Selective K+ recognition at the interface during self-assembly of a bis-podand thiol on a gold surface

A novel example of selective K+ recognition (over Na+, Cs+, Ba2+. and Ca2+) during the formation of a self-assembled monolayer (SAM) from a bis-podand thiol (4) on gold is reported. The ion incorporation in the monolayer was monitored by impedance spectroscopy measuring the capacitance as well as the change in the charge-transfer resistance (R-ct) of the modified gold electrode. Results suggest that two different binding states of K+ exist in the monolayer structure, one that is strongly bound (primary) and another that is weakly bound (secondary). The primary K+ ions can be removed irreversibly, but the process is very slow, in contrast to the nearly reversible and fast binding nature of the secondary type. K+ does not bind to preformed SAMs, only to monolayers assembled in the presence of K+. Reflection-absorption infrared spectroscopy (RAIRS) indicates a more compact and oriented monolayer when grown in the presence of K+, providing indirect evidence of the templating process.