Abstract: This specification describes a nanoparticle delivery agent for drugs such as chemotherapy drugs. Starch nanoparticles are internally crosslinked by a phosphate crosslinker such as sodium trimetaphosphate (STMP) using a phase inversion emulsion process. The particle size may be in the range of 80-500 nm. A wide variety of organic phosphates are present apart from the phosphodiester crosslinking. These included triphosphates, monophosphates and diphosphates. The nanoparticles are hydrogels and retain significant amounts of water when dispersed in solution possibly due to the electrostatic repulsion between the chains within the nanoparticle. The nanoparticles are, in general, non-toxic, for example to HeLa cancer cells. The nanoparticles display a high drug loading, with a maximum seen with about 20-40 mol % STMP. Drug release occurs more readily at lower pH. Exposure to typical cell culture environments induces significant release of drug compared to simple buffer environments.