Publications

N-doped graphene oxides (GO) are nanomaterials of interest as building blocks for 3D electrode architectures for vanadium redox flow battery applications. N- and O-functionalities have been reported to increase charge transfer rates for vanadium redox couples. However, GO synthesis typically yields heterogeneous nanomaterials, making it challenging to understand whether the electrochemical activity of conventional GO electrodes results from a sub-population of GO entities or sub-domains. Herein, single-entity voltammetry studies of vanadyl oxidation at N-doped GO using scanning electrochemical cell microscopy (SECCM) are reported. The electrochemical response is mapped at sub-domains within isolated flakes and found to display significant heterogeneity: small active sites are interspersed between relatively large inert sub-domains. Correlative Raman-SECCM analysis suggests that defect densities are not useful predictors of activity, while the specific chemical nature of defects might be a more important factor for understanding oxidation rates. Finite element simulations of the electrochemical response suggest that active sub-domains/sites are smaller than the mean inter-defect distance estimated from Raman spectra but can display very fast heterogeneous rate constants $>$1 cm s−1. These results indicate that N-doped GO electrodes can deliver on intrinsic activity requirements set out for the viable performance of vanadium redox flow battery devices.

Carbon porous materials containing nitrogen functionalities and encapsulated iron-based active sites have been suggested as electrocatalysts for energy conversion, however their applications to the hydrogenation of organic substrates via electrocatalytic hydrogenation (ECH) remain unexplored. Herein, we report on a Fe@C:N material synthesized with an adapted annealing procedure and tested as electrocatalyst for the hydrogenation of benzaldehyde. Using different concentrations of the organic, and electrolysis coupled to gas chromatography experiments, we demonstrate that it is possible to use such architectures for the ECH of unsaturated organics. Potential control experiments show that ECH faradaic efficiencies $>$70% are possible in acid electrolytes, while maintaining selectivity for the alcohol over the pinacol dimerization product. Estimates of product formation rates and turnover frequency (TOF) values suggest that these carbon-encapsulated architectures can achieve competitive performance in acid electrolytes relative to both base and precious metal electrodes.

A nitrate reducing microbial biocathode was developed through constant polarization at -0.5 V vs SCE using two types of inoculums: a pure culture of Thiobacillus denitrificans and water collected from the artificial wetland of Rampillon (France). The results show a clear increase of the nitrate removal efficiency for the Pilots with the natural water although no catalytic nitrate reduction can be evidenced by cyclic voltammetry. Further studies show a catalytic oxygen reduction through exo-electrogenic metabolism and a correlation between the cathode polarization at -0.5 V vs SCE and the nitrate remediation. 16S rRNA gene amplicon sequencing of the biofilm bacterial DNA shows a very large predominance of Pseudomonas, a genus that includes many species able to reduce nitrate and/or reduce dioxygen (O2) using electrons from a cathode. The increased nitrate reduction performance is hypothesized to arise from an indirect bioelectro-assistance that allows the emergence of local anoxic conditions caused by microbial endo-electrogenic pathway for oxygen reduction.

Polypyrrole films are commonly prepared as conductive electrode surfaces for a variety of applications. Recently, there has been increasing interest in improving the adhesive properties and biocompatibility of polypyrrole electrodes via the incorporation of bioinspired polydopamine within the polymer scaffold. However, very little is currently known about the structural effects of polydopamine incorporation during the electropolymerisation of hybrid films. In this work, we combine electrochemical quartz crystal microbalance studies, fundamental electrochemical characterisation, atomic force microscopy, and a suite of spectroscopic techniques in order to correlate changes in the structure and performance of polypyrrole–polydopamine films to the structural modifications of the nanostructure induced by polydopamine incorporation. The results indicate that polydopamine incorporation greatly increases the rate of hybrid film deposition, as well as improving adhesion, surface homogeneity, and wettability, with no compromise in charge transfer properties. Polydopamine incorporation is strongly suggested to occur in non-connected domains within a predominantly polypyrrole-like scaffold. We propose a two-step model of co-polymerisation and the subsequent surface adhesion of hybrid films. Results are expected to be of broad general interest to researchers utilizing polypyrrole and polydopamine to prepare tailor-made electrodes for biosensing and catalysis.

Iron is the second most abundant metal in the earth’s crust and among the most commonly used metals in industry. Iron is of fundamental importance in electrochemistry due to its rich redox chemistry, which has seen applications in bioelectrochemistry as well as materials in fuel cell electrocatalysts, batteries, and capacitors. This chapter briefly outlines the basic physical and chemical properties of iron including its fundamental thermodynamics and its redox behavior in solution. The electrochemistry of iron is briefly discussed with special emphasis on power sources, including nickel–iron, and iron–air batteries, FeS2 cathodes, iron-nitrogen electrocatalysts for fuel cells and microbial fuel cells based on iron redox proteins.

Conjugation of biomolecules on the surface of nanoparticles (NPs) to achieve active targeting is widely investigated within the scientific community. However, while a basic framework of the physicochemical processes underpinning bionanoparticle recognition is now emerging, the precise evaluation of the interactions between engineered NPs and biological targets remains underdeveloped. Here, we show how the adaptation of a method currently used to evaluate molecular ligand–receptor interactions by quartz crystal microbalance (QCM) can be used to obtain concrete insights into interactions between different NP architectures and assemblies of receptors. Using a model bionanoparticle grafted with oriented apolipoprotein E (ApoE) fragments, we examine key aspects of bionanoparticle engineering for effective interactions with target receptors. We show that the QCM technique can be used to rapidly measure construct–receptor interactions across biologically relevant exchange times. We contrast random adsorption of the ligand at the surface of the NPs, resulting in no measurable interaction with target receptors, to grafted oriented constructs, which are strongly recognized even at lower graft densities. The effects of other basic parameters impacting the interaction such as ligand graft density, receptor immobilization density, and linker length were also efficiently evaluated with this technique. Dramatic changes in interaction outcomes with subtle alterations in these parameters highlight the general importance of measuring the interactions between engineered NPs and target receptors ex situ early on in the construct development process for the rational design of bionanoparticles.

This article gives an up-to-date (2023) account on the bioinorganic basis for extracellular electron transfer (EET) in electroactive bacteria. These microorganisms connect their respiratory metabolism to extracellular solid electron acceptors or donors, typically metal oxides of iron or manganese. Thanks to this peculiar property, electroactive bacteria can develop as biofilms at electrodes, be studied electrochemically, and form the basis of diverse potential applications termed microbial electrochemical systems (MES). The metalloproteins forming the respiratory chain from NADH oxidation to the reduction of the terminal solid electron acceptor are described in detail for the most studied Gram-negative electroactive strains developed at anodes: Shewanella oneidensis MR-1 and Geobacter sulfurreducens. Although less efficient than their Gram-negative counterpart and sometimes referred to as weak electricigens, an example of electroactive anodophile Gram-positive bacteria, Thermincola sp., is also discussed. The key cytochromes involved in the electron transport chain are discussed such as outer membrane c-type cytochromes (Omc) and multiheme cytochromes, forming by self-assembly up to micrometer-long electron conductive extracellular pili or nanowires. The case of microorganisms that uptake electrons from solid extracellular electron donors is addressed with a highlight on photoferrotrophs and cathodic denitrifying bacteria. Finally, the common strategy developed by different bacteria to electrically connect different types of respiratory metabolism is stressed together with the apparent ubiquity of EET across life domains including archaea.

Grafting of aryldiazonium cations bearing a p-mannoside functionality over microbial fuel cell (MFC) anode materials was performed to investigate the ability of aryl-glycoside layers to regulate colonisation by biocatalytic biofilms. Covalent attachment was achieved via spontaneous reactions and via electrochemically-assisted grafting using potential step experiments. The effect of different functionalisation protocols on MFC performance is discussed in terms of changes in wettability, roughness and electrochemical response of modified electrodes. Water contact angle measurements (WCA) show that aryl-mannoside grafting yields a significant increase in hydrophilic character. Surface roughness determinations via atomic force microscopy (AFM) suggest a more disordered glycan adlayer when electrografting is used to facilitate chemisorption. MFCs were used as living sensors to successfully test the coated electrodes: the response of the MFCs in terms of start-up time was accelerated when compared to that of MFC equipped with non-modified electrodes, this suggests a faster development of a mature biofilm community resulting from aryldiazonium modifications, as confirmed by cyclic voltammetry of MFC anodes. These results therefore indicate that modification with glycans offers a bioinspired route to accelerating biofilm colonisation without any adverse effects on final MFC outputs.

The effect of glycan adlayers on the electrochemical response of glassy carbon electrodes was studied using standard redox probes and complex aqueous matrices. Aryldiazonium cations of aryl-lactoside precursors were used to modify glassy carbon via spontaneous and electrochemically assisted covalent grafting. Contact angle and fluorescence binding using Peanut Agglutinin (PNA) as a diagnostic lectin indicate that electrografting results in adlayers with greater glycan surface density than those obtained via spontaneous reaction. X-ray photoelectron spectroscopy with a fluorinated analog confirmed that electrografting results in multilayers of cross-linked aryl-lactosides. Adsorption studies with Bovine Serum Albumin (BSA) show that aryl-lactoside adlayers minimize unspecific protein adsorption. However, no significant differences were detected between spontaneous and electrografted layers in their ability to resist protein fouling despite their differences in coverage. Voltammetry studies show that spontaneous grafting has minimal effects on the response of standard redox probes in solution, whereas electrografting results in additional charge transfer impedance arising from increased electrode passivation. Bare and lactoside-modified carbon electrodes were tested for the detection of caffeine before and after prolonged exposure to coffee solutions. Spontaneous grafting was found to result in optimal properties by imparting antifouling performance in these complex matrices while preserving fast interfacial charge transfer.

Abstract Metal-free carbon electrodes with well-defined composition and smooth topography are prepared via sputter deposition followed by thermal treatment with inert and reactive gases. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy show that three carbons of similar N/C content that differ in N-site composition are thus prepared: an electrode consisting of almost exclusively graphitic-N (NG), an electrode with predominantly pyridinic-N (NP), and one with ≈1:1 NG:NP composition. These materials are used as model systems to investigate the activity of N-doped carbons in the oxygen reduction reaction (ORR) using voltammetry. Results show that selectivity toward 4e-reduction of O2 is strongly influenced by the NG/NP site composition, with the material possessing nearly uniform NG/NP composition being the only one yielding a 4e-reduction. Computational studies on model graphene clusters are carried out to elucidate the effect of N-site homogeneity on the reaction pathway. Calculations show that for pure NG-doping or NP-doping of model graphene clusters, adsorption of hydroperoxide and hydroperoxyl radical intermediates, respectively, is weak, thus favoring desorption prior to complete 4e-reduction to hydroxide. Clusters with mixed NG/NP sites display synergistic effects, suggesting that co-presence of these sites improves activity and selectivity by achieving high theoretical reduction potentials while facilitating retention of intermediates.

Metal-free nitrogenated amorphous carbon electrodes were synthesised via dc plasma magnetron sputtering and post-deposition annealing at different temperatures. The electrocatalytic activity of the electrodes towards the oxygen reduction reaction (ORR) was studied as a function of pH using cyclic voltammetry with a rotating disk electrode. The trends in onset potential were correlated to the carbon nanostructure and chemical composition of the electrodes as determined via Raman spectroscopy and X-ray photoelectron spectroscopy analysis. Results suggest that: 1) the ORR activity in acidic conditions is strongly correlated to the concentration of pyridinic nitrogen sites. 2) At high pH, the presence of graphitic nitrogen sites and a graphitized carbon scaffold are the strongest predictors of high ORR onsets, while pyridinic nitrogen site density does not correlate to ORR activity. An inversion region where pyridine-mediated activity competes with graphitic-N mediated activity is identified in the pH region close to the value of pKa of the pyridinium cation. The onset of the ORR is therefore determined by the activity of different sites as a function of pH and evidence for distinct reduction reaction pathways emerges from these results.

Abnormal levels of the neurotransmitter dopamine have been linked to a variety of neurochemical disorders including depression and Parkinson’s disease. Dopamine concentrations are often quantified electrochemically using biosensors prepared from carbon electrode materials such as carbon paste or glassy carbon. The charge transfer kinetics of dopamine is highly sensitive to carbon surface termination, including the presence of certain oxygen functional groups and adsorption sites. However, the nature of the binding sites and the effects of surface oxidation on the voltammetry of dopamine are both poorly understood. In this work the electrochemical response of dopamine at glassy carbon model surfaces was investigated to understand the effects of altering both the carbon nanostructure and oxygen surface chemistry on dopamine charge transfer kinetics and adsorption. Glassy carbon electrodes with low oxygen content and a high degree of surface graphitisation were prepared via thermal annealing at 900,°C, whilst highly oxidised glassy carbon electrodes were obtained through electrochemical anodisation at 1.8,V vs Ag/AgCl. The carbon surface structure and composition in each case was studied via X-Ray Photoelectron Spectroscopy. Voltammetry in solutions of dopamine at acidic pH confirmed that both annealing and anodisation treatments result in carbon surfaces with rapid charge transfer kinetics. However, dopamine adsorption occurs only at the low-oxygen, highly-graphitized carbon surface. Density functional theory studies on graphene model surfaces reveal that this behaviour is due to non-covalent interactions between the π-system of dopamine and the basal sites in the annealed surface. Simulations also show that the introduction of oxygen moieties disrupt these interactions and inhibit dopamine adsorption, in agreement with experiments. The results clarify the role of oxygen moieties and basal plane sites in facilitating both the adsorption of and charge transfer to DA at carbon electrodes.