High doses of nano-polystyrene aggravate the oxidative stress, DNA damage, and the cell death in onions.

Plastic pollution has been reported to negatively impact global biodiversity and ecosystem health. However, the molecular mechanisms of nano-plastics in plants are unidentified, especially their negative impacts on genomic stability. This study for the first time showed that nano-polystyrene leads to cell death in plants by subjugating the cellular antioxidant defence mechanisms through the aggravated production of ROS, which in turn could induce the DNA damage impairing the genetic regulation of the corresponding DNA repair pathway. To validate the proposed hypothesis, the DNA damage potential of nano-polystyrene and the expression levels of key genetic regulators of the DNA damage repair pathway (such as - CYCA/B, CDKA, SOG1, MYB transcription factors, and RAD51) have been assessed in onion roots after 72 h exposure with three ecologically relevant concentrations (25, 50, and 100 µg ml-1) of 100 nm nano-polystyrene. In addition, imbalance in redox homeostasis (oxidative stress), cell viability, and nuclear aberrations such as - the frequency of micronucleus and bi-nucleate cells that are directly linked to the DNA damages have been checked to point out the cause and effect of nano-polystyrene-induced DNA damage. Results showed a significant increase in oxidative stress in each treatment concentrations of nano-polystyrene. However, ROS generated at 100 µg ml-1 nano-polystyrene dose subdues the antioxidant defence system and induces cell death. These observations may be ascribed to the accumulation damaged DNA and the down-regulation of repair pathway-associated genes, as observed in this treatment group. Conversely, the observed DNA damage and the reduced expressions of genes would be a mere consequence of reduced cellular viability.

Toxicological impacts of nanopolystyrene on zebrafish oocyte with insight into the mechanism of action: An expression-based analysis.

Many studies have investigated the negative impacts of microplastics on teleost fishes with very little or no evidence of their mechanism of action. This scenario entreats us to investigate the toxicities of nanopolystyrene in zebrafish oocyte with emphasis on the mechanism of action. In the present study, the cellular levels of mRNA transcripts of different genetic markers (such as: sod, gpx, nrf2, inos, ucp2, and atp6 (redox-sensitive markers); nfkß, tnfα, il-10, ikß, gdf9, and bmp15 (immune markers); gadd45, rad51, p53 and bcl2 (DNA damage and apoptotic)) have been quantified by real-time PCR after 6 h of incubation of isolated oocyte with different doses of nanopolystyrene viz. P0 (control i.e. no polystyrene in culture medium), P1 (100 ng/ml), and P2 (400 ng/ml). Results showed that both the treatment concentrations of nanopolystyrene induce oxidative stress with % DPPH = 30.75, 31.61, and 32.43% for P0, P1, and P2, respectively. Increase in oxidative stress in oocytes with increasing doses of nanopolystyrene was also observed in TBARS assay with MDA content 0.12 and 0.21 µM for P1 and P2, respectively as compaired to the control 0.08 µM. This increased oxidative stress can regulate the expression pattern (upregulation/downregulation) of selected genes leading to different toxic effects like - oxidative stress, immunotoxicity, and apoptosis in oocytes, which suggests the impairment of reproductive functions by nanopolystyrene.

Cytogenotoxic potential of a hazardous material, polystyrene microparticles on Allium cepa L.

Plastic pollution represents a global concern for the biodiversity conservation, ecosystem and public health. The polystyrene is one of the dominant pollutants in both terrestrial and aquatic ecosystem. This work measured the hazardous nature of 100 nm micropolystyrene (MPS) using 25, 50, 100, 200, and 400 mg/L concentrations in terms of oxidative stress, morphotoxicity and cytogenotoxicity in Allium cepa. The results were compared with the positive control (PC) (400 mg/L chlorpyrifos). MPS significantly (p < 0.05) reduced the root length while induced the production of hydroxyl, superoxide radicals with a concomitant increase in DPPH scavenging activity and lipid peroxidation as compared to the negative control. The significant decrease in mitotic index with respect to the negative control (MI: 23.855 ±â€¯5.336 %; lowest MI: 3.88 ±â€¯1.042 %) showed the cytotoxic nature of MPS. Genotoxicity was assessed by various chromosomal and nuclear aberrations. The highest 3.029 ±â€¯0.403 % (PC: 3.09 ±â€¯0.535 %) chromosomal abnormality index and 2.31 ±â€¯0.338 % (PC: 1.178 ±â€¯0.095 %) nuclear abnormality index were observed. MPS down-regulated the expression of plant CDKA encoding gene: cdc2, an important cell cycle regulator. The overall results indicated that MPS could induce cytogenotoxicity through the exacerbation of ROS production and inhibition of cdc2.

The dependence of the optoelectrical properties of silver nanowire networks on nanowire length and diameter.

We have characterized the optoelectrical properties of networks of silver nanowires as a function of nanowire dimension by measuring transmittance (T) and sheet resistance (R(s)) for a large number of networks of different thicknesses fabricated from wires of different diameters (D) and lengths (L). We have analysed these data using both bulk-like and percolative models. We find the network DC conductivity to scale linearly with wire length while the optical conductivity is approximately invariant with nanowire length. The ratio of DC to optical conductivity, often taken as a figure of merit for transparent conductors, scales approximately as L/D. Interestingly, the percolative exponent, n, scales empirically as D², while the percolative figure of merit, Π, displays large values at low D. As high T and low R(s) are associated with low n and high Π, these data are consistent with improved optoelectrical performance for networks of low-D wires. We predict that networks of wires with D = 25 nm could give sheet resistance as low as 25 Ω/□ for T = 90%.

Percolation effects in supercapacitors with thin, transparent carbon nanotube electrodes.

We have explored the effects of percolation on the properties of supercapacitors with thin nanotube networks as electrodes. We find the equivalent series resistance, R(ESR), and volumetric capacitance, C(V), to be thickness independent for relatively thick electrodes. However, once the electrode thickness falls below a threshold thickness (∼100 nm for R(ESR) and ∼20 nm for C(V)), the properties of the electrode become thickness dependent. We show the thickness dependence of both R(ESR) and C(V) to be consistent with percolation theory. While this is expected for R(ESR), that the capacitance follows a percolation scaling law is not. This occurs because, for sparse networks, the capacitance is proportional to the fraction of nanotubes connected to the main network. This fraction, in turn, follows a percolation scaling law. This allows us to understand and quantify the limitations on the achievable capacitance for transparent supercapacitors. We find that supercapacitors with thickness independent R(ESR) and C(V) occupy a well-defined region of the Ragone plot. However, supercapacitors whose electrodes are limited by percolation occupy a long tail to lower values of energy and power density. For example, replacing electrodes with transparency of T = 80% with thinner networks displaying T = 97% will result in a 20-fold reduction of both power and energy density.

Transparent conducting films from NbSe3 nanowires.

We have developed methods to disperse and partially size separate NbSe(3) nanowires in aqueous surfactant solutions. These dispersions can easily be formed into thin films. Optical and electrical studies show these films to display sheet resistances and transmittances ranging from (460 Ω/□, 22%) to (12 kΩ/□, 79%) depending on thickness. For thicker films, we measured the transparent conducting figure of merit to be σ(DC, B)/σ(Op) = 0.32, similar to graphene networks. Thickness measurements gave individual values of σ(Op) = 17,800 S m(-1) and σ(DC, B) = 5700 S m(-1). Films thinner than ∼ 70 nm displayed reduced DC conductivity due to percolative effects.

Two-dimensional nanosheets produced by liquid exfoliation of layered materials.

If they could be easily exfoliated, layered materials would become a diverse source of two-dimensional crystals whose properties would be useful in applications ranging from electronics to energy storage. We show that layered compounds such as MoS(2), WS(2), MoSe(2), MoTe(2), TaSe(2), NbSe(2), NiTe(2), BN, and Bi(2)Te(3) can be efficiently dispersed in common solvents and can be deposited as individual flakes or formed into films. Electron microscopy strongly suggests that the material is exfoliated into individual layers. By blending this material with suspensions of other nanomaterials or polymer solutions, we can prepare hybrid dispersions or composites, which can be cast into films. We show that WS(2) and MoS(2) effectively reinforce polymers, whereas WS(2)/carbon nanotube hybrid films have high conductivity, leading to promising thermoelectric properties.

Size effects and the problem with percolation in nanostructured transparent conductors.

Much research is underway at present to develop nanostructured transparent conductors for use as electrodes. Transparent electrodes typically require high visible transmittances, T > 90%, and so must be very thin. We show that for most nanostructured films thin enough to display T > 90%, the conduction can be described by percolation theory. This means DC conductivities are lower than in bulk, giving correspondingly higher sheet resistances, R(s). To improve our understanding of the consequences of this, we develop a model which relates T to R(s) in the percolation regime. We define a percolative figure of merit, Π, for which high values result in high T and low R(s). High values of Π are achieved for high DC conductivity and low optical conductivity. In addition, the film thickness, t(min), where the DC conductivity first deviates from its bulk value and the percolation exponent, n, must both be as low as possible. We find that this model fits extremely well to much of the data in the literature. We demonstrate that t(min) scales linearly with the smallest dimension of the nanostructure in question (i.e., diameter for wires or thickness for flakes). This clearly confirms that low diameter nanowires or thin platelets are best for transparent conducting applications. We predict the properties of silver nanowire networks to improve as wire diameter is decreased. Networks of wires with D < 20 nm should display properties superior to the best ITO. We demonstrate the deficiencies of standard bulk theory and the importance of understanding percolation by measuring R(s) and T for networks of silver flakes. We measure the bulk ratio of DC to optical conductivity to be ∼35, suggesting R(s) = 100 Ω/◻ and T = 90% are attainable. However, the large flake thickness results in high t(min) and so low Π, resulting in actual values of T = 26% for R(s) = 100 Ω/◻. This makes this material completely unsuitable for transparent conductor applications.

Improvement of transparent conducting nanotube films by addition of small quantities of graphene.

We demonstrate a water-based method to prepare transparent, conducting graphene/single-walled nanotube hybrid films. While the transmittance decreases slightly with increasing graphene content, the DC conductivity, sigma(DC), and sheet resistance scale non-monotonically with film composition. We observe an optimum composition of approximately 3 wt % graphene, which results in a peak in the DC conductivity. We have calculated the figure of merit, the DC to optical conductivity ratio, sigmaDC/sigmaOp, which also shows a peak at this composition. We find that this effect is only present for small graphene flakes. In addition, acid treatment increases both the sigmaDC and sigmaDC/sigmaOp by x2.5. Interestingly, acid treatment is more effective for films close to the optimum composition. This has the effect of sharpening the peaks in both sigmaDC and sigmaDC/sigmaOp. For acid-treated films, addition of 3 wt % graphene results in a 40% increase in sigmaDC/sigmaOp compared to the nanotube-only film, from 12.5 to 18. Optimized, acid-treated films display transmittance of 80% coupled with a sheet resistance of 100 Omega/[square].

High-concentration, surfactant-stabilized graphene dispersions.

A method is presented to produce graphene dispersions, stabilized in water by the surfactant sodium cholate, at concentrations up to 0.3 mg/mL. The process uses low power sonication for long times (up to 400 h) followed by centrifugation to yield stable dispersions. The dispersed concentration increases with sonication time while the best quality dispersions are obtained for centrifugation rates between 500 and 2000 rpm. Detailed TEM analysis shows the flakes to consist of 1-10 stacked monolayers with up to 20% of flakes containing just one layer. The average flake consists of approximately 4 stacked monolayers and has length and width of approximately 1 mum and approximately 400 nm, respectively. These dimensions are surprisingly stable under prolonged sonication. However, the mean flake length falls from approximately 1 mum to approximately 500 nm as the centrifugation rate is increased from 500 to 5000 rpm. Raman spectroscopy shows the flake bodies to be relatively defect-free for centrifugation rates below 2000 rpm. The dispersions can be easily cast into high-quality, free-standing films. The method extends the scope for scalable liquid-phase processing of graphene for a wide range of applications.

Are there fundamental limitations on the sheet resistance and transmittance of thin graphene films?

From published transmittance and sheet resistance data, we have calculated a figure of merit for transparent, conducting graphene films; the DC to optical conductivity ratio, sigma(DC)/sigma(Op). For most reported results, this conductivity ratio clusters around the values sigma(DC)/sigma(Op) = 0.7, 4.5, and 11. We show that these represent fundamental limiting values for networks of graphene flakes, undoped graphene stacks, and graphite films, respectively. The limiting value for graphene flake networks is much too low for transparent-electrode applications. For graphite, a conductivity ratio of 11 gives R(s) = 377Omega/ for T = 90%, far short of the 10 Omega/ minimum requirement for transparent conductors in current driven applications. However, we suggest that substrate-induced doping can potentially increase the 2-dimensional DC conductivity enough to make graphene a viable transparent conductor. We show that four randomly stacked graphene layers can display T approximately 90% and 10 Omega/ if the product of carrier density and mobility reaches nmu = 1.3 x 10(17) V(-1) s(-1). Given achieved doping values and attainable mobilities, this is just possible, resulting in potential values of sigma(DC)/sigma(Op) of up to 330. This is high enough for any transparent conductor application.

High-concentration solvent exfoliation of graphene.

A method is demonstrated to prepare graphene dispersions at high concentrations, up to 1.2 mg mL(-1), with yields of up to 4 wt% monolayers. This process relies on low-power sonication for long times, up to 460 h. Transmission electron microscopy shows the sonication to reduce the flake size, with flake dimensions scaling as t(-1/2). However, the mean flake length remains above 1 microm for all sonication times studied. Raman spectroscopy shows defects are introduced by the sonication process. However, detailed analysis suggests that predominantly edge, rather than basal-plane, defects are introduced. These dispersions are used to prepare high-quality free-standing graphene films. The dispersions can be heavily diluted by water without sedimentation or aggregation. This method facilitates graphene processing for a range of applications.

Flexible, transparent, conducting films of randomly stacked graphene from surfactant-stabilized, oxide-free graphene dispersions.

Graphite is exfoliated in water to give dispersions of mono- and few-layer graphene stabilized by surfactant. These dispersions can be used to form thin, disordered films of randomly stacked, oxide-free, few-layer graphenes. These films are transparent with a direct current conductivity of up to 1.5 x 10(4) S m(-1). The conductivity is stable under flexing for at least 2000 cycles. The electrical properties are limited by disorder and aggregation suggesting future routes for improvement.

Electrical connectivity in single-walled carbon nanotube networks.

Transport in single-walled carbon nanotubes (SWCNTs) networks is shown to be dominated by resistance at network junctions which scale with the size of the interconnecting bundles. Acid treatment, known to dope individual tubes, actually produces a dramatic reduction in junction resistances, whereas annealing significantly increases this resistance. Measured junction resistances for pristine, acid-treated and annealed SWCNT bundles correlate with conductivities of the corresponding films, in excellent agreement with a model in which junctions control the overall network performance.

Silver Nanowire Networks as Flexible, Transparent, Conducting Films: Extremely High DC to Optical Conductivity Ratios.

We have used aqueous dispersions of silver nanowires to prepare thin, flexible, transparent, conducting films. The nanowires are of length and diameter close to 6.5 µm and 85 nm, respectively. At low thickness, the films consist of networks but appear to become bulk-like for mean film thicknesses above ∼160 nm. These films can be very transparent with optical transmittance reaching as high as 92% for low thickness. The transmittance (550 nm) decreases with increasing thickness, consistent with an optical conductivity of 6472 S/m. The films are also very uniform; the transmittance varies spatially by typically <2%. The sheet resistance decreases with increasing thickness, falling below 1 Ω/◻ for thicknesses above 300 nm. The DC conductivity increases from 2 × 10(5) S/m for very thin films before saturating at 5 × 10(6) S/m for thicker films. Similarly, the ratio of DC to optical conductivity increases with increasing thickness from 25 for the thinnest films, saturating at ∼500 for thicknesses above ∼160 nm. We believe this is the highest conductivity ratio ever observed for nanostructured films and is matched only by doped metal oxide films. These nanowire films are electromechanically very robust, with all but the thinnest films showing no change in sheet resistance when flexed over >1000 cycles. Such results make these films ideal as replacements for indium tin oxide as transparent electrodes. We have prepared films with optical transmittance and sheet resistance of 85% and 13 Ω/◻, respectively. This is very close to that displayed by commercially available indium tin oxide.

Liquid phase production of graphene by exfoliation of graphite in surfactant/water solutions.

We have demonstrated a method to disperse and exfoliate graphite to give graphene suspended in water-surfactant solutions. Optical characterization of these suspensions allowed the partial optimization of the dispersion process. Transmission electron microscopy showed the dispersed phase to consist of small graphitic flakes. More than 40% of these flakes had <5 layers with approximately 3% of flakes consisting of monolayers. Atomic resolution transmission electron microscopy shows the monolayers to be generally free of defects. The dispersed graphitic flakes are stabilized against reaggregation by Coulomb repulsion due to the adsorbed surfactant. We use DLVO and Hamaker theory to describe this stabilization. However, the larger flakes tend to sediment out over approximately 6 weeks, leaving only small flakes dispersed. It is possible to form thin films by vacuum filtration of these dispersions. Raman and IR spectroscopic analysis of these films suggests the flakes to be largely free of defects and oxides, although X-ray photoelectron spectroscopy shows evidence of a small oxide population. Individual graphene flakes can be deposited onto mica by spray coating, allowing statistical analysis of flake size and thickness. Vacuum filtered films are reasonably conductive and are semitransparent. Further improvements may result in the development of cheap transparent conductors.

Transparent, flexible, and highly conductive thin films based on polymer-nanotube composites.

We have prepared flexible, transparent, and very conducting thin composite films from poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), filled with both arc discharge and HIPCO single-walled nanotubes, at high loading level. The films are of high optical uniformity. The arc discharge nanotube-filled composites were significantly more conductive, demonstrating DC conductivities of >10(5) S/m for mass fractions >50 wt %. The ratio of DC to optical conductivity was higher for composites with mass fractions of 55-60 wt % than for nanotube-only films. For an 80 nm thick composite, filled with 60 wt % arc discharge nanotubes, this conductivity ratio was maximized at sigma(DC)/sigma(Op) = 15. This translates into transmittance (550 nm) and sheet resistance of 75 and 80 Omega/square, respectively. These composites were electromechanically very stable, showing <1% resistance change over 130 bend cycles.

High-yield production of graphene by liquid-phase exfoliation of graphite.

Fully exploiting the properties of graphene will require a method for the mass production of this remarkable material. Two main routes are possible: large-scale growth or large-scale exfoliation. Here, we demonstrate graphene dispersions with concentrations up to approximately 0.01 mg ml(-1), produced by dispersion and exfoliation of graphite in organic solvents such as N-methyl-pyrrolidone. This is possible because the energy required to exfoliate graphene is balanced by the solvent-graphene interaction for solvents whose surface energies match that of graphene. We confirm the presence of individual graphene sheets by Raman spectroscopy, transmission electron microscopy and electron diffraction. Our method results in a monolayer yield of approximately 1 wt%, which could potentially be improved to 7-12 wt% with further processing. The absence of defects or oxides is confirmed by X-ray photoelectron, infrared and Raman spectroscopies. We are able to produce semi-transparent conducting films and conducting composites. Solution processing of graphene opens up a range of potential large-area applications, from device and sensor fabrication to liquid-phase chemistry.

Mixed protonic-electronic conduction and dielectric response in layered vanadyl phosphate nanocomposites.

The layered vanadyl phosphate, VOPO(4)2H(2)O, is employed to prepare nanosized conducting polypyrrole by redox intercalation method. Transport and dielectric properties of various compositions have been investigated by impedance technique over a temperature range of 300-120 K. Grain boundary conductivity is larger than the bulk conductivity. The conductivity reveals a discontinuity at about 212-235 K. The conductivity is predominantly ionic at high temperature and electronic at low temperature. The dielectric spectra reveal a peak in the frequency range up to 2 MHz for higher concentration of intercalated polypyrrole. The activation energy of conductivity relaxation is different from that of total conductivity derived from the impedance plot.