Silicon is a high refractive index material. Consequently, silicon nanowires (SiNWs) with diameters on the order of the wavelengths of visible light show strong resonant field enhancement of the incident light, so this type of nanomaterial is a good candidate for all kinds of photonic devices. Surprisingly enough, a thorough experimental and theoretical analysis of both the polarization dependence of the absorption and the scattering behavior of individual SiNWs under defined illumination has not been presented yet. Here, the present paper will contribute by showing optical properties such as scattering and absorption of individual SiNWs experimentally in an optical microscope using bright- and dark-field illumination modes as well as in analytical Mie calculations. Experimental and calculation results are in good agreement, and both reveal a strong correlation of the optical properties of individual SiNWs to their diameters. This finding supports the notion that SiNWs can be used in photonic applications such as for photovoltaics or optical sensors.
Rapid and effective methods of pathogen identifications are of major interest in clinical microbiological analysis to administer timely tailored antibiotic therapy. Raman spectroscopy as a label-free, culture-independent optical method is suitable to identify even single bacteria. However, the low bacteria concentration in body fluids makes it difficult to detect their characteristic molecular fingerprint directly in suspension. Therefore, in this study, Raman spectroscopy is combined with dielectrophoresis, which enables the direct translational manipulation of bacteria in suspensions with spatial nonuniform electrical fields so as to perform specific Raman spectroscopic characterization. A quadrupole electrode design is used to capture bacteria directly from fluids in well-defined microsized regions. With live/dead fluorescence viability staining, it is verified, that the bacteria survive this procedure for the relevant range of field strengths. The dielectrophoretic enrichment of bacteria allows for obtaining high quality Raman spectra in dilute suspensions with an integration time of only one second. As proof-of-principle study, the setup was tested with Escherichia coli and Enterococcus faecalis, two bacterial strains that are commonly encountered in urinary tract infections. Furthermore, to verify the potential for dealing with real world samples, pathogens from patients' urine have been analyzed. With the additional help of multivariate statistical analysis, a robust classification model could be built and allowed the classification of those two strains within a few minutes. In contrast, the standard microbiological diagnostics are based on very time-consuming cultivation tests. This setup holds the potential to reduce the crucial parameter diagnosis time by orders of magnitude.
A new scheme for the detection of molecular interactions based on optical readout of nanoparticle labels has been developed. Capture DNA probes were arrayed on a glass chip and incubated with nanoparticle-labeled target DNA probes, containing a complementary sequence. Binding events were monitored by optical means, using reflected and transmitted light for the detection of surface-bound nanoparticles. Control experiments exclude significant influence of nonspecific binding on the observed contrast. Scanning force microscopy revealed the distribution of nanoparticles on the chip surface.
In this work, a multistep microcontinuous flow-through synthesis procedure for the generation of homogeneous, high-quality silver nanoprisms is presented. The particle synthesis is based on the wet chemical reduction of silver nitrate in the presence of the polyanionic polymer poly(sodium styrenesulphonate). To obtain a high yield of homogeneous prism-shaped Ag nanoparticles with a triangular base, two main experimental steps are necessary. The first step is the synthesis of seed particles. To match the quality criteria for small, homogeneous seed particles, the synthesis was carried out in a microcontinuous flow-through system. Constant residence times and an effective mixing of the reactants were realized by the application of the microsegmented flow technique. The advantage of good reactant mixing was also adapted in the second experimental step. The growth of silver nanoprisms by reduction of silver nitrate on the noncapped surfaces of the seed particles was again carried out within microfluid segments during a continuous flow-through synthesis. The obtained colloidal solutions of both, Ag seeds and Ag nanoprisms, were analyzed using differential centrifugal sedimentation, UV–vis spectrophotometry, and scanning electron microscopy. The size distributions of the product particles of the individual process steps were extremely narrow. For the Ag seed particles, an average particle diameter of 3.8 nm with a distribution half-width of 2.3 nm was found. The edge length of the Ag nanoprisms could be varied between 35 and 180 nm, while the size distribution remained narrow and the yield of particles of the desired shape high. Because of the strong sensitivity of the optical properties of the nanoprisms from the geometrical aspects, Ag nanoprisms promise a high potential for sensor applications. Constraints on nanoparticles presented by these applications, such as uniformity and narrow size distributions, can be met by microreaction technology. In particular, by applying a microsegmented flow, an improvement of the product quality can be achieved because of the enhanced segment-internal mixing and the suppression of a residence time distribution.
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