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Heteroaggregation of graphene oxide (GO) with nanometer- and micrometer-sized hematite colloids, which are naturally present in aquatic systems, is investigated in this study. The heteroaggregation rates between GO and hematite nanoparticles (HemNPs) were quantified by dynamic light scattering, while the heteroaggregation between GO and micrometer-sized hematite particles (HemMPs) was examined through batch adsorption and sedimentation experiments. The heteroaggregation rates of GO with HemNPs first increased and then decreased with increasing GO/HemNP mass concentration ratios. The conformation of GO-HemNP heteroaggregates at different GO/HemNP mass concentration ratios was observed through transmission electron microscopy imaging. Initially, GO underwent heteroaggregation with HemNPs through electrostatic attraction to form primary heteroaggregates, which were further bridged by GO to form bigger clusters. At high GO/HemNP mass concentration ratios where GO outnumbered HemNPs, heteroaggregation resulted in the formation of stable GO-HemNP nanohybrids that have a critical coagulation concentration of 308 mM NaCl at pH 5.2. In the case of HemMPs, GO adsorbed readily on the microparticles and, at an optimal GO/HemMP ratio of ∼0.002, the sedimentation of HemMPs was the fastest, most likely because of the formation of "electrostatic patches" leading to favorable aggregation of the microparticles.

The surface charge inversion (CI), critical for functions of many biological nanomachinery, is a counterintuitive phenomenon in which the net charge of a strongly charged surface bound with many counterions changes its sign. Many phenomena of CI cannot be explained in the framework of mean-field theories, such as linear Debye–Huckel and nonlinear Poisson–Boltzmann, and is so far best described by the strongly correlated liquid (SCL) theory for multivalent (Z ≥ 3) ions. However, the potential CI by divalent ions, which are more relevant in the biological environment, is much less studied and remains mostly illusive. In this study, we examine the divalent-ion-induced CI of a negatively charged bacterium’s cell membrane and reveal its underlying mechanism, using all-atom molecular dynamics simulations and free energy perturbation theory. Our simulation results are compared with theoretical predications from SCL, and noticeable discrepancies with SCL theory, originating from the fluidity of membrane lipids wh...

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.

The influence of solution chemistry and soft protein coronas on the interactions between citrate-coated silver nanoparticles (AgNPs) and model biological membranes was investigated by assembling supported lipid bilayers (SLBs) composed of zwitterionic 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) on silica crystal sensors in a quartz crystal microbalance with dissipation monitoring (QCM-D). Our results show that the deposition rates of AgNPs on unmodified silica surfaces increased with increasing electrolyte concentrations under neutral pH conditions. Similar trends were observed when AgNPs were deposited on SLBs, hence indicating that the deposition of AgNPs on model cell membranes was controlled by electrostatic interactions. In the presence of human serum albumin (HSA) proteins at both pH 7 and pH 2, the colloidal stability of AgNPs was considerably enhanced due to the formation of HSA soft coronas surrounding the nanoparticles. At pH 7, the deposition of AgNPs on SLBs was suppressed in the presence of HSA due to steric repulsion between HSA-modified AgNPs and SLBs. In contrast, pronounced deposition of HSA-modified AgNPs on SLBs was observed at pH 2. This observation was attributed to the reduction of electrostatic repulsion as well as conformation changes of adsorbed HSA under low pH conditions, resulting in the decrease of steric repulsion between AgNPs and SLBs.

In the semiconductor industry, chemical mechanical planarization (CMP) slurries are used in large quantities for the planarization of wafers and the release of CMP nanoparticles (NPs) into the natural environment may pose a threat to human health. In this study, the aggregation behavior of four model CMP NPs, namely, colloidal SiO2, fumed SiO2, CeO2, and Al2O3 NPs, was investigated through dynamic light scattering in the presence and absence of phosphate at pH 7.4. The colloidal and fumed SiO2 NPs were observed to have remarkable stability under neutral pH conditions both in the presence and absence of phosphate. Phosphate adsorption enhanced the negative surface charge of CeO2 NPs and reversed the charge of Al2O3 NPs, resulting in an increase in the stability of both NPs. In order to evaluate the propensity for the CMP NPs to attach to and disrupt cell membranes, the interactions of these NPs with supported lipid bilayers (SLBs) and lipid vesicles composed of zwitterionic 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) were examined using a quartz crystal microbalance. The attachment efficiencies of both SiO2 NPs on SLBs increased to 1 when the NaCl concentration was raised to 100 mM in the presence of phosphate. In contrast, favorable attachment of CeO2 and Al2O3 NPs to SLBs was not achieved in the presence of phosphate even at elevated NaCl concentrations, likely due to the enhancement of nanoparticle surface charge resulting from phosphate adsorption. When the supported vesicles were exposed to the SiO2 NPs, no disruption of vesicles was detected despite favorable conditions for nanoparticle–membrane association.

An intriguing polyvinylidene fluoride (PVDF) membrane with unique pillar-like structures was synthesized. The membrane was synthesized using the phase inversion method by adding the amphiphilic graft copolymer PVDF grafted with poly(ethylene glycol) methyl ether methacrylate (PEGMA) (PVDF-g-PEGMA) to the PVDF backbone material. It had high flux and high sodium alginate rejection ratio under low transmembrane pressure. However, the mechanisms for the formation of pillar-like structures are still unknown. In this paper, we explored the formation mechanism of pillar-like structures from aspects of solvent and additive. Based on the experimental results and analysis of ternary diagram, both NMP and PVDF-g-PEGMA must coexist in casting solution to form pillar-like structures. When NMP is in the solvent, PEGMA segments have enough time to migrate to the surface and repel each other during the phase inversion process. Finally, by using the target plot method, one membrane with the best performance was chosen as the proposed membrane from all membranes that casted under different conditions. The proposed membrane has a pure water flux of 2173 L/m2/h/bar. The total organic carbon (TOC, by sodium alginate) removal efficiency is 89%. This membrane may have a good potential in water treatment applications.

The surface of a polysulfone membrane was modified with a bioinspired polydopamine (PDA) film followed by the in situ formation of silver nanoparticles (AgNPs) to mitigate membrane biofouling. The PDA modification enhanced the membrane’s bacterial anti-adhesive properties by increasing the surface hydrophilicity, while AgNPs imparted strong antimicrobial properties to the membrane. The AgNPs could be generated on the membrane surface by simply exposing the membrane to AgNO3 solutions. Ag+ ions were reduced by the catechol groups in PDA; the AgNP mass loading increased with exposure time, and the AgNPs were firmly immobilized on the membrane through metal coordination. During leaching tests, the concentrations of Ag+ ions released were 2–3 orders of magnitude lower than the established contaminant limit for drinking water, thereby providing a safe antimicrobial technology. This novel membrane surface modification technique paves a way to mitigating biofouling by enhancing the membrane’s anti-adhesive and a...

This tutorial review focuses on aqueous slurries of dispersed engineered nanoparticles (ENPs) used in chemical mechanical planarization (CMP) for polishing wafers during manufacturing of semiconductors. A research consortium was assembled to procure and conduct physical, chemical, and in vitro toxicity characterization of four ENPs used in CMP. Based on input from experts in semiconductor manufacturing, slurries containing fumed silica (f-SiO2), colloidal silica (c-SiO2), ceria (CeO2), and alumina (Al2O3) were selected and subsequently obtained from a commercial CMP vendor to represent realistic ENPs in simplified CMP fluids absent of proprietary stabilizers, oxidants, or other chemical additives that are known to be toxic. ENPs were stable in suspension for months, had highly positive or negative zeta potentials at their slurry working pH, and had mean diameters measured by dynamic light scattering (DLS) of 46 ± 1 nm for c-SiO2, 148 ± 5 nm for f-SiO2, 132 ± 1 nm for CeO2, and 129 ± 2 nm for Al2O3, all of which were larger than the sub 100 nm diameter primary particle sizes measured by electron microscopy. The concentration of ENPs in all four slurries that caused 50% inhibition (IC-50) was greater than 1 mg mL−1 based on in vitro assays using bioluminescence of the bacterium Aliivibrio fischeri and the proliferation, viability, and integrity of human cells (adenocarcinomic human alveolar basal epithelial cell line A549). The general practice in the CMP industry is to dilute the slurry waste stream so actual abrasive concentrations are typically orders of magnitude smaller than 1 mg mL−1, which is lower than IC-50 levels. In contrast to recent reports, we observed similar toxicological characteristics between c-SiO2 and f-SiO2, and the materials exhibited similar X-ray diffraction (XRD) spectra but different morphology observed using electron microscopy. The ENPs and CMP slurries used in this study have been made available to a number of other research groups, and it is the intention of the consortium for this paper to provide a basis for characterizing and understanding human and environmental exposures for this important class of industrial ENPs.

Heteroaggregation between silver nanoparticles (AgNPs) and naturally occurring colloids is a critical process that controls the environmental impacts of AgNPs. In this study, the effect of heteroag...

In this study, the disaggregation behavior and the strength of heteroaggregates composed of multiwalled carbon nanotubes (CNTs) and hematite nanoparticles (HemNPs) were investigated in different solution chemistries. Negatively charged CNTs and positively charged HemNPs were first allowed to undergo favorable heteroaggregation at pH 5.5 and 0.1 mM NaCl before the CNT-HemNP heteroaggregates were disaggregated through the use of an ultrasonication bath. The heteroaggregate sizes before and after ultrasonication were measured by dynamic light scattering (DLS) in order to determine the degree of disaggregation. When the solution chemistry was unchanged, the heteroaggregates underwent partial disaggregation and heteroaggregate re-growth was observed after ultrasonication. Conversely, when the pH was raised to 11.0 immediately before ultrasonication, the heteroaggregates were almost completely disaggregated and no aggregation took place after disaggregation. Similarly, the introduction of humic acid led to a near complete disaggregation of the CNT-HemNP heteroaggregates. The elevated pH, as well as the adsorption of humic acid on the nanoparticles, was likely to weaken the particle-particle bonds within the heteroaggregates and hence increase the propensities of the heteroaggregates to undergo disaggregation.

Nanotoxicity studies have shown that both carbon-based and inorganic engineered nanoparticles can be toxic to microorganisms. Although the pathways for cytotoxicity are diverse and dependent upon the nature of the engineered nanoparticle and the chemical environment, numerous studies have provided evidence that direct contact between nanoparticles and bacterial cell membranes is necessary for cell inactivation or damage, and may in fact be a primary mechanism for cytotoxicity. The propensities for nanoparticles to attach to and disrupt cell membranes are still not well understood due to the heterogeneous and dynamic nature of biological membranes. Model biological membranes can be employed for systematic investigations of nanoparticle-membrane interactions. In this article, current and emerging experimental approaches to identify the key parameters that control the attachment of ENPs on model membranes and the disruption of membranes by ENPs will be discussed. This critical information will help enable the "safe-by-design" production of engineered nanoparticles that are nontoxic or biocompatible, and also allow for the design of antimicrobial nanoparticles for environmental and biomedical applications.

The bacterial anti-adhesive properties of polysulfone (PSU) microfiltration membranes modified with poly(allylamine hydrochloride) (PAH)/poly(acrylic acid) (PAA) polyelectrolyte multilayers (PEMs) were investigated in this study. Using a direct microscopic observation membrane filtration system, the deposition kinetics of Escherichia coli cells on the membrane surfaces, as well as the reversibility of bacterial deposition, were examined in the absence and presence of calcium. The PEM-modified membranes exhibited significantly improved bacterial anti-adhesive properties compared to the PSU base membranes in both the tested solution chemistries. Specifically, the bacterial deposition kinetics on the modified membranes were slower than the deposition kinetics on the base membranes. Furthermore, the bacterial removal efficiency was significantly enhanced from o10% to as high as 99% after PEM modification. Interaction force measurements conducted through atomic force microscopy revealed strong, long-ranged repulsive interactions between a carboxylate modified latex colloid probe and PEMmodified membranes, while attractive interactions were detected between the colloid probe and PSU base membranes. The bacterial anti-adhesive properties exhibited by the PEM-modified membranes were attributed to the highly swollen and hydrated PEMs that inhibit the direct contact or close approach of bacteria to the underlying PSU membranes.

The release of multiwalled carbon nanotubes (MWNTs) that were deposited on silica surfaces was investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D). MWNTs were deposited on silica surfaces at elevated NaCl and CaCl2 concentrations before being rinsed with eluents of different solution chemistries to induce their remobilization. Energetically speaking, the MWNTs were released from the primary energy minimum when the background NaCl or CaCl2 concentrations were decreased at pH 7.1. The increase in electrostatic repulsion between MWNTs and silica likely caused a reduction in the energy barrier, which enabled the release of MWNTs. The degree of release increased in a stepwise fashion when the nanotubes were sequentially exposed to eluents of decreasing electrolyte concentrations, possibly due to the heterogeneity in nanotube surface charge densities. The degree of release via a successive reduction in NaCl concentration was lower at pH 4.0 than at 7.1 due to MWNTs and silica surfaces exhibiting a less negative surface charge at pH 4.0. Most of the deposited MWNTs were released when the pH was decreased from 7.1 to 4.0 at 1.5 mM CaCl2. This was attributed to the elimination of calcium bridging between the carboxyl groups on MWNTs and silanol groups on silica surfaces.

The deposition kinetics of fullerene (C60) nanoparticles onto bare silica surfaces and surfaces precoated with humic acid and alginate are investigated over a range of monovalent (NaCI) and divalent (CaCl2) salt concentrations using a quartz crystal microbalance. Because simultaneous aggregation of the fullerene nanoparticles occurs, especially at higher electrolyte concentrations, we normalize the observed deposition rates by the corresponding favorable (transport-limited) deposition rates to obtain the attachment efficiencies, alpha. The deposition kinetics of fullerene nanoparticles onto bare silica surfaces are shown to be controlled by electrostatic interactions and van der Waals attraction, consistent with the classical particle deposition behavior where both favorable and unfavorable deposition regimes are observed. The presence of dissolved humic acid and alginate in solution leads to significantly slower deposition kinetics due to steric repulsion. Precoating the silica surfaces with humic acid and alginate exerts similar steric stabilization in the presence of NaCl. In the presence of CaCl2, the deposition kinetics of fullerene nanoparticles onto both humic acid- and alginate-coated surfaces are relatively high, even at relatively low (0.3 mM) calcium concentration. This behavior is attributed to the macromolecules undergoing complex formation with calcium ions, which reduces the charge and steric influences of the adsorbed macromolecular layers.

Injection of nanoscale zero-valent iron (NZVI) is potentially a promising technology for remediation of contaminated groundwaters. However, the efficiency of this process is significantly hindered by the rapid aggregation of the iron nanoparticles. The aim of this study was to enhance the colloidal stability of the nanoparticles through the addition of the "green" polymer guar gum. We evaluated the properties of guar gum and its influence on the surface properties, particle size, aggregation, and sedimentation of iron nanoparticles. Commercial iron nanoparticles were dispersed in guar gum solutions, and their aggregation and sedimentation behaviors were compared to those of bare iron nanoparticles and commercial nanoparticles modified with a biodegradable polymer (polyaspartate). High performance size exclusion chromatography, charge titration, and viscosity assessment showed that guar gum is a high molecular weight polymer which is nearly neutrally charged, rendering it suitable for steric stabilization of the iron nanoparticles. Electrophoretic mobility measurements demonstrated the ability of guar gum to adsorb on the nanoparticles, forming a slightly negatively charged layer. Dynamic light scattering experiments were conducted to estimate the particle size of the different nanoparticle suspensions and to determine the aggregation behavior at different ionic strengths. Guar gum effectively reduced the hydrodynamic radius of the bare nanoparticles from 500 nm to less than 200 nm and prevented aggregation of the nanoparticles even at very high salt concentrations (0.5 M NaCl and 3 mM CaCl(2)). Sedimentation profiles of the different nanoparticle suspensions confirmed the improved stability of the iron nanoparticles in the presence of guar gum. The results strongly suggest that guar gum can be used to effectively deliver stabilized zero-valent iron nanoparticles for remediation of contaminated groundwater aquifers.

The early stage aggregation kinetics of fullerene C60 nanoparticles were investigated in the presence of Suwannee River humic acid and common monovalent and divalent electrolytes through time-resolved dynamic light scattering (DLS). In the absence of humic acid, the aggregation behavior of the fullerene nanoparticles in the presence of NaCl, MgCl2, and CaCl2 was found to be consistent with the classic Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloidal stability. In the presence of humic acid and NaCl or MgCl2 electrolytes, the adsorbed humic acid on the fullerene nanoparticles led to steric repulsion, which effectively stabilized the nanoparticle suspension. This behavior manifested in a dramatic drop in the rate of aggregation, an increase in the critical coagulation concentration (CCC), and an attained value of less than unity for the inverse stability ratio (or attachment efficiency) at high MgCl2 concentrations. While the increase in the nanoparticle stability was similarly observed in the presence of humic acid at low CaCl2 concentrations, enhanced aggregation occurred at higher CaCl2 concentrations. Measurement of scattered light intensities over time indicated significant aggregation of the humic acid macromolecules in solutions of high CaCl2 concentrations. Transmission electron microscopy (TEM) imaging of the fullerene aggregate structures in the presence of humic acid revealed that bridging of the fullerene nanoparticles and aggregates by the humic acid aggregates is the likely mechanism for the enhanced aggregation at high CaCl2 concentrations.

Early-stage aggregation kinetics studies of alginate-coated hematite nanoparticles in solutions containing alkaline-earth metal cations revealed enhanced aggregation rates in the presence of Ca2+, Sr2+, and Ba2+, but not with Mg2+. Transmission electron microscopy (TEM) imaging of the aggregates provided evidence that alginate gel formation was essential for enhanced aggregation to occur. Dynamic light scattering (DLS) aggregation results clearly indicated that a much lower concentration of Ba2+ compared to Ca2+ and Sr2+ was required to achieve a similar degree of enhanced aggregation in each system. To elucidate the relationship between the alginate's affinities for divalent cations and the enhanced aggregation of the alginate-coated hematite nanoparticles, atomic force microscopy (AFM) was employed to probe the interaction forces between alginate-coated hematite surfaces under the solution chemistries used for the aggregation study. Maximum adhesion forces, maximum pull-off distances, and the work of adhesion were used as indicators to gauge the alginate's affinity for the divalent cations and the resulting attractive interactions between alginate-coated hematite nanoparticles. The results showed that alginate had higher affinity for Ba2+ than either Sr2+ or Ca2+. This same trend was consistent with the cation concentrations required for comparable enhanced aggregation kinetics, suggesting that the rate of alginate gel formation controls the enhanced aggregation kinetics. An aggregation mechanism incorporating the gelation of alginate is proposed to explain the accelerated aggregate growth in the presence of Ca2+, Sr2+, and Ba2+.

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 stability and aggregation behavior of iron oxide colloids in natural waters play an important role in controlling the fate, transport, and bioavailability of trace metals. Time-resolved dynamic light scattering experiments were carried out in a study of the aggregation kinetics and aggregate structure of natural organic matter (NOM) coated hematite colloids and bare hematite colloids. The aggregation behavior was examined over a range of solution chemistries, by adjusting the concentration of the supporting electrolyte-NaCl, CaCl2, or simulated seawater. With the solution pH adjusted so that NOM-coated and bare hematite colloids were at the same zeta potential, we observed a significant difference in colloid stability which results from the stability imparted to the colloids by the adsorbed NOM macromolecules. This enhanced stability of NOM-coated hematite colloids was not observed with CaCl2. Aggregate form expressed as fractal dimension was determined for both NOM-coated and bare hematite aggregates in both NaCl and CaCl2. The fractal dimensions of aggregates formed in the diffusion-limited regime indicate slightly more loosely packed aggregates for bare hematite than theory predicts. For NOM-coated hematite, a small decrease in fractal dimension was observed when the solution composition changed from NaCl to CaCl2. For systems in the reaction-limited regime, the measured fractal dimensions agreed with those in the literature. Colloid aggregation was also studied in synthetic seawater, a mixed cation system to simulate estuarine mixing. Those results describe the important phenomena of iron oxide aggregation and sedimentation in estuaries. When compared to field data from the Mullica Estuary, U.S.A., it is shown that collision efficiency is a good predictor of the iron removal in this natural system.

A predictive model was developed in this study for simulating the development of membrane fouling with time in full-scale RO processes. The increase in resistance was used as an indicator of membrane fouling and it enabled one to better describe membrane fouling taking place in a long membrane channel. In this model, fouling potential of a feed water was defined as the increment in membrane resistance due to a unit volume of permeate passing through the membrane, which was directly measurable with a simple filtration experiment. By employing the concept of fouling potential, fouling property of a feed water could be directly related to fouling rate on a RO membrane. The local variations in flow properties and parameters were explicitly taken into consideration in the model in order to provide a more realistic description of a full-scale RO process. The results of the simulation studies demonstrated that a full-scale RO system could maintain a constant average permeate flux for a period of time even though fouling development had occurred right from the start of operation. The effects of water fouling potential, channel length, and membrane resistance on fouling development in full-scale RO processes were investigated.

The antimicrobial and bacterial anti-adhesive properties of polysulfone (PSU) membranes modified with silver nanoparticles (AgNPs) and polyelectrolyte multilayers (PEMs) composed of poly(allylamine hydrochloride) and poly(acrylic acid) were investigated. The membranes' antimicrobial properties were evaluated using a colony forming unit (CFU) enumeration method, while the anti-adhesive properties of the membranes were examined using a direct microscopy observation membrane filtration system. The AgNP mass loading required for the inhibition of bacterial growth on the AgNP/PEM-modified membranes was significantly lower than the AgNP loadings reported in other studies for membranes with the nanoparticles dispersed within the membrane matrix. The immobilization of AgNPs on the membrane surface maximized the opportunities for bacteria-nanoparticle contact, which allowed for effective bacteria inactivation. Furthermore, in comparison to unmodified PSU membranes, the bacterial deposition kinetics on all the modified membranes were reduced by ca. 50% and the bacterial removal efficiencies were significantly increased from close to 0% to as high as over 90%. Three-cycle filtration and rinsing experiments were also performed to evaluate the effectiveness of the surface modification over an extended time period of use.

Understanding the kinetics of the release of carbon nanotubes (CNTs) from naturally occurring surfaces is crucial for the prediction of the environmental fate and transport of CNTs. In this study, the release kinetics of multiwalled CNTs (MWNTs) from silica surfaces was investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D). MWNTs were first deposited on silica surfaces under favorable deposition conditions (1.50 mM CaCl2 and pH 7.1) and the deposited MWNTs were then rinsed at different electrolyte solutions to induce the release of MWNTs from the primary energy minimum. The kinetics of MWNT release was shown to be first order with respect to the deposited MWNTs when complete release took place. A model that accounts for the releasable and unreleasable components of MWNTs was used to fit the experimental data in order to derive the release rate coefficients. When the CaCl2 concentration in the eluent was decreased, a larger fraction of deposited MWNTs was released and the release rate coefficient of the releasable MWNTs also increased. The rise in the surface charges of both MWNTs and silica surfaces with the drop in CaCl2 concentration likely resulted in the decrease in the height of the energy barrier, thus facilitating the release of the nanotubes. Moreover, when the initial surface concentrations of deposited MWNTs were over 1000 ng/cm(2), the release rate coefficient was lower than expected. The reduced release kinetics was likely due to the formation of large surface-bound MWNT clusters which had considerably lower diffusion coefficients than dispersed MWNTs or MWNT aggregates.

The influence of solution chemistry on the kinetics and reversibility of the deposition of multiwalled carbon nanotubes (MWNTs) on model biological membranes was investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D). Supported lipid bilayers (SLBs) comprised of zwitterionic 1,2-dioleoyl-sn-glyero-3-phosphocholine (DOPC), as well as DOPC vesicles, were used as model cell membranes. Under neutral pH conditions, the deposition kinetics of MWNTs on SLBs increased with increasing electrolyte (NaCl and CaCl2) concentrations. In the presence of NaCl, favorable deposition was not achieved even at a concentration of 1 M, which is attributed to the presence of strong repulsive hydration forces due to the highly hydrophilic headgroups of SLBs. Conversely, favorable deposition was observed at CaCl2 concentrations above 0.5 mM when the charge of SLBs was reversed from negative to positive through the binding of Ca(2+) cations to the exposed phosphate headgroups. Favorable nanotube deposition was also observed at pH 2, at which the DOPC SLBs exhibited positive surface charge, since the isoelectric point of DOPC is ca. 4. When MWNTs on SLBs were rinsed with low ionic strength solutions at pH 7.3, only ca. 20% of deposited nanotubes were released, indicating that nanotube deposition was mostly irreversible. The deposition of MWNTs on DOPC vesicles under favorable deposition conditions did not result in any detectable leakage of solution from the vesicles, indicating that MWNTs did not severely disrupt the DOPC bilayers upon attachment.

The heteroaggregation rates of negatively charged multiwalled carbon nanotubes (CNTs) and positively charged hematite nanoparticles (HemNPs) were obtained over a broad range of nanoparticle distributions using time-resolved dynamic light scattering (DLS). Binary systems comprising CNTs and HemNPs were prepared using low ionic strength solutions to minimize the concurrent occurrence of homoaggregation. To elucidate the mechanisms of heteroaggregation, the structures of CNT-HemNP aggregates were observed using cryogenic transmission electron microscopy (cryo-TEM). An initial increase in the CNT concentration, while keeping the HemNP concentration constant, resulted in a corresponding increase in the rate of heteroaggregation, which occurred through the bridging of HemNPs by CNT strands. At the optimal CNT/HemNP mass concentration ratio (CNT/HemNP ratio) of 0.0316, the heteroaggregation rate reached 3.3 times of the HemNP homoaggregation rate in the diffusion-limited regime. Increasing the CNT/HemNP ratio above the optimal value, however, led to a dramatic decrease in the growth rate of heteroaggregates, likely through a blocking mechanism. In the presence of humic acid, the trends in the variation of the heteroaggregation rate with CNT/HemNP ratio were similar to that in the absence of humic acid. However, as the humic acid concentration was increased, the maximum aggregate growth rate decreased due to the lessening in the available surface of the HemNPs that CNTs can attach to through favorable electrostatic interaction.

Carbon nanotubes (CNTs) are currently incorporated into various consumer products, and numerous new applications and products containing CNTs are expected in the future. The potential for negative effects caused by CNT release into the environment is a prominent concern and numerous research projects have investigated possible environmental release pathways, fate, and toxicity. However, this expanding body of literature has not yet been systematically reviewed. Our objective is to critically review this literature to identify emerging trends as well as persistent knowledge gaps on these topics. Specifically, we examine the release of CNTs from polymeric products, removal in wastewater treatment systems, transport through surface and subsurface media, aggregation behaviors, interactions with soil and sediment particles, potential transformations and degradation, and their potential ecotoxicity in soil, sediment, and aquatic ecosystems. One major limitation in the current literature is quantifying CNT masses in relevant media (polymers, tissues, soils, and sediments). Important new directions include developing mechanistic models for CNT release from composites and understanding CNT transport in more complex and environmentally realistic systems such as heteroaggregation with natural colloids and transport of nanoparticles in a range of soils.

The aggregation kinetics of silver nanoparticles (AgNPs) that were coated with two commonly used capping agents-citrate and polyvinylpyrrolidone (PVP)--were investigated. Time-resolved dynamic light scattering (DLS) was employed to measure the aggregation kinetics of the AgNPs over a range of monovalent and divalent electrolyte concentrations. The aggregation behavior of citrate-coated AgNPs in NaCl was in excellent agreement with the predictions based on Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, and the Hamaker constant of citrate-coated AgNPs in aqueous solutions was derived to be 3.7 × 10(-20) J. Divalent electrolytes were more efficient in destabilizing the citrate-coated AgNPs, as indicated by the considerably lower critical coagulation concentrations (2.1 mM CaCl(2) and 2.7 mM MgCl(2) vs 47.6 mM NaCl). The PVP-coated AgNPs were significantly more stable than citrate-coated AgNPs in both NaCl and CaCl(2), which is likely due to steric repulsion imparted by the large, noncharged polymers. The addition of humic acid resulted in the adsorption of the macromolecules on both citrate- and PVP-coated AgNPs. The adsorption of humic acid induced additional electrosteric repulsion that elevated the stability of both nanoparticles in suspensions containing NaCl or low concentrations of CaCl(2). Conversely, enhanced aggregation occurred for both nanoparticles at high CaCl(2) concentrations due to interparticle bridging by humic acid aggregates.

The aggregation and deposition kinetics of two multiwalled carbon nanotubes (MWNTs) with different degrees of surface oxidation are investigated using time-resolved dynamic light scattering (DLS) and quartz crystal microbalance with dissipation monitoring (QCM-D), respectively. Carboxyl groups are determined to be the predominant oxygen-containing surface functional groups for both MWNTs through X-ray photoelectron spectroscopy (XPS). The aggregation and deposition behavior of both MWNTs is in qualitative agreement with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The critical coagulation concentration (CCC) of the highly oxidized MWNTs (HO-MWNTs) is significantly higher than the lowly oxidized MWNTs (LO-MWNTs) in the presence of NaCl (210 and 53 mM, respectively) since HO-MWNTs have a higher surface charge density. In contrast, the aggregation inverse stability profiles of HO-MWNTs and LO-MWNTs are identical and yield comparable CCCs (0.9 and 1.0 mM, respectively) in the presence of CaCl(2). Similar to the results obtained from the aggregation study, HO-MWNTs are considerably more stable to deposition on silica surfaces compared to LO-MWNTs in the presence of NaCl. However, both MWNTs have the same propensity to undergo deposition in the presence of CaCl(2). The remarkable similarity in the aggregation and deposition kinetics of HO-MWNTs and LO-MWNTs in CaCl(2) may be due to Ca(2+) cations having a higher affinity to form complexes with adjacent carboxyl groups on HO-MWNTs than with isolated carboxyl groups on LO-MWNTs.

The fouling and subsequent cleaning of RO membranes fouled by a mixture of organic foulants simulating wastewater effluent has been systematically investigated. The organic foulants investigated included alginate, bovine serum albumin (BSA), Suwannee River natural organic matter, and octanoic acid, representing, respectively, polysaccharides, proteins, humic substances, and fatty acids, which are ubiquitous in effluent organic matter. After establishing the fouling behavior and mechanisms with a mixture of organic foulants in the presence and absence of calcium ions, our study focused on the cleaning mechanisms of RO membranes fouled by the mixture of organic foulants. The chemical cleaning agents used included an alkaline solution (NaOH), a metal chelating agent (EDTA), an anionic surfactant (SDS), and a concentrated salt solution (NaCl). Specifically, we examined the impact of cleaning agent type, cleaning solution pH, cleaning time, and fouling layer composition on membrane cleaning efficiency. Foulant–foulant adhesion forces measured under conditions simulating chemical cleaning of a membrane fouled by a mixture of the investigated organic foulants provided insights into the chemical cleaning mechanisms. It was shown that while alkaline solution (NaOH) alone is not effective in disrupting the complexes formed by the organic foulants with calcium, a higher solution pH can lead to effective cleaning if sufficient hydrodynamic shear (provided by crossflow) prevails. Surfactant (SDS), a strong chelating agent (EDTA), and salt solution (NaCl) were effective in cleaning RO membranes fouled by a mixture of foulants, especially if applied at high pH and for longer cleaning times. The observed cleaning efficiencies with the various cleaning agents were consistent with the related measurements of foulant–foulant intermolecular forces. Furthermore, we have shown that an optimal cleaning agent concentration can be derived from a plot presenting the percent reduction in the foulant–foulant adhesion force versus cleaning agent concentration.

A quartz crystal microbalance with dissipation monitoring is used to study the adsorption of linear plasmid DNA on silica surfaces and silica surfaces coated with poly-L-lysine (PLL) in solutions containing either alkaline earth (calcium and magnesium) or transition (cobalt, copper, and zinc) metals. Our results show that electrostatic attraction alone does not fully explain the significantly higher adsorption rate of DNA on the positively charged PLL layer in Cu(2+) solution compared to solutions containing Ca(2+), Mg(2+), Co(2+), or Zn(2+). Diffusion coefficients measured by dynamic light scattering reveal that the compactness of plasmid DNA molecules is greater in solutions containing Cu(2+) compared to that of DNA in other divalent electrolyte solutions. When the adsorption rate of plasmid DNA on silica is normalized to the corresponding adsorption rate on PLL-coated surfaces at the same solution condition, the attachment (adsorption) efficiencies are about 0.01 for Ca(2+) or Mg(2), but close to unity for Co(2+), Cu(2+), or Zn(2+). Results from viscoelastic modeling of adsorbed DNA layers suggest that the DNA layer formed in Cu(2+) solutions is thicker and more viscous compared to that formed in Co(2+) solutions. This study demonstrates that plasmid DNA has a strong affinity to Cu(2+), which results in a more compact conformation of DNA molecules compared to the case with the other divalent cations investigated.

The stability and aggregation kinetics of two different suspensions of fullerene (C60) nanoparticles and their relation to nanoparticle charge (electrokinetic) properties were investigated. The two synthesis methods employed--a solvent exchange method involving sonication of fullerene initially dissolved in toluene and prolonged stirring of bulk fullerene in water--produce negatively charged fullerene nanoparticles. With an increase in electrolyte (KCl) concentration, the electrophoretic mobilities of both fullerene nanoparticles became less negative, while the corresponding aggregation rates increased until maximum rates were reached at their respective critical coagulation concentrations. This behavior is consistent with the classic Derjaguin-Landau-Verwey-Overbeek (DLVO) theory for the stability of charged colloidal particles. The nanoparticles prepared by prolonged stirring of bulk fullerene in water were much more stable than those prepared by sonication in toluene, as evident from their significantly higher critical coagulation concentration (166 and 40 mM KCl, respectively). A comparison of the aggregation kinetics with predictions based on DLVO theory yielded the same Hamaker constant (8.5 x 10(-21) J) for both fullerene nanoparticles, indicating that they have the same material composition. Further investigation shows that both fullerene nanoparticles are more negatively charged and stable at higher pH conditions, suggesting that dissociation of surface functional groups contributes to surface charge for both nanoparticles. This hypothesis is further supported by oxidation which occurs on the surface of bulk fullerene that has been exposed to water over a prolonged period of time, as detected through X-ray photoelectron spectroscopy (XPS). However, since both nanoparticles remain negatively charged at pH 2, it is likely that there are other contributing factors to the surface charge of fullerene nanoparticles.

The adsorption kinetics of supercoiled and linear plasmid DNA onto a natural organic matter (NOM)-coated silica surface are acquired using a quartz crystal microbalance with dissipation monitoring (QCM-D) in the presence of common divalent electrolytes CaCl2 and MgCl2. The adsorption kinetics of both DNA are noticeably higher in the presence of CaCl2 compared to MgCl2. We hypothesize that specific bridging between the DNA phosphate groups and NOM carboxyl functional groups in the presence of Ca2+ cations may lead to more efficient attachmentthan in the presence of Mg2+ cations, which are only likely to allow for charge neutralization. The influence of background Na+ cations on the adsorption kinetics in the presence of CaCl2 is found to be insignificant, while the presence of Na+ leads to slower attachment kinetics in MgCl2. Rinsing the DNA layer adsorbed in the presence of CaCl2 with a solution of low NaCl concentration followed by deionized water does not result in observable detachment, indicating irreversibility of DNA adsorption. Instead, softening of the DNA layer adsorbed in the presence of CaCl2 with background Na+ occurs with the rinses due to the increase in electrostatic repulsion between the phosphate functional groups along the DNA backbone. In the case of the DNA layer adsorbed in the presence of CaCl2 without background Na+, softening of the layer does not occur with the rinses.

The aggregation and deposition kinetics of fullerene C60 nanoparticles have been investigated over a wide range of monovalent and divalent electrolyte concentrations by employing time-resolved dynamic light scattering (DLS) and quartz crystal microbalance (QCM), respectively. Aggregation kinetics of the fullerene nanoparticles exhibited reaction-limited (slow) and diffusion-limited (fast) regimes in the presence of both electrolytes, having critical coagulation concentrations (CCC) of 120 and 4.8 mM for the monovalent (NaCl) and divalent (CaCl2) salts, respectively. The measured stability ratios of the aggregating fullerene nanoparticles were in very good agreement with Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, with a derived Hamaker constant of 6.7 x 10-21 J for the fullerene nanoparticles in aqueous medium. For the deposition kinetics studies, the rate of fullerene nanoparticle deposition increased with increasing electrolyte concentrations, as was indicated in the aggregation kinetics results. However, at electrolyte concentrations approaching or exceeding the CCC, the rate of deposition dropped sharply due to significant concurrent aggregation of the fullerene nanoparticles. The deposition of the fullerene nanoparticles was further shown to be mostly irreversible, with immediate detachment of the nanoparticles observed only when exposed to a solution of high pH.

The early stage aggregation kinetics of bare and alginate-coated hematite nanoparticles are acquired through time-resolved dynamic light scattering (DLS). Varying concentrations of monovalent (NaCl) and divalent (MgCl2 and CaCl2) electrolytes are employed to induce aggregation. In the presence of NaCl and MgCl2, the alginate-coated hematite nanoparticles undergo aggregation through electrostatic destabilization as described by the classic Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. This is ascertained through examination of the favorable and unfavorable regimes of the stability curves depicting the attachment efficiency as a function of salt concentration. Additional evidence may be found in the aggregation kinetics of alginate-coated particles, which, under favorable aggregation conditions, are reasonably close to that of bare hematite nanoparticles. However, in the presence of CaCl2, the aggregate growth rate of alginate-coated hematite nanoparticles is much higher than that which conventional diffusive aggregation predicts. Dispersed hematite primary particles and lower-order aggregates enmeshed within extended alginate gel networks were observed under transmission electron microscope (TEM). The proposed mechanism for enhanced aggregation suggests an apparent increase in the collision radii of alginate-coated hematite nanoparticles through alginate gel network formation from the particle surface. Additionally, cross-linking between unadsorbed (suspended) alginate macromolecules may form bridges between hematite-alginate gel clusters. It is further established that the presence of background electrolyte NaCl in solution is detrimental to the calcium-induced enhanced aggregation.

Normalization of permeate flux data is widely used to characterize membrane fouling under different experimental conditions. The main intention of normalization is to allow a fair comparison of feed water fouling potentials by eliminating the effects of different operational parameters used in the experiments, such as net driving pressure and clean-membrane resistance. However, it was demonstrated that the commonly used intuitive normalization methods usually could not serve their intended purpose. In this study, a new normalization method was proposed for characterizing water-fouling potential based on fundamental principles of membrane fouling. The intention of this normalization method was to define a fouling potential for feed water that was independent of, or at least, not strongly affected by operational conditions. Laboratory-scale ultrafiltration fouling tests were conducted under different colloid sizes, concentrations, and driving pressures. The experiments showed that the fouling potentials defined by the newly proposed normalization method were linearly related to the colloid concentration of the feed water and that the effect of operational conditions used in the fouling experiments on the fouling potential was minimal.

Environmental context. The fate and bioavailability of engineered nanoparticles in natural aquatic systems are strongly influenced by their ability to remain dispersed in water. Consequently, understanding the colloidal properties of engineered nanoparticles through rigorous characterisation of physicochemical properties and measurements of particle stability will allow for a more accurate prediction of their environmental, health, and safety effects in aquatic systems. This review highlights some important techniques suitable for the assessment of the colloidal properties of engineered nanoparticles and discusses some recent findings obtained by using these techniques on two popular carbon-based nanoparticles, fullerene C60 and multi-walled carbon nanotubes. Abstract. The colloidal properties of engineered nanoparticles directly affect their use in a wide variety of applications and also control their environmental fate and mobility. The colloidal stability of engineered nanoparticles depends on their physicochemical properties within the given aqueous medium and is ultimately reflected in the particles’ aggregation and deposition behaviour. This review presents some of the key experimental methods that are currently used to probe colloidal properties and quantify engineered nanoparticle stability in water. Case studies from fullerene C60 nanoparticles and multi-walled carbon nanotubes illustrate how the characterisation and measurement methods are used to understand and predict nanoparticle fate in aquatic systems. Consideration of the comparisons between these two classes of carbon-based nanoparticles provides useful insights into some major current knowledge gaps while also revealing clues about needed future developments. Key issues to be resolved relate to the nature of near-range surface forces and the origins of surface charge, particularly for the reportedly unmodified or ‘pure’ carbon-based nanoparticles.