Protein binding on thermally grown silicon dioxide
Diagram of the chemical linking method of streptavidin protein binding. Streptavidin is a tetrameric protein with four biotin binding pockets. As indicated, more than one biotin binding pocket (and possibly all four pockets) of a single streptavidin tetramer could be occupied by the biotin molecules linked to the silica surface. The biotin to streptavidin interaction is one of the strongest noncovalent bonds known, so the combination of the covalent bond of the biotin to the silane (to the surface) and the streptavidin–biotin bond is much stronger than a streptavidin adsorption to the surface directly.
Flow chart of the generation and analysis of streptavidin-coated surfaces.
AFM evaluation of silica substrate after cleaning and treatment with water or a biological buffer (PBS). The top row images were collected in ambient air, and the bottom row images were collected in an aqueous environment [(d) in water, (e) in PBS].
Fluorescent microscopy images of streptavidin deposition on various silica substrates. (a) A typical flat silica surface after direct adsorption of streptavidin (from a stock) the dip rinse technique (, exposure); (b) a sample comparable to that shown in sample (a) after the more vigorous vacuum rinse. Note the loss of streptavidin from the surface at large and the retention of streptavidin in scratches (, exposure); and (c) a biotinylated surface coated with fluorescently labeled streptavidin from a stock after vacuum rinse (, exposure). Fluorescently labeled streptavidin appears white in these images.
Three dimensional AFM images in air of: (a) patterned sample before protein adsorption (plateau: , ; whole image: , ) and (b) AFM image in air of patterned wafer after protein adsorption (plateau: , ; whole image: , ).
Three-dimensional AFM images derived in PBS of streptavidin (deposited on the surfaces from a stock) for each of the three binding techniques.
Adhesive force measurements comparing the various surfaces investigated with both a clean AFM tip, and a AFM tip functionalized with streptavidin, both assayed in an aqueous environment (i.e., in PBS). Histogram bar heights represent the average values and the error bars indicate the range of results obtained. (a) Adhesion force measurements compare the three adsorptive silica surfaces employed in this study. Note that there was no detectable variation in the adhesion measurements of the biotin-coated samples with an unfunctionalized tip, so no error bar appears for that data. (b) Adhesion force measurements compare the cleaned thermally grown silicon oxide film to a hydrated surface (i.e., more similar to glass substrates).
Relative binding strength of chemical conjugation and direct adsorption is rank ordered by exposure to either a turbulent (vacuum) rinse or a gentle (dip) rinse. Following rinsing, the amount of streptavidin bound was measured by ELISA and results are presented in adsorption units at per area of protein sample surface (adsorption used both sides of sample, conjugation only used one). Higher values of adsorption units indicate more protein bound (measurements were taken in the linear range of the assay, data not shown).
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