High-Throughput Synthesis of Uniform Silver Seed Particles by a Continuous Microfluidic Synthesis Platform Controlled silver particle geometries require at least a synthesis in two steps which strongly differ in their reaction kinetics. For the first step, the very fast seed forma- tion, chaotic advection-based micromixers are tested in combination with a batch reactor for the growth step. Nanoparticles with narrow size distribution and excel- lent shape uniformity can be prepared in large batches. To achieve a highly repro- ducible and homogeneous particle solution, a microfluidic system containing three different micromixers for optimal mixing of the chemical precursors is es- tablished, allowing stringent control of every synthesis step. The produced silver particles can be used as seeds for forming anisotropic particles. Their further po- tential is demonstrated by preparation of anisotropic silver triangles. The thus generated seed particles are better suited for growing to triangles than those from conventional batch synthesis. Keywords: Continuous high-throughput synthesis, Microfluidics, Micromixer, Plasmonic nanoparticles, Silver nanoprisms Received: August 29, 2014; revised: November 28, 2014; accepted: March 05, 2015 DOI: 10.1002/ceat.201400524 1 Introduction Metal nanoparticles are becoming favorite objects in current plasmonics [1]. Their extraordinary optical properties in form of sharp optical resonances, i.e., localized surface plasmon reso- nances (LSPR), are based on coherent oscillations of free con- ductive electrons upon irradiating electromagnetic waves [2]. The position of the LSPR and, therefore, the potential applica- tion strongly depends on the material, size, and shape of the nanoparticles [3]. These factors can be adjusted by the chemi- cal synthesis. Different applications as biochip- or cell-label [4, 5], as optical antenna for manipulation of biomolecules [6, 7], or as sensor transducer [8] are based on this potential. The application of gold spheres (LSPR by 520 nm) in bioana- lytics is widely spread [8–10]. These particles are chemically relative stable, their synthesis and biofunctionalization is well- established, and they are well biocompatible. Unfortunately, these particles are limited by their LSPR wavelength range above 520 nm. Triangular-shaped silver nanoparticles offer much higher scattering and absorption efficiency and addition- ally a spectral range tunable over the whole visible and infrared spectral range from 400 to 1200 nm [11] but the chemical syn- thesis of silver triangles is more complex compared to spheres [12–14]. In classical batch synthesis, the resulting nanoparticles are often polydisperse and their plasmonic properties, based on their geometry, are difficult to control. The critical part in the two-step synthesis is the first step, i.e., generation of small crys- talline seed nanoparticles. In general, miniaturization in the synthesis can be helpful to optimize this step [15]. Different strategies of microfluidics starting from a small capillary up to a complex network of different fluid units have been proposed [16, 17]. The main advantage of microfluidic systems is the small dimension of the channels and structures which results in shorter diffusion lengths and a higher surface- to-volume-ratio. Consequently, microsystems allow for a fast exchange of material and energy [5, 13, 18, 19]. For an optimal synthesis, the mixing parameters are of key importance. Changes in mixing conditions can result in uncontrollable col- loid morphologies and/or various sizes. Therefore, with in- creasing complexity of the particle shape, more control during the synthesis is needed. Direct control of the synthesis parame- ters allows the immediate control during the synthesis to reach a homogeneous composition of the mixture [14]. This is the basis for the synthesis of nanoparticles with homogeneous par- ticle size distribution and well-defined particle geometry. A novel system is described with a combination of different microfluidic units to create small and monodisperse silver seed particles utilizing a rather fast reducing agent (NaBH 4 ). Three different micromixers allow optimal mixing conditions of the reaction adducts and overcome diffusion limitations of classical methods and, therefore, variations in particle size and crystal- linity. The resulting defined seed particles enable a defined Chem. Eng. Technol. 2015, 38, No. 7, 1131–1137 ª 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cet-journal.com Matthias Thiele 1 Andrea Knauer 2 Andrea Csa ´ki 1 Daniell Mallsch 1 Thomas Henkel 1 Johann Michael Ko ¨ hler 2 Wolfgang Fritzsche 1 1 Leibniz Institute of Photonic Technology, Jena, Germany. 2 Ilmenau University of Technology, Institute for Physics, Dept. of Physical Chemistry and Micro Reaction Technology, Ilmenau, Germany. – Correspondence: Dr. Andrea Csa ´ki (csaki@ipht-jena.de), Leibniz Institute of Photonic Technology, Albert-Einstein-Straße 9, 07745 Jena, Germany. Research Article 1131