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.
Chemical sensors based on highly mass sensitive QCM or SAW devices, coated with sensitive layers, are especially suited for the trace analysis of organic solvent vapors, such as halogenated or aromatic hydrocarbons. The paracyclophanes B44TOS and CP44 were used as sensitive coatings, employing the principles of host-guest chemistry. The application of the BET model to the incorporation of chloroform in the sensitive coatings proves intracavitative complexation according to a 1:1 stoichiometry. Beyond the mass-sensitive detection, the complexation was studied by FT-IR spectroscopy. The exhibited saturation behavior of the characteristic CDCl 3 bands confirms the results of the mass-sensitive measurements. The lowering of the C-D stretching frequency was compared with the shifts caused by several model compounds, such as N-butylaniline. All results described indicate the formation of host-guest complexes between chloroform and paracyclophanes.
Preorganized calix[4]resorcinarenes forming vaselike
cavities are tested with respect to their chemical sensing
capabilities as coatings on mass-sensitive devices. A
synthetically modified vaselike host molecule with a
definite cavity morphology forms densely packed layers
for measurements with a SAW oscillator. In contrast
to
this detection of surface phenomena with ultrathin layers,
QCM resonators require bulky microporous coatings with
a high vapor permeability. Thus, short response times
are attained even for layers using bulk effects, which are
necessary to compensate the smaller sensitivity of the
QCM. A 3-fold substitution of a basic
calix[4]resorcinarene
cavity with 2,3-dichloroquinoxaline provides a one-side-opened structure forming a molecular entrance for the
easy access of analyte molecules. In combination with
aliphatic spacers, this material fulfills the required demands of high sensitivity and short response times, and
the detection of solvents in the gas phase to 2.5 ppm is
realized. The sensor effects of the established
coatings
are correlated to host−guest stabilization enthalpies
which
were calculated by force field methods. An improved
thermodynamic model that considers entropies of condensation is tested successfully for the prediction of the
sensor behavior.
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