Solid phase unstirred layer

Phase-separation immunoassays have been reported, in which the solid phase particles are formed after the immunoreaction is completed.(42) Phase-separation immunoassays are advantageous since the unstirred layer of solution near a solid surface alters diffusion and binding kinetics at the surface in comparison with the properties of the bulk solution. In phase-separation assays for IgG and IgM, capture antibodies are bound with monomers suitable for styrene or acrylamide polymerization.(42) Monomer-labeled capture antibodies are reacted with analyte and with fluorescein- and/or phycoerythrin-labeled antibodies in a sandwich assay, followed by polymerization of the monomers. Fluorescence of the resulting particles is quantitated in a FACS IV flow microfluorometer, and is directly proportional to analyte concentration. 

The effects immobilization of the enzyme has on its activity have been neglected in EIA. Fortunately, a large body of information is now available based on studies on immobilized enzymes (Trevan, 1980 Sharma et al., 1982). The immediate vicinity of a solid-phase may profoundly affect the activity of the enzyme. The first noticeable effects are the partitioning of the substrate between the fluid phase and the charged-polymer solid-phase, due to the charges of ionic species, and the limitation of diffusion of the solute to the solid-phase due to an unstirred layer of about 1 pm (i.e., more than 100 times the diameter of an average protein). 

Noyes, and Whitney and Nernst were the first to introduce and develop the concept of unstirred layer (quoted in [23]). According to the theoretical views of Nernst, there is a layer of static fluid present at a solid-liquid interface. The concentration of a given substance in this layer is not equal to that in the bulk solution and diffusion is the primary mode of transport within the unstirred layer. The thickness of the unstirred layer, though undetermined, is critically dependent on the rate of stirring in the bulk phase. 

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