Unveiling the fast adsorption and desorption of heavy metals on/off nanoplastics by real-time in-situ potentiometric sensing

Nanoplastics (<1 μm) can serve as a transport vector of environmental pollutants (e.g., heavy metals) and change their toxicities and bioavailabilities. Up to date the behaviors of adsorption and desorption heavy metals on/off nanoplastics are largely unknown. Herein, polymeric membrane potentiometric ion sensors are proposed for in-situ assessment of the real-time kinetics of heavy metal adsorption and desorption on/off nanoplastics. Results show that nanoplastics can adsorb and release heavy metals in a fast manner, indicating their superior ability in transferring heavy metals. The adsorption behaviors are closely related to the characteristics of nanoplastics and background electrolytes. Particle aggregation and increases in salinity and acidity suppress the adsorption of heavy metals on nanoplastics. The desorption efficiencies of different heavy metals are Pb2+ (31 %) < Cu2+ (40 %) < Cd2+ (97 %). Our proposed method is applicable for the detection of the plastic pollutants with size <100 nm and of the samples with high salinities (e.g., seawater). This work would provide new insights into the assessment of environmental risks posed by nanoplastics and heavy metals.

Miniaturized inkjet-printed flexible ion-selective sensing electrodes with the addition of graphene in PVC layer for fast response real-time monitoring applications

In this letter, we propose a miniaturization scheme of inkjet printed ionic sensing electrodes by adding graphene into the ion-selective PVC film not only to reduce the impedance of the ionic liquid layer of the electrode but also to increase the electrode capacitance for the reduction of the response time. Based on the scheme, we present a fully inkjet-printed electrochemical ion-selective sensor comprising a working electrode and reference electrode, which are inkjet-printed Ag NPs/PEDOT:PSS-graphene/PVC-graphene and Ag/AgCl(s)/ionic liquid PVC-graphene layer structures, respectively. The printed ion-selective working electrode has been miniaturized to a size of 22,400 μm2 equivalent to a square shape of ∼150 × 150 μm2 comparable to the size of a human cell. By adding graphene to the ion selective PVC film, more than 90 % charge transfer resistance reduction can be achieved and the shunt capacitance is increased by 3.4-fold in shunt capacitance compared to the film without graphene, thereby more than 33 % reduction of the response time required to reach equilibrium. Meanwhile, these miniaturized potassium sensors using the working electrodes with/without adding graphene have been integrated with in-lab signal-processing and wireless-transmission module to yield similar results to the one measured by commercial electrochemical workstation showing a great potential for real-time monitoring in portable clinical trials. Specifically, the proposed sensor utilizing graphene-enhanced electrodes demonstrates a linearity uncertainty of 2.9 mV, which is approximately half of the uncertainty observed in the sensors lacking graphene integration.

Cu1.4Mn1.6O4 as a bifunctional transducer for potentiometric Cu2+ solid-contact ion-selective electrode

Current potentiometric Cu2+ sensors mostly rely on polymer-membrane-based solid-contact ion-selective electrodes (SC-ISEs) that constitute ion-selective membranes (ISM) and solid contact (SC) for respective ion recognition and ion-to-electron transduction. Herein, we report an ISM-free Cu2+-SC-ISE based on Cu-Mn oxide (Cu1.4Mn1.6O4) as a bifunctional SC layer. The starting point is simplifying complex multi-interfaces for Cu2+-SC-ISEs. Specifically, ion recognition and signal transduction have been achieved synchronously by an ion-coupled-electron transfer of crystal ion transport and electron transfer of Mn4+/3+ in Cu1.4Mn1.6O4. The proposed Cu1.4Mn1.6O4 electrode discloses comparable sensitivity, response time, high selectivity and stability compared with present ISM-based potentiometric Cu2+ sensors. In addition, the Cu1.4Mn1.6O4 electrode also exhibits near Nernstian responses toward Cu2+ in natural water background. This work emphasizes an ISM-free concept and presents a scheme for the development of potentiometric Cu2+ sensors.

Determination of benzoate in cranberry and lingonberry by using a solid-contact benzoate-selective electrode

Benzoic acid is used as a preservative in processed food, and occasionally in cosmetics and pharmaceuticals, while benzoic acid occurs naturally in, e.g., cranberry and lingonberry. Therefore, the determination of benzoate is of interest for product quality assurance, food safety, and personal health. In this work, a solid-contact benzoate-selective electrode (benzoate-ISE) was developed by utilising poly(3,4-ethylenedioxythiophene) (PEDOT) as solid contact and a solvent polymeric membrane containing a 1,3-bis(carbazolyl)urea derivative as ionophore. The benzoate-ISE was characterised in parallel with an ionophore-free control-ISE by electrochemical impedance spectroscopy and potentiometry. The presence of the ionophore in the membrane improved the selectivity to benzoate. Benzoate-ISEs and control-ISEs were used further to determine the benzoate concentration in cranberry and lingonberry by standard addition. The results obtained with both types of ISEs were compared with those obtained by ion chromatography. The results obtained with benzoate-ISEs were consistent with those obtained with ion chromatography. On the contrary, the control-ISE (without ionophore) gave significantly higher benzoate concentrations, especially in the case of cranberry where the benzoate concentration was low (ca 0.2 g kg-1) compared to lingonberry (ca 1 g kg-1). Hence, the benzoate-selectivity of the ionophore was crucial to obtain a benzoate-ISE that was practically applicable for determination of benzoate concentrations in cranberry and lingonberry.

Recent advances in nanomaterial-based solid-contact ion-selective electrodes

Solid-contact ion-selective electrodes (SC-ISEs) are advanced potentiometric sensors with great capability to detect a wide range of ions for the monitoring of industrial processes and environmental pollutants, as well as the determination of electrolytes for clinical analysis. Over the past decades, the innovative design of ion-selective electrodes (ISEs), specifically SC-ISEs, to improve potential stability and miniaturization for in situ/real-time analysis, has attracted considerable interest. Recently, the utilisation of nanomaterials was particularly prominent in SC-ISEs due to their excellent physical and chemical properties. In this article, we review the recent applications of various types of nanostructured materials that are composed of carbon, metals and polymers for the development of SC-ISEs. The challenges and opportunities in this field, along with the prospects for future applications of nanomaterials in SC-ISEs are also discussed.

A Biodegradable Fiber Calcium Ion Sensor by Covalently Bonding Ionophores on Bioinert Nanoparticles

Implantable sensors, especially ion sensors, facilitate the progress of scientific research and personalized healthcare. However, the permanent retention of implants induces health risks after sensors fulfill their mission of chronic sensing. Biodegradation is highly anticipated; while; biodegradable chemical sensors are rare due to concerns about the leakage of harmful active molecules after degradation, such as ionophores. Here, a novel biodegradable fiber calcium ion sensor is introduced, wherein ionophores are covalently bonded with bioinert nanoparticles to replace the classical ion-selective membrane. The fiber sensor demonstrates comparable sensing performance to classical ion sensors and good flexibility. It can monitor the fluctuations of Ca2+ in a 4-day lifespan in vivo and biodegrade in 4 weeks. Benefiting from the stable bonding between ionophores and nanoparticles, the biodegradable sensor exhibits a good biocompatibility after degradation. Moreover, this approach of bonding active molecules on bioinert nanoparticles can serve as an effective methodology for minimizing health concerns about biodegradable chemical sensors.

Highly Stable Solid Contact Calcium Ion-Selective Electrodes: Rapid Ion-Electron Transduction Triggered by Lipophilic Anions Participating in Redox Reactions of CunS Nanoflowers

Solid contact (SC) calcium ion-selective electrodes (Ca2+-ISEs) have been widely applied in the analysis of water quality and body fluids by virtue of the unique advantages of easy operation and rapid response. However, the potential drift during the long-term stability test hinders their further practical applications. Designing novel redox SC layers with large capacitance and high hydrophobicity is a promising approach to stabilize the potential stability, meanwhile, exploring the transduction mechanism is also of great guiding significance for the precise design of SC layer materials. Herein, flower-like copper sulfide (CunS-50) composed of nanosheets is meticulously designed as the redox SC layer by modification with the surfactant (CTAB). The CunS-50-based Ca2+-ISE (CunS-50/Ca2+-ISE) demonstrates a near-Nernstian slope of 28.23 mV/dec for Ca2+ in a wide activity linear range of 10-7 to 10-1 M, with a low detection limit of 3.16 × 10-8 M. CunS-50/Ca2+-ISE possesses an extremely low potential drift of only 1.23 ± 0.13 μV/h in the long-term potential stability test. Notably, X-ray absorption fine-structure (XAFS) spectra and electrochemical experiments are adopted to elucidate the transduction mechanism that the lipophilic anion (TFPB-) participates in the redox reaction of CunS-50 at the solid-solid interface of ion-selective membrane (ISM) and redox inorganic SC layer (CunS-50), thereby promoting the generation of free electrons to accelerate ion-electron transduction. This work provides an in-depth comprehension of the transduction mechanism of the potentiometric response and an effective strategy for designing redox materials of ion-electron transduction triggered by lipophilic anions.

A smartphone-based sensor for detection of iron and potassium in food and beverage samples

A novel approach for simultaneous detection of iron and potassium via a smartphone-based potentiometric method is proposed in this study. The screen printed electrodes were modified with carbon black nanomaterial and ion selective membrane including zinc (II) phtalocyanine as the ionophore. The developed Fe3+-selective electrode and K+-selective electrode exhibited detection limits of 1.0 × 10-6 M and 1.0 × 10-5 M for Fe3+ and K+ ions, respectively. The electrodes were used to simultaneously detect Fe3+ and K+ ions in apple juice, skim milk, soybean and coconut water samples with recovery values between 90%-100.5%, and validated against inductively coupled plasma-optical emission spectrometry. Due to the advantageous characteristics of the sensors and the portability of Near Field Communication potentiometer supported with a smartphone application, the proposed method offers sensitive and selective detection of iron and potassium ions in food and beverage samples at the point of need.

Tailored Polypyrrole Nanofibers as Ion-to-Electron Transduction Membranes for Wearable K+ Sensors

Conductive polymers are recognized as ideal candidates for the development of noninvasive and wearable sensors for real-time monitoring of potassium ions (K+) in sweat to ensure the health of life. However, the low ion-to-electron transduction efficiency and limited active surface area hamper the development of high-performance sensors for low-concentration K+ detection in the sweat. Herein, a wearable K+ sensor is developed by tailoring the nanostructure of polypyrrole (PPy), serving as an ion-to-electron transduction layer, for accurately and stably tracing the K+ fluctuation in human sweat. The PPy nanostructures can be tailored from nanospheres to nanofibers by controlling the supramolecular assembly process during PPy polymerization. Resultantly, the ion-to-electron transduction efficiency (17-fold increase in conductivity) and active surface area (1.3-fold enhancement) are significantly enhanced, accompanied by minimized water layer formation. The optimal PPy nanofibers-based K+ sensor achieved a high sensitivity of 62 mV decade-1, good selectivity, and solid stability. After being integrated with a temperature sensor, the manufactured wearable sensor realized accurate monitoring of K+ fluctuation in the human sweat.

Pinecone derived hierarchical carbon nanostructure as a transducer in a solid-state ion-selective electrode for in vivo analysis of calcium ion

Monitoring extracellular calcium ion (Ca2+) chemical signals in neurons is crucial for tracking physiological and pathological changes associated with brain diseases in live animals. Potentiometry based solid-state ion-selective electrodes (ISEs) with the assist of functional carbon nanomaterials as ideal solid-contact layer could realize the potential response for in vitro and in vivo analysis. Herein, we employ a kind of biomass derived porous carbon as a transducing layer to prompt efficient ion to electron transduction while stabilizes the potential drift. The eco-friendly porous carbon after activation (APB) displays a high specific area with inherit macropores, micropores, and large specific capacitance. When employed as transducer in ISEs, a stable potential response, minimized potential drift can be obtained. Benefiting from these excellent properties, a solid-state Ca2+ selective carbon fiber electrodes (CFEs) with a sandwich structure is constructed and employed for real time sensing of Ca2+ under electrical stimulation. This study presents a new approach to develop sustainable and versatile transducers in solid-state ISEs, a crucial way for in vivo sensing.

Molecularly Imprinted Polymer Biosensor Based on Nitrogen-Doped Electrochemically Exfoliated Graphene/Ti3 CNTX MXene Nanocomposite for Metabolites Detection

Rapid and accurate quantification of metabolites in different bodily fluids is crucial for a precise health evaluation. However, conventional metabolite sensing methods, confined to centralized laboratory settings, suffer from time-consuming processes, complex procedures, and costly instrumentation. Introducing the MXene/nitrogen-doped electrochemically exfoliated graphene (MXene@N-EEG) nanocomposite as a novel biosensing platform in this work addresses the challenges associated with conventional methods, leveraging the concept of molecularly imprinted polymers (MIP) enables the highly sensitive, specific, and reliable detection of metabolites. To validate our biosensing technology, we utilize agmatine as a significant biologically active metabolite. The MIP biosensor incorporates electrodeposited Prussian blue nanoparticles as a redox probe, facilitating the direct electrical signaling of agmatine binding in the polymeric matrix. The MXene@N-EEG nanocomposite, with excellent metal conductivity and a large electroactive specific surface area, effectively stabilizes the electrodeposited Prussian blue nanoparticles. Furthermore, increasing the content of agmatine-imprinted cavities on the electrode enhances the sensitivity of the MIP biosensor. Evaluation of the designed MIP biosensor in buffer solution and plasma samples reveals a wide linear concentration range of 1.0 nM-100.0 μM (R2 = 0.9934) and a detection limit of 0.1 nM. Notably, the developed microfluidic biosensor offers low cost, rapid response time to the target molecule (10 min of sample incubation), good recovery results for detecting agmatine in plasma samples, and acceptable autonomous performance for on-chip detection. Moreover, its high reliability and sensitivity position this MIP-based biosensor as a promising candidate for miniaturized microfluidic devices with the potential for scalable production for point-of-care applications.

Graphene Oxide-Poly(vinyl alcohol) Hydrogel-Coated Solid-Contact Ion-Selective Electrodes for Wearable Sweat Potassium Ion Sensing

Solid-contact ion-selective electrodes (SC-ISEs) with ionophore-based polymer-sensitive membranes have been the major devices in wearable sweat sensors toward electrolyte analysis. However, the toxicity of ionophores in ion-selective membranes (ISMs), for example, valinomycin (K+ ion carrier), is a significant challenge, since the ISM directly contacts the skin during the tests. Herein, we report coating a hydrogel of graphene oxide-poly(vinyl alcohol) (GO-PVA) on the ISM to fabricate hydrogel-based SC-ISEs. The hydrogen bond interaction between GO sheets and PVA chains could enhance the mechanical strength through the formation of a cross-linking network. Comprehensive electrochemical tests have demonstrated that hydrogel-coated K+-SC-ISE maintains Nernstian response sensitivity, high selectivity, and anti-interference ability compared with uncoated K+-SC-ISE. A flexible hydrogel-based K+ sensing device was further fabricated with the integration of a solid-contact reference electrode, which has realized the monitoring of sweat K+ in real time. This work highlights the possibility of hydrogel coating for fabricating biocompatible wearable potentiometric sweat electrolyte sensors.

Fully Integrated Multiplexed Wristwatch for Real-Time Monitoring of Electrolyte Ions in Sweat

Considerable progress has already been made in sweat sensors based on electrochemical methods to realize real-time monitoring of biomarkers. However, realizing long-term monitoring of multiple targets at the atomic level remains extremely challenging, in terms of designing stable solid contact (SC) interfaces and fully integrating multiple modules for large-scale applications of sweat sensors. Herein, a fully integrated wristwatch was designed using mass-manufactured sensor arrays based on hierarchical multilayer-pore cross-linked N-doped porous carbon coated by reduced graphene oxide (NPCs@rGO-950) microspheres with high hydrophobicity as core SC, and highly selective monitoring simultaneously for K+, Na+, and Ca2+ ions in human sweat was achieved, exhibiting near-Nernst responses almost without forming an interfacial water layer. Combined with computed tomography, solid-solid interface potential diffusion simulation results reveal extremely low interface diffusion potential and high interface capacitance (598 μF), ensuring the excellent potential stability, reversibility, repeatability, and selectivity of sensor arrays. The developed highly integrated-multiplexed wristwatch with multiple modules, including SC, sensor array, microfluidic chip, signal transduction, signal processing, and data visualization, achieved reliable real-time monitoring for K+, Na+, and Ca2+ ion concentrations in sweat. Ingenious material design, scalable sensor fabrication, and electrical integration of multimodule wearables lay the foundation for developing reliable sweat-sensing systems for health monitoring.

Functionalizing Carbon Substrates with a Covalently Attached Cobalt Redox Buffer for Calibration-Free Solid-Contact Ion-Selective Electrodes

With a view to potentiometric sensing with minimal calibration requirements and high long-term stability, colloid-imprinted mesoporous (CIM) carbon was functionalized by the covalent attachment of a cobalt redox buffer and used as a new solid contact for ion-selective electrodes (ISEs). The CIM carbon surface was first modified by electroless grafting of a terpyridine ligand (Tpy-ph) using diazonium chemistry, followed by stepwise binding of Co(II) and an additional Tpy ligand to the grafted ligand, forming a bis(terpyridine) Co(II) complex, CIM-ph-Tpy-Co(II)-Tpy. Half a molar equivalent of ferrocenium tetrakis(3-chlorophenyl)borate was then used to partially oxidize the Co(II) complex. Electrodes prepared with this surface-attached CIM-ph-Tpy-Co(III/II)-Tpy redox buffer as a solid contact were tested as K+ sensors in combination with valinomycin as the ionophore and Dow 3140 silicone or plasticized poly(vinyl chloride) (PVC) as the matrixes for the ion-selective membrane (ISM). This solid contact is characterized by a redox capacitance of 3.26 F/g, ensuring a well-defined interfacial potential that underpins the transduction mechanism. By use of a redox couple as an internal reference element to control the phase boundary potential at the interface of the ISM and the CIM carbon solid contact, solid-contact ion-selective electrodes (SC-ISEs) with a standard deviation of E° as low as 0.3 mV for plasticized PVC ISMs and 3.5 mV for Dow 3140 silicone ISMs were obtained. Over 100 h, these SC-ISEs exhibit an emf drift of 20 μV/h for plasticized PVC ISMs and 62 μV/h for silicone ISMs. The differences in long-term stability and reproducibility between electrodes with ISMs comprising either a plasticized PVC or silicone matrix offer valuable insights into the effect of the polymeric matrix on sensor performance.

A novel potentiometric screen-printed electrode based on crown ethers/nano manganese oxide/Nafion composite for trace level determination of copper ion in biological fluids

Copper levels in biological fluids are associated with Wilson's, Alzheimer's, Menke's, and Parkinson's diseases, making them good biochemical markers for these diseases. This study introduces a miniaturized screen-printed electrode (SPE) for the potentiometric determination of copper(II) in some biological fluids. Manganese(III) oxide nanoparticles (Mn2O3-NPs), dispersed in Nafion, are drop-casted onto a graphite/PET substrate, serving as the ion-to-electron transducer material. The solid-contact material is then covered by a selective polyvinyl chloride (PVC) membrane incorporated with 18-crown-6 as a neutral ion carrier for the selective determination of copper(II) ions. The proposed electrode exhibits a Nernstian response with a slope of 30.2 ± 0.3 mV/decade (R2 = 0.999) over the linear concentration range 5.2 × 10-9 - 6.2 × 10-3 mol/l and a detection limit of 1.1 × 10-9 mol/l (69.9 ng/l). Short-term potential stability is evaluated using constant current chronopotentiometry (CP) and electrochemical impedance spectroscopy (EIS). A significant improvement in the electrode capacitance (91.5 μF) is displayed due to the use of Mn2O3-NPs as a solid contact. The presence of Nafion, with its high hydrophobicity properties, eliminates the formation of the thin water layer, facilitating the ion-to-electron transduction between the sensing membrane and the conducting substrate. Additionally, it enhances the adhesion of the polymeric sensing membrane to the solid-contact material, preventing membrane delamination and increasing the electrode's lifespan. The high selectivity, sensitivity, and potential stability of the proposed miniaturized electrode suggests its use for the determination of copper(II) ions in human blood serum and milk samples. The results obtained agree fairly well with data obtained by flameless atomic absorption spectrometry.

A Multichannel Screen-Printed Carbon Electrode Based on Fluorinated Poly(3-octylthiophene-2,5-diyl) and Purified Mesoporous Carbon Black Simultaneously Detects Na+, K+, Ca2+, and NO2. ACS OMEGA 2024;9:18238-18248. [PMID: 38680364 PMCID: PMC11044230 DOI: 10.1021/acsomega.3c10471] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Preparation of nanocomposites based on fluorinated poly(3-octylthiophene-2,5-diyl) (POTF) and purified mesoporous carbon black (PMCB) as the solid-contact layer of a screen-printed carbon electrode (SPCE) is proposed. POTF is used as a dispersant for PMCB. The obtained nanocomposites possess unique characteristics including high conductivity, capacitance, and stability. The SPCE based on POTF and PMCB is characterized by electrochemical impedance spectroscopy and chronopotentiometry, demonstrating simultaneous detection of Na+, K+, Ca2+, and NO2- ions with detection limits of 10-6.5, 10-6.4, 10-6.7, and 10-6.3 M, respectively. Water layer and anti-interference tests revealed that the electrode has high hydrophobicity, and the static contact angle is >140°. The electrode shows excellent selectivity, repeatability, reproducibility, and stability and is not easily affected by light, O2, or CO2.

Recent Progress on Potentiometric Sensor Applications Based on Nanoscale Metal Oxides: A Comprehensive Review

Electrochemical sensors have been the subject of much research and development as of late, with several publications detailing new designs boasting enhanced performance metrics. That is, without a doubt, because such sensors stand out from other analytical tools thanks to their excellent analytical characteristics, low cost, and ease of use. Their progress has shown a trend toward seeking out novel useful nano structure materials. A variety of nanostructure metal oxides have been utilized in the creation of potentiometric sensors, which are the subject of this article. For screen-printed pH sensors, metal oxides have been utilized as sensing layers due to their mixed ion-electron conductivity and as paste-ion-selective electrode components and in solid-contact electrodes. Further significant uses include solid-contact layers. All the metal oxide uses mentioned are within the purview of this article. Nanoscale metal oxides have several potential uses in the potentiometry method, and this paper summarizes such uses, including hybrid materials and single-component layers. Potentiometric sensors with outstanding analytical properties can be manufactured entirely from metal oxides. These novel sensors outperform the more traditional, conventional electrodes in terms of useful characteristics. In this review, we looked at the potentiometric analytical properties of different building solutions with various nanoscale metal oxides.

Silk Fibroin-Regulated Nanochannels for Flexible Hydrovoltaic Ion Sensing

The evaporation-induced hydrovoltaic effect based on ion-selective nanochannels can theoretically be employed for high-performance ion sensing; yet, the indeterminate ion-sensing properties and the acquisition of high sensing performance are rarely explored. Herein, a controllable nanochannel regulation strategy for flexible hydrovoltaic devices with highly sensitive ion-sensing abilities is presented across a wide concentration range. By multiple dip-coating of silk fibroin (SF) on an electrospinning nylon-66 nanofiber (NNF) film, the surface polarity enhancement, the fibers size regulation with a precision of ≈25 nm, and the nanostructure firm binding are achieved simultaneously. The resultant flexible freestanding hydrovoltaic device exhibits an open circuit voltage up to 4.82 V in deionized water, a wide ion sensing range of 10-7 to 100 m, and ultrahigh sensitivity as high as 1.37 V dec-1, which is significantly higher than the sensitivity of the traditional solid-contact ion-selective electrodes (SC-ISEs). The fabricated flexible ion-sensitive hydrovoltaic device is successfully applied for wearable human sweat electrolyte sensing and for environmental trace-ion monitoring, thereby confirming the potential application of the hydrovoltaic effect for ion sensing.

A rational study of transduction mechanisms of different materials for all solid contact-ISEs

The new era of solid contact ion selective electrodes (SC-ISEs) miniaturized design has received an extensive amount of concern. Because it eliminated the requirement for ongoing internal solution composition optimization and created a two-phase system with stronger detection limitations. Herein, the determination of venlafaxine HCl is based on a comparison study between different ion- to electron transduction materials (such as; multiwalled carbon nanotubes (MWCNTs), polyaniline (PANi), and ferrocene) and illustrating their mechanisms in their applied sensors. Their different electrochemical features (such as bulk resistance (Rb**), double-layer capacitance (Cdl), geometric capacitance (Cg), and specific capacitance (Cp)) were evaluated and discussed by using the Electrochemical Impedance Spectroscopy (EIS), Chronopotentiometry (CP), and Cyclic Voltammetry (CV) experiments. The results indicated that each transducer's influence on the proposed sensor's electrochemical characteristics is determined by their unique chemical and physical properties. The electrochemical features vary for different solid contact materials used in transduction mechanisms. The results confirm that the MWCNT sensor revealed the best electrochemical behavior with the potentiometric response of a near-Nernestian slope of 56.1 ± 0.8 mV/decade with detection limits of 3.8 × 10-6 mol/L (r2 = 0.999) and a low potential drift (∆E/∆t) of 34.6 µV/s. Also, the selectivity study was performed in the presence of different interfering species either in single or complex matrices. This demonstrates excellent selectivity, stability, conductivity, and reliability as a VEN-TPB ion pair sensor for accurately measuring VEN in its various formulations. The proposed method was compared to HPLC reported technique and confirmed no significant difference between them. So, the proposed sensors fulfill their solutions' demand features for VEN appraisal.

Porous Conductive Textiles for Wearable Electronics

Over the years, researchers have made significant strides in the development of novel flexible/stretchable and conductive materials, enabling the creation of cutting-edge electronic devices for wearable applications. Among these, porous conductive textiles (PCTs) have emerged as an ideal material platform for wearable electronics, owing to their light weight, flexibility, permeability, and wearing comfort. This Review aims to present a comprehensive overview of the progress and state of the art of utilizing PCTs for the design and fabrication of a wide variety of wearable electronic devices and their integrated wearable systems. To begin with, we elucidate how PCTs revolutionize the form factors of wearable electronics. We then discuss the preparation strategies of PCTs, in terms of the raw materials, fabrication processes, and key properties. Afterward, we provide detailed illustrations of how PCTs are used as basic building blocks to design and fabricate a wide variety of intrinsically flexible or stretchable devices, including sensors, actuators, therapeutic devices, energy-harvesting and storage devices, and displays. We further describe the techniques and strategies for wearable electronic systems either by hybridizing conventional off-the-shelf rigid electronic components with PCTs or by integrating multiple fibrous devices made of PCTs. Subsequently, we highlight some important wearable application scenarios in healthcare, sports and training, converging technologies, and professional specialists. At the end of the Review, we discuss the challenges and perspectives on future research directions and give overall conclusions. As the demand for more personalized and interconnected devices continues to grow, PCT-based wearables hold immense potential to redefine the landscape of wearable technology and reshape the way we live, work, and play.

Signal transduction interfaces for field-effect transistor-based biosensors

Biosensors based on field-effect transistors (FETs) are suitable for use in miniaturized and cost-effective healthcare devices. Various semiconductive materials can be applied as FET channels for biosensing, including one- and two-dimensional materials. The signal transduction interface between the biosample and the channel of FETs plays a key role in translating electrochemical reactions into output signals, thereby capturing target ions or biomolecules. In this Review, distinctive signal transduction interfaces for FET biosensors are introduced, categorized as chemically synthesized, physically structured, and biologically induced interfaces. The Review highlights that these signal transduction interfaces are key in controlling biosensing parameters, such as specificity, selectivity, binding constant, limit of detection, signal-to-noise ratio, and biocompatibility.

A physicochemical-sensing electronic skin for stress response monitoring

Approaches to quantify stress responses typically rely on subjective surveys and questionnaires. Wearable sensors can potentially be used to continuously monitor stress-relevant biomarkers. However, the biological stress response is spread across the nervous, endocrine, and immune systems, and the capabilities of current sensors are not sufficient for condition-specific stress response evaluation. Here we report an electronic skin for stress response assessment that non-invasively monitors three vital signs (pulse waveform, galvanic skin response and skin temperature) and six molecular biomarkers in human sweat (glucose, lactate, uric acid, sodium ions, potassium ions and ammonium). We develop a general approach to prepare electrochemical sensors that relies on analogous composite materials for stabilizing and conserving sensor interfaces. The resulting sensors offer long-term sweat biomarker analysis of over 100 hours with high stability. We show that the electronic skin can provide continuous multimodal physicochemical monitoring over a 24-hour period and during different daily activities. With the help of a machine learning pipeline, we also show that the platform can differentiate three stressors with an accuracy of 98.0%, and quantify psychological stress responses with a confidence level of 98.7%.

Design Criteria for Nanostructured Carbon Materials as Solid Contacts for Ion-Selective Sensors

The ability to miniaturize ion-selective sensors that enable microsensor arrays and wearable sensor patches for ion detection in environmental or biological samples requires all-solid-state sensors with solid contacts for transduction of an ion activity into an electrical signal. Nanostructured carbon materials function as effective solid contacts for this purpose. They can also contribute to improved potential signal stability, reducing the need for frequent sensor calibration. In this Perspective, the structural features of various carbon-based solid contacts described in the literature and their respective abilities to reduce potential drift during long-term, continuous measurements are compared. These carbon materials include nanoporous carbons with various architectures, carbon nanotubes, carbon black, graphene, and graphite-based solid contacts. The effects of accessibility of ionophores, ionic sites, and other components of an ion-selective membrane to the internal or external carbon surfaces are discussed, because this impacts double-layer capacitance and potential drift. The effects of carbon composition on water-layer formation are also considered, which is another contributor to potential drift during long-term measurements. Recommendations regarding the selection of solid contacts and considerations for their characterization and testing in solid-contact ion-selective electrodes are provided.

Ion-Electron Transduction Layer of the SnS2-MoS2 Heterojunction to Elevate Superior Interface Stability for All-Solid Sodium-Ion Selective Electrode

The high selectivity and fast ion response of all-solid sodium ion selective electrodes were widely applied in human sweat analysis. However, the potential drift due to insufficient interfacial capacitance leads to the deterioration of its stability and ultimately affects the potential accuracy of ion analysis. Designing a novel ion-electron transduction layer between the electrode and the ion selective membrane is an effective method to stabilize the interfacial potential. Herein, the SnS2-MoS2 heterojunction material was constructed by doping Sn in MoS2 nanosheets and used as the ion electron transduction layers of an all-solid sodium ion selective electrode for the first time, achieving the stable and efficient detection of Na+ ions. The proposed electrode exhibited a Nernst slope of 57.86 mV/dec for the detection of Na+ ions with a detection limit of 10-5.7 M in the activity range of 10-6-10-1 M. Via the electronic interaction at the heterojunction interfaces between SnS2 and MoS2 materials, the micro-nanostructure of the SnS2-MoS2 heterojunction was changed and SnS2-MoS2 as the ion-electron transduction layer acquired excellent capacitance (699 μF) and hydrophobicity (132°), resulting in a long-term potential stability of 1.37 μV/h. It was further proved that the large capacitance and high hydrophobicity of the ion-electron transduction layer are primary reasons for the excellent stability of the all-solid sodium ion selective electrode toward Na+ ions.

Carbon fiber-based multichannel solid-contact potentiometric ion sensors for real-time sweat electrolyte monitoring

Solid-contact ion-selective electrodes (SC-ISEs) feature miniaturization and integration that have gained extensive attention in non-invasive wearable sweat electrolyte sensors. The state-of-the-art wearable SC-ISEs mainly use polyethylene terephthalate, gold and carbon nanotube fibers as flexible substrates but suffer from uncomfortableness, high cost and biotoxicity. Herein, we report carbon fiber-based SC-ISEs to construct a four-channel wearable potentiometric sensor for sweat electrolytes monitoring (Na+/K+/pH/Cl-). The carbon fibers were extracted from commercial cloth, of which the starting point is addressing the cost and reproducibility issues for flexible SC-ISEs. The bare carbon fiber electrodes exhibited reversible voltammetric and stable impedance performances. Further fabricated SC-ISEs based on corresponding ion-selective membranes disclosed Nernstian sensitivity and anti-interface ability toward both ions and organic species in sweat. Significantly, these carbon fiber-based SC-ISEs revealed high reproducibility of standard potentials between normal and bending states. Finally, a textile-based sensor was integrated with a solid-contact reference electrode, which realized on-body sweat electrolytes analysis. The results displayed high accuracy compared with ex-situ tests by ion chromatography. This work highlights carbon fiber-based multichannel wearable potentiometric ion sensors with low cost, biocompatibility and reproducibility.

Poly(vinyl chloride)/Nanocarbon Composites for Advanced Potentiometric Membrane Sensor Design

Polymer nanocomposites filled with carbon nanoparticles (CNPs) are a hot topic in materials science. This article discusses the current research on the use of these materials as interfacial electron transfer films for solid contact potentiometric membrane sensors (SC-PMSs). The results of a comparative study of plasticized poly (vinyl chloride) (pPVC) matrices modified with single-walled carbon nanotubes (SWCNTs), fullerenes-C60, and their hybrid ensemble (SWCNTs-C60) are reported. The morphological characteristics and electrical conductivity of the prepared nanostructured composite films are reported. It was found that the specific electrical conductivity of the pPVC/SWCNTs-C60 polymer film was higher than that of pPVC filled with individual nanocomponents. The effectiveness of this composite material as an electron transfer film in a new potentiometric membrane sensor for detecting phenylpyruvic acid (in anionic form) was demonstrated. Screening for this metabolic product of phenylalanine in body fluids is of significant diagnostic interest in phenylketonuria (dementia), viral hepatitis, and alcoholism. The developed sensor showed a stable and fast Nernstian response for phenylpyruvate ions in aqueous solutions over the wide linear concentration range of 5 × 10-7-1 × 10-3 M, with a detection limit of 10-7.2 M.

Modern Potentiometric Biosensing Based on Non-Equilibrium Measurement Techniques

Modern potentiometric sensors based on polymeric membrane ion-selective electrodes (ISEs) have achieved new breakthroughs in sensitivity, selectivity, and stability and have extended applications in environmental surveillance, medical diagnostics, and industrial analysis. Moreover, nonclassical potentiometry shows promise for many applications and opens up new opportunities for potentiometric biosensing. Here, we aim to provide a concept to summarize advances over the past decade in the development of potentiometric biosensors with polymeric membrane ISEs. This Concept article articulates sensing mechanisms based on non-equilibrium measurement techniques. In particular, we emphasize new trends in potentiometric biosensing based on attractive dynamic approaches. Representative examples are selected to illustrate key applications under zero-current conditions and stimulus-controlled modes. More importantly, fruitful information obtained from non-equilibrium measurements with dynamic responses can be useful for artificial intelligence (AI). The combination of ISEs with advanced AI techniques for effective data processing is also discussed. We hope that this Concept will illustrate the great possibilities offered by non-equilibrium measurement techniques and AI in potentiometric biosensing and encourage further innovations in this exciting field.

A Fiber Sensor for Long-Term Monitoring of Extracellular Potassium Ion Fluctuations in Chronic Neuropsychiatric Diseases

The extracellular potassium ion concentration in the brain exerts a significant influence on cellular excitability and intercellular communication. Perturbations in the extracellular potassium ion level are closely correlated with various chronic neuropsychiatric disorders including depression. However, a critical gap persists in performing real-time and long-term monitoring of extracellular potassium ions, which is necessary for comprehensive profiling of chronic neuropsychiatric diseases. Here, a fiber potassium ion sensor (FKS) that consists of a soft conductive fiber with a rough surface and a hydrophobic-treated transduction layer interfaced with a potassium ion-selective membrane is found to solve this problem. The FKS demonstrates stable interfaces between its distinct functional layers in an aqueous environment, conferring an exceptional stability of 6 months in vivo, in stark contrast to previous reports with working durations from hours to days. Upon implantation into the mouse brain, the FKS enables effective monitoring of extracellular potassium ion dynamics under diverse physiological states including anesthesia, forced swimming, and tail suspension. Using this FKS, tracking of extracellular potassium ion fluctuations that align with behaviors associated with the progression of depression over months is achieved, demonstrating its usability in studying chronic neuropsychiatric disorders from a new biochemical perspective.

DNA Chimeras as Electrochemical Biosensors for Host-Guest Measurements in Blood

Few sensing platforms have become ubiquitous to enable rapid and convenient measurements at the point-of-care. Those, however, are "one-off" technologies, meaning that they can only detect a single target and are hardly adaptable. In response, we plan to develop a sensing platform that can be extended to detect other classes of molecules and that affords rapid, convenient, continuous measurements directly in undiluted complex matrices. For this, we decided to rely on a host molecule that presents reversible interactions toward specific guest molecules to develop a new class of sensors that we coined "Electrochemical DNA-host chimeras". As a proof-of-concept for our sensor, we decided to use cyclobis(paraquat-p-phenylene) ("blue box") that we attached on an electrode-bound DNA to allow measurements of electron-rich guests such as dopamine and aspirin. Doing so allows to promote host-guest complex that could be quantified using blue box's electrochemistry. Because of this unique sensor architecture, we achieve, to our knowledge, the first reagentless, continuous and rapid (<5 min) host-guest measurements in undiluted whole blood. We envision that given the library of electroactive host molecules that this will allow the development of a sensing platform for measurements of several classes of molecules in complex matrices at the point-of-care.

Electrochemical Methods for the Analysis of Trace Tin Concentrations-Review

Tin determination allows for the monitoring of pollution and assessment of the impact of human activities on the environment. The determination of tin in the environment is crucial for the protection of human health and ecosystems, and for maintaining sustainability. Tin can be released into the environment from various sources, such as industry, transportation, and electronic waste. The concentration of tin in the environment can be determined by different analytical methods, depending on the form of tin present and the purpose of the analysis. The choice of an appropriate method depends on the type of sample, concentration levels, and the available instrumentation. In this paper, we have carried out a literature review of electrochemical methods for the determination of tin. Electrochemical methods of analysis such as polarography, voltammetry, and potentiometry can be used for the determination of tin in various environmental samples, as well as in metal alloys. The detection limits and linearity ranges obtained for the determination of tin by different electrochemical techniques are collected and presented. The influence of the choice of base electrolyte and working electrode on signals is also presented. Practical applications of the developed tin determination methods in analyzing real samples are also summarized.

Trace-level chronopotentiometric detection in the presence of a high electrolyte background using thin-layer ion-selective polymeric membranes

We propose here a pulsed galvanostatic control of a solid-contact ion-selective electrode coupled with a thin-layer ion-exchanger free membrane, which allows chronopotentiometric trace-level ion detection with a high-interfering background in a rapid and reversible way.

A comparative study of green solid contact ion selective electrodes for the potentiometric determination of Letrozole in dosage form and human plasma

Analysis of drugs clinically and their identification in biological samples are of utmost importance in the process of therapeutic drug monitoring, also in pharmacokinetic investigations and tracking of illicit medications. These investigations are carried out using a variety of analytical methods, including potentiometric electrodes. Potentiometric electrodes are a wonderful solution for researchers because they outperform other methods in terms of sustainability, greenness, and cost effectiveness. In the current study, ion-selective potentiometric sensors were assembled for the aim of quantification of the anticancer drug Letrozole (LTZ). The first step was fabrication of a conventional sensor based on the formation of stable host-guest inclusion complex between the cationic drug and 4-tert-butylcalix-8-arene (TBCAX-8). Two additional sensors were prepared through membrane modification with graphene nanocomposite (GNC) and polyaniline (PANI) nanoparticles. Linear responses of 1.00 × 10-5-1.00 × 10-2, 1.00 × 10-6-1.00 × 10-2 and 1.00 × 10-8-1.00 × 10-3 with sub-Nernstian slopes of 19.90, 20.10 and 20.30 mV/decade were obtained for TBCAX-8, GNC, and PANI sensors; respectively. The developed sensors were successful in determining the drug LTZ in bulk powder and dosage form. PANI modified sensor was used to determine LTZ in human plasma with recoveries ranging from 88.00 to 96.30%. IUPAC recommendations were followed during the evaluation of the electrical performance of the developed sensors. Experimental conditions as temperature and pH were studied and optimized. Analytical Eco-scale and Analytical GREEness metric were adopted as the method greenness assessment tools.

Application of Metal Oxide Nanoparticles in the Field of Potentiometric Sensors: A Review

Recently, there has been rapid development of electrochemical sensors, and there have been numerous reports in the literature that describe new constructions with improved performance parameters. Undoubtedly, this is due to the fact that those sensors are characterized by very good analytical parameters, and at the same time, they are cheap and easy to use, which distinguishes them from other analytical tools. One of the trends observed in their development is the search for new functional materials. This review focuses on potentiometric sensors designed with the use of various metal oxides. Metal oxides, because of their remarkable properties including high electrical capacity and mixed ion-electron conductivity, have found applications as both sensing layers (e.g., of screen-printing pH sensors) or solid-contact layers and paste components in solid-contact and paste-ion-selective electrodes. All the mentioned applications of metal oxides are described in the scope of the paper. This paper presents a survey on the use of metal oxides in the field of the potentiometry method as both single-component layers and as a component of hybrid materials. Metal oxides are allowed to obtain potentiometric sensors of all-solid-state construction characterized by remarkable analytical parameters. These new types of sensors exhibit properties that are competitive with those of the commonly used conventional electrodes. Different construction solutions and various metal oxides were compared in the scope of this review based on their analytical parameters.

A multi-ion interference decoupling model based on ion-selective electrode arrays

Compared to traditional liquid-junction ion-selective electrodes, solid-contact ion-selective electrodes (SC-ISEs) have attracted much attention and undergone rapid development due to their compactness and ease of integration. However, the application and widespread use of SC-ISEs are limited by their non-ideal selectivity and susceptibility to signal drift. Although the principles of artificial neural network (ANN) methods have shown significant progress in partially resolving the selectivity issue, they generally require extensive calibration steps and computational resources to implement. As a result, numerical computation models are more practical and economical, but existing approaches often overlook experimental phenomena and have relatively complex modeling principles. In this study, we propose a proportional factor model based on the trend of SC-ISEs affected by the multiple ions, along with a scalable dynamic correction procedure to improve its robustness. This model utilizes an estimated response surface method to solve nonlinear equations, requiring fewer calibration experiments. It accurately extracts the concentrations of multiple target ions in the presence of multi-ion interference and dynamically adjusts the model parameters for different types of ISEs. Additionally, we design a multi-channel SC-ISE array as a carrier for theoretical validation. In a case study, we demonstrate the feasibility of decoupling multiple ions using the SC-ISE array, obtaining concentrations of calcium, magnesium, sodium, and potassium ions, and verify the accuracy of the multi-ion detection system for real-world water samples.

Toward Understanding the Formation Mechanism and OER Catalytic Mechanism of Hydroxides by In Situ and Operando Techniques

Developing efficient and affordable electrocatalysts for the sluggish oxygen evolution reaction (OER) remains a significant barrier that needs to be overcome for the practical applications of hydrogen production via water electrolysis, transforming CO2 to value-added chemicals, and metal-air batteries. Recently, hydroxides have shown promise as electrocatalysts for OER. In situ or operando techniques are particularly indispensable for monitoring the key intermediates together with understanding the reaction process, which is extremely important for revealing the formation/OER catalytic mechanism of hydroxides and preparing cost-effective electrocatalysts for OER. However, there is a lack of comprehensive discussion on the current status and challenges of studying these mechanisms using in situ or operando techniques, which hinders our ability to identify and address the obstacles present in this field. This review offers an overview of in situ or operando techniques, outlining their capabilities, advantages, and disadvantages. Recent findings related to the formation mechanism and OER catalytic mechanism of hydroxides revealed by in situ or operando techniques are also discussed in detail. Additionally, some current challenges in this field are concluded and appropriate solution strategies are provided.

Electrochemically-selective electrode for quantification of dorzolamide in bulk drug substance and dosage form

Three smart carbon paste electrodes were fabricated to quantify dorzolamide hydrochloride DRZ, including conventional carbon paste I, modified carbon paste embedding Silica II, and modified carbon paste embedding β-cyclodextrin III. This study is based on the insertion of DRZ with phosphomolybdic acid to create an electroactive moiety dorzolamide-phosphomolybdate ion exchanger using a solvent mediator dibutyl phthalate. The three constructed carbon paste electrodes displayed Nernstian responses and linear concentration ranges with lower detection limits. The vital performance of the created electrodes was verified in relation to various parameters. The electrodes enhance the selective determination of DRZ in the presence of inorganic ions, a co-formulated drug in the dosage form timolol maleate, and the excipient benzalkonium chloride. The modified carbon paste electrode including Silica was utilized to detect DRZ in ophthalmic eye drop form utilizing the direct calibration curve and potentiometric titration methods. Satisfactory findings were achieved by comparing them to other reported methods.

Exploring the Potential Applications of Engineered Borophene in Nanobiosensing and Theranostics

A monolayer of boron known as borophene has emerged as a novel and fascinating two-dimensional (2D) material with exceptional features, such as anisotropic metallic behavior and supple mechanical and optical capabilities. The engineering of smart functionalized opto-electric 2D materials is essential to obtain biosensors or biodevices of desired performance. Borophene is one of the most emerging 2D materials, and owing to its excellent electroactive surface area, high electron transport, anisotropic behavior, controllable optical and electrochemical properties, ability to be deposited on thin films, and potential to create surface functionalities, it has recently become one of the sophisticated platforms. Despite the difficulty of production, borophene may be immobilized utilizing chemistries, be functionalized on a flexible substrate, and be controlled over electro-optical properties to create a highly sensitive biosensor system that could be used for point-of-care diagnostics. Its electrochemical properties can be tailored by using appropriate nanomaterials, redox mediators, conducting polymers, etc., which will be quite useful for the detection of biomolecules at even trace levels with a high sensitivity and less detection time. This will be quite helpful in developing biosensing devices with a very high sensitivity and with less response time. So, this review will be a crucial foundation as we have discussed the basic properties, synthesis, and potential applications of borophene in nanobiosensing, as well as therapeutic applications.

Electrochemical Visualization of an Ion-Selective Membrane Using a Carbon Nanoelectrode

Molecular and physical probes have been widely employed to investigate physicochemical properties and mechanisms of interfaces due to their ability to provide accurate measurements with temporal and spatial resolution. However, the direct measurement of electroactive species diffusion in ion-selective electrode (ISE) membranes and quantification of the water layer have been challenging due to the high impedance and optical opacity of polymer membranes. In the present work, carbon nanoelectrodes with ultrathin insulating encapsulation and good geometrical structure are reported as physical probes for direct electrochemical measurement of the water layer. The scanning electrochemical microscopy experiment exhibits positive feedback at the interface of the fresh ISE, and negative feedback after conditioning for 3 h. The thickness of the water layer was estimated to be ca. 13 nm. For the first time, we provide direct evidence that, during conditioning, the water molecules diffuse through the chloride ion selective membrane (Cl-ISM) until a water layer establishes at almost 3 h. Furthermore, the diffusion coefficient and concentration of oxygen molecules in the Cl-ISM are also directly electrochemical measured by introducing ferrocene (Fc) as a redox molecule probe. The oxygen concentration in the Cl-ISM decreases during conditioning, suggesting the diffusion of oxygen from ISM to the water layer. The proposed method can be used for the electrochemical measurement of solid contact, providing theoretical guidance and advice for the performance optimization of ISEs.

Ion-Selective Electrodes with Solid Contact Based on Composite Materials: A Review

Potentiometric sensors are the largest and most commonly used group of electrochemical sensors. Among them, ion-selective electrodes hold a prominent place. Since the end of the last century, their re-development has been observed, which is a consequence of the introduction of solid contact constructions, i.e., electrodes without an internal electrolyte solution. Research carried out in the field of potentiometric sensors primarily focuses on developing new variants of solid contact in order to obtain devices with better analytical parameters, and at the same time cheaper and easier to use, which has been made possible thanks to the achievements of material engineering. This paper presents an overview of new materials used as a solid contact in ion-selective electrodes over the past several years. These are primarily composite and hybrid materials that are a combination of carbon nanomaterials and polymers, as well as those obtained from carbon and polymer nanomaterials in combination with others, such as metal nanoparticles, metal oxides, ionic liquids and many others. Composite materials often have better mechanical, thermal, electrical, optical and chemical properties than the original components. With regard to their use in the construction of ion-selective electrodes, it is particularly important to increase the capacitance and surface area of the material, which makes them more effective in the process of charge transfer between the polymer membrane and the substrate material. This allows to obtain sensors with better analytical and operational parameters. Brief characteristics of electrodes with solid contact, their advantages and disadvantages, as well as research methods used to assess their parameters and analytical usefulness were presented. The work was divided into chapters according to the type of composite material, while the data in the table were arranged according to the type of ion. Selected basic analytical parameters of the obtained electrodes have been collected and summarized in order to better illustrate and compare the achievements that have been described till now in this field of analytical chemistry, which is potentiometry. This comprehensive review is a compendium of knowledge in the research area of functional composite materials and state-of-the-art SC-ISE construction technologies.

Large-scale fabrication of ion-selective electrodes for simultaneous detection of Na+, K+, and Ca2+ in biofluids using a smartphone-based potentiometric sensing platform

A significant bottleneck exists for mass-production of ion-selective electrodes despite recent developments in manufacturing technologies. Here, we present a fully-automated system for large-scale production of ISEs. Three materials, including polyvinyl chloride, polyethylene terephthalate and polyimide, were used as substrates for fabricating ion-selective electrodes (ISEs) using stencil printing, screen-printing and laser engraving, respectively. We compared sensitivities of the ISEs to determine the best material for the fabrication process of the ISEs. The electrode surfaces were modified with various carbon nanomaterials including multi-walled carbon nanotubes, graphene, carbon black, and their mixed suspensions as the intermediate layer to enhance sensitivities of the electrodes. An automated 3D-printed robot was used for the drop-cast procedure during ISE fabrication to eliminate manual steps. The sensor array was optimized, and the detection limits were 10^-5 M, 10^-5 M and 10^-4 M for detection of K⁺, Na⁺ and Ca²⁺ ions, respectively. The sensor array integrated with a portable wireless potentiometer was used to detect K⁺, Na⁺ and Ca²⁺ in real urine and simulated sweat samples and results obtained were in agreement with ICP-OES with good recoveries. The developed sensing platform offers low-cost detection of electrolytes for point-of-care applications.

All-Solid-State Carbon Black Paste Electrodes Modified by Poly(3-octylthiophene-2,5-diyl) and Transition Metal Oxides for Determination of Nitrate Ions

This paper presents new paste ion-selective electrodes for the determination of nitrate ions in soil. The pastes used in the construction of the electrodes are based on carbon black doped with transition metal oxides: ruthenium, iridium, and polymer-poly(3-octylthiophene-2,5-diyl). The proposed pastes were electrically characterized by chronopotentiometry and broadly characterized potentiometrically. The tests showed that the metal admixtures used increased the electric capacitance of the pastes to 470 μF for the ruthenium-doped paste. The polymer additive used positively affects the stability of the electrode response. All tested electrodes were characterized by a sensitivity close to that of the Nernst equation. In addition, the proposed electrodes have a measurement range of 10^-5 to 10^-1 M NO₃^- ions. They are impervious to light conditions and pH changes in the range of 2-10. The utility of the electrodes presented in this work was demonstrated during measurements directly in soil samples. The electrodes presented in this paper show satisfactory metrological parameters and can be successfully used for determinations in real samples.

Skin-Interfaced Wearable Sweat Sensors for Precision Medicine

Wearable sensors hold great potential in empowering personalized health monitoring, predictive analytics, and timely intervention toward personalized healthcare. Advances in flexible electronics, materials science, and electrochemistry have spurred the development of wearable sweat sensors that enable the continuous and noninvasive screening of analytes indicative of health status. Existing major challenges in wearable sensors include: improving the sweat extraction and sweat sensing capabilities, improving the form factor of the wearable device for minimal discomfort and reliable measurements when worn, and understanding the clinical value of sweat analytes toward biomarker discovery. This review provides a comprehensive review of wearable sweat sensors and outlines state-of-the-art technologies and research that strive to bridge these gaps. The physiology of sweat, materials, biosensing mechanisms and advances, and approaches for sweat induction and sampling are introduced. Additionally, design considerations for the system-level development of wearable sweat sensing devices, spanning from strategies for prolonged sweat extraction to efficient powering of wearables, are discussed. Furthermore, the applications, data analytics, commercialization efforts, challenges, and prospects of wearable sweat sensors for precision medicine are discussed.

Experimentally designed potentiometric sensor for green real-time and direct assay of hazardous bromate in bakery products

Bakeries add extra potassium bromate to the dough to make homogeneous, elastic, fluffy bread. Bromate causes renal damage and cancer. FAO/WHO stated that bromate residues shouldn't be in baked products. A potentiometric sensor's membrane recipe was optimized for sensitive and selective bromate assay. We planned a custom experimental design of 21 sensors that included numerical and categorical factors (NPPE: PVC, matrix%, membrane thickness, and ionophore type). We defined sensor performance outcomes (Nernstian slope, quantification limit, correlation coefficient, response time and selectivity), and each sensor's outcome was determined. The computer software developed a predictive model for each outcome and the desirability function suggested the optimum sensor recipe. The sensor achieved a slope of -63.54 mV/decade and detection limit of 2 × 10^-6 mol/L. The greenness profile was evaluated by the National Environmental Approach Index protocol. The developed sensor represents a reliable, fast, in-site tool for the assay of bromate in bakery products.

Regulating the Polypyrrole Ion-Selective Membrane and Au Solid Contact Layer to Improve the Performance of Nitrate All-Solid Ion-Selective Electrodes

With polymerization duration and Au³⁺ concentration of the electrolyte regulated, a desirable nitrate-doped polypyrrole ion-selective membrane (PPy(NO₃^-)-ISM) and Au solid contact layer of anticipate surface morphology were obtained, and the performance of nitrate all-solid ion-selective electrodes (NS ISEs) was improved. It was found that the roughest PPy(NO₃^-)-ISM remarkably increases the actual contact surface area of the PPy(NO₃^-)-ISMs with nitrate solution, which leads to better adsorption of NO₃^- ions upon the PPy(NO₃^-)-ISMs, and produces a larger number of electrons. The most hydrophobic Au solid contact layer avoids the formation of the aqueous layer at the interface between the PPy(NO₃^-)-ISM and Au solid contact layer, and ensures unimpeded transporting of the produced electrons. The PPy-Au-NS ISE for polymerization duration 1800 s and at Au³⁺ concentration 2.5 mM of the electrolyte displays an optimal nitrate potential response, including a Nernstian slope of 54.0 mV/dec, LOD of 1.1 × 10^-4 M, rapid average response time less than 1.9 s, and long-term stability of more than 5 weeks. This indicates that the PPy-Au-NS ISE is an effective working electrode for the electrochemical determination of NO₃^- concentration.

Highly hydrophobic TEMPO-functionalized conducting copolymers for solid-contact ion-selective electrodes

Solid-contact ion-selective electrodes (SCISEs) emerged as the best electrode embodiment for miniaturized, wearable and disposable sensors for ion/electrolyte measurements in body fluids. The commercialization of inexpensive single-use "calibration-free" electrodes requires large scale manufacturing of electrodes with reproducible calibration parameters, e.g. E⁰. This is perhaps the most important shortcoming of SCISEs, beside the many advantages over their conventional liquid-contact counterparts. However, adjusting the E⁰ value for optimal potential stability is challenging for all state-of-the-art solid-contact materials, which may combine several types of transducing mechanism (e.g. capacitive and redox materials or their combination) for enhanced potential stability and analytical performance. Therefore, here we introduce for the first time the galvanostatic intermittent titration technique (GITT) to determine the best preadjusment potential. The proof of concept is shown for a novel type of solid-contact based on the copolymerization of 3,4-ethylenedioxythiophene with perfluorinated alkyl side chain (EDOTF) and (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl modified 3,4-ethylenedioxythiophene (EDOT-TEMPO). Such materials that are compliant with local electrodeposition and provide multiple functionalities, i.e. high hydrophobicity by the perfluorinated alkyl side chain, electron-to-ion transduction by the conducting polymer (EDOT) backbone and the confinement of well-defined redox couple (TEMPO), are expected to prevail as solid-contacts.

Directly Using Ti3C2Tx MXene for a Solid-Contact Potentiometric pH Sensor toward Wearable Sweat pH Monitoring

The level of hydrogen ions in sweat is one of the most important physiological indexes for the health state of the human body. As a type of two-dimensional (2D) material, MXene has the advantages of superior electrical conductivity, a large surface area, and rich functional groups on the surface. Herein, we report a type of Ti₃C₂T_x-based potentiometric pH sensor for wearable sweat pH analysis. The Ti₃C₂T_x was prepared by two etching methods, including a mild LiF/HCl mixture and HF solution, which was directly used as the pH-sensitive materials. Both etched Ti₃C₂T_x showed a typical lamellar structure and exhibited enhanced potentiometric pH responses compared with a pristine precursor of Ti₃AlC₂. The HF-Ti₃C₂T_x disclosed the sensitivities of -43.51 ± 0.53 mV pH^-1 (pH 1-11) and -42.73 ± 0.61 mV pH^-1 (pH 11-1). A series of electrochemical tests demonstrated that HF-Ti₃C₂T_x exhibited better analytical performances, including sensitivity, selectivity, and reversibility, owing to deep etching. The HF-Ti₃C₂T_x was thus further fabricated as a flexible potentiometric pH sensor by virtue of its 2D characteristic. Upon integrating with a solid-contact Ag/AgCl reference electrode, the flexible sensor realized real-time monitoring of pH level in human sweat. The result disclosed a relatively stable pH value of ~6.5 after perspiration, which was consistent with the ex situ sweat pH test. This work offers a type of MXene-based potentiometric pH sensor for wearable sweat pH monitoring.

Solid-contact polymeric membrane ion-selective electrodes using a covalent organic framework@reduced graphene oxide composite as ion-to-electron transducer

A solid-contact ion-selective electrode (SC-ISE) based on a covalent organic framework@reduced graphene oxide (rGO) composite is proposed. The composite can be synthesized through the polycondensation of 1,3,5-triformylphloroglucinol (TFP) and 2,6-diaminoanthraquinone (DAAQ) on the rGO nanosheets, which shows high capacitance and good redox-active properties. By applying Cd²⁺-ISE as a model, the electrode exhibits a Nernstian slope of 29.7 ± 0.4 mV/decade in the activity range of 1.0 × 10^-7 - 7.9 × 10^-4 M and the limit of detection is 6.8 × 10^-8 M. Particularly, the electrode based on DAAQ-TFP@rGO exhibits a low potential drift of 1.2 ± 0.2 μV/h over 70 h due to the large capacitance of 2.0 mF. Moreover, the DAAQ-TFP@rGO-based Cd²⁺-ISE shows good reproducibility and the standard deviations of the standard potentials for single batch and batch-to-batch are 0.28 (n = 4) and 0.30 mV (n = 4), respectively. The developed SC-Cd²⁺-ISE is not disturbed by light or gas and no aqueous layer occurs between the sensing membrane and DAAQ-TFP@rGO layer. The DAAQ-TFP@rGO composite is highly promising for construction of calibration-free SC-ISEs.

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.

All-solid-state potentiometric salicylic acid sensor for in-situ measurement of plant

Using PEDOT as the conductive polymer, an innovative small-scale sensor for directly measuring salicylate ions in plants was developed, which avoided the complicated sample pretreatment of traditional analytical methods and realized the rapid detection of salicylic acid. The results demonstrate that this all-solid-state potentiometric salicylic acid sensor is easy to miniaturize, has a longer lifetime (≥1 month), is more robust, and can be directly used for the detection of salicylate ions in real samples without any additional pretreatment. The developed sensor has a good Nernst slope (63.6 ± 0.7 mV/decade), the linear range is 10^-2 ~ 10^-6 M, and the detection limit can reach (2.8 × 10^-7 M). The selectivity, reproducibility, and stability of the sensor were evaluated. The sensor can perform stable, sensitive, and accurate in situ measurement of salicylic acid in plants, and it is an excellent tool for determining salicylic acid ions in plants in vivo.