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.

Novel Lignin-Functionalized Waterborne Epoxy Composite Coatings with Excellent Anti-Aging, UV Resistance, and Interfacial Anti-Corrosion Performance.

Developing high-performance lignin anti-corrosive waterborne epoxy (WEP) coatings is conducive to the advancement of environmentally friendly coatings and the value-added utilization of lignin. In this work, a functionalized biomass waterborne epoxy composite coating is prepared using quaternized sodium lignosulfonate (QLS) as a functional nanofiller for mild carbon steel protection. The results showed that QLS has excellent dispersion and interface compatibility within WEP, and its abundant phenolic hydroxyl, sulfonate, quaternary ammonium groups, and nanoparticle structure endowed the coating with excellent corrosion inhibition and superior barrier properties. The corrosion inhibition efficiency of 100 mg L-1 QLS in carbon steel immersed in a 3.5 wt% NaCl solution reached 95.76%. Furthermore, the coating maintained an impedance modulus of 2.29 × 106 Ω cm2 (|Z|0.01 Hz) after being immersed for 51 days in the high-salt system. In addition, QLS imparted UV-blocking properties and thermal-oxygen aging resistance to the coating, as evidenced by a |Z|0.01 Hz of 1.04 × 107 Ω cm2 after seven days of UV aging while still maintaining a similar magnitude as before aging. The green lignin/WEP functional coatings effectively withstand the challenging outdoor environment characterized by high salt concentration and intense UV radiation, thereby demonstrating promising prospects for application in metal protection.

Beyond Nonactin: Potentiometric Ammonium Ion Sensing Based on Ion-selective Membrane-free Prussian Blue Analogue Transducers.

The determination of ammonium ions (NH4+) is of significance to environmental, agriculture, and human health. Potentiometric NH4+ sensors based on solid-contact ion selective electrodes (SC-ISEs) feature point-of-care testing and miniaturization. However, the state-of-the-art SC-ISEs of NH4+ during the past 20 years strongly rely on the organic ammonium ionophore-based ion selective membrane (ISM), typically by nonactin for the NH4+ recognition. Herein, we report a Prussian blue analogue of copper(II)-hexacyanoferrate (CuHCF) for an ISM-free potentiometric NH4+ sensor without using the ionophores. CuHCF works as a bifunctional transducer that could realize the ion-to-electron transduction and NH4+ recognition. CuHCF exhibits competitive analytical performances regarding traditional nonactin-based SC-ISEs of NH4+, particularly for the selectivity toward K+. The cost and preparation process have been remarkably reduced. The theoretical calculation combined with electrochemical tests further demonstrate that relatively easier intercalation of NH4+ into the lattices of CuHCF determines its selectivity. This work provides a concept of the ISM-free potentiometric NH4+ sensor beyond the nonactin ionophore through a CuHCF bifunctional transducer.

Solid-Contact Ion Sensing Without Using an Ion-Selective Membrane through Classic Li-Ion Battery Materials.

The solid-contact ion-selective electrodes (SC-ISEs) are a type of potentiometric analytical device with features of rapid response, online analysis, and miniaturization. The state-of-the-art SC-ISEs are composed of a solid-contact (SC) layer and an ion-selective membrane (ISM) layer with respective functions of ion-to-electron transduction and ion recognition. Two challenges for the SC-ISEs are the water-layer formation at the SC/ISM phase boundary and the leaking of ISM components, which are both originated from the ISM. Herein, we report a type of SC-ISE based on classic Li-ion battery materials as the SC layer without using the ISM for potentiometric lithium-ion sensing. Both LiFePO4- and LiMn2O4-based SC-ISEs display good Li+ sensing properties (sensitivity, selectivity, and stability). The proposed LiFePO4 electrode exhibits comparable sensitivity and a linear range to conventional SC-ISEs with ISM. Owing to the nonexistence of ISM, the LiFePO4 electrode displays high potential stability. Besides, the LiMn2O4 electrode shows a Nernstian response toward Li+ sensing in a human blood serum solution. This work emphasizes the concept of non-ISM-based SC-ISEs for potentiometric ion sensing.

Amplified Electrochemical Biosensing of Thrombin Activity by RAFT Polymerization.

As a serine protease, thrombin is a pivotal component in coagulation cascade and has been frequently screened as an informative biomarker for the diagnosis of coagulation disorder-related diseases. Herein, a "signal-on" electrochemical biosensor is described for the highly sensitive and selective detection of thrombin activity, by exploiting a thrombin-specific substrate peptide (Tb peptide) as the recognition element and reversible addition-fragmentation chain transfer (RAFT) polymerization for signal amplification. Specifically, the carboxyl-group-free Tb peptides are self-assembled onto gold electrode surface via the N-terminal cysteine residue and are used for the specific recognition of thrombin molecules. After the proteolytic cleavage of the Tb peptides, the carboxyl-group-containing RAFT agents (4-cyano-4-(phenylcarbonothioylthio)pentanoic acid, CPAD) are tethered to the free carboxyl termini of the truncated peptide fragments via the carboxylate-zirconium-carboxylate chemistry. The subsequent RAFT polymerization leads to the grafting of a polymer chain from each proteolytically cleaved site, enabling the recruitment of a large number of electroactive ferrocene (Fc) tags to the electrode surface when ferrocenylmethyl methacrylate (FcMMA) is used as the monomer. Under optimal conditions, the detection limit of the described thrombin biosensor is as low as 2.7 µU mL-1 (∼0.062 pM), with a linear response over the range of 10-250 µU mL-1 (R2 = 0.997). Results also indicate that the biosensor is highly selective and applicable to the detection of thrombin activity in complex serum samples and the screening of thrombin inhibitors. The described biosensor is low-cost and relatively easy in preparation and thus shows great promise for the highly sensitive and selective detection of thrombin activity.

Electrochemically Controlled ATRP for Cleavage-Based Electrochemical Detection of the Prostate-Specific Antigen at Femtomolar Level Concentrations.

As a single-chain glycoprotein with endopeptidase activity, the prostate-specific antigen (PSA) is valuable as an informative serum marker in diagnosing, staging, and prognosis of prostate cancer. In this report, an electrochemical biosensor based on the target-induced cleavage of a specific peptide substrate (PSA peptide) is designed for the highly selective detection of PSA at the femtomolar level, using electrochemically controlled atom transfer radical polymerization (eATRP) as a method for signal amplification. The PSA peptides, without free carboxyl sites, are attached to the gold surface via the N-terminal cysteine residue. The target-induced cleavage of PSA peptides results in the generation of carboxyl sites, to which the alkyl halide initiator α-bromophenylacetic acid (BPAA) is linked via the Zr(IV) linkers. Subsequently, the potentiostatic eATRP of ferrocenylmethyl methacrylate (FcMMA, as the monomer) leads to the surface-initiated grafting of high-density ferrocenyl polymers. As a result, a large amount of Fc redox tags can be recruited for signal amplification, through which the limit of detection (LOD) for PSA can be down to 3.2 fM. As the recognition element, the PSA peptide is easy to synthesize, chemically and thermally stable, and low-cost. Without the necessity of enzyme or nanoparticle labels, the eATRP-based amplification method is easy to operate and low-cost. Results also show that the cleavage-based electrochemical PSA biosensor is highly selective and applicable to PSA detection in complex biological samples. In view of these merits, the integration of the eATRP-based amplification method into cleavage-based recognition is believed to hold great promise for the electrochemical detection of PSA in clinical applications.

Construction of Bimetallic Selenides Encapsulated in Nitrogen/Sulfur Co-Doped Hollow Carbon Nanospheres for High-Performance Sodium/Potassium-Ion Half/Full Batteries.

Metallic selenides have been widely investigated as promising electrode materials for metal-ion batteries based on their relatively high theoretical capacity. However, rapid capacity decay and structural collapse resulting from the larger-sized Na+ /K+ greatly hamper their application. Herein, a bimetallic selenide (MoSe2 /CoSe2 ) encapsulated in nitrogen, sulfur-codoped hollow carbon nanospheres interconnected reduced graphene oxide nanosheets (rGO@MCSe) are successfully designed as advanced anode materials for Na/K-ion batteries. As expected, the significant pseudocapacitive charge storage behavior substantially contributes to superior rate capability. Specifically, it achieves a high reversible specific capacity of 311 mAh g-1 at 10 A g-1 in NIBs and 310 mAh g-1 at 5 A g-1 in KIBs. A combination of ex situ X-ray diffraction, Raman spectroscopy, and transmission electron microscopy tests reveals the phase transition of rGO@MCSe in NIBs/KIBs. Unexpectedly, they show quite different Na+ /K+ insertion/extraction reaction mechanisms for both cells, maybe due to more sluggish K+ diffusion kinetics than that of Na+ . More significantly, it shows excellent energy storage properties in Na/K-ion full cells when coupled with Na3 V2 (PO4 )2 O2 F and PTCDA@450 °C cathodes. This work offers an advanced electrode construction guidance for the development of high-performance energy storage devices.

Compactly Coupled Nitrogen-Doped Carbon Nanosheets/Molybdenum Phosphide Nanocrystal Hollow Nanospheres as Polysulfide Reservoirs for High-Performance Lithium-Sulfur Chemistry.

Lithium-sulfur (Li-S) batteries have been disclosed as one of the most promising energy storage systems. However, the low utilization of sulfur, the detrimental shuttling behavior of polysulfides, and the sluggish kinetics in electrochemical processes, severely impede their application. Herein, 3D hierarchical nitrogen-doped carbon nanosheets/molybdenum phosphide nanocrystal hollow nanospheres (MoP@C/N HCSs) are introduced to Li-S batteries via decorating commercial separators to inhibit polysulfides diffusion. It acts not only as a polysulfides immobilizer to provide strong physical trapping and chemical anchoring toward polysulfides, but also as an electrocatalyst to accelerate the kinetics of the polysulfides redox reaction, and to lower the Li2 S nucleation/dissolution interfacial energy barrier and self-discharge capacity loss in working Li-S batteries, simultaneously. As a result, the Li-S batteries with MoP@C/N HCS-modified separators show superior rate capability (920 mAh g-1 at 2 C) and stable cycling life with only 0.04% capacity decay per cycle over 500 cycles at 1 C with nearly 100% Coulombic efficiency. Furthermore, the Li-S battery can achieve a high area capacity of 5.1 mAh cm-2 with satisfied capacity retention when the cathode loading reaches 5.5 mg cm-2 . This work offers a brand new guidance for rational separator design into the energy chemistry of high-stable Li-S batteries.

Surface-Initiated-Reversible-Addition-Fragmentation-Chain-Transfer Polymerization for Electrochemical DNA Biosensing.

Sensitive detection of biomolecules is integral for biomarker screening and early diagnosis. Herein, surface-initiated reversible-addition-fragmentation-chain-transfer (SI-RAFT) polymerization is exploited as a novel amplification strategy for highly sensitive electrochemical biosensing of DNA. Briefly, thiol-terminated peptide nucleic acid (PNA) probes are first self-assembled onto a gold electrode for the specific capture of target-DNA fragments; the carboxyl-group-containing dithiobenzoate 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid (CPAD) is then tethered to the hybridized PNA-DNA heteroduplexes by means of the well-established carboxylate-Zr4+-phosphate chemistry and serves as the chain-transfer agent (CTAs) for subsequent SI-RAFT polymerization, which is thermally initiated in the presence of 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (VA-044) as the free-radical initiator and ferrocenylmethyl methacrylate (FcMMA) as the monomer. Through SI-RAFT polymerization, one target-DNA fragment can be labeled by a large number of electroactive Fc tags, giving rise to significant amplification of the electrochemical signal. The SI-RAFT-polymerization-based strategy does not involve the use of natural enzymes or complex nanomaterials, offering the benefits of low cost and easy operation. Under optimal conditions, the electrochemical signal is linearly related to the logarithm of the concentration of target DNA over the range from 10 aM to 10 pM ( R2 = 0.997), with a detection limit down to 3.2 aM, which is much lower than those of other amplification-by-polymerization-based methods. By virtue of its easy operation, low cost, and high efficiency, the SI-RAFT-polymerization-based amplification strategy is believed to have great application prospects in the sensitive detection of biomolecules.

Self-assembling graphene-anthraquinone-2-sulphonate supramolecular nanostructures with enhanced energy density for supercapacitors.

Boosting the energy density of capacitive energy storage devices remains a crucial issue for facilitating applications. Herein, we report a graphene-anthraquinone supramolecular nanostructure by self-assembly for supercapacitors. The sulfonated anthraquinone exhibits high water solubility, a π-conjugated structure and redox active features, which not only serve as a spacer to interact with and stabilize graphene but also introduce extra pseudocapacitance contributions. The formed nest-like three-dimensional (3D) nanostructure with further hydrothermal treatment enhances the accessibility of ion transfer and exposes the redox-active quinone groups in the electrolytes. A fabricated all-solid-state flexible symmetric device delivers a high specific capacitance of 398.5 F g-1 at 1 A g-1 (1.5 times higher than graphene), superior energy density (52.24 Wh kg-1 at about 1 kW kg-1) and good stability (82% capacitance retention after 10 000 cycles).

Electrochemically Driven Surface-Confined Acid/Base Reaction for an Ultrafast H(+) Supercapacitor.

We discovered an organic weak acid, 3,4,9,10-perylene tetracarboxylic acid (PTCA), confined on the electrode surface, revealing a reversible and ultrafast protonation/deprotonation non-Faradaic process but exhibiting analogous voltammetric peaks (capacitive peaks). A further synthesized PTCA-graphene supramolecular nanocomplex discloses a wide voltage window (1.2 V) and ultrahigh specific capacitance up to 143 F g(-1) at an ultrafast charge-discharge density of 1000 A g(-1) (at least 1 order of magnitude faster than present speeds). The capacitance retention maintained at 73% after 5000 cycles. This unique capacitive voltammetric behavior suggests a new type of charge-storage modes, which may offer a way for overcoming the present difficulties of supercapacitors.

Probing Bio-Nano Interactions between Blood Proteins and Monolayer-Stabilized Graphene Sheets.

Meeting proteins is regarded as the starting event for nanostructures to enter biological systems. Understanding their interactions is thus essential for a newly emerging field, nanomedicine. Chemically converted graphene (CCG) is a wonderful two-dimensional (2D) material for nanomedicine, but its stability in biological environments is limited. Systematic probing on the binding of proteins to CCG is currently lacking. Herein, we report a comprehensive study on the interactions between blood proteins and stabilized CCG (sCCG). CCG nanosheets are functionalized by monolayers of perylene leading to significant improvement in their resistance to electrolyte salts and long-term stability, but retain their core structural characteristics. Five types of model human blood proteins including human fibrinogen, γ-globulin, bovine serum albumin (BSA), insulin, and histone are tested. The main driving forces for blood protein binding involve the π-π interacations between the π-plane of sCCG and surface aromatic amonic acid (sAA) residues of proteins. Several key binding parameters including the binding amount, Hill coefficient, and binding constant are determined. Through a detailed analysis of key controlling factors, we conclude that the protein binding to sCCG is determined mainly by the protein size, the number, and the density of the sAA.

Intermixed adatom and surface-bound adsorbates in regular self-assembled monolayers of racemic 2-butanethiol on Au(111).

In situ scanning tunneling microscopy combined with density functional theory molecular dynamics simulations reveal a complex structure for the self-assembled monolayer (SAM) of racemic 2-butanethiol on Au(111) in aqueous solution. Six adsorbate molecules occupy a (10×√3)R30° cell organized as two RSAuSR adatom-bound motifs plus two RS species bound directly to face-centered-cubic and hexagonally close-packed sites. This is the first time that these competing head-group arrangements have been observed in the same ordered SAM. Such unusual packing is favored as it facilitates SAMs with anomalously high coverage (30%), much larger than that for enantiomerically resolved 2-butanethiol or secondary-branched butanethiol (25%) and near that for linear-chain 1-butanethiol (33%).

Engineered photoelectrochemical platform for rational global antioxidant capacity evaluation based on ultrasensitive sulfonated graphene-TiO2 nanohybrid.

Dietary antioxidants as health promoters for human beings have attracted much attention and triggered tremendous efforts in evaluation of the antioxidant capacity. Unfortunately, no versatile detection system has been designed to date. Due to the possible synergistic effect among antioxidant components in a diversified system, to isolate and quantify an individual antioxidant via a chromatography approach limits the scope for global antioxidant activity assay. Quality inspections with a spectroscopy strategy to any colored food are far from satisfactory. Herein, a photoelectrochemical (PEC) platform with an ultrasensitive titanium dioxide decorated sulfonated graphene (SGE-TiO2) based transducer was introduced for antioxidant monitoring. Under an open circuit potential (zero potential), with extraordinary response, excellent reproducibility and stability, this PEC sensor could be successfully applied for rational analysis of the global antioxidant capacity. Such a highly efficient strategy showed advantages such as simplicity, convenience, high sensitivity and universality, which were also applicable to the detection of colored system. Moreover, the PEC sensor could be employed for practical evaluation of antioxidant capacity of teas. The concerned mechanism was further proposed and adequately discussed. This straightforward yet powerful approach provides a general format for dietary antioxidant assessment in foodstuff industries.

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.

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.

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 Ti3C2Tx-based potentiometric pH sensor for wearable sweat pH analysis. The Ti3C2Tx 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 Ti3C2Tx showed a typical lamellar structure and exhibited enhanced potentiometric pH responses compared with a pristine precursor of Ti3AlC2. The HF-Ti3C2Tx 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-Ti3C2Tx exhibited better analytical performances, including sensitivity, selectivity, and reversibility, owing to deep etching. The HF-Ti3C2Tx 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.

Fast and sensitive coulometric signal transduction for ion-selective electrodes by utilizing a two-compartment cell.

Here, we propose a fast and sensitive coulometric signal transduction method for ion-selective electrodes (ISEs) by utilizing a two-compartment cell. A potassium ion-selective electrode (K+-ISE) was connected as reference electrode (RE) and placed in the sample compartment. A glassy carbon (GC) electrode coated with poly(3,4-ethylenedioxythiophene) (GC/PEDOT), or reduced graphene oxide (GC/RGO), was connected as working electrode (WE) and placed in the detection compartment together with a counter electrode (CE). The two compartments were connected with an Ag/AgCl wire. The measured cumulated charge was amplified by increasing the capacitance of the WE. The observed slope of the cumulated charge with respect to the change of the logarithm of the K+ ion activity was linearly proportional to the capacitance of the GC/PEDOT and GC/RGO, estimated from impedance spectra. Furthermore, the sensitivity of the coulometric signal transduction using a commercial K+-ISE with internal filling solution as RE and GC/RGO as WE allowed to decrease the response time while still being able to detect a 0.2% change in K+ concentration. The coulometric method utilizing a two-compartment cell was found to be feasible for the determination of K+ concentrations in serum. The advantage of this two-compartment approach, compared to the coulometric transduction described earlier, was that no current passed through the K+-ISE that was connected as RE. Therefore, current-induced polarization of the K+-ISE was avoided. Furthermore, since the GCE/PEDOT and GCE/RGO (used as WE) had a low impedance, the response time of the coulometric response decreased from minutes to seconds.

Effective solid contact for ion-selective electrodes: tetrakis(4-chlorophenyl)borate (TB-) anions doped nanocluster films.

We report on a novel material, tetrakis(4-chlorophenyl)borate (TB(-)) anion doped nanocluster films, as the solid contact (SC) for producing well-defined, electrochemically reversible, and nonpolarizable double interfaces on it. Detailed studies have unambiguously revealed that, for the first time, the developed SC can fully overcome all the signal stability problems of ion-selective electrodes (ISEs), offering a reliable and universal platform for the development of high quality SC-ISEs. As an exemplification, the developed monolayer-protected cluster (MPC) based K(+)-ISEs have advantages of excellent analytical performances, e.g., the low potential drift (10.1 ± 0.3 µV h(-1) over 72 h measured in 0.1 M KCl and 10.8 ± 0.5 µV h(-1) over 96 h rechecked in 0.1 M KCl after 1 month) and the stable and reproducible linear range, sensitivity, and standard potential (few changes within the first 6 weeks). This evidence suggests that the developed MPC films are the most promising SC transducers among all the reported ones to the best of our knowledge.

All-solid-state potassium-selective electrode using graphene as the solid contact.

Graphene sheets are used for the first time to fabricate a new type of solid-contact ion-selective electrode (SC-ISE) as the intermediate layer between an ionophore-doped solvent polymeric membrane and a glassy carbon electrode. The new transducing layer was characterized by transmission electron microscopy, scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy. The performance of the new K(+-)selective electrodes was examined by a potentiometric water layer test, potentiometric measurements, and current reversal chronopotentiometry. The obtained potentiometric sensors were characterized with a calibration line of slope close to Nernstian (59.2 mV/decade) within the activity from 10(-4.5) to 0.1 M. The high capacitance of the graphene solid contacts results in a signal that is stable over one week. The short response time is less than 10 s for activities higher than 10(-5) M. The potential drift of the electrodes was calculated from the slope of the curves at longer times (ΔE/Δt = 1.2 × 10(-5) V s(-1) (I = 1 nA) and ΔE/Δt = 5.5 × 10(-5) V s(-1) (I = 5 nA)). All the results indicate that graphene is a promising material for use as a transducer layer for SC-ISEs.