Optical chip immunoassay for hCG in human whole blood

We report on the development of an integrated optic chip sensor for perform ing rapid and sensitive immunoassays with human whole blood using human cho rionic gonadotropin (hCG) as the model system. The optical chip is based on the Hartman interferometer, which uses a single planar lightbeam to addres s multiple interferometers, each comprising a signal/reference pair of sens ing regions. The binding of antigen to specific capture antibodies on the s ignal sensing region causes a change in the refractive index of the surface layer, which is detectable by its effect on the evanescent field of the gu ided lightbeam. The reference-sensing region is coated with an irrelevant a ntibody, which optically cancels a large fraction of the non-specific adsor ption that occurs on the specific-sensing region when the sensor is tested with clinical specimens. This work extends previous experiments with buffer and human serum to measurements in undiluted whole human blood. Optical ch ips were stored dry after surface functionalization, and rehydrated with bl ood. Colloidal gold nanoparticles conjugated to a second anti-hCG monoclona l antibody were used to provide signal amplification, thereby enhancing ass ay sensitivity, in a one-step procedure with the gold conjugate added to th e test sample immediately prior to measurement. Background signals due to n on-specific binding (NSB) in blood were found to be higher than those previ ously reported with human serum. In addition, a high level of background si gnal was found with the gold conjugate, which had not been observed in expe riments with either buffer or serum. Nevertheless, hCG could be detected at 0.5 ng/ml within 10 min of sample application. The sensor response was lin ear over the concentration range 0.5-5 ng/ml hCG, as compared with the clin ically-relevant range 0.3-1.5 ng/ml. Detection at higher concentrations was affected by scattering from large amounts of bound gold nanoparticles. How ever, initial binding rate measurements could be used to maintain assay qua ntitation. (C) 2000 Elsevier Science S.A. All rights reserved.