Many synthetic microspheres are spherical and symmetrical because of the thermodynamical limitations
of the reaction systems. We developed modification techniques to prepare spherical microspheres with
intrinsic unsymmetry, for example, a microsphere with both an anionic and a cationic part. The modification
was performed at the liquid−solid interface. Reactive microspheres with p-nitrophenyl moieties were
settled onto the IgG-preadsorbed substrate, and the reaction between the activated ester moieties and IgG
molecules proceeded only at the interface upon attachment. After the reaction, the modified microspheres
were detached from the substrate with ultrasonication. In another modification at the air−water interface,
the dispersion of reactive microspheres in ethanol was first spread on a water surface to produce the
monolayer of microspheres, and then hydrolysis at the water side of the monolayer was performed by
adding a NaOH aqueous solution to the subphase. After this unsymmetrical hydrolysis, we carried out
IgG immobilization with the remaining reactive moieties. In both interface methods, the immobilized IgG
molecules were labeled with colloidal gold particles conjugated with anti-IgG antibodies. Transmission
electron microscopy indicated that gold particles could be observed on only the immobilized side. Small
microspheres were coupled to the reactive microsphere, and its unsymmetry was confirmed by scanning
electron microscopy. Such unsymmetrical microspheres will be applicable to the electrical rheology, diagnosis,
display technology, and creation of functional devices through the assembling of microspheres.
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