Use of agrochemicals, such as fertilizer and pesticide, is essential for agricultural production. However, the utilization rate of most chemical fertilizers in plants is less than 50%
[37], which not only limits efficient agricultural production but also results in environment pollution. Most fertilizers are water-soluble and are easily leached in soil, resulting in environmental pollution and increased costs due to possible multiple applications of fertilizers
[38]. Compared with their conventional counterparts, the efficacy gain of nanofertilizers is 18–29% higher
[39]. Nanoparticles containing one or more elements required for plant growth can be directly used as nanofertilizers
[40][41][42][43]. As a carrier to enable slow and/or targeted delivery of nutrients into plants, nanomaterials can also be used as nanofertilizers
[44]. Regarding the use of nanofertilizers in agriculture, please refer to previous good reviews
[45][46]. Here, the researchers mainly focus on CNMs as nanofertilizers in agriculture.
Having the advantages of stable molecular structure, good biocompatibility, less toxicity, and uniform dispersion in application medium, CNMs can be used as good fertilizer carriers. For example, GO nanomaterials are good carriers of trace elements
[47]. The oxygen-containing groups on its surface can electrostatically adsorb trace elements and also have two-phase release characteristics, showing quick release at the early stage, and then slow and continuous release. Indeed, GO sheets are able to deliver Zn and Cu elements more efficiently in wheat than zinc or copper salts
[44]. In addition to GO, negatively charged copper CNF can also be used as a slow-release carrier of micronutrient copper
[48]. CNF-Cu can transfer from roots to shoots through xylem and slowly release copper in plants, showing significantly improved water absorption capacity, germination rate, root/shoot ratio, and protein content of chicory. Negatively charged fullerenol can be used as a leaf slow-release fertilizer to promote the absorption of Fe
2+ in cucumber leaves and to alleviate the symptoms of iron deficiency
[49]. Furthermore, previous research showed that adding nanocarbon to slow-release fertilizer can significantly improve rice yield and nitrogen use efficiency, and reduce nitrogen loss, indicating that nanocarbon can be used as an environment-friendly slow-release fertilizer coating material
[50]. These results show that CNMs can be good carriers to deliver nutrients to plants or to improve the effect of fertilizers.
In addition to fertilizers, pesticides are another major component of agrochemicals for agriculture. However, public concerns exist regarding the biosafety and pollution issues of traditional pesticides due to their easy leaching, volatilization, and loss properties
[50]. Excessive use of pesticides has also caused many problems that need to be addressed urgently, such as plant disease resistance, destruction of soil biodiversity, and adverse effects on human health and the environment
[51]. Therefore, more efficient and environmentally friendly solutions regarding the use of pesticides are encouraged. Nano-pesticides (including nano-insecticides, nano-herbicides, and nano-fungicides) can reduce volatilization and degradation of pesticides, improve utilization efficiency, reduce the use of pesticides, and alleviate environmental risks
[52][53]. In addition to adsorbing harmful organic matter to reduce the solubility and bioavailability of organic matter, CNMs are promising materials that can be used as a pesticide carrier to improve the utilization efficiency of pesticides
[54][55][56][57]. Currently, due to the unique physical and chemical properties mentioned above, GO is one of the most widely used carriers in the field of nano-pesticides. For example, rGO has the advantages of high pesticide adsorption capacity (up to 1200 mg/g for chloropyrifos), low toxicity, good antibacterial performance, insensitivity to pH value change, and the ability to be reused
[57]. GO loaded with red spider insecticide can be completely adsorbed on the surface of the red spider and have a strong toxic effect
[58]. GO can carry different types of pesticides through surface modification. Tong et al. (2018) used polydopamine-modified GO as the carrier of water-soluble pesticides, which alleviated the issue of easy loss of water-soluble pesticides, enabled controlled release of pesticides, enhanced the adhesion between pesticides and plants, and thus improved the utilization efficiency
[59]. Hydrophobic pesticides can be loaded by polylactic acid-modified GO
[60]. However, GO also has disadvantages, such as low stability under acidic solution
[61]. Song et al. (2019) developed nano-biochar as the carrier of emamectin benzoate, and used carboxymethyl chitosan as the pH-responsive switch to control the delivery and release of emamectin benzoate. As a result, the water solubility, dispersion stability, and UV resistance of the delivery system were significantly improved, ensuring its long-term control of pests
[62]. In addition to GO, CNTs can also be used as a sustained-release system for pesticides. For example, MWCNTs grafted with polycitric acid (PCA) can deliver zineb, an antifungal pesticide. Compared with zineb in bulk, the novel CNT-PCA-Zineb hybrid material has better water solubility, higher stability, and stronger toxicity to Alternaria
[63].
Overall, CNMs are good candidates to deliver agrochemicals into plants with better efficiency than conventional fertilizers and pesticides. However, there is an urgent need to understand how plants respond to their exposure. Moreover, the addition of CNMs has increased the complexity of the agro-ecosystem; whether they represent a new pollutant or a new opportunity is discussed in detail by Kah
[64]. With regard to the future of nano-agrochemicals, it is necessary to fully consider the views in many fields of science, industry, and regulation, so that the agrochemicals sector can make use of nanotechnology and reduce its negative impact on human beings and the environment as much as possible.