Triazolophanes: a new class of halide-selective ionophores for potentiometric sensors

Development of the New Sensor Based on Functionalized Carbon Nanomaterials for Promethazine Hydrochloride Determination

Promethazine hydrochloride (PM) is a widely used drug so its determination is important. Solid-contact potentiometric sensors could be an appropriate solution for that purpose due to their analytical properties. The aim of this research was to develop solid-contact sensor for potentiometric determination of PM. It had a liquid membrane containing hybrid sensing material based on functionalized carbon nanomaterials and PM ions. The membrane composition for the new PM sensor was optimized by varying different membrane plasticizers and the content of the sensing material. The plasticizer was selected based on calculations of Hansen solubility parameters (HSP) and experimental data. The best analytical performances were obtained using a sensor with 2-nitrophenyl phenyl ether (NPPE) as the plasticizer and 4% of the sensing material. It had a Nernstian slope (59.4 mV/decade of activity), a wide working range (6.2 × 10^-7 M-5.0 × 10^-3 M), a low limit of detection (1.5 × 10^-7 M), fast response time (6 s), low signal drift (-1.2 mV/h), and good selectivity. The working pH range of the sensor was between 2 and 7. The new PM sensor was successfully used for accurate PM determination in a pure aqueous PM solution and pharmaceutical products. For that purpose, the Gran method and potentiometric titration were used.

A new sensor for direct potentiometric determination of thiabendazole in fruit peels using the Gran method

A new sensor for direct potentiometric determination of thiabendazole (TBZ) was prepared. The ionic pair of TBZ cation and the 5-sulfosalicylate anion was used as the new sensor material incorporated in liquid type of ion-selective electrode membrane for TBZ determination. For optimization of the membrane of the sensor for TBZ determination, six different plasticizers and the content of the sensor material in the membrane were varied. The chosen sensor with dibutyl sebacate (DS) as plasticizer and 1% of sensor material in the membrane was characterized with Nernstian response towards TBZ (62.2 mV/decade of activity), a wide working range (8.6∙10^-7-1.0∙10^-3 M), and a low limit of detection (3.2·10^-7 M). Also, it proved to be an accurate and reliable sensor for TBZ determination in pure and real samples (peel of oranges, lemons and bananas) where it was determined using direct potentiometry and Gran method.

Highly Selective Potentiometric Sensing of Biologically Relevant Pyrophosphate and Lysophosphatidic Acid Using N-Alkyl/Aryl Ammonium Resorcinarenes/Extended-Resorcinarenes as Ionophores

The ionophore properties of four kinds of N-alkyl/aryl ammonium resorcinarenes and extended-resorcinarenes were inspected for the first time to fabricate polymeric membrane electrodes for determination of biologically relevant pyrophosphate (PPi) and lysophosphatidic acid (LPA). The proposed ion selective electrodes (ISEs) showed significant preference for PPi and LPA with significant selectivity pattern differences from the Hofmeister series. To gain further insight into the performances of the developed ISEs, the binding constants of ionophore-anion complexes in the plasticized membrane phase were investigated, along with the optimized geometries and calculated electrostatic potential. Nernstian potential responses with good reversibility to target anions can be observed when shifting the optimized membranes in aqueous solutions in the concentration range from 10^-6.5 to 10^-2.3/10^-2.2 M. Moreover, potentiometric sensings of PPi and LPA in mineral water and artificial serum were achieved in low μM concentration range, demonstrating their promising real-world applications. These results provide a promising avenue for the development of polymeric membrane electrodes for biological relevant anions and will broaden the scope of potentiometric sensing.

Anchoring H-Bond Donating/Accepting Pyrrolic Derivatives on Preorganized Scaffolds: Conformationally Switchable Bipedal/Tripodal and Locked Molecular Cage Ionophores for Potentiometric Sensing of Phosphate and Fluoride

The ionophore properties of a myriad of conformationally switchable bipedal/tripodal receptors and locked molecular cages were evaluated here for the first time to fabricate potentiometric sensors for the determination of environmentally important phosphate and fluoride. Owing to the competent ionophore properties such as high binding selectivity and affinity, the developed ion-selective electrodes displayed response preference for phosphate and fluoride with a selectivity pattern that differs distinctly from traditional Hofmeister series. Binding constants of the ionophore-anion complexes are determined to underscore how modifications in the preorganization and H-bond donating/accepting ability of a given series of ionophores can be exploited to improve the performance for potentiometric sensing. While conformationally switchable bipedal/tripodal ionophores prefer tetrahedral oxyanions, locked molecular cages shift their preference to spherical halides gradually. Nernstian potential responses with good reversibility to target anions can be observed when shifting the optimized membrane electrodes in aqueous solutions within the concentration range of 10^-6.5-10^-2.0 M. Moreover, potentiometric determination of phosphate and fluoride in mineral water, soil, and tap water samples was achieved in a low μM concentration range with high accuracy, confirming their promising utility in real world applications.

Embedding of Functionalized Coordination Cages and a Molecular Knot in a Polymeric Membrane for Potentiometric Sensing of Environmentally Important Oxyanions and Halides

Three kinds of coordination cages and a molecular knot with inductively activated ⁺P-H, N-H, or C-H hydrogen bond donors anchoring in the functionalized cavities were inspected as ionophores to develop polymeric membrane ISEs for potentiometric sensing of environmentally important oxyanions and halides. The proposed ISEs displayed significant preference for perrhenate, phosphate, or chloride with a selectivity pattern distinctively different from the sequence depending on the Gibbs free energy of hydration owing to the high degree of shape, charge, and size selectivity originating from the rigidity and complementarity of the binding cavities. To gain further insight into the response characters of the proposed ISEs, the binding constants of ionophore-anion complexes in the membrane phase were investigated, and the binding affinity, together with the Hofmeister series, correlates well with the determined selectivity pattern of the proposed ISEs. Optimizing the composition of the membrane such as lipophilic additives and plasticizers produced ISEs displaying Nernstian/near-Nernstian potentiometric responses to primary anions with a wide linear range, improved detection limits, good reversibility, and satisfying lifetime. Potentiometric determination of perrhenate, phosphate, and chloride in river water, mineral water, and artificial serum samples was achieved with good recovery and accuracy using the proposed ISEs, demonstrating their potential for real-life applications. These results will shed new light on how novel ionophores could be designed for potentiometric sensing and broaden the scope of host-guest chemistry of coordination cages and molecular knots.

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.

Development of an Electrochemical Dual H2S/Ca2+ Microsensor and Its In Vivo Application to a Rat Seizure Model

A dual electrochemical microsensor was fabricated for concurrent monitoring of hydrogen sulfide (H₂S) and calcium ions (Ca²⁺), which are closely linked important signaling species involved in various physiological processes. The dual sensor was prepared using a dual recessed electrode consisting of two platinum (Pt) microdisks (50 μm in diameter). Each electrode was individually optimized for the best sensing ability toward a target analyte. One electrode (WE1, amperometric H₂S sensor) was modified with electrodeposition of Au and electropolymerized polyaniline coating. The other electrode (WE2, all-solid-state Ca²⁺-selective electrode) was composed of Ag/AgCl onto the recessed Pt disk formed via electrodeposition/chloridation, followed by silanization and Ca²⁺-selective membrane loading. The current of WE1 and the potential of WE2 in a dual sensor responded linearly to H₂S concentration and logarithm of Ca²⁺ concentration, respectively, without a crosstalk between the sensing signals. Both WE1 and WE2 presented excellent sensitivity, selectivity (log⁡KH2S,iAmp≤-3.5, i = CO, NO, O₂, NO₂^-, AP, AA, DA, and GABA; and log⁡KCa2+,jPot≤-3.2, j = Na⁺, K⁺, and Mg²⁺), and fast response time with reasonable stability (during ca. 6 h in vivo experiment). Particularly, WE2 prepared using a mixture of two ionophores (ETH1001 and ETH129) and two plasticizers (2-nitrophenyl octyl ether and bis(2-ethylhexyl) sebacate) showed a very shortened response time (t_R to attain the ΔE/Δt slope of 0.6 mV/min = 3.0 ± 0.2 s, n ≥ 10), a critically required factor for real-time analysis. The developed sensor was utilized for simultaneous real-time monitoring of H₂S and Ca²⁺ changes at the brain cortex surface of a living rat during spontaneous epileptic seizures induced by a cortical 4-aminopyridine injection. The dynamic changes of H₂S and Ca²⁺ were clearly observed in an intimate correlation with the electrophysiological recording of seizures, demonstrating the sensor feasibility of in vivo and real-time simultaneous measurements of H₂S and Ca²⁺.

Anion-Selective Electrodes Based On a CH-Hydrogen Bonding Bis-macrocyclic Ionophore with a Clamshell Architecture

CH-hydrogen bonding provides access to new building blocks for making macrocyclic ionophores with high degrees of preorganization and selective anion recognition. In this study, an anion-binding ionophore in the shape of a clamshell (ClS) was employed that is composed of two cyanostar (CNstar) macrocycles with preorganized cavities linked with a 12-carbon chain. This ionophore allows for anion complexation by CH-hydrogen bonding. The potentiometric performance of membrane-based ion-selective electrodes incorporating this ionophore was evaluated. Different membrane compositions were prepared to determine the optimum concentrations of the ionophore and lipophilic additive in the membrane. The optimized electrode had a slope of -58.2 mV/decade and demonstrated an anti-Hofmeister selectivity pattern toward iodide with a nanomolar detection limit. Electrospray ionization mass spectrometry was employed to study the relative association strengths of ClS with various anions. The observed mass peaks of the ion-ionophore complexes were found to be consistent with the potentiometric selectivity pattern of the corresponding electrodes. Overall, the selectivity of the electrode could be altered by using an ionophore in which the two CNstar macrocycles are linked together with a flexible 12-carbon chain to control the molecularity of the binding event.

Hydrogen Bond-Based Macrocyclic and Tripodal Neutral Ionophores for Highly Selective Polymeric Membrane Sulfate-Selective Electrodes

Four hydrogen bond-based macrocyclic and tripodal neutral receptors with increasing conformational complementarity with sulfate were used for the first time as ionophores to develop polymeric membrane sulfate-selective electrodes. Optimizing the membrane composition such as ionophores, lipophilic additives, and plasticizers yielded ISEs which showed Nernstian response to sulfate with the best selectivity so far and improved detection limits (a slope of -29.8 mV/dec in the linear range of 1 × 10^-6-1 × 10^-1 M with a detection limit of 5 × 10^-7 M), which led to the success of the determination of sulfate in drinking water samples and neomycine tablets. The anion-ionophore complex constants in the membrane phase were determined and correlated with the selectivity sequence of the ISEs. Studies on the influence of pH of the sample solution demonstrated that the developed ISEs can be operated in a wide pH range of 3-8 with fast response and rapid (in 1 min) and long lifetime. The success of these ionophores represents a feasible strategy for overcoming the "Hofmeister series" by employing a combination of complementarity and hydrogen bonds.

Polymeric Membrane Fluoride-Selective Electrodes Using Lewis Acidic Organo-Antimony(V) Compounds as Ionophores

Four Lewis acidic organo-antimony(V) compounds with strong binding affinity to fluoride were used for the first time as ionophores to fabricate polymeric membrane fluoride-selective electrodes. Improved detection limits and significant anti-Hofmeister selectivity could be achieved by optimizing ionophores, lipophilic additives, and plasticizers. Membrane electrodes fabricated with tetrakis-(pentafluorophenyl)stibonium (ionophore 2) performed best in detection limit, sensitivity, and selectivity. Optimal performance was obtained by fluoride with a slope of -59.5 mV/decade in the linear range of 1 × 10^-5 to 4 × 10^-2 M and a detection limit of 5 × 10^-6 M. Studies on the influence of sample solution pH demonstrate that the best pH for fluoride determination is pH 3.0. All of the electrodes studied respond rapidly (in 1 min) in different concentrations of fluoride solutions. The anion-ionophore complex constants in the membrane phase determined using the segmented sandwich membrane method correlate well with the solution-phase binding data and determined selectivity sequence of the ion-selective electrodes. The possibility of real life application of the optimized electrodes was assessed by determination of fluoride concentrations in tap water.

Electroactive Anion Receptor with High Affinity for Arsenate

Herein, we present the synthesis and characterization of a macrocyclic polyamide cage that incorporates redox-active 1,4-dithiin units. UV/vis titration experiments with eight anions in acetonitrile revealed high affinity for H₂AsO₄^- (log β₂ = 10.4_-0.4^+0.4) and HCO₃^- (log β₂ = 8.3_-0.4^+0.3) over other common anionic guests, such as Cl^- (log K_1:1 = 3.20_-0.02^+0.03), HSO₄^- (log K_1:1 = 3.57_-0.03^+0.02), and H₂PO₄^- (log K_1:1 = 4.24_-0.04^+0.05), by the selective formation of HG₂ complexes. The recognition of arsenate over phosphate is rare among both proteins and synthetic receptors, and though the origin of selectivity is not known, exploiting the difference in the binding stoichiometry represents an underexplored avenue toward developing receptors that can differentiate between the two anions. Additional analysis by ¹H NMR in 1:3 CD₂Cl₂/MeCN-d₃ found a strong dependence of anion binding stoichiometry with the solvent employed. Finally, titration experiments with cyclic voltammetry provided varying and complex responses for each anion tested, though reaction between the anion and receptors was observed in most cases. These results implicate 1,4-dithiins as interesting recognition moieties in the construction of supramolecular receptors.

Electrochemical Anion Sensing: Supramolecular Approaches

Anions play a vital role in a broad range of environmental, technological, and physiological processes, making their detection/quantification valuable. Electroanalytical sensors offer much to the selective, sensitive, cheap, portable, and real-time analysis of anion presence where suitable combinations of selective (noncovalent) recognition and transduction can be integrated. Spurred on by significant developments in anion supramolecular chemistry, electrochemical anion sensing has received considerable attention in the past two decades. In this review, we provide a detailed overview of all electroanalytical techniques that have been used for this purpose, including voltammetric, impedimetric, capacititive, and potentiometric methods. We will confine our discussion to sensors that are based on synthetic anion receptors with a specific focus on reversible, noncovalent interactions, in particular, hydrogen- and halogen-bonding. Apart from their sensory properties, we will also discuss how electrochemical techniques can be used to study anion recognition processes (e.g., binding constant determination) and will furthermore provide a detailed outlook over future efforts and promising new avenues in this field.

The road to aryl CHanion binding was paved with good intentions: fundamental studies, host design, and historical perspectives in CH hydrogen bonding

Throughout the design and development of supramolecular receptors for anion binding, many different non-covalent anion-binding motifs have been employed. One motif seen in many host-guest systems is the sometimes weaker, 'non-traditional' aryl CH hydrogen bond. From June Sutor's discovery of the interaction and its subsequent dismissal by the field in the 1960s to today's use of the aryl CH hydrogen bond in synthetic anion receptors, the path our lab took to begin studying this interaction has been influenced by many other researchers in the field. This feature article highlights the history and properties of the CH hydrogen bond, with a particular focus on aryl CH hydrogen bonds in anion recognition. We highlight select recent developments in the field of anion receptors utilizing aryl CH hydrogen bonds, with an emphasis on how this has influenced the evolution of our approach in designing fundamental studies on CH hydrogen bonding and exploiting this interaction in efforts aimed toward preferential anion binding.

Aromatization of 9,10-Dihydroacridine Derivatives: Discovering a Highly Selective and Rapid-Responding Fluorescent Probe for Peroxynitrite

As part of an effort to develop generally applicable strategies for creating probes suitable for detecting important molecular and ionic species, the oxidative aromatization of nonfluorescent 9,10-dihydroacridine derivatives triggered by peroxynitrite (ONOO^-) led to the identification of compound 2H, 9-phenyl-9,10-dihydroacridine-4-carboxylic acid, as a rapid-responding fluorescent probe capable of detecting ONOO^- with an extraordinary selectivity. Adding a little more than 1 equiv of ONOO^- to a solution of 2H resulted in over 100-fold fluorescence enhancement. In sharp contrast, treating 2H with excessive amounts of other oxidants that often interfere with the detection of ONOO^- failed to lead to noticeable fluorescence increase. The reaction of ONOO^- with 2H shows a similar efficiency in the pH range of 2-8. Low cytotoxicity was observed for 2H and its aromatized product. Bioimaging experiments revealed the promising potential of 2H as a new fluorescent probe for the selective detection of intracellular ONOO^-.

Living on the edge: Tuning supramolecular interactions to design two-dimensional organic crystals near the boundary of two stable structural phases

One of the benefits of supramolecular assemblies that form at dynamic interfaces is the opportunity to develop condensed phase systems that respond to environmental stimuli. A prerequisite of this responsive behavior is that the supramolecular system be designed to sit very near the stability of two or more crystal structures. We have created such a bi-phasic system with aryl-triazole oligomers by investigating how phase morphology is controlled by the interplay between interactions that involve the oligomer's dipolar cores (Δμ = 3.5 debye), van der Waals contacts of their pendant alkyl chains (C4-C18), and close-contact hydrogen bonding. Scanning tunneling microscopy experiments conducted at the solution-graphite interface allow sub-molecular resolution of the ordered monolayers to unambiguously determine the packing and structure of two principle phases, α and β. The system is balanced very near the edge of phase stability, evidenced by co-existent phases present over short time frames and by the changes in preference between the two 2D supramolecular assemblies that occur with small modifications to the molecular structure. We demonstrate that the bi-phasic behavior can be understood as a balance between electrostatic interactions and van der Waals contacts, two variables within a larger parameter space, allowing synthetic design to move this solution-surface system across the stability boundary of different condensed-phase structures. These findings are a foundation for the development of environmentally responsive 2D supramolecular arrays.

Binding studies and anion-selective electrodes with neutral isophthalamide-based receptors

The incorporation of neutral and structurally simple isophthalamide-based receptors into plasticised polymeric membrane anion-selective electrodes has afforded devices with a good sensing ability.

Measurement of the ground-state distributions in bistable mechanically interlocked molecules using slow scan rate cyclic voltammetry

In donor-acceptor mechanically interlocked molecules that exhibit bistability, the relative populations of the translational isomers--present, for example, in a bistable [2]rotaxane, as well as in a couple of bistable [2]catenanes of the donor-acceptor vintage--can be elucidated by slow scan rate cyclic voltammetry. The practice of transitioning from a fast scan rate regime to a slow one permits the measurement of an intermediate redox couple that is a function of the equilibrium that exists between the two translational isomers in the case of all three mechanically interlocked molecules investigated. These intermediate redox potentials can be used to calculate the ground-state distribution constants, K. Whereas, (i) in the case of the bistable [2]rotaxane, composed of a dumbbell component containing π-electron-rich tetrathiafulvalene and dioxynaphthalene recognition sites for the ring component (namely, a tetracationic cyclophane, containing two π-electron-deficient bipyridinium units), a value for K of 10 ± 2 is calculated, (ii) in the case of the two bistable [2]catenanes--one containing a crown ether with tetrathiafulvalene and dioxynaphthalene recognition sites for the tetracationic cyclophane, and the other, tetrathiafulvalene and butadiyne recognition sites--the values for K are orders (one and three, respectively) of magnitude greater. This observation, which has also been probed by theoretical calculations, supports the hypothesis that the extra stability of one translational isomer over the other is because of the influence of the enforced side-on donor-acceptor interactions brought about by both π-electron-rich recognition sites being part of a macrocyclic polyether.

Using ratiometric indicator-displacement assays in semi-quantitative colorimetric determination of chloride, bromide, and iodide anions

A ratiometric indicator-displacement assay (RIDA) array has been developed for the semi-quantitative colorimetric determination of chloride, bromide, and iodide anions. Determinations of these halide anions follow the displacement reaction using the chelate compound of (2-(3,5-dibromo-2-pyridylazo)-5-(diethylamino)phenol) (3,5-Br2-PADAP) and heavy metal salts as colorimetric reagent. Different from regular silver nitrate titrations, the chloride, bromide, and iodide anions compete with the 3,5-Br2-PADAP ligand and change the colour of the 3,5-Br2-PADAP-metal chelate compound dramatically. These clearer colour changes make the semi-quantitative colorimetric determination of chloride, bromide, and iodide anions possible. The colour changes are imaged using a conventional flatbed scanner, and digitized. After statistical analysis, these colour changes in the RIDA array provide facile identification of chloride, bromide, and iodide anions at a wide concentration range (10 μM to 10 mM) without any misclassification. The RIDA array is able to discriminate without misclassifications among seven concentrations of chloride, bromide, and iodide anions. No shelf life issue exists because the chelating compounds react with halide anions directly without any pre-immobilizations.