A liquid-junction-free reference electrode based on a PEDOT solid-contact and ionogel capping membrane

Response Patterns of Chromoionophore-Based Bulk Optodes Containing Lipophilic Electrolytes: Toward Background-Independent pH-Sensing

Despite a number of advantages of ion-selective optical sensors (optodes), their practical application is limited by their response mechanism, which leads to the dependence of the signal on the activity of two ions (analyte ion and the so-called reference ion) in the solution at the same time. Here, we show that the introduction of a lipophilic electrolyte into the polymeric optode membrane allows assessing the ionic activity of H+ cations regardless of the concentration of the background electrolyte containing a hydrophilic cation, with NaCl as an example of such an electrolyte. For the first time, the applicability of this approach is proven theoretically utilizing the numerical simulation of optode response. A correlation between the interfacial potential stability and the single-ion optical response is established. The predicted optical response is independent of background cation concentration to a significant extent. Theoretical conclusions are supported by experimental data obtained with chromoionophore-based optodes doped with various lipophilic electrolytes, including ionic liquids, by thin-film spectrophotometry and macrophotography coupled with digital color analysis. Most of the experimental sensor characteristics, such as the response range and its median, as well as its independence from the background electrolyte concentration are in quantitative agreement with the proposed theoretical description.

A Comprehensive Review of the Recent Developments in Wearable Sweat-Sensing Devices

Sweat analysis offers non-invasive real-time on-body measurement for wearable sensors. However, there are still gaps in current developed sweat-sensing devices (SSDs) regarding the concerns of mixing fresh and old sweat and real-time measurement, which are the requirements to ensure accurate the measurement of wearable devices. This review paper discusses these limitations by aiding model designs, features, performance, and the device operation for exploring the SSDs used in different sweat collection tools, focusing on continuous and non-continuous flow sweat analysis. In addition, the paper also comprehensively presents various sweat biomarkers that have been explored by earlier works in order to broaden the use of non-invasive sweat samples in healthcare and related applications. This work also discusses the target analyte's response mechanism for different sweat compositions, categories of sweat collection devices, and recent advances in SSDs regarding optimal design, functionality, and performance.

Microfluidics and materials for smart water monitoring: A review

Water quality monitoring of drinking, waste, fresh and seawaters is of great importance to ensure safety and wellbeing for humans, fauna and flora. Researchers are developing robust water monitoring microfluidic devices but, the delivery of a cost-effective, commercially available platform has not yet been achieved. Conventional water monitoring is mainly based on laboratory instruments or sophisticated and expensive handheld probes for on-site analysis, both requiring trained personnel and being time-consuming. As an alternative, microfluidics has emerged as a powerful tool with the capacity to replace conventional analytical systems. Nevertheless, microfluidic devices largely use conventional pumps and valves for operation and electronics for sensing, that increment the dimensions and cost of the final platforms, reducing their commercialization perspectives. In this review, we critically analyze the characteristics of conventional microfluidic devices for water monitoring, focusing on different water sources (drinking, waste, fresh and seawaters), and their application in commercial products. Moreover, we introduce the revolutionary concept of using functional materials such as hydrogels, poly(ionic liquid) hydrogels and ionogels as alternatives to conventional fluidic handling and sensing tools, for water monitoring in microfluidic devices.

Solid-Contact Electrode with Composite PVC-Based 3D-Printed Membrane. Optimization of Fabrication and Performance

Intense interest in reference electrode design and fabrication has recently been enriched with the application of 3D printing of electrodes with salt-loaded PVC membranes. This type of material is attractive in sensor technology and is challenging to implement in 3D. In this report, several improvements and simplifications in the technology were focused on and supported by a fundamental electrochemical characterization.

Potentiometric Sensor with High Capacity Composite Composed of Ruthenium Dioxide and Poly(3,4-ethylenedioxythiophene) Polystyrene Sulfonate

This work presents the first-time application of the ruthenium dioxide-poly(3,4-ethylenedioxythiophene) polystyrene sulfonate high-capacity composite material as a mediation layer in potassium selective electrodes, which turned out to significantly enhance the electrical and analytical parameters of the electrodes. The idea was to combine the properties of two different types of materials: a conducting polymer, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate, and a metal oxide, ruthenium dioxide, in order to obtain the material for a solid-contact layer of great electrical and physicochemical parameters. The preparation method for composite material proposed in this work is fast and easy. The mediation layer material was examined using a scanning electron microscope and chronopotentiometry in order to confirm that all requirements for mediation layers materials were fulfilled. Ruthenium dioxide-poly(3,4-ethylenedioxythiophene) polystyrene sulfonate nancomposite material turned out to exhibit remarkably high electrical capacitance (of approximately 17.5 mF), which ensured great performance of designed K+-selective sensors. Electrodes of electrical capacity equal to 7.2 mF turned out to exhibit fast and stable (with only 0.077 mV potential change per hour) potentiometric responses in the wide range of potassium ion concentrations (10-6 M to 10-1 M). The electrical capacity of ruthenium dioxide-poly(3,4-ethylenedioxythiophene) polystyrene sulfonate-contacted electrodes characterized by electrical capacitance parameters was the highest reported so far for this type of sensor.

A screen-printed three-electrode-type sticker device with an accurate liquid junction-type reference electrode

We developed a novel sticker device that can convert any metal or alloy into the working electrode of a three-electrode system, enabling simple and accurate measurement. The sticker, containing a counter electrode and a stable and accurate liquid junction-type reference electrode, is attached to the metal or alloy; meanwhile the surface exposed from a hole in the device functions as the working electrode. This sticker device was fabricated by screen-printing. The polarization curve of the copper and tin-plated copper measured using the sticker device exhibited approximately the same behavior as that obtained for the conventional three-electrode system. The characteristics of various materials can be easily evaluated using this system by only dropping a small amount of solution.

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.

A wearable sensor for the detection of sodium and potassium in human sweat during exercise

The SwEatch platform, a wearable sensor for sampling and measuring the concentration of electrolytes in human sweat in real time, has been improved in order to allow the sensing of two analytes. The solid contact ion-sensitive electrodes (ISEs) for the detection of Na⁺ and K⁺ have been developed in two alternative formulations, containing either poly(3,4-ethylenedioxythiophene) (PEDOT) or poly(3-octylthiophene-2,5-diyl) (POT) as a conductive polymer transducing component. The solution-processable POT formulation simplifies the fabrication process, and sensor to sensor reproducibility has been improved via partial automation using an Opentron® automated pipetting robot. The resulting electrodes showed good sensitivity (52.4 ± 6.3 mV/decade (PEDOT) and 56.4 ± 2.2 mV/decade (POT) for Na⁺ ISEs, and 45.7 ± 7.4 mV/decade (PEDOT) and 54.3 ± 1.5 mV/decade (POT) for K⁺) and excellent selectivity towards potential interferents present in human sweat (H⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺). The 3D printed SwEatch platform has been redesigned to incorporate a double, mirrored fluidic unit which is capable of drawing sweat from the skin through passive capillary action and bring it in contact with two independent electrodes. The potentiometric signal generated by the electrodes is measured by an integrated electronics board, digitised and transmitted via Bluetooth to a laptop. The results obtained from on-body trials on athletes during cycling show a relatively small increase in sodium (1.89 mM-2.97 mM) and potassium (3.31 mM-7.25 mM) concentrations during the exercise period of up to 90 min.

A miniaturized solid-contact potentiometric multisensor platform for determination of ionic profiles in human saliva

This paper describes a miniaturized multisensor platform (MP-ISES) consisting of electrodes: a reference one (RE) and ion-selective electrodes (ISEs) for monitoring Na+, K+, Ca2+, Mg2+, Cl−, and SCN− ions and pH in human saliva. Gold electrode surface was modified by deposition of two layers: electrosynthesized PEDOT:PSS forming an intermediate layer, and ion-selective membrane. The developed ISEs were characterized by a wide linear range and sensitivity consistent with the Nernst model. The entire MP-ISEs are characterized by satisfactory metrological parameters demonstrating their applicability in biomedical research, in particular in measurements concerning determination of ionic profiles of saliva. Saliva samples of 18 volunteers aged from 20 to 26 participating in a month experiment had been daily collected and investigated using the MP-ISEs assigned individually to each person. Personalized profiles of ions (ionograms) in saliva, such as Na+, K+, Ca2+, Mg2+, Cl−, SCN−, and H+, were obtained.