Dr. Abbott is a Professor of Chemical Engineering at the University of Wisconsin-Madison. His research interests span colloid, interface, and surfactant science. A particular focus is directed toward the design of new surfactants that can be placed under active control, the use of liquid crystalline materials to amplify biomolecular interactions, and the design of interfaces with chemistry and topography patterned on nanometer dimensions. His work has been recognized by honors, which include a Presidential Early Career Award in Science and Engineering, a Camille Dreyfus Teacher-Scholar Award and a Fulbright Fellowship. He has published in leading journals, including Science, Journal of the American Chemical Society, Physical Review Letters and Langmuir.
Gas-phase microactuation using kinetically controlled surface states of ultrathin catalytic sheets
Proc Natl Acad Sci U S A. 2023 May 9;120(19):e2221740120. doi: 10.1073/pnas.2221740120. Epub 2023 May 1.
Biological systems convert chemical energy into mechanical work by using protein catalysts that assume kinetically controlled conformational states. Synthetic chemomechanical systems using chemical catalysis have been reported, but they are slow, require high temperatures to operate, or indirectly perform work by harnessing reaction products in liquids (e.g., heat or protons). Here, we introduce a bioinspired chemical strategy for gas-phase chemomechanical transduction that sequences the elementary steps of catalytic reactions on ultrathin (<10 nm) platinum sheets to generate surface stresses that directly drive microactuation (bending radii of 700 nm) at ambient conditions (T = 20 °C; Ptotal = 1 atm). When fueled by hydrogen gas and either oxygen or ozone gas, we show how kinetically controlled surface states of the catalyst can be exploited to achieve fast actuation (600 ms/cycle) at 20 °C. We also show that the approach can integrate photochemically controlled reactions and can be used to drive the reconfiguration of microhinges and complex origami- and kirigami-based microstructures.
PMID:37126707 | PMC:PMC10175785 | DOI:10.1073/pnas.2221740120
Energy landscapes on polymerized liquid crystal interfaces
Soft Matter. 2023 May 30. doi: 10.1039/d3sm00356f. Online ahead of print.
We measure and model monolayers of concentrated diffusing colloidal probes interacting with polymerized liquid crystal (PLC) planar surfaces. At topological defects in local nematic director profiles at PLC surfaces, we observe time-averaged two-dimensional particle density profiles of diffusing colloidal probes that closely correlate with spatial variations in PLC optical properties. An inverse Monte Carlo analysis of particle concentration profiles yields two-dimensional PLC interfacial energy landscapes on the kT-scale, which is the inherent scale of many interfacial phenomena (e.g., self-assembly, adsorption, diffusion). Energy landscapes are modelled as the superposition of macromolecular repulsion and van der Waals attraction based on an anisotropic dielectric function obtained from the liquid crystal birefringence. Modelled van der Waals landscapes capture most net energy landscape variations and correlate well with experimental PLC director profiles around defects. Some energy landscape variations near PLC defects indicate either additional local repulsive interactions or possibly the need for more rigorous van der Waals models with complete spectral data. These findings demonstrate direct, sensitive measurements of kT-scale van der Waals energy landscapes at PLC interfacial defects and suggest the ability to design interfacial anisotropic materials and van der Waals energy landscapes for colloidal assembly.
PMID:37248864 | DOI:10.1039/d3sm00356f
Liquid Crystals as Multifunctional Interfaces for Trapping and Characterizing Colloidal Microplastics
Small. 2023 Mar 9:e2207802. doi: 10.1002/smll.202207802. Online ahead of print.
Identifying and removing microplastics (MPs) from the environment is a global challenge. This study explores how the colloidal fraction of MPs assemble into distinct 2D patterns at aqueous interfaces of liquid crystal (LC) films with the goal of developing surface-sensitive methods for identifying MPs. Polyethylene (PE) and polystyrene (PS) microparticles are measured to exhibit distinct aggregation patterns, with addition of anionic surfactant amplifying differences in PS/PE aggregation patterns: PS changes from a linear chain-like morphology to a singly dispersed state with increasing surfactant concentration whereas PE forms dense clusters at all surfactant concentrations. Statistical analysis of assembly patterns using deep learning image recognition models yields accurate classification, with feature importance analysis confirming that dense, multibranched assemblies are unique features of PE relative to PS. Microscopic characterization of LC ordering at the microparticle surfaces leads to predict LC-mediated interactions (due to elastic strain) with a dipolar symmetry, a prediction consistent with the interfacial organization of PS but not PE. Further analysis leads to conclude that PE microparticles, due to their polycrystalline nature, possess rough surfaces that lead to weak LC elastic interactions and enhanced capillary forces. Overall, the results highlight the potential utility of LC interfaces for rapid identification of colloidal MPs based on their surface properties.
PMID:36892170 | DOI:10.1002/smll.202207802
Non-equilibrium ordering of liquid crystalline (LC) films driven by external gradients in surfactant concentration
J Colloid Interface Sci. 2023 May;637:134-146. doi: 10.1016/j.jcis.2022.12.124. Epub 2022 Dec 24.
HYPOTHESIS: Gradients in the concentration of amphiphiles play an important role in many non-equilibrium processes involving complex fluids. Here we explore if non-equilibrium interfacial behaviors of thermotropic (oily) liquid crystals (LCs) can amplify microscopic gradients in surfactant concentration into macroscopic optical signals.
EXPERIMENTS: We use a milli-fluidic system to generate gradients in aqueous sodium dodecyl sulfate (SDS) concentration and optically quantify the dynamic ordering of micrometer-thick nematic LC films that contact the gradients.
FINDINGS: We find that the reordering of the LCs is dominated by interfacial shearing by Marangoni flows, thus providing simple methods for rapid mapping of interfacial velocities from a single optical image and investigating the effects of confinement of surfactant-driven interfacial flows. Additionally, we establish that surface advection and surfactant desorption are the two key processes that regulate the interfacial flows, revealing that the dynamic response of the LC can provide rapid and potentially high throughput approaches to measurement of non-equilibrium interfacial properties of amphiphiles. We also observe flow-induced assemblies of microparticles to form at the LC interface, hinting at new non-equilibrium approaches to microparticle assembly. We conclude that dynamic states adopted by LCs in the presence of surfactant concentration gradients provide new opportunities for engineering complex fluids beyond equilibrium.
PMID:36696789 | DOI:10.1016/j.jcis.2022.12.124
Optical Fingerprinting of Dynamic Interfacial Reaction Pathways Using Liquid Crystals
Langmuir. 2023 Feb 7;39(5):1793-1803. doi: 10.1021/acs.langmuir.2c02622. Epub 2023 Jan 24.
Reactions at interfaces between fluid phases are widely used to synthesize small molecules, polymers, and nanoparticles. In situ monitoring of the underlying dynamic reaction pathways remains challenging. Liquid crystals (LCs) have been used to detect simple chemical transformations at interfaces in situations where interface-bound reactants and products trigger distinct equilibrium orientations of LCs. However, whether or not LCs can be used to report complex reaction pathways via nonequilibrium states generated by reactions has not been explored. Here we explore this question using SN2' nucleophilic substitution reactions that involve a synthetic amphiphile and a series of amine-based nucleophiles with one to four reaction sites. Although all reactants and products generate the same equilibrium LC orientation, we find that each nucleophile defines a distinct set of possible reaction pathways with a characteristic spatial and temporal LC optical response unique to the nucleophile. Additional experiments reveal that the nonequilibrium orientational states of the LCs arise from a combination of dynamic interfacial processes that include adsorption/desorption of reactants, the presence of reaction intermediates on the LC interface, and the generation of interfacial tension gradients (Marangoni stresses). Overall, our results reveal that the spatiotemporal optical outputs of LCs ("optical fingerprints") can be a rich source of information regarding interfacial reactions.
PMID:36693164 | DOI:10.1021/acs.langmuir.2c02622
Interactions of SARS-CoV-2 and MERS-CoV fusion peptides measured using single-molecule force methods
Biophys J. 2023 Feb 21;122(4):646-660. doi: 10.1016/j.bpj.2023.01.016. Epub 2023 Jan 16.
We address the challenge of understanding how hydrophobic interactions are encoded by fusion peptide (FP) sequences within coronavirus (CoV) spike proteins. Within the FPs of severe acute respiratory syndrome CoV 2 and Middle East respiratory syndrome CoV (MERS-CoV), a largely conserved peptide sequence called FP1 (SFIEDLLFNK and SAIEDLLFDK in SARS-2 and MERS, respectively) has been proposed to play a key role in encoding hydrophobic interactions that drive viral-host cell membrane fusion. Although a non-polar triad (Leu-Leu-Phe (LLF)) is common to both FP1 sequences, and thought to dominate the encoding of hydrophobic interactions, FP1 from SARS-2 and MERS differ in two residues (Phe 2 versus Ala 2 and Asn 9 versus Asp 9, respectively). Here we explore whether single-molecule force measurements can quantify hydrophobic interactions encoded by FP1 sequences, and then ask whether sequence variations between FP1 from SARS-2 and MERS lead to significant differences in hydrophobic interactions. We find that both SARS-2 and MERS wild-type FP1 generate measurable hydrophobic interactions at the single-molecule level, but that SARS-2 FP1 encodes a substantially stronger hydrophobic interaction than its MERS counterpart (1.91 ± 0.03 nN versus 0.68 ± 0.03 nN, respectively). By performing force measurements with FP1 sequences with single amino acid substitutions, we determine that a single-residue mutation (Phe 2 versus Ala 2) causes the almost threefold difference in the hydrophobic interaction strength generated by the FP1 of SARS-2 versus MERS, despite the presence of LLF in both sequences. Infrared spectroscopy and circular dichroism measurements support the proposal that the outsized influence of Phe 2 versus Ala 2 on the hydrophobic interaction arises from variation in the secondary structure adopted by FP1. Overall, these insights reveal how single-residue diversity in viral FPs, including FP1 of SARS-CoV-2 and MERS-CoV, can lead to substantial changes in intermolecular interactions proposed to play a key role in viral fusion, and hint at strategies for regulating hydrophobic interactions of peptides in a range of contexts.
PMID:36650897 | PMC:PMC9841730 | DOI:10.1016/j.bpj.2023.01.016
Using Nanoscopic Solvent Defects for the Spatial and Temporal Manipulation of Single Assemblies of Molecules
Nano Lett. 2022 Sep 28;22(18):7506-7514. doi: 10.1021/acs.nanolett.2c02454. Epub 2022 Sep 12.
Here we report the use of defects in ordered solvents to form, manipulate, and characterize individual molecular assemblies of either small-molecule amphiphiles or polymers. The approach exploits nanoscopic control of the structure of nematic solvents (achieved by the introduction of topological defects) to trigger the formation of molecular assemblies and the subsequent manipulation of defects using electric fields. We show that molecular assemblies formed in solvent defects slow defect motion in the presence of an electric field and that time-of-flight measurements correlate with assembly size, suggesting methods for the characterization of single assemblies of molecules. Solvent defects are also used to transport single assemblies of molecules between solvent locations that differ in composition, enabling the assembly and disassembly of molecular "nanocontainers". Overall, our results provide new methods for studying molecular self-assembly at the single-assembly level and new principles for integrated nanoscale chemical systems that use solvent defects to transport and position molecular cargo.
PMID:36094850 | DOI:10.1021/acs.nanolett.2c02454
Two-Dimensional Mechanics of Atomically Thin Solids on Water
Nano Lett. 2022 Sep 14;22(17):7180-7186. doi: 10.1021/acs.nanolett.2c02499. Epub 2022 Sep 1.
Movement of a three-dimensional solid at an air-water interface is strongly influenced by the extrinsic interactions between the solid and the water. The finite thickness and volume of a moving solid causes capillary interactions and water-induced drag. In this Letter, we report the fabrication and dynamical imaging of freely floating MoS2 solids on water, which minimizes such extrinsic effects. For this, we delaminate a synthesized wafer-scale monolayer MoS2 onto a water surface, which shows negligible height difference across water and MoS2. Subsequently patterning by a laser generates arbitrarily shaped MoS2 with negligible in-plane strain. We introduce photoswitchable surfactants to exert a lateral force to floating MoS2 with a spatiotemporal control. Using this platform, we demonstrate a variety of two-dimensional mechanical systems that show reversible shape changes. Our experiment provides a versatile approach for designing and controlling a large array of atomically thin solids on water for intrinsically two-dimensional dynamics and mechanics.
PMID:36047815 | PMC:PMC9479134 | DOI:10.1021/acs.nanolett.2c02499
Ordering Transitions of Liquid Crystals Triggered by Metal Oxide-catalyzed Reactions of Sulfur Oxide Species
J Am Chem Soc. 2022 Sep 14;144(36):16378-16388. doi: 10.1021/jacs.2c03424. Epub 2022 Sep 1.
Liquid crystals (LCs), when supported on reactive surfaces, undergo changes in ordering that can propagate over distances of micrometers, thus providing a general and facile mechanism to amplify atomic-scale transformations on surfaces into the optical scale. While reactions on organic and metal substrates have been coupled to LC-ordering transitions, metal oxide substrates, which offer unique catalytic activities for reactions involving atmospherically important chemical species such as oxidized sulfur species, have not been explored. Here, we investigate this opportunity by designing LCs that contain 4'-cyanobiphenyl-4-carboxylic acid (CBCA) and respond to surface reactions triggered by parts-per-billion concentrations of SO2 gas on anatase (101) substrates. We used electronic structure calculations to predict that the carboxylic acid group of CBCA binds strongly to anatase (101) in a perpendicular orientation, a prediction that we validated in experiments in which CBCA (0.005 mol %) was doped into an LC (4'-n-pentyl-4-biphenylcarbonitrile). Both experiment and computational modeling further demonstrated that SO3-like species, produced by a surface-catalyzed reaction of SO2 with H2O on anatase (101), displace CBCA from the anatase surface, resulting in an orientational transition of the LC. Experiments also reveal the LC response to be highly selective to SO2 over other atmospheric chemical species (including H2O, NH3, H2S, and NO2), in agreement with our computational predictions for anatase (101) surfaces. Overall, we establish that the catalytic activities of metal oxide surfaces offer the basis of a new class of substrates that trigger LCs to undergo ordering transitions in response to chemical species of relevance to atmospheric chemistry.
PMID:36047705 | DOI:10.1021/jacs.2c03424
Sensing Gas Mixtures by Analyzing the Spatiotemporal Optical Responses of Liquid Crystals Using 3D Convolutional Neural Networks
ACS Sens. 2022 Sep 23;7(9):2545-2555. doi: 10.1021/acssensors.2c00362. Epub 2022 Aug 23.
We report how analysis of the spatial and temporal optical responses of liquid crystal (LC) films to targeted gases, when performed using a machine learning methodology, can advance the sensing of gas mixtures and provide important insights into the physical processes that underlie the sensor response. We develop the methodology using O3 and Cl2 mixtures (representative of an important class of analytes) and LCs supported on metal perchlorate-decorated surfaces as a model system. Although O3 and Cl2 both diffuse through LC films and undergo redox reactions with the supporting metal perchlorate surfaces to generate similar initial and final optical states of the LCs, we show that a three-dimensional convolutional neural network can extract feature information that is encoded in the spatiotemporal color patterns of the LCs to detect the presence of both O3 and Cl2 species in mixtures and to quantify their concentrations. Our analysis reveals that O3 detection is driven by the transition time over which the brightness of the LC changes, while Cl2 detection is driven by color fluctuations that develop late in the optical response of the LC. We also show that we can detect the presence of Cl2 even when the concentration of O3 is orders of magnitude greater than the Cl2 concentration. The proposed methodology is generalizable to a wide range of analytes, reactive surfaces, and LCs and has the potential to advance the design of portable LC monitoring devices (e.g., wearable devices) for analyzing gas mixtures using spatiotemporal color fluctuations.
PMID:35998611 | DOI:10.1021/acssensors.2c00362
Formation of versus Recruitment to RNA-Rich Condensates: Controlling Effects Exerted by Peptide Side Chain Identity
J Am Chem Soc. 2022 Jun 15;144(23):10386-10395. doi: 10.1021/jacs.2c02222. Epub 2022 May 31.
Liquid-liquid phase separation (LLPS), the spontaneous formation of contiguous liquid phases with distinct compositions, has been long known in chemical systems and more recently recognized as a ubiquitous feature of cell biology. We describe a system involving biologically relevant components, synthetic peptides, and total yeast RNA, that has enabled us to explore factors that underlie phase separation. Coulombic complementarity between a cationic peptide and anionic RNA is necessary but not sufficient for formation of a condensed phase in our system. In addition to a net positive charge, the peptide must present the proper type of cationic moiety. Guanidinium groups, as found in the Arg side chain, support phase separation, but ammonium groups, as found in the Lys side chain, or dimethylguanidinium groups, as found in post-translationally modified Arg side chains, do not support phase separation in our system. However, the cationic groups that do not support phase separation via interaction with RNA can nevertheless enable recruitment to a condensed phase, which reveals that the network of forces governing condensed phase formation can differ from the network of forces governing recruitment to such a phase. We introduce a new method for measuring the concentrations of components in condensed phases based on fluorine-containing additives and 19F NMR.
PMID:35639776 | PMC:PMC9746169 | DOI:10.1021/jacs.2c02222
Programming Solitons in Liquid Crystals Using Surface Chemistry
Langmuir. 2022 Mar 22;38(11):3575-3584. doi: 10.1021/acs.langmuir.2c00231. Epub 2022 Mar 9.
AC electric fields cause three-dimensional orientational fluctuations (solitons) to form and rapidly propagate in confined films of liquid crystals (LCs), offering the basis of a new class of active soft matter (e.g., for accelerating mixing and transport processes in microscale chemical systems). How surface chemistry impacts the formation and trajectories of solitons, however, is not understood. Here, we show that self-assembled monolayers (SAMs) formed from alkanethiols on gold, which permit precise control over surface chemistry, are electrochemically stable over voltage and frequency windows (<100 V; 1 kHz) that lead to soliton formation in achiral nematic films of 4'-butyl-4-heptyl-bicyclohexyl-4-carbonitrile (CCN-47). By comparing soliton formation in LC films confined by SAMs formed from hexadecanethiol (C16SH) or pentadecanethiol (C15SH), we reveal that the electric field required for soliton formation increases with the LC anchoring energy: surfaces patterned with regions of C16SH and C15SH SAMs thus permit spatially controlled creation and annihilation of solitons necessary to generate a net flux of solitons. We also show that solitons propagate in orthogonal directions when confined by obliquely deposited gold films decorated with SAMs formed from C16SH or C15SH and that the azimuthal direction of propagation of solitons within achiral LC films possessing surface-induced twists is not unique but reflects variation in the spatial location of the solitons across the thickness of the twisted LC film. Finally, discontinuous changes in LC orientation induced by patterned surface anchoring lead to a range of new soliton behaviors including refraction, reflection, and splitting of solitons at the domain boundaries. Overall, our results provide new approaches for the controlled generation and programming of solitons with complex and precise trajectories, principles that inform new designs of chemical soft matter.
PMID:35263108 | DOI:10.1021/acs.langmuir.2c00231
Stimuli-Responsive Liquid Crystal Printheads for Spatial and Temporal Control of Polymerization
Adv Mater. 2022 Mar;34(12):e2106535. doi: 10.1002/adma.202106535. Epub 2022 Feb 14.
Polymerization reactions triggered by stimuli play a pivotal role in materials science, with applications ranging from lithography to biomedicine to adaptive materials. However, the development of chemically triggered, stimuli-responsive systems that can confer spatial and temporal control on polymerization remains a challenge. Herein, chemical-stimuli-induced polymerization based on a liquid crystal (LC) printhead is presented. The LC responds to a local chemical stimulus at its aqueous interface, resulting in the ejection of initiator into the solution to trigger polymerization. Various LC printhead geometries are designed, allowing programming of: i) bulk solution polymerization, ii) synthesis of a thin surface-confined polymeric coating, iii) polymerization-induced self-assembly of block copolymers to form various nanostructures (sphere, worm-like, and vesicles), and iv) 3D polymeric structures printed according to local solution conditions. The approach is demonstrated using amphiphiles, multivalent ions, and biomolecules as stimuli.
PMID:35065542 | DOI:10.1002/adma.202106535
Strongly Chiral Liquid Crystals in Nanoemulsions
Small. 2022 Mar;18(10):e2105835. doi: 10.1002/smll.202105835. Epub 2022 Jan 13.
Liquid crystal (LC) emulsions represent a class of confined soft matter that exhibit exotic internal organizations and size-dependent properties, including responses to chemical and physical stimuli. Past studies have explored micrometer-scale LC emulsion droplets but little is known about LC ordering within submicrometer-sized droplets. This paper reports experiments and simulations that unmask the consequences of confinement in nanoemulsions on strongly chiral LCs that form bulk cholesteric and blue phases (BPs). A method based on light scattering is developed to characterize phase transitions of LCs within the nanodroplets. For droplets with a radius to the pitch ratio (Rv /p0 ) as small as 2/3, the BP-to-cholesteric transition is substantially suppressed, leading to a threefold increase of the BP temperature interval relative to bulk behavior. Complementary simulations align with experimental findings and reveal the dominant role of chiral elastic energy. For Rv /p0 ≈ 1/3, a single LC phase forms below the clearing point, with simulations revealing the new configuration to contain a τ-1/2 disclination that extends across the nanodroplet. These findings are discussed in the context of mechanisms by which polymer networks stabilize BPs and, more broadly, for the design of nanoconfined soft matter.
PMID:35023609 | DOI:10.1002/smll.202105835
Interfacial Polyelectrolyte-Surfactant Complexes Regulate Escape of Microdroplets Elastically Trapped in Thermotropic Liquid Crystals
Langmuir. 2022 Jan 11;38(1):332-342. doi: 10.1021/acs.langmuir.1c02580. Epub 2021 Dec 30.
Polyelectrolytes adsorbed at soft interfaces are used in contexts such as materials synthesis, stabilization of emulsions, and control of rheology. Here, we explore how polyelectrolyte adsorption to aqueous interfaces of thermotropic liquid crystals (LCs) influences surfactant-stabilized aqueous microdroplets that are elastically trapped within the LCs. We find that adsorption of poly(diallyldimethylammonium chloride) (PDDA) to the interface of a nematic phase of 4-cyano-4'-pentylbiphenyl (5CB) triggers the ejection of microdroplets decorated with sodium dodecylsulfate (SDS), consistent with an attractive electrical double layer interaction between the microdroplets and LC interface. The concentration of PDDA that triggers release of the microdroplets (millimolar), however, is three orders of magnitude higher than that which saturates the LC interfacial charge (micromolar). Observation of a transient reorientation of the LC during escape of microdroplets leads us to conclude that complexes of PDDA and SDS form at the LC interface and thereby regulate interfacial charge and microdroplet escape. Poly(sodium 4-styrenesulfonate) (PSS) also triggers escape of dodecyltrimethylammonium bromide (DTAB)-decorated aqueous microdroplets from 5CB with dynamics consistent with the formation of interfacial polyelectrolyte-surfactant complexes. In contrast to PDDA-SDS, however, we do not observe a transient reorientation of the LC when using PSS-DTAB, reflecting weak association of DTAB and PSS and slow kinetics of formation of PSS-DTAB complexes. Our results reveal the central role of polyelectrolyte-surfactant dynamics in regulating the escape of the microdroplets and, more broadly, that LCs offer the basis of a novel probe of the structure and properties of polyelectrolyte-surfactant complexes at interfaces. We demonstrate the utility of these new insights by triggering the ejection of microdroplets from LCs using peptide-polymer amphiphiles that switch their net charge upon being processed by enzymes. Overall, our results provide fresh insight into the formation of polyelectrolyte-surfactant complexes at aqueous-LC interfaces and new principles for the design of responsive soft matter.
PMID:34967209 | DOI:10.1021/acs.langmuir.1c02580
Surfaces Decorated with Enantiomorphically Pure Polymer Nanohelices via Hierarchical Chirality Transfer across Multiple Length Scales
Adv Mater. 2022 Mar;34(9):e2108386. doi: 10.1002/adma.202108386. Epub 2022 Jan 24.
Mesoscale chiral materials are prepared by lithographic methods, assembly of chiral building blocks, and through syntheses in the presence of polarized light. Typically, these processes result in micrometer-sized structures, require complex top-down manipulation, or rely on tedious asymmetric separation. Chemical vapor deposition (CVD) polymerization of chiral precursors into supported films of liquid crystals (LCs) are discovered to result in superhierarchical arrangements of enantiomorphically pure nanofibers. Depending on the molecular chirality of the 1-hydroxyethyl [2.2]paracyclophane precursor, extended arrays of enantiomorphic nanohelices are formed from achiral nematic templates. Arrays of chiral nanohelices extend over hundreds of micrometers and consistently display enantiomorphic micropatterns. The pitch of individual nanohelices depends on the enantiomeric excess and the purity of the chiral precursor, consistent with the theoretical model of a doubly twisted LC director configuration. During CVD of chiral precursors into cholesteric LC films, aspects of molecular and mesoscale asymmetry combine constructively to form regularly twisted nanohelices. Enantiomorphic surfaces permit the tailoring of a wide range of functional properties, such as the asymmetric induction of weak chiral systems.
PMID:34918392 | DOI:10.1002/adma.202108386
Coupling the chemical reactivity of bimetallic surfaces to the orientations of liquid crystals
Mater Horiz. 2021 Jul 1;8(7):2050-2056. doi: 10.1039/d1mh00035g. Epub 2021 May 28.
The development of responsive soft materials with tailored functional properties based on the chemical reactivity of atomically precise inorganic interfaces has not been widely explored. In this communication, guided by first-principles calculations, we design bimetallic surfaces comprised of atomically thin Pd layers deposited onto Au that anchor nematic liquid crystalline phases of 4'-n-pentyl-4-biphenylcarbonitrile (5CB) and demonstrate that the chemical reactivity of these bimetallic surfaces towards Cl2 gas can be tuned by specification of the composition of the surface alloy. Specifically, we use underpotential deposition to prepare submonolayer to multilayers of Pd on Au and employ X-ray photoelectron and infrared spectroscopy to validate computational predictions that binding of 5CB depends strongly on the Pd coverage, with ∼0.1 monolayer (ML) of Pd sufficient to cause the liquid crystal (LC) to adopt a perpendicular binding mode. Computed heats of dissociative adsorption of Cl2 on PdAu alloy surfaces predict displacement of 5CB from these surfaces, a result that is also confirmed by experiments revealing that 1 ppm Cl2 triggers orientational transitions of 5CB. By decreasing the coverage of Pd on Au from 1.8 ± 0.2 ML to 0.09 ± 0.02 ML, the dynamic response of 5CB to 1 ppm Cl2 is accelerated 3X. Overall, these results demonstrate the promise of hybrid designs of responsive materials based on atomically precise interfaces formed between hard bimetallic surfaces and soft matter.
PMID:34846482 | DOI:10.1039/d1mh00035g
Sculpting the shapes of giant unilamellar vesicles using isotropic-nematic-isotropic phase cycles
Soft Matter. 2021 Oct 20;17(40):9078-9086. doi: 10.1039/d1sm00910a.
Understanding how soft matter deforms in response to mechanical interactions is central to the design of functional synthetic materials as well as elucidation of the behaviors of biological assemblies. Here we explore how cycles of thermally induced transitions between nematic (N) and isotropic (I) phases can be used to exert cyclical elastic stresses on dispersions of giant unilamellar vesicles (GUVs) and thereby evolve GUV shape and properties. The measurements were enabled by the finding that I-N-I phase transitions of the lyotropic chromonic liquid crystal disodium cromoglycate, when conducted via an intermediate columnar (M) phase, minimized transport of GUVs on phase fronts to confining surfaces. Whereas I to N phase transitions strained spherical GUVs into spindle-like shapes, with an efflux of GUV internal volume, subsequent N to I transitions generated a range of complex GUV shapes, including stomatocyte, pear- and dumbbell-like shapes that depended on the extent of strain in the N phase. The highest strained GUVs were observed to form buds (daughter vesicles) that we show, via a cycle of I-N-I-N phase transitions, are connected via a neck to the parent vesicle. Additional experiments established that changes in elasticity of the phase surrounding the GUVs and not thermal expansion of membranes were responsible for the shape transitions, and that I-N-I transformations that generate stomatocytes can be understood from the Bilayer-Coupling model of GUV shapes. Overall, these observations advance our understanding of how LC elastic stresses can be regulated to evolve the shapes of soft biological assemblies as well as provide new approaches for engineering synthetic soft matter.
PMID:34558596 | DOI:10.1039/d1sm00910a
Structured Liquid Droplets as Chemical Sensors that Function Inside Living Cells
ACS Appl Mater Interfaces. 2021 Sep 15;13(36):42502-42512. doi: 10.1021/acsami.1c12667. Epub 2021 Sep 1.
We report that micrometer-scale droplets of thermotropic liquid crystals (LCs) can be positioned inside living mammalian cells and deployed as chemical sensors to report the presence of toxins in extracellular environments. Our approach exploits droplets of LC enclosed in semi-permeable polymer capsules that enable internalization by cells. The LC droplets are stable in intracellular environments, but undergo optical changes upon exposure of cells to low, sub-lethal concentrations of toxic amphiphiles. Remarkably, LC droplets in intracellular environments respond to extracellular analytes that do not generate an LC response in the absence of cellular internalization. They also do not respond to other chemical stimuli or processes associated with cell growth or manipulation in culture. Our results suggest that droplet activation involves the transport and co-adsorption of amphiphilic toxins and other lipophilic cell components to the surfaces of internalized droplets. This work provides fundamentally new designs of biotic-abiotic systems that can report sensitively and selectively on the presence of select chemical agents outside cells and provides a foundation for the design of structured liquid droplets that can sense and report on other biochemical or metabolic processes inside cells.
PMID:34469102 | DOI:10.1021/acsami.1c12667
Using Liquid Crystals for <em>In Situ</em> Optical Mapping of Interfacial Mobility and Surfactant Concentrations at Flowing Aqueous-Oil Interfaces
Langmuir. 2021 May 18;37(19):5810-5822. doi: 10.1021/acs.langmuir.1c00133. Epub 2021 May 5.
Flow-induced states of fluid interfaces decorated with amphiphiles underlie phenomena such as emulsification, foaming, and spreading. While past studies have shown that interfacial mass transfer, the kinetics of surfactant adsorption and desorption, interfacial mobility, and surfactant reorganization regulate the dynamic properties of surfactant-laden interfaces, few simple methods permit simultaneous monitoring of this interplay. Here, we explore the optical responses of micrometer-thick films of oils (4-cyano-4'-pentylbiphenyl, 5CB) with a liquid crystalline order in contact with flowing aqueous phases of soluble [e.g., sodium dodecyl sulfate (SDS)] or insoluble (e.g., 1,2-dilauroyl-sn-glycero-3-phosphocholine) amphiphiles. We observe the onset of flow of 0.5 mM SDS solutions within a millifluidic channel (area-average velocity of 200 mm/s) to transform a liquid crystal (LC) film with an alignment along the interface normal into a bright birefringent state (average LC tilt angle of 30°), consistent with an initially mobile interface that shears and thus tilts the LC along the flow direction. Subsequently, we observed the LC film to evolve to a steady state (over ∼10 s) with position-dependent optical retardance controlled by gradients in surfactant concentration and thus Marangoni stresses. For 0.5 mM SDS solutions, by using particle tracking and a simple hydrodynamic model, we reveal that the dominant role of the flow-induced interfacial surfactant concentration gradient is to change the mobility of the interface (and thus shear rate of LC) and not to change the easy axis (equilibrium orientation) or anchoring energy (orientation-dependent interfacial energy) of the LC. At lower surfactant concentrations (0.015 mM SDS), however, we show that the LC directly maps flow-induced interfacial surfactant concentration gradients via a change in the local easy axis of the LC. When combined with additional measurements obtained with simple salts and insoluble amphiphiles, these results hint that LC oils may offer the basis of general and facile methods that permit mapping of both interfacial mobilities and surfactant distributions at flowing interfaces.
PMID:33950693 | DOI:10.1021/acs.langmuir.1c00133
Cationic Side Chain Identity Directs the Hydrophobically Driven Self-Assembly of Amphiphilic β-Peptides in Aqueous Solution
Langmuir. 2021 Mar 23;37(11):3288-3298. doi: 10.1021/acs.langmuir.0c03255. Epub 2021 Mar 8.
Hydrophobic interactions mediated by nonpolar molecular fragments in water are influenced by local chemical and physical contexts in ways that are not yet fully understood. Here, we use globally amphiphilic (GA) β-peptides (GA-Lys and GA-Arg) with stable conformations to explore if replacement of β3-homolysine (βLys) with β3-homoarginine (βArg) influences the hydrophobically driven assembly of these peptides in bulk aqueous solution. The studies were conducted in 10 mM triethanolamine buffer at pH 7, where both βLys (ammonium) and βArg (guanidinium) side chains are substantially protonated. Comparisons of light scattering measurements and cryo-electron micrographs before and after the addition of 60 vol% MeOH indicate very different outcomes of the hydrophobically driven assembly of AcY-GA-Lys versus AcY-GA-Arg (AcY denotes an N-acetylated-β3-homotyrosine (βTyr) at each N-terminus). Nuclear magnetic resonance and analytical ultracentrifugation confirm that AcY-GA-Lys assembles into large aggregates in aqueous buffer, whereas AcY-GA-Arg at comparable concentrations forms only small oligomers. Titration of AcY-GA-Arg into aqueous solutions of AcY-GA-Lys reveals that the driving force for AcY-GA-Lys association is far stronger than that for AcY-GA-Arg association. We discuss these results in the light of past experimental observations involving single molecule force measurements with GA β-peptides and hydrophobically driven dimerization of conventional peptides that form a GA α-helix upon dimerization (but do not display the Lys versus Arg trend predicted by extrapolating from the earlier AFM studies with β-peptides). Overall, our results establish that the identity of proximal cationic groups, ammonium versus guanidinium, profoundly modulates the hydrophobically driven self-assembly of conformationally stable β-peptides in bulk aqueous solution.
PMID:33683138 | DOI:10.1021/acs.langmuir.0c03255
Design of Chemoresponsive Soft Matter Using Hydrogen-Bonded Liquid Crystals
Materials (Basel). 2021 Feb 24;14(5):1055. doi: 10.3390/ma14051055.
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.
PMID:33668152 | PMC:PMC7975976 | DOI:10.3390/ma14051055
Active motion of multiphase oil droplets: emergent dynamics of squirmers with evolving internal structure
Soft Matter. 2021 Mar 18;17(10):2985-2993. doi: 10.1039/d0sm01873b.
Synthetic soft matter systems, when driven beyond equilibrium by active processes, offer the potential to achieve dynamical states and functions of a complexity found in living matter. Emulsions offer the basis of a simple yet versatile system for identification of the physicochemical principles underlying active soft matter, but how multiple internal phases within emulsion droplets (e.g., Janus morphologies) organize to impact emergent dynamics is not understood. Here, we create multiphase oil droplets with ultralow interfacial tensions but distinct viscosities, and drive them into motion in aqueous micellar solutions. Preferential solubilization of select components of the oil both drives the droplet motion and yields a progression of internal phase morphological states with distinct symmetries. We find the active droplets to exhibit five dynamical states during morphogenesis. By quantifying microscopic flow fields, we show that it is possible to map the diverse droplet behaviors to squirmer models of spherical microswimmers in Stokes flow, thus showing that multiphase droplets offer the basis of a versatile platform with which to study and engineer the hydrodynamics of microswimmers.
PMID:33596294 | DOI:10.1039/d0sm01873b
Using machine learning and liquid crystal droplets to identify and quantify endotoxins from different bacterial species
Analyst. 2021 Feb 22;146(4):1224-1233. doi: 10.1039/d0an02220a.
Detection and quantification of bacterial endotoxins is important in a range of health-related contexts, including during pharmaceutical manufacturing of therapeutic proteins and vaccines. Here we combine experimental measurements based on nematic liquid crystalline droplets and machine learning methods to show that it is possible to classify bacterial sources (Escherichia coli, Pseudomonas aeruginosa, Salmonella minnesota) and quantify concentration of endotoxin derived from all three bacterial species present in aqueous solution. The approach uses flow cytometry to quantify, in a high-throughput manner, changes in the internal ordering of micrometer-sized droplets of nematic 4-cyano-4'-pentylbiphenyl triggered by the endotoxins. The changes in internal ordering alter the intensities of light side-scattered (SSC, large-angle) and forward-scattered (FSC, small-angle) by the liquid crystal droplets. A convolutional neural network (Endonet) is trained using the large data sets generated by flow cytometry and shown to predict endotoxin source and concentration directly from the FSC/SSC scatter plots. By using saliency maps, we reveal how EndoNet captures subtle differences in scatter fields to enable classification of bacterial source and quantification of endotoxin concentration over a range that spans eight orders of magnitude (0.01 pg mL-1 to 1 μg mL-1). We attribute changes in scatter fields with bacterial origin of endotoxin, as detected by EndoNet, to the distinct molecular structures of the lipid A domains of the endotoxins derived from the three bacteria. Overall, we conclude that the combination of liquid crystal droplets and EndoNet provides the basis of a promising analytical approach for endotoxins that does not require use of complex biologically-derived reagents (e.g., Limulus amoebocyte lysate).
PMID:33393547 | DOI:10.1039/d0an02220a
Dynamic and reversible shape response of red blood cells in synthetic liquid crystals
Proc Natl Acad Sci U S A. 2020 Oct 20;117(42):26083-26090. doi: 10.1073/pnas.2007753117. Epub 2020 Oct 2.
Mammalian cells are soft, and correct functioning requires that cells undergo dynamic shape changes in vivo. Although a range of diseases are associated with stiffening of red blood cells (RBCs; e.g., sickle cell anemia or malaria), the mechanical properties and thus shape responses of cells to complex viscoelastic environments are poorly understood. We use vapor pressure measurements to identify aqueous liquid crystals (LCs) that are in osmotic equilibrium with RBCs and explore mechanical coupling between RBCs and LCs. When transferred from an isotropic aqueous phase into a LC, RBCs exhibit complex yet reversible shape transformations, from initially biconcave disks to elongated and folded geometries with noncircular cross-sections. Importantly, whereas the shapes of RBCs are similar in isotropic fluids, when strained by LC, a large variance in shape response is measured, thus unmasking cell-to-cell variation in mechanical properties. Numerical modeling of LC and cell mechanics reveals that RBC shape responses occur at constant cell membrane area but with membrane shear moduli that vary between cells from 2 to 16 × 10-6 N/m. Temperature-dependent LC elasticity permits continuous tuning of RBC strains, and chemical cross-linking of RBCs, a model for diseased cells, leads to striking changes in shape responses of the RBCs. Overall, these results provide insight into the coupling of strain between soft mammalian cells and synthetic LCs, and hint at new methods for rapidly characterizing mechanical properties of single mammalian cells in a population and thus cell-to-cell variance.
PMID:33008877 | PMC:PMC7585021 | DOI:10.1073/pnas.2007753117
Biomolecular Binding at Aqueous Interfaces of Langmuir Monolayers of Bioconjugated Amphiphilic Mesogenic Molecules: A Molecular Dynamics Study
Langmuir. 2020 Oct 20;36(41):12281-12287. doi: 10.1021/acs.langmuir.0c02191. Epub 2020 Oct 9.
We report a molecular dynamics (MD) simulation study of protein binding at the aqueous-liquid crystal (LC) interfaces of bioconjugated mesogenic molecules. As a simple model of these interfaces, we use monolayers composed of biotin-conjugated or biotin-free amphiphilic mesogenic molecules and streptavidin in water. The all-atom MD simulations reveal that the binding of streptavidin to the biotin mesogenic monolayer is significantly stronger than that to biotin-free mesogenic monolayers. Although specific protein binding marginally increases the overall orientational order and the tilt of the biotin-conjugated mesogenic molecules of the monolayer, significant changes in tilt were observed near the bound protein (in contrast to the protein interaction with the monolayer without biotin). We also observe that specific protein binding changes the dynamic properties of the mesogens within the monolayer (e.g., lateral diffusion coefficients) and associated water. Overall, these MD simulations advance our understanding of the molecular-level phenomena involved in the binding of biomolecules and subsequent dynamic changes at the aqueous-LC interfaces. These results provide guidance to future molecular-level designs of biofunctional LC interfaces.
PMID:32970447 | DOI:10.1021/acs.langmuir.0c02191
Prolate and oblate chiral liquid crystal spheroids
Sci Adv. 2020 Jul 10;6(28):eaba6728. doi: 10.1126/sciadv.aba6728. eCollection 2020 Jul.
Liquid crystals are known to exhibit intriguing textures and color patterns, with applications in display and optical technologies. This work focuses on chiral materials and examines the palette of morphologies that arises when microdroplets are deformed into nonspherical shapes in a controllable manner. Specifically, geometrical confinement and mechanical strain are used to manipulate orientational order, phase transitions, and topological defects that arise in chiral liquid crystal droplets. Inspired by processes encountered in nature, where insects and animals often rely on strain and temperature to alter the optical appearance of dispersed liquid crystalline elements, chiral droplets are dispersed in polymer films and deformation induced by uniaxial or biaxial stretching. Our measurements are interpreted by resorting to simulations of the corresponding systems, thereby providing an in-depth understanding of the morphologies that arise in these materials. The reported structures and assemblies offer potential for applications in smart coatings, smart fabrics, and wearable sensors.
PMID:32832603 | PMC:PMC7439570 | DOI:10.1126/sciadv.aba6728
Structural and Optical Response of Polymer-Stabilized Blue Phase Liquid Crystal Films to Volatile Organic Compounds
ACS Appl Mater Interfaces. 2020 Sep 16;12(37):42099-42108. doi: 10.1021/acsami.0c11138. Epub 2020 Sep 8.
Engineering useful mechanical properties into stimuli-responsive soft materials without compromising their responsiveness is, in many cases, an unresolved challenge. For example, polymer networks formed within blue-phase liquid crystals (BPs) have been shown to form mechanically robust films, but the impact of polymer networks on the response of these soft materials to chemical stimuli has not been explored. Here, we report on the response of polymer-stabilized BPs (PSBPs) to volatile organic compounds (VOCs, using toluene as a model compound) and compare the response to BPs without polymer stabilization and to polymerized nematic and cholesteric phases. We find that PSBPs generate an optical response to toluene vapor (change in reflection intensity under crossed polars) that is sixfold greater in sensitivity than the polymerized nematic or cholesteric phases and with a limit of detection (140 ± 10 ppm at 25 °C) that is relevant to the measurement of permissible exposure limits for humans. Additionally, when compared to BPs that have not been polymerized, PSBPs respond to a broader range of toluene vapor concentrations (5000 vs <1000 ppm) over a wider temperature interval (25-45 vs 45-53 °C). We place these experimental observations into the context of a simple thermodynamic model to explore how the PSBP response reflects the effect of toluene on competing contributions of double-twisted LC cylinders, disclinations, and polymer network to the free energy that controls the PSBP lattice spacing. Overall, we conclude that the mechanical and thermal stability of PSBPs, when combined with their optical responsiveness to toluene, make this class of self-supporting LCs a promising one as the basis of passive and compact (e.g., wearable) sensors for VOCs.
PMID:32794738 | DOI:10.1021/acsami.0c11138
Binding of Organophosphorus Nerve Agents and Their Simulants to Metal Salts
ACS Appl Mater Interfaces. 2020 Jul 8;12(27):30941-30953. doi: 10.1021/acsami.0c05777. Epub 2020 Jun 25.
Nerve agents (NAs) pose a great threat to society because they are easy to produce and are deadly in nature, which makes developing methods to detect, adsorb, and destroy them crucial. To enable the development of these methods, we report the use of first principles electronic structure calculations to understand the binding properties of NAs and NA simulants on metal salt surfaces. We report calculated Gibbs free binding energies (GBE) for four NAs (tabun (GA), sarin (GB), soman (GD), and venomous X (VX)) and five NA simulants (dimethyl methylphosphonate (DMMP), dimethyl chlorophosphate (DMCP), trimethyl phosphate (TMP), methyl dichlorophosphate (MDCP), and di-isopropyl methylphosphonate (DIMP)) on metal perchlorate and metal nitrate salts using density functional theory. Our results indicate a general trend in the binding strength of NAs and NA simulants to metal salt surfaces: MDCP < DMCP < GA < GD ≈ GB < TMP < VX ≈ DMMP < DIMP. Based on their binding properties on salt surfaces, we identify the most effective simulant for each of the studied NAs as follows: DMCP for GA, TMP for GB and GD, and DMMP for VX. To illustrate the utility of the binding energies calculated in our study, we address the design of NA sensors based on the competitive binding of NAs and liquid crystalline compounds on metal salts. We compare our results with previous experimental findings and provide a list of promising combinations of liquid crystal and metal salt systems to selectively and sensitively detect NAs. Our study highlights the great value of computational chemistry for designing selective and sensitive NA sensors while minimizing the number of very dangerous experiments involving NAs.
PMID:32506901 | DOI:10.1021/acsami.0c05777
Liquid Crystal Emulsions That Intercept and Report on Bacterial Quorum Sensing
ACS Appl Mater Interfaces. 2020 Jul 1;12(26):29056-29065. doi: 10.1021/acsami.0c05792. Epub 2020 Jun 17.
We report aqueous emulsions of thermotropic liquid crystals (LCs) that can intercept and report on the presence of N-acyl-l-homoserine lactones (AHLs), a class of amphiphiles used by pathogenic bacteria to regulate quorum sensing (QS), monitor population densities, and initiate group activities, including biofilm formation and virulence factor production. The concentration of AHL required to promote "bipolar" to "radial" transitions in micrometer-scale droplets of the nematic LC 4'-pentyl-cyanobiphenyl (5CB) decreases with increasing carbon number in the acyl tail, reaching a threshold concentration of 7.1 μM for 3-oxo-C12-AHL, a native QS signal in the pathogen Pseudomonas aeruginosa. The LC droplets in these emulsions also respond to biologically relevant concentrations of the biosurfactant rhamnolipid, a virulence factor produced by communities of P. aeruginosa under the control of QS. Systematic studies using bacterial mutants support the conclusion that these emulsions respond selectively to the production of rhamnolipid and AHLs and not to other products produced by bacteria at lower (subquorate) population densities. Finally, these emulsions remain configurationally stable in growth media, enabling them to be deployed either in bacterial supernatants or in situ in bacterial cultures to eavesdrop on QS and report on changes in bacterial group behavior that can be detected in real time using polarized light. Our results provide new tools to detect and report on bacterial QS and virulence and a materials platform for the rapid and in situ monitoring of bacterial communication and resulting group behaviors in bacterial communities.
PMID:32484648 | PMC:PMC7343617 | DOI:10.1021/acsami.0c05792
Bacterial Quorum Sensing Signals Self-Assemble in Aqueous Media to Form Micelles and Vesicles: An Integrated Experimental and Molecular Dynamics Study
J Phys Chem B. 2020 May 7;124(18):3616-3628. doi: 10.1021/acs.jpcb.0c00496. Epub 2020 Apr 23.
Many species of common bacteria communicate and coordinate group behaviors, including toxin production and surface fouling, through a process known as quorum sensing (QS). In Gram-negative bacteria, QS is regulated by N-acyl-l-homoserine lactones (AHLs) that possess a polar homoserine lactone headgroup and a nonpolar aliphatic tail. Past studies demonstrate that AHLs can aggregate in water or adsorb at interfaces, suggesting that molecular self-assembly could play a role in processes that govern bacterial communication. We used a combination of biophysical characterization and atomistic molecular dynamics (MD) simulations to characterize the self-assembly behaviors of 12 structurally related AHLs. We used static light scattering and measurements of surface tension to characterize the assembly of four naturally occurring AHLs (3-oxo-C8-AHL, 3-oxo-C12-AHL, C12-AHL, and C16-AHL) in aqueous media and determine their critical aggregation concentrations (CACs). MD simulations and alchemical free energy calculations were used to predict thermodynamically preferred aggregate structures for each AHL. Those calculations predicted that AHLs with 10 or 12 tail carbon atoms should form spherical micelles and that AHLs with 14 or 16 tail carbon atoms should form vesicles in solution. Characterization of solutions of AHLs using negative stain transmission electron microscopy (TEM) and dynamic light scattering (DLS) revealed aggregates with sizes consistent with spherical micelles or small unilamellar vesicles for 3-oxo-C12-AHL and C12-AHL and the formation of large vesicles (∼250 nm) in solutions of C16-AHL. These experimental findings are in general agreement with our simulation predictions. Overall, our results provide insight into processes of self-assembly that can occur in this class of bacterial amphiphiles and, more broadly, provide a potential basis for understanding how AHL structure could influence processes that bacteria use to drive important group behaviors.
PMID:32271573 | PMC:PMC7331775 | DOI:10.1021/acs.jpcb.0c00496
Optical "Blinking" Triggered by Collisions of Single Supramolecular Assemblies of Amphiphilic Molecules with Interfaces of Liquid Crystals
J Am Chem Soc. 2020 Apr 1;142(13):6139-6148. doi: 10.1021/jacs.9b13360. Epub 2020 Mar 18.
We report that incubation of aqueous dispersions of supramolecular assemblies formed by synthetic alkyl triazole-based amphiphiles against interfaces of thermotropic liquid crystals (LCs; 4-cyano-4'-pentylbiphenyl) triggers spatially localized (micrometer-scale) and transient (subsecond) flashes of light to be transmitted through the LC. Analysis of the spatiotemporal response of the LC supports our proposal that each optical "blinking" event results from collision of a single supramolecular assembly with the LC interface. Particle tracking at the LC interface confirmed that collision and subsequent spreading of amphiphiles at the interface generates a surface pressure-driven interfacial flow (Marangoni flow) that causes transient reorientation of LC and generation of a bright optical flash between crossed polarizers. We also found that dispersions of phospholipid vesicles cause "blinks". When using vesicles formed from 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC), we measured the frequency of blinking to decrease proportionally with the number density of vesicles in the aqueous phase, consistent with single vesicle events, with the lifetime of each blink dependent on vesicle size (800 ± 80 nm to 150 ± 30 nm). For 100 μM of DLPC, we measured vesicles with a diameter of 940 ± 290 nm to generate 47 ± 9 blinks min-1 mm-2, revealing that the fraction of vesicle collisions resulting in fusion with the LC interface is ∼10-3. Overall, the results in this paper unmask new nonequilibrium behaviors of amphiphiles at LC interfaces, and provide fresh approaches for exploring the dynamic interactions of supramolecular assemblies of amphiphiles with fluid interfaces at the single-event level.
PMID:32085676 | DOI:10.1021/jacs.9b13360
Cuboidal liquid crystal phases under multiaxial geometrical frustration
Soft Matter. 2020 Jan 28;16(4):870-880. doi: 10.1039/c9sm02021g. Epub 2020 Jan 15.
Cuboidal liquid crystal phases - the so-called blue phases - consist of a network of topological defects arranged into a cubic symmetry. They exhibit striking optical properties, including Bragg reflection in the visible range and fast response times. Confining surfaces can interfere with the packing of such a network, leading to structures that have not been explored before. In this work, a Landau-de Gennes free energy formalism for the tensor alignment field Q is used to investigate the behavior of chiral liquid crystals under non-isotropic confinement. The underlying free energy functional is solved by relying on a Monte Carlo method that facilitates efficient exploration of configuration space. The results of simulations are expressed in terms of phase diagrams as a function of chirality and temperature for three families of spheroids: oblate, spherical, and prolate. Upon deformation, blue phases adapt and transform to accommodate the geometrical constraints, thereby resulting in a wider range of thermal stability. For oblate spheroids, confinement interferes with the development of a full blue phase structure, resulting on a combination of half skyrmions. For prolate spheroids, the blue phases are hybridized and exhibit features of blue phases I and II. More generally, it is shown that mechanical deformation provides an effective means to control, manipulate and stabilize blue phases and cholesterics confined in tactoids.
PMID:31938794 | DOI:10.1039/c9sm02021g
A New Strategy for Reporting Specific Protein Binding Events at Aqueous-Liquid Crystal Interfaces in the Presence of Non-Specific Proteins
ACS Appl Mater Interfaces. 2020 Feb 19;12(7):7869-7878. doi: 10.1021/acsami.9b16867. Epub 2020 Feb 7.
Aqueous-liquid crystal (LC) interfaces offer promise as responsive interfaces at which biomolecular recognition events can be amplified into macroscopic signals. However, the design of LC interfaces that distinguish between specific and non-specific protein interactions remains an unresolved challenge. Herein, we report the synthesis of amphiphilic monomers, dimers, and trimers conjugated to sulfonamide ligands via triazole rings, their assembly at aqueous-LC interfaces, and the orientational response of LCs to the interactions of carbonic anhydrase II (CAII) and serum albumin with the oligomer-decorated LC interfaces. Of six oligomers synthesized, only dimers without amide methylation were found to assemble at aqueous interfaces of nematic 4-cyano-4'-pentylbiphenyl (5CB) to induce perpendicular LC orientations. At dimer-decorated LC interfaces, we found that concentrations of CAII less than 4 μM did not measurably perturb the LC but prevented non-specific adsorption and penetration of serum albumin into the dimer-decorated interface that otherwise triggered bright, globular LC optical domains. These experiments and others (including competitive adsorption of CAII, BSA, and lysozyme) support our hypothesis that specific binding of CAII to the dimer prevents LC anchoring transitions triggered by non-specific adsorption of serum albumin. We illustrate the utility of the approach by reporting (i) the relative activity of two small-molecule inhibitors (6-ethoxy-2-benzothiazolesulfonamide and benzenesulfonamide) of CAII to sulfonamide and (ii) proteolytic digestion of a protein (CAII) by thermolysin. Overall, the results in this paper provide new insight into the interactions of proteins at aqueous-LC interfaces and fresh ideas for either blocking non-specific interactions of proteins at surfaces or reporting specific binding events at LC interfaces in the presence of non-specific proteins.
PMID:31825195 | PMC:PMC7368459 | DOI:10.1021/acsami.9b16867
Steering Active Emulsions with Liquid Crystals
Langmuir. 2020 Jun 30;36(25):6948-6956. doi: 10.1021/acs.langmuir.9b02975. Epub 2019 Dec 24.
Colloids dispersed in liquid crystals (LCs) diffuse preferentially along the LC director because this direction of displacement generates the lowest hydrodynamic drag. In this article, we report on the active transport of micrometer-sized nematic droplets of 4'-pentyl-4-biphenylcarbonitrile (5CB) propelled through a continuous LC phase formed from aqueous solutions of disodium cromoglycate (DSCG) by Marangoni stresses (generated through the addition of sodium dodecyl sulfate (SDS)). We observe the nematic droplets to exhibit motion guided by the continuous LC phase, but in contrast to passive diffusion, the LC droplets move preferentially in a direction perpendicular to the continuous-phase LC director. Our results suggest that the LC droplets, with internal symmetry broken by the Marangoni flow, interact through orientation-dependent van der Waals forces with the LC continuous phase, biasing the orientation of the droplets and the direction of propulsion orthogonal to the far-field director of the continuous LC phase. This proposal is supported by measurements of the orientations of droplets of 5CB and 4-ethoxy-4'-(6-acryloyloxyhexyloxy) azobenzene (RM257) polymerized in a preradial director configuration, which reveal the polymerized droplets to adopt orientations that are biased toward the perpendicular of the far-field DSCG director. Additionally, we demonstrate that preferential motion parallel to the continuous-phase LC director is recovered when using self-propelled isotropic oil droplets. We also observe periodic changes in the instantaneous velocities of LC droplets. We show the changes to correlate with the formation and detachment of satellite droplets, consistent with the solubilization of the nematic oil into surfactant assemblies near the trailing edge of the droplets and their accumulation near a stagnation region downstream of the droplet. Overall, our results provide fundamental insights into ways in which LC ordering can change the dynamics of active colloidal systems and hint at principles by which the motion of active colloids can be steered.
PMID:31804839 | DOI:10.1021/acs.langmuir.9b02975
Retention of Coiled-Coil Dimer Formation in the Absence of Ion Pairing at Positions Flanking the Hydrophobic Core
Biochemistry. 2019 Dec 3;58(48):4821-4826. doi: 10.1021/acs.biochem.9b00668. Epub 2019 Nov 18.
Hydrophobic interactions govern how proteins fold and interact with other molecules, but the impact of nearby polar functionality on the effective hydrophobicity of nonpolar surfaces remains unclear. Here we use a common protein quaternary structure motif, the parallel coiled-coil dimer, to ask whether the identity of basic residues (arginine vs lysine; guanidinium vs ammonium) arrayed along one side of the constituent α-helices influences the favorability of dimerization driven by burial of hydrophobic surface on the other side of each helix. Significant sequence redesign was necessary to achieve the desired juxtaposition of nonpolar and cationic functionality, because we needed to eliminate charged side chains from positions flanking the nonpolar helix surface. Natural and designed sequences that form coiled coils are almost universally rich in acidic and basic residues at these flanking positions. Our arginine coiled-coil dimer was moderately more stable than the lysine analogue, which contrasts with behavior previously observed with helical β-amino acid oligomers bearing guanidinium versus ammonium groups. We attribute this backbone-dependent difference to variations in the extent to which the helical propensities of α- and β-residues can be modulated by design. These findings highlight the challenge of identifying noncovalent forces that direct structure formed by a flexible backbone.
PMID:31738525 | DOI:10.1021/acs.biochem.9b00668
Reconfigurable Multicompartment Emulsion Drops Formed by Nematic Liquid Crystals and Immiscible Perfluorocarbon Oils
Langmuir. 2019 Dec 10;35(49):16312-16323. doi: 10.1021/acs.langmuir.9b02864. Epub 2019 Nov 25.
Liquid crystalline (LC) oils offer the basis of stimuli-responsive LC-in-water emulsions. Although past studies have explored the properties of single-phase LC emulsions, few studies have focused on complex multicompartment emulsions containing co-existing isotropic and LC domains. In this paper, we report a study of multiphase emulsions using LCs and immiscible perfluoroalkanes dispersed in water or glycerol (the latter continuous phase is used to enable characterization). We found that the nematogen 4'-pentyl-4-biphenylcarbonitrile (5CB) anchors homeotropically (perpendicularly) and weakly at liquid perfluorononane (F9) interfaces, consistent with the smectic layering of 5CB molecules. The proposed role of smectic layering is supported by experiments performed with 4-(trans-4-pentylcyclohexyl)benzonitrile, a nematogen that possesses a cyclohexyl group that frustrates the smectic packing and leads to tilted orientations at the F9 interface. By employing perfluorocarbon and hydrocarbon surfactants in combination with multiphase 5CB and F9 emulsion droplets dispersed in a continuous water or glycerol phase, we observe a range of emulsion droplet morphologies to form, including core-shell and Janus structures, with internal organizations that reflect an interplay of interfacial (anchoring energies; F9 and glycerol) and elastic energies within the confines of the geometry of the emulsion droplet. By comparing experimental observations to simulations of the LC-perfluorocarbon droplets based on a Landau-de Gennes model of the free energy, we place bounds on the orientation-dependent interfacial energies that underlie the internal ordering of these complex emulsions. Additionally, by forming core-shells emulsion droplets from 5CB (shell) and perfluoroheptane (cores), we demonstrate how a liquid-to-vapor phase transition in the perfluorocarbon core can be used to actuate the droplet and rapidly thin the nematic shell. Overall, the results reported in this paper demonstrate that multiphase LC emulsions formed from mixtures of perfluoroalkanes and LCs provide new opportunities to engineer hierarchical and stimuli-responsive emulsion systems.
PMID:31652070 | DOI:10.1021/acs.langmuir.9b02864
Amplification of Elementary Surface Reaction Steps on Transition Metal Surfaces Using Liquid Crystals: Dissociative Adsorption and Dehydrogenation
J Am Chem Soc. 2019 Oct 9;141(40):16003-16013. doi: 10.1021/jacs.9b08057. Epub 2019 Sep 25.
Elementary reaction steps, including adsorption and dissociation, of a range of molecular adsorbates on transition metal surfaces have been elucidated in the context of chemical catalysis. Here we leverage this knowledge to design liquid crystals (LCs) supported on ultrathin polycrystalline gold films (predominant crystallographic face is (111)) that are triggered to undergo orientational transitions by dissociative adsorption and dehydrogenation reactions involving chlorine and carboxylic acids, respectively, thus amplifying these atomic-scale surface processes in situ into macroscopic optical signals. We use electronic structure calculations to predict that 4'-n-pentyl-4-biphenylcarbonitrile (5CB), a room temperature nematic LC, does not bind to Au(111) in an orientation that changes upon dissociative adsorption of molecular chlorine, a result validated by experiments. In contrast, 4-cyano-4-biphenylcarboxylic acid (CBCA) is calculated to bind strongly to Au(111) in a perpendicular orientation via dehydrogenation of the carboxylic acid group, which we confirmed using polarization-modulation infrared reflection-absorption spectroscopy. A maximum coverage of 0.07 monolayer of CBCA on the gold surface is sufficient to cause a perpendicular orientation of the LC. Dissociative adsorption of Cl2 gas on the gold surface, resulting in 0.5 monolayer coverage of Cl, displaces CBCA from Au(111) and thus triggers a strikingly visible change in orientation of the LC. Infrared spectroscopy established the orientation of adsorbed CBCA to be parallel to the Cl covered surface, with the COOH plane perpendicular to the surface, as predicted by first-principles calculations. These results demonstrate the use of first-principles calculations and transition metal surfaces to design LCs that report in situ targeted atomic-scale surface processes.
PMID:31508958 | DOI:10.1021/jacs.9b08057
Soft matter from liquid crystals
Soft Matter. 2019 Sep 21;15(35):6913-6929. doi: 10.1039/c9sm01424a. Epub 2019 Aug 23.
Liquid crystals (LCs) are fluids within which molecules exhibit long-range orientational order, leading to anisotropic properties such as optical birefringence and curvature elasticity. Because the ordering of molecules within LCs can be altered by weak external stimuli, LCs have been widely used to create soft matter systems that respond optically to electric fields (LC display), temperature (LC thermometer) or molecular adsorbates (LC chemical sensor). More recent studies, however, have moved beyond investigations of optical responses of LCs to explore the design of complex LC-based soft matter systems that offer the potential to realize more sophisticated functions (e.g., autonomous, self-regulating chemical responses to mechanical stimuli) by directing the interactions of small molecules, synthetic colloids and living cells dispersed within the bulk of LCs or at their interfaces. These studies are also increasingly focusing on LC systems driven beyond equilibrium states. This review presents one perspective on these advances, with an emphasis on the discovery of fundamental phenomena that may enable new technologies. Three areas of progress are highlighted; (i) directed assembly of amphiphilic molecules either within topological defects of LCs or at aqueous interfaces of LCs, (ii) templated polymerization in LCs via chemical vapor deposition, an approach that overcomes fundamental challenges related to control of LC phase behavior during polymerization, and (iii) studies of colloids in LCs, including chiral colloids, soft colloids that are strained by LCs, and active colloids that are driven into organized states by dissipation of energy (e.g. bacteria). These examples, and key unresolved issues discussed at the end of this perspective, serve to convey the message that soft matter systems that integrate ideas from LC, surfactant, polymer and colloid sciences define fertile territory for fundamental studies and creation of future transformative technologies.
PMID:31441481 | DOI:10.1039/c9sm01424a
Understanding lipopolysaccharide aggregation and its influence on activation of Factor C
J Colloid Interface Sci. 2019 Sep 15;552:540-553. doi: 10.1016/j.jcis.2019.05.013. Epub 2019 May 16.
The quantification of lipopolysaccharide (LPS) shed by bacteria within aqueous samples is typically performed by binding LPS to a protein called Factor C within a lysate prepared from the blood of horseshoe crabs (Limulus amebocyte lysate (LAL)). How the state of aggregation of LPS impacts Factor C activation, however, is not understood, particularly in the presence of select salts and non-ionic surfactants that are commonly incorporated into pharmaceutical formulations. To address this open question, herein we report on the aggregation status of LPS in aqueous solution, characterized using angle-dependent static and dynamic light scattering with and without chelating salts and polysorbate surfactants, and its correlation with activation of Factor C. Because the aggregation status of LPS is kinetically controlled, care was taken to compare LPS aggregation and activity using identically prepared samples. By plotting LPS activity versus the LPS aggregate size distribution over varied solution conditions, we found a positive correlation between LPS aggregate sizes between 30 and 50 nm and LAL activity. Overall, our results support the hypothesis that activation of Factor C is dependent of LPS aggregate size, and that the modulating effects of salts and surfactants on activation of Factor C is associated with changes in the LPS aggregation.
PMID:31154247 | DOI:10.1016/j.jcis.2019.05.013
Thermally reconfigurable Janus droplets with nematic liquid crystalline and isotropic perfluorocarbon oil compartments
Soft Matter. 2019 Mar 20;15(12):2580-2590. doi: 10.1039/c8sm02600a.
We report that mixtures of perfluorocarbon oils and hydrocarbon mesogens can be used to prepare multi-compartment (Janus) emulsion drops comprising coexisting nematic liquid crystalline (LC) and isotropic oil phases. The droplets exhibit stable spherical shapes with internal Janus-type morphologies that can be tuned widely through changes in temperature or adsorbates. In particular, we observe evidence of preferential adsorption of hydrocarbon or fluorocarbon surfactants on the interfaces of nematic versus isotropic domains, respectively, providing added control over the droplet structure. Comparisons of experiments and numerical simulations using a Landau-de Gennes continuum model provide insight into the relative importance of the LC elasticity and orientational-dependent interfacial energies on droplet morphologies and properties. We show that the hierarchical organization of the LC compartments generates optical properties and responsiveness not found in emulsions of isotropic oils.
PMID:30816895 | DOI:10.1039/c8sm02600a
Molecular Order Affects Interfacial Water Structure and Temperature-Dependent Hydrophobic Interactions between Nonpolar Self-Assembled Monolayers
Langmuir. 2019 Feb 12;35(6):2078-2088. doi: 10.1021/acs.langmuir.8b03287. Epub 2019 Feb 1.
Understanding how material properties affect hydrophobic interactions-the water-mediated interactions that drive the association of nonpolar materials-is vital to the design of materials in contact with water. Conventionally, the magnitude of the hydrophobic interactions between extended interfaces is attributed to interfacial chemical properties, such as the amount of nonpolar solvent-exposed surface area. However, recent experiments have demonstrated that the hydrophobic interactions between uniformly nonpolar self-assembled monolayers (SAMs) also depend on molecular-level SAM order. In this work, we use atomistic molecular dynamics simulations to investigate the relationship between SAM order, water structure, and hydrophobic interactions to explain these experimental observations. The SAM-SAM hydrophobic interactions calculated from the simulations increase in magnitude as SAM order increases, matching experimental observations. We explain this trend by showing that the molecular-level order of the SAM impacts the nanoscale structure of interfacial water molecules, leading to an increase in water structure near disordered SAMs. These findings are consistent with a decrease in the solvation entropy of disordered SAMs, which is confirmed by measuring the temperature dependence of hydrophobic interactions using both simulations and experiments. This study elucidates how hydrophobic interactions can be influenced by an interfacial physical property, which may guide the design of synthetic materials with fine-tuned interfacial hydrophobicity.
PMID:30645942 | DOI:10.1021/acs.langmuir.8b03287
Templated nanofiber synthesis via chemical vapor polymerization into liquid crystalline films
Science. 2018 Nov 16;362(6416):804-808. doi: 10.1126/science.aar8449.
Extrusion, electrospinning, and microdrawing are widely used to create fibrous polymer mats, but these approaches offer limited access to oriented arrays of nanometer-scale fibers with controlled size, shape, and lateral organization. We show that chemical vapor polymerization can be performed on surfaces coated with thin films of liquid crystals to synthesize organized assemblies of end-attached polymer nanofibers. The process uses low concentrations of radical monomers formed initially in the vapor phase and then diffused into the liquid-crystal template. This minimizes monomer-induced changes to the liquid-crystal phase and enables access to nanofiber arrays with complex yet precisely defined structures and compositions. The nanofiber arrays permit tailoring of a wide range of functional properties, including adhesion that depends on nanofiber chirality.
PMID:30442805 | DOI:10.1126/science.aar8449
Machine Learning Algorithms for Liquid Crystal-Based Sensors
ACS Sens. 2018 Nov 26;3(11):2237-2245. doi: 10.1021/acssensors.8b00100. Epub 2018 Oct 19.
We present a machine learning (ML) framework to optimize the specificity and speed of liquid crystal (LC)-based chemical sensors. Specifically, we demonstrate that ML techniques can uncover valuable feature information from surface-driven LC orientational transitions triggered by the presence of different gas-phase analytes (and the corresponding optical responses) and can exploit such feature information to train accurate and automatic classifiers. We demonstrate the utility of the framework by designing an experimental LC system that exhibits similar optical responses to a stream of nitrogen containing either 10 ppmv dimethyl-methylphosphonate (DMMP) or 30% relative humidity (RH). The ML framework is used to process and classify thousands of images (optical micrographs) collected during the LC responses and we show that classification (sensing) accuracies of over 99% can be achieved. For the same experimental system, we demonstrate that traditional feature information used in characterizing LC responses (such as average brightness) can only achieve sensing accuracies of 60%. We also find that high accuracies can be achieved by using time snapshots collected early in the LC response, thus providing the ability to create fast sensors. We also show that the ML framework can be used to systematically analyze the quality of information embedded in LC responses and to filter out noise that arises from imperfect LC designs and from sample variations. We evaluate a range of classifiers and feature extraction methods and conclude that linear support vector machines are preferred and that high accuracies can only be achieved by simultaneously exploiting multiple sources of feature information.
PMID:30289249 | DOI:10.1021/acssensors.8b00100
Amphiphile-Induced Phase Transition of Liquid Crystals at Aqueous Interfaces
ACS Appl Mater Interfaces. 2018 Oct 31;10(43):37618-37624. doi: 10.1021/acsami.8b09639. Epub 2018 Oct 22.
Monolayer assemblies of amphiphiles at planar interfaces between thermotropic liquid crystals (LCs) and an aqueous phase can give rise to configurational transitions of the underlying LCs. A common assumption has been that a reconfiguration of the LC phase is caused by an interdigitation of the hydrophobic tails of amphiphiles with the molecules of the LC at the interface. A different mechanism is discovered here, whereby reorientation of the LC systems is shown to occur through lowering of the orientation-dependent surface energy of the LC due to formation of a thin isotropic layer at the aqueous interface. Using a combination of atomistic molecular dynamics simulations and experiments, we demonstrate that a monolayer of specific amphiphiles at an aqueous interface can cause a local nematic-to-isotropic phase transition of the LC by disturbing the antiparallel configuration of the LC molecules. These results provide new insights into the interfacial, molecular-level organization of LCs that can be exploited for rational design of biological sensors and responsive systems.
PMID:30285408 | DOI:10.1021/acsami.8b09639
Mechanical Genomic Studies Reveal the Role of d-Alanine Metabolism in Pseudomonas aeruginosa Cell Stiffness
mBio. 2018 Sep 11;9(5):e01340-18. doi: 10.1128/mBio.01340-18.
The stiffness of bacteria prevents cells from bursting due to the large osmotic pressure across the cell wall. Many successful antibiotic chemotherapies target elements that alter mechanical properties of bacteria, and yet a global view of the biochemistry underlying the regulation of bacterial cell stiffness is still emerging. This connection is particularly interesting in opportunistic human pathogens such as Pseudomonas aeruginosa that have a large (80%) proportion of genes of unknown function and low susceptibility to different families of antibiotics, including beta-lactams, aminoglycosides, and quinolones. We used a high-throughput technique to study a library of 5,790 loss-of-function mutants covering ~80% of the nonessential genes and correlated P. aeruginosa individual genes with cell stiffness. We identified 42 genes coding for proteins with diverse functions that, when deleted individually, decreased cell stiffness by >20%. This approach enabled us to construct a "mechanical genome" for P. aeruginosa d-Alanine dehydrogenase (DadA) is an enzyme that converts d-Ala to pyruvate that was included among the hits; when DadA was deleted, cell stiffness decreased by 18% (using multiple assays to measure mechanics). An increase in the concentration of d-Ala in cells downregulated the expression of genes in peptidoglycan (PG) biosynthesis, including the peptidoglycan-cross-linking transpeptidase genes ponA and dacC Consistent with this observation, ultraperformance liquid chromatography-mass spectrometry analysis of murein from P. aeruginosa cells revealed that dadA deletion mutants contained PG with reduced cross-linking and altered composition compared to wild-type cells.IMPORTANCE The mechanical properties of bacteria are important for protecting cells against physical stress. The cell wall is the best-characterized cellular element contributing to bacterial cell mechanics; however, the biochemistry underlying its regulation and assembly is still not completely understood. Using a unique high-throughput biophysical assay, we identified genes coding proteins that modulate cell stiffness in the opportunistic human pathogen Pseudomonas aeruginosa This approach enabled us to discover proteins with roles in a diverse range of biochemical pathways that influence the stiffness of P. aeruginosa cells. We demonstrate that d-Ala-a component of the peptidoglycan-is tightly regulated in cells and that its accumulation reduces expression of machinery that cross-links this material and decreases cell stiffness. This research demonstrates that there is much to learn about mechanical regulation in bacteria, and these studies revealed new nonessential P. aeruginosa targets that may enhance antibacterial chemotherapies or lead to new approaches.
PMID:30206169 | PMC:PMC6134093 | DOI:10.1128/mBio.01340-18
Oligomers as Triggers for Responsive Liquid Crystals
Langmuir. 2018 Aug 28;34(34):10092-10101. doi: 10.1021/acs.langmuir.8b01944. Epub 2018 Aug 20.
We report an investigation of the influence of aqueous solutions of amphiphilic oligomers on the ordering of micrometer-thick films of thermotropic liquid crystals (LCs), thus addressing the gap in knowledge arising from previous studies of the interactions of monomeric and polymeric amphiphiles with LCs. Specifically, we synthesized amphiphilic oligomers (with decyl hydrophobic and pentaethylene glycol hydrophilic domains) in monomer, dimer, and trimer forms, and incubated aqueous solutions of the oligomers against nematic films of 4'-pentyl-4-biphenylcarbonitrile (5CB). All amphiphilic oligomers caused sequential surface-driven orientational (planar to homeotropic) and then bulk phase transitions (nematic to isotropic) with dynamics depending strongly on the degree of oligomerization. The dynamics of the orientational transitions accelerated from monomer to trimer, consistent with the effects of an increase in adsorption free energy. The mechanism underlying the orientational transition, however, involved a decrease in anchoring energy and not change in the easy axis of the LC. In contrast, the rate of the phase transition induced by absorption of oligomers into the LC decreased from monomer to trimer, suggesting that constraints on configurational degrees of freedom influence the absorption free energies of the oligomers. Interestingly, the oligomer-induced transition from the nematic to isotropic phase of 5CB was observed to nucleate at the aqueous-5CB interface, consistent with surface-induced disorder underlying the above-reported decrease in anchoring energy caused by the oligomers. Finally, we provided proof-of-concept experiments of the triggering of LCs using a trimeric amphiphile that is photocleaved by UV illumination into monomeric fragments. Overall, our results provide insight into the rational design of oligomers that can be used as triggers to create responsive LCs.
PMID:30064213 | DOI:10.1021/acs.langmuir.8b01944
Redox-Triggered Orientational Responses of Liquid Crystals to Chlorine Gas
Angew Chem Int Ed Engl. 2018 Jul 26;57(31):9665-9669. doi: 10.1002/anie.201803194. Epub 2018 Jul 9.
Surface-supported liquid crystals (LCs) that exhibit orientational and thus optical responses upon exposure to ppb concentrations of Cl2 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 Cl2 gas. 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 MnO2 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 Cl2 gas. The response was also selective to Cl2 over other oxidizing agents such as air or NO2 and other chemical targets such as organophosphonates.
PMID:29931784 | DOI:10.1002/anie.201803194
Liquid Crystals with Interfacial Ordering that Enhances Responsiveness to Chemical Targets
Adv Mater. 2018 Jul;30(27):e1706707. doi: 10.1002/adma.201706707. Epub 2018 May 21.
The development of stimuli-responsive materials suitable for use in wearable sensors is a key unresolved challenge. Liquid crystals (LCs) are particularly promising, as they do not require power, are light-weight, and can be tuned to respond to a range of targeted chemical stimuli. Here, an advance is reported in the design of LCs for chemical sensors with the discovery of LCs that assume parallel orientations at free surfaces and yet retain their chemoresponsiveness. The resulting LC-based sensors are more sensitive and exhibit faster responses than previous LC sensor designs.
PMID:29782666 | DOI:10.1002/adma.201706707
Self-reporting and self-regulating liquid crystals
Nature. 2018 May;557(7706):539-544. doi: 10.1038/s41586-018-0098-y. Epub 2018 May 9.
Liquid crystals (LCs) are anisotropic fluids that combine the long-range order of crystals with the mobility of liquids1,2. This combination of properties has been widely used to create reconfigurable materials that optically report information about their environment, such as changes in electric fields (smart-phone displays) 3 , temperature (thermometers) 4 or mechanical shear 5 , and the arrival of chemical and biological stimuli (sensors)6,7. An unmet need exists, however, for responsive materials that not only report their environment but also transform it through self-regulated chemical interactions. Here we show that a range of stimuli can trigger pulsatile (transient) or continuous release of microcargo (aqueous microdroplets or solid microparticles and their chemical contents) that is trapped initially within LCs. The resulting LC materials self-report and self-regulate their chemical response to targeted physical, chemical and biological events in ways that can be preprogrammed through an interplay of elastic, electrical double-layer, buoyant and shear forces in diverse geometries (such as wells, films and emulsion droplets). These LC materials can carry out complex functions that go beyond the capabilities of conventional materials used for controlled microcargo release, such as optically reporting a stimulus (for example, mechanical shear stresses generated by motile bacteria) and then responding in a self-regulated manner via a feedback loop (for example, to release the minimum amount of biocidal agent required to cause bacterial cell death).
PMID:29743674 | DOI:10.1038/s41586-018-0098-y
Multi-Scale Responses of Liquid Crystals Triggered by Interfacial Assemblies of Cleavable Homopolymers
Chemphyschem. 2018 Aug 17;19(16):2037-2045. doi: 10.1002/cphc.201800106. Epub 2018 May 16.
Liquid crystals (LCs) offer the basis of stimuli-responsive materials that can amplify targeted molecular events into macroscopic outputs. However, general and versatile design principles are needed to realize the full potential of these materials. To this end, we report the synthesis of two homopolymers with mesogenic side chains that can be cleaved upon exposure to either H2 O2 (polymer P1) or UV light (polymer P2). Optical measurements reveal that the polymers dissolve in bulk LC and spontaneously assemble at nematic LC-aqueous interfaces to impose a perpendicular orientation on the LCs. Subsequent addition of H2 O2 to the aqueous phase or exposure of the LC to UV was shown to trigger a surface-driven ordering transition to a planar orientation and an accompanying macroscopic optical output. Differences in the dynamics of the response to each stimulus are consistent with sequential processing of P1 at the LC-aqueous interface (H2 O2 ) and simultaneous transformation of P2 within the LC (UV). The versatility of the approach is demonstrated by creating stimuli-responsive LCs as films or microdroplets, and by dissolving mixtures of P1 and P2 into LCs to create LC materials that respond to two stimuli. Overall, our results validate a simple and generalizable approach to the rational design of polymers that can be used to program stimuli-responsiveness into LC materials.
PMID:29682873 | DOI:10.1002/cphc.201800106