Nanostructure Molecular Assemblies Constructed Based on <i>Ex-Situ</i> and <i>In-Situ</i> Layer-by-Layer Ferrioxamation Characterized by Electrochemical and Scanning Tunneling Microscopy Methods

Abstract

Nanostructure assemblies based on ferrioxamation of gold-mercaptopropionic acid, Au-MPA-DFO-Fe(III), are prepared via in-situ and ex-situ approaches for the first time and described by cyclic voltammetry (CV), square wave voltammetry (SWV), electrochemical impedance spectroscopy (EIS) and scanning tunneling microscopy (STM) techniques. The assemblies formed via the two approaches show different structures, topologies, and electrochemical kinetics. The STM images show that the Au-MPA-DFO layer is formed in rodlike arrays with 3−4 nm peak-to-peak distances, while the arrays of the Au-MPA-DFO-Fe(III) layers formed by the in-situ and ex-situ methods, respectively with sharp and round ridges, have similar peak-to-peak distances of 2−3 nm. The CVs reveal that the Fe(III) ion immobilized by the ex-situ method is more stable by 14.48 kJ/mol. The relative amounts of the adsorbed Fe(III) ions via the in-situ and ex-situ methods, Γex-situ/Γin-situ, obtained by CV and EIS are 1.33 ± 0.13 and 1.37 ± 0.19, respectively. Faradaic and impedimetric surface titrations both predict a surface-pKa of ∼9.4 for the Au-MPA-DFO self-assembled monolayer (SAM).