Towards sustainable and humane dairy farming: A low-cost electrochemical sensor for on-site diagnosis of milk fever

Milk fever is a metabolic disorder that predominantly affects dairy animals during the periparturient period and within four weeks of calving. Milk fever is primarily attributed to a decrease in the animal's serum Ca2+ levels. Clinical milk fever occurs when Ca2+ concentration drops below 1.5 mM (6 mg/dL). Without prompt intervention, clinical milk fever leads to noticeable physical symptoms and health complications including coma and fatality. Subclinical milk fever is characterized by Ca2+ levels between 1.5 and 2.12 mM (6-8.48 mg/dL). Approximately 50% of multiparous dairy cows suffer from subclinical milk fever during the transition to lactation. The economic impact of milk fever, both direct and indirect, is substantial, posing challenges for farmers. To address this issue, we developed a low-cost electrochemical sensor that can measure bovine serum calcium levels on-site, providing an opportunity for early detection of subclinical and clinical milk fever and early intervention. This calcium sensor is a scalable solid contact ion sensing platform that incorporates a polymeric calcium-selective membrane and ionic liquid-based reference membrane into laser-induced graphene (LIG) electrodes. Our sensing platform demonstrates a sensitivity close to the theoretical Nernstian value (29.6 mV/dec) with a limit of detection of 15.6 μM and selectivity against the species in bovine serum. Moreover, our sensor can detect Ca2+ in bovine serum with 91% recovery.

Fueling the Future: The Emergence of Self-Powered Enzymatic Biofuel Cell Biosensors

Self-powered biosensors are innovative devices that can detect and analyze biological or chemical substances without the need for an external power source. These biosensors can convert energy from the surrounding environment or the analyte itself into electrical signals for sensing and data transmission. The self-powered nature of these biosensors offers several advantages, such as portability, autonomy, and reduced waste generation from disposable batteries. They find applications in various fields, including healthcare, environmental monitoring, food safety, and wearable devices. While self-powered biosensors are a promising technology, there are still challenges to address, such as improving energy efficiency, sensitivity, and stability to make them more practical and widely adopted. This review article focuses on exploring the evolving trends in self-powered biosensor design, outlining potential advantages and limitations. With a focal point on enzymatic biofuel cell power generation, this article describes various sensing mechanisms that employ the analyte as substrate or fuel for the biocatalyst's ability to generate current. Technical aspects of biofuel cells are also examined. Research and development in the field of self-powered biosensors is ongoing, and this review describes promising areas for further exploration within the field, identifying underexplored areas that could benefit from further investigation.

Fully Integrated Multiplexed Wristwatch for Real-Time Monitoring of Electrolyte Ions in Sweat

Considerable progress has already been made in sweat sensors based on electrochemical methods to realize real-time monitoring of biomarkers. However, realizing long-term monitoring of multiple targets at the atomic level remains extremely challenging, in terms of designing stable solid contact (SC) interfaces and fully integrating multiple modules for large-scale applications of sweat sensors. Herein, a fully integrated wristwatch was designed using mass-manufactured sensor arrays based on hierarchical multilayer-pore cross-linked N-doped porous carbon coated by reduced graphene oxide (NPCs@rGO-950) microspheres with high hydrophobicity as core SC, and highly selective monitoring simultaneously for K+, Na+, and Ca2+ ions in human sweat was achieved, exhibiting near-Nernst responses almost without forming an interfacial water layer. Combined with computed tomography, solid-solid interface potential diffusion simulation results reveal extremely low interface diffusion potential and high interface capacitance (598 μF), ensuring the excellent potential stability, reversibility, repeatability, and selectivity of sensor arrays. The developed highly integrated-multiplexed wristwatch with multiple modules, including SC, sensor array, microfluidic chip, signal transduction, signal processing, and data visualization, achieved reliable real-time monitoring for K+, Na+, and Ca2+ ion concentrations in sweat. Ingenious material design, scalable sensor fabrication, and electrical integration of multimodule wearables lay the foundation for developing reliable sweat-sensing systems for health monitoring.

Functionalizing Carbon Substrates with a Covalently Attached Cobalt Redox Buffer for Calibration-Free Solid-Contact Ion-Selective Electrodes

With a view to potentiometric sensing with minimal calibration requirements and high long-term stability, colloid-imprinted mesoporous (CIM) carbon was functionalized by the covalent attachment of a cobalt redox buffer and used as a new solid contact for ion-selective electrodes (ISEs). The CIM carbon surface was first modified by electroless grafting of a terpyridine ligand (Tpy-ph) using diazonium chemistry, followed by stepwise binding of Co(II) and an additional Tpy ligand to the grafted ligand, forming a bis(terpyridine) Co(II) complex, CIM-ph-Tpy-Co(II)-Tpy. Half a molar equivalent of ferrocenium tetrakis(3-chlorophenyl)borate was then used to partially oxidize the Co(II) complex. Electrodes prepared with this surface-attached CIM-ph-Tpy-Co(III/II)-Tpy redox buffer as a solid contact were tested as K+ sensors in combination with valinomycin as the ionophore and Dow 3140 silicone or plasticized poly(vinyl chloride) (PVC) as the matrixes for the ion-selective membrane (ISM). This solid contact is characterized by a redox capacitance of 3.26 F/g, ensuring a well-defined interfacial potential that underpins the transduction mechanism. By use of a redox couple as an internal reference element to control the phase boundary potential at the interface of the ISM and the CIM carbon solid contact, solid-contact ion-selective electrodes (SC-ISEs) with a standard deviation of E° as low as 0.3 mV for plasticized PVC ISMs and 3.5 mV for Dow 3140 silicone ISMs were obtained. Over 100 h, these SC-ISEs exhibit an emf drift of 20 μV/h for plasticized PVC ISMs and 62 μV/h for silicone ISMs. The differences in long-term stability and reproducibility between electrodes with ISMs comprising either a plasticized PVC or silicone matrix offer valuable insights into the effect of the polymeric matrix on sensor performance.

Design Criteria for Nanostructured Carbon Materials as Solid Contacts for Ion-Selective Sensors

The ability to miniaturize ion-selective sensors that enable microsensor arrays and wearable sensor patches for ion detection in environmental or biological samples requires all-solid-state sensors with solid contacts for transduction of an ion activity into an electrical signal. Nanostructured carbon materials function as effective solid contacts for this purpose. They can also contribute to improved potential signal stability, reducing the need for frequent sensor calibration. In this Perspective, the structural features of various carbon-based solid contacts described in the literature and their respective abilities to reduce potential drift during long-term, continuous measurements are compared. These carbon materials include nanoporous carbons with various architectures, carbon nanotubes, carbon black, graphene, and graphite-based solid contacts. The effects of accessibility of ionophores, ionic sites, and other components of an ion-selective membrane to the internal or external carbon surfaces are discussed, because this impacts double-layer capacitance and potential drift. The effects of carbon composition on water-layer formation are also considered, which is another contributor to potential drift during long-term measurements. Recommendations regarding the selection of solid contacts and considerations for their characterization and testing in solid-contact ion-selective electrodes are provided.

Selective Deionization of Thin-Layer Samples Using Tandem Carbon Nanotubes-Polymeric Membranes

Herein, we investigate the selective deionization (i.e., the removal of ions) in thin-layer samples (<100 μm in thickness) using carbon nanotubes (CNTs) covered with an ionophore-based ion-selective membrane (ISM), resulting in a CNT-ISM tandem actuator. The concept of selective deionization is based on a recent discovery by our group ( Anal. Chem. 2022, 94, 21, 7455-7459), where the activation of the CNT-ISM architecture is conceived on a mild potential step that charges the CNTs to ultimately generate the depletion of ions in a thin-layer sample. The role of the ISM is to selectively facilitate the transport of only one ion species to the CNT lattice. To estimate the deionization efficiency of such a process, a potentiometric sensor is placed less than 100 μm away from the CNT-ISM tandem, inside a microfluidic cell. This configuration helped to reveal that the selective uptake of ions increases with the capacitance of the CNTs and that the ISM requires a certain ion-exchanger capacity, but this does not further affect its efficiency. The versatility of the concept is demonstrated by comparing the selective uptake of five different ions (H+, Li+, Na+, K+, and Ca2+), suggesting the possibility to remove any cation from a sample by simply changing the ionophore in the ISM. Furthermore, ISMs based on two ionophores proved to achieve the simultaneous and selective deionization of two ion species using the same actuator. Importantly, the relative uptake between the two ions was found to be governed by the ion-ionophore binding constants, with the most strongly bound ion being favored over other ions. The CNT-ISM actuator concept is expected to contribute to the analytical sensing field in the sense that ionic interferents influencing the analytical signal can selectively be removed from samples to lower traditional limits of detection.

A high-performance, all-solid-state Na+ selective sensor printed with eco-friendly conductive ink

In recent years, the integration of flexible printed electronics and electrochemical sensors has emerged as a new approach for developing wearable biochemical detecting devices. Among the materials utilized in flexible printed electronics, carbon-based conductive inks are considered to be crucial. In this study, we propose a cost-effective, highly conductive, and environmentally friendly ink formulation utilizing graphite and carbon black (CB) as conductive fillers, resulting in a very low sheet resistance of 15.99 Ω sq^-1 (conductivity of 2.5 × 10³ S m^-1) and a printed film thickness of 25 μm. The unique "sandwich" structure of the working electrode (WE) printed with this ink enhances its electrical conductivity, leading to high sensitivity, selectivity, and stability, with almost no water film generated between the WE and the ion-selective membrane (ISM), strong ion selectivity, long-term stability, and anti-interference. The lower detection limit of the sensor for Na⁺ is 0.16 mM with a slope of 75.72 mV per decade. To validate the sensor's usability, we analyzed three sweat samples collected during physical activity, with Na⁺ concentrations within the typical range for human sweat (51 ± 4 mM, 39 ± 5 mM, and 46 ± 2 mM).

Utilizing Gradient Porous Graphene Substrate as the Solid-Contact Layer To Enhance Wearable Electrochemical Sweat Sensor Sensitivity

Wearable sweat monitoring represents an attractive opportunity for personalized healthcare and for evaluating sports performance. One of the limitations with such monitoring, however, is water layer formation upon cycling of ion-selective sensors, leading to degraded sensitivity and long-term instability. Our report is the first to use chemical vapor deposition-grown, three-dimensional, graphene-based, gradient porous electrodes to minimize such water layer formation. The proposed design reduces the ion diffusion path within the polymeric ion-selective membrane and enhances the electroactive surface for highly sensitive, real-time detection of Na⁺ ions in human sweat with high selectivity. We obtained a 7-fold enhancement in electroactive surface against 2D electrodes (e.g., carbon, gold), yielding a sensitivity of 65.1 ± 0.25 mV decade^-1 (n = 3, RSD = 0.39%), the highest to date for wearable Na⁺ sweat sensors. The on-body sweat sensing performance is comparable to that of ICP-MS, suggesting its feasibility for health evaluation through sweat.

Selective Ion Capturing via Carbon Nanotubes Charging

We present a phenomenon consisting of the synergistic effects of a capacitive material, such as carbon nanotubes (CNTs), and an ion-selective, thin-layer membrane. CNTs can trigger a charge disbalance and propagate this effect into a thin-layer membrane domain under mildly polarization conditions. With the exceptional selectivity and the fast establishment of new concentration profiles provided by the thin-layer membrane, a selective ion capture from the solution is expected, which is necessarily linked to the charge generation on the CNTs lattice. As a proof-of-concept, we investigated an arrangement based on a layer of CNTs modified with a nanometer-sized, potassium-selective membrane to conform an actuator that is in contact with a thin-layer aqueous solution (thickness of 50 μm). The potassium ion content was fixed in the solution (0.1–10 mM range), and the system was operated for 120 s at −400 mV (with respect to the open circuit potential). A 10-fold decrease from the initial potassium concentration in the thin-layer solution was detected through either a potentiometric potassium-selective sensor or an optode confronted to the actuator system. This work is significant, because it provides empirical evidence for interconnected charge transfer processes in CNT–membrane systems (actuators) that result in controlled ion uptake from the solution, which is monitored by a sensor. One potential application of this concept is the removal of ionic interferences in a sample by means of the actuator to enhance precision of analytical assessments of a charged or neutral target in the sample with the sensor.

A quantitative sensing system based on a 3D-printed ion-selective electrode for rapid and sensitive detection of bacteria in biological fluid

Bacterial infections, such as urinary tract infections, are crucial health problems. Here, we report a new potentiometric sensor to detect bacteria sensitively, accurately, and quickly. First, a customizable, 3D printed Ag⁺ selective electrode was fabricated as the probe. Our 3D printed electrode showed sensitive, linear, and selective responses to Ag⁺. Compared to commercial Ag⁺ selective electrodes, ours required less sample volume, shorter responding time, and lower costs. Next, a novel potentiometer was developed with Arduino to couple the electrode for data transducing and transferring, which was programmed to transfer results to cell phones wirelessly. Moreover, a filter was designed to quickly remove interfering species in a biofluid sample (e.g., Cl^-). By detecting the lost Ag⁺ taken by bacteria, the bacterial number could be elucidated. With this sensor system, bacteria numbers could be detected as low as 80 CFU/mL (LOD) within 15 min, which is sufficient for many diagnoses (e.g., urinary tract infection >1000 CFU/mL). An amplification method was presented for single-digit bacteria detection. Overall, we are presenting a bacteria detector with three innovative components: the electrode (signal transduction and detection), the potentiometer (transducer and data processing), and the 3D printed filter (sample preparation), which showed robust and improved (than previously reported ones) analytical merits. The low-cost and customizable (the electrode and the open-source coding) nature enhances the transnationality of the system, especially in underdeveloped areas.

Hydrogels Doped with Redox Buffers as Transducers for Ion-Selective Electrodes

Solid-contact ion-selective electrodes (ISEs) with an unintentional water layer between the sensing membrane and underlying electron conductor are well known to suffer from potential drift caused by the instability of the phase boundary potential between the sensing membrane and the water layer with its uncontrolled ionic composition. The reproducibility and long-term emf stability of ISEs with a miniaturized inner filling solution comprising a hydrogel and a hydrophilic electrolyte have not been studied as thoroughly. Here, such devices are discussed with a view to electrode-to-electrode reproducibility, using both hydrophilic ion-exchange and plasticized PVC membranes, along with a hydrophilic redox buffer composed of ferrocyanide and ferricyanide to control the potential between the hydrogel and the underlying electron conductor. With plasticized PVC sensing membranes, these electrodes showed an E⁰ reproducibility of ±1.1 mV or better, while with hydrophilic ion-exchange membranes, this variability was slightly larger. Long-term drifts were also assessed with both membranes, and the effect of osmotic pressure on drift was shown to be insignificant for the PVC membranes and very small at most for the hydrophilic membranes.

A critical review on the use of potentiometric based biosensors for biomarkers detection

Potentiometric-based biosensors have the potential to advance the detection of several biological compounds and help in early diagnosis of various diseases. They belong to the portable analytical class of biosensors for monitoring biomarkers in the human body. They contain ion-sensitive membranes sensors can be used to determine potassium, sodium, and chloride ions activity while being used as a biomarker to gauge human health. The potentiometric based ion-sensitive membrane systems can be coupled with various techniques to create a sensitive tool for the fast and early detection of cancer biomarkers and other critical biological compounds. This paper discusses the application of potentiometric-based biosensors and classifies them into four major categories: photoelectrochemical potentiometric biomarkers, potentiometric biosensors amplified with molecular imprinted polymer systems, wearable potentiometric biomarkers and light-addressable potentiometric biosensors. This review demonstrated the development of several innovative biosensor-based techniques that could potentially provide reliable tools to test biomarkers. Some challenges however remain, but these can be removed by coupling techniques to maximize the testing sensitivity.

Ion-to-electron capacitance of single-walled carbon nanotube layers before and after ion-selective membrane deposition

The capacitance of the ion-to-electron transducer layer helps to maintain a high potential stability of solid-contact ion-selective electrodes (SC-ISEs), and its estimation is therefore an essential step of SC-ISE characterization. The established chronopotentiometric protocol used to evaluate the capacitance of the single-walled carbon nanotube transducer layer was revised in order to obtain more reliable and better reproducible values and also to allow capacitance to be measured before membrane deposition for electrode manufacturing quality control purposes. The capacitance values measured with the revised method increased linearly with the number of deposited carbon nanotube-based transducer layers and were also found to correlate linearly before and after ion-selective membrane deposition, with correlation slopes close to 1 for nitrate-selective electrodes, to 0.7 and to 0.5 for potassium- and calcium-selective electrodes.

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).

Electrochemical recovery of tellurium from metallurgical industrial waste

Abstract

The current study outlines the electrochemical recovery of tellurium from a metallurgical plant waste fraction, namely Doré slag. In the precious metals plant, tellurium is enriched to the TROF (Tilting, Rotating Oxy Fuel) furnace slag and is therefore considered to be a lost resource—although the slag itself still contains a recoverable amount of tellurium. To recover Te, the slag is first leached in aqua regia, to produce multimetal pregnant leach solution (PLS) with 421 ppm of Te and dominating dissolved elements Na, Ba, Bi, Cu, As, B, Fe and Pb (in the range of 1.4–6.4 g dm−3), as well as trace elements at the ppb to ppm scale. The exposure of slag to chloride-rich solution enables the formation of cuprous chloride complex and consequently, a decrease in the reduction potential of elemental copper. This allows improved selectivity in electrochemical recovery of Te. The results suggest that electrowinning (EW) is a preferred Te recovery method at concentrations above 300 ppm, whereas at lower concentrations EDRR is favoured. The purity of recovered tellurium is investigated with SEM–EDS (scanning electron microscope–energy dispersion spectroscopy). Based on the study, a new, combined two-stage electrochemical recovery process of tellurium from Doré slag PLS is proposed: EW followed by EDRR.

Graphic abstract

Carbon allotropes grafted with poly(pyrrole) derivatives via living radical polymerizations: electrochemical analysis of nano-composites for energy storage

Carbon-based nanomaterials are key components in energy storage devices.