UV–Vis and NMR study of the formation of gold nanoparticles by citrate reduction: Observation of gold–citrate aggregates

• Citrate reduction of AuCl4- was studied by UV–Vis absorption and NMR spectroscopies.
• The protonation state of citrate plays a crucial role in the reaction mechanism.
• NMR diffusion measurements highlight the presence of gold–citrate aggregates.

The citrate reduction of gold(III) in water is one of the most commonly used synthetic pathways for the preparation of gold colloids. In order to gain insight into the formation of gold nanoparticles (GNPs) using this method, the synthesis of GNPs was undertaken under different experimental conditions and monitored in operando by UV–Vis spectroscopy. These experiments highlight that citrate should be polydeprotonated and that Au(III) should not be polyhydroxylated in order to obtain GNPs with a narrow size distribution. Samples taken during the reaction were also characterized by Nuclear Magnetic Resonance Spectroscopy (NMR) to monitor the various reaction products as a function of time. Diffusion Ordered SpectroscopY (DOSY) experiments allowed us to identify slow diffusing citrate – Au(I) or Au(0) complexes which could play a role in the formation of GNPs.

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