In electrochemical sensing, the miniaturisation of electrodes leads to enhanced characteristics, including higher signal-to-noise ratio and lower detection limits and sensitivity to external convection due to more efficient mass transport. In recent years, this has generated considerable interest in both the manufacturing and characterisation of nanoelectrodes. However, the high-volume, commercial fabrication of integratable, low cost nanoelectrodes remains a challenge. This work presents a nanoelectrode architecture that can be manufactured using established and well-characterised microfabrication methods. Vertical ring electrodes are fabricated at hole edges using thin film deposition and microlithography techniques. A metal layer of nanometre thickness is sandwiched between two insulators on a substrate followed by the etching of micron scale holes through the stack of layers. This leads to the exposure of a metal nanoband around the hole perimeter and thus a nanoelectrode with the area defined by the hole perimeter and the deposited metal layer thickness. This work first reports a simulation study, which investigates the in uence of design parameters such as band and insulator thicknesses and hole size on the diffusive current. The results show a relative independence of the current to the band thickness and a linear dependence on the hole perimeter with a steady state current comparable to that of a microelectrode. For example, a nanoband electrode with a band thickness of 50nm produces up to approximately half of the limiting current measured on a planar microsquare electrode and a 25 nm electrode produces 88% of the current of a 50 nm electrode. This information contributed to the development of a process for the fabrication of arrays of platinum nanoband electrodes in microsquare holes on a silicon substrate with control over the critical geometric parameters. Electrodes with band thicknesses of 5 nm to 50 nm and a range of square side lengths have been fabricated for experimental validation. Their performance has been compared to microsquare electrode arrays, and was shown to give a similar response to established microdisc and square electrodes. An analysis procedure has been developed and inherent nanoelectrode behaviour and effects have been investigated. The relative importance of a range of nanoeffects on the electrodes has been assessed, indicating a contribution of migration to mass transfer. It has been demonstrated that these nanoband electrodes can be used to detect rapid processes such as the measurement of large electrochemical rate constants, unlike microsquare array electrodes.

Self-assembled monolayers (SAMs) of 4-aminothiophenol (4-ATP) has been successfully deposited onto nanometer-sized gold (Au) electrodes. The cyclic voltammograms obtained on a 4-ATP SAMs modified electrode show peaks and the peak height is proportional to the scan rate, which is similar to that on an electroactive SAMs modified macro electrode. The electrochemical behavior and mechanism of outer-sphere electron transfer reaction on the 4-ATP SAMs modified nanometer-sized electrode has also been studied. The 4-ATP SAMs modified electrode is further modified with platinum (Pt) nanoparticles. Electrochemical behaviors show that there is electrical communication between Pt nanoparticles and Au metal on the Pt nanoparticles/4-ATP SAMs/Au electrode. However, scanning electron microscopic image shows that the Pt nanoparticles are not evenly covered the electrode.

Dans cette thèse, des réseaux de nano- et micro-capteurs électrochimiques et opto-électrochimiques sont fabriqués en utilisant la technique de microfabrication « template synthesis ». Dans une première partie, des ensembles de nanoélectrodes (NEEs) sont utilisés comme plate-forme pour obtenir un biocapteur. Les NEEs sont préparés par déposition d’or dans une membrane poreuse en polycarbonate. L’originalité de notre approche a été de modifier la membrane de polycarbonate entourant les NEEs et non les NEEs elles-mêmes. La peroxydase de raifort (HRP) qui est fixée sur un anticorps secondaire a servi comme marqueur. Cette enzyme catalyse la réduction de H2O2 qui est ajouté en solution. En utilisant un système de détection dérivé de l'approche ELISA (Enzyme- Linked Immuno-Sorbants Assay), le récepteur de la protéine HER2 a été pris comme analyte cible. Il s’agit d’une protéine très importante puisqu’elle permet de dépister le cancer du sein. Dans une seconde partie, un réseau ordonné de sondes opto-électrochimiques est développé sur la face distale d’un faisceau de fibres optiques qui a été attaquée sélectivement par voie humide. Une structure macroporeuse est fabriquée en utilisant un cristal colloïdal comme « template ». L’or est ensuite déposé dans les interstices avant de dissoudre les nanoparticules de latex formant le cristal colloïdal. Ce réseau de microcavités macroporeuses a été testé avec succès comme substrat pour des mesures de Raman exalté de surface (SERS). / In this thesis, arrays of nano- and microelectrodes are developed to obtain electrochemical and optoelectrochemical sensors, by using the template synthesis as a microfabrication technique. In the first part, ensembles of nanoelectrodes (NEEs), obtained using a track-etched polycarbonate membrane as template, are functionalised in order to obtain electrochemical immunosensors. A biorecognition chain, antigen-antibody, is immobilized on the wide polycarbonate membrane letting uncovered the gold nanodisk electrodes. A label redox enzyme, linked to the biorecognition chain, is recognized and quantified electrochemically. Two different detection schemes are developed and low protein detection limits are achieved. In the second part, a macroporous micrometer sized opto-electrochemical sensor is developed on the distal face of an imaging fiber (coherent optical fiber bundle). A microwell array is obtained by controlled chemical etching, by exploiting the different chemical composition between cores and clads. Colloidal templates are created inside the microcavities, using polystirene beads of 280 nm. Gold is deposited inside the cavities, filling the void in the colloidal template, exploiting electroless and electrochemical deposition techniques. The gold macroporous structure inside the wells is successfully tested as SERS substrate.

Doctor of Philosophy / Department of Chemistry / Jun Li / Reduction in electrode size down to nanometers dramatically enhances the detection sensitivity and temporal resolution. Here we explore nanoelectrode arrays (NEAs) and nanoparticles in building high performance biosensors. Vertically aligned carbon nanofibers (VACNFs) of diameter ~100 nm were grown on a Si substrate using plasma enhanced chemical vapor deposition. SiO[subscript]2 embedded CNF NEAs were then fabricated using techniques like chemical vapor deposition, mechanical polishing, and reactive ion etching, with CNF tips exposed at the final step. The effect of the interior structure of CNFs on electron transfer rate (ETR) was investigated by covalently attaching ferrocene molecules to the exposed end of CNFs. Anomalous differences in the ETR were observed between DC voltammetry (DCV) and AC voltammetry (ACV). The findings from this study are currently being extended to develop an electrochemical biosensor for the detection of cancerous protease (legumain). Preliminary results with standard macro glassy carbon electrodes show a significant decrease in ACV signal, which is encouraging. In another study, NEA was employed to capture and detect pathogenic bacteria using AC dielectrophoresis (DEP) and electrochemical impedance spectroscopy (EIS). A nano-DEP device was fabricated using photolithography processes to define a micro patterned exposed active region on NEA and a microfluidic channel on macro-indium tin oxide electrode. Enhanced electric field gradient at the exposed CNF tips was achieved due to the nanometer size of the electrodes, because of which each individual exposed tip can act as a potential DEP trap to capture the pathogen. Significant decrease in the absolute impedance at the NEA was also observed by EIS experiments. In a final study, we modified gold nanoparticles (GNPs) with luminol to develop chemiluminescence (CL) based blood biosensor. Modified GNPs were characterized by UV-Vis, IR spectroscopy and TEM. We have applied this CL method for the detection of highly diluted blood samples, in both intact and lysed forms, which releases Fe[superscipt]3[superscript]+ containing hemoglobin to catalyze the luminol CL. Particularly, the lysed blood sample can be detected even after 10[superscript]8 dilution (corresponding to ~0.18 cells/well). This method can be readily developed as a portable biosensing technique for rapid and ultrasensitive point-of-care applications.

Nanoelectrodes are defined as electrodes which have a critical dimension on the order of nanometres. Due to their smaller dimensions they have a reduced iR drop and enhanced mass transport, which results in the rapid establishment of an enhanced steady-state diffusion profile and a greater Faradaic current density, along with a smaller relative double layer capacitance, which together give a significantly increased signal to noise ratio compared to macroelectrodes. This potentially makes nanoelectrodes better sensors and analytical tools than macroelectrodes in terms of their having lower limits of detection and faster detection times. However, due to difficulties with fabrication most nanoelectrode designs are highly irreproducible which has inhibited their characterisation and commercial development. The Mount group has previously reported the design, fabrication and characterisation of a novel nanoelectrode design in conjunction with Engineers from the Scottish Microelectronic Centre (SMC). Microsquare Nanoband Edge Electrode arrays (MNEEs) consist of an array of cavities with nanoscale Pt bands (formed by sandwiching the metal between insulating layers) exposed around their perimeter. MNEEs are fabricated using a photolithographic process so can be reproducibly made in large quantities to high fidelity. The purpose of this work is to develop our understanding of the fundamental electrochemical behaviour of MNEEs for biosensing. First, a quantitative analysis of the cyclic voltammograms (CVs) and Electrochemical Impedance Spectroscopy (EIS) of macroelectrodes, microelectrodes and MNEE are compared and discussed. Second, their fundamental response is compared in terms of their biosensing properties by using a pre-established impedimetric biosensing protocol developed on macroelectrodes. This protocol uses a PNA probe to detect the mecA cassette of methicillin resistant staphylococcus aureus (MRSA). The procedure has been optimised and compared for macroelectrodes, microelectrodes and MNEE so as to compare their performances as biosensors. It was observed that MNEE’s: (a) form thiol films faster than electrodes with larger dimensions, determined by kinetic studies of 6-mercaptohexan-1-ol film formation (b) form films with different packing structures dependant on the electrode bulk to edge ratio (c) can detect the same concentration of target in less time than larger electrodes because of their increased sensitivity. The film packing has also been quantitatively investigated using EIS and it can be seen that films formed n MNEE were better able to incorporate target DNA into their more splayed out structure. Unique to this project has been the establishment of a protocol to form heterogeneous carbazole-alanine hydrogel matrices on nanoelectrodes, whose polymerisation is initiated by a pH swing at the electrode surface induced by the oxidation of hydroquinone. The gels growth pattern follows the diffusion field at the electrode and can be monitored using EIS. This also gives a measure of the permeability of the gel by fitting to the correct equivalent circuit. The gel structure has been imaged using light microscopy, confocal microscopy and scanning electrochemical microscopy (SEM). The results give a visual demonstration that MNEE has enhanced diffusion at the corners of the cavities, which is in agreement with previously published simulations, and give evidence as to the onset of hemispherical diffusion and the conditions at which the diffusion field between neighbouring electrodes begin to overlap, a phenomenon which can be observed visually and correlated to changes in the EIS data. Hydrogels have been grown chronopotentiometrically at different currents and the permittivity (through the diffusion coefficients) has been measured of redox couples through gels grown at different speeds. It was found that the hierarchical structure of the hydrogels can be tuned; potentially opening the door to a new breed of tuneable, biocompatible anti-biofouling matrices on bio-functionalised electrodes. The system was characterised using the same MRSA detection protocol as optimised for the MNEE and the target DNA was found to be able to permeate through the hydrogels and bind to the probe, which resulted in a significant change in impedance.

Orientadores: Stanislav Alexandrovich Moshkalev, Mario Antonio Bica de Moraes / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-11T00:23:59Z (GMT). No. of bitstreams: 1 LeonEras_JorgeAugusto_D.pdf: 28434284 bytes, checksum: 042fbff5bb19c08014235156fdc57ee4 (MD5) Previous issue date: 2007 / Resumo: Não informado / Abstract: Not informed. / Doutorado / Física da Matéria Condensada / Doutor em Ciências

There is a compelling need for the development of new sensory and neural prosthetic devices which are capable of more precise point stimulation. Current prosthetic devices suffer from the limitation of low spatial resolution due to the non-specific stimulation characteristics of electrical stimulation, i.e., the spread of electric fields generated. We present a visible light stimulation method for modulating the firing patterns of electrically-excitable cells using surface plasmon resonance phenomena. In in-vitro studies using gold (Au) nanoparticle-coated nanoelectrodes, we show that this method (substrate coated with nanoparticles) has potential for incorporating the technology into neural stimulation prosthetics, such as cochlear implants, with arbitrarily high spatial resolution. Au nanoparticles (NPs) were coated on micropipettes using aminosilane linkers; and these micropipettes were used for stimulating and inhibiting the action potential firing patterns of SH-SY5Y human neuroblastoma cells and neonatal cardiomyocytes. Our findings pave the way for development of biomedical implants and neural testing devices using nanoelectrodes capable of temporally and spatially precise excitation and inhibition of electrically-excitable cellular activity.

Nano-objects have unique properties due to their sizes, shapes and structure. When electronic properties of such nano-objects are used to build devices, the control of interfaces at atomic level is required. In this thesis, systems were built that can not only electrically characterize nano-objects, but also allow to analyze a large number of individual nano-objects statistically at the example of graphene and nanoparticle-molecule-nanoelectrode junctions. An in-situ electrical characterization system was developed for the analysis of free standing graphene sheets containing defects created by an acid treatment. The electrical characterization of several hundred sheets revealed that the resistance in acid treated graphene sheets decreased by 50 times as compared to pristine graphene and is explained by the presence of di-vacancy defects. However, the mechanism of defect insertion into graphene is different when graphene is bombarded with a focused ion beam and in this case, the resistance of graphene increases upon defect insertion. The defect insertion becomes even stronger at liquid N2 temperature. A molecular electronics platform with excellent junction properties was fabricated where nanoparticle-molecule chains bridge 15-30nm nanoelectrodes. This approach enabled a systematic evaluation of junctions that were assembled by functionalizing electrode surfaces with alkanethiols and biphenyldithiol. The variations in the molecular device resistance were several orders of magnitude and explained by variations in attachment geometries of molecules.  The spread of resistance values of different devices was drastically reduced by using a new functionalization technique that relies on coating of gold nanoparticles with trityl protected alkanedithiols, where the trityl group was removed after trapping of nanoparticles in the electrode gap. This establishment of a reproducible molecular electronics platform enabled the observation of vibrations of a few molecules by inelastic tunneling spectroscopy. Thus this system can be used extensively to characterize molecules as well as build devices based on molecules and nanoparticles.

Recent advances in nanofabrication techniques have opened up new avenues and numerous possible applications in both nanoscale electrochemistry and analytical nanoscience by enabling the fabrication of reproducible nanoelectrodes with different new geometries. Nanoelectrodes exhibit advantages including enhanced mass transport, higher current densities, improved signal-to-noise ratios, and lower ohmic drop. In this dissertation, the use of nanoelectrodes in the electrochemical response properties investigations or in the spectroelectrochemical studies is the unifying factor among all the chapters. First (in Chapter 4), we presented a direct comparison between the electrochemical characteristics of two finite nanoelectrodes arrays with different geometries: 6 × 6 recessed nanodiscs and nanorings microarrays. Using computational methods, it was demonstrated that the electrode geometry’s parameters have a drastic influence on the mass transport properties of the nanoelectrodes. The results presented here are the first combination of experimental and numerical studies that elucidate the transport on nanoring electrode arrays. The comparison of the electrochemical behavior between nanostructures using full 3D simulations is also unique. Second, we have provided a comprehensive numerical study on the redox cycling performance properties of a 6 × 6 recessed nanorings-ring electrode array configuration. The simulation results were in good agreement with the experimental data. After validating the model against experiments, a comprehensive computational investigation revealed avenues to optimize the performance of the structure in terms of geometric parameters and scan rates. The second half of this dissertation is comprised of the spectroelectrochemical studies. The combination of surface plasmon resonance with electrochemistry presents new paths to investigateredox reaction events at the electrode surface since it brings an additional dimension to the classical electrochemical approaches. Third, we have reported a novel active plasmonic device based on a new switching mechanism for the nanohole electrodes array to bridge between photonics and electronics at nanoscales. The inner surfaces of the nanohole electrodes in the array were coated with an electroconductive polymer, polypyrrole, (PPy). Then, it was shown that light transmitted through the PPy- modified nanohole electrodes can be easily tuned and controled by applying an external potential. We were also able to switch on and off the transmitted light intensity through the modified nanohole arrays by potential steps, demonstrating the potential of this platform to be incorporated into optoelectronic devices. Finally, we have fabricated larger area plasmonic periodic nanopillar 3D electrodes using a rapid, high-throughput, and cost-effective approach: the laser interference lithography. Then, the electrochemical behavior of these electrodes was investigated both experimentally and computationally. The properties were ‘compared with a flat electrode with an equivalent geometric area. Afterward, we have successfully probed the changes in the concentration of a reversible redox pair near the electrode surface induced by various applied potentials, in an in-situ EC-SPR experiment. / Graduate

Esta tese apresenta os achados e avanços obtidos na fabricação, caracterização e aplicação de pontas de prova eletroquímicas multifuncionais para a obtenção de informações eletroquímicas resolvidas no espaço em diversas superfícies/interfaces por meio de técnicas de microscopia de varredura de ponta de prova eletroquímica (SEPM, em inglês). Diferentes designs de pontas de prova multifuncionais foram investigados e, devido a natureza não convencional destas, novos métodos para fabricação e caracterização foram desenvolvidos. Os benefícios da utilização de pontas de prova multifuncionais para a obtenção de informações eletroquímicas resolvidas no espaço ficaram evidente durante a realização de experimentos \"prova de conceito\", onde a maior densidade de informação obtida permitiu o estudo de sistemas mais complexos e a aquisição de informações eletroquímicas livre de interferência topográfica mesmo em superfícies não planas. A hibridização de diferentes técnicas de microscopia de varredura de ponta de prova eletroquímica em uma única ponta de prova também foi investigada o que se provou extremamente útil para a aquisição de imagens eletroquímicas de alta resolução, livres de influências topográficas, quando utilizada a técnica de microscopia de condutividade iônica (SICM, em inglês) como sensor de topografia do substrato investigado. Por ultimo, uma nova técnica, baseada na microscopia de condutividade iônica, que se utiliza de pontas de prova eletroquímicas multifuncionais fabricadas a partir de uma nanopipeta de um único canal, foi desenvolvida. Esta nova técnica se mostrou extremamente ponderosa, capaz de obter informações a respeito da topografia e mapear sítios ativos sobre um substrato utilizando uma nanopipeta de um único canal com alta resolução especial e temporal a uma taxa de aproximadamente 4000 pixels por Segundo. / This thesis presents the findings and advances made on fabrication, characterization and application of multifunctional electrochemical probes to acquire space resolved electrochemical information on diverse surfaces/interfaces employing Scanning Electrochemical Probe Microscopy (SEPM) techniques. Different multifunctional probes designs were investigated and new and innovative methods for fabrication and characterization of those probes were developed, which was necessary due to the unconventional nature of most of the probes studied. The benefits of using multifunctional probes for space resolved electrochemical measurements was clear during \"proof-of-concept\" experiments, where the increased density of information allowed the study of complex systems and the acquisition of topography-free electrochemical information of rough surfaces. The hybridization of different SEPM techniques in a single probe tip was also investigated, and this was found to be extremely beneficial, especially for acquiring high-resolution, topography-free, electrochemical images employing Scanning Ion Conductance Microscopy as a topography feedback. Finally, a new SICM technique, based on the use of a multifunctional probe tip fabricated from a single barrel nanopipette, was developed. This new technique was found to be extremely powerful, capable of acquiring information on topography and map active sites over substrates using a single barrel pipette with high spatial and temporal resolution at a rate of approx. 4000 pixels per second.