Au nanoparticle conjugation for impedance and capacitance signal amplification in biosensors

Amplification of the electrochemical impedance and capacitance signals in a biosensor is demonstrated for the model fluorescein/anti-fluorescein system. Following immobilization of fluorescein onto Au through formation of a self-assembled monolayer, goat anti-fluorescein conjugated with 10-nm Au nanoparticles is introduced into the system. This results in an increase in the capacitance of similar to 400 nF/cm(2), whereas no change can be observed for goat anti-fluorescein without the Au nanoparticle conjugate. An even greater sensitivity is obtained by introduction of a redox probe, [Fe(CN)(6)](3-/4-), whereby the charge-transfer resistance (R-ct) is reduced to similar to 25% of its original value. This allows construction of high-sensitivity electrochemical impedance biosensors at a single low frequency, where the signal is sensitive to the interfacial R-ct. This change in the electrochemical impedance signal upon binding to goat anti-fluorescein conjugated with Au nanoparticles can be attributed to the much higher electrochemical activity of Au surfaces relative to the underlying organic layer.