Ion-Selective Electrodes Using Carbon Nanotubes as Ion-to-Electron Transducers

Reliable environmental trace heavy metal analysis with potentiometric ion sensors - reality or a distant dream

Over two decades have passed since polymeric membrane ion-selective electrodes were found to exhibit sufficiently lower detection limits. This in turn brought a great promise to measure trace level concentrations of heavy metals using potentiometric ion sensors at environmental conditions. Despite great efforts, trace analysis of heavy metals using ion-selective electrodes at environmental conditions is still not commercially available. This work will predominantly concentrate on summarizing and evaluating prospects of using potentiometric ion sensors in view of environmental determination of heavy metals in on-site and on-line analysis modes. Challenges associated with development of reliable potentiometric sensors to be operational in environmental conditions will be discussed and reasoning behind unsuccessful efforts to develop potentiometric on-site and on-line environmental ion sensors will be explored. In short, it is now clear that solely lowering the detection limit of the ion-selective electrodes does not guarantee development of successful sensors that would meet the requirement of environmental matrices over long term usage. More pressing challenges of the properties and the performance of the potentiometric sensors must be addressed first before considering extending their sensitivity to low analyte concentrations. These are, in order of importance, selectivity of the ion-selective membrane to main ion followed by the membrane resistance to parallel processes, such as water ingress to the ISM, light sensitivity, change in temperature, presence of gasses in solution and pH and finally resistance of the ion-selective membrane to fouling. In the future, targeted on-site and on-line environmental sensors should be developed, addressing specific environmental conditions. Thus, ion-selective electrodes should be developed with the intention to be suitable to the operational environmental conditions, rather than looking at universal sensor design validated in the idealized and simple sample matrices.

Anodic Stripping Voltammetry on a Carbon-based Ion-Selective Electrode

In this study, we demonstrated the unique capability of carbon-based ion-selective electrode (ISE) to perform highly sensitive square wave anodic stripping voltammetry, while maintaining all the properties of an ISE, in terms of sensitivity, detection limit, response time and selectivity. Square wave anodic stripping voltammetry involves deposition and dissolution steps of metal ions, which means adsorption and desorption of metal ions on the conductive ion-selective membrane without losing its ion-sensing property. To demonstrate this capability, we chose a Ca2+ ion-selective microelectrode (μISE) as a potentiometric method and Cu2+-stripping voltammetry as an amperometric method. The carbon-based ISE surface is capable of quantifying nanomolar to micromolar Cu2+ in both a standard acetate buffer and a complex water sample. The Ca2+-μISE also showed a Nernstian slope of 29 mV / log [Ca2+] and a detection limit of 1 μM within the linear range of 1 μM to 10 mM. It thus opens an opportunity to use the low detection limit of anodic stripping voltammetry and the high selectivity of ISE-based potentiometry.

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.

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.

A new solid-state anionic surfactant-selective sensor based on functionalized MWCNT

A new solid-state potentiometric sensor for anionic surfactants (AnS) determination was prepared. The sensor material in the liquid membrane was made of multi-walled carbon nanotubes (MWCNTs) chemically functionalized with a quaternary ammonium group and tetraphenylborate (TPB) anion (MWCNT-N⁺(CH₃)₃TPB^-). The response of the MWCNT-N⁺(CH₃)₃TPB^- sensor was Nernstian (59.3 mV/decade of activity) for both AnS investigated (sodium dodecyl sulfate (NaDDS) and sodium dodecylbenzenesulfonate (NaDBS)). The limits of detection were 2.0 ∙ 10^-7 and 1.5 ∙ 10^-7 for NaDDS and NaDBS, respectively, and the average response time was only 5 s. The new MWCNT-N⁺(CH₃)₃TPB^- sensor was very selective for NaDDS compared to anions usually contained in commercial products and is not affected by nonionic surfactants that can also be present in these products. It was tested to determine AnS concertation by the potentiometric titration method in a pH range between 3 and 12 and successfully applied for its determination in three-component mixtures and real systems.

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.

Textile-based wearable solid-contact flexible fluoride sensor: Toward biodetection of G-type nerve agents

Rising global concerns posed by chemical and biological threat agents highlight the critical need to develop reliable strategies for the real-time detection of such threats. While wearable sensing technology is well suited to fulfill this task, the use of on-body devices for rapid and selective field identification of chemical agents is relatively a new area. This work describes a flexible printed textile-based solid-contact potentiometric sensor for the selective detection of fluoride anions liberated by the biocatalytic hydrolysis of fluorine-containing G-type nerve agents (such as sarin or soman). The newly developed solid-contact textile fluoride sensor relies on a fluoride-selective bis(fluorodioctylstannyl)methane ionophore to provide attractive analytical performance with near-Nernstian sensitivity and effective discrimination against common anions, along with excellent reversibility and repeatability for dynamically changing fluoride concentrations. By using stress-enduring printed inks and serpentine structures along with stretchable textile substrates, the resulting textile-based fluoride sensor exhibits robust mechanical resiliency under severe mechanical strains. Such realization of an effective textile-based fluoride-selective electrode allowed biosensing of the nerve-agent simulant diisopropyl fluorophosphate (DFP), in connection to immobilized organophosphorus acid anhydrolylase (OPAA) or organophosphorus hydrolase (OPH) enzymes. A user-friendly portable electronic module transmits data from the new textile-based potentiometric biosensor wirelessly to a nearby smartphone for alerting the wearer instantaneously about potential chemical threats. While expanding the scope of wearable solid-contact anion sensors, such a textile-based potentiometric fluoride electrode transducer offers particular promise for effective discrimination of G-type neurotoxins from organophosphate (OP) pesticides, toward specific field detection of these agents in diverse defense settings.

High Capacity Nanocomposite Layers Based on Nanoparticles of Carbon Materials and Ruthenium Dioxide for Potassium Sensitive Electrode

This work presents the new concept of designing ion-selective electrodes based on the use of new composite materials consisting of carbon nanomaterials and ruthenium dioxide. Using two different materials varying in microstructure and properties, we could obtain one material for the mediation layer that adopted features coming of both components. Ruthenium dioxide characterized by high electrical capacity and mixed electronic-ionic transduction and nano-metric carbon materials were reportedly proved to improve the properties of ion-selective electrodes. Initially, only the materials and then the final electrodes were tested in the scope of the presented work, using scanning and transmission electron microscope, contact angle microscope, and various electrochemical techniques, including electrochemical impedance spectroscopy and chronopotentiometry. The obtained results confirmed beneficial influence of the designed nanocomposites on the ion-selective electrodes' properties. Nanosized structure, high capacity (characterized by the electrical capacitance value from approximately 5.5 mF for GR + RuO₂ and CB + RuO₂, up to 14 mF for NT + RuO₂) and low hydrophilicity (represented by the contact angle from 60° for GR+RuO₂, 80° for CB+RuO₂, and up to 100° for NT + RuO₂) of the mediation layer materials, allowed us to obtain water layer-free potassium-selective electrodes, characterized by rapid and stable potentiometric response in a wide range of concentrations-from 10^-1 to 10^-6 M K⁺.

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.

Amr A, A. Elsayed E, Sayed AYA, Kamel AH. Paper-based potentiometric sensing devices modified with chemically reduced graphene oxide (CRGO) for trace level determination of pholcodine (opiate derivative drug)

Robust, reliable and cost-effective paper-based analytical device for potentiometric pholcodine (opiate derivative drug) ion sensing has been prepared and characterized. A printed pholcodinium (PHL)²⁺/5-nitrobarbiturate (NB)⁻ ion-association complex as a sensory material-based all-solid-state ion-selective electrode (ISE) on a chemically reduced graphene oxide (CRGO) solid-contact, and a printed all-solid-state Ag/AgCl reference electrode, has been combined on a hydrophobic paper substrate coated with fluorinated alkyl silane (CF₃(CF₂)₇CH₂CH₂SiCl₃, C^F₁₀). The sensors revealed a potentiometric slope of 28.7 ± 0.3 mV dec⁻¹ (R² = 0.9998) over a linear range starting from 2.0 × 10⁻⁷ M to 1.0 × 10⁻² M and a detection limit of 0.04 μg mL⁻¹. The repeatability and stability of the pholcodine paper-based sensor was found to be 2.32%. The RSD% (n = 6) was found to be 2.67% when using five different paper-based sensors. The sensor revealed an excellent selectivity towards PHL over dextromethorphan, codeine, ephedrine, carbinoxamine, caffeine, ketamine, and K⁺, Na⁺ and Ca²⁺ ions. It showed a good recovery (94–104%) for the determination of PHL in different artificial serum samples. The presented paper-based analytical device was successfully introduced for PHL determination in different pharmaceutical formulations (i.e. syrups and suspensions) containing pholcodine. The current work can be considered as a promising possible analytical tool to obtain cost-effective and disposable paper-based potentiometric sensing devices. These devices can be potentially manufacturable at large scales in pharmaceutical, clinical and forensic applications for opiate drug assessment.

Robust, reliable and cost-effective paper-based analytical device for potentiometric pholcodine (opiate derivative drug) ion sensing has been prepared and characterized.

Real-time in situ auto-correction of K+ interference for continuous and long-term NH4+ monitoring in wastewater using solid-state ion selective membrane (S-ISM) sensor assembly

Potassium ions (K⁺) present in wastewater has caused severe interference for NH₄⁺ monitoring, over-estimation of NH₄⁺ concentration and ultimately leads to extra energy consumption. Past effort for enhancing the selectivity of NH₄⁺ over K⁺ were oftentimes complex, costly, or compromised the selectivity and accuracy of the NH₄⁺ ion selective membrane (ISM) sensors. This study targeted this imminent challenge by developing an integrated NH₄⁺/K⁺ auto-correction solid-state ISM (S-ISM) sensor assembly combined with a data-driven model to monitor [NH₄⁺] under different [NH₄⁺] and [K⁺] concentrations. The results showed that the interference of K⁺ was substantially alleviated for NH₄⁺ measurement. The accuracy was enhanced by over 70% when examined using real wastewater and energy consumption was expected to reduce by 26% for a wastewater treatment plant, especially for wastewater with high [K⁺]. Furthermore, the uniquely structured S-ISMs were made by embedding the ionophores in a robust polyvinyl chloride (PVC) matrix containing plasticizers and a layer of carbon nanotubes (CNT) as ion-to-electron transducer, which maintained the selectivity and accuracy of the S-ISM sensor for 4 weeks in wastewater. NH₄⁺/K⁺ sensor assembly integrated with data-driven correction models poses great potential in high-efficiency and energy-saving wastewater treatment and water reuse processes.

Unintended Changes of Ion-Selective Membranes Composition-Origin and Effect on Analytical Performance

Ion-selective membranes, as used in potentiometric sensors, are mixtures of a few important constituents in a carefully balanced proportion. The changes of composition of the ion-selective membrane, both qualitative and quantitative, affect the analytical performance of sensors. Different constructions and materials applied to improve sensors result in specific conditions of membrane formation, in consequence, potentially can result in uncontrolled modification of the membrane composition. Clearly, these effects need to be considered, especially if preparation of miniaturized, potentially disposable internal-solution free sensors is considered. Furthermore, membrane composition changes can occur during the normal operation of sensors—accumulation of species as well as release need to be taken into account, regardless of the construction of sensors used. Issues related to spontaneous changes of membrane composition that can occur during sensor construction, pre-treatment and their operation, seem to be underestimated in the subject literature. The aim of this work is to summarize available data related to potentiometric sensors and highlight the effects that can potentially be important also for other sensors using ion-selective membranes, e.g., optodes or voltammetric sensors.

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.

An electrochemical sensing platform to determine tetrahydrozoline HCl in pure form, pharmaceutical formulation, and rabbit aqueous humor

In the pharmaceutical industry, finding cost-effective and real-time analyzers that provide valid data is a good aim. The purpose of this work was to propose a link between the pharmaceutical industry and the recent innovations in solid-contact ion-selective electrodes (SC-ISEs) for the utilization of these electrodes as real-time analyzers to evaluate the concentration of tetrahydrozoline HCl in different matrices. The backbone of these new potentiometric sensors is the conjunction of calix[6]arene and (2-hydroxypropyl)-β-cyclodextrin as molecular recognition elements and a network of multi-walled carbon nanotubes as a solid transducer material between an ionophore-doped PVC membrane and microfabricated Cu electrodes. The proposed sensors were optimized to determine tetrahydrozoline, and their performances were assessed according to the IUPAC recommendations. The proposed solid-contact sensors were compared with liquid contact sensors, and the former sensors were found to be better than the latter sensors in terms of durability, handling, and easier adaptation to industry with comparable sensitivity. The measurements were implemented using phosphate buffer (pH: 6). The best obtained linearity range was 1 × 10-2 to 1 × 10-7 M, and the best LOD was 1 × 10-8 M. The sensors with the best performance were successfully applied to determine tetrahydrozoline in a pharmaceutical eye preparation and rabbit tears. The obtained results were statistically compared to those obtained by the official method of analysis, and no significant difference was obtained. The eco-score of the method was assessed using the eco-scale tool and also compared with that of the official method. The proposed approach was validated according to the International Council for Harmonisation (ICH) guidelines.

Flexible Electrochemical Bioelectronics: The Rise of In Situ Bioanalysis

The amalgamation of flexible electronics in biological systems has shaped the way health and medicine are administered. The growing field of flexible electrochemical bioelectronics enables the in situ quantification of a variety of chemical constituents present in the human body and holds great promise for personalized health monitoring owing to its unique advantages such as inherent wearability, high sensitivity, high selectivity, and low cost. It represents a promising alternative to probe biomarkers in the human body in a simpler method compared to conventional instrumental analytical techniques. Various bioanalytical technologies are employed in flexible electrochemical bioelectronics, including ion-selective potentiometry, enzymatic amperometry, potential sweep voltammetry, field-effect transistors, affinity-based biosensing, as well as biofuel cells. Recent key innovations in flexible electrochemical bioelectronics from electrochemical sensing modalities, materials, systems, fabrication, to applications are summarized and highlighted. The challenges and opportunities in this field moving forward toward future preventive and personalized medicine devices are also discussed.

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).

Potentiometric Sensors for the Determination of Anionic Surfactants - A Review

Anionic surfactants are important components of many products used in everyday life in all households. They are also applied in various industrial fields at a very large scale. Since they have a negative influence on the environment, it is an imperative to monitor their concentration in aquatic ecosystems. Therefore, it is of great importance to develop new methods for the determination of a wide spectra of anionic surfactants in complex environmental samples in a short time. A comprehensive review of potentiometric sensors for the determination of anionic surfactants in the last 50 years is given with special concern to papers published since 2000, but noting some earlier published important papers. The latest development in use of new ionophores, polymer formulations, and nanomaterials is presented. Additionally, the application of new potentiometric sensors in batch mode or in miniaturized microfluidic methods is discussed.

Single-Piece All-Solid-State Potential Ion-Selective Electrodes Integrated with Molecularly Imprinted Polymers (MIPs) for Neutral 2,4-Dichlorophenol Assessment

A novel single-piece all-solid-state ion-selective electrode (SC/ISE) based on carbon-screen printed is introduced. Polyaniline (PANI) is dissolved in a membrane cocktail that contains the same components used for making a conventional ion-selective polyvinyl chloride (PVC) matrix membrane. The membrane, having the PANI, is directly drop-casted on a carbon substrate (screen-printed-carbon electrode). PANI was added to act as an intermediary between the substrate and the membrane for the charge transfer process. Under non-equilibrium sensing mechanism, the sensors revealed high sensitivity towards 2,4-dichlorophenol (DCP) over the linearity range 0.47 to 13 µM and a detection limit 0.13 µm. The selectivity was measured by the modified separate solution method (MSSM) and showed good selectivity towards 2,4-DCP over the most commonly studied ions. All measurements were done in 30 mm Tris buffer solution at a pH 5.0. Using constant-current chronopotentiometry, the potential drift for the proposed electrodes was checked. Improvement in the potential stability of the SPE was observed after the addition of PANI in the sensing membrane as compared to the corresponding coated-wire electrode (membrane without PANI). The applicability of the sensor has been checked by measuring 2,4-DCP in different water samples and the results were compared with the standard HPLC method.

Multiwalled Carbon Nanotubes-Poly(3-octylthiophene-2,5-diyl) Nanocomposite Transducer for Ion-Selective Electrodes: Raman Spectroscopy Insight into the Transducer/Membrane Interface

An approach to overcome drawbacks of well-established transducer materials for all-solid-state ion-selective electrodes is proposed; it is based on the formulation of the nanocomposite of multiwalled carbon nanotubes (MWCNTs) and poly(3-octylthiophene-2,5-diyl) (POT), in which the polymer is used as a dispersing agent for carbon nanotubes. Thus, the obtained material is characterized with unique properties that are important for its application as solid contact in ion-selective electrodes, including high: electronic conductivity, capacitance, and lipophilicity. Performance of the obtained all-solid-state electrodes was studied using a standard approach as well as Raman spectroscopy to allow insight into distribution of the transducer material within the sensor phases: the membrane and the transducer. Application of the composite prevents unwanted partition of POT to the membrane phase, thus eliminating the risk of alteration of the sensor performance due to uncontrolled change in the membrane composition.

Potential Reproducibility of Potassium-Selective Electrodes Having Perfluorinated Alkanoate Side Chain Functionalized Poly(3,4-ethylenedioxytiophene) as a Hydrophobic Solid Contact

The irreproducibility of the standard potential (E°) is probably the last major challenge for the commercialization of solid-contact ion-selective electrodes (SCISEs) as single-use or wearable sensors. To overcome this issue, we are introducing for the first time a perfluorinated alkanoate side chain functionalized poly(3,4-ethylenedioxythiophene) (PEDOTF) as a hydrophobic SC in potassium-selective electrodes (K-SCISEs) based on plasticized poly(vinyl chloride). The SC incorporates the tetrakis(pentafluorophenyl)borate (TFAB^-) anion, which is also present as a lipophilic additive in the ion-selective membrane (ISM), thus ensuring thermodynamic reversibility at the SC/ISM interface and improving the potential reproducibility of the electrodes. We show here that the PEDOTF-TFAB solid contact, which was prepolarized prior to the ISM deposition to either its half or fully conducting form (i.e. different oxidation states) in acetonitrile containing 0.01 M KTFAB, had a very stable open-circuit potential and an outstanding potential reproducibility of only ±0.5 mV (n = 6) for 1 h in the same solution after the prepolarization. This shows that the oxidation state of the highly hydrophobic PEDOTF-TFAB film (water contact angle 133°) is stable over time and can be precisely controlled with prepolarization. The SC was also not light sensitive, which is normally a disadvantage of conducting polymer SCs. After the ISM deposition, the standard deviation of the E° of the K-SCISEs prepared on glassy carbon was ±3.0 mV (n = 5), which is the same as that for conventional liquid contact K-ISEs. This indicates that the ISM deposition is the main source for the potential irreproducibility of the K-SCISEs, which has been overlooked previously.

Wearable Sensors for Biochemical Sweat Analysis

Sweat is a largely unexplored biofluid that contains many important biomarkers ranging from electrolytes and metabolites to proteins, cytokines, antigens, and exogenous drugs. The eccrine and apocrine glands produce and excrete sweat through microscale pores on the epidermal surface, offering a noninvasive means for capturing and probing biomarkers that reflect hydration state, fatigue, nutrition, and physiological changes. Recent advances in skin-interfaced wearable sensors capable of real-time in situ sweat collection and analytics provide capabilities for continuous biochemical monitoring in an ambulatory mode of operation. This review presents a broad overview of sweat-based biochemical sensor technologies with an emphasis on enabling materials, designs, and target analytes of interest. The article concludes with a summary of challenges and opportunities for researchers and clinicians in this swiftly growing field.

Fast Procedures for the Electrodeposition of Platinum Nanostructures on Miniaturized Electrodes for Improved Ion Sensing

Nanostructured materials have attracted considerable interest over the last few decades to enhance sensing capabilities thanks to their unique properties and large surface area. In particular, noble metal nanostructures offer several advantages including high stability, non-toxicity and excellent electrochemical behaviour. However, in recent years the great expansion of point-of-care (POC) and wearable systems and the attempt to perform measurements in tiny spaces have also risen the need of increasing sensors miniaturization. Fast constant potential electrodeposition techniques have been proven to be an efficient way to obtain conformal platinum and gold nanostructured layers on macro-electrodes. However, this technique is not effective on micro-electrodes. In this paper, we investigate an alternative one-step deposition technique of platinum nanoflowers on micro-electrodes by linear sweep voltammetry (LSV). The effective deposition of platinum nanoflowers with similar properties to the ones deposited on macro-electrodes is confirmed by morphological analysis and by the similar roughness factor (~200) and capacitance (~18 μ F/mm 2 ). The electrochemical behaviour of the nanostructured layer is then tested in an solid-contact (SC) L i + -selective micro-electrode and compared to the case of macro-electrodes. The sensor offers Nernstian calibration with same response time (~15 s) and a one-order of magnitude smaller limit of detection (LOD) ( 2.6 × 10 - 6 ) with respect to the macro-ion-selective sensors (ISE). Finally, sensor reversibility and stability in both wet and dry conditions is proven.

Real-Time in Situ Monitoring of Nitrogen Dynamics in Wastewater Treatment Processes using Wireless, Solid-State, and Ion-Selective Membrane Sensors

Real-time, in situ accurate monitoring of nitrogen contaminants in wastewater over a long-term period is critical for swift feedback control, enhanced nitrogen removal efficiency, and reduced energy consumption of wastewater treatment processes. Existing nitrogen sensors suffer from high cost, low stability, and short life times, posing hurdles for their mass deployment to capture a complete picture within heterogeneous systems. Tackling this challenge, this study presents solid-state ion-selective membrane (S-ISM) nitrogen sensors for ammonium (NH₄⁺) and nitrate (NO₃^-) in wastewater that were coupled to a wireless data transmission gateway for real-time remote data access. Lab-scale test and continuous-flow field tests using real municipal wastewater indicated that the S-ISM nitrogen sensors possessed excellent accuracy and precision, high selectivity, and multiday stability. Importantly, autocorrections of the sensor readings on the cloud minimized temperature influences and assured accurate nitrogen concentration readings in remote-sensing applications. It was estimated that real-time, in situ monitoring using wireless S-ISM nitrogen sensors could save 25% of electric energy under normal operational conditions and reduce 22% of nitrogen discharge under shock conditions.

Bio-Integrated Wearable Systems: A Comprehensive Review

Bio-integrated wearable systems can measure a broad range of biophysical, biochemical, and environmental signals to provide critical insights into overall health status and to quantify human performance. Recent advances in material science, chemical analysis techniques, device designs, and assembly methods form the foundations for a uniquely differentiated type of wearable technology, characterized by noninvasive, intimate integration with the soft, curved, time-dynamic surfaces of the body. This review summarizes the latest advances in this emerging field of "bio-integrated" technologies in a comprehensive manner that connects fundamental developments in chemistry, material science, and engineering with sensing technologies that have the potential for widespread deployment and societal benefit in human health care. An introduction to the chemistries and materials for the active components of these systems contextualizes essential design considerations for sensors and associated platforms that appear in following sections. The subsequent content highlights the most advanced biosensors, classified according to their ability to capture biophysical, biochemical, and environmental information. Additional sections feature schemes for electrically powering these sensors and strategies for achieving fully integrated, wireless systems. The review concludes with an overview of key remaining challenges and a summary of opportunities where advances in materials chemistry will be critically important for continued progress.

Electrically-Transduced Chemical Sensors Based on Two-Dimensional Nanomaterials

Electrically-transduced sensors, with their simplicity and compatibility with standard electronic technologies, produce signals that can be efficiently acquired, processed, stored, and analyzed. Two dimensional (2D) nanomaterials, including graphene, phosphorene (BP), transition metal dichalcogenides (TMDCs), and others, have proven to be attractive for the fabrication of high-performance electrically-transduced chemical sensors due to their remarkable electronic and physical properties originating from their 2D structure. This review highlights the advances in electrically-transduced chemical sensing that rely on 2D materials. The structural components of such sensors are described, and the underlying operating principles for different types of architectures are discussed. The structural features, electronic properties, and surface chemistry of 2D nanostructures that dictate their sensing performance are reviewed. Key advances in the application of 2D materials, from both a historical and analytical perspective, are summarized for four different groups of analytes: gases, volatile compounds, ions, and biomolecules. The sensing performance is discussed in the context of the molecular design, structure-property relationships, and device fabrication technology. The outlook of challenges and opportunities for 2D nanomaterials for the future development of electrically-transduced sensors is also presented.