Elsevier

Journal of Hazardous Materials

Volume 264, 15 January 2014, Pages 98-106
Journal of Hazardous Materials

Foliar exposure of the crop Lactuca sativa to silver nanoparticles: Evidence for internalization and changes in Ag speciation

https://doi.org/10.1016/j.jhazmat.2013.10.053Get rights and content

Highlights

  • Ag-NPs are internalized inside lettuce leaves after foliar exposure to a suspension of Ag-NPs.

  • A classical washing process is inefficient at decreasing significantly Ag content.

  • Ag-NPs in plants undergo oxidation, and resulting ionic Ag are complexed with organic compounds including thiol-containing molecules.

  • Foliar exposure to Ag-NPs does not lead to detectable phytotoxicity symptoms.

Abstract

The impact of engineered nanomaterials on plants, which act as a major point of entry of contaminants into trophic chains, is little documented. The foliar pathway is even less known than the soil-root pathway. However, significant inputs of nanoparticles (NPs) on plant foliage may be expected due to deposition of atmospheric particles or application of NP-containing pesticides. The uptake of Ag-NPs in the crop species Lactuca sativa after foliar exposure and their possible biotransformation and phytotoxic effects were studied. In addition to chemical analyses and ecotoxicological tests, micro X-ray fluorescence, micro X-ray absorption spectroscopy, time of flight secondary ion mass spectrometry and electron microscopy were used to localize and determine the speciation of Ag at sub-micrometer resolution. Although no sign of phytotoxicity was observed, Ag was effectively trapped on lettuce leaves and a thorough washing did not decrease Ag content significantly. We provide first evidence for the entrapment of Ag-NPs by the cuticle and penetration in the leaf tissue through stomata, for the diffusion of Ag in leaf tissues, and oxidation of Ag-NPs and complexation of Ag+ by thiol-containing molecules. Such type of information is crucial for better assessing the risk associated to Ag-NP containing products.

Introduction

In the last decade, nanotechnology has emerged as a major field of research and investments for industries. Thanks to their unique properties, NPs are included in more and more consumer products. 313 consumer products containing Ag-NPs have been listed over the 1317 products of the database in 2011 [1]. Ag-NPs are mainly used as an anti-bacterial agent. Major products include cosmetics, paints, fabrics, food containers, medical equipment and pesticides. The production of Ag-NPs is estimated at 500 tons per year worldwide [2]. Eventually, most NPs will end up in the environment. Current knowledge on the fate of Ag-NPs in the environment has been recently reviewed [2] and their potential toxicological and ecotoxicological impact raises much concern [3], [4].

Plants play a critical role in the fate of NPs in the environment through their uptake, bioaccumulation and transfer to trophic chains [5]. Crop plants are more likely to be exposed to NPs than wild plants due to the application of sewage sludge on agricultural soils and use of Ag-NPs in plant protection products. After root exposure, uptake of Ag-NPs by roots was shown [6], [7], [8], as well as phytotoxic effects: reduced seed germination, perturbed root and shoot growth, and decreased evapotranspiration [6], [7], [9], [10], [11], [12]. Genotoxic effects were evidenced as well [7], [13], [14], [15]. The foliar exposure pathway has been much less investigated, although such information is necessary for a comprehensive risk assessment of nanomaterials. Indeed, NPs have been used in plant protection products for several years so far [16], [17]. 110 products containing Ag-NPs were registered as pesticides by the US-EPA in 2010 [16], and several patents for Ag-NP fungicides were deposited [18]. The use of such products is expected to expand in the future [18], [19], [20], [21]. Moreover, new applications of NPs such as controlled release of agrochemicals for nutrition and protection against pests and pathogens, delivery of genetic material and sensitive detection of plant disease are currently under investigation [19], [20]. Although these developments may be beneficial for the environment (decreased inputs), there is an urgent need of better evaluating the risks associated to these products [20], [22]. To the best of our knowledge, published studies on foliar exposure to NPs concerned TiO2-NPs [23], CeO2-NPs [24] and polystyrene-NPs [25], but none was realized on Ag-NPs.

The purpose of this study was to investigate the impact of a foliar exposure to Ag-NPs on a leafy crop plant in terms of phytotoxicity, to determine the proportion of Ag retained on/in the leaves after thorough washing, and also to get insights on the mechanisms of foliar uptake and possible transformations of Ag-NPs.

Lettuce (Lactuca sativa) was chosen as model species because of its widespread occurrence in kitchen gardens or farmlands and large foliar surface making it an ideal model to study the foliar transfer of atmospheric contaminants [26], [27]. Plants were exposed to 1–100 μg Ag-NPs per g fresh weight (FW) and to Ag+ ions. This latter treatment aimed at evaluating the ionic contribution resulting from the dissolution of Ag-NPs. Total Ag accumulation in shoots was measured before and after rinsing with slightly acidic water (as performed classically by consumers) [26], [27]. Impact of Ag-NP exposure on plant biomass, chlorophyll, protein, total glutathione and phytochelatin (PC) contents and oxidative stress were evaluated. We combined scanning electron microscopy observation of whole leaves and micro X-ray fluorescence (μXRF) mapping of leaf cryo-sections to localize Ag-NPs and their weathering products in leaf tissues. Changes in Ag-NP speciation were evaluated using Ag LIII-edge X-ray absorption near edge structure spectroscopy (XANES) and time of flight-secondary ion mass spectrometry (ToF-SIMS).

Section snippets

Nanoparticle characterization and dispersion

Uncoated Ag-NPs were provided by the paint company PPG (Pittsburgh, Pennsylvania, USA). These NPs were fully characterized: shape, nominal diameter (transmission electron microscopy TEM – TECNAI OSIRIS) and specific surface area (BET). Before each exposure, stock suspensions were prepared with NPs dispersed in ultrapure water (10, 100, 1000 mg L−1) and homogenized 3 min in an ultrasonic bath. NP hydrodynamic diameter was assessed by dynamic light scattering using a NanoZS (Malvern) and is

Nanoparticle characterization

Ag-NP physico-chemical characteristics are summarized in Figure S1 (supporting information). Briefly, Ag-NPs were mainly round shaped (68%) with an average nominal diameter of 38.6 nm. Another fraction (32%) of these NPs was elongated with dimensions of 38.2 nm × 57.8 nm (Figure S1A and B). The average specific surface area measured by BET was 4.1 m2 g−1.

When in suspension, Ag-NPs were well dispersed with a hydrodynamic diameter close to nominal diameter: 47.9 ± 29.2 nm (Figure S1A and C). Ag-NP

Discussion

If the internalization of Ag-NPs has been demonstrated after root [6], [7], [8], [36], [37] and fruit [38] exposure, nothing is known on their fate after foliar exposure. In the present study, Ag was found as micrometer-scale agglomerates on the surface of leaves, and in leaf tissues including epidermis, mesophyll and vascular tissues. Thus, Ag-NPs deposited on the surface were able to cross the epidermis and eventually to be transferred to the leaf tissue. The low solubility of Ag-NPs used in

Conclusion

This study gives first insights on the localization and speciation of Ag-NPs in a crop plant after foliar exposure. Results show that Ag-NPs were transferred in all types of tissues, and suggest that both stomatal and cuticular pathways were used. A washing procedure with slightly acidified water was not efficient at removing significant amount of contamination. Thus, Ag-NPs applied on crops may potentially be transferred to humans. Bioaccumulated Ag-NPs underwent dissolution and complexation

Acknowledgements

The authors would like to thank the FP7 of the European Union for the funding (Nanohouse project no. 247810) and French program LABEX Serenade (11-LABX-0064). TEM analyses were obtained with the TEM OSIRIS, Plateform Nano-Safety, CEA-Grenoble funded by the Agence Nationale de la Recherche, program ‘Investissements d’Avenir’, reference ANR-10-EQPX-39, and operated by François Saint Antonin. Synchrotron experiments were performed on the ID21 beamline at the European Synchrotron Radiation Facility

References (54)

  • M.M. Bradford et al.

    Sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein–dye binding

    Analytical Biochemistry

    (1976)
  • M. Dazy et al.

    Ecological recovery of vegetation on a coke-factory soil: role of plant antioxidant enzymes and possible implications in site restoration

    Chemosphere

    (2008)
  • V.A. Sole et al.

    A multiplatform code for the analysis of energy-dispersive X-ray fluorescence spectra

    Spectrochimica Acta Part B – Atomic Spectroscopy

    (2007)
  • G. Sarret et al.

    Use of synchrotron-based techniques to elucidate metal uptake and metabolism in plants

  • C. Larue et al.

    Accumulation, translocation and impact of TiO2 nanoparticles in wheat (Triticum aestivum spp.): influence of diameter and crystal phase

    Science of the Total Environment

    (2012)
  • C. Larue et al.

    Quantitative evaluation of multi-walled carbon nanotube uptake in wheat and rapeseed

    Journal of Hazardous Materials

    (2012)
  • K.B. Narayanan et al.

    Green synthesis of biogenic metal nanoparticles by terrestrial and aquatic phototrophic and heterotrophic eukaryotes and biocompatible agents

    Advances in Colloid and Interface Science

    (2011)
  • The project on emerging nanotechnologies,...
  • S.-j. Yu et al.

    Silver nanoparticles in the environment

    Environmental Science: Processes & Impacts

    (2013)
  • S.W.P. Wijnhoven et al.

    Nano-silver – a review of available data and knowledge gaps in human and environmental risk assessment

    Nanotoxicology

    (2009)
  • T. Faunce et al.

    Nanosilver and global public health: international regulatory issues

    Nanomedicine

    (2010)
  • J.D. Judy et al.

    Evidence for biomagnification of gold nanoparticles within a terrestrial food chain

    Environmental Science & Technology

    (2011)
  • L.Y. Yin et al.

    More than the ions: the effects of silver nanoparticles on Lolium multiflorum

    Environmental Science & Technology

    (2011)
  • Y.S. El-Temsah et al.

    Impact of Fe and Ag nanoparticles on seed germination and differences in bioavailability during exposure in aqueous suspension and soil

    Environmental Toxicology

    (2012)
  • J. Hawthorne et al.

    Accumulation and phytotoxicity of engineered nanoparticles to Cucurbita pepo

    International Journal of Phytoremediation

    (2012)
  • C. Musante et al.

    Toxicity of silver and copper to Cucurbita pepo: differential effects of nano and bulk-size particles

    Environmental Toxicology

    (2010)
  • A. Ravindran et al.

    Bovine serum albumin mediated decrease in silver nanoparticle phytotoxicity: root elongation and seed germination assay

    Toxicological & Environmental Chemistry

    (2011)
  • Cited by (0)

    View full text