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Peer-Review Record

A Test of Sol–Gel Incorporation of Organic Compounds as Translucent, Marine Biofouling-Resistant Windows

J. Mar. Sci. Eng. 2023, 11(4), 733; https://doi.org/10.3390/jmse11040733
by Malcolm Dale Stokes
Reviewer 1: Anonymous
Reviewer 2: Anonymous
Reviewer 3:
J. Mar. Sci. Eng. 2023, 11(4), 733; https://doi.org/10.3390/jmse11040733
Submission received: 10 March 2023 / Revised: 25 March 2023 / Accepted: 27 March 2023 / Published: 28 March 2023
(This article belongs to the Section Ocean Engineering)

Round 1

Reviewer 1 Report

This paper investigates the efficiency of composite materials containing biocides compounds intended to function as a biofouling-resistant window for the oceanographic instrumentation. It examines the influence of various biocides compounds (antimicrobial agents, azithromycin and hydrous allicin) incorporated in a transparent silicate sol-gel matrix.

This paper is original and creative. It presents a promising way for the fabrication of biofouling-resistant translucent materials based on environmentally safe molecules. The paper is well organized. The scientific approach adopted in this study is rigorous, and the results are well explained. 

Therefore, I am in favor for publication in “JMSE” after minor corrections.

 

Minor:

- The percentages reported on lines 250 and 253 (in p6) are not in accordance with data shown in Fig. 2. “had decreased approximately 6%” instead of “had decreased approximately 11%” and “had decreased approximately 5% from the gradual growth” instead of “had decreased approximately 10% from the gradual growth”

- The number of references is 94, which seems somewhat excessive for a research article. I suggest to the author to cite just the most relevant and recent articles.

- Legend of Fig. 1: “AZITH” instead of “AZIH”

Author Response

I would like to thank the Reviewer for their cogent review and suggestions for minor edits.   All suggested edits have been made in the revised manuscript and are itemized as follows:

"...Therefore, I am in favor for publication in “JMSE” after minor corrections."

"The percentages reported on lines 250 and 253 (in p6) are not in accordance with data shown in Fig. 2. “had decreased approximately 6%” instead of “had decreased approximately 11%” and “had decreased approximately 5% from the gradual growth” instead of “had decreased approximately 10% from the gradual growth” :  The percent decrease has been changed in the manuscript to align with the reviewers analysis and corrections.

"The number of references is 94, which seems somewhat excessive for a research article. I suggest to the author to cite just the most relevant and recent articles.": The number of references has been reduced by eliminating some of the older, possibly redundant references (20 references have been removed) and this has been updated in the manuscript.

"Legend of Fig. 1: “AZITH” instead of “AZIH” "  : The legend has been corrected as suggested. Thank you for noticing the error.

Reviewer 2 Report

This manuscript describes the incorporation of five different organic compounds in sol-gel derived materials for use as translucent, marine biofouling-resistant windows. These composite materials were tested for three weeks against the formation of marine biofilm. The amount of biofilm formed was estimated by the transmission of visible light and confirmed by twice weekly steromicroscopy, although no stereomicroscope pictures are shown in the manuscript.

This approach of incorporating active compounds in sol-gel coatings or structures is not new; a search in Google Scholar for the terms "sol-gel antifouling coatings" produces 16,300 hits. The author refers to some of this past work in L85-L90. Therefore the potentially novel aspect of this work is the proposed use of these materials as optical windows for underwater sensors. However, even this application has been previously reported (e.g. Regan 2017).

The degree of biofilm fouling is assessed by the change in transmission of visible light using a (self-constructed?) transmissometer. The author states (L200-L203) that the change in transmission is not due to "some other mechanism" but this is not proven. It is highly likely that the clarity of the materials will change, even in the absence of fouling, as the active compounds are leached out, and this could be a concern for the stability of optical sensor readings and this should be addressed.

The test period of three weeks immersion is extremely short and not of practical importance for underwater sensors that will be deployed for periods of many months. However it does provide proof of the concept, and is sufficient to demonstrate the different relative performance of the five compounds tested.

It would be very helpful to provide photographs or microscope images of the accumulated fouling.

In L69 write "metal or metalloid alkoxide" to include silicon (not a true metal).

There are a few minor typing and formatting errors ('ph' in L97 and subscript '2' is required in H2O throughout): the IUPAC symbol for millilitres is now mL, not ml, and the symbol for kelvin is uppercase 'K'.

REF: Regan, Fiona, Alan Barrett, Ciprian Briciu-Burghina, and Timothy Sullivan. "Antifouling studies and coating strategies for marine deployed structures." In OCEANS 2017-Aberdeen, pp. 1-6. IEEE, 2017.

Author Response

I would like to thank the reviewer and their experience for their extensive review and comments on the manuscript.   Their suggested revisions will clarify and improve the document.  The changes are itemized below:

"This approach of incorporating active compounds in sol-gel coatings or structures is not new; a search in Google Scholar for the terms "sol-gel antifouling coatings" produces 16,300 hits. The author refers to some of this past work in L85-L90. Therefore the potentially novel aspect of this work is the proposed use of these materials as optical windows for underwater sensors. However, even this application has been previously reported (e.g. Regan 2017).
" :

The reviewer has noted the extensive literature on sol-gel coatings and their use for underwater applications and I thank them for including the Regan et al. 2017 reference (this important reference has now been added to the literature cited) which explores the use of these coatings as effective optical windows for sensors. In the case of Regan et al. 2017, they reported tests on a commercial antifoulant, copper plates, PDMSe and Cu-based superhydrophobic sol-gels, and Cu nanoparticle doped sol-gels of varying effectiveness as instrument window antifoulants. In the case of study reported in this submission, although the use of a sol-gel as a antifoulant on an optical window is not unique, it is novel in its use of a allicin compound as a natural biocide incorporated into the sol-gel.

"The degree of biofilm fouling is assessed by the change in transmission of visible light using a (self-constructed?) transmissometer. The author states (L200-L203) that the change in transmission is not due to "some other mechanism" but this is not proven. It is highly likely that the clarity of the materials will change, even in the absence of fouling, as the active compounds are leached out, and this could be a concern for the stability of optical sensor readings and this should be addressed.
"

The reviewer is correct, and that stability and clarity of the material can change over time and with leaching, and this has now been clarified in the text as follows, "…that the light extinction being measured was primarily the result of biofilm formation and not the result of inorganic surface particle contamination. It should be noted that the methodology and experiment controls followed herein does not preclude the potential role of chemical leaching, discoloration during aging or other unknown factors that could cause changes to the light transmission stability of the sample disks over time."


With the reviewers prompting, some additional information on the transmission measurements using the transmissometer have been added to the text as follows, "Biofilm growth on the samples was estimated by measuring the transmission of light through each disk using a simple transmissometer consisting of an LED light (6500 K color temperature) and thermally-stable power source, collimator optics (Thorlabs), amplified photodiode (Hamamatsu S1087, peak sensitivity 560 nm ) with stable voltage reference and collecting optics (Thorlabs). The optical components and sample holding jig, were all enclosed in a light-proof box with a fixed pathway of light transmission (40 mm path-length)."

"The test period of three weeks immersion is extremely short and not of practical importance for underwater sensors that will be deployed for periods of many months. However it does provide proof of the concept, and is sufficient to demonstrate the different relative performance of the five compounds tested.
"

I agree with the reviewer that even though the present study was of relatively short duration, only 3 weeks, but, as noted, it does provide a test of the concept because biofilms critical to secondary fouling by more macroscopic organisms begin forming within minutes of seawater submersion. 

"It would be very helpful to provide photographs or microscope images of the accumulated fouling.
"

Unfortunately, it would be extremely difficult to provide images of fouled samples ex post facto and match the environmental conditions of the experiment reported in this manuscript.

"In L69 write "metal or metalloid alkoxide" to include silicon (not a true metal). "

Very true! This has been corrected in the manuscript to include metalloid

"There are a few minor typing and formatting errors ('ph' in L97 and subscript '2' is required in H2O throughout): the IUPAC symbol for millilitres is now mL, not ml, and the symbol for kelvin is uppercase 'K'."

Thank you for noticing these details; these units and typsetting errors have all been corrected in the manuscript.

"REF: Regan, Fiona, Alan Barrett, Ciprian Briciu-Burghina, and Timothy Sullivan. "Antifouling studies and coating strategies for marine deployed structures." In OCEANS 2017-Aberdeen, pp. 1-6. IEEE, 2017. "

As noted above, this critical  reference has been added to the literature cited. 

Reviewer 3 Report

Two suggestions for the authors:

1)Since the translucent gel with incorporation of biocides was already partially reported elsewhere by other people, readers will hope to see more detailed antifouling results from this study, for example, combinations of several antimicrobial reagents will produce very good anti-biofilm effects or very good effect specifically for some macrofoulers?

2) The references seem too many. The introductionitself has 71 references.

Author Response

I would like to thank the reviewer for their detailed comments to improve the manuscript.  The changes are itemized below.

"1) Since the translucent gel with incorporation of biocides was already partially reported elsewhere by other people, readers will hope to see more detailed antifouling results from this study, for example, combinations of several antimicrobial reagents will produce very good anti-biofilm effects or very good effect specifically for some macrofoulers?" : 

The reviewer makes an excellent point —  sol-gels with incorporated biocides have been reported earlier in the literature, but, to my knowledge this is the first incorporation of natural and easily-sourced allicin compounds which may make it an effective and safe antifouling agent. Combinations of multiple reagents is a very good idea and this comment has been expanded upon in the last paragraph of the discussion. This will be a focus for upcoming research beyond the scope of this initial publication.  The last paragraph now includes, “It is also conceivable that many different compounds, of similar molecular size, could be combined in a single composite sol-gel to provide broad-spectrum biofouling resistance that first slows biofilm formation and then acts as a biocide / deterrent to later multicellular fouling organisms. The combination of multiple quorum-blocking and compatible biocidal compounds into the one sol-gel silicate matrix is currently under active study. Ideally, an antifouling window of hybrid design incorporating environmentally safe biocidal molecules for in situ resistance to biofilm formation and organism settlement, as well as having the tunable surface wettability and roughness possible with sol-gels to reduce adhesion will provide as close to an ideal passive anti-fouling solution for marine instrumentation “

"2) The references seem too many. The introduction itself has 71 references.":

The literature on antifouling coatings is extensive, so it is difficult to limit the reference list and still give a complete accounting of the research, however the number of references has been reduced, eliminating some of the older, possibly redundant references (20 references have been removed).

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