The binding of metal ions to self-assembled nitrilotriacetic acid (NTA)-modified thioalkane monolayers was monitored by impedance spectroscopy via a capacitance change of the blocking gold electrode. Binding to a fluorescent NTA-derivative and the native NTA, measured in bulk by fluorescence quenching and isothermal titration calorimetry, was determined for comparison. Surface and bulk absolute dissociation constants K-abs of metal ion-NTA complexes for these three derivatives showed no significant differences. Cu2+ concentrations as low as 0.5 nM (30 ppt) could be detected by impedance spectroscopy. This concept of metal ion trace detection might be further extended to other chelating groups for use in future applications.