title: Real-time monitoring of copolymer stabilized growing gold nanoparticles.
abstract: A great challenge in the production of nanoparticles with defined sizes and properties is to control their growth in situ. We developed a dedicated combined small-angle X-ray scattering (SAXS) and X-ray absorption spectroscopy (XAS) setup to monitor nanoparticle formation in solution. The capabilities of simultaneously deriving particle sizes and oxidation states of atoms/ions are illustrated for the formation of spherical gold nanoparticles by the reduction of hydrogen tetrachloroaureate (HAuCl(4)). Particles with initial radii of 4.60 +/- 0.10 nm and final radii of 5.67 +/- 0.10 nm were produced in a levitated droplet with a volume of 4 microL. An ethylene oxide/propylene oxide triblock copolymer PEO-PPO-PEO (Pluronic F-127) functions as reducing agent and colloidal stabilizer. XANES shows in situ how the gold was reduced in the droplet from Au(III) to Au(0), and simultaneously SAXS recorded the size distribution of the formed nanoparticles. It is shown that the final particle number is reached quickly. Thereafter, only the particles' sizes increase. Comparison of XANES and SAXS shows that the quantity of Au(0) is higher than the quantity of gold located in the nanoparticles while the particles are growing. Finally, all the Au(0) is found in the nanoparticles. We tentatively attribute this finding to the pseudo crown ether effect of the polymer surfactant that kinetically stabilizes gold atoms when formed from gold ions within their protecting cavity. A simple "burst" mechanism for the gold nanoparticle formation is the consequence. The possibility of an inhomogeneous particles structure with an enhanced density near the particle surface is discussed.
authors.affiliation: BAM Federal Institute for Materials Research and Testing, Richard-Willstaetter-Strasse 11, 12489 Berlin, Germany.
authors.forename: Jörg
authors.initials: J
authors.lastname: Polte
authors.forename: Franziska
authors.initials: F
authors.lastname: Emmerling
authors.forename: Martin
authors.initials: M
authors.lastname: Radtke
authors.forename: Uwe
authors.initials: U
authors.lastname: Reinholz
authors.forename: Heinrich
authors.initials: H
authors.lastname: Riesemeier
authors.forename: Andreas F
authors.initials: AF
authors.lastname: Thünemann
mesh_terms: D006046:Gold / Q000737:chemistry*; D053768:Metal Nanoparticles / Q000737:chemistry*; D011092:Polyethylene Glycols / Q000737:chemistry; D011108:Polymers / Q000737:chemistry*; D011409:Propylene Glycols / Q000737:chemistry; D053838:Scattering, Small Angle; D056928:X-Ray Absorption Spectroscopy
chemical_list: C116176:PEO-PPO-PEO; D011108:Polymers; D011409:Propylene Glycols; D011092:Polyethylene Glycols; D006046:Gold