Figure -b shows the results of electrochemical aptamer sensors for vancomycin with MCH and MCO passivation layers repeatedly scanned in target-free serum at 37 ˚C for over 1 week. The data in Figure show the expected early stage foulant passivation of monolayer defects within the first few hours, but also show that after 24 hours, monolayer desorption for MCH sensors actually accelerated compared to those tested in PBS. This is evident by the excessive increase in sensor background currents such that redox tag currents became virtually indistinguishable by day 3 (Figure ). We speculate this is due to foulants creating an energetic bridge between the surface-bound and solubilized states for monolayer molecules (Figure ). For example, adjacent foulant and alkylthiolate molecules could interact through attractive inter-molecular interactions that in turn may energetically favor desorption compared to desorption in buffer conditions.