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Self-Assembled Monolayer-Modified Electrodes
The self-assembled monolayer (SAM) may be described as the ordered arrangement of spontaneously adsorbed molecules (such as thiol species) from solution directly onto the surface of an appropriate substrate (such as gold) resulting in the formation of an ultrathin film [1,41]. In 1946, four decades after Langmuir [42] introduced the concept of monolayers, Zisman [43] demonstrated the self-assembly of alkylamine monolayers onto a platinum substrate. Since then a number of adsorbate/substrate SAM forming combinations have been found that are able to form SAMs e.g., alkyltrichlorosilanes on glass [44] and fatty acids on metal oxides [45]. In 1983, Allara and Nuzzo [46] showed the adsorption of sulphur-containing compounds onto gold surfaces. However, it was only after the numerous articles published in 1987 and 1988 that there was an influx of thiol based SAM research [47-50]. Since then the fabrication of ultrathin, well-ordered selfassembled monolayer films of thiol-derived organic molecules on gold substrates have been a major research interest due to the potential ability of such ultrathin films to be used as scaffolds in a plethora of nanotechnological applications and fundamental studies including the immobilization of biomolecules (e.g., proteins, DNA) and redox-active functional materials for catalysis and sensing. For example, several potential applications of carbon nanotubes mean that some of their future applications in catalysis, sensing and electronics will require their integration on solid substrates as ultrathin nano-scaled molecular films. Sulphur containing compounds (eg. thiols) have a high affinity for gold and are used in the self assembly formation of the base monolayer on gold surfaces. Cysteamine is one such example which also allows for the covalent attachment of other species. In 1997, Caruso et al. [51] introduced a 2-dimethylaminoethanethiol (DMAET) SAM as a platform for integrating DNA on gold surfaces. Since then, no work has been reported on this important SAM. This motivated me to use this innovative SAM as a potential base monolayer for forming multilayer films. However, successful future applications of DMAET SAM is dependent on thorough understanding of the state of the amino head group from which further surface functionality can be derived. Therefore, in this work, I have used cyclic voltammetry and electrochemical impedance spectroscopy to probe the behaviour of the –NH+(CH3)2 head group of DMAET SAM in solutions containing outersphere redox probe (K4Fe(CN)6 / K3Fe(CN)6), different electrolytes and epinephrine.
From an electrochemistry point of view, the chemisorption of thiolates on gold depicted in Figure 1.10 is regarded [1] as the most important class of SAMs. The nature and the formation of the bond between gold surface and the thiol has been a subject of much interest [1,52-55] but it is often recognised that the gold-thiolate bond results from cleavage of the S-H bond. Gold substrates are preferred in thiol SAMs because of the strong interaction between gold and sulphur that allows the formation of monolayers in the presence of many other functional groups [54]. Generally, the central feature of all SAMs is the strong interaction between the functional group of the adsorbate and the bare substrate as well as the van der Waals interactions among the adsorbed molecules
Layer-by-layer Self Assembly
One of the elegant means of forming ultrathin films on solid substrates is the self-assembly used in forming molecular monolayers, and the layer-by-layer (LBL) self-assembly for forming molecular multi-layers. LBL has recently been elegantly reviewed by Zhang et al. [56]. LBL, first discovered in 1966 by Iler [57] and re-discovered in 1991 by Decher and Hong [58,59] may simply be described as controlled coordination or stepwise self-assembly strategy based on the alternating adsorption of materials containing complementary charged or functional groups to form integrated, well-organized arrays of ultrathin superstructure films on solid surfaces [56] that are often less than 1 mm [60,61], on solid surfaces. This strategy could provide a powerful bottom-up approach for the construction of nano-scaled 3D architectures with various properties. Indeed, the simplicity and versatility of this technique makes it admirable for producing high quality and consistent films.
After the introduction of the LBL technique based on physical adsorption driven by ionic attraction, other LBL methods emerged. Some of which were based on hydrogen bonds, step-by-step reactions, sol–gel processes, molecular recognition, charge-transfer, stepwise stereocomplex assembly and electrochemistry [56]. This work concentrates on the LBL method that uses electrostatic interaction as the main driving force. The basic principle in the assembly is “charge reversal”, where oppositely charged material referred to as bilayers are successively adsorbed. Each bilayer is approximately 1-100 nm thick [62] and can be tailored by adjusting the pH [63], counter ion [64], ionic strength [65], chemistry [66], molecular weight [67] and temperature [68]. The original construction of these multilayer films involved the use of polyelectrolytes, and thereafter extended to conjugated polymers [69-71], nanomaterials [72,73], dye and drug crystals [74], organic latex particles [75], inorganic particles [76], protein [77], biological cells [78], DNA [79], antigen-antibody pairs [80], CNTs [81] metallophthalocyanines [82] and monolayer-protected-clusters of gold 83]. This LBL technique produces high quality and consistent films that have a few advantages over other multilayer fabrication processes.
(a) The fabrication of multilayers is a simple method that does not require the use of complicated instruments.
(b) The deposition times and material used during the process are user controlled.
(c)The prospect of adjusting the layer thickness at the nanometer scale results in improved control of the mechanical properties.
(d) Increased versatility of applications subsequent to the development of LBL methods based on intermolecular interactions other than electrostatic.
Furthermore, the LBL assembly does not require only planar substrates as demonstrated by the step-wise build up on a spherical template [84].
However, it does require a surface with the lowest possible roughness since roughness leads to an assembly with poor uniformity and far less stability to electrochemical cycling [85] which inevitably has an adverse affect on the applications.
This relatively new technique already has applications [86] in a few areas which include drug and gene delivery, fuel cells, electrical conductors, photo-detection, biosensing and electrochemical sensing devices. However, this dissertation concentrates on the fabrication of electrochemical sensors via electrostatic interaction using either the self assembly or layer-by-layer assembly processes to incorporate carbon nanotubes, monolayer-protected clusters of gold nanoparticles and MPc complexes.
Given the plethora of potential technological applications of CNTs and MPc complexes and their hybrids, an emerging area of research is on the smart integration of CNTs with MPc complexes for enhancing electrocatalysis and sensing. Reports on the CNT-MPc nanohybrids or thin films have shown these nanohybrids to be more efficient in improving electrochemical responses compared to the bare electrode or the individual CNT or MPc species [87-91]. To improve the physicochemical properties of MPc complexes for potential nanotechnological applications, continued investigation on their nanostructures are essential. Although there are reports on the self-assembled monolayers (SAMs) of electroactive MPc complexes [92-95] and their CNT hybrids [20], there are limited studies on the use of LBL for electroactive MPc complexes. Indeed, the few reports on LBL involving MPc complexes have been those of Oliveira and co-workers [96-99] who only reported the LBL multilayer films of polyelectrolyte cations with the popular and commercially-available, water-soluble tetrasulphonated MPc complexes using ITO based electrode.
Transition metal tetrasulfophthalocyanine complexes, notably iron (II) tetrasulfophthalocyanine (FeTSPc), are highly water-soluble species and well recognized for their high catalytic activity in homogeneous electrocatalysis. The ease with which they are washed away from electrodes during electrochemical studies has long been a major setback and has limited their fundamental studies and potential applications for heterogeneous electrocatalysis in aqueous environment. To date, all available reports on the surface-confinement of water-soluble metallotetrasulfophthalocyanine (MTSPc, where M = central metal ion) complexes involved indium tin oxide (ITO)-coated glass electrodes and LBL strategy using polycationic and/or highlybranched polymeric complexes such as polyamidoamine (PAMAM) dendrimers [97], poly(diaallyldimethylammonium chloride) (PDDA) incorporating poly(2-(methacryloxy)ethyl)trimethylammonium chloride (PCM), poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(4- styrenesulfonate) (PSS) [100], chitosan [98] and PAMAM [99]. Bedioui and co-workers [87] reported the use of drop-casting to immobilize slurry of nickel (II) tetrasulfophthalocyanine (NiTSPc) and SWCNTs onto a glassy carbon electrode (GCE). The most common feature of the LBL strategies includes the use of a cocktail of relatively expensive reagents and the confinement of the MTSPc inside the thick multilayered polymeric films. The main problems usually associated with physical anchorage (i.e., drop-casting method) of such films onto a GCE surface are poor stability as well as the difficulty in controlling the amount of film or thickness deposited. These problems have the tendency to compromise on the application of the films. Therefore, it is without doubt very crucial to continue the search for other means of immobilizing them onto electrode surfaces as thin stable solid films without compromising on their electrocatalytic activity towards the detection of analytes in aqueous conditions. More importantly, integrating such water-soluble redox-active MPc complexes with CNTs (as electron transfer mediators) could provide an interesting synergistic means of further enhancing the electron transfer dynamics of MPc complexes.
Chapter One : Introduction
1.1 General Overview
1.2 Electrochemistry : An Overview
1.3 Modified Electrodes
1.4 Species Investigated as Probe Analytes
1.5 Microscopic Techniques
Reference
Chapter Two : Experimental
2.1 Materials and Reagents
2.2 Apparatus and Procedure
2.3 Electrode Modification and Pre-treatment
Reference
Chapter Three : Results and Discussion
3.2 Single Walled Carbon Nanotubes and nanosized Iron (II) Phthalocyanine modified Gold Electrodes
3.3 Colloidal Gold Nanoparticles and Iron (II) Phthalocyanine Modified Gold Electrodes
3.4 Single Walled Carbon Nanotubes and Iron (II) Tetrasulpho-Phthalocyanine Modified Gold Electrodes
3.5 Monolayer-Protected Clusters of Gold Nanoparticles Modified Gold Electrodes
Reference
Conclusion
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