A regenerable and ultrasensitive voltammetric biosensor is described for the determination of thrombin. It relies on a combination of (a) enzymatic catalysis, (b) a G-quadruplex/hemin DNAzyme system, and (c) AuPd nanoparticles for signal amplification. Poly(o-phenylenediamine) was decorated with AuPd nanoparticles and loaded with horseradish peroxidase (HRP) and thrombin aptamer (TBA), and the mixture was allowed to interact with hemin to form the G-quadruplex/hemin/HRP/AuPd/poly(o-phenylenediamine) bioconjugates. In the presence of thrombin, the bioconjugates remain immobilized on the surface of the modified glassy carbon electrode through a sandwich reaction. Poly(o-phenylenediamine) also acts as a redox mediator, and the electrochemical reaction of poly(o-phenylenediamine) in the presence of H2O2 is efficiently catalyzed by HRP, AuPd nanoparticles and G-quadruplex/hemin as the peroxidase mimics. Thus, a remarkably amplified electrochemical signal is obtained by the triple catalytic amplification. The biosensor has a dynamic range that spans the 100 f. to 20 nM thrombin concentration range, and the detection limit is 20 fM. The biosensor can be regenerated by applying an electrochemical desorption technique that breaks the gold-thiol bond and releases the components from the surface. In our perception, the mediator-free and signal-amplified biosensor demonstrated here has a large potential with respect to the quantitation of thrombin in clinical samples.
