Organosilane-modified (island-type) electrodes of 5 and 10 mu m were fabricated and used to detect trace amounts of metal ions using atomic force microscopy. The smaller electrode had a lower limit of detection due to the enhancement in electron tunneling through metal ions that are adsorbed between the conductive-tip (mobile) and the surface (fixed) electrode. To simulate the detection of metal ions at concentrations below 10(-3) mM, a sectional adsorption mechanism is proposed, which satisfactorily explains the adsorption of Cu(II) and Hg(II) to the amine and thiol moieties of the 5-mu m-sized electrode.