Assembly of organic layers onto carbon surfaces
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This thesis presents the study of organic layers covalently assembled onto carbon surfaces. As a result of their attachment, the properties of carbon surfaces were controllably adjusted so that these surfaces could be used for desired applications. In order that a wide range of properties were imparted onto the carbon surface, many different modifiers were attached and thoroughly characterised. Three applications that the modified carbon surfaces were used for were the subsequent coupling of molecular species, adsorption of protein and assembly of aldehyde/sulfate-functionalised polystyrene (PS) and citrate-capped gold nanoparticles (NPs). Finally, patterning of different organic layers at pre-determined spatially defined locations on the one carbon surface was also investigated. The carbon surfaces used in this work were glassy carbon (GC) and pyrolysed photoresist film (PPF) surfaces. For PPF, methods for the reproducible fabrication of electrochemically suitable surfaces were investigated. The properties of GC and PPF surfaces are very similar apart from the surface roughness. PPF has near atomic smoothness and has RMS roughness values that are approximately four times smaller than GC. The first series of modifier layers attached to the carbon surfaces was via the oxidation of seven different primary amines. The different layers allowed the modulation of the wettability of the surface. Both n-tridecylamine (TDA, monoamine) and 1,12-diaminododecane (DAD, diamine) are able to form multilayers. The stability of TDA and DAD layers were tested by scanning, soaking and sonicating the layers in different media. Changes in the layer were monitored by the probe response of ferrocene monocarboxylic acid (FCA). However, atomic force microscope (AFM) depth profiling experiments showed that changes in the probe response did not indicate cleavage of the covalently attached layer and mechanisms are proposed to account for the changes in the response of the probe. Surface concentrations of the amine modifiers were estimated by the coupling of an electrochemically active species, FCA and nitrobenzoyl chloride (NBC). The electrochemical reduction of the 4-nitrophenylethylamine (NPEA) layer in acid caused the layer to 'shrink'. Surface concentration estimates of NPEA from acid reduction of layers with different thicknesses suggested that only a limited fraction of the p-nitrophenyl groups were reduced in acid. However, in ACN (acetonitrile)/0.1 M [Bu4N]BF4 (tetrabutyl ammonium fluoroborate) the relationship between the concentration of electroactive surface groups and layer thickness was linear. The other series of modifiers that was attached to alter the surface properties was performed by the reduction of aryl diazonium salts. Subsequent coupling reactions of tetraethylene glycol diamine (TGD) to para methylene carboxylic acid phenyl (MCA) and NBC to electrochemically reduced para nitro phenyl (NPh) layers were carried out. Surface concentrations of NPh as estimated from reduction scans was higher when reduction was performed in ethanol/water compared to acid. Four peaks at N1s binding energies were observed in x-ray photoelectron spectroscopy (XPS) spectra for both acid and ethanol/water reduced layers. The ability of attached amine and aryl layers to modulate the adsorption of protein was investigated using fluorescently labelled protein, bovine serum albumin-fluorescein isothiocynate (BSA-FITC) and fluorescence microscopy. TGD, para methyl phenyl (MP), para hexyl phenyl (HP) and para polyethylene glycol phenyl (PEG)-modified GC surfaces promoted protein adsorption relative to as-prepared GC, whereas n-hexylamine (HA) and polyethylene glycol diamine (PGD) layers reduced protein adsorption. The assembly of two types of NPs, aldehyde/sulfate-functionalised PS and citrate-capped gold NPs, onto amine-containing modifiers layers was examined. Citrate-capped gold NPs were synthesised and characterised. The surface coverage of the gold NPs was controlled by using different modifiers of different chemical compositions, tuning the modification conditions and adjusting the immersion time, concentration and pH of gold NP solution. Approaches to creating patterns of modifiers in pre-determined spatially defined locations on GC and PPF surfaces using poly(dimethyl)siloxane (PDMS), poly(vinyl)alcohol (PVA) and thin metal films were investigated. With the "fill-in" approach using PDMS, the smallest pattern of modifiers was the parallel lines with a line width of 20 µm and straight edges and was created by performing electrochemistry in PDMS microchannels which has not been previously investigated. Visualisation techniques, based on optical and scanning electron microscopy, were demonstrated for the molecular patterns.