The role of nanoparticle structure and morphology in the dissolution kinetics and nutrient release of nitrate-doped calcium phosphate nanofertilizers

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The idea behind this paper is part of a larger multidisciplinary project combining nanotechnology concepts, green chemistry principles and forefront experimental/modelling techniques, for designing, preparing and testing efficient and sustainable nanofertilizers.

There are two major questions motivating this paper: why nanofertilizers? And why we consider calcium phosphate nanoparticles an ideal system for sustainable fertilization?

The Food and Agriculture Organization (FAO) of the United Nations (2017) is best answering the first question: according to the 2017 annual report1, the world population growth is expected to approach 10 billion by 2050. In this context, intensifying the agriculture productions to match the food demand while keeping them (and the whole supply chain) sustainable and safe remains a global challenge. Conventional fertilizers (provided as highly soluble salts) are rather inefficient. More than 50% are lost in the environment, which makes them among the major anthropogenic factors influencing climate changes, groundwater eutrophication, consumption of non-renewable resources.

The way for us to contribute to was fostering a sustainable and viable solution through Nanotechnology. This is strictly linked to the second question, for which we took inspiration from nature.

Calcium phosphate (CaP) minerals are the main inorganic component of bone tissue in vertebrates. Here, a complex biomineralization process assists the mineral with growth (likely from an amorphous metastable precursor) of highly defective, thin nanoplates (NPLs) of apatite, within a collagen matrix. The process is dynamic and dissolution/recrystallization phenomena are both at work, controlled by many diverse factors (pH, ionic strength, Ca/P ratio, nanoparticles size, structural defects and ionic substitutions, particularly carbonate ions).

Synthetic CaP nanoparticles (NPs) mimicking bone mineral structure and composition (so called “biomimetic”) are emergent materials for sustainable applications in agriculture. They are sparingly soluble salts, can be prepared via green chemistry routes from aqueous solutions and used as P-fertilizers from which nutrient release is (more efficiently than in conventional ones) controlled by the slow(er) NPs dissolution. An additional advantage of biomimetic apatite relies on its pronounced ability of incorporating a number of exogenous ions, making the doping with nitrate ions an appealing step, in view of potential applications as N,P-nanofertilizers.

In this work, we explored the role of size, defects and morphology in the solubility and dissolution rates of CaP NPs. Having a quantitative robust description of these features remains one of the weakest aspects towards the comprehension of dissolution-related phenomena that are of fundamental importance in nano fertilization through NPs. We prepared nitrate-free and nitrate-doped biomimetic CaP materials, both as amorphous NPs and amorphous-apatite hybrid NPLs and developed a complex atomic-to-nanometre scale modelling combining X-ray scattering experiments in the small- and wide-angle regions. The joint analysis suggested that the amorphous NPs are spherical in shape and that apatite and amorphous components are interconnected in a core-crown-like structure in the NPLs2.

The kinetic profiles of Ca2+ ions release (measuring the CaP dissolution) were found to differ for spherical NPs and NPLs, indicating the direct role of the particles nature and morphology in guiding the kinetics of dissolution. The study of nitrate release suggested that small amounts of nitrate ions can be incorporated in the apatite crystal structure (not obvious according to the state-of-art literature), in line with detectable variations of the apatite lattice parameters (beyond carbonation). In purely amorphous NPs, the nitrate ions were mostly released within the first ten hours, as expected for weakly adsorption on the NPs surface. In contrast, gradual release follows that of Ca2+ ions in the NPLs over days. This finding  supported the appealing hypothesis that nitrates are released from the crystalline core through the basal faces of the NPLs rather than from the amorphous crown.

Very promising results using urea-decorated CaP NPs have been obtained on durum wheat.3,4 New ones will be soon available for grapevines and hydroponic cultivation. Follow us at the project webpage!

 

Link to the manuscript: www.nature.com/articles/s41598-020-69279-2

 

References

1. The future of food and agriculture –Trends and challenges. Annu. Rep. 2017, 12.

2. Bertolotti, F. et al., On the amorphous layer in bone mineral and biomimetic apatite: A combined small- and wide-angle X-ray scattering analysis, Acta Biomater. (2020), doi:10.1016/j.actbio.2020.04.026.

3. Ramírez-Rodríguez, G. B. et al., Engineering Biomimetic Calcium Phosphate Nanoparticles: A Green Synthesis of Slow-Release Multinutrient (NPK) Nanofertilizers, ACS Appl. Bio Mater. 2020, 3, 3, 1344–1353.

4. Ramírez-Rodríguez, G. B. et al., Reducing Nitrogen Dosage in Triticum durum Plants with Urea-Doped Nanofertilizers, Nanomaterials, 2020, 10, 1043.

Antonietta Guagliardi

Researcher, CNR

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