Volume 3, Issue 1 - Special issue: Nanomaterials in microreactors / Issue editor: J. Michael Koehler

The possible diversity of nanoparticles is extremely high. This variability corresponds to a huge potential of possible functions in future materials and nanotechnical devices. Apart from rational designs, there is an urgent need for screening strategies for specific nanoparticle properties. Miniaturized screening techniques are challenged for efficient screening procedures. The review gives an overview on the possibilities of tuning and screening of nanoparticle properties and focuses on the application of microfluidic techniques for nanoparticle synthesis. Furthermore, the variation of parameters during the generation of nanoparticles and its connection with the resulting nanoparticle properties are highlighted as well. Among other microfluidic techniques, microsegmented flow is particularly promising for the synthesis of different types of homogeneous nanoparticles and offers interesting approaches for the screening of process parameters and nanoparticle properties. The tuning of reactant concentrations in the micro-flow-through synthesis of nonspherical silver particles and gold/silver core/shell particles and their effects on the shape, size, and optical parameters of the formed nanoparticles are given as examples for the application of microsegmented flow in the screening of nanoparticle properties.

In the field of microfluidic particle synthesis, significant progress has been made during the past decade. Excellent control of the process and flow conditions allows the synthesis of particulate materials with unique properties, which are not achievable with conventional techniques. In this paper, we give a brief overview of the main strategies for microfluidic syntheses of polymer particles and microcapsules on the basis of continuous droplet formation as the initial process step. Additionally, we outline the major challenges, which have to be overcome to allow the fabrication of large quantities of novel microparticular products.

Lab-on-a-chip (LOC) systems are extensively used in recent times for applications in nanotechnology ranging from synthesis of nanomaterials to their utilization in catalysis, biomedicine, and drug delivery. A variety of nanomaterials – inorganic materials such as metal, metal oxide, quantum dots, and organic materials based on polymers and biological molecules – have been synthesized and their applications explored based on LOC devices. Among several inorganic nanomaterials, the applications of LOC devices for gold-based nanomaterials have been widely investigated over the past couple of decades. Though the synthesis and applications of inorganic nanomaterials using these systems have been thoroughly reviewed earlier, including those from our group, there are only a few recent review articles that cover gold-based nanomaterials. As the promise of supported gold nanoparticles (NPs) as exceptionally effective catalysts is beginning to be realized, LOC-based approach for continuous flow gold catalysis has begun to be exploited. Here, in this review, we focus on synthesis and catalysis applications of nanostructured gold using the LOC systems. With millifluidics-based LOCs gaining traction, this review fulfills the need for a comprehensive analysis covering both traditional microfluidics as well as recent millifluidics for catalysis applications utilizing gold nanomaterials.

This paper reviews recent developments on the synthesis of noble metal nanoparticles in micro and millifluidic devices and their catalytic application in organic flow synthesis. A variety of synthesis methods using microfluidics is presented for gold, silver, palladium, platinum, and copper nanoparticles, including the formation in single-phase flows and multiphase flows. In the field of organic chemistry, metal nanoparticles can be used as catalysts. This can lead to remarkably improved reaction performance in terms of minimizing the reaction time and higher yields. In this context, various applications of those metal nanoparticles as catalysts in microfluidic devices are highlighted at selected examples. As a new direction and operational window, nanocatalysts may be synthesized in situ in flow and directly utilized in an organic synthesis. This allows making use of highly active, yet instable catalyst species, which may only have a very short life of a few seconds – a type of flashed nanocatalyst organic synthesis.

The optical, chemical, and catalytic properties of metallic nanoparticles (NPs) depend strongly on their particle size and shape. Therefore, the preparation of monodisperse metallic NPs is very important for fundamental studies and practical applications. However, the isolation of the different structures by separation from a polydisperse sample, especially in the size range below 10 nm, is not well applied so far. Here, the asymmetric flow field flow fractionation (AF4) is adapted for the preparative separation of the Pd NPs regarding their size and shape in the sub-10-nm size range. To prove the efficiency of the applied method, small-angle X-ray scattering (SAXS) and high-resolution transmission electron microscopy (TEM) were utilized to determine the particle size distribution at different stages of the separation process. A major benefit of this method compared to most of the other separation techniques, the removal of impurities during the separation process, was proven by proton nuclear magnetic resonance (NMR). The obtained results demonstrate that the AF4 is well suited for the rapid preparation of the purified uniform precious metal NPs at the applied size range. Single fractions of the different-sized and -shaped Pd NPs were deposited on titania (TiO 2 ) and tested in the catalytic hydrogenation of 2,5 hydroxymethylfurfural (HMF) in aqueous solution under mild conditions. While the spherical-shaped particles show a high activity, the separated agglomerated particles show a higher selectivity to the desired products.

Nanoparticles (NP) have specific catalytic properties, which are influenced by parameters like their size, shape, or composition. Bimetallic NPs, composed of two metal elements can show an improved catalytic activity compared to the monometallic NPs. We, herein, report on the selective aerobic oxidation of benzyl alcohol catalyzed by unsupported Pd/Au and Pd NPs at atmospheric pressure. NPs of varying compositions were synthesized and characterized by UV/Vis spectroscopy, transmission electron microscopy (TEM), and small-angle X-ray scattering (SAXS). The NPs were tested in the model reaction regarding their catalytic activity, stability, and recyclability in batch and continuous procedure. Additionally, in situ extended X-ray absorption fine structure (EXAFS) measurements were performed in order to get insight in the process during NP catalysis.