Use of self-assembled monolayers, metal ions and smectic liquid crystals to detect organophosphonates

Smectic liquid crystals form stable, micrometer-thick films when deposited onto the surfaces of solids. In this paper, we report that orientational transitions of thin films of the smectic liquid crystal (LC), 4′-octyl-4-biphenyl-carbonitrile (8CB) supported on surfaces presenting copper ions can be exploited to detect the presence of organophosphonate compounds at parts-per-billion (ppb) levels in gas phases. The orientational transition of the 8CB is driven by the competitive binding of the nitrile groups of 8CB and dimethyl methyl phosphonate (DMMP) for the surface-immobilized copper ions. Because the orientational transition of the LC can be observed by using a polarizing film and the naked eye, this detection scheme does not require complex instrumentation to transduce the signal. The film of LC responds to 10 ppbv of DMMP and the initial response time is approximately 20 s. The system is also demonstrated to exhibit molecular specificity. The LC does not respond to gaseous streams containing 1% of the saturated vapor concentrations of ethanol (770 ppm), hexane (1990 ppm) and acetone (3020 ppm) and 75% relative humidity of water.