Real-time monitoring of the dissolution of silver nanoparticles by using a solid-contact Ag+-selective electrode

Single-Frequency Effective Capacitance C ec and Membrane Resistance Z Readout for Solid-Contact Ion-Selective Electrodes

Here, we propose new single-frequency effective capacitance C ec and membrane resistance Z readout principle for solid-contact ion-selective electrodes (SCISEs). Conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) doped with polystyrenesulfonate (PSS-), i.e., PEDOT(PSS), as solid contact and valinomycin-based membrane were prepared for K+-SCISEs. At high frequencies, the membrane resistance of K+-SCISEs corresponding to impedance absolute value Z was recorded constantly as KCl aqueous solution diluted with water. The membrane resistance Z increases as the electrolyte concentration decreases. Under identical dilution steps, the linear slope of the logarithmic membrane resistance logZ vs loga K+ for K+-SCISEs with the spin-coated membrane is larger than that of the electrode covered with the drop-cast membrane. As the K+-SCISE resistance with the spin-coated membrane was reduced to hundreds of Ω, the logZ of K+-SCISEs is linearly proportional to loga K+ in the range of -1 to -3.4, providing a possibility of utilizing membrane resistance Z as a calibration-free analytical signal for SCISEs. The effective capacitance C ec of K+-SCISEs with the spin-coated membrane was performed in 0.1 M KCl applied with single frequency ranging from 1 MHz and decreases by a factor of 10 to 10 mHz. The obtained C ec of K+-SCISEs with the spin-coated membrane is linearly proportional to logfin the range of 1 MHz to 10 Hz with a slope of ca. -0.97, while at a low frequency ranging from 1 Hz to 10 mHz, the linear slope of logC ec vs logf is suppressed, where Warburg diffusion takes effect. Furthermore, the membrane resistance Z is independent of applied high frequencies, and the effective capacitance C ec is independent of the excitation amplitude.

Unveiling the fast adsorption and desorption of heavy metals on/off nanoplastics by real-time in-situ potentiometric sensing

Nanoplastics (<1 μm) can serve as a transport vector of environmental pollutants (e.g., heavy metals) and change their toxicities and bioavailabilities. Up to date the behaviors of adsorption and desorption heavy metals on/off nanoplastics are largely unknown. Herein, polymeric membrane potentiometric ion sensors are proposed for in-situ assessment of the real-time kinetics of heavy metal adsorption and desorption on/off nanoplastics. Results show that nanoplastics can adsorb and release heavy metals in a fast manner, indicating their superior ability in transferring heavy metals. The adsorption behaviors are closely related to the characteristics of nanoplastics and background electrolytes. Particle aggregation and increases in salinity and acidity suppress the adsorption of heavy metals on nanoplastics. The desorption efficiencies of different heavy metals are Pb2+ (31 %) < Cu2+ (40 %) < Cd2+ (97 %). Our proposed method is applicable for the detection of the plastic pollutants with size <100 nm and of the samples with high salinities (e.g., seawater). This work would provide new insights into the assessment of environmental risks posed by nanoplastics and heavy metals.

Recent advances in nanomaterial-based solid-contact ion-selective electrodes

Solid-contact ion-selective electrodes (SC-ISEs) are advanced potentiometric sensors with great capability to detect a wide range of ions for the monitoring of industrial processes and environmental pollutants, as well as the determination of electrolytes for clinical analysis. Over the past decades, the innovative design of ion-selective electrodes (ISEs), specifically SC-ISEs, to improve potential stability and miniaturization for in situ/real-time analysis, has attracted considerable interest. Recently, the utilisation of nanomaterials was particularly prominent in SC-ISEs due to their excellent physical and chemical properties. In this article, we review the recent applications of various types of nanostructured materials that are composed of carbon, metals and polymers for the development of SC-ISEs. The challenges and opportunities in this field, along with the prospects for future applications of nanomaterials in SC-ISEs are also discussed.

Polymeric membrane potentiometric antibiotic sensors using computer-aided screening of supramolecular macrocyclic carriers

Carrier-based polymeric membrane potentiometric sensors are an ideal tool for detecting ionic species. However, in the fabrication of these sensors, the screening of carriers still relies on empirical trial- and error-based optimization, which requires tedious and time-consuming experimental verification. In this work, computer-aided screening of carriers is applied in the preparation of polymeric membrane potentiometric sensors. Molecular docking is used to study the host-guest interactions between receptors and targets. Binding energies are employed as the standard to screen the appropriate carrier. As a proof-of-concept experiment, the antibiotic ciprofloxacin is selected as the target model. A series of supramolecular macrocyclic receptors including cyclodextrins, cucurbiturils and calixarenes are chosen as potential receptors. The proposed sensor based on the receptor calix[4]arene screened by molecular docking shows a lower detection limit of 0.5 μmol L-1 for ciprofloxacin. It can be expected that the proposed computer-aided screening technique of carriers can provide a simple but highly efficient method for the fabrication of carrier-based electrochemical and optical sensors.

Design Criteria for Nanostructured Carbon Materials as Solid Contacts for Ion-Selective Sensors

The ability to miniaturize ion-selective sensors that enable microsensor arrays and wearable sensor patches for ion detection in environmental or biological samples requires all-solid-state sensors with solid contacts for transduction of an ion activity into an electrical signal. Nanostructured carbon materials function as effective solid contacts for this purpose. They can also contribute to improved potential signal stability, reducing the need for frequent sensor calibration. In this Perspective, the structural features of various carbon-based solid contacts described in the literature and their respective abilities to reduce potential drift during long-term, continuous measurements are compared. These carbon materials include nanoporous carbons with various architectures, carbon nanotubes, carbon black, graphene, and graphite-based solid contacts. The effects of accessibility of ionophores, ionic sites, and other components of an ion-selective membrane to the internal or external carbon surfaces are discussed, because this impacts double-layer capacitance and potential drift. The effects of carbon composition on water-layer formation are also considered, which is another contributor to potential drift during long-term measurements. Recommendations regarding the selection of solid contacts and considerations for their characterization and testing in solid-contact ion-selective electrodes are provided.

Near-Infrared Laser Irradiation-Modulated High-Temperature Solid-Contact Ion-Selective Electrodes: Potentiometric Detection of Ca2+ in Seawater

The high-temperature potentiometry operated by nonisothermal heating is a promising way to break through the traditional potentiometric responses of ion-selective electrodes (ISEs) at room temperature. Herein, a locally heated strategy through near-infrared region (NIR) laser irradiation upon the photothermal mesoporous carbon material placed between the ion-selective membrane and the glassy carbon substrate is introduced to obtain the high-temperature potentiometric performance of a solid-contact Ca2+-ISE for detection of Ca2+ in seawater. Based on the light-to-heat conversion of the mesoporous carbon-based solid contact, the temperature of the solid-contact Ca2+-ISE upon continuous NIR laser irradiation can be increased from room temperature to 60-70 °C, and the slope of the electrode is promoted up to about 30% according to the thermodynamic steady-state potentiometric response. The pulsed potentiometric response of the solid-contact Ca2+-ISE upon a pulsed NIR laser irradiation of 5 s also shows a linear change as a function of Ca2+ activities, and the improved slope from 27.1 ± 0.6 to 38.1 ± 0.9 mV/dec can be obtained under dual control of the temperature of the electrode and the transient current induced by the pulsed NIR laser irradiation. As compared to the traditional potentiometric measurement under zero-current conditions at room temperature, the NIR laser-modulated high-temperature potentiometric response provides an alternative way for measurement of the solid-contact ISEs.

Ion-Selective Electrodes with Solid Contact Based on Composite Materials: A Review

Potentiometric sensors are the largest and most commonly used group of electrochemical sensors. Among them, ion-selective electrodes hold a prominent place. Since the end of the last century, their re-development has been observed, which is a consequence of the introduction of solid contact constructions, i.e., electrodes without an internal electrolyte solution. Research carried out in the field of potentiometric sensors primarily focuses on developing new variants of solid contact in order to obtain devices with better analytical parameters, and at the same time cheaper and easier to use, which has been made possible thanks to the achievements of material engineering. This paper presents an overview of new materials used as a solid contact in ion-selective electrodes over the past several years. These are primarily composite and hybrid materials that are a combination of carbon nanomaterials and polymers, as well as those obtained from carbon and polymer nanomaterials in combination with others, such as metal nanoparticles, metal oxides, ionic liquids and many others. Composite materials often have better mechanical, thermal, electrical, optical and chemical properties than the original components. With regard to their use in the construction of ion-selective electrodes, it is particularly important to increase the capacitance and surface area of the material, which makes them more effective in the process of charge transfer between the polymer membrane and the substrate material. This allows to obtain sensors with better analytical and operational parameters. Brief characteristics of electrodes with solid contact, their advantages and disadvantages, as well as research methods used to assess their parameters and analytical usefulness were presented. The work was divided into chapters according to the type of composite material, while the data in the table were arranged according to the type of ion. Selected basic analytical parameters of the obtained electrodes have been collected and summarized in order to better illustrate and compare the achievements that have been described till now in this field of analytical chemistry, which is potentiometry. This comprehensive review is a compendium of knowledge in the research area of functional composite materials and state-of-the-art SC-ISE construction technologies.

Chloride Ion-Selective Electrode with Solid-Contact Based on Polyaniline Nanofibers and Multiwalled Carbon Nanotubes Nanocomposite

Use of the nanocomposite of chloride-doped polyaniline nanofibers and multiwalled carbon nanotubes (PANINFs-Cl:MWCNTs) for construction of ion-selective electrodes with solid-contact sensitive to chloride ions has been described. Many types of electrodes were tested, differing in the quantitative and qualitative composition of the layer placed between the electrode material and the ion-selective membrane. Initial tests were carried out, including tests of electrical properties of intermediate solid-contact layers. The obtained ion-selective electrodes had a theoretical slope of the electrode characteristic curve (-61.3 mV dec^-1), a wide range of linearity (5 × 10^-6-1 × 10^-1 mol L^-1) and good potential stability resistant to changing measurement conditions (redox potential, light, oxygen). The chloride contents in the tap, mineral and river water samples were successfully determined using the electrodes.

Indirect Nano-sensing approach: A universal potentiometric silver ion selective sensor for inline quantitative profiling of the kinetics and thermodynamics of formation and decay of silver nanoparticles

Indirect Nano-sensing are indispensable chemical sensory points that make use of the unique properties of nanoparticles to derive information about it to our macroscopic world. Precious Silver nanoparticles have become more attractive in many areas of healthcare and life sciences leading to massive industrial production and increase of environmental exposure which may lead to Nanotoxicity accompanied by the release of Ag⁺ ions. A reversible silver selective screen-printed electrode was fabricated, optimized, and validated. A wide linearity range of 1 × 10^-6 - 1 × 10^-2 M was obtained, with a LOD that reaches 1.5 × 10^-7 M and a typical slope of monovalent cationic compounds of 59.6 mV/decade. It showed high selectivity towards the cationic Ag⁺ ion activity in presence of the negatively charged citrate capped silver nanoparticles (Cit-AgNPs). The fabricated sensor has been used for tracking the decrease of Ag⁺ activity during the reduction of AgNO₃ with tri-sodium citrate during the Bottom-up synthesis of Cit-AgNPs at different temperature (60, 70 and 80 °C). The kinetic parameters (Activation energy (E_a) and Reaction rate (K)) and the thermodynamic characteristics (free activation energy (ΔG), entropy (ΔS), enthalpy (ΔH)) have been calculated. Furthermore, it has been used for tracking the release of Ag+ during the spontaneous and stimulated decay of Cit-AgNPs. The present work could be a junction between nanotechnology and recent advances in design of a reproducible, portable real-time analyzer for in-process monitoring of the production of Cit-AgNPs and its environmental hazards with many advantages in comparison to the reported techniques in terms of portability, simplicity, cost-efficient, fast inline tracking, no sampling, real-time profiles at high temperatures and it does not need professional operators.

Fabrication of an electrochemical sensor based on metal-organic framework ZIF-8 for quantitation of silver ions: optimizing experimental conditions using central composite design (CCD)

ZIF-8 was synthesized and carbon paste electrodes (CPEs) modified with this metal-organic framework were utilized for quantitation of silver(i) by the differential pulse anodic stripping voltammetry (DPASV) technique.Prepared ZIF-8 and the matrix of the electrodes were distinguished by impedance spectroscopy (EIS), XRD, FT-IR spectroscopy, cyclic voltammetry (CV), TEM and SEM/EDX methods. To obtain the strongest stripping peak currents, several significant variables were optimized with response surface methodology (RSM), including the ligand amount (near 11% w/w), applied potential for preconcentration (approximately -1.36 V), pH of the preconcentration solution (about 8.5) and preconcentration time (about 275 s). A calibration curve was acquired in the limits from 1.0 × 10-10 to 5.0 × 10-7 M with the Pearson correlation coefficient R = 0.9993. The limiting detectable concentration (LDC) was determined to be 1.0 × 10-11 M. The developed sensor has high selectivity for mercury(ii). The excellent pH, potential and especially size-exclusion based selectivity of the prepared sensor are unique characteristics that are very important in the determination of silver ions. The developed method was effectively employed for the quantitation of silver(i) ions in environmental and industrial samples.

Recent advances in solid-contact ion-selective electrodes: functional materials, transduction mechanisms, and development trends

This article presents a comprehensive overview of recent progress in the design and applications of solid-contact ion-selective electrodes (SC-ISEs).