Cation exchange synthesis and optoelectronic properties of type II CdTe-Cu2-xTe nano-heterostructures

Rod-shaped CdTe-Cu2-xTe nano-heterostructures with tunable dimensions of both sub-units and a type II band alignment were prepared by Cd2+/Cu+ cation exchange. The light absorption properties of the heterostructures are dominated by the excitonic and plasmonic contributions arising, respectively, from the CdTe and the Cu2-xTe sub-units. These results were confirmed over a wide range of sub-unit length fractions through optical modelling based on the discrete dipole approximation (DDA). Although assuming electronically independent sub-units, our modelling results indicate a negligible ground state interaction between the CdTe exciton and the Cu2-xTe plasmon. This lack of interaction may be due to the low spectral overlap between exciton and plasmon, but also to localization effects in the vacancy-doped sub-unit. The electronic interaction between both sub-units was evaluated with pump-probe spectroscopy by assessing the relaxation dynamics of the excitonic transition. In particular, the CdTe exciton decays faster in the presence of the Cu2-xTe sub-unit, and the decay gets faster with increasing its length. This points towards an increased probability of Auger mediated recombination due to the high carrier density in the Cu2-xTe sub-unit. This indication is supported through length-fraction dependent band structure calculations, which indicate a significant leakage of the CdTe electron wavefunction into the Cu2-xTe sub-unit that increases along with the shortening of the CdTe sub-unit, thus enhancing the probability of Auger recombination. Therefore, for the application of type II chalcogenide-chalcogenide heterostructures based on Cu and Cd for photoenergy conversion, a shorter Cu-based sub-unit may be advantageous, and the suppression of high carrier density within this sub-unit is of high importance

Electrospinning of nanofibers in the presence of surfactants and surfactant aggregates

Electrospun nanofibers have improved physicochemical properties compared to macroscalar fibers and are therefore increasingly investigated for use in novel food packaging systems. The objectives of this study were to electrospin nanofibers and to evaluate the effect and feasibility of surfactants and surfactant aggregates to modulate properties and functionalities of electrospun nanofibers. Nanofibers were fabricated by electrospinning a mixture of cationic chitosan and noncharged poly(ethylene oxide) (PEO) in aqueous acetic acid. To improve spinnability and nanofiber morphologies, surfactants were added to biopolymer/polymer solutions and their influence was investigated. Solution properties were evaluated by rheological, surface tension, and conductivity measurements. Fibers were characterized by scanning and transmission electron microscopy, differential scanning calorimetry, and Fourier transform infrared spectroscopy. Addition of PEO and surfactants induced spinnability producing larger fibers with diameters ranging from 40 to 240 nm, while pure chitosan did not form fibers and was instead deposited as beads. Compositional analysis suggested that nanofibers consisted of all solution constituents with chitosan concentrations being significantly lower in fibers than in solution, indicating that surfactants may have decreased polymer-polymer interactions responsible for entanglement. Poly(vinyl alcohol) nanofibers were used as novel delivery system for eugenol carrying Surfynol®465 micelles (microemulsion). Solution properties were not significantly altered after addition of microemulsion regardless of surfactant and/or eugenol concentration. Transmission electron imaging revealed a homogeneous distribution of microemulsion throughout the nanofibers. Release studies suggested a burst release mechanism of encapsulated eugenol microemulsion, potentially due to hydrophilicity of the polymeric carrier, while faster release was observed in samples with a higher eugenol loading ratio in the microemulsion. Antimicrobial activity of produced nanofibers carrying phytophenol microemulsions was evaluated against two strains of Salmonella Typhimurium and Listeria monocytogenes. Overall, the functionalized nanofibers had higher antimicrobial efficacies against Gram-negative than Gram-positive bacterial strains and were also more effective than pure eugenol microemulsion added at respective concentrations to the test system possibly due to a faster exhaustion and loss of antimicrobial activity in free microemulsions. Results of this study suggest that composite solutions of biopolymers, synthetic polymers, and micellar surfactant solutions can be successfully electrospun potentially offering a new means to functionalize biopolymeric nanofibers

Modelling of a targeted nanotherapeutic ‘stroma’ to deliver the cytokine LIF, or XAV939, a potent inhibitor of Wnt–β-catenin signalling, for use in human fetal dopaminergic grafts in Parkinson’s disease

The endogenous reparative capacity of the adult human brain is low, and chronic neurodegenerative disorders of the central nervous system represent one of the greatest areas of unmet clinical need in the developing world. Novel therapeutic strategies to treat them include: (i) growth factor delivery to boost endogenous repair and (ii) replacement cell therapy, including replacing dopaminergic neurons to treat Parkinson’s disease (PD). However, these approaches are restricted not only by rapid degradation of growth factors, but also by the limited availability of cells for transplant and the poor survival of implanted cells that lack the necessary stromal support. We therefore hypothesised that provision of a transient artificial stroma for paracrine delivery of pro-survival factors could overcome both of these issues. Using leukaemia inhibitory factor (LIF) – a proneural, reparative cytokine – formulated as target-specific poly(lactic-co-glycolic acid) (PLGA) nano-particles (LIF-nano-stroma), we discovered that attachment of LIF-nano-stroma to freshly isolated fetal dopaminergic cells improved their survival fourfold: furthermore, in vivo, the number of surviving human fetal dopaminergic cells tended to be higher at 3 months after grafting into the striatum of nude rats, compared with controls treated with empty nanoparticles. In addition, we also analysed the effect of a novel nano-stroma incorporating XAV939 (XAV), a potent inhibitor of the developmentally important Wnt–β-catenin signalling pathway, to investigate whether it could also promote the survival and differentiation of human fetal dopaminergic precursors; we found that the numbers of both tyrosine-hydroxylase-positive neurons (a marker of dopaminergic neurons) and total neurons were increased. This is the first demonstration that LIF-nano-stroma and XAV-nano-stroma each have pro-survival effects on human dopaminergic neurons, with potential value for target-specific modulation of neurogenic fate in cell-based therapies for PD

Fabrication, Functionalization, and Application of Electrospun Biopolymer Nanofibers

The use of novel nanostructured materials has attracted considerable interest in the food industry for their utilization as highly functional ingredients, high-performance packaging materials, processing aids, and food quality and safety sensors. Most previous application interest has focused on the development of nanoparticles. However, more recently, the ability to produce non-woven mats composed of nanofibers that can be used in food applications is beginning to be investigated. Electrospinning is a novel fabrication technique that can be used to produce fibers with diameters below 100 nm from (bio-) polymer solutions. These nanofibers have been shown to possess unique properties that distinguish them from non-woven fibers produced by other methods, e.g., melt-blowing. This is because first the process involved results in a high orientation of polymers within the fibers that leads to mechanically superior properties, e.g., increased tensile strengths. Second, during the spinning of the fibers from polymer solutions, the solvent is rapidly evaporated allowing the production of fibers composed of polymer blends that would typically phase separate if spun with other processes. Third, the small dimensions of the fibers lead to very high specific surface areas. Because of this the fiber properties may be greatly influenced by surface properties giving rise to fiber functionalities not found in fibers of larger sizes. For food applications, the fibers may find uses as ingredients if they are composed solely of edible polymers and GRAS ingredients, (e.g., fibers could contain functional ingredients such as nutraceuticals, antioxidants, antimicrobials, and flavors), as active packaging materials or as processing aids (e.g., catalytic reactors, membranes, filters (Lala et al., 2007), and sensors (Manesh et al., 2007; Ren et al., 2006; Sawicka et al., 2005). This review is therefore intended to introduce interested food and agricultural scientists to the concept of nano-fiber manufacturing with a particular emphasis on the use of biopolymers. We will review typical fabrication set-ups, discuss the influence of process conditions on nanofiber properties, and then review previous studies that describe the production of biopolymer-based nanofibers. Finally we briefly discuss emerging methods to further functionalize fibers and discuss potential applications in the area of food science and technology