Adsorption mechanism and dispersion efficiency of three anionic additives [poly(acrylic acid), poly(styrene sulfonate) and HEDP] on zinc oxide

Relative Strength of Polycation Adsorption on Oxide Surfaces

Polyelectrolyte adsorption onto surfaces is widely employed in water treatment and mining. However, little is known of the relative interaction strengths between surfaces and polymer. This fundamental property is assumed to be dominated by electrostatics, i.e., attractive interactions between opposite charges, which are set by the overall ionic strength ("salt concentration") of the solution, and charge densities of the surface and the polymer. A common, counterintuitive finding is a range of salt concentrations over which the amount of adsorbed polyelectrolyte increases as electrostatic interactions are tempered by the addition of salt. After an adsorption maximum, higher salt concentrations then produce the expected gradual desorption of polyelectrolyte. In this work, the salt response of the adsorption of the same narrow molecular weight distribution polycation, poly(N-methyl-4-vinylpyridinium), PM4VP, to a variety of surfaces was explored. Oxide powders for adsorption included Al2O3, SiO2, Fe2O3, Fe3O4, TiO2, ZnO, and CuO. Planar surfaces included silicon wafers, mica, calcium carbonate, and CaF2 single crystals. The PM4VP was radiolabeled with 14C so that sensitive, submonolayer amounts could be detected. The position of the peak maximum, or the lack of a peak, in response to added salt was used to rank the electrostatic component of the interaction. The importance of charge regulation, a shift in the surface pKa in response to solution species, was highlighted as a mechanism for adsorption on the "wrong" side of the isoelectric point and also as a factor contributing to the difficulty of reaching the totally desorbed state even at the highest salt concentrations.

Electrodeposited Zinc Coatings for Biomedical Application: Morphology, Corrosion and Biological Behaviour

The improvement of biodegradable metals is currently an active and promising research area for their capabilities in implant manufacturing. However, controlling their degradation rate once their surface is in contact with the physiological media is a challenge. Surface treatments are in the way of addressing the improvement of this control. Zinc is a biocompatible metal present in the human body as well as a metal widely used in coatings to prevent corrosion, due to its well-known metal protective action. These two outstanding characteristics make zinc coating worthy of consideration to improve the degradation behaviour of implants. Electrodeposition is one of the most practical and common technologies to create protective zinc coatings on metals. This article aims to review the effect of the different parameters involved in the electrochemical process on the topography and corrosion characteristics of the zinc coating. However, certainly, it also provides an actual and comprehensive description of the state-of-the-art of the use of electrodeposited zinc for biomedical applications, focusing on their capacity to protect against bacterial colonization and to allow cell adhesion and proliferation.

Behavior of PBTC, HEDP, and Aminophosphonates in the Process of Wastewater Treatment

Ten times at intervals of 1–2 months, individual treatment stages of two wastewater treatment plants (WWTPs) were analyzed for the five quantitatively most widely used phosphonates. The total dissolved concentration of the investigated phosphonates in the influents was between 131 µg/L and 384 µg/L. The nitrogen-free phosphonates 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC) and 1-hydroxyethylidene(1,1-diphosphonic acid) (HEDP) accounted for an average proportion of 83–85%. Diethylenetriaminepenta(methylene phosphonic acid) (DTPMP) contributed with 13–14%, whereas aminotris(methylphosphonic acid) (ATMP) (≤15 µg/L) and ethylenediaminetetra(methylene phosphonic acid) (EDTMP) (≤11 µg/L) contents detected in the WWTP influents were comparatively low. The application of new analytical methods allowed the quantification of phosphonates in the solid fraction of the WWTP influents for the first time. High loads of phosphonates were determined (223–2555 mg/kg), indicating that 20%–80% of the phosphonates are present in the adsorbed state. The removal of total dissolved phosphonate by secondary clarification was between 69.7% and 92.4% (medians: 90.7% and 87.7%). In both WWTPs, HEDP (medians: 89.2% and 86.4%) was slightly better eliminated than PBTC (medians: 87.2% and 82.5%). In the sand filtration stage of a WWTP, the average removal was not further improved. In contrast, an additional removal of dissolved phosphonates could be achieved by activated carbon treatment (median: 96.4%). The proportion of phosphonate-P in the dissolved unreactive phosphorus fraction was consistently between 10% and 40% throughout all treatment stages.

Influence of water chemistry on the environmental behaviors of commercial ZnO nanoparticles in various water and wastewater samples

Zinc oxide nanoparticles (ZnO NPs) are widely used nanomaterials and their environmental impacts have received increasing attention. The fate and toxicity of ZnO NPs in the environment are determined by their stability and dissolution. In this study, the influence of water chemistry on aggregation, sedimentation, and dissolution of ZnO NPs was investigated. The stabilized ZnO NPs aggregated and precipitated when the aqueous pH closed to their zero point of charge (pH_zpc). Counter-ions neutralized the surface charge of NPs and promoted their destabilization. However, a high concentration of counter-ion (SO₄^2-, >10meq/L) made the NPs more stable because of the inverted surface potential. The stability of ZnO NPs was maintained by high concentration of Suwannee River humic acid (SRHA, 10mg/L) even the concentration of electrolytes was high. The influence of water chemistry on the stability and dissolution of ZnO NPs was further demonstrated in different wastewaters. In one wastewater sample, ZnO NPs was unexpectedly stable and with a high dissolution, which was due to the effects of pH value, organic matter concentration, as well as the concentration of counter ions. Our findings facilitate the predictions of the fate of stabilized ZnO NPs in the environment.

Physicochemical transformation and algal toxicity of engineered nanoparticles in surface water samples

Most studies on the behavior and toxicity of engineered nanoparticles (NPs) have been conducted in artificial water with well-controlled conditions, which are dramatically different from natural waters with complex compositions. To better understand the fate and toxicity of NPs in the natural water environment, physicochemical transformations of four NPs (TiO2, ZnO, Ag, and carbon nanotubes (CNTs)) and their toxicities towards a unicellular green alga (Chlorella pyrenoidosa) in four fresh water and one seawater sample were investigated. Results indicated that water chemistry had profound effects on aggregation, dissolution, and algal toxicity of the NPs. The strongest homoaggregation of the NPs was associated with the highest ionic strength, but no obvious correlation was observed between the homoaggregation of NPs and pH or dissolved organic matter content of the water samples. The greatest dissolution of ZnO NPs also occurred in seawater with the highest ionic strength, while the dissolution of Ag NPs varied differently from ZnO NPs. The released Zn(2+) and especially Ag(+) mainly accounted for the algal toxicity of ZnO and Ag NPs, respectively. The NP-cell heteroagglomeration occurred generally for CNTs and Ag NPs, which contributed to the observed nanotoxicity. However, there was no significant correlation between the observed nanotoxicity and the type of NP or the water chemistry. It was thus concluded that the physicochemical transformations and algal toxicities of NPs in the natural water samples were caused by the combined effects of complex water quality parameters rather than any single influencing factor alone. These results will increase our knowledge on the fate and effects of NPs in the aquatic environment.

The effect of electrolytes on the aggregation kinetics of three different ZnO nanoparticles in water

Nanoscale ZnO particles are receiving increasing attention because they are widely used in commercial products, but they do have potentially hazardous effects. The aggregation behavior of ZnO nanoparticles (NPs) in the environment contributes to the real risk assessment of nano-toxicity, and the real size of the nano-aggregates should be investigated. In this study, the influences of electrolytes on the stabilities of three ZnO NPs were compared: the commercial powder (NP1), the lab synthesized suspension (NP2) and the commercial suspension (NP3). The initial particle size of NP2 and NP3 in water was at a nanoscale whilst NP1 tended to form microscale aggregates. The capping reagents helped to retain their suspension. The stability of ZnO NPs depends on their zeta potential under specific pH value, ionic types and ionic strength. In general, neutralization plays a major role in aggregation. The effect of divalent counter-ions on ZnO NP aggregation was more than that of monovalent ones. The stabilities of NP2 and NP3 were confirmed by the large critical coagulation concentration (CCC) values of these particles. The experimental results also fit the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The aggregation of different ZnO NPs is relevant to their basic properties and is influenced by electrolytes, which decreases the possibility of the penetration of NPs into cells to cause toxicity in the environment. An understanding of the basic properties of NPs is crucial for assessing their fate in the environment as well as for setting up usage regulation and treatment strategy.

Temperature-dependent breakdown of hydrogen peroxide-treated ZnO and TiO2 nanoparticle agglomerates

Metal oxide nanoparticles (MONPs) are used in a variety of applications including drug formulations, paint, sensors and biomedical devices due to their unique physicochemical properties. One of the major problems with their widespread implementation is their uncontrolled agglomeration. One approach to reduce agglomeration is to alter their surface chemistry with a proper functionality in an environmentally friendly way. In this study, the influence of hydrogen peroxide (H2O2) treatment on the dispersion of ZnO and TiO2 nanoparticle (NP) agglomerates as a function of temperature is studied. The H2O2 treatment of the MONPs increases the density of hydroxyl (-OH) groups on the NP surface, as verified with FTIR spectroscopy. The influence of heating on the dispersion of H2O2-treated ZnO and TiO2 NPs is investigated using dynamic light scattering. The untreated and H2O2-treated ZnO and TiO2 NP suspensions were heated from 30 °C to 90 °C at 5 °C intervals to monitor the breakdown of large aggregates into smaller aggregates and individual nanoparticles. It was shown that the combined effect of hydroxylation and heating enhances the dispersion of ZnO and TiO2 NPs in water.

Synthesis of tripodal catecholates and their immobilization on zinc oxide nanoparticles

A common approach to generate tailored materials and nanoparticles (NPs) is the formation of molecular monolayers by chemisorption of bifunctional anchor molecules. This approach depends critically on the choice of a suitable anchor group. Recently, bifunctional catecholates, inspired by mussel-adhesive proteins (MAPs) and bacterial siderophores, have received considerable interest as anchor groups for biomedically relevant metal surfaces and nanoparticles. We report here the synthesis of new tripodal catecholates as multivalent anchor molecules for immobilization on metal surfaces and nanoparticles. The tripodal catecholates have been conjugated to various effector molecules such as PEG, a sulfobetaine and an adamantyl group. The potential of these conjugates has been demonstrated with the immobilization of tripodal catecholates on ZnO NPs. The results confirmed a high loading of tripodal PEG-catecholates on the particles and the formation of stable PEG layers in aqueous solution.

Interaction of L-Cysteine with ZnO: Structure, Surface Chemistry, and Optical Properties

Zinc oxide (ZnO) nanoparticles (NPs) were stabilized in water using the amino acid l-cysteine. A transparent dispersion was obtained with an agglomerate size on the level of the primary particles. The dispersion was characterized by dynamic light scattering (DLS), pH dependent zeta potential measurements, scanning transmission electron microscopy (STEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, photoluminescence (PL) spectroscopy, and X-ray absorption fine structure (EXAFS, XANES) spectroscopy. Cysteine acts as a source for sulfur to form a ZnS shell around the ZnO core and as a stabilizer for these core-shell NPs. A large effect on the photoluminescent properties is observed: the intensity of the defect luminescence (DL) emission decreases by more than 2 orders of magnitude, the intensity of the near band edge (NBE) emission increases by 20%, and the NBE wavelength decreases with increasing cysteine concentration corresponding to a blue shift of about 35 nm due to the Burstein-Moss effect.

Influence of phosphate and solution pH on the mobility of ZnO nanoparticles in saturated sand

The mobility of nanoparticles (NPs) strongly depends on the chemical characterization of the environmental medium. However, the influence of phosphate on NPs mobility was ignored by scientists despite the serious phosphate contamination in natural environments. Hence, the influence of phosphate and solution pH on the mobility of zinc oxide nanoparticles (ZnO-NPs) was investigated in water-saturated sand representative of groundwater aquifers, which encompassed a range of P/Zn molar ratios (P/Zn: 0-4) and pH (4.8-10.0). The transport of ZnO-NPs was dramatically enhanced in the presence of phosphate, even at a low P/Zn molar ratio namely 0.25, and the retention of ZnO-NPs in the saturated sand decreased with increasing P/Zn molar ratio. Moreover, attachment efficiencies (α) and deposition rates (kd) of ZnO-NPs rapidly decreased with increasing P/Zn molar ratio. In contrast, the solution pH had negligible effects on ZnO-NP transport behavior under phosphate-abundant condition (P/Zn: 4). The distinct effects may be explained by the energy interaction between ZnO-NPs and sand surface under different conditions. Interestingly, under phosphate-abundant condition (P/Zn: 4), solution pH could strongly affect the transport of Zn(2+) in the water-saturated sand. Overall, this study outlines the importance of taking account of phosphate into risk assessment of NPs in the environment.

Dispersion and surface functionalization of oxide nanoparticles for transparent photocatalytic and UV-protecting coatings and sunscreens

This review describes recent efforts on the synthesis, dispersion and surface functionalization of the three dominating oxide nanoparticles used for photocatalytic, UV-blocking and sunscreen applications: titania, zinc oxide, and ceria. The gas phase and liquid phase synthesis is described briefly and examples are given of how weakly aggregated photocatalytic or UV-absorbing oxide nanoparticles with different composition, morphology and size can be generated. The principles of deagglomeration are reviewed and the specific challenges for nanoparticles highlighted. The stabilization of oxide nanoparticles in both aqueous and non-aqueous media requires a good understanding of the magnitude of the interparticle forces and the surface chemistry of the materials. Quantitative estimates of the Hamaker constants in various media and measurements of the isoelectric points for the different oxide nanoparticles are presented together with an overview of different additives used to prepare stable dispersions. The structural and chemical requirements and the various routes to produce transparent photocatalytic and nanoparticle-based UV-protecting coatings, and UV-blocking sunscreens are described and discussed.

Dissolution and microstructural transformation of ZnO nanoparticles under the influence of phosphate

The toxicity and fate of nanoparticles (NPs) have been reported to be highly dependent on the chemistry of the medium, and the effects of phosphate have tended to be ignored despite the wide existence of phosphate contamination in aqueous environments. In the present study the influence of phosphate on the dissolution and microstructural transformation of ZnO NPs was investigated. Phosphate at a low concentration rapidly and substantially reduced the release of Zn(2+) into aqueous solution. Synchrotron X-ray absorption spectroscopy and X-ray diffraction analysis reveal that interaction between ZnO NPs and phosphate induced the transformation of ZnO into zinc phosphate. Transmission electronic microscopy observation shows that the morphology of the particles changed from structurally uniform nanosized spherical to anomalous and porous material containing mixed amorphous and crystalline phases of ZnO and zinc phosphate in the presence of phosphate. To our knowledge, this is the first study in which the detailed process of phosphate-induced speciation and microstructural transformation of ZnO NPs has been analyzed. In view of the wide existence of phosphate contamination in water and its strong metal-complexation capability, phosphate-induced transformations may play an important role in the behaviors, fate, and toxicity of many other metal-based nanomaterials in the environment.

Sodium-phosphorylated chitosan/zinc oxide complexes and evaluation of their cytocompatibility: An approach for periodontal dressing

The aim of this study was to evaluate the possibility of metal complex formation between sodium-phosphorylated chitosan (PCTS) and ZnO. The polymer–metal complex formation was investigated in terms of thermal degradation. The structure deduction of the PCTS/ZnO complex was investigated by means of Fourier transform infrared spectroscopy and X-ray diffraction (XRD). The PCTS/ZnO complexes were formed by the sharing of lone pairs of electrons from the N atoms in the amine groups and O atoms in the phosphate and hydroxyl groups of PCTS to the protonated hydroxyl species on the ZnO surface. Because complex formation occurred at the surface of ZnO particles, it did not change the ZnO crystalline structure. Cytotoxicity, evaluated by a direct contact test with primary human gingival fibroblast cells, revealed that PCTS was biocompatible and reduced the cytotoxicity of ZnO by complexation, making PCTS/ZnO complexes potentially biocompatible. Within the limits of these data, it appears that PCTS could be used as a reaction rate-modifying agent in periodontal dressings.

Aggregation and dissolution of 4 nm ZnO nanoparticles in aqueous environments: influence of pH, ionic strength, size, and adsorption of humic acid

Metal oxide nanoparticles are used in a wide range of commercial products, leading to an increased interest in the behavior of these materials in the aquatic environment. The current study focuses on the stability of some of the smallest ZnO nanomaterials, 4 ± 1 nm in diameter nanoparticles, in aqueous solutions as a function of pH and ionic strength as well as upon the adsorption of humic acid. Measurements of nanoparticle aggregation due to attractive particle-particle interactions show that ionic strength, pH, and adsorption of humic acid affect the aggregation of ZnO nanoparticles in aqueous solutions, which are consistent with the trends expected from Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Measurements of nanoparticle dissolution at both low and high pH show that zinc ions can be released into the aqueous phase and that humic acid under certain, but not all, conditions can increase Zn(2+)(aq) concentrations. Comparison of the dissolution of ZnO nanoparticles of different nanoparticle diameters, including those near 15 and 240 nm, shows that the smallest nanoparticles dissolve more readily. Although qualitatively this enhancement in dissolution can be predicted by classical thermodynamics, quantitatively it does not describe the dissolution behavior very well.