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Hubert Girault

Ecole Polytechnique Federale de Lausanne, Chemistry, Faculty Member

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Papers by Hubert Girault

Surprising acidity of hydrated lithium cations in organic solvents

Chemical Communications, 2014

Lithium cations are shown to have a significant role in catalyzing oxygen and proton reduction al... more Lithium cations are shown to have a significant role in catalyzing oxygen and proton reduction along with S N 1 reactions in biphasic systems. We propose that this catalytic effect is due to the surprising acidity of the hydrated cations; interactions between the cation and its surrounding solvation shell will make the constituent water molecules more acidic.

Parylene C coated microelectrodes for scanning electrochemical microscopy

Electrochimica Acta, 2013

Herein, we present a simple microelectrode preparation methodology consisting in coating a platin... more Herein, we present a simple microelectrode preparation methodology consisting in coating a platinum wire or a carbon fiber with a thin insulating Parylene C film (e.g. 1-10 m), to produce SECM probes with a small and constant probe RG (i.e. ratio between the radius of the insulating sheath and the radius of the active electrode area). After exposition of a fresh active electrode area by blade cutting, a disc shaped electrode is obtained thanks to a protective hot mounting wax layer that avoids Parylene C coating deformation and is easily removed with acetone. Stiffness and straightness of the probe can be tuned by modifying the Parylene C coating thickness and the length of the carbon fiber or platinum wire. This simple electrode preparation method is highly reproducible (c.a. > 90%). The prepared Parylene C coated microelectrodes were characterized by optical microscopy, cyclic voltammetry, scanning electrochemical microscopy (SECM) approach curves and finally applied to SECM imaging of Pt band structures in contactless and contact mode.

Time-Resolved Laser-Induced Fluorescence Study of Photoinduced Electron Transfer at the Water/1,2-Dichloroethane Interface

The Journal of Physical Chemistry a, 1997

Time-resolved laser-induced fluorescence in total internal reflection mode has been used to monit... more

Fixation and Permeabilization Approaches for Scanning Electrochemical Microscopy of Living Cells

Analytical Chemistry, 2016

Self-assembly and redox induced phase transfer of gold nanoparticles at the water-propylene carbonate interface

by Evgeny Smirnov and Hubert Girault

Citrate-stabilized gold nanoparticles were found to spontaneously self-assemble in a lusturous fi... more Citrate-stabilized gold nanoparticles were found to spontaneously self-assemble in a lusturous film at a bare water-propylene carbonate interface after vigorous shaking, due to the extremely low interfacial tension. The presence of the electron donor, tetrathiafulvalene, in the oil phase, led to the extraction of particles into the organic phase.

Photocurrent responses associated with heterogeneous electron transfer at liquid/liquid interfaces

by David Fermín and Hubert Girault

Chemical Communications, 1998

Fig. 2 Photocurrent spectra of cell 1 at various potentials in the presence of DFCET. The transmi... more

Surface plasmon enhanced second harmonic response from gold clusters embedded in an alumina matrix

by Hubert Girault and B. Prével

Journal of Applied Physics, 1998

The second order nonlinear response from 4 nm diam gold clusters embedded in an alumina matrix de... more The second order nonlinear response from 4 nm diam gold clusters embedded in an alumina matrix deposited on a pure silica substrate has been recorded as a function of the wavelength of the fundamental incident beam. The spectrum exhibits a narrow resonance band peaked at 520 nm as a result of the coupling of the second harmonic field with the surface plasmon of the particles. The nonlinear second harmonic response of gold clusters is found to be blueshifted compared to the bulk, due to the finite size effects on the cluster optical properties, as already observed with the linear response. Furthermore, this nonlinear response appears to be well described with a simple free electron model where the valence electrons only participate through the screening of the ionic cores, owing to the weakness of the interband transition contribution.

Boosting water oxidation layer-by-layer

Electrocatalysis of water oxidation was achieved using fluorinated tin oxide (FTO) electrodes mod... more Electrocatalysis of water oxidation was achieved using fluorinated tin oxide (FTO) electrodes modified with layer-by-layer deposited films consisting of bilayers of negatively charged citrate-stabilized IrO 2 NPs and positively charged poly(diallyldimethylammonium chloride) (PDDA) polymer. The IrO 2 NP surface coverage can be fine-tuned by controlling the number of bilayers. The IrO 2 NP films were amorphous, with the NPs therein being well-dispersed and retaining their as-synthesized shape and sizes. UV/vis spectroscopic and spectro-electrochemical studies confirmed that the total surface coverage and electrochemically addressable surface coverage of IrO 2 NPs increased linearly with the number of bilayers up to 10 bilayers. The voltammetry of the modified electrode was that of hydrous iridium oxide films (HIROFs) with an observed super-Nernstian pH response of the Ir(III)/Ir(IV) and Ir(IV)–Ir(IV)/Ir(IV)–Ir(V) redox transitions and Nernstian shift of the oxygen evolution onset potential. The overpotential of the oxygen evolution reaction (OER) was essentially pH independent, varying only from 0.22 V to 0.28 V (at a current density of 0.1 mA cm À2), moving from acidic to alkaline conditions. Bulk electrolysis experiments revealed that the IrO 2 /PDDA films were stable and adherent under acidic and neutral conditions but degraded in alkaline solutions. Oxygen was evolved with Faradaic efficiencies approaching 100% under acidic (pH 1) and neutral (pH 7) conditions, and 88% in alkaline solutions (pH 13). This layer-by-layer approach forms the basis of future large-scale OER electrode development using ink-jet printing technology.

Charge distribution and Fermi level in bimetallic nanoparticles

Upon metal–metal contact, a transfer of electrons will occur between the metals until the Fermi l... more Upon metal–metal contact, a transfer of electrons will occur between the metals until the Fermi levels in both phases are equal, resulting in a net charge difference across the metal–metal interface. Here, we have examined this contact electrification in bimetallic model systems composed of mixed Au–Ag nano-particles containing ca. 600 atoms using density functional theory calculations. We present a new model to explain this charge transfer by considering the bimetallic system as a nanocapacitor with a potential difference equal to the work function difference, and with most of the transferred charge located directly at the contact interface. Identical results were obtained by considering surface contacts as well as by employing a continuum model, confirming that this model is general and can be applied to any multimetallic structure regardless of geometry or size (going from nano-to macroscale). Furthermore, the equilibrium Fermi level was found to be strongly dependent on the surface coverage of different metals, enabling the construction of scaling relations. We believe that the charge transfer due to Fermi level equilibration has a profound effect on the catalytic, electrocatalytic and other properties of bimetallic particles. Additionally, bimetallic nanoparticles are expected to have very interesting self-assembly for large superstructures due to the surface charge anisotropy between the two metals.

Self-healing gold mirrors and filters at liquid–liquid interfaces

by Hubert Girault and Evgeny Smirnov

The optical and morphological properties of lustrous metal self-healing liquid-like nanofilms wer... more The optical and morphological properties of lustrous metal self-healing liquid-like nanofilms were systematically studied for different applications (e.g., optical mirrors or filters). These nanofilms were formed by a one-step self-assembly methodology of gold nanoparticles (AuNPs) at immiscible water–oil interfaces, previously reported by our group. We investigated a host of experimental variables and herein report their influence on the optical properties of nanofilms: AuNP mean diameter, interfacial AuNP surface coverage, nature of the organic solvent, and nature of the lipophilic organic molecule that caps the AuNPs in the interfacial nanofilm. To probe the interfacial gold nanofilms we used in situ (UV-vis-NIR spectroscopy and optical microscopy) as well as ex situ (SEM and TEM of interfacial gold nanofilms transferred to silicon substrates) techniques. The interfacial AuNP surface coverage strongly influenced the morphology of the interfacial nanofilms, and in turn their maximum reflectance and absorbance. We observed three distinct morphological regimes; (i) smooth 2D monolayers of " floating islands " of AuNPs at low surface coverages, (ii) a mixed 2D/3D regime with the beginnings of 3D nanostructures consisting of small piles of adsorbed AuNPs even under sub-full-monolayer conditions and, finally, (iii) a 3D regime characterised by the 2D full-monolayer being covered in significant piles of adsorbed AuNPs. A maximal value of reflectance reached 58% in comparison with a solid gold mirror, when 38 nm mean diameter AuNPs were used at a water–nitrobenzene interface. Meanwhile, interfacial gold nanofilms prepared with 12 nm mean diameter AuNPs exhibited the highest extinction intensities at ca. 690 nm and absorbance around 90% of the incident light, making them an attractive candidate for filtering applications. Furthermore , the interparticle spacing, and resulting interparticle plasmon coupling derived optical properties, varied significantly on replacing tetrathiafulvalene with neocuproine as the AuNP capping ligand in the nanofilm. These interfacial nanofilms formed with neocuproine and 38 nm mean diameter AuNPs, at monolayer surface coverages and above, were black due to aggregation and broadband absorbance.

Contact Potentials, Fermi Level Equilibration, and Surface Charging

This article focuses on contact electrification from thermodynamic equilibration of the electroch... more This article focuses on contact electrification from thermodynamic equilibration of the electrochemical potential of the electrons of two conductors upon contact. The contact potential difference generated in bimetallic macro-and nanosystems, the Fermi level after the contact, and the amount and location of the charge transferred from one metal to the other are discussed. The three geometries considered are spheres in contact, Janus particles, and core−shell particles. In addition, the force between the two spheres in contact with each other is calculated and is found to be attractive. A simple electrostatic model for calculating charge distribution and potential profiles in both vacuum and an aqueous electrolyte solution is described. Immersion of these bimetallic systems into an electrolyte solution leads to the formation of an electric double layer at the metal−electrolyte interface. This Fermi level equilibration and the associated charge transfer can at least partly explain experimentally observed different electrocatalytic, catalytic, and optical properties of multimetallic nanosystems in comparison to systems composed of pure metals. For example, the shifts in the surface plasmon resonance peaks in bimetallic core−shell particles seem to result at least partly from contact charging.

Characterization of Surface State of Inert Particles: Case of Si and SiC

Silicon and Silicon carbide particles have been investigated by the mean of infrared (IR) spectro... more Silicon and Silicon carbide particles have been investigated by the mean of infrared (IR) spectros-copy and X-ray photoelectron spectroscopy (XPS) to establish their surface states. The results of this research are based on the estimation of the area under the high resolution peaks by isosceles triangles. This approach leads to the repartition of the particles surfaces in term of atomic percentage and of type of bonds. The surface of silicon particles is divided up into 54.85% of Si-O bonds and 36.85% of Si-Si bonds. The remaining surface is constituted of zeolite, the raw material used to produce the silicon particles. The surface of silicon carbide particles consists of 50.44% of Si-C bonds, 24.01% of Si-O bonds and 25.55% of graphite. 10.01% of the graphite is derived from the oxidation of Si-C bonds while 11.48% is due to contamination. The zeta potential evolution versus pH confirms the distribution of chemical groups found.

All-vanadium dual circuit redox flow battery for renewable hydrogen generation and desulfurisation

An all-vanadium dual circuit redox flow battery is an electrochemical energy storage system able ... more An all-vanadium dual circuit redox flow battery is an electrochemical energy storage system able to function as a conventional battery, but also to produce hydrogen and perform desulfurization when a surplus of electricity is available by chemical discharge of the battery electrolytes. The hydrogen reactor chemically discharging the negative electrolyte has been designed and scaled up to kW scale, while different options to discharge the positive electrolyte have been evaluated, including oxidation of hydrazine, SO 2 and H 2 S. The system is well suited to convert sulfur dioxide and hydrogen sulfide to harmless compounds while producing hydrogen, with overall system efficiencies from 50 to 70% for hydrogen production.

CHINESE JOURNAL OF ANALYTICAL CHEMISTRY Antioxidant Assay Based on Quenching of Photocatalytically Generated Reactive Oxygen Species

A method based on photogeneration of OH radicals in water from TiO 2 nanoparticles was developed ... more A method based on photogeneration of OH radicals in water from TiO 2 nanoparticles was developed to study the kinetics of oxidation of organic molecules which were used as biological antioxidants. The kinetics of oxidation of terephthalic acid as a reference probe was monitored by fluorescence measurements of the concentration of its oxidized form, 2-hydroterephthalic acid (λ ex =315 nm, λ em = 425 nm). The kinetics of oxidation of other antioxidant molecules was then deduced from the radical scavenging competition. The antioxidant properties of normal antioxidants were compared based on this kinetic model. And the antioxidant kinetic decreased in the order: lipoic acid, gallic acid, glutathione, uric acid, vitamin C, vitamin E, trolox and bilirubin.

Gold Nanofilm Redox Catalysis for Oxygen Reduction at Soft Interfaces

A B S T R A C T Functionalization of a soft or liquid-liquid interface by a one gold nanoparticle... more A B S T R A C T Functionalization of a soft or liquid-liquid interface by a one gold nanoparticle thick " nanofilm " provides a conductive pathway to facilitate interfacial electron transfer from a lipophilic electron donor to a hydrophilic electron acceptor in a process known as interfacial redox catalysis. The gold nanoparticles in the nanofilm are charged by Fermi level equilibration with the lipophilic electron donor and act as an interfacial reservoir of electrons. Additional thermodynamic driving force can be provided by electrochemically polarising the interface. Using these principles, the biphasic reduction of oxygen by a lipophilic electron donor, decamethylferrocene, dissolved in a,a,a-trifluorotoluene was catalysed at a gold nanoparticle nanofilm modified water-oil interface. A recently developed microinjection technique was utilised to modify the interface reproducibly with the mirror-like gold nanoparticle nanofilm, while the oxidised electron donor species and the reduction product, hydrogen peroxide, were detected by ion transfer voltammetry and UV/vis spectroscopy, respectively. Metallization of the soft interface allowed the biphasic oxygen reduction reaction to proceed via an alternative mechanism with enhanced kinetics and at a significantly lower overpotential in comparison to a bare soft interface. Weaker lipophilic reductants, such as ferrocene, were capable of charging the interfacial gold nanoparticle nanofilm but did not have sufficient thermodynamic driving force to significantly elicit biphasic oxygen reduction.

Open channel-based microchip electrophoresis interfaced with mass spectrometry via electrostatic spray ionization

Chemical Science Sensitive and fast identification of bacteria in blood samples by immunoaffinity mass spectrometry for quick BSI diagnosis

Bloodstream infections rank among the most serious causes of morbidity and mortality in hospitali... more Bloodstream infections rank among the most serious causes of morbidity and mortality in hospitalized patients, partly due to the long period (up to one week) required for clinical diagnosis. In this work, we have developed a sensitive method to quickly and accurately identify bacteria in human blood samples by combining optimized matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MS) and efficient immunoaffinity enrichment/separation. A library of bacteria reference mass spectra at different cell numbers was firstly built. Due to a reduced sample spot size, the reference spectra could be obtained from as few as 10 to 10 2 intact bacterial cells. Bacteria in human blood samples were then extracted using antibodies-modified magnetic beads for MS fingerprinting. By comparing the sample spectra with the reference spectra based on a cosine correlation, bacteria with concentrations as low as 500 cells per mL in blood serum and 8000 cells per mL in whole blood were identified. The proposed method was further applied to positive clinical blood cultures (BCs) provided by a local hospital, where Escherichia coli and Staphylococcus aureus were identified. Because of the method's high sensitivity, the BC time required for diagnosis can be greatly reduced. As a proof of concept, whole blood spiked with a low initial concentration (10 2 or 10 3 cells per mL) of bacteria was cultured in commercial BC bottles and analysed by the developed method after different BC times. Bacteria were successfully identified after 4 hours of BC. Therefore, an entire diagnostic process could be accurately accomplished within half a day using the newly developed method, which could facilitate the timely determination of appropriate anti-bacterial therapy and decrease the risk of mortality from bloodstream infections.

Ion transfer battery: storing energy by transferring ions across liquid–liquid interfaces

Melanoma Diagnosis Monitoring Tyrosinase Expression in Non-metastatic and Metastatic MelanomaT issues by Scanning Electrochemical Microscopy

Although tremendous progress has been made in the diagnosis of melanoma, the identification of di... more Although tremendous progress has been made in the diagnosis of melanoma, the identification of different stages of malignancy in ar eliable wayr emains challenging.C urrent strategies rely on optical monitoring of the concentration and spatial distribution of specific biomarkers.S tate-of-the-art optical methods can be affected by background-color interference and autofluorescence.W eo vercame these shortcomings by employings canning electrochemical microscopy(SECM) to map the prognostic indicator tyrosinase (TyR) in non-metastatic and metastatic melanoma tissues by using soft-stylus microelectrodes.E lectrochemical readout of the TyRd istribu-tion was enabled by adapting an immunochemical method. SECM can overcome the limitations of optical methods and opens unprecedented possibilities for improved diagnosis and understanding of the spatial distribution of TyRi nd ifferent melanoma stages.

Melanomdiagnostik Untersuchung der Tyrosinase-Expression in nicht-metastatischen und metastatischen Melanomgeweben durch elektrochemische Rastersondenmikroskopie

by Hubert Girault and Alexandra Bondarenko

Trotz des enormen Fortschritts,der bei der Diagnose von Melanomen erzielt wurde,b leibt die zuver... more Trotz des enormen Fortschritts,der bei der Diagnose von Melanomen erzielt wurde,b leibt die zuverlässige Erken-nung der verschiedenen Malignomstadien eine Herausforde-rung. Derzeitige Strategien setzen auf die optischeE rfassung der Konzentration und räumlichen Verteilung spezifischer Biomarker.N eueste optische Methoden kçnnen allerdings durch Interferenzen mit Hintergrundfarben und durch Auto-fluoreszenzb eeinflusst werden. Wirb ewältigen diese Defizite hier unter Einsatz der elektrochemischen Rastersonden-mikroskopie (SECM), um den prognostischen Indikator Ty-rosinase (TyR) in nicht-metastatischen und metastatischen Melanomgeweben mithilfe einer weichen Mikroelektroden-sonde abzubilden. Das elektrochemischeA uslesen der TyR-Verteilung wurde durchd as Adaptieren eines immunchemi-schenV erfahrens ermçglicht. Wir zeigen, dass die SECM die erwähnten Beschränkungen optischer Methoden übertreffen kann und bisher nicht gekannte Mçglichkeiten für eine ver-besserte Diagnose und das Verständnis der räumlichen Ver-teilung von TyRinv erschiedenen Melanomstadien erçffnet.