37 research outputs found
Adsorbate-Induced Anchoring Transitions of Liquid Crystals on Surfaces Presenting Metal Salts with Mixed Anions
We report that metal salts composed of mixtures of anions of differing coordination strength can be used to increase the sensitivity and selectivity of adsorbate-induced anchoring transitions of liquid crystals (LCs) supported on surfaces decorated with the metal salts. Specifically, the dynamics of anchoring transitions triggered by the adsorbate dimethyl methylphosphonate (DMMP) on surfaces of aluminumÂ(III) salts were analyzed within the framework of a model for mass transport to reveal that the sensitivity of a nitrile-containing nematic LC to DMMP increased from 250 to 25 ppb when the composition of the (counter) anion was changed from 100% perchlorate to 90% nitrate and 10% perchlorate (by mole percent). To provide insight into these observations, polarization-modulation infrared reflectance-absorbance spectroscopy (PM-IRRAS) was used to show that the intensity of the absorption band in the IR spectrum corresponding to the coordinated state of the nitrile group (but not the position of the peak) decreased with the increase in the mole fraction of the strongly coordinating anion (nitrate) in the anion mixture, thus suggesting that the addition of the strongly coordinating anion decreased the number of coordination interactions (per unit area of the interface) but not the strength of the individual coordination interactions between the metal cation and the LC. We also measured the incorporation of the nitrate anion into the metal salt to decrease the effect of humidity on the dynamic response of the LC to DMMP, a result that is consistent with weaker interactions between the nitrate anion and water as compared to the perchlorate anion and water. Finally, the bidentate anion acetylacetonate was measured to cause a similar increase in sensitivity to DMMP when mixed with perchlorate in a 1:1 ratio (the resulting sensitivity of the system to DMMP was 100 ppb). Overall, these results suggest that tailoring the identity of the anion represents a general and facile approach for tuning the orientational response of LCs supported on metal salts to targeted analytes
Enantiomeric Interactions between Liquid Crystals and Organized Monolayers of Tyrosine-Containing Dipeptides
We have examined the orientational ordering of nematic liquid crystals (LCs) supported on organized monolayers of dipeptides with the goal of understanding how peptide-based interfaces encode intermolecular interactions that are amplified into supramolecular ordering. By characterizing the orientations of nematic LCs (4-cyano-4âČ-pentylbiphenyl and TL205 (a mixture of mesogens containing cyclohexane-fluorinated biphenyls and fluorinated terphenyls)) on monolayers of l-cysteine-l-tyrosine, l-cysteine-l-phenylalanine, or l-cysteine-l-phosphotyrosine formed on crystallographically textured films of gold, we conclude that patterns of hydrogen bonds generated by the organized monolayers of dipeptides are transduced via macroscopic orientational ordering of the LCs. This conclusion is supported by the observation that the ordering exhibited by the achiral LCs is specific to the enantiomers used to form the dipeptide-based monolayers. The dominant role of the âOH group of tyrosine in dictating the patterns of hydrogen bonds that orient the LCs was also evidenced by the effects of phosphorylation of the tyrosine on the ordering of the LCs. Overall, these results reveal that crystallographic texturing of gold films can direct the formation of monolayers of dipeptides with long-range order, thus unmasking the influence of hydrogen bonding, chirality, and phosphorylation on the macroscopic orientational ordering of LCs supported on these surfaces. These results suggest new approaches based on supramolecular assembly for reporting the chemical functionality and stereochemistry of synthetic and biological peptide-based molecules displayed at surfaces
A Practical Guide to the Preparation of Liquid Crystal-Templated Microparticles
We
provide a practical guide to methods and protocols that use polymer
networks templated from droplets of liquid crystal (LC) to synthesize
micrometer-sized polymeric particles that are chemically patchy, are
anisometric in shape, possess anisotropic optical properties, and/or
are mesoporous. We describe a range of methods that permit the preparation
of LC droplets (containing reactive monomers) as templates for polymerization,
including formation of LC-in-water emulsions by mechanical methods
(e.g., vortexing), encapsulation in polymeric shells, or microfluidics.
The relative merits of the methods, including ease of use and potential
pitfalls, and the resulting droplet size distributions, are described.
We also report a menu of approaches that can be used to control the
internal configurations of the LC droplets, including changes in composition
of the continuous solvent phases (e.g., addition of glycerol) and
adsorption of surfactants or colloids at the interfaces of the LC
droplets. Photopolymerization of the LC droplets in bipolar, radial,
axial, or preradial configurations and subsequent extraction of the
nonreactive mesogens generates polymeric particles that have spindle,
spherical, spherocylindrical, or tear shapes, respectively. Finally,
we describe how to characterize these polymeric particles, including
their shape, internal structure, optical properties, and porosity.
The methods described in this paper, which provide access to complex
microparticles with properties relevant to separation processes, drug
delivery, and optical devices, are general and versatile and can be
readily developed further (e.g., by changing the choice of LC) to
create an even greater diversity of microparticles
Influence of Self-Assembling Redox Mediators on Charge Transfer at Hydrophobic Electrodes
We
report an investigation of the influence of reversible self-assembly
of amphiphilic redox-mediators on interfacial charge transfer at chemically
functionalized electrodes. Specifically, we employed (11-ferrocenylundecyl)-trimethylammonium
bromide (FTMA) as a model self-assembling redox mediator and alkanethiol-modified
gold films as hydrophobic electrodes. By performing cyclic voltammetry
(CV, 10 mV/s) in aqueous solutions containing FTMA above its critical
micellar concentration (CMC), we measured anodic (<i>I</i><sub>a</sub>) and cathodic (<i>I</i><sub>c</sub>) peak
current densities of 18 ± 3 and 1.1 ± 0.1 ΌA/cm<sup>2</sup>, respectively, revealing substantial current rectification
(<i>I</i><sub>a</sub>/<i>I</i><sub>c</sub>= 17)
at the hydrophobic electrodes. In contrast, hydroxymethyl ferrocene
(a non-self-assembling redox mediator) at hydrophobic electrodes and
FTMA at bare gold electrodes, yielded relatively low levels of rectification
(<i>I</i><sub>a</sub>/<i>I</i><sub>c</sub>= 1.7
and 2.3, respectively). Scan-rate-dependent measurements revealed <i>I</i><sub>a</sub> of FTMA to arise largely from the diffusion
of FTMA from bulk solution to the hydrophobic electrode whereas <i>I</i><sub>c</sub> was dominated by adsorbed FTMA, leading to
the proposal that current rectification observed with FTMA is mediated
by interfacial assemblies of reduced FTMA that block access of oxidized
FTMA to the hydrophobic electrode. Support for this proposal was obtained
by using atomic force microscopy and quartz crystal microbalance measurements
to confirm the existence of interfacial assemblies of reduced FTMA
(1.56 ± 0.2 molecules/nm<sup>2</sup>). Additional characterization
of a mixed surfactant system containing FTMA and dodecyltrimethylammonium
bromide (DTAB) revealed that interfacial assemblies of DTAB also block
access of oxidized FTMA to hydrophobic electrodes; this system exhibited <i>I</i><sub>a</sub>/<i>I</i><sub>c</sub> > 80. These
results and others reported in this paper suggest that current rectification
occurs in this system because oxidized FTMA does not mix with interfacial
assemblies of reduced FTMA or DTAB formed at hydrophobic electrodes.
More broadly, these results show that self-assembling redox mediators,
when combined with chemically functionalized electrodes, offer the
basis of new principles for controlling charge transfer at electrode/solution
interfaces
Ordering Transitions Triggered by Specific Binding of Vesicles to Protein-Decorated Interfaces of Thermotropic Liquid Crystals
We report that specific binding of ligand-functionalized
(biotinylated)
phospholipid vesicles (diameter = 120 ± 19 nm) to a monolayer
of proteins (streptavidin or anti-biotin antibody) adsorbed at an
interface between an aqueous phase and an immiscible film of a thermotropic
liquid crystal (LC) [nematic 4âČ-pentyl-4-cyanobiphenyl (5CB)]
triggers a continuous orientational ordering transition (continuous
change in the tilt) in the LC. Results presented in this paper indicate
that, following the capture of the vesicles at the LC interface via
the specific binding interaction, phospholipids are transferred from
the vesicles onto the LC interface to form a monolayer, reorganizing
and partially displacing proteins from the LC interface. The dynamics
of this process are accelerated substantially by the specific binding
event relative to a protein-decorated interface of a LC that does
not bind the ligands presented by the vesicles. The observation of
the <i>continuous</i> change in the ordering of the LC,
when combined with other results presented in this paper, is significant,
as it is consistent with the presence of suboptical domains of proteins
and phospholipids on the LC interface. An additional significant hypothesis
that emerges from the work reported in this paper is that the ordering
transition of the LC is strongly influenced by the bound state of
the protein adsorbed on the LC interface, as evidenced by the influence
on the LC of (i) âcrowdingâ of the protein within a
monolayer formed at the LC interface and (ii) aging of the proteins
on the LC interface. Overall, these results demonstrate that ordering
transitions in LCs can be used to provide fundamental insights into
the competitive adsorption of proteins and lipids at oilâwater
interfaces and that LC ordering transitions have the potential to
be useful for reporting specific binding events involving vesicles
and proteins
Hierarchical Microstructures Formed by Bidisperse Colloidal Suspensions within Colloid-in-Liquid Crystal Gels
Past studies have reported that colloids
of a single size dispersed in the isotropic phase of a mesogenic solvent
can form colloid-rich networks (and gels) upon thermal quenching of
the system across the isotropicânematic phase boundary of the
mesogens. Herein we report the observation and characterization of
complex hierarchical microstructures that form when bidisperse colloidal
suspensions of nanoparticles (NPs; iron oxide with diameters of 188
± 20 nm or polyÂ(methyl methacrylate) with diameters of 150 ±
15 nm) and microparticles (MPs; polystyrene with diameters of 2.77
± 0.20 ÎŒm) are dispersed in the isotropic phase of 4-pentyl-4âČ-cyanobiphenyl
(5CB) and thermally quenched. Specifically, we document microstructuring
that results from three sequential phase separation processes that
occur at distinct temperatures during stepwise cooling of the ternary
mixture from its miscibility region. The first phase transition demixes
the system into coexisting MP-rich and NP-rich phases; the second
promotes formation of a particle network within the MP-rich phase;
and the third, which coincides with the isotropic-to-nematic phase
transition of 5CB, produces a second colloidal network within the
NP-rich phase. We quantified the dynamics of each demixing process
by using optical microscopy and Fourier transform image analysis to
establish that the phase transitions occur through (i) surface-directed
spinodal decomposition, (ii) spinodal decomposition, and (iii) nucleation
and growth, respectively. Significantly, the observed series of phase
transitions leads to a hierarchical organization of cellular microstructures
not observed in colloid-in-liquid crystal gels formed from monodisperse
colloids. The results of this study suggest new routes to the synthesis
of colloidal materials with hierarchical microstructures that combine
large surface areas and organized porosity with potential applications
in catalysis, separations, chemical sensing, or tissue engineering
Liquid Crystal-Based Emulsions for Synthesis of Spherical and Non-Spherical Particles with Chemical Patches
We
report the use of liquid crystal (LC)-in-water emulsions for
the synthesis of either spherical or non-spherical particles with
chemically distinct domains located at the poles of the particles.
The approach involves the localization of solid colloids at topological
defects that form predictably at surfaces of water-dispersed LC droplets.
By polymerizing the LC droplets displaying the colloids at their surface
defects, we demonstrate formation of both spherical and, upon extraction
of the mesogen, anisotropic composite particles with colloids located
at either one or both of the poles. Because the colloids protrude
from the surfaces of the particles, they also define organized, chemical
patches with functionality controlled by the colloid surface
Covalent Immobilization of Caged Liquid Crystal Microdroplets on Surfaces
Microscale droplets of thermotropic
liquid crystals (LCs) suspended
in aqueous media (e.g., LC-in-water emulsions) respond sensitively
to the presence of contaminating amphiphiles and, thus, provide promising
platforms for the development of new classes of droplet-based environmental
sensors. Here, we report polymer-based approaches to the immobilization
of LC droplets on surfaces; these approaches introduce several new
properties and droplet behaviors and thus also expand the potential
utility of LC droplet-based sensors. Our approach exploits the properties
of microscale droplets of LCs contained within polymer-based microcapsule
cages (so-called âcagedâ LCs). We demonstrate that caged
LCs functionalized with primary amine groups can be immobilized on
model surfaces through both weak/reversible ionic interactions and
stronger reactive/covalent interactions. We demonstrate using polarized
light microscopy that caged LCs that are covalently immobilized on
surfaces can undergo rapid and diagnostic changes in shape, rotational
mobility, and optical appearance upon the addition of amphiphiles
to surrounding aqueous media, including many useful changes in these
features that cannot be attained using freely suspended or surface-adsorbed
LC droplets. Our results reveal these amphiphile-triggered orientational
transitions to be reversible and that arrays of immobilized caged
LCs can be used (and reused) to detect both increases and decreases
in the concentrations of model contaminants. Finally, we report changes
in the shapes and optical appearances of LC droplets that occur when
immobilized caged LCs are removed from aqueous environments and dried,
and we demonstrate that dried arrays can be stored for months without
losing the ability to respond to the presence of analytes upon rehydration.
Our results address practical issues associated with the preparation,
characterization, storage, and point-of-use application of conventional
LC-in-water emulsions and provide a basis for approaches that could
enable the development of new âoff-the-shelfâ LC droplet-based
sensing platforms
Comparison of the Influence of Humidity and dâMannitol on the Organization of Tetraethylene Glycol-Terminated Self-Assembled Monolayers and Immobilized Antimicrobial Peptides
We
report the use of polarization-modulation infrared reflectionâabsorption
spectroscopy (PM-IRRAS) to characterize the effects of relative humidity
(RH) and d-mannitol on the conformations of tetraethylene
glycol (EG<sub>4</sub>)-terminated self-assembled monolayers (SAMs)
and immobilized antimicrobial peptides (Cecropin P1 and a hybrid of
Cecropin A (1â8) and Melittin (1â18)). These results
are used to assess the extent to which d-mannitol can substitute
for water in promoting conformational states of the SAMs and oligopeptides
similar to those induced by hydration. Our measurements reveal a red
shift of the COC asymmetric stretching vibration of the EG<sub>4</sub>-terminated SAMs with increasing humidity, consistent with a transition
from a mixed all-trans/helical (7/2 helix) conformation at 0% RH to
a predominantly helical conformation at 90% RH. Significantly, under
dry conditions, a thin (2 nm in thickness) overlayer of d-mannitol generated the COC spectroscopic signature of the EG<sub>4</sub>-terminated SAM measured at high humidity. Comparisons of
the effects of humidity and d-mannitol on the secondary structure
of the two oligopeptides also revealed both to cause the amide I peak
positions, which were measured in dry air (and without d-mannitol)
to correspond to α-helical conformations, to undergo red-shifts.
The magnitudes of the red-shifts, however, were more pronounced for
dry d-mannitol than for high RH, with Cecropin P1 and the
hybrid peptide exhibiting amide I peak positions under d-mannitol
consistent with bulk aqueous solution secondary structures (random
and ÎČ-sheet, respectively). These results are discussed in the
context of prior reports of the tendency of d-mannitol to
form glassy states in the absence of water. Overall, the results presented
in this paper support the hypothesis that d-mannitol can
substitute, in at least some ways, for the influence of water on the
conformational states of biologically relevant molecules at interfaces.
The results provide guidance for the design of interfaces for water-free
biologics
Influence of Specific Anions on the Orientational Ordering of Thermotropic Liquid Crystals at Aqueous Interfaces
We report that specific anions (of sodium salts) added
to aqueous
phases at molar concentrations can trigger rapid, orientational ordering
transitions in water-immiscible, thermotropic liquid crystals (LCs;
e.g., nematic phase of 4âČ-pentyl-4-cyanobiphenyl, 5CB) contacting
the aqueous phases. Anions classified as chaotropic, specifically
iodide, perchlorate, and thiocyanate, cause 5CB to undergo continuous,
concentration-dependent transitions from planar to homeotropic (perpendicular)
orientations at LCâaqueous interfaces within 20 s of addition
of the anions. In contrast, anions classified as relatively more kosmotropic
in nature (fluoride, sulfate, phosphate, acetate, chloride, nitrate,
bromide, and chlorate) do not perturb the LC orientation from that
observed without added salts (i.e., planar orientation). Surface pressureâarea
isotherms of Langmuir films of 5CB supported on aqueous salt solutions
reveal ion-specific effects ranking in a manner similar to the LC
ordering transitions. Specifically, chaotropic salts stabilized monolayers
of 5CB to higher surface pressures and areal densities (12.6 mN/m
at 27 Ă
<sup>2</sup>/molecule for NaClO<sub>4</sub>) and thus
smaller molecular tilt angles (30° from the surface normal for
NaClO<sub>4</sub>) than kosmotropic salts (5.0 mN/m at 38 Ă
<sup>2</sup>/molecule with a corresponding tilt angle of 53° for
NaCl). These results and others reported herein suggest that anion-specific
interactions with 5CB monolayers lead to bulk LC ordering transitions.
Support for the proposition that these ion-specific interactions involve
the nitrile group was obtained by using a second LC with nitrile groups
(E7; ion-specific effects similar to 5CB were observed) and a third
LC with fluorine-substituted aromatic groups (TL205; weak dipole and
no ion-specific effects were measured). Finally, we also establish
that anion-induced orientational transitions in micrometer-thick LC
films involve a change in the easy axis of the LC. Overall, these
results provide new insights into ionic phenomena occurring at LCâaqueous
interfaces, and reveal that the long-range ordering of LC oils can
amplify ion-specific interactions at these interfaces into macroscopic
ordering transitions