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

Ion-selective-membrane-free high-pressure potentiometric ammonium ion sensing

The state-of-the-art solid-contact ion-selective electrodes (SC-ISEs) for NH4+ primarily utilize organic carrier-based ion-selective membranes (ISM). However, they face challenges such as the water-layer effect at the SC/ISM interface and the weak mechanical strength of the ISM. In this work, we present an ISM-free, high-pressure potentiometric NH4+ sensor based on a bifunctional transducer, specifically a framework of copper hexacyanoferrate (CuHCF). CuHCF serves as both an ion-to-electron transducer and an NH4+ recognition element. The sensing mechanism involves electron transfer from the Fe redox center coupled with the ion transfer of NH4+ within its framework channels. To further develop an all-solid-state sensor, we integrated a solid-contact reference electrode of silver/silver tetraphenylborate electrode. This all-solid-state NH4+ sensor demonstrates Nernstian response sensitivity and comparable selectivity under 1 MPa pressure. Importantly, it avoids the generation of a water layer and exhibits long-term stability. This work highlights a concept for ISM-free high-pressure potentiometric NH4+ sensing.

Chainmail Structures of CoNi Alloys Encapsulated in Nitrogen-Doped Carbon Nanotubes Empowered Long-Term Stable Detection of Sodium Ions

Low potential drift is one of the performance criteria for designing all-solid-state sodium ion selective electrodes (Na+-SC-ISEs), which directly affects the stability and reliability of detection results. Currently, most attempts primarily focus on improving the hydrophobicity and capacitance of solid-contact (SC) layers to enhance the stability of Na+-SC-ISEs, while neglecting the important impact of the stability and capacitance retention rate of SC materials on the long-term stability of Na+-SC-ISEs. Herein, chainmail-structured nanomaterials are elaborately designed, where CoNi alloys are encapsulated in nitrogen-doped carbon nanotubes (NCNTs), as SC layers for the construction of all-solid sodium ion selective electrodes. The Na+-SC-ISEs based on CoNi-in-NCNTs (CoNi-in-NCNTs/Na+-ISEs) achieve a minimal potential drift of 1.14 µV h-1 during long-term stable detection for 4 days and a commendable capacitance retention rate of 92%. It is revealed by density functional theory (DFT) calculations and kinetic simulations that CoNi alloys continuously penetrate electrons to NCNTs surface, realizing the rapid ion-electron transduction at the SC interface. Besides, NCNTs both serve as physical barriers to the hydrophobic interface to prevent the water layer formation and provide more support sites to restrain CoNi nanoparticles aggregating. Such barrier protection and electron penetration effect of the CoNi-in-NCNTs significantly enhances the long-term stable detection of Na+.

Detection and Explanation of the Hidden Self-Discharge of Single-Walled Carbon-Nanotube Solid Contacts in Ion-Selective Electrodes

Solid contacts made of nonredox-active high-surface-area materials provide ion-selective electrodes comprising an ionophore-doped sensing membrane with a high capacitance. As emphasized in the literature, this minimizes changes in the measured potential that result from the minimal but unavoidable currents of real-life potentiometric measurements. However, as shown here for solid contacts made of single-walled carbon nanotubes (SWCNTs), solid contacts actively charged up over several minutes to voltages as small as ±100 mV do not hold this charge for longer than a few hours. Potential discharge occurs due to Faradaic processes and charge redistribution within the narrow confines of the SWCNT layer. The composition of the sensor membranes and atmospheric conditions have only a small impact on the kinetics of this spontaneous discharge, suggesting that redox reactions involving oxygen and the sensing membrane components do not play critical roles. Because both ion mobilities and the rate of redox reactions are expected to increase with temperature, the significant acceleration of discharge at higher temperature does not clarify whether charge redistribution or redox reactions dominate this discharge. However, contact angle measurements show that SWCNT-modified electrodes without an ion-selective membrane exhibit a substantial decrease in hydrophobicity after prolonged application of a bias potential as small as +100 mV, while application of a negative voltage had only a minor effect. This is consistent with very slow oxidation of the SWCNTs. These findings highlight the importance of optimizing the surface chemistry of high-surface-area solid contacts in view of high long-term stabilities. We propose quick charging of solid contacts to moderate potentials, followed by long-term potential monitoring under zero-current conditions, as a more thorough approach to characterize ISEs with high-surface-area solid contacts, offering insights not available with conventional chronopotentiometry measurements.

High-Reproducibility and -Stability All-Solid-Contact Nitrate Ion-Selective Electrode with CoWSe2 as Solid Contact for Nitrate Monitoring in Wetland Soil

The monitoring of nitrate ions is of great significance for human health, agricultural development, and environmental protection. All-solid-state nitrate ion-selective electrodes (ASS-NO3--ISEs), as an important NO3- analysis method, still have two challenges of poor stability and reproducibility due to the ill-defined phase boundary between the solid-contact (SC) layer and the ion-selective membrane (ISM). In this work, a novel strategy for constructing the ASS-NO3--ISEs based on CoWO4, CoWS4, CoWSe2, or CoSe2 as SC layers was reported for improving the stability and reproducibility. The result shows that the developed CoWSe2-based NO3--ISE exhibits a good Nernstian response slope of -61.9 ± 0.4 mV dec-1 in the activity range from 1.0 × 10-6 to 7.5 × 10-2 M and a detection limit of 1.0 × 10-6 M. A good long-term stability (as low as 2.3 ± 0.4 μV h-1) of the CoWSe2-based NO3--ISE is the primary reason for the high redox capacitance of the ternary selenide. Experimental results show a surprisingly good reproducibility of approximately 0.5 mV for five individual ASS-NO3--ISEs. Notably, electrochemical experiments and scanning electron microscopy mapping tests are used to predict the ion-electron transduction mechanism in which the lipophilic anion (tetrakis(4-chlorophenyl)borate) participates in the transduction process at the SC/ISM interface to stabilize the electrode potential and provide high reproducibility. It was further proved that the introduction of CoWSe2 as the SC layer maintains an excellent anti-interference to water layers, light, and gas. Hence, the CoWSe2-based ASS-NO3--ISEs achieve accurate detection for free NO3- in wetland soil and the estuary of the Yellow River delta.

Next-Generation Potentiometric Sensors: A Review of Flexible and Wearable Technologies

In recent years, the field of wearable sensors has undergone significant evolution, emerging as a pivotal topic of research due to the capacity of such sensors to gather physiological data during various human activities. Transitioning from basic fitness trackers, these sensors are continuously being improved, with the ultimate objective to make compact, sophisticated, highly integrated, and adaptable multi-functional devices that seamlessly connect to clothing or the body, and continuously monitor bodily signals without impeding the wearer's comfort or well-being. Potentiometric sensors, leveraging a range of different solid contact materials, have emerged as a preferred choice for wearable chemical or biological sensors. Nanomaterials play a pivotal role, offering unique properties, such as high conductivity and surface-to-volume ratios. This article provides a review of recent advancements in wearable potentiometric sensors utilizing various solid contacts, with a particular emphasis on nanomaterials. These sensors are employed for precise ion concentration determinations, notably sodium, potassium, calcium, magnesium, ammonium, and chloride, in human biological fluids. This review highlights two primary applications, that is, (1) the enhancement of athletic performance by continuous monitoring of ion levels in sweat to gauge the athlete's health status, and (2) the facilitation of clinical diagnosis and preventive healthcare by monitoring the health status of patients, in particular to detect early signs of dehydration, fatigue, and muscle spasms.

Continuous Monitoring of Lithium Ions in Lithium-Rich Brine Using Ion Selective Electrode Sensors Modified with Polyelectrolyte Multilayers of Poly(allylamine hydrochloride)/Poly(sodium 4-styrenesulfonate)

Monitoring lithium ions (Li+) in lithium-rich brine (LrB) is critical for metal recovery, yet challenges such as high ionic strength and gypsum-induced surface deterioration hinder the performance of potentiometric ion-selective electrode (ISE) sensors. This study advances the functionality of Li+ ISE sensors and enables continuous monitoring of Li+ concentration in LrB by introducing apolyelectrolyte multilayer (PEM) of poly(allylamine hydrochloride)/poly(sodium 4-styrenesulfonate) (PAH/PSS) that serves as an antigypsum scaling material to minimize nucleation on the sensor surface. With 5.5 bilayers of PAH/PSS coating, the Li+ ISE sensors possess a high Nernst slope (59.14 mV/dec), rapid response (<10 s), and superior selectivity against competitive ions (Na+, log Ks = -2.35; K+, log Ks = -2.47; Ca2+, log Ks = -4.05; Mg2+, log Ks = -4.18). The impedance (85.1 kΩ) of (PAH/PSS)5.5-coated sensors is 1 order of magnitude lower than that of electrospray ion-selective membrane (E-ISM) Li+ sensors (830 kΩ), attributed to the ultrathin (45.3 nm) and highly dielectric PAH/PSS bilayers. During a 15-day continuous monitoring test in LrB, the (PAH/PSS)5.5-coated Li+ ISE sensors with their superhydrophilic and smooth surface diminish nucleation sites for scaling agents (e.g., Ca2+ and SO42-) and consequently mitigate gypsum scaling. Moreover, a brine-tailored denoising data processing algorithm (bt-DDPA), coupled with the salinity-adjusted mathematical model with Lagrange interpolation, effectively captures Li+ fluctuation by filtering out anomalies and reducing sensor drift in brine. Bt-DDPA alleviates the discrepancy between the sensor readings and the lab-based validation results by 46.06%. This study demonstrates that the integration of material advancement (PAH/PSS coating) with sensor data processing (bt-DDPA) bolsters continuous and accurate Li+ monitoring in LrB, crucial for brine water treatment and resource recovery.

Advancing Multi-Ion Sensing with Poly-Octylthiophene: 3D-Printed Milker-Implantable Microfluidic Device

On-site rapid multi-ion sensing accelerates early identification of environmental pollution, water quality, and disease biomarkers in both livestock and humans. This study introduces a pocket-sized 3D-printed sensor, manufactured using additive manufacturing, specifically designed for detecting iron (Fe2+), nitrate (NO3 -), calcium (Ca2+), and phosphate (HPO4 2-). A unique feature of this device is its utilization of a universal ion-to-electron transducing layer made from highly redox-active poly-octylthiophene (POT), enabling an all-solid-state electrode tailored to each ion of interest. Manufactured with an extrusion-based 3D printer, the device features a periodic pattern of lateral layers (width = 80 µm), including surface wrinkles. The superhydrophobic nature of the POT prevents the accumulation of nonspecific ions at the interface between the gold and POT layers, ensuring exceptional sensor selectivity. Lithography-free, 3D-printed sensors achieve sensitivity down to 1 ppm of target ions in under a minute due to their 3D-wrinkled surface geometry. Integrated seamlessly with a microfluidic system for sample temperature stabilization, the printed sensor resides within a robust, pocket-sized 3D-printed device. This innovation integrates with milking parlors for real-time calcium detection, addressing diagnostic challenges in on-site livestock health monitoring, and has the capability to monitor water quality, soil nutrients, and human diseases.

Insights into solid-contact ion-selective electrodes based on laser-induced graphene: Key performance parameters for long-term and continuous measurements

This work aims to serve as a comprehensive guide to properly characterize solid-contact ion-selective electrodes (SC-ISEs) for long-term use as they advance toward calibration-free sensors. The lack of well-defined SC-ISE performance criteria limits the ability to compare results and track progress in the field. Laser-induced graphene (LIG) is a rapid and scalable method that, by adjusting the CO2 laser parameters, can create LIG substrates with tunable surface properties, including wettability, surface chemistry, and morphology. Herein, we fabricate laser-induced graphene (LIG) solid-contact electrodes using different laser settings and subsequently convert them into ion-selective sensors using a potassium-selective membrane. We measure the aforementioned tunable surface properties and correlate them with resultant low-frequency capacitance and water layer formation in an effort to pinpoint their effects on the sensitivity (Nernstian response), reproducibility (E°' variation), and potential stability of the LIG-based SC-ISEs. More specifically, we demonstrate that the surface wettability of the LIG substrate, which can be tuned by controlling the lasing parameters, can be modified to exhibit hydrophobic (contact angle > 90°) and even highly hydrophobic surfaces (contact angle ≈ 130°) to help reduce sensor drift. Recommendations are also provided to ensure proper and robust characterization of SC-ISEs for long-term and continuous measurements. Ultimately, we believe that a comprehensive understanding of the correlation between LIG tunable surface properties and SC-ISE performance can be used to improve the electrochemical behavior and stability of SC-ISEs designed with a wide range of materials beyond LIG.

Accounting for the Quantum Capacitance of Graphite in Constant Potential Molecular Dynamics Simulations

Molecular dynamics (MD) simulations at a constant electric potential are an essential tool to study electrochemical processes, providing microscopic information on the structural, thermodynamic, and dynamical properties. Despite the numerous advances in the simulation of electrodes, they fail to accurately represent the electronic structure of materials such as graphite. In this work, a simple parameterization method that allows to tune the metallicity of the electrode based on a quantum chemistry calculation of the density of states (DOS) is introduced. As a first illustration, the interface between graphite electrodes and two different liquid electrolytes, an aqueous solution of NaCl and a pure ionic liquid, at different applied potentials are studied. It is shown that the simulations reproduce qualitatively the experimentally-measured capacitance; in particular, they yield a minimum of capacitance at the point of zero charge (PZC), which is due to the quantum capacitance (QC) contribution. An analysis of the structure of the adsorbed liquids allows to understand why the ionic liquid displays a lower capacitance despite its large ionic concentration. In addition to its relevance for the important class of carbonaceous electrodes, this method can be applied to any electrode materials (e.g. 2D materials, conducting polymers, etc), thus enabling molecular simulation studies of complex electrochemical devices in the future.

Nanoporous Carbon Materials as Solid Contacts for Microneedle Ion-Selective Sensors

Continuous sensing of biomarkers, such as potassium ions or pH, in wearable patches requires miniaturization of ion-selective sensor electrodes. Such miniaturization can be achieved by using nanostructured carbon materials as solid contacts in microneedle-based ion-selective and reference electrodes. Here we compare three carbon materials as solid contacts: colloid-imprinted mesoporous (CIM) carbon microparticles with ∼24-28 nm mesopores, mesoporous carbon nanospheres with 3-9 nm mesopores, and Super P carbon black nanoparticles without internal porosity but with textural mesoporosity in particle aggregates. We compare the effects of carbon architecture and composition on specific capacitance of the material, on the ability to incorporate ion-selective membrane components in the pores, and on sensor performance. Functioning K+ and H+ ion-selective electrodes and reference electrodes were obtained with gold-coated stainless-steel microneedles using all three types of carbon. The sensors gave near-Nernstian responses in clinically relevant concentration ranges, were free of potentially detrimental water layers, and showed no response to O2. They all exhibited sufficiently low long-term potential drift values to permit calibration-free, continuous operation for close to 1 day. In spite of the different specific capacitances and pore architecture of the three types of carbon, no significant difference in potential stability for K+ ion sensing was observed between electrodes that used each material. In the observed drift values, factors other than the carbon solid contact are likely to play a role, too. However, for pH sensing, electrodes with CIM as a carbon solid contact, which had the highest specific capacitance and best access to the pores, exhibited better long-term stability than electrodes with the other carbon materials.

Potassium Ion-Selective Electrodes with BME-44 Ionophores Covalently Attached to Condensation-Cured Silicone Membranes

For ion-selective electrodes (ISEs) to be employed in wearable and implantable applications, the ion-selective membrane components should be biocompatible, and leaching of components, such as plasticizer or ionophore, out of the sensing membrane should be inhibited. To achieve this, we employed a plasticizer-free silicone as the membrane matrix and synthesized as the ionophore a derivative of the bis-crown ether based potassium ionophore BME-44, incorporating a triethoxysilyl functional group that covalently attaches to condensation-cured silicones during the curing process. Soxhlet extraction of these membranes with dichloromethane shows that up to 96% of the ionophore is attached to the silicone membrane during curing. We found that the covalently attachable BME-44 derivative can inadvertently adsorb onto high surface area carbon solid contacts before attaching to the silicone matrix if the curing of the silicone is performed in the presence of the high surface area carbon, resulting in depletion of ionophore from the membrane and yielding solid-contact ISEs with poor selectivity. In contrast, we observed Nernstian responses to K+ in plasticizer-free silicone-based K+ ISMs with either mobile BME-44 or the covalently attachable BME-44 derivative when the membranes were prepared on octane-thiol coated gold electrodes, where ionophore adsorption does not occur to a noticeable extent. As compared with ISMs doped with the mobile BME-44, ISMs prepared with the covalently attachable BME-44 derivative have better selectivity for K+ vs Na+ (log ⁡ K K + , N a + values of -3.54 and <- 4.05 for mobile and covalently attachable BME-44, respectively) and lower resistance. This can be explained by a more homogeneous incorporation of the covalently attachable BME-44 derivative into the silicone matrix than is the case for the mobile BME-44.

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.

Ion-Selective Potentiometry with Plasma-Initiated Covalent Attachment of Sensing Membranes onto Inert Polymeric Substrates and Carbon Solid Contacts

The physical delamination of the sensing membrane from underlying electrode bodies and electron conductors limits sensor lifetimes and long-term monitoring with ion-selective electrodes (ISEs). To address this problem, we developed two plasma-initiated graft polymerization methods that attach ionophore-doped polymethacrylate sensing membranes covalently to high-surface-area carbons that serve as the conducting solid contact as well as to polypropylene, poly(ethylene-co-tetrafluoroethylene), and polyurethane as the inert polymeric electrode body materials. The first strategy consists of depositing the precursor solution for the preparation of the sensing membranes onto the platform substrates with the solid contact carbon, followed by exposure to an argon plasma, which results in surface-grafting of the in situ polymerized sensing membrane. Using the second strategy, the polymeric platform substrate is pretreated with argon plasma and subsequently exposed to ambient oxygen, forming hydroperoxide groups on the surface. Those functionalities are then used for the initiation of photoinitiated graft polymerization of the sensing membrane. Attenuated total reflection-Fourier transform infrared spectroscopy, water contact angle measurements, and delamination tests confirm the covalent attachment of the in situ polymerized sensing membranes onto the polymeric substrates. Using membrane precursor solutions comprising, in addition to decyl methacrylate and a cross-linker, also 2-(diisopropylamino)ethyl methacrylate as a covalently attachable H+ ionophore and tetrakis(pentafluorophenyl)borate as ionic sites, both plasma-based fabrication methods produced electrodes that responded to pH in a Nernstian fashion, with the high selectivity expected for ionophore-based ISEs.