Development of a Cobinamide-Based Optical Sensor for Hydrogen Cyanide and Hydrogen Sulfide

In an occupational or military environment, a personal air-purifying respirator must be provided when breathing-air is contaminated by harmful dust, fumes, gases, aerosols, or vapors. Too often, a respirator user does not have enough information to know when to change his/her cartridge/canister and thus is potentially exposed to toxic gases. Currently, there is no definitive way to determine when respirators' carbon beds have begun to fail. When an end-of-service-life indicator (ESLI) is incorporated into the carbon bed, it informs one, in real-time, when imminent breakthrough is occurring and to replace the cartridge/canister. ESLIs are a more reliable and safer way to determine respirator end-of-service-life. To date, there are no commercially available active ESLIs for inorganic gases.;The objective of this research is to develop an inexpensive, optical sensor for the detection of hydrogen cyanide (HCN) and hydrogen sulfide (H 2S) gas, which can be used to determine the end-of-service-life of a respirator carbon bed. The sensor relies on diffuse reflectance from a paper substrate fixed with cobinamide, a Vitamin B12 derivative. Cobinamide undergoes a metal-ligand binding interaction with HCN, whereas both a binding and reduction reaction may occur with exposure to H2S. Characteristic and different spectral shifts rapidly occurred after exposure to HCN and H 2S, implying a dual ESLI could be developed to simultaneously detect both gases. Upon increasing the relative humidity from 25 to 85%, the sensitivity was found to increase 7x for cobinamide-immobilized cellulose fiber filter paper and 50x for glass fiber filter paper upon exposure to 5.0 parts-per-million (ppm) HCN---the NIOSH recommended exposure limit for HCN. The cobinamide-immobilized paper sensor successfully detected low concentrations of HCN and H2S upon imminent breakthrough of respirator canisters and cartridges (respectively). The breakthrough curves of the cobinamide paper sensor correlated well with commercial electrochemical detectors, implying that cobinamide may be used to detect both gases at a certain location in the respirator carbon bed and inform the user to replace his or her cartridge/canister.;Additionally, a low-power, inexpensive 3-color (RGB) sensor was prototyped to actively monitor the total change in color of the cobinamide complex on paper upon HCN exposure. The photodiodes detected, in real-time, a rapid change in the red, green, and blue values of the cobinamide compound upon exposure to HCN at various concentrations and relative humidity levels. Total change in color from initial cobinamide on paper increased as a function of HCN concentration, where faster reaction kinetics were observed at higher relative humidity. Response times at all relative humidity levels were within 20 seconds for 5.0 ppm HCN exposure. The color sensor offers an economical and more quantitative approach to determining color change compared to current, subjective end-of-service-life indicators.