Voltammetric lability of metal complexes at spherical microelectrodes with various radii

Abstract

The size of a microelectrode can have a dramatic impact on the relative importance of the diffusional and kinetic contributions to the voltammetric current of an electroactive metal ion in a complexing medium. Decreasing the radius enhances the diffusional contribution and, as a consequence, the complex system tends to move away from labile behaviour (where an equilibrium relationship holds). Therefore, sufficiently small microelectrodes (either or not combined with short measuring times) should be able to directly sense free metal concentration for not too fast association/dissociation kinetics.The particular case of steady state spherical (or hemispherical) diffusion under ligand excess (pseudo-first order kinetics) is solved analytically. The ensuing lability criterion is shown to be in accordance with a geometrical derivation based on an analysis of the Published in Journal of Electroanalytical Chemistry 2001, vol 505, p 85-94 Voltammetric lability.. 2/25random walk of the free metal ions produced by dissociation of the complex. It is shown that, for a generated metal ion, the probability of reaching the microelectrode surface can be quite different from the planar case. Alternatively, the classical reaction layer concept can be used in the derivation of the lability criterion for spherical geometry as it is shown in this work. All treatments quantitatively show how the lability of metal complexes is reduced with decreasing the dimension of the microelectrode. [85][86][87][88][89][90][91][92][93][94] Voltammetric lability.. inert total kin J J J + ,The thickness of the reaction layer µ is a kinetic parameter [34][35][36][37][38][39], defined such that:

Dynamics of Biouptake Processes: The Role of Transport, Adsorption and Internalisation

“…As physically expected, it can be shown [55,59] that the higher the degree of lability, the stronger the depletion of ML at the microorganism surface:…”

“…The linear steady-state equations associated with (51) and (53), under excess ligand conditions, can be solved analytically [59] in terms of the concentration of M at the surface c SS M . The resulting supply flux is:…”

Experimental verification of the metal flux enhancement in a mixture of two metal complexes: the Cd/NTA/glycine and Cd/NTA/citric acid systems

“…Moreover, the lability degree has also been introduced to indicate the percentage of the complex contribution to the uptake flux with respect to its maximum contribution obtained when the kinetics of the complexation processes were fast enough to reach equilibrium at any time and relevant spatial position. 3,4,7,[9][10][11] It has been shown that the lability degree depends on the kinetic constants, diffusion coefficients, size of the sensor, composition of the system, etc.…”

Lability of nanoparticulate metal complexes in electrochemical speciation analysis

“…its radius r 0 . Due to the greater magnitude of J diff the lability of metal complexes with simple ligands is lower at microelectrodes than that at conventional sized ones at which planar diffusion prevails [39][40][41].…”

“…The emergence of nano-sized electrodes in recent years [40,[42][43][44][45] generates the situation in which the reactive interface is more comparable in size to many NPs. That is, the spatial scale over which there is a concentration gradient in the surrounding medium will be comparable to or smaller than the particle body.…”