Gold nanoparticles (Au-NPs) are among the most intensely studied nanoscale materials. [1-2] The molecular mechanisms of Au-NPs formation remain unclear due first to the quantity of material analyzed (~mM), second to the relative short time scale for the nucleation and growth steps and third to the limited accessibility to in situ time-resolved characterization. To overcome those issues and regarding the experimental aspects, Synchrotron radiation related techniques are useful to characterize such rapid NPs formation. [3] In order to access to a direct in situ time-resolved characterization, we build and adapt a micro-fluidic system under continuous flow conditions, whereby we follow, via in situ XANES spectroscopy, the Au-NPS formation from a gold precursor (HAuCl4) with sodium citrate as reducing and stabilizer agent to final. The setup is constituted by two glass syringes, a syringe pump, and one tee-mixer for combining the reagents and a 1.5 m
7 200 \u3bcm internal diameter fused silica capillary as micro-reactor heated using a heating plate. By focusing the X-rays (analysis spot of 50*50 \ub5m), the reaction was followed by monitoring the gold LIII-edge XAS spectra at different locations along the micro-tube, temperature and flow rates. Thanks to this set up it is possible to convert a "time scale" reaction in a "length scale" reaction. We describe the embryonic state NPs formation by characterizing the oxidation state of gold, geometry modifications of the precursor and ligand substitutions. [1] M. C. Daniel, D. Astruc, Chem. Rev., 104 (2004) 293. [2] A. S. K. Hashmi, G. J. Hutchings, Angew. Chem. Int. Ed., 45, (2006), 7896. [3] J. Polte, , T. T. Ahner, F. Delissen, , S. Sokolov, F. Emmerling, A. F.Th\ufcnemann, R. Kraehnert, J. Am. Chem. Soc, 132, (2010), 1296
