Pyrene-based fluorescent supramolecular hydrogel: scaffold for nanoparticle synthesis

Article abstract of DOI:10.1002/poc.4026

The present work illustrates the design and development of pyrene-based fluorescent supramolecular hydrogel consisting of l-phenylalanine residue as the linker and ethyleneoxy unit with free primary amine at C-terminal. The gelator efficiently immobilized

Full text of DOI:10.1002/poc.4026

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R E S E A R C H A R T I C L E
Pyrene‐based fluorescent supramolecular hydrogel:
scaffold for nanoparticle synthesis
Tanmoy Kar | Nitai Patra
Pyrene containing intrinsically fluorescent amino acid based
hydrogelator has been developed. Free primary amine containing
hydrogelator has been exploited for in situ synthesis of AgNPs under
sunlight in absence of any external reducing agent.
Wiley Online Library Graphical TOC

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Received: 22 June 2019
DOI: 10.1002/poc.4026
Revised: 6 September 2019
Accepted: 27 September 2019
R E S E A R C H A R T I C L E
Pyrene‐based fluorescent supramolecular hydrogel: scaffold
for nanoparticle synthesis
Tanmoy Kar1
| Nitai Patra2
1 Department of Chemistry, Vivekananda
Mission Mahavidyalaya, Chaitanyapur,
Haldia, West Bengal, India
2 Centre for Surface Science, Physical
Chemistry Section, Department of
Chemistry, Jadavpur University, Kolkata,
West Bengal, India
Abstract
The present work illustrates the design and development of pyrene‐based fluo-
rescent supramolecular hydrogel consisting of L‐phenylalanine residue as the
linker and ethyleneoxy unit with free primary amine at C‐terminal. The gelator
efficiently immobilized aqueous media, and the critical gelation concentration
(CGC) was found at 2.8 mM of the compound. Interestingly, the presence of
pyrene moiety in the structure made the hydrogel intrinsically fluorescent.
The self‐aggregation properties of the thermo‐reversible hydrogel were investi-
gated using different microscopic and spectroscopic techniques such as high‐
resolution transmission electron microscopy (HRTEM), Fourier transform
infrared (FTIR) spectroscopy, fluorescence spectroscopy, and X‐ray diffraction
(XRD) spectroscopy. A balanced participation of noncovalent interactions like
hydrogen bonding, π‐π stacking, and van der Waals interaction was identified
as the driving force for gelation. Rheological experiments were performed to
confirm the viscoelastic nature of the prepared hydrogel. Furthermore, primary
amine (─NH2) group of hydrogelator was utilized for the in situ synthesis of
silver nanoparticles (AgNPs) within the soft material under sunlight in the
absence of any external reducing and stabilizing agent.
Correspondence
Tanmoy Kar, Department of Chemistry,
Vivekananda Mission Mahavidyalaya,
Chaitanyapur, Haldia. West Bengal
721645, India.
Email: taanmoykar@gmail.com
KEYWORDS
Amino acid, fluorescent, hydrogel, nanoparticle, self‐assemby
1 | INTRODUCTION
phase by the addition of very small amount of gelators.
LMW gelators form supramolecular assemblies (fibers,
Supramolecular gels, a class of soft materials,[1–20] have
drawn prevalent attention due to their potential applica-
tions in diversified fields such as template for sen-
sors,[16,21–23] template nanostructured materials,[24–26]
drug delivery,[27–29] tissue engineering,[30–32] removal of
pollutants,[33–35] and enzyme‐immobilization matri-
ces.[36–38] This ever‐increasing demand has augmented
the necessity for the rational design and synthesis of
gelator molecules having suitable functionality for task‐
specific applications. Low molecular weight (LMW)
gelators have the unique ability to immobilize a free
flowing solvent of different polarities into a semisolid
rods, ribbons, or other morphologies) where combination
of noncovalent interactions (hydrogen bonding, π‐π stack-
ing, and van der Waals interactions) shoulders the weight
of huge amount of solvents against gravity by forming
three‐dimensional (3‐D) self‐assembled fibrillar networks
(SAFINs).[39–44] LMW gelators are classified largely into
two categories according to the nature of solvent they
immobilize, hydrogelator and organogelator.[1–3] Among
the various types of supramolecular gels, hydrogels have
earned much attention particularly for biological applica-
tions due to their ability to imbibe water.[27–32] Although
there are a large number of reports in literature on LMW
J Phys Org Chem. 2019;e4026.
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© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/poc.4026

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KAR AND PATRA
molecules that have been studied for self‐assembled
gelation in water, still, there is a requirement of designing
simple hydrogelator molecules having task‐specific
applications. Ideally, a hydrogelator should have an
optimum balance between hydrophobicity and hydrophi-
licity that would lead to self‐assembled gelation. Due
to the amphiphilic nature of the SAFINs of supramolecu-
lar gels, it can host externally doped hydrophilic or
hydrophobic molecules inside the interstitial space
of the networks and arranges them along with the align-
ment of the gelators via various modes of noncovalent
interactions.[44–46]
2 | EXPERIMENTAL
2.1 | Materials
Amino acids, dicyclohexylcarbodiimide (DCC), 4‐N,N‐
(dimethyl)aminopyridine (DMAP), 1‐hydroxybenzotriazole
(HOBT), sodium hydroxide (NaOH), silver nitrate
(AgNO3), and all the solvents were purchased from
SRL, India. Pyrenebutyric acid, and 2,2′‐(ethylenedioxy)
bis(ethylamine) were bought from Sigma. All deuterated
solvents for nuclear magnetic resonance (NMR) and
Fourier transform infrared (FTIR) experiments were
obtained from Aldrich Chemical Co. 1H NMR spectra
were recorded in AVANCE 500‐MHz (Bruker) spectrom-
eter. Mass spectrometric data were acquired by electron
spray ionization (ESI) technique on a Q‐tof‐micro
quadruple mass spectrometer (Micromass). Elemental
analyses were performed on Perkin‐Elmer 2400 CHN
analyzer.
To this end, development of hybrid materials resulting
from the combination of gels with nanoparticles of differ-
ent origins has gathered momentum in modern‐day sci-
ence because of their superior properties.[44–46] The
common thread between the two constituents in soft
nanocomposites is that both are governed by colloidal
stability and weak forces provide stability from the
nanoscopic to microscopic domains. These gel‐
nanoparticle composites can be prepared either by syn-
thesizing nanoparticles in situ or by addition of the
preformed nanoparticles into the three‐dimensional
(3‐D) gel matrix. The conjugates of hydrogels and metal
nanoparticles (MNPs) have impending prospect in devel-
oping biomaterials, labeling agent, sensors, etc.[5,44–46]
In the present work, the synthesis and development of
pyrene‐based fluorescent supramolecular hydrogel (1,
Figure 1A) and its utilization in the synthesis and stabili-
zation of silver nanoparticles (AgNPs) have been
reported. Pyrenebutyric acid, bis‐ethyleneoxy unit, and
amino acid residue (L‐phenylalanine) have been used as
hydrophobic unit, hydrophilic unit, and a linker, respec-
tively (1, Figure 1A). The gelator was found to immobilize
aqueous media, and critical gelation concentration (CGC)
was 2.8 mM. The gel‐melting temperature (Tgel) of the
hydrogel was 37°C at CGC. Different spectroscopic and
microscopic techniques were employed to study self‐
aggregation properties of the thermo‐reversible hydrogel.
Additionally, AgNPs were synthesized within the hydro-
gel matrix in situ under sunlight without using any exter-
nal reducing agent. Rheological study of the composite
showed significant enhancement (almost threefold) in
mechanical strength compared with the native hydrogel.
2.2 | Synthetic procedure
Compound 1 was synthesized following well‐established
protection and deprotection chemistry used for peptide
synthesis (Figure S1).[45] Pyrenebutyric acid (a) was
coupled with methyl ester of protected L‐phenyl alanine
(b) in dichloromethane (DCM) using DCC (1.1 equiva-
lent), DMAP (1.1 equivalent), and HOBT (1.1 equivalent)
following the reported protocol. The coupled product (c)
was hydrolyzed using 1 N NaOH solution, followed by
acidification with 1 N HCl. The product with free carbox-
ylic acid at the terminal (d) was further coupled with
mono Boc‐protected 2,2′‐(ethylenedioxy)bis(ethylamine)
to get (e). The Boc‐protected compound was
subjected to deprotection by TFA (2 equiv) in dry DCM
under magnetic stirring for approximately 2 hours.
The purified product (f) was obtained by column
chromatography using 60 to 120 mesh silica gel and
methanol/chloroform as eluent.
Characterization data for 1 (Figures S2 and S3):
1H NMR (500 MHz, CDCl3, 25°C, TMS):
δ/ppm = 8.23‐8.21 [d, 1H], 8.17‐8.15 [m, 2H], 8.10‐8.07
[t, 2H], 8.02‐7.97 [m, 3H], 7.78‐7.76 [d, 1H], 7.44‐7.38
[m, 1H], 7.24‐7.15 [m, 5H], 6.57‐6.55 [d, 1H], 4.81‐4.77
[m, 1H], 3.55‐3.40 [m, 10H], 3.33‐3.24 [m, 4H], 3.14‐
3.09 [m, 1H], 3.01‐2.96 [m, 1H], 2.87‐2.81 [m, 2H],
2.16‐2.05 [m, 4H]; ESI‐MS: m/z calculated for
C35H39N3O4 (the 4° ammonium ion, 100%): 565.2941;
found: 565.4931; elemental analysis calculated (%) for
C35H39N3O4: C 74.31; H 6.95; N 7.43; found: C 74.46;
H 6.91; N 7.49.
FIGURE 1 A, structure of gelator. B, hydrogel of 1 under the
exposure of UV light

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KAR AND PATRA
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(TEM) images were taken on a JEOL JEM 2010 high‐
resolution microscope operating at 200 kV.
2.3 | Surface tension measurement
The critical aggregation concentration (CAC) of the
amphiphilic molecule was measured using a tensiometer
(Jencon, India) applying Du Noüy ring method at
25 0.1°C in water. The CAC value was determined by
plotting surface tension (γ) vs concentration of surfactant
with the accuracy of 2% in duplicate experiments.[47]
2.9 | Fluorescence microscopic study
Fluorescence microscopic images of the gel sample were
taken using a light microscope (BX61, Olympus). A dilute
gel sample was placed on a glass slide, which was sealed
with a glass cover slip, and images were recorded.
2.4 | Preparation of gel
2.10 | FTIR measurements
The requisite amount of compound was taken in a screw‐
capped vial having internal diameter (i.d.) of 10 mm. The
compound was slowly heated to dissolve in water,
followed by slow cooling (undisturbed) to room tempera-
ture. The gelation was checked by “stable to inversion” of
the aggregated material in the glass vial.
Fourier transform infrared measurements on gelator in
CHCl3 solution and D2O for hydrogel were carried out
in a Perkin‐Elmer Spectrum 100 FT‐IR spectrometer
using KBr pellets and CaF2 cell, respectively.
2.11 | Fluorescence spectroscopy study
2.5 | Dynamic light scattering study
The emission spectra of gelator at the different concentra-
tion were recorded on a Varian Cary Eclipse lumines-
cence spectrometer. The excitation wavelength was at
λex = 340 nm, whereas both of the excitation and emis-
sion slit width were at 5 nm.
Dynamic light scattering (DLS) measurements were
carried out in a Malvern Zetasizer NanoZS apparatus at
173° angle with a He─Ne laser of wavelength (λ)
632 nm at 25°C.
2.6 | Determination of gel‐melting
temperature (Tgel)
2.12 | Synthesis of silver nanoparticles
within the hydrogel
The gel‐to‐sol transition temperature (Tgel) was recorded
by gradually increasing the temperature (at a rate of
2°C/min) of the thermostated oil bath in which
hydrogel‐containing glass vial was placed. The tempera-
ture at which the gel melted and started to flow was
The required amount of gelator 1 (10 mg, in 830‐μL
water) was taken in a vial, and the mixture was gently
heated to produce a homogeneous solution. To this mix-
ture, freshly prepared aqueous solution of AgNO3
(170 μL, 10 mM) was added. Then, this solution was
allowed to cool to room temperature to form a gel. The
concentration ratio was maintained as gelator:
AgNO3 = 10:1. This hydrogel containing AgNO3 was kept
under sunlight. Within few minutes, the colorless gel
turned yellow, which indicates the formation of silver
nanoparticles. The formation of AgNPs was monitored
by UV‐Vis spectroscopy.
stated as Tgel
.
2.7 | Field emission scanning electron
microscopy study
A drop of hydrogel was placed on a piece of cover slip and
dried in vacuum for few hours before imaging. Then, it
was coated with platinum vapor to make them
conducting and analyzed on a JEOL‐6700F microscope.
2.13 | Rheology
The rheological experiments were performed in cone and
plate geometry (diameter was 40 mm) on the rheometer
plate using an Anton Paar, MCR 302. The native gel
was placed on the rheometer plate so that there was no
air gap with the cone. Frequency sweep experiment was
done as a function of angular frequency (0.1‐350 rad/s)
at fixed strain of 0.01% at room temperature, and the
2.8 | Transmission electron microscopy
study
A drop of diluted solution of hydrogel and AgNP‐
hydrogel composite was placed on a 300 mesh carbon‐
coated Cu grid and dried for few hours under vacuum
before imaging. The transmission electron microscopy