Aqueous dispersion and tuning surface charges of polytetrafluoroethylene particles by bioinspired polydopamine–polyethyleneimine coating via one-step method

We propose a surface modification of poorly dispersive polytetrafluoroethylene (PTFE) particles via bioinspired polydopamine–polyethyleneimine (PDA–PEI) which conferred PTFE particles a uniform dispersion in aqueous medium. With increasing dopamine concentration in the reaction solution, dispersity of PTFE particles improved and the surface charges of particles changed from negative to positive due to an increase of surface coverage of PDA–PEI layers. Simplicity of the method here outlines an attractive route for surface modification of inert surfaces useful for large-scale applications.


Recommendation?
Accept with minor revision (please list in comments)

Comments to the Author(s)
This paper describes the surface modification of superhydrophobic PTFE particles with polydopamine-polyethylenimine (PDA-PEI). Although there are many reports on the modification of the PTFE surface by conventional PDA coating, this study shows the modification of the PTFE particle surface in a single step in an aqueous system. The procedure is simple and may extend the range of use of PTFE particles. Therefore, the reviewer is recommended to publish this article with appropriate modifications. 1.
What is the difference between a conventional PDA coating and a PDA-PEI coating? Please mention the interaction between the PDA-PEI thin film and the surface of the PTFE particles. The large contact area of the coated film has been reported to induce strong adhesion of the film to the substrate (see Soft Matter 2011, 7, 1856, Macromol. Rapid Commun., 2013, 34, 1220. In addition to the chemical structure, physical effects can also affect the coating. Does the molecular weight of PEI affect the adhesive properties? 2.
The stability of the PDA-PEI coating is important. Is PDA-PEI detachment observed when reanalyzed after 1 week or 1 month?

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

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

Recommendation?
Major revision is needed (please make suggestions in comments) Comments to the Author(s) This paper described the surface modification of PTFE particles that can disperse in water uniformly. Precisely tuned concentration of DA and PEI successfully modified the surface charge between positive and negative of the PTFE particles. The reaction mechanism is very straightforward, and thus this kind of surface engineering onto polymers can be applied into the various practical applications. The reviewer accept this manuscript of RSOS-210582 after major revision as noted.
1. For Figure 1, the reviewer suggests that addition of water contact angle measurements of sample after spin coating or drop casting the (i), (ii), and (iii) onto glass substrate for reader's better understanding in surface charge density change part.
2. How much of the thickness of the wrapped PDA-PEI shells on the PTFE? 3. For Figure 2, the size distribution diagram of PTFE and PTFE@PDA-PEI particles analyzed and calculated directly by SEM images should be useful to compare the dynamic light scattering spectra. This reviewer feels that the size of particles are not matched well between SEM and DLS. Figure 2, is there any difference between PTFE and PTFE@PDA-PEI when conduct 1D wide angle X-ray diffraction (WAXD) measurements after lyophilization? 5. For Figures 3 and 4, they are hard to get information because of low resolution. Please fully revise them, particularly x-and y-axis.

For
6. The author stated that PTFE@PDA-PEI can be used for industrial and biomedical application. Could you please explain more in detail because the changing charge density of conventional materials is very effective to pave the new way for development of antifouling finish or filter membrane.
7. For Page 2 right column, space is missing for "2g" and "3g". The editor assigned to your manuscript has now received comments from reviewers. We would like you to revise your paper in accordance with the referee and Subject Editor suggestions which can be found below (not including confidential reports to the Editor). Please note this decision does not guarantee eventual acceptance.
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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 paper describes the surface modification of superhydrophobic PTFE particles with polydopamine-polyethylenimine (PDA-PEI). Although there are many reports on the modification of the PTFE surface by conventional PDA coating, this study shows the modification of the PTFE particle surface in a single step in an aqueous system. The procedure is simple and may extend the range of use of PTFE particles. Therefore, the reviewer is recommended to publish this article with appropriate modifications. 1. What is the difference between a conventional PDA coating and a PDA-PEI coating? Please mention the interaction between the PDA-PEI thin film and the surface of the PTFE particles. The large contact area of the coated film has been reported to induce strong adhesion of the film to the substrate (see Soft Matter 2011, 7, 1856, Macromol. Rapid Commun., 2013, 34, 1220). In addition to the chemical structure, physical effects can also affect the coating. Does the molecular weight of PEI affect the adhesive properties? 2. The stability of the PDA-PEI coating is important. Is PDA-PEI detachment observed when reanalyzed after 1 week or 1 month?
Reviewer: 2 Comments to the Author(s) This paper described the surface modification of PTFE particles that can disperse in water uniformly. Precisely tuned concentration of DA and PEI successfully modified the surface charge between positive and negative of the PTFE particles. The reaction mechanism is very straightforward, and thus this kind of surface engineering onto polymers can be applied into the various practical applications. The reviewer accept this manuscript of RSOS-210582 after major revision as noted.
1. For Figure 1, the reviewer suggests that addition of water contact angle measurements of sample after spin coating or drop casting the (i), (ii), and (iii) onto glass substrate for reader's better understanding in surface charge density change part.
2. How much of the thickness of the wrapped PDA-PEI shells on the PTFE? 3. For Figure 2, the size distribution diagram of PTFE and PTFE@PDA-PEI particles analyzed and calculated directly by SEM images should be useful to compare the dynamic light scattering spectra. This reviewer feels that the size of particles are not matched well between SEM and DLS. Figure 2, is there any difference between PTFE and PTFE@PDA-PEI when conduct 1D wide angle X-ray diffraction (WAXD) measurements after lyophilization? Figures 3 and 4, they are hard to get information because of low resolution. Please fully revise them, particularly x-and y-axis.

For
6. The author stated that PTFE@PDA-PEI can be used for industrial and biomedical application. Could you please explain more in detail because the changing charge density of conventional materials is very effective to pave the new way for development of antifouling finish or filter membrane.

Comments to the Author(s)
The revised manuscript is clearly written on the points suggested by reviewers. Thus, this manuscript is acceptable for publication in Royal Society Open Science.

Recommendation?
Accept as is

Comments to the Author(s)
The revised version and author's response is acceptable.

Decision letter (RSOS-210582.R1)
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Dear
Thank you for your fine contribution. On behalf of the Editors of Royal Society Open Science and the Royal Society of Chemistry, I look forward to your continued contributions to the Journal. We would like to thank the reviewer for the careful and thorough reading of this manuscript.
The comments are encouraging, and the reviewer appears to share our judgement about the present study. The reviewer's comments are in italics, and the revised sentences in the manuscript were coloured in red. Figure 1 Authors' response: The reviewer's concern on the standard deviation on the peaks, in this case, is quite understandable. The DLS graphs show the result as an intensity distribution.

For
In the same sample, due to possible presence of particles of different size. As a result, larger particles scatter much more light than small particles, the intensity of scattering of a particle is proportional to the sixth power of its diameter (from Rayleigh's approximation). The values obtained from dls measurements with change in concentration of dopamine as shown in the Table 2 are the overall range with uncertainty considered. The reason for such high tolerance is due to the Zetasizer system that determines the size by first measuring the Browian motion of particles in a sample using Dynamic Light Scattering (DLS) and then interpreting a size from this correlation function (based on Stokes-Einstein Equation). The Zetasizer software uses algorithms to extract the decay rates for a number of size classes (arising from the amount of PDA-PEI coating on PTFE particles) to produce a size distribution. Whereas SEM images shows the clear PTFE particles coated with PDA-PEI shells with thickness ranged from 100 nm to 500 nm. The authors prepared samples multiple times in each with DA concentration. Figure 2, is there any difference between PTFE and PTFE@PDA-PEI when conduct 1D wide angle X-ray diffraction (WAXD) measurements after lyophilization?

For
Authors' response: In the present study, An X-ray photoelectron spectroscopy (XPS, KRATOS AXIS-Ulta DLD, Shimazu, Japan) was used for determining an elemental composition, chemical state and electronic state of the elements within the composite materials. WAXD measurements of samples after freeze drying or lyophilization was not carried out but should present considerable 2θ differences or intensity. Figures 3 and 4, they are hard to get information because of low resolution. Please fully revise them, particularly x-and y-axis.

For
Authors' response: On reviewer's indication, the resolutions for Figures 3 and 4 were improved.
6. The author stated that PTFE@PDA-PEI can be used for industrial and biomedical application. Could you please explain more in detail because the changing charge density of conventional materials is very effective to pave the new way for development of antifouling finish or filter membrane.
Authors' response: Polytetrafluoroethylene (PTFE) is one of the most important candidate of engineering polymers owing to its high chemical inertness, high temperature resistance, low temperature resistance, excellent heat endurance and strong mechanical strength.
However strong hydrophobicity limits its wide applications. By coating PDA-PEI onto PTFE surface, its hydrophilicity and compatibility with other materials can be improved which makes it to be a good candidate for the applications in the fields of water treatment, chemical industry, textile, electronics, medical treatment, military, aerospace and so on. Further, specific functional groups can be designed and incorporated into the surface of PTFE@PDA-PEI particles to allow subsequent surface functionalization, such as enzyme and protein immobilization via covalent bonding.