Abstract: The presently disclosed subject matter provides a kinetically controlled mixing process, referred to herein as “flash nanocomplexation” or “(FNC),” to accelerate the mixing of a polyanion solution, for example, a plasmid DNA solution, with a polycation solution to match the polyelectrolyte complex (PEC) assembly kinetics through turbulent mixing in a microchamber, thus achieving explicit control of the kinetic conditions for nanoparticle assembly as demonstrated by the tunability of nanoparticle size, composition, hydrodynamic size, hydrodynamic density, surface charge, and polyanion payload.