Observation of removal of an Fmoc protecting group by scanning tunneling microscopy

We demonstrate here by imaging successive surface reactions in self-assembled monolayers (SAMs) on Au(1 1 1) at molecular scale with a scanning tunneling microscope (STM): (i) SAM matrices formation with 1-octanethiol on Au(1 1 1) in ethanol, (ii) insertion of N-Fmoc-aminooctanethiol into the SAM matrices in ethanol, and (iii) removal of the Fmoc protecting group with tris(2-aminoethyl)amine (TAEA). The total reaction is formation of SAMs containing a small amount of NH2 terminated molecules in the CH3 terminated SAM matrices. After the reaction of the protecting group with TAEA, STM imaging revealed the decrease in heights of the inserted molecules on average. We attributed this observation to removal of the protecting group by taking account of a convolution of electronic and topographic contributions to observed STM heights. Apparent areas of the terminal groups, however, became larger on removal. The increase in the areas was attributed to water adsorption to the NH2 terminal group under air.