A solid-contact Ca2+-selective electrode based on an inorganic redox buffer of Ag@AgCl/1-tetradecyl-3-methylimidazolium chloride as ion-to-electron transducer

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.

Recent Developments and Challenges in Solid-Contact Ion-Selective Electrodes

Solid-contact ion-selective electrodes (SC-ISEs) have the advantages of easy miniaturization, even chip integration, easy carrying, strong stability, and more favorable detection in complex environments. They have been widely used in conjunction with portable, wearable, and intelligent detection devices, as well as in on-site analysis and timely monitoring in the fields of environment, industry, and medicine. This article provides a comprehensive review of the composition of sensors based on redox capacitive and double-layer capacitive SC-ISEs, as well as the ion-electron transduction mechanisms in the solid-contact (SC) layer, particularly focusing on strategies proposed in the past three years (since 2021) for optimizing the performance of SC-ISEs. These strategies include the construction of ion-selective membranes, SC layer, and conductive substrates. Finally, the future research direction and possibilities in this field are discussed and prospected.

Highly Stable Solid Contact Calcium Ion-Selective Electrodes: Rapid Ion-Electron Transduction Triggered by Lipophilic Anions Participating in Redox Reactions of CunS Nanoflowers

Solid contact (SC) calcium ion-selective electrodes (Ca2+-ISEs) have been widely applied in the analysis of water quality and body fluids by virtue of the unique advantages of easy operation and rapid response. However, the potential drift during the long-term stability test hinders their further practical applications. Designing novel redox SC layers with large capacitance and high hydrophobicity is a promising approach to stabilize the potential stability, meanwhile, exploring the transduction mechanism is also of great guiding significance for the precise design of SC layer materials. Herein, flower-like copper sulfide (CunS-50) composed of nanosheets is meticulously designed as the redox SC layer by modification with the surfactant (CTAB). The CunS-50-based Ca2+-ISE (CunS-50/Ca2+-ISE) demonstrates a near-Nernstian slope of 28.23 mV/dec for Ca2+ in a wide activity linear range of 10-7 to 10-1 M, with a low detection limit of 3.16 × 10-8 M. CunS-50/Ca2+-ISE possesses an extremely low potential drift of only 1.23 ± 0.13 μV/h in the long-term potential stability test. Notably, X-ray absorption fine-structure (XAFS) spectra and electrochemical experiments are adopted to elucidate the transduction mechanism that the lipophilic anion (TFPB-) participates in the redox reaction of CunS-50 at the solid-solid interface of ion-selective membrane (ISM) and redox inorganic SC layer (CunS-50), thereby promoting the generation of free electrons to accelerate ion-electron transduction. This work provides an in-depth comprehension of the transduction mechanism of the potentiometric response and an effective strategy for designing redox materials of ion-electron transduction triggered by lipophilic anions.

Application of ionic liquids in ion-selective electrodes and reference electrodes: A review

Ionic liquids (ILs) are organic chemical compounds that are composed only of ions, a large organic cation and a smaller inorganic or organic anion. These are salts whose melting point is lower than the boiling point of water. ILs have many interesting properties, thanks to which they find great practical applications in analytics, electrochemistry, separation techniques, catalysis and others. One of the many areas of application of ionic liquids is sensors especially electrochemical sensors including ion-selective electrodes. In this case, the properties of ILs that are particularly useful include very good electrical conductivity, high electrochemical stability, good extraction properties, hydrophobic character and compatibility with other materials, e. g. polyvinyl chloride plasticizers or carbon nanomaterials. ILs were used as components of ion-selective membranes, both polymeric ones based on PVC and membranes in carbon paste electrodes. ILs performed various functions in these membranes, including lipophilic ionic additive, ionophore/ion exchanger, plasticizer, transducer media and matrix. They were also used as a component of the intermediate layer in solid contact ISEs. The last chapter presents examples of the use of ILs in reference electrodes. This review discusses the use of ionic liquids in ion-selective electrodes (ISEs) and reference electrodes over the last ten years.

Label-free and highly-sensitive detection of calcium ions using a silicon-on-sapphire light-addressable potentiometric sensor

BACKGROUND

Ionic calcium (Ca2+) plays a crucial role in maintaining normal physiological and biochemical functions within the human body. Detecting the concentration of Ca2+ is of utmost significance for various purposes, including disease screening, cellular metabolism research, and evaluating drug effectiveness. However, current detection approaches such as fluorescence and colorimetry face limitations due to complex labeling techniques and the inability to track changes in Ca2+ concentration. In recent years, extensive research has been conducted in this field to explore label-free and efficient approaches.

RESULTS

In this study, a novel light-addressed potentiometric sensor (LAPS) using silicon-on-sapphire technology, has been successfully developed for Ca2+ sensing. The Ca2+-sensitive LAPS achieved a wide-range detection of Ca2+, ranging from 10-2 M to 10-7 M, with an impressive detection limit of 100 nM. These advancements are attributed to the ultra-thin silicon layer, silicon dioxide layer, and solid-state silicon rubber sensitive membrane around 6 μm. Furthermore, the sensor demonstrated the ability to dynamically monitor fluctuations in Ca2+ concentration ranging from 10-9 M to 10-2 M within a solution. Its remarkable selectivity, specificity, and long-term stability have facilitated its successful application in the detection of Ca2+ in human serum and urine.

SIGNIFICANCE AND NOVELTY

This work presents a Ca2+-sensitive sensor that combines a low detection limit and a wide detection range. The development represents the emergence of a label-free and rapid Ca2+ detection tool with immense prospects in home-based health monitoring, community disease screening, as well as cellular metabolism, and drug screening evaluations.

Skin-Interfaced Wearable Sweat Sensors for Precision Medicine

Wearable sensors hold great potential in empowering personalized health monitoring, predictive analytics, and timely intervention toward personalized healthcare. Advances in flexible electronics, materials science, and electrochemistry have spurred the development of wearable sweat sensors that enable the continuous and noninvasive screening of analytes indicative of health status. Existing major challenges in wearable sensors include: improving the sweat extraction and sweat sensing capabilities, improving the form factor of the wearable device for minimal discomfort and reliable measurements when worn, and understanding the clinical value of sweat analytes toward biomarker discovery. This review provides a comprehensive review of wearable sweat sensors and outlines state-of-the-art technologies and research that strive to bridge these gaps. The physiology of sweat, materials, biosensing mechanisms and advances, and approaches for sweat induction and sampling are introduced. Additionally, design considerations for the system-level development of wearable sweat sensing devices, spanning from strategies for prolonged sweat extraction to efficient powering of wearables, are discussed. Furthermore, the applications, data analytics, commercialization efforts, challenges, and prospects of wearable sweat sensors for precision medicine are discussed.

A Co3O4 Nanoparticle-Modified Screen-Printed Electrode Sensor for the Detection of Nitrate Ions in Aquaponic Systems

In this study, a screen-printed electrode (SPE) modified with cobalt oxide nanoparticles (Co₃O₄ NPs) was used to create an all-solid-state ion-selective electrode used as a potentiometric ion sensor for determining nitrate ion (NO₃^-) concentrations in aquaculture water. The effects of the Co₃O₄ NPs on the characterization parameters of the solid-contact nitrate ion-selective electrodes (SC-NO₃^--ISEs) were investigated. The morphology, physical properties and analytical performance of the proposed NO₃^--ion selective membrane (ISM)/Co₃O₄ NPs/SPEs were studied by X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), potentiometric measurements, and potentiometric water layer tests. Once all conditions were optimized, it was confirmed that the screen-printed electrochemical sensor had high potential stability, anti-interference performance, good reproducibility, and no water layer formation between the selective membrane and the working electrode. The developed NO₃^--ISM/Co₃O₄ NPs/SPE showed a Nernstian slope of -56.78 mV/decade for NO₃^- detection with a wide range of 10^-7-10^-2 M and a quick response time of 5.7 s. The sensors were successfully used to measure NO₃^- concentrations in aquaculture water. Therefore, the electrodes have potential for use in aquaponic nutrient solution applications with precise detection of NO₃^- in a complicated matrix and can easily be used to monitor other ions in aquaculture water.

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.

Unlocking All-Solid Ion Selective Electrodes: Prospects in Crop Detection

This paper reviews the development of all-solid-state ion-selective electrodes (ASSISEs) for agricultural crop detection. Both nutrient ions and heavy metal ions inside and outside the plant have a significant influence on crop growth. This review begins with the detection principle of ASSISEs. The second section introduces the key characteristics of ASSISE and demonstrates its feasibility in crop detection based on previous research. The third section considers the development of ASSISEs in the detection of corps internally and externally (e.g., crop nutrition, heavy metal pollution, soil salinization, N enrichment, and sensor miniaturization, etc.) and discusses the interference of the test environment. The suggestions and conclusions discussed in this paper may provide the foundation for additional research into ion detection for crops.

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.

Using MoS2/Fe3O4 as Ion-Electron Transduction Layer to Manufacture All-Solid-State Ion-Selective Electrode for Determination of Serum Potassium

As an essential electrolyte for the human body, the potassium ion (K+) plays many physiological roles in living cells, so the rapid and accurate determination of serum K+ is of great significance. In this work, we developed a solid-contact ion-selective electrode (SC-ISE) using MoS2/Fe3O4 composites as the ion-to-electron transducer to determine serum K+. The potential response measurement of MoS2/Fe3O4/K+-ISE shows a Nernst response by a slope of 55.2 ± 0.1 mV/decade and a low detection limit of 6.3 × 10−6 M. The proposed electrode exhibits outstanding resistance to the interference of O2, CO2, light, and water layer formation. Remarkably, it also presents a high performance in potential reproducibility and long-term stability.

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).