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We report on the formation of polymer-stabilized superparamagnetic single-core and multi-core maghemite nanoparticles. The particle formation was carried out by coprecipitation of Fe(II) and Fe(III) sulfate in a continuous aqueous process using a micromixer system. Aggregates containing 50 primary particles with sizes of 2 nm were formed at a reaction temperature of 30 °C. These particles aggregated further with time and were not stable. In contrast, stable single-core particles with a diameter of 7 nm were formed at 80 °C as revealed by small-angle X-ray scattering (SAXS) coupled in-line with the micromixer for particle characterization. X-ray diffraction and TEM confirmed the SAXS results. X-ray absorption near-edge structure spectroscopy (XANES) identified the iron oxide phase as maghemite.
Continuous-flow process conditions for the reduction to gold nanoparticles were varied following a variation scheme aiming at impacting each of the elementary steps–mixing, reaction, polymer diffusion and polymer binding. While the last three variations are done by proper material choice (reducer; polymeric stabilizer), flow processing using a multilamination micromixer intensified the mass transfer in all these steps and particularly improved the first mixing step of the reactants. As reducers L-ascorbic acid (H2Asc) and sodium borohydride (NaBH4) were used and as gold particle precursor chloroauric acid (HAuCl4). All processing runs were analysed by plasmon absorption and transmission electron microscopy to reveal size, shape and polydispersity. For the HAuCl4-H2Asc system, the smallest particles were 0.84 nm (7.5 mL/min) and 0.63 nm (10.0 mL/min) for the HAuCl4-NaBH4 system. This is at the upper edge of performance reported for batch systems in literature, i.e. among the smallest gold particles generated. While the particles made for the HAuCl4-H2Asc system were polydisperse and polyshaped for most process conditions (however being spherical and relatively uniform at the two largest flow rates investigated), processing with the HAuCl4-NaBH4 system resulted in particles sizes with small mean deviation and almost ideal spherical shape. With polyvinyl alcohol (PVA) as stabilizer, stable colloids were obtained for one week (violet coloured) up to at least 2 months (pink coloured) in case of reaction with ascorbic acid, and for at least 1 month in case of reaction with NaBH4.