While slow release of chemicals has been widely applied for drug delivery, little work has been done on using this general nanotechnology-based principle for delivering nutrients to crops. In developing countries, the cost of fertilizers can be significant and is often the limiting factor for food supply. Thus, it is important to develop technologies that minimize the cost of fertilizers through efficient and targeted delivery. Urea is a rich source of nitrogen and therefore a commonly used fertilizer. We focus our work on the synthesis of environmentally benign nanoparticles carrying urea as the crop nutrient that can be released in a programmed manner for use as a nanofertilizer. In this study, the high solubility of urea molecules has been reduced by incorporating it into a matrix of hydroxyapatite nanoparticles. Hydroxyapatite nanoparticles have been selected due to their excellent biocompatibility while acting as a rich phosphorus source. In addition, the high surface area offered by nanoparticles allows binding of a large amount of urea molecules. The method reported here is simple and scalable, allowing the synthesis of a urea-modified hydroxyapatite nanohybrid as fertilizer having a ratio of urea to hydroxyapatite of 6:1 by weight. Specifically, a nanohybrid suspension was synthesized by in situ coating of hydroxyapatite with urea at the nanoscale. In addition to the stabilization imparted due to the high surface area to volume ratio of the nanoparticles, supplementary stabilization leading to high loading of urea was provided by flash drying the suspension to obtain a solid nanohybrid. This nanohybrid with a nitrogen weight of 40% provides a platform for its slow release. Its potential application in agriculture to maintain yield and reduce the amount of urea used is demonstrated.

Worldwide, there is an amplified interest in nanotechnology-based approaches to develop efficient nitrogen, phosphorus, and potassium fertilizers to address major challenges pertaining to food security. However, there are significant challenges associated with fertilizer manufacture and supply as well as cost in both economic and environmental terms. The main issues relating to nitrogen fertilizer surround the use of fossil fuels in its production and the emission of greenhouse gases resulting from its use in agriculture; phosphorus being a mineral source makes it nonrenewable and casts a shadow on its sustainable use in agriculture. This study focuses on development of an efficient P nutrient system that could overcome the inherent problems arising from current P fertilizers. Attempts are made to synthesize citric acid surface-modified hydroxyapatite nanoparticles using wet chemical precipitation. The resulting nanohybrids were characterized using powder X-ray diffraction to extract the crystallographic data, while functional group analysis was done by Fourier transform infrared spectroscopy. Morphology and particle size were studied using scanning electron microscopy along with elemental analysis using energy-dispersive X-ray diffraction spectroscopy. Its effectiveness as a source of P was investigated using water release studies and bioavailability studies using Zea mays as the model crop. Both tests demonstrated the increased availability of P from nanohybrids in the presence of an organic acid compared with pure hydroxyapatite nanoparticles and rock phosphate.

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Application of micronutrients through seed amendments is an interesting option for field crops, allowing a uniform distribution of fertilizers in a seed batch. This process can be further improved by developing advanced materials for seed coatings that have the ability to carry out the delivery of plant nutrients with greater precision leading to enhanced germination. Herein, we disclose a novel nanoseed coating based on zinc-doped urea−hydroxyapatite nanohybrids synthesized via a one-pot in situ sol−gel synthetic route. The formation of a substituted solid with an altered chemical environment and the lattice shrinkage of hydroxyapatite nanoparticles due to the successful doping of zinc and urea incorporation were evident from characterization studies. The developed nanoseed coating was effective in increasing the rate of growth of Zea mays seeds by ∼69% and the germination rate by 19%, claiming its suitability as a potential candidate for the delivery of nitrogen, phosphorus, and zinc at the seedling stage.

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A 2 : 1 urea • adipic acid cocrystal was obtained in two polymorphic forms (Form I reported earlier, and Form II synthesized in this study) using mechanochemistry as well as solution crystallization. Lower solubility and leaching study showed the newly synthesized urea • adipic acid 2 : 1 cocrystal to be an efficient sustained-release nitrogen fertilizer compared to commercially available urea.

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