Improving the Through-Thickness Thermal Conductivity of Carbon Fiber/Epoxy Laminates by Direct Growth of SiC/Graphene Heterostructures on Carbon Fibers.

Poor thermal conductivity in the through-thickness direction is a critical limitation in the performance of carbon fiber-reinforced polymer (CFRP) composites over a broad range of applications in the aviation industry, where heat dissipation is required (e.g., battery packs, electronic housing, and heat spreaders). In this work, it is demonstrated for the first time that a hierarchical network of vertically oriented graphene nanoflakes (GNFs), with nanoconfined silicon carbide (SiC) nanocrystals, self-assembled on carbon fibers (CFs) can provide significant improvement to the thermal conductivity (TC) of CFRPs in the through-thickness direction. The vertically aligned SiC/GNF heterostructures were grown directly on CFs for the first time by single-step plasma-enhanced chemical vapor deposition (PECVD) employing tetramethylsilane (TMS) and methane (CH4) gases at temperatures of 800 and 950 °C. At the deposition temperature of 950 °C, the controlled introduction of SiC/GNF heterostructures induced a 56% improvement in through-thickness TC over the bare CFRP counterparts while simultaneously preserving the tensile strength. The increase in thermal conductivity is accomplished by SiC nanocrystals, which serve as linkage thermal conducting paths between the vertical graphene layers, further enhancing the smooth transmission of phonons in the vertical direction. The work demonstrates for the first time the unique potential of novel SiC/GNF heterostructures for attaining strong and thermally conductive multifunctional CFRPs.

Impedimetric detection of miRNA biomarkers using paper-based electrodes modified with bulk crystals or nanosheets of molybdenum disulfide.

Paper-based electrodes modified with molybdenum disulfide (MoS2) in the form of bulk crystals or exfoliated nanosheets were developed and used as a biosensing platform for the impedimetric detection of miRNAs (miRNA-155 and miRNA-21) related to early diagnosis of lung cancer. For this purpose, MoS2 crystals or nanosheets were used for the modification of the working electrode area of paper-based platform for the first time in this study. The proposed assay offers sensitive and selective detection of microRNAs by electrochemical impedance spectroscopy (EIS) technique. The entire assay, both the electrode modification and the miRNA detection being completed in 30 min and a single sample droplet (5 µL) was enough to cover working electrode area which enabled analysis in low sample volumes. The limits of detection (LOD) for miRNA-21 and miRNA-155 were calculated both in buffer and fetal bovine serum media. It is found that the LOD is varying between 1 and 200 ng/mL. In comparison to nanosheets, a larger electroactive surface area was obtained with bulk MoS2 resulting in lower LOD values on miRNA detection. The paper-based electrodes showed high specificity towards their target sequences. Moreover, they effectively discriminated single base mismatched non-target sequences. The advantages of these MoS2 paper based electrodes include high sensitivity, and low-cost provide great potential for improved monitoring of miRNA biomarkers even in artificial serum media.

Inhibition of corrosion causing Pseudomonas aeruginosa using plasma-activated water.

AIMS: The cost of Microbiologically Influenced Corrosion (MIC) significantly affects a wide range of sectors. This study aims to assess the efficiency of a novel technology based on the use of plasma-activated water (PAW) in inhibiting corrosion caused by bacteria. METHODS AND RESULTS: This study evaluated the effectiveness of PAW, produced by a plasma bubble reactor, in reducing corrosion causing Pseudomonas aeruginosa planktonic cells in tap water and biofilms were grown onto stainless steel (SS) coupons. Planktonic cells and biofilms were treated with PAW at different discharge frequencies (500-1500 Hz) and exposure times (0-20 min). P. aeruginosa cells in tap water were significantly reduced after treatment, with higher exposure times and discharge frequencies achieving higher reductions. Also, PAW treatment led to a gradual reduction for young and mature biofilms, achieving >4-Log reductions after 20 min. Results were also used to develop two predictive inactivation models. CONCLUSIONS: This work presents evidence that PAW can be used to inactivate both planktonic cells and biofilms of P. aeruginosa. Experimental and theoretical results also demonstrate that reduction is dependent on discharge frequency and exposure time. SIGNIFICANCE AND IMPACT OF THE STUDY: This work demonstrates the potential of using PAW as means to control MIC.

Paper-Based Electrochemical Biosensors for Voltammetric Detection of miRNA Biomarkers Using Reduced Graphene Oxide or MoS2 Nanosheets Decorated with Gold Nanoparticle Electrodes.

Paper-based biosensors are considered simple and cost-efficient sensing platforms for analytical tests and diagnostics. Here, a paper-based electrochemical biosensor was developed for the rapid and sensitive detection of microRNAs (miRNA-155 and miRNA-21) related to early diagnosis of lung cancer. Hydrophobic barriers to creating electrode areas were manufactured by wax printing, whereas a three-electrode system was fabricated by a simple stencil approach. A carbon-based working electrode was modified using either reduced graphene oxide or molybdenum disulfide nanosheets modified with gold nanoparticle (AuNPs/RGO, AuNPs/MoS2) hybrid structures. The resulting paper-based biosensors offered sensitive detection of miRNA-155 and miRNA-21 by differential pulse voltammetry (DPV) in only 5.0 µL sample. The duration in our assay from the point of electrode modification to the final detection of miRNA was completed within only 35 min. The detection limits for miRNA-21 and miRNA-155 were found to be 12.0 and 25.7 nM for AuNPs/RGO and 51.6 and 59.6 nM for AuNPs/MoS2 sensors in the case of perfectly matched probe-target hybrids. These biosensors were found to be selective enough to distinguish the target miRNA in the presence of single-base mismatch miRNA or noncomplementary miRNA sequences.

Paper-based electrode assemble for impedimetric detection of miRNA.

In the present work, a paper-based electrode assemble was developed and implemented to detect target microRNA 155 (miRNA 155) via electrochemical impedance spectroscopy (EIS) measurements. In this concept, gold nanoparticles (AuNPs) modified paper based electrode assemble system (AuNP-PE) was designed, and characterized by scanning electron microscopy (SEM), cyclic voltammetry (CV) and EIS measurements. The impedimetric detection of miRNA 155 was performed by measuring the fractional change at the charge transfer resistance (Rct). The detection limits were found as 33.8 nM in PBS and 93.4 nM in fetal bovine serum (FBS) medium, respectively. The selectivity of the proposed assay was tested against to non-complementary (NC) and mismatch (MM) miRNA sequences in the presence of mixture sample containing miRNA:NC (1:1) and miRNA:MM (1:1) in PBS (pH 7.40) or FBS. The analytical performance and the selectivity of impedimetric biosensor were also tested in FBS.

Correction: Bismuthene nanosheets produced by ionic liquid assisted grinding exfoliation and their use for oxygen reduction reaction.

[This corrects the article DOI: 10.1039/D0RA09763B.].

Bismuthene nanosheets produced by ionic liquid assisted grinding exfoliation and their use for oxygen reduction reaction.

We report the simple synthesis of bismuthene nanosheets (BiNS) by ionic liquid assisted grinding exfoliation, followed by size selection sequential centrifugation steps for the first time. The exfoliation process results in the formation of self-assembled spherule-like superstructures with abundant edge sites, which are able to catalyze the oxygen reduction reaction (ORR) via a two-electron pathway, with a higher efficiency than the bulk Bismuth. We rationalize the enhanced ORR activity of the BiNS to: (i) the presence of 1 dimensional topological edge states, which provide strong conduction channels for electron hopping between the bismuth layers and (ii) the more active role of edge sites in facilitating O2 adsorption and dissociation of O-O bonds compared to the basal plane. The present study provides a pathway for employing 2D topological insulators as a new class of electrocatalysts for clean energy applications.

Size-dependent stability of ultra-small α-/ß-phase tin nanocrystals synthesized by microplasma.

Nanocrystals sometimes adopt unusual crystal structure configurations in order to maintain structural stability with increasingly large surface-to-volume ratios. The understanding of these transformations is of great scientific interest and represents an opportunity to achieve beneficial materials properties resulting from different crystal arrangements. Here, the phase transformation from α to ß phases of tin (Sn) nanocrystals is investigated in nanocrystals with diameters ranging from 6.1 to 1.6 nm. Ultra-small Sn nanocrystals are achieved through our highly non-equilibrium plasma process operated at atmospheric pressures. Larger nanocrystals adopt the ß-Sn tetragonal structure, while smaller nanocrystals show stability with the α-Sn diamond cubic structure. Synthesis at other conditions produce nanocrystals with mean diameters within the range 2-3 nm, which exhibit mixed phases. This work represents an important contribution to understand structural stability at the nanoscale and the possibility of achieving phases of relevance for many applications.

Sensitive Chronocoulometric Detection of miRNA at Screen-Printed Electrodes Modified by Gold-Decorated MoS2 Nanosheets.

Developing novel simple and ultrasensitive strategies for detecting microRNAs (miRNAs) is highly desirable because of their association with early cancer diagnostic and prognostic processes. Here a new chronocoulometric sensor, based on semiconducting 2H MoS2 nanosheets (MoS2 NSs) decorated with a controlled density of monodispersed small gold nanoparticles (AuNPs@MoS2), was fabricated via electrodeposition, for the highly sensitive detection of miRNA-21. The size and interparticle spacing of AuNPs were optimized by controlling nucleation and growth rates through the tuning of deposition potential and Au precursor concentration and by getting simultaneous feedback from morphological and electrochemical activity studies. The sensing strategy, involved the selective immobilization of the thiolated capture probe DNA (CP) at AuNPs and hybridization of CP to a part of the miRNA target, whereas the remaining part of the target was complementary to a signaling nonlabeled DNA sequence that served to amplify the target upon hybridization. Chronocoulometry provided precise quantification of nucleic acids at each step of the sensor assay by interrogating [Ru(NH3)6]3+ electrostatically bound to phosphate backbones of oligonucleotides. A detailed and systematic optimization study demonstrated that the thinnest and smallest MoS2 NSs improved the sensitivity of the AuNP@MoS2 sensor, achieving an impressive detection limit of ≈100 aM, which is 2 orders of magnitude lower than that of a bare Au electrode and also enhanced the DNA-miRNA hybridization efficiency by 25%. Such an improved performance can be attributed to the controlled packing density of CPs achieved by their self-assembly on AuNPs, large interparticle density, small size, and intimate coupling between AuNPs and MoS2. Alongside the outstanding sensitivity, the sensor exhibited an excellent selectivity down to femtomolar concentrations, for discriminating a complementary miRNA-21 target in a complex system composed of different foreign targets including mismatched and noncomplementary miRNA-155. These advantages make our sensor a promising contender in the point of care miRNA sensor family for medical diagnostics.

The effects of exfoliation, organic solvents and anodic activation on the catalytic hydrogen evolution reaction of tungsten disulfide.

The rational design of transition metal dichalcogenide electrocatalysts for efficiently catalyzing the hydrogen evolution reaction (HER) is believed to lead to the generation of a renewable energy carrier. To this end, our work has made three main contributions. At first, we have demonstrated that exfoliation via ionic liquid assisted grinding combined with gradient centrifugation is an efficient method to exfoliate bulk WS2 to nanosheets with a thickness of a few atomic layers and lateral size dimensions in the range of 100 nm to 2 nm. These WS2 nanosheets decorated with scattered nanodots exhibited highly enhanced catalytic performance for HER with an onset potential of -130 mV vs. RHE, an overpotential of 337 mV at 10 mA cm-2 and a Tafel slope of 80 mV dec-1 in 0.5 M H2SO4. Secondly, we found a strong aging effect on the electrocatalytic performance of WS2 stored in high boiling point organic solvents such as dimethylformamide (DMF). Importantly, the HER ability could be recovered by removing the organic (DMF) residues, which obstructed the electron transport, with acetone. Thirdly, we established that the HER performance of WS2 nanosheets/nanodots could be significantly enhanced by activating the electrode surface at a positive voltage for a very short time (60 s), decreasing the kinetic overpotential by more than 80 mV at 10 mA cm-2. The performance enhancement was found to arise primarily from the ability of a formed proton-intercalated amorphous tungsten trioxide (a-WO3) to provide additional active sites and favourably modify the immediate chemical environment of the WS2 catalyst, rendering it more favorable for local proton delivery and/or transport to the active edge site of WS2. Our results provide new insights into the effects of organic solvents and electrochemical activation on the catalytic performance of two-dimensional WS2 for HER.

Chemical Modification of Graphene Oxide by Nitrogenation: An X-ray Absorption and Emission Spectroscopy Study.

Nitrogen-doped graphene oxides (GO:Nx) were synthesized by a partial reduction of graphene oxide (GO) using urea [CO(NH2)2]. Their electronic/bonding structures were investigated using X-ray absorption near-edge structure (XANES), valence-band photoemission spectroscopy (VB-PES), X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS). During GO:Nx synthesis, different nitrogen-bonding species, such as pyrrolic/graphitic-nitrogen, were formed by replacing of oxygen-containing functional groups. At lower N-content (2.7 at%), pyrrolic-N, owing to surface and subsurface diffusion of C, N and NH is deduced from various X-ray spectroscopies. In contrast, at higher N-content (5.0 at%) graphitic nitrogen was formed in which each N-atom trigonally bonds to three distinct sp2-hybridized carbons with substitution of the N-atoms for C atoms in the graphite layer. Upon nitrogen substitution, the total density of state close to Fermi level is increased to raise the valence-band maximum, as revealed by VB-PES spectra, indicating an electron donation from nitrogen, molecular bonding C/N/O coordination or/and lattice structure reorganization in GO:Nx. The well-ordered chemical environments induced by nitrogen dopant are revealed by XANES and RIXS measurements.

Electrocatalytic Hydrogen Evolution Reaction on Edges of a Few Layer Molybdenum Disulfide Nanodots.

The design and development of inexpensive highly efficient electrocatalysts for hydrogen production underpins several emerging clean-energy technologies. In this work, for the first time, molybdenum disulfide (MoS2) nanodots have been synthesized by ionic liquid assisted grinding exfoliation of bulk platelets and isolated by sequential centrifugation. The nanodots have a thickness of up to 7 layers (∼4 nm) and an average lateral size smaller than 20 nm. Detailed structural characterization established that the nanodots retained the crystalline quality and low oxidation states of the bulk material. The small lateral size and reduced number of layers provided these nanodots with an easier path for the electron transport and plentiful active sites for the catalysis of hydrogen evolution reaction (HER) in acidic electrolyte. The MoS2 nanodots exhibited good durability and a Tafel slope of 61 mV dec(-1) with an estimated onset potential of -0.09 V vs RHE, which are considered among the best values achieved for 2H phase. It is envisaged that this work may provide a simplistic route to synthesize a wide range of 2D layered nanodots that have applications in water splitting and other energy related technologies.

Oxygen reduction reaction by electrochemically reduced graphene oxide.

We show that a partially reduced graphene oxide electrocatalyst, synthesized by electrochemical reduction of graphene oxide (GO), displays significantly enhanced catalytic activity towards the oxygen reduction reaction (ORR) in alkaline solutions compared to the starting GO. The electrochemical partial reduction of GO was confirmed by X-ray diffraction and X-ray photoelectron spectroscopy. Electrochemical impedance spectroscopy (EIS) verified the enhanced electron transfer ability of the electrochemically reduced graphene oxide (ErGO) compared to GO. The resultant ErGO electrode showed enhanced capacitance and an ORR onset potential of -0.11 V vs. Ag/AgCl, similar to that of a nitrogen doped reduced graphene oxide (NrGO) electrode produced by a hydrothermal process. However the ErGO exhibited considerably lower electron transfer numbers (2.0-3.3 at a potential range of -0.4 V to -1.0 V) indicating that although both catalysts operate under combined 4e(-) and 2e(-) ORR processes, ErGO follows a more predominant 2e(-) pathway. The ORR process in ErGO has been linked to the presence of quinone functional groups, which favour the 2e(-) ORR pathway.

Tuning the catalytic activity of graphene nanosheets for oxygen reduction reaction via size and thickness reduction.

Currently, the fundamental factors that control the oxygen reduction reaction (ORR) activity of graphene itself, in particular, the dependence of the ORR activity on the number of exposed edge sites remain elusive, mainly due to limited synthesis routes of achieving small size graphene. In this work, the synthesis of low oxygen content (<2.5±0.2 at. %), few layer graphene nanosheets with lateral dimensions smaller than a few hundred nanometers were achieved using a combination of ionic liquid assisted grinding of high purity graphite coupled with sequential centrifugation. We show for the first time that the graphene nanosheets possessing a plethora of edges exhibited considerably higher electron transfer numbers compared to the thicker graphene nanoplatelets. This enhanced ORR activity was accomplished by successfully exploiting the plethora of edges of the nanosized graphene as well as the efficient electron communication between the active edge sites and the electrode substrate. The graphene nanosheets were characterized by an onset potential of -0.13 V vs Ag/AgCl and a current density of -3.85 mA/cm2 at -1 V, which represent the best ORR performance ever achieved from an undoped carbon based catalyst. This work demonstrates how low oxygen content nanosized graphene synthesized by a simple route can considerably impact the ORR catalytic activity and hence it is of significance in designing and optimizing advanced metal-free ORR electrocatalysts.

Biosensor based on ultrasmall MoS2 nanoparticles for electrochemical detection of H2O2 released by cells at the nanomolar level.

Monodispersed surfactant-free MoS2 nanoparticles with sizes of less than 2 nm were prepared from bulk MoS2 by simple ultrasonication and gradient centrifugation. The ultrasmall MoS2 nanoparticles expose a large fraction of edge sites, along with their high surface area, which lead to attractive electrocatalytic activity for reduction of H2O2. An extremely sensitive H2O2 biosensor based on MoS2 nanoparticles with a real determination limit as low as 2.5 nM and wide linear range of 5 orders of magnitude was constructed. On the basis of this biosensor, the trace amount of H2O2 released from Raw 264.7 cells was successfully recorded, and an efficient glucose biosensor was also fabricated. Since H2O2 is a byproduct of many oxidative biological reactions, this work serves as a pathway for the application of MoS2 in the fields of electrochemical sensing and bioanalysis.

Size-Dependent Enhancement of Electrocatalytic Oxygen-Reduction and Hydrogen-Evolution Performance of MoS2 Particles.

MoS2 particles with different size distributions were prepared by simple ultrasonication of bulk MoS2 followed by gradient centrifugation. Relative to the inert microscale MoS2, nanoscale MoS2 showed significantly improved catalytic activity toward the oxygen-reduction reaction (ORR) and hydrogen-evolution reaction (HER). The decrease in particle size was accompanied by an increase in catalytic activity. Particles with a size of around 2 nm exhibited the best dual ORR and HER performance with a four-electron ORR process and an HER onset potential of -0.16 V versus the standard hydrogen electrode (SHE). This is the first investigation on the size-dependent effect of the ORR activity of MoS2, and a four-electron transfer route was found. The exposed abundant Mo edges of the MoS2 nanoparticles were proven to be responsible for the high ORR catalytic activity, whereas the origin of the improved HER activity of the nanoparticles was attributed to the plentiful exposed S edges. This newly discovered process provides a simple protocol to produce inexpensive highly active MoS2 catalysts that could easily be scaled up. Hence, it opens up possibilities for wide applications of MoS2 nanoparticles in the fields of energy conversion and storage.

Thermal stability study of nitrogen functionalities in a graphene network.

Catalyst-free vertically aligned graphene nanoflakes possessing a large amount of high density edge planes were functionalized using nitrogen species in a low energy N(+) ion bombardment process to achieve pyridinic, cyanide and nitrogen substitution in hexagonal graphitic coordinated units. The evolution of the electronic structure of the functionalized graphene nanoflakes over the temperature range 20-800 °C was investigated in situ, using high resolution x-ray photoemission spectroscopy. We demonstrate that low energy irradiation is a useful tool for achieving nitrogen doping levels up to 9.6 at.%. Pyridinic configurations are found to be predominant at room temperature, while at 800 °C graphitic nitrogen configurations become the dominant ones. The findings have helped to provide an understanding of the thermal stability of nitrogen functionalities in graphene, and offer prospects for controllable tuning of nitrogen doping in device applications.