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One-step vs. two-step deposition of CaF2 nanoparticles: From the perspective of confined water release and bonding
Jiajia Xu1, Yanji Lu1, Kai Zhang1
1Stomatology Hospital, School of Stomatology, Zhejiang University School of Medicine, Zhejiang Provincial Clinical Research Center for Oral Diseases, Key Laboratory of Oral Biomedical Research of Zhejiang Province, Cancer Center of Zhejiang University, Engineering Research Center of Oral Biomaterials and Devices of Zhejiang Province, Hangzhou 310000, China.
Objective:
To compare the bonding efficacy of one-step versus two-step calcium fluoride (CaF₂) nanoparticle deposition and to elucidate the underlying mechanisms. The null hypotheses tested are as follows: 1) Nanoparticles deposited via the one-step strategy are less uniformly distributed than those deposited via the two-step strategy; 2) The one-step strategy does not release more confined water compared to the two-step strategy; 3) The one-step strategy is inferior to the two-step strategy in enhancing dentin bonding durability.
Methods:
A stable amorphous calcium fluoride (ACF) precursor solution was prepared by mixing 10 mM CaCl₂ and 10 mM NaF in 20 mM HEPES buffer for 20 s, enabling one-step CaF₂ nanoparticle deposition through the mineralization-regulating function of non-collagenous proteins acting as pseudonuclei. The two-step strategy consisted of sequentially applying 0.1 M CaCl₂ and 0.2 M NaF, each for 20 s. Neutralization of matrix negative charges, reduced hydration capacity, and confined water release were assessed via cationic dye adsorption, zeta potential, water contact angle, ATR-FTIR spectroscopy, and thermogravimetric analysis. Interface characteristics were evaluated using SEM, TEM, and XPS. Bonding effectiveness was determined by adhesive infiltration depth, micro-tensile bonding strength (μTBS), and nanoleakage expression, both initially and after aging.
Results:
Our results demonstrate that the one-step strategy produces smaller, more homogeneous nanoparticles that penetrate collagen fibrils more effectively. We thus reject the null hypothesis 1. Compared to the unmodified demineralized dentin matrix (DDM), the DDM obtained with the two-step approach exhibits reduced interfacial water, which further reduced by the one-step strategy. The null hypothesis 2 is thus rejected. Compared to the conventional wet-bonding control (AW), bonding performed on these optimized DDM leads to enhanced adhesive infiltration. Both one-step (ACF) and two-step strategies (CW) improve the immediate bonding strength (43.00 ± 4.15 MPa and 36.81 ± 3.32 vs. 31.57 ± 2.02 MPa). after 12 months of storage in water at 37 °C, bonding strength of the AW group significantly decreased to 26.92 ± 2.35 MPa (p < 0.05), whereas the CW group decreased to 30.20 ± 4.66 MPa (p < 0.05) and the ACF group maintained a high level (39.22 ± 5.12 MPa, p > 0.05).
Significance:
This novel one-step dentin bonding strategy effectively addresses the challenge of dentin bonding durability. By simultaneously reduced electronegativity, lower hydration capacity, decreased confined water, enhanced mechanical properties, and reinforcement at the basal level, adhesive infiltration was improved, clinical procedure was simplified, and bond durability was sustained provided. The one-step dentin bonding strategy offers a clinically practical, time-efficient, and superior alternative for durable resin-dentin bonds.
