Adding surfactant is good method to avoid agglomeration in nanoparticles. If you don't want to add any surfactant, you can search for suitable plant extract. Some plant extracts acts as a reducing agent as well as capping agent. Other-hand, You can go for mild sonication, immediately do analysis (SEM, TEM), otherwise, nanoparticles easily agglomerate with time.
A surfactant may not work well with metal nanoparticles (perhaps with judcious choice as they are many different types?). A steric stabilizer such as starch will do it. One can still employ sonification at a later stage.