Lability of nanoparticulate metal complexes in electrochemical speciation analysis

Abstract

Lability concepts are elaborated for metal complexes with soft (3D) and hard (2D) aqueous nanoparticles. In the presence of a non-equilibrium sensor, e.g. a voltammetric electrode, the notion of lability for nanoparticulate metal complexes, M-NP, reflects the ability of the M-NP to maintain equilibrium with the reduced concentration of the electroactive free M 2+ in its diffusion layer. Since the metal ion binding sites are confined to the NP body, the conventional reaction layer in the form of a layer adjacent to the electrode surface is immaterial. Instead an intraparticulate reaction zone may develop at the particle/ medium interface. Thus the chemodynamic features of M-NP complexes should be fundamentally different from those of molecular systems in which the reaction layer is a property of the homogeneous solution (μ = (D M /k a ′ ) 1/2 ). For molecular complexes, the characteristic timescale of the electrochemical technique is crucial in the lability towards the electrode surface. In contrast, for nanoparticulate complexes it is the dynamics of the exchange of the electroactive metal ion with the surrounding medium that governs the effective lability towards the electrode surface.

Chemodynamics of Soft Nanoparticulate Metal Complexes: From the Local Particle/Medium Interface to a Macroscopic Sensor Surface

“…Because the release of free M aq z + is the relevant process at the NP/medium interface, a reaction layer should here take the form of an intraparticulate reactive outer shell (Figure ). Quantitative description of λ NP is not straightforward because steady-state diffusion does not hold within particle bodies in the nano size range. Instead, rather cumbersome transient conditions govern the release process; a couple of basic noncomplex cases for spherical bodies have been drawn to analytical release-time dependencies in Crank .…”

“…The reaction layer concept is based on the rate of reassociation of a free ion with a site S and therefore only has physical meaning in the presence of complexing sites. Accordingly, for the case of nanoparticulate complexing agents, in which the reactive sites are confined to the particle body, the true reaction layer is bound to be an intraparticulate feature . A quantitative description of the lability of nanoparticulate complexes toward a reactive macrosurface thus calls for a more differentiated approach that considers the role of the chemodynamics inside the NP body, in conjunction with the particle/medium exchange of the reactive target species .…”

“…Accordingly, for the case of nanoparticulate complexing agents, in which the reactive sites are confined to the particle body, the true reaction layer is bound to be an intraparticulate feature. 43 A quantitative description of the lability of nanoparticulate complexes toward a reactive macrosurface thus calls for a more differentiated approach that considers the role of the chemodynamics inside the NP body, 43 in conjunction with the particle/medium exchange of the reactive target species. 44 In the light of recent theoretical insights, we here develop an approach to couple the local particulate processes of association/dissociation and diffusion to/from the particle body with macroscopic fluxes at sensor/medium interfaces.…”

Lability of Nanoparticulate Metal Complexes at a Macroscopic Metal Responsive (Bio)interface: Expression and Asymptotic Scaling Laws

“…2,29 By its very nature, a reaction layer can only exist in the presence of complexing sites. 13,30 Accordingly, for the case of NP complexants, the reaction layer at the sensing surface is an operational one that is related to the time-averaged presence of particle body volume/surface area and the corresponding time-averaged complexing site concentration. Unlike molecular ligands, [25][26][27][28] the dissociation kinetic flux of M-NP complexes, or for that matter, the reaction layer thickness λ , generally depends on the degree of geometrical exclusion of the complexing NP's body from the reaction layer, i.e.…”

“…A convenient way to formulate the dissociative kinetic flux of M–NP complexes at the sensor surface is to invoke the concept of a reaction layer, originally developed by Heyrovský and Brdička − for molecular complexing ligands and recently detailed for nanoparticulate ligands . The thickness of this reaction layer, λ̅, basically derives from the relative mobilities of free and complexed metal species in the medium and their respective lifetimes as determined by the rate of re-association of M with NPs and the rate of M–NP dissociation. , In turn, the spatial zone lying within a distance λ̅ from the sensor defines a nonlabile region where metal release from the complexes to the macroscopic interface is operational, i.e., λ̅ denotes an effective reaction layer thickness at the macroscopic reactive interface. , By its very nature, a reaction layer can only exist in the presence of complexing sites. , Accordingly, for the case of NP complexants, the reaction layer at the sensing surface is an operational one that is related to the time-averaged presence of the particle body volume/surface area and the corresponding time-averaged complexing site concentration. Unlike molecular ligands, − the dissociation kinetic flux of M–NP complexes, or for that matter, the reaction layer thickness λ̅, generally depends on the degree of geometrical exclusion of the complexing NP’s body from the reaction layer, i.e., on the fraction of metal binding sites effectively involved in the dissociation of the complexes at the metal-sensing interface. ,, Recent successful confrontation between the abovementioned lability framework and electrochemical data collected for Cd­(II) in a dispersion of polymer NPs evidenced the paramount importance of NP exclusion effects for a proper assessment of the lability of the nanoparticulate metal complexes considered. , In contrast, the application of the conventional reaction layer approachwhich neglects such effects − was shown to overestimate the lability parameter of the complexes by more than 3 orders of magnitude …”