Liquid Sensor Research Articles

Development of a liquid crystal-based sensor utilizing EDTA-cyclodextrin polymer for real-time optical detection of methylene blue in natural water samples

The discharge of dyes in industrial wastewater poses significant environmental and health risks when released into natural water resources. In this study, we report the development of an EDTA-crosslinked cyclodextrin polymer (ECDP)-based sensor for the real-time, naked-eye detection of hazardous methylene blue (MB) dye in aqueous solutions and natural water samples. The sensor functions via a competitive host–guest inclusion mechanism involving sodium dodecyl sulphate (SDS) and ECDP, which modulates the alignment of liquid crystals (LCs). Initially, SDS induces homeotropic ordering at the fluid interface, but when complexed with ECDP, it causes a tilted LC alignment. Upon the introduction of MB, SDS is displaced from the ECDP cavity and re-adsorbs at the LC/aqueous interface, triggering an orientational transition from tilted to homeotropic. This transition is clearly observed as a distinct bright-to-dark shift under crossed polarizers. The host–guest (ECDP/MB) inclusion complexation mechanism was further confirmed by FT-IR, XRPD, and DSC analyses. The developed sensor exhibits high selectivity for MB in dye-contaminated natural water samples and sensitivity to MB concentrations upto 0.10 mM. This study demonstrates the potential of cyclodextrin-based polymers for liquid crystal sensing applications, offering promising pathways for future developments in environmental monitoring.

Femto-Laser Processed Metasurface With Fano Response: Applications to a High Performance Refractometric Sensor

The practical development of compact modern nanophotonic devices relies on the availability of fast and low-cost fabrication techniques applicable to a wide variety of materials and designs. We have engraved a split grating geometry on stainless steel using femtosecond laser processing. This structure serves as a template to fabricate efficient plasmonic sensors, where a thick gold layer is grown conformally on it. The scanning electron microscope (SEM) images confirm the generation of the split laser-induced periodic spatial structures. The optical reflectance of our sensors shows two dips corresponding to the excitation of surface plasmon resonances (SPRs) at two different wavelengths. Furthermore, the asymmetric shape of these spectral responses reveals a strong and narrow Fano resonance. Our computational electromagnetism models accurately reproduce the reflectivity of the fabricated structure. The spectral responses of both the simulated and fabricated structures are fitted to the Fano model that coherently combines the narrow SPRs with the broad continuum background caused by diffraction. The parameters extracted from the fitting, such as the resonance wavelengths and line widths, are used to evaluate the performance of our device as a refractometric sensor for liquids. The maximum sensitivity and figure of merit are 880 nm/RIU and 80 RIU−1, respectively. Besides the compact design of our sensing device, its performance exceeds the theoretical maximum sensitivity of a classical Kretschmann setup.

Saline based microfluidic soft pressure sensor utilizing a three-dimensional focused electric field for motion and healthcare monitoring

This paper introduces the ‘Spatially Focused Saline-based Pressure Sensor (SF-SaPS)', a novel soft microfluidic pressure sensor featuring a distinctive three-dimensional focusing structure. By critically reducing the cross-sectional area of the microchannel at the focused structure, the SF-SaPS achieves excellent sensitivity to pressure within the sensing region. With the spatially focused region, the SF-SaPS could detect a wide range of pressure from gentle touches to human weight, which is typically unachievable with low-conductivity sensing media such as saline, a medium inherently safe for human use. Beyond its sensitivity, the SF-SaPS exhibits sensing performance and stability comparable with conventional liquid metal-based pressure sensors. Our sensor demonstrated minimal signal drift, a rapid response time of 70 ms under cyclic loading, and 20-day long-term stability tests immersed in water. Additionally, the sensor possesses a transparency advantage unattainable by liquid metal sensors as we utilized transparent polymers and saline. A unique advantage of the SF-SaPS lies in its selective spatial and mechanical sensitivity; as the electrical resistance is highly dependent on changes in the cross-sectional area of the microchannels, the sensor has superior pressure sensitivity compared to bending and strain. Finally, various application examples highlight the SF-SaPS's advantages. By configuring the sensor in a two-axis array, the SF-SaPS facilitates pressure mapping across a plane. Additionally, it proves effective in healthcare monitoring, from radial pulse to finger movements. In conclusion, the SF-SaPS's combination of performance, stability, biocompatibility, and transparency positions this sensor as a versatile tool for applications extending beyond healthcare, as demonstrated in this study.

Ultrasensitive liquid sensor based on an embedded microchannel bulk acoustic wave resonator

The high-frequency and high-quality factor characteristics of bulk acoustic wave (BAW) resonators have significantly advanced their application in sensing technologies. In this work, a fluidic sensor based on a BAW resonator structure is fabricated and investigated. Embedded microchannels are formed beneath the active area of the BAW device without the need for external processes. As liquid flows through the microchannel, pressure is exerted on the upper wall (piezoelectric film) of the microchannel, which causes a shift in the resonant frequency. Using density functional theory, we revealed the intrinsic mechanism by which piezoelectric film deformation influences BAW resonator performance. Theoretically, the upwardly convex piezoelectric film caused by liquid flow can increase the resonant frequency. The experimental results obtained with ethanol solutions of different concentrations reveal that the sensor, which operates at a high resonant frequency of 2.225 GHz, achieves a remarkable sensitivity of 5.1 MHz/% (221 ppm/%), with an ultrahigh linearity of 0.995. This study reveals the intrinsic mechanism of liquid sensing based on BAW resonators, highlights the potential of AlN/Al0.8Sc0.2N composite film BAW resonators in liquid sensing applications and offers insights for future research and development in this field.

Effect of pH, acid catalyst, and aging time on pore characteristics of dried silica xerogel

ABSTRACT This study investigates the impact of pH and ageing time on silica xerogel pore characteristics prepared via the sol-gel method. Using tetraethyl orthosilicate (TEOS) and HCl as a precursor and acid catalyst, respectively, tailored pore size distributions in the meso- and macroporous range were achieved. Field emission scanning electron microscopy (FESEM) revealed interconnected porous structures, with pore size increasing linearly with ageing time. pH variation yielded average pore sizes ranging from ~ 31 nm (mesoporous) to ~ 59 nm (macroporous). Higher acid catalyst concentration accelerated gelation, resulting in smaller pores. Elemental analysis confirmed silicon and oxygen as primary constituents, while X-ray diffraction (XRD) confirmed amorphous nature. Sol-gel synthesis provides versatile control over microstructure and properties, enabling customisation for applications such as liquid filters, batteries, sensors, and bio-scaffolds. It offers a cost-effective, direct synthesis of high-purity multi-component materials, emphasising its potential for functional material development.

A reconfigurable and conformal liquid sensor for ambulatory cardiac monitoring

The severe mismatch between solid bioelectronics and dynamic biological tissues has posed enduring challenges in the biomonitoring community. Here, we developed a reconfigurable liquid cardiac sensor capable of adapting to dynamic biological tissues, facilitating ambulatory cardiac monitoring unhindered by motion artifacts or interference from other biological activities. We employed an ultrahigh-resolution 3D scanning technique to capture tomographic images of the skin on the wrist. Then, we established a theoretical model to gain a deep understanding of the intricate interaction between our reconfigurable sensor and dynamic biological tissues. To properly elucidate the advantages of this sensor, we conducted cardiac monitoring alongside benchmarks such as the electrocardiogram. The liquid cardiac sensor was demonstrated to produce stable signals of high quality (23.1 dB) in ambulatory settings.

Liquid crystal gel-based acetone sensor using correlated laser speckles

Acetone, a common volatile organic compound (VOC), poses health risks even at low concentrations. Current acetone sensors are costly and require specialized equipment and expertise. This work develops a novel vapor sensor for determining acetone vapor concentration using the speckle patterns generated by liquid crystal gels (LCGs). The vapor sensor comprises a LCG film prepared by the phase separation of a mixture containing polystyrene microspheres and liquid crystals (LCs). The orientation of the LC molecules changes when the LCG film is exposed to an acetone vapor environment, altering the equivalent refractive indices of the LC domains. This leads to a change in the scattering state of the LCG film under laser illumination, forming different speckle patterns. The concentration of acetone vapor is determined by calculating the correlation coefficient of the speckle images, where the sensitivity and limit of detection of the sensor are 4×10-4 ppm-1 and 754.05 ppm, respectively. The developed correlated laser speckle-based optical system is simpler, less expensive, and more stable than traditional LC film vapor sensors. This acetone gas sensor has potential applications in industrial and indoor air quality testing.

Experimental and theoretical study on liquid film thickness of upward annular flow in helically coiled tubes

The liquid film thickness is crucial for studying the thermal hydraulic mechanism of the annular flow region in helically coiled tubes (HCTs). This paper introduces a refined experimental study on the liquid film thickness of annular flow in HCTs, utilizing a newly developed liquid film sensor. The experimental results indicate that the smaller coil diameters and pitches result in thinner average liquid film thicknesses. The average liquid film thickness decreases with increasing superficial gas velocity and decreasing superficial liquid velocity. However, the liquid film thickness at different circumferential positions on the cross-section of the tube exhibits varying sensitivities to superficial gas and liquid velocities. The experimental data reveal that the existing typical correlation formula for the average liquid film thickness of annular flow in straight tubes does not apply to HCTs. Consequently, a new prediction model is proposed for the average liquid film thickness of the annular flow region in HCTs, based on the modified Froude number, modified Dean number, Ekman number, and Reynolds number. This model comprehensively incorporates the structural characteristics of HCTs and fluid properties, and its validity is verified through the utilization of available and current data in the literature.

Liquid Metal Fiber-Based High-Sensitivity Strain and Pressure Sensors Enhanced by Porous Structure

Liquid Metal Fiber-Based High-Sensitivity Strain and Pressure Sensors Enhanced by Porous Structure

A capillary fiber-based liquid metal pressure sensor

The capillary fibers can easily be prefabricated in the factory, and their production cost is reduced. Moreover, the liquid metal fibers have the advantages of good integrity, excellent electrical conductivity, inherent stretchability, easy phase transition, and can be woven or knitted into smart fabrics. To solve the problems of the complex manufacture process and low integrity of lithographic sensors, capillary fibers replace the lithographic microfluidic channel to fill liquid metal to manufacture the pressure sensor in this paper. The prefabricated fiber is poured directly to produce the flexible chip. The steel shell is employed to increase the sensor’s measuring range and to enhance its overall performance. Compression experiments on the developed sensor are conducted, and pressure-resistance curves of the developed pressure sensor are obtained. The analytical solution of the pressure for the developed sensor is derived, and the analytical results are in good agreement with the experimental data. The cyclic loading experimental result shows that the measuring range of the chip is from 0 kPa to 1900 kPa with a full-scale output value of 1644 mΩ, linearity varying from 0.14 to 1.22 mΩ kPa−1, curve coincidence of 48.2%, repeatability of 2.77% and hysteresis of 5.26%. The measuring range of the developed pressure sensor is from 0 MPa to 20 MPa with a full-scale output value of 1046 mΩ, linearity ranging from 35.63 to 70.20 mΩ MPa−1, curve coincidence of 7.5%, repeatability of 2.35% and hysteresis of 4.53%. The comparison of performance indexes shows that the capillary fiber-based chip has good measurement performance, and the introduction of steel shell further improves the measurement performance.

Real-time detection of Tau-381 protein using liquid crystal-based sensors for Alzheimer's disease diagnosis

Tau is a protein found in the central nervous system (CNS) and is involved in stabilizing microtubules in axons. Given the link between Tau levels in the body and Alzheimer's disease (AD), there is a demand for straightforward and precise strategies to detect Tau in body fluids. In this study, we report liquid crystal (LC)-based sensors for the real-time detection of Tau protein, a well-known AD biomarker. The sensor uses a detection method based on the orientation change of the LC because of the competitive biomolecular interaction between Tau and Tau aptamers with the cationic polymer poly-L-lysine (PLL). Tau and its aptamers form stable complexes through electrostatic interactions. Owing to the consumption of the aptamer, the positively charged PLL fails to interact with the aptamer but binds to the negatively charged 1.2-dioleoyl-sn-glycero-3-phospho-(1′-rac-glycerol) sodium salt (DOPG). The PLL and DOPG complex alters the orientation of the LC to ensure a planar anchoring of the 4-cyano-4′-pentylbiphenyl (5CB)/aqueous interface; this anchoring intensifies with increasing Tau concentration, thus enabling the observation of a bright optical image. Our LC-based sensor demonstrated a low detection limit of 2.77 pg/mL in phosphate buffered saline (PBS) and 10.86 pg/mL and 19.31 pg/mL in human serum and plasma, respectively. Moreover, it is anticipated to be suitable for point-of-care diagnosis of AD because it does not require specialized analytical equipment and only requires microliters of sample.

Highly Adhesive and Stretchable Epidermal Electrode for Bimodal Recording Patch.

For numerous biological and human-machine applications, it is critical to have a stable electrophysiological interface to obtain reliable signals. To achieve this, epidermal electrodes should possess conductivity, stretchability, and adhesiveness. However, limited types of materials can simultaneously satisfy these requirements to provide satisfying recording performance. Here, we present a dry electromyography (EMG) electrode based on conductive polymers and tea polyphenol (CPT), which offers adhesiveness (0.51 N/cm), stretchability (157%), and low impedance (14 kΩ cm2 at 100 Hz). The adhesiveness of the electrode is attributed to the interaction between catechol groups and hydroxyls in the polymer blend. This adhesive electrode ensures stable EMG recording even in the presence of vibrations and provides signals with a high signal-to-noise ratio (>25 dB) for over 72 h. By integrating the CPT electrode with a liquid metal strain sensor, we have developed a bimodal rehabilitation monitoring patch (BRMP) for sports injuries. The patch utilizes Kinesio Tape as a substrate, which serves to accelerate rehabilitation. It also tackles the challenge of recording with knee braces by fitting snugly between the brace and the skin, due to its thin and stretchable design. CPT electrodes not only enable BRMP to assist clinicians in formulating effective rehabilitation plans and offer patients a more comfortable rehabilitation experience, but also hold promise for future applications in biological and human-machine interface domains.

Dynamically tunable multi-band quasi-permittivity-asymmetric BIC in the GST225 metasurface.

Bound states in the continuum (BICs) on metasurfaces have garnered significant interest for their ultrahigh Q-factor potential in sensing applications. Reconfigurability and multi-band resonance are highly desirable for sensing systems. In this work, we introduce a metasurface comprising four nanocubes with different permittivity asymmetries, which can be dynamically adjusted using Ge2Sb2Te5 (GST225), a phase-change material, in the near-infrared (NIR) region. Additionally, a simulation for a liquid molecule sensor based on the metasurface shows a sensitivity of 1017 nm/RIU. This research introduces a novel, to the best of our knowledge, approach for designing multi-band, dynamically tunable quasi-BIC metasurfaces, which are good candidates for tunable, high-sensitivity biochemical sensing and nonlinear optics applications.

A Highly Sensitive Ethanol Liquid Sensor Based on No-Core Fiber Coated With Metal-Organic Framework

A Highly Sensitive Ethanol Liquid Sensor Based on No-Core Fiber Coated With Metal-Organic Framework

Microwave Dielectric Properties of Ge-substituted Nd(Ti0.5Mo0.5)O4 Ceramics for Application in Slot Antenna Liquid Sensor

Microwave Dielectric Properties of Ge-substituted Nd(Ti0.5Mo0.5)O4 Ceramics for Application in Slot Antenna Liquid Sensor

Facile barometric calibration of photoluminescence-based oxygen sensors on a standard rotary evaporator system with a digital vacuum pump

A simple setup and methodology for barometric calibration of quenched phosphorescence oxygen sensors are described, which utilize a standard rotary evaporator (Rotavap) system equipped with a digital vacuum pump. They were demonstrated with a panel of different sensing materials based on PtBP dye, including solid-state sensor coatings on different substrates (membrane inserts, stickers or inner wall of the vacuum flask), as well as liquid sensor formulations. The sensors inside the flask were exposed to different conditions created by the Rotavap system (dry or humid air, water, temperatures ranging 2°C-40°C, residual total pressure), and their intensity, lifetime or phase shift signals were measured with handheld sensor readers Optech (Mocon) and Firesting (PyroScience) in a contact-less manner to generate multi-point calibration curves. Several critical points of the method, which require careful consideration, have been revealed, including sensor mounting inside the Rotavap, its temperature control during calibration, choice of the reader. Finally, a detailed characterization of several sensor types was carried out and their key operational parameters (τ0, KSV, heterogeneity, T-coefficients), and calibration equations for the different conditions were determined and compared to each other and the results of conventional O2 calibration. Overall, barometric calibration method using Rotavap provides a useful alternative to conventional O2 calibration methods, being simple, accurate, convenient and versatile.

Symmetrical dual-D and dual-core single-mode fiber surface plasmon resonance liquid sensor

A symmetrical dual-D and dual-core single-mode fiber surface plasmon resonance (SPR) liquid sensor is designed for biological detection. The dual-core design optimizes the transmission path, improves the momentum matching between free electrons and photons, and facilitates bidirectional coupling, consequently amplifying the SPR effect and enabling sensitive monitoring of the refractive index changes of biological solutions. In this structure, a gold wire is placed in the middle of the polished surface of the double-D-shaped single-mode fiber (SMF) to produce high-quality free electrons and promote the mode-coupling excitation of the SPR effect. The characteristics of the sensor are analyzed by the finite element method, and the important structural parameters are optimized systematically. The sensor can be operated in the near-infrared region for a refractive index (RI) range of 1.31–1.40 with a maximum wavelength sensitivity (WS) of 21,000 nm/RIU, amplitude sensitivity of 586.62RIU−1, as well as resolution of 4.76×10−6RIU. Small changes in the refractive index can be detected by the sensor and it can be produced easily by conventional manufacturing techniques. The sensor thus has wide application prospects in biomedical and chemical analysis and related applications.

A Flexible, Architected Soft Robotic Actuator for Motorized Extensional Motion

To advance the design space of electrically‐driven soft actuators, a flexible, architected soft robotic actuator is presented for motor‐driven extensional motion. The actuator comprises a 3D printed, cylindrical handed shearing auxetic (HSA) structure and a deformable, internal rubber bellows shaft. The actuator linearly extends upon applying torque from a servo motor; the rubber bellows shaft is stretchable but resistant to torsional deflection, allowing it to transmit torque from the servo motor to the other end of the HSA. The high flexibility of the HSA and rubber bellows shaft enable the actuator to adaptively extend even when bent. The actuator's two components and its performance are mechanically characterized. Actuation strains of 45% elongation and a maximum blocked pushing force of about 8 N are demonstrated. The actuator's capabilities are showcased in two separate demonstrations: a crawling robot and a sensorized artificial muscle that integrates a microfluidic, liquid metal strain sensor. The architected material design approach for a robust, motor‐driven soft actuator provides several unique features—including a compact form factor and ease of use—over other motorized soft robotic actuators based on HSA assemblies or cable tendon mechanisms.

Electromigrated Gold Nanogap Tunnel Junction Arrays: Fabrication and Electrical Behavior in Liquid and Gaseous Media.

Tunnel junctions have been suggested as high-throughput electronic single molecule sensors in liquids with several seminal experiments conducted using break junctions with reconfigurable gaps. For practical single molecule sensing applications, arrays of on-chip integrated fixed-gap tunnel junctions that can be built into compact systems are preferable. Fabricating nanogaps by electromigration is one of the most promising approaches to realize on-chip integrated tunnel junction sensors. However, the electrical behavior of fixed-gap tunnel junctions immersed in liquid media has not been systematically studied to date, and the formation of electromigrated nanogap tunnel junctions in liquid media has not yet been demonstrated. In this work, we perform a comparative study of the formation and electrical behavior of arrays of gold nanogap tunnel junctions made by feedback-controlled electromigration immersed in various liquid and gaseous media (deionized water, mesitylene, ethanol, nitrogen, and air). We demonstrate that tunnel junctions can be obtained from microfabricated gold nanoconstrictions inside liquid media. Electromigration of junctions in air produces the highest yield (61-67%), electromigration in deionized water and mesitylene results in a lower yield than in air (44-48%), whereas electromigration in ethanol fails to produce viable tunnel junctions due to interfering electrochemical processes. We map out the stability of the conductance characteristics of the resulting tunnel junctions and identify medium-specific operational conditions that have an impact on the yield of forming stable junctions. Furthermore, we highlight the unique challenges associated with working with arrays of large numbers of tunnel junctions in batches. Our findings will inform future efforts to build single molecule sensors using on-chip integrated tunnel junctions.

Novel Green Screen-Printed Potentiometric Sensor for Monitoring Antihistamine Drug Chlorphenoxamine HCl in Various Matrices

The pharmaceutical sector is seeking cost-effective analyzers that deliver precise, real-time data. This study aims to establish a correlation between the pharmaceutical industry and advancements in solid-contact ion-selective electrodes for quantifying chlorphenoxamine hydrochloride (CPX) concentration in various matrices. A comparative analysis of the performance between solid contact and liquid contact sensors showed that solid contact sensors outperformed their liquid contact counterparts in terms of durability, handling, and ease of integration. A sensor was developed using MWCNT and calix[8]arene as ionophore, resulting in a Nernstian potentiometric response for CPX across a linear range of 5.0 × 10−5 to 1.0 × 10−8 M. The slope of the response was 57.89 ± 0.77 mV/decade, and the standard potential was determined to be 371.9 ± 0.8 mV. The developed sensor exhibits notable intrinsic advantages, such as a rapid response time of 12 ± 2 s and an extended lifespan of 3 months. The sensor exhibiting optimal performance has been effectively employed for the analysis of CPX in different matrices, including pharmaceutical formulations, urine, and plasma. The developed method underwent validation in compliance with ICH requirements. Finally, the method’s greenness and whiteness were evaluated using five different tools and successfully compared to those obtained from the established reported method.