Dear Shin

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.

Good luck

Hello

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

then Isn't the gold(Ⅲ) chloride suitable for synthesize AuNP using just Sodium citrate?

Hello, Everything depends on the reducing agent and stabilizing substance/ polymer. Some reactions of AuNPs formation won't proceed with HAuCL4 or gold(Ⅲ) chloride however It works using K(Na)AuCL4 salts. AuNPs formation and their shape, size, stability pH depended. Here is just an example a few examples. https://www.sciencedirect.com/science/article/pii/S0959652619339599

https://doi.org/10.1016/j.jclepro.2019.119089

https://pubs.acs.org/doi/abs/10.1021/bm5003834

Shin Hyeyoung

Many routes as stated above. Take a look at:

Metal colloids - their preparation, application and characterization

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W160218MetalColloids.html

Silver colloids and invisible ink

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W150902SilverColloidInvisibleInk.html

Dear Shin Hyeyoung, in general, any soluble Au3+ compound can be used as precursor for AuNPs. However, HAuCl4 x 3 H2O is very often employed as it is the most readily accessible gold(III) starting material. AuCl3 can be prepared e.g. by treatment of HAuCl4 x 3 H2O with thionylchloride. Thus AuCl3 is significantly more expensive than tetrachloroauric acid. On the other hand, AuCl3 offers the advantage that it can be used in non-aqueous solvents such as DMSO (see attached) or ionic liquids.