Avoiding Errors in Electrochemical Measurements: Effect of Frit Material on the Performance of Reference Electrodes with Porous Frit Junctions

Long-Term Stable Reference Electrodes with High-Pressure Tolerance and Salinity-Independence

The reference electrode's performance is essential for ensuring the accuracy of electrochemical sensors in marine environments. Yet, the many existing reference electrodes can exhibit sensitivity to salinity variations, potentially leading to inaccuracies in the measurement process. Herein, we have designed a reliable solid-state reference electrode by introducing SiOx-stabilized 1-methyl-3-octylimidazolium bis(trifluoromethyl sulfonyl)imide ([C8mim+] [Ntf2-]) into a P(VdF-co-HFP) matrix with a SPEEK/[C8mim+] [Ntf2-] coated Ag/AgCl as substrate. The SPEEK/[C8mim+] [Ntf2-] coating protects the AgCl substrate, and the incorporation of SiOx improves the compatibility of the IL with the polymer matrix, thereby increasing the electrode's resistance to interference and extending its long-term stability and lifespan. The developed reference electrode showed a stable and rapid response, with potential variations of less than 0.7 mV across various salinity solutions, including practical seawater, lake water, and their mixture samples. During extended periods of 18 days in deionized water and artificial seawater, the electrode demonstrated negligible potential drifts of 0.36 and 0.14 mV/d, respectively. Notably, the electrode could maintain a stable potential even after being stored in a preservative solution for 67 days. Furthermore, the electrode showed a stable response to withstand pressures of up to 100 MPa, covering the vast majority of the seafloor. This innovative reference electrode is capable of maintaining a stable reference potential across various salinities, ionic strength, and full ocean depth, making it versatile for use in diverse aquatic environments, underscoring its significant potential for advancing oceanographic research and enabling new insights into the unexplored depths of oceans.

Ion sensors based on organic semiconductors acting as quasi-reference electrodes

Thin-film devices that transduce the chemical activity of ions into electronic signals are essential components in various applications, including healthcare diagnostics and environmental monitoring. Combinations of organic semiconductors (OSCs) and ion-selective materials have been explored for developing solution-processable ion sensors. However, the necessity of reference electrodes (REs) and operational stability in ion-permeable OSCs have posed questions regarding whether reliable measurements with thin-film components are attainable with OSCs. Herein, we report electric double-layer transistors (EDLTs) with OSCs in single-crystal forms for ion sensing. Our EDLTs demonstrated high operational stability, with a one-to-one relationship between the source electrode potential and device resistance, and served as quasi-REs (qRE). When our EDLT is served as qRE, its drift was as small as 0.5 mV/h and comparable to that of commonly employed REs. In our system, the semiconductor-electrolyte interface is self-passivated by the alkyl chains of OSCs in single-crystal structures, with the two-dimensional transport layer appearing unaltered upon gating. EDLT arrays with ion-selective and nonselective liquid junctions enable ion concentration sensing without a conventional RE. These findings provide opportunities to develop thin-film devices based on OSCs for easy integration and reliable measurements.

Polymerization Induced Microphase Separation for the Fabrication of Nanostructured Materials

Polymerization induced microphase separation (PIMS) is a strategy used to develop unique nanostructures with highly useful morphologies through the microphase separation of emergent block copolymers during polymerization. In this process, nanostructures are formed with at least two chemically independent domains, where at least one domain is composed of a robust crosslinked polymer. Crucially, this synthetically simple method is readily used to develop nanostructured materials with the highly coveted co-continuous morphology, which can also be converted into mesoporous materials by selective etching of one domain. As PIMS exploits a block copolymer microphase separation mechanism, the size of each domain can be tightly controlled by modifying the size of block copolymer precursors, thus providing unparalleled control over nanostructure and resultant mesopore sizes. Since its inception 11 years ago, PIMS has been used to develop a vast inventory of advanced materials for an extensive range of applications including biomedical devices, ion exchange membranes, lithium-ion batteries, catalysis, 3D printing, and fluorescence-based sensors, among many others. In this review, we provide a comprehensive overview of the PIMS process, summarize latest developments in PIMS chemistry, and discuss its utility in a wide variety of relevant applications.

Tribotronic and electrochemical properties of platinum-nanofluid interfaces formed by aqueous suspensions of 5 and 40 nm TiO2 nanoparticles

Nanoparticles (NPs) can be highly beneficial as additives to lubricating fluids, and the tribotronic response of charged NPs tuned by external fields represents an area of great technological potential. Tribotronic response, however, is expected to be highly size dependent, which represents a significant design challenge. To explore this issue, quartz crystal microbalance and cyclic voltammetry were employed to characterize nanotribological and electrochemical behavior of platinum-nanofluid interfaces formed by aqueous suspensions of different-sized negatively charged titanium dioxide (TiO2) NPs. Suspensions of 5, 40, and 100 nm NPs were all observed to reduced interfacial frictional drag forces upon introduction into pure water in zero field conditions, with reductions for the 40 nm NPs about twice those of 5 nm particles at comparable concentrations. Suspensions of 100 nm NPs produced even greater reductions, but rapidly precipitated from the suspension when left unstirred. NPs were also driven to and from Pt electrode surfaces by applying external electric fields with varying amplitudes and modulation frequencies. For electric fields of sufficient amplitude and duration, the 40 nm TiO2 nanosuspension exhibited tribological properties consistent with a reversible electrophoretic deposition of the NPs, accompanied by changes in the electrochemical attributes and increasing interfacial drag. The 5 nm NP properties were consistent with progressive reductions in interfacial drag forces at the NP-suspension interface linked to field-induced increases in concentration.

On the mechanism of the bipolar reference electrode

Commercial silver/silver chloride (Ag/AgCl) reference electrodes are some of the most commonly used reference electrodes, but they suffer from a number of issues due to their porous frits. Such issues include difficulty miniaturizing, silver and chloride ion leakage, charge screening effects at low ionic strength, frit drying if left unattended in air, and incompatibility with organic solvents. To solve these issues, we recently designed a reference electrode that is leakless in principle by replacing the porous frit with a sealed, conductive wire, where the ends of the wire are exposed to the reference electrode solution and the working electrode solution. We hypothesized that the reference electrode operated like a closed, bipolar electrochemical cell, and we termed the name bipolar reference electrode (BPRE). Here, we provide evidence that the BPRE can either act as a reference electrode by operating through an ion transfer mechanism via leakage through the imperfect seal, or it can act as a highly stable quasi-reference electrode through a bipolar electron transfer mechanism (BPQRE). Finally, we demonstrate the effectiveness of the BPRE in other types of common electrochemical studies, including chronoamperometry, linear sweep voltammetry, differential pulse voltammetry, and electrochemical impedance spectroscopy.

Precise Cation Separations with Composite Cation-Exchange Membranes: Role of Base Layer Properties

Separation of specific ions from water could enable recovery and reuse of essential metals and nutrients, but established membrane technologies lack the high-precision selectivity needed to facilitate a circular resource economy. In this work, we investigate whether the cation/cation selectivity of a composite cation-exchange membrane (CEM), or a thin polymer selective layer on top of a CEM, may be limited by the mass transfer resistance of the underlying CEM. In our analysis, we utilize a layer-by-layer technique to modify CEMs with a thin polymer selective layer (∼50 nm) that has previously shown high selectivity toward copper over similarly sized metals. While these composite membranes have a CuCl₂/MgCl₂ selectivity up to 33 times larger than unmodified CEMs in diffusion dialysis, our estimates suggest that eliminating resistance from the underlying CEM could further increase selectivity twofold. In contrast, the CEM base layer has a smaller effect on the selectivity of these composite membranes in electrodialysis, although these effects could become more pronounced for ultrathin or highly conductive selective layers. Our results highlight that base layer resistance prevents selectivity factors from being comparable across diffusion dialysis and electrodialysis, and CEMs with low resistance are necessary for providing highly precise separations with composite CEMs.

Novel Inorganic Membranes Based on Magnetite-Containing Silica Porous Glasses for Ultrafiltration: Structure and Sorption Properties

Porous glasses (PGs) obtained from sodium borosilicate (NBS) phase-separated glasses via leaching are promising inorganic membranes. Introducing Fe₂O₃ into NBS glasses imparts ferrimagnetic properties due to magnetite crystallization. Leaching of such glasses leads to the formation of magnetic PGs with interesting electro-surface characteristics. This work aimed to investigate the process of obtaining magnetite-containing PGs from NBS glasses depending on silica content, using XRPD and Raman spectroscopy, studying the PG membranes' structural characteristics and their sorption properties with respect to methylene blue (MB). Obtained PGs were characterized by a polymodal distribution of mesopores and a small number of micropores with specific surface area values of 32-135 m²/g and an average mesopore diameter of 5-41 nm. The kinetic data were analyzed using pseudo-first-order, pseudo-second-order, and intra-particle diffusion equations. The equilibrium isotherms were fitted with Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich models. MB adsorption was found to be a complex process. The glass with the highest specific surface area demonstrated the maximum sorption capacity (10.5 mg/g). The pore size of PGs allowed them to be considered potential novel magnetic membranes for ultrafiltration.

Nonaqueous Li-Mediated Nitrogen Reduction: Taking Control of Potentials

The performance of the Li-mediated ammonia synthesis has progressed dramatically since its recent reintroduction. However, fundamental understanding of this reaction is slower paced, due to the many uncontrolled variables influencing it. To address this, we developed a true nonaqueous LiFePO4 reference electrode, providing both a redox anchor from which to measure potentials against and estimates of sources of energy efficiency loss. We demonstrate its stable electrochemical potential in operation using different N2- and H2-saturated electrolytes. Using this reference, we uncover the relation between partial current density and potentials. While the counter electrode potential increases linearly with current, the working electrode remains stable at lithium plating, suggesting it to be the only electrochemical step involved in this process. We also use the LiFePO4/Li+ equilibrium as a tool to probe Li-ion activity changes in situ. We hope to drive the field toward more defined systems to allow a holistic understanding of this reaction.

Evaluation of mesoporous borosilicate glass-ceramic composites as frits in reference electrodes

The development of new mesoporous frits for reference electrodes to overcome the limitations of cross-contamination and screening effect is essential for many electrochemical measurements. Available frit-based reference electrodes (e.g., mesoporous, microporous) still suffer from cross-contamination and/or errors in electrochemical measurements. In this work, a mesoporous glass-ceramic composite is prepared to mitigate such limitations. Mesoporous glass-ceramic frits were prepared from low-cost materials (i.e., borosilicate and kaolin) at relatively low temperatures (750-850 °C). The prepared glass-ceramic frits were characterized using scanning electron microscopy (SEM), impedance measurements, and nitrogen sorption isotherms. The developed mesoporous glass-ceramic composites are characterized by a high chemical resistance against corrosive materials and a low thermal expansion. Reference electrodes constructed with the developed mesoporous glass-ceramic frits exhibited a low flow rate of 0.002 ± 0.001 to 0.41 ± 0.06 μL h^-1 and high potential stability as well as very small potential drift of -2.4 ± 0.2 to -4.9 ± 0.2 μV h^-1. Mesoporous glass-ceramic based reference electrodes exhibited average potential variations of 13 ± 3 mV over the concentration range of 1 mM to 0.1 M KCl. This indicates that mesoporous glass-ceramic frit-based reference electrodes exhibited a much lower flow rate compared to available microporous frit-based reference electrodes. Moreover, the developed mesoporous ceramic-based reference electrodes exhibited a 4-15-fold improvement in potential variations and a large improvement in potential stability in comparison with the reported mesoporous-frit-based reference electrodes.

Prussian-Blue Analogue-Derived Hollow Structured Co3 S4 /CuS2 /NiS2 Nanocubes as an Advanced Battery-Type Electrode Material for High-Performance Hybrid Supercapacitors

The facile and cost-effective fabrication of hybrid nanostructures comprised of hollow mixed metallic chalcogenides has attracted growing interest in the development of high-performance energy storage devices. Herein, multi-component (nickel-cobalt-copper-sulfides/selenides (NCCS/NCCSe)) hollow nanocubes (HNCs) are prepared via a single-step sulfurization/selenization process. The NCCS material shows interior HNCs, and the NCCSe material exhibits slightly formed porous cubes. Both the prepared materials demonstrate higher charge storage performance than the precursor NCC NCs owing to the improved surface morphology and addition of sulfur and selenium ions. Particularly, the NCCS HNCs electrode reveals superior specific capacity (capacitance) (70.32 mAh g^-1 (666.20 F g^-1 ) at 5 mA cm^-2 ) along with excellent cycling stability of 108.6% even after 10 000 cycles. Interestingly, the electrode delivers a good rate capability of 83.5% at a high current density of 20 mA cm^-2 . The feasibility of the battery-type NCCS HNCs as a positive electrode is explored by constructing an aqueous electrochemical hybrid capacitor (AEHC). The AEHC exhibits maximum energy and power densities of 23.15 Wh kg^-1 and 7899.08 W kg^-1 , respectively. Remarkably, it demonstrates superior long-life cycling stability even after 10 000 cycles (120.6% retention). The suitability of AEHC for practical application is also tested by driving electronic devices.

Potential application of metallacarboranes as internal reference: An electrochemical comparative study to ferrocene

Ferrocene and its derivatives have been extensively used as internal reference in electrochemical processes. Yet, they possess limitations such as solvent restrictions that require chemical modifications. In this regard, we...

Pt/Polypyrrole Quasi-References Revisited: Robustness and Application in Electrochemical Energy Storage Research

Choosing reference electrodes for nonaqueous electrochemical measurements, especially in energy storage research, is challenging due to lengthy experiments (>1 day), the lack of alternatives to the commonly used Ag/Ag⁺ reference electrode (RE), the introduction of junction potentials, and the possibility of sample contamination. Often, quasi-reference electrodes (QREs) such as Ag wires and Li metal strips are used. However, small changes in electrolyte composition can cause large potential drifts, and their surfaces may be reactive to the solution. Here, we propose an alternative QRE based on polypyrrole electrodeposited on Pt wire (PPyQRE) encased in a glass tube with the open end sealed with commercial frits. While freestanding PPyQRE wires have been reported in the literature, simple encasing of the PPyQRE overcomes the above-mentioned drawbacks of QREs while providing a reliable reference potential that is closer to the performance of an RE. During cyclic voltammetric and bulk electrolysis testing of a redox mediator in solution, the encased PPyQRE exhibited stable reference potentials over multiple charge/discharge cycles with minimal drift (∼5 mV) after ∼2.25 days of operation. We also tested the reliability of our reference during the testing of multilayer graphene Li-ion anodes, which often involve cycling samples at highly reducing potentials (<-3 V vs Fc/Fc⁺) over long durations (>1 day). In the same testing conditions, the Ag/Ag⁺ electrode led to observable Ag deposits on the graphene and large potential drifts (∼50 mV), while the PPyQRE exhibited no measurable drift and revealed changes in voltammetric features that were obscured by reference drift when using Ag/Ag⁺. Minor reference drifts of ∼30 mV over long usage of the PPyQRE (∼2 months) can be addressed by calibration with a ferrocene couple at the end of experiments. These results highlight the advantages of using an encased PPyQRE as a simple and practical reference electrode for electrochemical measurements in the field of nonaqueous energy storage research.

Leakless, Bipolar Reference Electrodes: Fabrication, Performance, and Miniaturization

Reference electrodes must maintain a well-defined potential for long periods of time to be useful. The silver/silver chloride (Ag/AgCl) reference electrode is arguably the most widely used reference electrode, but it leaks silver and chloride ions into the sample solution through the porous frit over time. Further, the porous frit makes miniaturization to the micro- and nanoscale challenging. Here, we present an alternative, where the traditional Ag/AgCl reference electrode porous frit is replaced by a conductive wire, preventing ion leakage and allowing miniaturization to the microscale. Charge balance is maintained through a closed bipolar electrochemical mechanism, where faradaic processes occur on each end of the sealed wire. Using the above design, we demonstrate the efficacy of the leakless, bipolar reference electrode (BPRE) and miniaturize it to the microscale (μ-leakless BPRE). Importantly, we demonstrate that leakless and μ-leakless BPREs behave the same as commercial reference electrodes during potentiometric measurements and leakless BPREs perform similarly during voltammetric measurements on ultramicroelectrodes. We demonstrate that the drift during voltammetry using a leakless BPRE on a macroelectrode is slightly more appreciable compared to the drift seen with a commercial reference electrode. We detail design principles for the use of leakless BPREs in nonaqueous solvents and in sealing other conductive materials (e.g., gold and carbon). Using mass spectrometry, we show that the maximum leakage of methylene blue is 0.36 fmol/s, at least 2 orders of magnitude smaller than that of commercial reference electrodes. Finally, we demonstrate the efficacy of using leakless BPREs in potentiometric glucose sensing.

Utilizing RAFT Polymerization for the Preparation of Well-Defined Bicontinuous Porous Polymeric Supports: Application to Liquid Chromatography Separation of Biomolecules

Polymer-based monolithic high-performance liquid chromatography (HPLC) columns are normally obtained by conventional free-radical polymerization. Despite being straightforward, this approach has serious limitations with respect to controlling the structural homogeneity of the monolith. Herein, we explore a reversible addition-fragmentation chain transfer (RAFT) polymerization method for the fabrication of porous polymers with well-defined porous morphology and surface chemistry in a confined 200 μm internal diameter (ID) capillary format. This is achieved via the controlled polymerization-induced phase separation (controlled PIPS) synthesis of poly(styrene-co-divinylbenzene) in the presence of a RAFT agent dissolved in an organic solvent. The effects of the radical initiator/RAFT molar ratio as well as the nature and amount of the organic solvent were studied to target cross-linked porous polymers that were chemically bonded to the inner wall of a modified silica-fused capillary. The morphological and surface properties of the obtained polymers were thoroughly characterized by in situ nuclear magnetic resonance (NMR) experiments, nitrogen adsorption-desorption experiments, elemental analyses, field-emission scanning electron microscopy (FESEM), scanning electron microscopy-energy-dispersive X-ray (SEM-EDX) spectroscopy, and X-ray photoelectron spectroscopy (XPS) as well as time-of-flight secondary ion mass spectrometry (ToF-SIMS) revealing the physicochemical properties of these styrene-based materials. When compared with conventional synthetic methods, the controlled-PIPS approach affects the kinetics of polymerization by delaying the onset of phase separation, enabling the construction of materials with a smaller pore size. The results demonstrated the potential of the controlled-PIPS approach for the design of porous monolithic columns suitable for liquid separation of biomolecules such as peptides and proteins.

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.

Lifting the lid on the potentiostat: a beginner's guide to understanding electrochemical circuitry and practical operation

Students who undertake practical electrochemistry experiments for the first time will come face to face with the potentiostat. To many this is simply a box containing electronics which enables a potential to be applied between a working and reference electrode, and a current to flow between the working and counter electrode, both of which are outputted to the experimentalist. Given the broad generality of electrochemistry across many disciplines it is these days very common for students entering the field to have a minimal background in electronics. This article serves as an introductory tutorial to those with no formalized training in this area. The reader is introduced to the operational amplifier, which is at the heart of the different potentiostatic electronic circuits and its role in enabling a potential to be applied and a current to be measured is explained. Voltage follower op-amp circuits are also highlighted, given their importance in measuring voltages accurately. We also discuss digital to analogue and analogue to digital conversion, the processes by which the electrochemical cell receives input signals and outputs data and data filtering. By reading the article, it is intended the reader will also gain a greater confidence in problem solving issues that arise with electrochemical cells, for example electrical noise, uncompensated resistance, reaching compliance voltage, signal digitisation and data interpretation. We also include trouble shooting tables that build on the information presented and can be used when undertaking practical electrochemistry.

Impacts of Performing Electrolysis During Organocatalyzed Atom Transfer Radical Polymerization

An electrochemical variant of organocatalyzed atom transfer radical polymerization (O-ATRP) is developed and investigated. Inspired by electrochemically mediated atom transfer radical polymerization (eATRP), potentiostatic electrolysis is used to manipulate the catalyst's redox states in O-ATRP to understand whether deactivation in O-ATRP can be enhanced to improve polymerization control. During the course of this work, several possible side reactions are investigated, and the electrochemical apparatus is optimized to reduce side reactions at the counter electrode. This electrochemically modified O-ATRP method (eO-ATRP) is then studied at different applied potentials, under different irradiation conditions, and with two photoredox catalysts to understand the impact of electrolysis on polymerization control. Ultimately, although electrolysis was successfully used to improve polymerization control in O-ATRP, some additional challenges have been identified. Several key questions are postulated to guide future work in this area.

Valve-Actuator-Integrated Reference Electrode for an Ultra-Long-Life Rumen pH Sensor

We demonstrated a newly developed Ag/AgCl reference electrode- with a valve-actuator for two years or longer rumen pH monitoring. Previous studies on pH sensors reported that the short lifetime of Ag/AgCl reference electrodes is caused by an outflow of internal electrolyte. We introduced a valve-actuator into a liquid junction to reduce the outflow by intermittent measurement. The results indicated that the potential change when switching the liquid junction was less than 0.5 mV and its response time was less than 0.083 s. In the 24-h potential measurement with the valve-actuator-integrated reference electrode (VAIRE), the valve was actuated once every hour, and the standard deviation of the potential was 0.29 mV. The lifetime of the VAIRE was estimated at 2.0 years calculating from an electrolyte outflow, which is significantly longer than that of conventional reference electrodes. A pH sensor using the VAIRE was estimated to operate for 2.0 years with the pH error ≤0.1, which meets the requirement of cows' rumen pH monitoring.

Critical Comparison of Reference Electrodes with Salt Bridges Contained in Nanoporous Glass with 5, 20, 50, and 100 nm Diameter Pores

Porous glass frits are frequently used to contain the salt bridges through which reference electrodes interface samples. Prior work with widely used glass frits with 4 - 10 nm diameter pores showed that, when samples have a low electrolyte strength, electrostatic screening of sample ions by charged sites on the glass surface occurs. This creates an ion-specific phase-boundary potential at the interface between the sample and frit, and it biases the potential of the reference half-cell. Use of frits with much larger pores eliminates this problem but results in the need for frequent replenishing of the bridge electrolyte. A methodical study to determine the optimum pore size has been missing. We show here that charge screening of sample ions occurs when the pore size of nanoporous glass frits is on the order of 1 - 50 nm and samples have a low electrolyte strength. An increase in pores size to 100 nm eliminates charge screening in samples with ionic strengths in the 1.0 M to 3.3 × 10^-4 M range. However, the rates of electrolyte solution flow through frits with 1, 5, 20, 50, and 100 nm pores are still low, which makes diffusion the dominant mode of ion transport into and out of these frits. Consequently, the flow of bridge electrolyte into samples is not fast enough to prevent diffusion of ions and electrically neutral components from the sample diffusing into the salt bridge, which can result in cross contamination among samples.

Indirect Potentiometric Determination of Polyquaternium Polymer Concentrations by Equilibrium Binding to 1-Dodecyl Sulfate

Polyquaternium polymers are polycationic polymers that are contained in many hair shampoos and conditioners and are also often added to water to remove organic and inorganic anions by floc formation. While polyquaternium analysis is not trivial, electroanalytical methods have been proposed for their detection using either irreversible emf responses or reversible potential-driven extraction into and out of polymeric sensing membranes. We present here an alternative technique for the determination of a representative polyquaternium polymer, poly(dimethylamine-co-epichlorohydrin) chloride, by equilibrium binding with a singly charged anionic surfactant, 1-dodecyl sulfate. Binding of an anionic surfactant to the polyquaternium polymer simplifies electrochemical detection as the concentration of unbound surfactant can be monitored using the equilibrium Nernstian emf response of ion-exchanger membranes. The latter can be used to determine the nature of the binding interaction and allows for the straightforward determination of polyquaternium polymer concentrations by titration.

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.

Remediation of Perfluorooctylsulfonate Contamination by in Situ Sequestration: Direct Monitoring of PFOS Binding to Polyquaternium Polymers

In situ methods for the sequestration of perfluorooctyl-1-sulfonate (PFOS) that are based on PFOS binding to polyquaternium polymers were reported previously, providing an approach to immobilize and concentrate PFOS in situ. To apply these methods in real life, the concentrations of polymers that permit efficient sequestration must be determined. This is only possible if the stoichiometry and strength of PFOS binding to polyquaternium polymers are known. Here, we report on the use of fluorous-phase ion-selective electrodes (ISEs) to determine the equilibrium constants characterizing binding of PFOS to poly(dimethylamine-co-epichlorohydrin) and poly(diallyldimethylammonium) in simulated groundwater and in soil suspensions. We introduce a new method to interpret potentiometric data for surfactant binding to the charged repeat unit of these polyions by combining a 1:1 binding model with the ISE response model. This allows for straightforward prediction and fitting of experimental potentiometric data in one step. Data fit the binding model for poly(diallyldimethylammonium) and poly(dimethylamine-co-epichlorohydrin) chloride in soil-free conditions and in the presence of soil from Tinker Air Force Base. When the total PFOS concentration in a soil system is known, knowledge of these PFOS binding characteristics permits quantitative prediction of the mobile (free) and polymer-bound fractions of PFOS as a function of the concentrations of the polyquaternium polymer. Because the technique reported here is based on the selective in situ determination of the free ionic surfactant, we expect it to be similarly useful for determining the sequestration of a variety of other ionic pollutants.

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).

Self-Supporting, Hydrophobic, Ionic Liquid-Based Reference Electrodes Prepared by Polymerization-Induced Microphase Separation

Interfaces of ionic liquids and aqueous solutions exhibit stable electrical potentials over a wide range of aqueous electrolyte concentrations. This makes ionic liquids suitable as bridge materials that separate in electroanalytical measurements the reference electrode from samples with low and/or unknown ionic strengths. However, methods for the preparation of ionic liquid-based reference electrodes have not been explored widely. We have designed a convenient and reliable synthesis of ionic liquid-based reference electrodes by polymerization-induced microphase separation. This technique allows for a facile, single-pot synthesis of ready-to-use reference electrodes that incorporate ion conducting nanochannels filled with either 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide or 1-dodecyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide as ionic liquid, supported by a mechanically robust cross-linked polystyrene phase. This synthesis procedure allows for the straightforward design of various reference electrode geometries. These reference electrodes exhibit a low resistance as well as good reference potential stability and reproducibility when immersed into aqueous solutions varying from deionized, purified water to 100 mM KCl, while requiring no correction for liquid junction potentials.

Enhanced biodegradation of atrazine by bacteria encapsulated in organically modified silica gels

Biodegradation by cells encapsulated in silica gel is an economical and environmentally friendly method for the removal of toxic chemicals from the environment. In this work, recombinant E. coli expressing atrazine chlorohydrolase (AtzA) were encapsulated in organically modified silica (ORMOSIL) gels composed of TEOS, silica nanoparticles (SNPs), and either phenyltriethoxysilane (PTES) or methyltriethoxysilane (MTES). ORMOSIL gels adsorbed much higher amounts of atrazine than the hydrophilic TEOS gels. The highest amount of atrazine adsorbed by ORMOSIL gels was 48.91×10^-3μmol/mlgel, compared to 8.71×10^-3μmol/mlgel by the hydrophilic TEOS gels. Atrazine biodegradation rates were also higher in ORMOSIL gels than the TEOS gels, mainly due to co-localization of the hydrophobic substrate at high concentrations in close proximity of the encapsulated bacteria. A direct correlation between atrazine adsorption and biodegradation was observed unless biodegradation decreased due to severe phase separation. The optimized PTES and MTES gels had atrazine biodegradation rates of 0.041±0.003 and 0.047±0.004μmol/mlgel, respectively. These rates were approximately 80% higher than that measured in the TEOS gel. This study showed for the first time that optimized hydrophobic gel material design can be used to enhance both removal and biodegradation of hydrophobic chemicals.