Abstract
Cholesteric liquid crystals (CLCs) are used as sensitive coatings for the detection of organic solvent vapours for both polar and non-polar substances. The incorporation of different analyte vapours in the CLC layers disturbs the pitch length which changes the optical properties, i.e., shifting the absorption band. The engulfing of CLCs around non-polar solvent vapours such as tetrahedrofuran (THF), chloroform and tetrachloroethylene is favoured in comparison to polar ones, i.e., methanol and ethanol. Increasing solvent vapour concentrations shift the absorbance maximum to smaller wavelengths, e.g., as observed for THF. Additionally, CLCs have been coated on acoustic devices such as the quartz crystal microbalance (QCM) to measure the frequency shift of analyte samples at similar concentration levels. The mass effect for tetrachloroethylene was about six times higher than chloroform. Thus, optical response can be correlated with intercalation in accordance to mass detection. The mechanical stability was gained by combining CLCs with imprinted polymers. Therefore, pre-concentration of solvent vapours was performed leading to an additional selectivity.
Highlights
Monitoring of hazardous organic vapours is of great importance in different laboratories, industries and congested public places
The overall comparison of optical methods with mass sensitive technique offers an inside view of the changes in the pitch length and twisting angle of Cholesteric liquid crystals (CLCs) layers, which are very attractive for studying the morphological structure of analyte vapours
Using cholesteric liquid crystals a similar approach can be applied for the detection of volatile organic vapours
Summary
Monitoring of hazardous organic vapours is of great importance in different laboratories, industries and congested public places. The suitable selection of different cholesteric esters combinations had proven very effective to tune the optical properties of sensing material These studies provide the information about humidity effect and temperature dependence on absorbance of organic vapours detection by CLCs. Winterbottom et al [21] had employed a similar approach for the detection of organic vapours in air using a mixture of cholesteryl nonanoate and cholesteryl chloride. The variation in the results of two different transducing systems, i.e., optical and mass sensitive techniques using the same sensitive coating material for the very same analyte has been investigated In this way steric effects and pure mass effects can be compared. The overall comparison of optical methods with mass sensitive technique offers an inside view of the changes in the pitch length and twisting angle of CLCs layers, which are very attractive for studying the morphological structure of analyte vapours
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