Evaluation and origins of the difference between double-layer capacitance behaviour at Au-metal and oxidized Au surfaces

1.2 MV/cm pulsed electric fields promote transthyretin aggregate degradation

Numerous theoretical studies have been conducted on the effects of high-voltage electric fields on proteins, but few have produced experimental evidence. To acquire experimental data for the amyloid disassemble theory, we exposed transthyretin aggregates to 1,000 ns 1.26 MV/cm pulsed electric fields (PEFs) to promote transthyretin degradation. The process produced no changes in pH, and the resulting temperature increases were < 1 °C. We conclude that the physical effects of PEFs, rather than thermal or chemical effects, facilitate aggregate degradation.

Fabrication of magnetically assembled aptasensing device for label-free determination of aflatoxin B1 based on EIS

Aflatoxin B1 (AFB1), one of the most common mycotoxins in food matrixes, has been identified as the most toxic contaminant with mutagenic, teratogenic, immunosuppressive, and carcinogenic effects. In this work, a magnetically assembled aptasensing device has been designed for label-free determination of AFB1 by employing a disposable screen-printed carbon electrode (SPCE) covered with a designed polydimethylsiloxane (PDMS) film as the micro electrolytic cell. The magnetically controlled bio-probes were firstly prepared by immobilization of the thiolated aptamers on the Fe₃O₄@Au magnetic beads, which was rapidly assembled on the working electrode of SPCE within 10 s, by using a magnet placed at the opposite side. The PDMS film with a centered hole was covered on the SPCE surface to achieve a more practicable and flexible electrochemical measurement. In this effort, a label-free aptasensor for the sensitive and selective determination of AFB1 has been developed using electrochemical impedance spectroscopy upon the biorecognition between aptamers and the targets. The developed method had a wide linear range of 20 pg mL^-1-50 ng mL^-1 with a detection limit of 15 pg mL^-1 (S/N = 3) and succeeded in spiked samples of peanuts. The developed aptasensing device shows fantastic application prospect with simple design, easy operation, low cost, and high sensitivity and selectivity characteristics. This sensing strategy represents a promising path toward routine quality control of food safety and creates the opportunity to develop facile aptasensing device for other targets.

On the impact of capillarity for strength at the nanoscale

The interior of nanoscale crystals experiences stress that compensates for the capillary forces and that can be large, in the order of 1 GPa. Various studies have speculated on whether and how this surface-induced stress affects the stability and plasticity of small crystals. Yet, experiments have so far failed to discriminate between the surface contribution and other, bulk-related size effects. To clarify the issue, here we study the variation of the flow stress of a nanomaterial while distinctly different variations of the two capillary parameters, surface tension, and surface stress, are imposed under control of an applied electric potential. Our theory qualifies the suggested impact of surface stress as not forceful and instead predicts a significant contribution of the surface energy, as measured by the surface tension. The predictions for the combined potential-dependence and size-dependence of the flow stress are quantitatively supported by the experiment. Previous suggestions, favoring the surface stress as the relevant capillary parameter, are not consistent with our experiment.

Nanoscale crystals can experience lattice instability and a tension-compression asymmetry of their strength. Here, Mameka and colleagues scrutinize the proposition that these phenomena arise from surface-induced stress, finding that the surface tension contributes significantly, while the surface stress does not measurably impact the nanomechanical behaviour.

Electrocatalytic methanol oxidation with nanoporous gold: microstructure and selectivity

The properties of Nanoporous Gold (NPG) obtained by the selective dissolution of Ag from an Au-Ag alloy can be tuned by the details of its fabrication, and specifically the residual Ag content is correlated to the ligament size of the material. We link this correlation to methanol electro-oxidation. Specifically, two different NPG types (obtained by potentiostatic dealloying) are compared with one obtained by free corrosion. They show remarkable differences in activity. Quantitative product analysis reveals that NPG shows nearly selective oxidation of CH₃OH to HCOO^- when NPG is used as an active electrode in contrast to planar Au. This trend can further be enhanced when applying finer nanoporous structures that are linked to a higher Ag content. X-ray photoelectron spectroscopy (XPS) reveals changes in the nature of residual Ag from which we conclude that Ag is not a passive component in the methanol oxidation process.

Electrochemical tuning of the optical properties of nanoporous gold

Using optical in-situ measurements in an electrochemical environment, we study the electrochemical tuning of the transmission spectrum of films from the nanoporous gold (NPG) based optical metamaterial, including the effect of the ligament size. The long wavelength part of the transmission spectrum around 800 nm can be reversibly tuned via the applied electrode potential. The NPG behaves as diluted metal with its transition from dielectric to metallic response shifted to longer wavelengths. We find that the applied potential alters the charge carrier density to a comparable extent as in experiments on gold nanoparticles. However, compared to nanoparticles, a NPG optical metamaterial, due to its connected structure, shows a much stronger and more broadband change in optical transmission for the same change in charge carrier density. We were able to tune the transmission through an only 200 nm thin sample by 30%. In combination with an electrolyte the tunable NPG based optical metamaterial, which employs a very large surface-to-volume ratio is expected to play an important role in sensor applications, for photoelectrochemical water splitting into hydrogen and oxygen and for solar water purification.

Fabrication and Characterization of Ultrathin-ring Electrodes for Pseudo-steady-state Amperometric Detection

The fabrication of ultrathin-ring electrodes with a diameter of 2 mm and a thickness of 100 nm is established. The ultrathin-ring electrodes provide a large density of pseudo-steady-state currents, and realize pseudo-steady-state amperometry under quiescent conditions without a Faraday cage. Under the limiting current conditions, the current response at the ultrathin-ring electrode can be well explained by the theory of the microband electrode response. Cyclic voltammograms at the ultrathin-ring electrode show sigmoidal characteristics with some hysteresis. Numerical simulation reveals that the hysteresis can be ascribed to the time-dependence of pseudo-steady-state current. The performance of amperometry with the ultrathin-ring electrode has been verified in its application to redox enzyme kinetic measurements.

Optimized design of a nanostructured SPCE-based multipurpose biosensing platform formed by ferrocene-tethered electrochemically-deposited cauliflower-shaped gold nanoparticles

The demand for on-site nanodevices is constantly increasing. The technology development for the design of such devices is highly regarded. In this work, we report the design of a disposable platform that is structured with cauliflower-shaped gold nanoparticles (cfAuNPs) and we show its applications in immunosensing and enzyme-based detection. The electrochemical reduction of Au(III) allows for the electrodeposition of highly dispersed cauliflower-shaped gold nanoparticles on the surface of screen-printed carbon electrodes (SPCEs). The nanostructures were functionalized using ferrocenylmethyl lipoic acid ester which allowed for the tethering of the ferrocene group to gold, which serves as an electrochemical transducer/mediator. The bioconjugation of the surface with anti-human IgG antibody (α-hIgG) or horseradish peroxidase (HRP) enzyme yields biosensors, which have been applied for the selective electrochemical detection of human IgG (hIgG) or H2O2 as model analytes, respectively. Parameters such as the number of sweeps, amount of charge generated from the oxidation of the electrodeposited gold, time of incubation and concentration of the ferrocene derivatives have been studied using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). Selectivity and specificity tests have been also performed in the presence of potentially interfering substances to either hIgG or H2O2. Results showed that the devised immunosensor is endowed with good selectivity and specificity in the presence of several folds of competitive analytes. The enzyme-based platform showed a good catalytic activity towards H2O2 oxidation which predestined it to potential applications pertaining to enzymatic kinetics studies. The levels of hIgG in human serum and H2O2 in honey were successfully determined and served as assessment tools of the applicability of the platforms for real samples analysis.

Electrochemical properties and applications of nanocrystalline, microcrystalline, and epitaxial cubic silicon carbide films

Microstructures of the materials (e.g., crystallinitiy, defects, and composition, etc.) determine their properties, which eventually lead to their diverse applications. In this contribution, the properties, especially the electrochemical properties, of cubic silicon carbide (3C-SiC) films have been engineered by controlling their microstructures. By manipulating the deposition conditions, nanocrystalline, microcrystalline and epitaxial (001) 3C-SiC films are obtained with varied properties. The epitaxial 3C-SiC film presents the lowest double-layer capacitance and the highest reversibility of redox probes, because of its perfect (001) orientation and high phase purity. The highest double-layer capacitance and the lowest reversibility of redox probes have been realized on the nanocrystalline 3C-SiC film. Those are ascribed to its high amount of grain boundaries, amorphous phases and large diversity in its crystal size. Based on their diverse properties, the electrochemical performances of 3C-SiC films are evaluated in two kinds of potential applications, namely an electrochemical capacitor using a nanocrystalline film and an electrochemical dopamine sensor using the epitaxial 3C-SiC film. The nanocrystalline 3C-SiC film shows not only a high double layer capacitance (43-70 μF/cm(2)) but also a long-term stability of its capacitance. The epitaxial 3C-SiC film shows a low detection limit toward dopamine, which is one to 2 orders of magnitude lower than its normal concentration in tissue. Therefore, 3C-SiC film is a novel but designable material for different emerging electrochemical applications such as energy storage, biomedical/chemical sensors, environmental pollutant detectors, and so on.

Gold nanoparticles supported in zirconia-ceria mesoporous thin films: a highly active reusable heterogeneous nanocatalyst

Gold nanoparticles (NP) trapped in the mesopores of mixed zirconia-ceria thin films are prepared in a straightforward and reproducible way. The films exhibit enhanced stability and excellent catalytic activity in nitro-group reduction by borohydride and electrocatalytic activity in CO and ethanol oxidation and oxygen reduction.

Hierarchical nested-network nanostructure by dealloying

Applications of porous microstructures in functional materials often impose conflicting requirements on the pore size, which may be met by hierarchical structures that combine porosity on distinctly different length scales. Here we report an electrochemical dealloying strategy that yields bulk samples of porous gold with a hierarchical microstructure. A nanoscale network of solid ligaments forms the lower hierarchy level, which is nested within the geometrically similar, but much larger, network of the upper hierarchy level. Starting from a dilute solid solution of Au in Ag, controlled electrochemical corrosion yields nanoporous Ag-Au alloy as an intermediate product. Coarsening of the porous alloy creates the large ligaments of the upper hierarchy level. Those are then again dealloyed, which creates the fine ligaments of the lower hierarchy level. We show that the material exhibits enhanced charge transport kinetics while maintaining a large specific surface area.

Probing the electrochemical double layer of an ionic liquid using voltammetry and impedance spectroscopy: a comparative study of carbon nanotube and glassy carbon electrodes in [EMIM](+)[EtSO(4)](-)

Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) are compared as techniques for analyzing double layer capacitances of ionic liquids (ILs) at the surfaces of two carbon-based electrodes. These systems are relevant for energy storage supercapacitors and often are associated with unconventional electrochemical properties. Certain theoretical and experimental aspects of CV and EIS necessary for quantitative evaluation of the capacitance characteristics of such systems are explored. The experiments use 1-ethyl-3-methyl imidazolium ethylsulfate as a model IL electrolyte in combination with a porous electrode of carbon nanotubes (CNTs). The results are compared with those obtained with a nonporous glassy carbon (GC) electrode. The time is constant, and hence the power delivery characteristics of the experimental cell are affected by the electrolyte resistance and residual faradaic reactions of the IL, as well as by the spatially inhomogeneous electrode surfaces. It is shown that adequate characterization of these IL-electrode systems can be achieved by combining CV with EIS. A phenomenological framework for utilizing this combination is discussed.