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

Recent advances in nanomaterial-based solid-contact ion-selective electrodes

Solid-contact ion-selective electrodes (SC-ISEs) are advanced potentiometric sensors with great capability to detect a wide range of ions for the monitoring of industrial processes and environmental pollutants, as well as the determination of electrolytes for clinical analysis. Over the past decades, the innovative design of ion-selective electrodes (ISEs), specifically SC-ISEs, to improve potential stability and miniaturization for in situ/real-time analysis, has attracted considerable interest. Recently, the utilisation of nanomaterials was particularly prominent in SC-ISEs due to their excellent physical and chemical properties. In this article, we review the recent applications of various types of nanostructured materials that are composed of carbon, metals and polymers for the development of SC-ISEs. The challenges and opportunities in this field, along with the prospects for future applications of nanomaterials in SC-ISEs are also discussed.

A Review of the Carbon-Based Solid Transducing Layer for Ion-Selective Electrodes

As one of the key components of solid-contact ion-selective electrodes (SC-ISEs), the SC layer plays a crucial role in electrode performance. Carbon materials, known for their efficient ion-electron signal conversion, chemical stability, and low cost, are considered ideal materials for solid-state transducing layers. In this review, the application of different types of carbon materials in SC-ISEs (from 2007 to 2023) has been comprehensively summarized and discussed. Representative carbon-based materials for the fabrication of SC-ISEs have been systematically outlined, and the influence of the structural characteristics of carbon materials on achieving excellent performance has been emphasized. Finally, the persistent challenges and potential opportunities are also highlighted and discussed, aiming to inspire the design and fabrication of next-generation SC-ISEs with multifunctional composite carbon materials in the future.

Solid-contact polymeric membrane ion-selective electrodes using a covalent organic framework@reduced graphene oxide composite as ion-to-electron transducer

A solid-contact ion-selective electrode (SC-ISE) based on a covalent organic framework@reduced graphene oxide (rGO) composite is proposed. The composite can be synthesized through the polycondensation of 1,3,5-triformylphloroglucinol (TFP) and 2,6-diaminoanthraquinone (DAAQ) on the rGO nanosheets, which shows high capacitance and good redox-active properties. By applying Cd²⁺-ISE as a model, the electrode exhibits a Nernstian slope of 29.7 ± 0.4 mV/decade in the activity range of 1.0 × 10^-7 - 7.9 × 10^-4 M and the limit of detection is 6.8 × 10^-8 M. Particularly, the electrode based on DAAQ-TFP@rGO exhibits a low potential drift of 1.2 ± 0.2 μV/h over 70 h due to the large capacitance of 2.0 mF. Moreover, the DAAQ-TFP@rGO-based Cd²⁺-ISE shows good reproducibility and the standard deviations of the standard potentials for single batch and batch-to-batch are 0.28 (n = 4) and 0.30 mV (n = 4), respectively. The developed SC-Cd²⁺-ISE is not disturbed by light or gas and no aqueous layer occurs between the sensing membrane and DAAQ-TFP@rGO layer. The DAAQ-TFP@rGO composite is highly promising for construction of calibration-free SC-ISEs.

Carbon-Based Transducers for Solid-Contact Calcium Ion-Selective Electrodes: Mesopore and Nitrogen-Doping Effects

Solid-contact ion-selective electrodes (SC-ISEs) exhibit great potential in the detection of routine and portable ions which rely on solid-contact (SC) materials for the transduction of ions to electron signals. Carbon-based materials are state-of-the-art SC transducers due to their high electrical double-layer (EDL) capacitance and hydrophobicity. However, researchers have long searched for ways to enhance the interfacial capacitance in order to improve the potential stability. Herein, three representative carbon-based SC materials including nitrogen-doped mesoporous carbon (NMC), reduced graphene oxide (RGO), and carbon nanotubes (CNT) were compared. The results disclose that the NMC has the highest EDL capacitance owing to its mesopore structure and N-doping while maintaining high hydrophobicity so that no obvious water-layer effect was observed. The Ca²⁺-SC-ISEs based on the SC of NMC exhibited high potential stability compared with RGO and CNT. This work offers a guideline for the development of carbon-material-based SC-ISEs through mesoporous and N-doping engineering to improve the interfacial capacitance. The developed NMC-based solid-contact Ca²⁺-SC-ISE exhibited a Nernstian slope of 26.3 ± 3.1 mV dec^-1 ranging from 10 μM to 0.1 M with a detection limit of 3.2 μM. Finally, a practical application using NMC-based SC-ISEs was demonstrated through Ca²⁺ ion analysis in mineral water and soil leaching solutions.

All-Solid-State Polymeric Membrane Ion-Selective Electrodes Based on NiCo2S4 as a Solid Contact

The performance criteria for the design of all-solid-state ion-selective electrodes mainly include high electrode-to-electrode reproducibility and a low potential drift. Here, we introduce nickel cobalt sulfide (NiCo₂S₄) as a solid contact for ion-to-electron transduction based on multiple redox couples. NiCo₂S₄ materials with different morphologies can be prepared through a facile hydrothermal/solvothermal method. A NiCo₂S₄-based solid-contact Ca²⁺-ISE has been developed, which exhibits a Nernstian slope of 27.5 ± 0.2 mV/dec in the activity range from 1.0 × 10^-6 to 2.9 × 10^-2 M with a detection limit of 5.0 × 10^-7 M. A variation of the standard potential E° for eight individual solid-contact electrodes can be obtained as low as 0.35 mV. Due to the synergistic effect of cobalt and nickel ions in the ternary sulfide, an excellent redox capacitance (565 μF) of the buried solid contact coated with the ion-selective membrane can be achieved and is much larger than those obtained from other redox solid-contact materials reported so far, thus yielding a high potential stability of 2.2 ± 0.4 μV/h. In addition, the NiCo₂S₄-based solid-contact Ca²⁺-ISE shows a reduced water layer at the sensing membrane/NiCo₂S₄ interface and provides an excellent resistance to the interferences from light, O₂, and CO₂. The proposed strategy utilizing NiCo₂S₄ as a solid contact is a promising alternative for the fabrication of calibration-free ASS-ISEs.

Solid-Contact Potentiometric Anion Sensing Based on Classic Silver/Silver Insoluble Salts Electrodes without Ion-Selective Membrane

Current solid potentiometric ion sensors mostly rely on polymeric-membrane-based, solid-contact, ion-selective electrodes (SC-ISEs). However, anion sensing has been a challenge with respect to cations due to the rareness of anion ionophores. Classic metal/metal insoluble salt electrodes (such as Ag/AgCl) without an ion-selective membrane (ISM) offer an alternative. In this work, we first compared the two types of SC-ISEs of Cl- with/without the ISM. It is found that the ISM-free Ag/AgCl electrode discloses a comparable selectivity regarding organic chloride ionophores. Additionally, the electrode exhibits better comprehensive performances (stability, reproducibility, and anti-interference ability) than the ISM-based SC-ISE. In addition to Cl-, other Ag/AgX electrodes also work toward single and multi-valent anions sensing. Finally, a flexible Cl- sensor was fabricated for on-body monitoring the concentration of sweat Cl- to illustrate a proof-of-concept application in wearable anion sensors. This work re-emphasizes the ISM-free SC-ISEs for solid anion sensing.

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.

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

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

Solid-Contact Ion-Selective Electrodes: Response Mechanisms, Transducer Materials and Wearable Sensors

Wearable sensors based on solid-contact ion-selective electrodes (SC-ISEs) are currently attracting intensive attention in monitoring human health conditions through real-time and non-invasive analysis of ions in biological fluids. SC-ISEs have gone through a revolution with improvements in potential stability and reproducibility. The introduction of new transducing materials, the understanding of theoretical potentiometric responses, and wearable applications greatly facilitate SC-ISEs. We review recent advances in SC-ISEs including the response mechanism (redox capacitance and electric-double-layer capacitance mechanisms) and crucial solid transducer materials (conducting polymers, carbon and other nanomaterials) and applications in wearable sensors. At the end of the review we illustrate the existing challenges and prospects for future SC-ISEs. We expect this review to provide readers with a general picture of SC-ISEs and appeal to further establishing protocols for evaluating SC-ISEs and accelerating commercial wearable sensors for clinical diagnosis and family practice.

Recent advances in solid-contact ion-selective electrodes: functional materials, transduction mechanisms, and development trends

This article presents a comprehensive overview of recent progress in the design and applications of solid-contact ion-selective electrodes (SC-ISEs).

Thiol-functionalized reduced graphene oxide as self-assembled ion-to-electron transducer for durable solid-contact ion-selective electrodes

Thiol-functionalized reduced graphene oxide (TRGO) as a novel ion-to-electron transducing layer is firstly employed to develop durable solid-contact ion-selective electrodes (SC-ISEs) in this work. The performance of the sensors is evaluated by determining K⁺ and NO₃^- as an example of cation and anion. The covalent linkage of TRGO at golden electrode surface generates a stable transducing layer. No water films are observed in the proposed TRGO-based potassium (K⁺-TRGO-ISEs) and nitrate (NO₃^--TRGO-ISEs) selective SC-ISEs. The resultant electrodes exhibit Nernstian responses (60.0 ± 0.4 mV/decade for K⁺-TRGO-ISEs and -60.0 ± 0.5 mV/decade for NO₃^--TRGO-ISEs), low detection limits (2.5 × 10^-6 M for K⁺-TRGO-ISEs and 4.0 × 10^-6 M for NO₃^--TRGO-ISEs) and good selectivity behavior. More importantly, the TRGO-based SC-ISEs display a much longer lifetime of 2 weeks than that of reduced graphene oxide-based SC-ISEs in continuous flowing solutions using a longer peristaltic pump. These improvements push TRGO a general and reliable transducer for the development of durable SC-ISEs.

Gold Plate Electrodes Functionalized by Multiwall Carbon Nanotube Film for Potentiometric Thallium(I) Detection

Solid-contact potentiometric ion-selective electrodes (SC-ISEs) for thallium determination have been designed using multiwall carbon nanotubes (MWCNTs) as the ion-to-electron transducer. Dispersed MWCNTs were drop-casted over a gold plate electrode. Two different crown ethers were used in the sensing membrane for the recognition of thallium (I). Sensorsbased on dibenzo-18-crown-6 (DB18C6) as a neutral carrier and NaTPB as an anionic additive exhibited a near Nernstian response of 57.3 mV/decade towards Tl+ ions over the activity range 4.5 × 10-6-7.0 × 10-4 M, with a limit of detection of 3.2 × 10-7 M. The time required to achieve 95% of the steadyequilibrium potential was <10 s. The complex formation constant (log βML) between dibenzo-18-crown-6 and thallium (I) (i.e., 5.99) was measured using the sandwich membrane technique. The potential response was pH independent over the range 3.0-9.5. The introduction of MWCNTs as an electron-ion-transducer layer between gold plate and the sensing membrane lead to a smaller membrane resistance and a large double layer capacitance, which was proven using impedance spectra and chronopotentiometry (i.e., 114.9 ± 12 kΩ, 52.1 ± 3.3 pF, 200 ± 13.2 kΩ, and 50 ± 4.2 µF). Additionally, reduction ofthe water layer between the sensing membrane and the underlying conductor wastested. Thus, it is clear that MWCNTs can be used as a transducing layer in SC-ISEs. The proposed sensor was introduced as an indicator electrode for potentiometric titration of single and ternary mixtures of I-, Br-, and S2- anions.

Robust fabrication of nanomaterial-based all-solid-state ion-selective electrodes

Currently, nanomaterial-based all-solid-state ion-selective electrodes (ASS-ISEs) have become attractive tools for ion sensing in environmental and biological applications. However, nanomaterial solid contact can easily fall off the electrode surface owing to poor adhesion. This poses serious limits to the wide use of these sensors. Herein, we report a general and facile method for the robust fabrication of nanomaterial-based ASS-ISEs. It is based on the silver-based conductive adhesive (CA) with excellent electronic conductivity and strong adhesion ability as the binder to construct nanomaterial-based solid contact. The solid-contact Ca²⁺-ISE based on single-walled carbon nanotubes (SWCNTs) is chosen as a model. The proposed electrode based on CA-SWCNTs shows a linear response in the concentration range of 10⁻⁶ to 10⁻³ M with a slope of 25.96 ± 0.36 mV per decade and a detection limit of 1.7 × 10⁻⁷ M. In addition, the CA-SWCNT-based Ca²⁺-ISE exhibits an improved potential stability and reduced water film compared to the coated-wire ISE. Above all, experiments also show that the CA-SWCNT-based electrode exhibits nearly the same electrochemical characteristics as the classical only SWCNT-based electrode in term of resistance, capacitance and potential stability. We believe that CA-nanomaterial-based solid contacts provide an appealing substitute for traditional solid contacts based on nanomaterials.

For the first time, a general and facile approach for the robust fabrication of nanomaterial-based solid contact ISEs is reported.

Highly-stable Li+ ion-selective electrodes based on noble metal nanostructured layers as solid-contacts

Nowadays the development of stable and highly efficient Solid-Contact Ion-Selective Electrodes (SC-ISEs) attracts much attention in the research community because of the great expansion of portable analytical devices. In this work, we present highly stable Li⁺ all-solid-state ISEs exploiting noble metals nanostructures as ion-to-electron transducers. The detection of lithium is essential for therapeutic drug monitoring of bipolar patients. In addition, greater environmental exposure to this ion is occurring due to the large diffusion of lithium-ion batteries. However, only a limited number of SC Li⁺ ISEs already exists in literature based on Conductive Polymers (CPs) and carbon nanotubes. The use of noble metals for ion-to-electron transduction offers considerable advantages over CPs and carbon materials, including fast and conformal one-step deposition by electrochemical means, non-toxicity and high stability. We investigate for the first time the use of gold nanocorals obtained by means of a one-step electrodeposition process to improve sensor performance and we compare it to all-solid-state ISEs based on electrodeposited platinum nanoflowers. In addition, the effect of substrate electrode material, membrane thickness and conditioning concentration on the potentiometric response is carefully analysed. Scanning Electron Microscopy (SEM) and Current Reversal Chronopotentiometry (CRC) techniques are used to characterize the morphology and the electrochemical behaviour of the different ISEs. The use of nanostructured gold and platinum contacts allows the increase of the SC capacitance by one or two orders of magnitude, respectively, with respect to the flat metal, while the SC resistance is significantly reduced. We show that the microfabricated sensors offer Nernstian behaviour (58.7±0.8 mV/decade) in the activity range from 10^-5 to 0.1 M, with short response time (∼15 s) and small potential drift during CRC measurements (dEdt=3×10^-5±2×10^-5 V/s). The exceptional response stability is verified also when no potential is applied. The sensor shows high selectivity towards all clinically important ions, with values very similar to conventional ISEs. Furthermore, to our knowledge, the selectivity towards Ca⁺² is the best ever reported for SC-ISEs. In conclusion, the present study opens up new interesting perspectives towards the development of simple and reproducible fabrication protocols to obtain high-quality and high-stability all-solid-state ISEs.

Solid contact ion-selective electrodes with a well-controlled Co(II)/Co(III) redox buffer layer

Solid contact ion-selective electrodes (ISEs) typically have an intermediate layer between the ion-selective membrane and the underlying solid electron conductor that is designed to reduce the irreproducibility and instability of the measured electromotive force (emf). Nevertheless, the electrode-to-electrode reproducibility of the emf of current solid contact ISEs is widely considered to be unsatisfactory. To address this problem, we report here a new method of constructing this intermediate layer based on the lipophilic redox buffer consisting of the Co(III) and Co(II) complexes of 1,10-phenanthroline ([Co(phen)3](3+/2+)) paired with tetrakis(pentafluorophenyl)borate as counterion. The resulting electrodes exhibit emf values with an electrode-to-electrode standard deviation as low as 1.7 mV after conditioning of freshly prepared electrodes for 1 h. While many prior examples of solid contact ISEs also used intermediate layers that contained redox active species, the selection of a balanced ratio of the reduced and oxidized species has typically been difficult and was often ignored, contributing to the emf irreproducibility. The ease of the control of the [Co(phen)3](3+)/[Co(phen)3](2+) ratio explains the high emf reproducibility, as confirmed by the emf decrease of 58 mV per 10-fold increase in the ratio of the reduced and oxidized redox buffer species. Use of a gold electrode modified with a self-assembled 1-hexanethiol monolayer as underlying electron conductor suppresses the formation of a water layer and results in an electrode-to-electrode standard deviation of E° of 1.0 mV after 2 weeks of exposure to KCl solution.

Improved potentiometric response of all-solid-state Pb(2+)-selective electrode

Zero-current ion-flux has a great influence on the characteristics of the ion-selective electrodes. In this work the improvement of analytical performance of all-solid-state Pb(2+)-selective membrane electrodes was demonstrated by adjusting the transmembrane ion flux. The study is focused on the relationship between the conditioning solution and the linear working range of the obtained electrodes for different sample matrixes. Results show that the electrode with appropriate conditioning keeps good reproducibility within linear working range. The utility of the electrode has been tested by successfully determining Pb(2+) concentration in real water samples.