Study On Electrochemical Immunosensors Based On Ag@Au Nanorods

Electrochemical immunosensor, combining the specificity of immunoassay and advantages of electroanalysis, has become one of the most popular research fields of electroanalysis. Nanomaterials not only can serve as the ideal substrates for loading of the biomacromolecules onto electrode surface, but also can be applied to develop novel ideas and strategies for electrochemical immunosensors. Ag@Au nanorods is a kind of core-shell alloy nanomaterial with high particle stability, large specific surface, high catalytic activity and good biocompatibility. In this thesis, Ag@Au nanorods ?Ag@AuNRs? were prepared and characterized with techiniques including UV-visible spectrophotometry, transmission electron microscopy and X-ray diffraction. Three novel electrochemical immunosensors based on Ag@Au nanorods were constructed. Details contents were described as follows:?1? Ag@AuNRs were utilized to construct a novel sandwich-type electrochemical immunosensor. The sensor was prepared by immoblizing capture antibodies on the amine-terminated nanocomposite of carbon nanofibers-polyamidoamine dendrimer ?CNFs-PAMAM?. After a sandwich type immunoreaction, the Ab₂ labeled by Ag@AuNRs were captured onto an electrode surface and the thick layer of silver on nanorod resulted in strong electrochemical signal. The results of cyclic voltammetry indicated that CNFs-PAMAM nanocomposite with abundant amino-functional groups could enhance the oxidative peak current of silver dramatically. After the parameters optimization, with human IgG as a model analyte, the immunosensor displayed excellent analytical performance, the oxidative peak current of silver was proportional to the concentration of IgG in a wide linear range from 1fg·mL^-1 to 1000pg·mL^-1 with a low detection limit of 0.5 fg·mL^-1. Besides, compared with the traditional immunosensor based on silver enhancement, the sensor using the "built-in" silver in Ag@AuNRs as label avoided the non-specific and time-consuming silver deposition procedure.?2? Fe₃O₄ magnetic nanospheres bestrewed with Ag@AuNRs via the PDA coating were prepared and utilized as nanoelectrocatalysts to construct a novel sandwich-type electrochemical immunosensor. The immunosensor was prepared by immobilizing the capture antibodies on the amine-terminated nanocomposite of carbon nanofibers-chitosan, whilst the trace tag was prepared by loading detection antibodies on the Ag@Au-Fe₃O₄ nanocomposite. Comparing the electrocatalytic characteristic of the nanomaterial modified electrode using cyclic voltamogram, the nanohybrid Ag@Au-Fe₃O₄ exhibited much better electrocatalytic activity toward the reduction of hydrogen peroxide than Fe₃O₄ nanospheres or Ag@AuNRs due to the synergetic catalytic effect. After the parameter optimization, the amperometric signal increased linearly with human IgG concentration in the broad range from 0.1 pg·mL^-1 to 5 ?g·mL^-1 with a detection limit of 50 fg·mL^-1. The results showed that the recovery in human serum was in the range of 94.0 to 106.0%. Meanwhile, electrochemical immunosensor using the Ag@Au-Fe₃O₄ as an enzyme mimic nanocatalysts showed higher chemical stability when compared with the sensor based on enzymes.?3? A novel dual-signal and label-free immunosensor for sensitive detection of carcinoembryonic antigen ?CEA? was developed based on Ag@Au-CNFs-Chit nanocomposite modified electrode, which produced a highly sensitive silver oxidation peak and showed a high electrocatalytic activity towards the reduction of H₂O₂. The nanocomposite was prepared by directly mixing Ag@Au NRs and CNFs-Chit and utilized for the immobilization of anti-CEA antibodies on the electrode surface. The results of cyclic voltamograms showed that CNFs-Chit nanocomposite could enhance the oxidation of nanosilver of Ag@AuNRs, and Ag@Au-CNFs-Chit exhibited much better electrocatalytic activity toward the reduction of hydrogen peroxide than CNFs-Chit or Ag@AuNRs. The sensing mechanism was based on the variation of reduction current of H₂O₂ catalyzed by or the oxidation peak current of silver enhanced by Ag@Au-CNFs-Chit nanocomposite. Under the optimal conditions, a linear range of 0.01pg·mL^-1 to 1ng·mL^-1 and 0.01pg·mL^-1 to 10ng·mL^-1 were obtained, respectively, and a detection limit of 1 fg·mL^-1 was obtained from two kinds of signals. The sensor exhibited better analytical performance superior to several time-consuming sandwich-type immunosensors.