Klára Tóth

Electrochemical biosensors: recommended definitions and classification

Two Divisions of the International Union of Pure and Applied Chemistry (IUPAC), namely Physical Chemistry (Commission 1.7 on Biophysical Chemistry formerly Steering Committee on Biophysical Chemistry) and Analytical Chemistry (Commission V.5 on Electroanalytical Chemistry) have prepared recommendations on the definition, classification and nomenclature related to electrochemical biosensors: these recommendations could, in the future, be extended to other types of biosensors. An electrochemical biosensor is a self-contained integrated device, which is capable of providing specific quantitative or semi-quantitative analytical information using a biological recognition element (biochemical receptor) which is retained in direct spatial contact with an electrochemical transduction element. Because of their ability to be repeatedly calibrated, we recommend that a biosensor should be clearly distinguished from a bioanalytical system, which requires additional processing steps, such as reagent addition. A device that is both disposable after one measurement, i.e. single use, and unable to monitor the analyte concentration continuously or after rapid and reproducible regeneration, should be designated a single use biosensor. Biosensors may be classified according to the biological specificity-conferring mechanism or, alternatively, to the mode of physico-chemical signal transduction. The biological recognition element may be based on a chemical reaction catalysed by, or on an equilibrium reaction with macromolecules that have been isolated, engineered or present in their original biological environment. In the latter cases. equilibrium is generally reached and there is no further, if any, net consumption of analyte(s) by the immobilized biocomplexing agent incorporated into the sensor. Biosensors may be further classified according to the analytes or reactions that they monitor: direct monitoring of analyte concentration or of reactions producing or consuming such analytes; alternatively, an indirect monitoring of inhibitor or activator of the biological recognition element (biochemical receptor) may be achieved. A rapid proliferation of biosensors and their diversity has led to a lack of rigour in defining their performance criteria. Although each biosensor can only truly be evaluated for a particular application, it is still useful to examine how standard protocols for performance criteria may be defined in accordance with standard IUPAC protocols or definitions. These criteria are recommended for authors. referees and educators and include calibration characteristics (sensitivity, operational and linear concentration range, detection and quantitative determination limits), selectivity, steady-state and transient response times, sample throughput, reproducibility, stability and lifetime.

Electrochemical Biosensors: Recommended Definitions and Classification

Two Divisions of the International Union of Pure and Applied Chemistry (IUPAC), namely Physical Chemistry (Commission I.7 on Biophysical Chemistry formerly Steering Committee on Biophysical Chemistry) and Analytical Chemistry (Commission V.5 on Electroanalytical Chemistry) have prepared recommendations on the definition, classification and nomenclature related to electrochemical biosensors; these recommendations could, in the future, be extended to other types of biosensors. An electrochemical biosensor is a self-contained integrated device, which is capable of providing specific quantitative or semi-quantitative analytical information using a biological recognition element (biochemical receptor) which is retained in direct spatial contact with an electrochemical transduction element. Because of their ability to be repeatedly calibrated, we recommend that a biosensor should be clearly distinguished from a bioanalytical system, which requires additional processing steps, such as reagent addition. A device which is both disposable after one measurement, i.e., single use, and unable to monitor the analyte concentration continuously or after rapid and reproducible regeneration should be designated a single use biosensor. Biosensors may be classified according to the biological specificity-conferring mechanism or, alternatively, to the mode of physico-chemical signal transduction. The biological recognition element may be based on a chemical reaction catalysed by, or on an equilibrium reaction with macromolecules that have been isolated, engineered or present in their original biological environment. In the latter cases, equilibrium is generally reached and there is no further, if any, net consumption of analyte(s) by the immobilized biocomplexing agent incorporated into the sensor. Biosensors may be further classified according to the analytes or reactions that they monitor: direct monitoring of analyte concentration or of reactions producing or consuming such analytes; alternatively, an indirect monitoring of inhibitor or activator of the biological recognition element (biochemical receptor) may be achieved. A rapid proliferation of biosensors and their diversity has led to a lack of rigour in defining their performance criteria. Although each biosensor can only truly be evaluated for a particular application, it is still useful to examine how standard protocols for performance criteria may be defined in accordance with standard IUPAC protocols or definitions. These criteria are recommended for authors, referees and educators and include calibration characteristics (sensitivity, operational and linear concentration range, detection and quantitative determination limits), selectivity, steady-state and transient response times, sample throughput, reproducibility, stability and lifetime.

ELECTROCHEMICAL BIOSENSORS: RECOMMENDED DEFINITIONS AND CLASSIFICATION*

*A special report on the International Union of Pure and Applied Chemistry, Physical Chemistry Division, Commission I.7 (Biophysical Chemistry), Analytical Chemistry Division, Commission V.5 (Electroanalytical Chemistry).

Selectivity of ion-specific membrane electrodes

Abstract A theoretical interpretation is given for the selectivity constants of ionselective electrodes. The validity of the theory is proved for halide-selective electrodes. The high selectivity of the halide electrodes to individual halide ions offers many applications of these electrodes in analytical chemistry.

Injection techniques in dynamic flow-through analysis with electroanalytical sensors

Abstract The theoretical and practical aspects of injection techniques used in flow-through systems with ion-selective electrodes and voltammetric detectors are discussed. Mathematical descriptions of the measuring systems based on the use of mixing chambers in the analytical channel are briefly outlined. The effects of different parameters on the analytical signals are described theoretically and experimentally. The methods developed for the evaluation of the analytical signals are presented in detail.

Ion-transport and diffusion coefficients of non-plasticised methacrylic–acrylic ion-selective membranes

The ion-transport behaviour of methacrylic-acrylic-based polymers for ion-selective electrode (ISE) membranes was investigated by a spectrophotometric method to determine the apparent diffusion coefficient. By observing the degree of deprotonation of the chromoionophore or chromogenic ionophore, the extent of penetration of cations into the polymer films was determined. The transport of the cations into the optode films depended on the stoichiometry of complexation by the ionophores. The apparent diffusion coefficients, estimated from the deprotonation data were of the order of 10(-12) to 10(-11)cm(2)s(-1). These values indicate that the apparent ion mobility in the methacrylic-acrylic ISE membranes is approximately a thousand times lower than that in plasticised PVC ISE membranes. For some ionophores, the value of the apparent diffusion coefficient could be modulated according to the ionophore content in the membrane and the data obtained for a calixarene containing membrane were tested against a model for facilitated diffusion with chained carriers. The data did not fit a model where intramolecular diffusion was limiting, but were consistent with a first-order rate-limiting mechanism involving an intermediate 1:2 complex between ion and ionophore. In this instance, the lowest values for D(app) were thus not necessarily obtained for lowest ionophore loading and in the range examined, a trend of decreasing D(app) with increasing ionophore was noted.

Responses of H(+) selective solvent polymeric membrane electrodes fabricated from modified PVC membranes.

Potentiometric responses of a novel class of pH sensitive ionophores, namely several phenoxazine derivatives, were tested in different modified PVC matrices. The ionophores were compounded into liquid membranes as usual or were covalently coupled to the polymeric matrix. The general analytical performance of the membranes and other membrane characteristics (i.e., resistance and response time, as measures of membrane decomposition or structural changes) were followed in time. The transient responses of membranes with mobile ionophores in high molecular weight (HMW) and carboxylated PVC (PVC-COOH) were compared to those with immobilized ionophores. The response time of membranes with immobilized ionophores was found to be between those with mobile ionophores in HMW (fast response) and PVC-COOH (sluggish response). Accordingly, the rate of response was correlated primarily to the -COOH content of the membranes.

Donnan exclusion failure in low anion site density membranes containing valinomycin

Abstract The theory of Donnan Exclusion Failure of carrier-containing, low-fixed site density membranes has been derived, for high carrier loadings. The precarious character of commercial PVC samples for membrane applications, because of the low site densities is illustrated by the ease of Donnan Failure induced by bromide, nitrate, iodide, and thiocyanate. The theory is checked by a correlation between potentiometric response maxima (the onset of Failure), and independently determined salt extraction coefficients. Massive uptake of salt, under Donnan Failure conditions, was illustrated by comparing impedance spectra of KSCN with KCl as bathing electrolytes.

Transient phenomena of ion-selective membrane electrodes

Abstract A method of studying the response time of the ion-selective electrodes has been worked out. An exponential equation has been derived to describe the response of the electrodes. The potential of the silver halide electrodes reaches a constant value within a few hundred milliseconds, from which it follows that these electrodes can be used advantageously as sensors in flowing and control systems.

Novel polypyrrole based all-solid-state potassium-selective microelectrodes

Potassium-selective potentiometric microelectrodes with a polypyrrole solid internal contact were fabricated by the application of a potassium-selective bis-crown ether ligand based, plasticized poly(vinyl chloride) (PVC) membrane to the surface of conducting polymer modified Pt, Au or C micro disk electrodes. The selectivity and sensitivity of the new type of potentiometric potassium microelectrode were found to be comparable with those of the conventional macro ion-selective electrodes and of the micropipet type microelectrodes based on the same ionophore. The ease of preparation and robustness are the main advantages of this new electrode design, which can replace the classical micropipet type microelectrodes in many applications. The microelectrodes showed good dynamic characteristics and were used successfully in a wall-jet cell incorporated flow injection analysis system and as a measuring tip in scanning electrochemical microscopy. The polypyrrole modified substrate electrodes can be made selective for different ions just by changing the ionophore in the plasticized PVC membrane, which can extend their use to a wide range of applications.