Electrochemical impedance spectroscopy of the interaction of STMpep9 with recombinant CDK2. (a) EIS φ(ω) data for a gold surface functionalized with STMpep9 (orange circles) and subsequently exposed to recombinant CDK2 (blue circles). (b) As (a) but following exposure of STM to rCDK2. (c) Phase shift (Δφ(ω)) of STMpep9 and an STM layer on two independent gold electrodes following exposure to rCDK2. (d) Concentration dependence of the phase shift at 70 Hz. The solid line is a linear fit to the data (see text) provided as a guide to the eye.