Self-assembled monolayers (SAMs) of thiol-derivatized 4-acyl-5-pyrazolone on gold electrodes were used for ion-channel-mimetic sensing of inorganic cations. The SAMs on gold electrodes were characterized with reductive-desorption and capacitance measurements. The pH dependence of the cyclic voltammograms (CVs) of [Fe(CN)(6)](3-) and [Ru(NH3)(6)](3+) as electroactive markers suggested the protonation not only of the beta -diketone part but also of the nitrogen moiety in the pyrazolone. With regard to the voltammetric responses to metal cations, an increase and a decrease in redox current of [Fe(CN)(6)](3-/4-) and [Ru(NH3)(6)](3+/) 2+, respectively, were observed with increasing concentrations of the trivalent cations examined (La3+, Gd3+, Yb3+ or Al3+) from 10(-6)-10(-4) up to 10(-2) M at pH 5.5, at which the chelating group is present as its deprotonated form. In contrast, such responses were negligible in the presence of up to 10(-2) M divalent cation (Mg2+, Ca2+, Sr2+ or Ba2+), Li+ or Na+. The order of the magnitudes of the responses was Al3+ > Yb3+ approximate to Gd3+ C > La3+ >> divalent cations, which is quite similar to that of the stability of 1:1 and 1:2 complexes between a beta -diketonate-type chelate and the metal ions. The highly selective responses to trivalent cations seem to reflect the selectivity of the chelating group as well as the large change in the surface charge induced by the complexation.