Nano-impact Experiments Research Articles

Precise Electrochemical Sizing of Individual Electro-Inactive Particles.

Nanoimpact electrochemistry enables the time-resolved in situ characterization (e.g., size, catalytic activity) of single nanomaterial units, providing a means of elucidating heterogeneities that would be masked in ensemble studies. To implement this technique with redox inactive particles, a solution-phase redox reaction is used to produce a steady-state background current on a disk ultramicroelectrode. When a particle adsorbs onto the electrode, it produces a stepwise reduction in the exposed electrode area, which produces, in turn, a stepwise decrease in the current commensurate with the size of the adsorbing species. Historically, however, nanoimpact electrochemistry has suffered from "edge effects," in which the radial diffusion layer formed at the circumference of the ultramicroelectrodes renders the step size dependent not only on the size of the particle but also on where it lands on the electrode. The introduction of electrocatalytic current generation, however, mitigates the heterogeneity caused by edge effects, thus improving the measurement precision. In this approach, termed "electrocatalytic interruption," a substrate that regenerates the redox probe at the diffusion layer is introduced. This shifts the rate-limiting step of the current generation from diffusion to the homogeneous reaction rate constant, thus reducing flux heterogeneity and increasing the precision of particle sizing by an order of magnitude. The protocol described here explains the set-up and data collection employed in nanoimpact experiments implementing this effect for improved precision in the sizing of redox in-active materials.

Light-Controlled Nanoparticle Collision Experiments.

Electrochemical monitoring of catalytically amplified collisions of individual metal nanoparticles (NP) with ultramicroelectrodes (UME) has been extensively used to study electrocatalysis, mass-transport, and charge-transfer processes at the single NP level. More recently, photoelectrochemical collision experiments were carried out with semiconductive NPs. Here, we introduce two new types of light-controlled nanoimpact experiments. The first experiment involves localized photodeposition of catalyst (Pt) on TiO2 NPs with a glass-sheathed carbon fiber simultaneously serving as the light guide and collector UME. The collisions of in situ prepared Pt@TiO2 NPs with the carbon surface produced blips of water oxidation current, while the activity of pristine TiO2 NPs was too low to yield measurable signal. In another experiment, collisions of catalytic (Ir oxide) NPs with the semiconductor (Nb doped n-type TiO2 rutile single crystal) electrode are monitored by measuring the photocurrent of water oxidation.

Nano Impact Electrochemistry: Effects of Electronic Filtering on Peak Height, Duration and Area

AbstractNano impact electrochemistry is used to measure a transient signal while a nanoparticle (NP) hits an electrode due to its motion in a solution. A variety of information can be obtained from this current pulse, yet its accurate measurement is challenging due to its short duration (μs to s) and small amplitude (≤10 nA). A typically used low bandwidth low‐pass filter can improve the signal‐to‐noise ratio, but it may cost severely in the accuracy of the data. Here, we demonstrate the effects of electronic filters by using generated current impulses with duration from 125 μs to 8 ms. Initially, a system dedicated to measure short and low current impulses was employed. There, an 8th order Bessel filter was used and the effect of varying the cut‐off frequency between 50 Hz and 20 kHz on the impulse response is studied. Even though the charge is generally conserved by the filter, amplitude and duration of the pulse vary greatly in dependence of the cut‐off frequency. In comparison, the response of widely used potentiostats was tested and significant deviations of the measured signal from the input were detected. Supported by destructive nano impact experiments with Ag NPs in KCl(aq), we show how the filtering affects the experimentally determined size of Ag NPs and Cl− diffusion coefficient, using impact charges and duration, respectively. As a result, we suggest a general guideline to researchers for accurate electrochemical nano impact measurements, in particular with respect to current peak duration analysis.

Nanoparticle impacts in innovative electrochemistry

Recent developments in the use of nanoimpacts as an investigative electrochemical technique are discussed. Highlights include literature on the imaging of nanoimpacts themselves and developments in the application of nanoimpacts such as, surface analysis of single particles including surface coverage and sizing. Also included are factors to consider which may affect the outcome of nanoimpact experiments.

Immobilised Electrocatalysts: Nafion Particles Doped with Ruthenium(II) Tris(2,2'-bipyridyl).

Nafion particles doped with ruthenium(II) tris(2,2'-bipyridyl) are synthesized by using a re-precipitation method. Characterization including SEM sizing and quantification of Ru(bpy)32+ in the Nafion particles using UV/Vis spectroscopy was conducted. The synthesized Ru-Nafion particles were investigated electrochemically at both ensemble and single particle levels. Voltammetry of the drop-cast Ru-Nafion particles evidences the successful incorporation of Ru(bpy)32+ into the Nafion particle but only a small fraction of the incorporated Ru(bpy)32+ was detected due at least in part to the formation of the likely agglomerated and irregular "mat" associated with the dropcast technique. In contrast, nano-impact experiments provided a quantitative determination of the amount of Ru(bpy)32+ in single Ru-Nafion particles. Finally, oxidation of solution-phase oxalate mediated by Ru(bpy)32+ within individual Nafion particles was observed, showing the electrocatalytic properties of the Ru-Nafion particles.

Electrode-particle impacts: a users guide.

We present a comprehensive guide to nano-impact experiments, in which we introduce newcomers to this rapidly-developing field of research. Central questions are answered regarding required experimental set-ups, categories of materials that can be detected, and the theoretical frameworks enabling the analysis of experimental data. Commonly-encountered issues are considered and presented alongside methods for their solutions.

Electrochemistry of Single Nanodomains Revealed by Three-Dimensional Holographic Microscopy.

Interest in nanoparticles has vigorously increased over the last 20 years as more and more studies show how their use can potentially revolutionize science and technology. Their applications span many different academically and industrially relevant fields such as catalysis, materials science, health, etc. Until the past decade, however, nanoparticle studies mostly relied on ensemble studies, thus leaving aside their chemical heterogeneity at the single particle level. Over the past few years, powerful new tools appeared to probe nanoparticles individually and in situ. This Account describes how we drew inspiration from the emerging fields of nanoelectrochemistry and plasmonics-based high resolution holographic microscopy to develop a coupled approach capable of analyzing in operando (electro)chemical reaction over one single nanoparticle. A brief overview of selected optical strategies to image NPs in situ with emphasis on scattering based methods is presented. In an electrochemical context, it is necessary to track particle behavior both in solution and near a polarized electrode, which is why 3D optical observation is particularly appealing. These approaches are discussed together with strategies to track NPs beyond the diffraction limit, allowing a much finer description of their trajectories. Then, the holographic setup is used to study electrochemically triggered Ag NP oxidation reaction in the presence of different electrolytes. Holography is shown to be a powerful technique to track and analyze the trajectory of individual NPs in situ, which further sheds light on in operando behaviors such as electrogenerated NP transport, aggregation, or adsorption. We then show that spectroscopy and scattering-based optical methods are reliable and sensitive to the point of being used to investigate and quantify NP (electro)chemical reactions in model cases. However, since real chemical reactions usually take place in an inherently complex environment, approaches based exclusively on optical imaging only reach their limitations. The strategy is then taken one step further by merging together electrochemical nanoimpact experiments with 3D optical monitoring. Previous strategies are validated by showing that in simple cases, these two independent ways of probing NP size and reactivity yield the same results. For more complicated reactions (e.g., multistep reactions), one must go beyond either technique by showing that the two approaches are perfectly complementary and that the two signals contain information of different natures, thus providing a much better characterization of the reaction. This point is illustrated by studying Ag NP oxidation (single or agglomerates) in the presence of a precipitating agent, where the actual oxidation is uncoupled from the dissolution of the particle, thus proving the point of our symbiotic approach.

Understanding Nano-Impact Current Spikes: Electrochemical Doping of Impacting Nanoparticles

We demonstrate the generic value of a rigorous analysis of spike shapes observed in nanoimpact experiments. To this end, we investigate the electrochemical doping of insoluble nanoparticles impacting on a biased electrode surface and develop an analytical model of the doping process is developed that accounts for the diffusion of ions inside nanoparticles as well as a numerical model for the response characteristics of the analogue measurement circuitry. By this means, spike shapes that are experimentally observed in the electrode current are predicted and directly compared with experimental data, while appropriate fitting procedures allow detailed physical insights into the ionic mass transport within the particle. Using the oxidative doping of ferrocene nanoparticles with tetrafluoroborate anions as a paradigm case, we demonstrate that size distributions of particle populations showing excellent agreement with scanning electron microscopy measurements can be extracted from experimental data and ionic di...

Hydrogen Oxidation Reaction on Platinum Nanoparticles: Understanding the Kinetics of Electrocatalytic Reactions via “Nano-Impacts”

The kinetics of the hydrogen oxidation reaction (HOR) on platinum nanoparticles (PtNPs) is explored. The steady-state current for HOR on individual PtNPs is measured via the nanoimpact approach. Adopting the Tafel-Volmer mechanism, the adsorption rate constant for HOR on PtNPs is calculated as 0.020 ± 0.008 m s–1. The HOR is also studied on electrodes where PtNPs are immobilized via drop-casting on the supporting substrate prior to experimentation and the kinetic parameters contrasted with those from the nanoimpact experiments. The distinction in the kinetic parameters between the nanoimpact and the drop-casting approaches reveals that the measurement on an individual nanoparticle is required in order to correctly understand the electrochemical reaction catalyzed by nanoparticles, because the aggregation or loss of the nanoparticles drop-casted on the electrode leads to unavoidable errors in the measurement.

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.

Understanding nano-impacts: Reversible agglomeration and near-wall hindered diffusion

[Display omitted] The effects of reversible agglomeration and near-wall hindered diffusion are investigated in the context of nano-impact experiments. A significant reduction in the mass transport to the electrode is observed due to the formation of slow-moving agglomerates. It is further demonstrated that in the presence of a rapid agglomeration process, larger agglomerated species are virtually unobservable, since the monomer is the dominant flux carrier. This observation is consistent with previously reported experimental data.

Diffusional impacts of nanoparticles on microdisc and microwire electrodes: The limit of detection and first passage statistics

We derive approximate expressions for the average number of diffusive impacts/hits of nanoparticles on microdisc and microwire electrodes for the case where the impact leads to the loss of the nanoparticles from solution either via irreversible adsorption or complete electro-dissolution. The theory can also be applied to sub-micrometre size electrodes (nano-electrodes). The resulting equations can be utilised to analyse the number of impacts and its variance in the ‘nano-impact’ experiment. We also provide analytical expressions for the first passage time of an impact for dilute nanoparticle solutions in the continuum limit of Fickian diffusion. The expressions for the first passage times are used to estimate the lower limit of detection in ultra-dilute nanoparticle solutions for typical nano-impact experiments, and show the advantage of using microwire electrodes in ultra-dilute solutions or solutions containing larger nano-particles.

Are Nanoparticles Spherical or Quasi-Spherical?

The geometry of quasi-spherical nanoparticles is investigated. The combination of SEM imaging and electrochemical nano-impact experiments is demonstrated to allow sizing and characterization of the geometry of single silver nanoparticles.

Evaluation of Incoherent Interface Strength of Solid-State-Bonded Ti64/Stainless Steel Under Dynamic Impact Loading

Ti/steel interfaces are produced using field-assisted sintering technology, a technique known to bring about full consolidation of materials using much lower sintering temperatures and durations. The interface thickness is verified using the energy-dispersive x-ray analysis exhibiting the extent of diffusion in interface regions. The interface mechanical strength is characterized using dynamic indentation experiments at strain rates approaching 400 s−1. The experiments were conducted on the interfaces within the spatial error tolerance of less than 3 µm. The measurements of dynamic hardness values, strain rates, and plastic-residual depths were correlated to show the relation of interface mechanical strength with the bulk-phase mechanical strength properties of Ti and steel. The Johnson–Cook model is fitted to the obtained interface normal stress–normal strain data based on the nanoimpact experiments. The coefficient of restitution in the mechanical loading and its dependence on the interface dynamic hardness and interface impact velocity validate the experimental results. The results show that interfacial properties are affected by the rate of loading and are largely dependent upon the interface structural inhomogeneity.

Investigation of Single‐Drug‐Encapsulating Liposomes using the Nano‐Impact Method

Encapsulating liposomes are widely used for controlled drug delivery. We report the use of nano-impact experiments for the electrochemical attomolar quantification of the liposome load, uniquely at the single liposome level, using vitamin C encapsulated liposomes as a model. The size of the liposomes and their picomolar concentration are also determined in biological buffer in real time.

The use of cylindrical micro-wire electrodes for nano-impact experiments; facilitating the sub-picomolar detection of single nanoparticles

Electrochemical impact experiments can be used to detect and size single nanoparticles in suspension and at low concentrations. This is generally performed using a micro-disc working electrode; however, for the first time we report the use of cylindrical micro-wire electrodes for nanoparticle impact experiments. These electrodes provide much enhanced detection limits; specifically decreasing the concentration of nanoparticles measurable by over two orders of magnitude. In addition, the use of micro-wire electrodes reduces the shielding effect due to absorption of particles to the insulating sheath that surrounds a micro-disc electrode. Micro-wire electrodes are fabricated and their electrochemical response analysed via cyclic voltammetry experiments using molecular species. This provides a theoretical framework which is used to calculate the reduced concentration of nanoparticles required for an impact experiment at a micro-cylinder electrode in comparison to the micro-disc. Experimentally, it is demonstrated that impact experiments on the micro-cylinder electrodes can indeed be used for accurate characterisation of ultra-low concentrations (≈0.1pM) of silver nanoparticles.