Direct and homogeneous immunoassay for IgG analyses

Sol-gel-derived thick-film amperometric immunosensors

Sol-gel processing is used for the first time for the preparation of electrochemical immunosensors. One-step sensor fabrication, based on the coupling of sol-gel and screen-printing technologies, is employed. A low-temperature cured ink is prepared by dispersion of rabbit immunoglobulin G (RIgG), graphite powder, and a binder in the sol-gel solution. The enzyme-labeled antibody can readily diffuse toward the encapsulated antigen, which retains its binding properties, and the association reaction is easily detected at the dispersed graphite surface. Use of anti-RIgG labeled with alkaline phosphatase, naphthyl phosphate as the substrate, and amperometric detection at +400 mV (vs Ag/AgCl) results in a low detection limit of 5 ng/mL (32 pM) for the solution antigen. Tailoring the porosity of the ceramic-carbon matrix can be used for tuning the assay performance. The high sensitivity, low cost, durability, and simplicity of the new single-use immunosensors make them well suited for various on-site applications.

Homogeneous liposome immunoassay for insulin using phospholipase C from Clostridium perfringens

A new homogeneous liposome immunoassay system was developed in which analyte-phospholipase C conjugates are used instead of complement or melittin. This system was applied for the determination of insulin. Liposomes incorporated with calcein as a marker were prepared by the reverse-phase evaporation method. The lysis of liposomes was determined by measuring the fluorescence intensity of calcein released from liposomes and it was increased with increasing concentration of phospholipase C. Insulin was conjugated to phospholipase C by a three-step procedure with hetero-bifunctional crosslinking reagents, succinimidyl 4-(N-maleimidomethyl)cyclohexane-1-carboxylate and 3-(2-pyridyldithio)propionic acid N-hydroxysuccinimide ester. The lytic activity of phospholipase C was not affected by the reaction for conjugation. Both p-nitrophenylphosphatidylcholine hydrolytic activity and liposome lytic activity of insulin-phospholipase C conjugate were inhibited in the presence of insulin antiserum. However, lower concentration of conjugate and shorter incubation time were required when liposomes were used in the inhibition study. The antibody inhibition of conjugate-induced lysis could be reversed by addition of a competing amount of free insulin. The standard calibration curve was obtained in the range between 4 and 130 microIU/ml (r = 0.994). The detection limit (8 microIU/ml) was comparable with those of conventional heterogeneous enzyme immunoassays. This new assay approach offers a simple, sensitive, and inexpensive testing procedure for determining ultratrace amounts of bioactive substances.

A homogeneous fluorescence polarization assay for detection of antibody to Brucella abortus

A homogeneous fluorescence polarization (FP) assay (FPA) was developed for detection of antibody in bovine sera to Brucella abortus. The assay used O-polysaccharide prepared from B. abortus lipopolysaccharide in the molecular weight range of 20-30 kDa which was conjugated with fluorescein isothiocyanate and used as a tracer. Fluorescence polarization was measured with a FPM-1 fluorescence polarization analyzer. Sample (20 microliters) was added to 2.0 ml of diluent buffer at ambient temperature. A serum blank reading was taken and tracer (10 microliters) to yield approx. 1.5 nM fluorescein equivalents was added. The FP of the tracer was determined after a period of greater than 2 min. A positive reaction was indicated by a significant elevation of the FP reading over the negative control. In a blind study, 9480 bovine sera were tested in addition to sets of four controls which were included with each lot of 100 samples tested. The controls were a strong positive, a weak positive, a negative and a serum derived from a B. abortus strain 19 vaccinated cow. Test sera included 8669 sera from Canadian cattle which were negative by routine serological tests, 561 sera from cows from which B. abortus had been isolated either from tissues or milk and 250 sera from cattle previously vaccinated with B. abortus strain 19 at various times. One lot of O-polysaccharide tracer was used for all tests. The initial cut-off for negative samples in the fluorescence polarization assay was set at 107.2 mP. This resulted in a sensitivity estimate of 98.1 +/- 1.1% and the specificity was 99.8 +/- 0.09%. After decoding the samples and retesting false positive and negative reactions, the sensitivity estimate was 98.5 +/- 1.0% and the specificity was 100%. It became evident that the initial cut-off value was set too high and, using ROC analysis, a cut-off of 90 mP increased the sensitivity to 99.02% while the specificity decreased to 99.96%. Of the 250 sera from vaccinated cattle, 248 were negative giving a point specificity value of 99.2%.