Preparation of polyacrylonitrile-based fibres with chelated Ag ions for antibacterial applications

The need for an excellent antibacterial material that is sufficiently powerful to never develop bacterial resistance is urgent. In this study, a series of novel polyacrylonitrile-based fibres with chelated Ag ions (referred to as Ag-SH-PANF) were prepared by a two-step chemical modification process: grafting and chelating. The properties of the as-prepared Ag-SH-PANF were characterized by Fourier transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The antibacterial activities of Ag-SH-PANF were examined against pathogenic bacteria, and an antibacterial mechanism was explicated based on the release of Ag ions from the fibres' surfaces. The results showed that, although chelation occurred between the Ag ions and the grafted amino, sulfhydryl and disulfide groups, Ag-SH-PANF retained its fine microstructure and thermal stability. Moreover, Ag-SH-PANF displayed excellent antibacterial ability against pathogenic bacteria as well as good washing durability. In terms of the antibacterial mechanism, Ag ions are the main bactericidal agents in the role of catalysts and are not consumed in the antibacterial process. Nonetheless, a relatively higher concentration of Ag ions can accelerate the bactericidal process.


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Recommendation?
Major revision is needed (please make suggestions in comments)

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Comments to the Author(s) Authors have reported the synthesis of polyacrylonitrile-based fibres with chelated with Ag+ ions (Ag-SH-PANF) and its antibacterial activities has been studied against pathogenic bacteria. Although results are interesting, several areas need major modifications before this manuscript can be considered for publication. See the comments below: It is stated in the experimental section that the release of Ag amount from the Ag-SH-PANF was tested by treating with soaking solutions. What is the soaking solution? How long the treatment was carried out? How the authors are sure that all Ag ions are released from the polymer after this treatment?
Introduction is not written properly and too brief. Authors have functionalized polyacrylonitrile fibres with surface S-S sites for the chelation of Ag(I), largely due to soft acid-soft base interactions. Ag+/Hg2+ favourably bind with solid matrixes bearing soft sulphur containing ligand sites. This should be discussed in the background. See and include such interactions described by different research groups: ACS Sustainable Chem. Eng. 2019, 7, 7353-7361;Chem. Eng. J. 2018, 332, 387-397. Resolution of the electron microscopic images are quite low. Authors should provide more high resolution images of Ag-SH-PANF, which can provide better understanding on the surface nanostructure.

Decision letter (RSOS-200324.R0)
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Once again, thank you for submitting your manuscript to Royal Society Open Science and I look forward to receiving your revision. If you have any questions at all, please do not hesitate to get in touch. ********************************************** RSC Associate Editor: Comments to the Author: (There are no comments.) RSC Subject Editor: Comments to the Author: (There are no comments.) ********************************************** Reviewers' Comments to Author: Reviewer: 1 Comments to the Author(s) This manuscript reported a series of novel polyacrylonitrile-based fibres with chelated Ag ions and their antibacterial activities. However, the synthesized methods are not brand new, besides the biological activity research is not deep enough. Thus, I do not think it is suitable to publish on Royal Society Open Science in present status. The authors should consider correction of the following suggestions: 1. In the summary part and part 3.8, the author mentioned the antibacterial mechanism, however, according to the report the fibres' antibacterial activity entirely depends on the Ag ions. There is no real mechanism research part in this paper, just a relationship of antibacterial activities and the concentration of Ag. 2. In the part of 3.2, the absorption peak from 3150 to 3700 cm−1 was mainly due to the water in the sample. Why not dry the sample? In that case, it is not hard to figure out the peak of N-H. 3. The IR spectra of Ag-SH-PANF was almost the same as that of SH-PANF, how can the author confirm the Ag ion is chelated with the ligand? The slight red-shift of 3 cm-1 could not be a proof. 4. Where is the peak of S-S bond in IR spectra? 5. In the antibacterial part, the positive group of Ag ion should be compared. 6. Also, in part 2.5 how can the author confirm the release Ag ion is came from the chelated Ag ion, rather than the scatter ones in the firbes? 7. By the way, do the fibres produce antibacterial activity only in solution? In that case, they can release Ag ion. Thus, how can we use them as a new medical material? Could the release Ag ion in dry condition?
Reviewer: 2 Comments to the Author(s) The paper reports on fiber-silver complex for antibacterial application. The paper is interesting. Some points need be strengthened. There are also some concerns to be addressed. All these are stated as follows: Why was this material selected for antibacterial studies? There have been many others, this needs be defined and some recent typical papers should be cited: Bioactive compounds: antioxidant, antibacterial and antiproliferative activities in chloranthus henryi, Sci. Adv. Mater., 2020, 12, 144-151; Durably antibacterial and bacterially anti-adhesive cotton fabrics coated by cationic fluorinated polymers, ACS Appl. Mater. Interfaces, 2018, 10, 6124-6136; A new anti-biofilm strategy of enabling arbitrary surfaces of materials and devices with robust bacterial antiadhesion via spraying modified microspheres method, J. Mater. Chem. A, 2019, 7, 26039; Antifouling and antibacterial behaviors of capsaicin-based pH responsive smart coatings in marine environments, Mater. Sci. Eng. C, 2020, 108, 110361; The application of the PAN-based fibers should be introduced: Chemical modification of carbon fiber with diethylenetriaminepentaacetic acid/halloysite nanotube as a multifunctional interfacial reinforcement for silicone resin composites. Polym. Adv. Technol., 2020, 31, 527; Modification of renewable cardanol onto carbon fiber for the improved interfacial properties of advanced polymer composites. Polymers, 2020, 12, 45 The TGA part needs be discussed with more details and the following papers will be helpful: Investigation of Thermostability of Modified Graphene Oxide/Methylsilicone Resin Nanocomposites, Engineered Science, 2019, 5, 73-78; Micro-crack behavior of carbon fiber reinforced Fe3O4/graphene oxide modified epoxy composites for cryogenic application. Compos. Part A, 2018, 108, 12-22. The conclusion needs be more focused and should be revised. The language needs be polished for a precise presentation.

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Reviewer: 3 Comments to the Author(s) Authors have reported the synthesis of polyacrylonitrile-based fibres with chelated with Ag+ ions (Ag-SH-PANF) and its antibacterial activities has been studied against pathogenic bacteria. Although results are interesting, several areas need major modifications before this manuscript can be considered for publication. See the comments below: It is stated in the experimental section that the release of Ag amount from the Ag-SH-PANF was tested by treating with soaking solutions. What is the soaking solution? How long the treatment was carried out? How the authors are sure that all Ag ions are released from the polymer after this treatment?

Decision letter (RSOS-200324.R1)
We hope you are keeping well at this difficult and unusual time. We continue to value your support of the journal in these challenging circumstances. If Royal Society Open Science can assist you at all, please don't hesitate to let us know at the email address below.

Dear editor and reviewers:
Thanks for your valuable advices. In view of the reviewers' comments, we have made the necessary changes to the manuscript, which have been marked in red. In addition, the data deposited in Dryad has also been simultaneously updated. Besides, a point-by-point response to the reviewers' comments is provided below.

Referee: 1
Comments to the Author(s) This manuscript reported a series of novel polyacrylonitrile-based fibres with chelated Ag ions and their antibacterial activities. However, the synthesized methods are not brand new, besides the biological activity research is not deep enough. Thus, I do not think it is suitable to publish on Royal Society Open Science in present status. The authors should consider correction of the following suggestions: 1. In the summary part and part 3.8, the author mentioned the antibacterial mechanism, however, according to the report the fibres' antibacterial activity entirely depends on the Ag ions. There is no real mechanism research part in this paper, just a relationship of antibacterial activities and the concentration of Ag. 2. In the part of 3.2, the absorption peak from 3150 to 3700 cm−1 was mainly due to the water in the sample. Why not dry the sample? In that case, it is not hard to figure out the peak of N-H. 3. The IR spectra of Ag-SH-PANF was almost the same as that of SH-PANF, how can the author confirm the Ag ion is chelated with the ligand? The slight red-shift of 3 cm-1 could not be a proof. 4. Where is the peak of S-S bond in IR spectra? 5. In the antibacterial part, the positive group of Ag ion should be compared. 6. Also, in part 2.5 how can the author confirm the release Ag ion is came from the chelated Ag ion, rather than the scatter ones in the firbes? 7. By the way, do the fibres produce antibacterial activity only in solution? In that case, they can release Ag ion. Thus, how can we use them as a new medical material? Could the release Ag ion in dry condition? REPLY: Thank you for your valuable comments and suggestions. 1. In this study, a series of polyacrylonitrile-based fibres with chelated silver ions (Ag-SH-PANF) were prepared, and their excellent antibacterial performance was confirmed. To identify whether the fibres act as an in situ antibacterial or agent-releasing antibacterial material, the Ag ion release experiment was designed, and the relationship between the amount of silver ions released and the antibacterial performance was determined. Thus, it was shown that the antibacterial performance of the material (i.e. Ag-SH-PANF) was achieved through the antibacterial agent. Besides, the antibacterial mechanism of Ag-SH-PANF is consistent with that of silver ions. Moreover, the antibacterial mechanism of Ag ions has been investigated by many researchers (please see, references 18 and 34-37 in the manuscript).
2. We dried the fibres for 4 h at 60℃ before FT-IR test. PANF used in this study are commercial fibres, in which the addition of the second and third monomers improves their moisture absorption performance. However, the chemical modification of PANF introduces a large number of amino, sulfhydryl and disulfide groups on their surface. Further, the introduction of amino, sulfhydryl and disulfide groups further on the surface of PANF further enhances their moisture absorption performance. In addition, the as-obtained FT-IR spectra were collected using the attenuated total reflection (ATR) method, and the absorption peaks in the ATR-FT-IR spectra were not as strong as those in the FT-IR spectra collected using the KBr pellet method. Therefore, we obtained new FT-IR spectra for the three fibres using the KBr pellet method, and the as-obtained FT-IR spectra were replaced. Further, the corresponding results have also been modified accordingly in section 3.2, and are marked in red. Unfortunately, although we tried our best to avoid the influence of water, it was still observed in the new spectra.

MODIFICATION:
The new FT-IR spectra for the three fibres using the KBr pellet method were showed in Fig. 3. The related sentences in section 2.3 were modified as "FT-IR spectra were collected with a Nicolet IR200 spectrometer (Thermo Electron Scientific Instruments, USA) in the range of 4000-400 cm -1 ; samples were prepared by the KBr pellet method".
(1) Marked as 0 when the antibacterial ratio is negative.
(2) The used silver mass in AgNO3 is the same as the content of silver in 0.1g Ag-SH-PANF 10 Fig. 8 Antibacterial activities of Ag-SH-PANF against (a) E. coli, (b) S. aureus, and (c) C. albicans.
6. The XRD characterization confirmed that no silver crystals were produced on Ag-SH-PANF. Moreover, the prepared Ag-SH-PANF were washed with deionised water until no more Ag ions could be detected in washwater. Therefore, there were no scattered silver ions in the fibres.
7. The antibacterial activity test was performed according to Chinese national standard "Textiles -Evaluation for anti bacterial activity -Part 3: Shake flask method" (GB/T 20944.3-2008). It simulates human clothing and living conditions. In particular, it focuses on textiles such as bedding, underwear, and socks, which are often in contact with human skin and absorb sweat. Therefore, the fibres won't release silver ions in absolutely dry condition, but release in a humid condition.
Reviewer: 2 Comments to the Author(s) The paper reports on fiber-silver complex for antibacterial application. The paper is interesting. Some points need be strengthened. There are also some concerns to be addressed. All these are stated as follows: Why was this material selected for antibacterial studies? There have been many others, this needs be defined and some recent typical papers should be cited: The conclusion needs be more focused and should be revised. The language needs be polished for a precise presentation.

REPLY:
Thank you for your valuable comments and suggestions. The corresponding changes have been made in section 1, section 3.3, and section 4, and more papers have been cited. These changes are marked in red. We have polished the language for a precise presentation, and the relevant changes are marked in red.

MODIFICATION:
The sentences we added in section 1 are as follows: " Various antibacterial components are used for preparing antibacterial materials, such as quaternary ammonium groups [5], quaternary phosphonium groups [6,7], silver nanoparticles [8,9] Fig. 5 (a) and (b). It can be easily seen from Fig. 5(b) that the weight loss rate of PANF is much higher than those of other two fibres. According to Fig. 5(a), in the case of PANF, there are two distinct plateaus in the TGA curve, with the first decomposition temperature near 290 °C. The cyclisation of -CN groups contributes to the first decomposition plateau, and the formation of a cross-linked structure in response to the oxidation and dehydrogenation of the functionalities on the macromolecular chains contributes to the second decomposition plateau [44]. After the desorption of atmospheric gases and the loss of water near 120°C, only one prominent plateau is observed in the curve for SH-PANF, whereas two plateaus appear again for Ag-SH-PANF. The second plateaus in SH-PANF and Ag-SH-PANF correspond to the first plateau of PANF. However, the decomposition temperatures for SH-PANF and Ag-SH-PANF are much lower than that of PANF, which is ascribed to the consumption of most -CN groups and the simultaneous conversion to other functionalities (i.e. -C=N) during the grafting process. The third plateau reflects the behaviour of the grafted cysteamine molecules and the cross-linked, oxidised, and dehydrogenated macromolecular chains. However, after the whole process, Ag-SH-PANF gives a significantly higher char yield of 69.2% than SH-PANF (56.5%) and PANF (49.4%), which may be attributed to the large amounts of adsorbed silver. This finding accords with those of Duan W et al. [32], but are contrary to those stated by Sun C et al. [45], who observed that the char yield of the unmodified fibres was higher than that of heavy-metal-loaded fibres". The section 4 after modification is as follows: "In this study, we successfully prepared a series of new antibacterial fibres, Ag-SH-PANF, with different Ag contents by a grafting and chelation sequence. The presence of Ag ions on the surface of Ag-SH-PANF was confirmed by XPS and FT-IR spectroscopy, which showed that chelation takes place between Ag ions and amino, sulfhydryl and disulfide groups. Moreover, XRD analysis confirmed that no silver crystals were produced on Ag-SH-PANF. Antibacterial testing showed that Ag-SH-PANF are excellent antibacterial agents against S. aureus, but less effective against E. coli, which is ascribed to structural differences between the two bacteria. Furthermore, we observed that the antibacterial activity of the fibres increased with the contact time between bacteria and fibres, as well as with the initial Ag content of the fibres. In addition, the results of washing durability experiments indicated the good washing durability of Ag-SH-PANF, whose antibacterial efficiency against E. coli remained above 98% after washing the fibres 100 times, even though the Ag content of the fibre was only 2.40%. Finally, silver release experiments elucidated the antibacterial mechanism of Ag-SH-PANF was the same as silver ions, in which silver ions are the main antibacterial factors, acting as catalysts and therefore are not consumed in the antibacterial process. Nonetheless, a relatively high concentration of silver ions can accelerate bacterial cell death within a certain concentration range".