Abstract
Simulated body fluids (SBFs) that mimic human blood plasma are widely used media for in vitro studies in an extensive array of research fields, from biomineralization to surface and corrosion sciences. We show that these solutions undergo dynamic nanoscopic conformational rearrangements on the timescale of minutes to hours, even though they are commonly considered stable or metastable. In particular, we find and characterize nanoscale inhomogeneities made of calcium phosphate (CaP) aggregates that emerge from homogeneous SBFs within a few hours and evolve into prenucleation species (PNS) that act as precursors in CaP crystallization processes. These ionic clusters consist of ∼2 nm large spherical building units that can aggregate into suprastructures with sizes of over 200 nm. We show that the residence times of phosphate ions in the PNS depend critically on the total PNS surface. These findings are particularly relevant for understanding nonclassical crystallization phenomena, in which PNS are assumed to act as building blocks for the final crystal structure.
Highlights
We want to stress that we collectively term all calcium phosphate (CaP) aggregates that occur in solution during the early onset of CaP precipitation, for the sake of generality, prenucleation species or “PNS”
They revealed that PNS form within 5 h after preparing fresh Modified SBFs (mSBFs) and undergo continuous changes throughout a 15 h period
We found that the mSBF system passes from a slow to an intermediate to a fast exchange regime with growing PNS concentration
Summary
Biomineralization is defined as the ability of living organisms to produce mineral phases embedded mostly in calcified tissues such as bone and teeth of vertebrates.[1−3] Beyond its fundamental aspect, its understanding is of imminent interest for developing nature-inspired materials with tailored properties such as improved bone implants[4] or functional materials.[5,6] In this context, simulated body fluids (SBFs) that mimic the ionic composition of human blood plasma[7,8] are ubiquitous and used in a wide array of research fields, from the design of bone graft materials[9,10] and tissue engineering[11,12] to corrosion and biodegradation studies[13−15] to bioinspired material design.[16,17] In particular, SBF plays a crucial role as it is the most widespread biomimetic medium used to assess the bioactivity of materials.[18−20] Notwithstanding its frequent use, the “nanostructure” of SBF remains unsolved to large degrees. A deeper understanding of the structural dynamics is yet essential due to the influence of solvent properties on mineralization and crystallization phenomena.[21,22] environmental conditions often determine the morphology of solid phases precipitating from solution. The highly dynamic behavior of SBFs and its ionic constituents is challenging to characterize experimentally from both, structural and dynamical, viewpoints To overcome this bottleneck, we here present an integrative approach, including the first observation of PNS by real-time nuclear magnetic resonance (NMR) spectroscopy in solution combined with cryoelectron microscopy and calcium potentiometric measurements. Images were recorded on an UltraScan 4000 Gatan (USA) camera with a 4096 × 4096 charge-coupled device
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