Tunable Gas Sensing Gels by Cooperative Assembly

Effect of Polymer Hydrophobicity in the Performance of Hybrid Gel Gas Sensors for E-Noses

Relative humidity (RH) is a common interferent in chemical gas sensors, influencing their baselines and sensitivity, which can limit the performance of e-nose systems. Tuning the composition of the sensing materials is a possible strategy to control the impact of RH in gas sensors. Hybrid gel materials used as gas sensors contain self-assembled droplets of ionic liquid and liquid crystal molecules encapsulated in a polymeric matrix. In this work, we assessed the effect of the matrix hydrophobic properties in the performance of hybrid gel materials for VOC sensing in humid conditions (50% RH). We used two different polymers, the hydrophobic PDMS and the hydrophilic bovine gelatin, as polymeric matrices in hybrid gel materials containing imidazolium-based ionic liquids, [BMIM][Cl] and [BMIM][DCA], and the thermotropic liquid crystal 5CB. Better accuracy of VOC prediction is obtained for the hybrid gels composed of a PDMS matrix combined with the [BMIM][Cl] ionic liquid, and the use of this hydrophobic matrix reduces the effect of humidity on the sensing performance when compared to the gelatin counterpart. VOCs interact with all the moieties of the hybrid gel multicomponent system; thus, VOC correct classification depends not only on the polymeric matrix used, but also on the IL selected, which seems to be key to achieve VOCs discrimination at 50% RH. Thus, hybrid gels' tunable formulation offers the potential for designing complementary sensors for e-nose systems operable under different RH conditions.

Advanced Functional Liquid Crystals

Liquid crystals have been intensively studied as functional materials. Recently, integration of various disciplines has led to new directions in the design of functional liquid-crystalline materials in the fields of energy, water, photonics, actuation, sensing, and biotechnology. Here, recent advances in functional liquid crystals based on polymers, supramolecular complexes, gels, colloids, and inorganic-based hybrids are reviewed, from design strategies to functionalization of these materials and interfaces. New insights into liquid crystals provided by significant progress in advanced measurements and computational simulations, which enhance new design and functionalization of liquid-crystalline materials, are also discussed.

Technological Advancements in Bio‐recognition using Liquid Crystals: Techniques, Applications, and Performance

The application of liquid crystal (LC) materials has undergone a modern-day renaissance from its classical use in electronics industry as display devices to new-fangled techniques for optically detecting biological and chemical analytes. This review article deals with the emergence of LC materials as invaluable material for their use as label-free sensing elements in the development of optical, electro-optical and electrochemical biosensors. The property of LC molecules to change their orientation on perturbation by any external stimuli or on interaction with bioanalytes or chemical species has been utilized by many researches for the fabrication of high sensitive LC-biosensors. In this review article we categorized LC-biosensor based on biomolecular reaction mechanism viz. enzymatic, nucleotides and immunoreaction in conjunction with operating principle at different LC interface namely LC-solid, LC-aqueous and LC-droplets. Based on bimolecular reaction mechanism, the application of LC has been delineated with recent progress made in designing of LC-interface for the detection of bio and chemical analytes of proteins, virus, bacteria, clinically relevant compounds, heavy metal ions and environmental pollutants. The review briefly describes the experimental set-ups, sensitivity, specificity, limit of detection and linear range of various viable and conspicuous LC-based biosensor platforms with associated advantages and disadvantages therein.

Novel Aggregation-Enhanced PEC Photosensitizer Based on Electrostatic Linkage of Ionic Liquid with Protoporphyrin IX for Ultrasensitive Detection of Molt-4 Cells

Nowadays, aggregation quenching of most organic photosensitizers in aqueous media seriously restricts analytical and biomedical applications of photoelectrochemical (PEC) sensors. In this work, an aggregation-enhanced PEC photosensitizer was prepared by electrostatically bonding protoporphyrin IX (PPIX) with an ionic liquid of 1-butyl-3-methylimidazole tetrafluoroborate ([BMIm][BF₄]), termed as PPIX-[BMIm] for clarity. The resultant PPIX-[BMIm] showed weak photocurrent in pure dimethyl sulfoxide (DMSO, good solvent), while the PEC signals displayed a 44.1-fold enhancement in a water (poor solvent)/DMSO binary solvent with a water fraction (f_w) of 90%. Such PEC-enhanced mechanism was critically studied by electrochemistry and density functional theory (DFT) calculation in some detail. Afterward, a label-free PEC cytosensor was built for ultrasensitive bioassay of acute lymphoblastic leukemia (molt-4) cells by electrodepositing Au nanoparticles (Au NPs) on the PPIX-[BMIm] aggregates and sequential assembly of protein tyrosine kinase (PTK) aptamer DNA (aptDNA). The resultant cytosensor showed a wide linear range (300 to 3 × 10⁵ cells mL^-1) with a limit of detection (LOD) as low as 63 cells mL^-1. The aggregation-enhanced PEC performance offers a valuable and practical pathway for synthesis of advanced organic photosensitizer to explore its PEC applications in early diagnosis of tumors.

Nanoscale Events on Cyanobiphenyl-Based Self-Assembled Droplets Triggered by Gas Analytes

Liquid crystals (LCs) are prime examples of dynamic supramolecular soft materials. Their autonomous self-assembly at the nanoscale level and the further nanoscale events that give rise to unique stimuli-responsive properties have been exploited for sensing purposes. One of the key features to employ LCs as sensing materials derives from the fine-tuning between stability and dynamics. This challenging task was addressed in this work by studying the effect of the alkyl chain length of cyanobiphenyl LCs on the molecular self-assembled compartments organized in the presence of ionic liquid molecules and gelatin. The resulting multicompartment nematic and smectic gels were further used as volatile organic compound chemical sensors. The LC structures undergo a dynamic sequence of phase transitions, depending on the nature of the LC component, yielding a variety of optical signals, which serve as optical fingerprints. In particular, the materials incorporating smectic compartments resulted in unexpected and rich optical textures that have not been reported previously. Their sensing capability was tested in an in-house-assembled electronic nose and further assessed via signal collection and machine-learning algorithms based on support vector machines, which classified 12 different gas analytes with high accuracy scores. Our work expands the knowledge on controlling LC self-assembly to yield fast and autonomous accurate chemical-sensing systems based on the combination of complex nanoscale sensing events with artificial intelligence tools.

Tackling Humidity with Designer Ionic Liquid-Based Gas Sensing Soft Materials

Relative humidity is simultaneously a sensing target and a contaminant in gas and volatile organic compound (VOC) sensing systems, where strategies to control humidity interference are required. An unmet challenge is the creation of gas-sensitive materials where the response to humidity is controlled by the material itself. Here, humidity effects are controlled through the design of gelatin formulations in ionic liquids without and with liquid crystals as electrical and optical sensors, respectively. In this design, the anions [DCA]^- and [Cl]^- of room temperature ionic liquids from the 1-butyl-3-methylimidazolium family tailor the response to humidity and, subsequently, sensing of VOCs in dry and humid conditions. Due to the combined effect of the materials formulations and sensing mechanisms, changing the anion from [DCA]^- to the much more hygroscopic [Cl]^- , leads to stronger electrical responses and much weaker optical responses to humidity. Thus, either humidity sensors or humidity-tolerant VOC sensors that do not require sample preconditioning or signal processing to correct humidity impact are obtained. With the wide spread of 3D- and 4D-printing and intelligent devices, the monitoring and tuning of humidity in sustainable biobased materials offers excellent opportunities in e-nose sensing arrays and wearable devices compatible with operation at room conditions.

Quantitative volatile organic compound sensing with liquid crystal core fibers

Polymer fibers with liquid crystals (LCs) in the core have potential as autonomous sensors of airborne volatile organic compounds (VOCs), with a high surface-to-volume ratio enabling fast and sensitive response and an attractive non-woven textile form factor. We demonstrate their ability to continuously and quantitatively measure the concentration of toluene, cyclohexane, and isopropanol as representative VOCs, via the impact of each VOC on the LC birefringence. The response is fully reversible and repeatable over several cycles, the response time can be as low as seconds, and high sensitivity is achieved when the operating temperature is near the LC-isotropic transition temperature. We propose that a broad operating temperature range can be realized by combining fibers with different LC mixtures, yielding autonomous VOC sensors suitable for integration in apparel or in furniture that can compete with existing consumer-grade electronic VOC sensors in terms of sensitivity and response speed.

Stable and Oriented Liquid Crystal Droplets Stabilized by Imidazolium Ionic Liquids

Liquid crystals represent a fascinating intermediate state of matter, with dynamic yet organized molecular features and untapped opportunities in sensing. Several works report the use of liquid crystal droplets formed by microfluidics and stabilized by surfactants such as sodium dodecyl sulfate (SDS). In this work, we explore, for the first time, the potential of surface-active ionic liquids of the imidazolium family as surfactants to generate in high yield, stable and oriented liquid crystal droplets. Our results show that [C₁₂MIM][Cl], in particular, yields stable, uniform and monodisperse droplets (diameter 74 ± 6 µm; PDI = 8%) with the liquid crystal in a radial configuration, even when compared with the standard SDS surfactant. These findings reveal an additional application for ionic liquids in the field of soft matter.

Structured Liquid Droplets as Chemical Sensors that Function Inside Living Cells

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.

Ionogels Based on a Single Ionic Liquid for Electronic Nose Application

Ionogel are versatile materials, as they present the electrical properties of ionic liquids and also dimensional stability, since they are trapped in a solid matrix, allowing application in electronic devices such as gas sensors and electronic noses. In this work, ionogels were designed to act as a sensitive layer for the detection of volatiles in a custom-made electronic nose. Ionogels composed of gelatin and a single imidazolium ionic liquid were doped with bare and functionalized iron oxide nanoparticles, producing ionogels with adjustable target selectivity. After exposing an array of four ionogels to 12 distinct volatile organic compounds, the collected signals were analyzed by principal component analysis (PCA) and by several supervised classification methods, in order to assess the ability of the electronic nose to distinguish different volatiles, which showed accuracy above 98%.

Optical Gas Sensing with Liquid Crystal Droplets and Convolutional Neural Networks

Liquid crystal (LC)-based materials are promising platforms to develop rapid, miniaturised and low-cost gas sensor devices. In hybrid gel films containing LC droplets, characteristic optical texture variations are observed due to orientational transitions of LC molecules in the presence of distinct volatile organic compounds (VOC). Here, we investigate the use of deep convolutional neural networks (CNN) as pattern recognition systems to analyse optical textures dynamics in LC droplets exposed to a set of different VOCs. LC droplets responses to VOCs were video recorded under polarised optical microscopy (POM). CNNs were then used to extract features from the responses and, in separate tasks, to recognise and quantify the vapours exposed to the films. The impact of droplet diameter on the results was also analysed. With our classification models, we show that a single individual droplet can recognise 11 VOCs with small structural and functional differences (F1-score above 93%). The optical texture variation pattern of a droplet also reflects VOC concentration changes, as suggested by applying a regression model to acetone at 0.9-4.0% (v/v) (mean absolute errors below 0.25% (v/v)). The CNN-based methodology is thus a promising approach for VOC sensing using responses from individual LC-droplets.

Screening of Xanthine Oxidase Inhibitors by Liquid Crystal-Based Assay Assisted with Enzyme Catalysis-Induced Aptamer Release

Small-molecule drugs play an important role in the treatment of various diseases. The screening of enzyme inhibitors is one of the most important means in developing therapeutic drugs. Herein, we demonstrate a liquid crystal (LC)-based screening assay assisted with enzyme catalysis-induced aptamer release for screening xanthine oxidase (XOD) inhibitors. The oxidation of xanthine by XOD prevents the specific binding of xanthine and its aptamer, which induces a bright image of LCs. However, when XOD is inhibited, xanthine specifically binds to the aptamer. Correspondingly, LCs display a dark image. Three compounds are identified as potent XOD inhibitors by screening a small library of triazole derivatives using this method. Molecular docking verifies the occupation of the active site by the inhibitor, which also exhibits excellent biocompatibility to HEK293 cells and HeLa cells. This strategy takes advantages of the unique aptamer-target binding, specific enzymatic reaction, and simple LC-based screening assay, which allows high-throughput and label-free screening of inhibitors with high sensitivity and remarkable accuracy. Overall, this study provides a competent and promising approach to facilitate the screening of enzyme inhibitors using the LC-based assay assisted with the enzyme catalysis-induced aptamer release.

Fish Gelatin-based Films for Gas Sensing

Electronic noses (e-noses) mimic the complex biological olfactory system, usually including an array of gas sensors to act as the olfactory receptors and a trained computer with signal-processing and pattern recognition tools as the brain. In this work, a new stimuli-responsive material is shown, consisting of self-assembled droplets of liquid crystal and ionic liquid stabilised within a fish gelatin matrix. These materials change their opto/electrical properties upon contact with volatile organic compounds (VOCs). By using an in-house developed e-nose, these new gas-sensing films yield characteristic optical signals for VOCs from different chemical classes. A support vector machine classifier was implemented based on 12 features of the signals. The results show that the films are excellent identifying hydrocarbon VOCs (toluene, heptane and hexane) (95% accuracy) but lower performance was found to other VOCs, resulting in an overall 60.4% accuracy. Even though they are not reusable, these sustainable gas-sensing films are stable throughout time and reproducible, opening several opportunities for future optoelectronic devices and artificial olfaction systems.

Imidazole-based ionogel as room temperature benzene and formaldehyde sensor

A room temperature benzene and formaldehyde gas sensor system with an ionogel as sensing material is presented. The sensing layer is fabricated employing poly(N-isopropylacrylamide) polymerized in the presence of 1-ethyl-3-methylimidazolium ethyl sulfate ionic liquid onto gold interdigitated electrodes. When the ionogel is exposed to increasing formaldehyde concentrations employing N₂ as a carrier gas, a more stable response is observed in comparison to the bare ionic liquid, but no difference in sensitivity occurs. On the other hand, when air is used as carrier gas the sensitivity of the system towards formaldehyde is decreased by one order of magnitude. At room temperature, the proposed sensor exhibited in air higher sensitivities to benzene, at concentrations ranging between 4 and 20 ppm resulting, in a limit of detection of 47 ppb, which is below the standard permitted concentrations. The selectivity of the IL towards HCHO and C₆H₆ is demonstrated by the absence of response when another IL is employed. Humidity from the ambient air slightly affects the resistance of the system proving the protective role of the polymeric matrix. Furthermore, the gas sensor system showed fast response/recovery times considering the thickness of the material, suggesting that ionogel materials can be used as novel and highly efficient volatile organic compounds sensors operating at room temperature.Graphical abstract.

Sustainable plant polyesters as substrates for optical gas sensors

The fast and non-invasive detection of odors and volatile organic compounds (VOCs) by gas sensors and electronic noses is a growing field of interest, mostly due to a large scope of potential applications. Additional drivers for the expansion of the field include the development of alternative and sustainable sensing materials. The discovery that isolated cross-linked polymeric structures of suberin spontaneously self-assemble as a film inspired us to develop new sensing composite materials consisting of suberin and a liquid crystal (LC). Due to their stimuli-responsive and optically active nature, liquid crystals are interesting probes in gas sensing. Herein, we report the isolation and the chemical characterization of two suberin types (from cork and from potato peels) resorting to analyses of gas chromatography–mass spectrometry (GC-MS), solution nuclear magnetic resonance (NMR), and X-ray photoelectron spectroscopy (XPS). The collected data highlighted their compositional and structural differences. Cork suberin showed a higher proportion of longer aliphatic constituents and is more esterified than potato suberin. Accordingly, when casted it formed films with larger surface irregularities and a higher C/O ratio. When either type of suberin was combined with the liquid crystal 5CB, the ensuing hybrid materials showed distinctive morphological and sensing properties towards a set of 12 VOCs (comprising heptane, hexane, chloroform, toluene, dichlormethane, diethylether, ethyl acetate, acetonitrile, acetone, ethanol, methanol, and acetic acid). The optical responses generated by the materials are reversible and reproducible, showing stability for 3 weeks. The individual VOC-sensing responses of the two hybrid materials are discussed taking as basis the chemistry of each suberin type. A support vector machines (SVM) algorithm based on the features of the optical responses was implemented to assess the VOC identification ability of the materials, revealing that the two distinct suberin-based sensors complement each other, since they selectively identify distinct VOCs or VOC groups. It is expected that such new environmentally-friendly gas sensing materials derived from natural diversity can be combined in arrays to enlarge selectivity and sensing capacity.

Microfluidics in Gas Sensing and Artificial Olfaction

Rapid, real-time, and non-invasive identification of volatile organic compounds (VOCs) and gases is an increasingly relevant field, with applications in areas such as healthcare, agriculture, or industry. Ideal characteristics of VOC and gas sensing devices used for artificial olfaction include portability and affordability, low power consumption, fast response, high selectivity, and sensitivity. Microfluidics meets all these requirements and allows for in situ operation and small sample amounts, providing many advantages compared to conventional methods using sophisticated apparatus such as gas chromatography and mass spectrometry. This review covers the work accomplished so far regarding microfluidic devices for gas sensing and artificial olfaction. Systems utilizing electrical and optical transduction, as well as several system designs engineered throughout the years are summarized, and future perspectives in the field are discussed.

Protein Microgel-Stabilized Pickering Liquid Crystal Emulsions Undergo Analyte-Triggered Configurational Transition

Herein, we report a novel approach that involves Pickering stabilization of micometer-sized liquid crystal (LC) droplets with biocompatible soft materials such as a whey protein microgel (WPM) to facilitate the analysis of analyte-induced configurational transition of the LC droplets. The WPM particles were able to irreversibly adsorb at the LC-water interface, and the resulting WPM-stabilized LC droplets possessed a remarkable stability against coalescence over time. Although the LC droplets were successfully protected by a continuous network of the WPM layer, the LC-water interface was still accessible for small molecules such as sodium dodecyl sulfate (SDS) that could diffuse through the meshes of the adsorbed WPM network or through the interfacial pores and induce an LC response. This approach was exploited to investigate the dynamic range of the WPM-stabilized LC droplet response to SDS. Nevertheless, the presence of the unadsorbed WPM in the aqueous medium reduced the access of SDS molecules to the LC droplets, thus suppressing the configuration transition. An improved LC response to SDS with a lower detection limit was achieved after washing off the unadsorbed WPM. Interestingly, the LC exhibited a detection limit as low as ∼0.85 mM for SDS within the initial WPM concentration ranging from 0.005 to 0.1 wt %. Furthermore, we demonstrate that the dose-response behavior was strongly influenced by the number of droplets exposed to the aqueous analytes and the type of surfactants such as anionic SDS, cationic dodecyltrimethylammonium bromide (DTAB), and nonionic tetra(ethylene glycol)monododecyl ether (C₁₂E₄). Thus, our results address key issues associated with the quantification of aqueous analytes and provide a promising colloidal platform toward the development of new classes of biocompatible LC droplet-based optical sensors.

Detection of organophosphorus pesticides with liquid crystals supported on the surface deposited with polyoxometalate-based acetylcholinesterase-responsive supramolecular spheres

Here, we demonstrate use of acetylcholinesterase (AChE)-responsive polyoxometalate (POM)/surfactant supramolecular spheres to build a liquid crystal (LC)-based sensing platform for detection of organophosphorus pesticides. The self-assembled spheres are composed of hybrid materials of a POM, sodium dodecatungstophosphate (PW₁₂), and a surfactant, myristoylcholine (Myr). It displays dark appearance when the aqueous solution is in contact with LCs supported on the octadecyltrichlorosilane-treated glass deposited with the supramolecular spheres, suggesting perpendicular orientation of LCs at the aqueous/LC interface. In contrast, LCs show bright appearance when the surface-deposited supramolecular spheres are enzymatically hydrolyzed by AChE, corresponding to planar orientation of LCs at the aqueous/LC interface. Detection of organophosphates are successfully achieved as they are potent inhibitors of AChE. The detection limit of the sensing platform reached 0.9 ng/mL for dimethoate. This method can avoid disturbance of external interference with excellent specificity and sensitivity, which makes it very promise in detection of organophosphorus pesticides.

Seeing the Unseen: The Role of Liquid Crystals in Gas-Sensing Technologies

Fast, real-time detection of gases and volatile organic compounds (VOCs) is an emerging research field relevant to most aspects of modern society, from households to health facilities, industrial units, and military environments. Sensor features such as high sensitivity, selectivity, fast response, and low energy consumption are essential. Liquid crystal (LC)-based sensors fulfill these requirements due to their chemical diversity, inherent self-assembly potential, and reversible molecular order, resulting in tunable stimuliresponsive soft materials. Sensing platforms utilizing thermotropic uniaxial systems-nematic and smectic-that exploit not only interfacial phenomena, but also changes in the LC bulk, are demonstrated. Special focus is given to the different interaction mechanisms and tuned selectivity toward gas and VOC analytes. Furthermore, the different experimental methods used to transduce the presence of chemical analytes into macroscopic signals are discussed and detailed examples are provided. Future perspectives and trends in the field, in particular the opportunities for LC-based advanced materials in artificial olfaction, are also discussed.

Simultaneous Detection of Multiple Tumor Markers in Blood by Functional Liquid Crystal Sensors Assisted with Target-Induced Dissociation of Aptamer

Multiplex detection of tumor markers in blood with high specificity and high sensitivity is critical to cancer diagnosis, treatment, and prognosis. Herein, we demonstrate a strategy for simultaneous detection of multiple tumor markers in blood by functional liquid crystal (LC) sensors assisted with target-induced dissociation (TID) of an aptamer for the first time. Magnetic beads (MBs) coated with an aptamer (apt1) are employed to specifically capture target proteins in blood. After incubation of the obtained protein-coated MBs with duplexes of another aptamer (apt2) and signal DNA, sandwich complexes of apt1/protein/apt2 are formed on the MBs due to specific recognition of target proteins by apt2, which induces release of signal DNA into the aqueous solution. Subsequently, signal DNA is specifically recognized by highly sensitive DNA-laden LC sensors. Using this strategy, a 3D printed optical cell was employed to enable simultaneous detection of multiple tumor markers such as carcinoembryonic antigen (CEA), alpha-fetoprotein (AFP), and prostate specific antigen (PSA) with high specificity and high sensitivity. Overall, this effective and low-cost multiplex approach takes advantage of the easy separation of MBs, high specificity of aptamer-based recognition, and high sensitivity of functional LC sensors. Plus, it offers a performance that is competitive to that of commercial ELISA kits without potential interference from hemolysis, which makes it very promising in multiplex detection of tumor markers in clinical applications.

Identification of a biomarker panel for improvement of prostate cancer diagnosis by volatile metabolic profiling of urine

BACKGROUND

The lack of sensitive and specific biomarkers for the early detection of prostate cancer (PCa) is a major hurdle to improve patient management.

METHODS

A metabolomics approach based on GC-MS was used to investigate the performance of volatile organic compounds (VOCs) in general and, more specifically, volatile carbonyl compounds (VCCs) present in urine as potential markers for PCa detection.

RESULTS

Results showed that PCa patients (n = 40) can be differentiated from cancer-free subjects (n = 42) based on their urinary volatile profile in both VOCs and VCCs models, unveiling significant differences in the levels of several metabolites. The models constructed were further validated using an external validation set (n = 18 PCa and n = 18 controls) to evaluate sensitivity, specificity and accuracy of the urinary volatile profile to discriminate PCa from controls. The VOCs model disclosed 78% sensitivity, 94% specificity and 86% accuracy, whereas the VCCs model achieved the same sensitivity, a specificity of 100% and an accuracy of 89%. Our findings unveil a panel of 6 volatile compounds significantly altered in PCa patients' urine samples that was able to identify PCa, with a sensitivity of 89%, specificity of 83%, and accuracy of 86%.

CONCLUSIONS

It is disclosed a biomarker panel with potential to be used as a non-invasive diagnostic tool for PCa.

Enhanced Gas Sensing with Soft Functional Materials

The materials described in this work result from the self-assembly of liquid crystals and ionic liquids into droplets, stabilized within a biopolymeric matrix. These systems are extremely versatile gels, in terms of composition, and offer potential for fine tuning of both structure and function, as each individual component can be varied. Here, the characterization and application of these gels as sensing thin films in gas sensor devices is presented. The unique supramolecular structure of the gels is explored for molecular recognition of volatile organic compounds (VOCs) by employing gels with distinct formulations to yield combinatorial optical and electrical responses used in the distinction and identification of VOCs.

Detection of Biomarkers in Blood Using Liquid Crystals Assisted with Aptamer-Target Recognition Triggered in Situ Rolling Circle Amplification on Magnetic Beads

Detection of biomarkers in body fluids is critical to both diagnosing the life-threatening diseases and optimizing therapeutic interventions. We herein report use of liquid crystals (LCs) to detect biomarkers in blood with high sensitivity and specificity by employing in situ rolling circle amplification (RCA) on magnetic beads (MBs). Specific recognition of cancer biomarkers, such as platelet derived growth factor BB (PDGF-BB) and adenosine, by aptamers leads to formation of a nucleic acid circle on MBs preassembled with ligation DNA, linear padlock DNA, and aptamers, thereby triggering in situ RCA. LCs change from dark to bright appearance after the in situ RCA products being transferred onto the LC interface decorated with octadecy trimethylammonium bromide (OTAB), which is particularly sensitive to the amplified DNA on MBs. Overall, this label-free approach takes advantages of high specificity of aptamer-based assay, efficient enrichment of signaling molecules on MBs, remarkable DNA elongation performance of the RCA reaction, and high sensitivity of LC-based assay. It successfully eliminates the matrix interference on the LC-based sensors and thus achieves at least 4 orders of magnitude improvement in sensitivity for detection of biomarkers compared to other LC-based sensors. In addition, performance of the developed sensor is comparable to that of the commercial ones. Thus, this study provides a simple, powerful, and promising approach to facilitate highly sensitive, specific, and label-free detection of biomarkers in body fluids.

Microgel-stabilized liquid crystal emulsions enable an analyte-induced ordering transition

Microgels enable reversible stabilization of liquid crystal (LC) emulsions in ways that facilitate analysis of LC droplets that undergo an analyte-triggered conformational transition.

Effect of film thickness in gelatin hybrid gels for artificial olfaction

Artificial olfaction is a fast-growing field aiming to mimic natural olfactory systems. Olfactory systems rely on a first step of molecular recognition in which volatile organic compounds (VOCs) bind to an array of specialized olfactory proteins. This results in electrical signals transduced to the brain where pattern recognition is performed. An efficient approach in artificial olfaction combines gas-sensitive materials with dedicated signal processing and classification tools. In this work, films of gelatin hybrid gels with a single composition that change their optical properties upon binding to VOCs were studied as gas-sensing materials in a custom-built electronic nose. The effect of films thickness was studied by acquiring signals from gelatin hybrid gel films with thicknesses between 15 and 90 μm when exposed to 11 distinct VOCs. Several features were extracted from the signals obtained and then used to implement a dedicated automatic classifier based on support vector machines for data processing. As an optical signature could be associated to each VOC, the developed algorithms classified 11 distinct VOCs with high accuracy and precision (higher than 98%), in particular when using optical signals from a single film composition with 30 μm thickness. This shows an unprecedented example of soft matter in artificial olfaction, in which a single gelatin hybrid gel, and not an array of sensing materials, can provide enough information to accurately classify VOCs with small structural and functional differences.

Free-energy model for nanoparticle self-assembly by liquid crystal sorting

We modeled the experimentally observed self-assembly of nanoparticles (NPs) into shells with diameters up to 10 μm, via segregation from growing nematic domains. Using field-based Monte Carlo simulations, we found the equilibrium configurations of the system by minimizing a free-energy functional that includes effects of excluded-volume interactions among NPs, orientational elasticity, and the isotropic-nematic phase-transition energy. We developed a Gaussian-profile approximation for the liquid crystal (LC) order-parameter field that provides accurate analytical values for the free energy of LC droplets and the associated microshells. This analytical model reveals a first-order transition between equilibrium states with and without microshells, governed mainly by the competition of excluded-volume and phase-transition energies. By contrast, the LC elasticity effects are much smaller and mostly confined to setting the size of the activation barrier for the transition. In conclusion, field-based thermodynamic methods provide a theoretical framework for the self-assembly of NP shells in liquid crystal hosts and suggest that field-based kinetic methods could be useful to simulate and model the time evolution of NP self-assembly coupled to phase separation.

Tilapia fish microbial spoilage monitored by a single optical gas sensor

As consumption of fish and fish-based foods increases, non-destructive monitoring of fish freshness also becomes more prominent. Fish products are very perishable and prone to microbiological growth, not always easily detected by organoleptic evaluation. The analysis of the headspace of fish specimens through gas sensing is an interesting approach to monitor fish freshness. Here we report a gas sensing method for monitoring Tilapia fish spoilage based on the application of a single gas sensitive gel material coupled to an optical electronic nose. The optical signals of the sensor and the extent of bacterial growth were followed over time, and results indicated good correlation between the two determinations, which suggests the potential application of this simple and low cost system for Tilapia fish freshness monitoring.

Design and Evolution of an Opto-electronic Device for VOCs Detection

Electronic noses (E-noses) are devices capable of detecting and identifying Volatile Organic Compounds (VOCs) in a simple and fast method. In this work, we present the development process of an opto-electronic device based on sensing films that have unique stimuli-responsive properties, altering their optical and electrical properties, when interacting with VOCs. This interaction results in optical and electrical signals that can be collected, and further processed and analysed. Two versions of the device were designed and assembled. E-nose V1 is an optical device, and E-nose V2 is a hybrid opto-electronic device. Both E-noses architectures include a delivery system, a detection chamber, and a transduction system. After the validation of the E-nose V1 prototype, the E-nose V2 was implemented, resulting in an easy-to-handle, miniaturized and stable device. Results from E-nose V2 indicated optical signals reproducibility, and the possibility of coupling the electrical signals to the optical response for VOCs sensing.

Thermotropic Liquid Crystal-Assisted Chemical and Biological Sensors

In this review article, we analyze recent progress in the application of liquid crystal-assisted advanced functional materials for sensing biological and chemical analytes. Multiple research groups demonstrate substantial interest in liquid crystal (LC) sensing platforms, generating an increasing number of scientific articles. We review trends in implementing LC sensing techniques and identify common problems related to the stability and reliability of the sensing materials as well as to experimental set-ups. Finally, we suggest possible means of bridging scientific findings to viable and attractive LC sensor platforms.

Direct evidence of multichannel-improved charge-carrier mechanism for enhanced photocatalytic H2 evolution

In the field of photocatalysis, the high-charge recombination rate has been the big challenge to photocatalytic conversion efficiency. Here we demonstrate the direct evidence of multichannel-improved charge-carrier mechanism to facilitate electron-hole transfer for raising photocatalytic H₂ evolution activity. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and UV-Vis diffuse reflectance spectroscopy (DRS), were used to characterize the as-fabricated samples. The result shows that the present design of Au/Pt nanoparticles (NPs) decorated one-dimensional Z-scheme TiO₂/WO₃ heterostructure composite nanofibers have been fabricated, which even exhibited excellent light absorption in the visible region and greatly enhanced photocatalytic activities on H₂ generation comparing with pure TiO₂, TiO₂/WO₃ and Pt/WO₃/TiO₂ nanofibers. This greatpromotion is mainly on account of the photosynthetic heterojunction system, which include the surface plasmon resonance (SPR) of Au nanoparticles, low overpotential of Pt nanoparticles, and more importantly, the one-dimensional multichannel-improved charge-carrier photosynthetic heterojunction system with Pt as an electron collector and WO₃ as a hole collector. Transferring photoinduced electrons and holes at the same time, leading to effective charge separation was directly proved by ultraviolet photoelectron spectroscopy, electrochemical impedance spectroscopy, photocurrent analysis and incident photon-to-electron conversion spectrum.