- NSF-PAR ID:
- 10353947
- Date Published:
- Journal Name:
- ACS Sensors
- ISSN:
- 2379-3694
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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null (Ed.)Soft matter that undergoes programmed macroscopic responses to molecular analytes has potential utility in a range of health and safety-related contexts. In this study, we report the design of a nematic liquid crystal (LC) composition that forms through dimerization of carboxylic acids and responds to the presence of vapors of organoamines by undergoing a visually distinct phase transition to an isotropic phase. Specifically, we screened mixtures of two carboxylic acids, 4-butylbenzoic acid and trans-4-pentylcyclohexanecarboxylic acid, and found select compositions that exhibited a nematic phase from 30.6 to 111.7 °C during heating and 110.6 to 3.1 °C during cooling. The metastable nematic phase formed at ambient temperatures was found to be long-lived (>5 days), thus enabling the use of the LC as a chemoresponsive optical material. By comparing experimental infrared (IR) spectra of the LC phase with vibrational frequencies calculated using density functional theory (DFT), we show that it is possible to distinguish between the presence of monomers, homodimers and heterodimers in the mixture, leading us to conclude that a one-to-one heterodimer is the dominant species within this LC composition. Further support for this conclusion is obtained by using differential scanning calorimetry. Exposure of the LC to 12 ppm triethylamine (TEA) triggers a phase transition to an isotropic phase, which we show by IR spectroscopy to be driven by an acid-base reaction, leading to the formation of ammonium carboxylate salts. We characterized the dynamics of the phase transition and found that it proceeds via a characteristic spatiotemporal pathway involving the nucleation, growth, and coalescence of isotropic domains, thus amplifying the atomic-scale acid-base reaction into an information-rich optical output. In contrast to TEA, we determined via both experiment and computation that neither hydrogen bonding donor or acceptor molecules, such as water, dimethyl methylphosphonate, ethylene oxide or formaldehyde, disrupt the heterodimers formed in the LC, hinting that the phase transition (including spatial-temporal characteristics of the pathway) induced in this class of hydrogen bonded LC may offer the basis of a facile and chemically selective way of reporting the presence of volatile amines. This proposal is supported by exploratory experiments in which we show that it is possible to trigger a phase transition in the LC by exposure to volatile amines emitted from rotting fish. Overall, these results provide new principles for the design of chemoresponsive soft matter based on hydrogen bonded LCs that may find use as the basis of low-cost visual indicators of chemical environments.more » « less
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Abstract Surface‐supported liquid crystals (LCs) that exhibit orientational and thus optical responses upon exposure to ppb concentrations of Cl2gas are reported. Computations identified Mn cations as candidate surface binding sites that undergo redox‐triggered changes in the strength of binding to nitrogen‐based LCs upon exposure to Cl2gas. Guided by these predictions, μm‐thick films of nitrile‐ or pyridine‐containing LCs were prepared on surfaces decorated with Mn2+binding sites as perchlorate salts. Following exposure to Cl2, formation of Mn4+(in the form of MnO2microparticles) was confirmed and an accompanying change in the orientation and optical appearance of the supported LC films was measured. In unoptimized systems, the LC orientational transitions provided the sensitivity and response times needed for monitoring human exposure to Cl2gas. The response was also selective to Cl2over other oxidizing agents such as air or NO2and other chemical targets such as organophosphonates.
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Abstract Surface‐supported liquid crystals (LCs) that exhibit orientational and thus optical responses upon exposure to ppb concentrations of Cl2gas are reported. Computations identified Mn cations as candidate surface binding sites that undergo redox‐triggered changes in the strength of binding to nitrogen‐based LCs upon exposure to Cl2gas. Guided by these predictions, μm‐thick films of nitrile‐ or pyridine‐containing LCs were prepared on surfaces decorated with Mn2+binding sites as perchlorate salts. Following exposure to Cl2, formation of Mn4+(in the form of MnO2microparticles) was confirmed and an accompanying change in the orientation and optical appearance of the supported LC films was measured. In unoptimized systems, the LC orientational transitions provided the sensitivity and response times needed for monitoring human exposure to Cl2gas. The response was also selective to Cl2over other oxidizing agents such as air or NO2and other chemical targets such as organophosphonates.
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