ABSTRACT
A new photoacoustic (PA) cell design, which is particularly suitable for investigations of liquids, gels, and outgassing samples is presented. The setup is based on a PA cell of only 78.5 mm(3) volume, which is sealed on the sample side with either a 163 µm thick chemical vapor deposition diamond window or a 3.91 µm thin diamond membrane. This design offers great advantages compared to traditionally used open-ended PA cells especially when investigating volatile compounds. The new PA cell design is particularly interesting in the studies of biological samples characterized by a high water content. The performance was demonstrated with mid-infrared PA measurements of glucose in aqueous solutions using a tunable quantum-cascade laser as a light source. A detection limit of 100 mg/dl (SNR = 3) has been achieved. Furthermore, the spectral changes of glucose dissolved in water caused by mutorotation have been monitored time-resolved.
ABSTRACT
We study numerically two-dimensional nanoparticles with a non-regular shape and demonstrate that these particles can support many more plasmon resonances than a particle with a regular shape (e.g. an ellipse). The electric field distributions associated with these different resonances are investigated in detail in the context of near-field microscopy. Depending on the particle shape, extremely strong and localized near-fields, with intensity larger than 105 that of the illumination wave, can be generated. We also discuss the spectral dependence of these near-fields and show that different spatial distributions are observed, depending which plasmon resonance is excited in the particle.
ABSTRACT
We study the coupling induced by retardation effects when two plasmon-resonant nanoparticles are interacting. This coupling leads to an additional resonance, the strength of which depends on a subtle balance between particle separation and size. The scattering cross section and the near field associated with this coupled resonance are studied for cylindrical particles in air and in water. Implications for surface-enhanced Raman scattering and nano-optics are discussed.
ABSTRACT
We investigate the plasmon resonances of interacting silver nanowires with a 50 nm diameter. Both non-touching and intersecting configurations are investigated. While individual cylinders exhibit a single plasmon resonance, we observe much more complex spectra of resonances for interacting structures. The number and magnitude of the different resonances depend on the illumination direction and on the distance between the particles. For very small separations, we observe a dramatic field enhancement between the particles, where the electric field amplitude reaches a hundredfold of the illumination. A similar enhancement is observed in the grooves created in slightly intersecting particles. The topology of these different resonances is related to the induced polarization charges. The implication of these results to surface enhanced Raman scattering (SERS) are discussed.
ABSTRACT
We study the plasmon resonances of 10 (nm)--100 (nm) two-dimensional metal particles with a non-regular shape. Movies illustrate the spectral response of such particles in the optical range. Contrary to particles with a simple shape (cylinder, ellipse) non-regular particles exhibit many distinct resonances over a large spectral range. At resonance frequencies, extremely large enhancements of the electromagnetic fields occur near the surface of the particle, with amplitudes several hundred-fold that of the incident field. Implications of these strong and localized fields for nano-optics and surface enhanced Raman scattering (SERS) are also discussed.
ABSTRACT
A new method for the selective removal of traces of molybdenum from growth media of N2-fixing bacteria (Rhodobacter capsulatus and Klebsiella pneumoniae) was developed. This method is based on the filtration of nutrient solutions through a layer of activated carbon (pulverized charcoal). The adsorption of Mo (molybdate) to activated carbon was optimal if a charcoal suspension (50 g/liter) was degassed by boiling before use and if the pH of the solutions, which had to be purified, was adjusted to values between 1.5 and 4. In this pH region no or only negligible amounts of other metal ions were adsorbed. The activated carbon method was compared with other Mo-eliminating procedures, including 8-hydroxyquinoline/dichloromethane extraction, Chelex 100 chromatography, and treatment with Mo-starved Azotobacter vinelandii cells. The activated carbon filtration appeared to be the most effective, specific, and rapid method. Whereas the untreated Rhodobacter growth medium was contaminated with 1.2 ppb Mo, as analyzed by inductively coupled plasma mass spectrometry (ICP-MS), the activated carbon-treated medium was below the ICP-MS detection limit (less than 0.05 ppb). A similarly effective removal of Mo impurities was obtained by the Azotobacter treatment. Even at low optical densities (2-5 at 436 nm) Mo traces were removed very rapidly within 10-15 min. However, because the Mo uptake/Mo adsorption capacity of A. vinelandii depended on freshly cultivated cells and on the growth phase at which the cells were harvested, this microbiological method was generally more time-consuming and less reproducible than the activated carbon method.(ABSTRACT TRUNCATED AT 250 WORDS)
