The depletion effect on a concave microstructure for size-specific target particle collection was evaluated using a set of strictly size-controlled superparamagnetic hemispheres, "magcups", with mixtures of differently sized model target microbeads in the presence of dispersed polystyrene nanoparticles (PS-NPs), which are thought to enhance entropic depletion effects to capture model target beads having the size closest to those of the hemispherical microstructures of magcups. Magcups with diameters of 10 to 50μm were fabricated with the formation of three nickel layers (2nm thickness) inserted between silicon dioxide layers (15 nm) on polystyrene template spheres by vapor deposition and subsequent polystyrene removal by burning. Two different diameters of model target beads (10 and 20 μm) were mixed with the magcups in order to evaluate the contribution of the depletion effect to the frequency of target bead capture by the concave microstructures of magcups of various sizes. To evaluate the depletion effect caused by the presence of dispersed PS-NPs, we compared the frequencies of target bead-magcup conjugations with and without PS-NPs in reaction solvents, and found that (i) the frequency of 10-μm-diameter bead capture increased 3.0 times on average for 15- and 20-μm-diameter magcups, and (ii) the frequencies of 10-μm-diameter beads capture increased as almost the same level with that of 20-μm bead for 30-, 40-, and 50-μm-diameter magcups. These results suggest that conjugations of target beads with magcups were encouraged by the depletion effect due to the presence of PS-NPs.
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