A liquid crystal optical sensor for simple and quantitative determination of dimethyl methylphosphonate using laser speckle

A new method is reported for determining the concentration of dimethyl methylphosphonate (DMMP) vapor based on the capture of laser speckles from a liquid crystal (LC) sensor. The sensor comprises a film of the nematic LC, 4-cyano-4'-pentylbiphenyl, anchored to a carboxy-terminated self-assembled monolayer of 11-mercaptoundecanoic acid via interaction with copper ions. A linearly polarized laser beam is transmitted through the sensor and irradiated onto a rough diffuser. A charge-coupled device camera is used to capture speckle images from the LC sensor and calculate the correlation coefficient. DMMP vapor is selectively detected over a 500 ppb to 5 ppm concentration range, demonstrating the feasibility of the method for determining vapor phase concentrations of organophosphorus compounds. Compared with previous methods that use a polarizing optical microscope to image the LC film, our approach does not require complex instrumentation and reduces the size, complexity, and cost of the detection system. The detection methodology described in this study provides new ideas for LC-based sensing of organophosphorus vapors.