Mobility and transformation of CdSe/ZnS quantum dots in soil: Role of the capping ligands and ageing effect.

The increasing application of Quantum Dots (QDs) is cause of concern for the potential negative effects for the ecosystem, especially in soils that may act as a sink. In this study, soil leaching experiments were performed in quartz sand packed columns to investigate the behavior of core-shell CdSe/ZnS QDs coated with either small ligands (TGA-QDs) or more complex polymers (POAMA-QDs). Fluorescence emission was compared to mass spectrometric measurements to assess the nanoparticles (NPs) state in both the leachate (transported species) and porous media (deposited amounts). Although both QDs were strongly retained in the column, large differences were observed depending on their capping ligand stability. Specifically, for TGA-QDs elution was negligible and the retained fraction accumulated in the top-columns. Furthermore, 74% of the NPs were degraded and 38% of the Se was found in the leachate in non-NPs state. Conversely, POAMA-QDs were recovered to a larger extent (78.1%), and displayed a higher transport along the soil profile. Further experiments with altered NPs showed that homo-aggregation of the QDs prior injection determined a reduced mobility but no significant changes in their stability. Eventually, ageing of the NPs in the column (15 days) caused the disruption of up to 92% of the original QDs and the immobilization of NPs and metals. These results indicate that QDs will accumulate in top-soils, where transformations phenomena will determine the overall transport, persistency and degradation of these chemicals. Once accumulated, they may act as a source for potentially toxic Cd and Se metal species displaying enhanced mobility.

Tracing multi-isotopically labelled CdSe/ZnS quantum dots in biological media.

The strengths and limits of isotopically labelled Engineered Nanoparticles (spiked ENPs) spread in biological media have been assessed. Multi-spiked CdSe/ZnS quantum dots (QDs), measuring 7 nm and coated with thioglycolic acid (TGA), were synthesized and enriched in 68Zn, 77Se and 111Cd. These QDs were dispersed at very low concentrations (0.1 to 5000 ppt) in diverse biological matrices (synthetic saliva, synthetic urine, plasma and Dulbecco's phosphate buffered saline - DPBS growth medium) and the isotopic compositions were determined by HR-ICP-MS. The initial QDs concentrations were calculated to assess the limit of quantification (QD-LOQ) according to the matrix and the isotopically enriched element. The obtained results demonstrated the advantages of the isotopic labelling method in order to work at very low concentrations: the QD-LOQ values for the spiked Zn, Cd and Se originated from the QDs were 10, 0.3 and 6 ppt, respectively, which is below the conventional LOQ of the HR-ICP-MS used (30, 3 and 60 ppt for Zn, Cd and Se, respectively). Conversely, in complex matrices such as saliva, urine, plasma and DPBS growth medium, the QD-LOQ values increased significantly, with values ranging from 16 to 32 ppt for Cd, 446 to 10598 ppt for Zn and 1618 to 8317 ppt for Se. These QD-LOQs are dependent on factors as the elemental background concentration already present in the matrices, and the dilution factor. In this study, the QD-LOQs are expressed for the first time with respect to the background concentration in biological media (QD-RLOQ), which can be used to better assess and then predict the efficiency of the spiking method.

Isotopically Labeled Nanoparticles at Relevant Concentrations: How Low Can We Go? The Case of CdSe/ZnS QDs in Surface Waters.

Analytical barriers impose work at nanoparticles (NPs) concentrations orders of magnitude higher than the expected NPs concentrations in the environment. To overcome these limitations, the use of nontraditional stable isotope tracers incorporated in NPs (spiked-NPs) coupled with HR-ICP-MS has been proposed. The performance and efficiency of this analytical method was assessed in the case of quantum dots (QDs). Multi-isotopically labeled 111Cd77Se/68ZnS QDs were synthesized and their dissemination in natural aquatic matrices (river, estuarine and sea waters) was modeled at very low concentrations (from 0.1 to 5000 ppt). The QD limits of quantification (QD-LOQ) in each matrix were calculated according to the isotopic tracer. In ultrapure and simple medium (HNO3 2%), Zn, Cd, and Se originated from the QDs were quantifiable at concentrations of 10, 0.3, and 6 ppt, respectively, which are lower than the conventional HR-ICP-MS LOQs. In aquatic matrices, the QD-LOQs increase 10-, 130-, and 250-fold for Zn, Cd, and Se, respectively, but remain relevant of environmental concentrations (3.4 ppt ≤ QD-LOQs ≤ 2.5 ppb). These results validate the use of isotopically labeled ENPs at relevant concentrations in experimental studies related to either their fate, behavior, or toxicity in most aquatic matrices.

An Isotopic Exchange Kinetic Model to Assess the Speciation of Metal Available Pool in Soil: The Case of Nickel.

In this study an innovative approach is proposed to predict the relative contribution of each mineral phase to the total metal availability in soils, which, in other words, could be called the available metal fractionation. Through the use of isotopic exchange kinetics (IEK) performed on typical Ni bearing phases (i.e., two types of serpentines, chlorite, smectite, goethite, and hematite) the isotopic exchange and metal-solid interaction processes are connected, considering both the thermodynamic and kinetic aspects. Results of Ni IEK experiments on mineral phases are fitted with a pseudo-first order kinetic model. For each Ni bearing phase, this allows to (i) determine the number and size of exchangeable pools (ENi(i)), (ii) assess their corresponding kinetic constants (k(i)), and (iii) discuss the mechanism of Ni isotopic exchange at mineral surfaces. It is shown that all the phases investigated, with the only exception of hematite, present at least two distinct reactive pools with significantly different k(i) values. Results suggest also that metal involved in outer-sphere complexes would display isotopic exchange between 100 and 1000 times faster than metal involved in inner-sphere complexes, and that the presence of high and low affinity sites may influence the rate of isotopic exchange up to 1 order of magnitude. Moreover, the method developed represents a tool to predict and estimate Ni mobility and availability in natural soil samples on the basis of soil mineral composition, providing information barely obtained with other techniques.

Behavior and fate of industrial zinc oxide nanoparticles in a carbonate-rich river water.

The present study precisely describes the solubility patterns of commercial uncoated and organic coated ZnO NPs (nc-NPs and c-NPs, respectively) in a natural carbonate-rich water and the physicochemical processes involved. NPs transformation rates were determined with the Donnan Membrane approach (DMT, to obtain Zn(2+) concentration) and ultrafiltration (i.e. Zn(2+) and Zn bound to small organic ligands) and modeled with VMinteQ. XPS measurements evidenced the presence on native nc-NPs of a Zn(OH)2 layer which accounts for almost 22% of total Zn. This Zn(OH)2 phase is more soluble than ZnO, and could control the early dissolution steps of the nc-NPs in our system. Indeed, nc-NPs display a fast (<1 h) dissolution step reaching 19 µM Zn in solution (<1% of the total initial zinc concentration). Comparatively, c-NPs progressively release zinc during the first 48 h, to finally reach a maximum of 197 µM (10% of total Zn), which is 10 times the maximum value measured for nc-NPs. Over the long term, dissolved Zn concentrations decrease in both systems, corresponding to the neoformation of carbonate phases observed by TEM imaging. The kinetic modeling allows highlighting two different ranges of time, corresponding to (i) first 10h with a fast precipitation (kp(')=-182.10(-4)) related to a highly oversaturated solution with respect to carbonate zinc mineral and (ii) a second slower precipitation step (kp(')=-8.10(-4)), related to the embedding of NPs in the precipitated carbonate matrix. The steady state is reached after 3 months of interaction.