title: Diffusion and reaction in microbead agglomerates.
abstract: 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.
authors.affiliation: Carl von Ossietzky University of Oldenburg, Faculty of Mathematics and Natural Sciences, CIS-Center of Interface Science, Department of Pure and Applied Chemistry, D-26111 Oldenburg, Germany.
authors.forename: Carolina
authors.initials: C
authors.lastname: Nunes Kirchner
authors.forename: Markus
authors.initials: M
authors.lastname: Träuble
authors.forename: Gunther
authors.initials: G
authors.lastname: Wittstock
mesh_terms: D000627:Aminophenols / Q000737:chemistry; D004058:Diffusion; D005696:Galactosidases / Q000378:metabolism*; D007118:Immunoassay / Q000379:methods*; D007700:Kinetics; D008855:Microscopy, Electron, Scanning; D008863:Microspheres
chemical_list: D000627:Aminophenols; D005696:Galactosidases; C026729:4-aminophenol