La Siberia rosarina

Characterisation of amine layers

Instead of using direct methods (refer above) to characterise the modifier layer at a carbon surface, selected species have been assembled onto already attached modifiers with the sole intention of gaining information about the modifier layers. The characterisation of the assembled species provided useful information about the modifier layer. The other reason for subsequent assembly was to investigate the versatility of the surface-bound modifiers as tethers.

Assembly of electroactive species onto amine-modified carbon surfaces has been widely studied. Metal complexes were assembled onto the ethylene diamine-modified carbon fibres via electrostatic interactions. The positive charge on the protonated amine groups of the ethylene diamine-modified carbon fibres was initially compensated by chloride ions when immersed in concentrated HCl. The chloride anions were then exchanged with noble metal complexes such as [Au(III)Cl4]-, [PtCl4]2- and [PdCl4]2-, as

determined from the electrochemical behaviour of the noble metals in the presence of the anionic components when attached to the carbon fibres.35 3,5-dinitrobenzoyl chloride was

Chapter 1: Introduction

_________________________________________________________________________ also covalently coupled to the ethylene diamine-modified carbon fibres.35 Other examples of electroactive species that have been assembled onto terminal amines of diamine- modified GC surfaces are the heteropolyanion, P2W18O626-, which was attached to 1,7-

diaminododecane via electrostatic interactions with the protonated amines of the modifier layer and ferrocene acetic acid which was covalently coupled to 1,10-diaminododecane via amide bond formation.20

Instead of subsequent assembly to already attached modifiers, amine modifiers with electroactive functionalities have been attached to the surface and their electrochemistry analysed to determine surface concentrations. For example, the reductions of 4- nitrobenzylamine and aminomethyl-9-anthracene attached onto GC and carbon fibres have been observed.12 From the CV scans, the reduction of these surface-bound species in ACN/electrolyte solution was not fully reversible and upon repeat scans the response slowly disappeared. This was accounted for by the protonation of the radical anion by residual water.12 GC surfaces have also been modified with 3-nitrobenzylamine and 4- nitrobenzylamine and from their electrochemical behaviours in DMF and ACN/electrolyte solutions, their estimated surface concentrations were found to be close to those for a calculated compact layer.30

Rather than using electroactive species to gain information about the amine layers, using functionalities that can be observed by XPS was also a useful strategy for analysis of the layers. 4-fluorobenzoyl chloride was coupled to ethylene diamine attached to carbon fibres. The fluorine group was easily detected by XPS and it was suggested from the F : N ratio that 75% of the modifiers were in a looped arrangement where both amines were

Assembly of Organic Layers onto Carbon Substrates

_________________________________________________________________________ surface-bound and the remaining 25% were attached via one amine.12 Epichlorohydrin was also subsequently attached to ethylene diamine and hexamethylene tetramine-modified carbon fibres and the chlorine group was detected by XPS.12 2-amino-4-methyl thiazole- modified carbon fibres were also analysed using XPS before and after electrooxidation treatment. No cleavage of the layer was observed.12

Catalytic surfaces and sensors

Amine-modified surfaces have been used for the assembly of species that have good catalytic ability and are useful for the fabrication of sensor devices. The strategy commonly utilised for assembly has been the layer-by-layer deposition involving electrostatic interactions, where alternating layers were assembled on top of each other until the multilayers were formed. The deposition of zirconium-phosphonate multilayers was carried out on amine-modified GC surfaces. Zirconium-1,10-decanediylbis(phosphonic acid) multilayers were attached by firstly modifying surface attached butylamine, decylamine, octadecylamine and 6-aminohexanol with phosphoryl chloride. The resulting phosphate sites were subsequent reacted with zirconium acetylacetonate followed by alkylbis(phosphonic acid) and these two adsorption steps were repeated as desired.32 1,7- diaminoheptane-modified GC surfaces were used for assembly of Dawson-type tungstodiphosphate anion P2W18O626- and Ru(NH3)63+ multilayers, and the resulting surface

allowed for the detection of trace amounts of iodate.37

Another related example is the immobilisation of polyoxometalates (inorganic metal-oxygen cluster anionic compounds) onto a GC surface modified with 4-

Chapter 1: Introduction

_________________________________________________________________________ aminobenzoic acid. Alternating combinations of species have been assembled on the 4- aminobenzoic acid-modified surface where one layer was quaternised poly(4-vinylpyridine) complexed with [Os(bpy)2Cl]2+/+ (QPVP-Os) and the other layer were polyoxometalates

such as Pr(SiMo7W4O39)213-19 and SiW12O404-,18 giving multilayers. The SiW12O404-/QPVP-

Os/4-aminobenzoic acid/GC surface showed to have electrocatalytic abilities for the reductions of HNO2, BrO3- and H2O2. Instead of polyoxometalates, P2W15V3O629-,48 a

sandwich-type compound Fe4(H2O)2(As2W15O56)212- 47 and PdCl42- 43 were also attached

using an identical strategy. The P2W15V3O629-/QPVP-Os/4-aminobenzoic acid/GC electrode

possessed high catalytic ability for the electroreduction of BrO3- and NO2- and the

electrooxidation of ascorbic acid48 and the Fe4(H2O)2(As2W15O56)212-/QPVP-Os/4-

aminobenzoic acid/GC surface was able to catalyse the electroreduction of NO2- and

H2O2.47 The sandwiched PdCl42- anions were then electrochemically reduced to give

palladium nanoparticles (NPs) and the resulting NP surface was able to enhance the electroreduction of dissolved oxygen and the electrooxidation of hydrazine compounds in aqueous solutions.43 The layer-by-layer method has also been used to assemble platinum NPs to 4-aminobenzoic acid-modified GC electrodes. The [tetrakis(N-

methylpyridyl)porphyrinato]cobalt cation and PtCl62- were alternately assembled by

electrostatic interactions, forming multilayers. The sandwiched PtCl62- ion was

electrochemically reduced to yield the platinum NPs and the NP surface was able to catalyse the electroreduction of oxygen.38,39 Instead of the PtCl62- anion, SiW12O404- 40,41 and

Assembly of Organic Layers onto Carbon Substrates

_________________________________________________________________________ demonstrated catalytic activity for the hydrogen evolution reaction and the electroreduction of oxygen.40,41

Transition metal substituted polyoxometalates ZnW11M(H2O)O39n- (M = Mn, Cu, Fe,

Co, Cr, Ni, Zn)42 and bis-Keggin-type heteropolyanion Nd(SiMo7W4O39)213- 45 were

assembled directly onto 4-aminobenzoic acid-modified GC surfaces. These surfaces were shown to have electrocatalytic behaviour for the reduction of H2O2 and BrO3- and/or

HNO2.42,45 Copper hexacyanoferrate multilayers49 were also assembled onto 4-

aminobenzoic acid-modified GC surfaces.

The p-benzoquinone-modified electrode which was fabricated by subsequent coupling of p-benzoquinone to an ethylene diamine-modified GC surface was used for the detection of hydrogen peroxide in a flow injection system.34 Aminopyridyl compounds, 4- aminopyridine, 4-aminoethylpyridine and imidazole, acted as tethers for the attachment of cobalt(II) tetraphenylporphyrin. The nitrogen on the pyridyl group was the fifth ligand of the complex. The efficiency for the electroreduction of carbon dioxide was enhanced by having a tether group between the complex and electrode.24

Biosensors

Many different biological molecules have been immobilised onto amine-modified surfaces. The applications of the immobilised lipids, enzymes, protein and DNA as biosensors have typically been investigated. Potential biosensors, for the detection of K+ for example, were fabricated from the construction of a hybrid bilayer membrane on a GC surface by the interaction of the lipid monolayer with the hydrophobic surface. A

Chapter 1: Introduction

_________________________________________________________________________ hydrophobic carbon surface was achieved by electrochemical modification with octylamine, dodecylamine and octadecylamine.23 One method for immobilising enzymes such as glucose oxidase, horseradish peroxide and dehyrogenase at a GC surface was using a 4- aminomethylphenylboronic acid tether.25 The immobilised enzymes maintained good catalytic activity, as demonstrated by the electrocatalytic reduction of H2O2.25 The protein

avidin was immobilised onto N-(5-aminopentyl)biotinamide-modified GC surfaces which have potential uses in biosensing platforms.17

Calf thymus DNA was deposited onto 4-aminobenzoic acid-modified GC surfaces by layer-by-layer deposition using poly(diallyldimethylammonium chloride)46 or QPVP-Os

44

as the alternating layer. Noradrenaline and β-nicotinamide adenine dinucleotide have been coupled to ethylene diamine-modified GC surfaces using glutaraldehyde.34

The activities of biological molecules have also been shown to be increased at amine-modified electrode. The electrochemical oxidation of uric acid has been shown to be catalysed at a cysteine-modified GC electrode compared to an unmodified electrode.50 Di(n-aminopropyl)viologen and p-benzoquinone-modified GC electrodes mediated the electron transfer of the enzymes, desulfovibrio vulgaris hydrogenase and horseradish peroxidase.34 The p-benzoquinone-modified electrode was fabricated by subsequent coupling of p-benzoquinone to an ethylene diamine-modified GC surface.34 Oxidation of β- nicotinamide adenine dinucleotide was facilitated by dopamine immobilised at a GC electrode.17

Attachment of modifiers with selective capabilities for different analytes has shown to be useful for sensing devices. Examples included the ability of the aspartic acid-22 or β-

Assembly of Organic Layers onto Carbon Substrates

_________________________________________________________________________ alanine-modified21 GC electrodes to differentiate between dopamine and ascorbic acid, due to charge discrimination of the negatively charged carboxyl groups of the surface-bound modifiers for protonated dopamine over ascorbic acid.