Aleksandar Radu

Diagnostic of functionality of polymer membrane – based ion selective electrodes by impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) is a powerful tool for the analysis of various electrochemical systems because it allows the separation and characterization of individual kinetic processes. In this paper we investigate whether changes in the EIS characteristics can be used to distinguish between solid-state ISE membrane that have been subjected to physical damage, biofouling or leaching of active components. We conclude that with these relatively simple electronic measurements, we can effectively evaluate the functionality of the ISE membrane; i.e. we can predict whether the sensors are fully functional, in need for calibration or are completely non-functional. We believe this could form the basis of a simple but effective diagnostic tool for probing the condition of remotely deployed ISEs in widely distributed chemo-sensor networks (e.g. for environmental monitoring) and for enhancing the reliability of these devices. Our ultimate goal is to implement such tools in place of conventional approaches to ISE testing like calibration with standard solutions, which require the integration of complex and costly fluidics.

Ion selective electrodes in environmental analysis

An overview is given dealing with the application of ion-selective electrodes (ISEs) in environmental analysis. ISEs are placed into the context of the trend of development of sensors for extensive and frequent monitoring. Discussed are the issues such as sensing platforms and their mass-production, improvement of precision, diagnostic of sensor functionality, and development of reference electrodes. Several examples of real-life application of ISEs in environmental analysis are given. The main emphasis of this article is directed towards summarizing recent results of the authors during the past several years.

Wireless Ion-Selective Electrode Autonomous Sensing System

A paradigm shift in sensing methods and principles is required to meet the legislative demands for detecting hazardous substances in the molecular world. This will encompass the development of new sensing technologies capable of performing very selective and sensitive measurements at an acceptable cost, developed by multidisciplinary teams of chemists, engineers and computer scientists to harvest information from a multitude of molecular targets in health, food and within the environment. In this study we present the successful implementation of a low-cost, wireless chemical sensing system that employs a minimum set of components for effective operation. Specifically, our efforts resulted in a wireless, tri-electrode, ISE pH sensor for use in environmental monitoring. Sensor calibration and validated in situ field trials have been carried out and are presented in this paper.

Robust and Ultrasensitive Polymer Membrane-Based Carbonate-Selective Electrodes.

Quantitative analysis of the carbonate species within clinical and environmental samples is highly critical to the advancement of accurate environmental monitoring, disease screening, and personalized medicine. Herein we report the first example of carbonate detection using ultrasensitive ion selective electrodes (ISEs). The low detection limit (LDL) of these electrodes was at least 4 orders of magnitude lower than the best currently existing carbonate sensors. This was achieved by a simple alteration of the sensors conditioning protocol. This resulted in the reduction of ion fluxes across the membrane interface consequently lowering the LDL to picomolar levels. The proposed ISEs exhibited near-Nernstian potentiometric responses to carbonate ions with a detection limit of 80 pmol L(-1) (5 ppt) and was utilized for direct determination of carbonate in seawater. Moreover, the new methodology has produced electrodes with excellent reproducibility, robustness, and durability. It is anticipated that this approach may form the basis for the development of highly sensitive and robust ion selective electrodes capable of in situ measurements.

Low cost, calibration-free sensors for in situ determination of natural water pollution

One of the critical challenges for analytical sciences is the ability to perform reliable environmental monitoring at remote locations while significantly reducing per-sample and per-measurement cost. In this work, we demonstrate an approach that demonstrates production of ultra-sensitive sensors with almost identical response characteristics on a mass-scale, their integration with low-cost electronics for wireless data transmission and the use of such devices in environmental analysis

Influence of Ionic Liquids on the Selectivity of Ion Exchange-Based Polymer Membrane Sensing Layers

The applicability of ion exchange membranes is mainly defined by their permselectivity towards specific ions. For instance, the needed selectivity can be sought by modifying some of the components required for the preparation of such membranes. In this study, a new class of materials –trihexyl(tetradecyl)phosphonium based ionic liquids (ILs) were used to modify the properties of ion exchange membranes. We determined selectivity coefficients for iodide as model ion utilizing six phosphonium-based ILs and compared the selectivity with two classical plasticizers. The dielectric properties of membranes plasticized with ionic liquids and their response characteristics towards ten different anions were investigated using potentiometric and impedance measurements. In this large set of data, deviations of obtained selectivity coefficients from the well-established Hofmeister series were observed on many occasions thus indicating a multitude of applications for these ion-exchanging systems.

Chapter 2 Ion-selective electrodes in trace level analysis of heavy metals: Potentiometry for the XXI century

Publisher Summary Potentiometric sensors (ion-selective electrodes [ISEs]) are able to directly determine the activity of the ion of interest in the sample. ISEs are the method of choice for clinical analysis of ions of clinical relevance such as K+, Na+, Ca2+, Mg2+, and Cl–. In recent years, the field has undergone a renaissance when determination of selectivity coefficients and Pb2+-selective electrode with limit of detection in picomolar range are reported. A very important direction in the development of ISEs is their miniaturization. While microelectrodes have been known for a long time, there is only one report so far describing miniature ISEs with low limit of detection. Because of the relative simplicity of construction and optimization, solid-contact polymer-based membrane ISEs are very promising platforms for this research direction. Low-cost construction and simple data acquisition facilitate the integration of such platforms into sensor arrays and further into sensor communities based on networks of simple and accurate devices for widely distributed monitoring of—for example, water quality. In addition, microelectrodes can be used for spatial mapping of low chemical concentrations (e.g., in chemical microscopy or the study of ion uptake by the plant roots. Moreover, in contrast to most other analytical techniques, potentiometric sensors ideally do not perturb the sample, and such microelectrodes can therefore be used in very small sample volumes allowing determination of extremely small total quantities.

Circumventing Traditional Conditioning Protocols in Polymer Membrane-Based Ion-Selective Electrodes

Preparation of ion-selective electrodes (ISEs) often requires long and complicated conditioning protocols limiting their application as tools for in-field measurements. Herein, we eliminated the need for electrode conditioning by loading the membrane cocktail directly with primary ion solution. This proof of concept experiment was performed with iodide, silver, and sodium selective electrodes. The proposed methodology significantly shortened the preparation time of ISEs, yielding functional electrodes with submicromolar detection limits. Moreover, it is anticipated that this approach may form the basis for the development of miniaturized all-solid-state ion-selective electrodes for in situ measurements.

Single strip solid contact ion selective electrodes on a pencil-drawn electrode substrate

A simple and low-cost approach for the preparation of ion-selective electrodes (ISEs) is proposed as a favorable alternative to traditional paper-based electrodes. This involved the application of graphite from a simple household pencil via mechanical abrasion onto a modified acetate sheet. The resulting electrodes exhibited excellent sensing properties towards all tested ions, including a wide dynamic response range, fast response time and satisfactory long-term stability. The same methodology was used to produce stable and functional reference electrodes. These electrodes were then combined with other graphite-based ISEs to yield single strip solid-contact electrodes for simultaneous detection of cations and anions in aqueous solutions. The described approach, which is analogous to simply drawing on paper, opens new avenues for the development of sensing devices using very cheap and easily accessible components.

Photo-switchable surfaces: a new approach to chemical sensing

Here, we discuss an interesting concept that brings an added flexibility in chemo/bio sensing. We present system that can be switched photonically between two states, only one of which exhibits ion-binding behaviour. The system is based on molecular photoswitch spiropyran, which is probably the most studied compound exhibiting above characteristics. Upon irradiation with UV light the passive spiropyran (SP) molecule undergoes a heterocyclic ring cleavage that results with the formation of the merocyanine (MC) which is zwitterionic form capable of ion binding. In contrast to the uncharged and colourless spiropyran form, the merocyanine form is highly charged and can be utilized as ligand for other charged species. Moreover, it is strongly coloured, and the colour tells us which form is present. In addition it provides interesting information about the immediate environment of the merocyanine binding site (e.g. polarity, presence of certain ions etc.). In this work, we present a SP-based system in which SP is immobilized and protected within a polymeric matrix. Such system may be used for detection of metal ions in highly polar solvents, e.g. water. The response characteristics and kinetics of MC-Cr3+ complex formation and SP-MC switching within the polymer matrix have been determined. Simple light emitting diodes (LEDs) have been employed for photoswitching and colorimetric measurement of SP-MC switching and MC-Cr3+ complex formation as light sources and detectors.