Formation and modification of a binary self-assembled monolayer on a nano-structured gold electrode and its structural characterization by electrochemical impedance spectroscopy
By Park, Byung-Wook; Yoon, Do-Young & Kim, Dong-Shik
Published in Journal of Electroanalytical Chemistry
2011
Abstract
Formation and electrochemical characterization of uni- and binary-self-assembled monolayers (SAMs) are presented. One of the purposes of this study is to show the effects of a gold nanoparticle (AuNP)-modified surface on the reductive desorption of SAMs. The binary SAM consisted of 3-mercaptopropionic acid (MPA, HS–(CH2)2–COOH) and 1-tetradecanethiol (TDT, HS–(CH2)13CH3) was formed on a bare gold (Au) and AuNP-modified electrode. Then, MPA was removed off the electrode’s surface using selective desorption technique to form a uni-SAM of TDT with defect sites. Comparisons of structures and electrochemical characteristics were performed using electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). Electrochemical parameters investigated for the comparisons were charge transfer resistance (Rp), double layer capacitance (Cdl), electron transfer rate constant (ket), and the time constant (Ï„et). It was demonstrated that the AuNP-modified electrode not only enhanced the electrochemical properties of the electrode, but also it affected the further SAM formation on and desorption from the electrode tailored with a binary SAM. The obtained nano-structures of the gold electrodes provided higher electrocatalytic activity than a planar gold electrode as characterized by EIS. The electron transfer rate constant, ket, of the AuNP electrode (2.741 X 10-2/s) turned out to be about two times greater than the bare electrode. When both MPA and TDT were layered on the AuNP, ket lowered 10 times from the bare AuNP to 3.070 X 10-3/s. However, for the bare gold, the lowering effect of the binary SAM on ket was much greater than the AuNP, where ket (1.343 X 10-5/s) of the MPA + TDT SAM was three orders of magnitude lower than that of the bare gold. When only TDT remained on the surface after selective desorption of MPA, ket raised 27 times to 3.661 X 10-4/s for the bare gold, and for the AuNP, ket doubled from 3.070 X 10-3 of the binary SAM. For the each case, the AuNP showed higher ket than the bare gold by an order to three orders of magnitude. The effect of SAMs on ket was much more significant for the bare gold than for the AuNP, and a possible explanation was discussed. The TDT SAM-modified AuNP electrode had medium ket and Cdl, which indicates a fast and sensitive electrochemical electrode. Selective desorption for two different SAMs proved to be a valid technique to control the surface coverage of a binary SAM on the gold nano-structure.