mPEG-amine (mPEG-NH2) (MW 5000) is a modifier that can replace the sulfonic acid portion of dye molecules to enhance the water solubility of long-wavelength voltage-sensitive dyes (VSD) or Pittsburgh (PGH) dyes. It also forms amide bonds with carboxyl groups on microsphere surfaces in the presence of EDC and Sulfo-NHS, creating a PEG coating on fluorescent microspheres for large-scale rotational cytoplasmic flow studies.