Coulometric Response of H+-Selective Solid-Contact Ion-Selective Electrodes and Its Application in Flexible Sensors†

The analytical performance of H⁺-selective solid-contact ion-selective electrodes (SCISEs) based on solid contact polyaniline doped with chloride (PANI(Cl)) and poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT(PSS)) was characterized by a developed coulometric signal transduction method. PEDOT(PSS) solid contact is covered by PVC based H⁺-selective membrane. The obtained coulometric signal demonstrates that the cumulated charge can be amplified by increasing the capacitance of solid contact. SCISEs covered with spin-coated membrane behave faster amperometric response than electrodes with drop-cast membrane. In contrast to earlier works, the amperometric response and impedance spectrum demonstrates H⁺ transfer through SCISEs is independent from the thickness of membrane. The exceptional behavior of PANI(Cl) H⁺-SCISEs shows that the capacitance estimated from impedance spectrum at low frequency 10 mHz and coulometric signal of PANI(Cl) based SCISEs is influenced by the applied potentials, whereas PEDOT(PSS) solid contact is independent from the chosen applied potentials. Furthermore, preliminary investigations of coulometric signal transduction on flexible pH sensor implies its potential applications in wearable sensors for sweat ion concentration detection.      

Competitive binding of Na<Superscript>+</Superscript> and Ca<Superscript>2+</Superscript> ions to teichoic acid analogues

The competitive binding of monovalent and divalent counterions to poly(alkylene phosphate) related to bacterial teichoic acids and poly(styrenesulfonate) was studied experimentally by potentiometry with ion-selective electrodes. The binding of calcium ions and the release of sodium ions accompanying calcium ions binding in aqueous solutions of the polyelectrolytes was analysed and the mean exchange ratio Na⁺/Ca²⁺ was estimated. It was found that in the process of addition of calcium ions to sodium poly(alkylene phosphate) and sodium poly(styrenesulfonate) solutions all the Ca²⁺ ions added are bound to polyions and the initially condensed Na⁺ ions are released proportionally to the concentration of the added Ca²⁺ ions up to the critical concentration of the Ca²⁺ ions added. For a molar concentration ratio of calcium counterions to charged groups on the polyion up to 0.20 the exchange ratio was approximately equal to 1 or 2 for the sodium poly(alkylene phosphate)/CaCl₂ and sodium poly(styrenesulfonate)/CaCl₂ systems, respectively.     

An insight into aggregation kinetics of polystyrene nanoplastics interaction with metal cations

Once inevitably released into the aquatic environment, polystyrene nanoplastics (PS-NPs) will present complicated environmental behaviors, of which the aggregation is a key process determining their environmental fate and impact. In this study, the aggregation kinetics of different sizes (30 nm and 100 nm) of PS-NPs with metal cations (Na⁺, K⁺, Ca²⁺, Mg²⁺ and Pb²⁺) at different solution pH (3, 6 and 8) were investigated. The results showed that the aggregation of PS-NPs increased with cation concentration. Taking Pb²⁺ as an example, the adsorption behavior of cations onto PS-NPs was determined by transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) spectroscopy, which demonstrated Pb²⁺ could be adhered onto the surface of PS-NPs with the effect of charge neutralization. The critical coagulation concentrations (CCC) of smaller PS-NPs were higher than that of larger PS-NPs for monovalent cations, whereas a different pattern is observed for divalent cations. It was suggested that there were other factors that DLVO theory does not consider affect the stability of NPs with different particle sizes. In addition, it should be noted that PS-NPs had the capacity of adsorbing large amounts of heavy metal cations and carried them transport to a long distance, and the corresponding ecological risks need to further elucidate.     

Synthesis and characterisation of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) Zr(IV) monothiophosphate composite cation exchanger: analytical application as lead ion selective membrane electrode

A Pb²⁺ ion selective membrane electrode based on poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) Zr(IV) monothiophosphate composite cation exchange material was fabricated using solution casting method. The effect of membrane composition on the proton exchange capacity was investigated by using varying amounts of electroactive material. The membrane with 250 mg of electroactive material and 10 µL of plasticiser exhibited higher proton conductivity. The optimised membrane composition was used for the fabrication of ion selective membrane electrode which exhibited typical Nernstian response towards Pb²⁺ ions in the concentration range 20.70 gL^?1–20.7 µgL^?1 (1 × 10^–1–1 × 10^–7 mol L^?1) with a sub-Nernstian slope of 27.429 mV per decade change in Pb²⁺ ion concentration. The response time of the electrode under study for Pb²⁺ ions was found to be 11 s and the electrode can be used for 120 days without any considerable divergence in response potential. It can also be successfully used in the pH range from 3.0 to 6.5. It was found selective for Pb²⁺ ions in the presence of various monovalent, divalent and trivalent interfering metal ions. It was also employed as an indicator electrode in the potentiometric titration of Pb²⁺ ions using ethylenediaminetetraacetic acid, disodium salt, as a titrant.     

Qualitative and quantitative 1H NMR spectroscopy for determination of divalent metal cation concentration in model salt solutions,food supplements,and pharmaceutical products by using EDTA as chelating agent

This paper introduces an ¹H NMR method to identify individual divalent metal cations Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺, Zn²⁺, Cd²⁺, Hg²⁺, Sn²⁺, and Pb²⁺ in aqueous salt solutions through their unique signal shift and coupling after complexation with the salt of ethylenediaminetetraacetic acid (EDTA). Furthermore, quantitative determination applied for the divalent metal cations Ca²⁺, Mg²⁺, Hg²⁺, Sn²⁺, Pb²⁺, and Zn²⁺ (limit of quantification: 5–22 μg/ml) can be achieved using an excess of EDTA with aqueous model salt solutions. An internal standard is not required because a known excess of EDTA is added and the remaining free EDTA can be used to recalculate the quantity of chelated metal cations. The utility of the method is demonstrated for the analysis of divalent cations in some food supplements and in pharmaceutical products.     

Influence of divalent ions on composition and viscoelasticity of polyelectrolyte complexes

The addition of monovalent salts to polyelectrolyte complexes (PECs) comprising oppositely charged polyelectrolytes results in diminishing propensity for complexation, leading to complexes with higher water contents and lower moduli. However, the corresponding influence of multivalent ions on polyelectrolyte complexation has not yet been explored beyond enhanced screening effects. Here, we elucidate the significant impact of the valency of the salt cation on the composition, ion partitioning, and viscoelasticity of charge-matched PECs comprising sodium salt of poly(acrylic acid) and poly(allylamine hydrochloride). Notably, preferential partitioning of divalent cations (Ca²⁺ and Sr²⁺) into the complexes is observed, in stark contrast to the depletion of monovalent ions (Na⁺) from the complexes. Concomitantly, electrostatic bridging of polyanion chains by divalent ions is found to hinder their relaxation, manifesting as a non-monotonic evolution of the shear moduli of the complexes with increasing divalent salt concentrations. Relatedly, a failure of time-salt and time-ionic strength superposition approaches in presence of divalent ions is demonstrated, highlighting the nontrivial influence of these ions on chain relaxation behavior.     

How Divalent Cations Interact with the Internal Channel Site of Guanine Quadruplexes

The formation of guanine quadruplexes (GQ) in DNA is crucial in telomere homeostasis and regulation of gene expression. Pollution metals can interfere with these DNA superstructures upon coordination. In this work, we study the affinity of the internal GQ channel site towards alkaline earth metal (Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺), and (post-)transition metal (Zn²⁺, Cd²⁺, Hg²⁺, and Pb²⁺) cations using density functional theory computations. We find that divalent cations generally bind to the GQ cavity with a higher affinity than conventional monovalent cations (e. g. K⁺). Importantly, we establish the nature of the cation-GQ interaction and highlight the relationship between ionic and nuclear charge, and the electrostatic and covalent interactions. The covalent interaction strength plays an important role in the cation affinity and can be traced back to the relative stabilization of cations’ unoccupied atomic orbitals. Overall, our findings contribute to a deeper understanding of how pollution metals could induce genomic instability.     

Effect of counterion condensation on the acid-base equilibrium of a pH-sensitive merocyanine dye covalently attached to polyelectrolytes

Effects of monovalent and divalent counterions on the acid-base equilibrium of a pH-sensitive merocyanine dye covalently attached to copolymers of acrylic acid and acrylamide with varying charge densities (0.28 < ξ < 2.8) were investigated. Added chloride salts of Li⁺, Na⁺, K⁺, and NH⁺₄ (< 0.2 mM) had essentially no effect on pK observed (pK^obs) for the equilibrium. By contrast, the salts of Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ caused a significant decrease in pK^obs for the copolymers with larger ξ. With smaller ξ, most likely when ξ < 0.5, no decrease in pK^obs was observed upon addition of the salts of divalent cations. A competitive effect of Ca²⁺ and Na²⁺ ions on pK^obs in the presence of an excess of Na⁺ ions implied that Ca²⁺ ions at very low concentrations were preferentially, and therefore exhaustively, condensed on the polyanions with sufficiently large ξ probably until effective charge density was lowered to 0.5. The observed difference in the influence of the monovalent and divalent cations on pK^obs was discussed in terms of the difference in the microscopic behavior of the condensed monovalent and divalent cations. © 1993 John Wiley & Sons, Inc.     

Lead‐Free Silver‐Bismuth Halide Double Perovskite Nanocrystals

Lead‐free perovskite nanocrystals (NCs) were obtained mainly by substituting a Pb²⁺ cation with a divalent cation or substituting three Pb²⁺ cations with two trivalent cations. The substitution of two Pb²⁺ cations with one monovalent Ag⁺ and one trivalent Bi³⁺ cations was used to synthesize Cs₂AgBiX₆ (X=Cl, Br, I) double perovskite NCs. Using femtosecond transient absorption spectroscopy, the charge carrier relaxation mechanism was elucidated in the double perovskite NCs. The Cs₂AgBiBr₆ NCs exhibit ultrafast hot‐carrier cooling (<1 ps), which competes with the carrier trapping processes (mainly originate from the surface defects). Notably, the photoluminescence can be increased by 100 times with surfactant (oleic acid) added to passivate the defects in Cs₂AgBiCl₆ NCs. These results suggest that the double perovskite NCs could be potential materials for optoelectronic applications by better controlling the surface defects.     

Highly Selective All‐Plastic,Disposable, Cu2+‐Selective Electrodes

Conducting polymers (CP) remain a promising material to construct stable potential all‐solid‐state ion‐selective potentiometric electrodes. The unique properties of poly(3,4‐ethylenedioxythiophene) doped with poly(4‐styrenesulfonate) ions, PEDOT‐PSS: high CP stability and affinity of doping anions towards Cu²⁺ ions, make it highly attractive for construction of all‐solid‐state copper(II)‐selective electrodes with outstanding selectivity. The additional benefits can arise from solution processability of commercially available PEDOT‐PSS system. This material was highly promising for a new sensor arrangement, i.e. to obtain disposable, planar and flexible all‐plastic Cu²⁺‐selective electrodes. These sensors can be obtained by casting a commercially available dispersion of PEDOT‐PSS (Baytron P) on a plastic, non‐conducting support material. The CP being both electrical lead and ion‐to‐electron transducer, was covered with plastic, solvent polymeric Cu²⁺ selective membrane. This extremely simple arrangement, after conditioning in dilute Cu²⁺ solution, was characterized with linear Nernstian responses within the activities range from: 0.1 to 10^?4 M, followed by super‐Nernstian responses for lower activities. The latter result points to effective elimination of primary ions leakage from the plastic membrane / transducer phase and has resulted in significantly improved selectivities. Obtained log K values were equal to ?7.6 for Co²⁺, ?7.4 for Zn²⁺, ?7.2 for Ca²⁺ and ?6.8 for Na⁺, respectively.     

Interaction study of a lysozyme‐binding aptamer with mono‐ and divalent cations by ACE

Binding between an aptamer and its target is highly dependent on the conformation of the aptamer molecule, this latter seeming to be affected by a variety of cations. As only a few studies have reported on the interactions of monovalent or divalent cations with aptamers, we describe herein the use of ACE in its mobility shift format for investigating interactions between various monovalent (Na⁺, K⁺, Cs⁺) or divalent (Mg²⁺, Ca²⁺, Ba²⁺) cations and a 30‐mer lysozyme‐binding aptamer. This study was performed in BGEs of different natures (phosphate and MOPS buffers) and ionic strengths. First, the effective charges of the aptamer in 30 mM ionic strength phosphate and MOPS (pH 7.0) were estimated to be 7.4 and 3.6, respectively. Then, corrections for ionic strength and counterion condensation effects were performed for all studies. The effective mobility shift was attributed not only to these effects, but also to a possible interaction with the buffer components (binary or ternary complexes) as well as possible conformational changes of the aptamer. Finally, apparent binding constants were calculated for divalent cations with mathematical linearization methods, and the influence of the nature of the BGE was evidenced.     

A miniaturized solid-contact potentiometric multisensor platform for determination of ionic profiles in human saliva

This paper describes a miniaturized multisensor platform (MP-ISES) consisting of electrodes: a reference one (RE) and ion-selective electrodes (ISEs) for monitoring Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, and SCN⁻ ions and pH in human saliva. Gold electrode surface was modified by deposition of two layers: electrosynthesized PEDOT:PSS forming an intermediate layer, and ion-selective membrane. The developed ISEs were characterized by a wide linear range and sensitivity consistent with the Nernst model. The entire MP-ISEs are characterized by satisfactory metrological parameters demonstrating their applicability in biomedical research, in particular in measurements concerning determination of ionic profiles of saliva. Saliva samples of 18 volunteers aged from 20 to 26 participating in a month experiment had been daily collected and investigated using the MP-ISEs assigned individually to each person. Personalized profiles of ions (ionograms) in saliva, such as Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, SCN⁻, and H⁺, were obtained.

     

Multilayer polyelectrolyte films as nanofiltration membranes for separating monovalent and divalent cations

Alternating adsorption of polyanions and polycations on porous supports provides a convenient way to prepare ion-selective nanofiltration membranes. This work examines optimization of ultrathin, multilayer polyelectrolyte films for monovalent/divalent cation separations relevant to water softening. Membranes composed of five bilayers of poly(styrene sulfonate)/poly(allylamine hydrochloride) (PSS/PAH) on porous alumina supports allow a solution flux of 0.85 m³/(m² day) at 4.8 bar, and exhibit 95% rejection of MgCl₂ along with a Na⁺/Mg²⁺ selectivity of 22. Similar results were obtained in Na⁺/Ca²⁺ separations. PSS/poly(diallyl-dimethylammonium chloride) (PDADMAC) films permit higher fluxes than PSS/PAH systems due to the higher swelling of films containing PDADMAC, but the Mg²⁺ rejection by PSS/PDADMAC membranes is less than 45%. However, capping PSS/PDADMAC films with a bilayer of PSS/PAH yields Mg²⁺ rejections and Na⁺/Mg²⁺ selectivities that are typical of pure PSS/PAH membranes. Separation performance can be optimized through control over deposition conditions (pH and supporting electrolyte concentration) and the charge of the outer layer since Donnan exclusion is a major factor in monovalent/divalent cation selectivity. Streaming potential measurements demonstrate that the magnitude of positive surface charge increases with increasing concentrations of Mg²⁺ in solution or when the outer polycation layer is deposited from a solution of high ionic strength.     

A portable wireless-sensor system from all-in-one sensor array based hybrid solid contact layer for point-of-care ion monitoring in river basin

The sensor arrays are considered as a promising way to aim at point-of-care (POC) testing for the detection of different ions in environment through the use of solid-contact ion-selective electrodes (SC-ISEs). However, the exorbitant equipment and intricate process for the production of SC-ISEs arrays has limit its application. Herein, the Au with nano-branched structure (Au NBS) and poly (3,4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT : PSS) were decorated on the surface of bare pad of traditional printed circuit boards coated with Au (Au-PCB) to construct a hybrid solid-contact layer including double-layer capacitance and pseudocapacitance of SC-ISEs synergistically, thus the multichannel SC-ISEs exhibited high stability and specificity. Overall, based on portable multichannel sensor and homemade APP (Ion Analysis), a wireless all-in-one sensor system was constructed to detect ions with the limit of NO₃⁻, Cl⁻, Na⁺, K⁺, Ca²⁺ and Mg²⁺ as 0.047 mM, 0.039 mM, 0.022 mM, 0.066 mM, 0.045 mM and 0.024 mM, respectively. Besides, it can detect such ions in river basin successfully. With the cost of each electrode less than 0.5 $, this sensor arrays enable a wide range of hazardous ion monitoring in water environment.     

Chromogenic derivatives of new bis(phenylhydrazono-1H-tetrazol-5-yl-acetonitriles) – synthesis and properties

Derivatives of bis(phenylhydrazono-1H-tetrazol-5-yl-acetonitriles) with oxygen and sulphur atoms in the structure of aliphatic chains were successfully synthesised. The correlation between the ligand structure and its complexation properties was investigated by absorption spectroscopy. The formation of complexes of presented compounds with metal cations (Cu²⁺, Ni²⁺, Zn²⁺, Co²⁺, Fe²⁺ and Pb²⁺) was studied. Ligands 5–8 were additionally applied as ion carriers in ion-selective membrane electrodes. Membranes of ion-selective electrodes doped with these ligands are selective to Cu²⁺ and Pb²⁺ cations.     

The use of ESI-MS to probe the binding of divalent cations to calmodulin

Proteins have evolved with distinct sites for binding particular metal ions. This allows metalloproteins to perform a myriad of specialized tasks with conformations tailor-made by the combination of its primary sequence and the effect on this of the ligated metal ion. Here we investigate the selectivity of the calcium trigger protein calmodulin for divalent metal ions. This ubiquitous and highly abundant protein exists in equilibrium between its apo and its holo form wherein four calcium ions are bound. Amongst its many functions, calmodulin modulates the calcium concentration present in cells, but this functional property renders it a target for competition from other metal ions. We study the competition posed by four other divalent cations for the calcium binding sites in calmodulin using electrospray ionization mass spectrometry (ESI-MS). We have chosen two other group II cations Mg²⁺, Sr²⁺, and two heavy metals Cd²⁺, Pb²⁺. The ease with which each of these metals binds to apo and to holo CaM[4Ca] is described. We find that each metal ion has different properties with respect to calmodulin binding and competition with calcium. The order of affinity for apo CaM is Ca²⁺ ≫ Sr²⁺ ∼ Mg²⁺ > Pb²⁺ ∼ Cd²⁺. In the presence of calcium the affinity alters to Pb²⁺ > Ca²⁺ > Cd²⁺ > Sr²⁺ > Mg²⁺. Once complexes have been formed between the metal ions and protein (CaM:[xM]) we investigate whether the structural change which must accompanies calcium ligation to allow target binding takes place for a given CaM:[xM] system. We use a 20 residue target peptide, which forms the CaM binding site within the enzyme neuronal nitric-oxide synthase. Our earlier work (Shirran et al. 2005) [1] has demonstrated the particular selectivity of this system for CaM:4Ca²⁺. We find that along with Ca²⁺ only Pb²⁺ forms complexes of the form CaM:4M²⁺:nNOS. This work demonstrates the affinity for calcium above all other metals, but also warns about the ability of lead to replace calcium with apparent ease.     

Abrupt Structural Changes in Polyanionic Gels Evoked by Na-Ca Ion Exchange: Their Biological Implications

Anionic polyelectrolyte gels, synthetic or natural, undergo abrupt structural changes when the monovalent counterions in the gel, e.g., Na⁺, are replaced with divalent cations, e.g., Ca²⁺. These structural changes are caused by the preferential binding of Ca²⁺ to the polyelectrolyte chains and the ability of Ca²⁺ to form crosslinks between two neighboring chains. Strong lyotropic (Hofmeister) effects of anions, such as fluoride, phosphate, aspartate and chloride, on the repetitiveness of abrupt structural changes in the gel strands are demonstrated. These findings have important biological implications.     

Poly(vinyl chloride)-membrane ion-selective bulk optode based on 1,10-dibenzyl-1,10-diaza-18-crown-6 and 1-(2-pyridylazo)-2-naphthol for Cu and Pb ions

An ion-selective bulk optode (ISBO) for sensing Cu²⁺ and Pb²⁺ ions based on plasticized poly(vinyl chloride) containing 1,10-dibenzyl-1,10-diaza-18-crown-6 (DBzDA18C6) as ionophore and 1-(2-pyridylazo)-2-naphthol (PAN) as chromoionophore was prepared. The effects of DBzDA18C6/PAN and NaTPB/PAN mole ratios on the response behavior of the ISBO were investigated. The ISBO membrane shows enhanced selectivities for Cu²⁺ (at 530 nm) and Pb²⁺ (at 467 nm) over alkali, alkaline earth and other transition metal ions. The optical selectivity coefficients were measured using the separate solution method (SSM) in the two corresponding wavelengths at pH=5. The detection limit for Cu²⁺ and Pb²⁺ are 3.2×10⁻⁷ and 1.0×10⁻⁸ M, respectively.     

Stability constants for some divalent metal ion/crown ether complexes in methanol determined by polarography and conductometry

Stability constants in methanol at 25.0°C were evaluated for the complexes of the divalent cations Ca²⁺, Ni²⁺, Zn²⁺, Pb²⁺, Mg²⁺, Co²⁺ and Cu²⁺ with the macrocyclic polyethers 15-crown-5 (15C5), 18-crown-6 (18C6), dicyclohexyl-18-crown-6 (DC18C6) and dibenzo-24-crown-8 (DB24C8). The log K values of the 1:1 complexes were generally in the range 2.1–4.2, which is low in comparison to the values of the corresponding crown ether/alkali metal ion complexes. M₂L complexes were observed for the systems Pb²⁺/18C6, Pb²⁺/DC18C6, Ca²⁺/DC18C6 and Cu²⁺/D18C6, whereas ML₂ complexes were found for Ca²⁺/18C6 and Cu²⁺/18C6. Within the series of complexes studied, there was no clear relationship between cation diameter and hole size.     

Conductivity enhancement of PEDOT:PSS film via sulfonic acid modification: application as transparent electrode for ITO-free polymer solar cells

3-Hydroxy-1-propanesulfonic acid(HPSA)was applied as a modification layer on poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)(PEDOT:PSS)film via spin-coating,resulting in a massive boost of the conductivity of PEDOT:PSS film,and thus the as-formed PEDOT:PSS/HPSA bilayer film was successfully used as a transparent electrode for ITO-free polymer solar cells(PSCs).Under the optimized concentration of HPSA(0.2 mol L~(-1)),the PEDOT:PSS/HPSA bilayer film has a conductivity of 1020 S cm~(-1),which is improved by about 1400 times of the pristine PEDOT:PSS film(0.7 S cm~(-1)).The sheet resistance of the PEDOT:PSS/HPSA bilayer film was 98Ωsq~(-1),and its transparency in the visible range was over 80%.Both parameters are comparable to those of ITO,enabling its suitability as the transparent electrode.According to atomic force microscopy(AFM),UV-Vis and Raman spectroscopic measurements,the conductivity enhancement was resulted from the removal of PSS moiety by methanol solvent and HPSA-induced segregation of insulating PSS chains along with the conformation transition of the conductive PEDOT chains within PEDOT:PSS.Upon applying PEDOT:PSS/HPSA bilayer film as the transparent electrode substituting ITO,the ITO-free polymer solar cells(PSCs)based on poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)]:[6,6]-phenyl C71-butyric acid methyl ester(PC_(71)BM)(PCDTBT:PC_(71)BM)active layer exhibited a power conversion efficiency(PCE)of 5.52%,which is comparable to that of the traditional ITO-based devices.