Reference Electrodes with Salt Bridges Contained in Nanoporous Glass: An Underappreciated Source of Error

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.

Hot or Not? Reassessing Mechanisms of Photocurrent Generation in Plasmon-Enhanced Electrocatalysis

It is now widely accepted that certain effects arising from localised surface plasmon resonance, such as enhanced electromagnetic fields, hot carriers, and thermal effects, can facilitate electrocatalytic processes. This newly emerging field of research is commonly referred to as plasmon-enhanced electrocatalysis (PEEC) and is attracting increasing interest from the research community, particularly regarding harnessing the high energy of hot carriers. However, this has led to a lack of critical analysis in the literature, where the participation of hot carriers is routinely claimed due to their perceived desirability, while the contribution of other effects is often not sufficiently investigated. As a result, correctly differentiating between the possible mechanisms at play has become a key point of contention. In this review, we specifically focus on the mechanisms behind photocurrents observed in PEEC and critically evaluate the possibility of alternative sources of current enhancement in the reported PEEC systems. Furthermore, we present guidelines for the best experimental practices and methods to distinguish between the various enhancement mechanisms in PEEC.

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.

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.

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.

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.

Influence of nanoporosity on the nature of hydroxyapatite formed on bioactive calcium silicate model glass

For hard tissue regeneration, the bioactivity of a material is measured by its ability to induce the formation of hydroxyapatite (HA) under physiological conditions. It depends on the dissolution behavior of the glass, which itself is determined by the composition and structure of glass. The enhanced HA growth on nanoporous than on nonporous glass has been attributed by some to greater specific surface area (SSA), but to nanopore size distribution by others. To decouple the influence of nanopore size and SSA on HA formation, we have successfully fabricated homogeneous 30CaO-70SiO₂ (30C70S) model bioactive glass monoliths with different nanopore sizes, yet similar SSA via a combination of sol-gel, solvent exchange, and sintering processes. After incubation in PBS, HA, and Type-B carbonated HA (HA/B-CHA) form on nanoporous monoliths. The XPS, FTIR, and SEM analyses provide the first unambiguous demonstration of the influence of nanopore size alone on the formation pathway, growth rate, and microstructure of HA/CHA. Due to pore-size limited diffusion of PO₄ ^3- , two HA/CHA formation pathways are observed: HA/CHA surface deposition and/or HA/CHA incorporation into nanopores. HA/CHA growth rate on the surface of a nanoporous glass monolith is dominated by the pore-size limited transport of Ca²⁺ ions dissolved from nanoporous glass substrates. Furthermore, with increasing nanopore size, HA/CHA microstructures evolve from needle-like, plate-like, to flower-like appearance. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 886-899, 2019.

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.

Unconventional Electrochemistry in Micro-/Nanofluidic Systems

Electrochemistry is ideally suited to serve as a detection mechanism in miniaturized analysis systems. A significant hurdle can, however, be the implementation of reliable micrometer-scale reference electrodes. In this tutorial review, we introduce the principal challenges and discuss the approaches that have been employed to build suitable references. We then discuss several alternative strategies aimed at eliminating the reference electrode altogether, in particular two-electrode electrochemical cells, bipolar electrodes and chronopotentiometry.

Ionic Liquids as Electrolytes for Electrochemical Double-Layer Capacitors: Structures that Optimize Specific Energy

Key parameters that influence the specific energy of electrochemical double-layer capacitors (EDLCs) are the double-layer capacitance and the operating potential of the cell. The operating potential of the cell is generally limited by the electrochemical window of the electrolyte solution, that is, the range of applied voltages within which the electrolyte or solvent is not reduced or oxidized. Ionic liquids are of interest as electrolytes for EDLCs because they offer relatively wide potential windows. Here, we provide a systematic study of the influence of the physical properties of ionic liquid electrolytes on the electrochemical stability and electrochemical performance (double-layer capacitance, specific energy) of EDLCs that employ a mesoporous carbon model electrode with uniform, highly interconnected mesopores (3DOm carbon). Several ionic liquids with structurally diverse anions (tetrafluoroborate, trifluoromethanesulfonate, trifluoromethanesulfonimide) and cations (imidazolium, ammonium, pyridinium, piperidinium, and pyrrolidinium) were investigated. We show that the cation size has a significant effect on the electrolyte viscosity and conductivity, as well as the capacitance of EDLCs. Imidazolium- and pyridinium-based ionic liquids provide the highest cell capacitance, and ammonium-based ionic liquids offer potential windows much larger than imidazolium and pyridinium ionic liquids. Increasing the chain length of the alkyl substituents in 1-alkyl-3-methylimidazolium trifluoromethanesulfonimide does not widen the potential window of the ionic liquid. We identified the ionic liquids that maximize the specific energies of EDLCs through the combined effects of their potential windows and the double-layer capacitance. The highest specific energies are obtained with ionic liquid electrolytes that possess moderate electrochemical stability, small ionic volumes, low viscosity, and hence high conductivity, the best performing ionic liquid tested being 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide.

Potentiometric in Situ Monitoring of Anions in the Synthesis of Copper and Silver Nanoparticles Using the Polyol Process

Potentiometric sensors, such as polymeric membrane, ion-selective electrodes (ISEs), have been used in the past to monitor a variety of chemical processes. However, the use of these sensors has traditionally been limited to aqueous solutions and moderate temperatures. Here we present an ISE with a high-capacity ion-exchange sensing membrane for measurements of nitrate and nitrite in the organic solvent propylene glycol at 150 °C. It is capable of continuously measuring under these conditions for over 180 h. We demonstrate the usefulness of this sensor by in situ monitoring of anion concentrations during the synthesis of copper and silver nanoparticles in propylene glycol using the polyol method. Ion chromatography and a colorimetric method were used to independently confirm anion concentrations measured in situ. In doing so, it was shown that in this reaction the co-ion nitrate is reduced to nitrite.

Application of voltammetric techniques at microelectrodes to the study of the chemical stability of highly reactive species

The application of voltammetric techniques to the study of chemical speciation and stability is addressed both theoretically and experimentally in this work. In such systems, electrode reactions are coupled to homogeneous chemical equilibria (complexations, protonations, ion associations, ...) that can be studied in a simple, economical, and accurate way by means of electrochemical methods. These are of particular interest when some of the participating species are unstable given that the generation and characterization of the species are performed in situ and on a short time scale. With the above aim, simple explicit solutions are presented in this article for quantitative characterization with any voltammetric technique and with the most common electrode geometries. From the theoretical results obtained, it is pointed out that the use of square-wave voltammetry in combination with microelectrodes is very suitable. Finally, the theory is applied to the investigation of the ion association between the anthraquinone radical monoanion and the tetrabutylammonium cation in acetonitrile medium.

Paper-based potentiometric ion sensing

This paper describes the design and fabrication of ion-sensing electrochemical paper-based analytical devices (EPADs) in which a miniaturized paper reference electrode is integrated with a small ion-selective paper electrode (ISPE) for potentiometric measurements. Ion-sensing EPADs use printed wax barriers to define electrochemical sample and reference zones. Single-layer EPADs for sensing of chloride ions include wax-defined sample and reference zones that each incorporate a Ag/AgCl electrode. In EPADs developed for other electrolytes (potassium, sodium, and calcium ions), a PVC-based ion-selective membrane is added to separate the sample zone from a paper indicator electrode. After the addition of a small volume (less than 10 μL) of sample and reference solutions to different zones, ion-sensing EPADs exhibit a linear response, over 3 orders of magnitude, in ranges of electrolyte concentrations that are relevant to a variety of applications, with a slope close to the theoretical value (59.2/z mV). Ion-selective EPADs provide a portable, inexpensive, and disposable way of measuring concentrations of electrolyte ions in aqueous solutions.