Miniaturizable ion-selective arrays based on highly stable polymer membranes for biomedical applications

Novel paper-based potentiometric combined sensors using coumarin derivatives modified with vanadium pentoxide nanoparticles for the selective determination of trace levels of lead ions

Novel miniaturized Pb(II) paper-based potentiometric sensors are described using coumarin derivatives I and II as electroactive ionophores and nano vanadium pentoxide as a solid contact material for the sensitive and selective monitoring of trace lead ions. Density functional theory (DFT) confirms optimum geometries, electronic properties, and charge transfer behaviors of 1:2 Pb(II): coumarin complexes. The sensors are prepared by using two strips of 20 × 5 mm filter paper with two circular orifices. One orifice is coated with vanadium pentoxide (V2O5) nanoparticles in colloidal conductive carbon as a solid-contact, covered by a PVC membrane containing coumarin ionophore to act as a sensing probe. The other orifice is treated with Ag/AgCl in a polyvinyl butyral (PVB) film, to act as a reference electrode. Sensors with ionophores (I) and (II) exhibit Nernstian slopes of 27.7 ± 0.2 and 30.2 ± 0.2 mV/decade over the linear concentration range 4.5 × 10-7 to 6.2 × 10-3 M and 8.5 × 10-8 to 6.2 × 10-3 M, with detection limits of 1.3 × 10-7 M (26.9 ppb) and 2.1 × 10-8 M (4.4 ppb), respectively. The sensors are satisfactorily used for accurate determination of lead ions in drinking water, lead-acid battery wastewater, and electronic waste leachates. The results compare favourably well with data obtained by flameless atomic absorption spectrometry.

H+-Selective Electrodes Based on Amine-Type Ionophores: Generalized Theory and A Priori Quantification of Lower and Upper Detection Limits

A critical analysis of the known theories of functioning of H⁺-selective electrodes (H⁺-SEs) based on neutral amine-type carriers is given. A model of specific ion association is proposed, according to which, in membranes plasticized with 2-nitrophenyloctyl ether, the protonated ionophore and cation-exchanger form much stronger ion pairs with inorganic ions extracted from the sample solution than with each other, and simple equations that describe the lower and upper limit detection (pH_UDL and pH_LDL) are obtained. A feasible and reliable method for quantifying the pK_a values of ionophores in the membrane phase from potentiometric data is substantiated. The efficiency of using single-ion partition coefficients and ion pair formation constants for a priori quantitative description of the H⁺-SE response in solutions of various compositions has been demonstrated for the first time. It is shown that the width of the dynamic response range of such electrodes depends on the nature of the tertiary amino group, and the reasons for the observed effect are discussed.

Development of a Disposable Polyacrylamide Hydrogel-Based Semipermeable Membrane for Micro Ag/AgCl Reference Electrode

Currently, Ag/AgCl-based reference electrodes are used in most electrochemical biosensors and other bioelectrochemical devices. However, standard reference electrodes are rather large and do not always fit within electrochemical cells designed for the determination of analytes in low-volume aliquots. Therefore, various designs and improvements in reference electrodes are critical for the future development of electrochemical biosensors and other bioelectrochemical devices. In this study, we explain a procedure to apply common laboratory polyacrylamide hydrogel in a semipermeable junction membrane between the Ag/AgCl reference electrode and the electrochemical cell. During this research, we have created disposable, easily scalable, and reproducible membranes suitable for the design of reference electrodes. Thus, we came up with castable semipermeable membranes for reference electrodes. Performed experiments highlighted the most suitable gel formation conditions to achieve optimal porosity. Here, Cl^- ion diffusion through the designed polymeric junctions was evaluated. The designed reference electrode was also tested in a three-electrode flow system. The results show that home-built electrodes can compete with commercial products due to low reference electrode potential deviation (~3 mV), long shelf-life (up to six months), good stability, low cost, and disposability. The results show a high response rate, which makes in-house formed polyacrylamide gel junctions good membrane alternatives in the design of reference electrodes, especially for these applications where high-intensity dyes or toxic compounds are used and therefore disposable electrodes are required.

Biosensors Integration in Blood-Brain Barrier-on-a-Chip: Emerging Platform for Monitoring Neurodegenerative Diseases

Over the most recent decades, the development of new biological platforms to study disease progression and drug efficacy has been of great interest due to the high increase in the rate of neurodegenerative diseases (NDDs). Therefore, blood–brain barrier (BBB) as an organ-on-a-chip (OoC) platform to mimic brain-barrier performance could offer a deeper understanding of NDDs as well as a very valuable tool for drug permeability testing for new treatments. A very attractive improvement of BBB-oC technology is the integration of detection systems to provide continuous monitoring of biomarkers in real time and a fully automated analysis of drug permeably, rendering more efficient platforms for commercialization. In this Perspective, an overview of the main BBB-oC configurations is introduced and a critical vision of the BBB-oC platforms integrating electronic read out systems is detailed, indicating the strengths and weaknesses of current devices, proposing the great potential for biosensors integration in BBB-oC. In this direction, we name potential biomarkers to monitor the evolution of NDDs related to the BBB and/or drug cytotoxicity using biosensor technology in BBB-oC.

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.

Emulsion Doping of Ionophores and Ion-Exchangers into Ion-Selective Electrode Membranes

Ion-selective electrodes (ISEs) are widely used analytical devices to selectively measure ionic species. Despite significant advances in recent years, ion-selective membranes are still mostly prepared in the same manner, by preloading the selective components into a solvent that is subsequently cast into a membrane or film. This paper describes an alternative method to prepare ISE membranes by mass transfer of the sensing components from an emulsion phase. Specifically, blank (undoped) plasticized poly(vinyl chloride) (PVC) membranes mounted into an electrode body are immersed into an aqueous solution containing analyte ions and an appropriate emulsion of the desired sensing components to allow their transfer into the membrane. The concept is demonstrated with conventional membrane electrodes containing an inner solution as well as all-solid-state electrodes. It is shown to be universally useful for the realization of ISEs for K⁺, Na⁺, Ca²⁺, and NO₃^-.

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).

Disposable Multi-Walled Carbon Nanotubes-Based Plasticizer-Free Solid-Contact Pb2+-Selective Electrodes with a Sub-PPB Detection Limit †

Potentiometric plasticizer-free solid-contact Pb²⁺-selective electrodes based on copolymer methyl methacrylate-n-butyl acrylate (MMA-BA) as membrane matrix and multi-walled carbon nanotubes (MWCNTs) as intermediate ion-to-electron transducing layer have been developed. The disposable electrodes were prepared by drop-casting the copolymer membrane onto a layer of MWCNTs, which deposited on golden disk electrodes. The obtained electrodes exhibited a sub-ppb level detection limit of 10⁻¹⁰ mol·L⁻¹. The proposed electrodes demonstrated a Nernstian slope of 29.1 ± 0.5 mV/decade in the linear range from 2.0 × 10⁻¹⁰ to 1.5 × 10⁻³ mol·L⁻¹. No interference from gases (O₂ and CO₂) or water films was observed. The electrochemical impedance spectroscopy of the fabricated electrodes was compared to that of plasticizer-free Pb²⁺-selective electrodes without MWCNTs as intermediated layers. The plasticizer-free MWCNTs-based Pb²⁺-selective electrodes can provide a promising platform for Pb(II) detection in environmental and clinical application.