Behavior Of AgNPs Research Articles

Behaviour of silver nanoparticles in simulated saliva and gastrointestinal fluids

Continuously increasing application of silver nanoparticles (AgNPs) requires information on their safety and performance under biological conditions. Assessment of AgNPs in biological systems is also related to availability of robust toxicological methods for evaluation of toxic potential of AgNPs and information on their physicochemical state. Silver nanoparticles were subjected to action of simulated saliva, gastric and intestinal fluids, appropriately supplemented with digestive enzymes pepsin or pancreatin. The behaviour of AgNPs was determined using dynamic light scattering and transmission electron microscopy, and their toxicity as well as capability to induce inflammatory reactions were assessed using reconstructed human tissue models (EpiOral, EpiGingival, EpiIntestinal). The study revealed that during exposure to the fluids, AgNPs size and morphology changed and depended on composition and pH of the respective fluid. If present, the change in terms of growth of AgNPs size occurred immediately after contact of AgNPs with the respective fluid and continued with prolonged time of contact. A pilot study on reconstituted human tissue models revealed low toxicity and inflammatory effects of AgNPs and confirmed the suitability of 3-D models for toxicological studies including bioavailability.

Partitioning of nanoparticle-originated dissolved silver in natural and artificial sediments.

Sediments are believed to be a major sink for silver nanoparticles (AgNPs) in the aquatic environment, but there is a lack of knowledge about the environmental effects and behavior of AgNPs in sediments. The release of highly toxic Ag+ through dissolution of AgNPs is one mechanism leading to toxic effects in sediments. We applied an ultrasound-assisted sequential extraction method to evaluate the dissolution of AgNPs and to study the partitioning of dissolved Ag in sediments. Silver was spiked into artificial and 2 natural sediments (Lake Höytiäinen sediment and Lake Kuorinka sediment) as silver nitrate (AgNO3 ), uncoated AgNPs, or polyvinylpyrrolidone-coated AgNPs (PVP-AgNPs). In addition, the total body burdens of Ag in the sediment-dwelling oligochaete Lumbriculus variegatus were assessed over a 28-d exposure period. The dissolution rate was found to be similar between the uncoated AgNP and PVP-AgNP groups. In all sediments, dissolved Ag was mainly bound to the residual fraction of the sediment, followed by iron and manganese oxides or natural organic matter. In Lake Kuorinka sediment, dissolved Ag that originated from PVP-AgNPs was relatively more bioaccessible, also resulting in higher total body burden in L. variegatus than that from uncoated AgNPs or AgNO3 . In artificial sediment and Lake Höytiäinen sediment, AgNO3 was significantly more bioaccessible than AgNPs. Our results highlight the importance of sediment properties and AgNP surface chemistry when evaluating the environmental exposure of AgNPs. Environ Toxicol Chem 2017;36:2593-2601. © 2017 SETAC.

Interaction of silver nanoparticles with algae and fish cells: a side by side comparison

BackgroundSilver nanoparticles (AgNP) are widely applied and can, upon use, be released into the aquatic environment. This raises concerns about potential impacts of AgNP on aquatic organisms. We here present a side by side comparison of the interaction of AgNP with two contrasting cell types: algal cells, using the algae Euglena gracilis as model, and fish cells, a cell line originating from rainbow trout (Oncorhynchus mykiss) gill (RTgill-W1). The comparison is based on the AgNP behavior in exposure media, toxicity, uptake and interaction with proteins.Results(1) The composition of exposure media affected AgNP behavior and toxicity to algae and fish cells. (2) The toxicity of AgNP to algae was mediated by dissolved silver while nanoparticle specific effects in addition to dissolved silver contributed to the toxicity of AgNP to fish cells. (3) AgNP did not enter into algal cells; they only adsorbed onto the cell surface. In contrast, AgNP were taken up by fish cells via endocytic pathways. (4) AgNP can bind to both extracellular and intracellular proteins and inhibit enzyme activity.ConclusionOur results showed that fish cells take up AgNP in contrast to algal cells, where AgNP sorbed onto the cell surface, which indicates that the cell wall of algae is a barrier to particle uptake. This particle behaviour results in different responses to AgNP exposure in algae and fish cells. Yet, proteins from both cell types can be affected by AgNP exposure: for algae, extracellular proteins secreted from cells for, e.g., nutrient acquisition. For fish cells, intracellular and/or membrane-bound proteins, such as the Na+/K+-ATPase, are susceptible to AgNP binding and functional impairment.

Retention of silver nano-particles and silver ions in calcareous soils: Influence of soil properties

The rapid production and application of silver nanoparticles (AgNPs) have led to significant release of AgNPs into the terrestrial environments. Once released into the soil, AgNPs could enter into different interactions with soil particles which play key roles in controlling the fate and transport of these nanoparticles. In spite of that, experimental studies on the retention of AgNPs in soils are very scarce. Hence, the key objective of this research was to find out the retention behavior of AgNPs and Ag(I) ions in a range of calcareous soils. A second objective was to determine the extent to which the physico-chemical properties of the soils influence the Ag retention parameters. To this end, isothermal batch experiments were used to determine the retention of Poly(vinylpyrrolidinone)-capped AgNPs (PVP-AgNPs) and Ag(I) ions by nine calcareous soils with a diversity of physico-chemical properties. The results revealed that the retention data for both PVP-AgNPs and Ag(I) ions were well described by the classical Freundlich and Langmuir isothermal equations. The retention of PVP-AgNPs and Ag(I) ions was positively correlated to clay and organic carbon (OC) contents as well as electrical conductivity (EC), pH, and cation exchange capacity (CEC) of the soils. Due to multicolinearity among the soil properties, principal component analysis (PCA) was used to group the soil properties which affect the retention of PVP-AgNPs and Ag(I) ions. Accordingly, we identified two groups of soil properties controlling retention of PVP-AgNPs and Ag(I) ions in the calcareous soils. The first group comprised soil solid phase parameters like clay, OC, and CEC, which generally control hetero-aggregation and adsorption reactions and the second group included soil solution variables such as EC and pH as well as Cl− and Ca2+ concentrations, which are supposed to mainly affect homo-aggregation and precipitation reactions.

Particle coatings but not silver ions mediate genotoxicity of ingested silver nanoparticles in a mouse model

Incorporation of silver nanoparticles (AgNPs) in toothpaste, food containers, dietary supplements and other consumer products can result in oral exposure to AgNPs and/or silver ions (Ag+) released from the surface of AgNPs. To examine whether ingestion of AgNPs or Ag+ results in genotoxic damage and whether AgNP coatings modulate the effect, we exposed mice orally to 20nm citrate-coated AgNPs, polyvinylpyrrolidone (PVP)-coated AgNPs, silver acetate or respective vehicles at a 4mg/kg dose (equivalent to 800× the EPA reference dose for Ag) for 7days. Genotoxicity was examined in the systemic circulation and bone marrow at 1, 7, and 14days post-exposure. We found that citrate-coated AgNPs induced chromosomal damage in bone marrow and oxidative DNA damage and double strand breaks in peripheral blood. These damages persisted for at least 14days after exposure termination. Because oxidative DNA damage and strand breaks are repaired rapidly, their presence after exposure cessation indicates that citrate-coated AgNPs persist in the body. In contrast, PVP-coated AgNPs and silver acetate did not induce DNA or chromosomal damage at any time point measured. To determine whether coating-dependent genotoxicity is related to different AgNP changes in the gastrointestinal tract, we examined AgNP behavior and fate in an in vitro gastrointestinal digestion model using UV–visible spectroscopy and DLS. Citrate-coated AgNPs were more susceptible to agglomeration than PVP-coated AgNPs in digestive juices with or without proteins. In summary, AgNPs but not Ag+ are genotoxic following oral ingestion. Nanoparticle coatings modulate gastrointestinal transformation and genotoxicity of AgNPs, where higher agglomeration of AgNPs in gastrointestinal juices is associated with higher genotoxicity in tissues. Since genotoxicity is a strong indicator of cancer risk, further long-term studies focusing on cancer are warranted.

Simultaneous characterisation of silver nanoparticles and determination of dissolved silver in chicken meat subjected to in vitro human gastrointestinal digestion using single particle inductively coupled plasma mass spectrometry.

In this study, a chicken meat containing AgNPs (candidate reference material Nanolyse 14) has been used as a model matrix to study the fate and behaviour of AgNPs upon oral ingestion following an in vitro model that included saliva, gastric and intestinal digestions. The behaviour of a 40nm AgNPs standard solution during the three digestion steps was also evaluated. Sample preparation conditions were optimised to prevent AgNPs oxidation and/or aggregation and to ensure the representativeness of the reported results. Total silver released from the test sample and the evaluated AgNP standard was determined by inductively coupled plasma mass spectrometry (ICPMS). The presence of both AgNPs and dissolved silver in the extracts was confirmed by single particle (SP)-ICPMS analysis. AgNPs were sized and the particle number concentration determined in the three digestion juices. Experimental results demonstrated differentiated behaviours for AgNP from the standard solution and the meat sample highlighting the relevance of using physiological conditions for accurate risk assessment. In the most realistic scenario assayed (i.e., spiked chicken meat analysis), only 13% of the AgNPs present in the reference material would reach the intestine wall. Meanwhile, other bioaccessible dissolved forms of silver would account for as much as 44% of the silver initially spiked to the meat paste.

How test vessel properties affect the fate of silver nitrate and sterically stabilized silver nanoparticles in two different test designs used for acute tests with Daphnia magna.

The relation between test conditions such as medium composition or pH on silver nanoparticle (AgNP) behavior and its link to toxicity is one of the major topics in nanoecotoxicological research in the last years. In addition, the adaptation of the ecotoxicological standard tests for nanomaterials is intensely discussed to increase comparability and reliability of results. Due to the limitation of test material production volumes and the need for high-throughput screening, miniaturization has been proposed for several test designs. In the present study, the effect of a miniaturization of the acute Daphnia immobilization test on AgNP behavior was investigated. For this purpose, available, adsorbed, and dissolved silver fractions were measured using AgNP and silver nitrate in the following two test designs: a standard test (ST) design and a miniaturized test (MT) design with reduced test volume and less animals. Despite the increase in surface area in relation to the test volume in MT, more AgNP attached to the ST vessel surface, so that in this case, exposure concentrations were significantly lower compared to the MT assessment. Ionic silver concentrations resulting from AgNP dissolution were similar in both test designs. The same was observed for ionic silver concentrations in silver nitrate (AgNO3) treatments, but adsorbed silver was also higher in ST treatments. Assessing the structure-activity relationships revealed that surface properties such as hydrophobicity, potential binding sites, or surface roughness were of higher importance than surface:volume ratios for both test substances.

Unraveling the Complex Behavior of AgNPs Driving NP-Cell Interactions and Toxicity to Algal Cells.

While the importance of nanoparticle (NP) characterization under relevant test conditions is widely recognized in nanotoxicology, few studies monitor NPs behavior in the presence of exposed organisms. Here we studied the behavior of nine types of silver nanoparticles (AgNPs) during the 48 h algal toxicity test. In particular, we investigated NP aggregation and dissolution by time-resolved inductively coupled plasma mass spectrometry and ultrafiltration and performed mass balance measurements to study the distribution of Ag in the test system. We also determined the amount of extra- and intracellular Ag by chemically etching AgNPs on the surface of algal cells and used dark field microscopy for their imaging. We observed that positively charged branched polyethilenimine (bPEI)-coated AgNPs tend to aggregate in the presence of algae and interact with test vessels and algal cells, while citrate-coated AgNPs have a tendency to dissolve. On the other hand, with large variation of half-maximum effective concentration (EC50) across tested NPs (5.4 to 300 ngAg mL-1), Ag internalized by the algal cells at EC50 was similar (0.8 to 3.6 ngAg mL-1) for all AgNP types. These data show that while sorption to the vessels, dissolution, and aggregation impact on the distribution of AgNPs in the test system and on interactions with algal cells, AgNP toxicity is strongly correlated with the NP-cell surface interaction and internalization of Ag.

Green synthesis and physico-chemical study of silver nanoparticles extracted from a natural source Luffa acutangula

In this paper, silver nanoparticles (AgNPs) by the aqueous extract of Luffa acutangula were prepared by green synthesis and characterized using UV–Visible spectroscopy, Fourier transform infrared spectroscopy (FTIR), energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM). Antimicrobial activities of the synthesized AgNPs were also investigated against Escherichia coli and Saccharomyces cerevisiae. Further, the physico-chemical behaviour of AgNPs have been explained in order to explore the strength as well as nature of the interactions between AgNPs and various solvents by deriving thermodynamic parameters. A mathematical model is also proposed for the reliability of the experiment.

Effect of gemini surfactant (16-6-16) on the synthesis of silver nanoparticles: A facile approach for antibacterial application.

In this report, we describe the effect of Gemini surfactants1, 6-Bis (N, N-hexadecyldimethylammonium) adipate (16-6-16) on synthesis, stability and antibacterial activity of silver nanoparticles (AgNPs). The stabilizing effect of Gemini surfactant and aggregation behavior of AgNPs was evaluated by plasmonic property and morphology of the AgNPs were characterized by UV-vis spectroscopy, Dynamic Light Scattering (DLS), X-ray diffraction (XRD), High resolution transmission electron microscopy (HRTEM) and Energy dispersive X-ray analysis (EDX) techniques. Interestingly, the formation of quite mono-dispersed spherical particles was found. Apart from the stabilizing role, the Gemini surfactant has promoted the agglomeration of individual AgNPs in small assemblies whose Plasmon band features differed from those of the individual nanoparticles. The antibacterial activity of the synthesized AgNPs on Gram-negative and Gram-positive bacterium viz., E. coli and S. aureus was carried out by plate count, growth kinetics and cell viability assay. Furthermore, the mechanism of antibacterial activity of AgNPs was tested by Zeta potential and DLS analysis, to conclude that surface charge of AgNPs disrupts the cells causing cell death.

Silver nanoparticles in complex biological media: assessment of colloidal stability and protein corona formation

Engineered silver nanoparticles (AgNPs) are among the most used nanomaterials in consumer products, therefore concerns are raised about their potential for adverse effects in humans and environment. Although an increasing number of studies in vitro and in vivo are being reported on the toxicity of AgNPs, most of them suffer from incomplete characterization of AgNPs in the tested biological media. As a consequence, the comparison of toxicological data is troublesome and the toxicity evaluation still remains an open critical issue. The development of a reliable protocol to evaluate interactions of AgNPs with surrounding proteins as well as to assess their colloidal stability is therefore required. In this regard, it is of importance not only to use multiple, easy-to-access and simple techniques but also to understand limitations of each characterization methods. In this work, the morphological and structural behaviour of AgNPs has been studied in two relevant biological media, namely 10 % FBS and MP. Three different techniques (Dynamic Light Scattering, Transmission Electron Microscopy, UV–Vis spectroscopy) were tested for their suitability in detecting AgNPs of three different sizes (10, 40 and 100 nm) coated with either citrate or polyvinylpyrrolidone. Results showed that UV–Vis spectroscopy is the most versatile and informative technique to gain information about interaction between AgNPs and surrounding proteins and to determine their colloidal stability in the tested biological media. These findings are expected to provide useful insights in characterizing AgNPs before performing any further in vitro/in vivo experiment.

Impact of surface coating and environmental conditions on the fate and transport of silver nanoparticles in the aquatic environment

The role of surface coating (polyvinylpyrrolidone (PVP) and citrate) and water chemistry on the fate and behavior of AgNPs in aquatic microcosms is reported in this study. The migration and transformation of the AgNPs was examined in low (ultrapure water-UPW) and high ionic strength (moderately hard water – MHW) preparations, and in the presence of modeled natural organic matter (NOM) of Suwannee River Fulvic Acid (SRFA). The migration and fate of the AgNPs in the microcosms was validated using a sedimentation-diffusion model and the aggregation behavior was monitored by UV–visible spectrometry (UV–vis). Dissolved and particulate Ag concentrations (% Ag) were analyzed by ultrafiltration methods. Imaging of the AgNPs was captured using transmission electron microscopy (TEM).Results indicate that PVP-coated AgNPs (PVP-AgNPs) remained stable for 28days with similarly distributed concentrations of the PVP-AgNPs throughout the columns in each of the water conditions after approximately 96h (4days). The sedimentation-diffusion model confirmed PVP-AgNP stability in each condition, by showing diffusion dominated transport by using the original unaltered AgNP sizes to fit the parameters. In comparison, citrate AgNPs were largely unstable in the more complex water preparations (MHW). In MHW, aggregation dominated behavior followed by sedimentation/dissolution controlled transport was observed. The addition of SRFA to MHW resulted in small stabilizing effects, to the citrate coated AgNPs, producing smaller sized AgNPs (TEM) and mixed sedimentation and diffusion migration compared the studies absent of SRFA. The results suggest that surface coating and solution chemistry has a major impact on AgNP stability, furthermore the corresponding modeling will support the experimental understanding of the overall fate of AgNPs in the environment.

The concentration-dependent aggregation of Ag NPs induced by cystine

Cystine is widely used in cell culture media. Cysteine, the reduced form of cystine, is widely used to scavenge dissolved Ag in eco-toxicological studies to differentiate dissolved vs. nanoparticle uptake and toxicity. However, little is known about the impact of cysteine and cystine on the aggregation behavior of Ag NPs, in particular as a function of Ag NP concentration. Herein, we investigate how cystine (0–300μM) affects the stability of citrate-, polyvinylpyrrolidone-, and polyethylene glycol-coated silver nanoparticles (cit-Ag NPs, PVP-Ag NPs and PEG-Ag NPs, respectively) with and without Suwannee River fulvic acid (SRFA) as a function of Ag NPs concentration using UV–vis spectroscopy at environmentally and ecotoxicologically relevant Ag NP concentrations (ca. 125–1000μgL−1). The results demonstrate, for the first time, the concentration-dependent aggregation of cit-Ag NPs in the presence of cystine with a shift in the critical coagulation concentration (CCC) to lower cystine concentrations at lower cit-Ag NP concentrations. At the highest cit-Ag NP concentration (1000μgL−1), reaction limited aggregation was only observed and no CCC was measured. SRFA slowed the aggregation of cit-Ag NPs by cystine and aggregation occurred in reaction limited aggregation (RLA) regime only. No CCC value was measured in the presence of SRFA. Cystine replaces citrate, PVP and PEG coatings, resulting in aggregation of both electrostatically and sterically stabilized Ag NPs. These findings are important in understanding the factors determining the behavior of Ag NPs in cell culture media. Also due to the similarity between cystine and cysteine, these results are important in understanding the uptake and toxicity of Ag NPs vs. Ag ions, and suggest that the reduction of the toxicity of Ag NPs in the presence of cysteine could be due to a combined effect of scavenging Ag+ ions and Ag NP aggregation in the presence of cysteine.

Surface coating affects behavior of metallic nanoparticles in a biological environment.

Silver (AgNPs) and maghemite, i.e., superparamagnetic iron oxide nanoparticles (SPIONs) are promising candidates for new medical applications, which implies the need for strict information regarding their physicochemical characteristics and behavior in a biological environment. The currently developed AgNPs and SPIONs encompass a myriad of sizes and surface coatings, which affect NPs properties and may improve their biocompatibility. This study is aimed to evaluate the effects of surface coating on colloidal stability and behavior of AgNPs and SPIONs in modelled biological environments using dynamic and electrophoretic light scattering techniques, as well as transmission electron microscopy to visualize the behavior of the NP. Three dispersion media were investigated: ultrapure water (UW), biological cell culture medium without addition of protein (BM), and BM supplemented with common serum protein (BMP). The obtained results showed that different coating agents on AgNPs and SPIONs produced different stabilities in the same biological media. The combination of negative charge and high adsorption strength of coating agents proved to be important for achieving good stability of metallic NPs in electrolyte-rich fluids. Most importantly, the presence of proteins provided colloidal stabilization to metallic NPs in biological fluids regardless of their chemical composition, surface structure and surface charge. In addition, an assessment of AgNP and SPION behavior in real biological fluids, rat whole blood (WhBl) and blood plasma (BlPl), revealed that the composition of a biological medium is crucial for the colloidal stability and type of metallic NP transformation. Our results highlight the importance of physicochemical characterization and stability evaluation of metallic NPs in a variety of biological systems including as many NP properties as possible.

Nanoparticle generation inside Ag-doped LBG glass by femtosecond laser irradiation

Precipitation of silver nanoparticles (Ag-NPs) can occur in a silver-doped lanthanum borogermanate glass matrix, either under thermal annealing or femtosecond direct laser writing (DLW). Macroscopic transmission and localized µ-fluorescence emission spectra provide typical plasmon resonance behavior of Ag-NPs. Annealing leads to the spatially random homogeneous creation of small NPs at low concentrations, while DLW shows higher and broader plasmon peaks, proving highly concentrated 3D localized structured NPs. DLW at high fluencies shows photo-induced dichroism, potentially giving access to 3D dichroic photonic responses.

Separation of silver nanoparticles by hollow fiber flow field-flow fractionation: Addition of tannic acid into carrier liquid as a modifier

A homemade hollow fiber flow-field fractionation (Hf-FlFFF) coupled with inductively coupled plasma mass spectrometry (ICP-MS) was set-up for silver nanoparticles (AgNPs) separation by using polysulfone hollow fiber membrane (30,000 MW cutoff) as a separation channel. Tannic acid and citrate stabilized AgNPs were synthesized and introduced into Hf-FlFFF. The effects of carrier liquid and stabilizing agent on retention behavior of AgNPs were investigated. Different elution behaviors were observed as follows: with 0.02% (w/v) FL-70, all of AgNPs were eluted from Hf-FlFFF but differences in retention behaviors were observed for AgNPs with tannic acid and citrate stabilizing agents; and with 30mM TRIS buffer, only tannic acid stabilized AgNPs were eluted from Hf-FlFFF, whereas citrate stabilized AgNPs were not eluted. In this work, tannic acid addition into carrier liquid was proposed to modify the surface of AgNPs and the surface of the membrane, and thereby adjusting the retention behaviors of AgNPs. Various concentrations of tannic acid were added into FL-70 and TRIS buffer. With the use of 0.1mM tannic acid in 30mM TRIS buffer as the carrier liquid, retention behaviors of both tannic acid stabilized- and citrate stabilized-AgNPs were similar and with similar fractionation recovery.

Chloride-induced shape transformation of silver nanoparticles in a water environment

The effects of chloride on dissolution and toxicity of silver nanoparticles (AgNPs) have been well studied. However, their intermediate shapes during the transition have not been illustrated to-date. Herein, the chloride-induced shape transformation process of AgNPs under long-term, low-concentration conditions is explored. A unique triangular Ag–AgCl heterostructure is observed. The structure then evolves into a symmetric hexapod and finally into a smaller AgNP. This transformation process could be affected by other environmental conditions, such as 0.4 mg/mL humic acid, 5% surfactants and 1 mg/mL bovine serum albumin protein. Our results offer new knowledge regarding the shape transformation process of AgNPs in the presence of chloride, which can be valuable in relevant studies concerning the effect of water chemistry on the behavior of AgNPs.

Cellular uptake and toxicity effects of silver nanoparticles in mammalian kidney cells.

The rapid progress and early commercial acceptance of silver-based nanomaterials is owed to their biocidal activity. Besides embracing the antimicrobial potential of silver nanoparticles (AgNPs), it is imperative to give special attention to the potential adverse health effects of nanoparticles owing to prolonged exposure. Here, we report a detailed study on the in vitro interactions of citrate-coated AgNPs with porcine kidney (Pk15) cells. As uncertainty remains whether biological/cellular responses to AgNPs are solely as a result of the release of silver ions or whether the AgNPs themselves have toxic effects, we investigated the effects of Ag(+) on Pk15 cells for comparison. Next, we investigated the cellular uptake of both AgNPs and Ag(+) in Pk15 cells at various concentrations applied. The detected Ag contents in cells exposed to 50 mg l(-1) AgNPs and 50 mg l(-1) Ag(+) were 209 and 25 µg of Ag per 10(6) cells, respectively. Transmission electron microscopy (TEM) images indicated that the Pk15 cells internalized AgNPs by endocytosis. Both forms of silver, nano and ionic, decreased the number of viable Pk15 cells after 24 h in a dose-dependent manner. In spite of a significant uptake into the cells, AgNPs had only insignificant toxicity at concentrations lower than 25 mg l(-1) , whereas Ag(+) exhibited a significant decrease in cell viability at one-fifth of this concentration. The Comet assay suggested that a rather high concentration of AgNP (above 25 mg l(-1) ) is able to induce genotoxicity in Pk15 cells. Further studies must seek deeper understanding of AgNP behavior in biological media and their interactions with cellular membranes.

Transformations of citrate and Tween coated silver nanoparticles reacted with Na2S

Silver nanoparticles (Ag NPs) are susceptible to transformations in environmental and biological media such as aggregation, oxidation, dissolution, chlorination, sulfidation, formation/replacement of surface coatings following interaction with natural organic matter (NOM). This paper investigates the impact of surface coating and Suwannee River fulvic acid (SRFA) on the transformations and behavior of Ag NPs (citrate coated and Tween coated; cit-Ag NPs and Tween-Ag NPs, respectively), following reaction with different concentrations of Na2S solution (as a source of sulfide species, H2S and HS−). These transformations and the dominant mechanisms of transformations were investigated using UV–vis and scanning transmission electron microscopy coupled with electron energy loss spectroscopy.Here, we have shown that Ag NP surface coating impacts their dissolution following dilution in ultrahigh purity water, with higher extent of dissolution of Tween-Ag NPs compared with cit-Ag NPs. Tween-Ag NPs are susceptible to dissolution following their sulfidation at low S/Ag molar ratio. Suwannee River fulvic acid (SRFA) slows down the dissolution of Tween-Ag NPs at low sulfide concentrations and reduces the aggregation of cit-Ag NP in the presence of sodium sulfide. Sulfidation appears to occur by direct interaction of sulfide species with Ag NPs rather than by indirect reaction of sulfide with dissolved Ag species subsequent to dissolution. Furthermore, the sulfidation process results in the formation of partially sulfidized Ag NPs containing unreacted (metallic) subgrains at the edge of the NPs for Tween-Ag NPs in the presence of high sulfide concentration (2000nM Na2S), which occurred to less extent at lower Na2S concentration for Tween-Ag NPs and at all concentrations of Na2S for cit-Ag NPs. Thus, sulfidized Ag NPs may preserve some of the properties of the Ag NPs such as their potential to shed Ag+ ions and their toxic potential of Ag NPs.

Size-Dependent Electrochemical Stability of Silver Nanoparticles

Understanding the electrochemical stability or corrosion behavior of metallic nanoparticles in aqueous environments is of central importance in the fields of catalysis, sensing, nanoelectronics and photonics [1]. The advantages of nanoparticles, such as increased solubility of nanosuspensions and their low production costs have resulted in their rapid commercialisation for medical applications (e.g. drug delivery) [2]. In addition, metal ion release is a major pathway underlying their potential toxicity, and the cellular environment can be considered as an electrochemical cell. Therefore, stability against oxidation or loss of effective surface area is necessary for metallic nanoparticles (NPs) to retain their useful properties in long-term applications and also important for understanding the safety of NPs. The objective of this study was to investigate the electrochemical stability of silver nanoparticles (AgNPs) as a function of applied potential, pH and particle size. AgNPs with controlled size and size distribution were deposited on to boron-doped diamond electrodes via magnetron sputtering, with size-filtering capability. The anodic behavior of AgNPs in perchlorate buffer pH solutions (ranging from pH 2-12) were examined by linear sweep voltammetry. The surface chemistry and oxidation state of Ag after each treatment were characterized using X-ray photoelectron spectroscopy (XPS). The size distribution of AgNPs in response to electrochemical polarization was examined using atomic force microscopy (AFM). Pourbaix diagrams for bulk Ag suggest that the equilibrium condition is not pH-dependent in acidic conditions. However, we observe that, for nanoparticles with the average size of 4.4 ± 2.1 nm, the oxidation potential deviates from expectation in acidic conditions by approximately 0.015V / pH. By contrast, the relationship between potential and pH follows that of bulk Ag for alkaline solutions. XPS confirmed the expected electrochemical changes in surface chemistry, with binding energy (BE) of Ag3d5/2 photoelectron signal and O1s/C1s ratio correlating with the observed changes in the silver oxidation state. These direct voltammetric measurements of the Ag oxidation potential, as a function of size and pH, indicate the electrochemical stability of NPs is different from their bulk metal. This study supports the findings of Tang et.al., [3] suggesting that theoretically derived energy diagrams for a bulk material might not always accurate to NPs, especially AgNPs which has been shown in our studies.