A method is provided for making nanoparticles, including the steps of: combining a hydrocarbon solvent and an aprotic, polar co-solvent, a mono-vinyl aromatic monomer, polymerization initiator, a solution stabilizer, and a first charge of a cross-linking agent. Subsequently, a second charge of cross-linking agent is added. The nanoparticles have an average diameter of 5 nanometers to about 10,000 nanometers. Spherical nanoparticles are also provided that include a cross-linking agent comprising 30% to 60% by weight of the combined weight of a mono-vinyl aromatic species and the cross-linking agent. The spherical nanoparticles also meet the following equation: 0.90≦(D1/D2)≦1.1 wherein D1 is a first diameter of a nanoparticle and D2 is a second diameter of the nanoparticle, and D1 and D2 intersect at right angles.