Femtosecond laser induced synthesis of Au nanoparticles mediated by chitosan.

This paper reports the synthesis of Au nanoparticles by 30-fs pulses irradiation of a sample containing HAuCl4 and chitosan, a biopolymer used as reducing agent and stabilizer. We observed that it is a multi-photon induced process, with a threshold irradiance of 3.8 × 10(11) W/cm2 at 790 nm. By transmission electron microscopy we observed nanoparticles from 8 to 50 nm with distinct shapes. Infrared spectroscopy indicated that the reduction of gold and consequent production of nanoparticles is related to the fs-pulse induced oxidation of hydroxyl to carbonyl groups in chitosan.

Two-photon polymerization for fabricating structures containing the biopolymer chitosan.

Two-photon polymerization is a powerful tool for fabricating three-dimensional micro/nano structures for applications ranging from nanophotonics to biology. To tailor such structure for specific purposes it is often important to dope them. In this paper we report on the fabrication of structures, with nanometric surface features (resolution of approximately 700 nm), using two-photon polymerization of an acrylic resin doped with the biocompatible polymer chitosan using a guest-host scheme. The fluorescence background in the Raman spectrum indicates the presence of chitosan throughout the structure. Mechanical characterization reveals that chitosan does not affect the mechanical properties of the host acrylic resin and, consequently, the structures exhibit excellent integrity. The approach presented in this work can be used in the fabrication of micro- and nanostructures containing biopolymers for biomedical applications.

Excited state absorption spectrum of chlorophyll a obtained with white-light continuum.

The study of excited state properties of chlorophyll a is a subject of foremost interest, given that it plays important roles in biological process and has also been proposed for applications in photonics. This work reports on the excited state absorption spectrum of chlorophyll a solution from 460 to 700 nm, obtained through the white-light continuum Z-scan technique. Saturation of absorption was observed due to the ground state depletion, induced by the white-light continuum region that is resonant with the Q band of chlorophyll a. The authors also observed reverse saturation of absorption related to the excitation from the first excited state to a higher energy level for wavelengths below 640 nm. An energy-level diagram, based on the electronic states of chlorophyll a, was employed to interpret their results, revealing that more states than the ones related to the Q and B bands participate in the excited state absorption of this molecule.

Estimation of the dose to the nursing infant due to direct irradiation from activity present in maternal organs and tissues.

Radionuclides deposited internally in the mother will give rise to a radiation dose in the infant in two ways. The radionuclides may be transferred through milk and give rise to an internal dose in the infant, or the radionuclides may emit photons that are absorbed by the infant, giving rise to an external dose. In this paper, the external dose to the newborn infant caused by direct irradiation was estimated for monoenergetic photons. Voxel models (also called voxel phantoms) of the mother and infant were made in three geometries. These models, consisting of volume elements, or voxels, were designed so that the infant model was placed in the lap, at the breast and on the shoulder of the mother model. The Visual Monte Carlo (VMC) code was used to transport the photons through the voxel models. Source regions for the emitted photons, such as the whole body, the thyroid, the lung, the liver and the skeleton, were chosen. For the validation of the calculation procedure, VMC results were favourably compared with the results obtained by using other Monte Carlo programs and also with the previously published results for specific absorbed fractions. This paper provides estimates of the external dose per photon to the infant for photon energies between 0.05 and 2.5 MeV. The external dose per photon estimates were made for the three geometries and for the sources listed above. The results show that, for the geometry of the nursing infant model at the breast, the highest dose to the infant per photon comes from radionuclides deposited in the mother's liver. For the nursing infant model at the shoulder, the highest dose to the infant per photon comes from radionuclides deposited in the mother's thyroid, and for the nursing infant model in the lap, the highest dose to the infant per photon comes from radionuclides deposited uniformly in the whole body. The dose per photon results were then used to estimate the dose an infant might receive over the lactation period (6 months) due to the incorporation of 1 Bq of a radionuclide by the mother. This information may be used to provide external dose estimates to the infant in the case of a known or suspected radionuclide incorporation by the mother due to, for example, a nuclear medicine procedure.

In situ UV-vis absorbance measurements for Langmuir films of poly[4'-[[2-(methacryloyloxy)-ethyl]ethylamino]-2-chloro-4-nitroazobenzene] (HPDR13) azopolymer.

In situ UV-vis absorbance measurements are used to investigate aggregation in Langmuir films from the azopolymer poly[4'-[[2-(methacryloyloxy)-ethyl]ethylamino]-2-chloro-4-nitroazobenzene] (HPDR13), a methacrylate derivative of DR13. The level of aggregation in a Langmuir film is intermediate between that of HPDR13 in chloroform solution and in a deposited LB film, as expected. Absorption is negligible at large areas per monomer, and starts to increase at a critical area that is the same as the one obtained in surface potential isotherms, being close to twice the area per monomer for a condensed film. This indicates that the onset for light absorption coincides with a critical packing density where monolayer structuring occurs and there is a sharp change in the effective dielectric constant of the film/water interface. Consistent with a featureless pressure-area isotherm for HPDR13, denoting no significant molecular rearrangement upon film compression, the UV-vis spectra did not vary with the surface pressure. The intensity of absorbed light increased, though, as the film was compressed owing to a higher density of chromophores. At higher subphase temperatures, larger flexibility of HPDR13 chains led to a more compact arrangement, causing the area per monomer to decrease and the absorbed light to increase-with approximately opposite trends.

The validation of organ dose calculations using voxel phantoms and Monte Carlo methods applied to point and water immersion sources.

The Monte Carlo program 'Visual Monte Carlo-dose calculation' (VMC-dc) uses a voxel phantom to simulate the body organs and tissues, transports photons through this phantom and reports the absorbed dose received by each organ and tissue relevant to the calculation of effective dose as defined in ICRP Publication 60. This paper shows the validation of VMC-dc by comparison with EGSnrc and with a physical phantom containing TLDs. The validation of VMC-dc by comparison with EGSnrc was made for a collimated beam of 0.662 MeV photons irradiating a cube of water. For the validation by comparison with the physical phantom, the case considered was a whole body irradiation with a point 137Cs source placed at a distance of 1 m from the thorax of an Alderson-RANDO phantom. The validation results show good agreement for the doses obtained using VMC-dc and EGSnrc calculations, and from VMC-dc and TLD measurements. The program VMC-dc was then applied to the calculation of doses due to immersion in water containing gamma emitters. The dose conversion coefficients for water immersion are compared with their equivalents in the literature.