Capacitance Measurements of Antibody−Antigen Interactions in a Flow System

Abstract

Capacitive immunosensors were made by coupling monoclonal antibodies to thioctic acid, which had self-assembled on a gold electrode. Surface areas that were not covered were plugged with 1-dodecanethiol to make the layer dense and insulating. Cyclic voltammetry showed that the hexacyanoferrate redox reactions were blocked by this procedure. The capacitance of the electrode was evaluated from the current transients obtained when a potentiostatic step was applied. The immunosensor was placed in a flow system, and a capacitance decrease could be observed after injection of an unlabeled antigen. It was linear over almost three decades when plotted vs the logarithm of the antigen concentration. Human chorionic gonadotropin hormone could be determined in the range 1 pg/mL-1 ng/mL, with a detection limit of 0.5 pg/mL (15 10(-15) M). A similar response was obtained with immobilized F(ab)2 fragments. No cross-reactivity was observed with the thyrotropic hormone, which has one chain in common with gonadotropin. Monoclonal antibodies toward interleukin-2 immobilized on the immunosensor gave also a response over 1 pg/mL-1 ng/mL, with a detection limit of 1 pg/mL. An immunosensor with monoclonal antibodies toward human albumin gave a calibration curve with lower slope than the other proteins but still with a detection limit of 1 pg/mL.

A radioimmunoassay method for detection of DNA based on chemical immobilization of anti-DNA antibody

“…It is noteworthy that the concentration of antibody used in this study (~50 µg/ml) was about 20-40 times diluted, compared to those used in previous experiments (1.0-2.1 mg/ml) (Duan and Meyerhoff, 1994;Welsch et al, 1996;Berggren and Johansson, 1997). This is important since the antibody concentration used in previous methods is not realistic in practical applications due to the high cost of antibodies.…”

“…We chose anti-DNA anti-body as a model system for immobilization because of the increasing importance of detecting DNA in many research and clinical fields. We used a lower concentration of antibody in immobilization reaction compared to those used in other reports (Duan and Meyerhoff, 1994;Welsch et al, 1996;Berggren and Johansson, 1997), demonstrating higher efficiency of immobilization. Anti-DNA antibody was immobilized on a solid surface that was chemically derivatized by using self-assembled monolayer formation of thioctic acids, and we confirmed the activity of chemically immobilized anti-DNA antibody.…”

A Reusable Capacitive Immunosensor Based on a CuS Ultrathin Film Constructed by Using a Surface Sol-Gel Technique

“…However, a label-free immunosensor 6,7 has provoked more interest for many reasons, such as simplicity and real-time compared with the traditional immunoassay technology, like enzyme-linked immunosorbent assay (ELISA) 8 and radioimmunoassay. 9 Among those immunosensors, the capacitive immunosensor has attracted more interest with high sensitivity, rapid test time and common instrumentations [10][11][12][13][14][15] compared with other types of direct biosensors, such as surface plasmon resonance, 16,17 quartz crystal microbalance (QCM), 18 surface acoustic wave, 19 and surface scanning detection. 20 The fabrication of a biocompatible layer is more pivotal to a capacitive immunosensor.…”

“…Otherwise, the capacitance of the diffuse layer of the electrode might utmost contribute to the total capacitance of the immunosensor, which would induce a capacitance decrease, caused by the interaction of an antibody-antigen, which could not be prominently detected. 21,22 Self-assembled monolayers (SAMs) on noble-metal surfaces were often employed in biosensor technology based on capacitance measurements. 23,24 Compounds containing thiol, 25 sulfide 26 and disulfide 27 groups were often chosen to spontaneously form functional films to immobilize biomolecules.…”

“…So, to block any possible pinholes or defects on the surface, the electrode should be assembled once again with one kind of long-chain alkanethiol solution, such as n-dodecanethiol. 22 However, such a treatment might cause a loss of activity and sensitivity of the capacitance immunosensor. Recently, Jiang et al reported a capacitive immumosensor based on an antibody-embedded ultrathin γ-alumina sol-gel film for the direct determination of liver fibrosis markers.…”