Optical Monitoring of Gases with Cholesteric Liquid Crystals

Abstract

A new approach to optical monitors for gases is introduced using cholesteric liquid crystals doped with reactive chiral compounds. The approach is based on cholesteric pitch length changes caused by a change in helical twisting power (HTP) of the chiral dopants upon reaction with the analyte. The concept is demonstrated for monitoring carbon dioxide via reversible carbamate formation and for oxygen using the irreversible oxidation of a chiral dithiol to a disulfide. Monitoring of CO(2) was achieved by doping a commercial cholesteric liquid crystalline mixture (E7) with 1.6% mol of the 1:1 complex of an optically pure diamine with a TADDOL derivative. Upon exposure to carbon dioxide, the reflection band of a thin film of the mixture shifted from 637 to 495 nm as a consequence of dissociation of the complex after carbamate formation of the diamine. An O(2) monitor was obtained by doping E7 with a chiral binaphthyl dithiol derivative and a nonresponsive codopant. The reflection band of the oxygen monitor film changed from 542 to 600 nm, due to the conformational change accompanying oxidation of the dithiol to disulfide. These monitoring mechanisms hold promise for application in smart packaging, where carbon dioxide and oxygen are of special interest because of their roles in food preservation.

Non-electronic gas sensors from electrospun mats of liquid crystal core fibres for detecting volatile organic compounds at room temperature

“…We believe that a promising strategy is to sensitise the liquid crystal-filled polymer fibres with appropriate additives that give a local specific response to the desired analyte, in such a way that the local response is amplified by the liquid crystal into an optically detectable response. The viability of this approach has been demonstrated in non-fibrous LCbased gas sensors by the Abbott group [33,34,43,44] and by the team of Broer and Sijbesma [19].…”

“…Whether the gas sensors are derived from polymers [13], silicon [14], graphene oxide [15], carbon nanomaterials [10], or metal-oxide semiconductors [16], and whether they target the automotive industry [7,8,17,18], food preservation [19,20], wearable technology [21][22][23][24][25] or other fields of high societal importance, they should all fulfil the following basic requirements in order to function optimally: high sensitivity, high selectivity, fast response, low energy consumption and, ideally, low fabrication cost [2,26]. While metal-oxide semiconductor gas sensors are the most common, commercially available and displaying high levels of sensitivity [27][28][29], they operate at high temperatures, may not ensure sufficient selectivity, and the fabrication technique is often complex [2,30].…”

“…The response is due to the ability of certain gas molecules, even at low concentration, to strongly influence the liquid crystal self-assembly [40,[42][43][44][45], triggering a reorientation of the liquid crystal director [32,34,43,44], a change in period of the supramolecular helix of shortpitch cholesterics [19,36,42,[46][47][48], or even complete loss of long-range ordering [11]. In particular, Abbott and his group [35,39,[43][44][45] demonstrated the capability of nematic LCs as sensors for detecting nerve agents at concentrations as low as part per billion.…”

Optical Acetone Vapor Sensors Based on Chiral Nematic Liquid Crystals and Reactive Chiral Dopants

“…Such sensors have been previously described for alcohols, [11] amines, [12,13] VOCs [14][15][16] and gases like oxygen and carbon dioxide. [17] These optical sensors can have a high sensitivity but are often also cross-sensitive to similar molecules. These sensors act as real-time sensors, and measure the level of analyte that is present.…”

“…[19] A secondary use is as chiral dopants in chiral-nematic liquid crystals, where they act as dopants with exceptionally high HTP. [20] Previous work by Han et al [17] has demonstrated that R,R-TADDOLs can be complexed with amines to produce a reactive dopant complex capable for the real-time detecting of CO2.…”