Sensitive Electrochemical Immunoassay Research Articles

Gold nanoparticles-decorated amine-terminated poly(amidoamine) dendrimer for sensitive electrochemical immunoassay of brevetoxins in food samples

A sensitive electrochemical immunosensor for the fast screening of brevetoxin B (BTX-2) in food samples was developed by means of immobilizing BTX-2–bovine serum albumin conjugate (BTX-2–BSA) on the gold nanoparticles-decorated amine-terminated poly(amidoamine) dendrimers (AuNP–PAADs). The presence of gold nanoparticles greatly improved the conductivity of the PAADs, and three-dimensional PAADs increased the surface coverage of the biomolecules on the electrode. Under optimal conditions, three types of immunosensor, i.e. with AuNPs, PAADs, or AuNP–PAADs, were used for the determination of BTX-2 in a competitive-type immunoassay format using horseradish peroxidase-labeled anti-BTX antibodies (HRP– anti-BTX-2) as trace in the H 2O 2– o-phenylenediamine ( o-PD) system. A low detection limit (LOD) of 0.01 ng/mL and a wide dynamic working linear range of 0.03–8 ng/mL BTX-2 using AuNP–PAADs as matrices were obtained in comparison with those of only using AuNP or PAADs. Intra-batch assay precision was substantially improved by resorting to the AuNP–PAADs manifold. The proposed method features unbiased identification of negative (blank) and positive samples. No significant differences were encountered in the analysis of the spiking real samples between the electrochemical immunosensor and liquid chromatography for the determination of BTX-2. Importantly, this method provided a biocompatible immobilization and a promising immunosensing platform for analytes with small molecules in the analysis and detection of food safety.

Nanosilver-doped DNA polyion complex membrane for electrochemical immunoassay of carcinoembryonic antigen using nanogold-labeled secondary antibodies

A simple and sensitive electrochemical immunoassay protocol was developed for the detection of carcinoembryonic antigen (CEA) using nanosilver-doped DNA polyion complex membrane (PIC) as sensing interface. To construct such an immunosensor, double-stranded DNA was initially assembled onto the surface of thionine/Nafion-modified screen-printed carbon electrode to adsorb silver ions with positive charges, then silver ions were reduced to nanosilver particles with the aid of NaBH 4, and then anti-CEA antibodies were immobilized on the nanosilver surface. Gold nanoparticles conjugated with horseradish peroxidase-labeled anti-CEA were employed as signal antibodies for the detection of CEA with a sandwich-type assay format. Under optimal conditions, the immunosensor exhibited a dynamic range of 0.03–32 ng mL −1 with a low detection limit of 10 pg mL −1 CEA. Intra- and inter-assay imprecision (CVs) were <9.5% and 6.5%, respectively. The response could remain 90.1% of the original current at 30th day. 50 real samples were evaluated using the immunosensor and the enzyme-linked immunosorbent assay, respectively, and received in accordance with those two methods.

Detection of breast cancer cells specially and accurately by an electrochemical method

Breast cancer is one of the most common cancers to cause death in the world, and the accurate diagnosis is of great importance to determine the stage of the disease and then to design the suitable therapy. Compared with the traditional detection methods relying on the recognition of only one tumor marker, we herein propose a sensitive electrochemical immunoassay to detect breast cancer cells by simultaneously measuring two co-expressing tumor markers, human mucin-1 and carcinoembryonic antigen on the surface of the cancer cells, which may efficiently improve the accuracy of the detection as well as facilitate the classification of the cancer cells. The experimental results have revealed that well electrochemical response can be observed only under the condition that both of the tumor markers are identified on the surface of the tumor cells. With this method, breast cancer cell MCF-7 can be easily distinguished from other kinds of cells, such as acute leukemia cells CCRF-CEM and normal cells islet beta cells. Moreover, the prepared cytosensor can specially monitor breast cancer cell MCF-7 in a wide range from 10 4 to 10 7 cell mL −1 with well reproduction and low detection limit, which may have great potential in clinical applications.

Thionine/nanogold multilayer film for electrochemical immunoassay of alpha-fetoprotein in human serum using biofunctional double-codified gold nanoparticles

A simple and sensitive electrochemical immunoassay method was developed for the detection of alpha-fetoprotein (AFP) in human serum. The immunoassay is based on the use of an AFP-functionalized thionine/nanogold multilayer film ({Thi/AuNP}n) on a single-walled carbon nanotubes (CNT)-coated gold electrode and double-codified gold nanoparticles with horseradish peroxidase-labelled anti-AFP antibodies (HRP-anti-AFP-AuNPs). With a competitive immunoassay format, the assay was performed using HRP-anti-AFP-AuNP as trace and H2O2 as enzyme substrate. The electrochemical behavior of the multilayer {Thi/AuNP}n films was studied. The maximal signal was obtained at n = 3 (i.e. {Thi/AuNP}3). Under optimal conditions, the obtained electrochemical responses were proportional to the AFP levels in the sample. The dynamic range for AFP quantification was 0.25–45 ng mL−1. Both the intra- and inter-assay coefficients of variation were less than 9.5%. The proposed assay had a detection limit of 0.05 ng mL−1, which was 20-fold vs. 10-fold lower than that obtained using monolayer Thi/AuNP film vs. conventional HRP-labelled anti-AFP antibodies. Importantly, no significant differences at the 0.05 confidence level were encountered in the analysis of clinical serum samples between the proposed immunoassay and the commercially available Roche 2010 Electrochemiluminescent (ECL) Automatic Analyzer for AFP determination.

Aptamer-Au NPs conjugates-accumulated methylene blue for the sensitive electrochemical immunoassay of protein

In this paper, we combine the advantages of aptamer, nanomaterial and antibody to design an electrochemical sandwich immunoassay for the ultrasensitive detection of human immunoglobulin E (IgE) by using methylene blue (MB) as electrochemical indicator. The sandwich structure is fabricated by using goat anti-human IgE as capturing probe. Aptamer-Au nanoparticles (NPs) conjugates are used both as a sandwich amplification element as well as an accumulation reagent of MB. Once the aptamer-Au NPs conjugates specifically bind to electrode surface, MB molecules are accumulated on its surface by the specific interaction of MB with G base of aptamer-Au NPs conjugates. Therefore, with the increase of human IgE concentration, more aptamer-Au-NPs conjugates are bound, and thus, more MB molecules are accumulated. A good linear relationship is obtained for the detection of human IgE over a range of 1-10,000 ng/ml with a lowest detection limit of 0.52 ng/ml. In addition, by using BSA, human IgA and human IgM as contrast, the excellent specificity of this sensing system for the detection of human IgE is also demonstrated.

Carbon Nanoparticle-Enhanced Immunoelectrochemical Detection for Protein Tumor Marker with Cadmium Sulfide Biotracers

We have developed a sensitive electrochemical immunoassay system for the detection of a protein tumor marker, carcinoembryonic antigen (CEA), that is based on a carbon nanoparticle (CNP)/poly(ethylene imine) (PEI)-modified screen-printed graphite electrode (CNP-PEI/SPGE) covered with anti-CEA antibodies. The signal amplification strategy--using CdS nanocrystals as biotracers and CNPs to enhance electron transfer--improves the sensitivity and detection limit for CEA, suggesting that this system holds promise for development into a point-of-care or disposable home-care self-diagnostic tool. This biosensor is based on a sandwich complex immunoassay, which we assembled from sequential layers of the anti-CEA antibody (alphaCEA) on CNP-PEI/SPGE, the CEA sample, and the CdS nanocrystal quantum dots (QDs) sensitized with alphaCEA (alphaCEA-CdS QD). We used square wave anodic stripping voltammetry (SWASV) to amplify the signal current response obtained from the dissolved alphaCEA-CdS QDs. The calibration curve for CEA concentration was linear in the range of 0.032-10 ng/mL; the detection limit (estimated as the mean of the blank sample plus three times the standard deviation obtained on the blank sample) was 32 pg/mL (equivalent to 160 fg in a 5 microL sample). This method is suitably precise and sensitive to function as a means of determining urinary CEA, which is a better marker than serum CEA for the early detection of urothelial carcinoma.

Ferrocenyl-doped silica nanoparticles as an immobilized affinity support for electrochemical immunoassay of cancer antigen 15-3

The aim of this study is to elaborate a simple and sensitive electrochemical immunoassay using ferrocenecarboxylic (Fc-COOH)-doped silica nanoparticles (SNPs) as an immobilized affinity support for cancer antigen 15-3 (CA 15-3) detection. The Fc-COOH-doped SNPs with redox-active were prepared by using a water-in-oil microemulsion method. The use of colloidal silica could prevent the leakage of Fc-COOH and were easily modified with trialkoxysilane reagents for covalent conjugation of CA 15-3 antibodies (anti-CA 15-3). The Fc-COOH-doped SNPs were characterized by X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The fabrication process of the electrochemical immunosensor was demonstrated by using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques. Under optimal conditions, the developed immunosensor showed good linearity at the studied concentration range of 2.0–240 U mL −1 with a coefficient 0.9986 and a detection limit of 0.64 U mL −1 at S/N = 3.

Development of an electrochemical immunoassay for rapid detection of E. coli using anodic stripping voltammetry based on Cu@Au nanoparticles as antibody labels

A sensitive electrochemical immunoassay for rapid detection of Escherichia coli has been developed by anodic stripping voltammetry (ASV) based on core–shell Cu@Au nanoparticles (NPs) as anti-E. coli antibody labels. The characteristics of Cu@Au NPs before and after binding with antibody were confirmed by transmission electron microscopy (TEM). After Cu@Au-labeled antibody reacted with the immobilized E. coli on Polystyrene (PS)-modified ITO chip, Cu@Au NPs were dissolved by oxidation to the metal ionic forms, and the released Cu2+ ions were determined at GC/Nafion/Hg modified electrode by ASV. The utilization of GC/Nafion/Hg modified electrode could enhance the sensitivity for Cu2+ detection with a concentration as low as 9.0×10−12mol/L. Since Cu@Au NPs labels were only present when antibody reacted with E. coli, the amount of Cu2+ directly reflected the number of E. coli. The technique could detect E. coli with a detection limit of 30CFU/mL and the overall analysis could be completed in 2h. By introducing a pre-enrichment step, a concentration of 3CFU/10mL E. coli in surface water was detected by the electrochemical immunoassay.

Double‐Layer Nanogold and Double‐Strand DNA‐Modified Electrode for Electrochemical Immunoassay of Cancer Antigen 15‐3

AbstractVarious sensor‐based immunoassay methods have been extensively developed for the detection of cancer antigen 15‐3 (CA 15‐3), but most often exhibit low detection signals and low detection sensitivity, and are unsuitable for routine use. The aim of this work is to develop a simple and sensitive electrochemical immunoassay for CA 15‐3 in human serum by using nanogold and DNA‐modified immunosensors. Prussian blue (PB), as a good mediator, was initially electrodeposited on a gold electrode surface, then double‐layer nanogold particles and double‐strand DNA (dsDNA) with the sandwich‐type architecture were constructed on the PB‐modified surface in turn, and then anti‐CA 15‐3 antibodies were adsorbed onto the surface of nanogold particles. The double‐layer nanogold particles provided a good microenvironment for the immobilization of biomolecules. The presence of dsDNA enhanced the surface coverage of protein, and improved the sensitivity of the immunosensor. The performance and factors influencing the performance of the immunosensor were evaluated. Under optimal conditions, the proposed immunosensor exhibited a wide linear range from 1.0 to 240 ng/mL with a relatively low detection limit of 0.6 ng/mL (S/N=3) towards CA 15‐3. The stability, reproducibility and precision of the as‐prepared immunosensor were acceptable. 57 serum specimens were assayed by the developed immunosensor and standard enzyme‐linked immunosorbent assay (ELISA), respectively, and the results obtained were almost consistent. More importantly, the proposed methodology could be further developed for the immobilization of other proteins and biocompounds.

Salmonella typhi determination using voltammetric amplification of nanoparticles: A highly sensitive strategy for metalloimmunoassay based on a copper-enhanced gold label

A highly sensitive electrochemical amplification immunoassay for Salmonella typhi ( S. typhi) determination has been developed for the first time by using a copper-enhanced gold nanoparticle label coupled with anodic stripping voltammetry. Monoclonal antibodies for S. typhi were first immobilized on polystyrene microwells and then captured by S. typhi bacteria. After an immunoreaction occurred, a polyclonal, antibody–colloidal gold conjugate was added to bind to the S. typhi bacteria. Next, a copper-enhancer solution containing ascorbic acid and copper (II) sulfate was added into the polystyrene microwells. The ascorbic acid was employed to reduce the copper (II) ions to copper (0), which was subsequently deposited onto the gold nanoparticle tags. After the copper was dissolved in nitric acid, the released copper ions were detected by anodic stripping voltammetry. The amount of deposited copper was related to the amount of gold nanoparticle tag present, which was controlled by the amount S. typhi attached to the polyclonal antibody–colloidal gold conjugate. Therefore, the anodic stripping peak current was linearly dependent on the S. typhi concentration over concentration range of 1.30 × 10 2 cfu/mL to 2.6 × 10 3 cfu/mL in a logarithmic plot, with a detection limit as low as 98.9 cfu/mL. The influences of the relevant experimental variables, such as the concentration of copper and the reaction time of S. typhi with antibody, were investigated. We also successfully applied this method to determine the presence of S. typhi in human serum. Our results are a step towards developing more sensitive and reliable nanoparticle immunoassays.

Antibody-functionalized magnetic nanoparticles for electrochemical immunoassay of α-1-fetoprotein in human serum

A simple and sensitive electrochemical immunoassay protocol for the detection of α-1-fetoprotein (AFP) has been designed based on antibody-functionalized core–shell–shell nanocomposite particles. The core–shell CoFe2O4/(3-mercaptopropyl) trimethoxysilane (CoFe2O4–MPS) was initially synthesized by covalent conjugation, then gold nanoparticles were adsorbed onto the surface of CoFe2O4–MPS via an Au–S bond, and then anti-AFP antibodies were immobilized onto the nanogold surface. With the aid of the external magnet, the biomolecules-doped magnetic nanoparticles were attached to the electrode surface. The electrochemical response was based on the modification of potential (potentiometric method) due to the interfacial change of charges as a consequence of the antigen-antibody reaction. Under optimal conditions, the immunosensor exhibited rapid response and a linear range from 0.8 to 120 ng mL−1 AFP with a detection limit of 0.3 ng mL−1 (S/N = 3). Moreover, the precision, reproducibility, selectivity and stability of the immunosensor were acceptable. Analytical results, obtained for the clinical serum specimen by the immunosensor, were in accordance with those assayed by the standard ELISA.

Highly sensitive electrochemical detection of immunospecies based on combination of Fc label and PPD film/gold nanoparticle amplification

A highly sensitive electrochemical immunoassay strategy based on the combination of ferrocene (Fc) label and poly( o-phenylenediamine) (PPD) film/gold nanoparticle (GNP) amplification for the detection of immunospecies is proposed using human IgG as the model analyte. A gold electrode is firstly modified with an electropolymerized film of poly( o-phenylenediamine), which provides a stable matrix with abundant amino-groups for the fabrication of sensing interface. Using glutaraldehyde as a cross-linker, cystamine is coupled onto the modified electrode. Subsequently, gold nanoparticle monolayer is assembled onto the resulting surface. Making use of the unique properties of gold nanoparticles, antibodies can be self-assembled onto the surface-confined gold nanoparticles via amine–Au affinity with a high loading amount and reserve high immunological activity. After the introduction of model analyte, the ferrocene (Fc)-labeled antibody is immobilized on the sensing interface by antibody–antigen specific reaction, resulting in a redox current signal. The peak current is proportional to the amount of the analyte. Under the optimized experimental conditions, the proposed sensing strategy provides a wide linear dynamic range from 25 to 1000 pg/mL with a low detection limit of 10 pg/mL. In addition, good reproducibility, high selectivity and stability are achieved. In particular, the extremely high stability of both poly( o-phenylenediamine) and gold nanoparticle monolayer allows the designed biosensing interface to withstand harsh regeneration treatment, making it reusable.

An electrochemical impedimetric arrayed immunosensor based on indium tin oxide electrodes and silver-enhanced gold nanoparticles

A novel sensitive electrochemical impedance immunoassay based on metal nanoparticle labels and ITO electrodes has been developed. First, 2-aminobenzoic acid (2-ABA) was electropolymerized onto an indium tin oxide (ITO) electrode. The coupling reagents 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and N-hydroxysulfosuccinimide were then used to activate the electroconductive polymer to form an active ester layer which could react with amino groups of antigen. Subsequently, immunoreaction was carried out between antigen and antibody labeled with gold nanoparticles, followed by the addition of the silver enhancer solution. The charge transfer processes of [Fe(CN)6]4−/[Fe(CN)6]3− on the ITO surface were affected due to the formation of silver precipitation on the gold nanoparticles, which was determined by electrochemical impedance spectroscopy. The surface was characterized by scanning electron microscopy. Finally, a multiplexed arrayed immunosensor was is described and the samples of antibody and antibody mixture were assayed specifically. The experimental conditions such as the number of electropolymerization cycles and the time of silver enhancement were examined and optimized. The detection range of antibody labeled with gold nanoparticles was between 10.0 ng mL−1 and 10.0 µg mL−1.

Sensitive electrochemical immunoassay for 2,4,6-trinitrotoluene based on functionalized silica nanoparticle labels

We present a poly(guanine)-functionalized silica nanoparticle (NP) label-based electrochemical immunoassay for sensitively detecting 2,4,6-trinitrotoluene (TNT). This immunoassay takes advantage of magnetic bead-based platform for competitive displacement immunoreactions and separation, and use electroactive nanoparticles as labels for signal amplification. For this assay, anti-TNT-coated magnetic beads interacted with TNT analog-conjugated poly(guanine)-silica NPs and formed analog-anti-TNT immunocomplexes on magnetic beads. The immunocomplexes coated magnetic beads were exposed to TNT samples, which resulted in displacing the analog conjugated poly(guanine) silica NPs into solution by TNT. In contrast, there are no guanine residues releasing into the solution in the absence of TNT. The reaction solution was then separated from the magnetic beads and transferred to the electrode surface for electrochemical measurements of guanine oxidation with Ru(bpy) 3 2+ as mediator. The sensitivity of this TNT assay was greatly enhanced through dual signal amplifications: (1) a large amount of guanine residues on silica nanoparticles are introduced into the test solution by displacement immunoreactions and (2) a Ru(bpy) 3 2+-induced guanine catalytic oxidation further enhances the electrochemical signal. Some experimental parameters for the nanoparticle label-based electrochemical immunoassay were studied and the performance of this assay was evaluated. The method is found to be very sensitive and the detection limit of this assay is ∼0.1 ng mL −1 TNT. The electrochemical immunoassay based on the poly[guanine]-functionalized silica NP label offers a new approach for sensitive detection of explosives.

A new strategy for electrochemical immunoassay based on enzymatic silver deposition on agarose beads

A novel, sensitive electrochemical immunoassay in a homogeneously dispersed medium is described herein based on the unique features of agarose beads and the special amplified properties of biometallization. The immunochemical recognition event between human immunoglobulin G (IgG) and goat anti-human IgG antibody is chosen as the model system to demonstrate the proposed immunoassay approach. Avidin-agarose beads rapidly react with the biotinylated goat anti-human IgG antibody to form agarose beads-goat anti-human IgG conjugate (agarose bead-Ab). Agarose bead-Ab, alkaline phosphatase conjugated goat anti-human IgG antibody (ALP-Ab) and the human IgG analyte are mixed to form sandwich-type immunocomplex followed by the addition of the enzymatic silver deposition solution to deposit silver onto the surface of proteins and agarose beads. The silver deposited are dissolved and quantified by anodic stripping voltammetry. The influence of relevant experimental variables was examined and optimized. The logarithm of the anodic stripping peak current depended linearly on the logarithm of the concentration of human IgG in the range from 1 to 1000 ng/ml. A detection limit as low as 0.5 ng/ml human IgG was attained by 3 σ-rule. The R.S.D. of the approach is 9.65% for eight times determination of 10 ng/ml human IgG under same conditions. Optical microscope and TEM graphs were also utilized to characterize agarose beads and silver nanoparticles formed.

Sensitive electrochemical enzyme immunoassay microdevice based on architecture of dual ring electrodes with a sensing cavity chamber

A novel electrochemical detection architecture was investigated for enzyme immunoassay sensors. Microchips with dual-ring working and counter electrodes, and a sensing cavity chamber were made on glass slides. The glass surface of the microchip was coated by 3-aminopropyltriethoxysilane (APTES). Goat IgG, as a example, was covalently captured on APTES-modified glass surfaces through glutaraldehyde (GA) as a cross-linker. Enzyme substrate, p-aminophenyl phosphate (PAPP) was prepared by electrolysis. The enzyme conversion from home-synthetic PAPP to p-aminophenol (PAP) was examined by differential pulse voltammetry (DPV). A competitive inhibition enzyme-linked immunosorbant assay (ELISA) was designed to test the system. Experimental results demonstrate that a detection limit of 118 fg/ml of goat IgG and a dynamic range of 118 fg/ml to 1.18 ng/ml, up to five orders of magnitude could be achieved. Due to its novel architecture design and electronic detection scheme, the method can be used to fabricate portable electrochemical ELISA lab-on-chip systems. The technology could have great potential in clinical diagnostic applications.

Enzyme-catalyzed amplified immunoassay for the detection of Toxoplasma gondii-specific IgG using Faradaic impedance spectroscopy, CV and QCM

A highly sensitive electrochemical immunoassay for Toxoplasma gondii-specific IgG (Tg-IgG) in human serum has been developed that is based on an enzyme-catalyzed amplification due to the formation of an insoluble precipitate on the surface of a quartz crystal microbalance (QCM). T. gondii antigen (TgAg) was immobilized on the surface of a gold electrode in order to bind Tg-IgG, and this was followed by the addition of anti-Tg-IgG horseradish peroxidase conjugate (anti-Tg-IgG-HRP). Subsequent exposure to 3,3'-diaminobenzidine (DAB) led to the enzymatically-catalyzed amplified deposition of the oxidation products on the QCM surface in the presence of H2O2. The transduction methods electrochemical Faradaic impedance spectroscopy (EIS) and cyclic voltammetry (CV) were used to assay the resistance to electron transfer at the conductive support upon accumulation of the insoluble products. The precipitation process was monitored in real time by QCM. The assay conditions, including the concentration of immobilized TgAg and the dosage of anti-Tg-IgG-HRP conjugate, were optimized. It was found that the amount of precipitate that accumulated on the conductive QCM surface was determined by the concentration of the target analyte Tg-IgG and the time permitted for biocatalyzed precipitation. The technique was shown to give a linear electron transfer resistance response (as measured by EIS) for Tg-IgG dilutions ranging between 1:8000 and 1:200, and a detection limit of 1:9600 dilution.

An electrochemical stripping metalloimmunoassay based on silver-enhanced gold nanoparticle label

A novel, sensitive electrochemical immunoassay has been developed based on the precipitation of silver on colloidal gold labels which, after silver metal dissolution in an acidic solution, was indirectly determined by anodic stripping voltammetry (ASV) at a glassy-carbon electrode. The method was evaluated for a noncompetitive heterogeneous immunoassay of an immunoglobulin G (IgG) as a model. The influence of relevant experimental variables, including the reaction time of antigen with antibody, the dilution ratio of the colloidal gold-labeled antibody and the parameters of the anodic stripping operation, upon the peak current was examined and optimized. The anodic stripping peak current depended linearly on the IgG concentration over the range of 1.66 ng ml −1 to 27.25 μg ml −1 in a logarithmic plot. A detection limit as low as 1 ng ml −1 (i.e., 6 × 10 −12 M) human IgG was achieved, which is competitive with colorimetric enzyme linked immuno-sorbent assay (ELISA) or with immunoassays based on fluorescent europium chelate labels. The high performance of the method is attributed to the sensitive ASV determination of silver (I) at a glassy-carbon electrode (detection limit of 5 × 10 −9 M) and to the catalytic precipitation of a large number of silver on the colloidal gold-labeled antibody.

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).