Carbon nanodot-induced gelation of a histidine-based amphiphile: Application as a fluorescent ink, and modulation of gel stiffness

Article abstract of DOI:10.1039/c7cc09824c

This is a unique example of fluorescent carbon dot-induced hydrogelation of an amino acid-based amphiphile. The carbon dot-to-amphiphile ratio dictates the gel stiffness. Moreover, this hydrogel can be used as a prominent fluorescent ink and the dried gel shows a remarkable, unusual green fluorescence in the solid state.

Full text of DOI:10.1039/c7cc09824c

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Carbon nano-dots induced gelation of a histidine based amphiphile: Application
as a fluorescent Ink and modulation of Gel stiffness
Subir Paul, Kousik Gayen, Nibedita Nandi and Arindam Banerjee*,a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
This is a unique example of fluorescent carbon dot induced materials. Incorporation of different nanomaterials including
hydrogelation of an amino acid based amphiphile.Carbon dots to metal nano-cluster,15 metal nanoparticle, graphene
amphiphile ratio dictates the gel stifness. Moreover, this hydrogel oxide,16carbon nanotube,17C-dots18into the gel matrix has
can be used as a prominant fluorescent ink and the dried gel been achieved and studied extensively so far. Though there
shows a remarkable, unusual green fluoresceence in the solid are several examples of modulating gel properties by including
state.
nanomaterials into gel matrix, however, in most of the cases
the nanomaterials do not play any part in gelation process and
the gel network acts as a mere host for the nanomaterials. To
the best of our knowledge, there is no systematic studies of C-
dots induced two component hydrogelation of an amino acid
based amphiphile with tuneable mechanical strength and
fluorescence behaviour. A new type of two-component gel has
been constructed in such a way that the carboxylic acid
functionalities on the carbon dot can interact with the cationic
amphiphile to form a hydrogel. This hydrogel exhibits a strong
cyan fluorescence, while the xerogel obtained from the
hydrogel exhibits green fluorescence in solid state providing a
meaningful strategy to overcome the aggregation induced
quenching of fluorescence in C-dots. This type of solid state
fluorescence is quite unusual for C-dots containing materials
reported so far.11bThe retaining of fluorescence in the solid
state facilitates the potential use of this C-dots induced
hydrogel as a new type of fluorescent ink for various
information storage applications.
Carbon dots(C-dots)1is an emerging field in scientific research
from the time of their discovery due to the intrinsic
fluorescence, photo-stability, chemical robustness, low
toxicity, and biocompatibility.2 This makes them very useful
candidate for widespread applications in bioimaging,3 drug
delivery,4 photoelectronics,5photocatalysis6 and others.7Since
its discovery, carbon dots has become a good replacement for
semiconducting metal oxide, metal chalcogenides quantum
dots since the former does not suffer from intrinsic
cytotoxicity.8One of the major drawbacks of C-dots is that they
do not show significant fluorescence in solid state due to
aggregation induced quenching phenomenon9, except a few
rare examples.10 Solid state fluorescence is important and it is
highly demanding due to various applications of fluorescent
solid materials inlight emitting diodes (LEDs) and security
printing.11 So, it is important to discover fruitful strategies to
preserve the fluorescence of C-dots in the solid state.
Supramolecular gels12 derived from assembly of small
molecules are a fascinating class of soft materials due to their
diversified applications including drug delivery, tissue
engineering, catalysis, wound dressing, pollutant removal and
others.13 Two-component hydrogels14 consisting of two
different components provide the opportunity to regulate the
gelation property by varying one of the components or both
components. It is important to modulate the mechanical
stiffness of gels for the growth and development of smart
C-dots were synthesized via simple hydrothermal treatment of
2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Ina typical
experiment, 200 mg of DDQ was dissolved in 60 ml of Milli-Q
water and the solution was placed in a 100 ml teflon lined
autoclave followed by heating at 180⁰ C for 3 hours (Fig. S7,
ESI†). The resulꢀng brown coloured soluꢀon was then
centrifuged to separate larger particles and then it was
subjected to dialysis for further purification.
The FEG-TEM image of C-dots(Fig. 2a) shows that these C-dots
are less than 1 nm in size and show almost monodispersed
nature withan average particle size of 0.8 nm. Close inspection
reveals no crystallinity among these nanoparticles confirming
their amorphous nature and this coincides with the previously
reported carbon nanoparticles with such small sizes.19
a.Department of Biological Chemistry, Indian Association for the Cultivation of
Science, Jadavpur, Kolkata, 700032 (India) Fax: (+ 91) 33-2473-2805, E-mail:
bcab@iacs.res.in.
Electronic Supplementary Information (ESI) available: [Experimental section,
Instrumentation, Synthetic procedures, NMR, HRMS, Spectroscopic studies, FTIR,
Rheological studies.] See DOI: 10.1039/x0xx00000x
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observed that the amphiphile formed self-supportive two-
component hydrogel with trimesic acid. However, in presence
of TREN or catechol no gelation was observed (Fig. S9, ESI†)
under similar conditions and other tested conditions. This
observation is pretty instructive and points towards the
interaction between –COOH groups of C-dots and the
amphiphile for playing a crucial role in gelation. Moreover, a
strong interaction between the imidazole ring of histidine and
the carboxylic group also plays an important role.21To probe
the importance of the imidazole ring of the histidine moiety,
another amphiphile P2 (Fig. 1) has been designed and
synthesised by replacing the –His residue with a –Phe residue.
It is observed that the amphiphile P2 does not form any
hydrogel under similar and widely tested conditions. These
findings clearly suggest that the interaction between the
imidazole ring of histidine and the –COOH groups on carbon
dots is the driving force for this two component gelation.
Fig. 1 (a) Chemical structures of amphiphilesP1and P2. (b) Appearance of the hydrogel
and soluble aggregate obtained from P1 and P2respectively on treatment with C-dots
(derived fromDDQ) under UV light by irradiating at 365nm. (c) Appearance of the
xerogelfrom P2 hydrogel under UV light.
To get insight into the morphological behaviour of the
amphiphile aggregate and the amphiphile-C dots gel, field-
emission scanning electron microscopy (FE-SEM) and field-
emission transmission electron microscopy (FEG-TEM)
experiments were carried out. From the FE-SEM images (Fig.
S10, ESI†),it is clear that the two-component hydrogel is
composed of a much dense fibrillar network than that of the
aggregated moiety. FEG-TEM image of the gel shows the
presence of entangled fibres of width ranging from 16 to 25
nm. The FEG-TEM image (Fig. 2c) of the carbon dots inside the
gel matrix was found to be quite different from the FEG-TEM
images of as-prepared C-dots (Fig-2a). It is apparent from the
image that the size of the C-dots within the gel varies between
4.5 to 6 nm with an average size of 5 nm (Fig. 2d). This is in
sharp contrast to the results obtained from the as-prepared C-
dots with the average size of 0.8 nm (Fig. 2b). We envisioned
that within the self-assembled gel medium carbon dots also
undergo some aggregation and it results in bigger particle sizes
as it is evident from FEG-TEM images of C-dots in gel (Fig. 2d).
This aggregation of C-dots upon gelation may be responsible
for the red shift of the fluorescence of the carbon dots in the
gel matrix compared to the fluorescence of the as-prepared C-
The FT-IR spectra of C-dots show abundance of different
functional groups on the surface of these carbon dots (Fig. S8,
ESI†). Broad peaks at 3380 cm-1 and 3213 cm-1 are assigned to
O-H and N-H stretching vibrations. Peaks at 3033 cm-1, 2812
cm-1 and 2234 cm-1 arise due to =C-H, -C-H and C≡N stretching
vibrations. Peaks at 1715 cm-1 and 1622 cm-1 are assigned to
C=O and C=C stretching vibrations and the peaks at 1404 cm-1,
1204 cm-1 and 1057 cm-1 are assigned as O-H deformation
peak, C-OH stretching peak and C-O stretching peaks
respectively.20
At first we tested the gelation ability of the amphiphile P1 in
Milli-Q water (pH 6.6). In a typical procedure, 4 mg of P1 was
taken in a glass vial and it was dissolved in 1 ml water through
heating on a hot plate, the vial was then allowed to cool down.
As the system attained room temperature only a white
coloured dispersion was obtained and no gelation was found.
Thus the amphiphile itself does not have any gelation
efficiency in water. In another attempt we dissolved 4 mg of
P1 in 800 µl of water by heating and then 200 µl of C-dots
solution (containing 1.0 mg of C-dots) was added to the hot
amphiphile solution. As the mixture attained room
temperature, a brown coloured self-supportive hydrogel was
obtained (checked by vial inversion). Different compositions of
amino acid based amphiphile P1 and C-dots were taken to
obtain two component gels. Amphiphile to C-dots ratio was
varied from 1:1 to 8:1 to obtain a stable two component gel.
Outside this range of ratio limit, no gelation was observed. We
were curios to find out the driving force behind this gelation
phenomenon as the amphiphile could only form a gel in water
in the presence of carbon dots. As the surface of the carbon
dots is highly functionalized with different functional groups,
we envisaged that the interaction of the amphiphile with some
particular functionalities of the carbon dots surface must be
the driving force behind gelation. To investigate this
hypothesis, we tested for the probable gelation ability of the
amphiphile with common organic molecules bearing –COOH, -
OH and –NH2 functionalities. For this purpose trimesic acid,
Tris(2-aminoethyl)amine(TREN) and catechol were used. We
dots (Fig 3)
.
The visco-elastic property of these hydrogels has been
measured using frequency sweep experiments. It is observed
from the Fig. 2f that the storage modulus (G’) value is the
lowest for the two-component gel containing amhiphile to C-
dots ratio4:1 and it increases with an increase in the C-dots
proportion in the gel matrix. It is found that at an angular
frequency of 0.72 rad/s, for 4:1 (w/w) gel, G′ value is 1.9 ×
103Pa, for 2:1 (w/w) gel it is 2.9 × 103Pa and for 1:1 (w/w)gel it
is 5.0 × 103Pa. So, there is an increase of 69.23% in G’ value as
we move from 4:1 (w/w) to 2:1 (w/w) gel and an increase in
56.93% in G’ value is noticed as we move from 2:1 (w/w) gel to
1:1 (w/w) gel. So, it can be said that the mechanical strength of
the gel can be easily tuned by varying the amount of C-dots in
the gel matrix
.
As the gel was broken upon mechanical shaking and then
reformed upon resting (Fig. S11, ESI†), time dependent step-
strain experiment was performed to investigate its mechano-
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responsive nature. A 2 ml gel containing 8 mg of amphiphile dependent emission spectra of the hydrogel. It is apparent
and 4 mg of carbon dots was used for the time dependent step from Fig. 3athat there is a slight red shift of 5 nm as the
strain experiment (Fig. S12, ESI†).At first a low strain of 0.1% excitation wavelength is increased from 345 to 405 nm and it
was applied for 100 seconds and the values of G’ and G” were is accompanied by a steady increase in the emission intensity
measured. The G’ value was found to be higher than the G” as the excitation wavelength is increased from 345 to 385 nm.
value under this strain indicating the gel nature. Then the Thereafter, it drops slightly for the excitation at 405 nm. So, it
strain was increased to 100% at once and this strain was can be stated that at excitation of 385 nm, the maximum
maintained for the next 100 seconds. Under this increased emission is observed for the as-prepared C-dots and C-dots
strain the G’ attained a lower value than G” indicating the containing gel. In C-dots containing gel the emission is 25 nm
transformation of the gel to sol. To allow the gel to reform, the red shifted compared to that of the as prepared C-dots
strain was again reduced to 0.1 % and it was kept at that value solution in water. New surface states may be been
for next hundred seconds. The values of G’ and G” were incorporated upon interaction of histidine and carbon dots
increased and regained their initial values after 70 seconds.
surface24 during gelation and this leads to the red shift of the
carbon dots fluorescence. The carbon dots containing gel with
a composition of amphiphile to C-dots ratio of 8:1 (w/w) was
freeze dried to obtain the xerogel and this was subjected to
solid state fluorescence study. This xerogel emits a bright
bluish green fluorescence upon the irradiation at 365 nm by
illuminating through a hand held UV lamp (Fig. 1c). In Fig. 3c
shows the emission peak of the xerogel materials at different
excitation wavelengths. It is evident from the Fig. 3c that the
xerogel too exhibits excitation independent fluorescence peak
at 393 nm and it can be stated that this emission peak is 20 nm
red shifted from the emission of the C-dots containing
hydrogel. The fluorescence quantum yield of the xerogel (in
the solid state) is found to be 13%. Generally, the fluorescence
of the C-dots is quenched at solid state. However, in this study
we got a very bright and significant florescence that has
emerged from the xerogel. No fluorescence is observed from
the xerogel when the composition of C-dots to amphiphile
ratio is more than 1:2 (w/w).For achieving fluorescent xerogel
the highest loading fraction of carbon dots is found to be 33.33
(% w/w). This is quite high compared to the previously
reported materials11a and thus it is an efficient system for
conserving the fluorescence of carbon dots in its solid state.
C-dots have been extensively used as fluorescent inks.25For an
ideal fluorescent ink application, it is desirable that the
fluorescent species should show solid state fluorescence.11b
However, it is not the case for most of the carbon dots
reported so far as they suffer from aggregation induced
fluorescent quenching in the solid state. When the ink is dried,
there is every possibility that the ink will lose its fluorescence
(Fig. 3d) rendering it unsuitable for practical application. In our
system, the fluorescence of the carbon dots is effectively
conserved in the solid state. So, this two-component hydrogel
system can be used effectively as a fluorescent ink. Fig.
3ddemonstrates the writings with the carbon dots solution in
water fluoresce initially, after the evaporation of water with
time the fluorescence of the carbon dots disappears due to
aggregation induced quenching. So, the writings are no longer
visible. Interestingly, the writings with the gel remain
fluorescent even after the evaporation of water molecules
with time. This indicates the important application of this
material as a fluorescent ink in future.
Fig. 2 (a) FEG-TEM image and (b) size distribution of as prepared C-dots. (c) FEG-TEM
image and (d) size distribution of C-dots in gel. (e) FEG-TEM image shows nanofibrilar
network assembly of C-dots gel. (f) Frequency sweep data showing increase in gel
strength with increase in C-dots proportion in the gel matrix.
The C-dots are soluble in polar solvents like water, DMF, DMSO
and methanol and in water they show bright cyan fluorescence
upon irradiation with 365 nm illumination of a hand-held UV
lamp. Fig.3a shows fluorescence spectra of the as-prepared C-
dots at various excitation wavelengths. It is apparent from this
figure that the emission peak is centred at 446 nm showing no
change of emission by the variation of the excitation
wavelength from 345 nm to 415 nm. However, there is a
change in intensity of emission spectra by increasing the
excitation wavelength upto 385 nm and then it starts
decreasing slightly upto 415 nm. So it can be said that these
carbon dots show excitation independent emission in aqueous
medium and this is rare.22The UV-visible spectra of as-
prepared C-dots (Fig. S13, ESI†) illustrates a small peak at 294
nm corresponding to the π-πtransition in the carbogenic core,
while the peak at 388 nm with a shoulder at 385 nm can arise
due to the n- π transition of C=O and C=N23. The two-
component hydrogel containing the C-dots and the amino acid
based amphiphile shows a green emission upon the irradiation
of 365 nm by a UV lamp. Fig. 3a shows the excitation
This study convincingly demonstrates C-dots induced gelation
of
a histidine based amphiphile through non-covalent
interactions in water medium. Mechanical strength of this two
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Fig. 3Fluorescence spectra of (a) as prepared C-dots in water (0.1 (b) C-dots hydrogel
(c) xerogel obtained from hydrogel. (inset shows corresponding optical images under
UV illumination of 365 nm) (d) Fluorescentink application of C-dots gel showing its
superiority to as prepared c-dots.
component gel has been successfully tuned by varying the
ratio of C-dots to amphiphile. The hydrogel shows
a
remarkable cyan fluorescence, while the xerogel shows a
green fluorescence. This gel based soft material has been
utilized as a good fluorescent ink on the glass plate and it is
visible under UV light even after the evaporation of water
molecules, as the dried gel is also fluorescent. This holds the
future promise for using this material in device applications.
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