Consecutive Selective Adsorption of Pentamidine and Phosphate Biomolecules on a Self-Assembled Layer: Reversible Formation of a Chemically Selective Coating

In situ ellipsometric film thickness measurements, FT-IR external reflectance spectroscopy, and potentiometric
measurements indicated that the amphiphile pentamidine
(PAM), a bisbenzamidine, associated by self-assembly
with a preformed self-assembled monolayer of a mercaptoalkanoic acid on gold. The structural properties of
PAM
and the nature of the substrate were ideal for the formation of a densely packed monolayer. This process was
fully reversible, as demonstrated by changing the pH of
the surrounding medium. Thus, disassembly−reassembly occurred when the pH was cycled between
8.7 and 3. The bilayer structure, featuring a
positively
charged amidinium surface, was subsequently used for
selective adsorption of phosphate biomolecules. Thus,
selective binding of adenosine and inositol phosphates
(Ka
= 5 × 104 M-1 for ATP)
to the surface, as well as strong
binding of DNA−oligonucleotides, was monitored by in
situ ellipsometry. This system suggests a new
approach
to chemical sensing, based on the reversible formation
of a chemically selective coating.