Impact of sunlight and humic acid on the deposition kinetics of aqueous fullerene nanoparticles (nC60)

Unveiling the effects of protein corona formation on the aggregation kinetics of gold nanoparticles in monovalent and divalent electrolytes

Elucidation of the aggregation behaviors of gold nanoparticles (AuNPs) in water systems is crucial to understanding their environmental fate and transport as well as human health effects. We investigated the early-stage aggregation kinetics of AuNPs coated by human serum albumin (HSA) protein corona (PC) in NaCl and CaCl2 through time-resolved dynamic light scattering. We found that the aggregation of PC-AuNPs depended on the concerted effects of electrolyte concentration, valence, and HSA concentration. At low HSA concentration (≤0.005 g/L), the aggregation kinetics of PC-AuNPs was similar to that of bare AuNPs due to insignificant HSA adsorption. At intermediate HSA concentrations of 0.025-0.050 g/L, the aggregation of PC-AuNPs was retarded in both electrolytes due to steric repulsive forces imparted by the PCs. Additionally, HSA PCs had a weaker retardation effect on PC-AuNPs aggregation in divalent than in monovalent electrolytes. Quartz crystal microbalance measurements revealed that the presence of Ca2+ promoted additional HSA adsorption on PC-AuNPs likely via -COO-Ca2+ bond, and eventually enhanced the aggregation between PC-AuNPs. High-concentration HSA (>0.5 g/L) resulted in no PC-AuNPs aggregation regardless of electrolyte valence and concentrations. Finally, desorption of HSA barely occurred after adsorption on the gold surface, suggesting that the formation of PC-AuNPs is mostly irreversible.

The heteroaggregation and deposition behavior of nanoplastics on Al2O3 in aquatic environments

The ubiquitous Al₂O₃ is anticipated to interact with nanoplastics, affecting their fate and transport in aquatic environments. In this study, the heteroaggregation and deposition behaviors of polystyrene nanoplastics (PSNPs) on Al₂O₃ were systematically investigated under different conditions (ionic strength, pH, and natural organic matter). The results showed that significant heteroaggregation occurred between PSNPs and Al₂O₃ particles under acidic and neutral conditions. When the NaCl concentration was increased from 50 to 500 mM, the heteroaggregation ratio gradually increased. However, poly (acrylic acid) (PAA) inhibited the heteroaggregation of PSNPs-Al₂O₃ due to steric repulsion. The deposition of PSNPs on Al₂O₃ surfaces was inhibited as the NaCl concentration or pH values increased. Due to charge reversal and steric repulsion, humic acid (HA) and fulvic acid (FA) prevented the deposition of PSNPs onto Al₂O₃ surfaces, and the former was more effective in reducing the deposition rate. The interaction mechanism between PSNPs and Al₂O₃ was revealed by using various characterization techniques and density function theory (DFT) calculation. The results demonstrated that in addition to the dominant electrostatic interaction, there were also weak hydrogen bonds and van der Waals interactions. Our research is of great significance for predicting the migration and fate of PSNPs in aquatic environments.

The impacts of metal-based engineered nanomaterial mixtures on microbial systems: A review

The last decade has witnessed tremendous growth in the commercial use of metal-based engineered nanomaterials (ENMs) for a wide range of products and processes. Consequently, direct and indirect release into environmental systems may no longer be considered negligible or insignificant. Yet, there is an active debate as to whether there are real risks to human or ecological health with environmental exposure to ENMs. Previous research has focused primarily on the acute effects of individual ENMs using pure cultures under controlled laboratory environments, which may not accurately reveal the ecological impacts of ENMs under real environmental conditions. The goal of this review is to assess our current understanding of ENM effects as we move from exposure of single to multiple ENMs or microbial species. For instance, are ENMs' impacts on microbial communities predicted by their intrinsic physical or chemical characteristics or their effects on single microbial populations; how do chronic ENM interactions compare to acute toxicity; does behavior under simplified laboratory conditions reflect that in environmental media; finally, is biological stress modified by interactions in ENM mixtures relative to that of individual ENM? This review summarizes key findings and our evolving understanding of the ecological effects of ENMs under complex environmental conditions on microbial systems, identifies the gaps in our current knowledge, and indicates the direction of future research.

Influence of dissolved black carbon on the aggregation and deposition of polystyrene nanoplastics: Comparison with dissolved humic acid

Dissolved black carbon (DBC), widely found in soil and water environments is likely to affect the transport of nanoplastics in aquatic environments. The aggregation and deposition behaviors of fresh and aged polystyrene nanoplastics (PSs) with and without DBC in NaCl solution were investigated by time-resolved dynamic light scattering (DLS) and quartz crystal microbalance with dissipation monitoring equipment (QCM-D) techniques. The results suggest that DBC can screen the surface charges of PSs by interacting with PSs through hydrogen bonding, hydrophobic interactions and π-π interactions, although they were negatively charged. DBC promoted the aggregation of PSs under relatively low ionic strengths, and it minimally affected the stability of PSs under high ionic strength. Deposition experiments showed that both DBC in salt solution and DBC adsorption on silica surface facilitated the deposition of fresh PSs while HA inhibited both deposition processes. After aging, PSs were more stable, and the effects of DBC and HA were weakened. This study investigated the influence mechanism of DBC on the aggregation and deposition behaviors, which provides new insights into the stability and transport of PSs in complex aquatic environments.

Humic acid induced weak attachment of fullerene nC60 nanoparticles and subsequent detachment upon reduction of solution ionic strength in saturated porous media

Sand column experiments were performed under saturated conditions to investigate impact of humic acid (HA) on attachment of nC₆₀ nanoparticles (NPs) in NaCl and CaCl₂ at ionic strengths (ISs) from 1 mM to 100 mM and subsequent detachment via reducing solution IS. The attachment increased with increasing IS due to reduced repulsive Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction energy and accordingly increased retention in primary energy wells. More attachments occurred in CaCl₂ compared to NaCl because Ca²⁺ exhibited greater charge screen ability and served as a bridging agent between the NPs and sand surfaces. The presence of HA significantly reduced nC₆₀ NPs attachment on sand surfaces (especially on nanoscale physical heterogeneities) in 10 mM NaCl and 1 mM CaCl₂ because of enhanced electrostatic and steric repulsions. Interestingly, although the HA did not cause reduction of attachment in 100 mM NaCl and 10 mM CaCl₂ compared to the case in absence of HA, the HA caused weak attachment of nC₆₀ on sand surfaces and then much more significant detachment by decreasing IS. The HA did not alter both attachment and detachment in 100 mM CaCl₂, because the Ca²⁺ at the high concentration caused formation of very stable complex of HA and NPs, and strong interaction of the complex with the sand surfaces via cation bridge. Our study highlighted that the HA can not only enhance the transport of NPs by inhibiting attachment as revealed in the literature, but also by the continuous capture and release of the NPs from surfaces in subsurface environments.

Generation and properties of aqu/nC60: the combined effects of humic acid, sunlight, and agitation intensity

Once released into natural water, the environmental behavior and fate of C₆₀ could inevitably been affected by humic acid (HA), sunlight, and hydrodynamic conditions. However, the combined effects of these factors are not so clear. Therefore, in the present study, effects of HA, sunlight, and agitation intensity on generation and properties of aqu/nC₆₀ were investigated. The results indicated that HA could increase the concentration of aqu/nC₆₀ mainly through the steric hindrance effect. The higher agitation intensity led to higher concentrations of aqu/nC₆₀ and more efficient steric stabilization was formed by HA. Sunlight irradiation promoted the surface oxidization and consequently enhanced the dispersion of C₆₀. The relative order of the influence on the UV/vis concentration was sunlight > agitation intensity > HA. In addition, HA might not always enhance the dispersion of aqu/nC₆₀ due to light screening/ROS scavenging, over-coating, or chain-like bridging mechanism. Therefore, evaluating the environmental behavior and fate of C₆₀ should take these factors into account together.

Adsorption of Human Serum Albumin on Graphene Oxide: Implications for Protein Corona Formation and Conformation

The influence of solution chemistry on the adsorption of human serum albumin (HSA) proteins on graphene oxide (GO) was investigated through batch adsorption experiments and the use of a quartz crystal microbalance with dissipation (QCM-D). The conformation of HSA layers on GO was also examined with the QCM-D. Our results show that an increase in ionic strength under neutral pH conditions resulted in stronger binding between HSA and GO, as well as more compact HSA layers on GO, emphasizing the key role of electrostatic interactions in controlling HSA-GO interactions. Calcium ions also facilitated HSA adsorption likely through charge neutralization and bridging effect. At physiological ionic strength conditions (150 mM), maximum HSA adsorption was observed at the isoelectric point of HSA (4.7). Under acidic conditions, the adsorption of HSA on GO led to the formation of protein layers with a high degree of fluidity due to the extended conformation of HSA. Finally, the attachment of GO to a supported lipid bilayer that was composed of zwitterionic 1,2-dioleoyl-sn-glycero-3-phosphocholine, a model for cell membranes, was reduced in the presence of protein coronas. This reduction in GO attachment was influenced by the conformation of the protein coronas on GO.

Deposition of protein-coated multi-walled carbon nanotubes on oxide surfaces and the retention in a silicon micromodel

The aggregation, deposition and porous retention of bovine serum albumin treated multi-walled carbon nanotubes (BSA-MWCNTs) are investigated using dynamic light scattering (DLS), quartz crystal microbalance with dissipation (QCM-D) and 2-dimensional silicon micromodel, respectively. The aggregation of BSA-MWCNTs is consistent with Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The critical coagulation concentration (CCC) is 175 mM NaCl and 2.7 mM CaCl₂, suggesting that Ca²⁺ causes stronger aggregation. The BSA-MWCNT deposition on SiO₂ surface is unfavorable with critical deposition concentration (CDC) of 100 mM in NaCl and 0.9 mM in CaCl₂. The deposition on the Al₂O₃ surface is favorable. Deposition rate is dominated by electrostatic forces at low ionic strength (IS), but electrostatic interaction is eliminated when IS is above CDC. Therefore the deposition rate on SiO₂ or Al₂O₃ surface starts decreasing at the CDC point due to the reduced particle diffusion. In micromodel, the amount and position of attached BSA-MWCNTs in pore space can be observed by a microscope. The retention attachment efficiency increases at higher IS. The suspended BSA-MWCNTs approach to the collector through either diffusion or interception. The attached BSA-MWCNTs narrow the pore space and then clog the pore throats. The straining process happens on the clogged pore throats.

Cotransformation of Carbon Dots and Contaminant under Light in Aqueous Solutions: A Mechanistic Study

In this study, the photochemistry of carbon dots (CDs) and their effects on pollutant transformation were systematically examined. Diethyl phthalate (DEP) degradation was strongly enhanced by CDs under UV light, with the observed reaction rate constant ( k_obs) increased by 2.4-15.1-fold by CDs at a concentration of 0.5-10 mg/L. Electron paramagnetic resonance (EPR) spectrometry combined with free radical quenching experiments with various chemical probes indicated the production of reactive oxygen species (ROS), including hydroxyl radicals (^•OH), singlet oxygen (¹O₂), and superoxide radical anions (O₂^•-), and these contributed to the enhanced DEP degradation. Meanwhile, CDs were also degraded to low-molecular-weight species and partially mineralized to CO₂ by ROS, as evidenced by Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) and total organic carbon (TOC) analysis, and transformation of CDs was accelerated by DEP. Furthermore, CDs were degraded rapidly under natural sunlight, accompanied by the formation of ^•OH and ¹O₂. Anions such as CO₃^2-, NO₃^-, and Cl^- had limited effects on transformation of CDs, while humic substances greatly inhibited this process. Our results indicate that photoreactions of CDs play an important role in influencing the transformation of pollutants and CDs themselves in the natural aquatic environment. The findings provide invaluable information for evaluating risks associated with the release of CDs into the natural environment.

Effects of oxidation degree on photo-transformation and the resulting toxicity of graphene oxide in aqueous environment

Graphene oxide (GO) has been demonstrated to be key component for diverse applications. However, their potential environmental reactivity, fate and risk have not been fully evaluated to date. In this study, we investigated the photochemical reactivity of four types of GO with different oxidation degrees in aqueous environment, and their related toxicity to two bacterial models Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) was further compared. After UV-irradiation, a large amount of oxygen functional groups on GO were reduced and the electronic conjugations within GO were restored as indicated by UV-visible absorption spectra, X-ray photoelectron spectroscopy and Raman spectroscopy analysis. Moreover, the higher the oxidation degree of the pristine GO was, the more obvious of the photo-transformation changes were. In order to further reveal the photochemical reactivity mechanisms, the reactive oxygen species (ROS) generation of GO was monitored. The quantity of ROS including singlet oxygen (¹O₂), superoxide anions (O₂^·-), and hydroxyl radicals (·OH) increased with increasing oxidation degree of GO, which was in accordance with the previous characterization results. Scanning electron microscopy and cell growth analyses of E. coli and S. aureus showed that the photochemical transformation enhanced the toxicity of GO, which might be due to an increase in functional group density. The higher conductivity of the reduced graphene oxide (RGO) was responsible for its stronger toxicity than GO through membrane damage and oxidative stress to bacteria. This study revealed that the oxidation degrees play important roles in photochemical transformation and the resulting toxicity of GO, which is helpful for understanding the environmental behaviors and risks of GO in aquatic environments.

Deposition Kinetics of Colloidal Manganese Dioxide onto Representative Surfaces in Aquatic Environments: The Role of Humic Acid and Biomacromolecules

The initial deposition kinetics of colloidal MnO₂ on three representative surfaces in aquatic systems (i.e., silica, magnetite, and alumina) in NaNO₃ solution were investigated in the presence of model constituents, including humic acid (HA), a polysaccharide (alginate), and a protein (bovine serum albumin (BSA), using laboratory quartz crystal microbalance with dissipation monitoring equipment (QCM-D). The results indicated that the deposition behaviors of MnO₂ colloids on three surfaces were in good agreement with classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Critical deposition concentrations (CDC) were determined to be 15.5 mM NaNO₃ and 9.0 mM NaNO₃ when colloidal MnO₂ was deposited onto silica and magnetite, respectively. Both HA and alginate could largely retard the deposition of MnO₂ colloids onto three selected surfaces due to steric repulsion, and HA was more effective in decreasing the deposition rate relative to alginate. However, the presence of BSA can provide more attractive deposition site and thus lead to greater deposition behavior of MnO₂ colloids onto surfaces. The dissipative properties of the deposited layer were also influenced by surface type, electrolyte concentration, and organic matter characteristics. Overall, these results provide insights into the deposition behavior of MnO₂ colloids on environmental surfaces and have significant implications for predicting the transport potential of common MnO₂ colloids in natural environments and engineered systems.

Deposition of engineered nanoparticles (ENPs) on surfaces in aquatic systems: a review of interaction forces, experimental approaches, and influencing factors

The growing development of nanotechnology has promoted the wide application of engineered nanomaterials, raising immense concern over the toxicological impacts of nanoparticles on the ecological environment during their transport processes. Nanoparticles in aquatic systems may undergo deposition onto environmental surfaces, which affects the corresponding interactions of engineered nanoparticles (ENPs) with other contaminants and their environmental fate to a certain extent. In this review, the most common ENPs, i.e., carbonaceous, metallic, and nonmetallic nanoparticles, and their potential ecotoxicological impacts on the environment are summarized. Colloidal interactions, including Derjaguin-Landau-Verwey-Overbeek (DLVO) and non-DLVO forces, involved in governing the depositional behavior of these nanoparticles in aquatic systems are outlined in this work. Moreover, laboratory approaches for examining the deposition of ENPs on collector surfaces, such as the packed-bed column and quartz crystal microbalance (QCM) method, and the limitations of their applications are outlined. In addition, the deposition kinetics of nanoparticles on different types of surfaces are critically discussed as well, with emphasis on other influencing factors, including particle-specific properties, particle aggregation, ionic strength, pH, and natural organic matter. Finally, the future outlook and challenges of estimating the environmental transport of ENPs are presented. This review will be helpful for better understanding the effects and transport fate of ENPs in aquatic systems. Graphical abstract ᅟ.

Dissolved Mineral Ash Generated by Vegetation Fire Is Photoactive under the Solar Spectrum

Vegetation fire generates vast amounts of mineral ash annually that can be readily mobilized by water or wind erosion. Little is known about the photoactivity of dissolved mineral ash in aquatic systems and its ability to mediate redox reactions of environmental pollutants. This study reports that dissolved mineral ash derived from pyrolysis of biomass is photoactive under simulated sunlight, generating reactive oxygen species. It can mediate the photoreduction of hexavalent chromium (Cr(VI)) in the presence of electron donors; for example, phenols and dissolved organic matter, at pH 4.7. The reaction kinetics followed the Langmuir-Hinshelwood model, suggesting a heterogeneous photocatalytic reaction. The enhancement of reduction efficiency was linearly correlated with the one-electron reduction potential of phenols. The synergy between dissolved mineral ash and phenols is attributed to the inhibition of electron-hole recombination. The reduction rate decreases with increasing solution pH, owing to the decreased reduction potential and surface adsorption of Cr(VI). The silicon and silicon carbide components are most likely responsible for the photocatalytic activity of dissolved mineral ash. Our results suggest that dissolved mineral ash is a natural photocatalyst that can mediate redox reactions of pollutants in sunlit aquatic systems, playing an overlooked role in natural attenuation and aquatic photochemistry.

Interaction of polyhydroxy fullerenes with ferrihydrite: adsorption and aggregation

The rapid development of nanoscience and nanotechnology, with thousands types of nanomaterials being produced, will lead to various environmental impacts. Thus, understanding the behaviors and fate of these nanomaterials is essential. This study focused on the interaction between polyhydroxy fullerenes (PHF) and ferrihydrite (Fh), a widespread iron (oxyhydr)oxide nanomineral and geosorbent. Our results showed that PHF were effectively adsorbed by Fh. The adsorption isotherm fitted the D-R model well, with an adsorption capacity of 67.1mg/g. The adsorption mean free energy of 10.72kJ/mol suggested that PHF were chemisorbed on Fh. An increase in the solution pH and a decrease of the Fh surface zeta potential were observed after the adsorption of PHF on Fh; moreover, increasing initial solution pH led to a reduction of adsorption. The Fourier transform infrared spectra detected a red shift of C-O stretching from 1075 to 1062cm^-1 and a decrease of Fe-O bending, implying the interaction between PHF oxygenic functional groups and Fh surface hydroxyls. On the other hand, PHF affected the aggregation and reactivity of Fh by changing its surface physicochemical properties. Aggregation of PHF and Fh with individual particle sizes increasing from 2nm to larger than 5nm was measured by atomic force microscopy. The uniform distribution of C and Fe suggested that the aggregates of Fh were possibly bridged by PHF. Our results indicated that the interaction between PHF and Fh could evidently influence the migration of PHF, as well as the aggregation and reactivity of Fh.

Interactions between engineered nanoparticles and dissolved organic matter: A review on mechanisms and environmental effects

Dissolved organic matter (DOM) is ubiquitous in the environment and has high reactivity. Once engineered nanoparticles (ENPs) are released into natural systems, interactions of DOM with ENPs may significantly affect the fate and transport of ENPs, as well as the bioavailability and toxicity of ENPs to organisms. However, because of the complexity of DOM and the shortage of useful characterization methods, large knowledge gaps exist in our understanding of the interactions between DOM and ENPs. In this article, we systematically reviewed the interactions between DOM and ENPs, discussed the effects of DOM on the environmental behavior of ENPs, and described the changes in bioavailability and toxicity of ENPs caused by DOM. Critical evaluations of published references suggest further need for assessing and predicting the influences of DOM on the transport, transformation, bioavailability, and toxicity of ENPs in the environment.

Probing the interactions of organic molecules, nanomaterials, and microbes with solid surfaces using quartz crystal microbalances: methodology, advantages, and limitations

Quartz crystal microbalances (QCMs) provide a new analytical opportunity and prospect to characterize many environmental processes at solid/liquid interfaces, thanks to their almost real-time measurement of physicochemical changes on their quartz sensor. This work reviews the applications of QCMs in probing the interactions of organic molecules, nanomaterials (NMs) and microbes with solid surfaces. These interfacial interactions are relevant to critical environmental processes such as biofilm formation, fate and transport of NMs, fouling in engineering systems and antifouling practices. The high sensitivity, real-time monitoring, and simultaneous frequency and dissipation measurements make QCM-D a unique technique that helps reveal the interaction mechanisms for the abovementioned processes (e.g., driving forces, affinity, kinetics, and the interplay between surface chemistry and solution chemistry). On the other hand, QCM measurement is nonselective and spatially-dependent. Thus, caution should be taken during data analysis and interpretation, and it is necessary to cross-validate the results using complementary information from other techniques for more quantitative and accurate interpretation. This review summarizes the general methodologies for collecting and analyzing raw QCM data, as well as for evaluating the associated uncertainties. It serves to help researchers gain deeper insights into the fundamentals and applications of QCMs, and provides new perspectives on future research directions.

The attachment of colloidal particles to environmentally relevant surfaces and the effect of particle shape

Despite the prevalence of nonspherical colloidal particles, the role of particle shape in the transport of colloids is largely understudied. This study investigates the attachment of colloidal particles onto environmentally relevant surfaces while varying particle shape and ionic strength. Using quartz crystal microbalance and atomic force microscopy measurements, the role of particle shape was elucidated and possible mechanisms discussed. The attachment of both spherical and stretched polystyrene colloidal particles onto a smooth alginate-coated silica surface showed qualitative agreement with DLVO theory. Attachment onto a Harpeth humic acid (HHA) surface, however, significantly deviated from DLVO theory due to its high surface heterogeneity and extended confirmation from the silica surface. This extended confirmation provided increased potential for spherical particle entanglement, while the enlarged major axis of the stretched particles hindered their ability to attach. As ionic strength increased, the HHA layer condensed and provided less potential for spherical particle entanglement and therefore the selectivity for spherical particle attachment vanished. The findings presented in this study suggest that colloidal particle shape may play a complex and important role in predicting the transport of colloidal particles, especially in the presence of natural organic matter-coated surfaces.

Effects of charge and surface defects of multi-walled carbon nanotubes on the disruption of model cell membranes

The direct contact between multi-walled carbon nanotubes (MWCNTs) and cell membranes causes membrane disruption, potentially leading to cytotoxicity. However, the role of electrostatic forces and MWCNT properties is still open to debate. In this study, the influences of charge and MWCNT surface defects on membrane disruption were investigated by microscopy and a quartz crystal microbalance with dissipation monitoring (QCM-D). Positively/negatively charged giant unilamellar vesicles (GUVs) and supported lipid bilayers (SLBs) were made as model cell membranes. Negatively charged MWCNTs disrupted the GUVs containing positively charged lipids, which confirmed the electrostatically mediated interaction. However, the mass loss was detected from the negatively charged SLBs after MWCNT exposure, which suggests the extraction of phospholipids. The defect degree of MWCNTs correlated with their adhesion amount on the membranes. Both the oxygenated functional groups and unoxidized dangling carbon bonds were active sites for MWCNT-membrane interactions. The MWCNTs were observed to be engulfed inside the GUVs. The results clearly demonstrate that phospholipid extraction by MWCNTs could occur in electrostatically repulsive conditions, and MWCNT defects were active binding sites whether or not they were oxygenated. Our findings should be helpful in the design and safe applications of carbon nanomaterials.

Effects of C60 on the Photochemical Formation of Reactive Oxygen Species from Natural Organic Matter

Buckminsterfullerenes (C₆₀) are widely used nanomaterials that are present in surface water. The combination of C₆₀ and humic acid (HA) generates reactive oxygen species (ROS) under solar irradiation, but this process is not well understood. Thus, the present study focused on the photochemical formation of singlet oxygen (¹O₂), hydroxyl radical (HO^•)-like species, superoxide radicals (O₂^•-), hydrogen peroxide (H₂O₂), and triplet excited states (³C₆₀*/³HA*) in solutions containing both C₆₀ and HA. The quantum yield coefficients of excited triplet states (f_TMP) and apparent quantum yields of ROS were measured and compared to the calculated values, which were based on the conservative mixing model. Although C₆₀ proved to have only a slight impact on the ¹O₂ formation from HA, C₆₀ played a key role in the inhibition of O₂^•-. The photochemical formation of H₂O₂ followed the conservative mixing model due to the reaction of C₆₀^•- with HO₂^•/O₂^•-, and the biomolecular reaction rate constant has been measured as (7.4 ± 0.6) × 10⁶ M^-1 s^-1. The apparent f_TMP was significantly lower than the calculated value, indicating that the steric effect of HA was significant in the reaction of ³C₆₀* with the TMP probe. In contrast, C₆₀ did not have an effect on the photochemical formation of HO^• from HA, suggesting that HO^• is elevated from the hydrophilic surface of HA. The aforementioned results may be useful for predicting the photochemical influence of C₆₀ on aqueous environments.

QCM-D study of nanoparticle interactions

Quartz crystal microbalance with dissipation monitoring (QCM-D) has been proven to be a powerful research tool to investigate in situ interactions between nanoparticles and different functionalized surfaces in liquids. QCM-D can also be used to quantitatively determine adsorption kinetics of polymers, DNA and proteins from solutions on various substrate surfaces while providing insights into conformations of adsorbed molecules. This review aims to provide a comprehensive overview on various important applications of QCM-D, focusing on deposition of nanoparticles and attachment-detachment of nanoparticles on model membranes in complex fluid systems. We will first describe the working principle of QCM-D and DLVO theory pertinent to understanding nanoparticle deposition phenomena. The interactions between different nanoparticles and functionalized surfaces for different application areas are then critically reviewed. Finally, the potential applications of QCM-D in other important fields are proposed and knowledge gaps are identified.

Detachment of fullerene nC60 nanoparticles in saturated porous media under flow/stop-flow conditions: Column experiments and mechanistic explanations

This study was aimed at investigating the detachment of fullerene nC60 nanoparticles (NPs) in saturated sand porous media under transient and static conditions. The nC60 NPs were first attached at primary minima of Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction energy profiles in electrolyte solutions with different ionic strengths (ISs). The columns were then eluted with deionized water to initiate nC60 NP detachment by decreasing solution IS. Finally, the flow of the columns was periodically interrupted to investigate nC60 NP detachment under static condition. Our results show that the detachment of nC60 NPs occurred under both transient and static conditions. The detachment under transient conditions was attributed to the fact that the attractions acting on the nC60 NPs at primary minima were weakened by nanoscale physical heterogeneities and overcome by hydrodynamic drags at lower ISs. However, a fraction of nC60 NPs remained at shallow primary minima in low flow regions, and detached via Brownian diffusion during flow interruptions. Greater detachment of nC60 NPs occurred under both transient and static conditions if the NPs were initially retained in electrolyte solutions with lower valent cations due to lower attractions between the NPs and collectors. Decrease in collector surface chemical heterogeneities and addition of dissolved organic matter also increased the extent of detachment by increasing electrostatic and steric repulsions, respectively. While particle attachment in and subsequent detachment from secondary minima occur in the same electrolyte solution, our results indicate that perturbation in solution chemistry is necessary to lower the primary minimum depths to initiate spontaneous detachment from the primary minima. These findings have important implications for predicting the fate and transport of nC60 NPs in subsurface environments during multiple rainfall events and accordingly for accurately assessing their environmental risks.

Interactions of Graphene Oxide with Model Cell Membranes: Probing Nanoparticle Attachment and Lipid Bilayer Disruption

With the rapid growth in the application of graphene oxide (GO) in diverse fields, the toxicity of GO toward bacterial and mammalian cells has recently attracted extensive research attention. While several mechanisms have been proposed for the cytotoxicity of GO, the attachment of GO to cell membranes is expected to be the key initial process that precedes these mechanisms. In this study, we investigate the propensity for GO to attach to and disrupt model cell membranes using supported lipid bilayers (SLBs) and supported vesicular layers (SVLs) that are composed of zwitterionic 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). The deposition kinetics of GO on SLBs were determined using quartz crystal microbalance with dissipation monitoring and were observed to increase with increasing electrolyte (NaCl and CaCl2) concentrations, indicating that GO attachment to SLBs was controlled by electrostatic interactions. The GO deposition kinetics measured at elevated electrolyte concentrations were lower than mass-transfer-limited kinetics, likely due to the presence of hydration forces between GO and SLBs. Upon the attachment of GO to supported vesicles that were encapsulated with a fluorescent dye, dye leakage was detected, thus indicating that the lipid vesicles were disrupted. When the exposure of the SVL to the GO suspension was terminated, the leakage of dye decreased significantly, demonstrating that the pores on the lipid bilayers have a self-healing ability.

The role of attached phase soil and sediment organic matter physicochemical properties on fullerene (nC60) attachment

Attached phase soil and sediment organic matter is ubiquitous in the subsurface environment, with a tendency to strongly sorb contaminants, and therefore it may play an important role in contaminant transport. In this study, the deposition of C60 nanoparticles onto attached phase Harpeth Humic Acid and Harpeth Fulvic Acid (HHA and HFA) is explored by using a quartz crystal microbalance with dissipation monitoring and systematically varying thermal energy. By comparing the C60 attachment onto HHA and HFA surfaces to that of bare silica and DLVO predictions, we find that the HHA and HFA layers hinder attachment at low temperatures, while HHA enhances attachment at higher temperatures. Based on thermal characterization of the HHA and HFA layers compared to the corresponding attachment trends, the attachment efficiency is strongly correlated with hydration of the layer. Possible mechanisms explaining this phenomenon include water-assisted disruption of polar SOM contacts and hydration-induced swelling of the AP-SOM matrix. Since humic substances typically dominate subsurface organic matter, these results may prove crucial to understanding the complex interactions of engineered nanomaterials in both the natural and engineered environment.

Influence of surface heterogeneities on reversibility of fullerene (nC60) nanoparticle attachment in saturated porous media

This study systematically investigated influence of surface roughness and surface chemical heterogeneity on attachment and detachment of nC60 nanoparticles in saturated porous media by conducting laboratory column experiments. Sand and glass beads were employed as a model collectors to represent a different surface roughness. The two collectors were treated by washing with only deionized water or by using acids to extensively remove chemical heterogeneities. Results show that both attachment and detachment were more in the acid-treated sand than those in the acid-treated glass beads. The greater attachment and detachment were attributed to the reason that sand surfaces have much more nanoscale asperities, which facilitates particle attachment atop of them at primary minima and subsequent detachment upon reduction of ionic strength. No detachment was observed if the water-washed collectors were employed, demonstrating that the couple of chemical heterogeneity with nanoscale roughness causes irreversible attachment in primary minima. Whereas existing studies frequently represented surface rough asperities as regular geometries (e.g., hemisphere, cone, pillar) for estimating influence of surface roughness on Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction energies, our theoretical calculations indicate that the assumptions could underestimate both attachment and detachment because these geometries cannot account for surface curvature effects.

Cotransport of bismerthiazol and montmorillonite colloids in saturated porous media

While bismerthiazol [N,N'-methylene-bis-(2-amino-5-mercapto-1,3,4-thiadiazole)] is one of the most widely used bactericides, the transport of bismerthiazol in subsurface environments is unclear to date. Moreover, natural colloids are ubiquitous in the subsurface environments. The cotransport of bismerthiazol and natural colloids has not been investigated. This study conducted laboratory column experiments to examine the transport of bismerthiazol in saturated sand porous media both in the absence and presence of montmorillonite colloids. Results show that a fraction of bismerthiazol was retained in sand and the retention was higher at pH7 than at pH 4 and 10. The retention did not change with ionic strength. The retention was attributed to the complex of bismerthiazol with metals/metal oxides on sand surfaces through ligand exchange. The transport of bismerthiazol was enhanced with montmorillonite colloids copresent in the solutions and, concurrently, the transport of montmorillonite colloids was facilitated by the bismerthiazol. The transport of montmorillonite colloids was enhanced likely because the bismerthiazol and the colloids competed for the attachment/adsorption sites on collector surfaces and the presence of bismerthiazol changed the Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction energies between colloids and collectors. The transport of bismerthiazol was inhibited if montmorillonite colloids were pre-deposited in sand because bismerthiazol could adsorb onto the colloid surfaces. The adsorbed bismerthiazol could be co-remobilized with the colloids from primary minima by decreasing ionic strength. Whereas colloid-facilitated transport of pesticides has been emphasized, our study implies that transport of colloids could also be facilitated by the presence of pesticides.

Ambient water and visible-light irradiation drive changes in graphene morphology, structure, surface chemistry, aggregation, and toxicity

The environmental behaviors and risks associated with graphene have attracted considerable attention. However, the fundamental effects of ambient water and visible-light irradiation on the properties and toxicity of graphene remain unknown. This work revealed that hydration and irradiation result in the transformation of large-sheet graphene to long-ribbon graphene. The thickness of the treated graphene decreased, and oxides were formed through the generation of singlet oxygen. In addition, hydration and irradiation resulted in greater disorder in the graphene structure and in the expansion of the d-spacing of the structure due to the introduction of water molecules and modifications of the functional groups. Oxidative modifications with two-stage (fast and low) kinetics enhanced the number of negative surface charges on the graphene and enhanced graphene aggregation. The above property alterations reduced the nanotoxicity of graphene to algal cells by reducing the generation of reactive oxygen species, diminishing protein carbonylation and decreasing tail DNA. A comparative study using graphene oxide suggested that oxidative modifications could play an important role in inhibiting toxicological activity. This study provides a preliminary approach for understanding the environmental behaviors of graphene and avoids overestimating the risks of graphene in the natural environment.

Synergistic photogeneration of reactive oxygen species by dissolved organic matter and C60 in aqueous phase

We investigated the photogeneration of reactive oxygen species (ROS) by C60 under UV irradiation, when humic acid (HA) or fulvic acid (FA) is present. When C60 and dissolved organic matter (DOM) were present as a mixture, singlet oxygen ((1)O2) generation concentrations were 1.2–1.5 times higher than the sum of (1)O2 concentrations that were produced when C60 and DOM were present in water by themselves. When C60 and HA were present as a mixture, superoxide radicals (O2(•–)) were 2.2–2.6 times more than when C60 and HA were present in water by themselves. A synergistic ROS photogeneration mechanism involved in energy and electron transfer between DOM and C60 was proposed. Enhanced (1)O2 generation in the mixtures was partly due to (3)DOM* energy transfer to O2. However, it was mostly due to (3)DOM* energy transfer to C60 producing (3)C60*. (3)C60* has a prolonged lifetime (>4 μs) in the mixture and provides sufficient time for energy transfer to O2, which produces (1)O2. The enhanced O2(•–) generation for HA/C60 mixture was because (3)C60* mediated electron transfer from photoionized HA to O2. This study demonstrates the importance of considering DOM when investigating ROS production by C60.

Effects of sulfide reduction on adsorption affinities of colloidal graphene oxide nanoparticles for phenanthrene and 1-naphthol

The abiotic transformation of nanomaterials in the natural environment can significantly affect their fate,transport, and effects. We observed that the adsorption affinities of graphene oxide nanoparticles(GONPs) for both phenanthrene and 1-naphthol were profoundly enhanced in the presence of sulfide, an environmentally relevant reductant, with doses as low as 0.5 mM Na(2)S per 10 mg/L GONPs. For phenanthrene adsorption enhancement was predominantly caused by the increased surface hydrophobicity from Na(2)S treatment. For 1-naphthol, however, adsorption enhancement was caused mainly by the conversion of the epoxy/ether groups on the surface of graphene oxide (GO) to the phenolic hydroxyl and carbonyl groups, which allowed more significant H-bonding between 1-naphthol and GONPs. The findings of this study underline that abiotic transformation of GO not only affects the stability and mobility of GONPs, but also influences the adsorptive interactions between GONPs and environmental contaminants, and consequently, may increase the environmental risks of GONPs.

Transformation and destabilization of graphene oxide in reducing aqueous solutions containing sulfide

The colloidal stability of carbon nanomaterials is a key factor controlling their fate and bioavailability in natural aquatic systems. The authors report that graphene oxide nanoparticles could be destabilized in reducing aqueous solutions containing a low concentration (0.5 mM) of sulfide, a naturally occurring reductant. Spectroscopic characterization using combined X-ray photoelectron, Fourier-transform infrared, X-ray diffraction, and Raman analyses revealed that the surface oxygen-containing groups (mainly epoxy groups) of graphene oxide were significantly reduced after reacting with sodium sulfide. The destabilization of graphene oxide was likely caused by the enhanced surface hydrophobicity of the reduced graphene oxide, whereas electrostatic repulsion played a minimal role. Solution pH was found to affect both the deoxygenation process and the aggregation behavior of graphene oxide. Coexisting humic acid reduced the reaction efficiency and stabilized graphene oxide through steric hindrance. These findings suggest for the first time that the colloidal behavior of carbon nanomaterials might change drastically when they enter natural reducing environments containing sulfide such as anaerobic aquifers and sediments.