Electrochemical properties of self-assembled films of single-walled carbon nanotubes, monolayer-protected clusters of gold nanoparticles and iron (II) phthalocyanines at gold electrodes

Abstract

This dissertation investigates the heterogeneous electron transfer dynamics and electrocatalytic behaviour of the following molecules immobilized on gold electrode: (a) 2-dimethylaminoethanethiol (DMAET), with and without integration with poly (m-aminobenzenesulfonic acid) functionalised single-walled carbon nanotubes (SWCNT-PABS); (b) SWCNT-PABS and iron (II) phthalocyanine nanoparticles (nanoFePc); (c) Colloidal gold / Gold nanoparticles (AuNP) and nanoFePc (d) ; water-soluble iron (II) tetrasulfophtalocyanine (FeTSPc) and SWCNT-PABS, and (e) novel monolayer protected gold nanoparticles (MPCAuNPs) by means of either (i) layer-by-layer (LBL) self-assembly or (ii) self-assembled monolayer (SAM) fabrication strategy. Atomic force microscopy and electrochemical studies (cyclic voltammetry, and electrochemical impedance spectroscopic) were used to monitor the substrate build-up, via strong electrostatic interaction. The surface pKa of DMAET was estimated at 7.6, smaller than its solution pKa of 10.8. It is also shown that SWCNT-PABS is irreversibly attached to the DMAET SAM. For layered films involving SWCNT-PABS and nanoFePc (Au-DMAET- SWCNT-PABS-nanoFePc) n (n=1-5 layers) as the number of bilayers increase, the electron transfer kinetics of the [Fe(CN) 6]3-/4 redox probe decreases. On the contrary, LBL assembly involving AuNP and nanoFePc (Au-DMAET-AuNP-nanoFePc) n (n=1-4 layers) shows an increase followed by a decrease in electron transfer kinetics subsequent to the adsorption of nanoFePc and AuNP layers, respectively. For SAMs involving FeTSPc and SWCNT-PABS, the mixed hybrid (Au-DMAET-SWCNT-PABS/FeTSPc) exhibited fastest charge transport compared to other electrodes. For the novel MPCAuNPs, the protecting or stabilizing ligands investigated were the (1-sulfanylundec-11-yl) tetraethylene glycol (PEG-OH) and the (1-sulfanylundec-11-yl) polyethylene glycolic acid (PEG-COOH). Three different mass percent ratios (PEG-COOH : PEG-OH), viz. 1:99 (MPCAuNP-COOH1%), 50:50 (MPCAuNP-COOH50%) and 99:1 (MPCAuNP-COOH99%) were used to protect the gold nanoparticles. The impact of these different ratios on the electron transfer dynamics in organic and aqueous media was explored. The average electron transfer rate constants (ket / s-1) in organic medium decreased as the concentration of the surface-exposed –COOH group in the protecting monolayer ligand increased: MPCAuNP-COOH1% (~ 10 s-1) > MPCAuNP-COOH50% (~ 9 s-1) > MPCAuNP-COOH99% (~ 1 s-1). In aqueous medium, the trend is reversed. This behaviour has been interpreted in terms of the hydrophobicity (quasi-solid nature) and hydrophilicity (quasi-liquid nature) of the terminal –OH and –COOH head groups, respectively. The ionization constants of the terminal groups (i.e., surface pKa) was estimated as ~ 8.2 for the MPCAuNP-COOH1%, while both MPCAuNP-COOH50% and MPCAuNP-COOH99% showed two pKa values of about 5.0 and ~ 8.0, further confirming the hydrophilicity / hydrophobicity of these surface functional groups. Hydrogen peroxide (H2O2), epinephrine (EP) and ascorbic acid (AA) were used as model analytes to examine electrocatalytic ability of these nanostructured assemblies. The electrochemical reduction of H2O2 at a constant concentration was amplified upon increasing bilayer formation of SWCNT-PABS and nanoFePc, while SWCNT-PABS/FeTSPc showed the best response towards the detection of epinephrine. MPCAuNP-COOH99% showed an excellent suppression of the voltammetric response of the AA and an enhanced electrocatalytic activity towards the detection of EP compared to the other MPCAuNPs.