Tracking of photochemical Ostwald ripening of nanoparticles through voltammetric atom counting.

We report the tracking of atom count in individual nanoparticles during photochemical Ostwald ripening. The nano-impact technique, in conjunction with UV-Vis and TEM analysis, is used to follow the photochemical formation of silver nano-prisms from spherical seed particles. A mechanism of photochemical Ostwald ripening is deduced and key growth stages are identified.

Nanorod Aspect Ratios Determined by the Nano-Impact Technique.

The in situ electrochemical sizing of individual gold nanorods is reported. Through the combination of electrochemical dissolution and the use of a surface-bound redox tag, the volume and surface area of the nanorods are measured, and provide the aspect ratio and the size of the nanorods. Excellent independent agreement is found with electron microscopy analysis of the nanorods, establishing the application of nano-impact experiments for the sizing of anisotropic nanomaterials.

The Electrochemical Characterization of Single Core-Shell Nanoparticles.

We report the direct solution-phase characterization of individual gold-core silver-shell nanoparticles through an electrochemical means, with selectivity achieved between the core and shell components based on their different redox activities. The electrochemically determined core-shell sizes are in excellent agreement with electron microscopy-based results, successfully demonstrating the electrochemical characterization of individual core-shell nanoparticles.

Rapid electrochemical detection of single influenza viruses tagged with silver nanoparticles.

Using a state of the art nano-electrochemical technique, we show that a single virus 'tagged' with silver nanoparticles can be rapidly detected in real time at the single virus level. A solution containing a low concentration of influenza virus is exposed to silver nanoparticles which are adsorbed onto the virus surface, as revealed by UV-Vis spectroscopy and transmission electron microscopy. With sufficient potential applied to a carbon electrode introduced into the solution, current spikes are observed which correspond to the oxidation of the nanoparticles decorating the virus. The frequency of the current spikes and their magnitude are linearly proportional to the virus concentration and to the surface coverage of the nanoparticles, respectively. Differences observed from single bacterium detection are discussed and a comparison with existing detection methods is made, with emphasis on the favourability of the proposed technique towards the realization of point of care test devices.

Capping agent promoted oxidation of gold nanoparticles: cetyl trimethylammonium bromide.

Capping agents, key for nanoparticle stability, may hugely influence chemical behaviour. We show that differently capped gold nanoparticles, with either citrate or cetyl trimethylammonium bromide (CTAB) capping agents, show qualitatively different electron transfer properties. Specifically through cyclic voltammetry and nanoimpact studies the CTAB promoted dissolution of gold nanoparticles is shown, highlighting the active role which capping agents can play in charge transfer.

The fate of nano-silver in aqueous media.

Silver nanoparticles offer highly attractive properties for many applications, however concern has been raised over the possible toxicity of this material in environmental systems. While it is thought that the release of Ag(+) can play a crucial role in this toxicity, the mechanism by which the oxidative dissolution of nano-silver occurs is not yet understood. Here we address this through the electrochemical analysis of gold-core silver-shell nanoparticles in various solutions. This novel method allows the direct quantification of silver dissolution by normalisation to the gold core signal. This is shown to be highly effective at discriminating between silver dissolution and the loss of nanoparticles from the electrode surface. We evidence through this rigorous approach that the reduction of O2 drives the dissolution of nano-silver, while in the presence of Cl(-) this dissolution is greatly inhibited. This work is extended to the single nanoparticle level using nano-impact experiments.

Building with bubbles: the formation of high surface area honeycomb-like films via hydrogen bubble templated electrodeposition.

While the evolution of hydrogen gas is often a troublesome process accompanying electrodeposition, this feature can be exploited to template the growth of highly porous surfaces. This process, known as the dynamic hydrogen bubble template (DHBT) method, can be utilised to create a wide range of macroporous films with nanostructured pore walls. This feature article presents an overview of the status of the DHBT technique, highlighting preparation techniques and emerging applications.

The strong catalytic effect of Pb(II) on the oxygen reduction reaction on 5 nm gold nanoparticles.

Citrate-capped gold nanoparticles (AuNPs) of 5 nm in diameter are synthesized via wet chemistry and deposited on a glassy carbon electrode through electrophoresis. The kinetics of the oxygen reduction reaction (ORR) on the modified electrode is determined quantitatively in oxygen-saturated 0.5 M sulphuric acid solution by modelling the cathode as an array of interactive nanoelectrodes. Quantitative analysis of the cyclic voltammetry shows that no apparent ORR electrocatalysis takes place, the kinetics on AuNPs being effectively the same as on bulk gold. Contrasting with the above, a strong ORR catalysis is found when Pb(2+) is added to the oxygen saturated solution or when the modified electrode is cycled in lead alkaline solution such that lead dioxide is repeatedly electrodeposited and stripped off on the nanoparticles. In both cases, the underpotential deposition of lead on the gold nanoparticles is found to be related to the catalysis.

Probing the effect of charge transfer enhancement in off resonance mode SERS via conjugation of the probe dye between silver nanoparticles and metal substrates.

The charge transfer-mediated surface enhanced Raman scattering (SERS) of crystal violet (CV) molecules that were chemically conjugated between partially polarized silver nanoparticles and optically smooth gold and silver substrates has been studied under off-resonant conditions. Tyrosine molecules were used as a reducing agent to convert silver ions into silver nanoparticles where oxidised tyrosine caps the silver nanoparticle surface with its semiquinone group. This binding through the quinone group facilitates charge transfer and results in partially oxidised silver. This establishes a chemical link between the silver nanoparticles and the CV molecules, where the positively charged central carbon of CV molecules can bind to the terminal carboxylate anion of the oxidised tyrosine molecules. After drop casting Ag nanoparticles bound with CV molecules it was found that the free terminal amine groups tend to bind with the underlying substrates. Significantly, only those CV molecules that were chemically conjugated between the partially polarised silver nanoparticles and the underlying gold or silver substrates were found to show SERS under off-resonant conditions. The importance of partial charge transfer at the nanoparticle/capping agent interface and the resultant conjugation of CV molecules to off resonant SERS effects was confirmed by using gold nanoparticles prepared in a similar manner. In this case the capping agent binds to the nanoparticle through the amine group which does not facilitate charge transfer from the gold nanoparticle and under these conditions SERS enhancement in the sandwich configuration was not observed.

Decoration of active sites to create bimetallic surfaces and its implication for electrochemical processes.

The creation of electrocatalysts based on noble metals has received a significant amount of research interest due to their extensive use as fuel cell catalysts and electrochemical sensors. There have been many attempts to improve the activity of these metals through creating nanostructures, as well as post-synthesis treatments based on chemical, electrochemical, sonochemical and thermal approaches. In many instances these methods result in a material with active surface states, which can be considered to be adatoms or clusters of atoms on the surface that have a low lattice co-ordination number making them more prone to electrochemical oxidation at a wide range of potentials that are significantly less positive than those of their bulk metal counterparts. This phenomenon has been termed pre-monolayer oxidation and has been reported to occur on a range of metallic surfaces. In this work we present findings on the presence of active sites on Pd that has been: evaporated as a thin film; electrodeposited as nanostructures; as well as commercially available Pd nanoparticles supported on carbon. Significantly, advantage is taken of the low oxidation potential of these active sites whereby bimetallic surfaces are created by the spontaneous deposition of Ag from AgNO3 to generate Pd/Ag surfaces. Interestingly this approach does not increase the surface area of the original metal but has significant implications for its further use as an electrode material. It results in the inhibition or promotion of electrocatalytic activity which is highly dependent on the reaction of interest. As a general approach the decoration of active catalytic materials with less active metals for a particular reaction also opens up the possibility of investigating the role of the initially present active sites on the surface and identifying the degree to which they are responsible for electrocatalytic activity.

Electrochemical fabrication of metallic nanostructured electrodes for electroanalytical applications.

The use of electrodeposited metal-based nanostructures for electroanalytical applications has recently received widespread attention. There are several approaches to creating nanostructured materials through electrochemical routes that include facile electrodeposition at either untreated or modified electrodes, or through the use of physical or chemical templating methods. This allows the shape, size and composition of the nanomaterial to be readily tuned for the application of interest. The use of such materials is particularly suited to electroanalytical applications. In this mini-review an overview of recently developed nanostructured materials developed through electrochemical routes is presented as well as their electroanalytical applications in areas of biological and environmental importance.

The active site behaviour of electrochemically synthesised gold nanomaterials.

Even though gold is the noblest of metals, a weak chemisorber and is regarded as being quite inert, it demonstrates significant electrocatalytic activity in its nanostructured form. It is demonstrated here that nanostructured and even evaporated thin films of gold are covered with active sites which are responsible for such activity. The identification of these sites is demonstrated with conventional electrochemical techniques such as cyclic voltammetry as well as a large amplitude Fourier transformed alternating current (FT-ac) method under acidic and alkaline conditions. The latter technique is beneficial in determining if an electrode process is either Faradaic or capacitive in nature. The observed behaviour is analogous to that observed for activated gold electrodes whose surfaces have been severely disrupted by cathodic polarisation in the hydrogen evolution region. It is shown that significant electrochemical oxidation responses occur at discrete potential values well below that for the formation of the compact monolayer oxide of bulk gold and are attributed to the facile oxidation of surface active sites. Several electrocatalytic reactions are explored in which the onset potential is determined by the presence of such sites on the surface. Significantly, the facile oxidation of active sites is used to drive the electroless deposition of metals such as platinum, palladium and silver from their aqueous salts on the surface of gold nanostructures. The resultant surface decoration of gold with secondary metal nanoparticles not only indicates regions on the surface which are rich in active sites but also provides a method to form interesting bimetallic surfaces.

Honeycomb nanogold networks with highly active sites.

The formation of macroporous honeycomb gold using an electrochemically generated hydrogen bubble template is described. The synthesis procedure induces the formation of highly active surfaces with enhanced electrocatalytic and surface enhanced Raman scattering properties.