Markedly Enhanced Photoluminescence of Carbon Dots Dispersed in Deuterium Oxide

Round 1

Reviewer 1 Report

Dears  Corneliu S. Stan, Adina Coroaba, Conchi O. Ania, Cristina Albu and Marcel Popa.

 

Concerning to the your manuscript: Markedly Enhanced Photoluminescence of Carbon Dots Dispersed in Deuterium Oxide

 

In which I found substantial and interesting results about the synthesis of four types of carbon dots (CDs) in D2O which enhances their photoluminescence properties and a very interesting hypothesis pointed by you, which hat suggest that: “the switching H2O to D2O, as dispersion medium, is able to provide a more favorable environment which diminishes the non-radiative deactivation thus favoring the radiative relaxations through photon production”. Furthermore, I found a manuscript well supported, organized, wrote and discussed, which supports that interesting explanation of the deuterium roll in the emissive process.

By this reasons I suggest the acceptation of the manuscript in the present form for publication in Carbon/MPDI. I only provide a little suggestions in the annex, which I hope help you to improve the usefulness of its interesting work

 

Annex:

1- In Abstract it is confuse the reason of the enhancement of the photoluminescence of the Carbon Dots by the use of the deuterium water, which need by clarify.

2- Quality or size of the fluorescent samples images in the Figures 1 to 5 could be improved?

3- Maybe the number of the time-resolved fluorescence decay pathways shows in Figure 6, should reduce of presented as additional material.

 

With the best regards,

 

 

The Reviewer

Annex:

1- In Abstract it is confuse the reason of the enhancement of the photoluminescence of the Carbon Dots by the use of the deuterium water, which need by clarify.

2- Quality or size of the fluorescent samples images in the Figures 1 to 5 could be improved?

3- Maybe the number of the time-resolved fluorescence decay pathways shows in Figure 6, should reduce of presented as additional material.

 

With the best regards,

 

 

The Reviewer

Author Response

Dear Reviewer 1,
Many thanks for your helpful and constructive comments. Please find below our point to point reply to the comments. Also please see the revised manuscript which was updated.
1- In Abstract it is confuse the reason of the enhancement of the photoluminescence of the Carbon Dots by the use of the deuterium water, which need by clarify.
We updated the Abstract text with the following line: The investigations revealed the major
influence of the deuterium oxide dispersion medium over the PL emission properties of the
investigated C Ds. This might give a hint over the presented results in the manuscript clearing possible confusions but without det ailing results which are clearly descripted in “Conclusions ”. Thus we are trying to keep both the Abstract and Conclusions text logically constructed avoiding overlapping text.
2- Quality or size of the fluorescent samples images in the Figures 1 to 5 could be improved?
The photos embedded in the 1 5 Figures were taken at highest possible resolution of the camera
(50 MP , 4K respectively). The pictures were taken in dimmed light conditions and under UV light.
The Figures 1 5 (and their embedded photos) were revised and saved in bmp format at 300 dpi
resolution according to the journal requirements. We hope that improved the overall quality.
3- Maybe the number of the time-resolved fluorescence decay pathways shows in Figure 6, should reduce of presented as additional material.
Thank you for your remark. The decay profiles of each investigated sample presented in Fig ure 6
are important for overall inteligibility of the presented results. Indeed, they might be moved to
Supplemental Information but, in our opinion, this could alter the consistence of the r elated
investigation results discussion. But, if you consider this is important for the overall presentation, we can move them in Supplemental Information. Also, according to the Reviewer 2 more results should be included in the main text of the submit ed revised manuscript. Therefore, we are in a kind of dilemma...

Author Response File: Author Response.pdf

Reviewer 2 Report

This study describes the photoluminescent (PL) properties of Carbon Dots (CDs) dispersed in deuterium oxide (D2O). The investigations have demonstrated that the recorded emission intensity is at least 1.27 - 2.3 x higher in D2O dispersion at all investigated excitation wavelengths, and the most intense emission has been achieved at 350 nm excitation. This is a relevant result of the manuscript and worth to be outlined in publication.

Some Minor Revisions are suggested to improve manuscript:

1. additional characterizations of CDs to address morphology, structure, size, and so on should be discussed in the main text. Please, add more characterizations of CDs.

2. what is the stability of CDs dispersed in D2O? Please, comment on this aspect.

3. what is the repeatability of PL intensity in the time ? Please, comment on this as well.

4. additional references should be listed to give evidence of the state-of-art on CDs research.

- some new Figures on morphology, structure, size and other physical parameters are necessary to improve quality of manuscript.

- additional references to focus on the state-of-art on CDs research are highly desiderable.

- a claear and concise text should be added in the Introduction and Discussion to focus the advantages and disadvantages of the CDs at the current stage of research.

Author Response

Dear Reviewer 2,

Many thanks for your helpful and constructive comments. Please find below our point to point reply to the comments. Also please see the revised manuscript which was accordingly updated.

Reviewer 2

This study describes the photoluminescent (PL) properties of Carbon Dots (CDs) dispersed in deuterium oxide (D2O). The investigations have demonstrated that the recorded emission intensity is at least 1.27 - 2.3 x higher in D2O dispersion at all investigated excitation wavelengths, and the most intense emission has been achieved at 350 nm excitation. This is a relevant result of the manuscript and worth to be outlined in publication.

Some Minor Revisions are suggested to improve manuscript:

(1) additional characterizations of CDs to address morphology, structure, size, and so on should be discussed in the main text. Please, add more characterizations of CDs.

Thank you for your suggestion. As stated in the manuscript, all the investigated CDs types (AW-CDs, Fe-CDs, NHF-CDs and NHS-CDs) were thoroughly presented in our previous published works [17-20]. Their morpho-structural configuration was thoroughly investigated and presented in the respective works. Since, the preparation procedure is exactly the same, their structural configuration is not modified. However, the only significant difference is that at the end of the thermal processing of the precursors, D2O was used as dispersion mediums. Therefore, the only difference is their behavior in this new dispersion medium.

In order to clarify this aspect, we performed a DLS (dimensional analysis) investigation for each type of CDs (AW-CDs, Fe-CDs, NHF-CDs and NHS-CDs) dispersed in D2O. The investigations were performed on both freshly prepared samples and also on 1 week aged same dispersions to clarify the inevitable agglomeration process, which could provide information about their longer term stability.

The results and related discussion (Figures and text) were added in the main text of the manuscript as follows: “As stated above, the experimental procedure for the preparation of each type of investigated CDs was kept unchanged as detailed in our previous reported works [14-17] where in-depth mor-pho-structural investigation was performed and discussed. Since the only difference consist in the final dispersion medium (D2O) the investigations presented in this work were focused on the morphological aspects which could provide new information. Therefore DLS (dimensional analysis) was performed for each type of CDs dispersed in D2O.

3.2.1. Dimensional (DLS) investigation

Freshly prepared samples of each type of CDs dispersed in D2O were investigated. Also, for the evaluation of longer term stability of the dispersions, same samples were investigated again after 1 week. In Figure 7 (a-d) are presented the dimensional distributions for each type of CDs dispersed in D2O, freshly prepared and after 1 week of aging.

All the prepared samples present, as expected, an agglomeration tendency which was observed irrespective of the type of solvent used as dispersion medium. The freshly prepared NHF-CDs/D2O dispersion size distribution is mainly situated within 30-90 nm range. As demonstrated in our previous works, CDs present a clusterization tendency which became even more noticeable in case of the aged dispersion where the size distribution migrates within 40-140 nm range. In case of NHS-CDs, freshly prepared dispersion presents a narrower size distribution (40-120 nm) compared with the aged dispersion where a broad 120-320 nm distribution was observed most probably due to an even more clusterization tendency. This behavior is almost the same in case of AW-CDs where the freshly prepared dispersion presents a narrow distribution (25-90 nm) which became significantly broader for the aged dispersion (60-250 nm). The Fe-CDs/D2O dispersion behave differently compared with the rest of the investigated samples, with an initial broad distribution (60-400 nm) which remain also large but slightly translated to higher dimensional range (100-400 nm). This particular behavior could be a result of iron presence within the CDs structure. Overall, the stability of the dispersions is slightly better compared with the same CDs types but dispersed in water. Interestingly, in case of NHF-CDs, AW-CDs and Fe-CDs dispersions the PL intensity remained unchanged after 1 week, no notable differences being noticed both visual and instrumental which might additionally sustain the key role of the dispersion medium in achieving a better emissive intensity. The aged NHS-CDs/D2O dispersion presents a slightly fainter emission intensity. Com-pared with samples of same CDs type dispersed in water, the PL emission is markedly better pre-served.

 

(2) what is the stability of CDs dispersed in D2O? Please, comment on this aspect.

Text regarding this matter was added in the new DLS Chapter. Please see the last part of our response to the previous comment.

(3) what is the repeatability of PL intensity in the time ? Please, comment on this as well.

Keeping the preparation procedure unchanged, the intensity of the PL emission is preserved from a batch to another. During the investigation of each type of CDs dispersed in D2O, several batches were required in order to perform the analyses, no differences being observed. Regarding the stability of the PL emission intensity, we added in the text the results of our observations after an 1 week aging of the dispersions. Please see the last paragraph in the added text in Chapter 3.2. (for convenience we also included the added text in our response to (1).

(4) additional references should be listed to give evidence of the state-of-art on CDs research.

During the writing of the manuscript we did our best to include up-to-date references regarding carbon dots, ex. 1-4, 13. In order to comply to this comment we added some new references regarding CDs. The following references were added to additionally sustain the state-of-the-art on CDs research:

[3] Ullah, M.; Awan, U.A.; Ali, H.; Wahab, A.; Khan, S.U.; Naeem, M.; Ruslin, M.; Mustopa, A.Z.; Hasan, N. Carbon Dots: New Rising Stars in the Carbon Family for Diagnosis and Biomedical Applications. J. Nanotheranostics 2025, 6, 1. https://doi.org/10.3390/jnt6010001

[4] Majid, A.; Ahmad, K.; Tan, L.; Niaz, W.; Na, W.; Huiru, L.; Wang, J. The Advanced Role of Carbon Quantum Dots in Nano-Food Science: Applications, Bibliographic Analysis, Safety Concerns, and Perspectives. C 2025, 11, 1. https://doi.org/10.3390/c11010001

[7] Carbonaro, C.M.; Corpino, R.; Salis, M.; Mocci, F.; Thakkar, S.V.; Olla, C.; Ricci, P.C. On the Emission Properties of Carbon Dots: Reviewing Data and Discussing Models. C 2019, 5, 60. https://doi.org/10.3390/c5040060

- some new Figures on morphology, structure, size and other physical parameters are necessary to improve quality .of manuscript.

Additional Figures and related text were added to the revised manuscript.

- additional references to focus on the state-of-art on CDs research are highly desiderable.

Please see the above response (4)

- a claear and concise text should be added in the Introduction and Discussion to focus the advantages and disadvantages of the CDs at the current stage of research.

In order to summarize the potential of the presented results we added the following text in the “Introduction” chapter: “The reported results could bring additional evidence in the pursuit of definitive elucidation of the specific PL mechanisms of CDs and also an interesting approach for their applications in optoelectronics, sensors, medical imaging where as high as possible emission intensity is often required. Also, given the observed markedly enhanced PL emission in D2O medium, this study may trigger further interest in research related to deuteration and deuterated mediums.”

Author Response File: Author Response.pdf

Reviewer 3 Report

The paper presents interesting results on increased photoluminescence for carbon dots in heavy water. The motivation and the interest of the research topic are well presented. The methodology and materials are mainly clearly described. While the presentation of the results is done in broad detail the discussion should evaluate the results in more depth. The English is in good order.

I recommend the publication after addressing some questions.

General:

Why do you use the term “deuterium oxide” instead of the more common term “heavy water” – that you may explain in the introduction?

 

2.3 Preparation:

Lines 100 -103:

“… further photopolymerized through exposing to an UV-A (360-370 nm) source. The photopolymerization process is fast (under 60 s) depending on the UV-A source type (in our case a 365 nm LED array was used).”

This is a bit confusing. Is the 360-370 nm the general way to do it and the 365 nm the specific condition in this work?

 

3. Results and Discussion

Lines 105- 141:

These two paragraphs provide some interesting discussion about the influence of the heavy water on the photoluminescence of the samples. However, the reader expects to see first the results and then the discussion. Please, move these two paragraphs to the end of this section and show how they are linked to your results.

Various instances:

The PL measurements are done using UV-light with 330 nm, 350 nm and 370 nm. The last wavelength is close to the one used for the polymerization. Can you be sure that there is no further polymerization going on?

Various instances:

For the PL in AW-CD and NHS-CD there is a clear dependence of the PL emission wavelength on the UV excitation wavelength while for NHF-CD and the Fe-CD no effect is visible. Could you comment on this?

The maximum of the PL is observed for 370 nm (longest wavelength) in NHF-CD and AW-CD in contrast to NHS-CD and Fe-CD where it is at 350 nm (middle wavelength) – and never for 330 nm  (shortest). Any comment/explanation?

Figures 1 – 4: The figures have no labels/units on the axes.

Author Response

Dear Reviewer 3,

Many thanks for your helpful and constructive comments. Please find below our point to point reply to the comments. Also please see the revised manuscript which was accordingly updated.

Reviewer 3

General:

Why do you use the term “deuterium oxide” instead of the more common term “heavy water” – that you may explain in the introduction?

We preferred deuterium oxide instead of heavy water because “heavy water” is somehow a generic term used to define a kind of water where an isotope of hydrogen or oxygen is present. Heavy water could be equally a combination of hydrogen with a heavier oxygen isotope (ex. oxygen-18). In the end, both (deuterium oxide and O18 water) have same molecular mass (20) but different atomic (isotope) configuration.

2.3 Preparation:

Lines 100 -103:

“… further photopolymerized through exposing to an UV-A (360-370 nm) source. The photopolymerization process is fast (under 60 s) depending on the UV-A source type (in our case a 365 nm LED array was used).”

This is a bit confusing. Is the 360-370 nm the general way to do it and the 365 nm the specific condition in this work?

Any UV-A light source will work but depending on the UV radiation power provided by the source will affect the photopolymerization time. We specifically provide in the text some information (365 nm LED array) about the UV source, in order to suggest to the eventual researcher who want to replicate the results described in the manuscript, what type of UV source to use in order to obtain similar photopolymerization timing. Of course, a general UV-A (discharge lamp type) could be used but, the polymerization time may differ.

3. Results and Discussion

Lines 105- 141:

These two paragraphs provide some interesting discussion about the influence of the heavy water on the photoluminescence of the samples. However, the reader expects to see first the results and then the discussion. Please, move these two paragraphs to the end of this section and show how they are linked to your results.

Thank you for this observation. Indeed, lines 105-141 fits logically within the Fluorescence Investigation Chapter. Since they contain some general aspects and also prepare the reader to easily understand the results presented in each sub-chapter (Steady State, PLQY, LT) we added them in the beginning of the new inserted Chapter title. The modifications are highlighted in yellow.

Various instances:

The PL measurements are done using UV-light with 330 nm, 350 nm and 370 nm. The last wavelength is close to the one used for the polymerization. Can you be sure that there is no further polymerization going on?

The 3 chosen excitation wavelengths (for steady state and PLQY measurements) were chosen because they are significant for the PL behavior of the Carbon Dots since their PL is usually excitation dependent. The investigations were performed over the Carbon Dots (all 4 investigated types) dispersed in water and deuterium oxide, therefore no monomer (which might photopolymerize) is present. Therefore no concern regarding this matter, no interference in the measurements may be suspected since there is nothing to polymerize in the investigated samples. The photopolymerization is significant only in case of the demonstration presented in Figure 5 (please see the related text (liner 196-211) which strengthen our observations regarding the overwhelming role of the deuterium oxide dispersion environment over the PL emission of all investigated Carbon Dots.

Various instances:

For the PL in AW-CD and NHS-CD there is a clear dependence of the PL emission wavelength on the UV excitation wavelength while for NHF-CD and the Fe-CD no effect is visible. Could you comment on this?

The slight different location of the emission peaks for each type of investigated Carbon Dots are in our opinion not significant enough (in most cases only 1-2 nm difference) in order to suspect a different behavior arising from a change in the involved  radiative processes. It might be equally some inherent measuring errors due to the equipment limitations or equipment software reconstruction of the graphs from the acquired measurements.

The observations were already included in the manuscript text:

” In all cases, the observed unchanged location of the emission peaks (in H2O and D2O dispersions) are consistent with other reported works where the emission peaks in deuterated solvents show no significant shift [6].”

“Practically, there are no differences between emission peaks, the variations (+/_1 nm) could be safely attributed to the inherent measuring and/or results interpretation errors.

Author Response File: Author Response.pdf