Observation of Single Pt Nanoparticle Collisions: Enhanced Electrocatalytic Activity on a Pd Ultramicroelectrode

Single Pt nanoparticle (NP) collisions on an electrode surface were detected by using an electrocatalytic amplification method with a Pd ultramicroelectrode (UME). Pd is not a preferred material for UMEs for the detection of single Pt NP collisions, because Pd shows similar electrocatalytic activity compared with Pt for hydrazine oxidation, thus resulting in a high background current level. However, a Pt NP colliding on the Pd UME shows greatly enhanced activity compared with a Pt NP on an inert UME, such as a Au UME, which is usually used for the detection of single Pt NP collisions. The use of an electroactive UME material instead of an inert one facilitated the study of single‐NP activity on the various solid supports, which is important in many NP applications.     

DNA analysis by application of Pt nanoparticle electrochemical amplification with single label response

This study demonstrates a highly sensitive sensing scheme for the detection of low concentrations of DNA, in principle down to the single biomolecule level. The previously developed technique of electrochemical current amplification for detection of single nanoparticle (NP) collisions at an ultramicroelectrode (UME) has been employed to determine DNA. The Pt NP/Au UME/hydrazine oxidation reaction was employed, and individual NP collision events were monitored. The Pt NP was modified with a 20-base oligonucleotide with a C6 spacer thiol (detection probe), and the Au UME was modified with a 16-base oligonucleotide with a C6 spacer thiol (capture probe). The presence of a target oligonucleotide (31 base) that hybridized with both capture and detection probes brought a Pt NP on the electrode surface, where the resulting electrochemical oxidation of hydrazine resulted in a current response.     

Various Current Responses of Single Silver Nanoparticle Collisions on a Gold Ultramicroelectrode Depending on the Collision Conditions

Collisions of silver nanoparticles (NPs) with a more electrocatalytic gold or platinum ultramicroelectrode (UME) surface have been observed by using an electrochemical method. Depending on the applied potential to the UME, the current response to the collision of Ag NPs on the UME resulted in various shape changes. A staircase decrease, a blip decrease, and a blip increase of the hydrazine oxidation current were obtained at an applied potential of 0.33, 0.80, and 1.3 V, respectively. Different collision behaviors of Ag NPs on the UME surface were suggested for each shape of current response. Ag NP attachment, which hindered the diffusion flux to the UME, caused a staircase decrease of the electrocatalytic current. Instantaneous blocking of the hydrazine oxidation by Ag NP collision and, following recovery of the current by means of oxidation of Ag NP, caused a blip decrease of the electrocatalytic current. The formation of a higher oxidation state of Ag on the Ag NP and its electrocatalytic hydrazine oxidation resulted in a blip increase of the electrocatalytic current. The analysis of the current response of a single NP collision experiment can be a useful tool to understand the various behaviors of NPs on the electrode surface.     

A Snapshot of the Properties of Single Nanoparticles at the Moment of a Collision

Direct electrochemical characterization of freely moving nanoparticles (NPs) at the individual particle level is challenging. A method is presented that can achieve this goal based on the collision between a NP and an ultramicroelectrode (UME). By applying a sinusoidal potential to the UME and monitoring the current response in the frequency domain, a sudden change in the phase angle indicates the arrival of a NP at the UME. The response induced by the collision can be isolated and used to explore the properties of the NP. This method, analogous to a high‐speed camera, can obtain a snapshot of the properties of the single NP at the moment of a collision. The proposed method was employed to investigate the properties of both the hard catalytic Pt NP and soft electroactive emulsion droplets, and many new insights were revealed thereafter. The method also has the potential to be applied in many other fields, where the interested signals appear as discrete events.     

Analysis of diffusion-controlled stochastic events of iridium oxide single nanoparticle collisions by scanning electrochemical microscopy

We investigated the electrochemical detection of single iridium oxide nanoparticle (IrO(x) NP) collisions at the NaBH(4)-treated Pt ultramicroelectrode (UME) in a scanning electrochemical microscope (SECM) over an insulating surface. The NP collision events were monitored by observing the electrocatalytic water oxidation reaction at potentials where it does not take place on the Pt UME. These collisions occurred stochastically, resulting in a transient response ("blip") for each collision. The frequency of the collisions is proportional to the flux of NPs to the UME tip, and thus equivalent to the SECM current. A plot of collision frequency versus distance followed the theoretical approach curve behavior for negative feedback for a high concentration of mediator, demonstrating that the collisions were diffusion-controlled and that single-particle measurements of mass transport are equivalent to ensemble ones. When the SECM was operated with a Pt substrate at the same potential as the tip, the behavior followed that expected of the shielding mode. These studies and additional ones result in a model where the IrO(x) NP collision on the Pt UME is adsorptive, with oxygen produced by the catalyzed water oxidation causing a current decay. This results in a blip current response, with the current decay diminished in the presence of the oxygen scavenger, sulfite ion. Random walk and theoretical bulk simulations agreed with the proposed mechanism of IrO(x) NP collision, adsorption, and subsequent deactivation.     

结合光学成像技术研究单颗粒碰撞电化学

近年来,单颗粒碰撞技术在纳米电化学领域受到广泛关注. 该技术通常控制超微电极处于某一电位,检测单个纳米颗粒随机碰撞到电极表面后产生的瞬时电流. 通过分析电流信号,可以研究单个纳米颗粒的性质. 尽管该技术可以检测单个纳米颗粒的电化学或电催化电流,但是传统的单颗粒碰撞技术缺乏空间分辨率,难以识别和表征特定的纳米颗粒. 因此,结合光学成像技术研究单颗粒碰撞电化学来补充电化学技术缺失的空间信息已成为一种趋势. 本文首先简要综述了单颗粒碰撞技术的三种检测原理,主要介绍了近年来单颗粒碰撞技术与荧光显微镜、表面等离激元共振显微镜、全息显微镜和电致化学发光相结合的研究进展,最后展望了单颗粒碰撞技术未来的发展趋势.     

Combined Blip and Staircase Response of Ascorbic Acid‐Stabilized Copper Single Nanoparticle Collision by Electrocatalytic Glucose Oxidation

The current response of the collision of ascorbic acid‐stabilized copper (Cu) single nanoparticles (NPs) on a gold (Au) ultramicroelectrode (UME) surface was observed by using an electrocatalytic amplification method. Here, the glucose oxidation electrocatalyzed by oxidized Cu NPs was used as the indicating reaction. In this system, the NP collision signals were obtained simultaneously by both direct particle electrolysis and electrocatalytic amplification. For example, when the applied potential was high enough for Cu NP oxidation, a blip response combined with a staircase response was observed as a current signal. The blip part in the single Cu NP collision signal indicates the self‐oxidation of a Cu NP, and the staircase part indicates the steady‐state electrocatalytic reaction by oxidized Cu NP.     

Electrochemical detection of reduced graphene oxide nanoparticles in aqueous solution

We have demonstrated electrochemical detection of reduced graphene oxide (rGO) nanoparticles on an ultramicroelectrode (UME) in aqueous solution using rGO collision events. The collision phenomena are detected by monitoring a current–time transient. To attract the rGO to the UME surface, a positive electric field was developed near the UME using a redox reaction. As model systems, ferrocenemethanol and ferrocyanide oxidation reactions were adopted. Amperometric current measurements showed a staircase current response after attachment of rGO on the UME surface. The magnitude of the staircase current is given by the stepwise increase in current, which can provide insight into the size distribution of the rGO colliding with the UME. In the presence of higher concentrations of rGO, multiple collision events happened sequentially on the UME. In this case, an increasing current trend, rather than a single staircase current, was observed. The overall current increment for a given time is a measure of the concentration of rGO in solution. By using this method, charged conductive materials in an aqueous solution can be sensitively detected and/or accumulated.     

Electro-oxidation of hydrazine at gold nanoparticle functionalised single walled carbon nanotube network ultramicroelectrodes

Networks of pristine single walled carbon nanotubes (SWNTs) grown by catalysed chemical vapour deposition (cCVD) on an insulating surface and arranged in an ultramicroelectrode (UME) format are insensitive to the electro-oxidation of hydrazine (HZ) in aqueous solution, indicating a negligible metallic nanoparticle content. Sensitisation of the network towards HZ oxidation is promoted by the deliberate and controlled electrodeposition of "naked" gold (Au) nanoparticles (NPs). By controlling the deposition conditions (potential, time) it is possible to control the size and spacing of the Au NPs on the underlying SWNT network. Two different cases are considered: Au NPs at a number density of 250 ± 13 NPs μm(-2) and height 24 nm ± 5 (effective surface coverage, θ = 92%) and (ii) Au NPs of number density ~ 22 ± 3 NPs μm(-2) and height 43 nm ± 8 nm (θ = 35%). For both morphologies the HZ oxidation half-wave potential (E(1/2)) is shifted significantly negative by ca. 200 mV, compared to a gold disc UME of the same geometric area, indicating significantly more facile electron transfer kinetics. E(1/2) for HZ oxidation for the higher density Au NP-SWNT structure is shifted slightly more negative (by ~25 mV) than E(1/2) for the lower density Au NP electrode. This is attributed to the lower flux of HZ at NPs in the higher number density arrangement (smaller kinetic demand). Importantly, using this approach, the calculated HZ oxidation current density sensitivities for the Au NP-SWNT electrodes reported here are higher than for many other metal NP functionalised carbon nanotube electrodes.     

Single Potential Scan Methods for Nanoporous Gold Formation on Ultramicroelectrode Surfaces

Nanoporous gold (NPG) has been extensively investigated because of their applications. Here, we report a straightforward method for the preparation of NPG on micrometer-scale electrodes. Well-defined NPG structures were formed on Au surfaces by a single potential scan within 100 seconds. This method is applied to Au surfaces with small dimensions regardless of the electrode geometry, whereas it is not applicable to conventional millimeter-scale electrodes. The effects of electrode sizes and scan rates on NPG formation were systematically examined, and the amperometric glucose detection with 20-μL sample volumes using an ultramicroelectrode (UME) with NPG surfaces was demonstrated.     

Mathematical Modelling of the Influence of Ultra‐micro Electrode Geometry on Approach Curves Registered by Scanning Electrochemical Microscopy

Scanning electrochemical microscopy (SECM) is an emerging electroanalytical sensing technique, used to investigate the electrochemical properties of the sample by ultra‐micro‐electrode(UME) scanning probe. UME signal usually is the current, which depends not only on the properties of the evaluated system but also on UME characteristics such as geometry. Variations of UME geometry can decrease accuracy of the measurement, and then correct analysis of the SECM data becomes almost impossible. In the present work, we studied the precision of measurements with three different the most frequent types of defected UME's ((i) recessed‐UME, (ii) outwarded‐UME, (iii) cone‐UME). Measurement results were compared with that obtained with not defected standard‐plane‐UME. Computational experiment was performed with SECM model using diffusion equations with non‐rectangular border conditions to calculate estimated currents for these three types of defected UMEs and to compare them with that for standard‐plane‐UME. In order to test the correctness of the model, computations for recessed‐UME model were compared with data of real‐recessed‐UME experiment.     

Microelectrodes modification through the reduction of aryl diazonium and their use in scanning electrochemical microscopy (SECM)

This work is devoted to the study of the electrochemical grafting of nitrophenyl groups onto platinum ultramicroelectrode (UME). The grafting was made using the electrochemical reduction of nitrophenyldiazonium. Our results demonstrate the possibility to reduce the diazonium onto Pt UME. As consequence the electrochemical reduction leads to the attachment of nitrophenyl groups onto the UME surface. Following that, the modified UME was characterized using electrochemical techniques. In addition, the electrochemical response of the modified UME in the presence of reversible redox couple, ferrocene, has been studied. The main remark is that the steady state current observed at the UME is not affected by the presence of the nitrophenyl layers. Finally, from this last point we demonstrate the possibility to achieve scanning electrochemical microscopy (SECM) using modified platinum UME.     

Fabrication of Prussian Blue modified ultramicroelectrode for GOD imaging using scanning electrochemical microscopy

A Prussian Blue (PB) film modified disk ultramicroelectrode (UME) was fabricated by electrochemical deposition technique on a Pt-disk UME. The electrocatalytical reductions of hydrogen peroxide derived from glucose oxidase (GOD) on this modified UME were investigated. The enzymatic biochemical reactivity was imaged by scanning electrochemical microscopy (SECM) utilizing the PB film modified UME. It is evident that sensitivity and spatial resolution for hydrogen peroxide measurement were improved obviously. SECM images obtained clearly revealed the concentration profile of the reaction products around the enzymes. The PB film modified microelectrode is in the nature of simple preparation, high catalytic activity on hydrogen peroxide and substrate selectivity for SECM etc.     

Direct Electrolysis and Detection of Single Nanosized Water Emulsion Droplets in Organic Solvent Using Stochastic Collisions

We studied the electrochemical detection of single nanosized water emulsion droplets in organic solution using the electrochemical collision technique on an ultramicroelectrode (UME). In this experiment, the detection system for water droplets does not require any kind of redox species in organic solvent. Only water molecules in the water droplets were considered. When water droplets collided with the UME surface, anodic current spikes were observed in the chronoamperometry, resulting from the electrolysis of water molecules in the water droplets. From the collision frequency and integrated current spike, concentrations and size distributions of water droplets in organic solvent can be determined.     

单个Nafion@Ru和Au-Nafion@Ru纳米粒子的电化学和电化学发光碰撞行为研究

通过静电作用在Nafion和Au-Nafion纳米粒子(NPs)上负载钌联吡啶(Ru(bpy)32+)分别制得Nafion@Ru和Au-Nafion@Ru NPs.分析并比较了Au-Nafion@Ru和Nafion@Ru NPs在金超微电极(Au UME)上随机碰撞产生电流响应峰的平均峰大小、峰电量和单峰持续时间,建立...     

Biodistribution of colloidal gold nanoparticles after intravenous administration: effect of particle size

Purpose of the present research work was to evaluate the biological distribution of differently size gold nanoparticles (NP) up on intravenous administration in mice. Another objective was to study effect of particle size on biological distribution of gold NP to enable their diverse applications in nanotechnology. Gold NP of different particle sizes, mainly 15, 50, 100 and 200nm, were synthesized by modifying citrate ion concentration. Synthesized gold nanoparticles were characterized by SEM and their size distribution was studied by particle size analyzer. Gold NP was suspended in sodium alginate solution (0.5%, w/v) and administered to mice (1g/kg, intravenously) [n=3]. After 24h of administration of gold NP, blood was collected under light ether anesthesia, mice were sacrificed by cervical dislocation and various tissues/organs were removed. The tissues were then washed with saline, homogenized and lysed with aqua regia. The determination of gold in samples was carried out quantitatively by inductively coupled plasma mass spectrometry (ICP-MS). SEM study revealed spherical morphology of gold NP with narrow particle size distribution. Biodistribution study revealed gold NPs of all sizes were mainly accumulated in organs like liver, lung and spleen. The accumulation of gold NP in various tissues was found to be depending on particle size. 15nm gold NP revealed higher amount of gold and number of particles in all the tissues including blood, liver, lung, spleen, kidney, brain, heart, stomach. Interestingly, 15 and 50nm gold NP were able to pass blood-brain barrier as evident from gold concentration in brain. Two-hundred nanometers gold NP showed very minute presence in organs including blood, brain, stomach and pancreas. The results revealed that tissue distribution of gold nanoparticles is size-dependent with the smallest 15nm nanoparticles showing the most widespread organ distribution.     

超微电极技术的发展现状

本文在阐述超微电极技术相关原理的基础上,总结了超微电极的特点、类型及研究方法,综述近十年来超微电极的制备与应用进展,并介绍了超微电极的发展现状。     

A Nanoporous PdCo Alloy as a Highly Active Electrocatalyst for the Oxygen‐Reduction Reaction and Formic Acid Electrooxidation

A nanoporous (NP) PdCo alloy with uniform structure size and controllable bimetallic ratio was fabricated simply by one‐step mild dealloying of a PdCoAl precursor alloy. The as‐made alloy consists of a nanoscaled bicontinuous network skeleton with interconnected hollow channels that extend in all three dimensions. With a narrow ligament size distribution around 5 nm, the NP PdCo alloy exhibits much higher electrocatalytic activity towards the oxygen‐reduction reaction (ORR) with enhanced specific and mass activities relative to NP Pd and commercial Pt/C catalysts. A long‐term stability test demonstrated that NP PdCo has comparable catalytic durability with less loss of ORR activity and electrochemical surface area than Pt/C. The NP PdCo alloy also shows dramatically enhanced catalytic activity towards formic acid electrooxidation relative to NP Pd and Pd/C catalysts. The as‐made NP PdCo holds great application potential as a promising cathode as well as an anode electrocatalyst in fuel cells with the advantages of superior catalytic performance and easy preparation.     

Impacts of Forced Convection Generated via High Precision Stirring on Scanning Electrochemical Microscopy Experiments in Feedback Mode

In this study, the effects of forced convection on scanning electrochemical microscopy (SECM) experiments in feedback mode using ferrocenemethanol as redox mediator are presented. Forced convection, which enhances the mass transfer inside the system, was generated via an electrical high precision stirrer integrated into the SECM setup. A thin‐film interdigitated array electrode serving as model substrate was investigated with probe scan curves in z‐direction and SECM imaging in constant height mode utilizing ultramicroelectrodes (UME) with diameters (d_probe) of 25 μm and 12.5 μm. It was found that forced convection increased the overall current during SECM imaging without distorting distinctive features of the imaged structure when working with a 25 μm UME at substrate‐to‐tip distances of 14 μm and 11 μm. Furthermore, the electrochemical contrast was improved under hydrodynamic conditions for a substrate‐to‐tip distance of 11 μm and scan rates of 5 μm s^?1, 10 μm s^?1, 20 μm s^?1 and 40 μm s^?1. When further decreasing the gap between the UME and the substrate to 9 μm almost no effects of the forced convection were observed. Consequently, for a 25 μm UME, forced convection led to higher currents and improved performance during SECM experiments in feedback mode at substrate‐to‐tip distances of 14 μm and 11 μm, whereas no effects were observed for a 12.5 μm UME at a distance of 8 μm.     

NaCl对烷基苯磺酸盐Gemini表面活性剂水溶液性质的影响

采用旋滴法测定了烷基苯磺酸盐Gemini表面活性剂Ia水溶液与正庚烷的界面张力,用超微电极循环伏安法测定了水溶液中Ia的胶束扩散系数.结果发现,NaCl的加入能够使界面张力降低2倍左右,使Ia的胶束扩散系数变大;Ia浓度增大,胶束扩散系数变小,而NaCl浓度增大,胶束扩散系数变大.这些变化说明NaCI的加入使得Ia胶束大小发生了变化.这是由于NaCl的加入降低了Ia离子头基之间的静电斥力,使得Ia分子排列更加紧密,更有效降低了油水的界面张力,使胶束形状变小;同时胶束表面电荷得到中和,胶束的静电斥力相应减小,胶束流动性相应增加,从而表现出较大的胶柬扩散系数.