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.