Scanning Tunneling Microscopy Observations of Proton and Metal Cation Catching Behavior of Embedded Bipyridine Thiols in Alkanethiol Self-Assembled Monolayers on Au(111)

Abstract

We studied the ion catching behavior of nanoscale molecular domains of bipyridine thiol embedded in alkanethiol self-assembled monolayers (SAMs) on Au(111) by scanning tunneling microscopy (STM) to construct and demonstrate the possibility of a molecular sensor. We used a bromine-terminated bipyridine thiol molecule, in which a bipyridine unit is connected to a conjugated phenylene–ethynylene system, and then successfully observed the protonation response of the bipyridine molecule. When the Au substrate was immersed into dilute CF3SO3H solution, the apparent height of bipyridine spots became lower after this treatment. On the other hand, when the substrate was immersed into 0.1 mM Pd complex solution, the apparent height of bipyridine spots became higher than that in the initial state. These findings demonstrate the possibility of constructing cation sensor using several molecular domains, and the different ion catching behaviors of bipyridine molecules may lead to the development of highly selective molecular sensors.