Application of displacement principle for detecting heavy metal ions and EDTA using microcantilevers

A microcantilever sensor based on a reversible displacement reaction was developed. The demonstration was performed with the Ni–NTA–EDTA sample system. Ni ions, a typical heavy metal, were injected and adsorbed onto a NTA-functionalized AFM cantilever in a fluid cell. This adsorption resulted in tensile stress which was detected by monitoring the corresponding deflection of the microcantilever. In the next step, EDTA was injected into the cell to release the adsorbed Ni ions from the surface of the cantilever. Since EDTA molecules have a stronger affinity for Ni ions compared to NTA, they compete with and eventually replace the NTA molecules and take over the binding positions on Ni. The displacement of Ni ions from the cantilever by EDTA, followed by water flow removes the tensile stress and restores the cantilever deflection to its original value. In this way, both heavy metal ions and EDTA can be specifically recognized. From the cantilever deflection, the concentration of Ni ions and EDTA can be quantified. The detection limits are presently at the micromolar level. Another weaker Ni ion binder, imidazole, was unable to dissociate Ni ions from NTA and produce similar effects. The demonstrated principle can be used for microcantilever sensors which are capable of regeneration for multiple uses. The method presented can be generalized to detect other reactants.