Metallic Nanoparticle (TiO2 and Fe3O4) Application Modifies

Sajad Hussain , Nasir Iqbal , Marian Brestic , Muhammad Ali Raza , Ting Pang , Derald Ray Langham , Muhammad Ehsan Safdar , Shoaib Ahmed , Bingxiao ...
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Journal of Agricultural and Food Chemistry

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Metallic Nanoparticles (TiO2 and Fe3O4) Application Modify Rhizosphere Phosphorus

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Availability and Uptake by Lactuca sativa

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Zahra Zahra1, Muhammad Arshad1,*, Rafia Rafique1, Arshad Mahmood2, Amir Habib3,

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Ishtiaq A. Qazi1, Saud A. Khan1

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Engineering, National University of Sciences and Technology, Sector H-12, Islamabad, 44000,

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Pakistan

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National Institute of Laser and Optronics, Nilore, Islamabad, 45650, Pakistan

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School of Chemical and Materials Engineering, National University of Sciences and

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Institute of Environmental Sciences and Engineering, School of Civil and Environmental

Technology, Sector H-12, Islamabad, 44000, Pakistan

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ABSTRACT

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Application of engineered nanoparticles (NPs) with respect to nutrient uptake in plants is

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not yet well understood. The impacts of TiO2 and Fe3O4 NPs on the availability of naturally soil

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bound inorganic phosphorus (Pi) to plants were studied along with relevant parameters. For this

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purpose, Lactuca sativa (Lettuce) was cultivated on the soil amended with TiO2 and Fe3O4 (0,

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50, 100, 150, 200 and 250 mg kg-1) over a period of 90 days. Different techniques like SEM,

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EDX, Raman and FTIR were used to monitor translocation and understand the possible

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mechanisms for phosphorus (P) uptake. The trends for P accumulation were different for roots

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(TiO2>Fe3O4>Control) and shoots (Fe3O4>TiO2>Control). Cystine and Methionine were detected

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in the rhizosphere in Raman spectra. NPs’ affinities to adsorb phosphate ions, modifications in P

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speciation and NPs stress in the rhizosphere had possibly contributed to enhanced root exudation

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and acidification. All these changes led to improved P availability and uptake by the plants.

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These promising results can help to develop an innovative strategy for using NPs for improved

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nutrient management to ensure food security.

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Keywords: Titania, Magnetite, Nanoparticles, Phosphorus Phytoavailability, Lactuca sativa

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Journal of Agricultural and Food Chemistry

INTRODUCTION

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All macro– and micro–nutrients have their own importance and P is one of the key life-

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supporting elements in all the living organisms. It is an essential constituent of adenosine

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triphosphate (ATP), deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). It also facilitates

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phospholipids in forming cell membranes. The P deficiency can affect different plant functions,

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seed development, root structure and ultimately the crop yield.1 Therefore, P is considered to be

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the yield limiting factor in numerous soils.

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The largest challenge in agricultural management of P is its low solubility.2 In soils, high

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concentrations of organic and inorganic phosphates are present, of which about 88 to 99% of

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total inorganic phosphorus (Pi) is bound by Ca+2 and thus unavailable to plants. About 30% of

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the world’s agricultural land needs P fertilizers for good crop production.3 The phosphate

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fertilizers form insoluble complexes in soil and the bioavailability of applied P tends to be less

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due to the immobilization.4 In calcareous and alkaline soils, the added soluble P also gets fixed

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resulting in low crop productivity. So there is growing interest in developing methods to improve

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the accessibility of naturally bound P in soils which could save natural/rock P resources for

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sustainable production of crops. Enhanced root exudates production is an adaptive response to

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acquire poorly mobile soil resources,5 particularly P.

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In the recent years, researchers have incorporated nanotechnology in agriculture and

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worked on the effects of plant growth and possible mechanisms.6,7,8 For the present study, TiO2

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and Fe3O4 NPs were selected for application in soil to grow Lactuca sativa. It is one of the green

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leafy vegetables that are cultivated worldwide. It is a source of mineral nutrients that are

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important for human health and nutrition. It is consumed in raw form due to its high nutritive

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value (i.e., vitamin, fiber, and mineral input to a diet), good taste and low price.9 Moreover, it is

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among the most consumed vegetables, accounting with a mean consumption of 23.4 g per day in

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Europe, which is about 7.2% of the total nutritional intake of vegetables.10 In 2011, its global

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production was about 25 million tons.11

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Among the engineered nanomaterials, TiO2 is the most widely used metal oxide NPs with

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up to 10,000 tons of global production per year.12 In different studies, the effects of TiO2 and

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Fe3O4 NPs have been examined in various plant species

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to nutrient uptake is limited. Both positive and negative effects have been reported upon

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application to higher plants.16 In the soil, microbial ecosystem can also be impacted by the NPs.

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In a study, Fe2O3 and Fe3O4 were reported to have beneficial impacts on the soil bacteria

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community at certain concentrations.17 Ge et al. 18 treated a grassland soil with 0, 500, 1000 and

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2000 mg TiO2 NPs kg-1 soil in microcosms over 60 days. They reported that metal oxide NPs can

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negatively affect the soil bacterial communities. However, these values are very high as

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compared to the treatments used in the present study. There exists a need to explore ENPs effects

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on the rhizophere microbiome

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soils.

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13,14,15

but the information with respect

and their possible role in nutrient management in agricultural

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In plant nutrition, NPs are not often considered as a factor affecting phytoavailable P.

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However a recent study reported increased accumulation of P (3686 mg kg-1) in roots of wheat

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at 250 mg kg-1 of CeO2 NPs treatment as compared to the control (2417 mg kg-1) while CeO2

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NPs at 500 mg kg-1 improved plant growth, shoot biomass and grain yield by 9.0, 12.7, and

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36.6%, respectively.20 In another work, Al2O3 NPs bound P was reported to enhance P uptake by

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plants in aqueous solutions at very low concentration of even 10 mg L–1.21 Pradhan et al.22

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studied the effects of Mn NPs on nitrogen uptake, assimilation, and metabolism in mung bean

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plants. Majority of these studies are short period assays up to two weeks and in rare cases up to

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Journal of Agricultural and Food Chemistry

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months. In this context, the objectives of present study were; 1) Effects of TiO2 and Fe3O4 NPs

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application on phytoavailability of P to Lactuca sativa and plant growth, 2) Rhizosphere changes

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in response to NPs applications in soil and, 3) Localization of metallic NPs in plants. To the best

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of our knowledge, this is the first report on P availability trends over an extended contact period

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(90 days) with different kinds of NPs under soil conditions.

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MATERIALS AND METHODS

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Synthesis of TiO2 NPs by Sol-Gel Method

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TiO2 NPs were synthesized using a slightly modified sol–gel method that was developed

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earlier for synthesis of molecularly imprinted Titania.23 Titania precursor was added in 0.5 M

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acidic solution. As the TiCl4 was added, the color of the solution turned yellowish. When the

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solution became transparent, it was neutralized with 0.5M NH4OH till pH became 7.0 followed

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by stirring until the gel like network formed. The gel was allowed to settle. The supernatant was

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discarded and rest of the sol was dried in vacuum–oven at 105 ºC. Dry gel was ground and

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calcined at 400 ºC for 6 h.

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Synthesis of Fe3O4 NPs by Co-precipitation Method

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For preparation of Fe3O4 NPs, FeCl2 (0.1 M) was mixed with FeCl3 (0.2 M) and NaOH

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(0.1 M) was added to the mixture. The solution turned black and placed on heating till the slurry

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rest behind. It was washed several times with distilled water until the pH 7.0 achieved. The

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slurry was dried in an oven at 80 ºC. The dried magnetite clusters were ground to get NPs.24

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Characterization of Synthesized NPs

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The phase composition, crystal structure and crystallite size measurements for the TiO2

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and Fe3O4 NPs were performed by X-ray Diffractometer (XRD, Theta-Theta STOE, Germany) at

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40 keV and 40 mA. Processing of XRD results were done with X'Pert High Score software

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package (PANalytical B.V. Almelo, Netherland). The surface morphology of TiO2 and Fe3O4

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NPs was analyzed using Scanning Electron Microscopy (SEM, Jeol, JSM 6490 A, Japan)

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equipped with Energy Dispersive X-ray Spectroscopy (EDX, Jeol, JED 2300) and ion sputtering

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device (Jeol, JFC 1500). Suspensions of TiO2 and Fe3O4 NPs in ethanol were made-up on quartz

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slides. Sputtering technique was used to evaporate the volatile substances or moisture and

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stabilize the NPs on the substrate to avoid interference or preventing contamination. The

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technique can be used for several conductive materials.25 Then the slides were observed under

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SEM.

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Soil Preparation

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For the present study, sandy-loam soil was selected. The air-dried soil was ground by ball

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mill and passed through mechanical sieve (