Numerous different photonics and biomedical applications depend on the fluorescent polymer micro- and nanoparticles. Besides optical or spectroscopic properties, the performance of the polymer nanoparticles is determined by their size, size distribution, and surface charge. Moreover, in order to realize a very uniform performance, the functional polymer nanoparticles should be of high homogeneity and demand for the preparation in a minimum number of synthesis steps. Here, we present a microfluidic-assisted synthesis of different types of reproducible fluorescent polymer nanoparticles with tuned size (40 nm up to 600 nm) and surface charge (ζ potential=-52 mV up to +45 mV). Four different preparation strategies were introduced for fluorophore-functionalized nanoparticles: (a) noncovalent binding of fluorophores with high loading, (b) covalent linking of fluorophores with enhanced stability, (c) surface-anchored fluorophores by hydrophobic interactions for triple function at the same time, and (d) surface immobilization of biomolecules and fluorophore by ionic as well as secondary interactions. In this way, four different classes of nanoparticles suited for different applications were prepared with a spherical shape as a model system. Moreover, the principle has been extended to the different types of nonspherical and composite polymer nanoparticles.
In this work, a wet-chemical synthesis method for gold-silver core-shell particles with nanometer precise adjustable silver shell thicknesses is presented. Typically wet-chemical syntheses lead to relatively large diameter size distributions and losses in the yield of the desired particle structure due to thermodynamical effects. With the here explained synthesis method in micro fluidic segment sequences, a combinatorial in situ parameter screening of the reactant concentration ratios by programmed flow rate shifts in conjunction with efficient segment internal mixing conditions is possible. The highly increased mixing rates ensure a homogeneous shell deposition on all presented gold core particles while the amount of available silver ions was adjusted by automated flow rate courses, from which the synthesis conditions for exactly tunable shell thicknesses between 1.1 and 6.1 nm could be derived. The findings according to the homogeneity of size and particle structure were confirmed by differential centrifugal sedimentation (DCS), scanning and transmission electron microscopy (SEM, TEM) and X-ray photoelectron spectroscopy (XPS) measurements. In UV-Vis measurements, a significant contribution of the core metal was found in the shape of the extinction spectra in the case of thin shells. These results were confirmed by theoretical calculations.
The well-known concept of electrical charging for the stabilization of colloidal solutions is extended to a general concept for explanation of behavior of nanoparticles during their formation, in their response behavior on chemical or electrochemical stress, and in particle/ particle interactions. This concept helps to understand the formation of polynuclear sphere-like plasmonic nanoparticles of gold, the surprising high stability of extended flat silver nanoprisms, and their switch-like transformation as well as the formation of special types of nanoparticle assemblies. It can be used for controlling the size of nanoparticles, for the synthesis of composite micro and nanoparticles, and for the generation of different classes of nonspherical polymer particles by particle/particle interactions during the process of particle growth. The reported experimental examples were mainly obtained by microfluidic experiments, which supplied a high homogeneity of the product particles. This high homogeneity is an excellent basis for studying the nanoparticle behavior in analogy to molecular processes and helps to get new insights in to the importance of electrical effects for nanoparticle synthesis and nanoparticle assembling.
The residence time distribution (RTD) characteristics of three microreactors containing different passive mixing structures, namely, a three dimensional serpentine structure, a split-and-recombine structure and a staggered herringbone structure, were investigated and compared. An experimental input-response technique was applied which required deconvolution of the measured data by modeling of the RTD. The proposed technique provides useful information on optimized application and operation of microfluidic devices. The serpentine reactor and the split-and-recombine reactor show improvement in RTD behaviour, i.e., narrowing of RTD curves, at Re-numbers > 30 due to effective transversal mixing and therefore reduced axial dispersion. In the case of the staggered herringbone structure, dead volumes could be observed which considerably affect the RTD.
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.
A double-layer chemo-chip for the characterization of liquid analytes by rapid fluorimetric imaging is described. The chemo-chip consists of an array of polymeric micro-spots prepared on a glass slide. Each spot is composed of a thin indicator layer made of PVA doped with an immobilized fluorescence dye and a top layer polymer spot with different permeation properties. The analytes can be differentiated by variations in the optical response rate of the indicator dye after its application. Consequently, different cross-linker concentrations were applied using the Nano-Plotter((TM)) which formed top layers of varying permeability. The chemo-chips were tested with the aqueous solutions of two model liquids (aqueous solutions of malonic acid and phenanthroline hydrochloride). It was found that the transition time of response had changed considerably (up to a factor of about 10) depending on different local cross linking degrees. This has resulted in time-dependent fluorescent patterns of the fluorescence images of the micro-array. The response was fast and the transition times were in the range between a few seconds to 30 s.
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