A review of some papers published in the last fifty years that focus on the semiconducting metal oxide (SMO) based sensors for the selective and sensitive detection of various environmental pollutants is presented.

A new composite electrode has been fabricated using multiwall carbon nanotubes (MWCNT) and the ionic liquid n-octylpyridinum hexafluorophosphate (OPFP). This electrode shows very attractive electrochemical performances compared to other conventional electrodes using graphite and mineral oil, notably improved sensitivity and stability. One major advantage of this electrode compared to other electrodes using carbon nanotubes and other ionic liquids is its extremely low capacitance and background currents. A 10% (w/w) loading of MWCNT was selected as the optimal composition based on voltammetric results, as well as the stability of the background response in solution. The new composite electrode showed good activity toward hydrogen peroxide and NADH, with the possibility of fabricating a sensitive biosensor for glucose and alcohol using glucose oxidase and alcohol dehydrogenase, respectively, by simply incorporating the specific enzyme within the composite matrix. The marked electrode stability and antifouling features toward NADH oxidation was much higher for this composite compared to a bare glassy carbon electrode. While a loading of 2% MWCNT showed very poor electrochemical behavior, a large enhancement was observed upon gentle heating to 70 degrees C, which gave a response similar to the optimum composition of 10%. The ease of preparation, low background current, high sensitivity, stability, and small loading of nanotubes using this composite can create new novel avenues and applications for fabricating robust sensors and biosensors for many important species.

The gas-phase adsorption of the nerve gas simulant dimethyl methylphosphonate (DMMP) along with trimethyl phosphate (TMP), methyl dichlorophosphate (MDCP), and trichlorophosphate (TCP) on silica have been studied using infrared spectroscopy. Each phosphonate compound adsorbs through a different number of H-bonds of the methoxy and PdO moieties with the surface hydroxyl groups on silica. The strength of the adsorption depends on the number and type of the H-bonds and follows the order TCP < MDCP < DMMP < TMP. TCP is completely removed from silica by evacuation at room temperature, adsorbed MDCP is removed by evacuation at 150 °C, DMMP requires an evacuation temperature of 300 °C, and TMP is eliminated at 400 °C. All phosphonate compounds molecularly desorb, and the silica returns to its original state. The differences in the reactivity of phosphonate compounds on silica from other oxides demonstrate the potential use of silica in prefiltering/preconcentrating strategies for semiconductive metal oxide based sensing devices. Specifically, it is shown that silica can be used to selectively adsorb DMMP from a gas stream containing methanol/DMMP mixtures.

We have developed a two-step method using preadsorbed ethylenediamine (EDA) to catalyze the reaction of (3-aminopropyl)dimethylethoxysilane (APDMES) on silica surfaces. The amount of adsorbed APDMES is nearly twice the amount obtained when the same reaction is performed without the preadsorbed EDA or when monoamines such as propylamine or triethylamine are used instead of EDA. In addition, the use of preadsorbed EDA leads to an increase in the number of chemisorbed APDMES with free aminopropyl groups extending from the surface. Preadsorbed monoamines such as triethylamine, pyridine, or propylamine do not alter the adsorbed amount or change the number of free amine groups extending from the surface because they are easily displaced from the surface by APDMES. Infrared spectroscopy is used to follow the surface reactions on silica powder, and contact angle measurements show that this method also works on glass slides.