Overview of Chemical Sensing using Liquid Crystals

Nematic Technologies, a brand of Platypus Technologies LLC, commercializes chemical sensor devices based on innovative liquid crystal (LC) technology invented at the University of Wisconsin-Madison.  This chemical sensor technology comprises thin films of nitrile-containing LC supported on chemically-reactive surfaces that orient the LC molecules.  When exposed to toxic gases, the molecules of the toxic gas bind to the chemically-reactive surface and change the orientation of the LC molecules in contact with the surface, producing an abrupt change in the optical appearance of the LC films.  Thus, LCs amplify molecular binding events at chemically-reactive surfaces into optical transitions that can be observed with the naked eye (the outcome is similar to a LC display), providing an unambiguous response to the presence of specific toxic gases. 

 

 

Dosimeters for Passive Sampling

Dosimeters are passive sampling badges for monitoring personal exposure to toxic gases.  A wearable dosimeter comprises a thin film (5-μm) of LC supported on a functionalized substrate and enclosed between two parallel glass plates separated by well-defined spacers (25-μm thick for H2S dosimeter) (See Figure below). When the dosimeter is introduced into an atmosphere containing the analyte (e.g. H2S), the analyte diffuses inside the dosimeter and binds to the chemically-reactive surface supporting the LC. The diffusion length of the analyte inside the dosimeter, which depends only on the concentration of H2S and the exposure time, produces an optical front in the LC film. The length of the optical front follows a relationship of the form: L = α(C x T)β, where L is the length of the LC optical front [mm], C is the air concentration of H2S [ppm], T is the exposure time [hours], and α and β are best-fit parameters determined from experimental data of L vs. (C x T).

Mechanism for sampling H2S using LC-based dosimeter.  H2S diffuses inside the dosimeter and absorbs at the reactive surface, causing a reorientation of the LC molecules inside the dosimeter.    (b) Images of LC-based dosimeter (left) before and after exposure to 5 ppm H2S for (middle) 1 hr. and (right) 6 hrs.  The length of the optical front of the LC, L, increases with exposure time, T: L = 7.6 mm at 1 hr., and L = 16.6 mm at 6 hrs. of exposure to 5 ppm H2S.

The data plot in below depicts the measured response length of the sampling badges (in units of millimeters) as a function of exposure time to H2S at concentrations of 1 ppm, 5 ppm, 10 ppm, or 15 ppm (ppm = parts per million of H2S in air). As demonstrated in this plot, for a fixed concentration of H2S the length of the optical front increases with increasing exposure time (units of hours). Similarly, for a fixed exposure time the length of the optical front is larger for higher concentrations of H2S than for lower concentrations of H2S (units of ppm). These two observations demonstrate that the response length is a function of the concentration of H2S, C, and the exposure time, T. The explicit function L = f(C, T) can be deduced by plotting  L against the exposure dose, which is defined as the product C x T (the concentration of H2S, in ppm, multiplied by the exposure time, in hours). When data is plotted in this matter, the data sets for the four concentrations collapse into a single continuous curve. Using the least square method, the line of best fit for the data in Figure b was identified as a power function with the following form: L = α(C x T)β, where α =3.816 and β = 0.433 are the best-fit parameters.

(a) Response length of H2S dosimeters exposed to 1 ppm (blue circles), 5 ppm (red triangles), 10 ppm (black squares), or 15 ppm (green diamonds) of H2S for various time intervals. (b) Response length of H2S dosimeters plotted against the exposure dose, which is calculated as the H2S concentration (ppm) multiplied by the exposure time (hrs.).  The black dotted line represents the best fit to the data set.

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