Biological hard tissues such as bones always contain extremely high levels of citrate, which is believed to play an important role in bone formation as well as in osteoporosis treatments. However, its mechanism on biomineralization is not elucidated. Here, it is found that the adsorbed citrate molecules on collagen fibrils can significantly reduce the interfacial energy between the biological matrix and the amorphous calcium phosphate precursor to enhance their wetting effect at the early biomineralization stage, sequentially facilitating the intrafibrillar formation of hydroxyapatite to produce an inorganic-organic composite. It is demonstrated experimentally that only collagen fibrils containing ≈8.2 wt% of bound citrate (close to the level in biological bone) can reach the full mineralization as those in natural bones. The effect of citrate on the promotion of the collagen mineralization degree is also confirmed by in vitro dentin repair. This finding demonstrates the importance of interfacial controls in biomineralization and more generally, provides a physicochemical view about the regulation effect of small biomolecules on the biomineralization front.

Generally, a solution nucleation model is used to study biomineralization kinetics. However, we found that the amorphous calcium phosphate (ACP)-mediated hydroxyapatite (HAP) nucleation in simulated body fluids (SBF) had a different profile from the linear relationship between ln J and ln(-2) S (J, nucleation rate; S, supersaturation). This behaviour was alternatively explained by a developed heterogeneous nucleation theory, which indicated that HAP was nucleated at the ACP-solution interface via a polymorph transformation. Based upon this new model, we demonstrated experimentally that the embedded polymer molecules inside ACP were inert on HAP nucleation kinetics; rather, the polymers adsorbed on ACP surface could inhibit HAP nucleation from ACP. It further confirmed the heterogeneous nucleation pathway of HAP on the precursor phase. The present study provides an in-depth understanding of HAP formation for ACP-mediated crystallization.

Citrate controls nucleation by association with a precursor amorphous phase, which inhibits the surface reaction for nucleation.

Exosomal microRNAs (miRNAs) have been proved to be important biomarkers for the early diagnosis of cancers. However, the accurate quantification of exosomal miRNAs is hampered either by laborious exosome isolation and lysis or by RNA extraction and the amplification process. Here, we reported an in situ platform for direct exosomal miRNAs from serum samples. First, locked nucleic acid (LNA)-modified Au@DTNB (DTNB is the Raman reporter molecule 5,5′-dithiobis-(2-nitrobenzoic acid)) was synthesized as surface-enhanced Raman scattering (SERS) tags to enter into exosomes and assemble with target miRNAs to induce hot-spot SERS signals. Second, Fe 3 O 4 @TiO 2 nanoparticles were added to enrich the exosomes through affinity interaction of the TiO 2 shell for further SERS detection. Based on the platform, target miRNAs can be directly qualified in situ with a detection limit of 0.21 fM, which is better or comparable with quantitative reverse transcription polymerase chain reaction (qRT-PCR) and other in situ methods reported before. Moreover, neither capture antibody nor ultracentrifugation pretreatment was needed in the whole detection procedure. Using exosomal miRNA-10b as a proof of concept, pancreatic ductal adenocarcinoma (PDAC) patients can be recognized from normal controls (NCs) with an accuracy of 99.6%. The simple and sensitive in situ exosomal miRNA detection assay can be seen as a noninvasive liquid biopsy assay for clinical cancer diagnostic adaption. KEYWORDS: exosomal microRNA, in situ detection, hot-spot SERS, Fe 3 O 4 @TiO 2 , diagnosis of cancers

Many biomineralization systems start from transient amorphous precursor phases, but the exact crystallization pathways and mechanisms remain largely unknown. The study of a well-defined biomimetic crystallization system is key for elucidating the possible mechanisms of biomineralization and monitoring the detailed crystallization pathways. In this review, we focus on amorphous phase mediated crystallization (APMC) pathways and their crystallization mechanisms in bio-and biomimetic-mineralization systems. The fundamental questions of biomineralization as well as the advantages and limitations of biomimetic model systems are discussed. This review could provide a full landscape of APMC systems for biomineralization and inspire new experiments aimed at some unresolved issues for understanding biomineralization.

Faster nucleation of hydroxyapatite (HAP) at lower pH (with lower supersaturation) contradicts classical understanding. We find that the residue calcium ion in the mother liquor is the key to trigger ACP phase transformation, which gives an understanding of nonclassical nucleation kinetics of ACP-mediated crystallization and sheds light on biomineralization.

In the ACP-mediated HAP nucleation pathway, the nucleation rate of HAP increases when ACP is in the separated state.

Dilute silicate (0.05–0.5 mM) promoted the nucleation of hydroxyapatite (HAP) in simulated body fluids, while a higher level of silicate (3–8 mM) inhibited it.