Characterization and Luminescence of Eu³⁺- and Gd³⁺-Doped Hydroxyapatite Ca₁₀(PO₄)₆(OH)₂

Round 1

Reviewer 1 Report

In this work, the authors report the synthesis of Eu3+- and Gd3+-doped Ca10(PO4)6(OH)2 phosphor and characterizations of SEM, XRD, FTIR, and PL. In addition, the crystal structure was analyzed and luminescence property was found in the synthesized powder. The manuscript was well-written, explanation is reasonable, and significance is clear. Therefore, it is suggested to be published in Crystals after some minor revisions. 

1. For the Eu3+-doped HA, the author did not refer the recently published paper titled “Synthesis of Eu‐doped hydroxyapatite whiskers and fabrication of phosphor layer via electrophoretic deposition process” .       (1) Baldassarre, Francesco, et al. "Crystal Chemistry and Luminescence
   Properties of Eu-Doped Polycrystalline Hydroxyapatite Synthesized by
   Chemical Precipitation at Room Temperature." Crystals 10.4 (2020): 250.
   (2) Mondal, Sudip, et al. "Rare earth element doped hydroxyapatite
   luminescent bioceramics contrast agent for enhanced biomedical imaging
   and therapeutic applications." Ceramics International (2020).
   (3) Szyszka, Katarzyna, et al. "The comprehensive approach to
   preparation and investigation of the Eu3+ doped hydroxyapatite/poly
   (L-lactide) nanocomposites: Promising materials for theranostics
   application." Nanomaterials 9.8 (2019): 1146.
   Please compare the difference and/or correlation between your work and
   the above reports.
2. Why is Gd doping concentration fixed unlike the various concentrations of Eu3+ in this work.
3. In the FT-IR spectra, what is the reasonable explanation for the slight peak shift with increasing doping concentration.
4. Please reconsider the equations for explaining mechanisms of the replacement of Eu3+ into Ca2 site and charge compensation.
5. The emission (314 nm) of Gd-doped HA is in UV region. Please describe the potential application of this phosphor.

Author Response

1)

We thank Reviewer 1 for the helpful comments on recent issues on RE-hydroxyapatites. We feel that three of the four proposed articles are a bit different from ours, while the fourth, Baldassarre et al, 2020 was already quoted in our work, relating on Eu-HA phases prepared at low and high temperatures. Szyszka et al. (2019) provide a characterization based on XRD (qualitative), FTIR, DSC, and luminescence spectroscopy. Even if it is a well-written work, discussing also on mechanical properties of HA, we present Rietveld refinement study for Eu and Gd-phases at maximum concentrations. Zhang et al. 2020 and Mondal et al., 2020 are works not yet published, they both are in press. Also in this case the two papers deal on multi-methodological characterization of Eu-HA (Zhang et al., 2020) and mixed Eu/Gd HA (Mondal et al., 2020). We quoted all the papers in the Introduction, and in Luminescence paragraph.   2)

At the beginning of the study, we had 4 Eu-doped samples, and 4 Gd-doped samples: both the series presented the same RE-concentrations. While Eu³⁺luminescence was clearly observed at all investigated concentrations, for Gd³⁺ only the highest concentration (2%) yielded an observable emission. At lower doping amounts, the intensity was too weak, thus all other samples with lower Gd concentrations were not considered for the present study, lacking of some interesting issues on HA luminescence. We added a short comment in Materials and Methods, and better clarified the previous explanation in Luminescence paragraph.

3)

Honestly we did not appreciate a slight shift of peaks, if not those attributed to experimental conditions and in any case not depending on doping concentrations. We are very careful with these observations in phosphate IR results: in a recent past for example in rare-earth doped Ca₃(PO₄)₂ tricalcium phosphates we observed a consistent shift as a function of the replacing RE cation [El Khouri et al., 2017].

4)

The mechanism can be described according to: OH^- + Ca²⁺ ¬® Eu³⁺ + O^2- (1); on the other hand another charge compensation mechanism is possible:3Ca²⁺¬® 2Eu³⁺ + □ (vacancy)       (2), both according to Han et al. 2013.

We better clarified the 2 mechanisms in Luminescence paragraph.

5)

We better clarified Gd-doped HA possible role in bioimaging applications, adding some reference proposed by the reviewer, and some other proposed by us. In particular, [Ignjatović, et al., 2019] relate on Gd-doped HA magnetic properties and its luminescence efficiency upon gamma rays radiation, that in turn can be useful for dosimetry devices. More, its emission in the UV range can be used as sensitizer in co-doped systems [Mondal et al., 2020], but applications in many other fields such as phosphor [ Zhang & Lin, 2012] or antibacterial materials [Cates et al., 2011] can also be interesting. Thus we added some comments in the Introduction paragraph, and in Gd-doped HA sub-paragraph.

Reviewer 2 Report

The research manuscript describes several methodological approaches for detailed characterization and luminescence of Eu3+- and Gd3+-
3 doped hydroxyapatite Ca10(PO4)6(OH)2. The information provided by this research will be very important for the design of novel biocompatible synthetic materials with potential use in biomedical imaging.

Author Response

Dear Reviewer,

while revising the whole manuscript according to the other comments, we also tried to do our best to improve the English language of the work.

Reviewer 3 Report

The manuscript entitled “Characterization and luminescence of Eu 3+ - and Gd 3+ -  2 doped hydroxyapatite Ca 10 (PO 4 ) 6 (OH) 2” authored by  Veronica Paterlini et all described very good psysicochemical approach in crystals characterization. The obtained during this studies crystals were investigated through Scanning Electron Microscopy (SEM),  powder X-ray diffraction (PXRD), Fourier Transform Infrared (FTIR) and luminescence spectroscopy.

The introduction is good and provides readers the basic information about the crystals applications and productions. The short details how to control the process of the productions of the crystals was added. The advantage of this manuscripts is using different analytical methods for the crystals characterization and properties, and this part it well presented.  The SEM images are in good quality  and XRD are well presented.

 

Some more comments

1)The figure 5 shows FTIR spectra. Please correct figure 5.

The figure presents comparisons of different spectra.  Please correct the figure, provide baseline of the spectra which will lead to have one common point on the left and right line of spectra. The comparison without providing baseline is incorrect. From my point of view this will not influence the obtained results so much and provided conclusions might be even the same. Due to the correct and professional way of presented FTIR spectra the baseline should be added. After that the comparison of teh spectra has got more value.

 

2)Could you please explain why at 570 cm-1 bands at figure 5 panel 'a' the blue line (Ha-Eu3) has got two bands? The other samples eg Ha-Eu1, Ha-Eu4, do not have this band.

Author Response

1) We revised Figs 5a and 5b according to this useful comment, and we embedded both the Figures in the revised FTIR paragraph.

 

2) in HA-Eu3 sample, the weak peak at 460 cm^-1 is attributed to trace amounts of Ca₃(PO₄)₂ (TCP) that are occasionally associated with hydroxyapatite synthesized at high temperature [Baldassarre et al., 2020]. We added a short comment in FTIR paragraph.

Reviewer 4 Report

In this manuscript, the authors investigated the luminescence properties of hydroxyapatite (HAP) doped with europium and with gadolinium by X-ray diffraction, FTIR and luminescence. The characterization of the crystal structure including atomic positions was carried out. The analysis showed the presence of doping at the Ca2 site of the P63/m HAP unit cell and all the samples and showed strong luminescence. This is a very interesting manuscript; I find this work very interesting.

Author Response

Dear Reviewer,

while revising the whole manuscript according to the other comments, we also tried to do our best to improve the English language of the work.