Carrier-Based Ion-Selective Electrodes and Bulk Optodes. 1. General Characteristics

Recent improvements to the selectivity of extraction-based optical ion sensors

Optical sensors continue to demonstrate tremendous potential across a wide range of applications due to their high versatility and low cost. This feature article will focus on a number of recent advances made in improving the performance of extraction-based optical ion sensors within our group. This includes the progress of anchored solvatochromic transduction to provide pH and sample volume independent optical responses in nanoemulsion-based sensors. A recent breakthough is in polyion sensing in biological fluids that uses a novel indirect transduction mechanism that significantly improves the selectivity of dinonylnaphthalenesulfonate-based protamine sensors and its potential applications beyond polyion sensing. The role of particle stabilizers in relation to the response of emulsified sensors is shown to be important. Current challenges in the field and possible opportunities are also discussed.

Selectivity remains a constant challenge in the development of optical extraction-based sensors. Fortunately, there are several mechanistic and compositional changes with the potential to improve selectivity without developing new ionophores.

Electrochemically Imaging the Response of Ion-Selective Membranes with an Ultralow Detection Limit

The development of ion-selective membranes for the selective response of a particular ion has been studied for many years; however, imaging the response of the membrane with a low detection limit is challenging. Here, high spatial-resolution electrochemical imaging of this response down to picomolar is achieved using scanning ion conductive microscopy. The detection strategy relies on the exclusion of a small amount of counter ions from the membrane in the presence of a low concentration of target ions in the solution. These excluded counter ions are adsorbed at the membrane-solution interface, leading to more positive charges at the surface. The resultant elevation of the ionic current in the approach curve behaves as the response for the target ions down to 10^-11 M, which is much more sensitive than that using potentiometric measurement. The constant-current scanning of the membrane exhibits the fluctuation of the apparent surface height that is correlated with the ionic concentration, permitting the imaging of the response at the nanoscale. The achievement of highly sensitive and spatial-resolution imaging for the ionic response enable the collection of spatial response at the ion-selective membrane, which will greatly advance the study of ion-selective electrodes.

Ionophore-Based Ion-Selective Electrodes in Non-Zero Current Modes: Mechanistic Studies and the Possibilities of the Analytical Application

Abstract

This mini review briefly describes (i) literature data on the non-zero current measurements with ionophore-based ion-selective electrodes (ISEs) aimed at fundamental studies of the mechanism of their potentiometric response, and (ii) the data on the possibilities of analytical applications of ISEs in voltametric and constant potential chronoamperometric/coulometric modes, in particular the K+ ion assay in blood serum with the sensitivity of 0.1%. A special attention is paid to the basics of voltammetry and chronoamperometry/coulometry with the ionophore-based ISEs, and to how and why these methods differ from the classical voltammetry and coulometry.

Materials and Interface Designs of Waterproof Field-Effect Transistor Arrays for Detection of Neurological Biomarkers

The continuous, real-time, and concurrent detection of multiple biomarkers in bodily fluids is of high significance for advanced healthcare. While active, semiconductor-based biochemical sensing platforms provide levels of functionality exceeding those of their conventional passive counterparts, the stability of the active biosensors in the liquid environment for continuous operation remains a challenging topic. This work reports the development of a class of flexible and waterproof field-effect transistor arrays for multiplexed biochemical sensing. In this design, monolithic, ultrathin, dense, and low defect nanomembranes consisting of monocrystalline Si and thermally grown SiO₂ simultaneously serve as high-performance backplane electronics for signal transduction and stable biofluid barriers with high structural integrity due to the high formation temperature. Coupling the waterproof transistors with various ion-selective membranes through the gate electrode allows for sensitive and selective detection of multiple ions as biomarkers for traumatic brain injury. The study also demonstrates a similar encapsulation structure which enables the design of waterproof amperometric sensors based on this materials strategy and integration scheme. Overall, key advantages in flexibility, stability, and multifunctionality highlight the potential of using such electronic sensing platforms for concurrent, continuous detection of various neurological biomarkers, proving a promising approach for early diagnosis and intervention of chronic diseases.

Dibenzo-18-Crown-6-based Carbon Paste Sensors for the Nanomolar Potentiometric Determination of Daclatasvir Dihydrochloride: An Anti-HCV Drug and a Potential Candidate for Treatment of SARS-CoV-2

Daclatasvir dihydrochloride (DAC) is an anti-hepatitis C virus (HCV) drug that has recently proven to be a promising candidate for the treatment of SARS-CoV-2. Still, there is a lack of sensitive potentiometric methods for its determination. In this work, carbon paste sensors based on dibenzo-18-crown-6 (DB18C6) were fabricated and optimized for the sensitive and selective potentiometric determination of DAC in Daclavirocyrl® tablets, serum, and urine samples. The best performance was obtained by two sensors referred to as sensor I and sensor II. Both sensors exhibited a wide linear response range of 5×10⁻⁹ − 1×10⁻³ mol/L, and Nernstian slopes of 29.8 ± 1.18 and 29.5 ± 1.00 mV/decade, with limits of detection, 4.8×10⁻⁹ and 3.2×10⁻⁹ mol/L, for the sensors I and II, respectively. Sensors I and II displayed fast response times of 5–8 and 5–6 s, respectively, with great reversibility and no memory effect. Moreover, the sensors exhibited a lifetime of 16 days. For the study of sensors morphology and elucidation of the interaction mechanism, the scanning electron microscope (SEM), Fourier-transform infrared spectroscopy (FTIR), and nuclear magnetic resonance (¹H NMR) techniques were performed. A selectivity study was performed, and the proposed sensors exhibited good discrimination between DAC and potentially coexisting interferents with sensor II displaying better selectivity. Finally, sensor II was successfully applied for the determination of DAC in the above-mentioned samples, with recovery values ranging from 99.25 to 101.42%, and relative standard deviation (RSD) values ranging from 0.79 to 1.53% which reflected the high accuracy and precision.

Chronopotentiometric Evaluation of Ionization Degree and Dissociation Constant of Imidazolium-Based Ionic Liquid [C6Meim][NTf2] in Polymeric Plasticized Membranes

Ionic liquids (ILs) have a wide variety of applications in modern electrochemistry due to their unique electrolytic properties. In particular, they are promising candidates as dopants for polymeric membranes in potentiometric sensors and liquid-junction free reference electrodes. However, the effective use of ILs requires a comprehensive understanding of their electrolytic behavior in the polymeric phase. We report here the exploration of the electrolytic and diffusion properties of IL 1-hexyl-3-methyl-1H-imidazol-3-ium bis[(trifluoromethyl)sulfonyl]amide ([C6Meim][NTf2]) in a poly(vinyl chloride) matrix. Chronopotentiometry is utilized to determine the concentration of charge carriers, ionic diffusion coefficients and apparent dissociation constant of [C6Meim][NTf2] in PVC membranes plasticized with a mixture of [C6Meim][NTf2] and bis(2-ethylhexyl) sebacate (DOS) over a wide range of IL concentrations. The diffusion properties of [C6Meim][NTf2] are confirmed by NMR-diffusometry. The non-monotonic electrolytic behavior of the IL in PVC-DOS matrix is described for the first time. A maximum ionization degree and diffusion coefficient is observed at 30 wt.% of IL in the plasticizing mixture. Thus, it is shown that by varying the flexible parameter of the IL to plasticizer ratio in the polymeric phase one can tune the electrolytic and transport properties of sensing PVC membranes.

Recent Advances in Chemical Sensors for Soil Analysis: A Review

The continuously rising interest in chemical sensors’ applications in environmental monitoring, for soil analysis in particular, is owed to the sufficient sensitivity and selectivity of these analytical devices, their low costs, their simple measurement setups, and the possibility to perform online and in-field analyses with them. In this review the recent advances in chemical sensors for soil analysis are summarized. The working principles of chemical sensors involved in soil analysis; their benefits and drawbacks; and select applications of both the single selective sensors and multisensor systems for assessments of main plant nutrition components, pollutants, and other important soil parameters (pH, moisture content, salinity, exhaled gases, etc.) of the past two decades with a focus on the last 5 years (from 2017 to 2021) are overviewed.

Ionophore-Based Potassium Selective Fluorescent Organosilica Nano-Optodes Containing Covalently Attached Solvatochromic Dyes

Fluorescent nanoprobes containing ionophores and solvatochromic dyes (SDs) were previously reported as an alternative to chromoionophore-based nano-optodes. However, the small-molecular SDs are prone to leakage and sequestration in complex samples. Here, we chemically attached the SDs to the surface of organosilica nanospheres through copper-catalyzed Click chemistry to prevent dye leakage. The nano-optodes remained well responsive to K+ even after exposure to a large amount of cation-exchange resin, which acted as a sink of the SDs. The potassium nanoprobes exhibited a dynamic range between 1 μM to 10 mM and a good selectivity thanks to valinomycin. Preliminary sensing device based on a nylon filter paper and agarose hydrogel was demonstrated. The results indicate that the covalent anchoring of SDs on nanospheres is promising for developing ionophore-based nanoprobes.

Bypassed ion-selective electrodes – self-powered polarization for tailoring of sensor performance

The bypass circuit with zinc wire induces spontaneous charge flow: oxidation of zinc and reduction of the solid contact of K-ISE. This effect is helpful in the improvement of analytical parameters of K-ISE.

Ion-selective membrane modified microfluidic paper-based solution sampling substrates for potentiometric heavy metal detection

Ion-selective membrane modified paper substrates were used to control the unfavourable super-Nernstian response of Pb2+-ISEs when coupled with microfluidic paper-based solution sampling.

Chloride ion-selective dye liquid nanoemulsion: improved sensor performance due to intermolecular interactions between dye and ionophore

Ionophore-based dye liquid nanoemulsion sensors exhibiting rapid response, high selectivity, and high sensitivity to chloride were developed. Intermolecular interactions within emulsion contributed to the background suppression.

Chemosensory Optode Array Based on Pluronic-Stabilized Microspheres for Differential Sensing

Differential sensing techniques are becoming nowadays an attractive alternative to classical selective recognition methods due to the “fingerprinting” possibility allowing identifying various analytes without the need to fabricate highly selective binding recognition sites. This work shows for the first time that surfactant-based ion-sensitive microspheres as optodes in the microscale can be designed as cross-sensitive materials; thus, they are perfect candidates as sensing elements for differential sensing. Four types of the newly developed chemosensory microspheres—anion- and cation-selective, sensitive toward amine- and hydroxyl moiety—exhibited a wide range of linear response (two to five orders of magnitude) in absorbance and/or fluorescence mode, great time stability (at least 2 months), as well as good fabrication repeatability. The array of four types of chemosensitive microspheres was capable of perfect pattern-based identification of eight neurotransmitters: dopamine, epinephrine, norepinephrine, γ-aminobutyric acid (GABA), acetylcholine, histamine, taurine, and phenylethylamine. Moreover, it allowed the quantification of neurotransmitters, also in mixtures. Its selectivity toward neurotransmitters was studied using α- and β-amino acids (Ala, Asp, Pro, Tyr, taurine) in simulated blood plasma solution. It was revealed that the chemosensory optode set could recognize subtle differences in the chemical structure based on the differential interaction of microspheres with various moieties present in the molecule. The presented method is simple, versatile, and convenient, and it could be adopted to various quantitative and qualitative analytical tasks due to the simple adjusting of microspheres components and measurement conditions.

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.

Tuning the sumanene receptor structure towards the development of potentiometric sensors

Herein, we report for the first time the application of a sumanene derivative in potentiometric recognition of caesium cations. The structure of the sumanene receptor was carefully tuned to obtain its compatibility with polyvinyl chloride plasticized membranes of potentiometric sensors. The developed ion-selective electrodes based on membranes doped with octyloxybenzene sumanene exhibited a near theoretical sensitivity towards caesium cations, with a detection limit of 4 μM Cs⁺.

Protamine/heparin optical nanosensors based on solvatochromism

Optical nanosensors for the detection of polyions, including protamine and heparin, have to date relied upon ion-exchange reactions involving an analyte and an optical transducer. Unfortunately, due to the limited selectivity of the available ionophores for polyions, this mechanism has suffered from severe interference in complex sample matrices. To date no optical polyion nanosensors have demonstrated acceptable performance in serum, plasma or blood. Herein we describe a new type of nanosensor based on our discovery of a “hyper-polarizing lipophilic phase” in which dinonylnaphthalenesulfonate (DNNS⁻) polarizes a solvatochromic dye much more than even an aqueous environment. We have found that the apparent polarity of the organic phase is only modulated when DNNS⁻ binds to large polyions such as protamine, unlike singly charged ions that lack the cooperative binding required to cause a significant shift in the distribution of the polarizing DNNS⁻ ions. Our new sensing mechanism allows solvatochromic signal transduction without the transducer undergoing ion exchange. The result is significantly improved sensitivity and selectivity, enabling for the first time the quantification of protamine and heparin in human plasma using optical nanosensors that correlates with the current gold standard analysis method, the anti-Xa factor assay.

Novel optical nanosensors for the selective detection of the polycationic protamine based on solvatochromic signal change allow one to detect heparin in plasma.

Recent advances in polymeric nanostructured ion selective membranes for biomedical applications

Nano structured ion-selective membranes (ISMs) are very attractive materials for a wide range of sensing and ion separation applications. The present review focuses on the design principles of various ISMs; nanostructured and ionophore/ion acceptor doped ISMs, and their use in biomedical engineering. Applications of ISMs in the biomedical field have been well-known for more than half a century in potentiometric analysis of biological fluids and pharmaceutical products. However, the emergence of nanotechnology and sophisticated sensing methods assisted in miniaturising ion-selective electrodes to needle-like sensors that can be designed in the form of implantable or wearable devices (smartwatch, tattoo, sweatband, fabric patch) for health monitoring. This article provides a critical review of recent advances in miniaturization, sensing and construction of new devices over last decade (2011-2021). The designing of tunable ISM with biomimetic artificial ion channels offered intensive opportunities and innovative clinical analysis applications, including precise biosensing, controlled drug delivery and early disease diagnosis. This paper will also address the future perspective on potential applications and challenges in the widespread use of ISM for clinical use. Finally, this review details some recommendations and future directions to improve the accuracy and robustness of ISMs for biomedical applications.

Ten Years Progress of Electrical Detection of Heavy Metal Ions (HMIs) Using Various Field-Effect Transistor (FET) Nanosensors: A Review

Heavy metal pollution remains a major concern for the public today, in line with the growing population and global industrialization. Heavy metal ion (HMI) is a threat to human and environmental safety, even at low concentrations, thus rapid and continuous HMI monitoring is essential. Among the sensors available for HMI detection, the field-effect transistor (FET) sensor demonstrates promising potential for fast and real-time detection. The aim of this review is to provide a condensed overview of the contribution of certain semiconductor substrates in the development of chemical and biosensor FETs for HMI detection in the past decade. A brief introduction of the FET sensor along with its construction and configuration is presented in the first part of this review. Subsequently, the FET sensor deployment issue and FET intrinsic limitation screening effect are also discussed, and the solutions to overcome these shortcomings are summarized. Later, we summarize the strategies for HMIs' electrical detection, mechanisms, and sensing performance on nanomaterial semiconductor FET transducers, including silicon, carbon nanotubes, graphene, AlGaN/GaN, transition metal dichalcogenides (TMD), black phosphorus, organic and inorganic semiconductor. Finally, concerns and suggestions regarding detection in the real samples using FET sensors are highlighted in the conclusion.

Multifunctional dendritic molecular probe for selective detection of Cu2+ ions using potentiometric and fluorometric techniques

We have designed and synthesized a multifunctional dendritic molecular probe that selectively detects Cu2+ ions via potentiometric and fluorometric techniques with low detection limits (3.5 μM in potentiometry, 15 nM in fluorometry). The selective and reversible binding of the molecule with the Cu2+ ion was used to make a solid-state microsensor (diameter of 25 μm) by incorporating the molecular probe into the carbon-based membrane as an ionophore for Cu(II). The Cu(II) microelectrode has a broad linear range of 10 μM to 1 mM with a near Nernstian slope of 30 mV/log [aCu2+] and detection limit of 3.5 μM. The Cu(II) microsensor has a fast response time (1.5 s), and it has a broad working pH range from 3.5 to 6.0. The incorporation of the hydrophobic dendritic moiety makes the ionophore less prone to leaching in an aqueous matrix for potentiometric measurement. The cinnamaldehyde component of the molecule helps detection of Cu2+ ions fluorometrically, as indicated by a change in fluorescence upon selective and reversible binding of the molecular probe to the Cu2+ ions. The strategic design of the molecular probe allows us to detect Cu2+ ions in drinking water by using this novel dendritic fluoroionophore and solid-state Cu2+ - ion-selective microelectrode.

Fluoride Ion-Selective Electrode for Organic Solutions

Fluoride ions are used in battery electrolytes in fluoride shuttle batteries. Since organic solvents are used in battery electrolytes, there is a growing demand to develop appropriate methods for quantifying fluoride ion concentration in organic solvents. In this study, a fluoride ion-selective electrode (ISE) for organic solutions is proposed as an electrode of the second kind. A Ag|AgF electrode was made via the anodization of a silver wire in propylene carbonate (PC) containing dissolved fluoride ions. The resultant electrode exhibits a stable linear response of the open circuit potential to the logarithm of the fluoride ion concentration in PC solutions over a range of 10^-4-10^-2 mol dm^-3. The lower and upper limits of the linear response were interpreted in terms of the solubility and the formation of a silver fluoride complex. The use of this electrode of the second kind is suitable for the analysis of fluoride ions in organic solutions and is a promising concept for the development of ISEs for the detection of ions in organic solutions under highly restrictive conditions.

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.

Electrochemical sensor for tricyclic antidepressants with low nanomolar detection limit: Quantitative Determination of Amitriptyline and Nortriptyline in blood

Amitriptyline and its metabolite, Nortriptyline are commonly used tricyclic antidepressant (TCA) drugs that are electrochemically active. In this work, the performance characteristics of a plasticized PVC membrane-coated glassy carbon (GC) electrode are described for the voltammetric quantification of Amitriptyline and Nortriptyline in whole blood. The highly lipophilic Amitriptyline and Nortriptyline preferentially partition into the plasticized PVC membrane where the free drug is oxidized on the GC electrode. The concentrations of the drugs in the membrane are orders of magnitude larger than in the sample solution, resulting in superb limit of detection (LOD) of the membrane-coated voltammetric sensor: 3 nmol/L for Amitriptyline and 20 nmol/L for Nortriptyline. Conversely, hydrophilic components of the sample solution, e.g., proteins, the protein-bound fraction of the drugs, and electrochemically active small molecules are blocked from entering the membrane, which provides exceptional selectivity for the membrane-coated sensor and feasibility for the measurements of Amitriptyline in whole blood. In this work, the concentrations of Amitriptyline and Nortriptyline were determined in whole blood using the sensor and the results of our analysis were compared to the results of the standard HPLC-MS method. Based on our experience, the one-step voltammetric methods with the membrane-coated sensor may become a real alternative to the significantly more complex HPLC-MS analysis.

Dual Sensitivity─Potentiometric and Fluorimetric─Ion-Selective Membranes

Classical application of ion-selective membranes is limited to either electrochemical or optical experiments. Herein, the proposed ion-selective membrane system can be used in both modes; each of them offering competitive analytical parameters: high selectivity and linear dependence of the signal on logarithm of analyte concentration, high potential stability in potentiometric mode, or applicability for alkaline solutions in optical mode. Incorporation of analyte ions into the membrane results in potentiometric signals, as in a classical system. However, due to the presence of lipophilic positively charged ions, polymer backbones, full saturation of the membrane is prevented even for long contact time with solution. The presence of both positively charged and neutral forms of conducting polymers in the membrane results in high stability of potential readings in time. Optical signal generation is based on polythiophene particulates dispersed within the ion-selective membrane as the optical transducer. An increase of emission is observed with an increase of analyte contents in the sample.

A portable solid-state potentiometric sensor based on a polymeric ion-exchanger for the assay of a controversial food colorant (sunset yellow)

Food additives are chemicals added to enhance the appearance, taste, or lifetime of food products. Authorities continuously update the lists of the allowed additives and their daily intake limits. Thus, authorities and food suppliers strictly monitor additives in food products to guarantee their safety and compliance with national laws and safety criteria. The daily intake of the food colorant sunset yellow is banned in some countries and strictly controlled in others. Herein, a chemically modified solid-state potentiometric sensor was fabricated and used for the direct, fast, sensitive and selective assay of sunset yellow in soft drink and pharmaceutical formulation samples. The study optimized the sensor composition and the optimized carbon paste included a novel polymeric ion-exchanger, dioctyl phthalate, chitosan, and calix-[8]-arene and produced a rapid and near-Nernstian response of -32.9 ± 0.821 mV per decade for sunset yellow in the concentration range 7.94 × 10^-5 M to 1.0 × 10^-2 M and in the pH range 5-10. The sensor revealed good selectivity toward sunset yellow in the presence of commonly encountered ionic species. The method was validated according to the International Council for Harmonization guidelines and the results were statistically comparable to those of a reported method. The solid-state sensor represents a tool for fast and direct assay of sunset yellow in food products without sample pretreatment.

Fabrication of a paper-based sensor from graphene quantum dots coated with a polymeric membrane for the determination of gold(III) ions

A novel paper-based sensor using graphene quantum dots (GQDs) as a colorimetric probe for Au³⁺ determination has been developed. The paper sensor was fabricated by the adsorption of GQDs onto cellulose filter paper and then coating with a PVC membrane. The PVC membrane was plasticized with o-NPOE containing potassium tetrakis(4-chlorophenyl)borate (KTpClPB) as a lipophilic cation-exchanger. According to the ion-exchanged mechanism between the lipophilic phase and aqueous phase, Au³⁺ in the aqueous solution was extracted to the lipophilic phase on the paper layer. Then, adsorbed GQDs on the paper could selectively reduce Au³⁺ to elemental gold (Au⁰). The generated gold nanoparticles (AuNPs) resulted in the color of the paper turning from pale yellow to pink, which was recorded by using CIE L*a*b* color space. Under optimized conditions, the change in the color difference (ΔE) was related to the concentration of Au³⁺ in a working linear range of 200-1000 μM and the detection limit was found to be 70 μM. The proposed sensor was successfully applied to the determination of Au³⁺ in real water samples. The results were in favorable agreement with standard inductively coupled plasma-optical emission spectrometry (ICP-OES) results.

Exploitation of 2D Cu-MOF nanosheets as a unique electroactive material for ultrasensitive Cu(II) ion estimation in various real samples

Herein, hybrid carbon sensor has been developed with graphite sheets as a matrix, tricresyl phosphate (TCP) as a plasticizer and nanosheets of 2D Cu-MOF (metal-organic framework) as an electroactive material for the ultrasensitive Cu(II) ion detection in various real samples. Where, the present study proves the efficiency of 2D Cu-MOF as a promising sensing material for the development of Cu(II) ion selective carbon sensor. The developed 2D Cu-MOF nanosheets based sensor containing 2D Cu-MOF: TCP: graphite in the ratio of 2.67: 30.54: 66.79 (% wt/wt) displayed unique Nernstian behavior over two linearity ranges of 1.0 × 10^-11-1.0 × 10^-9 and 1.0 × 10^-5-1.0 × 10^-1 mol L^-1 with slopes of 29.5 ± 0.25 and 29.6 ± 0.13 mV decade^-1, respectively. The fabricated carbon sensor achieved a widely pH independency, fast response time and superior thermal stability with highly selective and ultrasensitive performance. Moreover, It has been efficiently applied for the Cu(II) ion potentiometric estimation in human hair, sesames seeds, two different tea infusions and tap water real samples.

Computational Insights into the Role of External and Local Electric Fields in Macrocyclic Chemical and Biological Systems

The investigation of the role of the electric field in systems of widespread interest employing computational techniques is an emerging area of research. The outcome of applying an oriented external electric field (OEEF) on the geometric and electronic properties of the chemically unique π-conjugated cyclic carbon ring compounds has been explored with density functional theory (DFT). Distinct changes in the structural and electronic features of such ring compounds are observed upon the application of OEEFs. Importantly, the calculations indicate that a mixed aliphatic-aromatic conjugated ring converts from a singlet to a triplet after the application of an OEEF, suggesting potential applications in optoelectronics for such molecules, without the need for photochemically induced change in the spin state. Furthermore, the influence of built-in local electric fields (LEFs) present in naturally occurring macrocyclic systems such as valinomycin has also been explored. Static and ab initio molecular dynamics (AIMD) calculations indicate that LEFs are the primary driving factor in determining the energetically favoured position of counter anions such as chloride (Cl^- ) in the potassium (K⁺ ) and sodium (Na⁺ ) coordinated valinomycin macrocycle structures: they exist inside the cage in the case of K⁺ sequestration by valinomycin and outside for Na⁺ . This divergence has been proposed to be the determining factor for the selectivity of the valinomycin macrocycle for binding a K⁺ cation over Na⁺ .

A new route to select the referable values of property indicators for normalization and its application in screening polymeric membrane heavy metal (Co, Ni, and Cr)-ion-selective electrodes with multiple high-response performances

Distance to target normalization may not be suitable to select the referable values, (RC)_refer, and responsive characteristic (RC) indicators, for the electrode comprehensive quality index (I_ECQ). (RC)_refer is selected as an excellent response property value in alliance with corresponding constants (e.g., 1.8 or 2.5) in this method. It is recently found that these constants are not appropriate to normalize indicators for other ion-selective electrodes (ISEs). The present study aimed to report a new and universal route to select good (RC)_refer values for the normalization without additional constants by only controlling the mean value ([Formula: see text]) being 0.90 < [Formula: see text] < 1.1. The route provided reliable results of both (RC)_refer for indicator normalization and I_ECQ values. It has been successfully applied to select the referable values (RC)_refer of each indicator for the I_ECQ of three heavy metal (HM) (Co, Ni, and Cr)-ISEs. The second aim of the work is to screen these heavy metal-ion-selective electrodes (HM-ISEs) with highly multiple response properties by using the I_ECQ. Twenty-four HM-ISEs with the top 3 I_ECQ values have been recommended. For example, the largest I_ECQ for three indicators, [Formula: see text], P_S, and P_RT, which was used as the main indicator (MI), was 1.461 for Co²⁺-ISE, 1.385 for Ni²⁺-ISE, and 1.561 for Cr³⁺-ISE, respectively. The results will be beneficial to meeting the special requirements to monitor/analyze HM ions in different water samples.

Multiplexed all-solid-state ion-sensitive light-addressable potentiometric sensor (ISLAPS) system based on silicone-rubber for physiological ions detection

Light-addressable potentiometric sensor (LAPS) has been widely used in biomedical applications since its advent. As a member of the potentiometric sensors, ion-sensitive LAPS (ISLAPS) can be obtained by modifying ion selective sensing membrane on the sensor surface. Compared with the conventional ion-selective electrodes (ISEs) with liquid contact, the all-solid-state ISEs have more advantages such as easy maintenance, more convenient for miniaturization and practical applications. However, the commonly used ion-sensitive membrane (ISM) matrix like PVC has many limitations such as poor adhesion to silicone-based sensor and easy overflow of the plasticizer from the membrane. In this work, LAPS was combined with a variety of ionophore-doped all-solid-state silicone-rubber ISMs for the first time, to establish a program-controlled multiplexed ISLAPS system for physiological ions (Na⁺, K⁺, Ca²⁺ and H⁺) detection. The silicone-rubber ISMs have better adhesion to silicon-based sensors without containing plasticizers, which can avoid the plasticizer pollution and improve the long-term stability. A layer of poly(3-octylthiophene-2,5-diyl) (P3OT) was pre-modified on the sensor surface to inhibit the formation of an aqueous layer and improve the sensor lifetime. With the aid of a translation stage, the light spot automatically illuminated the detection sites in sequence, and the response of the four ions could be obtained in one measurement within 1 min. The proposed multiplexed ISLAPS has good sensitivity with micromolar limit of detection (LOD), good selectivity and long-term stability (more than 3 months). The results of the real Dulbecco's Modified Eagle Medium (DMEM) sample detection proved that the ISLAPS system can be used for the physiological ions detection, and is promising to realize a multi-parameter microphysiometer.

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.

Potent Acrylamide Determination in Food Products Using Ion-Selective Electrode Technique

A potent selective acrylamide liquid sensor based on the reaction of acrylamide with 2-(5-Bromo-2-pyridylazo)-5-[N-n-Propyl-N-(3-Sulfopropyl) amino] aniline reagent is successfully designed. The characteristics slope (52.33 mV/decade), linearity usable range from 1.0 × 10⁻⁷–1.0 × 10⁻¹ molar, limit of detection (1.6 × 10⁻⁸) molar, selectivity attitude to several inorganic cations, amino acids and sugars, time of response (8 s), lifetime (four months), pH effect on the electrode potential and the basic validation parameters were studied. The desirable pH applicable range was 3.0–6.5, and the restraint of the developed sensor is independent on this working pH range. The deployed electrode was effectively applied for rapid inexpensive analysis of acrylamide cations in food products with comparison to high-performance liquid chromatographic method and the results were agreeable with each other. The obtained data by the suggested electrode were treated statistically and compared with the various recently published acrylamide sensors.

A colorimetric paper-based optode sensor for highly sensitive and selective determination of thiocyanate in urine sample using cobalt porphyrin derivative

In this work, a highly sensitive colorimetric paper-based optode for the determination of thiocyanate in urine samples was developed for the first time. The cocktail solution of the optode was composed of 5,10,15,20-tetrakis(4-octyloxyphenyl)porphyrin cobalt(II) complex (L), tridodecylmethylammonium chloride (TDMACl), 2-nitrophenyl octyl ether, and polyvinyl chloride as an ionophore, an ion exchanger, a plasticizer, and a polymer, respectively. The paper-based optode responded to thiocyanate by increasing the blue component in the RGB index and a visible change, with the naked-eye, of the optode color from pink to green was observed. From the central composite design, the optimized conditions that yielded the highest sensitivity were 4.70 mmol/kg TDMACl and 13.75 mmol/kg L. The developed optode sensor was highly selective and responded to thiocyanate over other anions, with a working range of 0.001-5 mM and with a coefficient of determination (R²) of 0.9915. The limits of detection using naked-eye and camera were determined to be 50.0 μM and 1.26 μM, respectively. In addition, the LOD and LOQ estimated from the standard deviation of the blank were 0.65 and 1.87 μM, respectively. Furthermore, this sensor was successfully applied to the detection of thiocyanate in urine samples from non-smokers and smokers. The results were in good agreement with the standard ion chromatography (IC) technique. This developed paper-based optode sensor was simple, low-cost, portable, and easy to use as a sensing device without any complicated instrument.

Inexpensive ion-selective electrodes for the simultaneous monitoring of potassium and nitrate concentrations in nutrient solutions

A fast, simple and inexpensive potentiometric method has been developed for the determination of the major ions potassium and nitrate in nutrient solutions, by means of ion-selective electrodes (ISEs) based on plasticized polyvinyl membranes containing an ion-exchanger. Tridodecylmethylammonium chloride (TDMACl) and potassium tetrakis(4-chlorophenyl)borate (KTClPB) were used as ion-exchangers for the nitrate and potassium electrodes, respectively. Electrode membranes built with different plasticizers, bis-[2-ethylhexyl]-sebacate (DOS), tricresyl phosphate (TCP) and 2-nitrophenyloctyl ether (NPOE), were tested, and NPOE was selected. The electrodes were calibrated over both wide and narrow concentration ranges and residual analysis was made. Based on the results of these calibrations, the method of standard addition was developed and found to be suitable for the simultaneous determination of potassium and nitrate in nutrient solutions. A large group of samples taken from different stages of hydroponic crops was analysed. Several approaches recommended for statistical comparisons of the results obtained by potentiometric and by reference methods were tested, obtaining satisfactory results. The potentiometric methodology developed is promising for monitoring the concentration of these essential nutrients in nutrient solutions.

Polymeric membrane-modified voltammetric sensors for lipophilic analytes with nanomolar detection limit: Key parameters influencing the response characteristics

Thin plasticized PVC membrane-coated glassy carbon working electrodes have been used for the voltammetric measurement of highly lipophilic, electroactive drugs. Compared to conventional working electrodes, these membrane-coated electrodes exhibit remarkable detection limit and selectivity and are less prone to electrode fouling. The unique performance characteristics of these sensors are related to the large partition coefficient of the analyte in the membrane coating where it is oxidized in a non-aqueous membrane phase. To analyze the influence of the key parameters of the response of membrane-coated sensors, we derived theoretical expressions on the voltammetric response of the sensors. In our analysis we considered 1) the partition coefficient (P_mw) of the analyte between the aqueous sample and the organic membrane, 2) the membrane volume to sample volume ratio (V_m/V_w), and 3) the binding constant of constituents in the sample that bind the analyte (K). The results of our theoretical analysis have been tested through voltammetric measurement of highly lipophilic analytes with logP_ow values (logarithm of the partition coefficient between octanol and water) ranging between 0.3 and 7.5. By understanding of the influence of the sensor design parameters on the overall sensor response, these parameters can be tuned for optimized response slope, detection limit, etc., for solving specific analytical tasks.

Characterization of the impact of mixing and droplet volumes on the behavior of microfluidic ion-selective droptodes

Droplet microfluidic optodes, or "droptodes", have emerged as a powerful technology for rapid detection of small ions in complex matrices. While using segmented aqueous phases provides the benefits of sample isolation, the influence of the liquid nature of the oil carrier phase has not yet been explored. In this paper, we examine the influence of microfluidic parameters on droptode efficiency, using potassium-sensitive droptodes as a model system. We found that while changing flow rates on device does not change droptode performance, both channel geometry and droplet size significantly impact droptode efficiency. Specifically, enhanced mixing of the droplets leads to faster equilibration on device and lowers limits of detection by about one order of magnitude. We also found that increasing the size of the sample droplet, at the expense of the size of the oil carrier/sensing phase, leads to higher sensitivity in the linear region of the droptode. These easily manipulated properties will allow one device to potentially be adapted for several different applications, based upon the type and concentration range of measurement required.

Paper-Based Optode Devices (PODs) for Selective Quantification of Potassium in Biological Fluids

This paper describes the design, fabrication, and feasibility of paper-based optode devices (PODs) for sensing potassium selectively in biological fluids. PODs operate in exhaustive mode and integrate with a handheld, smartphone-connected optical reader. This integrated measuring system provides significant advantages over traditional optode membranes and other paper-based designs, by obtaining a linear optical response to potassium concentration via a simple, stackable design and by harnessing a smartphone to provide an easy-to-use interface, thus enabling remote monitoring of diseases.