Ultralong Nanowires Research Articles

Fabrication of flexible ceramic membranes derived from hard Si3N4 and soft MnO2 nanowires

Flexible ceramic material, as a fundamental component of a toolset for the design of multifunctional materials, is very desirable, but a simple and effective preparation strategy is still missing. Herein, a flexible Si3N4/MnO2 ceramic membrane was fabricated by facile vacuum filtration progress using hard Si3N4 particles and soft ultralong MnO2 nanowires as building blocks. The Si3N4 particles were uniformly anchored into the porous flexible membrane, depending on the mechanical entanglement from ultralong MnO2 nanowires, resulting in the excellent interface stability between them. Thanks to the flexibility of MnO2 nanowires and interface stability, the as-prepared flexible membrane achieves superior flexibility including bending, folding, and twining. The flexible ceramic membrane can broaden the application range beyond the traditional ceramic use as hard structural materials. Moreover, the design strategy developed herein is applicable to other hard ceramic with similar issues.

Fabrication of Highly Flexible Copper Nanowires in Dual Surfactant Hydrothermal Process.

Highly flexible Cu nanowires were successfully synthesized by a dual-surfactant hydrothermal process using oleylamine and oleic acid. The ultra-long Cu nanowires have a mean diameter as low as 82.3 nm and lengths greater than 300 μm. It was found that reaction time and oleylamine concentration significantly influenced the morphology and phase composition of the Cu products. At a shorter reaction time (about 4 hours), pentagonal bipyramidal CU₂O particles were precipitated together with the Cu nanowires. A longer reaction time of 12 hours resulted in smooth and purely metallic Cu nanowires. It is possible that CU₂O served as an intermediate phase to control the activity of free CU2+ ions in the solution. On the other hand, a higher oleylamine concentration generally produced longer Cu nanowires. Cu nanowires conducting electrode with a sheet resistance of about 157.0 Ω/□ were fabricated. However, the optical transmittance of the electrode at 550 nm was very low (<20%) because of the agglomeration of the Cu nanowires. The addition of a small quantity of Ag nanowires in the Cu nanowire ink markedly improved the appearance and electrical properties of the resulting electrode.

Nano electromechanical approach for flexible piezoresistive sensor

Flexible sensor with superior sensitivity and widened working range is urgently demanded to cater for the rapid progress of wearable electronics and mechnosensational human-machine interfaces. However, the manufacturing of the wearable sensor with promising sensitivity in a broad working range is a challenge for the soft property of the sensing materials. Introducing a porous insulating spacer between the sensing and electrode materials recently demonstrated an effective and energy-conservation approach. In this study we directly employed the bio-derived leaf vein as the first avoiding layer to partially separate the sensing component and electrode part, along with the ultralong MnO2 nanowires (NWs) as the second spacer inside the graphite flakes (GFs), to significantly improve the sensitivity from 4.7 kPa−1 to 8.6 kPa−1 as well as the broader working range of 28.9 kPa. The in situ transmission electron microscope (TEM) and scanning electron microscope (SEM) was respectively used to further quantitatively analyze the sensing mechanism and deformation process for GFs and MnO2 NWs at nano scale. These results demonstrated the flexibility and imperative roles of the GFs’ intrinsic deformation-induced current change on the improvement of sensitivity and the fracture behavior of MnO2 NW. Further, the flexible sensor on the potential applications of health monitoring and human-machine interfaces were successfully demonstrated. We thus strongly believe our work, based on the bio-derived spacing layer and advanced in situ electromechanical characterizations of GFs and MnO2 NWs for the wearable sensor, could profoundly guide people to explore more opportunities to develop new-concept sensor device and smart systems.

Enhanced As(Ш) removal from aqueous solutions by recyclable Cu@MNM composite membranes via synergistic oxidation and absorption

Arsenic contamination threatens the safety of drinking water in many parts of the world, especially As (Ш), which is more toxic and more difficult to remove than As (Ⅴ). Hence, in terms of environmental protection and sustainable development, it is very important to remove As (Ш) from the environment to reduce the damage to ecosystems and human health. Since there is no effective method for removing As (Ш), it is essential to oxidize As (Ш) into easily removable As (Ⅴ) to achieve effective separation. Herein, a novel copper-coated MnO2 nanowires membrane (Cu@MNM) which combines the oxidation properties of MnO2 and the catalytic and absorption properties of nanoscale Cu (NSCu), was developed based on in situ chemical deposition NSCu on the surface of ultralong MnO2 nanowires. The as-prepared Cu@MNM shows excellent arsenic separation properties with the maximum rejection rate of 96%. The results of pH studies indicate that acidic conditions promote the separation of As (Ш) by Cu@MNM, while alkaline conditions are inhibitory due to deprotonation of Cu@MNM surface enhances electrostatic repulsion. The results of the interfering ions show that the phosphate ions have a strong inhibitory effect on arsenic separation. In addition, Cu@MNM has been shown to be remarkably recyclable and can still achieve a separation efficiency of 60% after five cycles. Therefore, the prepared Cu@MNM with the high arsenic retention efficiency and excellent recycling capabilities has the potential to become an excellent candidate for practical application in arsenic separation.

One-dimensional ZnO nanowires grown on three-dimensional scaffolds for improved photocatalytic activity

In this paper, ultralong ZnO nanowires were prepared on the surfaces of nickel foams and zinc foils by a hydrothermal method. The photocatalytic activities of ZnO nanowires grown on nickel foams have been tested by the degradation of Rhodamine B solution and using ZnO nanowires grown on zinc foils as the reference. The results indicate that the photocatalytic performance of ZnO nanowires grown on nickel foams is better than those grown on zinc foils. The results are mainly due to the fact that nickel foam is a three-dimensional scaffold structure, which provides large surface area for ZnO nanowire growth. More importantly, the photocatalytic circulation experiments indicate that ZnO nanowires grown on nickel foams are stable and reusable. In addition, the photocatalytic reaction rates can be enhanced by the use of multi-layer of nickel foams with ZnO nanowires grown on their surfaces.

Deformable Biomaterials Based on Ultralong Hydroxyapatite Nanowires.

Hydroxyapatite (HAP) biomaterials with high biocompatibility, bioactivity, and osteoconductivity/osteoinductivity have many applications in the biomedical fields. The traditional HAP materials usually feature high brittleness. Although hybridizing HAP with polymers can obtain deformable materials, their deformability is mainly contributed by polymers. Recently, research progress has been made in the deformable HAP biomaterials with high flexibility, softness, or elasticity based on ultralong HAP nanowires (UHANWs). This article provides a brief review on the recent progress on this research topic. Specifically, the strategy is proposed for solving the high brittleness of HAP biomaterials using UHANWs; the synthetic strategies for UHANWs are presented; and a variety of deformable biomaterials based on UHANWs are discussed. This review article provides a new concept of deformable biomaterials consisting of UHANWs and will stimulate new research works on deformable HAP biomaterials and further contribute to their development and applications in the biomedical fields such as bone regeneration, artificial periosteum, skin wound healing, biomedical paper, medical test paper, drug delivery, diagnosis, and therapy.

Bendable Network Built with Ultralong Silica Nanowires as a Stable Separator for High-Safety and High-Power Lithium-Metal Batteries

Separators are key safety components for electrochemical energy storage systems. However, the intrinsic poor wettability with electrolyte and low thermal stability of commercial polyolefin separators cannot meet the requirements of the ever-expanding market for high-power, high-energy, and high-safety power systems, such as lithium-metal, lithium-sulfur, and lithium-ion batteries. In this study, scalable bendable networks built with ultralong silica nanowires (SNs) are developed as stable separators for both high-safety and high-power lithium-metal batteries. The three-dimensional porous nature (porosity of 73%) and the polar surface of the obtained SNs separators endue a much better electrolyte wettability, larger electrolyte uptake ratio (325%), higher electrolyte retention ratio (63%), and ∼7 times higher ionic conductivity than that of commercial polypropylene (PP) separators. Moreover, the pore-rich structure of the SNs separator can aid in evenly distributing lithium and, in turn, suppress the uncontrollable growth of lithium dendrites to a certain degree. Furthermore, the pure inorganic structure endows the SNs separators with excellent chemical and electrochemical stabilities even at elevated temperatures, as well as excellent thermal stability up to 700 °C. This work underpins the utilization of SNs separators as a rational choice for developing high-performance batteries with a metallic lithium anode.

One-step fabrication of InxGa1−xSb nanowires by vapor transport method

We try to synthesize InxGa1−xSb nanowires on silicon (100) substrate using InSb and GaSb as source materials and we succeeded. Au film was used as catalysts. The experiment is simple and repeatable. The diameter of the grown nanowires is 60–200 nm and up to 10 microns in length. The grown nanowires have good crystallinity. Due to the doping of indium, we found that the XRD peaks of nanowires were shifted. We have fabricated fully nanostructured device using ultra-long Ag nanowires as electrodes for the I–V characteristic study, and found that the device has photoresponse characteristics and the I–V characteristic curve was asymmetric, we explain that it was due to the asymmetry of this fully nanostructured devices.

Fabrication of sandwich-structured cellulose composite membranes for switchable infrared radiation

Developing a feasible and efficient biomass-based membrane for energy-saving applications is critical for addressing greenhouse effect and energy crises. Herein, a sandwich-structured cellulose composite membrane with controllable thermal management properties was fabricated via the assembly of Ag nanoparticles/cellulose (ANPs/C), ultralong MnO2 nanowires (UMNWs), and MnO2 nanosheets/cellulose (MNSs/C). In this strategy, the UMNWs as building blocks were prepared by hydrothermal method. Then, the ANPs/C and MNSs/C were fabricated via magnetron sputtering and in situ growth method, respectively. Finally, the cellulose composite membrane (CCM) was fabricated via vacuum-filtration of cotton cellulose (CC), UMNWs and MNSs/C suspension, and subsequently the CC surfaces were covered with the ANPs by magnetron sputtering. The sandwich-structured CCM exhibits Janus infrared radiation properties due the asymmetrical surface structure and chemical compositions. The ANPs layer has high infrared reflectivity of 91.6%, however, the MNSs/C layer shows the low infrared reflectivity with 62.7%. The results indicate that the Janus sandwich CCM not only can efficiently reflect infrared heat back toward the body, in order to achieve highly thermal insulating under low temperature environment, but also can achieve highly passive cooling under high temperature environment. In addition, the CCM exhibits excellent antibacterial properties due to presence of ANPs, which is beneficial to enhance the lifespan of the personal thermal management membrane. The present work may give some insight to prepare sandwich-structured membranes with controllable thermal management properties for application in wearable textiles.

Fabrication of dynamic zero-valent iron/MnO2 nanowire membrane for efficient and recyclable selenium separation

Controllable selenium separation processes are essential from the viewpoint of human health, electronics industry and environmental protection due to the vital role of selenium in the human body and photovoltaic devices, and the adverse impacts emanating from selenium pollution in surface and groundwater by the highly toxic and high lethal at higher selenium concentrations. Herein, a dynamic zero-valent iron/MnO2 nanowire hybrid membranes (ZVINM) with high chemical activity and excellent recoverability have been developed based on in situ chemical deposition nanoscale zero-valent iron (NZVI) on the surface of ultralong MnO2 nanowires to self-assemble into to hybrid membrane, and the applicability of this hybrid membrane for selenium separation was evaluated. To investigate the retention efficiency, the selenium separation behavior on ZVINM was examined by various separation conditions such as operating pressures, selenium concentrations and NZVI content, solution pH, interfering ions and penetration times. The as-prepared ZVINM shows excellent separation performance with the maximum retention efficiency of 95%. The appropriate selenium separation on ZVINM was obtained at an acidic pH range of 2–6 with a high separation efficiency (over 85%), and the rejection rate was decreased by increasing solution pH, indicating that the selenium separation process was promoted under strong acid conditions and inhibited under strong alkali environment. In addition, it was found that the carbonate ions and higher selenium concentration have a strong inhibitory effect on selenium separation. More importantly, the ZVINM is demonstrated to have excellent selenium recoverability and regenerative because of its dynamic coating characteristics. Therefore, the high selenium retention efficiency and excellent recycling capabilities of ZVINM make the hybrid membrane as a potential candidate for practical application in selenium separation.

Synthesis of ultra-long boron nanowires as supercapacitor electrode material

The aim of the present study was to examine the electrochemical performance of boron nanowires (BNWs) grown using the vapor-liquid-solid (VLS) growth mechanism in the chemical vapor deposition (CVD) system at 1200 °C on a silicon wafer in the presence of iron (Fe) catalyst nanodroplets as a supercapacitor electrode material in 1 M Na2SO4 aqueous electrolyte solution. The structure and morphology of BNWs were examined using field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HR-TEM) analyses, while their electrochemical performances were determined by measuring the cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectra (EIS). BNWs showed great cycle stability with an areal capacitance of 30 mF cm−2 and 93.4% of the initial areal capacitance even after a 10,000 GCD cycle at a current density of 0.1 mA cm−2.

Multifunctional Janus fibrous hybrid membranes with sandwich structure for on-demand personal thermal management

Developing functional Janus membrane with asymmetric infrared emissivity for personal thermal management is of significance but challenging, due to the difficulty in controlling the infrared insulation and infrared dissipation to satisfy the personal thermal comfort. Herein, a multifunctional Janus Cu/MnO2/cellulose@Layered Double hydroxide fiber (CMCFL) membrane with sandwich structure has been fabricated by vacuum filtrating ultralong MnO2 nanowires and Cu nanowires sequentially on cellulose fiber@Layered Double Hydroxide (LDH) basement membrane. The obtained CMCFL membrane allows for an integration of low infrared emission, promising electrical conductivity, antibacterial property from Cu nanowire layer, and high infrared emission from cellulose@LDH layer. Moreover, the Janus CMCFL membrane shows the asymmetrical characteristics of infrared radiation for on-demand personal thermal management: the low-emissivity layer (Cu nanowires layer) is facing outside to reduce the human thermo radiation, and high-emissivity layer (cellulose@LDH layer) is facing outward to enhance the human thermo radiation. In addition, the promising electrical conductivity of Cu nanowires layer endows the superior Joule heating for extra warmth of 19 °C using a low supply voltage around 8.4 V to enhance the thermal comfort in the cold environment. Besides, the obtained Janus CMCFL membrane not only shows good breathability and flexibility, but also possesses other desired properties including excellent interfacial compatibility and antibacterial activity for wearability. The outstanding integrated properties and corresponding design strategy of Janus CMCFL membrane are expected to be applicable in the fields of personal thermal management, providing a promising direction for the development of wearable textile to enhance the adaptability of human skin to the environment.

Controlling Ferromagnetic Ground States and Solitons in Thin Films and Nanowires Built from Iron Phthalocyanine Chains

AbstractIron phthalocyanine (FePc) is a molecular semiconductor whose building blocks are 1D ferromagnetic chains. It is shown that its optical and magnetic properties are controlled by the growth strategy, obtaining extremely high coercivities of over 1 T and modulating the exchange constant between 15 and 29 K through switching from thin films with controlled orientations, to ultralong nanowires. Magnetization measurements are analyzed using concepts and formulas with broad applicability to all 1D ferromagnetic chains. They show that FePc is best described by a xy model with moments preferentially lying in the molecular planes. The chain Hamiltonian is very similar to that for the classic inorganic magnet CsNiF3, but with ferromagnetic rather than antiferromagnetic interchain interactions. The dominant degrees of freedom are topological excitations called solitons, namely moving 1D magnetic domain walls, and at low temperatures and fields a “super‐Curie–Weiss” law characteristic of nearly 1D xy and Heisenberg ferromagnets, where susceptibility scales as 1/(T2–θ2), is observed. The ability to control the molecular orientation and ferromagnetism of FePc systems, and produce them on flexible substrates, together with excellent transistor characteristics reported previously for phthalocyanine analogues, makes them potentially useful for magneto‐optical and spintronic devices.

Highly stretchable graphene nanoribbon springs by programmable nanowire lithography

Graphene nanoribbons are ideal candidates to serve as highly conductive, flexible, and transparent interconnections, or the active channels for nanoelectronics. However, patterning narrow graphene nanoribbons to <100 nm wide usually requires inefficient micro/nano fabrication processes, which are hard to implement for large area or flexible electronic and sensory applications. Here, we develop a precise and scalable nanowire lithography technology that enables reliable batch manufacturing of ultra-long graphene nanoribbon arrays with programmable geometry and narrow width down to ~50 nm. The orderly graphene nanoribbons are patterned out of few-layer graphene sheets by using ultra-long silicon nanowires as masks, which are produced via in-plane solid–liquid–solid guided growth and then transferred reliably onto various stiff or flexible substrates. More importantly, the geometry of the graphene nanoribbons can be predesigned and engineered into elastic two-dimensional springs to achieve outstanding stretchability of >30%, while carrying stable and repeatable electronic transport. We suggest that this convenient scalable nanowire lithography technology has great potential to establish a general and efficient strategy to batch-pattern or integrate various two-dimensional materials as active channels and interconnections for emerging flexible electronic applications.

Controllable fabrication of tendril-inspired hierarchical hybrid membrane for efficient recovering tellurium from photovoltaic waste

Fabrication of membrane-based adsorbent with outstanding functions for efficient recovering tellurium from photovoltaic waste has been intriguing but is still challenging. Inspired by the winding of tendrils, herein, a membrane-based adsorbent for tellurium separation has been fabricated via winding calcined micron-sized Mg–Al layer double hydroxides (LDHs) fibers with the acid of ultralong MnO2 nanowires (UL-MnO2-NWs). To obtain the calcined LDHs fibers, Al2O3 fibers was fabricated by template-directed synthesis employing cotton fibers as templates, and then the calcined LDHs fibers were prepared via crystal growth on Al2O3 fibers surface and subsequent calcination treatment. UL-MnO2-NWs were obtained by a hydrophobic route. The immobilized LDHs in the tendril-inspired membrane allow the tellurium to be adsorbed in matrix from the simulate leaching solution of CdTe photovoltaic waste with permeate flux of 34 m3/m2h. It is worth mentioning that the tendril-inspired membrane achieves ultrahigh removal efficiency up to 98.73%. The static soaking tests proved the excellent mechanical stability of the tendril-inspired membrane. More importantly, the tendril-inspired membrane can be regenerated via elution and calcination processes, which match well with the requirements for practical separation applications. Under the cyclic tests, the tendril-inspired membrane still maintains a high level of separation after 5 cycles. As a high-efficient, mechanically stable, and regenerated membrane-based adsorbent, tendrils-inspired membrane has tremendous potential to handle and recycle toxic tellurium containing compounds.

One step synthesis of silver nanowires using fructose as a reducing agent and its antibacterial and antioxidant analysis

Silver nanowires were synthesised using hydrothermal method by reducing silver nitrate (AgNO3) using fructose in the presence of poly-vinylpyrrolidone (PVP). The parameters such as the effect of process temperature, AgNO3 molarity, PVP and fructose (C6H12O6) concentration influencing the synthesis of silver nanowires (Ag NWs) were investigated. The scanning electron microscope (SEM) images showed that ultra-long, uniform and thin silver nanowires were obtained under optimized conditions; 0.02 M AgNO3, 0.016 g ml−1 of fructose, 0.16 g ml−1 of PVP at 160 °C within 22 h. The dynamic light scattering (DLS) analysis revealed that the silver nanowires obtained have an average diameter of 77 nm possessing high level of crystallinity with face centered cubic (fcc) phase that is evident from the x-ray diffraction (XRD) patterns peaked at (111), (200), (220), (311) and (222) planes. FT-IR (Fourier transform infrared spectroscopy) results suggested that there is adsorption of PVP molecules on the silver atoms. Ag NWs exhibited better antibacterial activity against Escherichia coli and high antioxidant activity against 2,2-diphenyl-1- picrylhydrazyl (DPPH) free radical scavenger. This work gives a green approach to the hydrothermal synthesis of Ag NWs using fructose with a promising antibacterial and antioxidant properties.

Synthesis of ultra-long aluminum nitride nanowires with excellent photoluminescent property by aluminum chloride assisted chemical vapor reaction technique

Abstract Aluminum nitride (AlN) nanowires were performed on iron substrate using aluminum powders as material precursor by anhydrous aluminum chloride (AlCl3) assisted chemical vapor reaction (CVR) method. Yield production of AlN nanowires was significantly enhanced with AlCl3 addition into Al powders due to generating of intermediate gaseous AlCl, which further reacted with N2 to form AlN nanowires. With AlCl3 addition, a thin Fe film on Fe substrate can be transformed into Fe nanoparticles, which served as a catalyst in promoting the formation of AlN nanowires. The length of single-crystalline AlN nanowires was up to hundreds of microns. The formation and growth of AlN nanowires were most likely controlled by vapor–liquid–solid growth mechanism and AlCl3 assisted chemical vapor transport mechanism. The luminescence bands in the range of violet-green luminescence suggest its promising application in light-emitting devices.

Morphology-Controlled Versatile One-Pot Synthesis of Hydrophobic Gold Nanodots, Nanobars, Nanorods, and Nanowires and Their Applications in Surface-Enhanced Raman Spectroscopy

Many previously reported syntheses of gold nanoparticles required lengthy reaction times, complicated operations, high temperatures, or multi-step manipulations. In this work, a morphology-controlled versatile one-pot synthesis of hydrophobic gold nanodots, nanobars, nanorods, and nanowires has been developed. A series of gold nanomaterials ranging from round nanodots, short nanobars, and long nanorods to ultrathin and ultralong nanowires (diameter &lt;2 nm, length &gt;2 μm) have been readily prepared by simply adjusting the feeding ratio of chloroauric acid to oleylamine, oleic acid, and triphenylsilane. The silk-like ultralong and ultrathin nanowires were found to have a single crystalline structure and may have significant potential applications in microelectronics and biosensors. Large sizes of gold spherical nanoparticles were obtained from gold nanodots via a seed-mediated growth approach. These nanoparticles and ultralong nanowires showed excellent surface-enhanced Raman scattering (SERS) activity in organic solvents and, therefore, were employed as efficient organic-soluble SERS substrates for the detection of hydrophobic food toxicants, such as 3,4-benzopyrene, and carcinogens, such as benzidine.

Humidity sensor based on mesoporous Al-doped NiO ultralong nanowires with enhanced ethanol sensing performance

Mesoporous Al-doped NiO ultralong nanowires with better uniformity, dispersity and well-defined morphologies were synthesized by a simple surfactant based hydrothermal treatment of aqueous solution of Al(NO3)3, NiCl2 and NaC2O4 in the presence of ethylene glycol and polyethylene glycol through continuous open air heating at 500 °C. The sensing performances of the gas sensor’s based on bare and Al-doped NiO nanowires had been investigated towards ethanol, methanol, acetone, xylene, toluene and benzene. The results specified that the 3.2 at% Al-doped NiO nanowires display better gas sensitivity characteristics as compare to that of undoped, 1.6 at%, 2.1 at% and 4.3 at% Al-doped NiO nanowires. It has been investigated that the sensitivity features were upgraded with an increment on Al concentration into the crystallites based on NiO nanowires. The assimilation of Al3+ ions within NiO crystals modifies the carrier concentration by inducing changes in the chemisorbed and deficient oxygen of NiO nanowires. Hence, the incorporation of Al3+ into NiO nanowires would be a capable way for scheming and devising significant gas sensors.

Ultra-long silver nanowires induced mitotic abnormalities and cytokinetic failure in A549 cells

Asbestos fiber has been associated with mesothelioma and lung cancer. However, the carcinogenic risks of other fiber nanomaterials with morphological similarities to asbestos have not been fully studied. Ultra-long silver nanowires (AgNWs) are increasingly used fiber-shaped nanomaterials with a high aspect ratio, but very few studies have investigated their health risks. Here, proliferation abnormalities of lung epithelial cells induced by ultra-long AgNWs were investigated. Ultra-long AgNW treatment induced dose- and diameter-dependent increase in the ratio of multinucleated cells. Further, proteins involved in mitosis and cytokinesis, including Aurora A, p-Histone 3 (ser10), RhoA, p-MLC, and myosin IIb, were significantly upregulated after an ultra-long AgNW treatment, leading to mitotic abnormalities and cytokinetic failure. Meanwhile, exposure to ultra-long AgNWs induced cell cycle arrest. Interestingly, a series of experiments demonstrated that ROS generation and Ag+ release were not responsible for the multinucleation induced by ultra-long AgNWs, but ultra-long AgNWs in the intercellular bridge might obstruct the contractile ring and inhibit abscission of the cytokinetic furrow by direct physical contact. Altogether, our findings indicate that ultra-long AgNWs can induce chromosomal instability, which has important consequences for the safety of ultra-long AgNWs to human health.