Continuous manufacturing of silver nanoparticles between 5 and 80 nm with rapid online optical size and shape evaluation

The physical and chemical properties of metal nanoparticles are strongly dependent on their size and shape. In this work, we present a flexible manufacturing approach for the synthesis of spherical silver nanoparticles with tuneable sizes between 5 to 80 nm. This unique size flexibility is enabled by rapid online characterisation coupling spectroscopy and a mathematical Mie theory-based algorithm for size and shape evaluation. While it is conventionally believed that narrow size distributions require a fast nucleation step, herein, we demonstrate that fast and controllable growth is also required. To achieve this, a combination of chemical and engineering approaches is presented to limit thermodynamically driven size focus, coalescence and secondary nucleation. We show that an optimum reducing agent to silver precursor to seeds ratio and pH range need to be maintained throughout the growth stage. Such demanding conditions can be achieved by accurate control of the feed points and fluid dynamics across a series of microfluidic helical reactors leading to low mixing times. In this way, particle sizes with narrow size distributions and spherical shapes can be easily tuned by just varying the reducing agent-to-precursor concentration in the growth stage in an approach directly applicable to other metal nanoparticles.