Ion sensing with thread-based potentiometric electrodes

Nanodot zirconium trisulfide modified conducting thread: A smart substrate for fabrication of next generation biosensor

In present work, we report an eco-friendly, flexible and highly conducting cotton thread (CT) as a smart substrate for the development of biosensing platform towards ultrasensitive detection of swine flu serum amyloid A (SAA) biomarker. The biosensor was fabricated by optimized coating of CT with poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) conductive ink followed by incorporation of nanodot zirconium trisulfide (nZrS3) which helped in enhancing the electrochemical properties and improving stability of PEDOT:PSS polymeric film. The fabricated nZrS3/PEDOT:PSS/CT electrode was then used for sequential immobilization of monoclonal antibodies of SAA (anti-SAA) and bovine serum albumin (BSA). The synthesized nanomaterials and fabricated electrodes were characterized through X-ray diffraction, Fourier-transform infrared spectroscopy, transmission electron microscopy, scanning electron microscopy and contact angle analyser techniques. The electrochemical response of the fabricated smart thread based biosensor (BSA/anti-SAA/ZrS3/PEDOT:PSS/CT) was recorded against SAA using chronoamperometry technique which revealed superior sensitivity {30.2 μA [log (μg mL-1)]-1 cm-2}, excellent lower detection limit (0.72 ng mL-1) and prolonged shelf life up to 48 days. The response of the biosensor was also validated by analysing the electrochemical response of SAA spiked serum samples and the obtained results showed good correlation with that of standard samples.

Toward Personalized Treatment of Depression: An Affordable Citalopram Test based on a Solid-Contact Potentiometric Electrode for at-Home Monitoring of the Antidepressant Dosage

Citalopram (CTLP) is one of the most common antidepressants prescribed worldwide. It has a narrow therapeutic window and can cause severe toxicity and mortality if the dosage exceeds the safe level. Reports indicated that at-home monitoring of citalopram dosage considerably benefits the patients, yet there are no devices capable of such measurement of citalopram in biofluids. This work presents an affordable citalopram test for at-home and point-of-care monitoring of citalopram levels in urine, ensuring a safe and effective drug compliance. Our platform consists of a citalopram-selective yarn-based electrode (CTLP-SYE) that uses polymeric sensing membranes to provide valuable information about drug concentration in urine. CTLP-SYE is noninvasive and has a response time of fewer than 10 s. The fabricated electrode showed near-Nernstian behavior with a 52.3 mV/decade slope in citalopram hydrobromide solutions ranging from 0.5 μM to 1.0 mM, with a detection limit of 0.2 μM. Results also indicated that neither interfering ions nor pH affects electrode performance. We showed that CTLP-SYE could accurately and reproducibly measure citalopram in human urine (RSD 2.0 to 3.2%, error <12%) at clinically relevant concentrations. This work paves the way for the personalized treatment of depression and accessible companion diagnostics to improve treatment efficacy and safety.

FAST (Flexible Acetylcholine Sensing Thread): Real-Time Detection of Acetylcholine with a Flexible Solid-Contact Potentiometric Sensor

Acetylcholine (ACh) is involved in memory and learning and has implications in neurodegenerative diseases; it is therefore important to study the dynamics of ACh in the brain. This work creates a flexible solid-contact potentiometric sensor for in vitro and in vivo recording of ACh in the brain and tissue homogenate. We fabricate this sensor using a 250 μm diameter cotton yarn coated with a flexible conductive ink and an ACh sensing membrane that contains a calix[4]arene ionophore. The exposed ion-to-electron transducer was sealed with a 2.5 μm thick Parylene C coating to maintain the flexibility of the sensor. The resulting diameter of the flexible ACh sensing thread (FAST) was 400 μm. The FAST showed a linear response range from 1.0 μM to 10.0 mM in deionized water, with a near-Nernstian slope of 56.11 mV/decade and a limit of detection of 2.6 μM. In artificial cerebrospinal fluid, the limit of detection increased to 20 μM due to the background signal of ionic content of the cerebrospinal fluid. The FAST showed a signal stability of 226 μV/h over 24 h. We show that FAST can measure ACh dynamics in sheep brain tissue and sheep brain homogenate after ACh spiking. FAST is the first flexible electrochemical sensor for monitoring ACh dynamics in the brain.

Biosensor integrated brain-on-a-chip platforms: Progress and prospects in clinical translation

Because of the brain's complexity, developing effective treatments for neurological disorders is a formidable challenge. Research efforts to this end are advancing as in vitro systems have reached the point that they can imitate critical components of the brain's structure and function. Brain-on-a-chip (BoC) was first used for microfluidics-based systems with small synthetic tissues but has expanded recently to include in vitro simulation of the central nervous system (CNS). Defining the system's qualifying parameters may improve the BoC for the next generation of in vitro platforms. These parameters show how well a given platform solves the problems unique to in vitro CNS modeling (like recreating the brain's microenvironment and including essential parts like the blood-brain barrier (BBB)) and how much more value it offers than traditional cell culture systems. This review provides an overview of the practical concerns of creating and deploying BoC systems and elaborates on how these technologies might be used. Not only how advanced biosensing technologies could be integrated with BoC system but also how novel approaches will automate assays and improve point-of-care (PoC) diagnostics and accurate quantitative analyses are discussed. Key challenges providing opportunities for clinical translation of BoC in neurodegenerative disorders are also addressed.

An Accessible Yarn-Based Sensor for In-Field Detection of Succinylcholine Poisoning

Succinylcholine (SUX) is a clinical anesthetic that induces temporary paralysis and is degraded by endogenous enzymes within the body. In high doses and without respiratory support, it results in rapid and untraceable death by asphyxiation. A potentiometric thread-based method was developed for the in-field and rapid detection of SUX for forensic use. We fabricated the first solid-contact SUX ion-selective electrodes from cotton yarn, a carbon black ink, and a polymeric ion-selective membrane. The electrodes could selectively measure SUX in a linear range of 1 mM to 4.3 μM in urine, with a Nernstian slope of 27.6 mV/decade. Our compact and portable yarn-based SUX sensors achieved 94.1% recovery at low concentrations, demonstrating feasibility in real-world applications. While other challenges remain, the development of a thread-based ion-selective electrode for SUX detection shows that it is possible to detect this poison in urine and paves the way for other low-cost, rapid forensic diagnostic devices.

Home-Based Electrochemical Rapid Sensor (HERS): A Diagnostic Tool for Bacterial Vaginosis

Bacterial vaginosis (BV) is the most frequently occurring vaginal infection worldwide, yet it remains significantly underdiagnosed as a majority of patients are asymptomatic. Untreated BV poses a serious threat as it increases one's risk of STI acquisition, pregnancy complications, and infertility. We aim to minimize these risks by creating a low-cost disposable sensor for at-home BV diagnosis. A clinical diagnosis of BV is most commonly made according to the Amsel criteria. In this method, a fish-like odor, caused by increased levels of trimethylamine (TMA) in vaginal fluid, is used as a key diagnostic. This paper outlines the development of a Home-Based Electrochemical Rapid Sensor (HERS), capable of detecting TMA in simulated vaginal fluid (sVF). Instead of odor-based detection of volatilized TMA, we identify TMA in trimethylammonium form by utilizing HERS and a potentiometric readout. We fabricated the ion selective electrode using a carbon-black-coated cotton string and a TMA-selective membrane consisting of calix[4]arene and sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate. When paired with a standard reference electrode, our device was able to quantify TMA concentration in deionized (DI) water, as well as sVF samples at multiple pH levels with a clinically relevant limit of detection (8.66 µM, and theoretically expected Nernstian slope of 55.14 mV/decade).

A Critical Review for Real-Time Continuous Soil Monitoring: Advantages, Challenges, and Perspectives

Most soil quality measurements have been limited to laboratory-based methods that suffer from time delay, high cost, intensive labor requirement, discrete data collection, and tedious sample pretreatment. Real-time continuous soil monitoring (RTCSM) possesses a great potential to revolutionize field measurements by providing first-hand information for continuously tracking variations of heterogeneous soil parameters and diverse pollutants in a timely manner and thus enable constant updates essential for system control and decision-making. Through a systematic literature search and comprehensive analysis of state-of-the-art RTCSM technologies, extensive discussion of their vital hurdles, and sharing of our future perspectives, this critical review bridges the knowledge gap of spatiotemporal uninterrupted soil monitoring and soil management execution. First, the barriers for reliable RTCSM data acquisition are elucidated by examining typical soil monitoring techniques (e.g., electrochemical and spectroscopic sensors). Next, the prevailing challenges of the RTCSM sensor network, data transmission, data processing, and personalized data management are comprehensively discussed. Furthermore, this review explores RTCSM data application for updating diverse strategies including high-fidelity soil process models, control methodologies, digital soil mapping, soil degradation, food security, and climate change mitigation. Finally, the significance of RTCSM implementation in agricultural and environmental fields is underscored through illuminating future directions and perspectives in this systematic review.

Carbon composite thermoplastic electrodes integrated with mini-printed circuit board for wireless detection of calcium ions

Here, a smartphone-based portable sensing system is developed for real-time detection of Ca²⁺ ions in a variety of biofluids. A solid-contact calcium-selective electrode (Ca²⁺-ISE) consisting of an ion-selective membrane (ISM), carbon black nanomaterial and polystyrene-graphite nanoplatelets as a solid contact was fabricated. The polyvinylchloride (PVC)-based ISM was optimized using different plasticizers and ion-exchangers. Under optimized conditions, the solid contacts were electrochemically characterized by electrochemical impedance spectroscopy (EIS), chronopotentiometric and potentiometric measurements. The Ca²⁺-ISE showed a Nernst response with a slope of 31.2 ± 0.6 mV/decade in the concentration range from 0.1 M to 10^-4 M Ca²⁺ with a limit of detection (LOD) of 1.0 × 10^-5 M. In addition, the ISEs exhibited good selectivity to Ca²⁺ ions over various interfering electrolytes and metabolites. The Ca²⁺-ISEs were applied in human urine and, artificial serum and cerebrospinal fluid samples. The ISEs showed good recoveries between 90 and 105%, indicating potential applicability of these electrodes in biological fluids. The portable lab-made potentiometer provides wireless data signaling and transmission to a smartphone and final Ca²⁺ concentration display due to its customized software. Therefore, the developed smartphone-based sensing platform offers low cost (< $25), rapid, user-friendly detection of Ca²⁺ especially in resource-limited areas.

Ion-Selective Organic Electrochemical Transistors: Recent Progress and Challenges

The charged species inside biofluids (blood, interstitial fluid, sweat, saliva, urine, etc.) can reflect the human body's physiological conditions and thus be adopted to diagnose various diseases early. Among all personalized health management applications, ion-selective organic electrochemical transistors (IS-OECTs) have shown tremendous potential in point-of-care testing of biofluids due to low cost, ease of fabrication, high signal amplification, and low detection limit. Moreover, IS-OECTs exhibit excellent flexibility and biocompatibility that enable their application in wearable bioelectronics for continuous health monitoring. In this review, the working principle of IS-OECTs and the recent studies of IS-OECTs for performance improvement are reviewed. Specifically, contemporary studies on material design and device optimization to enhance the sensitivity of IS-OECTs are discussed. In addition, the progress toward the commercialization of IS-OECTs is highlighted, and the recently proposed solutions or alternatives are summarized. The main challenges and perspectives for fully exploiting IS-OECTs toward future preventive and personal medical devices are addressed.

3D Printed Ion-Selective Membranes and Their Translation into Point-of-Care Sensors

This technical note describes a method for fabricating ion-selective membranes (ISMs) for use in potentiometric sensing by using 3D printing technology. Here, we demonstrate the versatility of this approach by fabricating ISMs and investigating their performance in both liquid-contact and solid-contact ion-selective electrode (ISE) configurations. Using 3D printed ISMs resulted in highly stable (drift of ∼17 μV/h) and highly reproducible (<1 mV deviation) measurements. Furthermore, we show the seamless translation of these membranes into reliable, carbon fiber- and paper-based potentiometric sensors for applications at the point-of-care. To highlight the modifiability of this approach, we fabricated sensors for bilirubin, an important biomarker of liver health; benzalkonium, a common preservative used in the pharmaceutical industry; and potassium, an important blood electrolyte. The ability to mass produce sensors using 3D printing is an attractive advantage over conventional methods, while also decreasing the time and cost associated with sensor fabrication.

On-Chip Optical Anodic Stripping with Closed Bipolar Cells and Cathodic Electrochemiluminescence Reporting

The aim of this work was to develop a simple, accessible, and point-of-use sensor to measure heavy metal ions in water in low-resource areas that cannot accommodate expensive or technical solutions. This report describes a new bipolar electrochemical sensor platform that reimagines conventional anodic stripping voltammetry in a wireless bipolar format with an optical electrochemiluminescent readout that can be quantified with any simple optical sensor like that found on most modern cell phone cameras. We call this technique as optical anodic stripping. Using a new nonlithographic fabrication process, devices could be produced rapidly and simply at <$1/sensor. The sensing scheme was developed, characterized, and optimized using electrochemical and optical methods. Quantitation of Pb²⁺ in both lab and natural water samples was rapid (2-3 min), accurate, precise, and highly linear in the 25-1000 ppb range and was shown to be sufficiently selective in the presence of other common heavy metal ions such as Cu²⁺, Cd²⁺, and Zn²⁺.

A colorimetric paper-based optode sensor for highly sensitive and selective determination of thiocyanate in urine sample using cobalt porphyrin derivative

In this work, a highly sensitive colorimetric paper-based optode for the determination of thiocyanate in urine samples was developed for the first time. The cocktail solution of the optode was composed of 5,10,15,20-tetrakis(4-octyloxyphenyl)porphyrin cobalt(II) complex (L), tridodecylmethylammonium chloride (TDMACl), 2-nitrophenyl octyl ether, and polyvinyl chloride as an ionophore, an ion exchanger, a plasticizer, and a polymer, respectively. The paper-based optode responded to thiocyanate by increasing the blue component in the RGB index and a visible change, with the naked-eye, of the optode color from pink to green was observed. From the central composite design, the optimized conditions that yielded the highest sensitivity were 4.70 mmol/kg TDMACl and 13.75 mmol/kg L. The developed optode sensor was highly selective and responded to thiocyanate over other anions, with a working range of 0.001-5 mM and with a coefficient of determination (R²) of 0.9915. The limits of detection using naked-eye and camera were determined to be 50.0 μM and 1.26 μM, respectively. In addition, the LOD and LOQ estimated from the standard deviation of the blank were 0.65 and 1.87 μM, respectively. Furthermore, this sensor was successfully applied to the detection of thiocyanate in urine samples from non-smokers and smokers. The results were in good agreement with the standard ion chromatography (IC) technique. This developed paper-based optode sensor was simple, low-cost, portable, and easy to use as a sensing device without any complicated instrument.

Microneedle-Based Potentiometric Sensing System for Continuous Monitoring of Multiple Electrolytes in Skin Interstitial Fluids

Electrolytes play a pivotal role in regulating cardiovascular functions, hydration, and muscle activation. The current standards for monitoring electrolytes involve periodic sampling of blood and measurements using laboratory techniques, which are often uncomfortable/inconvenient to the subjects and add considerable expense to the management of their underlying disease conditions. The wide range of electrolytes in skin interstitial fluids (ISFs) and their correlations with those in plasma create exciting opportunities for applications such as electrolyte and circadian metabolism monitoring. However, it has been challenging to monitor these electrolytes in the skin ISFs. In this study, we report a minimally invasive microneedle-based potentiometric sensing system for multiplexed and continuous monitoring of Na⁺ and K⁺ in the skin ISFs. The potentiometric sensing system consists of a miniaturized stainless-steel hollow microneedle to prevent sensor delamination and a set of modified microneedle electrodes for multiplex monitoring. We demonstrate the measurement of Na⁺ and K⁺ in artificial ISFs with a fast response time, excellent reversibility and repeatability, adequate selectivity, and negligible potential interferences upon the addition of a physiologically relevant concentration of metabolites, dietary biomarkers, and nutrients. In addition, the sensor maintains the sensitivity after multiple insertions into the chicken skin model. Furthermore, the measurements in artificial ISFs using calibrated sensors confirm the accurate measurements of physiological electrolytes in artificial ISFs. Finally, the skin-mimicking phantom gel and chicken skin model experiments demonstrate the sensor's potential for minimally invasive monitoring of electrolytes in skin ISFs. The developed sensor platform can be adapted for a wide range of other applications, including real-time monitoring of nutrients, metabolites, and proteins.

Microfabricated potentiometric sensor for personalized methacholine challenge tests during the COVID-19 pandemic

The methacholine challenge test is considered to be the gold standard bronchoprovocation test used to diagnose asthma, and this test is always performed in pulmonary function labs or doctors' offices. Methacholine (MCH) acts by inducing airway tightening/bronchoconstriction, and more importantly, MCH is hydrolyzed by cholinesterase enzyme (ChE). Recently, the American Thoracic Society raised concerns about pulmonary function testing during the COVID-19 pandemic due to recently reported correlation between cholinesterase and COVID-19 pneumonia severity/mortality, and it was shown that cholinesterase levels are reduced in the acute phase of severe COVID-19 pneumonia. This work describes the microfabrication of potentiometric sensors using copper as the substrate and chemically polymerized graphene nanocomposites as the transducing layer for tracking the kinetics of MCH enzymatic degradation in real blood samples. The in-vitro estimation of the characteristic parameters of the MCH metabolism [Michaelis-Menten constant (K_m) and reaction velocity (V_max)] were found to be 241.041 μM and 56.8 μM/min, respectively. The proposed sensor is designed to be used as a companion diagnostic device that can (i) answer questions about patient eligibility to perform methacholine challenge tests, (ii) individualize/personalize medical dosing of methacholine, (iii) provide portable and inexpensive devices allowing automated readouts without the need for operator intervention (iv) recommend therapeutic interventions including intensive care during early stages and reflecting the disease state of COVID-19 pneumonia. We hope that this methacholine electrochemical sensor will help in assaying ChE activity in a "timely" manner and predict the severity and prognosis of COVID-19 to improve treatment outcomes and decrease mortality.

Electrode-Integrated Textile-Based Sensors for In Situ Temperature and Relative Humidity Monitoring in Electrochemical Cells

Temperature and humidity measurements in electrochemical energy devices are essential for maximizing their overall performance under different operating conditions and avoiding hazardous consequences that may arise from the malfunction of these systems. Using sensors for in situ measurements of temperature and relative humidity (RH) is a promising approach for continuous monitoring and management of electrochemical power sources. Here, we report on the feasibility of using thread-based sensors for in situ measurements of temperature and RH in proton exchange membrane fuel cells (PEMFCs) as an example of electrochemical energy devices. Commodity threads are low-cost and flexible materials that hold great promise for the creation of complex three-dimensional (3D) circuits using well-established textile methods such as weaving, braiding, and embroidering. Ex situ and in situ characterization show that threads can be introduced in the gas diffusion layer (GDL) structure to inscribe water highways within the GDL with minimal impact on the GDL microstructure and transport properties. Fluorinated ethylene propylene (FEP) is coated on thread-based sensors to decouple the response to temperature and humidity; the resulting threads achieve a linear change of resistance with temperature (-0.31%/°C), while RH is monitored with a second thread coated with poly(dimethylsiloxane) (PDMS). The combination of both threads allows for minimally invasive and dynamically responsive monitoring of local temperature and RH within the electrode of PEMFCs.

Public-Health-Driven Microfluidic Technologies: From Separation to Detection

Separation and detection are ubiquitous in our daily life and they are two of the most important steps toward practical biomedical diagnostics and industrial applications. A deep understanding of working principles and examples of separation and detection enables a plethora of applications from blood test and air/water quality monitoring to food safety and biosecurity; none of which are irrelevant to public health. Microfluidics can separate and detect various particles/aerosols as well as cells/viruses in a cost-effective and easy-to-operate manner. There are a number of papers reviewing microfluidic separation and detection, but to the best of our knowledge, the two topics are normally reviewed separately. In fact, these two themes are closely related with each other from the perspectives of public health: understanding separation or sorting technique will lead to the development of new detection methods, thereby providing new paths to guide the separation routes. Therefore, the purpose of this review paper is two-fold: reporting the latest developments in the application of microfluidics for separation and outlining the emerging research in microfluidic detection. The dominating microfluidics-based passive separation methods and detection methods are discussed, along with the future perspectives and challenges being discussed. Our work inspires novel development of separation and detection methods for the benefits of public health.

Point-of-care diagnostics for infectious diseases: From methods to devices

The current widespread of COVID-19 all over the world, which is caused by SARS-CoV-2 virus, has again emphasized the importance of development of point-of-care (POC) diagnostics for timely prevention and control of the pandemic. Compared with labor- and time-consuming traditional diagnostic methods, POC diagnostics exhibit several advantages such as faster diagnostic speed, better sensitivity and specificity, lower cost, higher efficiency and ability of on-site detection. To achieve POC diagnostics, developing POC detection methods and correlated POC devices is the key and should be given top priority. The fast development of microfluidics, micro electro-mechanical systems (MEMS) technology, nanotechnology and materials science, have benefited the production of a series of portable, miniaturized, low cost and highly integrated POC devices for POC diagnostics of various infectious diseases. In this review, various POC detection methods for the diagnosis of infectious diseases, including electrochemical biosensors, fluorescence biosensors, surface-enhanced Raman scattering (SERS)-based biosensors, colorimetric biosensors, chemiluminiscence biosensors, surface plasmon resonance (SPR)-based biosensors, and magnetic biosensors, were first summarized. Then, recent progresses in the development of POC devices including lab-on-a-chip (LOC) devices, lab-on-a-disc (LOAD) devices, microfluidic paper-based analytical devices (μPADs), lateral flow devices, miniaturized PCR devices, and isothermal nucleic acid amplification (INAA) devices, were systematically discussed. Finally, the challenges and future perspectives for the design and development of POC detection methods and correlated devices were presented. The ultimate goal of this review is to provide new insights and directions for the future development of POC diagnostics for the management of infectious diseases and contribute to the prevention and control of infectious pandemics like COVID-19.

Electrochemical Sensing in 3D Cell Culture Models: New Tools for Developing Better Cancer Diagnostics and Treatments

Currently, conventional pre-clinical in vitro studies are primarily based on two-dimensional (2D) cell culture models, which are usually limited in mimicking the real three-dimensional (3D) physiological conditions, cell heterogeneity, cell to cell interaction, and extracellular matrix (ECM) present in living tissues. Traditionally, animal models are used to mimic the 3D environment of tissues and organs, but they suffer from high costs, are time consuming, bring up ethical concerns, and still present many differences when compared to the human body. The applications of microfluidic-based 3D cell culture models are advantageous and useful as they include 3D multicellular model systems (MCMS). These models have demonstrated potential to simulate the in vivo 3D microenvironment with relatively low cost and high throughput. The incorporation of monitoring capabilities in the MCMS has also been explored to evaluate in real time biophysical and chemical parameters of the system, for example temperature, oxygen, pH, and metabolites. Electrochemical sensing is considered as one of the most sensitive and commercially adapted technologies for bio-sensing applications. Amalgamation of electrochemical biosensing with cell culture in microfluidic devices with improved sensitivity and performance are the future of 3D systems. Particularly in cancer, such models with integrated sensing capabilities can be crucial to assess the multiple parameters involved in tumour formation, proliferation, and invasion. In this review, we are focusing on existing 3D cell culture systems with integrated electrochemical sensing for potential applications in cancer models to advance diagnosis and treatment. We discuss their design, sensing principle, and application in the biomedical area to understand the potential relevance of miniaturized electrochemical hybrid systems for the next generation of diagnostic platforms for precision medicine.

A thread/fabric-based band as a flexible and wearable microfluidic device for sweat sensing and monitoring

Flexible biosensors for monitoring systems have emerged as a promising portable diagnostics platform due to their potential for in situ point-of-care (POC) analytic devices. Assessment of biological analytes in sweat can provide essential information for human physiology. Conventional measurements rely on laboratory equipment. This work exploits an alternative approach for epidermal sweat sensing and detection through a wearable microfluidic thread/fabric-based analytical device (μTFAD). This μTFAD is a flexible and skin-mounted band that integrates hydrophilic dot-patterns with a hydrophobic surface via embroidering thread into fabric. After chromogenic reaction treatment, the thread-embroidered patterns serve as the detection zones for sweat transferred by the hydrophilic threads, enabling precise analysis of local sweat loss, pH and concentrations of chloride and glucose in sweat. Colorimetric reference markers embroidered surrounding the working dots provide accurate data readout either by apparent color comparison or by digital acquirement through smartphone-assisted calibration plots. On-body tests were conducted on five healthy volunteers. Detection results of pH, chloride and glucose in sweat from the volunteers were 5.0-6.0, 25-80 mM and 50-200 μM by apparent color comparison with reference markers through direct visual observation. Similar results of 5.47-6.30, 50-77 mM and 47-66 μM for pH, chloride and glucose were obtained through calibration plots based on the RGB values from the smartphone app Lanse®. The limit of detection (LOD) is 10 mM for chloride concentration, 4.0-9.0 for pH and 10 μM for glucose concentration, respectively. For local sweat loss, it is found that the forehead is the region of heavy sweat loss. Sweat secretion is a cumulating process with a lower sweat rate at the beginning which increases as body movement continues along with increased heat production. These results demonstrate the capability and availability of our sensing device for quantitative detection of multiple biomarkers in sweat, suggesting the great potential for development of feasible non-invasive biosensors, with a similar performance to conventional measurements.

Paper-Based Screen-Printed Electrodes: A New Generation of Low-Cost Electroanalytical Platforms

Screen-printed technology has helped considerably to the development of portable electrochemical sensors since it provides miniaturized but robust and user-friendly electrodes. Moreover, this technology allows to obtain very versatile transducers, not only regarding their design, but also their ease of modification. Therefore, in the last decades, the use of screen-printed electrodes (SPEs) has exponentially increased, with ceramic as the main substrate. However, with the growing interest in the use of cheap and widely available materials as the basis of analytical devices, paper or other low-cost flat materials have become common substrates for SPEs. Thus, in this revision, a comprehensive overview on paper-based SPEs used for analytical proposes is provided. A great variety of designs is reported, together with several examples to illustrate the main applications.

PHAIR: a biosensor for pH measurement in air-liquid interface cell culture

In many biological systems, pH can be used as a parameter to understand and study cell dynamics. However, measuring pH in live cell culture is limited by the sensor ion specificity, proximity to the cell surface, and scalability. Commercially available pH sensors are difficult to integrate into a small-scale cell culture system due to their size and are not cost-effective for disposable use. We made PHAIR-a new pH sensor that uses a micro-wire format to measure pH in vitro human airway cell culture. Tungsten micro-wires were used as the working electrodes, and silver micro-wires with a silver/silver chloride coating were used as a pseudo reference electrode. pH sensitivity, in a wide and narrow range, and stability of these sensors were tested in common standard buffer solutions as well as in culture media of human airway epithelial cells grown at the air-liquid interface in a 24 well cell culture plate. When measuring the pH of cells grown under basal and challenge conditions using PHAIR, cell viability and cytokine responses were not affected. Our results confirm that micro-wire-based sensors have the capacity for miniaturization and detection of diverse ions while maintaining sensitivity. This suggests the broad application of PHAIR in various biological experimental settings.

Determination of sodium and potassium ions in patients with SARS-Cov-2 disease by ion-selective electrodes based on polyelectrolyte complexes as a pseudo-liquid contact phase

Supramolecular assemblies based on polyelectrolyte complexes made it possible to create complex interfaces with predictable properties. Polyelectrolyte complexes serve as a pseudo-liquid contact in ion-selective electrodes.

Exploring carbon particle type and plasma treatment to improve electrochemical properties of stencil-printed carbon electrodes

Stencil-printing conductive carbon inks has revolutionized the development of inexpensive, disposable and portable electrochemical sensors. However, stencil-printed carbon electrodes (SPCEs) typically suffer from poor electrochemical properties. While many surface pretreatments and modifications have been tested to improve the electrochemical activity of SPCEs, the bulk composition of the inks used for printing has been largely ignored. Recent studies of other carbon composite electrode materials show significant evidence that the conductive carbon particle component is strongly related to electrochemical performance. However, such a study has not been carried out with SPCEs. In this work, we perform a systematic characterization of SPCEs made with different carbon particle types including graphite particles, glassy carbon microparticles and carbon black. The relationship between carbon particle characteristics including particle size, particle purity, and particle morphology as well as particle mass loading on the fabrication and electrochemical properties of SPCEs is studied. SPCEs were plasma treated for surface activation and the electrochemical properties of both untreated and plasma treated SPCEs are also compared. SPCEs displayed distinct analytical utilities characterized through solvent window and double layer capacitance. Cyclic voltammetry (CV) of several standard redox probes, FcTMA+, ferri/ferrocyanide, and pAP was used to establish the effects of carbon particle type and plasma treatment on electron transfer kinetics of SPCEs. CV of the biologically relevant molecules uric acid, NADH and dopamine was employed to further illustrate the differences in sensing and fouling characteristics of SPCEs fabricated with different carbon particle types. SEM imaging revealed significant differences in the SPCE surface microstructures. This systematic study demonstrates that the electrochemical properties of SPCEs can be tuned and significantly improved through careful selection of carbon particle type and plasma cleaning with a goal toward the development of better performing electrochemical point-of-need sensors.

Real Time Sensing of Soil Potassium Levels Using Zinc Oxide-Multiwall Carbon Nanotube-Based Sensors

There is a significant interest in the detection and monitoring of nutrient levels in agriculture farms. In this article, we report the fabrication of Zinc oxide (ZnO) modified multi-walled Carbon nanotube (F-MWCNT) sensor specifically developed for soil nutrient sensing. A thin layer of Valinomycin membrane was grown on the top of the F-MWCNT/ZnO nanocomposite active layer. The resulting composite structure Al/F-MWCNT/ZnO/Valinomycin was found to have a proportional impedance change with soil Potassium (K⁺) levels. The performance of the sensor was investigated in the 1- 5 kHz range. The impedance magnitude was found to linearly decrease from 2.5± 0.23 to [Formula: see text] range for K⁺ concentrations from 5 to 25 mM displaying a sensitivity of [Formula: see text]/mM with a correlation coefficient (R²) of 0.95744.

Characterization of nano-layered solid-contact ion selective electrodes by simultaneous potentiometry and quartz crystal microbalance with dissipation

Nano-layered solid-contact potassium-selective electrodes (K⁺-ISEs) were explored as model ion-selective electrodes for their practical use in clinical analysis. The ultra-thin ISEs ought to be manufactured in a highly reproducible manner, potentially making them suitable for mass production. Thus, their development is pivotal towards miniaturised sensors with simplified conditioning/calibration protocols for point-of-care diagnostics. To study nano-layered ISEs, the ultra-thin nature of ISEs for the first time enabled to combine potentiometry-quartz crystal microbalance with dissipation (QCM-D) to obtain value-added information on the ISE potentiometric response regarding their physical state such as mass/thickness/viscoelastic properties/structural homogeneity. Specifically, the studies were focused on real-time observations of the ISE potentiometric response in relation to changes of their physicochemical properties during the ISE preparation (conditioning) and operation (including biofouling conditions) to identify the occurring processes that may accordingly be critical for potential instability of the ISEs, impeding their practical application. The K⁺-ISEs were prepared on a QCM-D gold sensor by electrodepositing poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) layer serving as an ion-to-electron transducer subsequently covered by a spin-coated poly(vinyl chloride) based K⁺-ion selective membrane (K⁺-ISM). The studies demonstrated that the performance of the nano-layered design of K⁺-ISEs is detrimentally affected by such processes as water layer formation accordingly causing the instability of the electrode potential. The changes in the ISE physical state such mass/viscoelastic properties associated with water layer formation and origin of the potential instability was already observed at the ISE conditioning stage. The potential instability of nano-layered ISEs limits their practical applicability, indicating the need of new solutions in designing ISEs, for instance, exploiting new water-resistant materials and modifying preparation protocols.

Solid-Contact Ion-Selective and Reference Electrodes Covalently Attached to Functionalized Poly(ethylene terephthalate)

Numerous ion-selective and reference electrodes have been developed over the years. Following the need for point-of-care and wearable sensors, designs have transitioned recently from bulky devices with an aqueous inner filling solution to planarizable solid-contact electrodes. However, unless the polymeric sensing and reference membranes are held in place mechanically, delamination of these membranes from the underlying solid to which they adhere physically limits sensor lifetime. Even minor external mechanical stress or thermal expansion can result in membrane delamination and, thereby, device failure. To address this problem, we designed a sensing platform based on poly(ethylene terephthalate) substrates to which polyacrylate-based sensing and polymethacrylate-based reference membranes are attached covalently. Ion-selective membranes with covalently attached or freely dissolved ionophore- and ionic-liquid-doped reference membranes can be directly photopolymerized onto surface-functionalized poly(ethylene terephthalate), resulting in the formation of covalent bonds between the underlying substrate and the attached membranes. H⁺- and K⁺-selective electrodes thus prepared exhibit highly selective responses with the theoretically expected (Nernstian) response slope, and reference electrodes provide sample-independent reference potentials over a wide range of electrolyte concentrations. Even repeated mechanical stress does not result in the delamination of the sensing and reference membranes, leading to electrodes with much improved long-term performance. As demonstrated for poly(ethylene-co-cyclohexane-1,4-dimethanol terephthalate) (PETG), this approach may be expanded to a wide range of other polyester, polyamide, and polyurethane platform materials. Covalent attachment of sensing and reference membranes to an inert plastic platform material is a very promising approach to a problem that has plagued the field of ion-selective electrodes and field effect transistors for over 30 years.

USB multiplex analyzer employing screen-printed silver electrodes on paper substrate; A developed design for dissolution testing

Multiplex ion analyzers have been introduced recently for the assay of several inorganic ions, whilst electrochemists have extensively employed screen printed sensors for pharmaceutical analyses. This work aims to develop a USB pluggable sensor with a user-friendly design for multiplex analysis of oppositely charged co-formulated organic ions. The miniaturized screen-printed electrode was developed using silver ink on paper substrate. A compact sensor design was attained by including three electrodes, a single reference electrode along with an indicator electrode for each of the determined ions. Optimized PVC membranes were drop-casted over each of the indicator electrodes for the determination of phenylephrine HCl (PHE) and ibuprofen (IBU). The proposed multiplex potentiometric sensors exhibit Nernstian slopes of 59.2 ± 0.26 and -56.8 ± 0.16 mV/decade for PHE and IBU, respectively, with respective detection limits of 1.6 × 10^-7 and 6.53 × 10^-8 mol L^-1. The fast and stable response of the developed sensor enabled the real-time monitoring of the combined dosage form dissolution. The dissolution profiles obtained by this potentiometric analyzer and an off-line separation technique were compared favourably, albeit our proposed in-line sensor reduced waste and time of analysis. The developed method successfully complies with the most demanding stipulations of green analytical chemistry.

Instrument-free detection of polyphenols with a thread-based analytical device

We described an instrument-free method for quantitative analysis of the total content of tea polyphenols by measurement of the length of a coloured band. Polyphenols react with ferrous ions to form a colourless ferrous-polyphenols complex on cotton threads, which could be adsorbed on the threads. The complex was then oxidized to form a blue-black ferric-polyphenols complex, generating a blue-black band on the cotton thread. The length of this blue-black band was then measured to detect the total content of polyphenols. The advantages of this method include low cost, rapid analysis, low consumption, easy fabrication and operation. Furthermore, the digital instrument (scanner or camera) as well as the image processing software are not required. This proposed method was used to detect polyphenols in tea leaf extracts with an analytical result agreeing well with that obtained by a standard method, which demonstrates its potential in monitoring of tea leaf quality, especially in resource-limited regions and settings.

Recent advances in solid-contact ion-selective electrodes: functional materials, transduction mechanisms, and development trends

This article presents a comprehensive overview of recent progress in the design and applications of solid-contact ion-selective electrodes (SC-ISEs).

Highly Flexible Transistor Threads for All-Thread Based Integrated Circuits and Multiplexed Diagnostics

Physically intimate, real-time monitoring of human biomarkers is becoming increasingly important to modern medicine and patient wellness. Such monitoring is possible due to advances in soft and flexible materials, devices and bioelectronics systems. Compared to other flexible platforms, multifilament textile fibers or threads offer superior flexibility, material diversity, and simple ambient processing to realize a wide range of flexible devices such as sensors, electronics, and microfluidics. In this paper, we realize unique flexible transistors on threads and interconnect them to realize logic gates and small-scale integrated circuits. Compared to prior textile-based transistors, the proposed thread-based transistors (TBTs) are realized with a readily shaped, colloidally dispersed gel consisting of silica nanoparticles and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI TFSI) ionic liquid for all-around electrolyte gating of a carbon nanotube (CNT) semiconducting network assembled on the thread. We interconnect TBTs with thread-based electrochemical sensors (TBEs) to realize an all-thread based multiplexed diagnostic device. All-thread based platforms are thin, highly flexible and conformal, allowing them to be worn directly on the skin without any polymeric substrate, or sutured transdermally using a needle.

Dual-Analyte Chronopotentiometric Aptasensing Platform Based on a G-Quadruplex/Hemin DNAzyme and Logic Gate Operations

Conventional potentiometric ion sensors that rely on a specific ion carrier in a polymeric membrane can hardly achieve multianalyte detection. Inspired by the remarkable ability of built-in logic gate sensors for multianalyte detection, herein we report a potentiometric aptasensing platform based on a G-quadruplex/hemin DNAzyme and logic gate operations for determination of two analytes using a single membrane electrode. A bifunctional probe with two aptamer units and a signal reporter oligonucleotide with a DNAzyme sequence are assembled on the magnetic beads to form a DNA hybrid structure. The "OR" and "INHIBIT" logic functions can be performed by using the two aptamers and their targets as inputs, and using the chronopotentiometric response based on the G-quadruplex/hemin DNAzyme-H₂O₂-mediated oxidation of 3,3',5,5'-tetramethylbenzidine as output. Kanamycin and oxytetracycline, as commonly used antibiotics, have been employed as the models and successfully measured.