Resembling Graphene/Polymer Aerogel Morphology for Advancing the CO2/N2 Selectivity of the Postcombustion CO2 Capture Process.

The separation of CO2 from N2 remains a highly challenging task in postcombustion CO2 capture processes, primarily due to the relatively low CO2 content (3-15%) compared to that of N2 (70%). This challenge is particularly prominent for carbon-based adsorbents that exhibit relatively low selectivity. In this study, we present a successfully implemented strategy to enhance the selectivity of composite aerogels made of reduced graphene oxide (rGO) and functionalized polymer particles. Considering that the CO2/N2 selectivity of the aerogels is affected on the one hand by the surface chemistry (offering more sites for CO2 capture) and fine-tuned microporosity (offering molecular sieve effect), both of these parameters were affected in situ during the synthesis process. The resulting aerogels exhibit improved CO2 adsorption capacity and a significant reduction in N2 adsorption at a temperature of 25 °C and 1 atm, leading to a more than 10-fold increase in selectivity compared to the reference material. This achievement represents the highest selectivity reported thus far for carbon-based adsorbents. Detailed characterization of the aerogel surfaces has revealed an increase in the quantity of surface oxygen functional groups, as well as an augmentation in the fractions of micropores (<2 nm) and small mesopores (<5 nm) as a result of the modified synthesis methodology. Additionally, it was found that the surface morphology of the aerogels has undergone important changes. The reference materials feature a surface rich in curved wrinkles with an approximate diameter of 100 nm, resulting in a selectivity range of 50-100. In contrast, the novel aerogels exhibit a higher degree of oxidation, rendering them stiffer and less elastic, resembling crumpled paper morphology. This transformation, along with the improved functionalization and augmented microporosity in the altered aerogels, has rendered the aerogels almost completely N2-phobic, with selectivity values ranging from 470 to 621. This finding provides experimental evidence for the theoretically predicted relationship between the elasticity of graphene-based adsorbents and their CO2/N2 selectivity performance. It introduces a new perspective on the issue of N2-phobicity. The outstanding performance achieved, including a CO2 adsorption capacity of nearly 2 mmol/g and the highest selectivity of 620, positions these composites as highly promising materials in the field of carbon capture and sequestration (CCS) postcombustion technology.

Synthesis and Crystallization of Waterborne Thiol-ene Polymers: Toward Innovative Oxygen Barrier Coatings.

The synthesis of waterborne thiol-ene polymer dispersions is challenging due to the high reactivity of thiol monomers and the premature thiol-ene polymerization that leads to high irreproducibility. By turning this challenge into an advantage, a synthesis approach of high solid content film-forming waterborne poly(thioether) prepolymers is reported based on initiator-free step growth sonopolymerization. Copolymerization of bifunctional thiol and ene monomers diallyl terephthalate, glycol dimercaptoacetate, glycol dimercaptopropionate, and 2,2-(ethylenedioxy)diethanethiol gave rise to linear poly(thioether) functional chains with molar mass ranging between 7 and 23 kDa when synthesized at 30% solid content and between 1 and 9 kDa at increased solid content of 50%. To further increase the polymers' molar mass, an additional photopolymerization step was performed in the presence of a water-soluble photoinitiator, i.e., lithium phenyl-2,4,6-trimethylbenzoylphosphinate, leading to high molar mass chains of up to 200 kDa, the highest reported so far for step grown poly(thioethers). The polymer dispersions presented good film-forming ability at room temperature, yielding semicrystalline films with a high potential for barrier coating applications. Nevertheless, affected by the polymer chemical repeating structure, which includes an aromatic ring, these thiol-ene chains can only crystallize very slowly from the molten state. Herein, for the first time, we present the successful implementation of a self-nucleation (SN) procedure for these types of poly(thioethers), which effectively accelerates their crystallization kinetics.

One Step Closer to Coatings Applications Utilizing Self-Stratification: Effect of Rheology Modifiers.

Self-stratification of model blends of colloidal spheres has recently been demonstrated as a method to form multifunctional coatings in a single pass. However, practical coating formulations are complex fluids with upward of 15 components. Here, we investigate the influence of three different rheology modifiers (RMs) on the stratification of a 10 wt % 7:3 w:w blend of 270 and 96 nm anionic latex particles that do not stratify without RM. However, addition of a high molar mass polysaccharide thickener, xanthan gum, raises the viscosity and corresponding Péclet number enough to achieve small-on-top stratification as demonstrated by atomic force microscopy (AFM) measurements. Importantly, this was possible due to minimal particle-rheology modifier interactions, as demonstrated by the bulk rheology. In contrast, Carbopol 940, a microgel-based RM, was unable to achieve small-on-top stratification despite a comparable increase in viscosity. Instead, pH-dependent interactions with latex particles lead to either laterally segregated structures at pH 3 or a surface enrichment of large particles at pH 8. Strong RM-particle interactions are also observed when the triblock associative RM HEUR10kC12 is used. Here, small-on-top, large-enhanced, and randomly mixed structures were observed at respectively 0.01, 0.1, and 1 wt % HEUR10kC12. Combining rheology, dynamic light scattering, and AFM results allows the mechanisms behind the nonmonotonic stratification in the presence of associative RMs to be elucidated. Our results highlight that stratification can be predicted and controlled for RMs with weak particle interactions, while a strong RM-particle interaction may afford a wider range of stratified structures. This takes a step toward successfully harnessing stratification in coatings formulations.

Unraveling the Complex Polymorphic Crystallization Behavior of the Alternating Copolymer DMDS-alt-DVE.

A complex crystallization behavior was observed for the alternating copolymer DMDS-alt-DVE synthesized via thiol-ene step-growth polymerization. Understanding the underlying complex crystallization processes of such innovative polythioethers is critical for their application, for example, in polymer coating technologies. These alternating copolymers have polymorphic traits, resulting in different phases that may display distinct crystalline structures. The copolymer DMDS-alt-DVE was studied in an earlier work, where only two crystalline phases were reported: a low melting, L - Tm, and high melting, H - Tm phase. Remarkably, the H - Tm form was only achieved by the previous formation and melting of the L - Tm form. We applied calorimetric techniques encompassing seven orders of magnitude in scanning rates to further explore this complex polymorphic behavior. Most importantly, by rapidly quenching the sample to temperatures well below room temperature, we detected an additional polymorphic form (characterized by a very low melting phase, denoted VL - Tm). Moreover, through tailored thermal protocols, we successfully produced samples containing only one, two, or all three polymorphs, providing insights into their interrelationships. Understanding polymorphism, crystallization, and the resulting morphological differences can have significant implications and potential impact on mechanical resistance and barrier properties.

Polymer Colloids: Current Challenges, Emerging Applications, and New Developments.

Polymer colloids are complex materials that have the potential to be used in a vast array of applications. One of the main reasons for their continued growth in commercial use is the water-based emulsion polymerization process through which they are generally synthesized. This technique is not only highly efficient from an industrial point of view but also extremely versatile and permits the large-scale production of colloidal particles with controllable properties. In this perspective, we seek to highlight the central challenges in the synthesis and use of polymer colloids, with respect to both existing and emerging applications. We first address the challenges in the current production and application of polymer colloids, with a particular focus on the transition toward sustainable feedstocks and reduced environmental impact in their primary commercial applications. Later, we highlight the features that allow novel polymer colloids to be designed and applied in emerging application areas. Finally, we present recent approaches that have used the unique colloidal nature in unconventional processing techniques.

Experimental and theoretical study of the effect of different functionalities of graphene oxide/polymer composites on selective CO2 capture.

There is a constant need for versatile technologies to reduce the continuously increasing concentration of CO2 in the atmosphere, able to provide effective solutions under different conditions (temperature, pressure) and composition of the flue gas. In this work, a combination of graphene oxide (GO) and functionalized waterborne polymer particles was investigated, as versatile and promising candidates for CO2 capture application, with the aim to develop an easily scalable, inexpensive, and environmentally friendly CO2 capture technology. There are huge possibilities of different functional monomers that can be selected to functionalize the polymer particles and to provide CO2-philicity to the composite nanostructures. Density functional theory (DFT) was employed to gain a deeper understanding of the interactions of these complex composite materials with CO2 and N2 molecules, and to build a basis for efficient screening for functional monomers. Estimation of the binding energy between CO2 and a set of GO/polymer composites, comprising copolymers of methyl methacrylate, n-butyl acrylate, and different functional monomers, shows that it depends strongly on the polymer functionalities. In some cases, there is a lack of cooperative effect of GO. It is explained by a remarkably strong GO-polymer binding, which induced less effective CO2-polymer interactions. When compared with experimental results, in the cases when the nanocomposite structures presented similar textural properties, the same trends for selective CO2 capture over N2 were attained. Besides novel functional materials for CO2 capture and a deeper understanding of the interactions between CO2 molecules with various materials, this study additionally demonstrates that DFT calculations can be a shorter route toward the efficient selection of the best functionalization of the composite materials for selective CO2 capture.

Effect of Particle Interactions on the Assembly of Drying Colloidal Mixtures.

The effects of particle interactions on the size segregation and assembly of colloidal mixtures during drying were investigated. A cationic surfactant was added to a binary latex/silica colloidal dispersion that has been shown to self-stratify upon drying at room temperature. Atomic force microscopy was used to show that the change in particle interactions due to the presence of surfactants reduced the degree of stratification and, in some cases, suppressed the effect altogether. Colloidal dispersions containing higher surfactant concentrations can undergo a complete morphology change, resulting instead in the formation of armored particles consisting of latex particles coated with smaller silica nanoparticles. To further prove that armored particles are produced and that stratification is suppressed, cross-sectional images were produced with energy-dispersive X-ray spectroscopy and confocal fluorescence microscopy. The growth of armored particles was also measured using dynamic light scattering. To complement this research, Brownian dynamics simulations were used to model the drying. By tuning the particle interactions to make them more attractive, the simulations showed the presence of armored particles, and the size segregation process was hindered. The prevention of segregation also results in enhanced transparency of the colloidal films. Overall, this research proves that there is a link between particle interactions and size segregation in drying colloidal blends and provides a valuable tool to control the assembly of different film architectures using an extremely simple method.

Semi-Crystalline Poly(thioether) Prepared by Visible-Light-Induced Organocatalyzed Thiol-ene Polymerization in Emulsion.

A photocatalytic thiol-ene aqueous emulsion polymerization under visible-light is described to prepare linear semicrystalline latexes using 2,2'-dimercaptodiethyl sulfide as dithiol and various dienes. The procedure involves low irradiance (3 mW cm-2 ), LED irradiation source, eosin-Y disodium as organocatalyst, low catalyst loading (<0.05% mol), and short reaction time scales (<1 h). The resulting latexes have molecular weights of about 10 kg mol-1 , average diameters of 100 nm, and a linear structure consisting only of thioether repeating units. Electron-transfer reaction from a thiol to the triplet excited state of the photocatalyst is suggested as the primary step of the mechanism (type I), whereas oxidation by singlet oxygen generated by energy transfer has a negligible effect (type II). Only polymers prepared with aliphatic dienes such as diallyl adipate or di(ethylene glycol) divinyl ether exhibit a high crystallization tendency as revealed by differential scanning calorimetry, polarized optical microscopy, and X-ray diffraction. Ordering and crystallization are driven by molecular packing of poly(thioether) chains combining structural regularity, compactness, and flexibility.

Ionic Inter-Particle Complexation Effect on the Performance of Waterborne Coatings.

The performance of waterborne (meth)acrylic coatings is critically affected by the film formation process, in which the individual polymer particles must join to form a continuous film. Consequently, the waterborne polymers present lower performance than their solvent-borne counter-polymers. To decrease this effect, in this work, ionic complexation between oppositely charged polymer particles was introduced and its effect on the performance of waterborne polymer films was studied. The (meth)acrylic particles were charged by the addition of a small amount of ionic monomers, such as sodium styrene sulfonate and 2-(dimethylamino)ethyl methacrylate. Density functional theory calculations showed that the interaction between the selected main charges of the respective functional monomers (sulfonate-amine) is favored against the interactions with their counter ions (sulfonate-Na and amine-H). To induce ionic complexation, the oppositely charged latexes were blended, either based on the same number of charges or the same number of particles. The performance of the ionic complexed coatings was determined by means of tensile tests and water uptake measurements. The ionic complexed films were compared with reference films obtained at pH at which the cationic charges were in neutral form. The mechanical resistance was raised slightly by ionic bonding between particles, producing much more flexible films, whereas the water penetration within the polymeric films was considerably hindered. By exploring the process of polymer chains interdiffusion using Fluorescence Resonance Energy Transfer (FRET) analysis, it was found that the ionic complexation was established between the particles, which reduced significantly the interdiffusion process of polymer chains. The presented ionic complexes of sulfonate-amine functionalized particles open a promising approach for reinforcing waterborne coatings.

Toward enhanced catalytic activity of magnetic nanoparticles integrated into 3D reduced graphene oxide for heterogeneous Fenton organic dye degradation.

Composite Fenton nanocatalyst was prepared by water-based in situ creation of Fe3O4 nanoparticles integrated within the self-assembly 3D reduced graphene oxide (rGO) aerogel. The hybrid applied for the degradation of Acid Green 25 (AG-25) organic dye in an aqueous solution, in the presence of H2O2. By investigating the conditions that maximize the dye adsorption by the 3D composite, it was found that the pH of the solution should be adjusted between the pKa of the functional groups present on the rGO surface (carboxylic acid) and that of the dye (sulfonic acid) to promote electrostatic interactions dye-3D structure. Performed under these conditions, Fenton degradation of AG-25 in presence of H2O2 was completed in less than 30 min, including all the intermediate products, as demonstrated by MALDI-TOF-MS analysis of the aqueous solution after discoloration. Moreover, this was achieved in a solution with as high a dye concentration of 0.5 mg/mL, with only 10 mg of 3D composite catalyst, at room temperature and without additional energy input. The high performance was attributed to the creation of charge-transfer complex between rGO and Fe3O4 nanoparticles throughout covalent bond C-O-Fe, the formation of which was promoted by the in situ synthesis procedure. For the first time, up to the authors' knowledge, AG-25 degradation mechanism was proposed.

Effect of Graphene Characteristics on Morphology and Performance of Composite Noble Metal-Reduced Graphene Oxide SERS Substrate.

Graphene/noble metal substrates for surface enhanced RAMAN scattering (SERS) possess synergistically improved performance, due to the strong chemical enhancement mechanism accounted to graphene and the electromagnetic mechanism raised from the metal nanoparticles. However, only the effect of noble metal nanoparticles characteristics on the SERS performance was studied so far. In attempts to bring a light to the effect of quality of graphene, in this work, two different graphene oxides were selected, slightly oxidized GOS (20%) with low aspect ratio (1000) and highly oxidized (50%) GOG with high aspect ratio (14,000). GO and precursors for noble metal nanoparticles (NP) simultaneous were reduced, resulting in rGO decorated with AgNPs and AuNPs. The graphene characteristics affected the size, shape, and packing of nanoparticles. The oxygen functionalities actuated as nucleation sites for AgNPs, thus GOG was decorated with higher number and smaller size AgNPs than GOS. Oppositely, AuNPs preferred bare graphene surface, thus GOS was covered with smaller size, densely packed nanoparticles, resulting in the best SERS performance. Fluorescein in concentration of 10-7 M was detected with enhancement factor of 82 × 104. This work demonstrates that selection of graphene is additional tool toward powerful SERS substrates.

Uptake and effects of graphene oxide nanomaterials alone and in combination with polycyclic aromatic hydrocarbons in zebrafish.

Because of its surface characteristics, once in the aquatic environment, graphene could act as a carrier of pollutants, such as polycyclic aromatic hydrocarbons (PAHs), to aquatic organisms. In this study we aimed to (1) assess the capacity of graphene oxide (GO) to sorb PAHs and (2) to evaluate the toxicity of GO alone and in combination with PAHs on zebrafish embryos and adults. GO showed a high sorption capacity for benzo(a)pyrene (B(a)P) (98% of B(a)P sorbed from a nominal concentration of 100 µg/L) and for other PAHs of the water accommodated fraction (WAF) of a naphthenic North Sea crude oil, depending on their log Kow (95.7% of phenanthrene, 84.4% of fluorene and 51.5% of acenaphthene). In embryos exposed to different GO nanomaterials alone and with PAHs, no significant mortality was recorded for any treatment. Nevertheless, malformation rate increased significantly in embryos exposed to the highest concentrations (5 or 10 mg/L) of GO and reduced GO (rGO) alone and with sorbed B(a)P (GO-B(a)P). On the other hand, adults were exposed for 21 days to 2 mg/L of GO, GO-B(a)P and GO co-exposed with WAF (GO + WAF) and to 100 µg/L B(a)P. Fish exposed to GO presented GO in the intestine lumen and liver vacuolisation. Transcription level of genes related to cell cycle regulation and oxidative stress was not altered, but the slight up-regulation of cyp1a measured in fish exposed to B(a)P for 3 days resulted in a significantly increased EROD activity. Fish exposed to GO-B(a)P and to B(a)P for 3 days and to GO + WAF for 21 days showed significantly higher catalase activity in the gills than control fish. Significantly lower acetylcholinesterase activity, indicating neurotoxic effects, was also observed in all fish treated for 21 days. Results demonstrated the capacity of GO to carry PAHs and to exert sublethal effects in zebrafish.

Dry sonication process for preparation of hybrid structures based on graphene and carbon nanotubes usable for chemical sensors.

The combination of graphene (G) and multi-walled carbon nanotubes (MWCNTs) creates three-dimensional hybrid structures particularly suitable as next-generation electrical interface materials. Nevertheless, efficient mixing of the nanopowders is challenging, unless previous disaggregation and eventual surface modification of both is reached. To avoid use of solvents and multistep purification process for synthesis of stable G/MWCNTs hybrids, herein, a novel dry method based on an air sonication process was used. Taking advantage from the vigorous turbulent currents generated by powerful ultrasonication in air that induces strong thermal convection or radiation to and from the particles, it simultaneously ensures disentanglement of the large MWCNT bundles and G exfoliation and their only mild surface modifications. By changing the ratio between MWCNTs and G, a range of hybrids was obtained, different in surface morphology and chemistry. These hybrids have shown great potential as sensing material for designing mass-based sensors for toxic gases and chemiresistor for vapors detection.

Reduced Graphene Oxide/Polymer Monolithic Materials for Selective CO2 Capture.

Polymer composite materials with hierarchical porous structure have been advancing in many different application fields due to excellent physico-chemical properties. However, their synthesis continues to be a highly energy-demanding and environmentally unfriendly process. This work reports a unique water based synthesis of monolithic 3D reduced graphene oxide (rGO) composite structures reinforced with poly(methyl methacrylate) polymer nanoparticles functionalized with epoxy functional groups. The method is based on reduction-induced self-assembly process performed at mild conditions. The textural properties and the surface chemistry of the monoliths were varied by changing the reaction conditions and quantity of added polymer to the structure. Moreover, the incorporation of the polymer into the structures improves the solvent resistance of the composites due to the formation of crosslinks between the polymer and the rGO. The monolithic composites were evaluated for selective capture of CO2. A balance between the specific surface area and the level of functionalization was found to be critical for obtaining high CO2 capacity and CO2/N2 selectivity. The polymer quantity affects the textural properties, thus lowering its amount the specific surface area and the amount of functional groups are higher. This affects positively the capacity for CO2 capture, thus, the maximum achieved was in the range 3.56-3.85 mmol/g at 1 atm and 25 °C.

A green synthesis of nanocatalysts based on reduced graphene oxide/magnetic nanoparticles for the degradation of Acid Red 1.

The increasing amount of organic dye-polluted wastewater from the textile industry makes the development of techniques for the efficient purification and reuse of wastewater an urgent issue. Accordingly, solid adsorbents based on three-dimensional (3D) reduced graphene oxide (rGO) aerogels combined with magnetic nanoparticles (rGO@Fe3O4) appear to be potential materials, which offer fast and efficient discoloration of dye solutions by dye adsorption, simultaneously acting as Fenton reaction nanocatalysts, and thus may eliminate organic dyes. In this work, 3D rGO@Fe3O4 aerogel nanocatalysts were synthesized via a low-energy, simple, one-step in situ method, in which GO and FeSO4·7H2O were simultaneously reduced. Consequently, monolithic porous nanocatalyst 3D structures were obtained, with a specific surface area of 241 m2 g-1 and pore volume 0.39 cm3 g-1. The nanocatalysts were applied for the degradation of Acid Red 1 azo-dye in aqueous solution in the presence of hydrogen peroxide, without the need for external energy. The effect of the adsorbent dose, and concentration of dye and peroxide on the dye removal was studied. The degradation of the dye was monitored by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. It was found that an increase in the amount of peroxide allowed complete degradation of the dye together with high molar mass side-products with a conjugated aromatic structure. The good nanocatalyst performance was explained based on the charge-transfer complex established between rGO and the magnetic nanoparticles, allowing the regeneration of ferrous ions during the Fenton process.

Toward a Green Synthesis of Polyurethane/(Meth)acrylic Dispersions through Control of Colloidal Characteristics.

Polyurethane (PU)/acrylic waterborne hybrids are an attractive class of materials with wide application possibilities, but their synthesis typically requires significant quantities of solvent which has negative economic and environmental consequences. In this work, solvent-free and surfactant-free polyurethane (PU)/acrylic waterborne hybrids were obtained by synthesizing the PU prepolymer containing carboxylic groups directly in (meth)acrylic monomers that act as solvent. Then, the mixture is dispersed in water; the PU is chain-extended with diamines, and the (meth)acrylic monomers are polymerized. It was found that, against expectations, colloidal stability did not improve with the concentration of carboxylic groups that acted as stabilizing moieties. A combination of MALDI-TOF MS analysis and Monte Carlo simulations revealed that the highly heterogeneous compositions of the short chains of the PU prepolymer and their reaction with the chain-extender in the aqueous phase were responsible for lack of control of the colloidal properties. This problem was overcome by using more hydrophobic chain-extenders that decrease the fraction of PU chains in the water phase. In this way high-solid-content stable dispersions with controlled particle size were obtained. Finally, the properties of the PU/(meth)acrylic films were studied in terms of mechanical properties and water resistance.

Low-Energy Encapsulation of α-Tocopherol Using Fully Food Grade Oil-in-Water Microemulsions.

Encapsulation of active agents, such as vitamins and antioxidants, is one of the possibilities that allow their incorporation in beverages, food, or in pharmaceutical products. Simultaneously, encapsulation protects these active agents from oxidation, producing more stable active compounds. Formation of nanodroplets by spontaneously formed microemulsion (ME) offers, on one hand, a low-energy technology of encapsulation and, on the other hand, because of a small size of the droplets, it assures long-term stability even in harsher environments. In this study, oil-in-water MEs allowed the low-energy encapsulation of α-tocopherol (αToc) into an aqueous medium with the aid of fully food-grade ingredients, using isoamyl acetate as the dispersed oil phase, which was selected between three different types of oils. Both cosurfactant-free and cosurfactant-holder ME systems were formulated, in which Tween 20 and glycerol were employed as the surfactant and the cosurfactant, respectively. The ME monophasic area was determined through the construction of pseudoternary phase diagrams. The encapsulated αToc within 10-20 nm nanocapsules showed radical scavenging activity dependent on the encapsulated amount of αToc, as it was demonstrated by electron paramagnetic resonance spectroscopy. The radical scavenging activity slightly increased within the time investigated, indicating a slow release of the active compound from the nanodroplets, which is a promising result for their application, especially in pharmaceuticals.

Miniemulsion copolymerization of (meth)acrylates in the presence of functionalized multiwalled carbon nanotubes for reinforced coating applications.

Film forming, stable hybrid latexes made of methyl metacrylate (MMA), butyl acrylate (BA) and 2-hydroxyethyl methacrylate (HEMA) copolymer reinforced with modified multiwalled carbon nanotubes (MWCNTs) were synthesized by in situ miniemulsion polymerization. The MWCNTs were pretreated by an air sonication process and stabilized by polyvinylpyrrolidone. The presence of the MWCNTs had no significant effect on the polymerization kinetics, but strongly affected the polymer characteristics (Tg and insoluble polymer fraction). The performance of the in situ composites was compared with that of the neat polymer dispersion as well as with those of the polymer/MWCNT physical blends. The in situ composites showed the presence of an additional phase likely due to the strong interaction between the polymer and MWNCTs (including grafting) that reduced the mobility of the polymer chains. As a result, a substantial increase of both the storage and the loss moduli was achieved. At 60 °C, which is above the main transition region of the polymer, the in situ composites maintained the reinforcement, whereas the blends behaved as a liquid-like material. This suggests the formation of a 3D network, in good agreement with the high content of insoluble polymer in the in situ composites.

Intracellular localization and toxicity of graphene oxide and reduced graphene oxide nanoplatelets to mussel hemocytes in vitro.

Recently, graphene materials have attracted tremendous research interest due to their unique physicochemical properties that hold great promise in electronics, energy, materials and biomedical areas. Graphene oxide (GO) is one of the most extensively studied graphene derivatives. In order to improve GO electrical properties, nanoplatelets are chemically reduced, thus increasing nanoplatelet conductivity. This reduced GO (rGO) shows different properties and behavior compared to GO. Graphene-based wastes are expected to end up in the marine environment. Here we aimed to assess the potential toxic effects of GO and rGO to marine organisms by using in vitro assays with mussel (Mytilus galloprovincialis) hemocytes. Cells were exposed to a wide range of concentrations (up to 100mg/L) of GO (with and without polyvinylpyrrolidone-PVP as stabilizing agent: GO and GO-PVP) and rGO with PVP (rGO-PVP) to assess cytotoxicity and cell membrane integrity. Then, cells were exposed to sublethal concentrations of GO and rGO-PVP to assess their subcellular distribution through transmission electron microscopy (TEM) and to evaluate their effects on ROS production. GO, GO-PVP and rGO-PVP showed low and concentration-dependent cytotoxicity. rGO-PVP (LC50=29.902 and 33.94mg/L depending on the origin) was more toxic than GO (LC50=49.84 and 54.51mg/L depending on the origin) and GO-PVP (LC50=43.72mg/L). PVP was not toxic to hemocytes but increased bioavailability and toxicity of nanoplatelets. At TEM, GO and rGO-PVP nanoplatelets caused invaginations and perforations of the plasma membrane, which agrees with the observed decrease in cell membrane integrity. Nanoplatelets were internalized, at a higher extent for rGO-PVP than for GO, and found in the cytosol and in endolysosomal vesicles of hemocytes. Both GO and rGO-PVP increased ROS production at the highest sublethal concentration tested. In conclusion, GO, GO-PVP and rGO-PVP are not highly toxic to mussel cells but they cause membrane damage and their toxicity is ROS-mediated. Finally, in vitro assays with mussel hemocytes are sensitive tools to detect toxic effects of graphene-based nanomaterials.

Covalent-Bonded Reduced Graphene Oxide-Fluorescein Complex as a Substrate for Extrinsic SERS Measurements.

When graphene is used as SERS substrates, it contributes to the chemical mechanism (CM) of enhancement of Raman signal, owing to which the detection limit is very low (lower than mM content of probe molecules). The CM of enhancement depends largely on the interactions between the substrate and the probe molecules. Therefore, in this work, we have investigated the possibility of increasing the SERS activity of graphene by improving the interaction between the probe molecule and the graphene substrate by establishing exclusively strong covalent bonding between them. Fluorescein (Fl) was selected as a probe molecule because it is one of the most commonly used fluorophore in bioscience. As a graphene substrate, reduced graphene oxide (rGO) platelets were used. In addition, silver nanoparticles (AgNPs) were added onto the hybrids to further increase the enhancement by electromagnetic mechanism. Highly enhanced Raman signal of Fl onto neat rGO was achieved for micromolar concentration of the probe molecules. This was attributed to the covalent bonding between them, which introduced hole doping to rGO, decreasing the Fermi level of rGO and bringing it more closely to the LUMO of Fl. This induces aligning of their energy levels, resulting in higher contribution of the nonresonance effect to the charge transfer mechanism of enhancement, which, in this case, occurred intramolecularly. When AgNPs were added onto the rGO substrate, the expected enhancement performance was not observed. On the one hand, this was attributed to small size (∼20 nm) of AgNPs and lack of aggregates and, on the other, due to the unusually high contribution of CM determined.