Au NPs are usually synthesized through the reduction of Au(III) precursors, as far as the Au(III) source is soluble in water.See for example the following reference.
DOI: 10.1007/s10876-017-1178-0.
Obviously, the source of the Au(III) and the reaction conditions affect the obtained Au(III) NPs.
Please see these articles and I hope that they will be useful.
Successful Synthesis of Gold Nanoparticles through Ultrasonic Spray Pyrolysis from a Gold(III) Nitrate Precursor and Their Interaction with a High Electron Beam
A halogen-free synthesis of gold nanoparticles using gold(III) oxide