 45, 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 (n₂) and the refractive index of a medium through which the light travels before impinging on the sensing element (n₁) as set forth below;

${\mathrm{\beta <br><br>}}_B=\mathrm{arc}<br><br>\text{\hspace{1em}<br><br>}<br><br>\tan <br><br>\left(\frac{n_2}{n_1}\right).$