Aggregation of oxidized multi-walled carbon nanotubes: Interplay of nanomaterial surface O-functional groups and solution chemistry factors

A meta-analysis of nanomaterial and nanoplastic fate in small column experiments and implications for fate in soils

A long list of possible processes may simultaneously control retention of engineered nanomaterials (NMs) and nanoplastics (NPs) in soils, but there is little insight in which of these processes dominates and under which circumstances. Though not easily transferable to field situations, repacked saturated column tests compose the richest available dataset to explore for overall trends in the behaviour of NMs and NPs in soils. Global attachment efficiencies (αglobal) were calculated uniformly from metadata of 624 column experiments and correlated against metadata using partial least squares and linear regression analysis. αglobal values appeared to some extent operationally defined as they correlate with the experimental column flow rate and in some cases with the particle concentration used in the feedstock. Particle aggregation occurred more as the feedstock concentration increased, but this only had a limited effect on subsequent column retention. In homogeneous sandy media, attachment of particles obeyed well-known trends indicative of non-favourable electrostatic interactions, whereas interactions in non-sandy media were dominated by favourable attractions to positively charged sites on clay edges and/or oxides as well as hydrophobic interactions with soil organic matter. The results may help to prioritize further research such as the currently unclear role of hydrophobic interactions in the fate of particles in porous media and identify the most important transport processes in more complex field situations.

A How-To Approach to Estimating Surface Charge Density of Nano/Micro Particles through Aggregation Experiments Considering the Specific Ion Effect

Accurate estimation of the surface charge density of nano/micro particles is crucial for regulating their environmental fate and bioavailability in the fields of environmental science and materials engineering. However, existing analytical methods for accurately estimating surface charge density remain significant challenges. In this study, we proposed a method for estimating the surface charge density of nano/micro particles through aggregation experiments using dynamic light scattering technology. The specific ion effect had a significant effect on the accuracy of these estimations. Without considering the specific ion effect, this led to significantly different and unacceptable surface charge densities for montmorillonite particles with permanent charges. Conversely, when considering the specific ion effect, similar surface charge densities were obtained (0.1974, 0.1930, and 0.1718 C/m2), with a mean value of 0.1874 ± 0.0136 C/m2, which aligned well with the literature-reported values. Furthermore, the surface charge density of various nano/micro particles, such as microplastic polymers, graphene oxide, and nanosilver, was also estimated using this method. The migration behavior of these particles was determined by electrostatic repulsion between them, which was controlled by their surface charge density. The surface analysis method proposed herein provides a solid foundation for the directed adjustment and control of the migration dynamics of nano/micro particles in environmental systems.

Antibiofilm and Anticancer Activity of Multi-Walled Carbon Nanotubes Fabricated with Hot-Melt Extruded Astaxanthin-Mediated Synthesized Silver Nanoparticles

Purpose

Multi-walled carbon nanotubes (MWCNTs) were used as carriers for silver nanoparticles (AgNPs). In this process, MWCNTs were coated with mesoporous silica (MWCNT-Silica) for uniform and regular loading of AgNPs on the MWCNTs. In addition, astaxanthin (AST) extract was used as a reducing agent for silver ions to enhance the antioxidant, antibiofilm, and anticancer activities of AgNPs. In this process, AST was extracted from Haematococcus pluvialis (H. pluvialis) and processed by hot melt extrusion (HME) to enhance the AST content of H. pluvialis. AST has strong antioxidative properties, which leads to anticancer activity. In addition, AgNPs are well known for their strong antibacterial properties. The antibiofilm and anticancer effects were studied comprehensively by loading the AST AgNPs onto MWCNT-Silica.

Methods

AgNPs-loaded MWCNT-silica (MWCNT-Ag) was prepared through the binding reaction of TSD and silanol groups and the aggregation interaction of Ag and TSD. To enhance the antioxidant, antibiofilm, and anticancer activities of AgNPs, HME-treated H. pluvialis extract (HME-AST) was used as a reducing solution of silver ions. The increased AST content of HME-AST was confirmed by high-performance liquid chromatography (HPLC) analysis, and the total phenol and flavonoid content analysis confirmed that HME enhanced the active components of H. pluvialis. The antibiofilm activity of MWCNT-AST was investigated by biofilm inhibition and destruction test, SEM, and CLSM analysis, and the anticancer activity was investigated by WST assay, fluorescent staining analysis, and flow cytometry analysis.

Results

MWCNT-AST showed higher antioxidant activity and antibiofilm activity than MWCNT-Ag against E. coli, S. aureus, and methicillin-resistant S. aureus (MRSA). MWCNT-AST showed higher anticancer activity against breast cancer cells (MDA-MB-231) than MWCNT-Ag, and lower toxicity in normal cells HaCaT and NIH3T3.

Conclusion

MWCNT-AST exhibits higher antioxidant, antibiofilm, and anticancer activities than MWCNT-Ag, and exhibits lower toxicity to normal cells.

A Meta-Analysis to Revisit the Property-Aggregation Relationships of Carbon Nanomaterials: Experimental Observations versus Predictions of the DLVO Theory

Contradicting relationships between physicochemical properties of nanomaterials (e.g., size and ζ-potential) and their aggregation behavior have been constantly reported in previous literature, and such contradictions deviate from the predictions of the classic Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. To resolve such controversies, in this work, we employed a meta-analytic approach to synthesize the data from 46 individual studies reporting the critical coagulation concentration (CCC) of two carbon nanomaterials, namely, graphene oxide (GO) and carbon nanotube (CNT). The correlations between CCC and material physicochemical properties (i.e., size, ζ-potential, and surface functionalities) were examined and compared to the theoretical predictions. Results showed that the CCC of electrostatically stabilized carbon nanomaterials increased with decreasing nanomaterial size when their hydrodynamic sizes were smaller than ca. 200 nm. This is qualitatively consistent with the prediction of the DLVO theory but with a smaller threshold size than the predicted 2 μm. Above the threshold size, the material ζ-potential can be correlated to CCC for nanomaterials with moderate/low surface charge, in agreement with the DLVO theory. The correlation was not observed for highly charged nanomaterials because of their underestimated surface potential by the ζ-potential. Furthermore, a correlation between the C/O ratio and CCC was observed, where a lower C/O ratio resulted in a higher CCC. Overall, our findings rationalized the inconsistency between experimental observation and theoretical prediction and provided essential insights into the aggregation behavior of nanomaterials in water, which could facilitate their rational design.

N-Type Coating of Single-Walled Carbon Nanotubes by Polydopamine-Mediated Nickel Metallization

Single-walled carbon nanotubes (SWCNTs) have unique thermal and electrical properties. Coating them with a thin metal layer can provide promising materials for many applications. This study presents a bio-inspired, environmentally friendly technique for CNT metallization using polydopamine (PDA) as an adhesion promoter, followed by electroless plating with nickel. To improve the dispersion in the aqueous reaction baths, part of the SWCNTs was oxidized prior to PDA coating. The SWCNTs were studied before and after PDA deposition and metallization by scanning and transmission electron microscopy, scanning force microscopy, and X-ray photoelectron spectroscopy. These methods verified the successful coating and revealed that the distribution of PDA and nickel was significantly improved by the prior oxidation step. Thermoelectric characterization showed that the PDA layer acted as a p-dopant, increasing the Seebeck coefficient S of the SWCNTs. The subsequent metallization decreased S, but no negative S-values were reached. Both coatings affected the volume conductivity and the power factor, too. Thus, electroless metallization of oxidized and PDA-coated SWCNTs is a suitable method to create a homogeneous metal layer and to adjust their conduction type, but more work is necessary to optimize the thermoelectric properties.

Insights into the transport of pristine and photoaged graphene oxide-hematite nanohybrids in saturated porous media: Impacts of XDLVO interactions and surface roughness

The transport behaviors of nanomaterials, in especial multifunctional nanohybrids have not been well disclosed until now. In this study, environmentally relevant conditions, including cation types, ionic strength and pH, were selected to investigate the transport and retention of graphene oxide-hematite (GO-Fe₂O₃) nanohybrids and a photoaged product in saturated sandy columns. Results show that more hybridization of hematite led to decreased negative surface charge, while increased particle size and hydrophobicity of the nanohybrids, which depressed their transport according to extented Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. However, the inhibitory transport of photoaged nanohybrids was attributed to their distinct surface roughness caused by relatively high hybridization and photoirradiation. Notably the restrained transport was alleviated in the CaCl₂ saturated media, since the less surface O-functional groups of the corresponding nanohybrids reduced the cation bridging effect caused by Ca²⁺. Similarly, increasing pH promoted the transport of the nanohybrids in NaCl saturated media, particularly for the nanohybrids that contained rich O-functional groups, but exerted inconspicuous effect on mobility of the nanohybrids in CaCl₂ saturated media. These observations highlight that both XDLVO interactions and surface roughness may work together to impact the transport and fate of the burgeoning, versatile nanohybrids in the environment.

Computational investigation of enhanced properties in functionalized carbon nanotube doped polyvinyl alcohol gel electrolyte systems

Recently, functionalized carbon nanotubes (fCNTs) were shown to increase the mechanical strength, thermal stability, and ionic conductivity in polyvinyl alcohol (PVA) based gel electrolytes (GE) for Zn ion batteries. However, questions remain about the origin of the property enhancement and the interactions between components of GEs. In this work, we employ density functional theory calculations to analyze the interactions between fCNT, PVA, and Zn ions. CNTs with increasing numbers of carboxyl (-COOH) functional groups and PVA chains with varying lengths were studied. We found that increasing the number of -COOH on the CNTs enhanced the adsorption energies (E_ads) of PVA, and E_ads also increased as the number of monomers increased. We then modelled the coordination of a Zn ion in fCNT-PVA complexes. Our results suggest that strong fCNT-PVA interactions contribute to the enhanced mechanical strength, while the enhanced ionic conductivity is partly owing to weak Zn adsorption.

Colloidal stability of nanosized activated carbon in aquatic systems: Effects of pH, electrolytes, and macromolecules

Nanosized activated carbon (NAC) is a novel adsorbent with great potential for water reclamation. However, its transport and reactivity in aqueous environments may be greatly affected by its stability against aggregation. This study investigated the colloidal stability of NAC in model aqueous systems with broad background solution chemistries including 7 electrolytes (NaCl, NaNO₃, Na₂SO₄, KCl, CaCl₂, MgCl₂, and BaCl₂), pH 4-9, and 6 macromolecules (humic acid (HA), fulvic acid (FA), cellulose (CEL), bovine serum albumin (BSA), alginate (ALG), and extracellular polymeric substance (EPS)), along with natural water samples collected from pristine to polluted rivers. The results showed that higher solution pH stabilized NAC by raising the critical coagulation concentration from 28 to 590 mM NaCl. Increased cation concentration destabilized NAC by charge screening, with the cationic influence following Ba²⁺ > Ca²⁺ > Mg²⁺ >> Na⁺ > K⁺. Its aggregation behavior could be predicted with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory with a Hamaker constant (A_CWC) of 4.3 × 10^-20 J. The presence of macromolecules stabilized NAC in NaCl solution and most CaCl₂ solution following EPS > BSA > CEL > HA > FA > ALG, due largely to enhanced electrical repulsion and steric hindrance originated from adsorbed macromolecules. However, ALG and HA strongly destabilized NAC via cation bridging at high Ca²⁺ concentrations. Approximately half of NAC particles remained stably suspended for ∼10 d in neutral freshwater samples. The results demonstrated the complex effects of water chemistry on fate and transport of NAC in aquatic environments.

Effects of residual carbon materials on the disinfection byproduct formation in artificial and natural waters

As the effective adsorbents, carbon materials (CMs) are typically used in the removal of disinfection byproduct (DBP) precursors during the water treatment by adding CMs before disinfection procedure. However, after the separation of CMs from the treated water by flocculation, sedimentation, and filtration, a small amount of loaded activated carbon could be released into the water treatment system and affect the DBP formation in the following disinfection. In this study, three CMs, including coal-made activated carbon (CAC), sawdust charcoal (SCC), and hydroxylated multiwall carbon nanotubes (OH-MWCNT), were used to explore the effects of residual CMs in the formation of DBPs. The results indicated that some DBP precursors could be irreversibly adsorbed into the pore structure of CMs and hardly to be extracted and determined, then affected the DBP formation in the water system. In the chlorination process of surface water samples, CMs have similar effects on the formation of DBPs. However, given that water samples contain a variety of complex substances, the effects of residual CMs on the formation of DBPs were also slightly changed.