All-Solid-State Reference Electrodes Based on Colloid-Imprinted Mesoporous Carbon and Their Application in Disposable Paper-based Potentiometric Sensing Devices

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.

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.

Spontaneous Mesoporosity-Driven Sequestration of Ionic Liquids from Silicone-Based Reference Electrode Membranes

Nanopore-driven sequestration of ionic liquids from a silicone membrane is presented, a phenomenon that has not been reported previously. Reference electrodes with ionic liquid doped polydimethylsiloxane (PDMS) reference membranes and colloid-imprinted mesoporous carbon (CIM) as solid contact are not functional unless special attention is paid to the porosity of the solid contact. In the fabrication of such reference electrodes, a solution of a hydroxyl-terminated silicone oligomer, ionic liquid, cross-linking reagent, and polymerization catalyst is deposited on top of the carbon layer, rapidly filling the pores of the CIM carbon. The catalyzed polymerization curing of the silicone quickly results in cross-linking of the hydroxyl-terminated polydimethylsiloxane oligomers, forming structures that are too large to penetrate the CIM carbon pores. Therefore, as solvent evaporation from the top of freshly prepared membranes drives the diffusional transport of solvent toward that membrane surface, the solvent molecules that leave the CIM carbon pores can only be replaced by the ionic liquid. This depletes the ionic liquid in the reference membrane that overlies the CIM carbon solid contact and increases the membrane resistance by up to 3 orders of magnitude, rendering the devices dysfunctional. This problem can be avoided by presaturating the CIM carbon with ionic liquid prior to the deposition of the solution that contains the silicone oligomers and ionic liquid. Alternatively, a high amount of ionic liquid can be added into the membrane solution to account for the size-selective sequestration of ionic liquid into the carbon pores. Either way, a wide variety of ionic liquids can be used to prepare PDMS-based reference electrodes with CIM carbon as a solid contact. A similar depletion of the K⁺ ionophore BME-44 from ion-selective silicone membranes was observed too, highlighting that the depletion of active ingredients from polymeric ion-selective and reference membranes due to interactions with high surface area solid contacts may be a more common phenomenon that so far has been overlooked.

Trends in Paper-Based Sensing Devices for Clinical and Environmental Monitoring

Environmental toxic pollutants and pathogens that enter the ecosystem are major global issues. Detection of these toxic chemicals/pollutants and the diagnosis of a disease is a first step in efficiently controlling their contamination and spread, respectively. Various analytical techniques are available to detect and determine toxic chemicals/pathogens, including liquid chromatography, HPLC, mass spectroscopy, and enzyme-linked immunosorbent assays. However, these sensing strategies have some drawbacks such as tedious sample pretreatment and preparation, the requirement for skilled technicians, and dependence on large laboratory-based instruments. Alternatively, biosensors, especially paper-based sensors, could be used extensively and are a cost-effective alternative to conventional laboratory testing. They can improve accessibility to testing to identify chemicals and pollutants, especially in developing countries. Due to its low cost, abundance, easy disposal (by incineration, for example) and biocompatible nature, paper is considered a versatile material for the development of environmentally friendly electrochemical/optical (bio) sensor devices. This review presents an overview of sensing platforms constructed from paper, pointing out the main merits and demerits of paper-based sensing systems, their fabrication techniques, and the different optical/electrochemical detection techniques that they exploit.

A Solid-Contact Reference Electrode Based on Silver/Silver Organic Insoluble Salt for Potentiometric Ion Sensing

Solid-contact ion-selective electrodes are a type of ion measurement devices that have been focused in wearable biotechnology based on the features of miniaturization and integration. However, the solid-contact reference electrodes (SC-REs) remain relatively less focused compared with numerous working (or indicator) electrodes. Most SC-REs in wearable sensors rely on Ag/AgCl reference electrodes with solid electrolytes, for example, the hydrophilic electrolyte salts in polymer matrix, but face the risk of electrolyte leakage. Herein, we report a type of SC-REs based on the silver/silver tetraphenylborate (Ag/AgTPB) organic insoluble electrode. The SC-RE consists of a Ag substrate, a solid contact (AgTPB), and a plasticized poly(vinyl chloride) (PVC) membrane containing the hydrophobic organic salt of tetrabutylammonium tetraphenylborate (TBATPB). The potentiometric measurements demonstrated that the SC-RE of Ag/AgTPB/PVC-TBATPB showed a reproducible standard potential in various electrolytes and disclosed high long-term stability. This SC-RE was further fabricated on a flexible substrate and integrated into all-solid-state wearable potentiometric ion sensor for sweat Cl- monitoring.

The Effect of Paper on the Detection Limit of Paper-Based Potentiometric Chloride Sensors

While paper is an excellent material for use in many other portable sensors, potentiometric paper-based sensors have been reported to perform worse than conventional rod-shaped electrodes, in particular in view of limits of detection (LODs). Reported here is an in-depth study of the lower LOD for Cl^- measurements with paper-based devices comprising AgCl/Ag transducers. Contamination by Cl^- from two commonly used device materials─a AgCl/Ag ink and so-called ashless filter paper─was found to increase the concentration of Cl^- in paper-contained samples far above what is expected for the spontaneous dissolution of the transducer's AgCl, thereby worsening lower LODs. In addition, for the case of Ag⁺, the commonly hypothesized adsorption of metal cations onto filter paper was found not to significantly affect the performance of AgCl/Ag transducers. We note that in the context of chemical analysis, metal impurities of paper are often mentioned in the literature, but Cl^- contamination of paper has been overlooked.

Deep Eutectic Solvents Enhance Stability of Ag/AgCl Solid State Miniaturised Reference Electrode

A new class of solid-state miniaturised reference electrodes with a deep eutectic solvent as an alternate enhancement electrode system is described. A simple and accurate stable electrochemical sensor was prepared by developing a conventional reference electrode using an Ag/AgCl planar micro-reference electrode covered with a PVC polymer. A conductive deep eutectic solvent (DES), ethaline, was added in small quantities and mixed with an internal electrolyte to maintain the Cl− ion concentration in the constructed electrode. The fabricated microelectrode showed good stability, reproducibility, and long-term stability against varying concentrations of different ions. The potential response of the fabricated microelectrode was studied under varying concentrations of Cl− ions in the presence of 0.1 to 1.0% DES in a concentrated electrolyte system (20 mM Na2SO4). The stability of the fabricated microelectrode was addressed against Br− and Cl− ions using different inorganic salts, and the potential measurements were found to be insensitive toward all responsive ions. The stability response of the fabricated microelectrode against Cl− ions was optimised in the presence of 1.0% DES. The experimental data showed good agreement with the potential change of the fabricated electrode in the presence of the supporting DES electrolyte. The liquid junction-free PVC solid-state miniaturised reference electrode demonstrated a constant potentiometric measurement over a long period of time. The concentrated supporting DES electrolyte solution (20 mM) exhibited better stability values and was a more suitable fabricated microelectrode than other additive concentrations. The long-term stability of the developed microelectrode displayed a good lifetime and high stability of around 60 days.

TEMPO-Functionalized Carbon Nanotubes for Solid-Contact Ion-Selective Electrodes with Largely Improved Potential Reproducibility and Stability

Solid-contact ion-selective electrodes (SCISEs) can overcome essential limitations of their counterparts based on liquid contacts. However, attaining a highly reproducible and predictable E⁰, especially between different fabrication batches, turned out to be difficult even with the most established solid-contact materials, i.e., conducting polymers and large-surface-area conducting materials (e.g., carbon nanotubes), that otherwise possess excellent potential stability. An appropriate batch-to-batch E⁰ reproducibility of SCISEs besides aiding the rapid quality control of the electrode manufacturing process is at the core of their "calibration-free" application, which is perhaps the last major challenge for their routine use as single-use "disposable" or wearable potentiometric sensors. Therefore, here, we propose a new class of solid-contact material based on the covalent functionalization of multiwalled carbon nanotubes (MWCNTs) with a chemically stable redox molecule, (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO). This material combines the advantages of (i) the large double-layer capacitance of MWCNT layers, (ii) the adjustable redox couple ratio provided by the TEMPO moiety, (iii) the covalent confinement of the redox couple, and (iv) the hydrophobicity of the components to achieve the potential reproducibility and stability for demanding applications. The TEMPO-MWCNT-based SC potassium ion-selective electrodes (K⁺-SCISEs) showed excellent analytical performance and potential stability with no sign of an aqueous layer formation beneath the ion-selective membrane nor sensitivity toward O₂, CO₂, and light. A major convenience of the fabrication procedure is the E⁰ adjustment of the K⁺-SCISEs by the polarization of the TEMPO-MWCNT suspension prior to its use as solid contact. While most E⁰ reproducibility studies are limited to a single fabrication batch of SCISEs, the use of prepolarized TEMPO-MWCNT resulted also in an outstanding batch-to-batch potential reproducibility. We were also able to overcome the hydration-related potential drifts for the use of SCISEs without prior conditioning and to feature application for accurate K⁺ measurements in undiluted blood serum.

Microfluidic-based ion-selective thermoplastic electrode array for point-of-care detection of potassium and sodium ions

A microfluidic paper-based thermoplastic electrode (TPE) array has been developed for point-of-care detection of Na⁺ and K⁺ ions using a custom-made portable potentiometer. TPEs were fabricated using polystyrene as the binder and two different types of graphite to compare the electrode performance. The newly designed TPE array embedded in a polymethyl methacrylate chip consists of two working electrodes modified with carbon black nanomaterial and an ion-selective membrane, and an all-solid-state reference electrode modified with Ag/AgCl ink and poly(butyl methacrylate-co-methyl methacrylate) membrane via drop-casting. Ion-selective membrane compositions and conditioning steps were optimized. Under optimized conditions, ion-selective TPEs demonstrated fast response time (4 s) and good stability. The TPE array demonstrated a Nernstian behavior for K⁺ with a sensitivity of 59.2 ± 0.2 mV decade^-1 and near-Nernstian response for Na⁺ with a sensitivity of 54.0 ± 1.1 mV decade^-1 in the range 10^-1 - 10^-4 M and 1 -  10^-3 M, respectively. The detection limits were 1 × 10^-5 M and 1 × 10^-4 M for K⁺ and Na⁺, respectively. In addition, a K⁺ and Na⁺ selective microfluidic paper-based analytical device (µPAD) was applied to artificial serum analysis and found in good agreement with average recoveries of 101.3% and 99.7%, respectively, suggesting that the developed ISE array is suitable for detection of sodium and potassium in complex matrix.

Tuning the Structure, Conductivity, and Wettability of Laser-Induced Graphene for Multiplexed Open Microfluidic Environmental Biosensing and Energy Storage Devices

The integration of microfluidics and electrochemical cells is at the forefront of emerging sensors and energy systems; however, a fabrication scheme that can create both the microfluidics and electrochemical cells in a scalable fashion is still lacking. We present a one-step, mask-free process to create, pattern, and tune laser-induced graphene (LIG) with a ubiquitous CO₂ laser. The laser parameters are adjusted to create LIG with different electrical conductivity, surface morphology, and surface wettability without the need for postchemical modification. Such definitive control over material properties enables the creation of LIG-based integrated open microfluidics and electrochemical sensors that are capable of dividing a single water sample along four multifurcating paths to three ion selective electrodes (ISEs) for potassium (K⁺), nitrate (NO₃^-), and ammonium (NH₄⁺) monitoring and to an enzymatic pesticide sensor for organophosphate pesticide (parathion) monitoring. The ISEs displayed near-Nernstian sensitivities and low limits of detection (LODs) (10^-5.01 M, 10^-5.07 M, and 10^-4.89 M for the K⁺, NO₃^-, and NH₄⁺ ISEs, respectively) while the pesticide sensor exhibited the lowest LOD (15.4 pM) for an electrochemical parathion sensor to date. LIG was also specifically patterned and tuned to create a high-performance electrochemical micro supercapacitor (MSC) capable of improving the power density by 2 orders of magnitude compared to a Li-based thin-film battery and the energy density by 3 orders of magnitude compared to a commercial electrolytic capacitor. Hence, this tunable fabrication approach to LIG is expected to enable a wide range of real-time, point-of-use health and environmental sensors as well as energy storage/harvesting modules.

Reliable environmental trace heavy metal analysis with potentiometric ion sensors - reality or a distant dream

Over two decades have passed since polymeric membrane ion-selective electrodes were found to exhibit sufficiently lower detection limits. This in turn brought a great promise to measure trace level concentrations of heavy metals using potentiometric ion sensors at environmental conditions. Despite great efforts, trace analysis of heavy metals using ion-selective electrodes at environmental conditions is still not commercially available. This work will predominantly concentrate on summarizing and evaluating prospects of using potentiometric ion sensors in view of environmental determination of heavy metals in on-site and on-line analysis modes. Challenges associated with development of reliable potentiometric sensors to be operational in environmental conditions will be discussed and reasoning behind unsuccessful efforts to develop potentiometric on-site and on-line environmental ion sensors will be explored. In short, it is now clear that solely lowering the detection limit of the ion-selective electrodes does not guarantee development of successful sensors that would meet the requirement of environmental matrices over long term usage. More pressing challenges of the properties and the performance of the potentiometric sensors must be addressed first before considering extending their sensitivity to low analyte concentrations. These are, in order of importance, selectivity of the ion-selective membrane to main ion followed by the membrane resistance to parallel processes, such as water ingress to the ISM, light sensitivity, change in temperature, presence of gasses in solution and pH and finally resistance of the ion-selective membrane to fouling. In the future, targeted on-site and on-line environmental sensors should be developed, addressing specific environmental conditions. Thus, ion-selective electrodes should be developed with the intention to be suitable to the operational environmental conditions, rather than looking at universal sensor design validated in the idealized and simple sample matrices.

Solid-Contact Electrode with Composite PVC-Based 3D-Printed Membrane. Optimization of Fabrication and Performance

Intense interest in reference electrode design and fabrication has recently been enriched with the application of 3D printing of electrodes with salt-loaded PVC membranes. This type of material is attractive in sensor technology and is challenging to implement in 3D. In this report, several improvements and simplifications in the technology were focused on and supported by a fundamental electrochemical characterization.

Easy-to-Make Capillary-Based Reference Electrodes with Controlled, Pressure-Driven Electrolyte Flow

As solid-contact potentiometric sensors based on novel materials have reached exceptional stabilities with drifts in the low μV/h range and long-term and calibration-free potentiometric measurements gain more and more attention, reference electrode designs that used to be satisfactory for most users do not satisfy the needs of new challenging applications. It is important that the interface between a reference electrode and the sample, often provided by a salt bridge, remains constant in ion composition over time. Excessive restriction of the flow of the bridge electrolyte, e.g., by using nanoporous frits or gelled reference electrolyte solutions, can result in contamination of the salt bridge with sample components and depletion of the reference electrolyte by diffusion into samples. This can be avoided by using salt bridges that flow freely into the sample. However, commonly used reference electrodes with free-flowing junctions often suffer either from experimental difficulties in assuring a minimum flow rate or from excessive flow rates that require frequent replenishing of the bridge electrolyte. To this end, we developed a reference electrode that contains a concentrated electrolyte contacting samples through a 10.2 μm capillary. By applying a minimal pressure of 10.0 kPa, a flow rate of 100 nL/h is achieved. This maintains a constant liquid junction potential at the interface with the sample and avoids contamination of the reference electrode, as evidenced by a potential stability of 6 ± 3 μV/h over 21 days. With such a minimal flow rate, there is no need to refill the reference electrode electrolyte for years.

A screen-printed three-electrode-type sticker device with an accurate liquid junction-type reference electrode

We developed a novel sticker device that can convert any metal or alloy into the working electrode of a three-electrode system, enabling simple and accurate measurement. The sticker, containing a counter electrode and a stable and accurate liquid junction-type reference electrode, is attached to the metal or alloy; meanwhile the surface exposed from a hole in the device functions as the working electrode. This sticker device was fabricated by screen-printing. The polarization curve of the copper and tin-plated copper measured using the sticker device exhibited approximately the same behavior as that obtained for the conventional three-electrode system. The characteristics of various materials can be easily evaluated using this system by only dropping a small amount of solution.

Highly reproducible solid contact ion selective electrodes: Emerging opportunities for potentiometry - A review

The solid contact ion-selective electrodes (SC-ISEs) have been extensively studied in the field of ion sensing as they offer the possibility of miniaturization, are relatively inexpensive in comparison to other analytical techniques and allow straightforward and routine analyses of ions in a number of clinical, environmental and industrial process samples. In recent years, significant interest has grown in the development of SC-ISEs with well-defined interfacialpotentials at the membrane, solid contact, and substrate electrode interfaces. This has resulted in interesting SC-ISEs exhibiting high electrode-to-electrode potential reproducibility, for those made in a single batch of electrodes, some approaching or exceeding those observed in liquid-contact ISEs. The advancement in the potential reproducibility of SC-ISEs has been partially achieved by scrutinizing insufficiently reproducible fabrication methods of SC-ISEs, or by introducing novel control measures or modifiers to components of the ISEs. This paper provides an overview of the methods as well as the challenges in establishing and maintaining reproducible potentials during the fabrication and use of novel SC-ISEs. The rules outlined in the works reviewed may form the basis of further development of cost-effective, user-friendly, limited calibration or calibration-free potentiometric SC-ISEs to achieve reliable ion analyses here and now.

A New and All-Solid-State Potentiometric Aluminium Ion Sensor for Water Analysis

An all-solid-state potentiometric electrode system for aluminium ion determination was developed with a new aluminium ion sensor as the working electrode based on a new ionophore for aluminium ion, 1,1'-[(methylazanediyl)bis(ethane-2,1-diyl)]bis[3-(naphthalen-1-yl)thiourea] (ACH). The reference electrode was a potassium ion sensor, which acts as a pseudo-reference. Both electrodes were made from Ag/AgCl screen-print electrodes fabricated from a non-plasticized and photocurable poly(n-butyl acrylate) membrane that contained various other membrane components. The pseudo-reference potential based on the potassium ion sensor was fixed in 0.050 M KNO₃, and such concentration of K⁺ ion did not interfere with the measurement of the Al³⁺ ion using the aluminium sensor. With such a pseudo-reference and in the presence of 0.050 M KNO₃ as a background medium, the aluminium sensor measured changes of aluminium ion concentrations linearly from 10^-6 to 10^-2 M Al³⁺ ion with a Nernstian response of 17.70 ± 0.13 mV/decade. A low detection limit of 2.45 × 10^-7 M was achieved with this all-solid-state potentiometric system. The aluminium sensor was insensitive to pH effects from 2.0 to 8.0 with a response time of less than 50 s. Under optimum conditions, a lifetime of 49 days was achieved with good sensor selectivity, reversibility, repeatability, and reproducibility. The all-solid-state electrode system was applied to analyze the Al³⁺ ion content of water samples from a water treatment plant. Compared with the conventional potentiometric detection system for aluminium ions, the new all-solid-state aluminium ion sensor incorporating a pseudo-reference from the potassium sensor demonstrated similar analytical performance. It thus provided a convenient means of aluminium content analysis in water treatment plants.

Highly Sensitive Electrochemical Detection of Folic Acid by Using a Hollow Carbon Nanospheres@molybdenum Disulfide Modified Electrode

As a nutrient in body functions, folic acid (FA) plays a very important role for human health, and thus developing a highly sensitive method for its determination is of great significance. In the present work, carbon hollow nanospheres decorated with molybdenum disulfide nanosheets (CHN@MoS₂) nanomaterials were produced through a simple method and adopted to modify a glassy carbon electrode for assembling a highly sensitive electrochemical sensor of FA. After characterizing the prepared nanomaterials using scanning-/transmission-electron microscopy and Raman spectra, as well as optimizing various testing conditions, including the pH value of the buffer solution, the accumulation time and amount of nanomaterials on electrode surface, and the electrochemical determination of FA was carried out using a CHN@MoS₂ electrode. Owing to the coordinative advantages from CHN and MoS₂, the results show that CHN@MoS₂ exhibits excellent sensing responses for FA, and it has a wide linear range from 0.08 to 10.0 μM coupled with a low detection limit of 0.02 μM. Finally, the proposed method for FA detection was successfully applied in human urine analysis. The obtained results are satisfactory, revealing that the developed method based on CHN@MoS₂ nanomaterials has important applications for FA determination.

A high flexibility all-solid contact sulfide selective electrode using a graphene transducer

In this paper, a novel high flexibility all-solid contact ion selective electrode (ASC-ISE) based on reduced graphene sheets (RGSs) as the ion-to-electron transducer was developed for rapid detection of sulfide. A graphene layer was firstly electrodeposited on a flexible silver wire by direct reduction of graphene oxide, and nanostructured Ag2S was then prepared as the selective membrane by electrodeposition. Scanning electrochemical microscopy was performed for the characterization of the morphological properties of the RGSs and Ag2S membranes. The evaluation of the analytical performances, such as the linear range, selectivity, stability, and practical application, of the proposed ASC-ISEs for the rapid detection of sulfide was performed. The results showed that, the ASC-ISEs exhibited a linear relationship between the obtained potential signal and sulfide concentration in the range of 0.50 μM to 1.0 mM, with a detection limit of 0.18 μM. Moreover, the ASC-ISEs showed good selectivity towards sulfide over other common interfering ions, and maintained a stable electrochemical response over 7 days. These results demonstrated that graphene was a promising material as the ion-to-electron transducer layer in the development of ASC-ISEs for sulfide detection, and the results of practical applications in tap water and seawater samples showed that the ASC-ISEs held significant promise in a broad range of applications.

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.

Comparison of Different Commercial Conducting Materials as Ion-to-Electron Transducer Layers in Low-Cost Selective Solid-Contact Electrodes

Simple, robust, sensitive and low-cost all-solid-state ion-selective electrodes (SCISEs) are of interest in different fields, such as medicine, veterinary, water treatment, food control, environmental and pollution monitoring, security, etc. as a replacement for traditional ion-selective electrodes with liquid inner contact. In spite of their potential advantages, SCISEs remain mainly in the research laboratories. With the motivation of developing simple and low-cost SCISEs with possible commercial applications, we report a comparison study of six different commercial conducting materials, namely, polypyrrole-block-polycaprolactone (PPy-b-PCaprol), graphene/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) ink, poly(3,4-ethylenedioxythiophene):polyethylenglycol (PEDOT:PEG), high conductivity PEDOT:PSS, polyethylenimine (PEI) with PEDOT:PSS for their possible use as ion-to-electron transducer in polyurethane based pH-SCISEs. Among all studied pH-SCISES, PEDOT:PEG based electrodes exhibited the best results in terms of sensitivity, reproducibility and lifetime. Finally, these sensors were tested in different real samples showing good accuracy.

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).

Reference Electrodes with Ionic Liquid Salt Bridge: When Will These Innovative Novel Reference Electrodes Gain Broad Acceptance?

In this paper, we raise questions that researchers have to ask if they intend to replace a conventional reference electrode with an ionic liquid-based reference electrode and try to answer these questions based on our experiences and literature data. Among these questions, the most important is which ionic liquid should be used. However, beyond the chemical composition of the ionic liquid, to realize all the potential benefits of ionic-liquid based reference electrodes, there are additional, equally important considerations. Through examples we will show the importance of the (i) purity of the ionic liquid and the consequences of deviations from its stoichiometric salt composition, (ii) form of implementation of the ionic liquid-based reference electrode membrane (free-flowing salt bridge, or ionic liquid embedded in a membrane), (iii) membrane/gelling agent material and its composition, and (iv) experimental conditions (steady state or flowing conditions) under which it will be used. Finally, we recommend methods to test the performance criteria of the ionic liquid-based reference electrodes.

Ion sensing with thread-based potentiometric electrodes

Potentiometric sensing of ions with ion-selective electrodes (ISEs) is a powerful technique for selective and sensitive measurement of ions in complex matrices. The application of ISEs is generally limited to laboratory settings, because most commercially available ISEs and reference electrodes are large, delicate, and expensive, and are not suitable for point-of-use or point-of-care measurements. This work utilizes cotton thread as a substrate for fabrication of robust and miniaturized ISEs that are suitable for point-of-care or point-of-use applications. Thread-based ISEs selective for Cl-, K+, Na+, and Ca2+ were developed. The cation-selective ISEs were fabricated by coating the thread with a surfactant-free conductive ink (made of carbon black) and then coating the tip of the conductive thread with the ion-selective membrane. The Cl- ISE was fabricated by coating the thread with an Ag/AgCl ink. These sensors exhibited slopes (of electrical potential vs. log concentration of target ion), close to the theoretically-expected values, over four orders of magnitude in concentrations of ions. Because thread is mechanically strong, the thread-based electrodes can be used in multiple-use applications as well as single-use applications. Multiple thread-based sensors can be easily bundled together to fabricate a customized sensor for multiplexed ion-sensing. These electrodes require volumes of sample as low as 200 μL. The application of thread-based ISEs is demonstrated in the analysis of ions in soil, food, and dietary supplements (Cl- in soil/water slurry, K+ and Na+ in coconut water, and Ca2+ in a calcium supplement), and in detection of physiological electrolytes (K+ and Na+ in blood serum and urine, with sufficient accuracy for clinical diagnostics).

Internal Calibration Potentiometric Aptasensors for Simultaneous Detection of Hg2+, Cd2+, and As3+ Based on a Screen-Printed Carbon Electrodes Array

An all-solid-state potentiometric aptasensor array based on a multichannel disposable screen-printed carbon electrode (SPCE) was demonstrated for the simultaneous detection of Hg²⁺, Cd²⁺, and As³⁺ by open circuit potential (OCP) technology. The potential of the channel with an internal calibration DNA sequence (IC-DNA) was employed as the internal calibration potential (ICP) to subtract the background signal generated by the detection system, providing a built-in correction methodology. As a result, the developed aptasensor array showed high sensitivity and accuracy for detecting Hg²⁺, Cd²⁺, and As³⁺ without mutual interference or interference from other ions. The linear response ranged from 2.5 pM to 2.5 μM, and the detection limits for Hg²⁺, Cd²⁺, and As³⁺ were 2.0, 0.62, and 0.17 pM, respectively. Furthermore, the potentiometric aptasensor array was successfully applied for the simultaneous detection of three ions in real samples. The results obtained from the developed approach agreed well with the results obtained from inductively coupled plasma mass spectrometry.

Conductive Metal-Organic Frameworks as Ion-to-Electron Transducers in Potentiometric Sensors

This paper describes an unexplored property of conductive metal-organic frameworks (MOFs) as ion-to-electron transducers in the context of potentiometric detection. Several conductive two-dimensional MOF analogues were drop-cast onto a glassy carbon electrode and then covered with an ion-selective membrane to form a potentiometric sensor. The resulting devices exhibited excellent sensing properties toward anions and cations, characterized by a near-Nernstian response and over 4 orders of magnitude linear range. Impedance and chronopotentiometric measurements revealed the presence of large bulk capacitance (204 ± 2 μF) and good potential stability (drift of 11.1 ± 0.5 μA/h). Potentiometric water test and contact angle measurements showed that this class of materials exhibited hydrophobicity and inhibited the formation of water layer at the electrode/membrane interface, resulting in a highly stable sensing response with a potential drift as low as 11.1 μA/h. The property of ion-to-electron transduction of conductive MOFs may form the basis for the development of this class of materials as promising components within ion-selective electrodes.

Recent advances in functionalized micro and mesoporous carbon materials: synthesis and applications

Functionalized nanoporous carbon materials have attracted the colossal interest of the materials science fraternity owing to their intriguing physical and chemical properties including a well-ordered porous structure, exemplary high specific surface areas, electronic and ionic conductivity, excellent accessibility to active sites, and enhanced mass transport and diffusion. These properties make them a special and unique choice for various applications in divergent fields such as energy storage batteries, supercapacitors, energy conversion fuel cells, adsorption/separation of bulky molecules, heterogeneous catalysts, catalyst supports, photocatalysis, carbon capture, gas storage, biomolecule detection, vapour sensing and drug delivery. Because of the anisotropic and synergistic effects arising from the heteroatom doping at the nanoscale, these novel materials show high potential especially in electrochemical applications such as batteries, supercapacitors and electrocatalysts for fuel cell applications and water electrolysis. In order to gain the optimal benefit, it is necessary to implement tailor made functionalities in the porous carbon surfaces as well as in the carbon skeleton through the comprehensive experimentation. These most appealing nanoporous carbon materials can be synthesized through the carbonization of high carbon containing molecular precursors by using soft or hard templating or non-templating pathways. This review encompasses the approaches and the wide range of methodologies that have been employed over the last five years in the preparation and functionalisation of nanoporous carbon materials via incorporation of metals, non-metal heteroatoms, multiple heteroatoms, and various surface functional groups that mostly dictate their place in a wide range of practical applications.

Recent Trends on Electrochemical Sensors Based on Ordered Mesoporous Carbon

The past decade has seen an increasing number of extensive studies devoted to the exploitation of ordered mesoporous carbon (OMC) materials in electrochemistry, notably in the fields of energy and sensing. The present review summarizes the recent achievements made in field of electroanalysis using electrodes modified with such nanomaterials. On the basis of comprehensive tables, the interest in OMC for designing electrochemical sensors is illustrated through the various applications developed to date. They include voltammetric detection after preconcentration, electrocatalysis (intrinsically due to OMC or based on suitable catalysts deposited onto OMC), electrochemical biosensors, as well as electrochemiluminescence and potentiometric sensors.

Toward practical application of paper-based microfluidics for medical diagnostics: state-of-the-art and challenges

Microfluidic paper-based analytical devices (μPADs) have emerged as a promising diagnostic platform a decade ago. In contrast to highly active academic developments, their entry into real-life applications is still very limited. This discrepancy is attributed to the gap between research developments and their practical utility, particularly in the aspects of operational simplicity, long-term stability of devices, and associated equipment. On the basis of these backgrounds, this review attempts to: 1) identify the reasons for success of paper-based devices already in the market, 2) describe the current status and remaining issues of μPADs in terms of operational complexity, signal interpretation approaches, and storage stability, and 3) discuss the possibility of mass production based on established manufacturing technologies. Finally, the state-of-the-art in commercialisation of μPADs is discussed, and the "upgrades" required from a laboratory-based prototype to an end user device are demonstrated on a specific example.

A solvent-free microbial-activated air cathode battery paper platform made with pencil-traced graphite electrodes

We present the fabrication of an ultra-low cost, disposable, solvent-free air cathode all-paper microbial fuel cell (MFC) that does not utilize any chemical treatments. The anode and cathode were fabricated by depositing graphite particles by drawing them on paper with a pencil (four strokes). Hydrophobic parchment paper was used as a proton exchange membrane (PEM) to allow only H⁺ to pass. Air cathode MFC technology, where O₂ was used as an electron acceptor, was implemented on the paper platform. The bioelectric current was generated by an electrochemical process involving the redox couple of microbial-activated extracellular electron transferred electrons, PEM-passed H⁺, and O₂ in the cathode. A fully micro-integrated pencil-traced MFC showed a fast start-time, producing current within 10 s after injection of bacterial cells. A single miniaturized all-paper air cathode MFC generated a maximum potential of 300 mV and a maximum current of 11 μA during 100 min after a single injection of Shewanella oneidensis. The micro-fabricated solvent-free air cathode all-paper MFC generated a power of 2,270 nW (5.68 mW/m²). The proposed solvent-free air cathode paper-based MFC device could be used for environmentally-friendly energy storage as well as in single-use medical power supplies that use organic matter.