Applications of liquid crystals in biosensing and organic light-emitting devices: future aspects

ABSTRACT

This article summarises recent advances made in our laboratory towards the development of new technological applications, such as biosensors and organic light-emitting diodes (OLEDs) based on liquid crystals (LCs) other than LC displays. The study of biomolecular interaction using LC material relies on the specific interaction between the LC and the biomolecule of interest at interfaces that permit the biomolecular events to be amplified into easily measured signals for various sensing applications. In the first part, we emphases recent studies in the design and modulation of LC-based interfaces based on robust colloidal LC gels for biological amplification, qualitative and quantitative understanding of important biomolecular interactions at LC–aqueous interfaces for diagnostic and laboratory applications and design of LC droplets that hold promise to act as a marker for cells and cell-based interactions. In the second part, we described design of organic materials for application in OLEDs on various discotic monomers, dimers and oligomers. These molecules have the ability to transport charges, holes and electrons. In addition, because of the high conductivity and π–π stacking, they are considered as the advanced materials for practical applications. The technological advances in our laboratory using discotic LCs will be briefly presented in this article.     

Gas–gas experimental interfacial tension measurement

A thermodynamically stable interface can only exist between two immiscible fluids, nonetheless in this work an “immiscible interface” between two gases (CO₂–methane) has been observed and is documented within this paper. This work is the first ever attempt to measure the interfacial tension (IFT) in a gas–gas system, the interfacial tension of supercritical carbon dioxide (SCO₂–methane) was measured at temperatures of 368.15 and 433.15 K and pressures between 6.9 and 41.37 MPa, using a modified pendant drop method. This study was undertaken to investigate the effects of CO₂ injection into a gas reservoir or enhanced gas recovery.     

Detection of creatinine using surface-driven ordering transitions of liquid crystals

Determining creatinine levels in blood is of great importance in the detection of high risk for renal failure. Here, we report a simple methodology for real-time monitoring of creatinine employing surface-driven ordering transitions in liquid crystals (LCs) by changing pH in presence of creatinine deiminase enzyme. It is found that when 5CB (4-Cyano-4?-pentylbiphenyl) LC doped with 4?-hexyl-biphenyl-4-carboxylic acid, a bright optical appearance was observed (at aqueous–LC interface) which is not disturbed in presence of creatinine, consistent with a planar/tilted orientation of the LC molecules at those interface. Interestingly, in presence of creatinine deiminase, an ordering transition was observed resulting from enzymatic reactions (giving rise to NH₄⁺ ions) that can change the local pH values and lead to dark optical appearance of the LC. Presence of different amounts of creatinine would lead varied ordering transition that can be monitored in real time in presence of creatinine deiminase. Our approach could detect the creatinine levels as low as that of the healthy adult (~50 µM) and can be successfully applied to measure higher concentration of creatinine in real time using dynamic optical response of the LC.     

Microwave-assisted synthesis of novel mixed tail rufigallol derivatives

Microwave-assisted syntheses of five new series of rufigallol-based mesogens are reported with branched alkyl chains at the peripheral positions. The chemical structures of these newly synthesised compounds were determined by ¹H nuclear magnetic resonance (NMR), ¹³C NMR, infrared spectroscopy, ultraviolet spectroscopy and elemental analysis. The thermotropic liquid crystalline properties were investigated by polarising optical microscopy (POM), differential scanning calorimetry and X-ray diffractometry. Most of the derivatives were found to be liquid crystalline over a wide temperature range.     

Microwave‐assisted synthesis of disulfides using tetrathiomolybdate: A step toward greener synthesis

An eco‐friendly, efficient, and rapid synthetic procedure for disulfides using benzyl triethyl ammonium tetrathiomolybdate through microwave irradiation of solid support adsorbed reactants is reported. © 2012 Wiley Periodicals, Inc. Heteroatom Chem 23:373–376, 2012; View this article online at wileyonlinelibrary.com . DOI 10.1002/hc.21025     

A Simple Quantitative Method to Study Protein–Lipopolysaccharide Interactions by Using Liquid Crystals

The interaction of proteins with endotoxins has divergent effects on lipopolysaccharide (LPS)‐induced responses, which serve as a basis for many clinical and therapeutic applications. It is, therefore, important to understand these interactions from both theoretical and practical points of view. This paper advances the design of liquid crystal (LC)‐based stimuli‐responsive soft materials for quantitative measurements of LPS–protein binding events through interfacial ordering transition. Micrometer‐thick films of LCs undergo easily visualized ordering transitions in response to proteins at LPS–aqueous interfaces of the LCs. The optical response of the LC changes from dark to bright after aqueous solutions of hemoglobin (Hb), bovine serum albumin (BSA), and lysozyme proteins (LZM) are in contact with a LPS‐laden aqueous–LC interface. The effects of interactions of different proteins with LPS are also observed to cause the response of the LC to vary significantly from one to another; this indicates that manipulation of the protein–LPS binding affinity can provide the basis for a general, facile method to tune the LPS‐induced responses of the LCs to interfacial phenomena. By measuring the optical retardation of the 4′‐pentyl‐4‐cyanobiphenyl (5CB) LC, the binding affinity of the proteins (Hb, BSA, and LZM) towards LPS that leads to different orientational behavior at the aqueous interfaces of the LCs can be determined. The interaction of proteins with the LPS‐laden monolayer is highest for LPS–Hb, followed by LPS–BSA, and least for LPS–LZM; this is in correlation with their increasing order of binding constants (LPS‐Hb>LPS‐BSA>LPS‐LZM). The results presented herein pave the way for quantitative and multiplexed measurements of LPS–protein binding events and reveal the potential of the LC system to be used as quantitative LC‐based, stimuli‐responsive soft materials.     

Kinetic investigations into the synthesis of disulphides via tetrathiomolybdate as a sulphur-transfer reagent

Kinetic and mechanistic aspects of the conversion of halides to disulphides using benzyl triethyl ammonium tetrathiomolybdate as sulphur-transfer reagent were investigated. The reaction follows a 1:1 stoichiometry with overall second-order kinetics and involves the formation of monosulphides in addition to disulphides. In the light of our observations, we propose a nucleophilic substitution: carbon–metal–carbon (S_N-CMC) reaction mechanism. The proposed mechanism, besides accounting for all of our experimental observations, also explains many aspects of such reactions that have been reported earlier by various groups.     

Protein triggered ordering transitions in poly (L-lysine)-coated liquid crystal emulsion droplets

Here, we report a simple and label-free methodology for real-time monitoring of adsorption of proteins such as bovine serum albumin (BSA), concanavalin A (ConA) (a lectin) and cathepsin D (CathD) (a tumour marker) on micrometer-sized poly (L-lysine) (PLL) functionalised liquid crystal (LC) droplets dispersed in aqueous phases. Earlier, we had demonstrated that PLL, a positively charged natural peptide, can induce homeotropic ordering of LCs at LC-aqueous interface, and thus PLL-adsorbed LC droplets showed radial director configuration. Herein, it was observed that subsequent non-specific adsorption of anionic proteins such as BSA, ConA and CathD can trigger a quick transition in director configuration of PLL-LC droplets (primarily dominated by electrostatic interactions) to pre-radial or bipolar, thus acting as a simple optical probe for detection of these proteins up to μg/mL of concentrations. Further, the detection limits for these proteins are found to vary (BSA<ConA<CathD) which follow the similar order as their anionic charges, thus suggesting the role of different binding affinities of protein-PLL in realising the director configuration of LC droplets. Overall, this study offers new pathways utilising ordering transition in LC droplets which will strengthen the principles to recognise biomolecular interactions for various interfacial and sensing applications.