Redox‐Triggered Orientational Responses of Liquid Crystals to Chlorine Gas

Abstract

Surface-supported liquid crystals (LCs) that exhibit orientational and thus optical responses upon exposure to ppb concentrations of Cl gas 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 Cl gas. Guided by these predictions, μm-thick films of nitrile- or pyridine-containing LCs were prepared on surfaces decorated with Mn binding sites as perchlorate salts. Following exposure to Cl , formation of Mn (in the form of MnO microparticles) 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 Cl gas. The response was also selective to Cl over other oxidizing agents such as air or NO and other chemical targets such as organophosphonates.

Seeing the Unseen: The Role of Liquid Crystals in Gas‐Sensing Technologies

“…Surface functionalization studies have extended to perchlorate salts of metal ions [ 125,126 ] such as, gallium, [ 102 ] lead, [ 127 ] or manganese. [ 128 ] Hunter and Abbott [ 129 ] have also explored the possibility of tuning the anion in perchlorate‐salt decorated surfaces in order to manipulate the LC anchoring to the binding surface, and hence increase the selectivity and sensitivity of surface‐induced LC transitions triggered by gas analytes.…”

Areas of opportunity related to design of chemical and biological sensors based on liquid crystals

“…The development of stable and long-lived surface chemistries for alignment of LCs, such as achieved via use of surfaces coated with polyimides, underlies the success of the electrooptical LC display technology [3]. One broad class of strategies for the design of LC chemical sensors aims to build from this success by creating tailored surfaces that cause LCs to change orientation in the presence of a targeted chemical species (Figure 1(a)) [10,12,[15][16][17][18][19][20]. Approaches used to date to transduce and quantify surface-driven changes in the orientation of LCs in sensors include measurement of light intensity transmitted through polarisers, absorbance of guesthost systems, and electrical capacitance.…”

“…Consistent with experimental observations of anchoring transitions triggered by DMMP and nerve agents, electronic structure calculations predict that the binding energy of DMMP to Al 3+ (−1.14 eV) is stronger than that of 5CB to Al 3+ (−0.40 eV) and thus that the orientational transition of 5CB is driven by ligand exchange at the surface (nitrile of 5CB by phosphoryl of DMMP) [17,19,20,26]. Experiments demonstrate that this experimental design can be highly sensitive; for example, parts-per-billion concentrations of DMMP in the gas phase can trigger anchoring transitions of 5CB supported on Al 3+ functionalised surfaces [18,24]. Through judicious selection of the metal cation, its counter ions, and the ligands incorporated into mesogens, a wide range of chemically specific LC systems can be created [18,20].…”

Gewinner der ACS National Awards 2019

“…Seit 2008 ist er dort Paul A. Elfers Professor und seit 2015 Vilas Distinguished Achievement Professor. Mavrikakis beschrieb in einer Angewandte‐Chemie ‐Zuschrift eine durch Redoxprozesse ausgelöste Ausrichtung von Flüssigkristallen in Gegenwart von Chlorgas . Er ist Mitglied des Redaktionsbeirats von ChemPhysChem .…”