Biomimetic ion-binding monolayers on gold and their characterization by AC-impedance spectroscopy

Novel ion-binding monolayers on gold surfaces are presented where the molecular design is based upon the natural ion binder ferrichrome. The new ion binders possess hydroxamate coordinating groups arranged in C-2 symmetry (bishydroxamate binder, BHB) or C-3 symmetry (trishydroxamate binder, THB), and a separate dialkyl sulfide moiety, which serves as an anchor to the gold substrate. The separation between the ion-binding cavity and the attachment site to the gold allows each parameter to be controlled separately, namely, cavity size, its symmetry and external envelope. as well as,he functional group used for immobilization. The monolayers were characterized with respect to ellipsometric thickness, wettability (advancing and receding contact angles (CAs) for water), and surface coverage; the latter is determined by metal underpotential deposition (UPD). It is shown that the introduction of hydrophobic side chains (i-butyl) improves the CAs, thickness, and surface coverage of the monolayers. A detailed analysis of the alternating-current (AC) impedance spectra is presented for THB monolayers on gold electrodes, where the impedance data are fitted to an equivalent circuit model. It is shown that the AC response in a wide frequency range can be used to probe ion binding and release in monolayer systems on electrodes.