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

© 2015, Tsinghua University Press and Springer-Verlag Berlin Heidelberg. 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. [Figure not available: see fulltext.

Planar diffusion to macro disc electrodes—what electrode size is required for the Cottrell and Randles-Sevcik equations to apply quantitatively?

© 2014, Springer-Verlag Berlin Heidelberg. Simulations and experiments are reported which investigate the size of a macro disc electrode necessary to quantitatively show the chronoamperometric or voltammetric behaviour predicted by the Cottrell equation or the Randles-Sevcik equation on the basis of exclusive one-dimensional diffusional mass transport. For experimental time scales of several seconds, the contribution of radial diffusion is seen to be measurable even for electrodes of millimetres in radius. Recommendations on the size of macro electrodes for quantitative study are given and should exceed 4 mm radius in aqueous solution

Chemical analysis in saliva and the search for salivary biomarkers – a tutorial review

Biomarkers refer to analytes that can be used in the diagnosis of diseases or disorders. In saliva, there are many components that are potential biomarkers, and an increasingly research has focussed on the development of saliva as a diagnostic fluid. This review summarizes existing uses of salivary biomarkers and highlights the importance of the choice of saliva collection as well as the storage procedures. A case study on the effect of collection tools on the concentrations of one of the potential biomarkers, glutathione, is highlighted. Moreover, molecular diagnosis requires reliable measurement assays. This review presents electroanalytical methods for the detection of salivary biomarkers. It further reviews approaches that can be taken to improve the selectivity of the electroanalytical assays without the use of biologically selective materials, notably without the use of enzymes or antibodie

Planar diffusion to macro disc electrodes-what electrode size is required for the Cottrell and Randles-Sevcik equations to apply quantitatively?

Simulations and experiments are reported which investigate the size of a macro disc electrode necessary to quantitatively show the chronoamperometric or voltammetric behaviour predicted by the Cottrell equation or the Randles-Sevcik equation on the basis of exclusive one-dimensional diffusional mass transport. For experimental time scales of several seconds, the contribution of radial diffusion is seen to be measurable even for electrodes of millimetres in radius. Recommendations on the size of macro electrodes for quantitative study are given and should exceed 4 mm radius in aqueous solution

Planar diffusion to macro disc electrodes-what electrode size is required for the Cottrell and Randles-Sevcik equations to apply quantitatively?

Simulations and experiments are reported which investigate the size of a macro disc electrode necessary to quantitatively show the chronoamperometric or voltammetric behaviour predicted by the Cottrell equation or the Randles-Sevcik equation on the basis of exclusive one-dimensional diffusional mass transport. For experimental time scales of several seconds, the contribution of radial diffusion is seen to be measurable even for electrodes of millimetres in radius. Recommendations on the size of macro electrodes for quantitative study are given and should exceed 4 mm radius in aqueous solution

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 hence 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 short timescale 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 non-invasive saliva sensing

Multiwalled carbon nanotube modified electrodes for the adsorptive stripping voltammetric determination and quantification of curcumin in turmeric

A sensitive electrochemical method for the determination and quantification of curcumin using adsorptive stripping voltammetry (AdsSV) at a multiwalled carbon nanotube modified basal plane pyrolytic graphite electrode (MWCNT-BPPG electrode) is presented exploiting the high surface area of the latter. Next the voltammetric behaviour of curcumin on the modified electrode is examined and AdsSV shown to be a sensitive method for quantifying curcumin. The adsorption of curcumin on the electrode surface is evidenced to follow a Langmuir adsorption isotherm. Linear calibration for curcumin in the range of 2 – 100 µM was obtained with a detection limit of 0.45 µM and a limit of quantification of 1.49 µM. For application to real samples of turmeric, a one-step sample preparation in ethanol has developed providing a simple and rapid extraction procedure. The MWCNT-BPPG electrode with AdsSV allowed the determination of curcumin equivalent in turmeric powder sample with recoveries in the range of 92-108%. This facile and fast method will be useful for monitoring the quality of curcumin containing in commercial turmeric products

The fate of silver nanoparticles in authentic human saliva

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-hour 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-hour 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

The Copper(II)-Catalyzed Oxidation of Glutathione.

The kinetics and mechanisms of the copper(II)-catalyzed GSH (glutathione) oxidation are examined in the light of its biological importance and in the use of blood and/or saliva samples for GSH monitoring. The rates of the free thiol consumption were measured spectrophotometrically by reaction with DTNB (5,5'-dithiobis-(2-nitrobenzoic acid)), showing that GSH is not auto-oxidized by oxygen in the absence of a catalyst. In the presence of Cu(2+) , reactions with two timescales were observed. The first step (short timescale) involves the fast formation of a copper-glutathione complex by the cysteine thiol. The second step (longer timescale) is the overall oxidation of GSH to GSSG (glutathione disulfide) catalyzed by copper(II). When the initial concentrations of GSH are at least threefold in excess of Cu(2+) , the rate law is deduced to be -d[thiol]/dt=k[copper-glutathione complex][O2 ](0.5) [H2 O2 ](-0.5) . The 0.5(th) reaction order with respect to O2 reveals a pre-equilibrium prior to the rate-determining step of the GSSG formation. In contrast to [Cu(2+) ] and [O2 ], the rate of the reactions decreases with increasing concentrations of GSH. This inverse relationship is proposed to be a result of the competing formation of an inactive form of the copper-glutathione complex (binding to glutamic and/or glycine moieties)