Self-Exfoliated Guanidinium-Based Ionic Covalent Organic Nanosheets (iCONs)

Article abstract of DOI:10.1021/jacs.5b13533

Covalent organic nanosheets (CONs) have emerged as functional two-dimensional materials for versatile applications. Although π-π stacking between layers, hydrolytic instability, possible restacking prevents their exfoliation on to few thin layered CONs from crystalline porous polymers. We anticipated rational designing of a structure by intrinsic ionic linker could be the solution to produce self-exfoliated CONs without external stimuli. In an attempt to address this issue, we have synthesized three self-exfoliated guanidinium halide based ionic covalent organic nanosheets (iCONs) with antimicrobial property. Self-exfoliation phenomenon has been supported by molecular dynamics (MD) simulation as well. Intrinsic ionic guanidinium unit plays the pivotal role for both self-exfoliation and antibacterial property against both Gram-positive and Gram-negative bacteria. Using such iCONs, we have devised a mixed matrix membrane which could be useful for antimicrobial coatings with plausible medical benefits.

Full text of DOI:10.1021/jacs.5b13533

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Article
Self-Exfoliated Guanidinium-Based Ionic
Covalent Organic Nanosheets (iCONs)
Shouvik Mitra, Sharath Kandambeth, Bishnu P. Biswal, Abdul Khayum M., Chandan
Kumar Choudhury, Mihir Mehta, Gagandeep Kaur, Subhrashis Banerjee, Asmita
A. Prabhune, Sandeep Verma, Sudip Roy, Ulhas K. Kharul, and Rahul Banerjee
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b13533 • Publication Date (Web): 11 Feb 2016
Downloaded from http://pubs.acs.org on February 11, 2016
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Self-Exfoliated Guanidinium-Based Ionic Covalent Organic
Nanosheets (iCONs)
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Shouvik Mitra,a‡ Sharath Kandambeth,a,b‡ Bishnu P. Biswal,a,b‡ Abdul Khayum M.,a,b Chandan K.
Choudhury,a,b Mihir Mehta,c,b Gagandeep Kaur,d Subhrashis Banerjee,a,b Asmita Prabhune,c,b Sandeep
Verma,d Sudip Roy,a Ulhas K. Kharul,e,b and Rahul Banerjee*a,b
a Physical/Materials Chemistry Division, CSIR-National Chemical Laboratory, Pune, India.
b Academy of Scientific and Innovative Research (AcSIR), New Delhi, India.
c Biochemical Sciences Division, CSIR-National Chemical Laboratory, Pune, India.
d Department of Chemistry, Indian Institute of Technology, Kanpur, India.
e Polymer Science and Engineering Division, CSIR-National Chemical Laboratory, Pune, India.
ABSTRACT: Covalent organic nanosheets (CONs) have emerged as functional two dimensional materials for versatile applica-
tions. Although π-π stacking between layers, hydrolytic instability, possible restacking prevents their exfoliation on to few thin lay-
ered CONs from crystalline porous polymers. We anticipated rational designing of a structure by intrinsic ionic linker could be the
solution to produce self-exfoliated CONs without external stimuli. In an attempt to address this issue, we have synthesized three
self-exfoliated guanidinium halide based ionic covalent organic nanosheets (iCONs) with antimicrobial property. Self-exfoliation
phenomenon has been supported by molecular dynamics (MD) simulation as well. Intrinsic ionic guanidinium unit plays the pivotal
role for both self-exfoliation and antibacterial property against both gram-positive and gram-negative bacteria. Using such iCONs
we have devised mixed matrix membrane which could be useful for antimicrobial coatings with plausible medical benefits.
Nanosheets (iCONs) for antimicrobial applications. Positively
charged guanidinium units result in interlayer repulsion to
INTRODUCTION
self-exfoliate into few layered iCONs. These iCONs showed
permanent porosity, chemical stability and potent
Covalent organic nanosheets (CONs) are two dimensional
(2D) materials constructed by symmetrically arranged organic
antimicrobial
(Staphylococcus aureus) and gram-negative (Escherichia coli)
bacteria. Although substituted guanidinium-based
property
against
both
gram-positive
linkers, along two dimension, via strong covalent bonds.1 In
recent years, researchers have made considerable efforts to
synthesize monolayer or few layered CONs, which could
emerge as a porous functional material with a plethora of
applications owing to their easy and pre-designable
conventional polymeric materials are known to demonstrate
antimicrobial property;8 but the solubility of guanidinium-
based materials limits their usage in heterogeneous water
resistant antimicrobial coatings. Hence, the insolubility of
iCONs with antimicrobial property could be the key solution
in devising antibacterial coatings useful in biomedical sectors.
To validate this conception, we have strategically fabricated
iCONs@Polysulfone (PSF) mixed matrix membrane for
antimicrobial coatings.
1,
2
functionalization opportunity.
On this line, few layered
CONs have been synthesized from 2D crystalline porous
polymers (CPPs) often known as covalent organic frameworks
(COFs)3 and covalent triazine frameworks (CTFs),4 using
conventional exfoliation techniques such as ultrasonication1c
and mechanical delamination.1a,b,d However, the major reasons
that prevent the formation of monolayer or few layered CONs
by these methods include (i) strong π-π stacking between the
layers,1-4 (ii) hydrolytic instability for most of the CPPs,5 (iii)
possible restacking;6 and most importantly, (iv) challenging
synthetic procedures to obtain the exfoliated nanosheets.1e,7
However, these shortcomings could be surpassed easily by
rational design of chemically stable CONs with inbuilt ionic
character to induce self-exfoliation. Moreover, if the ionic
building block itself possesses biological significance, the
synthesized CONs could be directly used for biomedical
applications without further post synthetic modifications.
Nevertheless, the self-exfoliation of 2D CPPs and exploring its
biological significance in a single domain is unprecedented
and thus remains a daunting task. In the same pursuit, we have
introduced the concept of rational design to self-exfoliate
guanidinium halide based porous ionic Covalent Organic
EXPERIMENTAL SECTION
Three of the iCONs (TpTGCl, TpTGBr, and TpTGI) were syn-
thesized via Schiff base condensation reaction between 0.2
mmol 1,3,5-triformylphloroglucinol (Tp) (42 mg) and 0.2
mmol amines (TGX) (TGCl: 28 mg or TGBr: 37 mg or TGI: 47.4
mg) in a sealed Pyrex tube using dioxane: water in the ratio 2:
0.6 mL. The reaction mixtures were charged into Pyrex tube
and sonicated for 20 min. The mixtures were degassed under
liquid N2 (77K) by freeze-pump-thaw cycles for three times
and the Pyrex tube was then vacuum sealed. The reaction mix-
tures
were
allowed
to
attend
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Figure 1. (a) Synthetic scheme of TpTGCl monomer (SaTGCl) (left) and of iCONs (TpTGx) (right); where X=Cl–, Br–, I–; (b) ORTEP dia-
gram of SaTGCl at 50% probability ellipsoid level; (c) Stacking model of SaTGCl showing sandwiched anions between the layers (side
view); (d) Comparison of PXRD patterns of the three iCONs; (e) 13C CP MAS solid-state NMR of iCONs; (f) Modeling of TpTGCl in
eclipsed mode (top view); (g) Stacking model showing individual layers (side view).
on porous non-woven support fabrics (3329). TpTGCl (5% by
room temperature and then kept at 120 ˚C for 3 days. TpTGCl,
TpTGBr, and TpTGI were obtained as a brown colored precipi-
tate. The product was washed thoroughly with DMAc, water,
and acetone respectively and dried at 90˚C for overnight to
obtain in ~70 % isolated yield.
wt. of PSF) was added to the PSF solution in DMF, stirred
well for another 6 h and degassed for 3 min to remove the
trapped gases. TpTGCl@PSF membranes were prepared using
Sheen Auto-matic Film Applicator-1132 at 25 ºC and exposed
in air for 15 seconds prior to dipping in non solvent (water)
bath. The knife movement was set to 10 cm.sec-1 transverse
speed with a gap of 250 μm. The total thickness of the sup-
ported membrane was ~100 μm. These membranes were
washed with distilled water and stored in aqueous formalin
(0.5%) solution. Antibacterial activity of iCONs membrane
was evaluated by the growth of bacteria on to it (detailed in
Section S11 ESI).
To obtain detail insights into self-exfoliation of iCONs in
presence of water molecules, we performed all atom molecular
dynamics (MD) simulations (detailed in Section S8 ESI).The
synthesized iCONs were used for antibacterial studies using
three different concentrations (100 µgmL-1, 200 µgmL-1 and
500 µgmL-1). They were dispersed in autoclaved deionized
water and allowed to incubate with bacterial strains Staphylo-
coccus aureus and Escherichia coli for about 4 h. The result-
ing mixture was plated in Petri plates containing solidified
potato dextrose agar media. The Petri plates were incubated at
37 ºC for 24 h and the resulting CFU counts were measured
visually. Morphological study of control and treated bacterial
samples were carried out by SEM and TEM. Detailed
experimental procedures are provided in Section S10 ESI. We
further fabricated iCONs mixed matrix membrane using PSF
RESULTS AND DISCUSSIONS
In order to incorporate guanidinium units within a stable
iCONs framework, we have synthesized triaminoguanidinium
halide (TGCl, TGBr, and TGI) which assumes a C3 symmetric
planner
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matched well with that of experimentally observed PXRD
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3
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(Figure S6-S11). Although the monomeric units were planner,
but ionic layers with sandwiched halide ions in between them
created an extremely weak stacking (Figure 1g). This motivat-
ed us to believe that the as synthesized polymers would intrin-
sically self-exfoliate to covalent organic nanosheets.
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8
FTIR spectra of iCONs revealed characteristic C=C stretch-
ing frequency at 1597 cm-1 and C–N stretching frequency at
1291 cm-1,1d,
while characteristic carbonyl and primary
12
amine stretching frequency of the precursors was absent
(Figure S12). Similarly, 13C CP-MAS solid-state NMR
spectroscopy of TpTGX showed a very sharp peak at 100.1
ppm which signified the exocyclic C=C carbon12 adjacent to
the C=O carbon (Figure 1e). Signals representing de-shielded
C=C carbon attached to the N was obtained at 150 ppm. Car-
bon signal of guanidinium C=N appeared at ~162 ppm and a
small keto (–C=O) carbon signal was noted in between 180-
182 ppm.1d, 12 The sharp exocyclic double bond signal is the
strong evidence of irreversible enol to keto tautomerism in
iCONs.12 In addition, TGA profiles of the iCONs
demonstrated their thermal stability up to ~225 °C (Figure
S14). Permanent porosity of these iCONs was verified using
N2 adsorption isotherms of the activated samples at 77K,
where all three iCONs followed Type-II reversible adsorption
isotherm. The Brunauer–Emmett–Teller (BET) surface area of
TpTGCl, TpTGBr, and TpTGI were found to be 267, 305 and
298 m2g-1 respectively (Figure 2a). This lower surface area of
iCONs could be a result of poor layer stacking, small pore
diameter, and pore blocking by the counter anions. Pore size
distributions were calculated on the basis of nonlocal density
function theory (NLDFT). Due to the lack of proper
channelled pore structure and pore blocking by subsequent
counter anions, no sharp pore size distributions were obtained
(Figure 2a inset, S15). These were consistent with the previous
report where delamination of 2D-CPP layers into CONs
resulted in destruction of finite porosity.1d
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Self-exfoliation of all three iCONs resulted in micrometer to
sub-micrometer sized sheet directly as observed in SEM (Fig-
ure S19); meanwhile, EDX analysis signified C, N, O and
corresponding halogens (Cl–, Br– and I–) as major chemical
components in TpTGCl, TpTGBr, and TpTGI (Figure S22).
TEM images of TpTGCl, TpTGBr, and TpTGI also showed thin
transparent sheets and they were marginally rippled (Figure
3a, 3b, 3c, S20). Average d spacing value (~3.7 Å) obtained
from TEM analysis was consistent with corresponding PXRD
analysis (Figure S23). TEM analysis was well supported by
corresponding AFM measurements carried out in the tapping
mode. iCONs sheets, with high aspect ratio, showed that their
range of height profile was around 2-5 nm (Figure 3d, 3e, 3f;
S21). This indicates that the iCONs could be exfoliated into 3-
6 layers.1e, 1h, 7a In order to validate self-exfoliation of iCONs,
we further used MD simulation studies (Section S8, ESI). In-
terestingly, in aqueous medium TpTGCl layers were found to
be exfoliated without any external stimuli, as the average in-
terlayer π-π stacking distance was increased up to ~5.5-6 Å
compared to initial 3.338 Å stacking (Figure 3g, 3h). Hydra-
tion shell surrounding the iCONs and sandwiched halide ions
(Cl- for TpTGCl) further contributed to self-exfoliation mecha-
nism as evident from the simulation studies (Figure S30).
Although we have attempted to synthesize CPPs by using neu-
tral ligand (1, 2, 3-triaminoguanidine), but
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Figure 2. (a) Nitrogen adsorption isotherm and (b) water vapour
uptake of TpTGCl, TpTGBr and TpTGI respectively. Figure 2a
inset shows pore size distribution of TpTGCl.
structure (Section S2, S3, ESI).9 These C3 symmetric amines
resulted into three 2D iCONs (TpTGCl, TpTGBr, and TpTGI)
when reacted with C3 symmetric Tp (Figure 1a). PXRD
patterns of these three iCONs revealed low crystallinity with
the first broad peak at 2 = ~9.7° which corresponds to100
plane (Figure 1d). The major broad peak at 2 = ~27.3°
signified poor π-π stacking between the vertically stacked
layers (Figure 1d). In order to get precise structural insights,
monomeric unit of TpTGCl was synthesized and crystallized
(Figure 1a).10 The single crystal structure of the monomer
showed the presence of Cl– anion sandwiched between two
monomer units (~3.3 Å), which remain hydrogen bonded [N –
H•••Cl–; D= 3.29 Å, d=2.57 Å, =141.8°] with the
guanidinium nitrogen and hydroxyl group of salicylaldehyde
(Figure 1b).11 Presence of this loosely bound chloride ions and
intrinsic positive charge of guanidinium units (Figure 1c) dis-
turb the π-π stacking interactions among the layers in the ex-
tended iCONs structure, which we believe is the reason for
their low crystallinity. Based on the monomer structure, all
three iCONs (TpTGCl, TpTGBr, and TpTGI) were modeled in
possible 2D eclipsed and staggered structure (Figure 1f, Sec-
tion S4 ESI). The PXRD pattern of fully eclipsed AA structure
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Figure 3. (a), (b), (c) TEM images of TpTGCl, TpTGBr and TpTGI respectively, (inset shows zoomed in images, scale bar 20 nm); (d), (e),
(f) AFM images and height profile of TpTGCl, TpTGBr and TpTGI respectively (AFM scale bars represent 200 nm); MD simulation snap-
shots of TpTGCl in aqueous medium demonstrating self-exfoliation: (g) snapshot at initial state (0 ns), (h) snapshot at 500 ns.
exfoliated sheet like CONs was not obtained for the same as
evident from TEM imaging (Figure S25). Alternatively, we
mechanically grinded as-synthesized TpTGCl and analyzed the
PXRD pattern to support the self-exfoliation. No significant
alteration in PXRD patterns of as-synthesized and mechanical-
ly grinded one was noted, which could be a fare evidence of
S. aureus was found to be spherical in shape with smooth,
continuous and unbroken cell membranes; while TpTGCl treat-
ed bacterial sample showed rupture in morphology as evident
in SEM and TEM imaging (Figure 4a, 4b). The same observa-
tion was noted for E. coli where the control sample showed
uniform rod shaped morphology and the treated ones illustrat-
ed shrunken, twisted rod like morphology (Figure 4c, 4d).14
Planner guanidinium units are known to demonstrate strong
binding with the oxoanions of phosphate through two point
hydrogen bonding interactions.16 We speculate, positively
charged iCONs could interact with bacterial cells followed by
the disruption of negatively charged phospholipid bilayer
(Figure 4e).17 This would result leaching out of the cellular
content and subsequently death of the bacteria. Although elec-
trostatic interactions between positively charged guanidinium
units and phospholipid bilayers play the pivotal role for
antibacterial response of the iCONs; hydrogen bonding inter-
actions between the keto groups of iCONs and phospholipids
might have contributed to the antibacterial mechanism as
well.18 Morphological alteration and antibacterial response of
iCONs was further verified by live/dead cell imaging study
(Figure S36) and cell cycle analysis (Figure S37) of control
and treated bacterial cells.
self-exfoliation phenomenon of iCONs (Figure S18)1c, 1d
.
These iCONs displayed stable aqueous dispersion for at
least 20 days owing to their ionic backbone (Figure S16)13. A
characteristic Tyndall effect was noted signifying stable col-
loidal suspension during this time (Figure S16).1d Hydrophilic
environment within the iCONs back bone was reflected in
corresponding water adsorption isotherms as 320, 325 and 350
ccg-1 water vapour uptake was noted at 0.9 P/Po (STP), at 298
K for TpTGCl, TpTGBr, and TpTGI respectively (Figure 2b).
Chemical stability of these iCONs was ensured in different
commonly used solvents such as methanol, acetone, THF and
water for 7 days (Figure S32, S33). Moreover, these iCONs
showed the stability in 3N HCl for 7 days, which has been
verified by PXRD and FTIR analysis owing to their more sta-
ble tautomeric form (Figure S31).12
For antibacterial studies, TpTGCl was selected as the model
iCONs. In all TpTGCl treated sets, colony forming units (CFU)
were drastically reduced in a concentration dependant manner
for both gram-positive (Staphylococcus aureus) and gram-
negative (Escherichia coli) bacteria (Figure S34). 14, 15 There-
fore, TpTGCl could be used as antibacterial agent against a
broad spectrum of microbial system. Antibacterial activities of
TpTGBr and TpTGI were also promising (Figure S35; Table
T7, ESI). Hence, it could be concluded that the guanidinium
core of the iCONs is responsible for such antimicrobial activi-
ty. Meanwhile, morphological studies of control and treated
bacterial samples were evaluated by SEM and TEM. Control
Inspired by antibacterial property and good dispersibility of
iCONs we have attempted to device a mixed matrix antimi-
crobial membrane with PSF and TpTGCl on a porous support
(Section S11, ESI). The resultant membrane (TpTGCl@PSF)
was found to be robust, free standing, stable and highly flexi-
ble (Figure 4f, Figure S38). SEM image illustrates the cross
section of the fabricated composite membrane (Figure 4g); the
presence of iCONs materials was clearly observed as noted by
elemental mapping (Figure S39). TpTGCl was not leached out
from
the
composite
membrane
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Figure 4. (a) SEM images and (b) TEM images of control and TpTGCl treated S. aureus, (c) SEM images and (d) TEM images of control
and TpTGCl treated E. coli, (e) Schematic representation for its mode of action between bacteria and iCONs, (f) Digital image of
TpTGCl@PSF mixed matrix membrane, (g) SEM image of the TpTGCl@PSF mixed matrix membrane, Antibacterial property of
TpTGCl@PSF mixed matrix membrane by growth of (h) S. aureus and (i) E. coli on to it.
materials even after a prolonged period of water treatment
conveying good stability as noted by FTIR (Figure S40). It is
noteworthy to mention that soluble nature of guanidinium-
based compounds in aqueous/organic medium limit their pro-
cessability for heterogeneous membrane fabrication. Although
AgCl-PAF-50 composites have been used for antimicrobial
coatings, aqueous instability of such boronic acid based mate-
rials could not be ruled out.15 In contrast; insolubility, aqueous
stability and inherent antimicrobial property of iCONs without
further doping paved the way for heterogeneous membrane
formulation overcoming the drawbacks. Even iCONs are un-
likely to engender oxidative stress since electrostatic interac-
tion of guanidinium unit is responsible for antibacterial re-
sponse. Notably, the membrane was hydrophilic in nature with
a water contact angle of ~53˚ at the water-air interface (Figure
S38a inset) and allows the permeation of water through it (27
lm-2h-1). Prior to the antimicrobial evaluation of the membrane,
activity of PSF was also checked which showed no significant
antimicrobial activity (Figure S42). Antibacterial activity of
this TpTGCl@PSF membrane was evaluated by growth of the
bacteria on to it; in contrast, we took a normal filter paper as a
control.19 Interestingly in filter paper, both the S. aureus and E.
coli retained morphological integrity (Figure S43) which justi-
fied no significant antibacterial property. While in
TpTGCl@PSF membrane both showed shrunk and rupture of
the morphology (Figure 4h, 4i) demonstrating antibacterial
property.
In summary, we have rationally designed guanidinium-based
self-exfoliated antimicrobial porous iCONs. Judiciously de-
signed iCONs were comprised of few thin layers, thanks to the
intrinsic charge within the framework back bone and sand-
wiched anions in between the layers. Since synthesis of thin
layered intrinsically ionic 2D materials are challenging, self-
exfoliation by incorporation of ionic guanidinium linker could
be an alternative to obtain thin layered 2D material without
further post modifications. As synthesized iCONs demonstrat-
ed chemical stability and good antibacterial activity against
both gram positive (S. aureus) and gram-negative bacteria (E.
coli). We speculate electrostatic interactions between positive-
ly charged iCONs and negatively charged phospholipid bi-
layer of bacterial membrane resulted in such antibacterial re-
sponse. Taking into account these observations we fabricated
antibacterial TpTGCl@PSF membrane which could have po-
tential medical benefits. Considering the water permeance and
antibacterial property of this membrane, water purification
process is currently underway in our laboratory.
ASSOCIATED CONTENT
Supporting Information Synthesis and crystallographic details
of ligand, molecular dynamics simulation study, biological exper-
iments, membrane fabrication, characterization details are provid-
ed in Supporting Information file.
AUTHOR INFORMATION
Corresponding Author
*r.banerjee@ncl.res.in.
CONCLUSION
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2015, 137, 7817-7827. (j) Xu, H.; Giao, J.; Jiang, D. Nat. Chem.
2015, 7, 905-912.
Author Contributions
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‡S.M, S.K, B.P.B have contributed equally.
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J. H.; Fleischman, M. S.; Rodriguez-Santiago, V.; Piehlar, L. T.; Wil-
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The authors declare no competing financial interests.
ACKNOWLEDGMENT
RB acknowledges CSIR [CSC0122 and CSC0102], DST
(SB/S1/IC-32/2013), DST Indo-Singapore Project (INT/SIN/P-
05) and DST Nano-mission Project (SR/NM/NS-1179/2012G) for
funding. SM acknowledges CSIR Nehru Science PDF for finan-
cial help. SK acknowledges CSIR, BPB and AKM acknowledge
UGC for fellowship. We acknowledge Center for Advance
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Imaging, IIT Kanpur for AFM facility.
We sincerely
acknowledge Dr. T. G. Ajithkumar and Mr. Sanoop B. Nair for
providing solid-state NMR facility, Dr. Guruswamy Kumaraswa-
my for PXRD facility and Mr. Bikash Garai for SCXRD data
acquisition.
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