This work describes, for the first time, the electrochemical and electrocatalytic properties of self-assembled layers of single-walled carbon nanotubes (SWCNTs) intergrated with selected organo-iron complexes and Cysteamine (Cys) forming a base on gold electrodes. The organo-iron complexes selected for this study were octa(hydroxyethylthio)phthalocyaninatoiron(II) (FeOHETPc), tetraaminophthalocyninatoiron(II) (FeTAPc), tetraaminophthalocyninatocobalt(II) (CoTAPc), ferrocene monocarboxylic acid (FMCA), ferrocene dicarboxylic acid (FDCA) or a mixture of SWCNT and FMCA or FDCA. The successful fabrication of these electrodes were established using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and electrochemical techniques such as cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), square wave voltammetry (SWV) and chronoamperometry (CA) The Au-Cys-SWCNT-FeOHETPc electrode exhibited strong dependence on the reaction of the head groups and the pH of the working electrolytes. The high electron transfer capability of the Au-Cys-SWCNT-FeOHETPc electrode over other electrodes as the Au-Cys-SWCNT or the Au-Cys-FeOHETPc or the Au-FeOHETPc suggests that SWCNT greatly improves the electronic communication between FeOHETPc and the bare gold electrode. The electron transfer rate constant (kapp) of Au-Cys-SWCNT-FeOHETPc in pH 4.8 conditions (~1.7 x 10-2 cm s-1) over that of the electrode obtained from SWCNT integrated with tetraaminophthalocyninatocobalt(II) (Au-Cys-SWCNT-CoTAPc) (5.1 x 10-3 cm s-1) is attributed to the possible effect of both the central metal on the phthalocyanine core and subsituents on the peripheral positions of the phthalocyanine rings. This work clearly proved that the aligned SWCNTs arrays exhibit much faster electron transfer kinetics to redox-active species in solutions compared to the randomly dispersed (drop-dried) SWCNTs. The advantageous electron transfer properties of the aligned Au-Cys-SWCNT-FeOHETPc electrode, coupled with its ease of fabrication and electrochemical stability, could be found useful in electrochemical sensing and catalysis. Thiocyanate ion was used as an analytical probe to examine the electrocatalytic properties of these modified gold electrodes. This work shows that SWCNT-FeOHETPc hybrid exhibits excellent sensitivity towards the detection of thiocyanate compared to electrodes containing SWCNT or FeTAPc or FeOHETPc only, indicating the ability of the SWCNTs to function as effective conductive nanowires for the detection of this important analyte. The electrochemical response of the FeOHETPc based electrodes was greater than their FeTAPc-based electrode counterparts, indicative of the impact of peripheral substituents on the phthalocyanine core towards electrocatalytic behaviour of these types of hybrids. Nevertheless, the exchange of the central metal as seen with the case of CoTAPc (see chapter 3), provide useful comparative electrochemical activity of this complexes versus FeTAPc with the same chemical environment with an indication of iron being the best as metal centre. FMCA or FDCA were covalently attached to the base Cysteamine monolayer to form the Au-Cys-FMCA and Au-Cys-FDCA, respectively. The same covalent attachment strategy was used to form the mixed SWCNTs and ferrocene-terminated layers (i.e., Au-Cys-SWCNT/FMCA and Au-Cys-SWCNT/FDCA). The impact of neighbouring SWCNTs on the electron transfer dynamics of the ferrocene molecular assemblies in acidic medium (0.5 M H2SO4) and in a solution (pH 7.2) of an outer-sphere redox probe ([Fe(CN)6]4-/ [Fe(CN) 6]3-) was explored. The electron transfer rate constants in both solution media essentially decrease as Au-Cys-FMCA > Au-Cys-SWCNT/FDCA > Au-Cys-FDCA > Au-Cys-SWCNT/FMCA. This trend has been interpreted in terms of several factors such as the locations of the ferrocene species in a range of environments with a range of potentials, the proximity /interactions of the ferrocenes with one another, and electrostatic interaction or repulsion existing between the negatively-charged redox probe and the modified electrodes. Square wave voltammetry was used to examine the catalytic behaviour of the electrodes. Au-Cys-SWCNT/FDCA proved to be the best electrode, possibly due to the repulsive interactions between the negatively charged SCN- and high number of surface –COOH species at the SWCNT/FDCA. This novel study has provided some useful insights as to how CNTs co-assembled with ferrocene-terminated thiols could impact on the heterogeneous electron transfer kinetics as well as the electrocatalytic detection of the self-assembled ferrocene layers. / Thesis (PhD)--University of Pretoria, 2010. / Chemistry / unrestricted

An experiment has been completed which demonstrated quantum mechanical tunneling of electrons between two gold electrodes separated in vacuum. The tunneling current between the gold electrodes has been measured, for fixed voltages of 0.1 and 0.01 volts, as the electrode spacing was varied from a distance of approximately 2.0 nm down to a point where the electrodes touched. Current-voltage characteristics for fixed electrode spacing in the direct tunneling region have also been measured. Numerical calculations of the tunneling current based on the free-electron model of the electrodes and the barrier, an image-potential reduced barrier, and a WKB approximation for the tunneling probability have been performed and compared with Simmons' theory and with the experimental results. Within experimental error the results indicate that an image potential reduced barrier with the modifications suggested by Lang and Kohn gives a close approximation to the true barrier for metal-vacuum-metal tunneling. For the first time, the work function of the electrodes in a tunneling experiment has been deduced from experimental parameters independent of the tunneling device.

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. / Thesis (PhD)--University of Pretoria, 2010. / Chemistry / unrestricted

O presente trabalho descreve uma forma inédita e simples de construção de eletrodos de ouro (CDtrodos) a partir de discos compactos graváveis (CD-Rs). Os CD-Rs possuem em sua constituição uma película de ouro com uma espessura entre 50 e 100 nm e com área aproximada de 100 cm2. Esta superfície de ouro pode ser facilmente utilizada para construção de eletrodos convencionais usados em células estacionárias ou em células em fluxo. Além disso, a espessura nanométrica do filme de ouro também pode ser utilizada para construção de microeletrodos de banda com áreas inferiores a 10-6 cm2. Devido ao fato do CD-R ser produzido em escala comercial, o custo do mesmo é muito baixo, tornando possível sua utilização na forma de eletrodos descartáveis. Para comprovar a viabilidade da utilização destes filmes na confecção de eletrodos, seu desempenho foi comparado com o de eletrodo de ouro comercial, sendo observado um comportamento análogo. Diferentes maneiras de utilização do disco compacto gravável foram testadas, tanto em amostras sintéticas, como também, em amostras reais. Utilizando o método de redissolução potenciométrica à corrente constante (CCPSA), foram analisadas amostras de mercúrio em água potável e em água do mar, adaptando o CD-R a uma célula de injeção em fluxo. Nestes estudos, o limite de detecção de mercúrio situou-se em 250 ng L-1, utilizando-se tempo de deposição de 300 segundos. Estudos preliminares em células estacionárias levaram a limites de detecção inferiores. Em célula estacionária, foram realizadas análises de cobre em aguardente e em água potável. Em alguns casos, os resultados obtidos com as amostras reais foram comparados com outra técnica (E.A.A.), onde os resultados obtidos foram muito próximos a àqueles obtidos com os eletrodos de ouro. Análises em água de abastecimento, mostraram que em células estacionárias o limite de detecção para cobre situa-se em 30 ng L-1, para tempos de deposição de 600 segundos. / This work describes an original and novel form of construction of gold electrodes (CDtrodes) from recordable CDs (CD-Rs). The CD-Rs contain in its constitution a fine gold layer with a thickness from 50 to 100 nm and an approx. 100 cm2 of area. This surface can be easily used for construction of conventional electrodes for stationary or flow electrochemical cells. Moreover, the nanometer thickness of the gold film can be employed for the construction of microelectrodes with areas smaller than 10-6 cm2. Due to the fact that CD-Rs are produced on commercial scale, the cost of each unit is very low, a feature that enables the use of these electrodes in disposable form. To prove the suitability of these films to make gold electrodes, their performance was compared with the response of a commercial electrode, and a similar behavior was observed. Different applications of recordable compact disk were tested either with synthetic or real samples. Using the Constant-Current Potentiometric Stripping Analysis (CCPSA) method, mercury was analyzed in seawater and drinking water samples, fitting the CD-R to flow injection analysis cell. In these studies, the detection limit for mercury was found to be 250 ng L-1, with a deposition time of 300 seconds. Preliminary studies involving stationary cells allowed lower detection limits for mercury to be attained. With a stationary cell, the analyses of copper in sugar cane spirits and drinking water were carried out. In some instances, comparisons were made between this and another analytical technique (namely A.A.S.) where the results were very close with the ones obtained with the gold electrodes. Analysis of drinking water showed that for stationary cells the detection limit for copper is 30 ng L-1, with a deposition time of 600 s.

O presente trabalho descreve uma forma inédita e simples de construção de eletrodos de ouro (CDtrodos) a partir de discos compactos graváveis (CD-Rs). Os CD-Rs possuem em sua constituição uma película de ouro com uma espessura entre 50 e 100 nm e com área aproximada de 100 cm2. Esta superfície de ouro pode ser facilmente utilizada para construção de eletrodos convencionais usados em células estacionárias ou em células em fluxo. Além disso, a espessura nanométrica do filme de ouro também pode ser utilizada para construção de microeletrodos de banda com áreas inferiores a 10-6 cm2. Devido ao fato do CD-R ser produzido em escala comercial, o custo do mesmo é muito baixo, tornando possível sua utilização na forma de eletrodos descartáveis. Para comprovar a viabilidade da utilização destes filmes na confecção de eletrodos, seu desempenho foi comparado com o de eletrodo de ouro comercial, sendo observado um comportamento análogo. Diferentes maneiras de utilização do disco compacto gravável foram testadas, tanto em amostras sintéticas, como também, em amostras reais. Utilizando o método de redissolução potenciométrica à corrente constante (CCPSA), foram analisadas amostras de mercúrio em água potável e em água do mar, adaptando o CD-R a uma célula de injeção em fluxo. Nestes estudos, o limite de detecção de mercúrio situou-se em 250 ng L-1, utilizando-se tempo de deposição de 300 segundos. Estudos preliminares em células estacionárias levaram a limites de detecção inferiores. Em célula estacionária, foram realizadas análises de cobre em aguardente e em água potável. Em alguns casos, os resultados obtidos com as amostras reais foram comparados com outra técnica (E.A.A.), onde os resultados obtidos foram muito próximos a àqueles obtidos com os eletrodos de ouro. Análises em água de abastecimento, mostraram que em células estacionárias o limite de detecção para cobre situa-se em 30 ng L-1, para tempos de deposição de 600 segundos. / This work describes an original and novel form of construction of gold electrodes (CDtrodes) from recordable CDs (CD-Rs). The CD-Rs contain in its constitution a fine gold layer with a thickness from 50 to 100 nm and an approx. 100 cm2 of area. This surface can be easily used for construction of conventional electrodes for stationary or flow electrochemical cells. Moreover, the nanometer thickness of the gold film can be employed for the construction of microelectrodes with areas smaller than 10-6 cm2. Due to the fact that CD-Rs are produced on commercial scale, the cost of each unit is very low, a feature that enables the use of these electrodes in disposable form. To prove the suitability of these films to make gold electrodes, their performance was compared with the response of a commercial electrode, and a similar behavior was observed. Different applications of recordable compact disk were tested either with synthetic or real samples. Using the Constant-Current Potentiometric Stripping Analysis (CCPSA) method, mercury was analyzed in seawater and drinking water samples, fitting the CD-R to flow injection analysis cell. In these studies, the detection limit for mercury was found to be 250 ng L-1, with a deposition time of 300 seconds. Preliminary studies involving stationary cells allowed lower detection limits for mercury to be attained. With a stationary cell, the analyses of copper in sugar cane spirits and drinking water were carried out. In some instances, comparisons were made between this and another analytical technique (namely A.A.S.) where the results were very close with the ones obtained with the gold electrodes. Analysis of drinking water showed that for stationary cells the detection limit for copper is 30 ng L-1, with a deposition time of 600 s.

Orientador: Lauro Tatsuo Kubota / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Química / Made available in DSpace on 2018-08-17T19:48:10Z (GMT). No. of bitstreams: 1 Passos_RafaelaFernandaCarvalhal_D.pdf: 3256536 bytes, checksum: 0fc4c2467e2c741531641a57e7a10925 (MD5) Previous issue date: 2010 / Resumo: Este trabalho apresenta o esforço dispendido na construção e caracterização de transdutores eletroquímicos sobre poliéster e papel, e também demonstra o emprego destas células eletroquímicas descartáveis no desenvolvimento de um biossensor para análises rápidas de salicilato em sangue, bem como a criação de um dispositivo de separação associado à detecção eletroquímica em papel. Cada célula eletroquímica é composta por um conjunto de três eletrodos de filmes finos construídos em ouro sobre poliéster ou papel cromatográfico por meio das técnicas de sputtering e electron-beam, respectivamente. Foi realizada a caracterização voltamétrica dos sistemas eletródicos empregando sondas redox como hexacianoferrato(II) de potássio, hexacianoferrato(III) de potássio e ácido ferrocenomonocarboxílico em meio eletrolítico, a fim de verificar a eletroatividade dos mesmos. Foi possível verificar que mesmo apresentando maior área eletroativa, os eletrodos de filmes finos construídos sobre papel apresentam uma menor densidade de corrente para as sondas redox em comparação com a célula eletroquímica construída em poliéster. Isto se deve à retenção das espécies eletroativas na fibra de celulose, fato que diminui a disponibilidade da espécie na superfície eletródica. Foi desenvolvido um biossensor amperométrico para a determinação de salicilato em sangue. O biossensor se baseia no emprego da enzima Salicilato hidroxilase imobilizada sobre a célula eletroquímica plástica. As condições experimentais otimizadas consistem em utilizar uma solução eletrolítica de tampão fosfato em pH 7,6 com 0,5 mmol L de NADH e 300 mV vs. Au como potencial aplicado durante as medidas. O biossensor apresentou adequada sensibilidade (97,4 nA/mmol L de salicilato) e faixa linear de resposta para o analito (1,25 10 to 1,0 10 mol L). O desempenho do biossensor foi verificado na determinação de salicilato em amostras de sangue dopadas com o analito e os resultados foram estatisticamente equivalentes àqueles obtidos com o método espectrofotométrico de Trinder em um nível de confiança de 95%. O dispositivo de separação cromatográfica em papel associado à detecção eletroquímica foi desenvolvido empregando a célula eletroquímica plástica e a célula eletroquímica sobre papel. O desempenho dos dispositivos foi avaliado na separação e quantificação de ácido úrico e áscórbico presentes em mistura. O método desenvolvido é uma alternativa para a determinação de compostos eletroativos em que o baixo custo e a simplicidade são essenciais / Abstract: This paper presents the efforts to the construction and characterization of electrochemical transducers on polyester and paper, and also demonstrates the use of these disposable electrochemical cells in the development of a biosensor for rapid analysis of salicylate in blood as well as the creation of a separation device associated electrochemical detection on paper. Each electrochemical cell consists of a set of three electrodes made of gold thin films on polyester or chromatographic paper using sputtering and electron-beam techniques, respectively. The electrochemical characterization of the systems with redox probes as potassium hexacyanoferrate(II), potassium hexacyanoferrate(III) and ferrocene monocarboxylic acid was performed in the electrolyte solution in order to evaluate the electroactivity of them. It was verified that even with higher electroactive area, the electrodes of thin films built on paper have a lower current density for the redox probes in comparison with the electrochemical cell constructed on polyester. This is due to retention of electroactive species in the cellulose fiber, a fact that reduces the availability of those species on the transducer surface. We developed an amperometric biosensor for the determination of salicylate in blood. The biosensor is based on the use of the salicylate hydroxylase enzyme immobilized on plastic electrochemical cell. The determined optimized experimental conditions are: an electrolyte solution of phosphate buffer at pH 7.6 with 0.5 mmol L of NADH and 300 mV vs. Au as the applied potential during the measurements. The biosensor showed adequate sensitivity (97.4 nA / mmol L salicylate) and linear response range for the analyte (1.25 10 to 1.0 10 mol L). The performance of the biosensor was found in the determination of salicylate in blood samples spiked with the analyte and the results were statistically equivalent to those obtained with the Trinder¿s spectrophotometric method, with a 95% confidence level. Prototypes of microfluidic paper-based separation devices with amperometric detection were developed and evaluated. The chromatographic separation on paper associated with electrochemical detection was developed using the plastic electrochemical cell and the gold electrochemical cell on paper. The performance of both devices was evaluated for separation and quantification of uric acid and ascorbic acid presented in the mixtures. The method is an alternative for the determination of electroactive compounds when low cost and simplicity are essential / Doutorado / Quimica Analitica / Doutor em Ciências

Orientador: Lauro Tatsuo Kubota / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Química / Made available in DSpace on 2018-08-06T00:45:50Z (GMT). No. of bitstreams: 1 Carvalhal_RafaelaFernanda_M.pdf: 708037 bytes, checksum: ed89f8b11b63131a7f452b5191b27432 (MD5) Previous issue date: 2005 / Mestrado / Quimica Analitica / Mestre em Química

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This MSc. Thesis deals with electrochemical studies of the analytical utilization of self-assembled monolayers of thiolated calix[4]arene (C4A) and/or undecanthiole (C11SH) on the surface of a polycrystalline gold electrode. Hydroquinone, nitrobenzene, 2-nitrophenole and 4-nitrophenole have been used as tested model compounds. The real surface area of the electrode was determined using these two methods: From the charge of desorption peak of α-gold oxides (5.72±0.48 mm2 ) and using the Randles- Ševčík equation. (6.14±0.59 mm2 ). The preparation of the electrode consisted of polishing on alumina, immersing into piranha solution and cyclic voltammetry. The area of electrode surface was well reproducible. Stability of thiol monolayer in the area of negative potentials was tested. The C11SH layer is very stable; the desorption peak was recorded in basic solutions only (at −1.3 V vs. 3M Ag/AgCl). The position of the desorption peak of C4A is more pH dependent and it was recorded at −1.08 V at pH 13. The properties of the electrode modified by C11SH and or C4A were investigated using the model compound hydroquinone. It is assumed that the oxidation of the hydroquinone does not take place inside the C4A cavity; hence the hydroquinone molecule is too big to pass through the cavity. C11SH and C4 + C11SH act...

This thesis presents the development and redesign of a commercial electrochemical quartz crystal microbalance with dissipation (E–QCM–D). This was used to study factors affecting the efficiency of the four electron reduction catalysed by the fuel cell enzyme bilirubin oxidase from Myrothecium verrucaria immobilised on thiol modified gold surfaces. Within this thesis, the E–QCM–D was used to show that application of a constant potential to bilirubin oxidase adsorbed to thiol-modified gold surfaces causes activity loss that can be attributed to a change in structural arrangement. Varying the load by potential cycling distorts the enzyme by inducing rapid mass loss and denaturation. Attaching the enzyme covalently reduces the mass loss caused by potential cycling but does not mitigate activity loss. Covalent attachment also changes the orientation of the surface bound enzyme as verified by the position of the catalytic wave (related to the overpotential for catalysis) and reactive labelling followed by mass spectrometry analysis. The E–QCM–D was used to show how electrostatic interactions affect enzyme conformation where high pH causes a reduction in both mass loading at the electrode and a reduction in activity. At pH lower than the enzyme isoelectric point, there is a build up of multilayers in a clustered adsorption. When enzyme adsorbs to hydrophobic surfaces there is a rapid denaturation which completely inactivates the enzyme. Changing the surface chemistry from carboxyl groups to hydroxyl and acetamido groups shows that catalysis is shifted to more negative potentials as a result of an enzyme misorientation. Further to this, increasing the chain length of the thiol modifier indicates that an increased distance between surface and enzyme reduces activity, enzyme loading and results in a conformational rearrangement that permits electron transfer over longer distances.