Encyclopedia
Please note this is a comparison between Version 1 by Martin Šebesta and Version 3 by Jason Zhu.
Engineered nanoparticles (ENPs) have potential application in precision farming and sustainable agriculture. Studies have shown that ENPs enhance the efficiency of the delivery of agrochemicals and thus, have the potential to positively affect the environment, thereby improving the growth and health of the crops.
- Engineered nanoparticles
- ZnO
- TiO2
- Field Application
1. Introduction
Nanotechnology plays an increasingly important role in most areas of human activity. Engineered nanoparticles (ENPs) have catalytic, photovoltaic, energetic, and sensory applications in diverse industries [1][2][3][4][5][6][7][8][1,2,3,4,5,6,7,8]. Moreover, biomedicine utilises ENPs as part of nano-vaccines, drug delivery, and diagnostic systems [9][10][11][12][9,10,11,12].
The interaction of ENPs with plants has been studied for about two decades. The initial research articles were mostly focused on the toxicity of ENPs on the plants; nevertheless, there were also few articles discussing their potential beneficial effects on crops [13][14][15][16][13,14,15,16]. At the same time, the first articles about the biosynthesis of nanomaterials by plants or plants extracts were published [17], which were partially inspired by the observations that NPs naturally form in the rhizosphere of plants [18][19][18,19]. At first, toxicity tests focused mostly on the short-term effects of the ENPs in seeds, seedlings, and young plants [20]. Early reviews concerned with ecotoxicology towards plants were published around the year 2008 [21][22][23][24][21,22,23,24] and were mostly concerned with research on the plant toxicity and interactions that were lacking for the higher plants at that time. The early studies on beneficial effects showed that, at optimum concentrations, ENPs might improve enzyme activities, photosynthesis, nitrogen absorption, and growth parameters of early seedlings [14][15][25][26][27][14,15,25,26,27].
Moreover, the preliminary reports on the effect of ENPs on plants grown in fields were published between the years 2010 and 2015 [28]. These studies showed the need to explore further the effects and interaction of ENPs under more realistic conditions as the underlying trend from laboratory experiments involved the application of higher doses of the nanoparticles which were toxic to the plants. In contrast, at appropriate lower concentrations, many ENPs were found to positively affect the plants’ growth, health, and quality [28][29][28,29]. For example, TiO2 NPs applied on barley during stem elongation and a second time during the four-leaf stage at concentrations of 0.01 to 0.03% increased grain yield and the weight of 1000 grains [30]. Peanut plants also responded positively to low concentrations of ZnO NPs, and higher concentrations of 2000 mg Zn∙L−1 revealed inhibitory effects [29]. Mostly, both ZnO and TiO2 NPs are only toxic at high concentrations, i.e., concentrations higher than 2000 mg∙L−1 [29]. Thus, both types of nanoparticles were found to be interesting for further field application, and their properties were also studied in this context. In recent studies on the interaction of ENPs with plants, the application of low, yet still effective, concentrations of ZnO and TiO2 NPs was investigated [31][32][31,32], and a new avenue of research was opened, where these nanomaterials can be applied not only to promote growth and agricultural productivity but also to alleviate abiotic and biotic stresses [33][34][35][33,34,35]. Both ZnO and TiO2 NPs were found to alleviate stresses caused by drought and heavy metals such as Cd. Further, these studies were performed under field conditions [33][34][35][33,34,35].
2. ZnO NPs and Application in Agriculture
ZnO NPs are an amphoteric semiconductive material with a wide band gap (Eg = 3.37 eV) [36]. Because of their unique properties, such as high binding energy, refractive index, thermal conductivity, piezoelectric nature, high absorbance of UV light, and antibacterial properties, these are widely used in various applications [37]. Moreover, as an added advantage, the above-mentioned properties are highly tuneable. Their size can be altered from a few nanometres to the upper limit of nanoparticle size definition (100 nm), and their shape can be easily adjusted by selecting the appropriate method of synthesis [37]. Different synthesis techniques have been used to produce ZnO NPs, including mechanochemical processes, controlled precipitation, sol-gel, solvothermal, hydrothermal methods, methods using emulsions and microemulsions, growing from a gas phase, pyrolysis spray methods, and others [37]. A broad range of shapes, such as flowerlike structures, nanorods, nanotubes and spherical or oblong nanoparticles, can be easily synthesised [37][38][37,38]. The surface of ZnO NPs is often modified to enhance their stability in colloidal suspension, to improve their positive effects on plants and to reduce their potential toxicity. The modification of their surface can be obtained by treatment with the inorganic compounds such as SiO2, Al2O3, etc., simple organic compounds, e.g., silanes or organic acids, and by more complex polymeric matrices [37]. Often biosynthesis of ZnO NPs is selected for agricultural applications since it is anticipated to create eco-benign nanomaterial [39]. Bare and surface-modified ZnO NPs were used in the laboratory, greenhouse, and field experiments on crop plants due to their UV protective, and antimicrobial properties besides their nutritional role as slow releasing Zn source for plants [31][38][39][40][41][42][31,38,39,40,41,42]. ZnO NPs easily dissolve compared with some other ENPs [43], such as TiO2 NPs, which affect plant health partly by their nano-specific properties, but also in larger part by the release of the Zn, which is essential to many processes on the cellular level [44]. In addition, ZnO NPs are reported to have an ability to decrease the effect of environmental stresses on plants, such as drought [45], temperature [46], metals, metalloids [47][48][47,48], and salt [49]. When applied at suitable concentrations, ZnO NPs increase plants’ seed germination [50], growth [51], the activity of antioxidants and protein production [52][53][52,53], chlorophyll content [54] and photosynthesis [55], production of oils and seeds [31][32][31,32], and uptake of essential elements [56].
3. TiO2 NPs and Application in Agriculture
TiO2 NPs are insoluble semiconductive material with a high refractive index, UV absorption, photocatalytic, and antimicrobial properties. These have highly tuneable properties partially because these ENPs exhibit diverse crystal symmetries represented by mineral phases such as anatase, brookite, or rutile. Each crystal structure has unique features that can benefit its application; most commonly, the suitable mineral form is selected for its lower or higher photocatalytic ability [57]. The size can be adjusted from a few nanometres up to 100 nm in any dimension, and the shape of TiO2 NPs can be tuned during their synthesis to obtain both nanorods and spherical nanoparticles [58]. Different types of synthesis protocols have been used for the production of TiO2 NPs to create nanomaterials of specific properties, e.g., sol, sol-gel, micelle, solvothermal, and hydrothermal methods, vapour deposition, and many others [59]. Because of their properties, TiO2 NPs have a wide range of applications in diverse fields of human activity, including agriculture. Similar to ZnO NPs, surface properties of TiO2 NPs are often modified to help with their stability or to increase their positive effects and decrease their toxicity [57][58][59][57,58,59]. Their environmental applications include water purification, degradation of pollutants, antimicrobial coating, biosensing, and drug delivery [60][61][62][63][64][60,61,62,63,64]. TiO2 NPs have been applied to protect seeds, enhance plant growth and germination, control crop diseases [65], degrade pesticides and detect their residues [66]. In addition, these NPs have been reported to increase root and shoot growth, seed or produce yield, and improve plant health. An increase in chlorophyll production, soluble leaf protein [67], and carotenoid content [68], and an increase in uptake of several essential elements [69] was also reported. Environmental stresses, such as drought in wheat [70] and high Cd levels in maize [71], were also alleviated significantly with the use of TiO2 NPs.
