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Cite This: Chem. Rev. 2020, 120, 269−287
Nanoscale Inorganic Motors Driven by Light: Principles,
Realizations, and Opportunities
Hana Š ípova-́ Jungova,́ * Daniel Andreń , Steven Jones, and Mikael Käll*
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Department of Physics, Chalmers University of Technology, S-412 96 Göteborg, Sweden
ABSTRACT: The prospect of self-propelled artificial machines small enough to navigate
within biological matter has fascinated and inspired researchers and the public alike since
the dawn of nanotechnology. Despite many obstacles toward the realization of such
devices, impressive progress on the development of its basic building block, the
nanomotor, has been made over the past decade. Here, we review this emerging area with
a focus on inorganic nanomotors driven or activated by light. We outline the distinct
challenges and opportunities that differentiate nanomotors from micromotors based on a
discussion of how stochastic forces influence the active motion of small particles. We
introduce the relevant light−matter interactions and discuss how these can be utilized to
classify nanomotors into three broad classes: nanomotors driven by optical momentum
transfer, photothermal heating, and photocatalysis, respectively. On the basis of this
classification, we then summarize and discuss the diverse body of nanomotor literature.
We finally give a brief outlook on future challenges and possibilities in this rapidly
evolving research area.
CONTENTS
1. Introduction
2. Light−Matter Interactions at the Nanoscale
2.1. Momentum Transfer
2.2. Photothermal Heating
2.3. Photocatalytic Reactions
3. Stochastic Forces Affecting Nanomotors
3.1. Brownian Motion
3.2. Hot Brownian motion
3.3. Active Motion of Brownian Particles
4. Examples of Light-Driven Nanomotors Classified
According to the Physical Driving Mechanism
4.1. Nanomotors Driven by Optical Forces and
Torques
4.1.1. Principle of Operation
4.1.2. Examples of Nanomotors Driven by
Optical Forces
4.2. Nanomotors Driven by Photothermal
Effects
4.2.1. Principle of Operation
4.2.2. Examples of Light-Driven Thermophoresis
4.3. Nanomotors Driven by Photocatalytic
Effects
4.3.1. Principle of Operation
4.3.2. Examples of Light-Driven Self-Electrophoresis
4.3.3. Examples of Light-Driven Diffusiophoresis
4.3.4. Examples of Light-Driven Bubble Propulsion
5. Conclusions and Outlook
© 2019 American Chemical Society
Author Information
Corresponding Authors
ORCID
Notes
Biographies
Acknowledgments
References
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1. INTRODUCTION
The prospect of man-made machines, far too small to be
visible to the naked eye but able to perform various functional
tasks, is often attributed to Dr. Richard Feynman in his famous
“There’s Plenty of Room at the Bottom” lecture from 1959.1
That Feynman’s idea is in principle achievable is proven by
Nature, which provides many examples of highly efficient
molecular nanomotors able to carry out functions essential to
cell reproduction and survival.2 With Eric Drexler’s inspirational but controversial book “Engines of Creation: The
Coming Era of Nanotechnology”, published in 1986,3 the
concept of artificial “nanobots” became a hot topic in science
and popular culture, and it has remained so ever since. A recent
example is Ray Kurzweil’s prophesy that nanobots will be
streaming through our blood already in the 2030s. Artificial
nanomotors, possible building blocks and predecessors of
much more advanced nanoscopic robots, are currently being
realized in laboratories across the world.
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Special Issue: Molecular Motors
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Received: June 24, 2019
Published: December 23, 2019
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the object’s size, shape, and material-specific dielectric
response, as well as by the refractive index of the surrounding
medium. For deeply subwavelength objects, i.e., nano-objects,
the magnitude of the dipole polarizability scales linearly with
its volume.
Photons incident on a nanoparticle can be either absorbed
or scattered out in any direction. These processes are
quantified by two separate cross sections (i.e., the effective
areas over which the nanoparticle interacts with light), which
scale differently with the nanoparticle size.21 The absorption
cross section (σabs) of a subwavelength particle scales linearly
with its volume, because σabs ∝ Im(α⃗ ). A 10 nm object will
thus absorb of the order a 1000 times less light than a 100 nm
object. Meanwhile, the scattering cross section (σscat) scales
quadratically with volume because σscat ∝ |α⃗ |2, implying a
million times less scattered light for a 10 nm object as
compared to a 100 nm object. Absorption is therefore the
dominant process for very small particles, while scattering
becomes more pronounced as the particle size approaches the
illumination wavelength.22
One way to counter the small size effect and significantly
boost the optical cross sections for nanoscale objects is to
utilize optical resonances, the most well-known being the
localized surface plasmon resonances (LSPRs) that occur in
noble metal nanoparticles.23,24 In a plasmonic resonance,
electromagnetic radiation drives coherent oscillations of the
conduction electrons, and this results in significant amplification of the light−matter interaction. Plasmonic nanoparticles
therefore act as optical antennas that concentrate electromagnetic radiation to highly confined subwavelength dimensions. At the peak of the LSPR, the optical cross sections of the
nanoparticle can in fact be increased by up to an order of
magnitude compared to its physical size. Other (non-LSPR)
resonance phenomena are also possible. In particular, recent
research efforts in the nanophotonics community have
demonstrated that nanoparticles made from dielectric materials
with high refractive index, such as silicon or germanium, can
support Mie (or geometrical) scattering resonances that can be
just as pronounced as the LSPR’s but with significantly
reduced absorption.25,26
As indicated in Figure 1, the three most important
nanomotor driving mechanisms used to date are (1) photon
momentum transfer/optical forces, (2) photothermal heating,
and (3) photocatalysis. In the following, we use these three
categories to classify different types of inorganic light-driven
nanomotors accordingly. However, there is a further important
distinction to be made between the three categories, namely
that photon momentum transfer induces real external forces
able to move a particle against friction while propulsion based
on photothermal heating and photocatalysis utilizes phoretic
effects, like thermophoresis or diffusiophoresis, that work
because of friction. Specifically, phoretic motion is due to local
gradients in the solution surrounding the particle that result in
hydrodynamic flows that propel the particle forward. As a
thought experiment, we can imagine what would happen if the
nanoparticle was placed in a vacuum: For the case of external
optical forces the particle would accelerate unimpeded, while
for phoretic effects no motion would occur because it is
specifically the interaction with the surrounding medium that
results in phoretic motion.
In the following subsections we briefly delineate the
fundamental light−matter interaction mechanisms that can
be utilized to drive nanomotors.
Development of artificial nanomotors is to a large extent
motivated by the possibilities of transformative future
applications in areas like drug delivery,4,5 cancer treatment,6
nanolithography,7 and environmental remediation,8 to name a
few. However, it is probably fair to say that many researchers in
the field are driven by pure curiosity. To quote Feynman:
“What are the possibilities of small but movable machines?
They may or may not be useful, but they surely would be fun
to make.” The first autonomous inorganic nanomotor (370 nm
in diameter but 2 μm in length) was constructed of platinum
and gold segments that could catalytically react with diluted
hydrogen peroxide in water to induce self-propelled motion.9
Since then, the development of nano- and micromotors has
gained momentum and a range of driving mechanisms has
been investigated, including optical,10 acoustic,11,12 and
magnetic13 sources of energy and motion control.
In this review, we endeavor to summarize the current state
of research in a subfield of nanomotor research, that is,
nanomotors made primarily from inorganic materials and
driven or activated by light. Light is a particularly attractive
source for autonomous movement of nanomotors, as it is a
clean energy source that provides facile remote control of the
nanomotor movement. Near-infrared (NIR) light is especially
appealing because it is able to penetrate biological tissue with
minimal absorption.14 However, when the size of the motor is
on the order of or smaller than the wavelength, as is the case
with nanomotors, its interaction with light decreases
significantly. Moreover, thermal fluctuations, resulting in
random Brownian motion, become increasingly pronounced
as the motor size decreases toward nanometric dimensions.
These effects pose significant challenges to the development of
functional nanomotors driven by light.
Several comprehensive reviews of microscopic light-driven
motors have been published recently.10,15−20 However, most of
these do not focus on the significant challenges and
opportunities that appear at the nanoscale and that distinguish
nanomotors from micromotors. In this review, we aim to
specifically cover the current state of research in light-driven
and light-activated inorganic nanomotors. We thus focus on
phenomena that are particularly important at the nanoscale
such as enhanced stochastic forces and resonant light−matter
interactions. We review case studies of inorganic motors with
at least two submicrometer dimensions, and we categorize
these nanomotors based on the physical mechanism that is
utilized to generate directed motion. In the concluding section,
we discuss potential applications of light-driven nanomotors
and provide an outlook on possible future developments.
2. LIGHT−MATTER INTERACTIONS AT THE
NANOSCALE
Recent advances in fundamental research and applications have
allowed researchers to control light−matter interactions at the
nanoscale with rapidly increasing precision. This research field,
often called nanophotonics or nano-optics, is a subject of high
current interest and importance because it enables a wide
range of applications, including the propulsion of nanomotors.
The optical response of a nanoscale object can, in a first
approximation, be described as that of an induced electric
point dipole. The oscillating electric field E of the incident
electromagnetic light induces a dipole moment p = α⃗ E in the
nanoobject, with an amplitude, phase, and direction governed
by the object’s complex and wavelength-dependent polarizability tensor α⃗ . The polarizability is in turn determined by
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2.3. Photocatalytic Reactions
Nanoparticles composed of or containing photocatalysts can
convert energy from light and chemical fuel to movement. For
instance, in the case of a semiconducting particle illuminated at
photon energies higher than the bandgap, absorbed light
pumps electrons to the conduction band and creates holes in
the valence band (Figure 1, bottom right). If these photoexcited charge carriers are sufficiently long-lived to be able to
diffuse to the surface of the particle, they can participate in
oxidation/reduction of surrounding molecules, which then act
as a chemical fuel. If the particle is asymmetric, for example, in
terms of composition or shape, this process will in turn create
an asymmetric distribution of reaction products, such as ions,
molecules, or gas, which can propel the particle through selfelectrophoresis, diffusiophoresis, or bubble recoil.
Figure 1. Optical driving mechanisms. Light carries energy and
momentum (top left) that can be transferred to a nanoparticle via
absorption and scattering, thereby causing the particle to move and
function as a nanomotor. The three main driving mechanisms are
linear and angular momentum transfer (top right), photothermal
heating (bottom left), and photocatalytic processes (bottom right).
3. STOCHASTIC FORCES AFFECTING NANOMOTORS
Small particles in solution are strongly influenced by random
collisions with molecules in the surrounding medium, and the
resulting Brownian motion will be much more pronounced for
nanoparticles than for microparticles because of the former
smaller size. Here we outline some of the fundamental
concepts of Brownian motion with specific emphasis on how
it affects the driven motion of nanomotors.
2.1. Momentum Transfer
Despite having no mass, a photon has mechanical properties
and can exert small forces on an object either via scattering or
absorption (top right panel, Figure 1). In the small particle
limit, transfer of linear momentum from a plane wave results in
a force on the particle approximately proportional to (σabs +
σscat)I, where I is the light intensity. This so-called radiation
pressure force points in the light propagation direction. If the
light is focused, an additional force, F ∝ Re[α]∇I, proportional
to the intensity gradient, appears. This force for example allows
particles to be trapped in the high intensity region of a focused
laser beam (given that the real part of the particle polarizability
is positive), an effect that is utilized in optical tweezers.27
Moreover, circularly polarized light carries angular momentum28 that can drive a particle to act as a rotary nanomotor.
This effect comes from the torque, τ ∝ p × E, exerted on the
induced particle dipole from the incident light field. More
exotic effects, such as “nonconservative” optical forces,29
optical binding between particles,30 and optical torques due
to transfer of orbital angular momentum,31 exist for certain
types of light fields or particle constellations. Momentum
transfer from light is a weak effect, and thus high light
intensities (on the order of at least 1 mW/μm2) are therefore
typically required to cause a nanoparticle to move and act as a
motor in a liquid environment based on this type of lightmatter interaction.
3.1. Brownian Motion
A particle immersed in a fluid is constantly undergoing random
positional and orientational fluctuations due to thermal
agitation from neighboring fluid molecules. Even in the
absence of any macroscopically applied force, the position of
the particle will change over time, causing it to explore the
surrounding space. Such a trajectory will appear essentially
chaotic in nature and as such is colloquially termed the
“random walk”. While this process occurs for all particles, it
becomes significant only for particle sizes of several micrometers or less, in which case it is known as Brownian motion.
A particle in a homogeneous medium will explore the available
space in a Gaussian manner, such that the probability
distribution of the particle position follows:
ρ(xi , t ) =
2
1
ji x zy
expjjj− i zzz
j 4Dt z
4πDt
k
{
where xi (i = x,y,z) is the displacement from the initial position
(at time t = 0) and D is the particle diffusivity. The standard
deviation of the probability distribution grows according to
σPDF = 2Dt along each axis. Particles with higher diffusivity
will therefore randomly explore the available space faster.
As shown by Einstein in 1905,38 D is proportional to the
thermal energy within the system and inversely proportional to
the drag that the particle experiences while moving through the
fluid. Thus, D = kBT/γ, where kB is Boltzmann’s constant, T is
the absolute temperature, and γ is the drag coefficient, which
depends on the shape and size of the particle as well as on the
viscous properties of the surrounding fluid. In the case of a
spherical particle with radius R, Stokes’ law gives that γ = 6πηR,
where η is the viscosity of the fluid. Thus,
2.2. Photothermal Heating
Light energy that is absorbed by a nanoparticle is typically
rapidly converted into heat through electron−phonon
coupling. This leads to a temperature increase of the particle
relative to its environment, ΔT ∝ σabsI, as well as dissipation of
this heat into the surroundings (bottom left panel, Figure 1).
Absorptive heating of plasmonic particles can result in a
temperature increase of hundreds of degrees for moderate light
intensities32 and has successfully been used for applications like
photothermal cancer therapy33 and photothermal microscopy.34,35 If an asymmetry in the temperature distribution
around the particle surface exists, this can be used to drive the
particle to move via thermophoresis.36,37
σPDF ∝ D ∝ R−1 , which implies that the position
probability distribution function for a 100 nm diameter
particle will grow 10 times faster with time than for a 10 μm
diameter particle. This observation indicates a major challenge
of creating motors on the nanoscale: due to their small size,
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Figure 2. Characteristic translational motion of active Brownian particles. (a) For a Brownian particle undergoing active motion, four distinct time
regimes can be distinguished. For very short times, the inertia of the particle will dominate the trajectory as well as hydrodynamic memory effects.
For a longer time scale, the particle is in the overdamped regime, where inertial effects are negligible. In this regime, the particle will exhibit
trajectories characteristic for Brownian, active, or effective Brownian motion. The time scales for these regimes depend on the magnitude of the
internal driving force and the rotational diffusion coefficient. Shown here is the mean squared displacement (MSD) of a 200 nm polystyrene sphere
in water at 300 K, with an internal driving force of 10 pN. (b) Because of the decreased rotational diffusion of larger particles, the duration of the
“active motion” regime increases for larger particle diameters. (c) Similarly, as the particle aspect ratio is increased, the rotation diffusion decreases
leading to later onset of t2. Shown here are MSD of three prolate spheroids with the equivalent volume of a 200 nm diameter sphere but with
different aspect ratios. The internal driving force is fixed along the long axis of the spheroid.
“active motion,” and it is common to other swimming entities
such as bacteria44 and certain algae.45 The dynamics of an
active Brownian particle can be decomposed into random
Brownian motion and directed self-propelled motion. In the
same way that the position of a Brownian particle fluctuates
over time, so too does the orientation. This implies that the
direction of propulsion can also fluctuate and lead to
interesting non-Gaussian dynamics in the evolution of the
position probability distribution.
As an exemplary model, consider a spherical particle that
propels itself along a direction fixed with respect to its own
frame of reference (see ref 46 for the full theoretical
description employed here, including the extension to
constrained motion and ellipsoidal particles). This model
particle undergoes unconstrained motion in three dimensions
and can rotationally diffuse about any axis. A useful way to
characterize the stochastic motion of such a particle is through
the mean squared displacement, i.e., MSD(t) = ⟨(x(t) −
x(0))2⟩. The MSD of this active Brownian particle exhibits
four distinct time scales as illustrated in Figure 2a.
On very short time scales, the particle exhibits ballistic
motion where the current velocity of the particle depends on
the velocity a moment prior. In this short time scale regime,
interesting hydrodynamical memory effects can also be
present. Although this time scale is traditionally very difficult
to experimentally observe, some novel experimental methods
have arisen to help illuminate this regime in recent years.47
However, due to the high viscosity of liquids, the “memory”
will quickly be lost. In general, the effects at this time scale are
their motion will be heavily influenced by the thermal
agitations from the surrounding medium, which means that
they will be more difficult to steer and control compared to
micromotors.
3.2. Hot Brownian motion
The description above assumes that the particle is in thermal
equilibrium with its surroundings. However, if the particle is
significantly heated by light, strong thermal and viscous
gradients in the particle’s surroundings will appear. Because
the time scale for heat conduction is significantly faster than
the characteristic diffusion time of the particle, the induced
gradients will move together with the particle. It turns out that
the dynamics of such a particle can be well described as
diffusive behavior in a homogeneous medium of effective
temperature and viscosity. This effect is known as “hot
Brownian motion” (HBM). The effective temperature and
viscosity (giving rise to an effective diffusion coefficient) is
derived and discussed for translational motion in refs 39 and
40 and for rotational motion in ref 41. Because rotational and
translational motion sample the environment differently, the
temperatures associated with translational (THBM,t) and
rotational (THBM,r) Brownian motions will be different. They
relate to each other and the nanoparticle surface temperature
(Tsurf) and ambient temperature (T0) according to T0 ≤ THBM,t
≤ THBM,r ≤ Tsurf.42,43
3.3. Active Motion of Brownian Particles
Nanomotors have a source of momentum, and therefore their
motion displays properties that differ strongly from passive
motion of Brownian particles. This type of behavior is termed
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The same concepts used above to discuss a freely rotating
active particle46 can also be extended to include cases where
particle motion is constrained by other factors such as the
shape of the confining area or external potentials. In any case, it
is the rate at which the active particle changes its orientation
(i.e., direction of internal force) that determines how efficiently
it explores the available space.
This also hints at the difficulty in decreasing the size of an
active motor down to the nanoscale. Because the overall size of
the particle has such a significant impact on the rotational
diffusivity, a nanoparticle will leave the active motion regime
more quickly than a microparticle. This effect is highlighted in
Figure 3 where the effective enhancement of diffusion over
long time scales for active nanospheres is explored for a driving
force such that the driven particle velocity is maintained
constant regardless of particle size. Here we can clearly see that
for larger particles this driving velocity results in significantly
enhanced long-term diffusivities while for smaller nanoparticles
the effect is essentially negligible.
In terms of the ability of a motor to explore its surroundings
with enhanced efficacy, the challenge with nanomotors is thus
not only about how to induce an effective force but also about
how to ensure that the active motion is not terminated too
soon by particle rotation.
Figure 3. Challenges of nanomotors at the nanoscale. Long-term
effective diffusion constant for an active spherical particle normalized
to the corresponding undriven diffusion constants versus particle
radius. All particles here have a self-driving force such that their driven
velocity is maintained constant at the value indicated by the legend.
often insignificant for the purposes of analyzing the behavior of
active particles.
After a time t0 ≈ m/γ (where m is the mass of the particle),
the particle enters the overdamped regime where inertial
effects are inconsequential. Within this second regime, the
particle initially exhibits primarily Brownian motion. After a
time t1, which depends on the magnitude of the driving force,
the particle trajectory is instead dominated by active motion,
where the internal driving force dictates the displacement of
the particle.46 Finally, due to rotational fluctuations of the
particle, the active motion will begin to explore all orientations,
eventually exhibiting Gaussian behavior again as t → ∞, albeit
with an increased “effective” diffusivity compared to a similar
passive particle. The time scale at which this final regime
emerges is dictated only by the rotational diffusivity of the
particle, i.e. t2 = Dr−1.
The key parameter that dictates how an active particle
explores the surrounding space is how quickly its orientation
changes. As shown in Figure 2b,c, this parameter can be
significantly influenced by both the size and aspect ratio of the
particle. Increasing either parameter tends to decrease the
rotational diffusion coefficient, thereby extending the active
motion regime and increasing the long-term effective translational diffusion coefficient.
4. EXAMPLES OF LIGHT-DRIVEN NANOMOTORS
CLASSIFIED ACCORDING TO THE PHYSICAL
DRIVING MECHANISM
As mentioned above, we classify light-driven inorganic
nanomotors into the three classes indicated in Figure 1. This
is motivated by the fact that there tends to be one dominant/
desired driving mechanism in most cases, even though more
than one physical effect may in principle contribute to the
propulsion of any given nanomotor. In the following, we
summarize and discuss the literature according to this
classification.
4.1. Nanomotors Driven by Optical Forces and Torques
Since 1986, when Arthur Ashkin first observed trapped
particles with a focused laser beam,48 light-induced gradient
and radiation pressure forces have been widely used to trap
and transport micro/nanoparticles using optical tweez-
Table 1. Summary of Light-Driven Nanomotors Driven by Momentum Transfer
material
shape
dimensionsa
type of movement
Au
Au
vaterite
Au
Ag
Ag
Au
Ag
round
rod
sphere
sphere
wire
wire
rod
sphere
D, 400 nm
L, 100 nm
D, 900 nm
D, 100−250 nm
D, 80 nm; L, 10 μm
D, 100 nm; L, 5 μm
L, 100 nm
D, 150 nm
Ag/Au
sphere
D, 150/200 nm
Au
gammadion
D, 100 nm
rotational
rotational
rotational
orbital (Rl = 2 μm)
orbital (Rl = 6.5 μm)
rotational
rotational
orbital/quasi-1D
(Rl = 9 μm)
orbital/quasi-1D
(Rl = 7 μm)
rotational
Au/PS
disk/sphere
D, 300/200 nm
orbital
light source
light power
rate/velocity
ref
SAM
SAM
SAM
OAM
OAM
SAM
SAM
OAM
driving momentum
NIR (830 nm)
Vis-NIR
Vis (532 nm)
Vis-NIR
NIR (800 nm)
NIR (830 nm)
NIR (1064 nm)
NIR (800 nm)
50 mWb
10−30 mWb
3188 mWb
<100 mWc
100 mWc
100 mWb
62 mWb
50 mWc
3 kHz
<42 kHz
4.9 kHz
86 Hz
1 Hz
1 Hz
5 kHz
150 μm/s
88
89−91
75
31,93
95
82
101
102,188
asymmetric
scattering
asymmetric
scattering
SAM
NIR (790 nm)
100 mWc
15 μm/s
105
NIR (810 nm)
1 mWb
0.3 Hz
107
NIR (974 nm)
2.5 mWb
6 Hz
108
L, length; D, diameter. bFocused approximately to the diffraction limit. Light power of 1 mW corresponds to an intensity ∼1 mW/μm2. cFocused
P
to an optical vortex beam. The intensity I relates to the light power (P) as I ≈ 2πλR , where λ is the wavelength of light and Rl is the radius of the
a
l
bright ring.
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ers.27,48−53 As a result, a wide range of interesting applications
has been explored within areas including thermodynamics,42,54−57 nanofabrication,58 the study of biological systems,59,60 and enhanced chemical reactions,61 to name a few.
We note that optical tweezers can be used to mechanically
move a trapped nanoparticle to a desired position via stage
manipulation (shifting the surrounding) or by shifting of the
focal position (translating the object). However, because this
simply involves translating a macroscopic movement of the
excitation laser beam to movement of the nanoparticle, it does
not qualify as a nanomotor. Hence, these types of studies are
excluded from this review. Below, we describe rotary
nanomotors driven by transfer of optical angular momentum
(a summary is given in Table 1).
4.1.1. Principle of Operation. Apart from the linear
momentum that is the basis for optical tweezing, a light beam
can carry angular momentum via its intrinsic polarization state
and/or via its spatial phase and intensity distribution (Figure
4). The two types of light angular momentum are referred to as
plasmonic nanostructures are beneficial to enhance optical
torques as well. Hence, the vast majority of the recent reports
on rotary nanomotors involves plasmonic structures, typically
made of gold or silver, including nanowires,82,83 nanorod
aggregates,84 as well as single nanoparticles.42,82,85
4.1.2. Examples of Nanomotors Driven by Optical
Forces. The optical gradient force induced by a single focused
laser beam is often insufficient to confine a plasmonic particle
in the direction of the optical axis because the particle
experiences strong radiation pressure from the laser beam.
However, the radiation pressure can be counteracted by
Coulomb repulsion from a cover glass surface to form a stable
trapping location about a particle-diameter away from the
surface while allowing the particle to rotate freely.86 In this
two-dimensional (2D) trapping geometry, colloidal gold
nanoparticles can be driven to rotate with extremely high
rotation frequencies,87 several kHz for nanospheres,88 and
several tens of kHz for nanorods,89 creating a high-speed rotary
nanomotor system. In 2015, it was shown that single plasmonic
nanorods could be rotated at frequencies above 40 kHz (2 500
000 rpm), which is likely to be the fastest rotation of any manmade or natural motor in aqueous solution recorded to date89
(Figure 5a).
Both the Brownian dynamics and the LSPR properties of the
trapped nanorods are highly sensitive to the surrounding
environment, which means that they can be used as tiny probes
and localized sensors. Reported experiments to date includes
measurements of local temperature,90 viscosity, viscoelasticity,
molecular attachment,91 and nonequilibrium themodynamics.92 Note that for these types of nanomotors the goal is
typically not to effectively explore a larger geometric space but
instead to utilize the local rotary motion either as a sensor or
actuator in a highly localized environment.
Interestingly, experimental results indicated that the temperature of the nanorod can reach more than 250 °C while the
surrounding water remains in a superheated liquid state.90 At
these temperatures, reshaping of the gold nanorods is possible
even at relatively low laser powers. 90 However, the
experimental parameters can also be chosen to keep photothermal heating low enough to be compatible with sensitive
biological systems.89 As an example of this, gold nanorod
motors were employed to probe photothermally induced
release of DNA from functionalized nanomotors and detailed
information on layer thicknesses and conformational changes
could be extracted.91
In a similar 2D trapping configuration, particles have been
confined and rotated using beams carrying orbital angular
momentum (i.e., optical vortex beams). Dienerowitz et al.93
utilized the repulsive nature of the gradient force on the bluewavelength side of the plasmon resonance of gold nanospheres94 to confine them in the low-intensity center of an
Laguerre−Gaussian beam with a donut shaped intensity
profile. Rotation is in this case induced by transfer of OAM
to the particle, while the temperature of the object is claimed
to remain low because the particle is confined to the dark
region of the beam. For applications that are not sensitive to
elevated temperatures, one can instead confine a particle to the
bright ring of the beam in order to reach significantly higher
speed of movement. This was demonstrated by Lehmuskero et
al., who showed that gold nanoparticles can be driven to orbit
around the optical axis at rotation frequencies up to ∼86 Hz.31
Similarly, nanowires can be driven to rotate in vortex beams95
Figure 4. Optically induced torques for driving nanomotors. Light
carrying either spin angular momentum (SAM) or orbital angular
momentum (OAM) can be used to drive the motion of a
nanoparticle. Left: A beam of circularly polarized light can transfer
SAM to a particle via light absorption or polarized scattering, causing
the particle to rotate around its own axis. Right: A beam of structured
light that carries OAM can cause a particle to orbit around the optical
axis.
spin angular momentum (SAM)28 and orbital angular
momentum (OAM).62 Both can be transferred to an optically
trapped nanoparticle, causing it to rotate around its own axis or
around the optical axis of the beam of light, respectively. The
particle thus actively rotates without any changes to the
external potential. Hence, a nanoparticle subject to lightinduced optical torque constitutes a light-driven rotary
nanomotor.
The first studies combining optical tweezers and optical
rotation involved microparticles driven by SAM63,64 and/or
OAM.65−67 There are also several reports on engineered
micromachines that convert linear momentum to angular
momentum through anisotropic scattering.68−70 These, and a
range of follow-up studies on optically driven rotation, were
thus concerned with microscopic objects.71−79 Works attempting to reduce the size of the trapped objects to reach the
nanoscale involved rotation of optically trapped dielectric
particles, such as glass nanowires,80,81 and quite recently
birefringent nanospheres, which were rotated up to almost 5
kHz by SAM transfer under special conditions.77 However,
during studies of plasmon-enhanced optical forces, it was
recognized that the advantageous optical properties of
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Figure 5. Techniques for nanomotor control using light-induced torques and forces. (a) Transfer of spin angular momentum from circularly
polarized light in a 2D trapping configuration. A spherical nanomotor can only be driven by absorption optical torque, while asymmetric
nanomotors are also driven by scattering torque (left), resulting in higher rotation speeds of asymmetric nanomotors, as measured by
autocorrelation spectroscopy (right). Adapted with permission from refs 87,88. Copyright 2018 John Wiley and Sons and 2015 American Chemical
Society, respectively. (b) Illustration of transfer of OAM from a trapping beam to a ∼10 μm silver nanowire. Inset: Dark-field images of the
nanowire. Reproduced with permission from ref 95. Copyright 2013 American Chemical Society. (c) Rotation of a gold nanorod in a counter
propagating beam geometry. A focused laser beam is divided into two foci by an anisotropic crystal. The second focus is reflected onto the first one
by a gold mirror. Reproduced with permission from ref 101. Copyright 2018 John Wiley and Sons. (d) Rotation of gold nanoparticles in a nodal
ring-trap, formed by an OAM beam interfering with its reflection from a gold mirror. Reproduced with pemission from ref 102. Copyright 2017
American Physical Society. (e) SEM image of a silica disk including a plasmonic motor that scatters linearly polarized light anisotropically and
hence induces a torque. Inset: Magnified top view of the nanomotor. Reproduced with permission from ref 107. Copyright 2010 Springer Nature.
(f) Surface bound plasmonic pillars that enhance the electrical near-field and create a trapping potential. Left: calculation of the y-component of the
Poynting vector around a nanopillar for left circularly polarized illumination. Right: Centroid tracking of a nanosphere trapped and rotated around
the nanopillar. Reproduced with permission from ref 108. Copyright 2011 Springer Nature.
(Figure 5b), albeit not at as high rotation frequencies due to
their increased Stokes drag.
A trapping configuration based on two equal counterpropagating beams cancels out radiation pressure and therefore
allows a wide range of particles to be stably trapped even in
three dimensions, enabling a more versatile probe.48,96−99
Although an established method, few studies have attempted to
apply torques in such trap geometries. One such attempt was
reported by Xu et al.,100 who showed that silver nanowires
(∼600 nm wide and ∼6.5 μm in length) could be optically
aligned between two optical fibers pointing toward each other.
However, this method does not allow continuous rotation of
the trapped object. A different approach was demonstrated by
Karpinski et al.,101 using a single focused laser beam and a
birefringent crystal. The crystal generates a primary and a
secondary focus displaced vertically from each other by a few
micrometers (Figure 5c). The secondary focus is reflected by a
mirror on the primary one, effectively resulting in a counterpropagating beam trap within a single-beam tweezers
configuration. Rotation of plasmonic nanorods at frequencies
up to several kHz due to SAM transfer was demonstrated in
this setup.101
A conceptually quite different nanomotor platform has been
investigated by the Scherer group.102 A so-called optical ring
trap, in which the phase front of light is engineered to create an
annulus several μm in diameter, was used to confine objects
and direct their motion around the ring. In the pioneering
work by Figliozzi et al.,102 spherical gold nanoparticles and
polystyrene beads could be induced to travel around the ring
trap driven by transfer of OAM (Figure 5d). They also
observed optical binding between adjacent particles and
managed to characterize periodic modulation of interparticle
separation. In follow-up studies with optically bound quasi-1D
nanomotors, the particle dynamics was used to model
molecular reactions, where the two-step process of the
nanoparticle passing in the optical trap reproduced the
statistics of biomolecular exchange reactions103 and to
elucidate the driving and binding strength in relation to the
phase properties of the trapping laser light.104 Furthermore, in
a recent paper,105 the ring trap platform helped verify the
theoretically predicted106 in-plane translational motion induced by a light field carrying no such momentum component.
By introducing differently sized nanoparticle into the ring trap,
Yifat et al. managed to demonstrate directed motion through
directional scattering with the small particle trailing.105
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By exploiting exotically shaped metal nanostructures, OAM
can be imparted to scattered light without the incoming beam
carrying it. The rotation of the nanostructure is then driven by
conservation of angular momentum. Liu et al. constructed
gammadion-shaped gold nanostructures on silica microdisks107
(Figure 5e) and made them rotate continuously using linearly
polarized light. Even though the rotation frequency is low due
to the large viscous drag of the microdisk, the optical torque
stemming from the nanostructure is impressively high and
sufficient to rotate the large structure. Higher rotation
frequencies are likely to be obtained by reducing the physical
size of the hybrid structure.
Alternatively, surface-bound plasmonic structures can be
used to generate fields that allow trapping and transfer of
angular momentum to confined nanomotors. One such
approach was demonstrated by Wang et al.,108 where a
template-stripped gold nanodisk forms a near-field enhancement that can confine and drive rotary nanomotors below the
diffraction limit of far-field radiation (Figure 5f). Their
geometry also efficiently removes heat, as the plasmonic
structure is coupled to a heat sink.
An additional system of potential interest as a nanomotor is
that of optically bound matter.109,110 Theoretical studies
indicate an interesting and intricate rotary dynamics111 and
even something as counterintuitive as “negative optical
torque”.112 When certain conditions are met, related to the
symmetry of the bound objects and their interaction with the
incoming light field, the resulting torque on the optical matter
system turns negative and rotation opposite to the handedness
of the incident light is possible both for simple dimers113 and
even particle arrays.114
Another more exotic conceivable light-induced nanomotor
driving mechanism under investigation is extraordinary lateral
forces on particles near interfaces. Theoretically it has been
demonstrated that under specific circumstances nanoparticles
can experience forces perpendicular to the propagation of the
exciting light. Both linearly polarized light on chiral nanoparticles115 and circularly polarized light on nonchiral
particles116 can in principle induce such lateral forces.
Conceivably, this could yield a platform for nonlocalized
directed motion of nanomotors. To date the effect has
experimentally been demonstrated as a detectable force on a
nanocantilever in an AFM system;117 however, the force
appears small and may not suffice for nanomotor propulsion.
A recent theoretical study indicates that it is also possible to
drive rotary Janus particle nanomotors in linearly polarized
plane waves by exploiting the topology of the phase space of
the light−particle interaction.118 Practical implications of this
finding need to be explored in further studies. However, it
could potentially serve as a link between rotary nanomotors
and the translational nanomotors discussed below.
Figure 6. Asymmetric light absorption by a mobile particle results in
thermophoretic drift. When light is locally absorbed on one side of a
micro/nanoparticle, a local temperature gradient is formed across the
particle surface. This temperature gradient results in a corresponding
osmotic pressure gradient (inset) which induces fluid flow at the
particle solvent interface. For a mobile particle under homogeneous
illumination, this implies that the particle will thermophoretically drift
along the rotational symmetry axis (typically away from the hot/
absorptive end).
nanomotors are therefore often constructed as Janus particles
made of a low-absorbing material with a partially absorptive
coating119,120 or as a composite particle composed of two
nanoobjects with different absorptive properties.121,122
A uniform illumination of the Janus particle will create an
asymmetric heat source density, thereby generating a temperature gradient across its surface that leads to self-induced
thermophoretic motion.123 On the nanoscale, the efficiency of
converting optical power into thermophoretically driven
movement increases with the reciprocal size of the motor.124
However, for very small nanomotors, the actual geometrical
parameters of the metal cap become important and may cause
a reduction in the efficiency.124
The thermophoretic motion of these particles physically
arises due to the shell of counterions that naturally exists in the
proximity of the surface of the nanomotor to counteract its
surface charge. When a temperature gradient is formed along
the interface between the solvent and the nanomotor, an
osmotic pressure gradient parallel to the interface and
antiparallel to the temperature gradient is created (Figure
6).125
This counterion concentration gradient results in a thermoosmotic fluid flow from high- to low-osmotic pressure regions
along the interface.126 As the fluid is stationary within the
laboratory frame of reference, this implies that the particle has
to move along the temperature gradient opposite to the
interfacial fluid flow. This phenomenon was shown to affect
the distribution of analytes near photothermally heated
plasmonic structures,127 and it has also been proposed as a
novel method for particle manipulation and confinement.128,129
While thermo-osmotic flows tend to cause thermophobic
behavior of colloids, a high salt content of the solvent can
significantly alter the colloid dynamics via thermoelectric
effects. Specifically, positive and negative ions can respond
differently to an imposed temperature gradient, thereby
creating an electrostatic thermopotential that can cause
4.2. Nanomotors Driven by Photothermal Effects
Over the past decade, numerous types of artificial swimmers
driven by self-thermophoretic, self-diffusiophoretic, or selfelectrophoretic surface flows have been investigated. These
self-propelled particles are driven by an osmotic pressure
difference across the particles surface caused by symmetry
breaking realized by an asymmetric material composition.
4.2.1. Principle of Operation. In the case of thermophoretic nanomotors, the idea is to create a nanoscopic object that
can convert light to heat locally on part of its surface, thereby
generating a temperature gradient (Figure 6). Thermophoretic
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Table 2. Summary of Thermophoretic Nanomotors Driven by Light
base/absorbing material
dimensions
[nm]
light source
polystyrene sphere/Au half-shell
500−1250
Vis (532 nm)
mesoporous silica sphere/Au
half-shell
bis-pyrene nanoaggregate/Au star
50, 80, 120
Al2O3 nanowire/Au sphere
D, 80; L, 500a
NIR
(808 nm)
NIR
(808 nm)
Vis (532 nm)
a
227
light intensity
20−160
kW/cm2b
3−70 W/cm2
max local temp increase
[K]
application
3−25
≈ 300d
7−40 mWc
14−56 mW
velocity
[μm/s]
5−50
1−2
3, (480 with bubble)
ref
120
cancer cell
ablation
cancer cell
ablation
drug delivery
139,141
121
122
b
D, diameter; L, length. ms pulsed excitation. cExcitation intensity for the experiments that corresponded to the particle velocities in the table
above was not specified. dTemperature increase at 3 W/cm2.
Figure 7. Light-driven nanomotors based on self-thermophoresis. (a) Schematic picture of adaptive steering process based on “photon nudging”.
The particle is heated and propelled only if it is oriented within a certain acceptance angle toward a defined target location. Reproduced from ref
120. Copyright 2014 American Chemical Society. (b) Schematic of positioning and injection of functionalized plasmonic Janus nanoparticles
(JNPs) into cells. JNPs are positioned on a cell membrane using an off-resonant NIR focused laser beam (left) and then injected with a green laser
that is close to the particle plasmon resonance (right). Reproduced with permission from ref 122. Copyright 2018 American Chemical Society. (c)
Application of gold-coated mesoporous silica Janus nanoparticles to cancer treatment. Top: Schematic illustration of the functionalization with a
macrophage cell membrane (MPCM). Bottom left: Cancer cells treated with the nanomotors after 150 s of laser illumination (dead cells are
colored in red). Bottom right: Therapeutic efficacy. Reproduced with permission from ref 141. Copyright 2018 John Wiley and Sons. (d)
Schematic illustration of fabrication and principle of operation of nanomotors composed of gold nanostars and bispyrene (BP) aggregates coated
with pegylated silica shells (SP) for cancer treatment under NIR laser irradiation. Reproduced with permission from ref 121. Copyright 2016 John
Wiley and Sons.
thermally induced electrophoresis.130 By varying the type and
concentration of the dissolved salt, the thermophoretic
diffusion coefficient can be varied by several orders of
magnitude and even change between thermophobic and
thermophilic behavior.131 This thermoelectric variant of
thermophoresis has been demonstrated as a versatile method
of optical manipulation.132
Self-propulsion of laser-heated Janus microparticles due to
thermophoresis has been studied intensely as a strategy to
control their movement.133,134 An early realization of microscale thermophoretic motors was created with polystyrene
beads half-coated in gold.37 An alternative approach for
nanomotor fabrication was described by Herms et al.,135 who
proposed gold spheres with attached DNA origami. In this
proposed nanomotor design, the gold nanoparticles would
absorb laser light and convert it into heat, generating a
symmetric temperature gradient around the nanoparticle, while
the DNA origami breaks the symmetry of the system and a slip
flow is induced due to the temperature gradient along its
surface.
Self-propelled Janus particles have shown potentials in
cancer therapy as smart drug delivery systems136,137 and in
stretching DNA molecules.138 Self-induced thermophoretic
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Janus particle through. The Al2O3 shaft served as a carrier for
genetic material, which was thus delivered to the cell. The
extended length of the nanowire, as well as the low thermal
conductivity of Al2O3, ensured that the genetic cargo was
thermally insulated from the heated gold nanoparticle.
Yang et al. described Janus particles composed of gold
nanostars and bispyrene aggregates and coated with PEGylated
silica shell (Figure 7d).121 The diameter of the whole structure
was approximately 230 nm. The Janus nanomotors were more
effective than pure gold nanostars in probing their surrounding
space and in increasing the temperature of solution. They were
also more successful in tumor removal, although the origin of
this effect requires further investigation.
forces can also act as an additional handle on optically trapped
particles to perturb the particle position within the trap.122
4.2.2. Examples of Light-Driven Thermophoresis. A
summary of light-driven nanomotors propelled by thermophoretic effects is given in Table 2. One of the first realizations of
thermophoretically driven motors smaller than one micrometer
was demonstrated by Bregulla et al.,120 who applied a
technique of “photon nudging”134 to propel micro- and
nanomotors (Figure 7a). Photon nudging implies tracking
the orientation of a Janus micro/nanoparticle in real time to
switch the propulsion on or off depending on its orientation
toward a target location. In their work,120 polystyrene spheres
with diameters from 0.5 to 1.25 μm were capped with a 50 nm
thick hemisphere of gold to act as the absorptive medium. The
particles were confined in 2D but allowed to freely rotate. A
micro/nanomotor was transported to a desired location by
illuminating the particle only when the gold cap was directly
opposite the target location. In this study, as well as in a
subsequent work,124 it was observed that the nanoparticle’s
thermophoretic drift velocity under laser illumination was
independent of particle size over a wide range of sizes;
however, the authors note that as the particle radius
approaches the thickness of the capping layer, the efficiency
of the nanomotor decreases significantly due to a reduction in
the temperature gradient across the particle.
Xuan et al. have employed Janus mesoporous silica
nanoparticle motors with diameters of 50, 80, and 120 nm
that were half-coated with a 10 nm Au layer.139 The gold
hemispherical cap was heated with a fs-pulsed near-infrared
laser to create a self-thermophoretic force. The nanomotors
were operated in phosphate buffer solution (PBS), fetal bovine
serum (FBS), as well as water. The images and accompanying
videos of this work show highly coherent trajectories for
different nanomotors, which is unexpected for randomly
oriented active particles. This may indicate some external
force contributing to the motion or orientation of the
nanoparticles. The temperatures claimed in this work should
also be taken with a degree of caution.
Mesoporous silica nanomotors have potential as drug
delivery systems due to their high loading capacity.140 Xuan
et al. employed 70 nm mesoporous silica Janus nanoparticles to
perforate cell membranes and inject molecular cargo.141 The
gold hemisphere was functionalized with a passivating
methoxy-PEG layer, while the mesoporous silica hemisphere
was modified with macrophage cell membrane to specifically
recognize and adhere to cancer cells (Figure 7c). The active
motion of the nanomotor increased the number of nanomotors
adhered to cancer cells compared to passively diffusing
particles. Heating of the nanomotors by laser illumination
was then used to open pores in the cell membrane to achieve
molecular delivery and, eventually, cell death.
Balancing of optical and thermophoretic forces in optical
tweezers in order to inject single Janus nanoparticles into living
cell was described in a recent work by Maier et al.122 The Janus
particles were composed of a gold nanoparticle (∼80 nm
diameter) attached to a dielectric shaft of Al2O3 (∼500 nm in
length). The nanowire was aligned with the optical axis of the
trapping laser, which allowed the opposing thermophoretic
force to directly control the axial position of the nanowire
within the trap and position the Janus nanowire just above a
specific cell (Figure 7b). A second laser that overlapped with
the plasmonic resonance of the gold nanoparticle was used to
heat it and create a hole in the cell membrane to push the
4.3. Nanomotors Driven by Photocatalytic Effects
Photocatalysis involves the facilitation of a chemical reaction
by light. Although light is typically absorbed in the process, it is
the chemical reaction that is the dominant energy source
utilized for propulsion of photocatalytic nanomotors. The light
intensities required to drive photocatalytic nanomotors are
thus usually much lower than for the two preceding nanomotor
classes. On the other hand, operation requires the presence of
a chemical “fuel” that will be consumed in the catalytic process.
4.3.1. Principle of Operation. Light-driven photocatalytic
motors convert light and chemical energy obtained from redox
reactions on the surface of a semiconducting particle into
movement. If the photon energy is larger than the semiconductor band gap, absorption excites electrons to the
conduction band and hence generates holes in the valence
band. The energetic holes and electrons can migrate to the
particle interface with an electrolyte and participate in redox
reactions with surrounding reactants.142 This process can result
in propulsion through one or more of three processes, namely
self-electrophoresis, self-diffusiophoresis, and bubble propulsion. Light-induced self-electrophoresis is caused by an
asymmetric distribution of ions around the asymmetric
nanomotor, resulting in an electric field that drives its motion
in a fashion similar to electrophoresis (Figure 8). Similarly,
Figure 8. Principle of operation of light-driven self-electrophoretic
nanomotors. Light absorption generates electron−hole pairs in the
semiconductor part of a composite nanoparticle. The photoinduced
charges migrate to the particle surface, where they participate in redox
reactions of available fuel. This leads to an asymmetric distribution of
ions around the nanoparticle and creation of an electric field that
drives its motion. The direction of the motion will depend on the
total electrostatic charge of the nanomotor.
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279
170
∼4.5
collective
wire
cap
cube
peanut
Au, Fe2O3
TiO2/Au
α-Fe2O3
α-Fe2O3
D, 600 nm; L,
1.5 μm
Janus sphere
TiO2, Pt
a
D, diameter; L, length; PEG-b-PS, poly(ethylene glycol)-block-poly(styrene); SE, self-electrophoresis; SD, self-diffusiophoresis. b“Light-assisted” nanomotors. Illumination causes aggregation of the
nanomotors and hence decrease of motion speed from 33.6 μm/s to 3.5 μm/s. cEstimate based on the reported increase in diffusivity and our calculations shown in Figure 2.
magnetic steering
magnetic steering, cargo
transport
SD
1% H2O2
146
159
168,169
magnetic steering
SE
SE
SD
2.5% H2O2
H2O
0.1−3% H2O2
3.3
0.1
30
∼10c
Vis (>380 nm)
Vis
UV, Vis (360−410 nm,
430−490 nm)
Vis (430−490 nm)
144
1
collective,
linear
D, 300 nm; L, 3 μm axial
175 nm
linear
500 nm
collective
wire
Si
700 nm
stomatocyte
D, 800 nm; L,
10.3 μm
UV (368 nm)
12
21
0.5% Q/QH
H2O
SE
0.1
Vis-NIR (>450 nm)
38
0.5% H2O2
SE
degradation of pollutants
147
160
cancer cell ablation
SE
collective,
linear
linear,
rotational
NIR (808 nm)
3.5
b
PEG-b-PS, Pt
Ag, Pt
a
wire
D, 400 nm; L,
2−3 μm
150 nm
linear
UV−Vis (250−800 nm)
40
2
0.03% H2O2
SE
113 μM I2
light intensity
(W/cm2)
velocity
(μm/s)
light source
type of
movement
dimensionsa
shape
material
Table 3. Summary of Self-Electrophoretic and Self-Diffusiophoretic Nanomotors Driven or Assisted by Light
fuel
mechanisma
application
ref
thrust by self-diffusiophoretic propulsion results from gradients
of redox reaction products around asymmetric Janus-type
nanoparticles. Bubble-propelled nanomotors are instead driven
by recoil from asymmetrically distributed dislodged bubbles
created by photochemical reactions. We summarize lightdriven nanomotors propelled by light-induced self-electrophoresis and self-diffusiophoresis in Table 3.
The efficiency of photocatalytic propulsion depends on the
materials involved in the process, available surface area, fuel
etc. Because of its high photocatalytic activity, TiO2 is the most
common photoactive material utilized in light-driven photocatalytic micro- and nanomotors.143−145 TiO2 is also a low cost
and biocompatible material. However, new photoactive
materials have recently been employed to improve the
efficiency of photocatalytic nanomotors, such as iron oxide146
and silicon.147 The three crystalline forms of TiO2 (rutile,
anatase, and brookite) have distinct photocatalytic properties148 that can be further improved by changing the synthesis
parameters and postfabrication treatment.149 Other ways to
increase the photocatalytic efficiency of TiO2 include coating
with metallic layers and/or doping with metallic (Au, Ag, Pt
etc.)148 or nonmetallic clusters.150 Combining photocatalytic
materials with large band gaps, like TiO2, with a plasmonic
material can greatly enhance the catalytic process due to
plasmon-induced photocatalytic effects.151,152
H2O2 has been by far the most frequently used fuel in
photocatalytic nanomotors due to its high efficiency. Other
fuels, such as iodine or water, can also be used to propel
nanomotors by the same mechanism, although with significantly reduced efficiency.147 Because of the ionic screening of
the induced electric field, nanomotors based on self-electrophoresis and diffusiophoresis suffer from reduced propulsion
speed in media with high electrolyte concentration, such as
biological media. Theoretical153 as well as experimental works
have shown a reciprocal dependence between migration speed
and solution conductivity.147
4.3.2. Examples of Light-Driven Self-Electrophoresis.
Zhou et al. reported synthesis and properties of rod-shaped
gold/iron oxide nanowire motors powered by visible light in
dilute H2O2 (Figure 9a).146 The nanomotor speed could be
modulated through the light intensity but an external magnetic
field, acting on the iron, had to be used to control the direction
of motion. Compared to typical bimetallic motors, which show
a high background rate of catalytic decomposition of H2O2, the
gold/iron oxide motors have relatively high fuel efficiency.
An alternative route toward decreasing fuel consumption
was shown by Wong and Sen, who used light to increase the
longevity and speed of self-electrophoretic nanomotors.154 A
bimetallic Ag−Pt motor (400 nm in width and 2−3 μm in
length) was propelled by the reversible reaction of silver with
iodine and simultaneous reduction of I2 at the platinum tail
(Figure 9b). Light assists the movement of the nanomotor by
decomposing AgI back to elemental silver (Ag) and I2, while
increasing the speed and longevity of nanomotors at low iodine
concentrations. The calculated power-conversion efficiency of
these Ag−Pt nanomotors was on the order of 10−5, which is 4
orders of magnitude better than self-electrophoretically
propelled Au−Pt/H2O2 motors.155
Tang and co-workers investigated Pt decorated core−shell Si
nanowires with overall lengths ∼10.3 μm but submicrometer
widths of ∼800 nm.147 These wires self-propel under low
intensity (≈3 mW/cm−2) visible or near-infrared illumination
using 1,4-benzoquinone/hydroquinone (Q/QH2) or H2O2 as
154
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Figure 9. Light-driven nanomotors based on self-electrophoresis (a) Left: Illustration of the propulsion mechanism of gold/iron oxide nanomotors
activated by visible light in hydrogen peroxide solutions. Right: Change in trajectory of the nanomotor during light switching. Reproduced with
permission from ref 146. Copyright 2017 Royal Society of Chemistry. (b) Ag−Pt nanowires that utilize light to increase the nanomotor speed and
lifetime. Reproduced with permission from ref 154. Copyright 2016 American Chemical Society. (c) Top: schematic diagram a nanomotor
composed of n+-Si shell (green), p-Si core (red), and platinum nanoparticles (yellow). The p-Si is only exposed at one end of the nanowire.
Bottom: Different motion trajectories induced by different nanowire end surface morphology. Reproduced with permission from ref 147. Copyright
2017 John Wiley and Sons. (d) Schematic of catalytic TiO2−Au Janus micromotors powered by UV light in water. Inset shows tracking lines
illustrating the distances traveled by three micromotors over 1 s. Reproduced with permission from ref 157. Copyright 2015 American Chemical
Society. (e) Schematic of Au/TiO2 nanocap motors propelled by visible light in water. Photocatalysis is enhanced by plasmon resonance in gold.
Reproduced with permission from ref 159. Copyright 2015 John Wiley and Sons. (f) Top: Schematic diagram of the propulsion mechanism of the
water-fueled TiO2/Pt motor. Bottom: the aggregation and separation of the motors through light-switchable electrostatic interactions. Reproduced
with permission from ref 144. Copyright 2016 RSC Publishing.
particle solution. Instead, the propulsion is interpreted as due
to a UV-induced charge imbalance between the two sides of
the Janus construct: photoexcited electrons generated in the
TiO2 transfer to the Pt layer, driving water reduction, while the
holes engage in water oxidation on the exposed TiO2 surface.
Slow reduction of water on the Pt side and accumulation of
electrons therein gives the particle a total negative charge. The
nanomotor then moves against the proton gradient, i.e., toward
the TiO2 side. Electrostatic attraction between the oppositely
charged hemispheres of the motors resulted in formation of
aggregates under continuous UV-light illumination, but
separation occurred when illumination was turned off. The
photoinduced redox process can also be utilized to degrade
organic molecules, as demonstrated for the case of Rhodamine
B acting as a model pollutant.144
In recent years, light-driven self-electrophoretic nanomotors
based on TiO2−Au constructs have also been explored. These
systems are claimed to be more efficient than Au−Pt Janus
motors, and they can generate a self-induced electric field in
pure water. Dong et. al reported TiO2 microspheres (∼1 μm in
diameter) coated with 40 nm-thick gold film157 that were
propelled in water at relatively low light intensities of UV light
(Figure 9d). Further improvement in active motion was
fuel, where, as expected, the latter was found to be more
effective. Interestingly, distinct migration trajectories were
observed depending on the shape of the nanowire tip (Figure
9c), and the speed of movement varied with illumination
wavelength similar to the absorption spectrum. This effect is
due to geometrical optical resonances within the wires, which
boost the light−matter interaction for certain wavelengths. The
same group also reported on intricate Janus nanotree
structures, in which TiO2 nanowire “brushes” are grown on a
p-type silicon nanowire trunk decorated with Pt.143 Although
these objects are clearly in the micrometer-regime, the study is
interesting as it demonstrates phototaxis, i.e., the microswimmers can self-align and move toward or away from the
light source depending on the nanotree’s zeta potential,
allowing optical steering of the swimmer. Moreover, a followup study showed that loading the motor with light-sensitive
dyes can change its spectral response and allow independent
propulsion and navigation of groups of micromotors loaded
with different dyes.156
Light-controlled propulsion, aggregation, and separation of
water-fueled TiO2/Pt Janus nanomotors (∼800 nm in
diameter) was reported by Mou et al.144 (Figure 9f). In this
case, there is thus no specific chemical fuel added to the
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achieved by Sridhar et al.,158 who reported that hollow
mesoporous TiO2/Au Janus particles (∼1 μm in diameter)
move 2−5 times faster than uniform TiO2/Au microparticles
under the same illumination intensity. Both works showed that
sandwiching a ferromagnetic material (Ni157 or Co158)
between the TiO2 sphere and the Au layer can provide a
means to control the direction of motion using a magnetic
field. Truly nanoscale motors composed of Au/TiO2 nanocaps
(∼175 nm in diameter) and propelled under broad-band
visible illumination were studied by Wang et al. (Figure 9e).159
The diffusivity of the nanocaps increased approximately 1.7
times under focused 100 mW illumination due to selfelectrophoresis enhanced by the plasmon resonance in the
gold layer.
Choi et al. reported light-assisted nanomotors160 based on a
previously published concept that utilized Pt particles
entrapped selectively within submicrometer artificial polymer
“stomatocytes”.161,162 The Pt catalyzes decomposition of H2O2
and the resulting O2 diffuse out through the narrow opening of
the bowl-shaped structure, resulting in propulsion. Choi et al.
used “stomatocytes” based on PEG copolymers encapsulating a
NIR absorbing dye (napthtalocyanine) and Pt. The nanomotors were found to aggregate under near-IR illumination
and separate in the dark. Similar to the original work by Wilson
and co-workers,163 the nanomotors tended to move toward
H2O2 released from cancer cells and to trigger photothermal
ablation of the cancer cells under NIR light illumination, thus
indicating potential use of artificial nanomotors in medicine.
4.3.3. Examples of Light-Driven Diffusiophoresis. If a
colloidal particle secretes ions due to redox surface reactions,
neighboring particles can respond to the induced concentration gradient and move through diffusiophoresis. This effect
can be utilized to construct “active matter”, i.e., coherent
collective motion in which the particles may align and form
swarms or other complex patterns.164 AgCl decompose in
water under UV illumination, and this process can be used to
create light-driven diffusiophoretic motors.165,166 Directional
movement necessitates asymmetric photodecomposition of the
AgCl due to either particle surface heterogeneity, nonuniform
light exposure, or the addition of inert and positively charged
particles that electrostatically attach to the AgCl particles.167
“Schooling” behavior from nonequilibrium driving forces can
be achieved by a suspension of photoactivated colloidal
particles of other materials as well. For example, Palacci et
al. have reported autonomous motion and pattern formation of
hematite nanoparticles driven by conversion of H2O2 by UV
and blue light.168,169 Peanut-shaped hematic particles (∼600
nm wide and 1.5 μm in length) were shown to be able to dock
a microparticle, transport it to desired location, and then
release it. The cargo attachment and transport was enabled by
a concentration gradient of H2O2 around the hematite particle,
induced by its photocatalytic activity under blue light.170 A
weak uniform magnetic field (or nanoscale tracks on a
substrate, alternatively) was used to steer the motor to the
desired location.
Phototactic microscale diffusiophoretic motors were realized
based on isotropic particles that consisted of TiO2171 or carbon
nitride decorated with platinum nanoparticles.172 While the
motion directionality of those motors is always along the
incident light direction, it is not interfered by their rotational
Brownian diffusion. However, this principle is yet to be
implemented in nanoscale motors.
Recently, several diffusiophoretic motors were reported in
proof-of-principle demonstrations of environmental remediation.173−175 Micrometer-sized Janus motors based on colloidal
carbon WO3 nanoparticle composite spheres were reported to
achieve a speed of 10 μm/s in water under UV illumination.
WO3 catalyzed photodecomposition of Rhodamine B and
sodium-2,6-dichloroindophenol (DCIP). 174 Mallick and
Roy173 used TiO2 nanoparticles and chemically decorated
them with Au clusters to increase their photoconversion
efficiency and shift the band gap of TiO2 to visible
wavelengths. Methylene blue and benzyl bromide were
photodecomposed as model pollutants. The nanomotors
achieved speed of 10 μm/s in water, but surprisingly, light
with lower energy than the modified TiO2 band gap was used,
so there might be additional mechanisms contributing to the
propulsion of the nanomotors in this case.
4.3.4. Examples of Light-Driven Bubble Propulsion.
Photochemical reactions on particles can result in tiny gas
bubbles forming on the particle surface. If such a bubble is
expelled from the surface in a certain direction, conservation of
linear momentum requires that the particle recoils and moves
in the opposite direction.176 The most widely used mechanism
for light-induced bubble formation utilizes photocatalytic
H2O2 decomposition over TiO2.177 As the gaseous reaction
products from this process in general dissolve into the
surrounding medium rather than form bubbles, nanomotors
with confined spaces, such as tubular structures,145 are needed.
Correlation of the geometrical factors with bubble size and
ejection frequency with propulsion velocity was investigated by
Wang et al.,178 who also showed that higher concentrations of
H2O2 is needed to activate platinum nanotubes with
nanometer-sized openings compared to microtubes.
Enachi et al. reported a UV-light-powered motor based on
arrays of TiO2 nanotubes (50−120 nm inner diameter) that
show directed motion both in presence and absence of
H2O2.145 Under UV-illumination, the arrays move due to both
diffusiophoresis and bubble propulsion, the latter being much
more efficient. The authors also argue that formation and
migration of nanobubbles inside the nanotubes creates an
inner liquid flow that can result in attachment of a model
object (an SiO2 microsphere in this case) to the nanotube
inlet. This object can then be transported to different locations
and released by switching off the UV light.
An alternative approach for nanobubble formation is to
create water vapor bubbles by photothermal heating of
absorbing particles. This has been demonstrated for plasmonic
nanoparticles using both continuous179,180 and pulsed181
illumination with wavelength tuned to the particle LSPR
absorption peak. Recently, femtosecond light pulses were used
to generate nanobubbles181 on gold nanoparticles (∼80 nm in
diameter), causing them to ballistically translate at the
picosecond scale. The diffusion constant, measured through
direct imaging in an electron microscope, followed a power-law
dependence on the laser fluence and a linear increase with the
laser repetition rate, respectively.
5. CONCLUSIONS AND OUTLOOK
Nanomotors present an intriguing concept for performing
tasks on the nanoscale. Even though we are far from creating
artificial “nanobots” that fulfill the prophecies encountered in
popular culture, several important steps toward this goal have
been made. Autonomous micromotors have been explored
over the last 15 years, and reduction of the size to below 1 μm
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or reach their target, for example, cancer cells located within
biological tissue or the interiors of artificial micro- and
nanochannels. In nanomedicine, the motor size is particularly
important, as it plays a key role in the circulation time and
biodistribution of nanoparticles in living organisms185 because
smaller particles have reduced probability of being cleared by
the immune system.186 In such applications, nanomotors
become the only viable option.
In the second category of applications, the nanomotors
predominantly benefit from the inherently greater spatial
precision they provide. For instance, rotary nanomotors have
been demonstrated as effective nanometric probes of environmental factors such as viscosity and temperature. Other
applications may involve drug delivery, where thermal effects
can be utilized to selectively release the target cargo while the
nanomotor is capable of monitoring this release91 or precisely
positioning itself near the target site.122
There is a large body of literature on light-driven microscale
motors based on utilizing various advanced materials,
functionalization strategies, etc., and many of these have not
yet been translated to and tested at the nanoscale. In our
opinion, future endeavors should be focused in harnessing the
potential of nanomotors to operate in highly confined
environments, in particular, within biological tissue and single
cells, and on the development of new functionalization
methods that can expand the range of applications within
molecular cargo delivery, environmental remediation, and
biosensing.187 There are also many avenues for applying lightdriven nanomotors in novel fundamental science, in particular,
in active matter and nonequilibrium thermodynamics research.
Finally, by decreasing the size of artificial inorganic nanomotors toward molecular length scales, new possibilities to
couple to, influence, and probe biological molecular motors
will arise.
will give access to new phenomena and open new avenues for
applications at the molecular scale.
Light as a power source has several advantages compared to
other types of fuels. It provides remote control over the
nanomotor motion and facile means to adjust the velocity by
change in light intensity. Optical control indeed offers
enormous flexibility, as light can be modulated not only
through intensity but also through its direction, wavelength,
and polarization. Light can be utilized not only as a power
source but also as a control signal for nanomotor navigation,
which promises a new dimension for nanomotor/nanorobot
design.143,182
However, the hoped-for advantages afforded by nanomotors
are accompanied by drawbacks and limitations that need to be
carefully considered. The first challenge is how to impart a
sufficiently strong force to drive the motion of the nanomotors.
This challenge can be partly addressed by using resonant
particles that enhance the light−matter interaction. However,
strong absorption comes at a cost of potentially substantial
temperature gradients near the nanomotor. This needs to be
taken into consideration while designing the nanomotor
functional coatings and may be of high importance for
biological applications. The second challenge is how to ensure
that any directed active motion is not terminated too soon by
thermal particle rotation. In this sense, high aspect ratio
structures can be advantageous as well as external fields able to
orient a particle. These factors will impact the rate at which an
individual nanomotor can explore its environment. The third
challenge of nanoscale motors is that it is technologically still
very difficult and costly to fabricate complex nanoscale
structures that might be needed to construct actual nanorobots. Hopefully this will change with advancements in
nanofabrication technology, part of which we have witnessed in
recent years. Finally, before widespread application of
nanomotors, one need to carefully evaluate the potential
toxicity of these nanomaterials to organisms and the
environment.183
Light-driven nanomotors are based on very diverse physical
phenomena. This makes it difficult to construct a common
framework for comparison of their performance. The more
technology steers toward applications, the higher need there
will be for designing application-specific figures of merit.
However, due to the extreme breadth of different types of
nanomotors and physical origin of their directed motion, it is
difficult to define a figure of merit able to characterize all types
of nanomotors in a meaningful way.
As we have shown in this review, the range of prospective
applications and level of advancement vary greatly between
different kinds of light-driven nanomotors. The potential
applications discussed in the published literature can be
divided into two main categories: those where the nanomotors
serve as autonomous vehicles able to explore and influence
their environment and those where nanomotors are used as
local probes or actuators.
In the first category, the active motion of the motor allows it
to explore its environment more effectively than a similarly
sized passive particle. However, as mentioned above, nanomotors are at a disadvantage when compared to micromotors
in this respect due to their enhanced rotational diffusion. Yet,
phototactic behavior184 or magnetic steering might help to
overcome these limitations. Moreover, there are many
potential applications in confined and crowded environments
where micromotors are simply too large to be able to function
AUTHOR INFORMATION
Corresponding Authors
*M.K.: Phone, +46-31-7723119; E-mail, mikael.kall@
chalmers.se.
*H.Š .-J.: E-mail, hana.sipova@chalmers.se.
ORCID
Hana Š ípová-Jungová: 0000-0002-5383-9120
Daniel Andrén: 0000-0003-0682-5129
Steven Jones: 0000-0003-2813-1945
Mikael Käll: 0000-0002-1163-0345
Notes
The authors declare no competing financial interest.
Biographies
Hana Š ı ́pová-Jungová received her Ph.D. in biophysics and chemical
and macromolecular physics in 2014 at Charles University in Prague,
Czech Republic. In January 2016, she joined the Bionanophotonics
group at Chalmers University of Technology as a postdoctoral fellow.
Her work focuses on applications of light-driven rotary nanomotors,
plasmonic biosensors, and single-molecule biophysics.
Daniel Andrén received his M.Sc. in engineering physics at Chalmers
University of Technology in 2016. In June 2016, he began his Ph.D.
studies in the Bionanophotonics group, where he focuses on optically
driven nanomotors and nano-optical light manipulation.
282
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Steven Jones received his M.Sc. (2016) in electrical engineering at the
University of Victoria, Canada. Since September 2016, he is pursuing
a Ph.D. in physics in the Bionanophotonics Group at Chalmers
University of Technology. His current research is in photothermal
effects involving plasmonics and how these effects can be either
beneficial or detrimental for applied microfluidic systems.
Mikael Käll is a professor of physics and head of the Bionanophotonics Group at the Department of Physics, Chalmers University of
Technology. After receiving a Ph.D. on spectroscopy of high-Tc
superconductors at Chalmers in 1995, he spent a postdoc year on
diffraction analysis of cuprates at Risø National Laboratory, Denmark.
He then moved back to Chalmers, where he has been a full professor
since 2006. Since the end of the 1990s, his main body of work
concerns nanophotonics, in particular plasmonics, where he has
contributed to a large number of influential publications on various
topics of fundamental and applied research. His current research
interests include light-induced forces on nanoparticles and the
application of resonant nanoparticles and metasurfaces in biomolecular sensing and actuation.
ACKNOWLEDGMENTS
We acknowledge the financial support from the Knut and Alice
Wallenberg Foundation, the Swedish Research Council, and
Chalmers Excellence Initiative Nano.
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