International audienceNanocrystalline calcium phosphate apatites constitute the main inorganic part of hard tissues, and a growing focus is devoted to prepare synthetic analogs, so-called "biomimetic", able to precisely mimic the morphological and physico-chemical features of biological apatite compounds. Both from fundamental and applied viewpoints, an accurate characterization of nanocrystalline apatites, including their peculiar surface features, and a deep knowledge of crystallization aspects are prerequisites to attempt understanding mineralization phenomena in vivo as well as for designing innovative bioactive materials that may then find applications in bone tissue engineering, either as self-supported scaffolds and fillers or in the form of coatings, but also in other domains such as drug delivery or else medical imaging. Also,interfacial phenomena are of prime importance for getting a better insight of biomineralization and for following the behavior of biomaterials in or close to their final conditions of use. In this view,both adsorption and ion exchange represent essential processes involving the surface of apatite nanocrystals, possibly doped with foreign elements or functionalized with organic molecules of interest. In this review paper, we will address these various points in details based on a large literature survey. We will also underline the fundamental physico-chemical and behavioral differences that exist between nanocrystalline apatites (whether of biological origin or their synthetic biomimetic analogs) and stoichiometric hydroxyapatite

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Demineralization of dental hard tissue is a widespread problem and the main responsible for dental caries and dentin hypersensitivity. The most promising strategies to induce the precipitation of new mineral phase are the application of materials releasing gradually Ca2+ and PO43− ions or mimicking the mineral phase of the host tissue. However, the design of formulations covering both processes is so far a challenge in preventive dentistry. In this work, we have synthesized innovative biomimetic amorphous calcium phosphate (ACP), which has been, for the first time, doped with fluoride ions (FACP) to obtain materials with enhanced anti-caries and remineralizing properties. Significantly, the doping with fluoride (F) did not vary the physico-chemical features of ACP but resulted in a faster conversion to the crystalline apatite phase in water, as observed by in-situ time-dependent Raman experiments. The efficacy of the as synthesized ACP and FACP samples to occlude dentinal tubules and induce enamel remineralization has been tested in vitro in human molar teeth. The samples showed good ability to partially occlude the tubules of acid-etched dentin and to restore demineralized enamel into its native structure. Results demonstrate that ACP and FACP are promising biomimetic materials in preventive dentistry to hinder demineralization of dental hard tissues.

Bio-inspired apatite nanoparticles precipitated in the presence of citrate ions at increasing maturation times are characterized in terms of structure, size, morphology, and composition through advanced X-ray total scattering techniques. The origin of the platy crystal morphology, breaking the hexagonal symmetry, and the role of citrate ions is explored. By cross-coupling the size and shape information of crystal domains with those obtained by atomic force microscopy on multidomain nanoparticles, a plausible mechanism underlying the amorphous-to-crystal transformation is reconstructed. In the present study, citrate plays the distinct roles of inducing the platy morphology of the amorphous precursor and controlling the thickness of the Ca-defi cient apatite nanocrystals. These fi ndings can open new scenarios also in bone mineralization, where citrate might have a broader role to play than has been thought to date.

Biomimetic calcium phosphate nanoparticles (CaP) have been actively used in biomedicine, due to their high biodegradability and biocompatibility. However, much less progress has been made regarding their application in precision agriculture, i.e., for the controlled delivery of active species to plants. Herein, we report a straightforward and green synthetic method to dope CaP with potassium (K) and nitrogen (N, as nitrate and urea). By modulating the synthetic conditions in terms of maturation time (at 37 °C) and doping, we prepared K-and N-doped nanoparticles in the form of either amorphous calcium phosphate (ACP) or nanocrystalline apatite (Ap) and studied the impact of the dopants on the ACP-to-Ap transformation pathways. Importantly, we found out that ACP, isolated at low maturation times, incorporates nitrogen (in the form of nitrate and urea) to a larger extent than Ap (2.6 vs 1.1 wt %, respectively). Multinutrient nanofertilizers (so-called nanoU-NPK) with the following composition (wt %) were obtained: Ca (23.3), P (10.1), K (1.5), NO 3 (2.9), and urea (4.8). The nanoU-NPK provides a slow and gradual release of the most important plant macronutrients (NPK), with N in two chemical forms, and different kinetics. The concentration of nutrients supplied by 10 g L −1 of nanoU-NPK to the media after 1 week (in mg L −1 ) was Ca (27.0), P (6.2), K (41.0), NO 3 (134.0), and urea (315.0). Preliminary tests on durum wheat have shown that the application of nanoU-NPK allows reducing the amount of nitrogen supplied to the plants by 40% with respect to a conventional treatment, without affecting the final kernel weight per plant. The application of these slow-release NPK nanofertilizers is a promising strategy toward enhancing the efficiency of the fertilization, complying with the concept of precision agriculture.