I am making polystyrene nanoparticles with carboxylic groups on the surface by emulsion polymerization of styrene and acrylic acid at the ratio of 4:1 or 20% acrylic acid (with the surfactant SDS).

This ratio is used in many journal papers. However, I found there are many problems in the particles that are supposed to conjugate with proteins via EDC chemistry to be used as the reporters in diagnostic assays. For example, it requires a large amount of proteins to get enough signal intensity. After the protein conjugation, the particles would aggregate gradually with time. I heard that some people only use 5% acrylic acid. Is it a better ratio?