Defined Synthesis of Plasmonic Nanoparticles Using Microfluidics
Plasmonic nanoparticles play an important role in photonic bioanalytics. Their optical properties depend strongly, among others, on the particle size distribution and particle shape which can be adjusted by a multistage microfluidic synthesis. With this method, it is possible to reproducibly manufacture shape-anisotropic nanoparticles.
Plasmonic nanoparticles play an increasingly important role in medicine and bioanalytics. In particular their optical properties (plasmon resonance) allow their usage as optical markers or as a sensor component. The so-called localized surface plasmon resonance (LSPR) – based on a collective oscillation of the conduction band electrons – is a property adjustable by chemical (colloidal) synthesis. In addition to the material, the geometry of the particles plays an important role. The production of nanoparticles of precise size and shape is a complex process that depends on many factors. Therefore, a uniform production of the particles is possible only to a limited extent in classic batch processes. Microfluidic synthesis allows a better and more reproducible manufacturing of shape-anisotropic nanoparticles. These particles allow an improved performance in terms of optical sensor technology with higher sensitivities1.
To achieve this goal, the department linked the competencies in microfluidics and microsystems technology with the expertise in the chemical synthesis of plasmonic nanoparticles. For this purpose, various micro flow reactors (in particular static mixers with channel widths in the micrometer range) were combined in order to implement more precise reaction conditions. Various micromixers were selected and tested for the different fluid streams and reaction kinetics and combined with each other for the individual reactions. In addition, the continuous microfluidic process control was compared to the microfluid segment technique and the classic batch process.
For the synthesis of various types of anisotropic nanoparticles, different combinations of the three above-mentioned approaches emerged as the optimum. While the two-step synthesis of silver triangular prisms requires a microfluidic (continuous or segmented) approach only in the first stage2, the production of gold nanocubes and its three-stage process is an even more complex procedure3. In general, the first stage of the multistage syntheses is a critical step that can be reproducibly implemented only by the application of microfluidics. The so-called seed particles must be small, defined, and preferably monocrystalline to enable the growth of defined anisotropic particles. Here, microfluidic synthesis offers a more homogeneous and faster mixing of the reactants and thermal management, lower material requirements, and shorter diffusion paths, thereby making reaction control easier and faster. This achieves a higher efficiency in the synthesis yield, for example, in silver triangular prisms from <70% to > 90% during the growth phase as compared to batch synthesis.
By means of optimized microfluidic synthesis, better sensors can be implemented based on the LSPR of the particles. Furthermore, particles prepared in this way exhibit higher sensitivities. In order to minimize material-related but undesired aging processes, a thin silica coating of the particles has been developed which stabilizes the particles4. This affects the sensitivity of the particles, but the sensory properties remain permanent.
1 A. Csáki, M. Thiele, J. Jatschka, A. Dathe, D. Zopf, O. Stranik and W. Fritzsche, Eng. Life Sci., 2015, 15, 266-275.
2 M. Thiele, A. Knauer, A. Csáki, D. Mallsch, T. Henkel, J. M. Köhler and W. Fritzsche, Chemical Engineering & Technology, 2015, 38, 1131-1137.
3 M. Thiele, J. Z. E. Soh, A. Knauer, D. Malsch, O. Stranik, R. Müller, A. Csáki, T. Henkel, J. M. Köhler and W. Fritzsche, Chemical Engineering Journal, 2016, 288, 432-440.
4 M. Thiele, I. Götz, S. Trautmann, R. Müller, A. Csáki, T. Henkel and W. Fritzsche, Materials Today: Proceedings, 2015, 2, 33-40.