Synthesis and grafting of diazonium tosylates for thermoplastic electrode immunosensors

For electrochemical immunosensors, inexpensive electrodes with fast redox kinetics, and simple stable methods of electrode functionalization are vital. However, many inexpensive and easy to fabricate electrodes suffer from poor redox kinetics, and functionalization can often be difficult and/or unstable. Diazonium tosylates are particularly stable soluble salts that can be useful for electrode functionalization. Recently developed thermoplastic electrodes (TPEs) have been inexpensive, moldable, and highly electroactive carbon composite materials. Herein, the synthesis and grafting of diazonium tosylate salts were optimized for modification of TPEs and used to develop the first TPE immunosensors. With diazonium tosylates, TPEs were amine functionalized either directly through grafting of p-aminophenyl diazonium salt or indirectly through grafting p-nitrophenyl diazonium salt followed by electrochemical reduction to an amine. Diazonium tosylates were synthesized in situ as a paste in 6 min. Once the reaction paste was spread over the electrodes, near monolayer coverage (1.0 ± 0.2 nmol cm-2) was achieved for p-nitrophenyl diazonium salt within 5 min. Amine functionalized electrodes were conjugated to C-reactive protein (CRP) antibodies. Antibody-modified TPEs were applied for the sensitive detection of CRP, a biomarker of cardiovascular disease using electrochemical enzyme-linked immunosorbent assays (ELISA). LODs were determined to be 2 ng mL-1 in buffer, with high selectivity against interfering species for both functionalization methods. The direct p-aminophenyl modification method had the highest sensitivity to CRP and was further tested in spiked serum with an LOD of 10 ng mL-1. This low-cost and robust TPE immunosensor platform can be easily adapted for other analytes and multiplexed detection.

Combining Porous Magnetic Ni@C Nanospheres and CaCO3 Microcapsule as Surface-Enhanced Raman Spectroscopy Sensing Platform for Hypersensitive C-Reactive Protein Detection

In this work, we have designed an efficient and rapid surface-enhanced Raman spectroscopy (SERS) immunosensor for a supersensitive analysis of hypersensitive C-reactive protein (hs-CRP) with a label-free method by combining porous magnetic Ni@C nanospheres to aggregate together for simplifying the experiment operation and CaCO₃ microcapsule to encapsulate rhodamine B as the Raman signal. The final solution containing the signal molecule was dropped on the Ag nanoparticle substrate, and the signal could be enhanced by Ag particles. First, rhodamine B was encapsulated in the CaCO₃ microcapsule when it precipitated to form microcubes. Subsequently, the porous CaCO₃ microcapsule was assembled layer by layer with poly(ether imide) (PEI) and a second antibody to obtain rhodamine B@CaCO₃@PEI@Ab₂. Then, the functionalized magnetic Ni@C nanospheres were prepared to immobilize the primary antibody (Ab₁). Finally, the immunosensor was fabricated by the sandwiched antibody-antigen interactions. Compared to the DNA hydrogel, the low-cost CaCO₃ microcapsule would be rapidly dissolved by ethylene diamine tetraacetic acid, releasing rhodamine B to produce a strong Raman signal for a rapid and efficient detection of hs-CRP. With the hs-CRP concentration ranging from 0.1 pg mL^-1 to 1 μg mL^-1, the SERS intensity of the platform has a linear relationship with the logarithm of hs-CRP concentration, and the detection limit was 0.01 pg mL^-1. By this smart design, this work can give a direction for a rapid label-free SERS analysis.

Lab on a chip for in situ diagnosis: From blood to point of care

As the point of care diagnosis devices are becoming ever more popular, this paper suggest a miniaturized testing device from a drop of blood to diagnosis of disease for the global healthcare. The minimal requirements for the POC blood-testing device are blood microsampling, blood separation, immunoassay, and detection and communication of the signals. The microsampling of the blood can be achieved by specialized needle, which can be connected to the microchip or analytical devices. The sampled blood is then separated using either a filter (weir or pillar type), or by the phenomena unique to microfluidic system. The separated blood should then go through sandwich, homogeneous non-competitive, or competitive immunoassay, which can effectively diagnose diverse diseases. Lastly, the device should detect and translate the immune-signals to readable, and clinically significant signals. The development of such device will play a great role for improving healthcare technology.

Mass Spectrometry and Integrated Virus Detection System Characterization of MS2 Bacteriophage

ABSTRACT In this study, we demonstrate the effect of sample matrix composition of MS2 virus on its characterization by ESI-MS and IVDS. MS2 samples grown and purified using various techniques showed different responses on ESI-MS than that on IVDS. The LC-MS of the specific biomarker of MS2 bacteriophage from an infected Escherichia coli sample was characterized by the presence of E. coli proteins. The significant impact of sample matrix was observed upon identification of MS2 using a database search. Infected E. coli with MS2 showed a matching score indifferent from uninfected ones. Only purified MS2, using CsCl and analyzed by LS-MS, showed a positive match using the database search. However, the variation in MS2 sample matrix had no effect on the deification of MS2.

Bead-based electrochemical immunoassay for bacteriophage MS2

Viruses are one of four classes of biothreat agents, and bacteriophage MS2 has been used as a simulant for biothreat viruses, such as smallpox. A paramagnetic bead-based electrochemical immunoassay has been developed for detecting bacteriophage MS2. The immunoassay sandwich was made by attaching a biotinylated rabbit anti-MS2 IgG to a streptavidin-coated bead, capturing the virus, and then attaching a rabbit anti-MS2 IgG-beta-galactosidase conjugate to another site on the virus. beta-Galactosidase converts p-aminophenyl galactopyranoside (PAPG) to p-aminophenol (PAP). PAPG is electroinactive at the potential at which PAP is oxidized to p-quinone imine (PQI), so the current resulting from the oxidation of PAP to PQI is directly proportional to the concentration of antigen in the sample. The immunoassay was detected with rotating disk electrode (RDE) amperometry and an interdigitated array (IDA) electrode. With an applied potential of +290 mV vs Ag/AgCl and a rotation rate of 3000 rpm, the detection limit was 200 ng/mL MS2 or 3.2 x 10(10) viral particles/mL with RDE amperometry. A trench IDA electrode was incorporated into a poly(dimethyl siloxane) channel, within which beads were collected, incubated with PAPG, and PAP generation was detected. The two working electrodes were held at +290 and -300 mV vs Ag/AgCl, and electrochemical recycling of the PAP/PQI couple by the IDA electrode lowered the limit of detection to 90 ng/mL MS2, or 1.5 x 10(10) MS2 particles/mL.

Microbead-based electrochemical immunoassay with interdigitated array electrodes

The objective of this study was to develop a sensitive and miniaturized immunoassay by coupling a microbead-based immunoassay with an interdigitated array (IDA) electrode. An IDA electrode amplifies the signal by recycling an electrochemically redox-reversible molecule. The microfabricated platinum electrodes had 25 pairs of electrodes with 1.6-microm gaps and 2.4-microm widths. An enzyme-labeled sandwich immunoassay on paramagnetic microbeads with mouse IgG as the analyte and beta-galactosidase as the enzyme label was used as the model system. beta-Galactosidase converted p-aminophenyl beta-D-galactopyranoside to p-aminophenol (PAP). This enzyme reaction was measured continuously by positioning the microbeads near the electrode surface with a magnet. Electrochemical recycling occurred with PAP oxidation to p-quinone imine (PQI) at +290 mV followed by PQI reduction to PAP at -300 mV vs Ag/AgCl. Dual-electrode detection amplified the signal fourfold compared to single-electrode detection, and the recycling efficiency reached 87%. A calibration curve of PAP concentration vs anodic current was linear between 10(-4) and 10(-6)M. A signal from 1000 beads in a 20-microL drop was detectable and the immunoassay was complete within 10 min with a detection limit of 3.5x10(-15)mol mouse IgG.

Bilirubin Oxidase Label for an Enzyme-Linked Affinity Assay with O2 as Substrate in a Neutral pH NaCl Solution

Laccase, a copper enzyme catalyzing the four-electron reduction of O(2) to water, has been shown by others to be a useful label in enzyme-linked immunoassays, in which the substrate is ambient O(2) instead of an added chemical, such as hydrogen peroxide, or a phosphate ester of a phenol. Laccase-catalyzed O(2) reduction is, however, inhibited by halides, which complex the enzyme's copper ions. Replacement of laccase by bilirubin oxidase, a copper enzyme retaining its maximal activity at high chloride concentrations and at pH 7.2, allows enzyme-amplified affinity assays with O(2) as the substrate in neutral-pH chloride solutions, exemplified here by the assay of DNA, the duplexes of which are unstable at low ionic strength but are stable in strong NaCl solutions.

An electrochemical metalloimmunoassay based on a colloidal gold label

A novel, sensitive electrochemical immunoassay has been developed using a colloidal gold label that, after oxidative gold metal dissolution in an acidic solution, was indirectly determined by anodic stripping voltammetry (ASV) at a single-use carbon-based screen-printed electrode (SPE). The use of disposable electrodes allows for simplified measurement in 35 microL of solution. The method was evaluated for a noncompetitive heterogeneous immunoassay of an immunoglobulin G (IgG) and a concentration as low as 3 x 10(-12) M was determined, which is competitive with colorimetric ELISA or with immunoassays based on fluorescent europium chelate labels. The high performance of the method is related to the sensitive ASV determination of gold(III) at a SPE (detection limit of 5 x 10(-9) M) and to the release of a large number of gold(III) ions from each gold particle anchored on the immunocomplex (e.g., 1.7 x 10(5) gold atoms are theoretically contained in a 18-nm spherical gold particle).

Development of sub-micron patterned carbon electrodes for immunoassays

Sub-micron sized domains of a carbon surface are derivatized with antibodies using biotin/avidin technology. These sites are spatially-segregated from, and directly adjacent to, electron transfer sites on the same electrode surface. The distance between these electron transfer sites and enzyme-loaded domains are kept to a minimum (e.g. less than a micron) to maintain the high sensitivity required for the measurement of enzyme-linked cofactors in an enzyme-linked immunoassay (ELISA). This is accomplished through the use of photolithographic attachment of photobiotin using an interference pattern from a UV laser generated at the electrode surface. This allows the construction of microscopic arrays of active ELISA sites on a carbon substrate while leaving other sites underivatized to facilitate electron transfer reactions of redox mediators; thus maximizing sensitivity and detection of the enzyme mediator. The carbon electrode surface is characterized with respect to its chemical structure and electron transfer properties following each step of the antibody immobilization process. The characterization of specific modifications of micron regions of the carbon surface requires analytical methodology that has both high spatial resolution and sensitivity. We have used fluorescence microscopy with a cooled CCD imaging system to visualize the spatial distribution of enzyme immobilization sites (indicated by fluorescence from Texas-Red labeled antibody) across the carbon surface. The viability of the enzyme attached to the surface in this manner was demonstrated by imaging the distribution of an insoluble, fluorescent product.

Capillary electrochemical enzyme immunoassay (CEEI) for phenobarbital in serum

A competitive heterogeneous capillary enzyme immunoassay with electrochemical detection has been developed for phenobarbital in serum. The oxidized primary antibody was attached covalently to the modified interior surface of a microcapillary (22 microl). The competition between analyte phenobarbital and alkaline phosphatase labeled phenobarbital for a limited number of antibody binding sites was complete in 1.5 h. The enzymatic product (p-aminophenol) from the catalytic conversion of the substrate (p-aminophenyl phosphate) was detected by amperometric flow injection analysis. The calibration curve for phenobarbital had a detection limit of 30 microg l(-1) (2.8 pmoles or 0.65 ng) and a range of 30-3000 microg l(-1). The assay could be used to determine the phenobarbital serum concentration in a 4 microl clinical serum sample without pretreatment.

Biosensors based on flow-through systems

When combined with biosensors as the sensing element microdialysis and flow injection analysis (FIA) systems become sophisticated tools for handling analytical processes. In particular a FIA system offers a high degree of automation together with high reproducibility and small sample volumes, whereas the biosensor, allows selective and sensitive measurements of the various analytes. Here we describe first a miniaturised microdialysis flow-through system developed for glucose determination, then we focus on amperometric immunosensors and on microbial sensors. In the former, antibodies against low molecular weight environmental contaminants or against high molecular weight proteins are responsible for analyte detection, whereas the latter use immobilised microorganisms as the recognising element for monitoring water pollutants.

Zeptomole-detecting biosensor for alkaline phosphatase in an electrochemical immunoassay for 2,4-dichlorophenoxyacetic acid

A bienzyme substrate-recycling biosensor in a flow injection analysis system is described for the sensitive measurement of alkaline phosphatase (ALP) and applied to the fast readout of a competitive immunoassay for the widely used pesticide 2,4-dichlorophenoxyacetic acid (2,4-D). The phenol-indicating biosensor consists of a Clark-type electrode covered by a membrane with coentrapped tyrosinase and quinoprotein glucose dehydrogenase. ALP dephosphorylates phenyl phosphate to phenol (K(m) = 36 microM) outside the flow system. Phenol is oxidized in the sensor membrane by the oxygen-consuming tyrosinase via catechol to o-quinone. The quinone is reconverted to catechol by glucose dehydrogenase. This substrate cycling results in a 350-fold amplified sensor response to phenol. The oxygen consumption of the enzyme couple in the presence of phenol is monitored as a decrease in current. A total of 3.2 fM ALP (320 zmol/ 100 microL) has been detected after a 57.5 min incubation with phenyl phosphate. All involved reagents are stable over the time of measurement. The sensor does not produce any measurable blank signals. The immunoassay detects 0.1 microgram/L 2,4-D, the maximum concentration for pesticides allowed in drinking water by European Community regulations. The applicability of this biosensor for fast immunoassay readout is demonstrated by a 2 min incubation. By comparison, a standard photometric method (p-nitrophenyl phosphate) requires overnight incubation.

Applications of electrochemical immunosensors to environmental monitoring

This paper discusses basic electrochemical immunoassay technology. Factors limiting the practical application of antibodies to analytical problems are also presented. It addresses the potential use of immunoassay methods based on electrochemical detection for the analysis of environmental samples. It provides examples for the detection and quantitation of environmental samples using conducting electroactive polymers (CEPs). CEP-based immunosensing systems are compared with conventional environmental immunoassay procedures. The advantages of using these types of sensors for rapid, sensitive, and cost-effective analysis of pesticides and toxic chemicals are analysed and discussed. CEP-based immunosensing technology might eventually be used for continuous monitoring of effluents such as waste streams to determine compliance with regulations. CEP-based sensors are suitable for monitoring ground-water, waste stream effluents, agricultural run-offs and for monitoring the effectiveness of remediation, or for other situations where a real-time monitoring capability is desired.

Avidin-biotin based electrochemical immunoassay for thyrotropin

A two site electrochemical enzyme immunoassay for thyrotropin (TSH) has been developed. This assay is based on the use of an immobilized capture antibody and a biotinylated second antibody. Detection is achieved via avidin labelled with alkaline phosphatase. The substrate 1-naphthyl phosphate was used and the product 1-naphthol was detected at disposable screen-printed carbon 8-electrode combs using specially designed instrumentation.

A rapid and sensitive heterogeneous immunoelectrochemical assay using disposable electrodes

In this novel enzyme-tagged immuoelectrochemical assay, disposable carbon felt discs serve both as electrodes and as the heterogeneous solid phase. Antibodies are immobilized on the carbon felt via a diaminoalkane-biotin-avidin-biotin bridge. Alkaline phosphatase is used as a label. Bound antibodies are monitored by following the electro-oxidation of aminophenol, produced enzymatically from p-amino-phenyl phosphate by the immobilized alkaline phosphatase at the electrode surface. A model system designed for determination of mouse IgG concentration yielded a calibration curve ranging from 10 pg/ml to 100 micrograms/ml. This assay can be performed rapidly and a single determination completed within 20 minutes. The system is useful also for rapid quantitation of a small number (approximately 80 organisms per ml) of bacteria.

An improved ELISA with linear sweep voltammetry detection

An improved ELISA combined with linear sweep voltammetry detection of p-nitrophenol generated by an enzyme has been investigated in this study. p-nitrophenol, produced from alkaline phosphatase catalysing p-nitrophenyl phosphate, yielded an oxidative peak at 1.06 V (vs. Ag/AgCl) with a wax-impregnated tubular graphite anode. Without separation, the small three-electrode system was directly inserted in the well of an ELISA plate for detection. The detection limit for p-nitrophenol was 1 x 10(-6) M, lower than that obtained by measuring the absorbance of p-nitrophenol. The feasibility of utilizing linear sweep voltammetry as a detection scheme was demonstrated by determining metallothionein, granulocyte-colony stimulating factor and Xenopus laevis keratin using the above new system. The method was simple, reproducible and much more sensitive than traditional spectrophotometry.

Electrochemical enzyme immunoassay for detection of toxic substances

Sensors that provide reliable, rapid measurement of toxic substances are needed to solve significant human health and safety problems. We developed a new biosensor design that combines the advantages of immunoassay with electrochemical response. We established that this enzyme-linked immunosensor measures toxic substances in biological samples. The biosensor consists of two major elements: (1) an electrical conducting layer having immobilized enzyme, polyclonal or monoclonal antibodies, and other necessary reagents, and (2) the electronic components used in the signal readout. The result is an amperometric immunoassay based on coupling the immunochemical reaction to the enzyme electrode response by using a soluble, electrochemically active mediator. The specific question addressed was: Does the system's immunochemical detection reliably respond at sufficiently low analyte concentrations? We present our results in these areas: (1) enzyme immobilization on colloidal gold; (2) colloidal gold-enzyme deposition on the electrode surface; (3) mediator-antigen conjugate synthesis; (4) antibody incorporation at the electrode surface; (5) bioelectrode characterization and optimization; and (6) immunosensor demonstration to detect antigen. Sensors that employ immunochemical detection will have broad applicability to detect/diagnose toxic substances in biological samples such as blood and urine and in environmental samples such as wastewater and drinking water.

An improved amperometric immunosensor for the detection and enumeration of protein A-bearing Staphylococcus aureus

An amperometric immunosensor specific to the protein A of Staphylococcus aureus, was developed using the direct electrochemical detection of phenol produced by alkaline phosphatase from phenylphosphate. The immunosensor could detect protein A at 0.01 ng/ml and could reliably detect and quantify pure cultures of protein A-bearing Staph. aureus above 10(3) cfu/ml. A similar sensitivity of detection was obtained with samples of beef and milk.

A semi-homogeneous amperometric immunosensor for protein A-bearing Staphylococcus aureus in foods

A semi-homogeneous amperometric immunosensor specific to the protein A of Staphylococcus aureus was developed using direct electrochemical detection of phenol produced by alkaline phosphatase from phenyl phosphate. The immunosensor could reliably detect strains of protein A-bearing S. aureus in pure cultures at ca. 10(4) cfu/ml, and at ca. 10(5) cfu/g or ml in various food samples. Due to its semi-homogeneous nature, the system was very simple, easy to operate, and labour-saving. The good correlation between the amperometric current generated by the immunosensor and plate counts illustrated the potential usefulness of this simple system. It proved to be a reliable 24-h detection method for food samples containing very low numbers of protein A-bearing S. aureus after pre-enrichment, as it was able to detect cells that could not directly be enumerated by plate counts.

A simple theoretical treatment of a competitive enzyme-linked immunosorbent assay (ELISA) and its application to the detection of human blood group antigens

A theory has been developed to explain the behaviour of a competitive enzyme-linked immunosorbent assay (ELISA) in which an immobilized antigen competes with a liquid-phase antigen for a limiting amount of antibody. The binding of antibody to antigen on a solid surface (microtitre well) is described in terms of Langmuirian adsorption with a binding constant kappa. Two equations are presented to describe the behaviour of the ELISA signal as a function of competing antigen concentration; an exact equation and an approximate equation which can be used when the surface coverage of the immobilized antigen is not known. It is shown how curves of ELISA signal vs. competing antigen concentration depend on K/kappa [antibody]. The theory has been tested using several immobilized blood group A antigens competing with ovarian cyst fluid A substance and found to adequately describe these competitive ELISAs which have a detection limit of approximately 1 ng of blood group antigen.

Solid-phase electrochemical enzyme immunoassay with attomole detection limit by flow injection analysis

A sandwich electrochemical enzyme immunoassay with flow injection analysis for the model antigen mouse IgG has been developed with alkaline phosphatase as the enzyme label. The enzyme substrate, 4-aminophenyl phosphate and its enzymatic reaction product, 4-aminophenol have been studied by cyclic and hydrodynamic voltammetry. The determination of 4-aminophenol by flow injection analysis with electrochemical detection (FIAEC) has a linear range of 5.0 x 10(-8) to 1.0 x 10(-5) M, a detection limit of 2.4 x 10(-8) M, and a sample throughput of 72 samples/h. The detection limit is set by a background capacitance response, which depends on the ionic strength difference between the sample and the mobile phase. The sandwich immunoassay has been characterized with respect to substrate concentration for the enzymatic reaction, detection limit, dynamic range and sources of error. Mouse IgG can be determined with a detection limit of 0.81 pg ml-1 by a 30-min substrate incubation time and a six orders of magnitude linear dynamic range.

Amperometric enzyme-amplified immunoassays

Enzyme-amplified immunoassays have been adapted for electrochemical measurement, using an NAD+/NADH redox cycle coupled to an electrode via the active site of diaphorase. Two amperometric methods are described, the first employs an organic conducting salt electrode, NMP+/TCNQ-; the second a platinum wire with ferricyanide as electron transfer mediator. In an immunoenzymometric assay for human prostatic acid phosphatase the sensitivities of the electrochemical methods were comparable to that achieved with the existing optical technique, but the dynamic range of the electrochemical assays was increased by at least two orders of magnitude. It is proposed that electrochemical enzyme-amplified immunoassays may eventually replace their optical counterparts.

Immunological-chromatographic analysis

Tandem immunoaffinity and conventional high-performance liquid chromatography columns, coupled with a switching valve, were used for the analysis of single and multicomponent antigen samples. Immunoaffinity columns were prepared by hydrophobic adsorption or covalent immobilization of poly- or monoclonal antibodies on macroporous poly(styrene-divinylbenzene) packing materials. Capacities of these columns were constant through at least 77 cycles. Macromolecular antigens were analyzed at submicrogram levels. Antigens bound to the immunoaffinity column were desorbed and concentrated on a conventional analytical column. Gradient elution on the analytical column separated the desorbed antigen(s) from interfering species and permitted the analysis of all species which bound to the immunoaffinity column. Immunological-chromatographic analysis was useful for purification and discrimination of polypeptides of similar three dimensional structures, such as several lysozyme variants.

Extending the detection limit of solid-phase electrochemical enzyme immunoassay to the attomole level

Electrochemical enzyme immunoassay methodology has been developed to take advantage of the selectivity of antibody reactions, the amplification feature of an enzyme-based assay, and the ease with which small amounts of the enzyme-generated product can be detected electrochemically. A heterogeneous sandwich enzyme immunoassay was used in this work as the model assay. In this type of assay, the antigen is sandwiched between the enzyme conjugate and a primary antibody that is adsorbed to the solid phase. Alkaline phosphatase is a suitable enzyme for electrochemical assays since it catalyzes the conversion of electroinactive phenyl phosphate to electroactive phenol. The product, phenol, is then quantitated by liquid chromatography with electrochemical detection in a thin-layer flow cell with a carbon paste electrode at 0.895 V vs Ag/AgCl. The current produced by the oxidation of phenol is directly proportional to the analyte (antigen) concentration. The problem associated with these types of solid-phase immunoassays is that the adsorption of the primary antibody is desired while the adsorption of other assay proteins is not. The detection limits are generally defined by the ability to control this nonspecific adsorption. The detection limit of a previous electrochemical assay for rabbit IgG was 100 pg/ml and was limited by a large background current observed in the absence of antigen. In the present study, each step of the assay was examined in order to determine the sources of this background current, and it was found that the major contribution was from the nonspecific adsorption of the enzyme conjugate. Using combinations of Tween 20 and bovine serum albumin as blocking agents, the level of nonspecific adsorption was reduced by 96%.(ABSTRACT TRUNCATED AT 250 WORDS)

Enzyme-amplified immunoassays

The sensitivity of enzyme immunoassays may be enhanced by the use of enzyme-amplification. This technique uses the enzyme label in the immunoassay to provide a trigger substance for a secondary system that can generate a large quantity of coloured product. Two examples of enzyme amplifiers are described, using either a substrate cycle with phosphorylated hexose sugars, or a redox cycle involving the coenzyme NAD(+). The redox enzyme-amplifier has a detection limit of less than one attomole for the enzyme label, alkaline phosphatase. The limited dynamic range of enzyme-amplified immunoassays may be overcome by kinetic analysis of the colour development in the enzyme-amplifier, to add at least a further order of magnitude to the range of directly measured analyte concentrations in the immunoassay. This is illustrated in an enzyme-amplified immunoassay for human thyroid stimulating hormone. Amperometric measurement of the enzyme-amplifier provides a method to extend the dynamic range still further and compares favourably with the performance of a gamma counter, a luminometer or a fluorimeter.