Tuning the magnetoresistance properties of phosphorene with periodic magnetic modulation.

Periodic superlattices constitute ideal structures to modulate the transport properties of two-dimensional materials. In this paper, we show that the tunneling magnetoresistance (TMR) in phosphorene can be tuned effectively through periodic magnetic modulation. Deltaic magnetic barriers are arranged periodically along the phosphorene armchair direction in parallel (PM) and anti-parallel magnetization (AM) fashion. The theoretical treatment is based on a low-energy effective Hamiltonian, the transfer matrix method and the Landauer-Büttiker formalism. We find that the periodic modulation gives rise to oscillating transport characteristics for both PM and AM configurations. More importantly, by adjusting the electrostatic potential appropriately we find Fermi energy regions for which the AM conductance is reduced significantly while the PM conductance keeps considerable values, resulting in an effective TMR that increases with the magnetic field strength. These findings could be useful in the design of magnetoresistive devices based on magnetic phosphorene superlattices.

Active brain targeting of a fluorescent P-gp substrate using polymeric magnetic nanocarrier system.

Magnetic nanoparticles (NP) were developed for the active brain targeting of water-soluble P-glycoprotein (P-gp) substrate rhodamine 123 (Rh123). The NP matrix of poly(lactide-co-glycolide) (PLGA) and methoxy poly(ethyleneglycol)-poly(lactic acid) (M-PEG-PLA) was prepared by single emulsion solvent evaporation of polymers with oleic acid-coated magnetic nanoparticles (OAMNP) and Rh123. All formulations were characterized in terms of morphology, particle size, magnetic content and Rh123 encapsulation efficiency. The maximum encapsulation efficiency of Rh123 was 45 ± 3% and of OAMNP was 42 ± 4%. The brain targeting and biodistribution study was performed on Sprague Dawley rats (3 groups, n = 6). Rh123 (0.4 mg kg(-1)) was administered in saline form, NP containing Rh123, and NP containing Rh123 in the presence of a magnetic field (0.8 T). The fluorimetric analysis of brain homogenates revealed a significant uptake (p < 0.05) of Rh123 in the magnetically targeted group relative to controls. These results were supported by fluorescence microscopy. This study reveals the ability of magnetically targeted nanoparticles to deliver substances to the brain, the permeation of which would otherwise be inhibited by the P-gp system.

The formulation, characterization and in vivo evaluation of a magnetic carrier for brain delivery of NIR dye.

This work reports the targeting of the near infrared (NIR) dye indocyanine green (ICG) to the brain using composite nanoparticles. Thermal decomposition of iron pentacarbonyl was used to synthesize monodisperse oleic acid coated magnetic nanoparticles (OAMNP). Synthesized OAMNP and ICG were encapsulated in a poly (lactide-co-glycolide) matrix using an emulsion evaporation method. Different batches containing OAMNP:PLGA ratios (1:4, 1:2 and 3:4) were prepared with ICG (group B-1, 2, 3) and without ICG (group A-1, 2, 3) loading. All the formulations were characterized in terms of morphology, particle size, zeta potential, magnetic content, ICG encapsulation efficiency and the spectral properties of ICG. The optimized formulation showed an encapsulation efficiency of 56 +/- 4.6% for ICG and 57 +/- 1.37% for OAMNP. The biodistribution and brain targeting study involved three groups of six animals, each with 0.4 mg kg(-1) equivalent of ICG, given as neat ICG solution, composite nanoparticles without the aid of a magnetic field, and composite nanoparticles under the influence of a magnetic field (8000 G) to groups 1, 2 and 3 respectively. The tissue analysis and microscopy images revealed a significantly higher brain concentration of ICG (p < 0.05) for group 3 than the two control groups. These results are encouraging for the brain delivery of hydrophilic dyes/drugs using this method for biomedical applications.

Investigation of the micellar properties of the tocopheryl polyethylene glycol succinate surfactants TPGS 400 and TPGS 1000 by steady state fluorometry.

This study has investigated the micellar properties of the d-alpha-tocopheryl polyethylene glycol succinate esters 400 (TPGS 400) and 1000 (TPGS 1000) in terms of critical micellar concentration (CMC), apparent aggregation number (N(agg)), microviscosity and micropolarity using steady state fluorescence techniques and fluorescent probes. In addition it has compared these properties against those of a Triton-type surfactant such as Triton X-100. The CMC values for TPGS 400, TPGS 1000 and Triton X-100 were 1.51 mM, 0.02 mM and 0.19 mM, respectively. The N(agg) values for TPGS 1000 and Triton X-100 were, respectively, 10 and 63. Due to the cloudiness of its aqueous solutions, no attempt was made to evaluate the microviscosity and to obtain the N(agg) for TPGS 400. Microviscosity at the probe site for all three surfactants was high and rather similar in spite of differences among their CMC values. The ability of TPGS 400 and TPGS 1000 to provide an environment for the fluorescence quenching of micelle-solubilized pyrene appears to be associated with their hydrophobic aromatic domains. To our knowledge the present study is the first attempt at systematically studying the micellar properties of the two most widely used TPGS compounds and of comparing these properties against each other and against those of an established nonionic surfactant.

Analytical models for time resolved fluorescence spectroscopy in tissues.

Laser induced fluorescence is a rapidly growing technique for diagnostics and imaging in scattering material, most notably in in vivo biomedical testing. Most previous applications have relied on the measurements of the steady-state emission spectrum, with subsequent analysis of the spectrum for relative concentrations of potential fluorophores. Only recently a few investigators have explored the use of the fluorescence lifetimes as a diagnostic tool by taking advantage of the perturbation of the lifetime by multiple scattering of the excitation and emission light in the tissue. We have developed a model to study the fluorescence signal generated by fluorophores distributed in a scattering medium. This model is based on two coupled time-dependent photon migration phenomena: the transport of the pulsed source laser light and the transport of the induced fluorescent light excited by the source. The coupling of these two is through the source for the induced fluorescence where the strength of the local fluorescence emission depends on the absorption of the laser intensity at that location. Whereas previous research focused mainly on the fluorescence properties of various dyes, compounds and materials, transport phenomena have only recently been addressed by researchers. We have presented general analytical and numerical solutions for finite, infinite, cylindrical and spherical geometries.

Nanowire array composites.

Long, nanometer-size metallic wires can be synthesized by injection of the conducting melt into nanochannel insulating plates. Large-area arrays of parallel wires 200 nanometers in diameter and 50 micrometers long with a packing density of 5 x 10(8) per square centimeter have been fabricated in this way. When charged, the ends of the wires generate strong, short-range electric fields. The nanowire electric fields have been imaged at high spatial resolution with a scanning force microscope.