Shape-Dependent Transformation and Translocation of Ceria

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Shape-dependent Transformation and Translocation of Ceria Nanoparticles in Cucumber Plants Peng Zhang, Changjian Xie, Yuhui Ma, Xiao He, Zhiyong Zhang, Yayun Ding, Lirong Zheng, and Jing Zhang Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.7b00359 • Publication Date (Web): 29 Aug 2017 Downloaded from http://pubs.acs.org on August 30, 2017

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Environmental Science & Technology Letters

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Shape-dependent Transformation and Translocation of Ceria Nanoparticles in

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Cucumber Plants

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Peng Zhang,†,* Changjian Xie,† Yuhui Ma,† Xiao He,† Zhiyong Zhang,†,* Yayun Ding,† Lirong Zheng,#

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Jing Zhang#†

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†

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Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China;

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#

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Chinese Academy of Sciences, Beijing 100049, China;

Key Laboratory for Biological Effects of Nanomaterials and Nanosafety,

Beijing Synchrotron Radiation Facility, Institute of High Energy Physics,

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* Corresponding author.

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E-mail addresses: [email protected]; [email protected]

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Tel: +86-10-88233215; Fax: +86-10-88235294

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ACS Paragon Plus Environment


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Abstract

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The association of physicochemical properties of CeO2-NPs per se with their transformation is not well

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understood. This study for the first time compared the translocation and transformation of octahedral,

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cubic, rod, and irregular shaped CeO2-NPs in hydroponic cucumber plants. Cerium contents in roots

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were close between different treatments, while shoots accumulated the highest (153 mg/kg) amount of

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Ce in Rod like CeO2-NPs treatments. TEM and X-ray absorption near edge spectroscopy (XANES)

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results all show that rod CeO2-NPs transformed faster and more than other CeO2-NPs, with nearly 40%

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of Ce was in the form of Ce(III) species in roots (CePO4) and shoots (Ce carboxylates). Rod like CeO2-

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NPs transformed to a higher degree than the other CeO2-NPs in solutions simulating the plant exudates,

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indicating that rod like CeO2-NPs have the highest chemical reactivity. These results suggest that the

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intrinsically different chemical reactivity of differently shaped CeO2-NPs resulted in their different

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transformation and translocation capacity in plants. This study provides new insight into the

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understanding of plant-NPs interaction, highlighting the significance of nanoparticle shape in assessing

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their environmental behavior and impacts. We suggest that the shape influence should be also considered

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for other nanomaterials and systems to accurately understanding the nano-bio interactions.

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Introduction

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CeO2-NPs are of great interest for industrial, agricultural, and biomedical application due to their unique

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redox-cycle property between oxidation states Ce(III) and Ce(IV).1,2,3 It was estimated that the global

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production of CeO2-NPs will be 1000 tons per year.4 The release of CeO2-NPs to the environment is

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inevitable which may affect the biota and environment.5,6 Exposure modeling has suggested that

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terrestrial systems are important sinks for nanomaterials (NMs).7 As a result, NMs will interact with

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plants, and may accumulate in plants and pose adverse effects to plant growth.6 Furthermore, transfer of

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NMs from plants to high-trophic level organisms through food chain is also possible.8,9

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CeO2-NPs were previously considered to be stable in the environment; however, recent studies have

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shown that CeO2-NPs are prone to transform in plants, releasing Ce3+ ions and further transforming into

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CePO4 or Ce carboxylates.10 Consequently, Ce will accumulate in plants in various forms rather than

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only CeO2, and the transformation products may or at least partially contribute to the toxic effects of

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CeO2.11

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Transformation of CeO2-NPs is highly affected by their surroundings. CeO2-NPs can release

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significant amount of Ce3+ ions in plant-free growth media depending on the media composition.12

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Presence of organic matters such as arabic gum prevent agglomeration of CeO2-NPs thus maintain a

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large reaction surface, which could promote the dissolution.12 Organic ligands such as EDTA and citrate

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promote the dissolution by complexation with the surface Ce(III).10,12 Reducing substances such as

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Fe(II) and ascorbic acids (Vc) can reduce the Ce(IV) to Ce(III) and release Ce3+.10,12 Plant root exudates

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containing organic acids, reducing sugars and phenols promote the dissolution and reduction of CeO2-

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NPs.10

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Physicochemical properties of NMs also have significant impacts on their transformation. It was

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reported that CeO2-NPs with smaller size released more Ce3+ ions in Lactuca plant roots.11 Citrate-

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functionalized CeO2-NPs transformed less than pristine CeO2-NPs in activated sludge reactor due to that

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citrate functionalization may act as a barrier against the interaction of CeO2-NPs with bacteria.13

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CeO2-NPs are usually manipulated into different shapes to achieve high catalytic capacity.14 However,

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how shape of CeO2-NPs influence their transformation has not been studied. In fact, studies on other

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NMs have shown that shape discrepancy may result in different biological effects and behavior. Fan et

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al. found that octahedral Cu2O-NPs induced more severe oxidative stress to Daphnia magna than cubic

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Cu2O-NPs.15 Oh et al. found that the viability of human lung fibroblast (IMR90) and mouse alveolar

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macrophage (J774A.1) cells treated with poly(3,4-ethylenedioxythiophene) (PEDT) nanomaterials with

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aspect ratio of 1.3 was nearly 20% less than that treated with PEDT with aspect ratio of 4.5.16 Syu at al.

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found that decahedral silver NPs promoted Arabidopsis root growth, while spherical silver NPs showed

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no effects.17 All these studies suggest that shape is a critical factor that should be considered when

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assessing the environmental risk and safety of NMs.

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Here, we exposed hydroponic cucumber plants to octahedral, cubic, rod-like, and commercial

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irregularly-shaped CeO2-NPs, and compared their transformation and uptake in plants. The goal of this

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study was to provide a deep insight into the behavior and fate of CeO2-NPs in plants and the link with

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their physicochemical properties, and provide basis for safer design of CeO2-NPs.

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Experimental

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Synthesis and characterization of CeO2-NPs

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All the chemicals used were of analytical purity. Octahedral CeO2-NPs (O-CeO2-NPs) was synthesized

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by a precipitation method. Cubic and rod like CeO2-NPs (C-CeO2-NPs and R-CeO2-NPs) were

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synthesized by hydrothermal methods. See details of CeO2-NPs syntheses in SI, Section 1, 1.1.

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Commercial CeO2-NPs (S-CeO2-NPs) were purchased from Sigma-Aldrich. Purity of the nanoparticles

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was determined by ICP-MS. Primary particle sizes and morphology were determined by TEM (JEM

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200CX, Japan). Zeta potential and hydrodynamic sizes of CeO2 NPs were analyzed by Zetasizer Nano

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ZS90 (Malvern, UK).

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Seedling culture and CeO2-NPs application

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Cucumber seeds were purchased from Chinese Academy of Agricultural Sciences. Seeds were sterilized,

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germinated and transferred into 250 mL beakers, and allowed to grow in 1/4 Hoagland solutions for 10

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days in an artificial climate chamber. Each seedling was then exposed to 100 ml 2000 mg/L CeO2-NPs

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suspensions in nutrient solutions. After 14 days, the seedlings were harvested for further analyses (see

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details in SI, 1.2). 2000 mg/L was the maximum exposure concentration established by US EPA

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guideline.18 According to our previous experiences, Ce uptake in shoots of cucumber at exposure

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concentration less than 2000 mg/L is too low to acquire XANES spectra with good signal.

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Quantification, localization and chemical species analysis of Ce in plant tissues

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For TEM observation, fresh root apices were collected, fixed, dehydrated, embedded, and sectioned

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following the standard procedures. Sections were then observed on a JEM-1230 (JEOL, Japan)

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transmission electron microscope operating at 80 kV. Freeze-dried roots and shoots were digested with

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nitric acid and hydrogen peroxide mixture, and the total Ce contents in roots and shoots were analyzed

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by ICP-MS (Thermo, USA). For XANES analyses, dry roots and shoots were ground to fine powders,

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pressed into thin slices (~2 mm). Ce LIII-edge (5723 eV) spectra were collected at ambient temperature

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in the fluorescence mode at beamline 1W1B of Beijing Synchrotron Radiation Facility. XANES spectra

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of reference compounds including CeO2-NPs, CePO4, Ce2(C2O4)3 as well as Ce(CH3COO)3 were also

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collected. See details in SI, 1.3.

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Transformation of CeO2-NPs in simulated solutions

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To compare the reactivity of differently shaped CeO2-NPs, CeO2-NPs were added into four solutions to

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a concentration of 2000 mg/L: KH2PO4+citric acid (CA) + Vc, KH2PO4 + EDTA + Vc, KH2PO4 + CA +

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catechol and KH2PO4 + EDTA + CAT. CA and EDTA represent organic acids, while Vc and CAT

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represent reducing agents, both of which simulate the key components of plant root exudates. After 21

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days, CePO4 and Ce3+ in the solutions were determined by FTIR and ICP-MS. See details in SI, 1.4.

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Results and discussion

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As shown in Figure S1 and Table S1, S-CeO2-NPs show irregular shape and the average particle size

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was 26 ± 18 nm. Primary size of O-CeO2-NPs and C-CeO2-NPs were 25.2 ± 2.3 nm and 30.9 ± 12.4 nm.

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R-CeO2-NPs show uniform sizes with diameter of 8.9 ± 0.9 nm and length of 106 ± 9 nm. All the CeO2-

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NPs agglomerated fast in 1/4 Hoagland solution (Figure S2). Sizes of agglomerates for R-CeO2-NPs

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were much larger than that for other CeO2-NPs.

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Figure S3 reveals that all the CeO2-NPs showed no adverse effects on shoot and root biomass. Ce

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contents in roots are not significantly different between different treatments (Figure 1). However, the Ce

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contents in shoots in the R-CeO2-NPs treatment (153 mg/kg) were much higher than those (54 ~ 69

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mg/kg) in the other treatments. This suggests that translocation of Ce in R-CeO2-NPs treated plant was

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more efficient than that in other treatments (Figure S3).

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Agglomeration is a critical factor that may affect the uptake and translocation of NPs in plants.

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Formation of agglomerates would reduce the uptake by plants.19 However, DLS results suggest that R-

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CeO2-NPs formed larger agglomerates than other CeO2-NPs (Table S1), which did not correlate with the

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Ce uptake in shoots, indicating that there are other factors leading to the high Ce uptake in shoots in R-

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CeO2-NPs treatment.

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(Figure 1 here)

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Previous studies showed that PO43- can reduce the dissolved Ce concentrations by forming CePO4

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precipitates,12 which consequently limits the translocation of Ce to shoots.20 This indicates that

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translocation of Ce is highly associated with the transformation of CeO2. Figure 2 shows the temporal

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transformation of CeO2-NPs on root surfaces. Transformation of S-CeO2-NPs, O-CeO2-NPs and C-

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CeO2-NPs are overall synchronous. At the 7th day, a few needle- or rod-like clusters were visible among

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the CeO2-NPs aggregates on the root surfaces (Figure 2A1-2A3). These clusters are CePO4 that have

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been reported in previous studies.10 A few needle-like clusters were also observed in the intercellular

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spaces in S-CeO2-NPs and O-CeO2-NPs treatments (Figure 2B1 and 2B2), whereas no particles were

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found in C-CeO2-NPs treatments. In comparison, R-CeO2-NPs transformed faster than the other CeO2-

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NPs. At the 7th day, most of the R-CeO2-NPs degraded into small particles on the root surfaces (Figure

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2A4), and many CePO4 clusters were visible in the intercellular spaces (Figure 2B4). The amount of

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CePO4 clusters on the root surface increased over time for all the treatments (Figure 2C1-2C4 and 2E1-

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2E4). However, the amount of CePO4 in intercellular regions did not change significantly over time

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(Figure 2D1-D3 and 2F1-F3) except for R-CNPs treatment, where large amount of need-like clusters

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were visible in the intracellular spaces at 21th day (Figure 2F4).

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The XANES spectra of roots in S-CeO2-NPs, O-CeO2-NPs and C-CeO2-NPs treatments show double

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white lines (Figure 3A), indicating that the Ce predominantly remained as CeO2. In contrast, spectra of

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roots in R-CeO2-NPs treatment presented a mixed feature of Ce(III) and Ce(IV), suggesting that R-

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CeO2-NPs partially transformed to Ce(III) species. In shoots (Figure 3B), CeO2-NPs transformed in all

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the treatments, as indicated by the presence of feature a.

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(Figure 2 here)

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(Figure 3 here)

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Linear combination fitting using standard references (Figure S4) revealed that more than 79% of the

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Ce remained as CeO2 in roots in S-CeO2-NPs, O-CeO2-NPs and C-CeO2-NPs treatments; while in R-

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CeO2-NPs treatments, more than 40% of the Ce presented as Ce(III) species, most of which were CePO4

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and a small part were carboxylates (Figure 3C). More than 80% of Ce remained as CeO2 in shoots in S-

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CeO2-NPs, O-CeO2-NPs and C-CeO2-NPs treatments, while only 62% remained as CeO2 in R-CeO2-

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NPs treatment (Figure 3D). These results agree with the TEM results, suggesting that R-CeO2-NPs

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transformed more than other CeO2-NPs.

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We further calculated the absolute Ce(IV) and Ce(III) contents by multiplying their proportion by the

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total Ce contents. Roots accumulated the lowest amount of Ce(IV) in R-CeO2-NPs treatment, which is

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likely due to that R-CeO2-NPs formed large agglomerates thus reduced the adsorption/absorption in

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roots. Ce(III) contents in roots were the highest in R-CeO2-NPs treatment, which agrees with the TEM

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and LCF results. Interestingly, Ce(IV) and Ce(III) contents in shoots were both higher in R-CeO2-NPs

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treatment than in other treatments. Degradation of R-CeO2-NPs into small particles on root surfaces

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(Figure 2A4) may facilitate the movement of CeO2 and result in the high Ce(IV) uptake in shoots. Since

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the transformation of CeO2-NPs occurs on the root surface rather than inside the plants,21 the high

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Ce(III) contents in shoots could be attributed to high dissolving capacity of R-CeO2-NPs outside the

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roots.

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In a follow-up study, we incubated CeO2-NPs in simulated solutions which are composed of KH2PO4,

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reducing agents, and low molecule organic acids. These components have been proved to have

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significant impacts on the transformation of CeO2-NPs.9,12,19 We found that the reactivity of CeO2-NPs

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follows the order: R-CeO2-NPs > S-CeO2-NPs > O-CeO2-NPs > C-CeO2-NPs (see detailed results and

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discussion in SI, section 2, 2.5). The amount of CePO4 ((Figure S5) or Ce3+ (Figure S6) transformed

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from R-CeO2-NPs were higher than those for other CeO2-NPs.

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CeO2-NPs with predominant facets of {100}/{110} planes are considered to be more catalytically

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active than those with {111} planes.22 In this study, O-CeO2-NPs enclosed by eight {111} planes are

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stable (Figure S7). S-CeO2-NPs exposed {111}, {200}, and {220} (Figure S8); however, S-CeO2-NPs

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containing large amount of small particles have high surface area, thus also presenting high reactivity.

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C-CeO2-NPs exposed {100} or {111} planes (Figure S9); besides, the sizes of C-CeO2-NPs are larger

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than the other CeO2-NPs. This may account for the low reactivity and transformation of C-CeO2-NPs.

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R-CeO2-NPs were enclosed by six active planes, i.e. four {110} and two {100} (Figure S10), thus

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presenting the highest reactivity.

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Taken together, our results suggest that the shape of CeO2-NPs determine their intrinsic chemical

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reactivity, thus affecting their transformation and translocation in plants. Studies regarding the particle

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shapes should be also introduced to other NMs and systems. This is not only important for

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comprehensively understanding of the nano-bio interaction, but also for safer-design of NMs.

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Acknowledgement

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This work was financially supported by the National Natural Science Foundation of China (Grant No.

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11405183, 11375009, 11575208, 11675190, 21507153), Ministry of Science and Technology of China

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(Grant No. 2016YFA0201600).

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Supporting Information

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Supporting Information Available: Additional materials and methods information (CeO2-NPs syntheses,

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seedling culture and CeO2-NPs application, sample preparation for TEM, ICP-MS, and XANES

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analyses, transformation of CeO2-NPs in simulated solutions); complete results of characterization of

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CeO2-NPs (Figure S1 and Table S1); photos of agglomeration of CeO2-NPs in nutrient solutions (Figure

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S2); effects of CeO2-NPs on biomass production (Figure S3); translocation factors of CeO2-NPs in

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plants (Figure S4); XANES spectra of reference compounds (Figure S5); complete results of simulation 9



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studies (Figure S6 and S7); HRTEM of CeO2-NPs (Figure S8-S11). This material is available free of

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charge via the Internet at http://pubs.acs.org.

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Notes

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The authors declare no competing financial interest.

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References

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Figure Captions

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Figure 1. Ce contents in root and shoot of cucumber treated with CeO2-NPs.

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Figure 2. TEM images of root sections of cucumber treated with S-CeO2-NPs (A1-F1), O-CeO2-NPs

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(A2-F2), C-CeO2-NPs (A3-F3) and R-CeO2-NPs (A4-F4) for 7d (A and B), 14d (C and D), and 21d (E

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and F). Surface and inside indicate that the images were collected at surface and inside of the root

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sections, respectively. Red arrows indicate the CePO4 on the root surface or intracellular spaces.

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Figure 3. Ce LIII edge XANES spectra of root (A) and shoot (B). Vertical dotted line marked the feature

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of Ce(III) (a) and Ce(IV) (b and c), respectively. Percentage of Ce(IV) and Ce(III) species in root (C)

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and shoot (D) Ce contents. Calculated contents of Ce(III) and Ce(IV) in root (E) and shoot (F).

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TOC 40x39mm (600 x 600 DPI)



Figure 1. Ce contents in root and shoot of cucumber treated with CeO2-NPs. 51x33mm (600 x 600 DPI)


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Figure 2. TEM images of root sections of cucumber treated with S-CeO2-NPs (A1-F1), O-CeO2-NPs (A2-F2), C-CeO2-NPs (A3-F3) and R-CeO2-NPs (A4-F4) for 7d (A and B), 14d (C and D), and 21d (E and F). Surface and inside indicate that the images were collected at surface and inside of the root sections, respectively. Red arrows indicate the CePO4 on the root surface or intracellular spaces. 199x351mm (300 x 300 DPI)



Figure 3. Ce LIII edge XANES spectra of root (A) and shoot (B). Vertical dotted line marked the feature of Ce(III) (a) and Ce(IV) (b and c), respectively. Percentage of Ce(IV) and Ce(III) species in root (C) and shoot (D) Ce contents. Calculated contents of Ce(III) and Ce(IV) in root (E) and shoot (F). 182x245mm (600 x 600 DPI)


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Shape-Dependent Transformation and Translocation of Ceria

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