Dual-Responsive Photocatalytic Polymer Nanogels

Article abstract of DOI:10.1002/anie.201903309

Selective activation of photocatalysts under constant light conditions has recently been targeted to produce multi-responsive systems. However, controlled activation, with easy recovery of the photocatalysts, induced by external stimuli remains a major challenge. Mimicking the responsiveness of biological systems to multiple triggers can offer a promising solution. Herein, we report dual-responsive polymer photocatalysts in the form of nanogels consisting of a cross-linked poly-N-isopropylacrylamide nanogel, copolymerised with a photocatalytically active monomer. The dual-responsive polymer nanogels undergo a stark decrease in diameter with increasing temperature, which shields the photocatalytic sites, decreasing the activity. Temperature-dependent photocatalytic formation of NAD+ in water demonstrates the ability to switch photocatalysis on and off. Moreover, the photocatalysed syntheses of several fine chemicals were carried out to demonstrate the utility of the designed material.

Full text of DOI:10.1002/anie.201903309

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Accepted Article
Title: Dual-Responsive Photocatalytic Polymer Nanogels
Authors: Calum T. J. Ferguson, Niklas Huber, Katharina Landfester,
and Kai A. I. Zhang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201903309
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Dual-Responsive Photocatalytic Polymer Nanogels
Calum T. J. Ferguson,[a] Niklas Huber,[a] Katharina Landfester*[a] and Kai A. I. Zhang*[a]
Abstract: Selective activation of photocatalysts under constant light
conditions has recently been targeted to produce multi-responsive
systems. However, controlled activation, with easy recovery of the
photocatalysts, induced by external stimuli remains a major challenge.
Mimicking the responsibilities of biological systems by multiple
triggers can offer a promising solution. Herein, we report dual-
responsive polymer photocatalysts in form of nanogels consisting of
a cross-linked poly-N-isopropylacrylamide nanogel, copolymerised
with a photocatalytically active monomer. Dual-responsive polymer
nanogels undergo a stark reduction in diameter with the increase in
temperature, above the lower critical solution temperature, shielding
photocatalytic sites retarding activity. Temperature-dependent
photocatalytic formation of the enzyme cofactor nicotinamide adenine
dinucleotide (NAD+) in water demonstrates the ability to switch on/off
photocatalysis. Moreover, photocatalysis of fine chemicals
stimulus could undergo a conformational change, allowing easy
extraction and recycling. A polymer nanogel system that can
expand and contract, transitioning between gel and solid phases,
could be utilised as a matrix for photocatalytic sites. This
expansion and contraction may lead to the controlled activation or
deactivation of the photocatalytic species. Moreover, the phase
transitioning of such a material may aid in recovery after synthesis.
To ensure control and efficient photocatalysis, it is important that
the photocatalytic species are evenly distributed within the
polymer matrix. If the photocatalytic species is concentrated at the
surface of the nanogel, it still may be even catalytically active in
the contracted state. Whereas, if the photocatalytic species are
concentrated within the core of the nanogel, they may be even
inaccessible in the swollen state.
Here, dual-responsive polymer photocatalysts have been
synthesised in form of nanogels, where photocatalysis can be
controlled by both light and temperature. These photoactive
polymer nanogels undergo a stark transition in size upon
elevation in temperature, as the polymer network is no longer
effectively wetted by the aqueous media and phase separates.
Phase separation of the polymer material at elevated temperature
facilitates efficient recovery from the reaction solution, allowing
efficient recyclability. Reversible transitions in size and solubility
lead to a switchable deactivation/activation of the photocatalytic
sites. This enables controllable photocatalysis, where reactions
can only proceed within a certain temperature range. Furthermore,
dual-responsive photocatalytic polymer nanogels can be utilised
to selectively stop/start catalysis, by simply controlling
temperature. At ambient temperature, polymer nanogels can be
used for the regeneration of the enzyme co-factor NAD+, as well
as dye degradation. Whereas, at elevated temperatures above
the lower critical solution temperature (LCST) of the nanogel
catalytic activity is retarded. Moreover, photocatalytic reactions
can be undertaken under visible light at room temperature for the
production of industrially relevant small molecules including
synthesis of benzaldehyde from styrene, oxidation of
phenylmethanamine to benzyl-1-phenylmethanimine and the
formation of disulfide bridges between octanethiol molecules.
(benzaldehyde,
benzyl-1-phenylmethanimine
and
1,2-
dioctyldisulfane) has been conducted in the nanogels swollen state,
demonstrating the utility of the designed material.
The employment of water-compatible heterogeneous polymer
photocatalysts under visible light irradiation is of high research
interest with applications in energy, environmental, and biological
systems. Namely, water splitting,[1] aqueous pollutant
remediation,[2] photodynamic therapy,[3] and chemical redox
reactions.[4] The main challenges within these photocatalytic
systems is maintaining efficient catalysis whilst producing a
material that can be easily recovered. To achieve this,
photocatalytic materials can be designed to transition between
soluble and insoluble states, in response to external stimuli.
Altering the materials solubility results in controlled activation or
deactivation of the catalyst by controlling access of reagents to
the catalytic sites.[5]
In biological systems, functions or activities can be enabled or
disabled by triggers, which lead to structural change in
conformation and therefore activity control. For artificial materials,
multiple stimuli have previously been reported that can be utilized
to modify the physical properties of classical polymers most
notably: change by light,[6] pH value,[7] and temperature.[8]
However, only a few examples of multi-responsive photoactive
polymers have been reported. In particular, integration of pH-
responsive moieties into the polymer structure was an efficient
method to gain control of the water-compatibility of the polymer
photocatalytic systems.[9] Recently, our group has reported a
CO2-triggered system, where enhanced wettability of
a
photocatalytic conjugated polymer can be triggered by the
addition of CO2 into the solvent.[10]
Ideally, the photocatalytic species would be easily accessible,
enabling efficient catalysis, but after the application of an external
[a]
Dr. C.T.J. Ferguson, N. Huber, Prof. K. Landfester, Prof. K. A. I.
Zhang
Max Planck Institute for Polymer Research, Ackermannweg 10,
55128 Mainz, Germany
E-mail: landfester@mpip-mainz.mpg.de, kai.zhang@mpip-
mainz.mpg.de
Supporting information for this article is given via a link at the end of
the document.
Scheme 1. Dual-responsive photocatalysts that can be selectively switched on
or off as a function of temperature. Inclusion of photocatalytic units into a
thermo-responsive nanogel matrix, which allows selective photocatalysis at low
temperature in the swollen state but not at elevated temperatures in the
contracted state.
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Dual-responsive thermo- and light-sensitive polymer
nanogels were designed so that upon application of temperature
photocatalysis could be quenched. The contraction of the
photocatalytic sites within the polymer gel network is
demonstrated in Scheme 1, where upon elevation in temperature
the gel network becomes insoluble and precipitates shielding the
photocatalytic units and retarding reagent partioning into the
network.[5] Furthermore, the condensation of the nanogel network
upon temperature elevation may limit light penetration into the gel
network. Reduction in temperature to ambient conditions results
in a rewetting of the gel network increasing the rate of transfer of
reagents from the continuous phase to the active regions. This
reversible ‘switchable’ behaviour of the photocatalytic nanogel
enables photocatalysis to be turned on or off depending on
temperature in a manor analogous to enzymatic functions within
the body.
nanogel network. The nanogel was synthesised in an aqueous
environment and evaluated by 1H-NMR spectroscopy (Figure S3).
To confirm that the catalytic behaviour is due to the photocatalytic
moieties and not residual Pd catalyst, inductively coupled plasma
optical emission spectrometry (ICP-OES) was undertaken, with
detected levels of Pd below 0.05 ppm (equivalent to nanogels
without the photocatalytic monomer).
The diameter of the temperature dependent polymer nanogel
was determined by light scattering (Figure 1b). Elevating the
temperature above 30 °C lead to a sharp reduction in the diameter
from around 190 to 70 nm, as expected. The data was fitted with
a sigmoidal decay to yield a LCST of 31.6 °C, slightly lower than
a pure PNIPAM system (LCST = 32 °C). Furthermore, the decay
seems to be stretched compared to a non-doped system. These
modifications to the nanogels behaviour may be due to the
inclusion of the photocatalytic sites within the gel network, which
do not contribute to the secondary bonding system.
To
demonstrate the reversible temperature dependent transition
from a single phase to two phase system images were taken after
heating and cooling the nanogels in a petri dish (Figure 1c). At
ambient temperature the nanogel is translucent and upon
elevation of the temperature the transmission significantly
decreases producing an opaque suspension. Cooling of the
system returns the translucent nature of the liquid as the nanogels
rehydrate, which in turn can be cycled back to opaque by raising
the temperature. The UV/Vis spectra of the dual-responsive
nanogels have broad absorption peak in the visible light region up
to 575 nm (Figure 1d).
Figure 1. Dual-responsive photocatalytic nanogels. (a) Molecular structure of
designed photocatalytic nanogels. (b) Nanogel diameter as a function of
temperature measured by DLS. (c) Variation in transmission of nanogel solution,
demonstrating reversibility. (d) Absorbance (black) and emission (red) spectra.
Dual-responsive photocatalytic polymer nanogels were
synthesised using
a
similar surfactant free emulsion
polymerisation route to literature.[11] The nanogel was designed to
encompass a hydrophilic gel network comprising of poly(N-
isopropylacrylamide) (PNIPAM) crosslinked with PEGDMA that is
copolymerised with
a
photocatalytic monomer N-(4-(7-
phenylbenzo[c][1,2,5]thiadiazol-4-yl)phenyl)acrylamide
(PhBTPhAM) (Figure 1a). The photocatalytic monomer was
specifically designed to encompass photocatalytic donor (phenyl)
and acceptor (benzothiadiazole) units similar to species
previously reported and with an acrylamide vinyl functional group,
to enable polymerisation (Figure. S1 & 2). The acrylamide
functional group was selected to ensure that the reactivity rate of
the photocatalytic monomer was in the same order of magnitude
as NIPAM. Equating the reactivity of both monomers results in a
statistical distribution of photocatalytic sites throughout the
Figure 2. Temperature dependent regeneration of enzyme cofactor NAD+ by
dual-responsive photocatalytic nanogels, conversion determined by 1H-NMR
spectroscopy. (a) Regeneration of NAD+ at 25 °C but not at 40 °C. (b)
Controllable on/off conversion of NADH by periodic fluctuations in temperature.
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The reversible activation/deactivation of photocatalytic sites
as a function of temperature was investigated by monitoring dye
degradation under visible light at constant ambient or elevated
temperature (Figure. S4). In a typical experiment, the dual-
responsive nanogel was dispersed in aqueous Rhodamine B
solution, samples were irradiated under white light at 25 or 40 °C.
At 25 °C, linear degradation of the dye molecule by the dual-
responsive photocatalytic polymer nanogel is observed with over
90% of the dye degraded after 4 h. Conversely, at an elevated
temperature of 40 °C, where the gel network is contracted, after
4 h irradiation less than 20% of the dye molecules have been
degraded. Dye degradation at 40 °C can be attributed to the
normal dye degradation caused by light irradiation.
monitoring NADH conversion over multiple additions, where no
loss in activity was observed after 4 cycles (Figure S7).
The versatility of dual-responsive nanogels was further
examined to catalyse a number of transformation reactions in an
aqueous environment. Photo-oxidation of styrene, benzylamine
and octanethiol was investigated. The photo-oxidation of styrene
yields valuable carbonyl compounds, such as benzaldehyde, by
carbon-carbon double bond cleavage. Typically, this reaction is
undertaken with molecular and transition metal based
photocatalysts, but recently metal-free photocatalysis has been
conducted using conjugated mesoporous polymers.[4a] However,
this metal-free catalysis could not be conducted in a pure aqueous
phase and required the addition of acetonitrile. Photocatalysis of
styrene using dual-responsive photocatalytic nanogels at ambient
temperature under visible light for 20 h led to conversion over 99%.
Furthermore, an 84% selectivity for the production of
benzaldehyde was achieved (Table 1, Figure S8 & S9).
Conversely, when the experiment was repeated at 40 oC no
conversion of styrene was observed (Figure S10). Using the dual
responsive photocatalyst at ambient temperature yielded a higher
conversion than previously reported for TiO2 catalysed reactions
whilst maintaining a similar level of selectivity.[14] Similar levels of
conversion have been previously reported for heterogeneous
organo-photocatalysts but the reactions must be conducted in
acetonitrile.[15] Moreover, the photocatalytic content used in both
systems was significantly increased compared to the system
presented. Indeed, this further demonstrates the enhanced
efficiency of the photocatalytic units in the nanogel network.
The regeneration of the enzyme co-factor nicotinamide
adenine dinucleotide (NAD) was utilised to further investigate the
temperature dependent photocatalytic properties of the nanogel
material produced. Within living cells NAD can be formed in either
its oxidised (NAD+) or reduced state (NADH). Dehydrogenases
within the cell remove electrons from their substrates and in turn
reduce NAD+ to NADH.[12] The reduced form of the cofactor can
then be utilised in further enzymatic reactions by reductases,
oxidising the NADH to NAD+. The temperature dependent
oxidation of NADH was investigated at both 25 and 40 °C, where
NADH (25 mM) was combined with the nanogel (1 mg/ml nanogel,
50 µg/ml photocatalyst) in an aqueous solution. This ratio of
photocatalytic units to reagents is significantly lower than
previously reported demonstrating the efficacy of the nanogel
matrix for photocatalysis. Oxidation of NADH was monitored, in
1
triplicate, by H-NMR spectroscopy at both temperatures for 8 h
Table 1. Photocatalytic conversion using dual-responsive polymer nanogels
at room temperature in aqueous media. All reactions undertaken under blue
light irradiation ( = 460 nm).
(Figure S5). At ambient temperature, linear conversion of NADH
as a function of time is observed, with over 70% of the NADH
converted after
8 h (Figure 2a, red). Here, the NADH
concentration converted was much higher than previously
reported using a much lower concentration of photocatalyst.[13] At
elevated temperatures the oxidation reaction is severely limited
with only around 5% of the enzyme cofactor oxidised (Figure 2a,
blue). The stark differences in the conversion of NADH
demonstrates the efficient deactivation of the photocatalytic sites
at elevated temperatures, where the nanogels have phase
separated and contracted.
Conversion > 99 % , selectivity 84 %[a]
Additionally, the reversibility of the activation and deactivation
of the photocatalytic sites was investigated by monitoring
conversion of NADH under a periodically cycling temperature
regime (Figure 2b). Initially, under ambient conditions, the gel is
swollen and the NADH is converted. No further conversion of
NADH is observed in the subsequent period of elevated
temperature. Upon reduction in the temperature, the conversion
of NADH is resumed. This process of cycling activation and
deactivation of the photocatalytic sites demonstrates fine control
of photocatalysis using dual-responsive nanogels. Control
scavenger experiments confirmed that 1O2, •OH, h+ and •O2
species are generated under light illumination at ambient
temperature and are involved in NADH oxidation (Figure S6).
From the control experiment, it can be observed that scavenging
of singlet oxygen from the system has the biggest impact on
NADH oxidation, resulting in less than 10% conversion over 20 h.
The recyclability of the photocatalyst was investigated by
Conversion > 87 %, selectivity > 99 % [b]
Conversion >88 %, selectivity 52 % [c]
Reaction conditions: [a] Styrene (50 mM), nanogel (0.05 mg/ml nanogel, 2.5
µg/ml photocatalytic units), 10 ml H2O, RT, 20 h, conversion determined by
GC-MS. [b] Benzylamine (50 mM), nanogel (0.5 mg/ml nanogel, 25 µg/ml
photocatalytic units), 20 ml H2O, RT, 20 h. conversion determined by GC-
MS. [c] 1-Octanethiol (100 mM), nanogel (5 mg/ml nanogel, 0.25 mg/ml
photocatalytic units), 1 ml H2O, RT, 40 h, conversion determined by GC-MS.
Secondary product formed was dioctylsulfane.
The selective oxidation of amines to imines was undertaken
using the dual responsive photocatalyst at room temperature in
water. Previously, this reaction has been undertaken using both
transition metal complexes and TiO2, with varying levels of
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conversion and selectivity. After 20 h using the dual responsive at
ambient temperature photocatalyst over 87% of the benzyl amine
was converted to the imine (Table 1, Figure S11 & S12).
Whereas, less than 15 % of the imine was produced when the
experiment was repeated at elevated temperature (Figure S13).
Similar to the styrene oxidation reaction at ambient temperature,
this reaction proceeded with a much higher concentration ratio of
reagents to photocatalyst than previously reported.
Furthermore, reactions can be rapidly quenched by changes in
temperature and recovery of the photocatalyst can be undertaken.
This new material and synthetic route produces highly effective
polymeric photocatalytic nanogels in water with greater efficiency
and tunability.
Acknowledgements
To further demonstrate the flexibility of the dual responsive
photocatalytic material produced a biologically relevant oxidative
reaction was undertaken. The formation of disulfide bridges within
cells is one of the most common post translational modifications
of peptides that is undertaken. To mimic this the disulfide bridges
N.H. acknowledges the Kekulé fellowship of Fonds der
chemischen Industrie (FCI) and the Gutenberg-Akademie of the
Johannes Gutenberg University of Mainz. This work is part of the
research conducted by the MaxSynBio consortium that is jointly
funded by the Federal Ministry of Education and Research of
Germany (BMBF) and the Max Planck Society (MPG).
were formed between
a model compound, 1-octanethiol.
Previously, a similar reaction has been photocatalysed using
Mn(CO)5Br; however, the disulfide bridge formation was
undertaken in organic media (cyclohexane or benzene).[16] Here,
in a pure aqueous solution 1-octanethiol is converted to 1,2-
dioctyldisulfane and dioctylsulfane by the dual-responsive
photocatalytic nanogels at room temperature. Furthermore, this is
the first example of disulfide bridge formation utilising a pure
organic photocatalytic system, with over 88% of the thiol groups
converted after 20 h (Table 1, Figure S14 & S15). Conversely, at
elevated temperature only 10 % of the thiol was converted
(Figure S16).
Keywords: dual-responsive • polymer photocatalysts •
heterogeneous photocatalysis • temperature dependent • NADH
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Entry for the Table of Contents
COMMUNICATION
Stimuli-induced polymer for
Dr. C.T.J. Ferguson, N. Huber,
Prof. K. Landfester, Prof. K. A. I.
Zhang*
photocatalytic reactions: Inclusion
of photocatalytic sites in a temperature
responsive polymer nanogel network
created a dual-responsive polymer
photocatalyst that can be utilised to
selectively catalyse numerous
Page No. – Page No.
Dual-Responsive
Photocatalytic Polymer
Nanogels
reactions at specific temperatures.
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