Carolina Nunes Kirchner

Diffusion and Reaction in Microbead Agglomerates

Scanning electrochemical microscopy has been used to analyze the flux of p-aminonophenol (PAP) produced by agglomerates of polymeric microbeads modified with galactosidase as a model system for the bead-based heterogeneous immunoassays. With the use of mixtures of enzyme-modified and bare beads in defined ratio, agglomerates with different saturation levels of the enzyme modification were produced. The PAP flux depends on the intrinsic kinetics of the galactosidase, the local availability of the substrate p-aminophenyl-beta-D-galactopyranoside (PAPG), and the external mass transport conditions in the surrounding of the agglomerate and the internal mass transport within the bead agglomerate. The internal mass transport is influenced by the diffusional shielding of the modified beads by unmodified beads. SECM in combination with optical microscopy was used to determine experimentally the external flux. These data are in quantitative agreement with boundary element simulation considering the SECM microelectrode as an interacting probe and treating the Michaelis-Menten kinetics of the enzyme as nonlinear boundary conditions with two independent concentration variables [PAP] and [PAPG]. The PAPG concentration at the surface of the bead agglomerate was taken as a boundary condition for the analysis of the internal mass transport condition as a function of the enzyme saturation in the bead agglomerate. The results of this analysis are represented as PAP flux per contributing modified bead and the flux from freely suspended galactosidase-modified beads. These numbers are compared to the same number from the SECM experiments. It is shown that depending on the enzyme saturation level a different situation can arise where either beads located at the outer surface of the agglomerate dominate the contribution to the measured external flux or where the contribution of buried beads cannot be neglected for explaining the measured external flux.

Nonlinear Boundary Conditions in Simulations of Electrochemical Experiments Using the Boundary Element Method.

The use of the boundary element method (BEM) in simulating steady‐state experiments of scanning electrochemical microscopy in feedback mode and in generation‐collection mode using complex three dimensional geometries has been shown in previous papers. In the context of generation‐collection mode experiments, catalytic reaction mechanisms of immobilized enzymes are of great interest. Due to the catalytic reaction behaviour, which can be described by nonlinear Michaelis‐Menten kinetics, the modelling of such systems results in solving a diffusion equation with nonlinear boundary conditions. In this article it is described how such nonlinear reaction mechanisms can be treated with the BEM.

Chapter 37 Scanning electrochemical microscopy in biosensor research

Publisher Summary Scanning electrochemical microscopy (SECM) allows one to record spatially resolved maps of chemical reactivities, i.e. images that reflect the rate of heterogeneous chemical reactions. This technique lends itself to the characterization of surfaces at which substances are locally released into the solution. It can be applied to a large variety of interfaces including solid–liquid, liquid–liquid, and liquid–gas interfaces. The sample can be conductive, semiconductive or insulating. The signal in SECM is based on an electrochemical signal specific for a certain chemical compound. In this respect, the scanning probe can also be regarded as a positionable chemical microsensor. SECM is not just suitable to measure local solute concentrations but also, and more importantly, represents a tool to map local (electro) chemical reactivities, to induce localized electrochemical surface modifications, or to investigate heterogeneous and homogeneous kinetics. The SECM image provides a direct representation of interfacial reactivity even in those cases where the topography of the interface does not change during the reaction, e.g. during an electron transfer from an electrode to a dissolved compound without accompanying deposition or dissolution processes.

Procedure 51 Kinetic analysis of titanium nitride thin films by scanning electrochemical microscopy

Publisher Summary This chapter presents a procedure for analysis of the standard electron transfer rate constant (k1) of titanium nitride thin film as an example of a new electrode material. Steps are provided for finding the distance offset of the measurements using theoretical curve. The approach curves are recorded at different potentials applied to the titanium number sample. The obtained effective rate constants are fitted to the Butler–Volmer equation. The mean standard rate constant was k˚ = (2.1±0.2)×10 –3 cms –1 and showed that scanning electrochemical microscope is a powerful method to determine the rate constant. The curve fitting and calculation of the offset are crucial for reproducible result. The special advantage of the method is its relative immunity to inaccuracies introduced by uncompensated resistance or limited rise time of potentiostats because the analysis occurs under steady-state conditions and very low total currents.