Nanofibers are a matter of perspective: effects of methodology and subjectivity on diameter measurements.

Nanofibers are currently among the most researched nanomaterials in materials science. Various high-resolution microscopy techniques are used for morphological investigations, with the diameter as primary characteristic. Since methodological factors influencing the diameter distribution are usually ignored, numerical values can hardly be compared across different or even within single studies. Here, we investigate influencing factors such as microscopy technique, degree of magnification, eventual coatings, and the analysts' bias in the image selection and evaluation. We imaged a single nanofiber sample using scanning electron microscopy (SEM), helium ion microscopy (HIM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). These techniques yield significant methodological variations between the diameter distributions. We further observed a strong influence of analysts' subjectivity, with a consistent average deviation between 4 different analysts of up to 31%. The average deviation between micrographs within each category was 14%, revealing a considerable influence of micrograph selection and strong potential for cherry picking. The mean values were mostly comparable with the results using automated image analysis software, which was more reproducible, much faster, and more accurate for images with lower magnification. The results demonstrate that one of the most frequently measured characteristics of nanofibers is subject to strong systematic fluctuations that are rarely if ever addressed.

Electrospinning of Magnetite-Polyacrylonitrile Composites for the Production of Oxygen Reduction Reaction Catalysts.

In this study, electrospun carbon fiber electrodes were prepared by the carbonization of PAN-Fe3O4 electrospun fibers at 800 °C for their use as catalysts in the oxygen reduction reaction in an alkaline electrolyte. Magnetic nanofiber mats were fabricated using a needle-free electrospinning method by incorporating magnetic nanoparticles into a polymer solution. Electrochemical tests revealed that the oxygen reduction reaction (ORR) activity is optimized at an intermediate magnetite loading of 30% wt. These catalysts not only show better performance compared to their counterparts but also achieve high selectivity to water at low potentials. The onset and half-wave potentials of 0.92 and 0.76 V shown by these samples are only slightly behind those of the commercial Pt 20%-carbon black ORR catalyst. The obtained results point out that the electrospinning of PAN-Fe3O4 solutions allows the preparation of advanced N-Fe ORR catalysts in fibrillar morphology.

Electrospinning Nanofiber Mats with Magnetite Nanoparticles Using Various Needle-Based Techniques.

Electrospinning can be used to produce nanofiber mats containing diverse nanoparticles for various purposes. Magnetic nanoparticles, such as magnetite (Fe3O4), can be introduced to produce magnetic nanofiber mats, e.g., for hyperthermia applications, but also for basic research of diluted magnetic systems. As the number of nanoparticles increases, however, the morphology and the mechanical properties of the nanofiber mats decrease, so that freestanding composite nanofiber mats with a high content of nanoparticles are hard to produce. Here we report on poly (acrylonitrile) (PAN) composite nanofiber mats, electrospun by a needle-based system, containing 50 wt% magnetite nanoparticles overall or in the shell of core-shell fibers, collected on a flat or a rotating collector. While the first nanofiber mats show an irregular morphology, the latter are quite regular and contain straight fibers without many beads or agglomerations. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) reveal agglomerations around the pure composite nanofibers and even, round core-shell fibers, the latter showing slightly increased fiber diameters. Energy dispersive X-ray spectroscopy (EDS) shows a regular distribution of the embedded magnetic nanoparticles. Dynamic mechanical analysis (DMA) reveals that mechanical properties are reduced as compared to nanofiber mats with smaller amounts of magnetic nanoparticles, but mats with 50 wt% magnetite are still freestanding.

Magnetic Carbon Nanofiber Mats for Prospective Single Photon Avalanche Diode (SPAD) Sensing Applications.

Electrospinning enables simple and cost-effective production of magnetic nanofibers by adding nanoparticles to a polymer solution. In order to increase the electrical conductivity of such nanofibers, the carbonization process is crucial. In this study, the chemical and morphological properties of magnetic nanofiber mats prepared from polyacrylonitrile (PAN)/magnetite were investigated. In our previous studies, PAN/magnetite nanofiber mats were carbonized at 500 °C, 600 °C, and 800 °C. Here, PAN/magnetite nanofiber mats were carbonized at 1000 °C. The surface morphology of these PAN/magnetite nanofiber mats is not significantly different from nanofiber mats thermally treated at 800 °C and have remained relatively flexible at 1000 °C, which can be advantageous for various application fields. The addition of nanoparticles increased the average fiber diameter compared to pure PAN nanofiber mats and improved the dimensional stability during thermal processes. The high conductivity, the high magnetization properties, as well as shielding against electromagnetic interference of such carbonized nanofibers can be proposed for use in single photon avalanche diode (SPAD), where these properties are advantageous.

Magnetic Properties of Electrospun Magnetic Nanofiber Mats after Stabilization and Carbonization.

Magnetic nanofibers are of great interest in basic research, as well as for possible applications in spintronics and neuromorphic computing. Here we report on the preparation of magnetic nanofiber mats by electrospinning polyacrylonitrile (PAN)/nanoparticle solutions, creating a network of arbitrarily oriented nanofibers with a high aspect ratio. Since PAN is a typical precursor for carbon, the magnetic nanofiber mats were stabilized and carbonized after electrospinning. The magnetic properties of nanofiber mats containing magnetite or nickel ferrite nanoparticles were found to depend on the nanoparticle diameters and the potential after-treatment, as compared with raw nanofiber mats. Micromagnetic simulations underlined the different properties of both magnetic materials. Atomic force microscopy and scanning electron microscopy images revealed nearly unchanged morphologies after stabilization without mechanical fixation, which is in strong contrast to pure PAN nanofiber mats. While carbonization at 500 °C left the morphology unaltered, as compared with the stabilized samples, stronger connections between adjacent fibers were formed during carbonization at 800 °C, which may be supportive of magnetic data transmission.

Orientation of Electrospun Magnetic Nanofibers Near Conductive Areas.

Electrospinning can be used to create nanofibers from diverse polymers in which also other materials can be embedded. Inclusion of magnetic nanoparticles, for example, results in preparation of magnetic nanofibers which are usually isotropically distributed on the substrate. One method to create a preferred direction is using a spinning cylinder as the substrate, which is not always possible, especially in commercial electrospinning machines. Here, another simple technique to partly align magnetic nanofibers is investigated. Since electrospinning works in a strong electric field and the fibers thus carry charges when landing on the substrate, using partly conductive substrates leads to a current flow through the conductive parts of the substrate which, according to Ampère's right-hand grip rule, creates a magnetic field around it. We observed that this magnetic field, on the other hand, can partly align magnetic nanofibers perpendicular to the borders of the current flow conductor. We report on the first observations of electrospinning magnetic nanofibers on partly conductive substrates with some of the conductive areas additionally being grounded, resulting in partly oriented magnetic nanofibers.

Growth of Pleurotus Ostreatus on Different Textile Materials for Vertical Farming.

The mycelium of the edible mushroom Pleurotus ostreatus can be used for diverse technical applications, such as packaging materials or wastewater treatment, besides the more obvious use for nutrition. While P. ostreatus usually grows on sawdust, wood or similar materials, a former study investigated mycelium growth on different nanofiber mats. Here, we report on growing P. ostreatus on fabrics knitted from different materials, enabling the use of this mushroom in textile-based vertical farming. Our results underline that P. ostreatus grows similar on natural fibers and on synthetic fibers. The agar medium used to provide nutrients was found to support mycelium growth optimally when applied by dip-coating, suggesting that, in this way, P. ostreatus can also be grown on vertically aligned textile fabrics for vertical farming.

Comparative Study of Pleurotus ostreatus Mushroom Grown on Modified PAN Nanofiber Mats.

Pleurotus ostreatus is a well-known edible mushroom species which shows fast growth. The fungus can be used for medical, nutritional, filter, or packaging purposes. In this study, cultivation experiments were carried out with Pleurotus ostreatus growing on polyacrylonitrile (PAN) nanofiber mats in the presence of saccharose and Lutrol F68. The aim of this study was to find out whether modified PAN nanofiber mats are well suited for the growth of fungal mycelium, to increase growth rates and to affect mycelium fiber morphologies. Our results show that Pleurotus ostreatus mycelium grows on nanofiber mats in different morphologies, depending on the specific substrate, and can be used to produce a composite from fungal mycelium and nanofiber mats for biomedical and biotechnological applications.

Magnetic Nanofiber Mats for Data Storage and Transfer.

Electrospun nanofiber mats may serve as new hardware for neuromorphic computing. To enable data storage and transfer in them, they should be magnetic, possibly electrically conductive and able to respond to further external impulses. Here we report on creating magnetic nanofiber mats, consisting of magnetically doped polymer nanofibers for data transfer and polymer beads containing larger amounts of magnetic nanoparticles for storage purposes. Using magnetite and iron nickel oxide nanoparticles, a broad range of doping ratios could be electrospun with a needleless technique, resulting in magnetic nanofiber mats with varying morphologies and different amounts of magnetically doped beads.

Fixing PAN Nanofiber Mats during Stabilization for Carbonization and Creating Novel Metal/Carbon Composites.

Polyacrylonitrile (PAN) is one of the materials most often used for carbonization. PAN nanofiber mats, created by electrospinning, are an especially interesting source to gain carbon nanofibers. A well-known problem in this process is fixing the PAN nanofiber mats during the stabilization process which is necessary to avoid contraction of the fibers, correlated with an undesired increase in the diameter and undesired bending. Fixing this issue typically results in breaks in the nanofiber mats if the tension is too high, or it is not strong enough to keep the fibers as straight as in the original state. This article suggests a novel method to overcome this problem by electrospinning on an aluminum substrate on which the nanofiber mat adheres rigidly, stabilizing the composite and carbonizing afterwards either with or without the aluminum substrate to gain either a pure carbon nanofiber mat or a metal/carbon composite.