Galactose-Grafted 2D Nanosheets from the Self-Assembly of Amphiphilic Janus Dendrimers for the Capture and Agglutination of Escherichia coli

Article abstract of DOI:10.1002/chem.201905228

High aspect ratio, sugar-decorated 2D nanosheets are ideal candidates for the capture and agglutination of bacteria. Herein, the design and synthesis of two carbohydrate-based Janus amphiphiles that spontaneously self-assemble into high aspect ratio 2D sh

Full text of DOI:10.1002/chem.201905228

A Journal of
Accepted Article
Title: Galactose-Grafted 2D Nanosheets from the Self-assembly of
Amphiphilic Janus Dendrimers for the Capture and Agglutination
of Escherichia coli
Authors: Nithiyanandan Krishnan, Devanathan Perumal, Siriki
Atchimnaidu, Kaloor S. Harikrishnan, Murali Golla, Nilima
Manoj Kumar, Jemshiya Kalathil, Jithu Krishna, Dileep K.
Vijayan, and Reji Varghese
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using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201905228
Link to VoR: http://dx.doi.org/10.1002/chem.201905228
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10.1002/chem.201905228
Chemistry - A European Journal
COMMUNICATION
Galactose-Grafted 2D Nanosheets from the Self-assembly of
Amphiphilic Janus Dendrimers for the Capture and Agglutination
of Escherichia coli
Nithiyanandan Krishnan, Devanathan Perumal, Siriki Atchimnaidu, Kaloor S. Harikrishnan, Murali
Golla, Nilima Manoj Kumar, Jemshiya Kalathil, Jithu Krishna, Dileep K. Vijayan, and Reji Varghese*
Abstract: High aspect ratio, sugar-decorated 2D nanosheets are the
ideal candidate for the capture and agglutination of bacteria. Herein
the design and synthesis of two carbohydrate-based Janus
amphiphiles that spontaneously self-assemble into high aspect ratio
2D sheets are reported. The unique structural features of the sheet
include the extremely high aspect ratio and dense display of galactose
on the surface. These structural characteristics allow the sheet to act
2
as a supramolecular 2D platform for the capturing and agglutination
of E. coli through specific multivalent non-covalent interactions, which
1
significantly reduces the mobility of the bacteria and leads to the
3
inhibition of their proliferation. Our results suggest that the design
strategy demonstrated here can be applied as a general approach for
the crafting of biomolecule-decorated 2D nanosheet, which can
perform as 2D platform for their interaction with specific targets.
self-assembly
E. coli
Considerable attention has been paid by different research
groups to understand and mimic the bacterial adhesion processes
as this is the major pathway for many pathogenic bacteria to infect
capturing of E. coli
the host cell through multivalent host-guest interactions.[1] Recent
years have witnessed the emergence of several carbohydrates
coated supramolecular nanostructures for bacterial capture. Zero-
dimensional (0D) nanostructures such as dendrimers,[2]
nanoparticles[3] and fullerenes[4] have applied as scaffolds for the
carbohydrate display and used for the interaction with bacteria.
galactose-decorated sheet
E. coli bound nanosheet
As expected, 1D nanostructures (e.g. carbon nanotubes[5] and
fibrous assemblies[6]) were found to be more effective as scaffolds
compared to 0D nanostructures. Lee et al. reported the efficient
capturing of Escherichia coli (E. coli) by carbohydrate coated
nanofiber and demonstrated the regulation of bacterial
agglutination by tuning the fiber length.[7] Undoubtedly, more
efficient would be 2D nanosheets because of their extremely large
surface area. Nevertheless, bacterial agglutination and inhibition
of cell proliferation using 2D sheet as a scaffold received very little
attention. Recently, Seeberger and Haag et al. demonstrated a
multi-step strategy for the design of mannose functionalized
graphene oxide as a 2D platform for the selective wrapping and
agglutination of E. coli.[8] Even more desirable would be a simple
design that allows the creation of carbohydrate decorated 2D
sheet in a single step self-assembly process. Zuckermann et al.
reported an interesting case of self-assembly of carbohydrate
functionalized peptoid into sheets having site-specific display of
carbohydrates, which exhibited selective
Scheme 1. Chemical structures of amphiphiles 1-3 and schematic
representation depicting the self-assembly of the amphiphile into sugar
decorated, multi-layered 2D nanosheet. Capture and agglutination of E. coli
onto the sheet through specific multivalent interaction are also shown. The
benzene rings of TPE need not be fully planar in the aggregated state as shown.
binding to target proteins.[9] However, the design of
a
supramolecular building block that spontaneously self-assembles
into carbohydrate- coated, high aspect ratio, 2D nanosheet that
can act as a multivalent ligand for the capturing, agglutination and
inhibition of bacterial proliferation has not yet been achieved.
Amphiphilicity-driven self-assembly is a simple yet efficient
bottom-up approach for the creation of soft nanostructures of
defined morphology with tailored functionalities.[10] Our group is
interested in the self-assembly of amphiphiles, particularly DNA-
based amphiphiles.[11] We have shown that the incorporation of a
large p-surface such as tetraphenylethylene (TPE)[12] or
hexabenzocoronene (HBC)[13] as hydrophobic domains in the
design of a DNA amphiphile drove the self-assembly into DNA-
decorated 2D nanosheets. Motivated from these findings, we
envisioned that the incorporation of a carbohydrate moiety as the
hydrophilic segment to a large p-surface could drive the self-
assembly of the amphiphile into carbohydrate-decorated 2D
sheet, and hence they could perform as a potential multivalent
ligand for the interaction with specific biomolecules.
[a]
N. Krishnan, D. Perumal, S. Atchimnaidu, K. S. Harikrishnan, M.
Golla, N. M. Kumar, J. Kalathil, J. Krishna, D. K. Vijayan and Dr. R.
Varghese
School of Chemistry, Indian Institute of Science Education and
Research (IISER) Thiruvananthapuram
Thiruvananthapuram-695551, India. E-mail: reji@iisertvm.ac.in
Supporting information for this article is given via a link at the end of
the document. ((Please delete this text if not appropriate))
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16
8
Herein we report the design and synthesis of three (1-3)
Janus amphiphiles that spontaneously self-assemble into high
aspect ratio 2D sheets. The amphiphiles 1 and 2 consist of
galactose dendron as the hydrophilic segment, which is
conjugated to a hydrophobic tetraphenylethylene (TPE) dendron.
Whereas the amphiphile 3 has tetraethylene glycol chains as the
hydrophilic segment instead of galactose. TPE was selected in
our study as the hydrophobe due to their unique optical properties
and straightforward synthesis.[14] Owing to the large p-surface of
TPE dendron, the amphiphiles undergo strong p-p stacking in
aqueous medium and spontaneously self-assemble into 2D sheet.
The unique structural features of the sheets of 1 and 2 include the
extremely high aspect ratio and dense display of galactose on
their surface. These structural characteristics allow the sheet to
act as a 2D platform for the efficient and selective capturing of E.
coli and their agglutination through multivalent interaction
between galactose and glycoproteins, protein expressed on the
bacterial cell surface. This interaction significantly reduces the
mobility of the bacteria and eventually leads to the inhibition of
their proliferation. On the other hand, sheet derived from the self-
assembly of 3 that has tetraethylene glycol on the surface shows
no interaction with the bacteria, indicating that the specific
interaction between the galactose and glycoproteins is solely
responsible for the bacteria capture.
a)
b)
DMSO
water:DMSO (98:2)
0
1.0
0.5
0.8
1.6
µm
I
em
0.0
400
500
l / nm
600
500 nm
c)
d)
1 μm
e)
e)
f)
The only structural difference between the amphiphiles 1 and
2 is the difference in the hydrophobic dendron part. The
amphiphile 1 has two TPE moieties, whereas 2 has four TPE
moieties as the hydrophobic segments, and hence the
hydrophobic surface area is larger for 2 compared to 1. Self-
assembly of the amphiphile was achieved by the gradual addition
of water into a DMSO solution of the amphiphile (water:DMSO =
98:2). All experiments were done at a concentration of 10 µM.
Absorption spectra of 1 and 2 in DMSO showed a broad band with
lmax at 307 nm, which corresponds to the p-p* transition of TPE
(Figure S1). Interestingly, red-shifts of 13 nm and 10 nm were
observed with the addition of water into the DMSO solution of 1
and 2, respectively. These observations suggest that 1 and 2 exist
as monomeric species in DMSO and undergo aggregation with
the addition of water mainly through p-p stacking interactions.
5 μm
Figure 1. a) Normalized fluorescence spectra of 1 in DMSO (black) and
water:DMSO (red). b) AFM, c) TEM, d) SEM, e) confocal microscopic and f) HR-
TEM images of sheet of 1. The inset of AFM image shows the corresponding
section analysis of the sheet. A possible bilayer organization of the amphiphile
is shown on the HR-TEM image.
sheets through H-bonding interactions of the galactose moieties
protruded on the sheet surface. Even at a low concentration of 1
µM, multilayered sheets were dominated. However, we could
observe population of sheets with thickness of ~3 and ~4 nm for
1 and 2, respectively (Figure S5). Since the observed heights are
approximately matching with the corresponding bilayer distances
of 1 and 2, these results support a lamellar assembly for the
amphiphiles. Fourier-transform infrared spectroscopy analyses of
the sheets showed significant red-shifts and broadening of O−H
stretching vibration of galactose when compared to the
corresponding monomeric species. Red-shifts of 272 cm-1
(3693®3421 cm-1) and 281 cm-1 (3695®3414 cm-1) were
respectively observed for 1 and 2 sheets (Figure S6 and S7),
which can be attributed to the H-bonding interaction between
galactose. This confirms the layer-by-layer assembly of the
sheets. Scanning electron (SEM) and transmission electron
microscopic (TEM) analyses also confirmed the formation of high
aspect ratio sheets for 1 (Figure 1c and d) and 2 (Figure S9 and
S11). Interestingly, confocal microscopic analyses revealed that
the lateral width of the sheets are extremely large and are in the
range of 10-100 µm (Figure 1e).
Moreover,
1 and 2 are weakly emissive in DMSO with
fluorescence quantum yields of 0.05 and 0.04, respectively.
Interestingly, a dramatic increase in emission intensity with a
concomitant red-shift of 100 nm was observed for the aggregated
species of 1 and 2 (376®476 nm) (Figure 1a and Figure S2). The
fluorescence quantum yields for the aggregated species of 1 and
2 are 0.4 and 0.5, respectively. These results imply that the weak
emission for the monomeric species of 1 and 2 is due to the
intramolecular C(sp2)-C(sp2) bond rotational relaxation of TPE,
whereas restriction of rotational relaxation in the aggregated state
enhances the emission.[14]
Atomic force microscopic (AFM) analyses revealed that the
aggregates formed in solution are high aspect ratio 2D sheets
(Figure 1b). Section analyses showed that the lateral width of the
sheets of 1 and 2 are in the range of several hundred nanometers
to micrometers (Figure 1b inset). Heights of the sheets are in the
range of 12-150 nm and 16-270 nm for 1 and 2, respectively,
which is significantly larger than the calculated bilayer
thicknesses of 1 (~3.2 nm) and 2 (~4.6 nm). Large thickness of
the sheet points to a possible layer-by-layer assembly of the
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a)
Tollens’ reaction
b)
a)
b)
1.2
0.6
E.Coli alone
E.Coli + sheet 1
E.Coli + sheet 2
E.Coli + sheet 3
c)
500 nm
0
7
14
Time / h
21
28
50 nm
1.0 µm
d)
c)
e)
1
2
3
4
d)
10 µm
2.0 µm
agglutinated E. coli + 2
Figure 2. a) TEM image of the in-situ formed AgNPs on the sheet of 1 using the
Tollens’ reaction and a schematic for this reaction is also shown. b) TEM and c)
confocal microscopic images of E. coli captured onto the sheet of 1. d) TEM
image of the interaction of E. coli with sheet of 3.
Figure 3. a) Changes of optical density at 600 nm for E. coli alone and E. coli
treated with 1-3 sheets. Images of E. coli bacterial colonies: b) E. coli alone
(control), c) E. coli treated with sheet of 1 and d) E. coli treated with sheet of 2.
e) Images of E. coli contaminated water treated with 2 sheets. The vials 1-4
represent E. coli, sheet of 2, E. coli treated with sheets of 2 (0.2 mM) and E. coli
treated with sheets of 2 (0.3 mM), respectively.
Remarkably, high resolution TEM image of sheet of 1 clearly
resolved the lamellar and layer-by-layer assembly of the
amphiphiles (Figure 1f) and the observed bilayer distance of ~3
nm is approximately matching with the calculated distance (~3.2
nm). However, we were unable to molecularly resolve the
structure of sheet of 2. Furthermore, broad diffraction peaks at 2θ
= 17−24° (d = 5.2-3.7 Å) and 16-23° (d = 5.5-3.8 Å) were
observed for the sheets of 1 (Figure S14a) and 2 (Figure S14b) in
the powder X-ray diffraction analyses, respectively. These peaks
could be assigned to different p-p stacking distance of TPE
segments in the multilayered sheet. Based on all these analyses,
a plausible model for the self-assembly of the amphiphiles is
depicted in Scheme 1.
The surface display of galactose on the sheet was
unambiguously confirmed using the well-known Tollens’ reaction,
which involves the reduction of Ag+ to silver nanoparticles
(AgNPs) using galactose as the reducing agent. To this end,
Tollens’ reagent ([Ag(NH3)2]+) (100 µM) was added to the
galactose decorated sheets of 1 or 2 (10 µM), vortexed for 2 h in
dark and filtered. The reaction was monitored using electronic
spectroscopy and it was observed that within an hour, a new band
started appearing at 330-670 nm, corresponding to the surface
plasmonic absorption band of AgNPs (Figure S15). A gradual
increase in intensity of this band was observed with time and got
saturated at 2 h. These results imply that the galactose moieties
protruded on the sheet surface act as a reducing agent for the
reduction of Ag+ to AgNPs.[15] In support of this, a significant
quenching of fluorescence of TPE was observed due to the
electronic interaction between AgNPs on the sheet and TPE. This
is possible as the distance separating the NPs and TPE is only ~2
nm (for 1). Nanoparticle formation was further confirmed by TEM
analyses, which showed the formation of NPs (~5 nm) that are
randomly deposited on the surface of the sheet (Figure 2a).
Motivated from the large surface area of the sheet and dense
display of galactose on the surface, we then explored the potential
of the sheet as a multivalent 2D scaffold for the capturing and
agglutination of bacteria. Cytotoxicity of the sheets was studied
using
3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium
bromide (MTT) assay. For this purpose, sheets (c = 10 µM) were
treated with A549 (cancer) and HEK 293 (normal) cell lines for 24
h and the subsequent MTT assay revealed that no toxicity was
associated with the sheets (Figure S18). After confirming the
biocompatibility of the sheets, their potential as a multivalent
ligand for the capturing of gram-negative E. coli (DH5a strain) was
studied. Nanosheets of 1 was treated with E. coli cells for 24 h.
TEM analyses revealed the clustering of bacteria on the sheet
(Figure 2b). It is also worth noting that the sheet morphology was
found to be intact even after binding of the bacteria, proving the
structural robustness of the sheet. Interestingly, confocal laser
scanning microscopic (CLSM) analyses clearly showed the
agglutination of bacteria, where bacteria appear as dark features
on blue emissive sheet (Figure 2c). Agglutination was further
confirmed with SEM (Figure S21) and AFM (Figure S22) analyses.
Interestingly, nanosheet derived from the self-assembly of
amphiphile 3 (Figure S25), which has tetraethylene glycol as the
hydrophilic segment instead of galactose, showed no interaction
with the bacteria as evident from TEM analyses (Figure 2d). This
shows that specific multivalent interaction of E. coli with galactose
is responsible for the capturing of bacteria onto the sheet.
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The ability of the sheet to capture and inhibit the growth of the
bacteria was studied by monitoring the changes of optical density
at 600 nm using a microplate reader. For this, E. coli was cultured
alone (control) and along with 1/2/3 nanosheets for 24 h. As
expected, normal bacterial growth was observed for E. coli alone
and E. coli treated with sheets of 3. On the other hand, a
significant reduction in the growth of the bacteria was observed
for E. coli treated nanosheets of 1 and 2. Agglutination Index (AI)
calculated from 20 random fields of microscopic images (TEM and
CLSM) showed high AI values of 60 and 62 for the sheets of 1
and 2, respectively, which are higher than those reported for
similar supramolecular systems.[6c] Inhibition of bacterial growth
was further supported by plate-counting technique. For this, E.
coli was cultured alone (control) and along with 1/2 nanosheets
for 24 h, spread over agar plate and the colony-forming ability was
monitored over a period of time. Colony counting showed a
normal growth for E. coli (Figure 3b), whereas significant inhibition
of bacterial growth was observed for E. coli treated with sheets of
1 and 2 (Figure 3c and 3d). These results fully support our
hypothesis that the galactose decorated sheet allow the efficient
capturing of bacteria through specific multivalent interaction
between galactose protruded on the sheet and galactose binding
protein in the bacterial pili of DH5a strain. This leads to the
agglutination of bacteria that prevent the motility and further
growth of the bacteria. We have also demonstrated the potential
of the sheet to capture and remove bacteria from water that was
artificially contaminated with E. coli. This was shown by treating
the bacteria contaminated water with sheets of 2 at two different
concentrations (0.2 and 0.3 mM) in PBS buffer (pH 7.5) and kept
undisturbed for 4 h. Agglutination followed by sedimentation of the
bacteria was clearly observed for both the concentrations of 2
(Figure 3e, vials 3 and 4), and as expected the sheets of high
concentration exhibited efficient bacterial capture (Figure 3e, vial
4). Furthermore, turbid bacteria solution became very clear after
bacterial capture, whereas no sedimentation of bacteria was
observed for E. coli alone (Figure 3e, vial 1).
In summary, we have reported the design and synthesis of a
new class of carbohydrate-based Janus amphiphiles that
spontaneously self-assemble into high aspect ratio, galactose-
decorated nanosheets. We have also shown the potential of 2D
sheet to act as a multivalent ligand for the capturing and
agglutination of bacteria through specific host-guest interaction.
To our knowledge, this is the first report demonstrating the crafting
of sugar-grafted, high aspect ratio, 2D sheet from the self-
assembly of sugar-based Janus amphiphile that efficiently
capture, agglutinate and inhibit the growth of bacteria. This study
indisputably suggests that the carbohydrate-decorated 2D sheets
having large surface area are indeed a promising class of
nanostructures for the interaction with bacteria, and our results
are expected to open up further research interest towards the
development of antibacterial nanomaterials.
Keywords: self-assembly • amphiphiles • 2D materials •
bacterial capture • fluorescence
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Acknowledgements
Financial supports from DBT and KSCSTE are gratefully
acknowledged. We thank UGC and CSIR for research fellowships.
D. K. V. thanks IISER for a post-doc fellowship.
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Entry for the Table of Contents (Please choose one layout)
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N. Krishnan, D. Perumal, S.
Atchimnaidu, K. S. Harikrishnan, M.
Golla, N. M. Kumar, J. Kalathil, J.
Krishna, D. K. Vijayan, R. Varghese*
Page No. – Page No.
Galactose-Grafted 2D Nanosheets
from the Self-assembly of
Amphiphilic Janus Dendrimers for
the Capture and Agglutination of
Escherichia coli
Amphiphilicity-driven self-assembly of galactose-based Janus amphiphiles into
galactose-decorated 2D nanosheet, which can efficiently capture and agglutinate E.
coli through specific multivalent interactions between galactose and galactose
binding protein in the bacterial pili of E. coli DH5a strain is reported.
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