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.

Heteroaggregation of Graphene Oxide with Nanometer- and Micrometer-Sized Hematite Colloids: Influence on Nanohybrid Aggregation and Microparticle Sedimentation.

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.

Influence of Solution Chemistry and Soft Protein Coronas on the Interactions of Silver Nanoparticles with Model Biological Membranes.

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.

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.

Imparting antimicrobial and anti-adhesive properties to polysulfone membranes through modification with silver nanoparticles and polyelectrolyte multilayers.

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.

Disaggregation of heteroaggregates composed of multiwalled carbon nanotubes and hematite nanoparticles.

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.

Release kinetics of multiwalled carbon nanotubes deposited on silica surfaces: quartz crystal microbalance with dissipation (QCM-D) measurements and modeling.

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.

Nanoparticles meet cell membranes: probing nonspecific interactions using model membranes.

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.

Influence of solution chemistry on the release of multiwalled carbon nanotubes from silica surfaces.

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.

Interaction of multiwalled carbon nanotubes with supported lipid bilayers and vesicles as model biological membranes.

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.

Heteroaggregation of multiwalled carbon nanotubes and hematite nanoparticles: rates and mechanisms.

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.

Potential release pathways, environmental fate, and ecological risks of carbon nanotubes.

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.

Aggregation kinetics of citrate and polyvinylpyrrolidone coated silver nanoparticles in monovalent and divalent electrolyte solutions.

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.

Influence of surface oxidation on the aggregation and deposition kinetics of multiwalled carbon nanotubes in monovalent and divalent electrolytes.

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.

Adsorption kinetics and reversibility of linear plasmid DNA on silica surfaces: influence of alkaline earth and transition metal ions.

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.

Relating colloidal stability of fullerene (C60) nanoparticles to nanoparticle charge and electrokinetic properties.

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.

Interaction of fullerene (C60) nanoparticles with humic acid and alginate coated silica surfaces: measurements, mechanisms, and environmental implications.

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.

Reduced aggregation and sedimentation of zero-valent iron nanoparticles in the presence of guar gum.

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.

Role of divalent cations in plasmid DNA adsorption to natural organic matter-coated silica surface.

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.