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Tracking silver nanoparticles: ultra-small silver refunctionalizable with fluorescent biopolymers
(2016)
We report on the synthesis of ultra-small silver nanoparticles and their quantitative characterization by small-angle X-ray scattering. The size distribution was derived by utilizing a Monte-Carlo data evaluation procedure reported by Pauw et al. Mean volume-weighted sizes are 3 nm with a size distribution width of 18 %. The particles should be used as reference materials for comparison of the result of different analytical methods among which are field-flow fractionation (FFF), dynamic light scattering (DLS), nanoparticle tracking analysis (NTA) and electron microscopy (EM). In addition further use of the particles is foreseen for comparison of studies on the toxicology of nanoparticles. Therefore the silver nanoparticles are transfunctionalized with fluorescent marked albumin (BSA-FITC) and also thoroughly characterized. With this it is possible to track silver nanoparticles and their behavior in interaction with cells.
The use of silver nanoparticles in consumer related products has significantly increased over the last decade, especially due to their antimicrobial properties. Today they are used in a variety of products, which range from textiles over children toys and dietary supplements. Therefore, research on silver in a nanoscale form becomes increasingly important for a high amount of studies. Unfortunately the results of these studies are extremely diverse and do not lead to a consistent evaluation of the toxicity of silver nanoparticles. The central problem lies in the utilization of a wide range of silver nanoparticles, which show a broad size distribution. To overcome this problem we synthesized ultra-small core-shell silver nanoparticles by an up-scaled modification of the polyol process. The particles are highly stable and show no aggregation for more than six months. Small-angle X-ray scattering (SAXS) analysis reveal a narrow size distribution of the silver cores with a mean radius of RC = 3.0 nm and a distribution width of 0.6 nm. Dynamic light scattering (DLS) provides a hydrodynamic radius of RH = 10.0 nm and a PDI of 0.09. The surface of the particles is covered with poly(acrylic acid) (PAA) forming a shell with a thickness of 7.0 nm, which provides colloidal stability lasting for more than six months at ambient conditions. It is foreseen to use these thoroughly characterized particles as reference material to compare the catalytic and biological properties of functionalized silver nanoparticles. The stabilizing ligand PAA can be easily exchanged by biomolecules to modify the surface functionality. Replacements of PAA with glutathione (GSH) and bovine serum albumin (BSA) have been performed as examples. We demonstrate that the particles effectively catalyze the reduction of 4-nitrophenol to 4-aminophenol with sodium borohydride. With PAA as stabilizer, the catalytic activity of (436 ± 24) L g-1 s-1 is the highest reported in literature for silver nanoparticles. GSH and BSA passivate the surface substantially resulting in lower catalytic activities of (77.6 ± 0.9) L g-1 s-1 and (3.47 ± 0.50) L g-1 s-1, respectively. The ultra-small particles were already used in the Nano Define project. Due to their small size it is possible to investigate the detection limits of different analytical techniques like electron microcopy, field flow fractionation or single particle tracking. In this project they serve as the calibration standard with the smallest radius.