Galactose Grafted Two-Dimensional Nanosheets as a Scaffold for the In Situ Synthesis of Silver Nanoparticles: A Potential Catalyst for the Reduction of Nitroaromatics

Article abstract of DOI:10.1002/chem.202102421

Two major hurdles in NP-based catalysis are the aggregation of the NPs and their recycling. Immobilization of NPs onto a 2D support is the most promising strategy to overcome these difficulties. Herein, amphiphilicity-driven self-assembly of galactose-hexaphenylbenzene-based amphiphiles into galactose-decorated 2D nanosheet is reported. The extremely dense decoration of reducing sugar on the surface of the sheets is used for the in situ synthesis and immobilization of ultrafine catalytically active AgNPs by using Tollens’ reaction. The potential of the system as a catalyst for the reduction of various nitroaromatics is demonstrated. Enhanced catalytic activity is observed for the immobilized AgNPs when compared to the corresponding discrete AgNPs. Recovery of the catalytic system from the reaction mixture by ultrafiltration and its subsequent recycling for several cycles without dropping its activity is shown. This is the first report demonstrating the in situ synthesis and immobilization of ultrafine AgNPs onto a 2D nanosheet that exhibits excellent catalytic performance for the reduction of nitroaromatics.

Full text of DOI:10.1002/chem.202102421

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Accepted Article
Accepted Article
Title: Galactose Grafted Two-Dimensional Nanosheets as a Scaffold
for the In-situ Synthesis of Silver Nanoparticles: A Potential
Catalyst for the Reduction of Nitroaromatics
Authors: Kaloor S. Harikrishnan, Nithiyanandan Krishnan, Nilima
Manoj Kumar, Anusree Krishna, Gowtham Raj, Devanathan
Perumal, Jemshiya Kalathil, Jithu Krishna, and Reji Varghese
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202102421
Link to VoR: https://doi.org/10.1002/chem.202102421
01/2020

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Galactose Grafted Two-Dimensional Nanosheets as a Scaffold for
the In-situ Synthesis of Silver Nanoparticles: A Potential Catalyst
for the Reduction of Nitroaromatics
Kaloor S. Harikrishnan,† Nithiyanandan Krishnan,† Nilima Manoj Kumar, Anusree Krishna, Gowtham
Raj, Devanathan Perumal, Jemshiya Kalathil, Jithu Krishna and Reji Varghese*
[a]
Kaloor S. Harikrishnan,† Dr. Nithiyanandan Krishnan,† Nilima M. Kumar, Anusree Krishna, Gowtham Raj, Devanathan Perumal, Jemshiya Kalathil, Jithu
Krishna and Dr. R. Varghese*
School of Chemistry
Indian Institute of Science Education and Research (IISER) Thiruvananthapuram
Thiruvananthapuram-695551, Kerala, India
E-mail: reji@iisertvm.ac.in
† These authors contributed equally
Supporting information for this article is given via a link at the end of the document.
Abstract: Two major hurdles in NP-based catalysis are
the
(iii) possibility of modification of the electron density of the
adsorbates and (iv) can favorably influence the kinetics of
catalysis. In addition to graphene-based materials, metal-organic
frameworks (MOFs), which can provide large surface area,
tunable pore structures and tailorable functionalities, have also
proven to be a promising scaffold for the organization of NPs for
catalytic applications.[8] Other 2D supports including metal
oxides,[9] hydrogels,[10] self-assembled nanostructures[11] etc.
have also shown to be efficient as scaffolds for the immobilization
catalytically active NPs. However, in all these cases
immobilization of NPs onto the 2D support involves multiple steps
and hence are highly laborious. Therefore, it is extremely
desirable to have a simple design strategy that allows in-situ
synthesis of NPs and their immobilization onto a 2D support in a
single-step process.
aggregation of the NPs and their recycling. Immobilization of NPs onto
a 2D support is the most promising strategy to overcome these
difficulties. Herein, amphiphilicity-driven self-assembly of galactose-
hexaphenylbenzene based amphiphiles into galactose-decorated 2D
nanosheet is reported. The extremely dense decoration of reducing
sugar on the surface of the sheets is used for the in-situ synthesis and
immobilization of ultrafine catalytically active AgNPs by using Tollens’
reaction. The potential of the system as a catalyst for the reduction of
various nitroaromatics is demonstrated. Enhanced catalytic activity is
observed for the immobilized AgNPs when compared to the
corresponding discrete AgNPs. Recovery of the catalytic system from
the reaction mixture by ultrafiltration and its subsequent recycling for
several cycles without dropping its activity is shown. This is the first
report demonstrating the in-situ synthesis and immobilization of
ultrafine AgNPs onto a 2D nanosheet that exhibits excellent catalytic
performance for the reduction of nitroaromatics.
Amphiphilicity-driven
self-assembly
is
an
efficient
supramolecular approach for the crafting of self-assembled
nanostructures of p-conjugated systems with well-defined
morphology.[12] Sugar-based amphiphilic systems are particularly
attractive as the nanostructures obtained from their self-assembly
in aqueous medium offers a shell made of hydrophilic sugar
moieties.[13] This subsequently permits sugar-directed chemistry
Introduction
High surface area to volume ratio provides remarkable chemical
and physical properties to noble metal nanoparticles (NPs). By
virtue of their unique properties they have found potential
applications in various fields such as catalysis,[1] disease
diagnosis,[2] therapy[3] and photonics.[4] Particularly, the area of
heterogeneous catalysis has been greatly blessed by the
advancements in the field of metal nanoparticle research.[5]
Nevertheless, two major hurdles that are yet to be fully resolved
in this field are the prevention of NPs aggregation during the
reaction and their recyclability. Immobilization of NPs onto a 2D
support is the most promising strategy for the prevention of NP
aggregation and in many cases, this allows the recycling of the
catalyst for several cycles without compromising the catalytic
performance.[6] For example, graphene and graphene-based
materials such as graphene oxide (GO) and reduced graphene
oxide (rGO) have proven as excellent scaffolds for the
immobilization of metal nanoparticles and remarkable catalytic
performance have been achieved.[7] This is because they offer (i)
extremely large surface area, (ii) strong interaction with the NPs
on
the
surface
of
the
nanostructures.
Similarly,
hexaphenylbenzene (HPB), a hydrophobe having large p-surface,
have extensively explored for the design of amphiphiles owing to
its high propensity to self-assemble via p-p stacking interaction.[14]
Inspired from these examples, we envisaged that the
incorporation of sugar as the hydrophilic unit to HPB could drive
the self-assembly of the amphiphile in a lamellar fashion through
the strong p-p stacking interaction of HPB to sugar-decorated 2D
nanosheets. The protruding sugar moieties on the surface of the
sheet then potentially permits sugar-guided chemistry on the
sheet surface.
Herein, we report the self-assembly of a new class of
amphiphiles having HPB as the hydrophobic segment and
galactose as the hydrophilic units into galactose-decorated 2D
nanosheet and demonstrates a single-step strategy for the in-situ
synthesis and immobilization of catalytically active AgNPs as a
catalyst for the reduction of nitroaromatics. Galactose is selected
as the hydrophilic unit due to its ability to reduce Ag(I) ions to
1
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OH
OH
=
self-assembly via
pi-stacking
NO2
∼5 nm
NO2
NaBH4
1a
1
Tollens’ reaction
[Ag(NH3)2]+
self-assembly via
pi-stacking
self-assembly
∼6 nm
=
2a
2
Scheme 1. Scheme for the syntheses of 1 and 2 and the schematic representation depicting the self-assembly of the amphiphiles into galactose-decorated 2D
nanosheets. The in-situ synthesis of AgNPs using Tollens’ reaction, their immobilization onto the sheet and the recycling of the catalytic system are also shown.
Ag(0), known as the Tollens’ reaction. This reaction has been
widely explored for the synthesis of AgNPs.[15] Accordingly, we
a)
b)
designed two amphiphiles: (i) a bolaamphiphile having HPB as
the hydrophobic segment and two galactose moieties
symmetrically tethered as the hydrophilic units (1) and HPB as the
hydrophobic segment and galactose as the hydrophilic segment
(2). Both the amphiphiles undergo spontaneous self-assembly in
aqueous medium via p–p stacking interaction of HPB into
galactose-decorated 2D sheets. The protruding reducing sugar
on the surface of the sheet is explored for the in-situ synthesis
and immobilization of ultrafine catalytically active AgNPs by using
Tollens’ reaction. The potential of the system as a catalyst for the
reduction of various nitroaromatics is demonstrated. Since the
lateral dimension of the sheet is in the order of several
micrometers, the catalytic system can easily be recovered from
the reaction mixture by mere ultrafiltration, and the subsequent
recycling of the catalyst for several cycles without dropping its
activity is also demonstrated.
c)
d)
Figure 1. (a) UV-Vis absorption spectra of 1 in DMSO and water:DMSO (98:2).
(b) Fluorescence spectral changes of 1 with the addition of water into its DMSO
solution. (c) Temperature dependent fluorescence changes of 1 aggregate.
Inset shows the comparison of spectral changes of the aggregates
(water:DMSO) and monomeric species (DMSO) of 1 with respect to different
temperatures. (d) DLS analysis of the aggregates of 1 and 2 in water:DMSO
(98:2).
Results and Discussion
Syntheses of the amphiphiles
The amphiphiles 1 and 2 were synthesized using multistep
organic reactions by
a following reported procedure with
appropriate modifications.[16] Initially, alcohol functionalized HPB
derivatives (1a and 2a) were synthesized through multistep
organic reactions. The corresponding alcohol derivatives of HPB
were then alkynylated (1b and 2b) with propargyl bromide using
NaH as base in DMF as the solvent. On the other hand, azide
functionalized galactose (3) was synthesized by following a
reported procedure. Subsequently, copper-catalyzed alkyne-
azide cycloaddition (CuAAC) reaction between azide
functionalized galactose and alkyne modified HPB derivative
yielded the protected form of the amphiphiles. Deprotection using
TFA:H2O (1:1) furnished the corresponding amphiphiles 1 and 2
in reasonable yields (Scheme 1) . Experimental procedures and
characterization details of 1, 2 and all intermediates are provided
in the Supporting Information.
Self-assembly of the amphiphiles
Self-assembly of the amphiphiles (1 and 2) was achieved by
adding water (980 µL) into a DMSO solution of 1 or 2 (100 µM, 20
µL), followed by annealing the solution at 90 °C for 10 minutes
and subsequent slow cooling to room temperature. UV-visible
absorption spectra of 1 (Figure 1a) and 2 (Figure S1) in DMSO
showed the characteristic monomeric absorption of HPB with
maximum centered at 270 nm, which corresponds to the p-p
transition of HPB. Both the amphiphiles 1 and 2 exhibited very
weak emission in DMSO λex = 260 nm). Interestingly, an increase
in emission intensity was observed with addition of water into their
2
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DMSO solution with fluorescence quantum yields of 17 % and
18 % for 1 and 2, respectively (Figure 1b and Figure S1). These
results imply that 1 and 2 exist as monomeric species in DMSO
and undergoes amphiphilicity-driven self-assembly with the
addition of water into their DMSO solution mainly through p-p
stacking of the HPB units. The nearly non-emissive nature of the
monomeric species is due to the intramolecular C(sp2)-C(sp2)
bond rotational relaxation of the excited states of 1 and 2.
Whereas the intramolecular bond rotation is completely arrested
in the aggregated state, which facilitate the radiative decay
channel (fluorescence) of the excited states and thereby
enhances the emission in the aggregated state. This
phenomenon is known as aggregation induced enhanced
emission (AIEE) and is very typical of HPB-based systems.[17]
Temperature-dependent emission studies of the aggregates of 1
(Figure 1c) and 2 (Figure S2) in water:DMSO showed a decrease
in emission intensity with the increase in temperature from 20 °C
to 90 °C, and become almost non-emissive at high temperature.
Moreover, emission intensity of 1 in water:DMSO (aggregated
species) at 90 °C is nearly matching with the emission intensity of
1 in DMSO at 20 °C (monomeric species) (Figure 1c inset). These
results confirm that the amphiphiles exist as aggregated species
in water:DMSO at 20 °C and gradually breaks into the
corresponding monomeric species upon rise in temperature. It is
also to be noted a decrease in emission intensity of 1 in DMSO at
90 °C when compared to its emission intensity at 20 °C and this
can be attributed to the mere effect of temperature on
fluorescence (Figure 1c inset).[18] In support of this, dynamic light
scattering (DLS) analyses of 1 and 2 solution in water:DMSO
revealed the formation of aggregates with size distributions in the
ranges of 250 nm-5 µm and 50 nm-6 µm for 1 and 2, respectively
(Figure 1d).
Detailed microscopic analyses were then carried out to
understand the morphology of the aggregated species of 1 and 2.
Atomic force microscopic (AFM) analyses of the aggregates of 1
(Figure 2a) and 2 showed the formation of nanosheets with lateral
dimensions in the range of several micrometers. Section analyses
revealed that height of sheets of 1 is in the range of 20-100 nm
(Figure 2a inset). Height of the sheet is significantly larger than
the corresponding monolayer distance of 1 sheet (~5 nm),
suggesting that the sheets are multilayered in nature. Scanning
electron microscopic analyses of the aggregates of 1 (Figure 2b)
and 2 (Figure S4) also showed the formation of multilayered
sheets with lateral dimensions of several micrometers, which is in
good agreement with the AFM analyses. Sheet morphology was
further supported by transmission electron microscopic (TEM)
analyses, which revealed the formation of highly transparent
nanosheets for 1 (Figure 2c) and 2 (Figure S6). Furthermore, TEM
analyses clearly disclosed the multilayered nature of the sheets
(Figure 2d). All these results collectively conclude that both the
amphiphiles undergo amphiphilicity-driven self-assembly mainly
through p-p stacking interaction of HPB to micrometer-sized
b)
a)
500 nm
1 µm
d)
c)
multi layer
Figure 2. (a) AFM height, (b) SEM and (c) & (d) TEM images of 1 aggregates
in water:DMSO (98:2). Inset of (a) shows the section analysis of the sheet along
the line drawn on the image. [1] = 2 μM for all the experiments.
Synthesis of AgNPs and their immobilization onto the sheet
The most noteworthy structural feature of the sheets is the
extremely dense display of reducing sugar on the surface of the
sheets, which means that they offer a unique opportunity for the
in-situ synthesis of AgNPs by using the well-known Tollens’
reaction. Tollens’ reaction involves the reduction of Ag(1) to Ag(0)
by a reducing sugar such as galactose, which on the other hand
undergoes oxidation of the aldehyde group to the corresponding
carboxylic acid group. Subsequently, Ag(0) is acting as the seed
for the growth of NPs of defined size and shape. For this purpose,
Tollens’ reagent ([Ag(NH3)2]+) (20 mM) was added to the sheets
of 1 or 2 (20 μM) in water:DMSO (98:2), vortexed for 2 h in dark.
The in-situ formation of AgNPs was then followed by monitoring
the emergence of surface plasmon absorption band of the NPs at
400 nm. Interestingly, UV-visible absorption spectrum of the
solution of 1 sheet after 2h of reaction disclosed a strong
absorption band with maximum centered at 400 nm, which clearly
implies the in-situ formation of AgNPs (Figure 3a). In support of
this, the colorless reaction mixture turned into pale yellow (Figure
3b). Furthermore, a significant quenching of fluorescence was
observed for the sheets due to the electronic interaction between
AgNPs formed on the sheet and HPB (Figure 3c), which is very
much feasible as the distance separating them is only ~2.5 nm.
Similar observations were seen for 2 sheets as well (Figure S7–
S9).
multilayered sheets. Accordingly, sheet of 1, which is
a
bolaamphiphile, consists of monolayer assembly of 1 having p-
stacked HPB as the hydrophobic core with hydrophilic galactose
protruding on both the faces of the sheet as shown in Scheme 1.
Similarly, sheet of 2, which is an amphiphile, consist of bilayer
assembly of HPB through p-p stacking as the hydrophobic core
with hydrophilic galactose protruding on the faces of the sheet
(Scheme 1).
To gain better insights into the structural characteristics of
the NPs, detailed TEM analyses were carried out on the NPs
obtained after the reactions of 1 and 2 sheets with Tollens’
reagent. The TEM images of NPs in the case of 1 sheet revealed
that the NPs are spherical in shape and nearly monodisperse
(Figure 3d). The size of the NPs is in the range of 5-10 nm, which
falls in the regime of ultrafine NPs (Figure 3d inset).[19] It is also
obvious from the TEM images that the NPs are well-dispersed
3
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