Glucose-cored poly(aryl ether) dendron based low molecular weight gels: PH controlled morphology and hybrid hydrogel formation

Article abstract of DOI:10.1039/c3cc43391a

We designed poly(aryl ether) dendron based transparent hydrogels containing glucose moiety, which undergoes in situ transition from nanofibers to spherical aggregates, upon pH variation. The process is reversible and the assembled structures have been characterized by DLS and SEM. More importantly, efficient dispersion of graphene oxide results in lower CGC (0.08 wt%) value and higher mechanical strength, compared to the native gel.

Full text of DOI:10.1039/c3cc43391a

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Glucose-cored poly(aryl ether) dendron based low
molecular weight gels: pH controlled morphology and
hybrid hydrogel formation†
Cite this: Chem. Commun., 2013,
49, 6758
Received 7th May 2013,
Accepted 4th June 2013
P. Rajamalli, Partha Sarathi Sheet and Edamana Prasad*
DOI: 10.1039/c3cc43391a
www.rsc.org/chemcomm
We designed poly(aryl ether) dendron based transparent hydro-
gels containing glucose moiety, which undergoes in situ transition
from nanofibers to spherical aggregates, upon pH variation. The
process is reversible and the assembled structures have been
characterized by DLS and SEM. More importantly, efficient disper-
sion of graphene oxide results in lower CGC (0.08 wt%) value and
higher mechanical strength, compared to the native gel.
Organogels and hydrogels are thermoreversible, viscoelastic
materials consisting of low molecular weight gelators which self-
assemble into three-dimensional networks.1 They have wide appli-
cations in biomedicine, cell cultures, drug delivery, enzyme immo-
bilizations, biosensors, regenerative tissue engineering, and hybrid
Fig. 1 Structure of glucose attached poly(aryl ether) dendrons.
functional materials.2 Among the various types of gel systems, Recently, our group has explored the light emitting properties
hydrogels have been widely recognized due to their increased of poly(aryl ether) dendron based gels containing polycyclic aromatic
compatibility, which makes them suitable for a wide range of hydrocarbons.8 In the present study, we demonstrate the design and
applications.3 While the design and synthesis of hydrogelators synthesis of biocompatible low molecular weight hydrogels based on
have been extensively explored, most reported work has been glucose functionalized poly(aryl ether) dendron derivatives (Fig. 1).
confined to polymer based gel systems.4 Low-molecular-weight- The synthesis is relatively easy and highly cost effective. Although
gelators (LMWGs) are emerging as the preferred choice over their there are a few reports present in the literature describing glucose
polymeric counterparts, because of their rapid response to external based amphiphiles forming hydrogels and organogels,9 dendron
stimuli such as pH, light, pressure and temperature.5 LMWGs also based hydrogels with a glucose moiety have not been reported, to
exhibit ready clearance from the body due to the highly feasible the best of our knowledge. The present supramolecular design
nature of the gel–sol reversibility in such systems. These attractive brings two important classes of compounds together which is
features make supramolecular hydrogels based on LMWGs promis- expected to deliver synergistic functional properties.
ing alternatives to polymeric hydrogels.
Owing to their commercial availability and eco-friendly
Dendrimer/dendron based gels have received considerable behaviour, carbohydrates have been selected as a component for
attention during the last decade.6 Among the various types designing LMWGs. We have synthesized glucose based poly(aryl
of dendrimeric scaffolds available, poly(aryl ether) dendron ether) dendrons (compounds I and II) in 490% yield with detailed
derivatives has been extensively studied for gelation purposes synthetic procedure and characterization data given in ESI.†
due to their optimum stability and flexibility, along with Compound I was dissolved completely in DMSO–water mixture
the feasibility of peripheral modification with numerous by heating, followed by sonication for a few seconds. The system
functional groups for enhancing intermolecular interactions.7 suddenly turned to a gel upon cooling to room temperature. The
gel formation was confirmed by inverted vial method. The lowest
critical gel concentration (CGC) value obtained for 1 in DMSO–
water systems (1 : 9) was 0.1% (w/v). It is worth noting that the
gelation processes are thermo-reversible in the present system. We
Department of Chemistry, Indian Institute of Technology Madras (IITM), Chennai,
600036, India. E-mail: pre@iitm.ac.in; Fax: +91-44-2257-4202
† Electronic supplementary information (ESI) available: Synthesis and character-
expected that compound II would be the most efficient gelator
in solvent mixtures due to a higher number of benzyl units
ization of dendron, CGC table, SEM images, FT-IR, DLS and powder-XRD
experiments. See DOI: 10.1039/c3cc43391a
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6758 Chem. Commun., 2013, 49, 6758--6760
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similar to our previous reports.8 However, compound II did not
show gelation in the above mixture of solvents. Conversely,
compound II shows translucent organogel formation only in
alcoholic solvents. This is presumably because the required
optimum balance between the hydrophilic and hydrophobic
interactions is achieved in alcoholic solvents, compared to
DMSO–water. The gels are stable at room temperature and
no noticeable change was observed when stored in a closed
container for more than 6 months.
Fig. 3 Photograph of gel from compound I (a) before addition of KOH, (b) after
addition KOH and corresponding TEM (left) and SEM (right) images. The scale bar
is 700 nm.
The FT-IR spectrum of compound I shows a single amide
band at 1646 cmꢀ1 and an intense hydrogen-bonded –OH and
–NH stretching broad vibration peaks at 3435 cmꢀ1 (Fig. S1,
ESI†). These data are an unambiguous signature of a network of the gel systems also changes to a spherical morphology as a
hydrogen-bonded hydroxyl group. function of pH (Fig. 3). The spherical aggregate formation at
The morphological properties of the glucose dendron organogels basic pH was confirmed by dynamic light scattering (DLS) and
were obtained through scanning electron microscopy (SEM) and SEM experiments (Fig. S5 and S6, ESI†). SEM and DLS analysis
transmission electron microscopy (TEM). Fig. 2 depicts selected together suggest that the spherical aggregates have diameters
TEM images of the xerogel of compound I in a DMSO–water in the nanometer ranges. The results indicate that the molecules
mixture. These xerogel consists of 3D entangled fiber aggregates self-assemble to spherical aggregates in the presence of base.
with diameter range 100–150 nm and micrometer scale length. Most This could be due to the increased aqueous environment, where
likely, the fibers observed in the electron micrographs consist of the aromatic rings tend to club together due to the increased
bundles of gelator aggregates and similar structures have also been hydrophobic interactions.
observed for compound II gel systems (Fig. S2, ESI†).
Next, this gelator was utilized to prepare a graphene oxide
To gain additional insight into the structures that comprise (GO)-incorporated hybrid hydrogel. Graphene and graphene
the gel formed from the compounds, small-angle X-ray diffraction oxide have been emerged as a fascinating research area during
(XRD) was employed. XRD of the xerogel gave a reflection ratio the last two decades, due to their attractive physical, chemical and
1 : 2 : 3 : 4, suggesting that molecules are arranged in a lamellar material properties.10 Studies of graphene oxide-based nanocompo-
pattern (Fig. S3, ESI†). The energy minimized structure of compound site materials have also attracted significant attention because of
I indicate end-to-end length of 20 Å, which is different from the their excellent mechanical properties and potential applications.11
observed d-spacing value (31.43 Å) from X-ray diffraction study. Since There are a few examples of hybrid hydrogels based on graphene
the d spacing value is in-between the mono and double layer oxide with DNA, amino acid, glucono-d-lactone, hemoglobin and
thickness formed by the molecular length, the molecules are likely poly(vinyl alcohol).12 However, little is known regarding composite
to be arranged in an interdigitated fashion with respect to the sugar hydrogels based on graphene oxide and dendrons, even though a
moiety. A schematic representation of molecular arrangement in the variety of dendron based stable gel systems have been reported. This
gel phase is given in ESI† (Fig. S4).
report represents the first example of a dendron based hydrogel with
It is worth mentioning that while the gels are highly stable in graphene oxide dispersed in the medium. The gelator was dissolved
neutral phase for long time, a change in pH from neutral in DMSO and graphene oxide/water (graphene dispersed in water)
condition to basic condition (pH = 10, by addition of KOH) leads was added and heated. The well-dispersed solution was then cooled
to the conversion of the gel to a solution (Fig. 3). Thus, pH change down to room temperature and followed by sonication for few
operates as a distinguishable input to produce a macroscopic seconds, to produce graphene oxide containing highly stable hybrid
output: gel–sol transition of the supramolecular hydrogel.
gel in DMSO–water (Fig. S7, ESI†). To investigate the morphology of
This could be due to the disruption of the H-bonding network the graphene oxide incorporated hybrid gel, TEM and SEM experi-
in the gel system since the presence of KOH might lead to a ments were performed for compound I hybrid gel in DMSO–water
deprotonation of the alcoholic protons of the glucose moiety. (1 : 10). Results are shown in Fig. 4 and Fig. S5 (ESI†), which reveal
More interestingly, we found that the fibrillar morphology of the presence of both graphene oxide nanosheets and cross-linked
gel nanofibers within the hybrid gel system. As evident from the
image, an entangled nanofibrillar structure is maintained within the
hybrid gel. Thin graphene oxide nanosheets are observable, and
nanofibrils are found to be paved on the surface.
This indicates that the graphene oxide sheets are interacting
well with the glucose-cored dendron gel nanofibers. Dendrons
form gels with lower CGC in the presence of GO than in the
absence of GO. This suggests that the presence of the glucose
moiety in the gelator can interact strongly, presumably with the
carboxylic acid units of graphene oxide through H-bonding
interactions. Interestingly, in this procedure, sonication plays
Fig. 2 TEM image of compound I in DMSO–water (1 : 10; v/v).
an important role to disperse the GO as well as the gel formation.
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In conclusion, we have designed and synthesized a new
family of linear sugar based poly(aryl ether) dendrons that self-
assembled into gels at concentrations as low as 0.1% (w/v). The
gel fibers obtained from compounds I and II self-assembled
and stabilized most likely through the extensive H-bonding and
p–p interactions that are available in the gelator. The morphol-
ogies and microstructures of the gel systems strongly depend
upon the solvent milieu. We have presented a general method
to increase the mechanical strength of the glucose containing
poly(aryl ether) based gel by incorporating GO. The organogels
reported herein may find potential applications in pharmaceutical
science, in particular for designing drug delivery systems where
glucose based gels are routinely used.
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We thank CSIR (02-0094-12-EMR-II), Govt. of India for
financial support.
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This journal is The Royal Society of Chemistry 2013