Recognition of cations by self-assembled monolayers of crown ethers

Self-assembled monolayers (SAMs) of crown ether adsorbates on gold reversibly bind metal ions from aqueous solutions. The resulting changes of the electrochemical properties of the monolayers were monitored by impedance spectroscopy. The increased dielectric constant of the layer due to the complexation of ions results in an increase of the monolayer capacitance (CML). Analysis of the response curves with a Langmuir isotherm enables the determination of association constants of the SAMs with various metal ions. The cation binding also influences the charge-transfer resistance (RCT) of a redox couple Ru(NH3)62+/3+ in the electrolyte. Comparison of both responses allows an accurate interpretation of the origin of the resistive response. Furthermore, the association constants enable the quantitative determination of interactions between SAMs and metal ions, using either capacitive or resistive responses.