Electrochemical Quartz Crystal Microbalance Study of Azurin Adsorption onto an Alkanethiol Self-Assembled Monolayer on Gold

A quartz crystal microbalance coupled with electrochemistry was used to examine the adsorption of azurin on a gold electrode modified with a self-assembled monolayer of octanethiol. Azurin adsorbed irreversibly to form a densely packed monolayer. The rate of azurin adsorption was related to the bulk concentration of azurin in solution within the concentration range studied. At a high azurin concentration (2.75 íM), adsorption was rapid with a stable adsorption maximum attained in 2-3 min. At a lower azurin solution concentration (0.35 íM), the time to reach a stable adsorption maximum was approximately 30 min. Interestingly, the maximum surface concentration attained for all solution concentrations studied by theQCMmethod was 25(1 pmol cm-2, close to that predicted for monolayer coverage. The dissipation was monitored during adsorption, and only small changes were detected, implying a rigid adsorption model, as needed when using the Sauerbrey equation. Cyclic voltammetric data were consistent with a one-electron, surface-confined CuII/CuI azurin process with fast electron-transfer kinetics. The electroactive surface concentration calculated using voltammetry was 7(1 pmolcm-2. The differences between theQCMand voltammetrically determined surface coverage values reflect, predominantly, the different measurement methods but imply that all surface-confined azurin is not electrochemically active on the time scale of cyclic voltammetry.