Copper and copper nanoparticles: role in management of insect-pests and pathogenic microbes

1 Introduction

The global human population is estimated to be seven billion, and it is projected to be more than eight billion by 2025. About 60% of the people of this population (i.e. 4.43 billion) are living in Asia [1], [2]. It is well known that the progress rate of the world is accelerated due to globalization, but as far as the agriculture is concerned, still a large population living in developing countries is facing the shortage of food due to enormous food wastage. According to the Food and Agriculture Organization (FAO), every year, around 1.3 billion tons of the edible food materials produced for human use is wasted globally. It was reported that wastage of food occurs at various stages of the food supply chain, i.e. from initial agricultural production to final household consumption. However, food wastage is more common in medium and high-income countries [3]. Because of these problems, it is a great challenge to produce enough quantity of food, which can feed seven billion people.

Food crop production can be increased by the development of drought- and insect-pest-resistant crop varieties [4], or alternatively, there is a necessity to develop novel fungicides, fertilizers, and pesticides, etc., which are target specific and efficient than their chemical counterparts available in the markets [5]. Chellaram et al. [6] rightly stated that nanotechnology is the only magical spell, which has a potential to revolutionize the current food and agriculture industries. It can be achieved by the development of various nanotechnological strategies including the nano-based products to overcome the problems associated with food and agricultural loss [6]. The remarkable antimicrobial potential of various metal nanoparticles may be applied for the development of novel nanoantimicrobials (antiviral, antibacterial, fungicidal, and pesticidal agents, etc.), which can be used as an effective tool for the management of plant diseases [5]. Similarly, nano-fertilizers can be formulated, which are reported to be easily absorbed by plants [7]. Moreover, the impact of nanotechnology in the food industry has become more apparent over the last few years. The important fields in which nanotechnology has demonstrated prominent applications include smart food packaging, food manufacturing, processing, and food preservations [8], [9], [10]. Among the various metal nanoparticles, silver (Ag), copper (Cu), and zinc (Zn) nanoparticles are preferentially used as antimicrobial agents. Out of these, silver is expensive metal, and hence, the cost involved in the preparation of silver nanoparticle-based products would be higher. On the contrary, Cu is comparatively cheaper and ubiquitously available. Therefore, the use of copper nanoparticles (CuNPs) in various agricultural applications is cost effective [11].

The main aim of this review is to discuss the different applications of CuNPs in agriculture and food sectors. Here, we have focused on antimicrobial and plant protection properties of CuNPs. Other important aspects such as mechanism involved in the interaction of nanoparticles with plant pathogens, CuNPs as nutrient for plant growth and their phytotoxicity, etc., are also discussed.

2 Copper through the ages

The archaeological evidence suggests that Cu was initially used between 8000 and 5000 B.C., most probably in the regions of Iran, Turkey, Iraq, and India. Archaeologists have also found proof of mining as well as annealing of abundant native Cu in the Upper Peninsula of Michigan in the US dating back to 5000 B.C. [12]. The Sumerians and the Chaldeans existing in prehistoric Mesopotamia are believed to be the leading community to make ample use of Cu, and their Cu crafting acquaintance was introduced to the ancient Egyptians. The Egyptians mined Cu from Sinai and used it to create agriculture and forestry tools such as sickles, hoes, chisels, saws, knives, and also utensils [13]. As per the chronology presented by the British Museum, their supreme epoch ranged amid 2800 and 2000 B.C. where Sumerians used bronze pots along with mixing trays originated in al’Ubaid, near Ur (circa 2600 B.C.) along with the silver-sprouted bronze jugs, saucer as well as drinking vessels, which were meant for traditional ceremonial purpose [14]. In ancient Ayurveda, Cu nanopowder (named as “Tamra Bhasma”) was used for the preparation of traditional medicines [15]. Cu and its compounds possess remarkable bactericidal and fungicidal activity, and therefore, are immensely used by the ancient farmers to control crop diseases. Copper sulfate was used to treat cereal seeds by many farmers in 1761 [16]. Nevertheless, it was not until the 1880s that copper sulfate fungicide was developed in an “accidental” invention of the Bordeaux mixture [17]. The farmers of the Bordeaux region, France, were using a paste of copper sulfate and lime mixture onto the grapes, which were infected with downy mildew. The French botany Professor Pierre-Marie-Alexis Millardet from the Bordeaux University observed that the grapes were free of disease [14]. By 1885, Prof. Millardet completed his experiments, which established the applicability of the mixture against downy mildew disease. By this time, the Bordeaux mixture was known globally as a fungicide [18].

3 Strategic role of CuNPs in food protection

The consumption of food contaminated with toxins of bacteria leads to various food-borne diseases such as campylobacteriosis, listeriosis, hemorrhagic colitis, and salmonellosis. In the US, it was estimated that more than 5000 people died and 76 million people suffered from food-borne illness [19]. Hence, it is essential to search for novel antimicrobial substance, which can solve the problem of food spoilage by microbes. Cu is present in green vegetables, meat, and fish (less than 2 mg). When Cu is present in a low concentration, it acts as a cofactor for metalloproteins and enzymes, whereas at a higher concentration, it performs as an antimicrobial agent against common food-borne pathogens such as Salmonella enterica and Campylobacter jejuni [20] yeast and moulds [21]. Al-Holy et al. [22] reported that copper along with lactic acid can be used in the preservation of infant food. Because of the antimicrobial efficacy of copper, researchers focused on its nanoparticles as an antimicrobial agent against food spoilage microorganisms.

4 CuNPs as a boon to sustainable agriculture

4.2 Management of insect-pests and diseases

The use of Cu in agriculture is the well-known application as a plant protector against diseases caused by fungi and bacteria. As mentioned earlier, it is anticipated that each year, approximately one-third of the global plant harvest vanishes as a consequence of various plant diseases caused by viruses, bacteria, fungi, and insects. To control the menace of pathogens, copper was used in several formulations since ancient times. Copper sulfate is one of those compounds, which has antifungal properties and is a key ingredient in most of the commercially available fungicides for farm and garden. In a typical formulation, copper sulfate mixed with lime or soda ash in water was sprayed onto the plants. Cu-based fungicides create extremely reactive hydroxyl radicals that may damage lipids, DNA, proteins, as well as other bio-molecules, and thus, play a vital role for the prevention of disease occurring in huge diversity of plant species [41]. Copper oxide is also used as a fungicidal agent in the protection of tea, banana, cocoa, citrus, coffee, and other important plant species from major fungal leaf and fruit diseases, for instance, blight, downy or powdery mildew, and rust, etc. [42], [43]. Cu is also used in organic farming as a plant protectant. More than the last 15–20 years, there have been substantial efforts made by the organic farming sector both in search for the copper substitute in crop protection and also in minimizing the amounts of applied Cu. However, until now, there are no active substances, techniques, or methods that could serve as a substitute for Cu. Irrespective of the difficulties for the exploration of Cu substitutes, the research for superior methods and agronomic Cu-minimizing measures need to be intensified in the upcoming years [44]. The overuse of the chemical formulations in the field may lead to environmental hazards as well as soil damage.

With an effective application of emerging technologies like micro-emulsions, liposomes and nano-emulsions in agrochemical formulations reduced the use of petrochemicals in effective delivery of pesticides, herbicides, and fungicides [45]. Therefore, in the recent years, the use of nanotechnology for plant protection has emerged with a great impact [46]. At nanoscale, the active ingredients of the products are able to provide increased efficiency or better penetration of essential components to the plants [47]. Moreover, soda lime glass powder of low melting point containing Cu nanoparticles demonstrated effective antimicrobial activity against bacteria and fungi [48]. The enhanced antimicrobial activity is due to the inhibitory synergistic effect of the Ca²⁺ lixiviated from the glass. The comparative antifungal effect of Cu-based nanoparticles (11–55 nm) and other commercial agrochemicals on Phytophthora infestans infected tomato (Solanum lycopersicum) led to the conclusion that synthesized Cu-based nanoparticles possess more activity than the commercial agrochemicals at a low concentration [49]. In another study, the nano-copper was used against Xanthomonas axonopodis pv. punicae, a causative agent of bacterial blight in pomegranate [50]. Nano-copper inhibited the growth of bacterium at 0.2 ppm only, i.e. >10,000 times lesser than it is generally suggested for Cu-oxychloride. Cu with chitosan complex nano-gels was reported to inhibit the growth of cereal plant pathogenic fungus Fusarium graminearum due to their synergistic effect. These nano-hydrogels can be considered as a new generation of Cu-based bio-pesticides because of their bio-compatibility [51]. In vitro antifungal activity of CuNPs synthesized by chemical method was studied against plant pathogens, namely, F. oxysporum, Alternaria alternata, Curvularia lunata, and Phoma destructiva [52]. The study demonstrated a remarkable antifungal activity against the test fungi and recommended application of CuNPs as an antifungal agent in nano-formulation. Shende et al. [53] also reported the inhibitory effect of biogenically synthesized CuNPs on the development of F. oxysporum, Fusarium culmorum, and F. graminearum. Chemically synthesized CuNPs also inhibited the growth of F. culmorum, F. oxysporum, and Fusarium equiseti [54]. The nano-based products such as nano-pesticides, nano-fungicides, nano-insecticides, etc., are already in the market, while many others are under the developing stage [55]. Hence, in the agriculture sector, CuNPs will be the most demanding nano-formulations, which could be used in plant protection in the near future by various modes. Figure 1 shows a schematic illustration for various modes by which CuNPs can protect the plants and also help in its growth promotion (Cajanus cajan plant is taken as the model plant). Moreover, the possible mechanism for the antimicrobial action of CuNPs was discussed in detail in the subsequent section.

Figure 1:

Schematic illustration of CuNPs application for protection of plants and its growth promotion. CuNPs provide protection to crop plants against (A) fungal infections (B) bacterial infections (C) by promoting plant growth.

5 Mechanism of CuNPs-microbe interaction

As previously mentioned, the antimicrobial activity of CuNPs was well studied, but the mechanism of antimicrobial action is not well understood. A few reports are available on the mechanism concerning the antimicrobial action of CuNPs. CuNPs interact with the microbial cell wall because of its affinity toward the carboxyl group present on the microbial surface [11], [56]. Generation of reactive oxygen species (ROS), membrane damage, loss of enzyme activity, protein dysfunction, etc., are accountable for the antimicrobial action of nanoparticles [57], [58]. Raffi et al. [59] investigated the antibacterial behavior of CuNPs. It was revealed that when CuNPs come in contact with a bacterial cell, it releases Cu ions, which are absorbed on the cell wall leading to the generation of ROS and loss of membrane integrity [59]. Similarly, CuNPs are also responsible for the disruption of cellular metabolic pathways, formation of pits in a membrane, development of oxidative stress, which eventually cause cell death [27], [53], [60]. It was proposed that Cu polymer nanocomposites can be an effective antibacterial agent. The authors studied that the bactericidal effect of nanocomposites was due to the release of Cu ions and CuNPs. The released Cu ions, upon interaction with an outer bacterial membrane, interact with amines and carboxyl groups in the peptidoglycan layer as well as with sulfhydryl groups, which leads to denaturation of the protein. Cu ions (Cu²⁺) bind to DNA and involve in cross-linking of nucleic acid strands, resulting into disorganization of the helical structure. In a similar way, the released CuNPs stick to the cell membrane and penetrate into the bacterium via endocytosis [24]. The hitherto known mechanisms for the bactericidal and fungicidal action of CuNPs are illustrated in Figure 2. The susceptibility of microbes to the microbicidal action of CuNPs mainly depends upon the particle size, electrostatic attraction between microbial cell and nanoparticles, composition of microbial cell wall and membrane, and hydrophobic or hydrophilic nature of the nanoparticles.

Figure 2:

Schematic illustration for possible mechanism of CuNPs on microbes: CuNPs act on microbial cell wall and disturbs its components, which leads to membrane damage. Membrane damage decreases the electrochemical potential, which affects membrane integrity. In addition, CuNPs target DNA, interferes with protein synthesis, and cause damage leading to death of microbial cell.

6 Emerging concerns of CuNPs toxicity

As discussed in the earlier section, CuNPs are a boon to the agricultural sector. However, on the other side, the concerns associated with them are also important particularly the accumulation, biomagnification, and biotransformation of nanoparticles in food crops. These issues warrant more attention and needs in-depth investigation [61]. The concentration of the nanoparticles in the surrounding environment is the major factor contributing to the harmful effects to food crops. Higher accumulation of nanoparticles into the soil will result in their higher uptake through plant roots, thus, showing enhanced harmful effects to the plants. For instance, the concentration of CuNPs in the range of 200–1000 mg/l was reported to exert toxic effects on Triticum aestivum, Cucurbita pepo, and Phaseolus radiatus seeds [62], [63]. Under microscopic study, it was found to penetrate into plant cell membrane. In a 14-day exposure study, at 1000 mg/l concentration of CuNPs, reduction in biomass of C. pepo was recorded [64]. Most probably, due to accumulation of CuNPs in the plant tissues, it interacts with all of its components, and consequently, it disturbs the normal functioning of plant tissues and cells [65]. Bradfield et al. [66] demonstrated the probable effect of CuNPs on sweet potato (Ipomoea batatas). The study claimed that CuNPs show adverse effects on the tuber biomass of sweet potato and were found to be accumulated at the higher concentration in their peel compared to the flesh. The group claimed that prior to the accumulation, nanoparticles underwent dissolution to release the ions [66].

The above studies proved that, if nanoparticles are used in an uncontrolled manner for increasing the plant productivity or in food industries, they may harm the ecosystem. However, concentration, size, shape, and types of nanoparticles are the key factors, which play an important role in their toxicity. As discussed earlier, in agriculture, nanoparticles can be used in the form of foliar spray as described in Figure 1 or supplied through the roots of the plant. In case of foliar application of nanoparticles, these can enter into plant cells through the organs and tissues such as cuticles, trichomes, stomata, stigma, and hydathodes. When nanoparticles are supplied through the roots, they enter into the plant cells through the root tips, lateral roots, root hairs, root wounds, and root junctions (Figure 3) [67].

Figure 3:

Probable entry points for nanoparticles in plants (Reproduced from Wang et al. [67] with copyright permission from Elsevier).

However, the entry of the different types of nanoparticles in various plant tissues and cells occur through different ways, which mainly include binding to carrier proteins, through aquaporins, ion channels, or by endocytosis, by creating new pores, or by binding to organic chemicals present in the environment. Apart from these, the increased surface area-to-mass ratio enhances the reactivity of nanoparticles with their surroundings, and hence, they are able to form complexes with membrane transporters or root exudates, which facilitate their entry into the plant cells (Figure 4) [65].

Figure 4:

Probable avenues of cellular uptake of the nanoparticles in a plant cell (Reproduced from Rico et al. [65] with permission from American Chemical society).

Once nanoparticles enter into the plant cells, they may travel apoplastically or symplastically from one cell to the other through the plasmodesmata. Wang et al. [26] reviewed the uptake and translocation of engineered nanoparticles (ENPs). They proposed that nanoparticles have to cross a series of chemical and physiological barriers for their transport from one cell to another, which strictly depends on their size generally referred to as size exclusion limits (SELs). It means nanoparticles having a specific size range can enter into the specific cells. In apoplastic transport pathway, transport of nanoparticles is controlled by the SEL of the cell walls, which allows entry of nanoparticles within the size range of 5–20 nm [68], [69], [70]. The Casparian strip allows nanoparticles having an SEL of <1 nm [71]. Moreover, in the case of the symplastic transport pathway, the transport of ENPs depend upon the SEL of the plasmodesmata, which is about 3–50 nm in diameter [72]. The SELs for other plant cells and tissues are provided in Figure 5.

Figure 5:

Size exclusion limits (SELs) of various plant tissues (barriers) for the uptake of nanoparticles (Reprinted from Wang et al. [67] with copyright permission from Elsevier).

The entry of nanoparticles in plant cells result into their accumulation in cells. The presence of nanoparticles negatively affects the plant’s metabolism and development. The toxicity of nanoparticles to plant tissues occurs due to many chemical or physical effects. There are some possible ways proposed by the researchers to explain how nanoparticles interact with plant cells and cause toxicity [67], [70]. These include the (i) chemical effects due to dissolution and release of toxic Cu²⁺ ions from CuNPs, (ii) binding and interaction of nanoparticles with biomolecules such as proteins and nucleic acid (DNA) either by non-covalent or covalent mechanisms. These interactions release surface free energy, leading to surface reconstruction of biomolecular structures [73], [74], (iii) the production of excess ROS through redox cycling and the Fenton reaction in the presence of Fe²⁺ [67], (iv) oxidation and catalysis of surface biomolecules through catalytic reactions [75], (v) size- or shape-dependent mechanical damage [76]. Considering these facts, we proposed the hypothetical mechanism to understand the toxicity of CuNPs to plants. Figure 6 presents the proposed mechanisms by which CuNPs can cause phytotoxicity.

Figure 6:

Schematic representation of several proposed mechanisms by which CuNPs cause phytotoxicity: (1) dissolution of nanoparticles and release of toxic ions, (2) binding and interaction of nanoparticles with biomolecules, such as proteins and DNA, (3) production of ROS through redox cycling and Fenton reaction, (4) oxidation and surface catalytic reactions, and (5) size-or- shape-dependent mechanical damage.

The aforementioned studies confirmed the phytotoxicity of CuNPs at a higher concentration. The studies performed in the past proposed a possible mechanism leading to harmful effects of CuNPs on plants. Moreover, the exact toxicity mechanisms involved in seed germination, growth of the roots and shoots, and other changes occurring in plants at the cellular and molecular level due to stress warrant further investigation.

7 Future perspectives

CuNPs possess potent antimicrobial activity against food spoilage microbes. The bimetallic nanoparticles such as Zn-Cu and Ti-Cu can be synthesized, which can inhibit food-borne pathogens and enhance the shelf life of food. Both zinc oxide nanoparticles and titanium dioxide nanoparticles are FDA approved, and therefore, can be used as a food additive for its preservation along with CuNPs. Bimetallic nanoparticles can be used as an effective plant protector. They can be utilized as a vehicle for the delivery of antioxidants, enzymes, and anti-browning agents in order to improve the quality of food. CuNPs can help in the targeted delivery of pesticides and can be stabilized with polyvinylpyrrolidone to enhance its antimicrobial activity. Incorporation of CuNPs in polymer silicate nanocomposite improves the antimicrobial efficacy, physical strength, and thermal stability of the film. CuNP-loaded naturally occurring antimicrobial peptides may be used in enhancing the shelf-life of food. Nisin and pediocin are some antimicrobial peptides used in the preservation of food.

The combination of CuNPs or bimetallic nanoparticles such as Ag-Cu, Cu-Zn, Cu-Ti along with essential oil will be novel antimicrobial agents for the inhibition of food spoilage microbes. The biopolymeric film containing CuNPs along with essential oil can be used in the preservation of food. Further, CuNP-impregnated paper can be a prospective candidate for packaging of food and fruits, by preventing food spoilage and off-odor of the food.

Cu is one of the most essential elements required by the plants. CuNPs showed potential activity against fungi and insect-pests of crop plants and, hence, can be used in the novel formulation such as CuNP-based nanopesticides, nanoherbicides, and nanofertilizers, which will be required in lower quantity. It can also minimize the toxicity problem due to excessive use of pesticides. CuNP-based biosensors can be used for the management of pests and also in the detection of pathogens responsible for food spoilage. Finally, the use of CuNPs may revolutionize the field of food and agriculture industries.

8 Conclusions

The pathogens and insect-pests are responsible for the reduced crop yield, and therefore, their management to minimize the yield loss is essentially required. During the course of time, many researchers have developed remedies to control them to some extent. Cu is a micronutrient, essential for plant growth, whereas it is also used as a fungicide and bactericide. However, with such benefits, there are some harmful effects to the soil ecosystem.

Various scientific groups across the globe have demonstrated that CuNPs are better plant protectors. Compared to bulk copper, nanocopper (CuNP) has been proved to be more efficient in controlling various common fungal and bacterial infections. The CuNPs deteriorate the cell wall and also alter the functioning of the cell organelle, leading to the death of pathogens. Therefore, such properties of nanoscale copper make it a better candidate for use in combating plant diseases. Moreover, it can also be applied for the packaging of food and food products to enhance its shelf-life.

Finally, many reports have claimed that CuNPs also demonstrated harmful effects on plant when used beyond a certain concentration. Application of CuNPs in a higher concentration can affect the plant metabolism, thereby, affecting the yield. Therefore, for safer use of CuNPs in agriculture, there is an urgent need to develop a clear understanding of nanoparticle-plant interaction. If it is done in the right direction, CuNPs will be a great boon for agriculture and the food industry to produce and store food for a longer period of time with the negligible loss to feed growing global human population.