Dynamic light scattering. The size distribution of CPICs in ethanol solution was determined at 25°C by a Zetasizer Nano S (ZEN 3600, Malvern, England).

NMR spectroscopy. Four samples were prepared as follows: H3PO4 (7 μl) was added to ethanol (10 ml); CaCl2·2H2O (0.02 g) and TEA (0.38 ml) were added to ethanol (10 ml); H3PO4 (7 μl) and TEA (0.38 ml) were added to ethanol (10 ml); and the CPIC ethanol solution (10 ml; 2 mg/ml) was centrifuged at 30,427 Rcf and redispersed in ethanol (10 ml). Each sample (0.4 ml) was added to an NMR tube containing DMSO-d6 sealed in a capillary tube as a deuterated solvent and immediately characterized by 1H and 31P DirectDrive2 600 MHz NMR spectrometer (Agilent Technologies Inc., USA).

FTIR spectroscopy. The gel-like CPICs and TEA [3.8% (v/v) in ethanol] were sealed between two KBr plates for characterization. The CPIC-induced bulk ACP was thoroughly blended with KBr powder to prepare a pellet. FTIR (IRAffinity-1, Shimadzu, Japan) was performed with 30 scans at 4 cm−1 resolution from 4000 to 400 cm−1. The background was determined using blank KBr plates.

Gas chromatography–mass spectrometry. CPIC-induced ACP (0.020 g) was dissolved by adding a definite volume of 5 M HCl and then adding a definite volume of methanol making a total volume of 5 ml. A stock standard solution (10 mg/ml) was prepared by dissolving 0.100 g of TEA into 10 ml of methanol solution, and a working standard solution was obtained by dilution. GC-MS analysis was performed on a Shimadzu GCMS-QP2010 system (Japan) using a DB-5ms capillary column (30 m by 0.25 mm; film thickness, 0.25 μm).

X-ray diffraction. The as-prepared samples were measured by using XRD (X’Pert3 Powder, Malvern Panalytical, Netherlands) with Cu Kα radiation (λ = 0.1542 nm) operating at an acceleration voltage of 40 kV and a current of 40 mA. The diffraction intensity data were scanned with a sampling step of 0.02° in the 2θ range from 10° to 70°.

Transmission electron microscopy. TEM was performed with a Hitachi HT-7700 (Japan) operating at 120 kV. HRTEM was carried out on an FEI Tecnai G2 F20 microscope (USA) or a JEOL JEM-2100F microscope (Japan), and both were operated at 200 kV. EDXS spectra were collected using JEM-2100F equipped with an energy dispersive x-ray spectrometer (Oxford-T80, NanoLab Technologies Inc., USA).

Scanning electron microscopy. SEM images were acquired by using a Hitachi SU8010 scanning electron microscope (Japan). Specimens were mounted on an aluminum stage with a carbon tape. The tooth surfaces were sputtered with Pt and observed under an accelerating voltage of 5 kV. Both top-down and side views of the sectioned tooth samples were observed using SEM. EDXS spectra were collected using SU8010 equipped with an energy dispersive x-ray spectrometer (Model 550i, IXRF Systems).

Atomic force microscopy. AFM images of enamel specimens and CPIC-induced bulk ACP were collected under ambient conditions (temperature, 25°C; relative humidity, 40%) by using a multimode atomic force microscope (NanoScope IVa, Veeco, USA) in tapping mode. All images were analyzed with an image analysis software (NanoScope Analysis version 1.7).

Confocal laser scanning microscopy. The samples were prepared by cutting repaired whole teeth along the longitudinal direction. The repaired layer of the whole tooth was labeled with calcein. Fluorescence images were obtained using an inverted confocal laser scanning microscope (IX81-FV1000, Olympus, Japan). Specimens were illuminated with a 488-nm laser. All images were captured and analyzed with an image analysis software (Olympus FluoView version 4.1 viewer).

Nanoindentation. Nanoindentation testing was carried out in a nanoindenter (G200, Agilent Technologies, CA, USA) with a Berkovich tip (tip radius of approximately 20 nm). Native enamel and acid-etched enamel were used as controls. Each sample was stored, and the data were obtained at 25°C and a relative humidity of 40%. For the test, the tip was calibrated with fused silica before evaluation. The hardness and elastic modulus of the specimens were measured using a continuous stiffness measurement technique. During the loading process, the constant strain rates were controlled at 0.2 nm s−1. The applied load force and the depth of penetration into the samples during the indentation were continuously monitored by the computer. Twenty points were indented for each specimen, and three different specimens of native enamel, acid-etched enamel, and repaired enamel were tested. The data were recorded and processed by TestWorks 4 software (MTS Systems Corporation, Eden Prairie, MN, USA), which obtained the nanohardness and elastic modulus by calculating the mean value from 200 nm to 2000 nm, and these data were presented as force-displacement curves.

Microscratch tests. Microscratch tests of sound enamel, acid-etched enamel, and repaired enamel were performed using a microscratch tester (UNHT/MCT/MST, Anton Paar GmbH, Austria). A Rockwell diamond tip with a radius of 20 μm was used for all tests. All scratch testing of the samples were conducted at a constant normal load of 500 mN with a velocity of 200 μm/min. The scratch distance was 200 μm. At least three scratches were made in each test region. Each scratch was at least 50 μm away from the next scratch.

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