Prospects of nanocarbons in agriculture

8

Prospects of nanocarbons in agriculture

Sumit Kumar Sonkar*, Sabyasachi Sarkar† *Department of Chemistry, Malaviya National Institute of Technology, Jaipur, India, † Nanoscience and Synthetic Leaf Laboratory at Downing Hall, Center for Healthcare Science and Technology, Indian Institute of Engineering Science and Technology, Shibpur, India

Chapter Outline 8.1 Introduction 287 8.2 Biochar 289 8.2.1 Biowaste-based charred carbon 289 8.2.2 Effect of biochar-derived nanocarbons on plant growth 291

8.3 Nanocarbons on plant growth 293 8.3.1 Water-soluble nanocarbons 294

8.4 Effect of nanocarbons on soil microenvironments 8.5 Conclusion 317 Acknowledgments 318 References 318

8.1

315

Introduction

Curiosity about the spectroscopic signature of interstellar dust in the universe led to the discovery of the third allotrope of carbon as fullerenes [1]. The discovery of fullerenes triggered the exploration of carbon in the nanodomain, leading to the characterization of diverse versions of nanocarbons in different shapes and sizes. This constitutes a family of nanocarbons, having the multiwalled carbon nanotubes (MWCNTs) [2–4], carbon nanoonions (CNOs) [5–10], single-walled carbon nanotubes (SWCNTs) [11], graphene/graphene nanosheets [12–15], photoluminescent carbon dots (CDs) [16–19], graphene quantum dots (GQDs) [20], carbon nanorods [21–23], carbon nanodiamonds [24], carbon nanohorns [25], and carbon nanocubes [26]. Such material in gram quantity was mostly synthesized by graphitic arc-based experiments [2, 5, 11, 17, 27, 28] and was further modified, decorated with hydrophilic surface functionalities to investigate the properties of its soluble form. Nanocarbons comprising only elemental carbon can be differentiated based on possessing the varied structure and bonding between the carbon atoms. As all the nanocarbons are comprised of the hydrophobic carbon network or cluster, therefore not being soluble in water, these versions are constrained to be used in biology. Smalley predicted that if these nanocarbons can have Nanocarbon and its Composites. https://doi.org/10.1016/B978-0-08-102509-3.00008-0 © 2019 Elsevier Ltd. All rights reserved.

288

Nanocarbon and its Composites

hydrophilic groups attached, then such derivatives of nanocarbons may have significant applications in the field of biological sciences [29, 30]. Since their discovery, it has been a prevalent practice to purify the crystalline nanocarbons with dilute hydrochloric acid [31–33] and nitric acid [34–36] to free these from contaminants such as metallic impurities and amorphous carbon. On longer treatment with dilute nitric acid, the crystalline form also got sporadic incorporation of carboxylic acid groups on their surfaces, which are normally removed by thermal annealing treatment. Thus, the robust hydrophobic nanocarbons have been shown to incorporate hydrophilic groups by the simple process of oxidative derivatization [6–8, 14, 15, 18, 21, 23, 37–43]. Such attempts with subsequent variations in the concentration of nitric acid yielded high density carboxylated group attachments on the surface or periphery of nanocarbon materials, rendering these fairly dispersible or even soluble in water [6–8, 14, 15, 18]. Such creation of soluble derivatives opened immense possibilities for the use of these nanocarbons to probe living objects of diverse kinds. The use of water-soluble or dispersible nanocarbons in exploring fauna and flora kingdoms slowly started to begin with cautionary steps mainly based on the issue of toxicity [44–55]. There were sporadic attempts to use nanocarbon derivatives in biomedical fields, mainly for imaging purposes. This is because of the surface passivation of these nanocarbons introducing hydrophilic groups that made them self-fluorescent [8, 15, 16, 56, 57]. However, out of curiosity, people used pure CNTs in cell studies and concluded that CNTs are toxic to living cells. The problem that was never addressed at that time is the nature of cell damage caused by these pure CNTs. The toxic effect could be chemical, physical, or biochemical in nature and in most cases, the physical interaction between hard CNTs with cell membranes caused cell damage. Such physical damage could be avoided by using soluble and soft versions of nanocarbons. These are in general named as defective nanocarbons possessing the high density surface defects [9, 15, 21, 34, 42]. Based on such criteria, the use of nanocarbons in the plant kingdom has been slowly developed. This chapter is based on such attempts to introduce nanocarbons in assisting the growth of the plant kingdom [34, 55, 58–65]. It is very relevant now as everyone understands that the increase in agricultural production for feeding a fastgrowing global population is the most significant challenge today. For the present and future demands of food [66–71], the use of newer sustainable materials and technologies [10, 72–77] is very desirable for increasing crop or plant productivity. To achieve this goal, presently we are in the state of using synthetic fertilizers [78–84] for improving crop productivity. But it is now known that the use of synthetic fertilizers is remarkable only for a short period of time. The long-run applications of synthetic fertilizers are showing adverse effects compared to organic based fertilizer like biochar [78, 81, 85–89]. Especially in deteriorating the natural composition of the soil [80, 82–84] and killing valuable microorganisms [90–92] and causing water pollution after discharge into water streams [79]. Presently, the use of sustainable organic-based fertilizers or manure is gaining popularity as some people believe that food grown with such a natural product would be better. Compared to synthetic fertilizers [78, 81, 91–94], the organic-based fertilizers show biocompatibity [95–97] and higher efficiencies, which make them more suitable materials for long-term agriculture applications as fertilizers. So organic-based

Prospects of nanocarbons in agriculture

289

fertilizers are being explored to increase the yield of agricultural output. However, organic fertilizers or manures are relatively costly compared with synthetic fertilizers. Along these, recent studies pointed out the benefits of age-old practices regarding the use of charred carbon materials from biowaste as a promoter of soil health in the name of “biochar” [86–89, 98–121]. Principally, biochar is a carbonaceous material mainly composed of carbon and partly with oxygen that has a good level of biological and chemical stability [104, 107, 120]. This is made by the simplest method of charring the residual waste biomass in open and in air-exposed atmospheres [101, 102, 115, 122, 123]. Thus swapping from synthetic chemical-based fertilizers [73, 78, 81, 91–94] to organic fertilizers [85, 124] or carbon-based [86–89, 111, 112, 125] to nanocarbon-based fertilizers [10, 36, 58, 61, 64, 126–130] could be a worthy option. As biochar is in principle of plant origin and it has recently been shown that it predominately contains graphene oxide [131], it can be extended to natural-based fertilizers. Also, with it being nanosized, this could handle the future positive consequences of carbon nanotechnology [10, 36, 65, 73, 74, 126] in the fields of agriculture and plant biotechnology [68, 74] to enhance crop health and increase productivity [10, 126]. So, the soluble versions of nanocarbons may challenge organically grown plant products, once the myth of toxicity can be completely erased. Such nanocarbons can be the solution that not only boosts plant growth but also helps generate the concept of nanofertilizers [132]. It has been argued that the slow and sustained release of micro and essential nutrients to young plant saplings by nanocarbons and nanocarbonbased composites may lead to plant growth, resulting in good and healthy flowering that yields the best possible fruit or crop. To achieve this, the negative impacts of nanocarbons in plants concerning its dose-dependent toxicity [44–55] must be addressed.

8.2

Biochar

The significant uses of biochar in agriculture are very simple to understand, as the precise information is available from long-known ancient agricultural practices concerned with the use of biochar [86–88, 100, 120] in the soil for increasing fertility [98, 101, 102], sequestration [98, 102, 113], restoration of soil carbons, and removal of hazardous chemicals [118, 121].

8.2.1 Biowaste-based charred carbon At present, biochar has been explored in various fields based on its immense potential [98, 105, 107, 120, 121, 133], as displayed in Fig. 8.1A that showing the environmental applications of biochar [87, 107, 114, 120]. During the process of charring or pyrolysis of biowaste in open air, many oxygen-bearing groups were impregnated on the carbonaceous surface of biochar, particularly the carboxylic acid and hydroxyl groups. There are several participating sites for the ion-exchange type of responses displayed by these charred species, as shown in Fig. 8.1B [87]. Chen et al. demonstrated in their representative work that the surface carboxylic acid groups were close

290

Nanocarbon and its Composites

Fig. 8.1 A schematic illustrating (A) biochar as a platform carbon material for various potential applications [107]; (B) pH-dependent dissociation of acid/base groups on the biochar surface. Reprinted with permission from (A) Liu W-J, Jiang H, Yu H-Q. Development of biochar-based functional materials: toward a sustainable platform carbon material. Chem Rev 2015;115 (22):12251–12285; (B) Chen Z, Xiao X, Baoliang Chen, Zhu L. Quantification of chemical states, dissociation constants and contents of oxygen-containing groups on the surface of biochars produced at different temperatures. Environ Sci Technol 2015;49:309–317.

Prospects of nanocarbons in agriculture

291

3
to the negative moieties (ClO
4 [87, 134] and PO4 [87]) via hydrogen bonding and once the species became deprotonated due to the change in soil pH, these groups then combined with the metal ions (Al, Cd, and Pb) [108, 116]. The study from Sarkar and coworkers [73] described the nanosignificance of the biochar where they have shown the presence of spherical nanocarbons in the biochar.They correlated the benefit of the biochar by high degree of surface functionalization which create defects on its surface and hence makes it highly porous. The presence of such ionic materials provides the active sites for the ion exchange reactions to control the uptake and release of several ions [73]. These studies successfully correlate the positive effects of the conventional technique of “charring the waste” (residual crop, waste root ends, straws, etc.) after harvesting the main crop; such burning to create biochar has been directly implicated with nanocarbons [73]. The elemental compositions of nanocarbons are very similar to the biochar and the difference could be related to the variation in the presence of hydrophilic groups per unit mass of the product. Being loaded with limited hydrophilic groups, biochar remains sparsely soluble in water and thus it remains in the soil for years with very slow aerial oxidation under a natural environment. So, to extend the extraordinary results with biochar, nanocarbons can achieve more if the simplest surface modification introducing more units of electrophilic groups per unit mass renders these appreciably soluble in an aqueous medium. Essentially, the introduction of more electrophilic groups is related to more changes of sp2 hybridization of carbon centers to the sp3 type of hybridization in the nanocarbon framework. This would allow more volume with porosity on the carbon network, which would help to absorb nutrients or even water in appreciable amounts. In comparison to synthetic fertilizers, nanocarbon-based fertilizers cause a slow and steady nutrient release that results in more sustained availability of nutrients, which can replenish soil structure and plant growth [98, 102], Such nanocarbon-based fertilizers show excellent properties related to balancing the soil pH [87, 108] and helping in the retention of water [102] and nutrients [73, 98, 102]. Because of a very high surface area [102, 121] to volume ratio, such material can significantly retain more water that could be released when needed by the plant [102]. So, the charred carbon deals with both the classical (black carbonaceous mass left in the field after burning the plant waste) [100, 104, 120, 135] and the laboratory made nanocarbons, both are showing the enhancement in the crop productivity.

8.2.2 Effect of biochar-derived nanocarbons on plant growth The surface of biochar and biochar-derived nanocarbons acts like a storage house of nutrients, including micronutrients. It was further reported that biochar derived CNPs show controlled release of nutrients with essential time of retention [73]. The surface defects associated with the presence of functional groups are directly involved with controlling the release of the nitrogen-comprising nutrient, like the cationic (NH+4 ) and anionic (NO
3 ) in the plant. This representative study is then correlated with the results in accordance with the use of biochar for the controlled release of the nutrients [73]. As described in Fig. 8.2. A-F the wheat seeds were germinated under different concentration (10, 20, 40, 50, 80, 65, 100 and 150 mg L
1) of soluble CNPs and the maximum growth was noticed in the plants treated with 50 mg L
1 concentration

292

Nanocarbon and its Composites

(C)

(D)

(E)

(F)

3

rCNPs wsCNPs

2

1

0

(G)

24

72

120 Time (h)

168

240

Concentration of [NO3]− ions released (10−5 M)

(B)

Concentration of [NH4]+ ions released (10−5 M)

(A)

3

rCNPs wsCNPs

2

1

0

(H)

24

72

120 Time (h)

168

240

Fig. 8.2 Germinated seeds (A) control (0 mg L
1) with wsCNPs of concentration; (B) 10 mg L
1; (C) 20 mg L
1; (D) 40 mg L
1; (E) 50 mg L
1; and (F) 150 mg L
1 (these are not imaged from the same distance and the figures compare only the variation of shoots in germination); Release of adsorbed ions from rCNPs and wsCNPs at different time intervals: (G) NH+4 ; (H) NO
3 ions. Reprinted with permission from Saxena M, Maity S, Sarkar S. Carbon nanoparticles in ‘biochar’ boost wheat (Triticum aestivum) plant growth. RSC Adv. 2014;4:39948–39954.

wsCNPs. So, the exact concentration-dependent studies are needed to understand the dose-dependent effect. They proposed a threshold concentration from the variable concentration window for wsCNPs to show the prominent optimized concentration. Exceeding this concentration, the effect of high concentration of carbon-based fertilizers leading to plant senescence is observed. Importantly, they showed by their model experiment that both the materials as raw CNPs (rCNPs) (just like as collected biochar) and the water-soluble version of CNPs as wsCNPs were capable of absorbing the nutrient molecules [73]. The slow release with time of cationic and anionic nutrients from wsCNPs is displayed in Fig. 8.2G and H.

Prospects of nanocarbons in agriculture

8.3

293

Nanocarbons on plant growth

Presently, the nature of nanocarbon–plant interaction is well supported by several reports. The simplest versions of hydrophilic nanocarbons, also known as watersoluble versions of nanocarbons [10, 36, 73, 130], can easily enter the seeds of the gram [10, 36] and wheat plants [126, 136, 137] to show the increase in the overall growth. Tripathi et al. described [132] that the wood wool (basically a wood) on charring formed carbon nanoanions (CNOs) and its water-soluble version is capable enough to increase the overall yield of the first-generation gram seeds with the additional advantage of having higher concentrations of micronutrients in the seeds. Such effort would be helpful to enrich cereal food grain with desired micronutrients for better health. Studies [30, 60, 63] have shown the physical presence of nanocarbons inside the tracheal elements of the xylem vessels, which are responsible for the conduction of water inside the plants. Scanning electron and transmission microscopic images [10, 36, 129, 130] showed that the growth-stimulating effects of the watersoluble nanocarbons are related to their water solubilization and high density surface defects [10, 36, 73, 130, 137]. Such a high degree of surface defects can easily be correlated with losing crystalline nature, resulting in high surface area like biochar. Khodakovskaya et al. reported that the increase in the gene expression can be correlated to the activity of the aquaporin [61, 126, 128, 138, 139]. Such a study complimented the belief that nanocarbons in the xylem vessels help the water conduction efficiency in plants. Thus the water-soluble and functionalized versions of nanocarbons can smoothly travel through the microchannels in plants whereas nonfunctionalized nanocarbons cannot be easily transported inside. Being insoluble, they are stuck and by physical force pierce the channel lines and cell lines of living plants manifesting toxic properties. For the CNTs, both SWCNTs and MWCNTs [136, 140, 141] can enter the seed coat of the tomato [126, 138, 139, 142]; these were shown to increase the growth of plants (tomato) [126] and tobacco cells [128]. However, the initial interaction of these crystalline nanocarbons with a swelled seed coat may assist in the germination process. Giraldo et al. reported a significant study related to the increase in the photosynthetic efficiency in the chloroplasts in spinach [65]. A few more reports are also available for the potential uses of several nanocarbons on other plants [34, 55, 58–65]. Studies related to the continuous tracking of the movement of nanocarbons inside the plant body and, at the end, their accumulation are available. Presently, the physical presence of nanocarbons inside plant parts is seen by various spectroscopic and microscopic techniques. The much-used spectroscopic techniques include Raman [61, 142], FTIR [34, 58], and microscopic techniques including fluorescence [36, 130, 136], confocal [74, 136], transmission electron microscopy [90, 143, 144], scanning electron microscopy (SEM) [62], atomic force microscopy (AFM) [145], and confocal scanning laser microscopy (CSLM) [130, 145]. Because of the presence of the extra cell wall in a plant, the internalization and the movement of the nanocarbons inside the plant system are not straightforward to trace like the approach used for mammalian cells. The uptake, movements, or translocation of different shaped nanocarbons purely depends on several factors. Among these, the important factors are the shape and size of the nanocarbons nature of surface

294

Nanocarbon and its Composites

functionalization, growth media, aggregation of nanocarbons, exposure time, water solubility and most importantly on the plant species. The type of nanoparticles, their physical presence, and the technique used to characterize those with positive impact inside plants are thoroughly reviewed, as briefed in Table 8.1. Generally, the nanocarbons suspended in aqueous growth medium are easily taken up by the plant via routine activity, such as the uptake of other essential nutrients. Nanocarbons taken by the plants were mostly been found inside the xylem vessels. Significant observations were made with the use of surface-functionalized nanospecies that showed reduced toxic effects of nanocarbons in plants [10, 36, 73, 126, 138–140, 142]. The surface-functionalized nanocarbons can easily cross the cell walls because of the softness and fluidity in the aqueous medium to utilize channels defined to allow the transport of materials [136, 140, 141]. On the other side, hard nonfunctionalized versions of the nanocarbons would physically interact with softer cell membranes of plants and eventually pierce the cell walls, causing cell death. Comparatively, the functionalized nanocarbons are fairly soluble or dispersed in the water, showing enough softness due to high surface functionalization. Thus nanocarbons can easily cross plant cell walls and significantly articulate in the overall growth of the treated plants by promoting the process in seed germination [61, 126, 127, 138, 139], gene regulation [10, 34, 35, 60, 61, 126, 128, 138, 139], and water uptake [10, 36, 130], leading to an increase in biomass resulting in the overall productivity of the plants [10, 126] and in enriching soil microenvironments [126, 132] as well. Such positive effects of carbon-based fertilizers such as biochar are shown in Fig. 8.3.

8.3.1 Water-soluble nanocarbons 8.3.1.1 Water-soluble carbon nanotubes Sarkar and coworkers were the first to document the positive impact of the watersoluble version of MWCNTs as wsCNTs on gram plants [36]. They used hard MWCNT and made them heavily derivatized, introducing peripheral carboxylic and hydroxyl groups to create water-soluble CNTs (wsCNTs). The gram plants treated with such wsCNTs showed increased overall growth with an increase in the length of shoot, the number of roots, and the branches. The increase in the overall growth was related to the increase in the water uptake efficiency of treated plants compared to control. Between the control and treated plant, this observation can be related to the better supply of nutrients and micronutrients in the treated plant with the uptake of more water. This can thus refer to the enhanced movement of the water-soluble nutrients with wsCNTs. They proposed a possible mechanism (Fig. 8.4A) related to the formation of smaller capillaries via the embedding of wsCNTs inside the larger capillaries as the tracheal elements of xylem vessels. This is somewhat like aligning the wsCNTs one on another from the head because of the large potential drift generated at the process of transpiration. The newer capillaries generated by the wsCNTs could help in enhancing the water uptake and retention properties of the plant. That can directly facilitate the movements of water-soluble nutrients required by the plant. They have successfully confirmed the proposed mechanism by using fluorescence

Table 8.1 Effects of nanocarbons on plant growth

Type of nanocarbons Water soluble

wsCNTs Or wsMWCNTs

Presence of nanocarbons inside the plant parts

Functionalized/ nonfunctionalized

Concentration of nanocarbons

Growth medium

Gram

Functionalized with conc. nitric acid

3 and 6 μg mL
1

DI water

Mustard

Functionalized with 2 M nitric acid

2.3 and 6.9 μg mL
1

DI water

Lumen of tracheal elements of xylem vessels Shoots and roots

Alfa-alfa, Wheat

Acid functionalized

40–2560 mg L
1

DI water

Root surface

Tomato

Functionalized with different groups (COOH
, acetone and CH2,3, poly-ethylene glycol (PEG))

40 μg mL
1

MS medium

Root cells

Wheat

Oxidization with hydrochloric acid (HCl)

10, 20, 40, 80, and 160 μg mL
1

DI water

Cellular cytoplasm of root cells

Plants/cells

Effect on plants Increased 1. overall growth 2. water uptake Increased 1. seed germination rate 2. shoot and root length Increased 1. seed germination 2. root elongation Increased 1. fresh biomass 2. seed germination 3. expression of water channel genes Increased 1. root growth 2. vegetative biomass

References [36]

[34]

[60]

[137]

Continued

Table 8.1 Continued

Type of nanocarbons

Plants/cells

Functionalized/ nonfunctionalized

Concentration of nanocarbons

Growth medium

Presence of nanocarbons inside the plant parts

Rice

Functionalized with nitric acid

50 μg mL
1

MS basal medium

Root surface

Barley, corn, soybean

Functionalized with carboxyl groups

100 and 200 μg mL
1

MS medium

Seed surface and can even penetrate seed coat of all plants, corn leafs, and shoots of soybean

Effect on plants 3. cell elongation 4. concentration dependent increase in dehydrogenase activity Increased 1. root and shoot length 2. seedling growth 3. seed germination rate Increased 1. seed germination rate 2. expression of water channel genes (aquaporins, PIP, TIP, SIP) 3. in corn highest (i) total fresh shoot weight

References

[35]

[61]

SWCNTs

Hybrid Bt cotton

Oxidized with H2SO4 + HNO3 (3:2)

20, 40, 60, 80, and 100 μg mL
1

MS medium

–

Fusarium graminearum and Fusarium poae

Functionalized with carboxyl groups

62.5–500 μg mL
1

–

Spore surface

Nicotiana tobacum L.cv. Bright Yellow (BY-2) cells

Functionalized with a concentrated mixture of H2SO4/HNO3

0.08 mg mL
1

DI water

Vacuoles

(ii) leaf length 4. in soybean longer root system Increase in 1. shoot and root length 2. height 3. number of roots, leaves, and bolls per plant 4. boll size Antifungal activity– inhibition of 1.water uptake 2 induced plasmolysis SWCNTs enter via fluidicphase endocytosis

[59]

[146]

[136]

Continued

Table 8.1 Continued

Type of nanocarbons

Effect on plants

Functionalized/ nonfunctionalized

Concentration of nanocarbons

Growth medium

Functionalized with a concentrated mixture of H2SO4/ HNO3 (3:1) Functionalized with mild nitric acid treatment

0.4 mg mL
1

DI water

Vacuoles, cytoplasm, and cell nucleus

SWCNT-FITC enters the cell wall

[141]

50 μg mL
1

MS medium

Root, leaves, and fruits

[138]

Fusarium graminearum and Fusarium poae

Functionalized

62.5–500 μg mL
1

–

Spore surfaces covered with SWCNT

Gram

Functionalized with conc. nitric acid

10, 20, and 30 μg mL
1

DI water

Tracheal elements of xylem vessels.

Increased 1. vegetative biomass 2. expression of various genes Antifungal activity 1. reduced water uptake 2. caused plasmolysis Increased 1. vegetative mass 2. fruit production Increased 1. yield per plant 2. individual weight, size of seeds

Plants/cells Catharanthus roseus

Tomato

wsCNOs

Presence of nanocarbons inside the plant parts

Cotyledon/seed coat

References

[146]

[10]

[132]

wsCDs

Wheat

wsCNPs

Wheat

CSCNTs

Arabidopsis thaliana

Functionalized with conc. nitric acid Functionalized with HNO3 and water (in 1:1 ratio)

Functionalized with H2SO4 + HNO3 (3:1)

150 μg mL
1

DI water

Root region of plants

10, 20, 40, 50, 80, 100, and 150 mg L
1

DI water

50 μg mL
1

MS medium with 1 μM of 2,4-diphenoxy acetic acid and kinetin.

Xylem vessels, show controlled release of nutrients and ions Structures of plant tracheids

3. protein content 4. stored electrolytes 5. metallic micronutrients Increased in shoot and root length Increased in overall growth of plant

Building the heterogeneous deposition of tracheary elements of plants and participate in nonenzymatic polymerization process

[73]

[130]

[145]

Continued

Table 8.1 Continued

Type of nanocarbons

Plants/cells

Functionalized/ nonfunctionalized

Concentration of nanocarbons

Growth medium

Presence of nanocarbons inside the plant parts

GO sheets

Wheat (Triticum durum cv. Yallaroi)

Functionalized

20 mg L
1

DI water

Desired slow and sustained release of micronutrients (Zn and Cu) by GO micronutrient fertilizers

GN, GO

–

–

0.05–0.5% w/w

–

Cogranulation of GN and GO sheets with MAP fertilizer enhanced mechanical strength, resistant to abrasion, impact resistance, diffusion rate of phosphorous

Effect on plants Grain dry mass was higher only for soil treated with Zn–GO, and nutrient uptake was higher for soil treated with Zn– GO and Cu– GO, slow release rate of Zn and Cu ions (55%) after 72 h –

References [147]

[148]

Functionalized

C60(OH)24–26 Fullerol

Arabidopsis thaliana

Synthesized through an alkali route

100 and 200 mg L
1

MWCNTs

Catharanthus roseus

Functionalized

0.04 mg mL
1

Tomato

Slightly functionalized

50 μg mL
1

Minimal medium (1 mM KCl, 1 mM CaCl2; solidified with 1% phytoagar) MS medium with minimal organics supplemented with 1 μM 2,4-diphenoxy acetic acid and 1 μM kinetin MS medium

lower with MAP-GN and MAP-GO fertilizers –

Shorterplastids, nucleus, and vacuoles Largerendoplasmic reticulum Fruits, root, and leaves

Increased in the length of hypocotyls

[51]

Plant protoplasts adopted endosomeescaping uptake mode (nontoxic internalization) of MWCNTs Increased 1. seed germination 2. biomass 3. relative transcriptive 4. water channel protein,

[140]

[138]

Continued

Table 8.1 Continued

Type of nanocarbons

Non functionalized

MWCNTs

Plants/cells

Functionalized/ nonfunctionalized

Concentration of nanocarbons

Growth medium

Presence of nanocarbons inside the plant parts

Tomato

Slightly oxidized

50 and 200 μg mL
1

Agar MS medium

Surface of fruits and flowers.

Tobacco

Purified with HCl

5100 and 500 μg mL
1

MS medium with 0.8% agar

Cells

Ryegrass, rape, corn, cucumber

Pristine

2000 mg L
1

DI water and Zn+2 solution

Different seed soaking techniques resulted in different effect

Effect on plants Les.564.1.S1heat shock protein 90, and stress protein Increased 1. number of seeds 2. flowers 3. fruits 4. plant height Increased 1. cell growth 2. fresh biomass 3. expression of aquaporin genes (NtPIP1), cell division genes (CycB), and cell wall extension genes (NtLRX1) Increased in the root length

References

[126]

[128]

[55]

SWCNTs

Maize

Pristine

10, 20, and 40 mg L
1

Nutrient agar gel medium

Seed surface

Corn

Pristine

500, 1000, and 5000 mg kg
1

DI water

–

Onion, Cucumber

Non functionalized

56, 315, and 1750 mg mL
1

DI water

Tomato

Non functionalized

50 μg mL
1

1% agar (Murashige and Skoog) MS medium

Surface of secondary roots and root hairs as compare to main roots Leaf

Rice

Purified and pristine form

50 μg mL
1

MS basal medium

Germination site, in root surface and shoot tissues

Increase 1. water absorption 2. plant biomass 3. concentration of Fe, and Ca nutrients Increased in the 1. net growth 2. biomass Increased in the root elongation

[64]

Increased 1. vegetative biomass 2. seedling growth Increased 1. root and shoot length 2. seedling growth 3. seed germination rate

[142]

[149]

[62]

[35]

Continued

Table 8.1 Continued

Type of nanocarbons

Plants/cells

Functionalized/ nonfunctionalized

Concentration of nanocarbons
1

Fullerene (C60)

Rice

Pristine form

50 μg mL

[C60(OH)20] Fullerol

Bitter melon

Pristine form

0.943, 4.72, 9.43, 10.88, and 47.2 nM

Growth medium

Presence of nanocarbons inside the plant parts

MS basal medium

Roots surface

Milli-Q water

Stem, leaf, petiole, flower, and fruit

Effect on plants Increased 1. root and shoot length 2. seedling growth 3. seed germination rate Increased 1. biomass yield 2. fruit number 3. fruit weight 4. fruit length 5. plant water content, two anticancer phytomedicine and two antidiabetic phytomedicine

References [35]

[58]

Prospects of nanocarbons in agriculture

305

Fig. 8.3 Schematic diagram supporting effects of the organic charred carbon as biochar and different types of nanocarbons in the plants and soil environment.

microscopic images, as shown in Fig. 8.4B and C and by SEM microscopic pictures, as illustrated in Fig. 8.4D and E showing the absence and presence of well-aligned and embedded wsCNTs in the control and treated samples, respectively.

8.3.1.2 Water-soluble carbon nanoonions and carbon dots Sonkar et al. described the beneficial role of wsCNOs in gram plants in terms of the increased productivity regarding harvesting more fruits (gram seeds) per plant, as shown in Fig. 8.5. The overall growth of gram plants was related to the increased water conduction properties of the plant facilitating the transport of micronutrients. Recently, Tripathi et al. reported a detailed comparative analysis of the micronutrient content in the first-generation gram seeds (FGSs) obtained just after harvesting [132]. Gram plants were treated with three different concentrations of wsCNOs for a short period of time to restrict the accumulation of wsCNOs in fruits.

306

Nanocarbon and its Composites

By the force of absorption

CNT forms the “large capillaries”

Carbon nanotube in random arrangement

Introduction of ‘‘large capillaries” inside tracheary elements

= Water conduction

(A)

= Water molecule

Conduction takes place

(B)

+

Arrangement of CNT inside tracheary elements

Tracheid Vessel Tracheary elements

Tracheid + vessel

(C)

200 nm

(D)

Mag = 50.00 K X

EHT = 10.00 kV

Date :2 Nov 2010

WD =

Signal A = InLens

Time :11:04:41

4mm

100 nm

Mag = 100.00 K X

EHT = 10.00 kV

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Fig. 8.4 (A) Schematic diagrams of alignment of wsCNTs inside roots. (B and C) Fluorescence images showing the transverse section (T.S.) and longitudinal section (L.S.) [128] of CdS-treated root; FESEM images of the L.S. of root; (D) without and (E) with wsCNTs, white arrows show its alignment inside the xylem vessels of root; inset shows zoomed of nicely aligned wsCNTs. Reprinted with permission from Tripathi S, Sonkar SK, Sarkar S. Growth stimulation of gram (Cicer arietinum) plant by water soluble carbon nanotubes. Nanoscale 2011;3:1176–1181.

In a continuation of their earlier findings, a full life cycle analysis related to the “seed to seed” cycle study was carried out and the result is shown in Fig. 8.6 [132]. A significant and substantial increase was observed in the concentrations of stored micronutrients (copper, zinc, molybdenum, iron, manganese, and nickel) in seeds obtained from the wsCNO treated plants compared to the seeds obtained from control plants. Tripathi et al. demonstrates the interaction of water-soluble carbon dots

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Fig. 8.5 (i) Phenotypic images of gram plants treated with different concentration of wsCNOs after 10 days; (A) control; (B–D) plants treated with 10, 20, and 30 mgmL
1 wsCNOs, respectively. (ii) Phenotypes of gram seeds (fruits) for the control (A), and under varying concentrations of wsCNOs (B) 10, (C) 20, and (D) 30 mgmL
1 of wsCNOs. (iii) Shows the weight of fruits per plant in terms of SE (Reprinted with permission from Sonkar SK, Roy M, Babara DG, Sarkar S. Water soluble carbon nano-onions from wood wool as growth promoters for gram plants. Nanoscale 2012;4(24):7670–7675). Histogram: (iii) effect of wsCNOs concentrations; (iv) total protein contents of FGSs with  SE (number of seeds—25); (v) electrical conductivity from stored electrolytes in FGSs with  SE (number of seeds—15) along with the given significance (p < 0.005). Reprinted with permission from Tripathi KM, Bhati A, Singh A, Sonker AK, Sarkar S, Sonkar SK. Sustainable changes in the contents of metallic micronutrients in first generation gram seeds imposed by carbon nano-onions: a life cycle seed to seed study. ACS Sustainable Chem Eng 2017;5(4);2906–2916.

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Fig. 8.6 Schematic diagram showing the effect of wsCNOs on the growth as overall productivity and metallic micronutrient concentration compared to control. Reprinted with permission from Tripathi KM, Bhati A, Singh A, Sonker AK, Sarkar S, Sonkar SK. Sustainable changes in the contents of metallic micronutrients in first generation gram seeds imposed by carbon nano-onions: a life cycle seed to seed study. ACS Sustainable Chem Eng 2017;5:2906–2916.

(wsCDs) with wheat plants under the influence of light and dark conditions of its growth. They also showed the increase in the growth of the root and shoot of plants using wsCDs [130].

8.3.1.3 Water-soluble fullerenes In comparison to CNTs, there are only limited reports available showing the positive interactions of plants with fullerenes (functionalized versions as fullerol [C60(OH)20]). Kole et al. used the seeds of a bitter melon and showed the significant effects of fullerol on the growth of those seeds [58]. They observed that the fullerol-treated plants were shown to have increased in the biomass and water content of the seeds while the phytomedicine content of the fruit also gets changed. The fullerol-treated fruits show improved length, weight, and amount of fruit with the increase in the overall yield (in terms of fruits). Nair et al. showed the improvement in the germination rate of 60% with rice when treated with C60 [35]. As discussed earlier, hydrophobic nanocarbons (in the present case C60) physically interact with the swelled seed coat to facilitate germination. Another study described by Gao et al. showed the positive interactions of polyhydroxyfullerols on Arabidopsis thaliana seeds [51].

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8.3.1.4 Multiwalled carbon nanotubes Being the first version of nanocarbons with low cost and easy access, MWCNTs have been the most explored nanocarbons used for the study on plant–nanocarbon interaction. Serag et al. reported the differential penetrating ability of MWCNTs into the cell membranes and protoplasts of Catharanthus roseus, using confocal microscopy and HRTEM analysis [140]. They reported that the penetration ability of MWCNTs was considerably dependent upon their size, as shown in Fig. 8.7A. The shorter FITC-labeled MWCNTs (30–100 nm) showed an increased infiltration capacity and were found distributed in the vacuole, plastids, and nucleus of the cells, as demonstrated in Fig. 8.7B and C while larger FITC-labeled MWCNTs (>200 nm) were found in other subcellular structures such as the endoplasmic reticulum and mitochondria of the Catharanthus cells. MWCNTs were internalized by labeling the endosomal membrane with FM4–64 dye, which is a robust marker for endocytosis. Only a few endosomes having a larger size were observed to entrap MWCNTs-FITC and show fluorescence. TEM observations (Fig. 8.7D–F) clearly indicated the presence of isolated MWCNT-FITC conjugates inside the cells while the presence of larger aggregates or bundles was observed outside the cells [140]. The uptaking of MWCNT-FITC conjugates via the endosome escaping route was suggested to endow with a method for further use in delivery techniques of biomolecules within the cells in a more precise way. Khodakovskaya and coworkers studied the interaction of MWCNTs with diverse plant cells [10, 61, 126, 128, 138, 139]. In their study they reported that the plants treated with MWCNTs exhibited a significant gain in overall growth, such as plant height, the number of leaves, flowers, and fruits, and the overall size of the fresh fruit (Fig. 8.8). It was proposed that the plant reproductive system was activated by the use of MWCNTs, which consequently led to overall increased productivity [126]. In another case, MWCNTs were supplied to plants by two different ways, either added via an air sprayer or directly mixed in the growth medium [61]. The MWCNTs exhibited more pronounced stimulating effects for treated tomato plants associated with the faster rate of germination, growth activation, and phenotype variation. MWCNTs have also been reported to activate the expression of many stressresponsive tomato genes [126, 138, 139]. It was explained that MWCNTs stimulated the water channel gene expressions that have a critical role in the seed germination process. MWCNT-exposed tomato plants showed the upregulation of stress-related and water-channel protein (LeAqp2) genes in roots and leaves. Significant differences were identified in 91 and 49 transcripts in leaves and roots, respectively, for 16 genes of known functions in MWCNT-exposed plants [138]. Genetic analysis was adapted to confirm the activation of LeAqp2 gene expression and enhanced the expressions of mitogen-activated protein kinase with MWCNT-exposed seedlings. The surface functionalization of MWCNTs significantly influenced their ability to cross the cellular membrane. For instance, seed germination and plant growth rates were more pronounced in oxidized MWCNT (o-MWCNT)-treated seeds than seeds treated only with nonfunctionalized MWCNTs [34]. Zhai et al. demonstrated the effect of neutral (p-MWCNTs), positively charged (NH2-MWCNTs), and negatively charged (COOH-MWCNTs) MWCNTs on the growth of maize and soybean plants [63]. The growth effects were found to be more significant for negatively

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Fig. 8.7 Uptake of MWCNTs-FITC by the protoplasts of Catharanthus roseus: (A) confocal microscopy images of the protoplasts incubated with MWCNTs-FITC conjugate, scale bar is 20 μm; (B) histogram showing the length dependent distribution of MWCNTs-FITC inside the protoplast after 3 h incubation; (C) schematic model of cell showing the uptake and distribution of MWCNTs-FITC based upon length of nanotubes; (D–F) TEM characterization showing the cellular distribution of MWCNTs-FITC inside the protoplasts; (D) localization of MWCNTs-FITC inside the cell vacuole (black arrows); (E) inside the plastids (green arrow). The dark rounded structure represents the starch granules; (F) inside the cell nucleus (red arrow) and blue arrow indicates the nuclear membrane. The scale bars are of 500 nm for (D–F). Reprinted with permission from Serag MF, Kaji N, Gaillard C, Okamoto Y, Terasaka K, Jabasini M, et al. Trafficking and subcellular localization of multiwalled carbon nanotubes in plant cells. ACS Nano 2011;5:493–499.

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Number of fruits

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Fig. 8.8 Effect of carbon nanotubes watering (50 and 200 μg mL
1) on development (CNT) supplied with tomato fruits: (A) average number of flowers; (B) average number of fruits; (C) size of fruits, and (D) number of seeds per fruit were measured for CNT-exposed plants (CNT 50 and CNT 200), plants exposed to activated carbon (AC), and unexposed tomato plants (control) at stage of mature (red) fruits. Each data point is the average of 20 individual measurements. Thus, vertical bars indicate SE (n ¼ 20). Reprinted with permission from Khodakovskaya MV, Kim B-S, Kim JN, Alimohammadi M, Dervishi E, Mustafa T, et al. Carbon nanotubes as plant growth regulators: effects on tomato growth, reproductive system, and soil microbial community. Small 2013;9:115–123.

charged MWCNTs compared to neutral or positively charged MWCNTs. Longer (50–1000 nm) MWCNTs were reported to accumulate in the roots while shorter ones (50–100 nm) accumulated in the stem and leaves. The significant effect of surface functionalities of nanocarbons with five different types of MWCNTs (MW1 to MW5) were investigated with tomato plants, as shown schematically in Fig. 8.9A [139]. The change in phenotypes for MWCNT-exposed plants was observed continuously up to 41 days and photographed sequentially after 1 week, 4 weeks, and 41 days, as shown in Fig. 8.9B–D, respectively. MW2- and MW3-treated plants showed two times higher in the biomass than control plants while an increase in the biomass of tomato plants treated with MW4 and MW5 was less pronounced, but higher than the control and MW1. That can be correlated with the enhanced expression of water channel protein (LeAqp1), as shown in Fig. 8.9E. They reported that MWCNTs activated the water channels (aquaporin) and major gene regulators responsible for cell division and extension [139]. Enhancement (55%–64%) in the fresh biomass and overall growth of tobacco cells was observed for MWCNTs in comparison with the activated carbon (AC), which stimulates only 16% cell

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Fig. 8.9 (A) Schematic diagrams showing the experimental conditions of various surface functionalities of MWCNTs; (B) Phenotypic effects of tomato seedlings treated with different surface functionalities (MW1–MW5) of MWCNTs (40 μg mL
1) with control sample after 1 week; (C) after 4 weeks; (D) after 41 days of incubation; (E) water channel protein (LeAqp1) expression analysis by Western blot after 8 and 41 days of incubation of tomato plants grown on standard MS medium (control). Reprinted with permission from Villagarcia H, Dervishi E, Silva KD, Biris AS, Khodakovskaya MV. Surface chemistry of carbon nanotubes impacts the growth and expression of water channel protein in tomato plants. Small 2012;8:2328–2334.

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growth. MWCNT-treated tobacco cells showed a 35-fold increase of the CycB (cell cycle progression) gene expression within 6 h of incubation. Additionally, the study showed a significant increase in the expressions of the aquaporin gene (NtPIP1) with MWCNT-exposed tobacco cells [128]. Similarly, Mondal et al. reported a comparative study for the beneficial effect of MWCNTs before and after oxidation (o-MWCNTs) with nitric acid on the germination and growth of mustard plants [34]. On comparison between o-MWCNTs and MWCNTs, o-MWCNTs showed more prominent effects, even at lower concentrations, because of an increase in moisture content and water uptake.

8.3.1.5 Single-walled carbon nanotubes Strano and coworkers reported the passive transport and localization ability (irreversibly inside the lipid envelope) of SWCNTs inside the chloroplast of spinach cells (Spinacia oleracea L.), as shown in Fig. 8.10A [65]. Once assembled inside chloroplasts, SWCNTs were found to add new functional properties in chloroplast-based photocatalytic complexes. Assimilations or interactions of the SWCNTs with chloroplasts is of great significance. Chloroplasts are a unique source of chemical energy and exhibit great potential as an alternative energy source if they can be appropriately engineered for long-term, stable photosynthesis ex vivo. The physical presence of SWCNTs inside the chloroplasts was detected by using near infrared optical imaging. An SWCNT-coupled chloroplast showed three times increased activity of photosynthetic machinery via increasing the rate of electron transport. An increase in the electron transport process was immediately connected with the absorption of more photons [65]. SWCNT-coupled chloroplast provides suitable electronic band gaps for the enhanced conversion of absorbed energies into excitations, which facilitated the electron transport mechanism and consequently improved the power of solar energy conversion. SWCNTs at 2.5 mg L
1 concentration significantly increased the electron transport rates up to 49% in vitro. In vivo 2.5 mg L
1 and 5 mg L
1 of SWCNT-treated leaves showed 27% and 31% enhancement of the electron transport process, respectively, in contrast to control leaves, as displayed in Fig. 8.10B and C. SWCNTs coupled with ceria significantly decreased the concentration of reactive oxygen species (ROS) 21.4% by converting the hydroxides and superoxide radicals into their respective ions that consequently prevent the damage of photo pigments and photoproteins. Fang and coworkers reported the ability of SWCNTs as nanotransporters (concerning intracellular labeling, imaging, and genetic transformation) for intact plant cells [136]. They investigated the temperature-dependent endocytosis effect of oxidized SWCNTs (SWCNTs) on tobacco (Nicotiana tobacum, bright yellow (BY-2)) cells. Conjugates of SWCNTs with fluorescein isothiocyanate (FITC) and FITClabeled single-stranded DNA (FITC-ssDNA) were fabricated via sonication and the efficiency of SWCNT-FITC and SWCNTs-FITC-ssDNA conjugates was further investigated. BY-2 cells, when incubated with SWCNT-FITC, exhibited better fluorescence in contrast to cells treated with only FITC molecules. More importantly, the ss-DNA-FITC conjugate molecule alone was not able to penetrate the cell walls. But

Nanocarbon and its Composites

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Fig. 8.10 (A) Mechanism of SWCNT trapping; SWCNTs transport through the chloroplast double membrane envelope via kinetic trapping by lipid exchange; (B) maximum electron-transport rates in extracted chloroplasts and leaves were quantified by the yield of chlorophyll fluorescence; (C) electron-transport-rate light curves indicated enhanced photosynthesis above 100 μmol m
2 s
1 for 5 mg L
1 SWCNTs leaves. Reprinted with permission from Giraldo JP, Landry MP, Faltermeier SM, McNicholas TP, Iverson NM, Boghossian AA, et al. Plant nanobionics approach to augment photosynthesis and biochemical sensing. Nat Mater 2014;13:400–408.

conjugated with SWCNTs, as SWCNT-ss-DNA-FITC, they can cross the cellular barrier, confirming the ability of SWCNTs in the delivery of macromolecules. SWCNT conjugates showed their preferences over the differential localization within a cell because fluorescence signals from SWCNT-FITC were localized at the vacuoles and cytoplasmic localization for the SWCNT-ss-DNA-FITC conjugates. Neither conjugate exhibited any toxicity (in terms of cell death and changes in normal cytoplasmic fluidity), even after an incubation period of 24 h. Serag et al. demonstrated the tracking of SWCNTs in various cell components based on the fluorescence recovery measurements after photo bleaching (FRAP) [141]. Discrimination between fluorophores and fluorophore-labeled SWCNTs was proposed as

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attributed on the basis of relative differences in mobility. Short SWCNTs and Catharanthus roseus cells were used to explain the facilitation and inhibition of SWCNTs across the cell wall. It was proposed that the intake, consumption, and distribution of SWCNTs inside cells can be easily controlled via the surface functionalities and provide their eliminations at minimum toxicity. SWCNTs in this study were used as carrier-mediated transport (CMT) to target the specific molecules inside cells. The FITC-functionalized SWCNTs (SWCNTs-FITC) were localized inside cell vacuoles, based on the photo-bleaching measurements by fluorescence recovery of suspended cells.The vacuolar uptake of SWCNTs-FITC conjugate were restricted by the probenecid. Then the cytoplasm-accumulated SWCNT-FITC conjugates, which further increases the possibility of the conjugate being taken as the nuclear uptake. After the FRAP experiment, FITC was released from the SWCNT-FITC conjugate. The basis for releasing the FITC molecule is a strong bond, formed by negatively charged SWCNTs and positively charged nucleoplasmic protein. The free FITC accumulated inside the nucleolus. intracellular transport of SWCNTs-FITC is shown inCanas et al. reported that seeds with smaller sizes were greatly affected by nanocarbons, more so than larger seeds [62]. Khodakovskaya et al. reported the effect of SWCNTs, QDs, and mixed conjugates such as SWCNT-QD composites on tomato plants [142]. Only SWCNT-treated plants were able to reach a larger biomass in comparison with QDs and SWCNT-QD treated plants. The chlorophyll content was observed to decrease 1.5-fold along with a 4-fold reduction in root system biomass in SWCNT-QD treated plants [142].

8.4

Effect of nanocarbons on soil microenvironments

Apart from offering many positive solutions, the persistent impact of nanocarbons on the soil microenvironment is crucial for their long-term applications. More significantly, under optimized conditions, nanocarbons can directly be used in the soil by merely watering to benefit the concerned plants. Studies on the impact of nanocarbons on a soil microenvironment (soil microbial community) related to their different shapes, sizes, and surface functionalities are very limited but showed huge differences [146–148, 150]. Alterations in soil microenvironments directly affect the nutrient cycle and its ability toward the sensor for heavy metal contamination and antimicrobial agents [149]. Studies related to enzyme activities are assumed as potential indicators of soil microbial function and are applied for the investigation of soil microenvironments [151, 152]. Shrestha et al. reported the impact of MWCNTs with varying concentration from 10 to 10,000 mg kg
1 on soil microorganisms [153]. Oleszczuk et al. reported the phytotoxicity of sewage sludge containing CNTs at various levels [154]. Chung et al. proposed the short-term effects of MWCNTs on the activity and biomass of microorganisms in two different types of soil via an incubation study [151]. Li and coworkers studied the effect of MWCNTs on the structure and diversity of the bacterial community in activated sludge [155]. A study by Khodakovskaya et al. revealed that tomato plants grown in soil supplemented with CNTs via watering have a substantial effect on plant phenotype and on the composition of soil microbiota [126]. A comparative analysis of species-level phenotypes (OTUs) for treated CNTs and control experiments is shown in Fig. 8.11A. The effect of CNT exposure with varying

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Fig. 8.11 (A) The analysis of common and different OTUs among control and CNT-treated samples obtained from pyrosequencing data. The upper numbers indicate the number of OTUs and numbers in the blank are total read numbers of each OTU. Most of the sequences were common among the three CNT-treated soil samples. Compositions of the bacterial community in control soils compared with those of CNT-treated soils. Relative abundances of phyla (B) and classes (C) were calculated from pyrosequencing data. The phyla and classes were selected by the abundance >0.5% in any of three soil samples. Reprinted with permission from Khodakovskaya MV, Kim B-S, Kim JN, Alimohammadi M, Dervishi E, Mustafa T, et al. Carbon nanotubes as plant growth regulators: effects on tomato growth, reproductive system, and soil microbial community. Small 2013;9:115–123.

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concentration on the composition of bacterial communities at each phylogenetic level in three different types of soil is shown in Fig. 8.11B and C. Likewise, Goyal et al. reported the impact of raw SWCNTs in a soil microbial community, showing that even at short exposure, they significantly affected the microbial community in complex environmental systems [156]. In a similar study, Jin et al. reported that high concentrations of SWCNTs significantly reduced the soil’s microbial enzyme activity and biomass [157]. Turco and coworkers reported the essence of raw (contains metal impurities) and purified SWCNTs functionalized with polyethylene glycol (PEG-SWCNTs) or m-poly aminobenzene sulfonic acid (PABS-SWCNTs) on microorganisms in two dissimilar types of soil [158]. The effects of carboxylic group-functionalized SWCNTs on fungal and bacterial microbial communities presented in soil were investigated by Rodrigues et al. [159]. Similarly, the impact of fullerenes on soil microorganism has been investigated by several researchers with varying results. Tong et al. studied the impact of fullerenes on soil microorganisms and observed that a dose of 1000 μg g
1 of granular C60 slightly affected both gram-positive and gram-negative bacteria in the soil [152]. The proportion of gram-negative bacteria was 5% higher than the control sample. Johansen et al. reported the impact of pristine C60 on soil bacteria and protozymes via the analysis of total respiration, biomass, number, and diversity of bacteria and protozymes [160]. Nyberg et al. investigated the effect of C60 fullerenes on anaerobic bacteria based upon methanogenesis by monitoring the production of CO2 and CH4 [78]. A possible explanation for these differential interactions can be ascribed on the basis of different synthetic routes, which cause changes in various physiochemical properties and thus show varying results based on soil microorganisms.

8.5

Conclusion

The beneficial effect of biochar in its use in the plant kingdom has been known for >1000 years. Recent research has shown that the microsized biochar also contains appreciable amounts of the oxidized derivative of assorted nanocarbons comprised of graphene oxide, oxidized CNTs, carbon nanoonions, and smaller versions of these such as carbon quantum dots. The surface derivatization, under burning of the biowaste in air, incorporates several oxo functionalities in this hydrophobic carbon to transform these to being hydrophilic in nature. Thus pyrolyzed carbonaceous material becomes porous and partially soluble in water. These have a great property to trap nutrients and micronutrients available in the soil. On entering the root of young saplings, these release essential ingredients to facilitate healthy plant growth. Also, these can trap heavy metals under favorable pH of the soil and thus, by arresting such toxic metal ions, may prevent these from interacting with safe plant life. Along with this property, these carbon materials can absorb water to retain it and can slowly release this to young plants when needed; they are therefore beneficial in arid zones or places with water scarcity. Therefore, nanocarbon derivatives as a composite with an optimum fertilizer level can be used as promoters for the plant. Such a composite can compete with natural fertilizers, manure, or organic fertilizers, as here the release of

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nutrients and micronutrients will be slow and as per the need of the young sapling to facilitate its normal and healthy growth. It is to be noted that the commercial production of nanocarbon derivatives will be dirt cheap from biowaste to be used in the composite. This will avoid the excessive use of synthetic chemical fertilizers to save plants and more so will protect the soil structure and its inhabitants. Furthermore, by exploiting the self-fluorescence property of some of these nanocarbons in the domain beyond a plant’s autofluorescence, the health of a plant can be monitored using the selective fluorescence of the nanocarbon as a sensor. In this process, specific drugs in effective doses can also be delivered to the affected part of a plant for its healthy recovery. Finally, it is to be clearly understood that nanocarbons such as carbon nanotubes, when initially used to probe a plant system, have been procured as defect-free carbon nanotubes. These are very hard in nature and readily pierce the cell wall of any biomaterial. The gross toxicity of nanocarbons is thus based on such physical damage. The use of soft, derivative nanocarbons that are dispersible or even soluble in water has recently been used to avoid such physical damage in the plants. Furthermore, the dose-specific use of these hydrophilic nanocarbons could avoid any deleterious effects.

Acknowledgments S.K.S. thanks DST (SB/EMEQ-383/2014) and CSIR (01(2854)/16/EMR-II) for the financial support and S. S. thanks SERB-DST for the Ramanna Fellowship.

References [1] Kroto HW, Heath JR, O’brien S, Curl RF, Smalley RE. C60: buckminsterfullerene. Nature 1985;318:162–3. [2] Iijima S. Helical microtubules of graphitic carbon. Nature 1991;354:56–8. [3] Roy M, Sonkar SK, Tripathi S, Saxena M, Sarkar S. Non-toxicity of water soluble multiwalled carbon nanotube on Escherichia-coli colonies. J Nanosci Nanotechnol 2012;12: 1754–9. [4] Banerjee S, Tripathi SN, Das U, Ranjan R, Jadhav N, Singh VP, et al. Enhanced persistence of fog under illumination for carbon nanotube fog condensation nuclei. J Appl Phys 2012;112:024901. [5] Ugarte DM. Curling and closure of graphitic networks under electron-beam irradiation. Nature 1992;359:707–9. [6] Sonkar SK, Ghosh M, Roy M, Begum A, Sarkar S. Carbon nano-onions as nontoxic and high-fluorescence bioimaging agent in food chain—an in vivo study from unicellular E. coli to multicellular C. elegans. Mater Express 2012;2:105–14. [7] Ghosh M, Sonkar SK, Saxena M, Sarkar S. Carbon nano-onions for imaging the life cycle of Drosophila melanogaster. Small 2011;7:3170–7. [8] Tripathi KM, Bhati A, Singh A, Gupta NR, Verma S, Sarkar S, et al. From the traditional way of pyrolysis to tunable photoluminescent water soluble carbon nano-onions for cells imaging and selective sensing of glucose. RSC Adv 2016;6:37319–29. [9] Dubey P, Tripathi KM, Sonkar SK. Gram scale synthesis of green fluorescent watersoluble onion-like carbon nanoparticles from camphor and polystyrene foam. RSC Adv 2014;4:5838–44.

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