5CB Droplets Research Articles

Solute induced jittery motion of self-propelled droplets

The intriguing role of the presence of solutes in the activity of a self-propelling droplet is investigated. A system of self-propelling micrometer-sized 4-Cyano-4′-pentylbiphenyl (5CB) droplets in an aqueous solution of tetradecyltrimethylammonium bromide (TTAB) as the surfactant is considered. It is shown that the addition of glycerol causes the active 5CB droplet to exhibit a transition from smooth to jittery motion. The motion is found to be independent of the droplet size and the nematic state of 5CB. Analogous experiments with Polyacrylamide (PAAm), Polyvinylpyrrolidone (PVP), and Polyvinyl Alcohol (PVA), as solutes, confirm that such a transition cannot merely be explained solely based on the viscosity or Peclet number of the system. We propose that the specific nature of physicochemical interactions between the solute and the droplet interface is at the root of this transition. The experiments show that the timescales associated with the influx and redistribution of surfactants at the interface are altered in the presence of solutes. Glycerol and PVP significantly enhance the rate of solubilization of the 5CB droplets resulting in a quicker re-distribution of the adsorbed TTAB molecules on the interface, causing the droplet to momentarily stop and then restart in an independent direction. On the other hand, low solubilization rates in the presence of PAAm and PVA lead to smooth trajectories. Our hypothesis is supported by the time evolution of droplet size and interfacial velocity measurements in the presence and absence of solute. Overall, our results provide fundamental insights into the complex interactions emerging due to the presence of solutes.

Reconfigurable Multicompartment Emulsion Drops Formed by Nematic Liquid Crystals and Immiscible Perfluorocarbon Oils.

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.

Anionic-cationic surfactant mixture providing the electrically controlled homeotropic surface anchoring of liquid crystals

In search of a substance able to function as an anionic surfactant applied for the electrically-induced anchoring transitions in liquid crystals, the compound 4-heptyloxybenzoate benzyl dodecyldimethylammonium (HOBBDDA) has been synthesized. Its orienting influence on nematic liquid crystal 4-pentyl-4′-cyanobiphenyl (5CB) has been tested. HOBBDDA dissolved up to 1.4% in 5CB droplets dispersed in polymer film does not change the tangential surface anchoring inherent to polyvinyl alcohol matrix used. Homeotropic surface anchoring is realized if the HOBBDDA content exceeds 1.7%. The molecules of the surfactant in 5CB dissociate into anions and cations. The anions modify the surface anchoring under the action of DC electric field both in the normal and inverse mode. The mixture of HOBBDDA and cetyltrimethylammonium bromide can function as a binary ionic-cationic surfactant which significantly expands the prospects for using the ionic-surfactant method to control liquid crystal materials.

Construction of liquid crystal droplet-based sensing platform for sensitive detection of organophosphate pesticide

Alkaline phosphatase (ALP), as an essential disease biomarker, widely distributes in various tissues, and plays a significant role in diagnosis and treatment of some diseases (e.g. cancers and liver dysfunction). Dichlorvos (DDVP), as one of typical organophosphate pesticide, could be hydrolyzed by ALP. In this work, we constructed a simple but robust 4-cyano-4′-pentylbiphenyl (5CB) droplet sensing platform for sensitive and convenient detection of DDVP based on its hydrolysis by ALP. The optical responses of liquid crystals (LCs) originated from controlling orientations of 5CB by using an ALP cleavable surfactant, sodium monododecyl phosphate (SMP). A dark crossed optical image of 5CB was observed due to formation of the SMP monolayer at the aqueous/LC droplet interface. Subsequently, 5CB turned to bright fan-shaped optical appearance upon addition of ALP and SMP mixture. Interestingly, LCs remained dark crossed optical images in contact with the pre-incubated mixture of DDVP and ALP. The detection limit of DDVP reached ~0.1 ng/mL. Isothermal titration calorimetry (ITC) and humam ALP ELISA kit provided strong evidence for the detection mechanism. This strategy was also used to evaluate whether the maximum pesticide residue limits of tomatoes conform to the relevant national standard requirement or not. Our study holds promise for real-time and label-free detection of pesticides with high sensitivity by employing portable LC-based sensing platform.

Surface modified liquid crystal droplets as an optical probe for the detection of bile acids in microfluidic channels

We report the surface modification of 4-n-pentyl-4’-cyanobiphenyl (5CB) droplets in aqueous solution by the adsorption of sulfated β-CD/tetradecyl sulfate sodium (SC14S) complexes at the 5CB-aqueous interface, followed by the coating of poly(diallyldimethylammonium chloride) (PDADMAC) through electrostatic interaction. The PDADMAC/sulfate β-CD/SC14S complex-coated 5CB droplets are highly stable in aqueous solution. We show that bile acids are able to penetrate into the PDADMAC coating and displace the SC14S from the cavity of the β-CD immobilized at the surface of the 5CB droplets through the competitive host-guest recognition, consequently inducing the radial-to-bipolar configuration transition of the 5CB droplets. The integration of PDADMAC/sulfate β-CD/SC14S complex-coated 5CB droplets in microfluidic channels allows the selective detection of bile acids in a small sample volume (1 μL) in the presence of ascorbic acid, uric acid, creatinine and urea by observing the configuration transition of the 5CB droplets. The detection limit of the miniaturized 5CB droplet-based sensor platform for bile acids can be tuned by the number density of 5CB droplets.

Liquid-crystal droplets functionalized with a non-enzymatic moiety for glucose sensing

3-Aminophenyl boronic acid (APBA)-decorated 4-cyano-4′-pentylbiphenyl (5CB) microdroplets were utilized for glucose detection with high specificity and sensitivity. APBA decoration was performed by firstly coating poly(acrylic acid) (PAA) on the 5CB droplet with amphiphilic poly(4-cyanobiphenyl-4′-undecylacrylate-b-acrylicacid) (LCP-b-PAA) and performing covalent bonding of APBA to the PAA chains by EDC coupling between the acrylic groups of PAA and the amine groups of APBA. The binding events between APBA and glucose were translated by configurational change (from radial to bipolar) of the 5CB droplets, observed under a polarized optical microscope with crossed polarizers. The liquid crystal (LC) droplet-based non-enzymatic (NE) biosensor for glucose detection exhibited high sensitivity (detection limit of 0.05mM) even in complex serum sample, high selectivity against cholesterol, uric acid, and acetaminophen, and good stability for 30 d. This NE-based glucose LC biosensor is cost-effective and highly stable compared to enzyme-based LC glucose biosensors and thus, could replace such enzyme-based biosensors.

SURFACTANT INDUCED INTERFACIAL ANCHORING TRANSITIONS IN NEMATIC LIQUID CRYSTAL DROPLETS ON GLASS SURFACES

The interfacial adsorption of surfactants at planar nematic liquid crystal (NLC)–water interface induces an ordering transition from a tilted to perpendicular anchoring with the increase in surfactant concentration at [Formula: see text], where [Formula: see text] is the Critical Micelle Concentration of surfactants in water. In this study, we show that depending upon the surfactant structure a tilted to perpendicular NLC anchoring transition is observed at [Formula: see text] in 5CB droplets of size 50–70[Formula: see text][Formula: see text]m. Micrometer sized 5CB droplets are deposited on glass surfaces using flow coating of 5CB-in-ethanol solutions. When placed on 5CB drop decorated glass surfaces, the aqueous surfactant solutions of aliphatic chain surfactants (SDS, CTAB and CPBr) at [Formula: see text], result in an optical transition to a bright-cross texture attributed to the tilted anchoring of 5CB molecules at 5CB–water interface. At [Formula: see text], perpendicular anchoring of 5CB molecules at 5CB–water interface results in a droplet texture with a hedgehog defect. In contrast, aqueous solutions of SDBS lead to 5CB droplets with a bright-cross texture regardless of the surfactant concentration in the aqueous phase. These results indicate that the orientation of 5CB molecules is independent of the nature of the surfactant headgroup. In addition, 5CB droplet decorated OTS treated glass substrates show a hedgehog texture which disappears completely on exposure to organic vapors with the response time-dependent on the polarity of the vapor molecules.

Optical biosensor based on liquid crystal droplets for detection of cholic acid

A highly sensitive cholic acid biosensor based on 4-cyano-4′-penthlbiphenyl (5CB) Liquid crystal droplets in phosphate buffer saline solution was reported. A radial-to-bipolar transition of 5CB droplet would be triggered during competitive reaction of CA at the sodium dodecyl sulfate surfactant-laden 5CB droplet surface. Our liquid crystal droplet sensor is a low-cost, simple and fast method for CA detection. The detection limit (5μM) of our method is 2.4 times lower than previously report by using liquid crystal film to detection of CA.

Optical imaging of cholylglycine by using liquid crystal droplet patterns on solid surfaces

Determining cholylglycine (CG) levels is of great importance in the detection of liver abnormalities. In this study, we present a novel liquid–crystal (LC)-based method assisted with cholylglycine hydrolase (CGH) for the optical detection of CG levels. The detection method is based on the disruption of the orientations of a nematic LC, 4-cyano-4′-pentylbiphenyl (5CB), doped with dodecyl aldehyde. Aldehyde-doped 5CB droplets were placed on pre-treated glass slides and patterned with solutions of interest. When in contact with a small drop of a CG aqueous solution, bright fan-shaped LC droplet patterns were observed under polarizing optical microscopy, indicating a planar orientation of LC at the aqueous/LC interface. However, aldehyde with short-alkyl chain couples with glycine released from the enzymatic reaction between CG and CGH forming amphiphilic Schiff bases, which display dark cross patterns of LC, suggesting a homeotropic orientation of the LC. This system may offer a highly sensitive and methodologically simple approach to determine CG levels for clinical diagnostics and commercial applications.

Specific detection of avidin–biotin binding using liquid crystal droplets

Poly(acrylicacid-b-4-cynobiphenyl-4′-undecylacrylate) (PAA-b-LCP)-functionalized 4-cyano-4′-pentylbiphenyl (5CB) droplets were made by using microfluidic technique. The PAA chains on the 5CB droplets, were biotinylated, and used to specifically detect avidin–biotin binding at the 5CB/aqueous interface. The avidin–biotin binding was characterized by the configurational change (from radial to bipolar) of the 5CB droplets, as observed through a polarized optical microscope. The maximum biotinylation was obtained by injecting a >100μg/mL biotin aqueous solution, which enabled a limit of detection of 0.5μg/mL avidin. This droplet biosensor could specifically detect avidin against other proteins such as bovine serum albumin, lysozyme, hemoglobin, and chymotrypsinogen solutions. Avidin detection with 5CBPAA-biotin droplets having high sensitivity, specificity, and stability demonstrates new applications of the functionalized liquid crystal droplets that can detect specific proteins or other analytes through a ligand/receptor model.

Tailoring the surface of liquid crystal droplets with chitosan/surfactant complexes for the selective detection of bile acids in biological fluids

The surface of 5CB droplets dispersed in aqueous solution is tailored by the adsorption of CHI/SC14S complexes at the 5CB/aqueous interface. The CHI/SC14S complex-coated 5CB droplets can be used as an optical probe to detect CA in biological fluids without dilution.

Design of β-CD–surfactant complex-coated liquid crystal droplets for the detection of cholic acid via competitive host–guest recognition

β-CD-C14TAB complex-coated 5CB droplets are designed by the adsorption of β-CD-C14TAB complexes at the 5CB/aqueous interface. We show that the 5CB droplets can be used as an optical probe for the selective detection of cholic acid in aqueous solution containing uric acid and urea via competitive host-guest recognition.

General Liquid-crystal droplets produced by microfluidics for urea detection

pH-sensitive 4-cyano-4′-pentylbiphenyl (5CB) droplets were produced for urea detection by coating with poly(acrylicacid-b-4-cyanobiphenyl-4-oxyundecylacrylate) (PAA-b-LCP) and covalently immobilizing urease on the PAA chains on the 5CB droplets using microfluidics. The functionalized 5CB droplets detected urea based on a bipolar-to-radial orientational change of the droplets observed by polarized optical microscopy using crossed polarizers. The detection limit was as low as 3mM with a response time of 180s. Urine was evaluated as a real sample for urea prescreening. This new and sensitive liquid crystal droplet-based urea biosensor has the merits of facile visual detection and is prospectively useful for urea prescreening for human subjects.

Protein-induced configuration transitions of polyelectrolyte-modified liquid crystal droplets.

Liquid crystal (LC) droplets dispersed in aqueous solution have emerged as an optical probe for sensing the adsorption and interaction of biological species at the LC/aqueous interface. In this paper, we modify the surface of 4-n-pentyl-4'-cyanobiphenyl (5CB) LC droplets by the adsorption of positively charged poly(diallyldimethylammonium chloride) (PDADMAC) and poly(ethylenimine) (PEI) with different molecular weights at the 5CB/water interface. The PDADMAC and PEI-modified 5CB droplets show a radial director configuration in aqueous solution with salt concentrations above 150 mM. The adsorption of negatively charged bovine serum albumin (BSA) on the positively charged PDADMAC and PEI-modified 5CB droplets through electrostatic interaction can induce the radial-to-bipolar configuration transition of the 5CB inside the droplets. We find that the concentration of BSA required to induce the configuration transition increases linearly with the decrease of the molecular weight of PDAMAC and PEI. Our results highlight the capability of the director configuration of LC droplets as an optical probe for sensing the interaction between proteins and polyelectrolytes at the LC/aqueous interface.

Fabrication of temperature- and pH-sensitive liquid-crystal droplets with PNIPAM-b-LCP and SDS coatings by microfluidics.

Dual responsive (temperature and pH) 4-cyano-4'-pentylbiphenyl (5CB) droplets were fabricated by coating with PNIPAM-b-LCP (PNIPAM: poly(N-isopropylacrylamide) and LCP: poly(4-cyanobiphenyl-4'-oxyundecylacrylate)) and sodium dodecyl sulfate (SDS) using a microfluidic method, and were tested for protein detection. The PNIPAM-b-LCP/SDS-functionalized 5CB droplets were effective in detecting proteins in water through a radial-to-bipolar (R-B) orientational change, with detection limits of 0.95, 1.1, 0.12, and 0.07 μM for bovine serum albumin (BSA), lysozyme (LYZ), hemoglobin (Hb), and chymotrypsinogen (ChTg), respectively. The R-B change of the 5CB droplet occurred above the lower critical solution temperature (LCST) of PNIPAM and at pH values below the pI values of the tested proteins, and was reversible by heating/cooling at the LCST of PNIPAM. These sensitive 5CB droplets are simple to prepare, cost-effective, easily detectable, and re-usable (by temperature control) for protein detection. Further, they can be applied to a biosensor after employing the selective units such as aptamers, antibodies, single-stranded DNA, proteins, peptides, and aptamer-binding RNA on the droplet.

Glucose Sensor using Liquid-Crystal Droplets Made by Microfluidics

Micrometer-sized, 4-cyno-4-pentylbiphenyl (5CB) droplets were developed for glucose detection in an aqueous medium by coating with poly(acrylicacid-b-4-cynobiphenyl-4-oxyundecylacrylate) (PAA-b-LCP) at the 5CB/water interface and covalently immobilizing glucose oxidase (GOx) to the PAA chains. This functionalized liquid-crystal (LC) droplet detected glucose from a radial to bipolar configurational change by polarized optical microscopy under crossed polarizers at concentrations as low as 0.03 mM and response times of ~3 min and showed the selective detection of glucose against galactose. This new and sensitive LC-droplet-based glucose biosensor has the merits of low production cost and easy detection by the naked eye and might be useful for prescreening the glucose level in the human body.

Imaging catalase reactions through interactions between liquid crystals and oil-in-water emulsions

In this study, a simple label-free technique for imaging catalase reactions through interactions between liquid crystals (LCs) and oil-in-water emulsions was developed. We prepared a dodecanal-in-water emulsion, and mixed the emulsion with 4-cyano-4′-pentylbi-phenyl (5CB) droplets. In the absence of hydrogen peroxide, a dark image was observed between crossed polarisers because the 5CB and dodecanal droplets coalesced resulting in the formation of a population of droplets comprised of 5CB and dodecanal that are isotropic. In contrast, if hydrogen peroxide was present, a bright appearance was observed, because some fraction of the dodecanal was converted to dodecanoic acid, which prevented coalescence of 5CB and dodecanal, and thus the 5CB droplets retained their nematic order. Since catalase could decompose hydrogen peroxide, we observed a dark image after LC made contact with a dodecanal–hydrogen peroxide mixture that had been pre-incubated with catalase. Control experiments were conducted to confirm the specificity of the enzymatic reaction. The detection limit of catalase in our system was 0.1 μg/mL. These results demonstrate that LC with oil-in-water emulsion can be used for the detection of relevant enzymatic reactions.

Optical Detection of Lithocholic Acid with Liquid Crystal Emulsions

The concentration level of bile acids is a clinical biomarker for the diagnosis of intestinal diseases because individuals suffering from intestinal diseases have a sharply increased concentration of bile acids at micromolar levels. Here, we report the detection of lithocholic acid (LCA) in aqueous solution by using surfactant-stabilized 4-n-pentyl-4'-cyanobiphenyl (5CB) liquid crystal droplets as an optical probe. We find that the surfactant adsorbed at the 5CB/water interface can be replaced by LCA, triggering a radial-to-bipolar configuration transition of the 5CB in the droplets. By simply observing the LCA-triggered transition with a polarizing optical microscope, micromolar levels of LCA in aqueous solution can be detected. The detection limit and selectivity of surfactant-stabilized 5CB droplets for LCA depend on the chain length and headgroup of the surfactants used for stabilizing 5CB droplets.

Director Configuration Transitions of Polyelectrolyte Coated Liquid-Crystal Droplets

Liquid-crystal droplets are of great interest because of their large surface areas, rich phases, and tunable optical properties. The director configuration of liquid-crystal droplets provides a unique optical sign to detect the events occurring at the droplet surface. In this article, we report the alternating bipolar/radial configuration transitions of 4-n-pentyl-4'-cyanobiphenyl (5CB) droplets triggered by the layer-by-layer coating of negatively charged poly(styrenesulfonate sodium (PSS) and positively charged poly(diallyldimethylammonium chloride) (PDADMAC) on the droplet surface. The alternating configuration transitions are due to the interactions of the 5CB with polar versus nonpolar PDADMAC/PSS multilayer coatings. Furthermore, we find that the coating of PDADMAC/PSS multilayers makes the director configuration of the 5CB in the droplets sensitive to environmental salt concentrations.

Microfluidic formation of pH responsive 5CB droplets decorated with PAA-b-LCP

We are reporting for the first time the pH responsiveness of liquid crystal (LC) microdroplets decorated with an amphiphilic block copolymer of PAA-b-LCP. We successfully demonstrated the adsorption of block copolymer on LC droplets by fluorescence microscopy and pH response to the radial-to-bipolar orientational change of the LC droplets by changing pH from 12 to 2 through the polarized optical microscope (POM). We believe that our results may pave the way for the generation of monodisperse droplets decorated by various amphiphilic block copolymers which respond to several kinds of the external stimuli. These developments may be important for potential applications of the LC droplets in sensing and encapsulation fields.