Fast and selective fluoride ion conduction in sub-1-nanometer metal-organic framework channels
Authors: Peiyao Wang
Benny D Freeman
Xingya Li
Lei Jiang
Jun Lu
Christopher D Easton
Anita J Hill
Jefferson Zhe Liu
Aaron W Thornton
Matthew R Hill
Huanting Wang
Huacheng Zhang
Xiwang Zhang
Jue Hou
Journal: Nature Communications (2019)
Discipline: Multidisciplinary
Keywords: Biological fluoride ion channels; synthetic F- channels; metal-organic frameworks; UiO-66-X; F- conductivity
Affiliations: Department of Chemical Engineering, Monash University, Clayton, VIC, 3800, Australia., Department of Chemical Engineering, Monash University, Clayton, VIC, 3800, Australia. ., Department of Mechanical Engineering, The University of Melbourne, Parkville, VIC, 3010, Australia., Manufacturing, CSIRO, Clayton, VIC, 3168, Australia., Department of Mechanical Engineering, The University of Melbourne, Parkville, VIC, 3010, Australia. ., Department of Chemical Engineering, The University of Texas at Austin, Austin, TX, 78712, USA., Key Laboratory of Bioinspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, 100190, Beijing, People's Republic of China.

Abstract:  Biological fluoride ion channels are sub-1-nanometer protein pores with ultrahigh F- conductivity and selectivity over other halogen ions. Developing synthetic F- channels with biological-level selectivity is highly desirable for ion separations such as water defluoridation, but it remains a great challenge. Here we report synthetic F- channels fabricated from zirconium-based metal-organic frameworks (MOFs), UiO-66-X (X = H, NH2, and N+(CH3)3). These MOFs are comprised of nanometer-sized cavities connected by sub-1-nanometer-sized windows and have specific F- binding sites along the channels, sharing some features of biological F- channels. UiO-66-X channels consistently show ultrahigh F- conductivity up to ~10 S m-1, and ultrahigh F-/Cl- selectivity, from ~13 to ~240. Molecular dynamics simulations reveal that the ultrahigh F- conductivity and selectivity can be ascribed mainly to the high F- concentration in the UiO-66 channels, arising from specific interactions between F- ions and F- binding sites in the MOF channels. 

Retracted: No

PubPeer comments:
 This paper claims the UiO-66 MOFs could fast, selectively transport fluoride ions. , 
 In this work, Figure. S6-A shows the authors interpretation of the fluoride interaction with UiO-66 as follows: F- can replace the -OH in defect sites; F- can bond the Zr even this Zr-O has been coordinated by ligands; But F- cannot replace the u00b53-OH. There is no evidence in this paper to support this understanding. , 
 I agree with the comments by the unregistered submission above. The authors claimed the binding of F- with the zirconium nodes contributes to the selectively fast F- movement in Zr-MOF channels. But, what the nature of this selective binding refers to herein? In the introduction part, there is one sentence u201cUiO-66(Zr) derivative MOFs display the highest F- adsorption capacity due to the specific binding of F- through the hydroxyl and open zirconium sitesu201d. This ambiguously description signify the authors seem to deliberately ignored the binding nature of F- with MOFs, i.e. ion exchange of F- with -OH (Zr-OH and u00b53-OH) of Zr nodes of MOFs. Consequently, these massively produced hydroxide ions are disregarded on purpose. The released OH- definitely cause the ion contamination, one serious issue in nanofluidic studies and make all I-V data for KF in MOFs in this work questionable. In addition, in this work, the stability of Zr-MOFs in KF solutions was obviously overestimated. For instance, put 20 mg UiO-66 MOFs in 20 mL 1.0 M KF solution for 24 hours, no MOF will be left in the end. , 
 #2 We have just found these comments about our paper published in 2019. Our response is listed below. Thank you for your comments. In this work, F$u2212$ ions can specifically bind to unsaturated Zr sites through specific Zru2013F interactions (including replacing some u2013OH groups on Zr6 nodes), which has been concluded from other references, including Microporous Mesoporous Mater. 185, 72-78 (2014), J. Colloid Interface Sci. 461, 79-87 (2016), and J. Colloid Interface Sci. 484, 162u2013172 (2016). We included these references as Ref. 34, 35, 36 in the paper. F$u2212 $cannot replace u00b53-OH because u00b53-OH is the skeleton of Zr6 nodes. If the u00b53-OH is replaced, the UiO-66 framework structure will collapse. From the stability test (Figure. S18-19), where UiO-66 powder was immersed in KF solution (0.1 M, pH 5.7) for 3 days, the XRD pattern and the pore size distribution demonstrate the integrity of the MOF framework. , 
 #3 We have just found these comments about our paper published in 2019. Our response is listed below. Thank you for your comments. The effect of the released OH- on the I-V data was ruled out in our work. For the I-V test, we collected the data until the current reached a stable value after at least three tests. In addition, the salt solution (e.g., KF) was replaced with the fresh solution after each test, so the released OH- should be negligible. In our work, the MOF concentration was ~0.5 mg mL in the stability test of the MOF powder in 0.1 M KF solution for 3 days. For the I-V test, the concentration of KF solution ranged from 0.01 to 1.0 M, and the scanning time is 20 s. Therefore, the collected data should be reliable under the above experimental conditions. , 
 Fig.18 in Supporting info. Panels a and b seem to be identical copies. The resolution is too low to see real noise but similarity between patterns in left and right panels is too strong.. ![file] ,