Self-assembled monolayers (SAMs) of 2-[(6-mercaptohexyl)oxy]methyl-12-crown-4 ( 1 ) and 2-[(6-mercaptohexyl)oxy]methyl-15-crown-5 ( 2 ) on gold are able to bind cations reversibly from aqueous solutions. This binding of the electrochemically inactive cations is monitored by cyclic voltammetry and impedance spectroscopy using the redox couple Ru(NH 3 ) 6 2+/3+ as “reporter ion”. Gold electrodes modified with 1 detect Na + even in the presence of a 100-fold excess of K +; SAMs of 2 are K + selective. As a result of the high surface density of crown ethers, sandwich complexes are formed. Reduced surface concentrations of receptors in mixed monolayers of 2 with heptanethiol prevent the sandwich complexation and decreases the K + /Na + selectivity of the SAM.
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Flink et al. (1998) studied this question.