Formation kinetics and stability of phosphonate self-assembled monolayers on indium-tin oxide

Graphical abstract: Display Omitted Highlights: → Formation kinetics and stability of phosphonate monolayers on indium-tin oxide. → Electrochemical impedance spectroscopic and cyclic voltammetric measurements. → Three carboxy-terminated phosphonate monolayers were tested. → The apparent formation rate constant was determined by a Langmuir adsorption model. → The rate increases with increasing phosphonate concentration. - Abstract: Phosphonate self-assembled monolayers (SAMs) have been widely used for the surface modification of indium-tin oxide (ITO) electrodes; however, their formation kinetics and stability are not well understood. In this paper, we describe our electrochemical studies of the formation kinetics and stability of a series of phosphonate SAMs on ITO electrodes. In particular electrochemical impedance spectroscopic (EIS) and cyclic voltammetric (CV) measurements have been carried out on three carboxy-terminated phosphonate SAMs by using Fe(CN)63-/4- as redox indicators. The dependence of the charge-transfer resistance (obtained from the EIS plots) on the incubation time allows an estimation of the apparent fractional surface coverage of phosphonate SAMs. The apparent formation rate constant (kobs) was determined by fitting the experimental data to a Langmuir adsorption model. For 3-phosphonopropanoic acid (PPA), the kobs value increases when the PPA concentration increases in the deposition solution, and is smaller than those of thiolate SAMs on Au. The stability of phosphonate SAMs was investigated in three different media (pure water, phosphate-buffered saline (PBS) solution, and ambient air condition). It has been shown that the phosphonate SAMs are rather stable in either PBS solution or ambient air condition.