Self-propelled supramolecular nanomotors with temperature-
responsive speed regulation
Citation for published version (APA):
Tu, Y., Peng, F., Sui, X., Men, Y., White, P. B., van Hest, J. C. M., & Wilson, D. A. (2017). Self-propelled
supramolecular nanomotors with temperature-responsive speed regulation.
Nature Chemistry
,
9
(5), 480-486.
https://doi.org/10.1038/nchem.2674
DOI:
10.1038/nchem.2674
Document status and date:
Published: 09/05/2017
Document Version:
Accepted manuscript including changes made at the peer-review stage
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Download date: 26. Aug. 2022
Self-propelled Supramolecular Nanomotors with
Temperature-Responsive Speed Regulation
This work has been published in:
Tu, Y.; Peng, F.; Sui, X.; Men, Y.; White, P. B.; van Hest, J. C. M.; Wilson, D. A. Self-propelled
supramolecular nanomotors with temperature-responsive speed regulation. Nat. Chem. 2016,
doi:10.1038/nchem.2674
Abstract
Self-propelled catalytic micro- and nanomotors have been the subject of intense study over the
past few years, but it remains a continuing challenge to build in an effective speed-regulation
mechanism. Movement of these motors is generally fully dependent on the concentration of
accessible fuel, with propulsive movement only ceasing when the fuel consumption is complete.
In this chapter we report a demonstration of control over the movement of self-assembled
stomatocyte nanomotors via a molecularly built, stimulus-responsive regulatory mechanism. A
temperature-sensitive polymer brush is chemically grown onto the nanomotor, whereby the
opening of the stomatocytes is enlarged or narrowed on temperature change, which thus controls
the access of hydrogen peroxide fuel and, in turn, regulates movement. To the best of our
knowledge, this represents the first nanosized chemically driven motor for which motion can be
reversibly controlled by a thermally responsive valve/brake. We envision that such artificial
responsive nanosystems could have potential applications in controllable cargo transportation.
1 Introduction
Recent advances in artificial micro- and nanomotors
1-3
have brought their potential applications
in the biomedical sciences closer
4-9
. Starting from the first centimeter-scale motors
10
, micro- and
nano-tubular engines
11-14
, wires
15,16
, helices
17,18
, rods
19-21
, Janus motors
22-24
and self-assembled
polymeric motors
8,25-27
scientists used both top-down or bottom-up approaches to design motors
with high speeds and improved efficiency. These classes of motors can convert chemical fuel
(such as hydrogen peroxide
21,28-30
, hydrazine
31
, acid
32,33
, water
34
, glucose
27,35
and urea
24
) or
external energy such as magnetic fields
17,36,37
, ultrasound
19,38
, electricity
39,40
, light
41-43
or even
organisms
44
into mechanical motion
45
. Recently, new avenues to control the directionality of the
nanomotors by mimicking taxis behavior inspired by nature were shown. These types of systems
are however still based on external factors for the directional control of motion such as the
presence of a gradient
8
. One of the limitations of current micro- and nanomotor systems is
therefore still the limited control over their speed
46-49
. Some level of manipulation of the
movement of micron-sized motors was previously achieved either by disassembling the whole
micromotor under a thermal stimulus
47
or by chemically inhibiting the catalytic enzymatic
system
49
. The latter required sequential steps of inhibition and reactivation via addition of
chemicals followed by multiple washings, which is not very practical for biomedical applications.
Motor systems would be more versatile if equipped with a molecularly built stimuli-responsive
valve or brake
50
, thus controlling and regulating the motion under the stimuli without changing
the shape or assembly of the motor itself or by affecting its catalytic activity. Such property is
particularly desirable for applications in the biomedical field and nanorobotics.
In our previous work, we demonstrated the formation of self-assembled nanomotors, based on
bowl-shaped polymer vesicles, known as stomatocytes, in which catalytic platinum nanoparticles
were entrapped
25
. The narrow opening of the bowl shape structures serves as an outlet for the
oxygen generated during the catalytic decomposition of the hydrogen peroxide fuel. Hydrogen
peroxide is found naturally in the human body, especially in diseased areas such as tumor tissue
and sites of inflammation. According to the literature
51
, human tumor cell lines can produce
hydrogen peroxide at rates of up to 0.5 nmol/10
4
cells/h which is significant when related to the
size of the tumor. Therefore nanomotor systems running on low concentrations of hydrogen
peroxide with further ability to sense changes in the environment and regulate their speed and
behavior via a stimuli-responsive valve or brake would be very attractive for biomedical
applications.
In this chapter we demonstrate the first nanomotor system with complete control over its speed
by chemically attaching a stimulus-responsive valve system (polymer brush) to our engine that
allows control of the motion of the nanovesicles without changing the catalyst activity or shape
of the motor (Fig. 1). This doesn’t require the addition of chemicals into the system but instead
the nanomotor is able to probe the environment and change its behavior by sensing the change
in the outside temperature. Stimulus-responsive polymer brushes
52
made of surface-tethered
macromolecules are commonly known and have been widely applied in many areas, including
the biomedical field
53-55
. Changes in the external environment (e.g., temperature, pH, light or
redox states) can generally trigger a sharp and large response in the structure and properties of
these grafted polymer layers
56
. Various polymer brushes have been synthesized via the SI-ATRP
approach on different substrates using surface-attached initiators
57
, which allows accurate control
of the structure and properties of the polymer brushes.
By functionalizing the surface of the stomatocytes with a poly(N-isopropyl acrylamide)
(PNIPAM) polymer brush via SI-ATRP a temperature-responsive polymer layer is introduced.
Due to PNIPAM’s well-known LCST behavior
58
, increasing the temperature above its transition
temperature leads to the collapse of the brushes, producing a hydrophobic layer on top of the
small opening of the stomatocytes (less than 5 nm); this closes the aperture and prevents easy
access of the fuel (hydrogen peroxide) inside the nanomotor (Fig. 1c, d). Due to the lack of fuel,
the propelling movement of the motor will cease. The long molecularly built brushes function as
a reversible brake system onto the nanomotors by controlling and regulating the access of the
fuel inside the catalytic bowl shape structures with temperature. As the LCST behavior is
reversible, by adjusting the temperature, the collapse of the PNIPAM brushes can be switched on
and off, functioning thus as a regulatory mechanism to control the speed of the nanomotor (Fig.
1). This is in our view an elegant example of a brake system that doesn’t affect the catalytic
activity or the shape of the motor but only its motion. It is also the closest mimic of a brake as
found in automated cars from the macroscopic world.