Recognition of cations by self-assembled monolayers of crown ethers

Self-assembled monolayers (SAMs) of crown ether adsorbates on gold reversib ly bind metal ions from aqueous solutions. The resulting changes of the ele ctrochemical properties of the monolayers were monitored by impedance spect roscopy. The increased dielectric constant of the layer due to the complexa tion of ions results in an increase of the monolayer capacitance (C-ML). An alysis of the response curves with a Langmuir isotherm enables the determin ation of association constants of the SAMs with various metal ions. The cat ion binding also influences the charge-transfer resistance (R-CT) Of a redo x couple Ru(NH3)(6)(2+/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 determinati on of interactions between SAMs and metal ions, using either capacitive or resistive responses.