Do you have an extinction coefficient for the gold nano particles? And, what was the path length for your OD measurements? I'd like to convert the concentration to molarity
Please refer to certificate of analysis for the UV spectrum. extinction coefficient at for 50 nm gold partical is at 531-532 nm for the nanoparticle. It’s 1.5*e10. 1 OD means the absorption = 1 at 1 cm path length for 1 mL solution.