Scanning Electrochemical Microscopy and Finite Element Modeling of Structural and Transport Properties of Electrochemical Systems

Understanding the fundamental properties of electrochemical systems, especially at the microscopic level, requires advanced tools combining electrochemical microscopy and scanning probe techniques with mathematical modeling for the quantification of the physicochemical characteristics of these environments. Comparison, analysis and sometimes calibration of the experimental results with theoretically predicted values is an indispensable strategy of any scientific method. In this thesis, experimental investigations are complemented with computations and numerical simulations. Novel microfluidic probes for scanning electrochemical microscopy (SECM) were developed. These soft SECM probes are equipped with an ultramicroelectode and integrated counter-reference electrode for characterizing the electrochemical reactivity of the substrate. They also incorporate microfluidic channels for delivery and removal of electrolyte solutions to and away from the probe tip allowing SECM experiments to be carried out on initially dry substrates. Additional integration of this concept with a chemical detection of the circulating electrolyte by means of mass-spectrometric analysis demonstrated the possibility to turn these microfluidic probes into a scanning tool for localized stimulation, control and read-out of chemical events occurring at interfaces. This work was also accomplished with two- and three- dimensional finite element modeling of the amperometric probe response under various geometrical arrangements taking into account microfluidic perturbations. Theoretical investigations of the capacitive properties of liquid/liquid interfaces were carried out taking into account the effect of smooth variation of permittivity across the liquid/liquid boundary. The width and symmetry of the resulting interphase were shown to be crucial characteristics for the variation of ion solvation within the interfacial region and fitting the experimental data with a finite element model was used to estimate the thickness of liquid/liquid junctions. Diode-like behavior of tapered charged nanopores was investigated by means of computing ion fluxes using a simple perm-selective approximation and more complicated finite element models taking into account a uniform surface charge density inside the conical pipettes. These simulations provided an insight on the mechanisms governing the rectifying behavior of such systems under various circumstances, i.e. geometrical arrangement, electrolyte concentrations and different electrical potential/voltage scan rate conditions, proving the existence of a perm-selective barrier attributed to diffuse layer overlapping inside the charged nanopore. Finally, theoretical investigations of the analytical characteristics of solid phase extraction-gradient elution-mass spectrometry (SPE-GEMS) technique for proteomics were performed. The computations were used to optimize the experimental conditions and to estimate performance aspects of this microchip-based strategy by following sorption-desorption equilibria under gradient elution conditions, i.e. by varying solvent strength.

Of gels, offgel and off with the gel

Hoang-Trang Lam

In the context of prefractionation methods for proteomics, this work deals mainly with the development of electrophoretic tools for isoelectric focusing of peptides and proteins for the analysis of biological mixtures. In the light of existing devices for isoelectric focusing (IEF), the objective has been to develop multicompartment devices, designed for the IEF of peptides and proteins. The first choice of the OFFGEL for IEF among other techniques is justified by the easy recovery of liquid fractions of peptides, of small volumes, further amenable to liquid chromatography (as a second dimension separation) or mass spectrometry analyses. The resolution is a key point to consider in the design of separation units. Finite element simulation of the isoelectric focusing of peptides by OFFGEL has allowed the design of a multicompartment OFFGEL device for high resolution separation of peptides. The numerical simulations have highlighted the importance of the mobility near pI for the IEF kinetics and the final peak shape. The calculation of the distribution of peptides mobility near pI for three proteomes has allowed concluding on the optimal width of the well to obtain best separation. This mathematical study has also illustrated the high focusing power of the OFFGEL technique as a separation tool for shotgun proteomics application. The design of a multicompartment OFFGEL device was then done, based on the results of the simulations. The reproducibility of the pH gradient was validated, the loading capacity was evaluated for proteins, and a demonstration of the high resolution separation of peptides and proteins from a complex biological mixture was performed. The OFFGEL separation was then integrated in a workflow combining chemical tagging of the cysteine residues. This approach showed that the high resolution of peptide OFFGEL and the added information on the sequence of the peptides permitted a more confident and accurate protein identification. In the context of gel-free proteomics, another separation cell has been designed, that enables performing isoelectric focusing without the need of an immobilized pH gradient (IPG) gel. The novel device has been characterized in terms of performances and has been be applied to a biological sample of Escherichia coli, showing a more rapid separation of proteins than OFFGEL IEF, thus demonstrating its potential for fast proteome prefractionation purposes. An electrochemical cell has also been designed, for the transfer of ionizable species by electrochemistry at the micro-interface of two immiscible electrolytes (µ-ITIES), supported by an IPG gel as the aqueous phase and a small drop of organic phase. This study was initially motivated by the aim of performing online extraction of proteins/peptides during IEF separation. The use of this device for the transfer of model molecules was demonstrated, opening the door to further developments concerning the electrochemical transfer of proteins.

EPFL2008

Préparation et caractérisation de nanoparticules bimétalliques (Pt-Au) sur un substrat en diamant dopé au bore

Bahaa El Roustom

Using petrol as a main source of energy, during the 20th century, has caused considerable amounts of damage among which are atmospheric pollution and global warming. At the same time, many geologists and economists expect that the discovery rate of new petrol sources will not follow the market's demand causing a serious shortage of petrol in the near future. These alarming perspectives have motivated scientists around the world to develop new clean and renewable energy sources. It is in that objective that electrochemists intensified their research in order to develop the fuel cell technology. This latter consists in directly converting chemical energy into electricity. The main advantage of this technology over traditional energy production is that the fuel cell energy efficiency is Carnot cycle independent. The theoretical efficiency of such an energy source is nearly 90%. Direct Alcohol Fuel Cell (DAFC), fed by ethanol on one hand and by air (oxygen) in the other hand, is a very interesting option for such kind of energy production. Ethanol is a renewable energy source of vegetal origin whose CO2 production is consumed by plants during the cycle. Even though Pt has long been known to be a perfect catalyst for DAFC, it has some principal limitations, the most important ones being poisoning by intermediate products (OH on the cathodic side and CO on the anodic side) and a relatively high cost compared to other materials. These problems have pushed the experts in the field to focalize their research on the development of more suitable and better performing materials. Gold nanoparticles have shown a surprising catalytic activity, very different from the bulk, making them a preferential candidate to replace Pt. A system based on using two or more metals as catalysts (example Au-Pt) dispersed on an appropriate substrate seems to be also an interesting candidate to enhance the cell's efficiency. The substrate should be chemically inert, non-oxydable and should not present any interference with the electrochemical and electrocatalytic behaviour of the nanoparticles. In the first part, the employed substrate, which is boron doped diamond (BDD) is characterized. The substrate's main characteristics are its chemical inertness, its weak residual current and its resistance to corrosion. These characteristics contribute to making the substrate an ideal support for nanoparticles. In this work, the substrate is a diamond film, but for fuel cell applications diamond should be used as powder or dispersed particles. In chapters III and IV, we studied the effect of boron incorporation on the morphology and the behaviour of the diamond film. For this purpose, samples with different ratios of boron/carbon (B/C) in the vapour phase during preparation were characterized (B/C = 300, 1000, 2500 and 6000 ppm). Results showed that the grain size decreases with increasing boron content in the film. This is accompanied by a transition towards a quasi-metallic character. It was also found that graphitic impurities in the diamond film increase when the B/C ratio increases. We also established some graphitic impurities increase in the diamond film when the ratio of B/C increases. The identity of these impurities was established in chapter III as being surface redox couples (semi-quinone/semi-hydroquinone). Using cyclic voltametry we were able to observe a total elimination (B/C=300 ppm) or partial elimination (B/C=6000 ppm) of these surface redox couples. We also found in chapter IV that these surface redox couples are responsible of the electrochemical activity of the BDD electrode. BDD electrodes with different doping level B/C=300, 1000 and 6000 ppm) in the vapour phase are modified with IrO2 nanoparticles using the method of thermal decomposition. Our results showed that, at low potentials, the behaviour of the composite electrode IrO2/BDD with respect to the reaction of oxygen evolution is independent of the doping level. On the other hand, at high potentials, the weakly doped electrode (B/C=300 ppm) presents a supplementary overpotential due to the interface BDD-IrO2. An approach based on the double junction is developed in this work describing two distinct behaviours depending on the boron concentration in the diamond film. A new method for Au nanoparticles deposition on highly doped BDD electrodes (B/C= 2500 ppm) is presented in chapter VI. This method involves two steps: the first step consists on sputtering the equivalent of few nanolayers of Au on the diamond surface; the second step is a heat treatment in air at high temperatures. Several parameters that could influence the dispersion, the stability as well as the diameter of the Au nanoparticles are studied. In chapter VII, the behaviour of the composite electrode Au/BDD is studied in a solution containing redox couples in acid media. This electrode was simulated as an arrangement of active Au microelectrodes dispersed on a less active BDD substrate. The overall behaviour of the composite electrode (Au/BDD) has been related to the electrochemical rate constants on each material. In chapter VIII, another new synthesis method for a bimetallic system formed by Au and Pt nanoparticles dispersed on a BDD substrate is developed. This method consists in electrodepositing a small amount of Pt on a composite electrode Au/BDD. Our results showed that Pt was preferentially deposited on Au covering its whole surface. In fact, Au nanoparticles serve as germ of nucleation for Pt. In the following step, the bimetallic PtbAu/BDD is heated to 400°C in air to enhance the interaction between Au and Pt. The experiments showed that Au reappeared on the surface. The electrochemical behaviour of the bi-metallic electrode is characterized by a negative shift in the peak potential of reduction of Pt oxides. This is not the case of heterogeneous alloys obtained by the fusion of two metals at high temperatures. The composite electrode Au/BDD as well as the bi-metallic electrode Pt-Au/BDD are used for electrocatalytic applications. The studied reaction in chapter IX is the oxygen reduction reaction (ORR) in an acid medium. We found that the number of exchanged electrons increases with increasing the amount of Pt at the surface. Oxygen is reduced into H2O2 on the Au/BDD electrode and into H2O on the Pt-Au/BDD electrode. In chapter X we studied the reaction of oxidation of oxalic acid. The behaviour of both Au and Pt electrodes seemed very similar for this reaction. The bi-metallic electrode containing both Pt and Au on its surface has shown an intermediate behaviour between the behaviour of the two metals.

EPFL2006

Electrochemical imaging of metabolic activity in tissue and biofilms

Sorour Darvishi

Non-invasive and semi-invasive bioimaging is of great interest for the rapid diagnosis and efficient as well as specific treatments of biological systems affecting human health, such as cancer tissue and biofilms. Electrochemical bioimaging is one of the diverse methods that can reach high sensitivities in small liquid volumes. When using (sub)micrometric electrodes, it can reach a high spatial-temporal resolution. The electrochemical processes can be based but are not limited to electron transfer reactions and the detection of metabolic activity indicators. An electrochemical scanning probe platform can be used, such as Scanning Electrochemical Microscopy (SECM), in which a micrometric electrochemical probe is translated in close proximity to a sample immersed in an electrolyte solution. In most SECM modes, the SECM tip detects locally electro-active species that are generated or consumed by active surface sites allowing to create surface reactivity maps of biological entities. It has in particular found applications for studying normal and cancer cells, yeast, and bacteria. In this thesis it was looked at utilizing Soft SECM probes as an analytical tool for sensing and analyzing complex biological samples, such as skin and three-dimensional biofilms in an as less as possible invasive manner, potentially forming the basis for the development of point of care devices for early detection and treatment. Electrochemical analysis was completed by fluorescence imaging as a state-of-the-art methodology. The study of melanoma and bacterial biofilms as the two major targets in this thesis was divided into five main chapters. The electrochemical detection of melanoma was based on the specific detection of tyrosinase, a melanoma protein marker. In Chapter 3, a non-invasive electrochemical melanoma detection approach is based on using adhesive tapes to collect and fix cells from a suspicious skin area and transfer the cells into an SECM for their analysis with soft probes. The enzyme was tagged with antibodies that contained a peroxidase label for the production of an electrochemical probe. In Chapter 4, tyrosinase was addressed with a microneedle type electrochemical biosensor containing a TYR-sensitive redox molecule for TYR sensing. Thereafter, Soft-Probe-SECM was used to investigate the electrochemical surface reactivity of Escherichia coli (E. coli) biofilms (Chapter 5) and to follow the effects of various antibiofilm treatments, such as gentamicin, sodium azide, silver nanoparticles, and flashlight (Chapter 6). Finally, in Chapter 7, an antibiofilm nanocarrier based on a DNA origami nanostructure for E. Coli biofilm treatment was studied. The DNA origami was loaded with doxorubicin (DOX) to enable the transport of DOX into the three-dimensional structure of the biofilm. In this way, the biofilm protection barrier against DOX, as generated by the extracellular polymeric matrix, was overcome. The biofilms were studied by confocal laser scanning microscopy and microelectrode arrays.

EPFL2022