Atomic force microscopy evidence of citrate displacement by 4-mercaptopyridine on gold in aqueous solution

The displacement adsorption process of 4-mercaptopyridine (4-MP) on trisodi um citrate preadsorbed gold surface in aqueous solution was studied using a tomic force microscopy (AFM). The measurement was done by monitoring the in teraction force between a gold-coated AFM tip and a gold substrate, both pr esaturated by trisodium citrate. The addition of 4-MP was found to result i n a remarkable change of tip-substrate adhesion force, which showed a gradu al increase and finally reached a maximum. Using the chemical force titrati on technique, we estimated the surface pK(1/2) of thus-formed 4-MP displace ment film and compared with pure 4-MP self-assembled monolayer (SAM:) forme d on gold in ethanol solution. Our results indicate that the 4-MP molecules are more strongly adsorbed on gold than citrate anions, mostly via Au-S bo nding, forming a less dense and mixed displacement film. The displacement a dsorption kinetics seems very fast and it takes only ca. 2 min to establish more than 50% surface coverage of 4-MP molecules.