Adsorbate-Induced Anchoring Transitions of Liquid Crystals on Surfaces Presenting Metal Salts with Mixed Anions

We report that metal salts composed of mixtures of anions of differing coordination strength can be used to increase the sensitivity and selectivity of adsorbate-induced anchoring transitions of liquid crystals (LCs) supported on surfaces decorated with the metal salts. Specifically, the dynamics of anchoring transitions triggered by the adsorbate dimethyl methylphosphonate (DMMP) on surfaces of aluminum­(III) salts were analyzed within the framework of a model for mass transport to reveal that the sensitivity of a nitrile-containing nematic LC to DMMP increased from 250 to 25 ppb when the composition of the (counter) anion was changed from 100% perchlorate to 90% nitrate and 10% perchlorate (by mole percent). To provide insight into these observations, polarization-modulation infrared reflectance-absorbance spectroscopy (PM-IRRAS) was used to show that the intensity of the absorption band in the IR spectrum corresponding to the coordinated state of the nitrile group (but not the position of the peak) decreased with the increase in the mole fraction of the strongly coordinating anion (nitrate) in the anion mixture, thus suggesting that the addition of the strongly coordinating anion decreased the number of coordination interactions (per unit area of the interface) but not the strength of the individual coordination interactions between the metal cation and the LC. We also measured the incorporation of the nitrate anion into the metal salt to decrease the effect of humidity on the dynamic response of the LC to DMMP, a result that is consistent with weaker interactions between the nitrate anion and water as compared to the perchlorate anion and water. Finally, the bidentate anion acetylacetonate was measured to cause a similar increase in sensitivity to DMMP when mixed with perchlorate in a 1:1 ratio (the resulting sensitivity of the system to DMMP was 100 ppb). Overall, these results suggest that tailoring the identity of the anion represents a general and facile approach for tuning the orientational response of LCs supported on metal salts to targeted analytes

Enantiomeric Interactions between Liquid Crystals and Organized Monolayers of Tyrosine-Containing Dipeptides

We have examined the orientational ordering of nematic liquid crystals (LCs) supported on organized monolayers of dipeptides with the goal of understanding how peptide-based interfaces encode intermolecular interactions that are amplified into supramolecular ordering. By characterizing the orientations of nematic LCs (4-cyano-4′-pentylbiphenyl and TL205 (a mixture of mesogens containing cyclohexane-fluorinated biphenyls and fluorinated terphenyls)) on monolayers of l-cysteine-l-tyrosine, l-cysteine-l-phenylalanine, or l-cysteine-l-phosphotyrosine formed on crystallographically textured films of gold, we conclude that patterns of hydrogen bonds generated by the organized monolayers of dipeptides are transduced via macroscopic orientational ordering of the LCs. This conclusion is supported by the observation that the ordering exhibited by the achiral LCs is specific to the enantiomers used to form the dipeptide-based monolayers. The dominant role of the −OH group of tyrosine in dictating the patterns of hydrogen bonds that orient the LCs was also evidenced by the effects of phosphorylation of the tyrosine on the ordering of the LCs. Overall, these results reveal that crystallographic texturing of gold films can direct the formation of monolayers of dipeptides with long-range order, thus unmasking the influence of hydrogen bonding, chirality, and phosphorylation on the macroscopic orientational ordering of LCs supported on these surfaces. These results suggest new approaches based on supramolecular assembly for reporting the chemical functionality and stereochemistry of synthetic and biological peptide-based molecules displayed at surfaces

A Practical Guide to the Preparation of Liquid Crystal-Templated Microparticles

We provide a practical guide to methods and protocols that use polymer networks templated from droplets of liquid crystal (LC) to synthesize micrometer-sized polymeric particles that are chemically patchy, are anisometric in shape, possess anisotropic optical properties, and/or are mesoporous. We describe a range of methods that permit the preparation of LC droplets (containing reactive monomers) as templates for polymerization, including formation of LC-in-water emulsions by mechanical methods (e.g., vortexing), encapsulation in polymeric shells, or microfluidics. The relative merits of the methods, including ease of use and potential pitfalls, and the resulting droplet size distributions, are described. We also report a menu of approaches that can be used to control the internal configurations of the LC droplets, including changes in composition of the continuous solvent phases (e.g., addition of glycerol) and adsorption of surfactants or colloids at the interfaces of the LC droplets. Photopolymerization of the LC droplets in bipolar, radial, axial, or preradial configurations and subsequent extraction of the nonreactive mesogens generates polymeric particles that have spindle, spherical, spherocylindrical, or tear shapes, respectively. Finally, we describe how to characterize these polymeric particles, including their shape, internal structure, optical properties, and porosity. The methods described in this paper, which provide access to complex microparticles with properties relevant to separation processes, drug delivery, and optical devices, are general and versatile and can be readily developed further (e.g., by changing the choice of LC) to create an even greater diversity of microparticles

Influence of Self-Assembling Redox Mediators on Charge Transfer at Hydrophobic Electrodes

We report an investigation of the influence of reversible self-assembly of amphiphilic redox-mediators on interfacial charge transfer at chemically functionalized electrodes. Specifically, we employed (11-ferrocenylundecyl)-trimethylammonium bromide (FTMA) as a model self-assembling redox mediator and alkanethiol-modified gold films as hydrophobic electrodes. By performing cyclic voltammetry (CV, 10 mV/s) in aqueous solutions containing FTMA above its critical micellar concentration (CMC), we measured anodic (<i>I</i>_a) and cathodic (<i>I</i>_c) peak current densities of 18 ± 3 and 1.1 ± 0.1 μA/cm², respectively, revealing substantial current rectification (<i>I</i>_a/<i>I</i>_c= 17) at the hydrophobic electrodes. In contrast, hydroxymethyl ferrocene (a non-self-assembling redox mediator) at hydrophobic electrodes and FTMA at bare gold electrodes, yielded relatively low levels of rectification (<i>I</i>_a/<i>I</i>_c= 1.7 and 2.3, respectively). Scan-rate-dependent measurements revealed <i>I</i>_a of FTMA to arise largely from the diffusion of FTMA from bulk solution to the hydrophobic electrode whereas <i>I</i>_c was dominated by adsorbed FTMA, leading to the proposal that current rectification observed with FTMA is mediated by interfacial assemblies of reduced FTMA that block access of oxidized FTMA to the hydrophobic electrode. Support for this proposal was obtained by using atomic force microscopy and quartz crystal microbalance measurements to confirm the existence of interfacial assemblies of reduced FTMA (1.56 ± 0.2 molecules/nm²). Additional characterization of a mixed surfactant system containing FTMA and dodecyltrimethylammonium bromide (DTAB) revealed that interfacial assemblies of DTAB also block access of oxidized FTMA to hydrophobic electrodes; this system exhibited <i>I</i>_a/<i>I</i>_c > 80. These results and others reported in this paper suggest that current rectification occurs in this system because oxidized FTMA does not mix with interfacial assemblies of reduced FTMA or DTAB formed at hydrophobic electrodes. More broadly, these results show that self-assembling redox mediators, when combined with chemically functionalized electrodes, offer the basis of new principles for controlling charge transfer at electrode/solution interfaces

Ordering Transitions Triggered by Specific Binding of Vesicles to Protein-Decorated Interfaces of Thermotropic Liquid Crystals

We report that specific binding of ligand-functionalized (biotinylated) phospholipid vesicles (diameter = 120 ± 19 nm) to a monolayer of proteins (streptavidin or anti-biotin antibody) adsorbed at an interface between an aqueous phase and an immiscible film of a thermotropic liquid crystal (LC) [nematic 4′-pentyl-4-cyanobiphenyl (5CB)] triggers a continuous orientational ordering transition (continuous change in the tilt) in the LC. Results presented in this paper indicate that, following the capture of the vesicles at the LC interface via the specific binding interaction, phospholipids are transferred from the vesicles onto the LC interface to form a monolayer, reorganizing and partially displacing proteins from the LC interface. The dynamics of this process are accelerated substantially by the specific binding event relative to a protein-decorated interface of a LC that does not bind the ligands presented by the vesicles. The observation of the <i>continuous</i> change in the ordering of the LC, when combined with other results presented in this paper, is significant, as it is consistent with the presence of suboptical domains of proteins and phospholipids on the LC interface. An additional significant hypothesis that emerges from the work reported in this paper is that the ordering transition of the LC is strongly influenced by the bound state of the protein adsorbed on the LC interface, as evidenced by the influence on the LC of (i) “crowding” of the protein within a monolayer formed at the LC interface and (ii) aging of the proteins on the LC interface. Overall, these results demonstrate that ordering transitions in LCs can be used to provide fundamental insights into the competitive adsorption of proteins and lipids at oil–water interfaces and that LC ordering transitions have the potential to be useful for reporting specific binding events involving vesicles and proteins

Hierarchical Microstructures Formed by Bidisperse Colloidal Suspensions within Colloid-in-Liquid Crystal Gels

Past studies have reported that colloids of a single size dispersed in the isotropic phase of a mesogenic solvent can form colloid-rich networks (and gels) upon thermal quenching of the system across the isotropic–nematic phase boundary of the mesogens. Herein we report the observation and characterization of complex hierarchical microstructures that form when bidisperse colloidal suspensions of nanoparticles (NPs; iron oxide with diameters of 188 ± 20 nm or poly­(methyl methacrylate) with diameters of 150 ± 15 nm) and microparticles (MPs; polystyrene with diameters of 2.77 ± 0.20 μm) are dispersed in the isotropic phase of 4-pentyl-4′-cyanobiphenyl (5CB) and thermally quenched. Specifically, we document microstructuring that results from three sequential phase separation processes that occur at distinct temperatures during stepwise cooling of the ternary mixture from its miscibility region. The first phase transition demixes the system into coexisting MP-rich and NP-rich phases; the second promotes formation of a particle network within the MP-rich phase; and the third, which coincides with the isotropic-to-nematic phase transition of 5CB, produces a second colloidal network within the NP-rich phase. We quantified the dynamics of each demixing process by using optical microscopy and Fourier transform image analysis to establish that the phase transitions occur through (i) surface-directed spinodal decomposition, (ii) spinodal decomposition, and (iii) nucleation and growth, respectively. Significantly, the observed series of phase transitions leads to a hierarchical organization of cellular microstructures not observed in colloid-in-liquid crystal gels formed from monodisperse colloids. The results of this study suggest new routes to the synthesis of colloidal materials with hierarchical microstructures that combine large surface areas and organized porosity with potential applications in catalysis, separations, chemical sensing, or tissue engineering

Covalent Immobilization of Caged Liquid Crystal Microdroplets on Surfaces

Microscale droplets of thermotropic liquid crystals (LCs) suspended in aqueous media (e.g., LC-in-water emulsions) respond sensitively to the presence of contaminating amphiphiles and, thus, provide promising platforms for the development of new classes of droplet-based environmental sensors. Here, we report polymer-based approaches to the immobilization of LC droplets on surfaces; these approaches introduce several new properties and droplet behaviors and thus also expand the potential utility of LC droplet-based sensors. Our approach exploits the properties of microscale droplets of LCs contained within polymer-based microcapsule cages (so-called “caged” LCs). We demonstrate that caged LCs functionalized with primary amine groups can be immobilized on model surfaces through both weak/reversible ionic interactions and stronger reactive/covalent interactions. We demonstrate using polarized light microscopy that caged LCs that are covalently immobilized on surfaces can undergo rapid and diagnostic changes in shape, rotational mobility, and optical appearance upon the addition of amphiphiles to surrounding aqueous media, including many useful changes in these features that cannot be attained using freely suspended or surface-adsorbed LC droplets. Our results reveal these amphiphile-triggered orientational transitions to be reversible and that arrays of immobilized caged LCs can be used (and reused) to detect both increases and decreases in the concentrations of model contaminants. Finally, we report changes in the shapes and optical appearances of LC droplets that occur when immobilized caged LCs are removed from aqueous environments and dried, and we demonstrate that dried arrays can be stored for months without losing the ability to respond to the presence of analytes upon rehydration. Our results address practical issues associated with the preparation, characterization, storage, and point-of-use application of conventional LC-in-water emulsions and provide a basis for approaches that could enable the development of new “off-the-shelf” LC droplet-based sensing platforms

Liquid Crystal-Based Emulsions for Synthesis of Spherical and Non-Spherical Particles with Chemical Patches

We report the use of liquid crystal (LC)-in-water emulsions for the synthesis of either spherical or non-spherical particles with chemically distinct domains located at the poles of the particles. The approach involves the localization of solid colloids at topological defects that form predictably at surfaces of water-dispersed LC droplets. By polymerizing the LC droplets displaying the colloids at their surface defects, we demonstrate formation of both spherical and, upon extraction of the mesogen, anisotropic composite particles with colloids located at either one or both of the poles. Because the colloids protrude from the surfaces of the particles, they also define organized, chemical patches with functionality controlled by the colloid surface

Influence of Simple Electrolytes on the Orientational Ordering of Thermotropic Liquid Crystals at Aqueous Interfaces

We report orientational anchoring transitions at aqueous interfaces of a water-immiscible, thermotropic liquid crystal (LC; nematic phase of 4′-pentyl-4-cyanobiphenyl (5CB)) that are induced by changes in pH and the addition of simple electrolytes (NaCl) to the aqueous phase. Whereas measurements of the zeta potential on the aqueous side of the interface of LC-in-water emulsions prepared with 5CB confirm pH-dependent formation of an electrical double layer extending into the aqueous phase, quantification of the orientational ordering of the LC leads to the proposition that an electrical double layer is also formed <i>on the LC-side of the interface</i> with an internal electric field that drives the LC anchoring transition. Further support for this conclusion is obtained from measurements of the dependence of LC ordering on pH and ionic strength, as well as a simple model based on the Poisson–Boltzmann equation from which we calculate the contribution of an electrical double layer to the orientational anchoring energy of the LC. Overall, the results presented herein provide new fundamental insights into ionic phenomena at LC–aqueous interfaces, and expand the range of solutes known to cause orientational anchoring transitions at LC–aqueous interfaces beyond previously examined amphiphilic adsorbates

Comparison of the Influence of Humidity and d‑Mannitol on the Organization of Tetraethylene Glycol-Terminated Self-Assembled Monolayers and Immobilized Antimicrobial Peptides

We report the use of polarization-modulation infrared reflection–absorption spectroscopy (PM-IRRAS) to characterize the effects of relative humidity (RH) and d-mannitol on the conformations of tetraethylene glycol (EG₄)-terminated self-assembled monolayers (SAMs) and immobilized antimicrobial peptides (Cecropin P1 and a hybrid of Cecropin A (1–8) and Melittin (1–18)). These results are used to assess the extent to which d-mannitol can substitute for water in promoting conformational states of the SAMs and oligopeptides similar to those induced by hydration. Our measurements reveal a red shift of the COC asymmetric stretching vibration of the EG₄-terminated SAMs with increasing humidity, consistent with a transition from a mixed all-trans/helical (7/2 helix) conformation at 0% RH to a predominantly helical conformation at 90% RH. Significantly, under dry conditions, a thin (2 nm in thickness) overlayer of d-mannitol generated the COC spectroscopic signature of the EG₄-terminated SAM measured at high humidity. Comparisons of the effects of humidity and d-mannitol on the secondary structure of the two oligopeptides also revealed both to cause the amide I peak positions, which were measured in dry air (and without d-mannitol) to correspond to α-helical conformations, to undergo red-shifts. The magnitudes of the red-shifts, however, were more pronounced for dry d-mannitol than for high RH, with Cecropin P1 and the hybrid peptide exhibiting amide I peak positions under d-mannitol consistent with bulk aqueous solution secondary structures (random and β-sheet, respectively). These results are discussed in the context of prior reports of the tendency of d-mannitol to form glassy states in the absence of water. Overall, the results presented in this paper support the hypothesis that d-mannitol can substitute, in at least some ways, for the influence of water on the conformational states of biologically relevant molecules at interfaces. The results provide guidance for the design of interfaces for water-free biologics