Metallic nanoparticles of tailored shape and size are important building blocks in several nascent but promising applications ranging from in-vivo cancer detection and therapy to those bordering on the realm of science fiction, such as plasmonic invisibility cloaks. The success of most such applications strongly depends on uniformity in particle size and shape. Bottom-up solution-phase techniques, involving the reduction of metal salts in the presence of stabilizing and/or shape-directing molecules are the most commonly used methods to obtain metallic nanoparticles. Particle formation in such reactions is typically kinetically controlled. Batch reactors such as stirred vials are widely employed for solution-phase synthesis, and are prone to severe reproducibility problems. Poorly controlled reactant addition, mixing, and temperature profiles commonly lead to polydisperse particle populations, and necessitate several time and cost-intensive classification steps.

Continuous-flow synthesis methods can potentially overcome several critical drawbacks of batch methods, leading to favorable process economics and product viability. However, new and significant engineering challenges arise in the implementation of continuous-flow processes to manufacture metallic nanoparticles. In this paper, we first outline the principles of reaction engineering relevant to the design of flow reactors for nanoparticle synthesis, with a focus on microreactor processes. We describe the design and fabrication of both single and multiphase microreactors, and compare their salient features for the synthesis of gold nanoparticles of tailored size and shape. We also outline how reactor materials selection is a critical issue on several fronts, and can dictate the final process strategy employed. Finally, we will present our perspective on the current state-of-art and the road ahead for microreactor processes for nanoscale products.