Aqueous aggregation and stability of graphene nanoplatelets, graphene oxide, and reduced graphene oxide in simulated natural environmental conditions: complex roles of surface and solution chemistry

Kinetics of Direct Reaction of Vanadate, Chromate, and Permanganate with Graphene Nanoplatelets for Use in Water Purification

Oxometalates of vanadium(V), chromium(VI), and manganese(VII) have negative impacts on water resources due to their toxicity. To remove them, the kinetics of 0.04 mM oxometalates in natural and synthetic water were studied using graphene nanoplatelets (GNP). The GNP were dispersible in water and formed aggregates >15 µm that could be easily separated. Within 30 min, the GNP were covered with ~0.4 mg/g vanadium and ~1.0 mg/g chromium as Cr(OH)3. The reaction of 0.04 mM permanganate with 50 mg of GNP resulted in a coverage of 10 mg/g in 5 min, while the maximum value was 300 mg/g manganese as Mn2O3/MnO. TEM showed a random metal distribution on the surfaces; no clusters or nanoparticles were detected. The rate of disappearance in aerated water followed a pseudo second-order adsorption kinetics (PSO) for V(V), a pseudo second-order reaction for Cr(VI), and a pseudo first-order reaction for Mn(VII). For Cr(VI) and Mn(VII), the rate constants were found to depend on the GNP mass. Oxygen sorption occurred with PSO kinetics as a parallel slow process upon contact of GNP with air-saturated water. For thermally regenerated GNP, the rate constant decreased for V(V) but increased for Cr(VI), while no effect was observed for Mn(VII). GNP capacity was enhanced through regeneration for V(V) and Cr(VI); no effect was observed for Mn(VII). The reactions are well-suited for use in water purification processes and the reaction products, GNP, decorated with single metal atoms, are of great interest for the construction of sensors, electronic devices, and for application in single-atom catalysis (SAC).

Predicting Free Energies of Exfoliation and Solvation for Graphitic Carbon Nitrides Using Machine Learning

As a metal-free and visible-light-responsive photocatalyst, graphitic carbon nitride (g-C3N4) has emerged as a new research hotspot and has attracted broad attention in the field of solar energy conversion and thin-film transistors. Liquid-phase exfoliation (LPE) is the best-known method for the synthesis of 2D g-C3N4 nanosheets. In LPE, bulk g-C3N4 is exfoliated in a solvent via high-shear mixing or sonication in order to produce a stable suspension of individual nanosheets. Two parameters of importance in gauging the performance of a solvent in LPE are the free energy required to exfoliate a unit area of layered materials into individual sheets in the solvent (ΔGexf) and the solvation free energy per unit area of a nanosheet (ΔGsol). While approximations for the free energies exist, they are shown in our previous work to be inaccurate and incapable of capturing the experimentally observed efficacy of LPE. Molecular dynamics (MD) simulations can provide accurate free-energy calculations, but doing so for every single solvent is time- and resource-consuming. Herein, machine learning (ML) algorithms are used to predict ΔGexf and ΔGsol for g-C3N4. First, a database for ΔGexf and ΔGsol is created based on a series of MD simulations involving 49 different solvents with distinct chemical structures and properties. The data set also includes values of critical descriptors for the solvents, including density, surface tension, dielectric constant, etc. Different ML methods are compared, accompanied by descriptor selection, to develop the most accurate model for predicting ΔGexf and ΔGsol. The extra tree regressor is shown to be the best performer among the six ML methods studied. Experimental validation of the model is conducted by performing dispersibility tests in several solvents for which the free energies are predicted. Finally, the influence of the selected descriptors on the free energies is analyzed, and strategies for solvent selection in LPE are proposed.

Removal of methylene blue using MnO2@rGO nanocomposite from textile wastewater: Isotherms, kinetics and thermodynamics studies

In this study, the adsorptive removal of methylene blue dye, which is commonly used in textile industries, was investigated using the MnO₂@reduced graphene oxide (rGO) adsorbent. The sonication-assisted synthesis from rGO nanosheets and MnO₂ nanoparticles resulted to the MnO₂@rGO nanocomposite with improved physicochemical properties. The characterization results showed the improved surface area, porous structure and adsorption sites from the nitrogen adsorption-desorption studies, improved morphology from the Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) and the improved crystal structure from X-ray powder diffraction (XRD). The improved physicochemical properties on the MnO₂@rGO nanocomposite played a significant role in enhancing the dye removal in textile wastewater. The equilibrium experimental data was best described by the Langmuir isotherm model with a maximum adsorption capacity of 156 mg g^-1, suggesting a monolayer adsorption. The kinetic data best fitted the pseudo-second order kinetic model, suggesting a chemisorption adsorption process. The thermodynamic data (ΔG°, ΔH° and ΔS°) confirmed the feasibility, randomness and spontaneous nature of the adsorption process. The mechanism of adsorption involved the hydrogen bonding, π-π interactions and electrostatic interactions. The removal of methylene blue using MnO₂@rGO nanocomposite in spiked textile wastewater yielded a 98-99% removal. The method demonstrated competitiveness when compared with literature reported results, paving way for further investigations towards industrial scale applications.

Exposure of Midge Larvae (Chironomus riparius) to Graphene Oxide Leads to Development Alterations

Despite the fast-growing use and production of graphene-based nanomaterials (GBMs), data concerning their effects on freshwater benthic macroinvertebrates are scarce. This study aims to investigate the effects of graphene oxide (GO) on the midge Chironomus riparius. Mortality, growth inhibition, development delay and teratogenicity, assessed using mentum deformity analysis, were investigated after a 7-day static exposure of the first instar larvae under controlled conditions. The collected data indicated that the survival rate was not impacted by GO, whereas chronic toxicity following a dose-dependent response occurred. Larval growth was affected, leading to a significant reduction in larval length (from 4.4 to 10.1%) in individuals reaching the fourth instar at any of the tested concentrations (from 0.1 to 100 mg/L). However, exposure to GO is not associated with an increased occurrence of mouthpart deformities or seriousness in larvae. These results highlight the suitability of monitoring the larval development of C. riparius as a sensitive marker of GO toxicity. The potential ecological consequences of larval size decrease need to be considered for a complete characterization of the GO-related environmental risk.

Enhanced Specificity in Loop-Mediated Isothermal Amplification with Poly(ethylene glycol)-Engrafted Graphene Oxide for Detection of Viral Genes

Loop-mediated isothermal amplification (LAMP) is a nucleic acid amplification method that allows the simple, quick, and low-cost detection of various viral genes. LAMP assays are susceptible to generating non-specific amplicons, as high concentrations of DNA primers can give rise to primer dimerization and mismatched hybridizations, resulting in false-positive signals. Herein, we reported that poly(ethylene glycol)-engrafted nanosized graphene oxide (PEG-nGO) can significantly enhance the specificity of LAMP, owing to its ability to adsorb single-stranded DNA (ssDNA). By adsorbing surplus ssDNA primers, PEG-nGO minimizes the non-specific annealing of ssDNAs, including erroneous priming and primer dimerization, leading to the enhanced specificity of LAMP. The detection of complementary DNAs transcribed from the hepatitis C virus (HCV) RNA was performed by the PEG-nGO-based LAMP. We observed that the inclusion of PEG-nGO significantly enhances the specificity and sensitivity of the LAMP assay through the augmented difference in fluorescence signals between the target and non-target samples. The PEG-nGO-based LAMP assay greatly facilitates the detection of HCV-positive clinical samples, with superior precision to the conventional quantitative real-time PCR (RT-qPCR). Among the 20 clinical samples tested, all 10 HCV-positive samples are detected as positive in the PEG-nGO-based LAMP, while only 7 samples are detected as HCV-positive in the RT-qPCR. In addition, the PEG-nGO-based LAMP method significantly improves the detection precision for the false-positive decision by 1.75-fold as compared to the LAMP without PEG-nGO. Thus, PEG-nGO can significantly improve the performance of LAMP assays by facilitating the specific amplification of target DNA with a decrease in background signal.

A critical review on the role of abiotic factors on the transformation, environmental identity and toxicity of engineered nanomaterials in aquatic environment

Engineered nanomaterials (ENMs) are at the forefront of many technological breakthroughs in science and engineering. The extensive use of ENMs in several consumer products has resulted in their release to the aquatic environment. ENMs entering the aquatic ecosystem undergo a dynamic transformation as they interact with organic and inorganic constituents present in aquatic environment, specifically abiotic factors such as NOM and clay minerals, and attain an environmental identity. Thus, a greater understanding of ENM-abiotic factors interactions is required for an improved risk assessment and sustainable management of ENMs contamination in the aquatic environment. This review integrates fundamental aspects of ENMs transformation in aquatic environment as impacted by abiotic factors, and delineates the recent advances in bioavailability and ecotoxicity of ENMs in relation to risk assessment for ENMs-contaminated aquatic ecosystem. It specifically discusses the mechanism of transformation of different ENMs (metals, metal oxides and carbon based nanomaterials) following their interaction with the two most common abiotic factors NOM and clay minerals present within the aquatic ecosystem. The review critically discusses the impact of these mechanisms on the altered ecotoxicity of ENMs including the impact of such transformation at the genomic level. Finally, it identifies the gaps in our current understanding of the role of abiotic factors on the transformation of ENMs and paves the way for the future research areas.

Silanization of nanographene platelets improves interaction with the dentin bonding resin matrix and enhances interfacial bond integrity to dentin

This study synthesized and characterized graphene nanoplatelets silanized with 3-(trimethoxysilyl)propyl methacrylate (MPS-GNP) for morphological and chemical characteristics. In addition, we modified a dentin bonding agent using different concentrations of MPS-GNP to study its interaction within the resin matrix of the adhesive, degree of conversion (DC), biological, and mechanical properties after bonding to tooth. Both 0.25% and 0.5% MPS-GNP-modified bonding agents showed comparable DC values to the unmodified control adhesive (range: 41%-43%). However, a statistically significant reduction in the DC was found when 0.25% and 0.5% non-silanized GNP was doped with the adhesive (<38%) (p < 0.05). On day 30, the bacterial viability of 0.5% GNP and MPS-GNP groups remained very low under 22% with the highest dead cell count (p < 0.05). GNP incorporated within the resin matrix of the dentin bonding agent showed clear evidence of several interfacial gap formations and non-union between the GNP surface and resin matrix, while the MPS-GNP modified dentin bonding agent showed MPS-GNP with no gap formation with complete union between the graphene surface and resin matrix. The decrease in the μTBS was least pronounced for 0.25% and 0.5% MPS-GNP groups. After 12 months of ageing, the groups 0.25% and 0.5% MPS-GNP also showed the highest BS as compared to the rest of the groups. Statistically significant reduction was seen in nanohardness at the hybrid layer and adhesive layer for GNP groups after 4 months of storage. The addition of up to 0.5% MPS-GNP showed optimized DC, antibiofilm activity, and micro-tensile bond strength without affecting the standard adhesion characteristics as compared to GNP alone.

Role of solution chemistry in the attachment of graphene oxide nanoparticles onto iron oxide minerals with different characteristics

Given the ubiquity and abundance of the iron oxide minerals and their important roles in affecting the environmental fate of graphene oxide (GO) nanoparticles, the attachment of GO onto three iron oxide minerals (i.e., hematite, goethite, and ferrihydrite) under different solution chemistry conditions was investigated in this study. The main mechanism of the attachment of GO was electrostatic interaction. Calculations based on the DLVO theory showed that the attachment was a favorable process. Interestingly, the affinity of GO towards three iron oxide minerals was in the order of ferrihydrite > goethite > hematite. This result indicates that different characteristics of various iron oxides (e.g., specific surface area, crystal structure, and surface charge, and surface hydroxyl densities) can influence their attachment capacities for GO. The attachment of GO depended on the solution pH and ionic strength. Electrostatic attraction and hydrogen bonding were the important retention mechanisms for GO attachment when pH < pH_PZC (the point of zero charge) and pH > pH_PZC, respectively. The attachment capacities of iron oxides decreased with increasing ionic strength at lower pH because of the decrease of the electrostatic attraction. Meanwhile, the presence of divalent cations (i.e., Ca²⁺ and Cu²⁺) could significantly promote GO attachment mainly by the surface-bridging mechanism. Meanwhile, the enhancement effect of Cu²⁺ was greater than Ca²⁺ due to the greater complexation affinity of Cu²⁺. Furthermore, attachment isotherms showed that the presence of phosphate could inhibit the attachment of GO onto minerals obviously. Because phosphate could form inner-sphere surface complex on the iron oxide surface, and consequently decreased the electrostatic attraction between nanoparticles and minerals. Our study has important implications for predicting the fate of GO in natural environment where amounts of iron oxide minerals are present.

Recent advances in graphene-family nanomaterials for effective drug delivery and phototherapy

INTRODUCTION

Owing to the unique properties of graphene, including large specific surface area, excellent thermal conductivity, and optical absorption, graphene-family nanomaterials (GFNs) have attracted extensive attention in biomedical applications, particularly in drug delivery and phototherapy.

AREAS COVERED

In this review, we point out several challenges involved in the clinical application of GFNs. Then, we provide an overview of the most recent publications about GFNs in biomedical applications, including diverse strategies for improving the biocompatibility, specific targeting and stimuli-responsiveness of GFNs for drug delivery, codelivery of drug and gene, photothermal therapy, photodynamic therapy, and multimodal combination therapy.

EXPERT OPINION

Although the application of GFNs is still in the preclinical stage, rational modification of GFNs with functional elements or making full use of GFNs-based multimodal combination therapy might show great potential in biomedicine for clinical application.

Comparing the effects of different types of inorganic nanoparticles on 17β-estradiol adsorption by graphene oxide

This study compared the effects of five types of inorganic nanoparticles (INPs) on the 17β-estradiol (E2) adsorption to graphene oxide (GO). The results showed that INPs increased the equilibrium time for the adsorption of E2 to GO. Higher Brunauer-Emmett-Teller (BET) surface area of INPs resulted in lower diffusion rate of E2, and thus the adsorption rate constant (k₂) calculated from pseudo-second-order kinetic model negatively correlated with the BET surface area of INPs (p = 0.037). In addition, INPs decreased the adsorption amount of E2 to GO, and the inhibition effects declined in the order of Al₂O₃ > ZnO > TiO₂ > SiO₂ > Fe₂O₃. This is determined by the interactions between GO and INPs. The positively charged ZnO and Al₂O₃ strongly heteroaggregate with GO via electrostatic attraction, and then significantly inhibited E2 adsorption to GO. In contrast, the homoaggregation of GO was superior to its heteroaggregation with negatively charged SiO₂ and TiO₂, and then lower inhibition of E2 adsorption to GO was induced. Fe₂O₃ with less negative charge (-8.48 mV) led to the lowest inhibition effect on E2 adsorption to GO because of its preferable homoaggregation. The results were further confirmed by Derjaguin-Landau-Verwey-Overbeek calculation, transmission electron microscopy, and sedimentation experiments. This study revealed how the properties of INPs influence their effects on the adsorption of E2 by GO, and the findings are critical to understand the behavior and fate of GO and pollutants in natural aquatic environment.

Toxic effects of different-sized graphene oxide particles on zebrafish embryonic development

Graphene oxide (GO) has broad application potential in many fields, such as biomedicine and energy. Due to the wide-ranging GO applications, its entry into the environment is inevitable along with the potential for ecological and environmental risks. In the present study, we systematically investigated the dose-dependent effects of three different-sized GO particles (50-200 nm, <500 nm, and >500 nm) on zebrafish during the very early developmental stages (4-124 h post-fertilization). The results showed that GOs could accumulate in the eyes, heart, yolk sac, and blood vessels of fish larvae. Consequently, their effects on multiple toxic endpoints were observed, including delayed hatching times, shortened body lengths, alterations in heart rate and blood flow, changes in swimming activity and responses to photoperiod stimulation, and the enhanced activity of total superoxide dismutase, inducible nitric oxide synthase, acetylcholinesterase, caspase-3, and induction of apoptosis-related gene expression. As a result, the occurrence of oxidative stress and the induction of apoptosis are suggested in fish larvae exposed to all three different-sized GO particles. In addition, our results highlight the impacts of waterborne-GO exposure on zebrafish during early development, which were not merely dependent on GO concentration but also on the associated GO sizes. This study hereby provides a basis for the potential ecological and health risks of GO exposure.

Effect of dissolved organic matter on adsorption of sediments to Oxytetracycline: An insight from zeta potential and DLVO theory

To reveal the adsorption mechanism of sediment to antibiotics with the presence of dissolved organic matter (DOM), batch experiments were carried out by oxytetracycline (OTC) on sediments with decayed plants (PDOM) and composted chicken manure (MDOM), and the zeta potential in the system before and after adsorption was measured. Results showed that the PDOM promoted the adsorption process, while the MDOM inhibited the adsorption. Adding PDOM, the change of zeta potential (Δζ) increased by 40.08% for first terrace sediments (FT) and 63.98% for riverbed sediments (RB), respectively; meanwhile, MDOM decreased by 20.04% for FT and 28.39% for RB, respectively. The results of kinetic fitting models of replacing the adsorption amount with Δζ were consistent with the initial. It indicated that there was a positive correlation between the adsorption amount and Δζ, and the zeta potential can be used to quickly judge the degree of adsorption process. The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory describes the interactions of sediment particles. In terms of adsorption amount, zeta potential (absolute value) and total interaction energy all followed the order: RB > FT, RB-PDOM > FT-PDOM, and RB-MDOM > FT-MDOM. The more negative the zeta potential is, the better the dispersion of the particles is. Stronger repulsion is more conducive to adsorbing positively charged OTC. The site energy distribution theory further explained that the distribution of adsorption site in the various states of sediments increased while adding the PDOM and decreased while adding the MDOM.

Adsorption kinetics and aggregation for three classes of carbonaceous adsorbents in the presence of natural organic matter

In this study, adsorption kinetics of phenanthrene (PNT) and trichloroethylene (TCE) by a graphene nanosheet (GNS), a graphene oxide nanosheet (GO), a single-walled carbon nanotube (SWCNT), a multi-walled carbon nanotube (MWCNT), and two coal based activated carbons (ACs) (F400 and HD3000) were examined in distilled and deionized water (DDW) and under natural organic matter (NOM) preloading conditions. The results showed the times needed for the adsorption of PNT and TCE to reach apparent equilibrium (i.e., ≤3% change per day) followed the order of GO ≥ MWCNT > GNS > SWCNT ∼ HD3000∼F400 and SWCNT > GNS ∼ HD3000 > F400 ∼ MWCNT > GO, respectively. The pseudo second order model successfully represented kinetics data for three classes of carbonaceous adsorbents. The Weber-Morris intraparticle diffusion model indicated three steps adsorption process for PNT and two step adsorption for TCE. In addition, the times needed to reach apparent equilibrium for the adsorption of PNT and TCE in the presence of hydrophobic (HPO) and hydrophilic (HPI) NOM solutions increased for all adsorbents (except for GO). In general, both NOM showed similar impacts on the adsorption rates of PNT and TCE. Aggregation of both GNS and CNTs rapidly occurred during initial couple hours of contact time during preloading, and spiking both PNT and TCE further increased their aggregation.

Adsorption of phenanthrene and 1-naphthol to graphene oxide and L-ascorbic-acid-reduced graphene oxide: effects of pH and surfactants

In this study, reduced graphene oxide (RGO) was synthesized by _L-ascorbic acid reduction, which was a relatively mild and environmental friendly reduction method, and the adsorption of organic contaminants was compared to graphene oxide (GO) to probe the potential adsorption mechanisms. The morphology properties of GO and RGO were characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared transmission (FTIR), Raman spectrometer, transmission electron microscope (TEM), and scanning electron microscopy (SEM). The adsorption affinities of GO and RGO for phenanthrene and 1-naphthol were studied in batch experiments. The effects of pH and surfactants were also assessed. The results demonstrated that RGO reduced by _L-ascorbic acid show significantly greater adsorption affinity for both phenanthrene and 1-naphthol than GO, and even greater than most of RGOs that reduced by the strong reductive reagents. This was mainly attributed to the hydrophobic interaction, π-π interaction, and H-bonding between graphene sheets and organic contaminants. Both GO and RGO showed stronger adsorption to phenanthrene than to 1-naphthol. The adsorption of 1-naphthol increased with decreasing pH and reached a maximum around pH = 7.34. The surfactants, sodium dodecyl benzene sulfaonate (SDBS) and cetyltrimethyl ammonium bromide (CTAB), had negligible influence on adsorption to GO. Note that CTAB significantly inhibited the adsorption of phenanthrene/1-naphthol on RGO, which could be attributed to the pore blockage effect. In addition, RGO could be regenerated and reused with high recyclability over five cycles. The present study suggests that RGO obtained via _L-ascorbic acid reduction can be deemed as a promising material for organic contaminated wastewater treatment.

Effects of solution chemistry on the attachment of graphene oxide onto clay minerals

With the increase in production and wide application of graphene oxide (GO), colloidal GO particles are expectantly released into soil and groundwater, where a large number of mineral particles exist. In addition, the porewater chemistry (e.g. organic acid, valence of cation) is a neglected but important aspect to comprehensively investigate the fate of GO. The interactions of GO with three ubiquitous clay minerals (i.e., montmorillonite, kaolinite and diatomite) have been systematically investigated through batch experiments across different solution chemistry conditions. In general, the affinity towards GO is in the order of montmorillonite > kaolinite > diatomite under the same experimental conditions. This observation can be explained by the characteristics of different clay minerals, such as surface charge and surface area. The results indicated that increasing the ionic strength or decreasing the pH enhanced the attachment of GO nanoparticles onto clay minerals as a result of electrostatic interactions. With the increase in concentration of Ca2+, more GO particles were attached onto clay mineral particles. This is caused by complexation between the surface oxygen functional groups of both GO nanoparticles and clay minerals. The presence of 0.1 mM tartaric acid significantly inhibited the attachment of GO onto clay minerals. This is possibly linked to the increased negative charges of the organic acids and the competition between organic acids and GO. The interaction energies were also calculated by applying the classical DLVO theory. The results of this study have helped to understand the behavior and fate of GO in subsurface formations.