Hello,
I want a detailed synthesis protocol for 13-15 nm citrate-stabilized gold nanoparticles using Frens' method. I followed the exact same protocol mentioned in Frens' method, but unfortunately, I could not get monodisperse nanoparticles. Here is the protocol I followed:
1. All glassware and stir bars were washed with aqua regia and finally using MQ water.
2. I used 50 mL of 0.01 g% hydrogen tetrachloroaurate (III) trihydrate and heated to boil under reflux.
3. I added 1.2-2.0 mL of 1 g% sodium citrate tribasic dihydrate.
4. After ~3 min, the color changed to red. I left it for another 5 and 10 min boiling under stirring before I switched off the heating and kept it under stirring to cool down.
During the 5-10 min boiling after the red color appeared, the color changed to purple (see attached photo) and ultimately to bluish-purple, and the stir bar was stained with a bluish stain (I suppose this is kind of aggregate). After one round of centrifugation, there was obvious aggregates as indicated by the black stuff formed on the wall of the Falcon tubes. The UV-vis showed a peak at ~520 nm, which is ok for such a size, but it also showed another distinct shoulder/hump peak (see attached photo marked with an arrow). The DLS showed multiple peaks in the intensity-weighted curves, with some peaks around 1000 nm (with PDI values >0.6).
I did the reactions multiple times, and I got the same results. All glassware, the condenser, and co. were carefully washed and very clean. I used MQ water (18.2 MΩ·cm).
I do not know where the problem is. For how long should the boiling after the appearance of the wine-red color continue? Why does the color change in the first place? The protocol says that there should be no color change after the appearance of the red color.
I am very disappointed with such results, as I followed the protocol as described. I want to exclusively use the Frens' method and not any other synthesis protocol.
Has anyone encountered such a problem? How could I fix it and get monodisperse 13, 14, or 15 nm monodisperse nanoparticles? Are there any tricks?
I would appreciate your help.
Ali Fayed
Hi !
Your observations suggest that the deviation from monodispersity and the post-red color shift toward bluish-purple likely result from secondary nucleation and uncontrolled growth or aggregation due to over-reduction and/or excessive citrate. In Frens’ method, precise control over the citrate-to-gold molar ratio is critical—typically, to obtain ~13–15 nm monodisperse AuNPs, a citrate:HAuCl₄ molar ratio of ~3.5–4 is optimal (for your case, ~1.4–1.6 mL of 1% citrate should suffice). Excess citrate (>2.0 mL) can lead to excessive reduction, inducing secondary nucleation or bridging aggregation. After the red color appears (~3 min post citrate addition), boiling should continue only for 5 more minutes maximum; prolonged heating promotes Ostwald ripening and fusion of particles. Also, rapid and consistent stirring is crucial—slow or uneven stirring can lead to polydispersity. The blue stain and shoulder in UV-Vis at ~600–700 nm confirm aggregation or formation of larger anisotropic particles. To mitigate this, ensure immediate and uniform mixing upon citrate addition, keep total reflux time short (~8–10 min total), and cool rapidly post-reaction. Finally, ensure all reagents are freshly prepared, as aged HAuCl₄ or citrate can affect reduction kinetics.
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