Structural and compositional features of amorphous calcium phosphate at the early stage of precipitation

Abstract

Precipitates formed at an early stage (during the first 6 h) of the hydroxyapatite crystallization of a solution were studied. A nitrous synthesis was used (0.583M (NH(4))(2)HPO(4) and 0.35 M Ca(NO(3))(2).4H(2)O solutions at pH 11-12, 21 degrees C, fast mixing, lyophilization of aliquots). Although XRD patterns indicated an amorphous calcium phosphate (ACP), IR spectra revealed apatite nanocrystals in the precipitates. Some amount of free calcium was found in the mother solution by mass spectrometrical analysis of the aliquots. This amount considerably decreased as the synthesis proceeded, however, the decrease had a slight effect on the crystallinity of the precipitates. A new suggestion on the nature of delayed crystallization (under conditions as those in the present study) was proposed. The free calcium adsorbed by the nanoparticles from the solution formed a shell around a particle because the calcium diffusion into the bulk was poor at the low synthesis temperature. As such, the encapsulation delayed the crystallization of the nanoparticles. Evidence for this suggestion was given. New possibilities were proposed for preparation of bioactive materials of desired composition based on the structural and compositional peculiarities of the X-ray diffraction-amorphous calcium phosphates.

Nanodimensional and Nanocrystalline Calcium Orthophosphates

“…Thus, with the exception of a hydrothermal synthesis [256][257][258], in aqueous solutions only CDHA might be prepared [148,[204][205][206][259][260][261][262][263]. As apatites (CDHA, HA and FA) belong to the sparingly soluble compounds (Table 1), simple mixing of calcium-and orthophosphate-containing aqueous solutions at pH > 9 results in formation of extremely supersaturated solutions and, therefore, a very fast precipitation of the tremendous amounts of very fine crystals [264], initially of ACP, those afterwards are re-crystallized into apatites [204][205][206][265][266][267][268]. The dimensions of the precipitated nano-sized crystals might be slightly increased by the Ostwald ripening approach (maturation), that is, by boiling and/or ambient aging in the mother liquid (Fig.…”

“…The dimensions of the precipitated nano-sized crystals might be slightly increased by the Ostwald ripening approach (maturation), that is, by boiling and/or ambient aging in the mother liquid (Fig. 4) [156,169,203,235,257,[267][268][269][270][271][272]. Heat treatment of ACP might be applied as well [273].…”

Amorphous dysprosium carbonate: characterization, stability, and crystallization pathways

“…These amorphous precursors are thermodynamically unstable and often transform into crystalline, but metastable intermediate phases before finally crystallizing into the thermodynamically stable solid endproducts (Meldrum and Cölfen 2008). Most of the research on such amorphous precursors has been focused on their compositional-dependent stabilities and on their formation and transformation conditions (i.e., amorphous calcium carbonates (Radha et al 2010;Goodwin et al 2010;Bolze et al 2002;Rodriguez-Blanco et al 2008) and amorphous calcium phosphates (Zyman et al 2010;Combes and Rey 2010). However, there are also salt systems where such amorphous precursors do either not crystallize even after very long time periods (Roncal-Herrero et al 2009;Tobler et al 2009) or where the formation of the thermodynamically stable end phase does not require an amorphous precursor as an initiators for the crystallization process (e.g., calcium sulfate, Van Driessche et al 2012;La and Nd-phosphates, Roncal-Herrero et al 2011).…”

Nano‐Sized Calcium Phosphate (CaP) Carriers for Non‐Viral Gene/Drug Delivery

“…Figure 6.3 shows a schematic representation of multiple types of CaP NPs for drug delivery application [82]. Synthesis of such CaP NPs encapsulating gene/drug can be achieved through various methods such as wet precipitation [83], sol-gel [84,85], spray-drying [86], micelle mediated [87,88], and double emulsion mediated synthesis [89].…”

“…In the study of Zyman, et al [83], solutions of Ca(N0 3 )»4H 2 0 and (NH 4 )HP0 4 are used for wet precipitation methods. Following the reactions between the solutions of Ca(N0 3 )»4H 2 0 and (NH 4 )HP0 4 , the aliquots were frozen at -23°C in order to stop the reaction.…”