New insights on energetic properties of graphene oxide (GO) materials and their safety and environmental risks.

Graphene oxide (GO) has been recognized as a thermally unstable and energetic material, but surprisingly its environmental and safety risks were not fully explored, defined, and regulated. In this study, systematic explosivity and flammability characterizations of commercial GO materials were conducted to evaluate the influence of key parameters such as physical forms (paste, powders, films, and aerogels), temperature, heating rate, mass, and heating environment, as well as their potential safety and environmental impacts. Results based on thermogravimetric analysis (TGA) showed that GO in paste and powder forms have lower temperature thresholds (>180-192 °C) to initiate micro-explosions compared to GO film and aerogels (> 205 °C and 213 °C) regardless of the environment (inert, air, or oxygen). The observed explosive behavior can be explained by thermal runaway reactions as a result of thermal deoxygenation and decomposition of oxygen functional groups. Flammability rating and limiting oxygen index (LOI) results confirmed that GO films are flammable materials that can spontaneously propagate flame in a low oxygen environment (~11 %). These results provided new insights about potential safety and environmental risks of GO materials, which somehow were not considered, suggesting urgent actions to improve current safety protocols for labeling, handling, transporting, and storage practices from manufacturers to the end-users.

Fractal Design for Advancing the Performance of Chemoresistive Sensors.

The rapid advancement of internet of things (IoT)-enabled applications along with connected automation in sensing technologies is the heart of future intelligent systems. The probable applications have significant implications, from chemical process monitoring to agriculture, mining, space, wearable electronics, industrial manufacturing, smart cities, and point-of-care (PoC) diagnostics. Advancing sensor performance such as sensitivity to detect trace amounts (ppb-ppm) of analytes (gas/VOCs), selectivity, portability, and low cost is critical for many of these applications. These advancements are mainly achieved by selecting and optimizing sensing materials by their surface functionalization and/or structural optimization to achieve favorable transport characteristics or chemical binding/reaction sites. Surprisingly, the sensor geometry, shapes, and patterns were not considered as critical parameters, and most of these sensors were designed by following simple planar and interdigitated electrode geometry. In this study, we introduce a new bioinspired fractal approach to design chemoresistive sensors with fractal geometry, which grasp the architecture of fern leaves represented by the geometric group of space-filling curves of fractal patterns. These fractal sensors were printed by an extrusion process on a flexible substrate (PET) using specially formulated graphene ink as a sensing material, which provided significant enhancement of the active surface area to volume ratio and allowed high-resolution fractal patterning along with a reduced current transportation path. To demonstrate the advantages and influence of fractal geometry on sensor performance, here, three different kinds of sensors were fabricated based on different fractal geometrics (Sierpinski, Peano, and Hilbert), and the sensing performance was explored toward different VOC analytes (e.g., ethanol, methanol, and acetone). Among all these fractal-designed sensors including interdigitate sensors, the Hilbert-designed printed sensor shows enhanced sensing properties in terms of fast response time (6 s for 30 ppm), response value (14%), enhanced detection range (5-100 ppm), high selectivity, and low interference to humidity (up to RH 80%) for ethanol at room temperature (20 °C). Moreover, a significant improvement of this sensor performance was observed by applying the mechanical deformation (positive bending) technique. The practical application of this sensor was successfully demonstrated by monitoring food spoilage using a commercial box of strawberries as a model. Based on these presented results, this biofractal biomimetic VOC sensor is demonstrated for a prospective application in food monitoring.

Comparative antibacterial activity of 2D materials coated on porous-titania.

Plasma electrolytic oxidation (PEO) is a well-established technique for the treatment of titanium-based materials. The formed titania-PEO surface can improve the osseointegration properties of titanium implants. Nevertheless, it can not address bacterial infection problems associated with bone implants. Recently, 2-dimensional (2D) materials such as graphene oxide (GO), MXene, and hexagonal boron nitride (hBN) have received considerable attention for surface modifications showing their antibacterial properties. In this paper, a comparative study on the effect of partial deposition of these three materials over PEO titania substrates on the antibacterial efficiency and bioactivity is presented. Their partial deposition through drop-casting instead of continuous film coating is propsed to simultaneously address both antibacterial and osseointegration abilities. Our results demonstrate the dose-dependent nature of the deposited antibacterial agent on the PEO substrate. GO-PEO and MXene-PEO samples showed the highest antibacterial activity with 70 (±2) % and 97 (±0.5) % inactivation of S. aureus colonies in the low concentration group, respectively. Furthermore, only samples in the higher concentration group were effective against E. coli bacteria with 18 (±2) % and 17 (±4) % decrease in numbers of colonies for hBN-PEO and GO-PEO samples, respectively. Moreover, all antibacterial samples demonstrated acceptable bioactivity and good biocompatibility, making them a considerable candidates for the next generation of antibacterial titanium implants.

Biosensing platform on ferrite magnetic nanoparticles: Synthesis, functionalization, mechanism and applications.

Ferrite magnetic nanoparticles (FMNPs) are gaining popularity to design biosensors for high-performance clinical diagnosis. The fusion of information shows that FMNPs based biosensors require well-tuned FMNPs as detection probes to produce large and specific biological signals with minimal non-specific binding. Nevertheless, there is a noticeable lacuna of information to solve the issues related to suitable synthesis route, particle size reduction, functionalization, sensitivity towards targeted intercellular biological tiny particles, and lower signal-to-noise ratio. Therefore it allows exploring unique characteristics of FMNPs to design a suitable sensing device for intracellular measurements and diseases detection. This review focuses on the extensively used synthesis routes, their advantages and limitations, crystalline structure, functionalization, along with recent applications of FMNPs in biosensors, taking into consideration their analytical figures of merit and range of linearity. This work also addresses the current progress, key factors for sensitivity, selectivity and productivity improvement along with the challenges, future trends and perspectives of FMNPs based biosensors.

Graphene ink for 3D extrusion micro printing of chemo-resistive sensing devices for volatile organic compound detection.

Printed electronic sensors offer a breakthrough in the availability of low-cost sensor devices for improving the quality of human life. Conductive ink is the core of printing technology and also one of the fastest growing sector among all ink industries. Among many developed conductive inks, graphene-based inks are especially recognized as very promising for future fabrication of devices due to their low cost, unique properties, and compatibility with various platforms such as plastics, textiles, and paper. The development of graphene ink formulations for achieving high conductivity and high resolution printing is highly realized in 2D inkjet printing. Unfortunately, the ongoing development of graphene inks is possibly hampered by the non-uniform particle size and structures (e.g., different shapes and number of layers), which adversely affect printing resolution, conductivity, adhesion, and structural integrity. This study presents an environmentally sustainable route to produce graphene inks specifically designed for 3D extrusion-printing. The application of the prepared ink is demonstrated by mask-free automatic patterning of sensing devices for the detection of volatile organic compounds (VOCs). The sensing devices fabricated with this new ink display high-resolution patterning (average height/thickness of ∼12 µm) and a 10-fold improvement in the surface area/volume (SA/V) ratio compared to a conventional drop casting method. The extrusion printed sensors show enhanced sensing characteristics in terms of sensitivity and selectivity towards trace amount of VOC (e.g. 5 ppm ethanol) at room temperature (20 °C), which highlights that our method has highly promising potential in graphene printing technology for sensing applications.

Multiple applications of bio-graphene foam for efficient chromate ion removal and oil-water separation.

This paper presents the synthesis of bio-graphene foams (bGFs) from renewable sources, and the application of bGFs as new adsorbents in removal of chromate ions and oil contaminants from waste water. A two-step synthetic method was developed to produce bGFs with unique porous architecture and high specific surface area (up to 805 m2 g-1) that is highly desirable for environmental applications. The adsorption performance of prepared bGFs for removal of chromate ions from water was studied in relation to CrO42- concentration, adsorbent load, pH, and contact time to confirm adsorption capacity, kinetics and pH dependence. The adsorption isotherms of chromate ions were consistent with the Langmuir model, revealing an outstanding adsorption capacity of 245 mg of Cr(VI)/g bGFs (pH∼7). bGFs were capable of reducing Cr(VI) in water below the maximum permissible level (0.050 mg dm-3) for human consumption (WHO). In a second application, our results convincingly showed excellent performance of bGFs in separating organic solvents and oils from water in a continuous oil-water separation process showing 99.1% and 98.8% separation efficiency for toluene and petroleum, respectively. Our findings confirm that the outstanding performance of bGFs, and suggest their use as efficient adsorbents for environmental remediation.

Functional inks and extrusion-based 3D printing of 2D materials: a review of current research and applications.

Graphene and related 2D materials offer an ideal platform for next generation disruptive technologies and in particular the potential to produce printed electronic devices with low cost and high throughput. Interest in the use of 2D materials to create functional inks has exponentially increased in recent years with the development of new ink formulations linked with effective printing techniques, including screen, gravure, inkjet and extrusion-based printing towards low-cost device manufacturing. Exfoliated, solution-processed 2D materials formulated into inks permits additive patterning onto both rigid and conformable substrates for printed device design with high-speed, large-scale and cost-effective manufacturing. Each printing technique has some sort of clear advantages over others that requires characteristic ink formulations according to their individual operational principles. Among them, the extrusion-based 3D printing technique has attracted heightened interest due to its ability to create three-dimensional (3D) architectures with increased surface area facilitating the design of a new generation of 3D devices suitable for a wide variety of applications. There still remain several challenges in the development of 2D material ink technologies for extrusion printing which must be resolved prior to their translation into large-scale device production. This comprehensive review presents the current progress on ink formulations with 2D materials and their broad practical applications for printed energy storage devices and sensors. Finally, an outline of the challenges and outlook for extrusion-based 3D printing inks and their place in the future printed devices ecosystem is presented.

Magnetic iron oxide nanoparticles decorated graphene for chemoresistive gas sensing: The particle size effects.

We report a synthesis of magnetic nanoparticles chemically immobilized onto reduced graphene oxide sheets (referred to as rGO-Fe3O4 NPs) as a gas and vapor sensing platform with precisely designed particle size of 5, 10 and 20 nm to explore their influence of particle size on sensing performance. The rGO-Fe3O4 NP sensors have been investigated their responses to different gases and volatile organic compounds (VOCs) at part-per-million (ppm) levels. Results show that the Fe3O4 NPs with smaller size (5 and 10 nm) on the rGO surface led to a lower sensitivity, while particles of a size of 20 nm have a significant enhancement of sensitivity compared to the bare rGO sensor. The rGO-Fe3O4 NP20 sensor can detect trace amounts of NO2 gas and ethanol vapor at the 1 ppm and is highly selective to the NO2 and ethanol among other tested gases and VOCs, respectively. The particle size causes different distribution behaviour of NPs over rGO surface and interspaced between them, which results in deceased or increased the surface interactions between gas and graphene. The NPs themselves contained different defects level and the charge depletion layer that affect their adsorption gas/vapor molecules, which are explained for different sensing responses.

Electrocatalytic Activity of a 2D Phosphorene-Based Heteroelectrocatalyst for Photoelectrochemical Cells.

Research into efficient synthesis, fundamental properties, and potential applications of phosphorene is currently the subject of intense investigation. Herein, solution-processed phosphorene or few-layer black phosphorus (FL-BP) sheets are prepared using a microwave exfoliation method and used in photoelectrochemical cells. Based on experimental and theoretical (DFT) studies, the FL-BP sheets are found to act as catalytically active sites and show excellent electrocatalytic activity for triiodide reduction in dye-sensitized solar cells. Importantly, the device fabricated based on the newly designed cobalt sulfide (CoSx ) decorated nitrogen and sulfur co-doped carbon nanotube heteroelectrocatalyst coated with FL-BP (FL-BP@N,S-doped CNTs-CoSx ) displayed an impressive photovoltaic efficiency of 8.31 %, outperforming expensive platinum based cells. This work paves the way for using phosphorene-based electrocatalysts for next-generation energy-storage systems.

Effects of surface coating on reducing friction and wear of orthopaedic implants.

Coatings such as diamond-like carbon (DLC) and titanium nitride (TiN) are employed in joint implants due to their excellent tribological properties. Recently, graphite-like carbon (GLC) and tantalum (Ta) have been proven to have good potential as coating as they possess mechanical properties similar to bones-high hardness and high flexibility. The purpose of this systematic literature review is to summarize the coating techniques of these four materials in order to compare their mechanical properties and tribological outcomes. Eighteen studies published between January 2000 and February 2013 have met the inclusion criteria for this review. Details of their fabrication parameters, material and mechanical properties along with the tribological outcomes, such as friction and wear rate, were identified and are presented in a systematic way. Although experiment conditions varied, we conclude that Ta has the lowest wear rate compared to DLC, GLC and TiN because it has a lower wear rate with high contact pressure as well as higher hardness to elasticity ratio. However, a further tribology test is needed in an environment which replicates artificial joints to confirm the acceptability of these findings.