Layer-by-Layer Construction of Novel Biofunctional Fluorescent Microparticles for Immunoassay Applications

Abstract

A novel class of biofunctional fluorescent microparticles for application in immunoassays was constructed by using the layer-by-layer self-assembly method to deposit multiple layers of fluorescently labeled polyelectrolytes onto colloidal particles, followed by deposition of a protein (immunoglobulin G, IgG) layer. Microelectrophoresis experiments revealed alternating negative and positive ζ-potentials with deposition of each successive polyelectrolyte layer, indicating that the alternate electrostatic adsorption of polyelectrolytes of opposite charge was successfully achieved. Transmission electron microscopy images showed a change of the particle surface texture after polyelectrolyte multilayer deposition. Fluorescence microscopy image (FMI) analysis provided direct measurement of the fluorescence intensity of single microparticles. The observed systematic increase of the fluorescence intensity of individual microparticles with increasing polyelectrolyte layer number from FMI analysis further demonstrated the controlled regular adsorption of polyelectrolyte layers onto the polystyrene (PS) particles. Protein immobilization onto the polyelectrolyte multilayer-coated particles was verified by the different surface properties of the microparticles with respect to surface charge under pH conditions above and below the isoelectric point of the proteins. The assembly of IgG and fluorescein isothiocyanate-labeled IgG onto polyelectrolyte multilayer-coated PS microparticles and their potential use was ultimately confirmed by a solid phase immunotest.