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 +, Ba 2+, and Ca 2+ ) 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.