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Spectroscopic and morphological data assessing the apatite forming ability of calcium hydroxide-releasing materials for pulp capping

A pulp capping material must perform as a barrier and protect the dental pulpal complex by inducing the formation of a new dentin bridge or dentin-like tissue. Being a favorable condition for the healing process, the apatite forming ability of TheraCal (light-curable Portland-dimethacrylate cement) and Dycal (calcium hydroxide-based) pulp capping materials was studied in two simulated body fluids, i.e. Dulbecco’s Phosphate Buffered Saline (DPBS) and Hank’s Balanced Salt Solution (HBSS). The cements were analyzed before and after soaking in these media for different times (1–28 days) by ESEM-EDX, micro-Raman and IR spectroscopy. This data article refers to “An in vitro study on dentin demineralization and remineralization: collagen rearrangements and influence on the enucleated phase” (Di Foggia et al., 2019).


Data
The Raman and IR spectra of the fresh TheraCal cement (i.e. upon light-curing, Fig. 1) were in agreement with its formulation (Table 1); ESEM-EDX analysis showed a granular surface displaying many element peaks (Ca, Si, W, Ba, Al, S) (Fig. 2). The comparison with the IR spectrum of the resin (Fig. 3) allowed to assign the bands due to the organic phase (methacrylate-based resin; bisphenol A-based component [2], photoinitiator) and inorganic components (barium sulphate, alite, belite, tricalcium aluminate [3,4], silica [5] and calcium tungstate [6]. The spectra of fresh Dycal (i.e. upon setting, Fig. 4) showed the presence of a salicylate ester, calcium hydroxide, zinc oxide, calcium tungstate, titanium oxide as anatase, and calcium phosphate. If compared with the IR spectra of the catalyst and base pastes (Fig. 5), the IR spectrum of the cement (i.e. upon setting) shows the disappearance of the phenolic OH stretching band at 3170 cm À1 , the shift of the C]O stretching mode from 1725 to 1669 cm À1 to 1660 cm À1 , and the appearance of the COOstretching mode at 1538 cm À1 , according to the cement hardening mechanism proposed in the literature [6]. Freshly prepared Dycal samples (Fig. 6) showed a flat homogeneous surface and displayed Ca, S, P, Zn, Ti and W peaks. Fig. 1a and b shows the IR and Raman spectra recorded on TheraCal after ageing in DPBS. After one day of ageing, a thick deposit of nanospherulites (showing Ca and P peaks at EDX, Fig. 2) formed, and the bands due to the underlying cement became undetectable in both IR and micro-Raman spectra. The position and broadening of the 952 cm À1 Raman band and the lack of resolution of the 549 cm À1 IR component suggested that calcium phosphate deposits on TheraCal were poorly crystalline. However, from 7 days onwards, the deposits attained a higher degree of crystallinity (Raman band at 959 cm À1 and IR bands at 600-560 cm À1 ) and the bands typical of a B-type carbonated apatite [7] were observed. Value of the data Vibrational spectroscopy proved suitable to characterize the different phases present in the cement formulations.
Spectroscopic techniques can provide useful information on material surface architecture of pulp capping materials as well as its evolution with ageing. Raman and IR spectra allow for evaluation of the relative thickness and maturation of the calcium phosphate phase formed upon soaking in simulated body fluids.  Fig. 1c shows the IR spectra recorded on TheraCal upon ageing in HBSS; after one day, the spectral features typical of ACP (amorphous calcium phosphate) [8], calcite and aragonite [4,9] phases were detected, superimposed to the band of the silicate hydrate gel (CSH) [4]. At this stage, calcite appeared the most abundant phase. At 28 days of ageing, the bands of ACP increased their relative intensity,  together with an increase in Ca and P weight% (Fig. 2), suggesting a thickening of the deposit, which at this stage appeared prevalently formed by ACP; the band of the CSH phase below 500 cm À1 was still detected. The spectra recorded on Dycal samples ( Fig. 4a and b) at different ageing times in DPBS showed a progressive strengthening of the bands assignable to calcium phosphates (due to apatite deposition) and a weakening of those due to the material components (salicylate bands), which became undetectable in the IR spectra since 14 days. This trend suggests that calcium phosphate deposits (with granules measuring around 0.3e0.5 mm, Fig. 6) were formed on Dycal and covered the material components still present in the deeper cement composition. At micro-Raman analysis (Fig. 4b), the material components were still detectable after 28 days, suggesting the formation of a thin layer of apatite agglomerates (Fig. 6).
The IR spectra recorded on Dycal after 1 and 28 days of soaking in HBSS (Fig. 4c) showed with progressively increased intensity the bands due to the crystalline calcium phosphate component already present in the fresh cement, together with those of calcite and aragonite; the marker bands of the cement progressively weakened.

Materials preparation and apatite forming ability tests
Dycal and TheraCal disks (8 mm diameter, 1.6 mm thick) were prepared according to the manufacturers' directions. Dycal was cured (37 C, 98% relative humidity) for a period corresponding to 70% of the final setting time, i.e. 2 min. TheraCal samples were light-cured with a halogen lamp for 20 s on both surfaces.
To test the apatite forming ability of the materials, the disks were soaked in 10 mL of DPBS or HBSS at 37 C, for 1e28 days. The soaking medium was renewed every week. The samples were analyzed before and after ageing by ESEM-EDX, micro-Raman and ATR-IR spectroscopic techniques.

Vibrational spectroscopy
Micro-Raman spectra were obtained using a Jasco NRS-2000C instrument connected to a microscope with 20 Â magnification. All the spectra were recorded in back-scattering conditions with 5 cm À1 spectral resolution (l exc ¼ 514 nm, 50 mW) using a 160K-frozen charge coupled detector. IR spectra were recorded on a Thermo Electron Nicolet 5700 Fourier Transform FT-IR, equipped with a smart orbit diamond ATR accessory and a deuterated triglycine sulphate detector; the spectral resolution was 4 cm À1 . To minimize problems deriving from the possible sample inhomogeneity, at least five Raman and IR spectra were recorded on the upper surface of each specimen.
The degree of conversion (DC%, i.e. the percentage of double bond vinyl conversion, Fig. 7) of TheraCal formulation, TheraCal control paste and control resin was calculated from the IR spectra (Figs. 3, 8 and 9 where (I C ¼ C aliphatic /I C ¼ O ) polymer and (I C ¼ C aliphatic /I C ¼ O ) monomer were the ratios between the intensities of the aliphatic C¼C and C¼O IR bands (at 1637 and 1716 cm À1 , respectively) calculated as peak heights from the IR spectra of the photopolymerized sample (i.e. TheraCal formulation or TheraCal control paste or control resin after light-curing) and unphotopolymerized sample (i.e. TheraCal formulation or TheraCal control paste or control resin before light-curing), respectively.