Kamonwad Ngamchuea

Advancing from Rules of Thumb: Quantifying the Effects of Small Density Changes in Mass Transport to Electrodes. Understanding Natural Convection

Understanding mass transport is prerequisite to all quantitative analysis of electrochemical experiments. While the contribution of diffusion is well understood, the influence of density gradient-driven natural convection on the mass transport in electrochemical systems is not. To date, it has been assumed to be relevant only for high concentrations of redox-active species and at long experimental time scales. If unjustified, this assumption risks misinterpretation of analytical data obtained from scanning electrochemical microscopy (SECM) and generator-collector experiments, as well as analytical sensors utilizing macroelectrodes/microelectrode arrays. It also affects the results expected from electrodeposition. On the basis of numerical simulation, herein it is demonstrated that even at less than 10 mM concentrations and short experimental times of tens of seconds, density gradient-driven natural convection significantly affects mass transport. This is evident from in-depth numerical simulation for the oxidation of hexacyanoferrate (II) at various electrode sizes and electrode orientations. In each case, the induced convection and its influence on the diffusion layer established near the electrode are illustrated by maps of the velocity fields and concentration distributions evolving with time. The effects of natural convection on mass transport and chronoamperometric currents are thus quantified and discussed for the different cases studied.

Magnetic control： Switchable ultrahigh magnetic gradients at Fe3O4 nanoparticles to enhance solution-phase mass transport

Enhancing mass transport to electrodes is desired in almost all types of electrochemical sensing, electrocatalysis, and energy storage or conversion. Here, a method of doing so by means of the magnetic gradient force generated at magnetic-nanoparticle-modified electrodes is presented. It is shown using Fe3O4-nanoparticle-modified electrodes that the ultrahigh magnetic gradients (>108 T·m–1) established at the magnetized Fe3O4 nanoparticles speed up the transport of reactants and products at the electrode surface. Using the Fe(III)/Fe(II)-hexacyanoferrate redox couple, it is demonstrated that this mass transport enhancement can conveniently and repeatedly be switched on and off by applying and removing an external magnetic field, owing to the superparamagnetic properties of magnetite nanoparticles. Thus, it is shown for the first time that magnetic nanoparticles can be used to control mass transport in electrochemical systems. Importantly, this approach does not require any means of mechanical agitation and is therefore particularly interesting for application in micro- and nanofluidic systems and devices.

Single Oxidative Collision Events of Silver Nanoparticles: Understanding the Rate‐Determining Chemistry

The oxidative dissolution of citrate-capped silver nanoparticles (AgNPs, ∼50 nm diameter) is investigated herein by two electrochemical techniques: nano-impacts and anodic stripping voltammetry. Nano-impacts or single nanoparticle-electrode collisions allow the detection of individual nanoparticles. The technique offers an advantage over surface-immobilized methods such as anodic stripping voltammetry as it eliminates the effects of particle agglomeration/aggregation. The electrochemical studies are performed in different electrolytes (KNO3 , KCl, KBr and KI) at varied concentrations (≤20 mm). In nano-impact measurements, the AgNP undergoes complete oxidation upon impact at a suitably potentiostated electrode. The frequency of the nanoparticle-electrode collisions observed as current-transient spikes depends on the electrolyte identity, its concentration and the potential applied at the working electrode. The frequencies of the spikes are significantly higher in the presence of halide ions and increase with increasing potentials. From the frequency, the rate of AgNP oxidation as compared with the timescale the AgNP is in electrical contact with the electrode can be inferred, and hence is indicative of the relative kinetics of the oxidation process. Primarily based on these results, we propose the initial formation of the silver (I) nucleus (Ag+ , AgCl, AgBr or AgI) as the rate-determining process of silver oxidation on the nanoparticle.

Electrochemical Detection of Ultratrace (Picomolar) Levels of Hg2+ Using a Silver Nanoparticle-Modified Glassy Carbon Electrode

Ultratrace levels of Hg2+ have been quantified by undertaking linear sweep voltammetry with a silver nanoparticle-modified glassy carbon electrode (AgNP-GCE) in aqueous solutions containing Hg2+. This is achieved by monitoring the change in the silver stripping peak with Hg2+ concentration resulting from the galvanic displacement of silver by mercury: Ag(np) + 1/2Hg2+(aq) → Ag+(aq) + 1/2Hg(l). This facile and reproducible detection method exhibits an excellent linear dynamic range of 100.0 pM to 10.0 nM Hg2+ concentration with R2 = 0.982. The limit of detection (LoD) based on 3σ is 28 pM Hg2+, while the lowest detectable level for quantification purposes is 100.0 pM. This method is appropriate for routine environmental monitoring and drinking water quality assessment since the guideline value set by the US Environmental Protection Agency (EPA) for inorganic mercury in drinking water is 0.002 mg L-1 (10 nM).

In Situ Detection of Particle Aggregation on Electrode Surfaces

Partially blocked electrodes (PBEs) are important; many applications use non-conductive nanoparticles (NPs) to introduce new electrode functionalities. As aggregation is a problem in NP immobilization, developing an in situ method to detect aggregation is vital to characterise such modified electrodes. We present chronoamperometry as a method for detection of NP surface aggregation and semi-quantitative sizing of the formed aggregates, based on the diffusion limited current measured at PBEs as compared with the values calculated numerically for different blocking feature sizes. In contrast to voltammetry, no approximations on electrode kinetics are needed, making chronoamperometry a more general and reliable method. Sizing is shown for two modification methods. Upon drop casting, significant aggregation is observed, while it is minimized in electrophoretic NP deposition. The aggregate sizes determined are in semi-quantitative agreement with ex situ microscopic analysis of the PBEs.

Rapid Method for the Quantification of Reduced and Oxidized Glutathione in Human Plasma and Saliva

A new method is developed to determine the concentrations of reduced (GSH) and oxidized glutathione (GSSG), and it enables the calculation of the GSH:GSSG ratios in human plasma and saliva samples. The assay is based on the masking of GSH in a GSH and GSSG mixture via a 1,4-addition reaction with p-benzoquinone (BQ), followed by enzymatic kinetic measurement. The enzyme, glutathione reductase, is highly specific to glutathione. Excess BQ can thus be easily removed by the addition of non-GSH thiols. The assay takes less than 2 min, is suitable for a short-time-scale study, and minimizes the in vitro underestimation of the GSH:GSSG ratio arising from the degradation of GSH and formation of GSSG. We further show in this paper that the stability of the total glutathione content (GSH + GSSG) and GSH in saliva is significantly greater than in plasma, encouraging the development of noninvasive saliva sensing.

Improving Single-Carbon-Nanotube–Electrode Contacts Using Molecular Electronics

We report the use of an electroactive species, acetaminophen, to modify the electrical connection between a carbon nanotube (CNT) and an electrode. By applying a potential across two electrodes, some of the CNTs in solution occasionally contact the electrified interface and bridge between two electrodes. By observing a single CNT contact between two microbands of an interdigitated Au electrode in the presence and absence of acetaminophen, the role of the molecular species at the electronic junction is revealed. As compared with the pure CNT, the current magnitude of the acetaminophen-modified CNTs significantly increases with the applied potentials, indicating that the molecule species improves the junction properties probably via redox shuttling.

Dynamics of silver nanoparticles in aqueous solution in the presence of metal ions

Using a combined UV-vis, DLS, and electrochemical approach, this work experimentally studies the physical origin of the observed colorimetric sensitivity of aqueous silver nanoparticles toward divalent metal ions. In the presence of Pb2+, AgNPs are slow to reversibly form agglomerates (the time scale of the reverse deagglomeration process is of the order of hours). This agglomeration is shown to be induced by complex formation between Pb2+ and citrate groups localized on the AgNPs, reducing surface charges (zeta-potential) and hence electrostatic repulsion between the AgNPs. Other divalent metal ions including Ca2+, Cd2+, Zn2+, Ni2+, Co2+, and Sn2+ are also studied, and the resulting sizes of the AgNPs clusters and the extents of the UV-vis spectrum red-shift in λmax have a strong positive correlation with the metal-ligand (citrate) complex formation constant (Kf). This work thus serves as a guide for the selection of capping agents on the basis of Kf and demonstrates the correlation between sizes and spectrophotometric as well as electrochemical responses of the AgNPs clusters. Importantly, we give further physical insights into the size-dependent properties of AgNPs and emphasize the difference between theoretical and experimental values of extinction coefficients, where the latter is affected by the angle-dependent scattering intensities and the measurement technique used.

Coupled Optical and Electrochemical Probing of Silver Nanoparticle Destruction in a Reaction Layer

Abstract The oxidation of silver nanoparticles is induced to occur near to, but not at, an electrode surface. This reaction at a distance from the electrode is studied through the use of dark‐field microscopy, allowing individual nanoparticles and their reaction with the electrode product to be visualized. The oxidation product diffuses away from the electrode and oxidizes the nanoparticles in a reaction layer, resulting in their destruction. The kinetics of the silver nanoparticle solution‐phase reaction is shown to control the length scale over which the nanoparticles react. In general, the new methodology offers a route by which nanoparticle reactivity can be studied close to an electrode surface.

The fate of silver nanoparticles in authentic human saliva

Abstract The physicochemical properties of silver nanoparticles (AgNPs) in human whole saliva are investigated herein. In authentic saliva samples, AgNPs exhibit a great stability with over 70% of the nanomaterial remaining intact after a 24-h incubation in the presence of ∼0.3 mM dissolved oxygen. The small loss of AgNPs from the saliva sample has been demonstrated to be a result of two processes: agglomeration/aggregation (not involving oxygen) and oxidative dissolution of AgNPs (assisted by oxygen). In authentic saliva, AgNPs are also shown to be more inert both chemically (silver oxidative dissolution) and electrochemically (electron transfer at an electrode) than in synthetic saliva or aqueous electrolytes. The results thus predict based on the chemical persistence (over a 24-h study) of AgNPs in saliva and hence the minimal release of hazardous Ag+ and reactive oxygen species that the AgNPs are less likely to cause serious harm to the oral cavity but this persistence may enable their transport to other environments.