Rute F. Domingos

Determining the environmental risk of metals requires an in-depth understanding of the environmental matrices composition, which currently also includes the presence of manufactured metallic nanoparticles (NPs) usually, stabilized by a polymer surface coating. As a consequence, is necessary to take into account effects of the NP core, the polymer surface coating and their mutual interaction as well as with other environmental components. The release of metal ions from metallic NPs is a well-known outcome, however, the effect of the presence of the NP polymer coating in the NPs solubilization mechanism is not well understood. In this study the dynamic speciation of Cd and Pb in presence of a polyacrylic acid (PAA)-stabilized CdTe/CdS NP was quantified by scanned stripping chronopotentiometry (SSCP). It was found that although the NP solubility was 5.8× larger at pH 8.5 than at pH 6.0, the amount of free Cd ions was much smaller (2.4 % at pH 8.5 vs. 57 % at pH 6.0). The concentration of free Cd and Pb ions in solution was larger in presence of the PAA-shells than when in presence of the same amount of polymer but when this is at the surface of the NP. This effect is attributed to the metal ions interaction with the particle itself. The effect is notably larger for Pb ions and might results from the exchange of Cd by Pb ions at the particle surface since PbS is less soluble than CdS.

The widespread use of nanoparticles (NPs) raises concern over their potential toxicological effects in humans and ecosystems. Here we used transcriptome sequencing (RNA-seq) to evaluate the effects of exposure to four different metal-based NPs, nano-Ag (nAg), nano-TiO ₂ (nTiO ₂ ), nano-ZnO (nZnO), and CdTe/CdS quantum dots (QDs), in the eukaryotic green alga Chlamydomonas reinhardtii . The transcriptome was characterized before and after exposure to each NP type. Specific toxicological effects were inferred from the functions of genes whose transcripts either increased or decreased. Data analysis resulted in important differences and also similarities among the NPs. Elevated levels of transcripts of several marker genes for stress were observed, suggesting that only nZnO caused nonspecific global stress to the cells under environmentally relevant conditions. Genes with photosynthesis-related functions were decreased drastically during exposure to nTiO ₂ and slightly during exposures to the other NP types. This pattern suggests either toxicological effects in the chloroplast or effects that mimic a transition from low to high light. nAg exposure dramatically elevated the levels of transcripts encoding known or predicted components of the cell wall and the flagella, suggesting that it damages structures exposed to the external milieu. Exposures to nTiO ₂ , nZnO, and QDs elevated the levels of transcripts encoding subunits of the proteasome, suggesting proteasome inhibition, a phenomenon believed to underlie the development and progression of several major diseases, including Alzheimer's disease, and used in chemotherapy against multiple myeloma.

Environmental context: The number of nano-enabled products reaching consumers is growing exponentially, inevitably resulting in their release to the environment. The environmental fate and mobility of nanomaterials will depend on their physicochemical form(s) under natural conditions. For ZnO nanoparticles, determinations of agglomeration and dissolution under environmentally relevant conditions of pH, ionic strength and natural organic matter content will provide insight into the potential environmental risk of these novel products. The increasing use of engineered nanoparticles (ENPs) in industrial and household applications has led to their release into the environment and increasing concern about their effects. Proper assessment of the ecological risks of ENPs will require data on their bioavailability, persistence and mobility over a broad range of physicochemical conditions, including environmentally relevant pH, ionic strength and concentrations of natural organic matter (NOM). In this study, fluorescence correlation spectroscopy was used to determine the agglomeration of a ZnO ENP (nZnO) with a nominal size of 20 nm. Particle dissolution was followed using scanned stripping chronopotentiometry. The effects of Suwannee River fulvic acid (SRFA, 0–60 mg L–1) and the roles of pH (4–10) and ionic strength (0.005–0.1 M) were carefully evaluated. Agglomeration of the bare nZnO increased for pH values near the zero point of charge, whereas the dissolution of the particles decreased. At any given pH, an increase in ionic strength generally resulted in a less stable colloidal system. The role of SRFA was highly dependent upon its concentration with increased agglomeration observed at low SRFA : nZnO mass ratios and decreased agglomeration observed at higher SRFA : nZnO mass ratios. The results indicated that in natural systems, both nZnO dispersion and dissolution will be important and highly dependent upon the precise conditions of pH and ionic strength.

The improvement of knowledge about the toxicity and even processability, and stability of quantum dots (QD) requires the understanding of the relationship between the QD binding head group, surface structure, and interligand interaction. The scanned stripping chronopotentiometry and absence of gradients and Nernstian equilibrium stripping techniques were used to determine the concentration of Cd dissolved from a polyacrylate-stabilized CdTe/CdS QD. The effects of various concentrations of small organic ligands such as citric acid, glycine, and histidine and the roles of pH (4.5–8.5) and exposure time (0–48 h) were evaluated. The highest QD dissolution was obtained at the more acidic pH in absence of the ligands (52 %) a result of the CdS shell solubility. At pH 8.5 the largest PAA ability to complex the dissolved Cd leads to a further QD solubility until the equilibrium is reached (24 % of dissolved Cd vs. 4 % at pH 6.0). The citric acid presence resulted in greater QD dissolution, whereas glycine, an amino acid, acts against QD dissolution. Surprisingly, the presence of histidine, an amino acid with an imidazole functional group, leads to the formation of much strong Cd complexes over time, which may be non-labile, inducing variations in the local environment of the QD surface.

In order to properly assess the environmental risk of engineered nanoparticles (ENP), it is necessary to determine their fate (including dissolution, aggregation, and bioaccumulation) under representative environmental conditions. CdTe/CdS quantum dots (QD), such as those used in medical imaging, are known to release Cd²⁺ due (mainly) to the dissolution of their outer shell. In this study, Chlamydomonas reinhardtii was exposed to either a soluble Cd salt or QD at similar concentrations of total Cd. Free Cd concentrations were measured using the Absence of Gradients and Nernstian Equilibrium Stripping technique. QD dissolution increased with decreasing pH and with increasing QD concentration. When exposed to QD, bioaccumulation was largely accounted for by dissolved Cd. Nonetheless, QD were shown to be taken up by the cells and to provoke unique biological effects. Whole transcriptome screening using RNA-Seq analysis showed that the free Cd and the QD had distinctly different biological effects.

Environmental context. The increasing use of nanomaterials in consumer products has led to increased concerns about their potential environmental and health impacts. TiO2 is a widely used nanoparticle found in sunscreens and electronic products. In order to understand and predict the mobility of TiO2 in the natural environment, it is essential to determine its state of aggregation under environmentally relevant conditions of pH, ionic strength, ion and natural organic matter content. Aggregation is likely to lead to both reduced mobility and bioavailability in soils and natural waters. The physicochemical characterisation of nanomaterials is crucial to predict their environmental and health impacts. Ion adsorption is known to influence the surface properties of nano-metal oxides in natural systems. The role of calcium and phosphate adsorption on aggregation was examined in the presence and absence of natural organic matter. Fluorescence correlation spectroscopy (FCS) was performed in order to determine the diffusion coefficients of TiO2 nanoparticles having a nominal size between 3 to 5 nm. Based upon FCS and electrophoretic mobility measurements, the presence of calcium resulted in a destabilisation and aggregation of the TiO2 nanoparticles, even in presence of Suwannee River fulvic acid (SRFA). Conditions which favoured phosphate adsorption also resulted in a destabilisation of the TiO2 sample but for low SRFA concentrations only.