Richard A. Durst

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.

Mediator compounds for the electrochemical study of biological redox systems: a compilation

Abstract Many biological compounds exhibit irreversible redox behavior as a result of slow heterogeneous electron transfer at electrode surfaces. In order to study the electrochemical behavior of these biocomponents, redox mediators are used to facilitate the electron transfer process. In this review the characteristics of ideal mediators are discussed and structural information on previously reported mediator compounds is provided. The electrochemical literature has been extensively surveyed to provide an up-to-date compilation of mediators suitable for use in potentiometric and coulometric titrations and in various types of voltammetric studies of biological redox systems. The compilation provides information on the formal potentials of the mediators as well as their previous applications and references. This review is intended to provide a current survey of compounds having suitable redox mediation characteristics.

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).

CHEMICALLY MODIFIED ELECTRODES: RECOMMENDED TERMINOLOGY AND DEFINITIONS

Chemically modified electrodes (CMEs) comprise a relatively modern approach to electrode systems that finds utility in (1) a wide spectrum of basic electrochemical investigations, including the relationship of heterogeneous electron transfer and chemical reactivity to electrode surface chemistry, electrostatic phenomena at electrode surfaces, and electron and ionic transport phenomena in polymers, and (2) the design of electrochemical devices and systems for applications in chemical sensing, energy conversion and storage, molecular electronics, electrochromic displays, corrosion protection, and electro-organic syntheses. Compared with other electrode concepts in electrochemistry, the distinguishing feature of a CME is that a generally thin film of a selected chemical is bonded or coated onto the electrode surface to endow the electrode with the chemical, electrochemical, optical, electrical, transport, and other desirable properties of the film in a rational, chemically designed manner. In this report, we have attempted to identify and define the most widely used terminology in the growing field of CMEs and to recommend a particular term in cases where a multiplicity of terms has arisen over the past several years or where previously defined terms have taken on broadened meanings for the special cases of CMEs. It is expected that additional terms will be added to this lexicon in the future as new research directions evolve.

Liposome immunomigration field assay device for Alachlor determination

Abstract The feasibility of a simple, single-use immunomigration system has been demonstrated, using Alachlor as a model environmental contaminant. In the device, capillary action causes Alachlor and Alachlor-tagged, dye-containing liposomes to migrate through an anti-Alachlor antibody zone, on a plastic-backed nitrocellulose strip, where competitive binding occurs. Unbound liposomes continue migration to a liposome capture zone, where they are quantified either visually or by densitometry. The amount of liposome-entrapped dye that is measured in this zone is directly proportional to the Alachlor concentration in the sample.

Liposome Behavior in Capillary Electrophoresis

The behavior of liposomes in capillary electrophoresis is studied for the purpose of developing a potential method for characterizing liposomes prepared for use in industrial and analytical applications. This study characterizes the electrophoretic behavior of liposomes under various conditions to provide information about electrophoretic mobility and liposome-capillary surface interactions. The results of this method are compared with the results obtained using traditional laser light-scattering methods to obtain size information about liposome preparations. Additionally, reactions of liposomes and the surfactant n-octyl-β-d-glucopyranoside are performed off-line in bulk solution experiments and on-line in the capillary. Automated delivery of lysis agents by multiple electrokinetic injections is demonstrated as a general method for inducing on-capillary reactions between liposomes and other reagents. Furthermore, some preliminary evidence on the use of liposomes as a hydrophobic partitioning medium for analytical separations is presented.

Simultaneous detection of Escherichia coli O157:H7, Salmonella spp. and Listeria monocytogenes with an array-based immunosorbent assay using universal protein G-liposomal nanovesicles

A novel universal reagent for immunoassays, protein G-liposomal nanovesicles has been developed and successfully used in an immunomagnetic bead sandwich assay for the detection of Escherichia coli O157:H7 [C.-S. Chen, A.J. Baeumner, R.A. Durst, Talanta 67 (2005) 205]. To demonstrate the universal capability of protein G-liposomal nanovesicles, this reagent was used to develop an array-based immunosorbent assay for the simultaneous detection of E. coli O157:H7, Salmonella, and Listeria monocytogenes. Both direct and competitive immunoassay formats were used to demonstrate the feasibility of detecting multiple analytes in a single test by using universal protein G-liposomal nanovesicles. Both pure and mixed cultures were examined in the direct immunoassay format. Results indicate that the limits of detection (LODs) of the direct assay for E. coli O157:H7, Salmonella enterica serovar Typhimurium and L. monocytogenes in pure cultures were approximately 100, 500 and 1.5 x 10(4)CFU/ml, respectively. In mixed cultures, the LODs were approximately 3.1 x 10(3), 7.8 x 10(4), and 7.9 x 10(5)CFU/ml. In the competitive assay format, the LODs for E. coli O157:H7, S. enterica serovar Typhimurium, and L. monocytogenes were approximately 1.5 x 10(4), 5 x 10(4), and 1.2 x 10(5)CFU/ml for the pure cultures. These results showed that protein G-liposomal nanovesicles can be successfully used in a simultaneous immunoassay for several food-borne pathogens, thereby demonstrating that they are effective universal reagents for use in immunoassays.

Generic liposome reagent for immunoassays.

We have derivatized liposomes with antibodies by using avidin to crosslink biotinylated phospholipid molecules in the liposome membranes with biotinylated antibody molecules. A comparison of the biotin binding activity of avidin in solution and avidin associated with liposomes shows that avidin bound to biotinylated phospholipid in liposome membranes retains full binding activity for additional biotin molecules. Changes in the fluorescence spectrum of avidin have been used to characterize the binding capacity of avidin for biotin in solution, and change in intensity of light scattered due to aggregation of liposomes was used to measure the biotin binding activity of avidin associated with liposomes. Relative amounts of the biotinylated phospholipid, avidin, and biotinylated antibody have been optimized to produce stable liposomes which are derivatized with up to 1.7 nmol of antibody/mumol of lipid. These derivatized liposomes are highly reactive to immunospecific aggregation in the presence of multivalent antigen. A linear increase in light scattering was recorded between 1 and 10 pmol of antigen. This work shows that liposomes containing biotinylated phospholipid can be a successful generic reagent for immunoassays.

Development of an immunomagnetic bead-immunoliposome fluorescence assay for rapid detection of Escherichia coli O157:H7 in aqueous samples and comparison of the assay with a standard microbiological method

ABSTRACT The objective of this study was to develop and optimize a protocol for the rapid detection of Escherichia coli O157:H7 in aqueous samples by a combined immunomagnetic bead-immunoliposome (IMB/IL) fluorescence assay. The protocol consisted of the filtration or centrifugation of 30- to 100-ml samples followed by incubation of the filter membranes or pellet with anti-E. coli O157:H7 immunomagnetic beads in growth medium specific for E. coli O157:H7. The resulting E. coli O157:H7-immunomagnetic bead complexes were isolated by magnetic separation, washed, and incubated with sulforhodamine B-containing immunoliposomes specific for E. coli O157:H7; the final immunomagnetic bead-E. coli O157:H7-immunoliposome complexes were again isolated by magnetic separation, washed, and lysed with a n-octyl-β-d-glucopyranoside to release sulforhodamine B. The final protocol took less than 8 h to complete and had a detection limit of less than 1 CFU of E. coli O157:H7 per ml in various aqueous matrices, including apple juice and cider. To validate the protocol at an independent facility, 100-ml samples of groundwater with and without E. coli O157:H7 (15 CFU) were analyzed by a public health laboratory using the optimized protocol and a standard microbiological method. While the IMB/IL fluorescence assay was able to identify E. coli O157:H7-containing samples with 100% accuracy, the standard microbiological method was unable to distinguish E. coli O157:H7-spiked samples from negative controls without further extensive workup. These results demonstrate the feasibility of using immunomagnetic beads in combination with sulforhodamine B-encapsulating immunoliposomes for the rapid detection of E. coli O157:H7 in aqueous samples.