Electrochemical Sensors for Field Measurements of Gases and Vapors

Abstract

Sensors that utilize signals produced by redox of a targeted species (analyte) at an electrode under controlled potential, the conductance changes produced by contact of a chemical species with an electrolyte, or a change in the equilibrium potential of an electrode upon contact with an analyte are being employed in studies of gaseous samples of environmental, biomedical, and industrial interest. Changes in the work function of chemically modified field effect transistors (FET) also is a strategy related to both conductivity and potentiometry. Electrochemical sensors are fundamentally advantageous over other types in that they directly convert information from the chemical to the electrical domain, rather than requiring another transducer. Yet, few have found commercial application. The common types that are employed for field measurements of gaseous samples include devices to quantify oxygen, hydrogen sulfide, sulfur dioxide, oxides of nitrogen, hydrogen cyanide, hydrogen chloride, and chlorine. The limitation is not sensitivity; by this measure electrochemistry is among the best of the analytical measurement methods. Amperometric sensors accrue selectivity by control of the electrode potential along with control of the process by which the analyte partitions from the gas to the electrolyte. The latter is the primary means by which conductivity and potentiometry systems achieve sensitivity. These methods are not as selective as those in which a chemical reaction is incorporated in the sensor. The most selective electrochemical sensors employ an enzymatic reaction as part of signal production, but these are applicable to only a few analytes, such as alcohols, of importance in the gas phase.

In-vehicle Exposure to Carbon Monoxide Emissions from Vehicular Exhaust: A Critical Review

“…52 Solids prepared by sol-gel chemistry (products of which include silica, vanadia, alumina, titania) are useful alternatives for Nafion R and are insensitive to humidity in cases where CO is the analyte. 24 With regard to NDIR technology, several manufacturers have miniaturized it for portable and hand-held operation (e.g., Engelhard, Horiba, Fuji Electric Systems). However, a major limitation remains the relatively high cost of the latter technology.…”

“…6,24 Nafion R (containing tetrafluoroethylene, side-chains of perfluorinated vinyl ethers with terminal sulfonic acid functionalities) is the most widely used SPE because it is highly stable in aqueous solution under a wide range of acid-base and redox environments. 24 A major limitation of Nafion R is its sensitivity to humidity. 52 Solids prepared by sol-gel chemistry (products of which include silica, vanadia, alumina, titania) are useful alternatives for Nafion R and are insensitive to humidity in cases where CO is the analyte.…”