In situ ATR-FTIR spectroscopic imaging of PVC, plasticizer and water in solvent-polymeric ion-selective membrane containing Cd2+-selective neutral ionophore

Generalized Adaptive Cation-Selective Interfaces Exhibiting High Stability: Integrating Transduction Materials and Uniform Ion-Selective Membranes via One-Step Fabrication

Rapidly advancing all-solid-state ion-selective electrodes are promising candidates as key components in intelligent biological and chemical sensors. Ionics sensing performance, essential for sensor stability and reliability, is influenced not only by interface compositions but by often-overlooked overall interface structures. This work develops a one-step adaptive integrated interface structure (AIIS) with high interfacial stability for analyzing general cations (K+, Na+, Ca2+, Mg2+, Pb2+, Cd2+, and Cu2+), showcasing exceptional near-Nernst response across wide linear ranges. AIIS, based on cetyltrimethylammonium-regulated lipophilic molybdenum disulfide (2.0 CTA-MoS2), forms single-piece ISM on top and bottom transduction layers over time due to THF volatilization in ISM solutions, ensuring performance adaptability. A kinetic model developed through electrochemical numerical simulation confirms the optimal theoretical stability of an AIIS based on maximum transduction layer charge current and minimal diffusion current. The mixed capacitive transduction mechanism driven by the adsorption of TFPB- on the 2.0 CTA-MoS2 surface is elucidated. Adaptive integrated cadmium ion-selective electrodes, as a case study, exhibit excellent interfacial stability (potential drift of 5.51 ± 0.32 µV h-1 for 24 h and sensitivity loss rate of 4.77% for 30 days) and selectivity. This study proposes a promising strategy for constructing extendable interface structures, providing valuable insights for advancing sensor chip development.

Aging of drinking water transmission pipes during long-term operation as a potential source of nano- and microplastics

Microplastics (MPs) and nanoplastics (NPs) released into drinking water from transmission pipes can pose a potential health risk to consumers. This paper presents the results of a comprehensive study of PE and PVC pipes after long-term operation in drinking water distribution networks, which confirmed that degradable polymers can be a significant source of MPs. Both plastics age relatively quickly, and the degree of damage to the pipe surface depends on the time and operating conditions. During aging, polymer chains deteriorate, leading to a weakening of the structure and increased amorphousness of the plastics. As a result, the surfaces of PE and PVC crack and peel, resulting in the formation of particles with sizes corresponding to NP and MP with high potential for release into water. The magnitude of the phenomenon increases as the diameter of the pipes decreases, indicating that the most vulnerable customers are those at the ends of the network to which drinking water is supplied through small-diameter pipes. Aging PE and PVC pipes should be considered a real and very important source of MPs and NPs in drinking water, and water quality in this aspect should be monitored by manufacturers.

Real-time analysis and prediction method of ion concentration using the effect of O-H stretching bands in aqueous solutions based on ATR-FTIR spectroscopy

Analyzing the concentration of ions in aqueous solutions in real-time plays an important role in the fields of chemistry and biology. Traditional methods for measuring ion concentrations, such as concentration analysis by measuring electrical conductivity, inductively coupled plasma mass spectrometry, and ion chromatography, have been used in many research fields. However, these methods are limited in determining ion concentrations instantaneously. Fourier-transform infrared-attenuated total reflectance (ATR-FTIR) spectroscopy provides a new approach for determining ion concentrations in aqueous solutions. This allows for fast analysis without pretreatment and is scalable for real-time measurements. In this study, we present a method for measuring ion concentrations by examining ion-water interactions in the O-H stretching band of aqueous solutions using ATR-FTIR spectroscopy. Five aqueous solutions, namely LiCl + HCl, LiOH + HCl, LiOH, Li3PO4, and NaCl were used in the experiments and prepared at concentrations between 0.5-2 M. The ion concentrations in the prepared aqueous solutions were measured using ATR-FTIR spectroscopy. We observed that the difference in absorbance increased and decreased linearly with changes in concentration. The concentration of ions in the aqueous solution could be measured by validating the designed linear regression analysis function model. In this study, we proposed five linear regression analysis function models, all of which showed high coefficients of determination above 0.9, with the highest coefficient of determination reaching 0.9969. These results show that ATR-FTIR spectroscopy has the potential to be applied as a rapid and simple concentration analysis system.

Transfer of Sodium Ion across Interface between Na+-Selective Electrode Membrane and Aqueous Electrolyte Solution: Can We Use Nernst Equation If Current Flows through Electrode?

Electrochemical impedance and chronopotentiometric measurements with Na+-selective solvent polymeric (PVC) membranes containing a neutral ionophore and a cation exchanger revealed low-frequency resistance, which is ascribed to Na+ ion transfer across the interface between the membrane and aqueous solution. The attribution is based on the observed regular dependence of this resistance on the concentration of Na+ in solutions. The respective values of the exchange current densities were found to be significantly larger than the currents flowing through ion-selective electrodes (ISEs) during an analysis in non-zero-current mode. This fact suggests that the interfacial electrochemical equilibrium is not violated by the current flow and implies that the Nernst equation can be applied to interpret the data obtained in non-zero-current mode, e.g., constant potential coulometry.

Electrochemical Visualization of an Ion-Selective Membrane Using a Carbon Nanoelectrode

Molecular and physical probes have been widely employed to investigate physicochemical properties and mechanisms of interfaces due to their ability to provide accurate measurements with temporal and spatial resolution. However, the direct measurement of electroactive species diffusion in ion-selective electrode (ISE) membranes and quantification of the water layer have been challenging due to the high impedance and optical opacity of polymer membranes. In the present work, carbon nanoelectrodes with ultrathin insulating encapsulation and good geometrical structure are reported as physical probes for direct electrochemical measurement of the water layer. The scanning electrochemical microscopy experiment exhibits positive feedback at the interface of the fresh ISE, and negative feedback after conditioning for 3 h. The thickness of the water layer was estimated to be ca. 13 nm. For the first time, we provide direct evidence that, during conditioning, the water molecules diffuse through the chloride ion selective membrane (Cl-ISM) until a water layer establishes at almost 3 h. Furthermore, the diffusion coefficient and concentration of oxygen molecules in the Cl-ISM are also directly electrochemical measured by introducing ferrocene (Fc) as a redox molecule probe. The oxygen concentration in the Cl-ISM decreases during conditioning, suggesting the diffusion of oxygen from ISM to the water layer. The proposed method can be used for the electrochemical measurement of solid contact, providing theoretical guidance and advice for the performance optimization of ISEs.

Advances in ATR-FTIR Spectroscopic Imaging for the Analysis of Tablet Dissolution and Drug Release

One of the major challenges in the development of effective pharmaceutical formulations for oral administration is the poor solubility of active pharmaceutical ingredients. For this reason, the dissolution process and drug release from solid oral dosage forms, such as tablets, is usually thoroughly studied in order to understand the dissolution behaviour under various conditions and optimize the formulation accordingly. Standard dissolution tests used in the pharmaceutical industry provide information on the amount of drug released over time; however, these do not allow for a detailed analysis of the underlying chemical and physical mechanisms of tablet dissolution. FTIR spectroscopic imaging, by contrast, does offer the ability to study these processes with high spatial and chemical specificity. As such, the method allows us to see the chemical and physical processes which occur inside the tablet as it dissolves. In this review, the power of ATR-FTIR spectroscopic imaging is demonstrated by presenting a number of successful applications of this chemical imaging technique to dissolution and drug release studies for a range of different pharmaceutical formulations and study conditions. Understanding these processes is essential for the development of effective oral dosage forms and optimization of pharmaceutical formulations.

In Situ Time-Dependent Attenuated Total Reflection Fourier Transform Infrared (ATR FT-IR) Spectroscopy of a Powdered Specimen in a Controlled Atmosphere: Monitoring Sorption and Desorption of Water Vapor

Attenuated total reflection Fourier transform infrared (ATR FT-IR) spectroscopy is a powerful instrumental method of chemical analysis of solids and liquids. The majority of published studies by in situ ATR FT-IR spectroscopy describe analysis of homogeneous samples, such as liquid solutions under circulation, or films on the ATR crystal that react with the gas of interest. The in situ ATR FT-IR spectroscopic studies of specimens in physical shape of crystals or powder that react with a gas or vapor are rare. This work describes a modification of in situ time-dependent ATR FT-IR spectroscopy to allow monitoring heterogeneous reaction "solid-gas" of powder in controlled atmosphere and in the time domain. Also, we describe a new facile gas flow chamber attachment to ATR FT-IR spectrometer which allows creating controlled atmosphere surrounding the specimen on the ATR crystal. Additionally, the capabilities of the described in situ time-dependent ATR FT-IR spectroscopy experiment in controlled atmosphere are enhanced by the sensor for in situ time-dependent monitoring the relative humidity (RH) of air surrounding the specimen. The operation of the setup for in situ time-dependent ATR FT-IR spectroscopy in controlled atmosphere is demonstrated by monitoring reaction of gradual desorption of water vapor from color-indicating molecular sieves under controlled low air humidity. Further, the described spectroscopic method and apparatus is applied to monitor the reverse process, namely sorption of water vapor by color-indicating molecular sieves under controlled elevated air humidity. Water molecules are found to reversibly interact with two distinct sorption sites in the sorbent: the Si-O backbone and the color-indicating Co(II) centers. The reported variant of in situ time-dependent ATR FT-IR spectroscopy in controlled atmosphere is powerful, yet facile and straightforward. It is promising for mechanistic, in situ studies of sorption, desorption, chemosensing, heterogeneous catalysis and photocatalysis, and analysis of chemical kinetics of various "solid-gas" reactions.

Voltammetric Ion Sensing with Ionophore-Based Ion-Selective Electrodes Containing Internal Aqueous Solution, Improving Lifetime of Sensors

The possibility of voltammetric ion sensing is demonstrated, for the first time, for ion-selective electrodes (ISEs) containing an internal aqueous solution. ISEs selective to calcium, lithium and potassium ions are used as model systems. The internal solution of the ISEs contains a chloride salt of the respective cation and a ferrocenemethanol or ferrocyanide/ferricyanide redox couple. A platinum wire is used as the internal reference electrode. It is shown, theoretically and experimentally, that the dependence of oxidation and reduction peak potentials on the sample composition obeys the Nernst law, while the peak currents virtually do not depend on the sample composition. Thus, the electrode behavior is similar to that reported by Bakker's group for solid contact ISEs with ultra-thin membranes (200-300 nm). It is shown that the use of classical ISEs with relatively thick membranes (100-300 µm) and internal aqueous solution allows for the sensor lifetime of about one month. It is also shown that use of a suitable background electrolyte allows for improvement of the detection limits in voltammetric measurements with ISEs.

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.

Constant Potential Coulometric Measurements with Ca2+-Selective Electrode: Analysis Using Calibration Plot vs. Analysis Using the Charge Curve Fitting

The possibility of analysis using charge curve fitting in constant potential coulometric mode instead of using a calibration plot is explored, for the first time. The results are compared with the analysis based on the use of a calibration plot. A Ca²⁺ ion-selective electrode, with and without an electronic capacitor in series, is used as a model system in pure solutions of CaCl₂. Both techniques delivered good results (error within 2%) when the final and the initial concentration values differed by not more than three times. Larger differences result in 10-25% error. The presence of an electronic capacitor in the measurement circuit and in series with the electrode, allows for significantly faster response.

The Origin of the Non-Constancy of the Bulk Resistance of Ion-Selective Electrode Membranes within the Nernstian Response Range

The dependence of the bulk resistance of membranes of ionophore-based ion-selective electrodes (ISEs) on the composition of mixed electrolyte solutions, within the range of the Nernstian potentiometric response, is studied by chronopotentiometric and impedance measurements. In parallel to the resistance, water uptake by the membranes is also studied gravimetrically. The similarity of the respective curves is registered and explained in terms of heterogeneity of the membranes due to the presence of dispersed aqueous phase (water droplets). It is concluded that the electrochemical equilibrium is established between aqueous solution and the continuous organic phase, while the resistance refers to the membrane as whole, and water droplets hamper the charge transfer across the membranes. In this way, it is explained why the membrane bulk resistance is not constant within the range of the Nernstian potentiometric response of ISEs.