Avidin-biotin based electrochemical immunoassay for thyrotropin

Biochemical Testing in Thyroid Disorders

This article summarizes the main principles for the appropriate use of laboratory testing in the diagnosis and management of thyroid disorders, as well as controversies that have arisen in association with some of these biochemical tests. To place a test in perspective, its sensitivity and accuracy should be taken into account. Ordering the correct laboratory tests facilitates the early diagnosis of a thyroid disorder and allows for timely and appropriate treatment. This article focuses on a comprehensive update regarding thyroid-stimulating hormone, thyroxine/triiodothyronine, thyroid autoantibodies, thyroglobulin, and calcitonin. Clinical uses of these biochemical tests are outlined.

Biosensors based on screen-printing technology, and their applications in environmental and food analysis

This review summarizes scientific research activity on biosensors, especially screen-printed, electrode-based biosensors. The basic configurations of biosensors based on screen-printing technology are discussed and different procedures for immobilization of the biorecognition component are reviewed. Theoretical aspects are exemplified by practical environmental and food-analysis applications of screen-printed, electrode-based biosensors.

Electrochemical immunosensor array using a 96-well screen-printed microplate for aflatoxin B1 detection

A novel analytical immunosensor array, based on a microtiter plate coupled to a multichannel electrochemical detection (MED) system using the intermittent pulse amperometry (IPA) technique, is proposed for the detection of aflatoxin B1 (AFB1). In the present work, the electrochemical behaviour and electroanalytical performance of the thick-film carbon sensors (also designated as screen-printed electrodes) incorporated in the multichannel electrochemical plate were first evaluated. Then the 96-well screen-printed microplate was modified in accord with a competitive indirect enzyme-linked immunoassay (ELISA) format for aflatoxin B1 detection. The measurements were performed using both spectrophotometric and electrochemical procedures and the results of the calibration curves, detection limit (LOD), sensitivity and reproducibility of the respective assay systems were evaluated. The immunoassay was then applied for analysis of corn samples spiked with AFB1 before and after the extraction treatment, in order to study the extraction efficiency and the matrix effect, respectively. These studies have shown that using this system, AFB1 can be measured at a level of 30 pg/mL and with a working range between 0.05 and 2 ng/mL. Good recoveries (103+/-8%) were obtained, demonstrating the suitability of the proposed assay for accurate determination of the AFB1 concentration in corn samples. The specificity of the assay was assessed by studying the cross-reactivity of PAb relative to AFB1. The results indicated that the PAb could readily distinguish AFB1 from other aflatoxins, with the exception for AFG1.

Labeling strategies for bioassays

Different labeling strategies for enzymatic assays and immunoassays are reviewed. Techniques which make use of direct detection of a label, e.g. radioimmunoassays, are discussed, as are techniques in which the label is associated with inherent signal amplification. Examples of the latter, e.g. enzyme-linked immunosorbent assays or nanoparticle-label based assays, are presented. Coupling of the bioassays to chromatographic separations adds selectivity but renders the assays more difficult to apply. The advantages and drawbacks of the different analytical principles, including future perspectives, are discussed and compared. Selected applications from clinical, pharmaceutical, and environmental analysis are provided as examples.

Hydroquinone diphosphate: an alkaline phosphatase substrate that does not produce electrode fouling in electrochemical immunoassays

Hydroquinone diphosphate (HQDP) was synthesized and compared to phenyl phosphate (PP) and 1-naphthyl phosphate (NP) as a substrate for alkaline phosphatase (AP) under electrochemical immunoassay (EIA) conditions. Voltammetric and amperometric experiments showed that electrochemical oxidation of hydroquinone (HQ), which is the AP hydrolysis product of HQDP, did not produce electrode passivation, even with repeated biosensor use. In contrast, phenol and 1-naphthol, the hydrolysis products of PP and NP, respectively, were shown to be irreversibly oxidized on the electrode surfaces, and produced rapid electrode passivation, resulting in complete loss of electrode signal. When employed as AP substrate in an iridium oxide based EIA, HQDP produced significantly larger amperometric responses (117 microA/cm2) compared to PP (31 microA/cm2) and NP (27 microA/cm2). The results presented in this paper show that HQDP is an attractive alternative to commonly used AP substrates such as NP and PP. The substrate shows excellent hydrolytic stability, produces larger amperometric responses (than PP or NP), and does not produce sensor passivation.

An analysis of avidin, biotin and their interaction at attomole levels by voltammetric and chromatographic techniques

The electroanalytical determination of avidin in solution, in a carbon paste, and in a transgenic maize extract was performed in acidic medium at a carbon paste electrode (CPE). The oxidative voltammetric signal resulting from the presence of tyrosine and tryptophan in avidin was observed using square-wave voltammetry. The process could be used to determine avidin concentrations up to 3 fM (100 amol in 3 microl drop) in solution, 700 fM (174 fmol in 250 microl solution) in an avidin-modified electrode, and 174 nM in a maize seed extract. In the case of the avidin-modified CPE, several parameters were studied in order to optimize the measurements, such as electrode accumulation time, composition of the avidin-modified CPE, and the elution time of avidin. In addition, the avidin-modified electrode was used to detect biotin in solution (the detection limit was 7.6 pmol in a 6 mul drop) and to detect biotin in a pharmaceutical drug after various solvent extraction procedures. Comparable studies for the detection of biotin were developed using HPLC with diode array detection (HPLC-DAD) and flow injection analysis with electrochemical detection, which allowed biotin to be detected at levels as low as 614 pM and 6.6 nM, respectively. The effects of applied potential, acetonitrile content, and flow rate of the mobile phase on the FIA-ED signal were also studied.

Silver electrodeposition catalyzed by colloidal gold on carbon paste electrode: application to biotin-streptavidin interaction monitoring

A new electrochemical method to monitor biotin-streptavidin interaction on carbon paste electrode, based on silver electrodeposition catalyzed by colloidal gold, was investigated. Silver reduction potential changed when colloidal gold was attached to an electrode surface through the biotin-streptavidin interaction. Thus, the direct reduction of silver ions on the electrode surface could be avoided and therefore, they were only reduced to metallic silver on the colloidal gold particle surface, forming a shell around these particles. When an anodic scan was performed, this shell of silver was oxidized and an oxidation process at + 0.08 V was recorded in NH3 1.0 M. Biotinylated albumin was adsorbed on the pretreated electrode surface. This modified electrode was immersed in colloidal gold-streptavidin labeled solutions. The carbon paste electrode was then activated in adequate medium (NaOH 0.1 M and H2SO4 0.1 M) to remove proteins from the electrode surface while colloidal gold particles remained adsorbed on it. Then, a silver electrodeposition at -0.18 V for 2 min and anodic stripping voltammetry were carried out in NH3 1.0 M containing 2.0 x 10(-5) M of silver lactate. An electrode surface preparation was carried out to obtain a good reproducibility of the analytical signal (5.3%), using a new electrode for each experiment. In addition, a sequential competitive assay was carried out to determine streptavidin. A linear relationship between peak current and logarithm of streptavidin concentration from 2.25 x 10(-15) to 2.24 x 10(-12) M and a limit of detection of 2.0 x 10(15) M were obtained.

Colloidal gold as an electrochemical label of streptavidin-biotin interaction

A new electrochemical method to monitor biotin-streptavidin interaction, based on the use of colloidal gold as an electrochemical label, is investigated. Biotinylated albumin is adsorbed on the pretreated surface of a carbon paste electrode (CPE). This modified electrode is immersed in colloidal gold-streptavidin labelled solutions. Adsorptive voltammetry is used to monitor colloidal gold bound to streptavidin, obtaining a good reproducibility of the analytical signal (R.S.D. = 3.3%). A linear relationship between peak current and streptavidin concentration from 2.5 x 10(-9) to 2.5 x 10(-5) M is obtained when a sequential competitive assay between streptavidin and colloidal gold-labelled streptavidin is carried out. On the other hand, the adsorption of streptavidin on the electrode surface was performed, followed by the reaction with biotinylated albumin labelled with colloidal gold. In this way, a linear relationship between peak current and colloidal gold labelled biotinylated albumin concentration is achieved with a limit of detection of 7.3 x 10(9) gold particles per ml (5.29 x 10(-9) M in biotin).

Comparative electrochemical behaviour of biotin hydrazide and photobiotin. Importance in the development of biosensors

The cyclic voltammetric behaviour of biotin hydrazide and photobiotin on carbon paste electrodes has been studied. Biotin hydrazide presents an anodic and irreversible process, meanwhile photobiotin presents two, adsorptive in nature. This characteristic makes photobiotin desirable for following the interaction between biotin and streptavidin, being possible to detect a streptavidin concentration of 10(-12) M. The evidence of this reaction has been shown either directly in solution or on the electrode surface. Photobiotin as the molecule portable of analytical information and carbon paste as the solid support could be applied to the development of sensors based on the oxidation of this molecule.

A comparison of 1-naphthyl phosphate and 4 aminophenyl phosphate as enzyme substrates for use with a screen-printed amperometric immunosensor for progesterone in cows' milk

4-Aminophenyl phosphate (4-APP) and 1-naphthyl phosphate (1-NP) were compared as enzyme substrates for an amperometric milk progesterone biosensor utilising progesterone-conjugated alkaline phosphatase in a competitive immunoassay format. Cyclic voltammetry of the corresponding hydrolysis products, 4-aminophenol and 1-naphthol, at the surface of screen-printed carbon base transducers, uncoated or coated with anti-progesterone monoclonal antibody (mAb) showed well-defined anodic responses for both species, with the more sensitive being 4-aminophenol. Scan rate studies produced evidence that surface mAb could impede the diffusion of 4-aminophenol, but not 1-naphthol, toward the electrode surface. This was supported by computer simulation for the electrochemical rate constant (khet) using 4-aminophenol, which gave values at uncoated and mAb-coated electrodes of 6.5 x 10(-4) and 3.0 x 10(-4) cm s-1, respectively. The applied potential for oxidation of 4-aminophenol was 230 mV lower than for 1-naphthol. Nevertheless, by operating below +400 mV versus a saturated calomel reference electrode, it was possible to obtain a chronoamperometric signal for 1-naphthol in the absence of electrochemical interference from milk. Using mAb-coated SPCEs, calibration curves were obtained for progesterone in oestrus whole cow's milk spiked with standard concentrations over the range 0-50 ng/ml, using either 4-APP or 1NP as enzyme substrate. Precision values for triplicate sensors were 5.3-18.3% for 4-APP and 4.1-12.4% for 1-NP. An assay of real whole milk samples from different cows at various stages of the oestrus cycle produced correlations against a commercial EIA of r = 0.840 and 0.946 for 4-APP and 1-NP, respectively, 1-NP possesses the advantages over 4-APP of being inexpensive, easy to obtain and soluble (1-naphthol cf. 4-aminophenol) at high pH. From these observations, it is concluded that 1-NP is the preferred substrate for use with our proposed milk progesterone biosensor.

Adsorption of immunoglobulin G on carbon paste electrodes as a basis for the development of immunoelectrochemical devices

The attachment of Immunoglobulin G (IgG) to a carbon paste electrode is investigated in this work. Studies of the immobilization of the immunoglobulin on this electrode were carried out. Alkaline phosphatase (AP) has been used as an enzyme label, linked to the antibody, for the determination of the adsorbed immunoglobulin. Various substrates for this enzyme have been tested and alpha-naphthyl phosphate was found to be the best because of the lower oxidation potential of the enzymatic product, alpha-naphthol, and greater sensitivity under the same experimental conditions. An electrodic pretreatment based on an anodic oxidation of the electrode surface in a phosphate media pH 9 was carried out prior to the adsorption step. This activation is also suitable for the removal of the IgG layer and the regeneration of the electrodic surface after each determination, with the intention of using the same electrode in the subsequent assays. A good reproducibility of the signal was achieved in this way (Relative Standard Deviation, R.S.D. = 3.7%). Using cyclic voltammetry, IgG labelled with AP has been quantified in a range from 2 x 10(-12) to 7 x 10(-11) M and a detection limit of 2.3 x 10(-12) M (signal-to-noise ratio = 3) was found. Finally, human IgG was quantified under non-optimized conditions on the electrodic surface through the reaction with AP labelled IgG using two different immunological designs.

Amperometric detection of alkaline phosphatase activity at a horseradish peroxidase enzyme electrode based on activated carbon: potential application to electrochemical immunoassay

Amperometric detection of alkaline phosphatase activity has been achieved using 5-bromo-4-chloro-3-indolyl phosphate (BCIP) as the enzyme substrate. The production of hydrogen peroxide from the dephosphorylation of BCIP was measured using an activated carbon electrode with horseradish peroxidase immobilised to its surface by simple passive adsorption. This method was easily capable of measuring 10(-12) M alkaline phosphatase and had a calculated detection limit of 2.2 x 10(-14) M. The horseradish peroxidase electrode system was investigated further as a method for non-competitive electrochemical enzyme immunoassay using thyrotropin (TSH) as the model analyte. This was realised by co-immobilization to the electrode surface of both horseradish peroxidase and an anti-thyrotropin monoclonal antibody. After addition of the analyte, a second biotinylated anti-thyrotropin monoclonal antibody and the substrate, streptavidin-labelled alkaline phosphatase was added and the current (generated by enzyme channelling of hydrogen peroxide) measured as a function of TSH concentration. Thus, the activated carbon electrode was used as a combined immunological capture phase and amperometric detection system.

Amplified electrochemical immunoassay for thyrotropin using thermophilic beta-NADH oxidase

The use of the highly stable, pH insensitive flavoenzyme, reduced nicotinamide adenine dinucleotide oxidase (NADH oxidase) from the thermophilic organism Thermus aquaticus in combination with alcohol dehydrogenase in an amperometric amplified immunoassay for thyrotropin (TSH) is described. NADH oxidase catalyses the oxidation of reduced nicotinamide adenine dinucleotide (NADH) with concomitant two electron reduction of di-oxygen to hydrogen peroxide. Hydrogen peroxide can be detected by oxidation at a platinum electrode poised at +650mV vs. Ag/AgCl. The enzyme amplification system described has advantages over existing amplification techniques in terms of sensitivity, specificity and operational pH dependence. The electrochemical enzyme-amplified assay for TSH was compared with a spectrophotometric enzyme-amplified system and with a non-amplified electrochemical immunoenzymometric TSH assay. The dynamic range of the electrochemical enzyme-amplified TSH immunoassay was 0.2-100 mIU/l, which was four times that of the enzyme-amplified spectrophotometric assay while the detection limits of both techniques were comparable.