Synthesis of novel mesoporous carbon nanoparticles ...

Journal of Saudi Chemical Society (2018) xxx, xxx–xxx

King Saud University

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ORIGINAL ARTICLE

Synthesis of novel mesoporous carbon nanoparticles and their phytotoxicity to rice (Oryza sativa L.) Yi Hao a,1, Bolong Xu b,1, Chuanxin Ma c,d, Jianying Shang e, Wenqian Gu a, Wei Li a, Tianqi Hou a, Yuxi Xiang a, Weidong Cao f, Baoshan Xing c, Yukui Rui a,c,* a Beijing Key Laboratory of Farmland Soil Pollution Prevention and Remediation, College of Resources and Environmental Sciences, China Agricultural University, Beijing 100193, China b Beijing Key Laboratory of Bioprocess, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing Laboratory of Biomedical Materials, College of Life Science and Technology, Beijing University of Chemical Technology, Beijing 100029, China c Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA 01003, United States d Department of Analytical Chemistry, The Connecticut Agricultural Experiment Station, New Haven, CT 06504, United States e Department of Soil and Water Sciences, China Agricultural University, Beijing 100193, China f Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, Ministry of Agriculture Key Laboratory of Crop Nutrition and Fertilization, Beijing 100081, China

Received 18 January 2018; revised 30 April 2018; accepted 1 May 2018

KEYWORDS Phytotoxicity; Rice; Mesoporous carbon nanoparticles; Synthesis; Phytohormones

Abstract Despite the remarkable number of investigations on the potential risks of the engineered nanomaterials (ENMs) to terrestrial plants, there was limited knowledge regarding the effects of mesoporous carbon nanoparticles (MCNs) with different sizes on crops. The objective of our study was to evaluate the toxicity of MCNs to rice (Oryza sativa L.) seedlings. Two novel MCNs with different particle sizes (MCN1: 150 and MCN2: 80 nm) were synthesized using the high-temperature pyrolysis method and characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy, and Raman spectra. Phytotoxicity of two MCNs was then comparatively evaluated using rice as a model plant. The rice seedlings were hydroponically exposed to both MCN suspensions with concentrations of 0, 10, 50, 150 mg/L for 20 days. Exposure to 150 mg/L MCN1 resulted in more than 21% and 29% decrease in root length and in shoot length, respectively. MCN2 significantly reduced the root and shoot lengths by approximately 70% and 57% at the concentration of 150 mg/L. Additionally, the concentrations of three endogenous

* Corresponding author at: Beijing Key Laboratory of Farmland Soil Pollution Prevention and Remediation, College of Resources and Environmental Sciences, China Agricultural University, Beijing 100193, China. E-mail address: [email protected] (Y. Rui). 1 These authors contributed equally to this work. Peer review under responsibility of King Saud University.

Production and hosting by Elsevier https://doi.org/10.1016/j.jscs.2018.05.003 1319-6103 Ó 2018 King Saud University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Please cite this article in press as: Y. Hao et al., Synthesis of novel mesoporous carbon nanoparticles and their phytotoxicity to rice (Oryza sativa L.), Journal of Saudi Chemical Society (2018), https://doi.org/10.1016/j.jscs.2018.05.003

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Y. Hao et al. phytohormones, including brassinolide (BR), indole propionic acid (IPA), and dihydrozeatinriboside (DHZR) in plant shoots were increased significantly compared with the control. Our findings illustrated that size-effects of MCNs contributed greatly in causing phytotoxicity to plants, which should have drawn our attention to the use of these novel ENMs in agriculture given the evidence of their potential risks to crops. More importantly, this is the first study on assessment of the phytotoxicity of MCNs to rice seedlings from the perspective of plant hormones. Ó 2018 King Saud University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction Nanotechnology has been experiencing a rapid and unremitting development and has been applied in a variety of fields. Based on metal-organic frameworks (MOF), the processed metal coordination center of a metal (Fe, Zn, Co, etc) and nitrogen co-doped mesoporous carbon nanoparticles were extensively studied [1–5]. Due to their high surface area, stable mesoporous structure and excellent catalytic performance, mesoporous carbon nanomaterials (MCNs) derived from MOF have a wide range of applications, including catalysis, energy storage, drug delivery and photodynamic therapy [6]. In addition, the size is an important factor affecting the properties and activities of MOF-derived carbon nanomaterials. For example, Zhang et al. (2017) reported that Fe-doped carbon nanoparticles exhibited size-dependent oxygen reduction reaction (ORR) activity, and more active sites were found when the size decreased from 1000 nm to 50 nm, indicating Fe-doped carbon nanoparticles with small size could have higher catalytic performance [7]. Chen et al. (2017) found optimizing the size of MCNs could reduce threats to environmental health as large size of MCNs exhibited more pronounced cytotoxicity and significantly inhibited the biophysical potentials toward natural pulmonary surfactant [8]. With the developments of these applications, it will be inevitable that an increasing number of mesoporous carbon nanoparticles (MCNs) could be released into the environment via wastewater, accumulate in sewage sludge, and eventually enter into agricultural field. Previous investigations indicated that carbon nanomaterials could interact with soil, microorganisms and plants; subsequently affect plant growth and development by accumulating in plant tissues especially in the edible portion, which could consequently cause agricultural economic losses in terms of crop yield and quality and pose a potential threat to human health [9–12]. Therefore, the studies on MCNs induced toxicity to crop plants are warranted. Rice (Oryza sativa L.) is one of the most vital worldwide grain crops. Considering the huge planting scale and great significance in food security, recent studies have paid close attention to the potential toxicity of carbon materials on rice. For example, carbon nanotubes (CNTs) promoted root growth via regulating related genes and increasing global histone acetylation in the meristem zones of rice roots [13]. Singlewalled CNTs (SWCNTs) were mainly found in the intercellular space while multi-walled CNTs (MWCNTs) penetrated cell walls in rice roots [13]. Single-walled carbon nanohorns (SWCNHs) activated rice seed germination and promoted rice growth [14]. SWCNTs and MWCNTs could accelerate leaf growth and development of rice at the concentration of 20 mg/L, and chlorophyll content and net photosynthetic rate

promoted via up-regulating expression of related genes [15]. Additionally, single-walled and multi-walled CNTs could decrease abscisic acid content, increase gibberellin content, as well as antioxidant enzyme activities [16]. However, multiwalled CNTs inhibited rice seedling growth at high concentrations (1000 and 2000 mg/L) resulting from cell death and electrolyte leakage [17]. 100 and 200 mg/L graphene delayed rice seed germination, suppressed the growth of rice radicle and plumule, and inhibited morphogenesis of rice seedlings [18]. Single-walled CNTs decreased the number of hydrogen bonds between water and DNA nucleobases in rice [16]. Under soil cultivated conditions, MWCNTs, fullerene (C60), and reduced graphene oxide (rGO) inhibited rice growth after 30-day exposure via altering and antioxidant enzyme activities and changed the soil bacterial community compositions in rhizosphere [19]. CNTs filled with alloys inhibited the rice seedling growth by influencing the carbon nitrogen ratio and mainly located in intercellular space and cell wall [20]. Zhao et al assessed the MWCNT content in rice via 14C labeled MWCNTs and indicated that MWCNT content in rice root was more than 2fold that of the sheath, 10-fold that of rice leaf [21]. GO nanosheets inhibited root growth, cell wall synthesis and plant endogenous hormone contents result from specific structures and properties [22]. Taking all these investigations into account, rice was sensitive to the bio-stimulation of carbon nanomaterials, and could be considered as a proper model plant to assess the phytotoxicity of MCN in our present study. Phytohormones play crucial roles in plant growth and development, mediate many metabolic processes in vivo, and are sensitive to the environmental changes for adaption. Previous study indicated that nanomaterials had effects on plant hormones, such as indole-3-acetic acid (IAA), brassinolide (BR), indole propionic acid (IPA), abscisic acid (ABA) and dihydrozeatinriboside (DHZR), despite various exposure methods [20]. Additionally, exposure to 500 mg/kg C60, rGO, and MWCNTs greatly increased the contents of BR, IAA, and gibberellin acid 4 (GA4) in plant roots compared with the control [19]. The metal-based nanomaterials could also alter the level of phytohormones. For example, in the hydroponic system, 1000 mg/L CuO nanoparticles (NPs) suppressed the growth and development of transgenic and conventional cotton, altered IAA and ABA concentrations [23]. Similarly, 1000 mg/kg Fe2O3 NPs promoted the growth of peanut by down-regulating the level of ABA [12]. Taken together, the phytohormone content as affected by different types of nanomaterials has become a crucial index in the assessment of NMinduced phytotoxicity. In the present study, the hybrid rice line Y Liangyou 1928 was used as a test plant and separately exposed to different concentrations of metal, nitrogen co-doped MCNs with two

Please cite this article in press as: Y. Hao et al., Synthesis of novel mesoporous carbon nanoparticles and their phytotoxicity to rice (Oryza sativa L.), Journal of Saudi Chemical Society (2018), https://doi.org/10.1016/j.jscs.2018.05.003

Synthesis of mesoporous carbon nanoparticles and their phytotoxicity to rice sizes (MCN1: 150 nm; MCN2: 80 nm). MCN induced phytotoxicity to rice seedlings was comparatively determined by plant growth and hormone levels. To our best knowledge, this is the first investigation on the influences of MCNs with different sizes on rice seedlings in the perspective of phytohormones. By synthesizing, characterising and comparatively assessing the phytotoxicity of MCNs, our investigation sheds light on the property and potential environmental risk of MCNs to plants and points out the size-controlled strategy to reduce MCN phytotoxicity. 2. Methods

Table 1

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The Composition of 1/2 Kimura nutrient solution.

Composition

Concentration (mmol/L)

KNO3 Ca(NO)24H2O MnSO4H2O KH2PO4 (NH4)2SO4 MnSO4H2O H3BO3 (NH4)6Mo7O245H2O ZnSO47H2O CuSO45H2O Fe-EDTA

0.091 0.183 0.274 0.1 0.183 1 10 3 10 1 10 1 10 2 10 6 10

3 3 3 3 4 2

2.1. MCN synthesis and characterization 2-mehtylimidazole, Zn(NO3)26H2O and cetyltrimethylammonium bromide (CTAB) were purchased from Macklin and tetraethyl orthosilicate (TEOS) was purchased from Sigma. All reagents were analytical reagents (A.R.). A SiO2-protected calcination method was used to prepare MCNs, and the size of MCNs was controlled by adjusting the concentration of metal salt solution in methanol. During the synthesis, the molar ratio between 2-mehtylimidazole and Zn(NO3)26H2O was kept at 1:4.22 constantly. Specifically, ZIF-8 crystals with 150 nm and 80 nm were prepared by keeping the molar radio of methanol to zinc at 728:1 and 1400:1, respectively. In a typical synthesis for 150 nm MCNs, 32 mmol Zn(NO3)26H2O was dissolved in 500 mL methanol, and mixed with 500 mL methanol solution containing 135 mmol 2-mehtylimidazole. Then the mixture was stirred for 2 h at ambient temperature. The white precipitates were collected after ethanol washing and then dispersed in 240 mL 10% methanol solution, and the pH of the suspension was adjusted to 11 by sodium hydroxide aqueous followed by adding 0.2 g CTAB. 1.2 mL TEOS was added to the suspension, which was stirred at 25 °C for a half hour. Afterward, ZIF-8@SiO2 was obtained by adequate washing with ethanol and dried at 60 °C followed by carbonized at 800 °C for 2 h under N2 atmosphere. Finally, the pyrolyzed samples were soaked into 3 mol/L NaOH aqueous phase to remove SiO2 shell. The morphology and size of MCNs were determined by transmission electron microscope (TEM, JEM-2100, JEOL, Japan) prior to use in plant experiments. The synthesized MCNs were dispersed in ethanol using a sonicator (KQ3200DE, Shumei, China), then the suspension was mounted onto a Cu grids and observed at 200 kV. The elemental surface compositions of MCNs were analyzed by a nonmonochromatized Al-Ka X-ray source (hm = 1486.6 eV) by an X-ray photoelectron spectrometer (XPS, ESCALab220iXL, Thermo Scientific, USA). The Raman spectra were taken on Renishaw at 514 nm excitation in Via-reflex spectrometer system. Sample powders were prepared on a standard microscope glass slide, and the excitation power of all samples remained constant at 150 lW.

tivator (DRP-9052, Peiyin, China) in dark at 25 °C for 5 days. Then, uniform sized seedlings were transplanted into 50 mL centrifuge tubes filled with 45 mL 1/2 strength Kimura nutrient solution (pH 6.5; Table 1). At the 5th day of transplanting, different concentrations (0, 10, 50, and 150 mg/L) of MCN1 and MCN2 were separately suspended in the 1/2 strength Kimura nutrient solution at 25 °C for 30 min using an ultrasonic bath sonicator (KQ3200DE, Shumei, China). Afterward, the prepared MCN suspensions were used to replace the old plant growth solution for MCN exposure study. All the NM suspensions were well stirred using a glass rod at a 12-hour interval during the exposure. The rice seedlings were cultivated in the greenhouse of China Agricultural University [23]. 2.3. Measurements of plant physiological parameter After 20 days of exposure to MCNs, the rice seedlings were harvested, rinsed with tap water and deionized water for three times, respectively. Root length was determined from the root base to the root tip. And shoot length was measured from the leaf base to the leaf tip. The fresh weight of rice was then measured using analytical balance [24]. 2.4. Phytohormone analysis The concentration of IPA, BR and DHZR in rice was determined using the enzyme-linked immunosorbent assay (ELISA) methods as described by previous reports [25,26]. 2.5. Data analysis All the data were statistically analyzed by SPSS 19.0 (one-way analysis of variance and Duncan’s test), provided as mean ± standard deviation of triplicate samples, and shown significant difference among treatments when the P value was less than 0.05. 3. Results 3.1. Characterization of MCNs

2.2. Hydroponic plant cultivation Rice seeds were firstly sterilized by 5% H2O2 for 30 min, and rinsed with deionized water for three times. Then the sterilized seeds were placed on culture dishes with moist filter paper (100 mm 15 mm), and germinated in a constant temperature cul-

To reveal size-dependent phytotoxicity of mesoporous carbon nanoparticles, MCNs with two different sizes were prepared and characterized using TEM and X-ray photoelectron spectroscopy (XPS). As shown in Fig. 1, the morphology of MCN1 and MCN2 is an irregular polygon, and the diameter


4 was approximately 150 nm and 80 nm, respectively. Both MCN1 and MCN2 tended to aggregate. The XPS spectrum identified that the main elements of MCN1 and MCN2 were C, N, O, and Zn, whose atom ratios were 69.91%, 19.59%, 7.7%, 0.97% and 69.15%, 14.84%, 12.43%, 2.39% for MCN1 and MCN2, respectively. For Raman spectra (Fig. 1E and F), two typical peaks are observed at 1352 and 1585 cm 1, indicating the existence of disordered sp3 carbon structure (D-band) and graphite sp2 carbon structure (G-band), respectively.

Y. Hao et al. 3.2. Effects of MCNs on rice root and shoot length After 20-day treatment with two types of MNCs, the growth of rice roots was significantly suppressed by 50 and 150 mg/L MCNs while the treatment of 10 mg/L MCNs had no impact in terms of root length (Fig. 2A, C and D). Exposure to 50 and 150 mg/L MCN1 resulted in more than 26 and 21% reduction in root length, respectively, as compared to the control. Similar to MCN1, 10 mg/L MCN2 did not inhibit root development, with the concentration increasing to 50 and 150 mg/L,

Fig. 1 TEM images and XPS spectrum of MCN1 and MCN2. Scale bar in Figures A and B represent 100 nm, XPS survey and corresponding element peak assignments including Zn2p, N1s, O1s, C1s for MCN1 (C) and MCN2 (D), and Raman spectra of MCN1 (E) and MCN2 (F).


Synthesis of mesoporous carbon nanoparticles and their phytotoxicity to rice approximately 70% reduction was evident as compared to the control. Regarding shoot length, MCN1 and MCN2 had no obvious influence on rice shoot length at the concentration of 10 mg/L, while severe inhibition was evident at the other two concentrations (Fig. 2B). Exposure to 50 and 150 mg/L MCN1 reduced the rice shoot length by approximately 35.9% and 28.9%, respectively, relative to the control, while 56.1% and 57.1% more reduction of MCN2 treated shoot length were found at the equivalent concentrations of MCN1. 3.3. Effects of MCNs on rice fresh weight Both sized MCNs caused notable impacts on rice seedlings over 20-day exposure. Growth inhibition was evident at the concentration of 50 and 150 mg/L. In Fig. 3A, the root fresh weight treated with 50 and 150 mg/L MCN1 was reduced by approximately 18% and 24%, respectively, as compared to the control. Similarly, more than 34% and 33% reduction of root fresh weight were observed upon exposure to 50 and 150 mg/L MCN2, respectively. While no obvious change was found at 10 mg/L of both MCN1 and MCN2.

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Regarding the effects of both MCNs on the shoot fresh weight, MCN1 and MCN2 had no impact on shoot weight at the relative low concentration (10 mg/L). MCN1 significantly reduced fresh weight by approximately 18% and 17% upon exposure to the concentration of 50 and 150 mg/L, respectively (Fig. 3B). Similarly, more than 45% and 41% decrease in shoot weight were found in the MCN2 treatments with the concentration of 50 and 150 mg/L. Exposure to MCN2 significantly reduced the seedling length and lowered the fresh weight as compared to the respective one in the MCN2 treatment, suggesting that MCN2 performed more toxicity to rice than MCN1. 3.4. Effects of MCNs on phytohormone concentrations in rice seedlings In Fig. 4A, the contents of three major phytohormones, including IPA, BR and DHZR, were regulated by the two MCNs after 20-day exposure. In rice shoots, the concentrations of IPA and DHZR were significantly suppressed by MCN1 at the concentration of 50 and 150 mg/L, while BR was only inhibited upon exposure to 150 mg/L MCN1.

Fig. 2 Rice root length (A), shoot length (B) and phenotypic images of rice seedlings (C and D) treated with MCN1 and MCN2, respectively. Data are averaged of three replicates and the error bars represent standard errors. Bars with asterisk are statistically different at p < 0.05, when compared to the control. Significant difference (p < 0. 05) between different MCNs in same concentrations was marked with asterisk above the line segment.


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Fig. 3 Fresh weight of rice roots (A) and shoots (B) treated with different concentrations of MCN1 and MCN2. Data are averaged of three replicates and the error bars represent standard errors. Bars with asterisk are statistically different at p < 0.05, when compared to the control. Significant difference (p < 0. 05) between different MCNs in same concentrations was marked with asterisk above the line segment.

Additionally, 10 mg/L MCN1 had no impact on the levels of three phytohormones in comparison with the control. In the treatments with MCN2 (Fig. 4B), the contents of IPA and DHZR were significantly reduced upon exposure to MCN2 at the concentration of 50 and 150 mg/L. While only 150 mg/L MCN2 inhibited the BR level in rice shoots. Similar to the MCN1 treatments, low exposure dose of MCN2 had no impact on the level of phytohormones. 4. Discussion BR is an important plant hormone, which mediates plant growth, development and stress tolerance such as low temperature stress, drought stress, salt stress and virus infection [27– 29]. IPA also played key roles in a series of metabolic processes in plants [30]. DHZR could stimulate the cellular division, regulate the formation of bands and stem and promote the leaf elongation [31]. All these three endogenous hormones positively regulated the growth and development of plants. In our study, the negative effect of MCN1 and MCN2 on fresh weight was significant at the concentration of 50 and 150 mg/L. Aligning with our findings on fresh weight, both MCNs also greatly decreased the levels of phytohormones at the same

Y. Hao et al.

Fig. 4 Contents of IPA, BR and DHZR in rice shoots treated with MCN1 (A) and MCN2 (B). Data are averaged of three replicates and error bars represent standard error. Bars with asterisk are statistically different at p < 0.05, when compared to the control.

exposure doses. Our previous reports also demonstrated that MWCNTs, C60, and rGO could inhibit rice seedling growth, affect plant metabolism processes, and further induce phytotoxicity via compromising the balance of the levels of phytohormones, including BR, jasmonic acid (JA), and ABA [19,20]. Zhang et al. (2017) pointed out that excessive amounts of reactive oxygen species (ROS) induced by nanomaterial exposure resulted in the changes of phytohormones, as the ROS played vital roles in hormone perception and transduction [15,32]. Syu et al. (2014) reported that NP exposure could manipulate the expression of genes, which involved in hormone signaling regulation, and subsequently alter the signaling transduction, and eventually imbalance the levels of the phytohormones [33]. Physiological responses of plants to CNM-exposure varies or are even opposite upon exposure to different doses. MWCNTs at the concentration of 10 and 50 mg/L significantly enhanced rice root growth, while 300 mg/L MWCNTs markedly inhibited rice growing by altering the carbon nitrogen ratio and plant hormone concentrations [20]. In addition, our previous studies also suggested that well controlled exposure dose of NMs increased crop yield and had the potential


Synthesis of mesoporous carbon nanoparticles and their phytotoxicity to rice to be a novel efficient fertilizer [12]. Regarding the different sizes of MCNs, 10 mg/L MCNs had no clear effect on rice physiologically, and might be considered as a relative safe exposure concentration in a life cycle study in our next-step study. Size effects also greatly contributed to plant growth inhibition. In the present study, MCNs with smaller diameter showed a more negative influence on rice biomass and phytohormone levels. It was reported that the structural properties of NMs, especially the particle size, were the primary factors in NM induced phytotoxicity [17,34]. NMs could be selectively absorbed, translocated and accumulated in specific plant tissues and cellular substructures, consequently resulted in various growing effects. For example, soybean roots took up various sizes of multiple-walled CNTs, but only the one with 50–100 nm in length were translocated to the aerial parts [35]. MWCNTs with the length less than 100 nm tended to accumulate in the nucleus, plastids, and vacuoles [36]. Similar results were also evident in metal-based NPs. Ag NPs with the diameter of 20–50 nm showed more substantial upward transport ability in crop species compared to bulk Ag (1–3 lm in diameter), and exhibited more toxicity to plants [37]. The toxicity of ZnO NPs to Chinese cabbage was significantly determined by particle sizes as measured by the Zn accumulation and generate excessive amounts of free hydroxyl groups (OH) in plants shoots and roots [38]. Approximately 47% increases in the Ce accumulation in 1000 mg/kg nano-CeO2 treated kidney bean roots was found as compared to the one treated with the equivalent concentration of bulk-CeO2 [39]. Unlike the element accumulation and ion release of metalbased NMs, carbon-based NMs with different sizes induced phytotoxicity mainly as a result of the effects on NM movement along with the route of membranes, xylem, phloem and finally into different plant tissues and cell cytoplasm, further inducing different plant physiological effects [35]. Considering the more obvious phytotoxicity of MCN with smaller size in our study, MCN with the diameter of 150 nm was relatively safe to plant growth. More importantly, our study suggested that the precise size control of CNMs could effectively reduce the environmental risk in premise of attaining desired material properties. Besides the toxicity assessment of NMs on plant, the application of NMs in agriculture field has also been conducted in recent years. For example, Sun et al. (2018) reported glutathione responsive mesoporous silicon nanoparticles could be used as an efficient tool to deliver ABA into plants [40]. MWCNTs significantly improved the antifungal effects of rose while had no obvious side effect on cut flower preservation [11]. Once exposed to NM, the interaction of plants with NMs will be inevitable; as a result, NM uptake, translocation, and accumulation in plants need to be fully characterized. Upon NM root exposure, most of NMs were tightly absorbed in root hair, while a portion could move along with symplastic or apoplastic pathways, and subsequently accumulate in root cell intercellular spaces and cell membranes [20], and eventually transport by xylem and phloem to the aboveground part of plants [35,41]. As for NM exposure via foliage spray, NMs mainly existed in leaf surface [42], internalized in guard cells associated with epidermal cells and the sub-stomatal chamber [40,42]. The aggregates of NMs were also observed in various leaf tissues such as epidermis, parenchyma and vascular bundles [42]. Considering the increasing demands for NM-applications in agriculture, the risk of NM bioaccumula-

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tion along the food chains should be well controlled and comprehensively assessed. 5. Conclusions Taken together, our findings suggest that exposure to 50 and 150 mg/L MCNs negatively affect rice growth via altering the levels of vital phytohormones and nanomaterial induced toxicity was also particle size-dependent. These results illustrate the potential risk of MCNs on crops, and contribute greatly in applications of nanomaterials or nanomaterial incorporated products in agriculture. Acknowledgments The project was supported by National Key R&D Program of China (SQ2017YFNC060064), National Key R&D Program of China (2017YFD0801300), the NSFC-Guangdong Joint Fund (U1401234), the National Natural Science Foundation of China (No. 41371471) and the Key National Natural Science Foundation of China (No. 41130526). Author Contributions Y.H and BL.X contributed equally to this work. Y.R, W.C and BS.X conceived the experiments. Y.H and Y.R designed the research. Y.H, W.G, W.L, Y.X and T.H performed the experiments and analyzed the data. Y.H, W.G, C.M, J.S, and Y.R wrote the manuscript. C.M. and BS.X revised the manuscript. All authors have read and approved the final manuscript. References [1] M. Hu, J. Reboul, S. Furukawa, L. Radhakrishnan, Y. Zhang, P. Srinivasu, H. Iwai, H. Wang, Y. Nemoto, N. Suzuki, Direct synthesis of nanoporous carbon nitride fibers using Al-based porous coordination polymers (Al-PCPs), Chem. Commun. 47 (2011) 8124–8126. [2] L. Radhakrishnan, J. Reboul, S. Furukawa, P. Srinivasu, S. Kitagawa, Y. Yamauchi, Preparation of microporous carbon fibers through carbonization of Al-based porous coordination polymer (Al-PCP) with furfuryl alcohol, Chem. Mater. 23 (2011) 1225–1231. [3] W. Chaikittisilp, N.L. Torad, C. Li, M. Imura, N. Suzuki, S. Ishihara, K. Ariga, Y. Yamauchi, Synthesis of nanoporous carbon-cobalt-oxide hybrid electrocatalysts by thermal conversion of metal-organic frameworks, Chem. – A Eur. J. 20 (2014) 4217–4221. [4] J. Tang, Y. Yamauchi, Carbon materials: MOF morphologies in control, Nat. Chem. 8 (2016) 638–639. [5] R.R. Salunkhe, C. Young, J. Tang, T. Takei, Y. Ide, N. Kobayashi, Y. Yamauchi, Correction: a high-performance supercapacitor cell based on ZIF-8-derived nanoporous carbon using an organic electrolyte, Chem. Commun. 52 (2016) 4764– 4767. [6] S. Wang, L. Shang, L. Li, Y. Yu, C. Chi, K. Wang, J. Zhang, R. Shi, H. Shen, G.I. Waterhouse, Metal-organic-frameworkderived mesoporous carbon nanospheres containing porphyrin-like metal centers for conformal phototherapy, Adv. Mater. 28 (2016) 8379–8387. [7] H. Zhang, S. Hwang, M. Wang, Z. Feng, S. Karakalos, L. Luo, Z. Qiao, X. Xie, C. Wang, D. Su, Single atomic iron catalysts for


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