Evidence for a surface confined ion-to-electron transduction reaction in solid-contact ion-selective electrodes based on poly(3-octylthiophene)

A smartphone-based sensor for detection of iron and potassium in food and beverage samples

A novel approach for simultaneous detection of iron and potassium via a smartphone-based potentiometric method is proposed in this study. The screen printed electrodes were modified with carbon black nanomaterial and ion selective membrane including zinc (II) phtalocyanine as the ionophore. The developed Fe3+-selective electrode and K+-selective electrode exhibited detection limits of 1.0 × 10-6 M and 1.0 × 10-5 M for Fe3+ and K+ ions, respectively. The electrodes were used to simultaneously detect Fe3+ and K+ ions in apple juice, skim milk, soybean and coconut water samples with recovery values between 90%-100.5%, and validated against inductively coupled plasma-optical emission spectrometry. Due to the advantageous characteristics of the sensors and the portability of Near Field Communication potentiometer supported with a smartphone application, the proposed method offers sensitive and selective detection of iron and potassium ions in food and beverage samples at the point of need.

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.

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.

Light-induced Delivery of Charged Species using Ion-selective Core-Shell Nanoparticles

Controlled release systems have gained considerable attention owing to their potential to deliver molecules, including ions and drugs, in a customized manner. We present a light-induced ion-transfer platform consisting of a dispersion of nanoparticles (NPs, ~300 nm) with the conductive polymer poly(3-octylthiophene-2,5-diyl) (POT) in the core and a potassium (K+)-selective membrane in the shell. Owing to the photoactive nature of POT, POT NPs can be used for a dual purpose: as a host for positively charged species and as an actuator to trigger the subsequent release. POT0 and doped POT+ coexist in the core, allowing K+ encapsulation in the shell. As POT0 is photo-oxidized to POT+, K+ is released to the (aqueous) dispersion phase to preserve the neutrality of the NPs. This process is reversible and can be simultaneously assessed using the native fluorescence of POT0 and via potentiometric measurements. The NP structure and its mechanism of action were thoroughly studied with a series of control experiments and complementary techniques. Understanding the NP and its surrounding interactions will pave the way for other nanostructured systems, facilitating sophisticated applications. The delivery of ionic drugs and interference/pollutant catching for advanced sensing/restoration will be considered in future research.

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.

Ion-selective membrane plasticizer leakage in all-solid-state electrodes – an unobvious way to improve potential reading stability in time

The effect of leakage of the plasticizer from the ion-selective membrane into the ion-to-electron transducer of all-solid-state potentiometric sensors is considered for the first time.

Solid-Contact pH Sensor with Covalent Attachment of Ionophores and Ionic Sites to a Poly(decyl methacrylate) Matrix

With a view to improving the sensor lifetime, solid-contact ion-selective electrodes (ISEs) were prepared with a plasticizer-free and cross-linked poly(decyl methacrylate) matrix, to which only the ionic sites, only the ionophore, or both the ionic sites and ionophore were covalently attached. In earlier work with covalently attached ionophores or ionic sites, it was difficult to discount the presence of ionophores or ionic site impurities that were not covalently attached to the polymer backbone because the reagents used to introduce the ionophore or ionic sites had high hydrophobicities. In this work, we deliberately chose readily available hydrophilic reagents for the introduction of covalently attached H⁺ ionophores with tertiary amino groups and covalently attached sulfonate groups as ionic sites. This simplified the synthesis and made it possible to thoroughly remove ionophores and ionic sites not covalently attached to the polymer backbone. Our results confirm the expectation that hydrophobic ISE membranes with both covalently attached ionophores and ionic sites have impractically long response times. In contrast, ISEs with either covalently attached H⁺ ionophores or covalently attached ionic sites responded to pH with quick Nernstian responses and high selectivity. Both conventional plasticized poly(vinyl chloride) (PVC)-based ISEs and the new poly(decyl methacrylate) membranes were exposed to 90 °C heat for 2 h, 10% ethanol for 1 day, or undiluted blood serum for 5 days. In all three cases, the poly(decyl methacrylate) ISEs exhibited properties superior to conventional PVC-based ISEs, confirming the advantages of the covalent attachment.

Integration of an Aerosol-Assisted Deposition Technique for the Deposition of Functional Biomaterials Applied to the Fabrication of Miniaturised Ion Sensors

Ion-selective electrodes are at the forefront of research nowadays, with applications in healthcare, agriculture and water quality analysis among others. Despite multiple attempts of miniaturization of these polyvinyl chloride (PVC) gel-based ion sensors, no ion-sensing devices with a thickness below the micrometer range, and operating using open circuit potential, have been developed so far. This work reports the causes of this thickness limitation in potassium-selective sensors. Highly homogeneous ion-sensing films were fabricated by a method based on aerosol assisted chemical vapour deposition, leading to smooth surfaces with 27 ± 11 nm of roughness. Such homogeneity allowed the systematic study of the performance and ionic diffusion properties of the sensing films at sub-micrometer scales. Sensitivities below the Nernst response were found at low thicknesses. The nature of this reduction in sensitivity was studied, and a difference in the superficial and bulk compositions of the films was measured. An optimal configuration was found at 15 µm, with a good selectivity against Na+ (KK+, Na+ = -1.8) a limit of detection in the range of 10-4 M and esponse time below 40 s. The stability of sensors was improved by the deposition of protective layers, which expanded the lifespan of the ion sensors up to 5 weeks while preserving the Nernst sensitivity.

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.

Solid-Contact Ion-Selective and Reference Electrodes Covalently Attached to Functionalized Poly(ethylene terephthalate)

Numerous ion-selective and reference electrodes have been developed over the years. Following the need for point-of-care and wearable sensors, designs have transitioned recently from bulky devices with an aqueous inner filling solution to planarizable solid-contact electrodes. However, unless the polymeric sensing and reference membranes are held in place mechanically, delamination of these membranes from the underlying solid to which they adhere physically limits sensor lifetime. Even minor external mechanical stress or thermal expansion can result in membrane delamination and, thereby, device failure. To address this problem, we designed a sensing platform based on poly(ethylene terephthalate) substrates to which polyacrylate-based sensing and polymethacrylate-based reference membranes are attached covalently. Ion-selective membranes with covalently attached or freely dissolved ionophore- and ionic-liquid-doped reference membranes can be directly photopolymerized onto surface-functionalized poly(ethylene terephthalate), resulting in the formation of covalent bonds between the underlying substrate and the attached membranes. H⁺- and K⁺-selective electrodes thus prepared exhibit highly selective responses with the theoretically expected (Nernstian) response slope, and reference electrodes provide sample-independent reference potentials over a wide range of electrolyte concentrations. Even repeated mechanical stress does not result in the delamination of the sensing and reference membranes, leading to electrodes with much improved long-term performance. As demonstrated for poly(ethylene-co-cyclohexane-1,4-dimethanol terephthalate) (PETG), this approach may be expanded to a wide range of other polyester, polyamide, and polyurethane platform materials. Covalent attachment of sensing and reference membranes to an inert plastic platform material is a very promising approach to a problem that has plagued the field of ion-selective electrodes and field effect transistors for over 30 years.

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).

From Molecular and Emulsified Ion Sensors to Membrane Electrodes: Molecular and Mechanistic Sensor Design

Selective molecular ion probes are often insoluble in water and require a hydrophobic solvent environment for strong and selective binding, which runs counter to the desire of utilizing them in a homogeneous solution. This Account aims to guide the reader on how such molecules, often coined ionophores, can be harnessed to design exceptionally useful optical and electrochemical sensors. We start here with some historical context on the design of such ionophores and continue with the explanation of the response mechanism of optical and potentiometric sensors and the role of combined components to build a robust ion sensor. This Account is addressed to nonspecialist readers and for this reason avoids extensive use of equations or theoretical considerations. The interested reader should turn to the original literature for further reading. Emulsified optical sensors are introduced as an initial example. Here, multiple reagents are confined in an attoliter sensing nanodroplet of the organic phase, immiscible with the aqueous sample phase. In this case, the ionophore molecules may retain their high affinity and selectivity to the target ion and the aqueous sample phase does not have to be modified. Emulsified optical sensors allow one to achieve the selective chemical sensing of ions, even with optically silent ionophores. Such ionophore-based nanodroplets are also discussed as a useful novel class of complexometric titration reagents and optical end point indicators with unique selectivities. We then turn our attention to potentiometric sensing probes and briefly discuss the unique opportunity of a direct characterization of ion-ionophore complexation properties offered by membrane electrodes. A carbonate-selective membrane electrode containing a highly selective tweezer-type ionophore with trifluoroacetophenone functional groups is then used as an example for the construction of a robust all-solid-state sensor. This potentiometric probe, in combination with a pH electrode, can directly measure PCO₂ in freshwater lakes, demonstrating a dramatically improved response time relative to traditional sensors equipped with a gas-permeable membrane. In recent years, new sensing modes and electrode designs have been introduced to expand the application scope of ionophore-based potentiometric sensors. Membrane electrodes containing ionophores are placed under dynamic electrochemistry control to give important progress in the field. We specifically highlight our recent works by membranes that are controlled by chronopotentiometry (controlled current) for speciation analysis, by ion transfer voltammetry on thin sensing films for multianalyte detection, by exhaustive coulometry for potentially calibration-free sensors and with coulometric membrane pumps for the selective delivery of reagents.

Ion-Selective Electrodes for Detection of Lead (II) in Drinking Water: A Mini-Review

Despite the fact that the adverse health effects due to the intake of lead have been well studied and widely recognized, lead contamination in drinking water has been reoccurring worldwide, with some incidents escalating into a public drinking water crisis. As lead contamination is often related to lead-based pipes close to or inside homes, it is not realistic, at least in the near term, to remove and replace all lead connection pipes and lead-based plumbing. Effective monitoring of lead concentration at consumers’ water taps remains critical for providing consumers with first-hand information and preventing potential wide-spread lead contamination in drinking water. This review paper examines the existing common technologies for laboratory testing and on-site measuring of lead concentrations. As the conventional analytical techniques for lead detection require using expensive instruments, as well as a high time for sample preparation and a skilled operator, an emphasis is placed on reviewing ion-selective electrode (ISE) technology due to its superior performance, low cost, ease of use, and its promising potential to be miniaturized and integrated into standalone sensing units. In a holistic way, this paper reviews and discusses the background, different types of ISEs are reviewed and discussed, namely liquid-contact ISEs and solid-contact ISEs. Along with the potential opportunities for further research, the limitations and unique challenges of ISEs for lead detection are also discussed in detail.

All-Solid-State Sodium-Selective Electrode with a Solid Contact of Chitosan/Prussian Blue Nanocomposite

Conventional ion-selective electrodes with a liquid junction have the disadvantage of potential drift. All-solid-state ion-selective electrodes with solid contact in between the metal electrode and the ion-selective membrane offer high capacitance or conductance to enhance potential stability. Solution-casted chitosan/Prussian blue nanocomposite (ChPBN) was employed as the solid contact layer for an all-solid-state sodium ion-selective electrode in a potentiometric sodium ion sensor. Morphological and chemical analyses confirmed that the ChPBN is a macroporous network of chitosan that contains abundant Prussian blue nanoparticles. Situated between a screen-printed carbon electrode and a sodium-ionophore-filled polyvinylchloride ion-selective membrane, the ChPBN layer exhibited high redox capacitance and fast charge transfer capability, which significantly enhanced the performance of the sodium ion-selective electrode. A good Nernstian response with a slope of 52.4 mV/decade in the linear range from 10-4-1 M of NaCl was observed. The stability of the electrical potential of the new solid contact was tested by chronopotentiometry, and the capacitance of the electrode was 154 ± 4 µF. The response stability in terms of potential drift was excellent (1.3 µV/h) for 20 h of continuous measurement. The ChPBN proved to be an efficient solid contact to enhance the potential stability of the all-solid-state ion-selective electrode.

Evidence of double layer/capacitive charging in carbon nanomaterial-based solid contact polymeric ion-selective electrodes

We report on the first direct spectroscopic evidence of the capacitive ion-to-electron transducing mechanism of carbon nanomaterials in solid contact polymeric ion-selective electrodes (SC-ISEs).

All-solid-state potentiometric sensors with a multiwalled carbon nanotube inner transducing layer for anion detection in environmental samples

While ion to electron transducing layers for the fabrication of potentiometric membrane electrodes for the detection of cations have been well established, similar progress for the sensing of anions has not yet been realized. We report for this reason on a novel approach for the development of all-solid-state anion selective electrodes using lipophilic multiwalled carbon nanotubes (f-MWCNTs) as the inner ion to electron transducing layer. This material can be solvent cast, as it conveniently dissolves in tetrahydrofuran (THF), an important advantage to develop uniform films without the need for using surfactants that might deteriorate the performance of the electrode. Solid contact sensors for carbonate, nitrate, nitrite, and dihydrogen phosphate are fabricated and characterized, and all exhibit comparable analytical characteristics to the inner liquid electrodes. For example, the carbonate sensor exhibits a Nernstian slope of 27.2 ± 0.8 mV·dec(-1), a LOD = 2.3 μM, a response time of 1 s, a linear range of four logarithmic units, and a medium-term stability of 0.04 mV·h(-1) is obtained in a pH 8.6 buffered solution. Water layer test, reversibility, and selectivity for chloride, nitrate, and hydroxide are also reported. The excellent properties of f-MWCNTs as a transducer are contrasted to the deficient performance of poly(3-octyl-thiophene) (POT) for carbonate detection. This is evidenced both with a significant drift in the potentiometric measures as well as a pronounced sensitivity to light (either sunlight or artificial light). This latter aspect may compromise its potential for environmental in situ measurements (night/day cycles). The concentration of carbonate is determined in a river sample (Arve river, Geneva) and compared to a reference method (automatic titrator with potentiometric pH detection). The results suggest that nanostructured materials such as f-MWCNTs are an attractive platform as a general ion-to-electron transducer for anion-selective electrodes.

Ion-selective electrodes with colloid-imprinted mesoporous carbon as solid contact

A new type of solid-contact ion-selective electrode (SC-ISE) has been developed that uses colloid-imprinted mesoporous (CIM) carbon with 24 nm diameter, interconnected mesopores as the intermediate layer between a gold electrode and an ionophore-doped ISE membrane. For a demonstration, valinomycin was used as K(+) ionophore, and a good Nernstian response with a slope of 59.5 mV/decade in the range from 10(-5.2) to 10(-1.0) M was observed. The high purity, low content of redox-active surface functional groups and intrinsic hydrophobic characteristics of CIM carbon prepared from mesophase pitch lead to outstanding performance of these sensors, with excellent resistance to the formation of a water layer and no interference caused by light, O2, and CO2. When a redox couple is introduced as an internal reference species, calibration-free SC-ISEs can be made with a standard deviation of E° as low as 0.7 mV. Moreover, the interconnected mesopore structure of ISE membrane-infused CIM carbon facilitates both ion and electron conduction and provides a large interfacial area with good ion-to-electron transduction. Because of the large double layer capacitance of CIM carbon, CIM carbon-based SC-ISEs exhibit excellent potential stability, as shown by chronopotentiometry and continuous potentiometric measurements. The capacitance of these electrodes as determined by chronopotentiometry is 1.0 mF, and the emf drift over 70 h is as low as 1.3 μV/h, making these electrodes the most stable SC-ISEs reported so far.