Monitoring the activity of tyrosinase on a tyramine/dopamine-functionalized surface by force microscopy.

Tyrosinase activity is monitored by pi-donor-acceptor force interactions between a bipyridinium-modified AFM tip and the biocatalytic reaction product generated on a tyramine- (or dopamine-) modified surface. Upon oxidation of the surface to dopaquinone as a result of tyrosinase activity, force interactions are switched "OFF". After reduction of the resulting surface with ascorbic acid, forces are quantitatively reestablished as a result of the formation of the dopamine-functionalized surfaces. The method provides a general approach to design biosensors using force interactions as the readout signal.