Communication
doi.org/10.1002/chem.202100450
Chemistry—A European Journal
microscopy) images also suggested formation of higher degree
of self-assembled GNR upon addition of NBI (Figure 3i–k).
fold) of GNR-bead conjugated particles in presence of NBI in
comparison with CNP, with NBI in water and in only buffer
(Figure 4b+4c, Figure S27-S29 and Table S2 in the Supporting
Information). We believe this behavior is due to self-electro-
phoretic effect as uncharged reactant results slightly charged
product resulting momentary imbalance of electrical charge.[4d]
It is noteworthy that micron sized bead which initially remained
catalytically inactive, can enhance the diffusion of the whole
conjugate, just by its surface functionalization with active
nanometer-sized GNR (Supporting video, SV1 in the Supporting
Information). In fact, we were able to visualize formation of
dimer/trimer of microbeads coated with GNR under catalytic
condition (Figure S28 in the Supporting Information). We
presume that faster diffusion under catalytic condition can play
additional role in higher assembly under catalytic condition as
mentioned in earlier literature reports.[26]
Finally, we were curious to explore if this self-assembly of
GNR during KE catalysis can lead to the emergence of additional
functionality not directly related to the KE catalysis. We argue
that the hydrophobic cavities formed during assembly forma-
tion of gold nanorods can be utilized as a nanoreactor where
hydrophobic reactants can be trapped and made to react.[12] To
study this dual catalytic behavior of gold nanorods, we chose
an aromatic nucleophilic substitution reaction between octyl
amine and 4-Chloro-7-nitrobenzofurazan (NBD-chloride) which
form a fluorescent adduct (NBD-C8), where reaction can be
monitored easily by fluorescence techniques (Figure S33 in the
Supporting Information). This reaction does not proceed in
aqueous buffer system, however, in presence of GNR substantial
reactivity was observed as the hydrophobic bilayer zone of
CTAB on its surface helped the solubilization of the substrates.
Interestingly, presence of NBI (100 μM) leads to more NBD-C8
formation where clustering of GNR is more (Figure 5a+b and
S35 in supporting information). Fluorescence microscopic
images also showed more and larger sized fluorescent particles
(NBD-C8 adduct formed inside GNRs or in the cavity of its
aggregates) in presence of NBI (Figure 5c+d). Interestingly,
here also, we have observed an increase in adduct formation
with increase in pH from 6 to 8. (Figure S35 in the supporting
information). Maximum NBD-C8 adduct formation was observed
for pH 8 in presence of NBI, which again corroborates previous
results of GNR aggregation. The formation of NBD-C8 adduct
was ensured by using mass spectrometry (Figure S36 in the
Supporting Information).
We have also studied the reusability of GNR for KE catalysis.
For this purpose, we added NBI in batches of 50 μM and looked
at the changes in size of GNR system by using DLS measure-
ments and product formation kinetics by using UV-Vis spectro-
scopy (Figure S25 in the Supporting Information). The efficiency
of KE catalysis in repetitive cycle decreased after addition of
each batch, presumably because of the product inhibition effect
(Figure S16 in the Supporting Information). It suggests that the
product also have affinity on the cationic GNR surface owing to
its negative charge along with the hydrophobic residue.
Further, we did motion analysis by optical video recording
and MSD (mean squared displacement) calculations to under-
stand the catalytic effect in diffusion.[24,25] It is worthy to
mention that enhanced diffusion of catalyst during reaction are
one of the fascinating phenomenon and discussed issues since
last decades among interdisciplinary sciences.[4] Mostly, self-
phoretic mechanism (e.g. self-electrophoretic, self-thermopho-
retic, self-diffusiophoretic etc.) play the prime role behind this
effect, although the exact reason still remain dubious.[4f] One of
the most reliable technique to measure this phenomenon is by
using tracking motion of the particle by optical/fluorescence
microscopy.[24,25] For this, we have used a micro-sized replica of
our system formed by using carboxylate modified polystyrene
bead (d=2 μm) GNR conjugate (PS-GNR) as only GNR (dimen-
sion of ~25 nm) cannot be observed under optical microscope
(Figure 4a and S26 in the Supporting Information). Encourag-
ingly, we have observed significantly enhanced diffusion (~5–7
In summary, we have shown that synthetic catalysts can
show enhanced assembly which is driven by the formation of
transition state during catalytic conversion. In KE catalysis,
uncharged reactant leading to anionic TS and thereby product
on the cationic nanocatalyst (GNR) surface leads to decrease in
surface potential which results in loss of dispersibility of the
colloidal system inducing the aggregation phenomenon. Nota-
bly, here neither substrate nor product plays any direct
interactive role towards aggregation. Additionally, we have
Figure 4. (a) Schematic representation of hybridization chamber (containing
eight units in one strip) containing PS-GNR solution with microscope setup.
The movement of the PS-GNR conjugate was observed under the optical
microscope at a resolution of 100× and scan rate of 10 frames/second. (b)
Trajectory of PS-GNR conjugate in absence and presence of NBI (100 μM)
over 10 sec in the XY plane observed under optical microscope and analyzed
using Tracker software. (b) Diffusion co-efficient of PS-GNR conjugate in
absence and presence of NBI (100 μM) and the catalyzed product, CNP
(100 μM) as obtained from the slope of the MSD curves using MSD=4DΔt.
CI=95% with 8 PS-GNR conjugates from 4 sets of experiment.
demonstrated
a catalytic phenomenon occurring at the
surfactant bilayer of the nanoparticle surface can lead to
cascading of other reactions, absolutely unrelated to the
original catalytic reaction (KE). We believe apart from unraveling
Chem. Eur. J. 2021, 27, 1–7
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