Graphene-based nanomaterials for the removal of emerging contaminants of concern from water and their potential adaptation for point-of-use applications.

Considering the plethora of work on the exceptional environmental performance of 2D nanomaterials, there is still a missing link in addressing their practical application in point-of-use (POU) water treatment. By reviewing the exceptional environmental performance of 2D nanomaterials with specific emphasis on graphene and its derivatives, this review aims at inspiring further discussions and research in graphene-based POU water treatment with particular focus on the removal of emerging contaminants of concern (ECCs), which is largely missing in the literature. We outlined the prevalence of ECCs in the environment, their health effects both on humans and marine life, and the potential of efficiently removing them from water using three-dimensional graphene-based macrostructures to ensure ease of adsorbent recovery and reuse compared to nanostructures. Given various successful studies showing superior adsorption capacity of graphene nanosheets, we give an account of the recent developments in graphene-based adsorbents. Moreover, several cost-effective materials which can be easily self-assembled with nanosheets to improve their environmental performance and safety for POU water treatment purposes were highlighted. We highlighted the strategy to overcome challenges of adsorbent regeneration and contaminant degradation; and concluded by noting the need for policy makers to act decisively considering the conservative nature of the water treatment industry, and the potential health risks from ingesting ECCs through drinking water. We further justified the need for the development of advanced POU water treatment devices in the face of the growing challenges regarding ECCs in surface water, and the rising cases of drinking water advisories across the world.

Green synthesis of carbon dots and their applications.

Carbon dots (CDs) are nanoparticles with tunable physicochemical and optical properties. Their resistance to photobleaching and relatively low toxicity render them attractive alternatives to fluorescent dyes and heavy metal-based quantum dots in the fields of bioimaging, sensing, catalysis, solar cells, and light-emitting diodes, among others. Moreover, they have garnered considerable attention as they lend themselves to green synthesis methods. Increasingly, one-pot syntheses comprising exclusively of renewable raw materials or renewable refined compounds are gaining favor over traditional approaches that rely on harsh chemicals and energy intensive conditions. The field of green CD synthesis is developing rapidly; however, challenges persist in ensuring the consistency of their properties (e.g., fluorescence quantum yield) relative to conventional preparation methods. This has mostly limited their use to sensing and bioimaging, leaving opportunities for development in optoelectronic applications. Herein, we discuss the most common green CD synthesis and purification methods reported in the literature and the renewable precursors used. The physical, chemical, and optical properties of the resulting green-synthesized CDs are critically reviewed, followed by a detailed description of their applications in sensing, bioimaging, biomedicine, inks, and catalysis. We conclude with an outlook on the future of green CD synthesis. Future research efforts should address the broad knowledge gap between CDs synthesized from renewable versus non-renewable precursors, focusing on discrepancies in their physical, chemical, and optical properties. The development of cost effective, safe, and sustainable green CDs with tunable properties will broaden their implementation in largely untapped applications, which include drug delivery, photovoltaics, catalysis, and more.

Reply to the 'Comment on "Hierarchically porous, ultra-strong reduced graphene oxide-cellulose nanocrystal sponges for exceptional adsorption of water contaminants"' by J. Ma, Y. Xiong and F. Yu, Nanoscale, 2019, 11, DOI: 10.1039/C8NR08780F.

We respond to the comments of Ma et al. and clarify the rationale behind our methodology and why it is suitable for our work.

Environmental performance of graphene-based 3D macrostructures.

Three-dimensional macrostructures (3DMs) of graphene and graphene oxide are being developed for fast and efficient removal of contaminants from water and air. The large specific surface area, versatile surface chemistry and exceptional mechanical properties of graphene-based nanosheets enable the formation of robust and high-performance 3DMs such as sponges, membranes, beads and fibres. However, little is known about the relationship between the materials properties of graphene-based 3DMs and their environmental performance. In this Review, we summarize the self-assembly and environmental applications of graphene-based 3DMs in removing contaminants from water and air. We also develop the critical link between the materials properties of 3DMs and their environmental performance, and identify the key parameters that influence their capacities for contaminant removal.

Antimicrobial Hierarchically Porous Graphene Oxide Sponges for Water Treatment.

Water treatment technologies based on graphene oxide (GO) sponges show promise due to their high surface area and versatile chemistry, yielding an excellent adsorption affinity for different contaminants. However, the bacteria removal capacity and the intrinsic antimicrobial properties of GO sponges are not well understood. While GO has been successfully functionalized with antibiotics or metal biocides, these antimicrobials present cytotoxicity concerns. Natural antimicrobial agents such as antimicrobial enzymes, peptides, and polymers hold promise in this respect due to their relatively low cost, biocompatibility, and ability to readily functionalize GO by covalent bond formation with oxygen-containing functional groups. In this work, the antimicrobial enzyme lysozyme, antimicrobial peptide nisin, and antimicrobial polyamide ε-poly-l-lysine were used to covalently functionalize the surface of a hierarchically porous GO sponge. The antimicrobial activity of the functionalized material was demonstrated against two model organisms: the Gram-positive B. subtilis and Gram-negative E. coli. The performance of the porous material in a simulated filtration context was evaluated using packed column experiments, and an improved bacterial retention of both strains by the functionalized sponge was demonstrated. Furthermore, samples of spent sponge after filtration were evaluated with a membrane integrity assay demonstrating antimicrobial activity in a continuous flow mode.

Hierarchically porous, ultra-strong reduced graphene oxide-cellulose nanocrystal sponges for exceptional adsorption of water contaminants.

Self-assembly of graphene oxide (GO) nanosheets into porous 3D sponges is a promising approach to exploit their capacity to adsorb contaminants while facilitating the recovery of the nanosheets from treated water. Yet, forming mechanically robust sponges with suitable adsorption properties presents a significant challenge. Ultra-strong and highly porous 3D sponges are formed using GO, vitamin C (VC), and cellulose nanocrystals (CNCs) - natural nanorods isolated from wood pulp. CNCs provide a robust scaffold for the partially reduced GO (rGO) nanosheets resulting in an exceptionally stiff nanohybrid. The concentration of VC as a reducing agent plays a critical role in tailoring the pore architecture of the sponges. By using excess amounts of VC, a unique hierarchical pore structure is achieved, where VC grains act as soft templates for forming millimeter-sized pores, the walls of which are also porous and comprised of micron-sized pores. The unique hierarchical pore structure ensures the interconnectivity of pores even at the core of large sponges as evidenced by micro and nano X-ray computed tomography. The unique pore architecture translates into an exceptional specific surface area for adsorption of a wide range of contaminants, such as dyes, heavy metals, pharmaceuticals and cyanotoxin from water.

Probing the Interaction between Nanoparticles and Lipid Membranes by Quartz Crystal Microbalance with Dissipation Monitoring.

There is increasing interest in using quartz crystal microbalance with dissipation monitoring (QCM-D) to investigate the interaction of nanoparticles (NPs) with model surfaces. The high sensitivity, ease of use and the ability to monitor interactions in real-time has made it a popular technique for colloid chemists, biologists, bioengineers, and biophysicists. QCM-D has been recently used to probe the interaction of NPs with supported lipid bilayers (SLBs) as model cell membranes. The interaction of NPs with SLBs is highly influenced by the quality of the lipid bilayers. Unlike many surface sensitive techniques, by using QCM-D, the quality of SLBs can be assessed in real-time, hence QCM-D studies on SLB-NP interactions are less prone to the artifacts arising from bilayers that are not well formed. The ease of use and commercial availability of a wide range of sensor surfaces also have made QCM-D a versatile tool for studying NP interactions with lipid bilayers. In this review, we summarize the state-of-the-art on QCM-D based techniques for probing the interactions of NPs with lipid bilayers.

Toward More Free-Floating Model Cell Membranes: Method Development and Application to Their Interaction with Nanoparticles.

Identifying the mechanisms of nanoparticle (NP) interactions with cell membranes is key to understanding potential NP cytotoxicity and applications as nanocarriers for targeted drug delivery. To elucidate these mechanisms of interaction, supported phospholipid bilayers (SPBs) are commonly used as models of cell membranes. However, SPBs are soft thin films, and, as such, their properties can be significantly affected by the underlying substrate. Free-floating cell membranes would be best modeled by weakly adhered SPBs; thus, we propose a method for tailoring the interfacial interaction of an electrically charged SPB-substrate system based on modulations in the solution chemistry. Using the dissipation signal of the quartz crystal microbalance with dissipation monitoring (QCM-D), we show that the method can be used to tailor SPB-substrate interactions without the loss of its structural integrity. To demonstrate the application of the method, SPBs are exposed to cationic and anionic polystyrene latex NPs. These studies reveal that the bilayer response to the modulations in the interfacial interaction with its underlying substrate can be used as a sensitive tool to probe the integrity of SPBs upon exposure to NPs. As expected, anionic NPs tend to impart no significant damage to the anionic bilayers, whereas cationic NPs can be detrimental to bilayer integrity. This is the first report of a QCM-D based method to probe bilayer integrity following exposure to NPs. Importantly, the degree of SPB interaction with its underlying substrate is shown to be a critical factor in the kinetics of bilayer disruption by cationic NPs, whereby weakly adhered bilayers are prone to significantly faster breakup. Since free-floating cell membranes are better represented by a weakly adhered SPB, the results of this work critically influence paradigms in experimental studies involving SPBs as models for cell membranes.

Highly aligned graphene/polymer nanocomposites with excellent dielectric properties for high-performance electromagnetic interference shielding.

Nanocomposites that contain reinforcements with preferred orientation have attracted significant attention because of their promising applications in a wide range of multifunctional fields. Many efforts have recently been focused on developing facile methods for preparing aligned graphene sheets in solvents and polymers because of their fascinating properties including liquid crystallinity and highly anisotropic characteristics. Self-aligned in situ reduced graphene oxide (rGO)/polymer nanocomposites are prepared using an all aqueous casting method. A remarkably low percolation threshold of 0.12 vol% is achieved in the rGO/epoxy system owing to the uniformly dispersed, monolayer graphene sheets with extremely high aspect ratios (>30000). The self-alignment into a layered structure at above a critical filler content induces a unique anisotropy in electrical and mechanical properties due to the preferential formation of conductive and reinforcing networks along the alignment direction. Accompanied by the anisotropic electrical conductivities are exceptionally high dielectric constants of over 14000 with 3 wt% of rGO at 1 kHz due to the charge accumulation at the highly-aligned conductive filler/insulating polymer interface according to the Maxwell-Wagner-Sillars polarization principle. The highly dielectric rGO/epoxy nanocomposites with the engineered structure and properties present high performance electromagnetic interference shielding with a remarkable shilding efficiency of 38 dB.

Fabrication of highly-aligned, conductive, and strong graphene papers using ultralarge graphene oxide sheets.

This study demonstrates that large-size graphene oxide (GO) sheets can impart a tremendous positive impact on self-alignment, electrical conductivity, and mechanical properties of graphene papers. There is a remarkable, more than 3-fold improvement in electrical conductivity of the papers made from ultralarge GO sheets (with an average area of 272.2 µm(2)) compared to that of the small GO counterpart (with an average area of 1.1 µm(2)). The corresponding improvements in Young's modulus and tensile strength are equally notable, namely 320% and 280%, respectively. These improvements of bulk properties due to the large GO sheets are correlated to multiscale elemental and structural characteristics of GO sheets, such as the content of carboxyl groups on the GO edge, C/O ratio and Raman D/G-band intensity ratio of GO on the molecular-scale, and the degree of dispersion and stacking behavior of GO sheets on the microscale. The graphene papers made from larger GO sheets exhibit a closer-stacked structure and better alignment as confirmed by the fast Fourier transform analysis, to the benefits of their electrical conductivity and mechanical properties. The molecular dynamics simulation further elucidates that the enhanced intersheet interactions between large GO sheets play a key role in improving the Young's modulus of GO papers. The implication is that the said properties can be further improved by enhancing the intersheet stress transfer and electrical conduction especially through the thickness direction.

Transparent conductive films consisting of ultralarge graphene sheets produced by Langmuir-Blodgett assembly.

Monolayer graphene oxide (GO) sheets with sizes ranging from a few to ∼200 µm are synthesized based on a chemical method and are sorted out to obtain four different grades having uniform sizes. Transparent conductive films are produced using the ultralarge graphene oxide (UL-GO) sheets that are deposited layer-by-layer on a substrate using the Langmuir-Blodgett (LB) assembly technique. The density and degree of wrinkling of the UL-GO monolayers are turned from dilute, close-packed flat UL-GO to graphene oxide wrinkles (GOWs) and concentrated graphene oxide wrinkles (CGOWs) by varying the LB processing conditions. The method demonstrated here opens up a new avenue for high-yield fabrication of GOWs or CGOWs that are considered promising materials for hydrogen storage, supercapacitors, and nanomechanical devices. The films produced from UL-GO sheets with a close-packed flat structure exhibit exceptionally high electrical conductivity and transparency after thermal reduction and chemical doping treatments. A remarkable sheet resistance of ∼500 Ω/sq at 90% transparency is obtained, which outperforms the graphene films grown on a Ni substrate by chemical vapor deposition. The technique used in this work to produce transparent conductive UL-GO thin films is facile, inexpensive, and tunable for mass production.