Martin Edwards - Academia.edu (original) (raw)
Papers by Martin Edwards
Chemical Communications, 2016
Faraday Discussions, organised by the Royal Society of Chemistry (RSC), bring the concept of the ... more Faraday Discussions, organised by the Royal Society of Chemistry (RSC), bring the concept of the flipped classroom to scientific conferences, promoting lively debate and discussion with the opportunity to hear complementary and contrasting views on topics spanning physical chemistry. The Faraday Discussion on the topic of single entity electrochemistry, in the picturesque setting of the University of York (31 st August to 2 nd September, 2016), presented an opportunity for researchers pushing at the lower limits of electrochemistry to engage in such lively discourse. The topic single entity was chosen to be as inclusive as possible, and proved to be so, with researchers probing single nanoparticles, nanopores, nanorods, vesicles and even single molecules, with a wide range of electrochemical techniques, both experimental and theoretical.
bioRxiv (Cold Spring Harbor Laboratory), Aug 18, 2022
Solid-state nanopores have been widely employed in the detection of biomolecules, but low signal-... more Solid-state nanopores have been widely employed in the detection of biomolecules, but low signal-to-noise ratios still represent a major obstacle to enable the discrimination of short nucleic acid and protein sequences. The addition of 50% polyethylene glycol (PEG) to the bath solution was recently demonstrated as a simple way to enhance the detection of such biomolecules translocating through a model solid-state nanopore. Here, we provide a comprehensive description of the physics describing a nanopore measurement carried out in 50% PEG that is supported by finite-element modelling and experiments. We demonstrate that the addition of PEG to the external solution introduces a strong imbalance in the transport properties of cations and anions, drastically affecting the characteristic current response of the nanopore. We further show that the strong asymmetric current response is due to a polarity-dependent ion distribution and transport at the nanopipette tip region, leading to either ion depletion or enrichment for few tens of nanometers across the aperture. Under negative potential, when double-stranded DNA molecules translocate, the depleted region (sensing region) significantly improves the sensitivity compared to systems without PEG. We then introduce a displacement of the interface between pore and external solution to simulate the mechanical interactions between analyte and PEG molecules. We found that this displacement affects the ion distribution in the sensing region, enhancing the detection current during the translocation of biomolecules. .
A finite-element model has been developed to simulate the cyclic voltammetric (CV) response of a ... more A finite-element model has been developed to simulate the cyclic voltammetric (CV) response of a planar electrode for a 1e outer-sphere redox process, which fully accounts for cell electrostatics, including ohmic potential drop, ion migration, and the structure of the potentialdependent electric double layer. Both reversible and quasi-reversible redox reactions are treated. The simulations compute the time-dependent electric potential and ion distributions across the entire cell during a voltammetric scan. In this way, it is possible to obtain the interdependent faradaic and non-faradaic contributions to a CV and rigorously include all effects of the electric potential distribution on the rate of electron transfer and the local
ABSTRACTNanopore sensing is a technique based on the Coulter principle to analyze and characteriz... more ABSTRACTNanopore sensing is a technique based on the Coulter principle to analyze and characterize nanoscale materials with single entity resolution. However, its use in nanoparticle characterization has been constrained by the need to tailor the nanopore aperture size to the size of the analyte, precluding the analysis of heterogenous samples. Additionally, nanopore sensors often require the use of high salt concentrations to improve the signal-to-noise ratio, which further limits their ability to study a wide range of nanoparticles that are unstable at high ionic strength.Here, we report the development of nanopore sensors enhanced by a polymer electrolyte system, enabling the analysis of heterogenous nanoparticle mixtures at low ionic strength. We present a finite element model to explain the anomalous conductive/resistive pulse signals observed and compare these results with experiments. Furthermore, we demonstrate the wide applicability of the method by characterizing metallic ...
Bipyridine complexes of Ni are used as catalysts in a variety of reductive transformations. Here,... more Bipyridine complexes of Ni are used as catalysts in a variety of reductive transformations. Here, the electroreduction of [Ni(Mebpy)3]2+ (Mebpy = 4,4’-dimethyl-2,2’-bipyridine) in dimethylformamide is reported, with the aim of determining the redox mechanism and oxidation states of products formed under well-controlled electrochemical conditions. Results from cyclic voltammetry, steady-state voltammetry (SSV) and chronoamperometry demonstrate that [Ni(Mebpy)3]2+ undergoes two sequential 1e reductions at closely separated potentials (E0’1 = -1.06 ± 0.01 V and E0’2 = -1.15 ± 0.01 V vs Ag/AgCl (3.4 M KCl)). Homogeneous comproportionation to generate [Ni(Mebpy)3]+ is demonstrated in SSV experiments in low ionic strength solutions. The comproportionation rate constant is determined to be > 106 M-1s-1, consistent with rapid outer-sphere electron transfer. Consequentially, on voltammetric time scales, the 2e reduction of [Ni(Mebpy)3]2+ results in formation of [Ni(Mebpy)3]1+ as the predo...
Chemical Science, 2020
Nanoparticles mimicking the three-dimensional architecture of enzymes where the reaction occurs d... more Nanoparticles mimicking the three-dimensional architecture of enzymes where the reaction occurs down a channel isolated from bulk solution, referred here as nanozymes, were used to explore the impact of nano-confinement on electrocatalytic reactions.
Journal of the American Chemical Society, 2019
Below is the automatically generated Comsol "model report", which describes in detail the boundar... more Below is the automatically generated Comsol "model report", which describes in detail the boundary conditions, geometry, mesh, solver configuration, etc., used in the finite element simulations of the 3-phase interface reported in this work and described in Supporting Information, section 6.
C-N cross-coupling is one of the most valuable and widespread transformations in organic synthesi... more C-N cross-coupling is one of the most valuable and widespread transformations in organic synthesis. Largely dominated by Pd-and Cu-based catalytic systems, it has proven to be a staple transformation for those in both academia and industry. The current study presents the development and mechanistic understanding of an electrochemically driven, Ni-catalyzed method for achieving this reaction of high strategic importance. Through a series of electrochemical, computational, kinetic, and empirical experiments the key mechanistic features of this reaction have been unraveled, leading to a second generation set of conditions that is applicable to a broad range of aryl halides and amine nucleophiles, including complex examples on oligopeptides, medicinally-relevant heterocycles, natural products, and sugars. Full disclosure of the current limitations as well as procedures for both batch and flow scale-ups (100 gram) are also described.
Faraday Discussions, 2018
The stochastic nature of very fast single-entity events challenges current electrochemical method... more The stochastic nature of very fast single-entity events challenges current electrochemical methods and modern electronics, as illustrated using recent experiments from the authors’ laboratory.
ACS nano, Jan 27, 2018
The oxidation of catecholamine at a microelectrode, following its release from individual vesicle... more The oxidation of catecholamine at a microelectrode, following its release from individual vesicles, allows interrogation of the content of single nanometer vesicles with vesicle impact electrochemical cytometry (VIEC). Previous to this development, there were no methods available to quantify the chemical load of single vesicles. However, accurate quantification of the content is hampered by uncertainty in the proportion of substituent molecules reaching the electrode surface (collection efficiency). In this work, we use quantitative modeling to calculate this collection efficiency. For all vesicles except those at the very edge of the electrode, modeling shows that ∼100% oxidation efficiency is achieved when employing a 33 μm diameter disk microelectrode for VIEC, independent of the location of the vesicle release pore. We use this to experimentally determine a precise distribution of catecholamine in individual vesicles extracted from PC12 cells. In contrast, we calculate that when...
The Journal of Physical Chemistry Letters, 2017
Herein, we use Pt nanodisk electrodes (apparent radii from 4 to 80 nm) to investigate the nucleat... more Herein, we use Pt nanodisk electrodes (apparent radii from 4 to 80 nm) to investigate the nucleation of individual O 2 nanobubbles generated by electrooxidation of hydrogen peroxide (H 2 O 2). A single bubble reproducibly nucleates when the dissolved O 2 concentration reaches ∼0.17 M at the Pt electrode surface. This nucleation concentration is ∼130 times higher than the equilibrium saturation concentration of O 2 and is independent of electrode size. Moreover, in acidic H 2 O 2 solutions (1 M HClO 4), in addition to producing an O 2 nanobubble through H 2 O 2 oxidation at positive potentials, individual H 2 nanobubbles can also be generated at negative potentials. Alternating generation of single O 2 and H 2 bubbles within the same experiment allows direct comparison of the critical concentrations for nucleation of each nanobubble without knowing the precise size/geometry of the electrode or the exact viscosity/ temperature of the solution.
Chemical Communications, 2016
Faraday Discussions, organised by the Royal Society of Chemistry (RSC), bring the concept of the ... more Faraday Discussions, organised by the Royal Society of Chemistry (RSC), bring the concept of the flipped classroom to scientific conferences, promoting lively debate and discussion with the opportunity to hear complementary and contrasting views on topics spanning physical chemistry. The Faraday Discussion on the topic of single entity electrochemistry, in the picturesque setting of the University of York (31 st August to 2 nd September, 2016), presented an opportunity for researchers pushing at the lower limits of electrochemistry to engage in such lively discourse. The topic single entity was chosen to be as inclusive as possible, and proved to be so, with researchers probing single nanoparticles, nanopores, nanorods, vesicles and even single molecules, with a wide range of electrochemical techniques, both experimental and theoretical.
Journal of the American Chemical Society, Jan 22, 2016
The dynamic collision behavior of the electro-oxidation of single Ag nanoparticles is observed at... more The dynamic collision behavior of the electro-oxidation of single Ag nanoparticles is observed at Au microelectrodes using stochastic single-nanoparticle collision amperometry. Results show that an Ag nanoparticle collision/oxidation event typically consists of a series of 1 to ∼10 discrete "sub-events" over an ∼20 ms interval. Results also show that the Ag nanoparticles typically undergo only partial oxidation prior to diffusing away from the Au electrode into the bulk solution. Both behaviors are characterized and shown to exist under a variety of experimental conditions. These previously unreported behaviors suggest that nanoparticle collision and electro-dissolution is a highly dynamic process driven by fast particle-electrode interactions and nanoparticle diffusion.
The journal of physical chemistry letters, Jan 6, 2016
An experimental system for controlling and interrogating the collisions of individual nanoparticl... more An experimental system for controlling and interrogating the collisions of individual nanoparticles at electrode/electrolyte interfaces is described. A nanopipet positioned over a 400 nm radius Pt ultramicroelectrode is used to deliver individual nanoparticles, via pressure-driven solution flow, to the underlying electrode, where the nanoparticles undergo collisions and are detected electrochemically. High-velocity collisions result in elastic collisions of negatively charged polystyrene nanospheres at the Pt/water interface, while low-velocity collisions result in nanoparticle adsorption ("sticky" collisions). The ability to position the nanopipet with respect to the underlying ultramicroelectrode also allows the time between particle release from the nanopipet and electrode collision to be investigated as a function of nanopipet-electrode separation, d. The time between release and collision of the nanoparticle is found to be proportional to d(3), in excellent agreement ...
Nano letters, Jan 12, 2016
The Young-Laplace equation is central to the thermodynamic description of liquids with highly cur... more The Young-Laplace equation is central to the thermodynamic description of liquids with highly curved interfaces, e.g., nanoscale droplets and their inverse, nanoscale bubbles. The equation relates the pressure difference across an interface to its surface tension and radius of curvature, but the validity in using the macroscopic surface tension for describing curved interfaces with radii smaller than tens of nanometers has been questioned. Here we present electrochemical measurement of Laplace pressures within single H2 bubbles between 7 and 200 nm radius (corresponding, respectively, to between 200 and 7 atm). Our results demonstrate a linear relationship between a bubble's Laplace pressure and its reciprocal radius, verifying the classical thermodynamic description of H2 nanobubbles as small as ∼10 nm.
Journal of The Electrochemical Society, 2016
The lifetimes of individual H 2 and N 2 nanobubbles, electrochemically generated at Pt nanoelectr... more The lifetimes of individual H 2 and N 2 nanobubbles, electrochemically generated at Pt nanoelectrodes (7-85 nm-radius), have been measured using a fast-scan electrochemical technique. To measure lifetime, a stable single H 2 or N 2 bubble is first generated by reducing protons or oxidizing hydrazine, respectively, at the Pt nanoelectrode. The electrode potential is then rapidly stepped (<100 μs) to a value where the bubble is unstable and begins to dissolve by gas molecule transfer across the gas/water interface and diffusion. The electrode potential is immediately scanned back to values where the bubble was initially stable. Depending on the rate of this second voltammetric scan, the initial bubble may or may not have time to dissolve, as is readily determined by the characteristic voltammetric signature corresponding to the nucleation of a new bubble. The transition between these regimes is used to determine the bubble's lifetime. The results indicate that dissolution of a H 2 or N 2 nanobubble is, in part, limited by the transfer of molecules across the gas/water interface. A theoretical expression describing mixed diffusion/kinetic control is presented and fit to the experimental data to obtain an interfacial gas transfer rate of ∼10 −9 mol N −1 s −1 .
ACS nano, Jan 8, 2015
Coulter counters measure the size of particles in solution by passing them through an orifice and... more Coulter counters measure the size of particles in solution by passing them through an orifice and measuring a resistive pulse, i.e., a drop in the ionic current flowing between two electrodes placed either side of the orifice. The magnitude of the pulse gives information on the size of the particle; however, accuracy is limited by variability in the path of the translocation, due to the Brownian motion of the particle. We present a simple-yet-powerful modified Coulter counter that uses programmable data acquisition hardware to switch the voltage after sensing the resistive pulse of a nanoparticle passing through the orifice of a nanopipette. Switching the voltage reverses the direction of the driving force on the particle, and when this detect-switch cycle is repeated, allows us to pass an individual nanoparticle through the orifice thousands of times. By measuring individual particles more than one hundred times per second we rapidly determine the distribution of the resistive puls...
Analytical chemistry, Jan 7, 2014
Iontophoresis uses a current to eject solution from the tip of a barrel formed from a pulled glas... more Iontophoresis uses a current to eject solution from the tip of a barrel formed from a pulled glass capillary and has been employed as a method of drug delivery for neurochemical investigations. Much attention has been devoted to resolving perhaps the greatest limitation of iontophoresis, the inability to determine the concentration of substances delivered by ejections. To further address this issue, we evaluate the properties of typical ejections such as barrel solution velocity and its relation to the ejection current using an amperometric and liquid chromatographic approach. These properties were used to predict the concentration distribution of ejected solute that was then confirmed by fluorescence microscopy. Additionally, incorporation of oppositely charged fluorophores into the barrel investigated the role of migration on the mass transport of an ejected species. Results indicate that location relative to the barrel tip is the primary influence on the distribution of ejected s...
ACS nano, Jan 28, 2014
We quantified the electrical polarization properties of single bacterial cells using electrostati... more We quantified the electrical polarization properties of single bacterial cells using electrostatic force microscopy. We found that the effective dielectric constant, ε(r,eff), for the four bacterial types investigated (Salmonella typhimurium, Escherchia coli, Lactobacilus sakei, and Listeria innocua) is around 3-5 under dry air conditions. Under ambient humidity, it increases to ε(r,eff) ∼ 6-7 for the Gram-negative bacterial types (S. typhimurium and E. coli) and to ε(r,eff) ∼ 15-20 for the Gram-positive ones (L. sakei and L. innocua). We show that the measured effective dielectric constants can be consistently interpreted in terms of the electric polarization properties of the biochemical components of the bacterial cell compartments and of their hydration state. These results demonstrate the potential of electrical studies of single bacterial cells.
The Journal of Physical Chemistry C, 2011
Intrinsic kinetics of gypsum and calcium sulfate anhydrite dissolution : surface selective studie... more Intrinsic kinetics of gypsum and calcium sulfate anhydrite dissolution : surface selective studies under hydrodynamic control and the effect of additives.
Chemical Communications, 2016
Faraday Discussions, organised by the Royal Society of Chemistry (RSC), bring the concept of the ... more Faraday Discussions, organised by the Royal Society of Chemistry (RSC), bring the concept of the flipped classroom to scientific conferences, promoting lively debate and discussion with the opportunity to hear complementary and contrasting views on topics spanning physical chemistry. The Faraday Discussion on the topic of single entity electrochemistry, in the picturesque setting of the University of York (31 st August to 2 nd September, 2016), presented an opportunity for researchers pushing at the lower limits of electrochemistry to engage in such lively discourse. The topic single entity was chosen to be as inclusive as possible, and proved to be so, with researchers probing single nanoparticles, nanopores, nanorods, vesicles and even single molecules, with a wide range of electrochemical techniques, both experimental and theoretical.
bioRxiv (Cold Spring Harbor Laboratory), Aug 18, 2022
Solid-state nanopores have been widely employed in the detection of biomolecules, but low signal-... more Solid-state nanopores have been widely employed in the detection of biomolecules, but low signal-to-noise ratios still represent a major obstacle to enable the discrimination of short nucleic acid and protein sequences. The addition of 50% polyethylene glycol (PEG) to the bath solution was recently demonstrated as a simple way to enhance the detection of such biomolecules translocating through a model solid-state nanopore. Here, we provide a comprehensive description of the physics describing a nanopore measurement carried out in 50% PEG that is supported by finite-element modelling and experiments. We demonstrate that the addition of PEG to the external solution introduces a strong imbalance in the transport properties of cations and anions, drastically affecting the characteristic current response of the nanopore. We further show that the strong asymmetric current response is due to a polarity-dependent ion distribution and transport at the nanopipette tip region, leading to either ion depletion or enrichment for few tens of nanometers across the aperture. Under negative potential, when double-stranded DNA molecules translocate, the depleted region (sensing region) significantly improves the sensitivity compared to systems without PEG. We then introduce a displacement of the interface between pore and external solution to simulate the mechanical interactions between analyte and PEG molecules. We found that this displacement affects the ion distribution in the sensing region, enhancing the detection current during the translocation of biomolecules. .
A finite-element model has been developed to simulate the cyclic voltammetric (CV) response of a ... more A finite-element model has been developed to simulate the cyclic voltammetric (CV) response of a planar electrode for a 1e outer-sphere redox process, which fully accounts for cell electrostatics, including ohmic potential drop, ion migration, and the structure of the potentialdependent electric double layer. Both reversible and quasi-reversible redox reactions are treated. The simulations compute the time-dependent electric potential and ion distributions across the entire cell during a voltammetric scan. In this way, it is possible to obtain the interdependent faradaic and non-faradaic contributions to a CV and rigorously include all effects of the electric potential distribution on the rate of electron transfer and the local
ABSTRACTNanopore sensing is a technique based on the Coulter principle to analyze and characteriz... more ABSTRACTNanopore sensing is a technique based on the Coulter principle to analyze and characterize nanoscale materials with single entity resolution. However, its use in nanoparticle characterization has been constrained by the need to tailor the nanopore aperture size to the size of the analyte, precluding the analysis of heterogenous samples. Additionally, nanopore sensors often require the use of high salt concentrations to improve the signal-to-noise ratio, which further limits their ability to study a wide range of nanoparticles that are unstable at high ionic strength.Here, we report the development of nanopore sensors enhanced by a polymer electrolyte system, enabling the analysis of heterogenous nanoparticle mixtures at low ionic strength. We present a finite element model to explain the anomalous conductive/resistive pulse signals observed and compare these results with experiments. Furthermore, we demonstrate the wide applicability of the method by characterizing metallic ...
Bipyridine complexes of Ni are used as catalysts in a variety of reductive transformations. Here,... more Bipyridine complexes of Ni are used as catalysts in a variety of reductive transformations. Here, the electroreduction of [Ni(Mebpy)3]2+ (Mebpy = 4,4’-dimethyl-2,2’-bipyridine) in dimethylformamide is reported, with the aim of determining the redox mechanism and oxidation states of products formed under well-controlled electrochemical conditions. Results from cyclic voltammetry, steady-state voltammetry (SSV) and chronoamperometry demonstrate that [Ni(Mebpy)3]2+ undergoes two sequential 1e reductions at closely separated potentials (E0’1 = -1.06 ± 0.01 V and E0’2 = -1.15 ± 0.01 V vs Ag/AgCl (3.4 M KCl)). Homogeneous comproportionation to generate [Ni(Mebpy)3]+ is demonstrated in SSV experiments in low ionic strength solutions. The comproportionation rate constant is determined to be > 106 M-1s-1, consistent with rapid outer-sphere electron transfer. Consequentially, on voltammetric time scales, the 2e reduction of [Ni(Mebpy)3]2+ results in formation of [Ni(Mebpy)3]1+ as the predo...
Chemical Science, 2020
Nanoparticles mimicking the three-dimensional architecture of enzymes where the reaction occurs d... more Nanoparticles mimicking the three-dimensional architecture of enzymes where the reaction occurs down a channel isolated from bulk solution, referred here as nanozymes, were used to explore the impact of nano-confinement on electrocatalytic reactions.
Journal of the American Chemical Society, 2019
Below is the automatically generated Comsol "model report", which describes in detail the boundar... more Below is the automatically generated Comsol "model report", which describes in detail the boundary conditions, geometry, mesh, solver configuration, etc., used in the finite element simulations of the 3-phase interface reported in this work and described in Supporting Information, section 6.
C-N cross-coupling is one of the most valuable and widespread transformations in organic synthesi... more C-N cross-coupling is one of the most valuable and widespread transformations in organic synthesis. Largely dominated by Pd-and Cu-based catalytic systems, it has proven to be a staple transformation for those in both academia and industry. The current study presents the development and mechanistic understanding of an electrochemically driven, Ni-catalyzed method for achieving this reaction of high strategic importance. Through a series of electrochemical, computational, kinetic, and empirical experiments the key mechanistic features of this reaction have been unraveled, leading to a second generation set of conditions that is applicable to a broad range of aryl halides and amine nucleophiles, including complex examples on oligopeptides, medicinally-relevant heterocycles, natural products, and sugars. Full disclosure of the current limitations as well as procedures for both batch and flow scale-ups (100 gram) are also described.
Faraday Discussions, 2018
The stochastic nature of very fast single-entity events challenges current electrochemical method... more The stochastic nature of very fast single-entity events challenges current electrochemical methods and modern electronics, as illustrated using recent experiments from the authors’ laboratory.
ACS nano, Jan 27, 2018
The oxidation of catecholamine at a microelectrode, following its release from individual vesicle... more The oxidation of catecholamine at a microelectrode, following its release from individual vesicles, allows interrogation of the content of single nanometer vesicles with vesicle impact electrochemical cytometry (VIEC). Previous to this development, there were no methods available to quantify the chemical load of single vesicles. However, accurate quantification of the content is hampered by uncertainty in the proportion of substituent molecules reaching the electrode surface (collection efficiency). In this work, we use quantitative modeling to calculate this collection efficiency. For all vesicles except those at the very edge of the electrode, modeling shows that ∼100% oxidation efficiency is achieved when employing a 33 μm diameter disk microelectrode for VIEC, independent of the location of the vesicle release pore. We use this to experimentally determine a precise distribution of catecholamine in individual vesicles extracted from PC12 cells. In contrast, we calculate that when...
The Journal of Physical Chemistry Letters, 2017
Herein, we use Pt nanodisk electrodes (apparent radii from 4 to 80 nm) to investigate the nucleat... more Herein, we use Pt nanodisk electrodes (apparent radii from 4 to 80 nm) to investigate the nucleation of individual O 2 nanobubbles generated by electrooxidation of hydrogen peroxide (H 2 O 2). A single bubble reproducibly nucleates when the dissolved O 2 concentration reaches ∼0.17 M at the Pt electrode surface. This nucleation concentration is ∼130 times higher than the equilibrium saturation concentration of O 2 and is independent of electrode size. Moreover, in acidic H 2 O 2 solutions (1 M HClO 4), in addition to producing an O 2 nanobubble through H 2 O 2 oxidation at positive potentials, individual H 2 nanobubbles can also be generated at negative potentials. Alternating generation of single O 2 and H 2 bubbles within the same experiment allows direct comparison of the critical concentrations for nucleation of each nanobubble without knowing the precise size/geometry of the electrode or the exact viscosity/ temperature of the solution.
Chemical Communications, 2016
Faraday Discussions, organised by the Royal Society of Chemistry (RSC), bring the concept of the ... more Faraday Discussions, organised by the Royal Society of Chemistry (RSC), bring the concept of the flipped classroom to scientific conferences, promoting lively debate and discussion with the opportunity to hear complementary and contrasting views on topics spanning physical chemistry. The Faraday Discussion on the topic of single entity electrochemistry, in the picturesque setting of the University of York (31 st August to 2 nd September, 2016), presented an opportunity for researchers pushing at the lower limits of electrochemistry to engage in such lively discourse. The topic single entity was chosen to be as inclusive as possible, and proved to be so, with researchers probing single nanoparticles, nanopores, nanorods, vesicles and even single molecules, with a wide range of electrochemical techniques, both experimental and theoretical.
Journal of the American Chemical Society, Jan 22, 2016
The dynamic collision behavior of the electro-oxidation of single Ag nanoparticles is observed at... more The dynamic collision behavior of the electro-oxidation of single Ag nanoparticles is observed at Au microelectrodes using stochastic single-nanoparticle collision amperometry. Results show that an Ag nanoparticle collision/oxidation event typically consists of a series of 1 to ∼10 discrete "sub-events" over an ∼20 ms interval. Results also show that the Ag nanoparticles typically undergo only partial oxidation prior to diffusing away from the Au electrode into the bulk solution. Both behaviors are characterized and shown to exist under a variety of experimental conditions. These previously unreported behaviors suggest that nanoparticle collision and electro-dissolution is a highly dynamic process driven by fast particle-electrode interactions and nanoparticle diffusion.
The journal of physical chemistry letters, Jan 6, 2016
An experimental system for controlling and interrogating the collisions of individual nanoparticl... more An experimental system for controlling and interrogating the collisions of individual nanoparticles at electrode/electrolyte interfaces is described. A nanopipet positioned over a 400 nm radius Pt ultramicroelectrode is used to deliver individual nanoparticles, via pressure-driven solution flow, to the underlying electrode, where the nanoparticles undergo collisions and are detected electrochemically. High-velocity collisions result in elastic collisions of negatively charged polystyrene nanospheres at the Pt/water interface, while low-velocity collisions result in nanoparticle adsorption ("sticky" collisions). The ability to position the nanopipet with respect to the underlying ultramicroelectrode also allows the time between particle release from the nanopipet and electrode collision to be investigated as a function of nanopipet-electrode separation, d. The time between release and collision of the nanoparticle is found to be proportional to d(3), in excellent agreement ...
Nano letters, Jan 12, 2016
The Young-Laplace equation is central to the thermodynamic description of liquids with highly cur... more The Young-Laplace equation is central to the thermodynamic description of liquids with highly curved interfaces, e.g., nanoscale droplets and their inverse, nanoscale bubbles. The equation relates the pressure difference across an interface to its surface tension and radius of curvature, but the validity in using the macroscopic surface tension for describing curved interfaces with radii smaller than tens of nanometers has been questioned. Here we present electrochemical measurement of Laplace pressures within single H2 bubbles between 7 and 200 nm radius (corresponding, respectively, to between 200 and 7 atm). Our results demonstrate a linear relationship between a bubble's Laplace pressure and its reciprocal radius, verifying the classical thermodynamic description of H2 nanobubbles as small as ∼10 nm.
Journal of The Electrochemical Society, 2016
The lifetimes of individual H 2 and N 2 nanobubbles, electrochemically generated at Pt nanoelectr... more The lifetimes of individual H 2 and N 2 nanobubbles, electrochemically generated at Pt nanoelectrodes (7-85 nm-radius), have been measured using a fast-scan electrochemical technique. To measure lifetime, a stable single H 2 or N 2 bubble is first generated by reducing protons or oxidizing hydrazine, respectively, at the Pt nanoelectrode. The electrode potential is then rapidly stepped (<100 μs) to a value where the bubble is unstable and begins to dissolve by gas molecule transfer across the gas/water interface and diffusion. The electrode potential is immediately scanned back to values where the bubble was initially stable. Depending on the rate of this second voltammetric scan, the initial bubble may or may not have time to dissolve, as is readily determined by the characteristic voltammetric signature corresponding to the nucleation of a new bubble. The transition between these regimes is used to determine the bubble's lifetime. The results indicate that dissolution of a H 2 or N 2 nanobubble is, in part, limited by the transfer of molecules across the gas/water interface. A theoretical expression describing mixed diffusion/kinetic control is presented and fit to the experimental data to obtain an interfacial gas transfer rate of ∼10 −9 mol N −1 s −1 .
ACS nano, Jan 8, 2015
Coulter counters measure the size of particles in solution by passing them through an orifice and... more Coulter counters measure the size of particles in solution by passing them through an orifice and measuring a resistive pulse, i.e., a drop in the ionic current flowing between two electrodes placed either side of the orifice. The magnitude of the pulse gives information on the size of the particle; however, accuracy is limited by variability in the path of the translocation, due to the Brownian motion of the particle. We present a simple-yet-powerful modified Coulter counter that uses programmable data acquisition hardware to switch the voltage after sensing the resistive pulse of a nanoparticle passing through the orifice of a nanopipette. Switching the voltage reverses the direction of the driving force on the particle, and when this detect-switch cycle is repeated, allows us to pass an individual nanoparticle through the orifice thousands of times. By measuring individual particles more than one hundred times per second we rapidly determine the distribution of the resistive puls...
Analytical chemistry, Jan 7, 2014
Iontophoresis uses a current to eject solution from the tip of a barrel formed from a pulled glas... more Iontophoresis uses a current to eject solution from the tip of a barrel formed from a pulled glass capillary and has been employed as a method of drug delivery for neurochemical investigations. Much attention has been devoted to resolving perhaps the greatest limitation of iontophoresis, the inability to determine the concentration of substances delivered by ejections. To further address this issue, we evaluate the properties of typical ejections such as barrel solution velocity and its relation to the ejection current using an amperometric and liquid chromatographic approach. These properties were used to predict the concentration distribution of ejected solute that was then confirmed by fluorescence microscopy. Additionally, incorporation of oppositely charged fluorophores into the barrel investigated the role of migration on the mass transport of an ejected species. Results indicate that location relative to the barrel tip is the primary influence on the distribution of ejected s...
ACS nano, Jan 28, 2014
We quantified the electrical polarization properties of single bacterial cells using electrostati... more We quantified the electrical polarization properties of single bacterial cells using electrostatic force microscopy. We found that the effective dielectric constant, ε(r,eff), for the four bacterial types investigated (Salmonella typhimurium, Escherchia coli, Lactobacilus sakei, and Listeria innocua) is around 3-5 under dry air conditions. Under ambient humidity, it increases to ε(r,eff) ∼ 6-7 for the Gram-negative bacterial types (S. typhimurium and E. coli) and to ε(r,eff) ∼ 15-20 for the Gram-positive ones (L. sakei and L. innocua). We show that the measured effective dielectric constants can be consistently interpreted in terms of the electric polarization properties of the biochemical components of the bacterial cell compartments and of their hydration state. These results demonstrate the potential of electrical studies of single bacterial cells.
The Journal of Physical Chemistry C, 2011
Intrinsic kinetics of gypsum and calcium sulfate anhydrite dissolution : surface selective studie... more Intrinsic kinetics of gypsum and calcium sulfate anhydrite dissolution : surface selective studies under hydrodynamic control and the effect of additives.