Naturally Derived Melanin Nanoparticle Composites with High Electrical Conductivity and Biodegradability

The development of electronic devices from naturally derived materials is of enormous scientific interest. Melanin, a dark protective pigment ubiquitous in living creatures, may be particularly valuable because of its ability to conduct charges both electronically and ionically. However, device applications are severely hindered by its relatively poor electrical properties. Here, the facile preparation of conductive melanin composites is reported in which melanin nanoparticles (MNPs), directly extracted from squid inks, form electrically continuous junctions by tight clustering in a poly(vinyl alcohol) (PVA) matrix. Prepared as freestanding films and patterned microstructures by a series of precipitation, dry casting, and post‐thermal annealing steps, the percolated composites show electrical conductivities as high as 1.17 ± 0.13 S cm^?1 at room temperature, which is the best performance yet obtained with biologically‐derived nanoparticles. Furthermore, the biodegradability of the MNP/PVA composites is confirmed through appetitive ingestion by Zophobas morios larvae (superworms). This discovery for preparing versatile biocomposites suggests new opportunities in functional material selections for the emerging applications of implantable, edible, green bioelectronics.     

The effect of surface properties on the strength of attachment of fungal spores using AFM perpendicular force measurements

Polymeric substrata may be biodegraded by fungal species resulting in damaged, weakened and unsightly materials. This process typically begins with fungal spore attachment to the surface. In order to better understand the processes that precedes a biofouling event, fungal spore attachment to a range of surfaces, was determined using perpendicular force measurements. This was carried out using atomic force microscope cantilevers modified with fungal spores from Aspergillus niger 1957 (5μm diameter, non-wettable, spherical), Aspergillus niger 1988 (5μm diameter non-wettable, spikey) or Aureobasidium pullulans (5μm-10μm sized, wettable, ellipsoidal). The strength of attachment of the spores was determined in combination with seven surfaces (nitric acid cleaned glass, cast poly(methylmethacrylate) sheet [c-PMMA], polytetrafluoroethylene [PTFE], silicon wafers spin coated with poly(3-methacryloxypropyltrimethoxy silane (γ-MPS)-co-methylmethacrylate (MMA)) [p(γ-MPS-co-MMA)], poly (γ-MPS-co-lauryl methacrylate) [p(γ-MPS-co-LMA)] [both in a ratio of 10-90], PMMA dissolved in a solvent [PMMAsc] and silicon wafers). Perpendicular force measurements could not be related to the R(a) values of the surfaces, but surface wettability was shown to have an effect. All three spore types interacted comparably with the surfaces. All spores attached strongly to c-PMMA and glass (wettable surfaces), and weakly to PTFE, (p(γ- MPS-co-LMA)) (non-wettable) and (p(γ-MPS-co-MMA)). Spore shape also affected the strength of attachment. Aureobasidium pullulans spores attached with the widest range of forces whilst A. niger 1957 attached with the smallest. Findings will inform the selection of surfaces for use in environments where biofouling is an important consideration.     

Magnetic properties of nanocrystalline CoFe2O4 synthesized by modified citrate-gel method

Nanocrystalline CoFe₂O₄ with an average grain size of about 40 nm was successfully prepared by a modified citrate-gel method. At temperatures of 3 and 300 K, the measured coercive fields are 0.43 and 0.07 T and the magnetizations at 7 T are 89 and 83 emu/g, respectively. At room temperature, the longitudinal and transversal magnetostriction values are −130 and 70 ppm, respectively. The contribution of a disordered magnetic phase was detected by the occurrence of a peak in the ac-susceptibilities curves at around 250 K. The temperature dependence of the field-cooled and zero field-cooled low-field magnetization showed a larger irreversibility below this temperature. This disordered phase behaves like a spin-glass, which is coexisting with the ferrimagnetically ordered main phase     

The thermo-oxidative degradation of metallocene polyethylenes. Part 1: Long-term thermal oxidation in the solid state

The long-term thermo-oxidative degradation of different metallocene polyethylenes (mPEs) was investigated in the solid state (oven ageing in air at 90 °C). The mPEs differed essentially according to their initial melt index, molar mass distribution, density and ash content. Two Phillips-type HDPEs were also studied for comparison. The rate of oxidation was assessed by carbonyl index measurements. Initial vinyl unsaturation and short chain branch contents were correlated with induction times. The hydroperoxide concentrations of the aged materials and the levels of metal catalyst residues were also evaluated. Outstanding oxidative stability was exhibited for all the mPEs in the solid state relative to the Phillips HDPEs. This was believed to be due to the low concentration of both adventitious metal catalyst residues and initial vinyl unsaturation. Degree of short chain branching also appeared to influence the oxidative stability of the mPEs. Besides inherent properties, the density/crystallinity appeared to be the main factor influencing their oxidative stability. In the case of the Phillips HDPEs, the high level of vinyl unsaturation together with chromium catalyst residues may contribute to their poor oxidative stability.     

Formation of ion damage tracks

The formation of damage tracks in insulators from the passage of energetic (MeV/amu) ions indicates that the energy lost by an ion to electronic excitation is partially transferred to atomic motion. It is known that a track consists of localized regions of extended defects that are separated by lengths that exhibit only point defects. The utility of tracks for selective detection of various types of ions arises because of preferential chemical etching along the track as compared to etching the bulk material. In this letter we propose a new model to explain both the localized damage regions and the preferential etching of damage tracks. The formation of each region of extended defects is initiated by the Auger decay of a vacancy produced in an inner electronic shell of an atom of the insulator by the incident ion. This decay produces an intense source of ionization within a small volume around the decaying atom, which causes decomposition of the material in a manner similar to that observed in pulsed laser irradiation. The resulting chemical or crystalline modification of the material is the latent track, which because of its changed structure can be preferentially etched.