Tetraphenylethylene-Substituted Bis(thienyl)imidazole (DTITPE), An Efficient Molecular Sensor for the Detection and Quantification of Fluoride Ions

Round 1

Reviewer 1 Report

In this manuscript, the authors synthesized an efficient molecular sensor for the quantification of fluoride ions, and revealed the detection mechanism of this sensor for fluoride ions. In general, this work is interesting, and I recommend its acceptance for publication in the journal.

Here are some of my suggestions:

1. The part of introduction has not highlighted the novelty and idea, which should be displayed as clear and concise as possible
2. It is suggested to compare this provided molecular sensor with other sensors based on imidazole rings containing fluorophores for detecting F^-, and explain the strengths of this provided sensor.
3. In the part of 3.2, the optical studies and F^­- detection of the molecular sensor were mixed and disordered. So please make it clear with well-ordered.
4. It is suggested to add practical sample analysis for F^‑ by this sensor.
5. Some grammatical and typographical errors should be revised, such as “however, Upon the”, “with the naked eye (Figure 3.13b)”.

Author Response

We thank the reviewer for the valuable suggestions. We have gone through all comments point-by-point and provided responses, please find the revised manuscript and responses to the reviewer's comments. 

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript by Kumar et al. describes an experimental/computational study on an efficient molecular sensor for fluoride ions by DTITPE. I found the study very interesting and of potential practical use. I am a theoretical chemist, I have the following suggestions:

1. Can the authors provide a few more examples where the detection of extremely low concentration of F- is important in the biological systems and in environmental sciences where the current available methods fail.

2. The computational methods should be described and justified in more detailed in section 2. The basis set is 6-31+G(d)in section 2 but 6-31+g(d,p) in section 3? It should be written as 6-31+G(d,p). The TD-DFT methods should be mentioned.

3. Can the authors compare the experimental UV-Vis spectra with computed ones?

4. The molecular structures in Figures 6 and 7 are vertically distorted.

5. The discussion on FMO, electron transfer, and band gaps is problematic. The purpose of using electronic structure calculation is to describe the systems more accurately with quantum mechanics. The FMO, HOMO-LUMO gaps are very crude concept. Since the DFT and TD-DFT methods have been used, the electron density distribution should be computed and compared on DTITPE, DTITPE.F-, and DTITPE-. The color changes, the absorption at 305 and 405 nm and emission at 510 nm should be discussed based on the excited state properties obtained by TD-DFT calculation.

6. Why not also calculate the molecular structures using IEFPCM?

7. Line 342, "within section"? Do the authors mean "within seconds?"

Author Response

We thank the reviewer for the valuable suggestions. We have gone through all comments point-by-point and provided responses, please find the revised manuscript and responses to the reviewer's comments. 

Author Response File: Author Response.pdf

Reviewer 3 Report

Refer to the compounds in Figure 1 in the text. 

As can be qualitatively seen from Fig. 3, if the first concentration of F- is 3 x 10-7 M, then the detection limit  is close to this concentration, as the signal is barely visible. It thus cannot be 1 x 10-13 M. The fluorescence detection limit needs to be recalculated.    

Compound concentration needs to be given in Figure 4. 

The band gap for the compound-F- complex should be 2.38 eV. 

For the strips, the F- concentration needs to be given in the text and figure. 

If the mechanism of fluorescence is H+ loss, then OH- should do the same thing as F-. This should be tested and the results included in the paper.  

Author Response

We thank the reviewer for the valuable suggestions. We have gone through all comments point-by-point and provided responses, please find the revised manuscript and responses to the reviewer's comments. 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

accept

Author Response

Many thanks to the reviewer for taking the time to review my manuscript.

Reviewer 2 Report

The authors made some effort to improve the theoretical description. However, the acronyms are still not used consistently. The basis set is written as 6-31+G(d,p) on page 3 but is 6-31+g(d,p) on page 9 and in SI. B3LYP(6-31+G(d,p))/CPCM, DFT/B3LYP/6-31+G(d,p) are just  very confusing.

The excited state properties including the electron density is not shown, and the discussion is still mainly based on the old fashioned band gap.  The TDDFT calculation did not seem to include enough states. The calculated spectra are only listed in the table and figures but not well described or discussed.  Figures 6 and 7 still seem to be distorted.

I am afraid that the excited-state calculation is still not properly done and discussed.

Author Response

Prof Enza Panzardi,

Guest Editor, Chemosensors.

Subject: Submission of original manuscript to Chemosensors journal.

Dear Enza Panzardi,

Many thanks for your invitation (June 28, 2021) to contribute a paper to the Special Issue entitled "Gas Sensors: Simulation, Modeling, and Characterization” of Chemosensors journal, and this manuscript “Tetraphenylethylene-substituted bis(thienyl)imidazole (DTITPE), an efficient molecular sensor for the detection and quantification of fluoride ions” has been reviewed and commented by three referees in the first round. We have resubmitted the revised manuscript including responses to the three reviewers on Sep 13, 2021.

Moreover, in a second round, we have received an acceptance from the first reviewer, while the second reviewers comments regarding the theoretical studies have been clearly addressed (see below). Furthermore, the second reviewer requested us to perform a few more DFT calculations, but the current pandemic scenario at our university will cause too much delay in receiving results. As a consequence, we feel that the current version of the manuscript provides adequate experimental results and preliminary DFT data for a wide range of readers.

We have no conflicts of interest to disclose and confirm that this work is original and has not been published elsewhere, nor is it currently under consideration for publication elsewhere.

Please address all correspondence concerning this manuscript to me at [email protected].

Thank you for your kind consideration of this manuscript.

Yours Sincerely,

Suresh K. Bhargava

Distinguished Professor and Director -Centre for Advanced Materials & Industrial Chemistry

Foreign Fellow of AAAS and INAE,

STEM college,

RMIT University, Melbourne

Here is a point-by-point response to the reviewers' comments and concerns

1. The authors made some effort to improve the theoretical description. However, the acronyms are still not used consistently. The basis set is written as 6-31+G(d,p) on page 3 but is 6-31+g(d,p) on page 9 and in SI. B3LYP(6-31+G(d,p))/CPCM, DFT/B3LYP/6-31+G(d,p) are just  very confusing.

Response: We have revised this manuscript carefully and modified it in a consistent manner of all acronyms.

2. The excited state properties including the electron density is not shown, and the discussion is still mainly based on the old-fashioned band gap.  The TDDFT calculation did not seem to include enough states. The calculated spectra are only listed in the table and figures but not well described or discussed.  Figures 6 and 7 still seem to be distorted. I am afraid that the excited-state calculation is still not properly done and discussed.

Response: Thanks for your constructive comment. We have taken a DFT optimized structure for TD-DFT calculations, where we have done the first 10 singlet transitions. Based on these outputs, we have used GaussSum Version 2.2.5 for analyzing the results. Obtained UV-Vis and electronic transitions are presented and discussed accordingly. We agree with your statement, i,e in-depth excited-state calculations like electron density, etc., (including our aims like binding energies etc.) could not be conducted due to the pandemic situation. We believe as the presented preliminary results would support experimental data and strengthen the manuscript. Also, we have modified the figures 6 and 7 to avoid the distortion.

Author Response File: Author Response.pdf