Photocatalytic inactivation of Escherischia coli under UV light irradiation using large surface area anatase TiO2 quantum dots

In this study, high specific surface areas (SSAs) of anatase titanium dioxide (TiO2) quantum dots (QDs) were successfully synthesized through a novel one-step microwave–hydrothermal method in rapid synthesis time (20 min) without further heat treatment. XRD analysis and HR-TEM images showed that the as-prepared TiO2 QDs of approximately 2 nm size have high crystallinity with anatase phase. Optical properties showed that the energy band gap (Eg) of as-prepared TiO2 QDs was 3.60 eV, which is higher than the standard TiO2 band gap, which might be due to the quantum size effect. Raman studies showed shifting and broadening of the peaks of TiO2 QDs due to the reduction of the crystallite size. The obtained Brunauer–Emmett–Teller specific surface area (381 m2 g−1) of TiO2 QDs is greater than the surface area (181 m2 g−1) of commercial TiO2 nanoparticles. The photocatalytic activities of TiO2 QDs were conducted by the inactivation of Escherischia coli under ultraviolet light irradiation and compared with commercially available anatase TiO2 nanoparticles. The photocatalytic inactivation ability of E. coli was estimated to be 91% at 60 µg ml−1 for TiO2 QDs, which is superior to the commercial TiO2 nanoparticles. Hence, the present study provides new insight into the rapid synthesis of TiO2 QDs without any annealing treatment to increase the absorbance of ultraviolet light for superior photocatalytic inactivation ability of E. coli.

(2) p 2. The authors state that a wider bandgap can prevent recombination of photo-generated electrons and holes in TiO2. This does not ring true for me: For molecules, at least, transitions of higher energy tend to have shorter lifetimes. Recombination is usually prevented by trap sites in semiconductors. The statement should be supported by a reference or deleted.
(3) Experimental section. The authors should provide more detail about their reagents and grades, including water and NaOH. They should also state the source of the commercial TiO2 that they used as a comparator and provide the concentration used in the experiment shown in fig. 9(a). I am confused by the description of the E. coli experiments: "Afterwards, 10 mL of TiO2 solution and 10% fresh standard inoculums of E. coli (~ 10^8 cfu/mL) were added into 80 mL sterilised normal saline." Was "10%" meant to read "10 mL", or does it mean that 10^8 cfu/mL represents 10% of the 10^9 cfu/mL standard inoculum, or is it something else? Is 10^8 cfu/mL the starting concentration or the final concentration? This needs to be clearer in the description. More information is needed about the UV light. What kind of light (e.g. low-pressure Hg), what power (e.g. 300 W), what is the power of the lamp at the target, what is the distance between the lamp and the target? This may not be a complete list; in essence, the authors need to provide enough information so that another person can reproduce the results.
(4) The reflections specified by the JCPDS card should be shown on fig. 1 so that the standard can be compared to the experimental diffractograms.
(5) The authors state that the crystalline peaks in fig. 1 indicate complete crystallisation. Is that so? Could a similar diffractogram not be produced from a mixture of crystallites and amorphous particles?
(6) p 6. The authors state that phonon confinement results in a shift of the Raman peaks to lower frequencies. The shift should be to higher frequencies, and that is what fig. 6 shows, particularly for the 399 cm^-1 (Eg) resonance.
(7) There are numerous language errors, but I assume that they will be picked up by the editors before publishing.

Review form: Reviewer 2
Is the manuscript scientifically sound in its present form? Yes

Are the interpretations and conclusions justified by the results? No
Is the language acceptable? No

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? No

Comments to the Author(s)
The authors present some information about the synthesis of anatase TiO2 quantum dots by a one-step microwave-hydrothermal method and application in the photocatalytic inactivation of E. coli under UV irradiation. However, there is a lack of research in the formation process of TiO2 quantum dots, leading to no breakthrough or real novelty in this work, because there are many reports on the application of TiO2 in sterilization. In addition, some of the statements within the manuscript are not very clear and contradictory between the figure and the results discussion. Therefore, I would not support its publication in Royal Society Open Science. More comments and suggestions are as follows: >1. In the abstract, some nouns appear only once and do not require abbreviation, such as SSA, XRD, HRTEM, etc. >2. Based on the XRD analysis and HRTEM image, the average crystallite size of TiO2 quantum dots is ~2 nm. However, the authors mention 'X-ray diffraction (XRD) analysis and highresolution transmission electron microscopy (HR-TEM) images showed that the as-prepared TiO2 quantum dots have high crystallinity with anatase phase and size varies from 2 to 4 nm.' in the abstract, it's a contradiction. >3. In the introduction, the authors should provide literature reviews about the preparation of TiO2 quantum dots at present. >4. In the introduction, some words are wrong, for example, the 'tome', 'synthetic routs', 'template routs' should be changed to 'time', 'synthetic routes', 'template routes', etc. >5. Moreover, the authors mention 'The wider band gap of TiO2 can prevent the charge recombination effect hence recombination of pair electron hole life time is longer.' in the introduction, please provide strong evidence. >6. The authors used microwave-hydrothermal method to prepare TiO2 quantum dots, we are expected to add the advantages of microwave-hydrothermal method over other methods for preparation of TiO2 quantum dots. >7. For the determination of TiO2 band gap, the figure should be provided about the relationship between absorption and wavelength. >8. In Fig. 6, the authors state that the peak corresponding to the B1g mode, A1g and Eg modes of TiO2 quantum dots shows a small shift toward the lower frequencies as compared with the commercial TiO2, but we find that these peaks are migrating to higher frequencies, the authors should give a reasonable explanation. >9. To make the results clear, the concentration of commercial TiO2 nanoparticles for photocatalytic inactivation of E. coli should also be marked in Fig. 9a.

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? I do not feel qualified to assess the statistics

Comments to the Author(s)
The article "Photocatalytic Inactivation of E. coli under UV Light Irradiation using Large Surface Area Anatase TiO2 Quantum Dots" was carefully reviewed. The overall outlay and quality of paper is not good for publication in Royal Society of Open Science. Hence, I recommend for its rejection.

30-Apr-2019
Dear Dr Ahmed: Title: Photocatalytic Inactivation of E. coli under UV Light Irradiation using Large Surface Area Anatase TiO2 Quantum Dots Manuscript ID: RSOS-190537 I am writing to you in regards to your manuscript above which you submitted to Royal Society Open Science.
In view of the criticisms of the reviewers and editors, found at the bottom of this letter, your manuscript has been rejected for publication.
Thank you for considering Royal Society Open Science for the publication of your research. I hope this decision will not discourage you from submitting manuscripts in the future. ********************************************** RSC Associate Editor: Comments to the Author: Although Reviewer 1 supported the publication of the manuscript after revisions, because Reviewer 2 and Reviewer 3 (Adjudicator) felt that the manuscript should be rejected and raised a number of technical concerns with the work, your manuscript has been rejected.

RSC Subject Editor:
Comments to the Author: (There are no comments.) ********************************************** Reviewers' Comments to Author: Reviewer: 1 Comments to the Author(s) The authors describe a simple, hydrothermal synthesis of TiO2 (anatase) quantum dots without calcining. The synthesis results in very small (2-4 nm) photo-active particles with a demonstrated ability to kill E. coli. The manuscript should be of interest to the community and the data convincingly support the conclusions. I have the following suggestions for revisions, which should be addressed before publishing: (1) p 2. The authors state that the band-gap "decreases" due to the quantum size effect. This should read "increases," as is consistent with both the literature their data.
(2) p 2. The authors state that a wider bandgap can prevent recombination of photo-generated electrons and holes in TiO2. This does not ring true for me: For molecules, at least, transitions of higher energy tend to have shorter lifetimes. Recombination is usually prevented by trap sites in semiconductors. The statement should be supported by a reference or deleted.
(3) Experimental section. The authors should provide more detail about their reagents and grades, including water and NaOH. They should also state the source of the commercial TiO2 that they used as a comparator and provide the concentration used in the experiment shown in fig. 9(a). I am confused by the description of the E. coli experiments: "Afterwards, 10 mL of TiO2 solution and 10% fresh standard inoculums of E. coli (~ 10^8 cfu/mL) were added into 80 mL sterilised normal saline." Was "10%" meant to read "10 mL", or does it mean that 10^8 cfu/mL represents 10% of the 10^9 cfu/mL standard inoculum, or is it something else? Is 10^8 cfu/mL the starting concentration or the final concentration? This needs to be clearer in the description. More information is needed about the UV light. What kind of light (e.g. low-pressure Hg), what power (e.g. 300 W), what is the power of the lamp at the target, what is the distance between the lamp and the target? This may not be a complete list; in essence, the authors need to provide enough information so that another person can reproduce the results.
(4) The reflections specified by the JCPDS card should be shown on fig. 1 so that the standard can be compared to the experimental diffractograms.
(5) The authors state that the crystalline peaks in fig. 1 indicate complete crystallisation. Is that so? Could a similar diffractogram not be produced from a mixture of crystallites and amorphous particles?
(6) p 6. The authors state that phonon confinement results in a shift of the Raman peaks to lower frequencies. The shift should be to higher frequencies, and that is what fig. 6 shows, particularly for the 399 cm^-1 (Eg) resonance.
(7) There are numerous language errors, but I assume that they will be picked up by the editors before publishing.

Reviewer: 2
Comments to the Author(s) The authors present some information about the synthesis of anatase TiO2 quantum dots by a one-step microwave-hydrothermal method and application in the photocatalytic inactivation of E. coli under UV irradiation. However, there is a lack of research in the formation process of TiO2 quantum dots, leading to no breakthrough or real novelty in this work, because there are many reports on the application of TiO2 in sterilization. In addition, some of the statements within the manuscript are not very clear and contradictory between the figure and the results discussion. Therefore, I would not support its publication in Royal Society Open Science. More comments and suggestions are as follows: >1. In the abstract, some nouns appear only once and do not require abbreviation, such as SSA, XRD, HRTEM, etc. >2. Based on the XRD analysis and HRTEM image, the average crystallite size of TiO2 quantum dots is ~2 nm. However, the authors mention 'X-ray diffraction (XRD) analysis and highresolution transmission electron microscopy (HR-TEM) images showed that the as-prepared TiO2 quantum dots have high crystallinity with anatase phase and size varies from 2 to 4 nm.' in the abstract, it's a contradiction. >3. In the introduction, the authors should provide literature reviews about the preparation of TiO2 quantum dots at present. >4. In the introduction, some words are wrong, for example, the 'tome', 'synthetic routs', 'template routs' should be changed to 'time', 'synthetic routes', 'template routes', etc. >5. Moreover, the authors mention 'The wider band gap of TiO2 can prevent the charge recombination effect hence recombination of pair electron hole life time is longer.' in the introduction, please provide strong evidence.
>6. The authors used microwave-hydrothermal method to prepare TiO2 quantum dots, we are expected to add the advantages of microwave-hydrothermal method over other methods for preparation of TiO2 quantum dots. >7. For the determination of TiO2 band gap, the figure should be provided about the relationship between absorption and wavelength. >8. In Fig. 6, the authors state that the peak corresponding to the B1g mode, A1g and Eg modes of TiO2 quantum dots shows a small shift toward the lower frequencies as compared with the commercial TiO2, but we find that these peaks are migrating to higher frequencies, the authors should give a reasonable explanation. >9. To make the results clear, the concentration of commercial TiO2 nanoparticles for photocatalytic inactivation of E. coli should also be marked in Fig. 9a.

Reviewer: 3
Comments to the Author(s) The article "Photocatalytic Inactivation of E. coli under UV Light Irradiation using Large Surface Area Anatase TiO2 Quantum Dots" was carefully reviewed. The overall outlay and quality of paper is not good for publication in Royal Society of Open Science. Hence, I recommend for its rejection.

16-Sep-2019
Dear Dr Ahmed: Title: Photocatalytic Inactivation of E. coli under UV Light Irradiation using Large Surface Area Anatase TiO2 Quantum Dots Manuscript ID: RSOS-191444 It is a pleasure to accept your manuscript in its current form for publication in Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
The comments of the reviewer(s) who reviewed your manuscript are included at the end of this email.

Reply to reviewer comments on the manuscript-RSOS-190537
We are very grateful to the reviewer for his beneficial and constructive comments, questions as well as suggestions to further improve the quality of the manuscript. We have tried our best efforts to revise the manuscript as per their suggestions and guidelines. The changes done in the manuscript are highlighted by red color text. The replies to the reviewer comments are given below: I could not find the part of the paper in which the relationship between recombination rate and band gap energy is discussed. Perhaps the authors can direct me to it or provide another reference (and direct me to the passage). Alternatively, they could delete the line.

Answer
We are thankful to the reviewer for this valuable comment. We have deleted the line in the revised manuscript.

Reviewer # 2
The authors have revised the manuscript according to the reviewers' comments. The revised manuscript can be accepted for publication after major revision. But the following contents should be addressed.

Comment # 1
The authors are encouraged to give the high-magnification and low-magnification of TiO2 quantum dots TEM images.

Answer
We are thankful to the reviewer. High-magnification and low-magnification TEM images of TiO2 quantum dots have been provided in Fig. 3 in the revised manuscript.

Comment # 2
High-magnification of TiO2 quantum dots FESEM image should be given.

Answer
High-magnification FESEM image of TiO2 quantum dots has been provided in the inset of Fig. 2 in the revised manuscript.

Comment # 3
TiO2 is insoluble in water, the UV-Vis DRS of TiO2 quantum dots should be given, not the UV-Vis absorption spectra in fig. 5. Eg should be recalculated and analyzed in detail.

Answer
We are thankful to the reviewer for their valuable comments. UV-Vis DRS of TiO2 quantum dots and commercial nanoparticles have been provided in fig. 5(a) and fig. 5(b). Also, Eg has been recalculated and analyzed in detail in the revised manuscript.

Faheem Ahmed
Physics department, College of Science King Faisal University, Hofuf, Saudi Arabia