We describe a facile approach for the synthesis of micrometer-sized (similar to 3.5 mu m), pH-responsive microgel particles, which have functional carboxylic acid groups concentrated in the shell. The large size offers the possibility to directly study the interactions between individual, isolated microgel particles with active ingredients by optical microscopy. Our results show that the synthesized microgel particles can load and release active ingredients via changing pH values. The complexation of Ca2+ with the -COOH functional groups located at the microgel surfaces not only regulates the active ingredient's uptake efficiency, but also provides a novel way to reveal the spatial distribution of the functional groups inside the microgel particles.