Biofunctionalization
of Si(111)7×7 by “Renewable” l‑Cysteine
Transitional Layer
- Publication date
- 2014
- Publisher
Abstract
Surface functionalization of an inorganic
surface with bio-organic
molecules is often aimed at creating a “permanent” bio-organic
surface with receptor functional groups. We show here that l-cysteine can be used to transform a highly reactive Si(111)7×7
surface to not just a permanent bio-organic surface but also a semipermanent
(or renewable) and a temporary bio-organic surfaces by manipulating
the exposure. In the early growth stage, the strong bonding between
the first cysteine adlayer and the Si substrate through Si–N
or Si–S linkages in unidentate or bidentate arrangement provides
permanent biofunctionalization by this interfacial layer. This interfacial
layer can be used to build a transitional layer (second adlayer) mediated
by interlayer vertical hydrogen bonding between an amino group and
a carboxylic acid group. Further exposure of cysteine eventually leads
to a zwitterionic multilayer film involving electrostatic interactions
between cation (−NH<sub>3</sub><sup>+</sup>) and anion moieties
(−COO<sup>–</sup>). The interlayer hydrogen bonding
therefore provides temporary trapping of bio-organic molecules as
the second transitional layer that is stable up to 175 °C. This
transitional layer can be easily removed by annealing above this temperature
and then regenerated with the same molecular layer or a different
one by “renewing” the interlayer hydrogen bonds. We
also illustrate coverage-dependent adsorption structures of cysteine,
from bidentate to unidentate attachments and to self-assembled multimers,
involving formation of intralayer horizontal N···H–O
hydrogen bonds, by combining our X-ray photoemission data with the
local density-of-state images obtained by scanning tunnelling microscopy