Complementary applications of Scanning Kelvin Nanoprobe microscopy and electrochemical techniques

Abstract

Scanning Kelvin Nanoprobe (SKN) microscopy is a new technique, based on Lord Kelvin's theory of contact electrification, which is able to measure changes in the work function of a surface with nanometre- scale precision. This technique has great potential in the analysis of surface chemistry, especially that of self-assembled monolayers and biochemical interactions. This thesis examines the potential of SKN microscopy in analytical chemical applications. SKN microscopy is used and contrasted against a range of other analytical techniques, including atomic force microscopy (AFM), confocal Raman spectroscopy and common electrochemical techniques such as cyclic voltammetry and electrochemical impedance spectroscopy. A short background is given on SKN microscopy, as well as the other analytical techniques used. This is followed by a chapter discussing the theory behind basic electrochemistry and the electroanalytical techniques used in the thesis. Work is then presented on the hydrogen bonding environment in mixtures of dimethylsulfoxide (DMSO) in water. Confocal Raman spectroscopy showed that the hydrogen bonding environment in DMSO:water mixtures had a profound effect on the S-H (thiol) group of the amino acid cysteine, as well as on the thiol groups of the small-molecule protein analogue BMC. As molecular biologists use DMSO:water mixtures extensively in studies, this work brings up important issues concerning these experiments. SKN microscopy was used with a range of other analytical techniques including AFM, Raman spectroscopy and cyclic voltammetry to probe the formation and characteristics of a new derivative of phthalocyanine synthesised to avoid crystalline formation and to naturally form an amorphous thin-film. Films were deposited on a variety of substrates, including gold, high-order pyrolytic graphite and glassy carbon, and were compared with a range of other phthalocyanine compounds. Simple computer modelling was also carried out on the compound. The derivative was found to form nanoporous films which allowed the passage of positively-charged molecules less then 7Å in diameter. Self-assembling monolayers of organothiols on gold were then probed using the SKN and electrochemical impedance spectroscopy. Selections of linear, branched, cyclic, aromatic and biological organothiols were tested. The SKN was capable of directly measuring the length of a linear alkanethiol from the change in work function of the monolayer. The SKN also proved capable of measuring the degree of organisation of the monolayer - branched and cyclic alaknethiols, which are expected to form looser-packed layers, recorded more significant changes in work function. These results were confirmed by the use of electrochemical impedance spectroscopy to measure the effects of a monolayer on an electrode surface.