James Behan | Trinity College Dublin (original) (raw)

Papers by James Behan

Bioconjugate Chemistry

The progress achieved over the last three decades in the field of bioconjugation has enabled the ... more The progress achieved over the last three decades in the field of bioconjugation has enabled the preparation of sophisticated nanomaterial−biomolecule conjugates, referred to herein as bionanoconstructs, for a multitude of applications including biosensing, diagnostics, and therapeutics. However, the development of bionanoconstructs for the active targeting of cells and cellular compartments, both in vitro and in vivo, is challenged by the lack of understanding of the mechanisms governing nanoscale recognition. In this review, we highlight fundamental obstacles in designing a successful bionanoconstruct, considering findings in the field of bionanointeractions. We argue that the biological recognition of bionanoconstructs is modulated not only by their molecular composition but also by the collective architecture presented upon their surface, and we discuss fundamental aspects of this surface architecture that are central to successful recognition, such as the mode of biomolecule conjugation and nanomaterial passivation. We also emphasize the need for thorough characterization of engineered bionanoconstructs and highlight the significance of population heterogeneity, which too presents a significant challenge in the interpretation of in vitro and in vivo results. Consideration of such issues together will better define the arena in which bioconjugation, in the future, will deliver functional and clinically relevant bionanoconstructs.

Carbon materials have prompted great interest in the biomedical field due to their good performan... more Carbon materials have prompted great interest in the biomedical field due to their good performance as coating for prosthetics and medical devices. However to realize their potential it is critical to control formation and composition of the protein corona in biological media. Mimicking the antifouling properties of the glycocalyx, found in certain cell membranes, offers a promising strategy to prevent clinical problems associated with nonspecific adsorption of plasma proteins on implants. Herein protein adsorption studies were carried out at carbon surfaces functionalized with aryldiazonium layers bearing mono-and di-saccharide glycosides. Localized Surface Plasma Resonance (LSPR) and Quartz Crystal Microbalance (QCM) were used for in situ determinations of the dynamic of protein fouling at bare and modified amorphous carbon surfaces. Surface IR reflectance absorption spectroscopy was used to study ex situ adsorption of albumin, lysozyme and fibrinogen. Protein adsorption at carbohydrate layers was found to decrease by 30-90% with respect to bare carbon surfaces. Finally, Multisolvent contact angle measurements were used to calculate surface free energy and acid-base polar components of bare and modified surfaces based on the van Oss-Chaudhury-Good model. May 22 nd , 2017 Oral ZGG9V

Nanoscale Advances

We describe how magnetic nanoparticles can be used to study intracellular nanoparticle traffickin... more We describe how magnetic nanoparticles can be used to study intracellular nanoparticle trafficking, and how magnetic extraction may be integrated with downstream analyses to investigate nanoscale decision-making events.

Frontiers in Chemistry

Nitrogen-free amorphous carbon thin films prepared via sputtering followed by graphitization, wer... more Nitrogen-free amorphous carbon thin films prepared via sputtering followed by graphitization, were used as precursor materials for the creation of N-doped carbon electrodes with varying degrees of amorphization. Incorporation of N-sites was achieved via nitrogen plasma treatments which resulted in both surface functionalization and amorphization of the carbon electrode materials. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were used to monitor composition and carbon organization: results indicate incorporation of predominantly pyrrolic-N sites after relatively short treatment cycles (5 min or less), accompanied by an initial etching of amorphous regions followed by a slower process of amorphization of graphitized clusters. By leveraging the difference in the rate of these two processes it was possible to investigate the effects of chemical N-sites and C-defect sites on their electrochemical response. The materials were tested as metal-free electrocatalysts in the oxygen reduction reaction (ORR) in alkaline conditions. We find that the introduction of predominantly pyrrolic-N sites via plasma modification results in improvements in selectivity in the ORR, relative to the nitrogen-free precursor material. Introduction of defects through prolonged plasma exposure has a more pronounced and beneficial effect on ORR descriptors than introduction of N-sites alone, leading to both increased onset potentials, and reduced hydroperoxide yields relative to the nitrogen-free carbon material. Our results suggest that increased structural disorder/heterogeneity results in the introduction of carbon sites that might either serve as main activity sites, or that enhance the effects of N-functionalities in the ORR via synergistic effects.

Carbon

The effect of glycan adlayers on the electrochemical response of glassy carbon electrodes was stu... more 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.

ACS Applied Bio Materials

A mild and efficient surface modification protocol for the preparation of β-cyclodextrin (βCD) mo... more A mild and efficient surface modification protocol for the preparation of β-cyclodextrin (βCD) modified surfaces through aryldiazonium mediated grafting is reported. Mono substituted 6-Oaminophenol-β-Cyclodextrin (amβCD) was synthesized through a three step protocol. This compound was found to form supramolecular aggregates in aqueous solutions at relatively low concentrations via cavity-directed self-assembly. Disruption of these supramolecular structures through judicious choice of solvent was found to be essential for the formation of the react ive 2 aryldiazonium species from the amino-phenolic precursor and for spontaneous surface grafting from aqueous solutions. Cyclodextrin thin films were prepared on carbon macroscopic substrates and electrodes and were characterized via infrared reflectance absorption spectroscopy (IRRAS), cyclic voltammetry and water contact angle measurements. Protein adsorption studies demonstrated that βCD adlayers reduced non-specific protein adsorption. βCD moieties in adlayers can be used nonetheless for specific host-guest complexation and are grafted at the surface with monolayer coverage (1.2 × 10-10 mol cm-2) as demonstrated via experiments using ferrocene, a redox probe. Finally, cyclodextrin covalent immobilization was demonstrated also on stainless steel and polyamide samples, two substrates with wide ranging technological applications.

Small

Metal-free carbon electrodes with well-defined composition and smooth topography were prepared vi... more Metal-free carbon electrodes with well-defined composition and smooth topography were prepared via sputter deposition followed by thermal treatment with inert and reactive gases. XPS and Raman spectroscopies show that three carbons of similar N/C content that differ in Nsite composition were thus prepared: an electrode consisting of almost exclusively graphitic-N (NG), an electrode with predominantly pyridinic-N (NP) and one with ca. 1:1 NG:NP composition. These materials were used as model systems to investigate activity of N-doped carbons in the oxygen reduction reaction (ORR) using voltammetry. Results show that selectivity towards 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 were carried out to elucidate the effect of N-site homogeneity on the reaction pathway. Calculations show that for pure NGdoping 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.

Carbon

Metal-free nitrogenated amorphous carbon electrodes were synthesised via dc plasma magnetron sput... more 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 pK a 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.

Electrochimica Acta

Abnormal levels of the neurotransmitter dopamine have been linked to a variety of neurochemical d... more 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 are 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 o 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. Surface ΔE / mV j p,c / j p,a α eff Polished GC

RSC Advances

Nitrogen incorporation into carbon increases metallic character and capacitance, however high con... more Nitrogen incorporation into carbon increases metallic character and capacitance, however high concentrations are instead disruptive and decrease interfacial capacitance.

The Journal of Physical Chemistry C

Oxygen electrochemistry is at the core of several emerging energy conversion technologies. The ro... more Oxygen electrochemistry is at the core of several emerging energy conversion technologies. The role of carbon nanostructures in the electrocatalysis of the oxygen reduction reaction is not well understood. Herein we report an investigation of the role of graphitic edges in oxygen electrochemistry. A new synthetic method was used to create all-carbon model electrode materials with controlled morphology. Electron microscopy results show that synthesized materials possess a high density of graphitic edges. Electrochemical intercalation experiments however indicate that the density of electroactive edges does not correlate positively with microscopy results. The materials were then characterized as electrodes for the oxygen reduction reaction in alkaline media. Results suggest that electrochemical determinations of edge and defect density more accurately predicts electrocatalytic activity thus suggesting that in situ characterization techniques are needed to understand the carbon/electrolyte interface.

The Journal of Physical Chemistry C

The modification of carbon materials via the incorporation of nitrogen has received much attentio... more The modification of carbon materials via the incorporation of nitrogen has received much attention in recent years due to their performance as electrodes in applications ranging from electroanalysis to electrocatalysis for energy storage technologies. In this work we synthesized nitrogen-incorporated amorphous carbon thin film electrodes (a-C:N) with different degrees of nitrogenation via magnetron sputtering. Electrodes were characterized using a combination of spectroscopic and electrochemical methods, including X-ray photoelectron spectroscopy, ellipsometry, voltammetry and impedance spectroscopy. Results indicate that low levels of nitrogenation yield carbon materials with narrow optical gaps and semimetallic character. These materials displayed fast electron-transfer kinetics to hexammine ruthenium(II)/(III), an outersphere redox couple that is sensitive to electronic properties near the Fermi level in the electrode material. Increasing levels of nitrogenation first decrease the metallic character of the electrodes and increase the impedance to charge transfer and, ultimately, yield materials with optical and electrochemical properties consistent with disordered cluster aggregates rather than amorphous solids. A positive correlation was found between the resistance to charge transfer and the optical gap when using the outer sphere redox couple. Interestingly, the use of ferrocyanide as a surfacesensitive redox probe resulted in a monotonic increase of the impedance to charge transfer vs. nitrogen content. This result suggests that surface chemical effects can dominate the electrochemical response, even when nitrogenation results in enhanced metallic character in carbon electrodes.

Surface and Interface Analysis

Carbon materials are widely used in a range of applications from biomaterials to sensing and elec... more Carbon materials are widely used in a range of applications from biomaterials to sensing and electronics. Many of these applications rely on the ability to control carbon/water interfacial properties, in particular surface charge density. This work reports a study of the electrokinetic properties of amorphous carbon thin films as a function of pH and surface chemistry. Surface ζpotential (SZP) and isoelectric point were determined using the tracer particle method. Initially the use of sulfonated and amine-terminated latex bead suspensions as tracer particles for the determination of SZP of reference polymer surfaces was validated. The tracer particle method was then applied to the determination of SZP and isoelectric point of macroscopic carbon surfaces with different surface chemistry. Highly graphitic and sp 3-rich hydrogenated carbon surfaces were found to display negative SZP, as expected for hydrophobic surfaces. The isoelectric point of the most highly graphitic surface was found to be pH iso = 3.7. Surface oxidation of these films resulted in a decrease of SZP at all pH values and in a downshift of pH iso to values lower than 1.5, consistently with the presence of surface acidic groups arising from oxidation. Results indicate that the specific choice of acid/base chemistry for the tracer particles does not significantly affect either SZP or pH iso determinations. These results show that the tracer particle method in combination with widely available latex beads as tracers can be applied for the determination of carbon SZP as a function of pH.

Langmuir : the ACS journal of surfaces and colloids, May 2, 2017

Various forms of carbon are known to perform well as biomaterials in a variety of applications an... more Various forms of carbon are known to perform well as biomaterials in a variety of applications and an improved understanding of their interactions with biomolecules, cells, and tissues is of interest for improving and tailoring their performance. Nanoplasmonic sensing (NPS) has emerged as a powerful technique for studying the thermodynamics and kinetics of interfacial reactions. In this work, the in situ adsorption of two proteins, bovine serum albumin and fibrinogen, were studied at carbon surfaces with differing chemical and optical properties using nanoplasmonic sensors. The carbon material was deposited as a thin film onto NPS surfaces consisting of 100 nm Au nanodisks with a localized plasmon absorption peak in the visible region. Carbon films were fully characterized by X-ray photoelectron spectroscopy, atomic force microscopy, and spectroscopic ellipsometry. Two types of material were investigated: amorphous carbon (a-C), with high graphitic content and high optical absorptiv...

Scientific Reports, 2016

Carbon materials and nanomaterials are of great interest for biological applications such as impl... more Carbon materials and nanomaterials are of great interest for biological applications such as implantable devices and nanoparticle vectors, however, to realize their potential it is critical to control formation and composition of the protein corona in biological media. In this work, protein adsorption studies were carried out at carbon surfaces functionalized with aryldiazonium layers bearing mono-and disaccharide glycosides. Surface IR reflectance absorption spectroscopy and quartz crystal microbalance were used to study adsorption of albumin, lysozyme and fibrinogen. Protein adsorption was found to decrease by 30-90% with respect to bare carbon surfaces; notably, enhanced rejection was observed in the case of the tested di-saccharide vs. simple mono-saccharides for near-physiological protein concentration values. ζ-potential measurements revealed that aryldiazonium chemistry results in the immobilization of phenylglycosides without a change in surface charge density, which is known to be important for protein adsorption. Multisolvent contact angle measurements were used to calculate surface free energy and acid-base polar components of bare and modified surfaces based on the van Oss-Chaudhury-Good model: results indicate that protein resistance in these phenylglycoside layers correlates positively with wetting behavior and Lewis basicity. Much effort towards the design and fabrication of biomaterials and medical devices is dedicated to the attainment of desirable surface chemistry and surface physical properties, as these can often determine the biological response to materials in vivo 1. There is therefore a strong interest in investigating surface modification strategies that enable a degree of control over interfacial biointeractions. Protein-surface interactions are thought to be of particular importance due to the abundance of these molecules in tissues and biological fluids and due to the central role of peptides and proteins in cell adhesion and signalling. Depending on the specific biomaterial and its application (e.g. biosensor, implant) it might be desirable to either promote protein adsorption or repel protein build-up in order to modulate performance 2-5. Therefore, much effort has been devoted to developing surface modification strategies to modulate protein-surface interactions. Various forms of carbon find multiple applications as biomaterials; coatings such as pyrocarbon and amorphous carbons (e.g. a-C, a-C:Si, a-C:H, ta-C) 6,7 , are promising for biomedical applications because of their frictional and mechanical properties, their corrosion resistance and chemical inertness, and their bio-and hemocompatibility. Carbon nanomaterials, such as nanotubes and nanodiamonds, have also received much attention as delivery agents for in vivo imaging and sensing 8,9. Finally, materials such as diamond electrodes, carbon coatings and carbon nanofibers are routinely used for in vivo and in vitro bioanalytical chemistry 10,11. For all of these applications it is critical to achieve control over interfacial interactions of the carbon solid surface with proteins in solution, to avoid unspecific adsorption that might result in undesirable cell-surface events, or in blocking of sensing/binding sites 12-15 .

Bioconjugate Chemistry

The progress achieved over the last three decades in the field of bioconjugation has enabled the ... more The progress achieved over the last three decades in the field of bioconjugation has enabled the preparation of sophisticated nanomaterial−biomolecule conjugates, referred to herein as bionanoconstructs, for a multitude of applications including biosensing, diagnostics, and therapeutics. However, the development of bionanoconstructs for the active targeting of cells and cellular compartments, both in vitro and in vivo, is challenged by the lack of understanding of the mechanisms governing nanoscale recognition. In this review, we highlight fundamental obstacles in designing a successful bionanoconstruct, considering findings in the field of bionanointeractions. We argue that the biological recognition of bionanoconstructs is modulated not only by their molecular composition but also by the collective architecture presented upon their surface, and we discuss fundamental aspects of this surface architecture that are central to successful recognition, such as the mode of biomolecule conjugation and nanomaterial passivation. We also emphasize the need for thorough characterization of engineered bionanoconstructs and highlight the significance of population heterogeneity, which too presents a significant challenge in the interpretation of in vitro and in vivo results. Consideration of such issues together will better define the arena in which bioconjugation, in the future, will deliver functional and clinically relevant bionanoconstructs.

Carbon materials have prompted great interest in the biomedical field due to their good performan... more Carbon materials have prompted great interest in the biomedical field due to their good performance as coating for prosthetics and medical devices. However to realize their potential it is critical to control formation and composition of the protein corona in biological media. Mimicking the antifouling properties of the glycocalyx, found in certain cell membranes, offers a promising strategy to prevent clinical problems associated with nonspecific adsorption of plasma proteins on implants. Herein protein adsorption studies were carried out at carbon surfaces functionalized with aryldiazonium layers bearing mono-and di-saccharide glycosides. Localized Surface Plasma Resonance (LSPR) and Quartz Crystal Microbalance (QCM) were used for in situ determinations of the dynamic of protein fouling at bare and modified amorphous carbon surfaces. Surface IR reflectance absorption spectroscopy was used to study ex situ adsorption of albumin, lysozyme and fibrinogen. Protein adsorption at carbohydrate layers was found to decrease by 30-90% with respect to bare carbon surfaces. Finally, Multisolvent contact angle measurements were used to calculate surface free energy and acid-base polar components of bare and modified surfaces based on the van Oss-Chaudhury-Good model. May 22 nd , 2017 Oral ZGG9V

Nanoscale Advances

We describe how magnetic nanoparticles can be used to study intracellular nanoparticle traffickin... more We describe how magnetic nanoparticles can be used to study intracellular nanoparticle trafficking, and how magnetic extraction may be integrated with downstream analyses to investigate nanoscale decision-making events.

Frontiers in Chemistry

Nitrogen-free amorphous carbon thin films prepared via sputtering followed by graphitization, wer... more Nitrogen-free amorphous carbon thin films prepared via sputtering followed by graphitization, were used as precursor materials for the creation of N-doped carbon electrodes with varying degrees of amorphization. Incorporation of N-sites was achieved via nitrogen plasma treatments which resulted in both surface functionalization and amorphization of the carbon electrode materials. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were used to monitor composition and carbon organization: results indicate incorporation of predominantly pyrrolic-N sites after relatively short treatment cycles (5 min or less), accompanied by an initial etching of amorphous regions followed by a slower process of amorphization of graphitized clusters. By leveraging the difference in the rate of these two processes it was possible to investigate the effects of chemical N-sites and C-defect sites on their electrochemical response. The materials were tested as metal-free electrocatalysts in the oxygen reduction reaction (ORR) in alkaline conditions. We find that the introduction of predominantly pyrrolic-N sites via plasma modification results in improvements in selectivity in the ORR, relative to the nitrogen-free precursor material. Introduction of defects through prolonged plasma exposure has a more pronounced and beneficial effect on ORR descriptors than introduction of N-sites alone, leading to both increased onset potentials, and reduced hydroperoxide yields relative to the nitrogen-free carbon material. Our results suggest that increased structural disorder/heterogeneity results in the introduction of carbon sites that might either serve as main activity sites, or that enhance the effects of N-functionalities in the ORR via synergistic effects.

Carbon

The effect of glycan adlayers on the electrochemical response of glassy carbon electrodes was stu... more 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.

ACS Applied Bio Materials

A mild and efficient surface modification protocol for the preparation of β-cyclodextrin (βCD) mo... more A mild and efficient surface modification protocol for the preparation of β-cyclodextrin (βCD) modified surfaces through aryldiazonium mediated grafting is reported. Mono substituted 6-Oaminophenol-β-Cyclodextrin (amβCD) was synthesized through a three step protocol. This compound was found to form supramolecular aggregates in aqueous solutions at relatively low concentrations via cavity-directed self-assembly. Disruption of these supramolecular structures through judicious choice of solvent was found to be essential for the formation of the react ive 2 aryldiazonium species from the amino-phenolic precursor and for spontaneous surface grafting from aqueous solutions. Cyclodextrin thin films were prepared on carbon macroscopic substrates and electrodes and were characterized via infrared reflectance absorption spectroscopy (IRRAS), cyclic voltammetry and water contact angle measurements. Protein adsorption studies demonstrated that βCD adlayers reduced non-specific protein adsorption. βCD moieties in adlayers can be used nonetheless for specific host-guest complexation and are grafted at the surface with monolayer coverage (1.2 × 10-10 mol cm-2) as demonstrated via experiments using ferrocene, a redox probe. Finally, cyclodextrin covalent immobilization was demonstrated also on stainless steel and polyamide samples, two substrates with wide ranging technological applications.

Small

Metal-free carbon electrodes with well-defined composition and smooth topography were prepared vi... more Metal-free carbon electrodes with well-defined composition and smooth topography were prepared via sputter deposition followed by thermal treatment with inert and reactive gases. XPS and Raman spectroscopies show that three carbons of similar N/C content that differ in Nsite composition were thus prepared: an electrode consisting of almost exclusively graphitic-N (NG), an electrode with predominantly pyridinic-N (NP) and one with ca. 1:1 NG:NP composition. These materials were used as model systems to investigate activity of N-doped carbons in the oxygen reduction reaction (ORR) using voltammetry. Results show that selectivity towards 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 were carried out to elucidate the effect of N-site homogeneity on the reaction pathway. Calculations show that for pure NGdoping 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.

Carbon

Metal-free nitrogenated amorphous carbon electrodes were synthesised via dc plasma magnetron sput... more 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 pK a 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.

Electrochimica Acta

Abnormal levels of the neurotransmitter dopamine have been linked to a variety of neurochemical d... more 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 are 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 o 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. Surface ΔE / mV j p,c / j p,a α eff Polished GC

RSC Advances

Nitrogen incorporation into carbon increases metallic character and capacitance, however high con... more Nitrogen incorporation into carbon increases metallic character and capacitance, however high concentrations are instead disruptive and decrease interfacial capacitance.

The Journal of Physical Chemistry C

Oxygen electrochemistry is at the core of several emerging energy conversion technologies. The ro... more Oxygen electrochemistry is at the core of several emerging energy conversion technologies. The role of carbon nanostructures in the electrocatalysis of the oxygen reduction reaction is not well understood. Herein we report an investigation of the role of graphitic edges in oxygen electrochemistry. A new synthetic method was used to create all-carbon model electrode materials with controlled morphology. Electron microscopy results show that synthesized materials possess a high density of graphitic edges. Electrochemical intercalation experiments however indicate that the density of electroactive edges does not correlate positively with microscopy results. The materials were then characterized as electrodes for the oxygen reduction reaction in alkaline media. Results suggest that electrochemical determinations of edge and defect density more accurately predicts electrocatalytic activity thus suggesting that in situ characterization techniques are needed to understand the carbon/electrolyte interface.

The Journal of Physical Chemistry C

The modification of carbon materials via the incorporation of nitrogen has received much attentio... more The modification of carbon materials via the incorporation of nitrogen has received much attention in recent years due to their performance as electrodes in applications ranging from electroanalysis to electrocatalysis for energy storage technologies. In this work we synthesized nitrogen-incorporated amorphous carbon thin film electrodes (a-C:N) with different degrees of nitrogenation via magnetron sputtering. Electrodes were characterized using a combination of spectroscopic and electrochemical methods, including X-ray photoelectron spectroscopy, ellipsometry, voltammetry and impedance spectroscopy. Results indicate that low levels of nitrogenation yield carbon materials with narrow optical gaps and semimetallic character. These materials displayed fast electron-transfer kinetics to hexammine ruthenium(II)/(III), an outersphere redox couple that is sensitive to electronic properties near the Fermi level in the electrode material. Increasing levels of nitrogenation first decrease the metallic character of the electrodes and increase the impedance to charge transfer and, ultimately, yield materials with optical and electrochemical properties consistent with disordered cluster aggregates rather than amorphous solids. A positive correlation was found between the resistance to charge transfer and the optical gap when using the outer sphere redox couple. Interestingly, the use of ferrocyanide as a surfacesensitive redox probe resulted in a monotonic increase of the impedance to charge transfer vs. nitrogen content. This result suggests that surface chemical effects can dominate the electrochemical response, even when nitrogenation results in enhanced metallic character in carbon electrodes.

Surface and Interface Analysis

Carbon materials are widely used in a range of applications from biomaterials to sensing and elec... more Carbon materials are widely used in a range of applications from biomaterials to sensing and electronics. Many of these applications rely on the ability to control carbon/water interfacial properties, in particular surface charge density. This work reports a study of the electrokinetic properties of amorphous carbon thin films as a function of pH and surface chemistry. Surface ζpotential (SZP) and isoelectric point were determined using the tracer particle method. Initially the use of sulfonated and amine-terminated latex bead suspensions as tracer particles for the determination of SZP of reference polymer surfaces was validated. The tracer particle method was then applied to the determination of SZP and isoelectric point of macroscopic carbon surfaces with different surface chemistry. Highly graphitic and sp 3-rich hydrogenated carbon surfaces were found to display negative SZP, as expected for hydrophobic surfaces. The isoelectric point of the most highly graphitic surface was found to be pH iso = 3.7. Surface oxidation of these films resulted in a decrease of SZP at all pH values and in a downshift of pH iso to values lower than 1.5, consistently with the presence of surface acidic groups arising from oxidation. Results indicate that the specific choice of acid/base chemistry for the tracer particles does not significantly affect either SZP or pH iso determinations. These results show that the tracer particle method in combination with widely available latex beads as tracers can be applied for the determination of carbon SZP as a function of pH.

Langmuir : the ACS journal of surfaces and colloids, May 2, 2017

Various forms of carbon are known to perform well as biomaterials in a variety of applications an... more Various forms of carbon are known to perform well as biomaterials in a variety of applications and an improved understanding of their interactions with biomolecules, cells, and tissues is of interest for improving and tailoring their performance. Nanoplasmonic sensing (NPS) has emerged as a powerful technique for studying the thermodynamics and kinetics of interfacial reactions. In this work, the in situ adsorption of two proteins, bovine serum albumin and fibrinogen, were studied at carbon surfaces with differing chemical and optical properties using nanoplasmonic sensors. The carbon material was deposited as a thin film onto NPS surfaces consisting of 100 nm Au nanodisks with a localized plasmon absorption peak in the visible region. Carbon films were fully characterized by X-ray photoelectron spectroscopy, atomic force microscopy, and spectroscopic ellipsometry. Two types of material were investigated: amorphous carbon (a-C), with high graphitic content and high optical absorptiv...

Scientific Reports, 2016

Carbon materials and nanomaterials are of great interest for biological applications such as impl... more Carbon materials and nanomaterials are of great interest for biological applications such as implantable devices and nanoparticle vectors, however, to realize their potential it is critical to control formation and composition of the protein corona in biological media. In this work, protein adsorption studies were carried out at carbon surfaces functionalized with aryldiazonium layers bearing mono-and disaccharide glycosides. Surface IR reflectance absorption spectroscopy and quartz crystal microbalance were used to study adsorption of albumin, lysozyme and fibrinogen. Protein adsorption was found to decrease by 30-90% with respect to bare carbon surfaces; notably, enhanced rejection was observed in the case of the tested di-saccharide vs. simple mono-saccharides for near-physiological protein concentration values. ζ-potential measurements revealed that aryldiazonium chemistry results in the immobilization of phenylglycosides without a change in surface charge density, which is known to be important for protein adsorption. Multisolvent contact angle measurements were used to calculate surface free energy and acid-base polar components of bare and modified surfaces based on the van Oss-Chaudhury-Good model: results indicate that protein resistance in these phenylglycoside layers correlates positively with wetting behavior and Lewis basicity. Much effort towards the design and fabrication of biomaterials and medical devices is dedicated to the attainment of desirable surface chemistry and surface physical properties, as these can often determine the biological response to materials in vivo 1. There is therefore a strong interest in investigating surface modification strategies that enable a degree of control over interfacial biointeractions. Protein-surface interactions are thought to be of particular importance due to the abundance of these molecules in tissues and biological fluids and due to the central role of peptides and proteins in cell adhesion and signalling. Depending on the specific biomaterial and its application (e.g. biosensor, implant) it might be desirable to either promote protein adsorption or repel protein build-up in order to modulate performance 2-5. Therefore, much effort has been devoted to developing surface modification strategies to modulate protein-surface interactions. Various forms of carbon find multiple applications as biomaterials; coatings such as pyrocarbon and amorphous carbons (e.g. a-C, a-C:Si, a-C:H, ta-C) 6,7 , are promising for biomedical applications because of their frictional and mechanical properties, their corrosion resistance and chemical inertness, and their bio-and hemocompatibility. Carbon nanomaterials, such as nanotubes and nanodiamonds, have also received much attention as delivery agents for in vivo imaging and sensing 8,9. Finally, materials such as diamond electrodes, carbon coatings and carbon nanofibers are routinely used for in vivo and in vitro bioanalytical chemistry 10,11. For all of these applications it is critical to achieve control over interfacial interactions of the carbon solid surface with proteins in solution, to avoid unspecific adsorption that might result in undesirable cell-surface events, or in blocking of sensing/binding sites 12-15 .