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.

X-ray scattering characterisation of nanoparticles

“…To avoid beam-induced fouling of the cell windows, it may be possible in some instances to create a 'windowless' cell using a liquid jet [312] or levitating droplet. [313] SAXS experiments have also been performed on nanoparticles formed in flames. [314,315] Time-resolved studies have been performed for nanoparticles in a variety of solutions and using various apparatus.…”

“…[313] The droplet acted as a 'wall-free reaction vessel' and its position could be controlled to ± 20 µm. Evaporation of solvent (water) was countered using piezoelectric nozzles to replace the solvent volume by feeding additional solvent to the droplet.…”

Nanomorphology characterization of sterically stabilized polypyrrole-palladium nanocomposite particles

“…This method is particularly powerful for quantitative monitoring of the temporal growth of nanoparticles; this non-destructive technique is the only method that provides the opportunity to analyze the timeevolution of structures or characterize dynamic aspects of nanoparticles. SAXS has previously been applied as a tool to evaluate the diameter and size distribution of nanoparticles to monitor the temporal growth of metal nanoparticles, 16 such as gold, [17][18][19] silver, 20 palladium 21,22 and rhodium. 21 Recently, SAXS was also used to characterize the morphologies of vinyl polymer-silica core-shell nanocomposite particles.…”

“…21 Recently, SAXS was also used to characterize the morphologies of vinyl polymer-silica core-shell nanocomposite particles. 23,24 In these studies, the size distribution was determined by assuming Gaussian 17,18,22 or Schulz-Zimm distribution functions 19,20 to analyze the SAXS results (particle scattering). Direct evaluation of the size distribution, which can be determined by TEM, 20,21 has not yet been performed using SAXS.…”

Mechanistic Investigation of Seeded Growth in Triblock Copolymer Stabilized Gold Nanoparticles

“…The observation of sub-10 nm GNPs has precedent. For example, F127 (EO 99 -PO 65 -EO 99 ) mediated Au(III) reduction nanostructures of ~ 6 nm were detected by Polte and co-workers 24 using quantitative small-angle X-ray scattering (SAXS) and X-ray absorption spectroscopy (XAS). We assume that such primary nanocrystallites are formed during the “burst” nucleation process.…”