Small volume bead assay for ovalbumin with electrochemical detection

High-sensitivity electrochemical enzyme-linked assay on a microfluidic interdigitated microelectrode

A novel enzyme-linked DNA hybridization assay on an interdigitated array (IDA) microelectrode integrated into a microfluidic channel is demonstrated with sub-nM detection limit. To improve the detection limit as compared to conventional electrochemical biosensors, a recyclable redox product, 4-aminophenol (PAP) is used with an IDA microelectrode. The IDA has a modest and easily fabricated inter-digit spacing of 10 μm, yet we were able to demonstrate 97% recycling efficiency of PAP due to the integration in a microfluidic channel. With a 70 nL sample volume, the characterized detection limit for PAP of 1.0 × 10⁻¹⁰ M is achieved, with a linear dynamic range that extends from 1.0 × 10⁻⁹ to 1.0 × 10⁻⁵ M. This detection limit, which is the lowest reported detection limit for PAP, is due to the increased sensitivity provided by the sample confinement in the microfluidic channel, as well as the increased repeatability due to perfectly static flow in the microchannel and an additional anti-fouling step in the protocol. DNA sequence detection is achieved through a hybridization sandwich of an immobilized complementary probe, the target DNA sequence, and a second complementary probe labeled with β-galactosidase (β-GAL); the β-GAL converts its substrate, 4-aminophenyl-d-galactopyranoside (PAPG), into PAP. In this report we present the lowest reported observed detection limit (1.0 × 10⁻¹⁰ M) for an enzyme-linked DNA hybridization assay using an IDA microelectrode and a redox signaling paradigm. Thus, we have demonstrated highly sensitive detection of a targeted DNA sequence using a low-cost easily fabricated electrochemical biosensor integrated into a microfluidic channel.

Magnetic microbead-based electrochemical immunoassays

This review provides a summary of recent works concerning electrochemical immunoassays using magnetic microbeads as a solid phase. Recent research activity has led to innovative and powerful detection strategies that have been resulted in sensitive electrochemical detection. Coupling of magnetic microbeads with highly sensitive electrochemical detection provides a useful analytical method for environmental evaluation and clinical diagnostics, etc. The huge surface area and high dispersion capability of magnetic microbeads strongly contributes towards the development of new sensitive, rapid, user-friendly, and miniaturized electrochemical immunoassay systems. Moreover, the immunocomplexes formed on the magnetic microbead surface can be easily detected without pretreatment steps such as preconcentration or purification, which are normally required for standard methods. The discussion in this review is organized in two main subjects that include magnetic-microbead-based assays using enzyme labels and nanoparticle tags.

Sandwich electrochemical immunoassay for the detection of Staphylococcal enterotoxin B based on immobilized thiolated antibodies

A new approach for the sensitive detection of Staphylococcal enterotoxin B (SEB) is presented based upon an electrochemical enzymatic immunoassay that utilizes thiolated antibodies immobilized on a gold surface. This method relies on the use of amine- or sulfhydryl-reactive heterobifunctional cross-linkers for the introduction of 2-pyridyl-disulfide groups to the antibody. The disulfide-containing linkages are subsequently cleaved with a suitable reducing agent, such as dithiothreitol (DTT), and the thiolated antibody-gold bond is covalently formed on a gold working electrode. Various cross-linking agents for immobilization of the capture antibody onto the gold electrode were investigated and compared. Factors influencing the thiolation and immobilization were investigated and optimized. The feasibility of such antibody immobilization and the subsequent sandwich enzyme immunoassay is demonstrated for the sensitive detection of SEB. The detection limit estimated from a representative dose-response curve is 1 ng/mL, corresponding to 5 pg in a 5-microL sample. Coupling the specificity of immunoassays with the sensitivity and low detection limits of electrochemical detection shows real promise for future sensing technology in enabling the development of single-use disposable devices.

Electrochemical microbead-based immunoassay using an (eta5-cyclopentadienyl)tricarbonylmanganese redox marker bound to bovine serum albumin

A first example of the solid-phase immunoassay of a high-weight antigen bovine serum albumin (BSA) using an (eta(5)-cyclopentadienyl)tricarbonylmanganese (cymantrene) redox probe is presented. The electrochemical detection is based on the impedance measurements of a one-electron reversible reduction of the organometallic probe. The microbead-based immunoassay is discussed for two types of microbeads with different diameters (2.5 and 90 microm) and capabilities to bind the immunoglobulins (2.4 and 10 microg/mg of beads). The use of larger agarose microbeads allows the formation of an antigen-antibody complex at the surface of microbeads directly dispersed in the analyzed solution. No additional separation step is necessary for the electrochemical competitive immunoassay analysis of BSA. The presence of agarose beads in the analyzed solution has no effect on the electrochemical signal from labeled BSA released from the antigen-antibody complex.

Amperometric determination of live Escherichia coli using antibody-coated paramagnetic beads

Detecting and enumerating fecal coliforms, especially Escherichia coli, as indicators of fecal contamination, are essential for the quality control of supplied and recreational waters. We have developed a sensitive, inexpensive, and small-volume amperometric detection method for E. coli beta-galactosidase by bead-based immunoassay. The technique uses biotin-labeled capture antibodies (Ab) immobilized on paramagnetic microbeads that have been functionalized with streptavidin (bead-Ab). The bead-Ab conjugate captures E. coli from solution. The captured E. coli is incubated in Luria Bertani (LB) broth medium with the added inducer isopropyl beta-D: -thiogalactopyranoside (IPTG). The induced beta-galactosidase converts p-aminophenyl beta-D: -galactopyranoside (PAPG) into p-aminophenol (PAP), which is measured by amperometry using a gold rotating disc electrode. A good linear correlation (R2 = 0.989) was obtained between log cfu mL(-1) E. coli and the time necessary to product a specific concentration of PAP. Amperometric detection enabled determination of 2x10(6) cfu mL(-1) E. coli within a 30 min incubation period, and the total analysis time was less than 1 h. It was also possible to determine as few as 20 cfu mL(-1) E. coli under optimized conditions within 6-7 h. This process may be easily adapted as an automated portable bioanalytical device for the rapid detection of live E. coli.

Immunoassay for B. globigii spores as a model for detecting B. anthracis spores in finished water

The 2001 anthrax alarm in the US raised concerns about the Nation's preparedness to the threat of bioterrorism, and the demand for early warning systems that might be used in the case of a biological attack continues to grow. Here we develop an ultra-sensitive rapid detection method for B. globigii(BG) spores, the simulant of B. anthracis(BA) spores. BG spores were detected by a bead-based sandwich immunoassay with fluorescence detection. Paramagnetic Dynal beads were used as a solid support, primary antibody was attached to the beads by streptavidin-biotin coupling and the secondary antibody had an alkaline phosphatase (AP) enzyme label. Enzymatic conversion of fluorescein diphosphate (FDP) to fluorescein by AP was measured in real time with lambda(ex)= 490 nm and lambda(em)= 520 nm. The assay was linear from 2.6 x 10(3)-5.6 x 10(5) BG spores mL(-1), and the detection limit was 2.6 x 10(3) spores mL(-1) or 78 spores. All reagent concentrations and incubation times were optimized. The assay time from the moment the spores were introduced to the system was 30 min, and real-time fluorescence detection was done in less than 1 min. Formation of the BG spores-capture beads complex was confirmed by environmental scanning electron microscopy (ESEM). BG spores were detected successfully when doped into Cincinnati tap water to demonstrate the applicability of the developed method to detect the spores in non-buffered media.

Microfabricated On-Chip-Type Electrochemical Flow Immunoassay System for the Detection of Histamine Released in Whole Blood Samples

This paper describes an on-chip-type electrochemical flow immunoassay system with a multichanneled matrix column. The multichanneled matrix column was functionally coated with cation-exchange resin and used for separation of proteins. Antihistamine immunoglobulin G (IgG) antibody conjugated with ferrocenemonocarboxylic acid (Fc) was also prepared and used as a novel analytical reagent. Antibody-antigen complexes were separated from free Fc-conjugated IgG antibody (Fc-IgG) on the basis of differences in isoelectric point (pI) using the multichanneled matrix column coated with cation-exchange resin. The assay yields a good relationship between current and histamine concentration in the range of 200-2000 ng/mL. This simple technique enables the assay of histamine released in whole blood within 2 min. Furthermore, a good correlation was found between the response of the electrochemical immunoassay described in this paper and the conventional RIA (radioimmunoassay). This on-chip-type electrochemical flow immunoassay requires only minute quantities of whole blood samples and generates highly reproducible results.

Electrochemical enzyme immunoassays on microchip platforms

A microfluidic device for conducting electrochemical enzyme immunoassays is described. The new "lab-on-a-chip" protocol integrates precolumn reactions of alkaline phosphatase-labeled antibody (anti-mouse IgG) with the antigen (mouse IgG), followed by electrophoretic separation of the free antibody and antibody-antigen complex. The separation is followed by a postcolumn reaction of the enzyme tracer with the 4-aminophenyl phosphate substrate and a downstream amperometric detection of the liberated 4-aminophenol product Factors influencing the reaction, separation, and detection processes were optimized, and the analytical performance was characterized. An applied field strength of 256 V/cm results in free antibody and antibody-antigen complex migration times of 125 and 340 s, respectively. A remarkably low detection limit of 2.5 x 10(-16) g/mL (1.7 x 10(-18) M) is obtained for the mouse IgG model analyte. Such combination of a complete integrated immunoassay, an attractive analytical performance, and the distinct miniaturization/portability advantages of electrochemical microsystems offers considerable promise for designing self-contained and disposable chips for decentralized clinical diagnostics or on-site environmental testing.