Alternating silica/polymer tetra- and penta-layer hybrid microspheres were first prepared via combined inorganic sol−gel reaction and distillation-precipitation polymerization. pH-responsive poly(methacrylic acid) (PMAA) hollow microspheres with asymmetric double-shells were produced after HF etching of the silica layers in the SiO 2 /PMAA tetra-layer microspheres with different degrees of cross-linking in the two PMAA layers. On the other hand, silica “core−double shell” hollow microspheres were obtained by calcination of the alternating silica/PMAA penta-layer microspheres. The resulting hollow polymer and silica microspheres with hierarchical structures were characterized by field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) analysis, dynamic laser scattering (DLS), and confocal laser scanning microscopy (CLSM) measurements.
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Li et al. (2010) studied this question.