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Vapor-Driven Propulsion of
Catalytic Micromotors
Renfeng
Dong
1,2,*
, Jinxing
Li
1,*
, Isaac
Rozen
1
, Barath
Ezhilan
3
, Tailin
Xu
1
,
Caleb Christianson
1
, Wei
Gao
1
, David
Saintillan
3
, Biye
Ren
2
& Joseph
Wang
1
Chemically-powered micromotors offer exciting opportunities in diverse fields, including therapeutic
delivery, environmental remediation, and nanoscale manufacturing. However, these nanovehicles
require direct addition of high concentration of chemical fuel to the motor solution for their
propulsion. We report the efficient vapor-powered propulsion of catalytic micromotors without direct
addition of fuel to the micromotor solution. Diffusion of hydrazine vapor from the surrounding
atmosphere into the sample solution is instead used to trigger rapid movement of iridium-gold
Janus microsphere motors. Such operation creates a new type of remotely-triggered and powered
catalytic micro/nanomotors that are responsive to their surrounding environment. This new
propulsion mechanism is accompanied by unique phenomena, such as the distinct off-on response
to the presence of fuel in the surrounding atmosphere, and spatio-temporal dependence of the
motor speed borne out of the concentration gradient evolution within the motor solution. The
relationship between the motor speed and the variables affecting the fuel concentration distribution
is examined using a theoretical model for hydrazine transport, which is in turn used to explain the
observed phenomena. The vapor-powered catalytic micro/nanomotors offer new opportunities in
gas sensing, threat detection, and environmental monitoring, and open the door for a new class of
environmentally-triggered micromotors.
The development of micro/nanoscale synthetic motors that convert energy into movement has been a
fascinating research area of considerable fundamental and practical interest
1–10
. A variety of artificial
micro/nanomotors propelled by chemical reactions or external stimuli has been developed over the past
decade to overcome the challenges of propulsion at low Reynolds numbers and the effects of Brownian
motion
11
. Particular attention has been given to chemically-powered catalytic motors, including bimetal
nanowires
12–17
, spherical Janus micromotors
18–22
, and tubular microengines
6,7,23–28
, that exhibit autono-
mous self-propulsion in the presence of hydrogen peroxide fuel. Unfortunately, the requirement of the
hydrogen peroxide fuel greatly impedes many practical applications of such catalytically propelled micro/
nanomotors, and has led to the exploration of alternative chemical fuels. Diatomic halogens have thus
been proposed to power bisegment Cu-Pt nanowires
29
, while acidic and chloride-rich water solutions
have been used to power Zn microengines and Mg Janus micromotors
30–33
, respectively. Gao
et al.
have
shown recently that ppb–ppm levels of hydrazine can also be used as an extremely efficient fuel to power
iridium-based Janus micromotors
34
. Such hydrazine levels have been shown to exert negligible toxicity
on different animals
35,36
. However, a common drawback of all chemically-powered micro/nanomotors is
that they must operate in solutions prepared with such additional fuel materials. There are no previous
reports about using the atmospheric environment to trigger the motion of micro/nanomotors.
This paper demonstrates the first example of catalytic micromotors powered by vapor-phase chemi-
cals present in their surrounding atmosphere. The new concept of micromotors harvesting the chemical
fuel from their own surrounding atmospheric environment is illustrated using catalytic iridium-gold
1
University of California, San Diego, Nanoengineering, La Jolla, 92093, United States.
2
South China University
of Technology, Research Institute of Materials Science, Guangzhou, 510640, China.
3
University of California, San
Diego, Department of Mechanical and Aerospace Engineering, La Jolla, 92093, United States.
*
These authors
contributed equally to this work. Correspondence and requests for materials should be addressed to D.S. (email:
dstn@ucsd.edu) or B.R. (email: mcbyren@scut.edu.cn) or J.W. (email: josephwang@ucsd.edu)
Received: 09 April 2015
Accepted: 20 July 2015
Published: 18 August 2015
OPEN
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(Ir-Au) Janus microsphere motors in the presence of hydrazine vapor. The self-electrophoretic movement
of Ir-Au micromotors in the presence of hydrazine fuel has been shown to propel these microsphere
motors even at very low (nanomolar to micromolar) fuel concentrations
34
. In the present work, we use
this behavior to demonstrate the first example of remotely triggered high-speed micromotors. Partition
of hydrazine from the surrounding atmosphere into the micromotor solution is shown to trigger the effi
-
cient movement of Ir-Au microsphere motors without direct addition of the hydrazine fuel to the sample
solution. The resulting vapor-driven catalytic micromotors demonstrate distinct ‘Off-On’ response to
fuel molecules in the surrounding atmosphere and move at remarkable speeds of 30 body lengths per
second. This operation obviates the need for mixing together the fuel and micromotor solutions, and cre
-
ates a new type of remotely-triggered and powered catalytic micromotors that are responsive to volatile
species in their surrounding environment. Such ability of catalytic micromotors to swim in response to
changes in surrounding atmosphere or by sensing a remote chemical source holds considerable promise
for diverse environmental and defense applications.
Results
The Janus micromotors consist of gold particles (1.15
μ
m diameter) with one hemisphere coated with irid
-
ium metal. Iridium metal catalyst beds are commonly utilized to decompose hydrazine fuel. Such Janus
motors are easily fabricated by a directional Ir sputter deposition onto a monolayer of gold microparti
-
cles dispersed onto a glass slide. The scanning electron microscopy (SEM) image of the fabricated Janus
motor and corresponding energy-dispersive X-ray (EDX) characterization are displayed in Fig. 1.
To demonstrate the vapor-powered propulsion of Ir-Au micromotors, we rely on the volatilization of
hydrazine from a nearby source and its diffusion through the surrounding atmosphere into the motor
droplet (Fig. 2A). The self-propulsion of such Ir-Au Janus micromotors is efficient enough that they can
be triggered by hydrazine molecules diffusing through the atmosphere from a remote hydrazine source
(Fig. 2A). Such large-scale micromotor motion, remotely triggered by a hydrazine droplet, placed at a
1 cm distance, is demonstrated in SI Video 1. In our experiments, the two droplets - containing the fuel
and motors - were placed nearby each other on a glass slide within a containment enclosure atop the
microscope stage, in order to minimize the potential effects of atmospheric convective streams. The rapid
diffusion of hydrazine from the surface of the fuel source droplet leads to the formation of a vapor-phase
hydrazine concentration field in the surrounding air. The dissolution of vapor-phase hydrazine into the
micromotor droplet and subsequent internal diffusion create a gradient of the hydrazine fuel within the
motor droplet. Such gradients in the surrounding air and within the motor droplet are illustrated in the
simulated profiles, shown in Fig. 2B, based on a theoretical model for hydrazine transport (discussed
in detail in Supporting Information). The resulting gradient within the motor droplet leads to distinct
position-concentration dependent speed variations.
Figure 2(C,D) displays tracking lines, taken from SI Video 2, illustrating the Brownian movement of
the Ir-Au micromotors in the fuel-free sample droplet prior to the placement of the hydrazine fuel source
(Fig. 2C), and their acceleration to an average speed of 10
μ
m/s 15
seconds after placing the hydrazine
droplet (concentration 20%) at a separation distance of 0.5
cm (Fig. 2D). Unlike previous experiments
involving direct fuel mixing, where micromotors immediately and uniformly reach their maximum
speed upon adding the fuel, the present vapor-based propulsion mechanism leads to the formation of
concentration gradients within the micromotor droplet (simulated below), and hence to several new
phenomena such as a distinct spatio-temporal speed dependence. These include gradual acceleration of
the micromotors as well as spatial speed variation for motors located at different positions within the
droplet. In addition, the new vapor propulsion mechanism allows control of the micromotor speed by
changing the source-sample separation distance or the hydrazine concentration in the source.
The rate of hydrazine vapor diffusion into the micromotor sample is a function of both the
hydrazine-source concentration and the source-sample separation distance. Evaporation of hydrazine
Figure 1.
(
A
) Scanning electron microscopy (SEM) image of the Au/Ir motor. (
B
,
C
) Energy-dispersive
X-ray (EDX) spectroscopy images illustrating the distribution of the gold inner core and iridium catalytic
patch, respectively.
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from the source droplet creates a concentration
c
s
of vapor phase hydrazine on the surface of the fuel
(source) droplet.
c
s
is directly proportional to the level of hydrazine inside the source droplet. Because of
the high diffusivity of hydrazine in air, its vapor concentration quickly reaches steady state in the vicinity
of the source. The steady state vapor phase hydrazine concentration immediately surrounding the sample
droplet
c
s
a
(
L
) is solved as
cL
cc
R
L
;
1
a
s
ss
()
=
()
where
R
is the source droplet radius and
L
is the separation distance between the two droplets containing
the fuel and micromotor solution (see supporting information)
37
.
A lower source concentration leads to a lower hydrazine vapor concentration in the vicinity of the
motor droplet
c
s
a
(
L
) due to their linear relationship shown in equation (1). Our experiments demonstrated
that a decrease in the hydrazine source concentration indeed leads to a decrease in the average motor
speed over time (Fig. 3A), although the speeds are still markedly higher than those due to Brownian fluc
-
tuations over the entire 5
minutes period. The dependence of the micromotor speed on the hydrazine fuel
concentration was examined by directly adding the fuel solution to the sample (supporting Figure S1).
The significantly faster motor speeds due to higher hydrazine concentrations are illustrated in the track
lines of the motor movement Fig. 3A(a–c) and the corresponding SI Video 3. Ordinarily, when the Ir-Au
micromotors are directly mixed with high concentrations of the hydrazine fuel (
≥
0.001% level), a strong
inhibiting effect arises due to the high concentration of the ionic conjugate species of both hydrazine and
Figure 2.
Catalytic micromotors powered by a remote fuel source.
(
A
) Propulsion of Ir-Au micromotors
by external hydrazine diffusing from the fuel source droplet through the surrounding atmosphere into the
motor droplet. (
B
) Simulation of the hydrazine concentration gradient produced by the hydrazine source
(left) and the hydrazine concentration gradient generated within the sample droplet (right) based on a
theoretical model for hydrazine transport. (
C
,
D
) Track lines of the motion of Ir-Au motors over 2
seconds,
taken from SI Video 2, before (
C
) and 15
seconds after (
D
) placing the 20% hydrazine droplet (1
μ
L;
diameter: 2.5
mm) 0.5
cm away from the micromotor droplet (1
μ
L; diameter: 2.5
mm). Scale bar, 10
μ
m.
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water (N
2
H
5
+
and OH
−
, respectively)
34
. In the case of vapor propulsion, however, no deceleration due
to this inhibiting ionic-strength effect has been observed. Our results indicate that the local hydrazine
concentration within the motor sample due to vapor dissolution lies solely below the 0.001% level, even
upon exposure to a high source concentration of 30% at close proximity to the micromotor droplet.
The source-sample separation distance has a profound effect upon the micromotor speed, as well as
on the acceleration behavior of the micromotors. The average motor speed decreases significantly upon
increasing the separation distance
L
between fuel source and motor droplet (Fig. 3B), reflecting the
lower steady-state concentration of hydrazine over larger distances due to their inverse linear relationship
(eq. 1). At short separation distances, the Janus micromotors are able to continuously accelerate over a
five-minute period to maximum speeds of 26
μ
m/s. This acceleration is greatly diminished using larger
separation distances. For example, at the largest separation distance the motors were found to accelerate
to their peak speeds of 7
μ
m/s only after 30
seconds. The effect of different separation distances on the
motor speed is illustrated from the 1
second motion track lines of Fig. 3B(a–c), following a five minute
exposure to the fuel droplet. At all separation distances examined, the motors still moved faster than
due to Brownian motion alone, highlighting the sensitivity of these micromotors to ultralow levels of
hydrazine.
An interesting phenomenon that arises due to the diffusion involved in the vapor propulsion mech
-
anism is that for a short time after exposure to the fuel droplet, the motor speed depends upon the
location of the motor within the sample droplet itself, and specifically on its distance from the free sur
-
face of the droplet. Although the environmental hydrazine immediately surrounding the motor droplet
reaches a steady-state concentration within approximately two seconds after the start of diffusion, it
takes approximately 15
minutes to reach a uniform concentration within the 2.5
mm diameter water
droplet containing the micromotors, owing to its 20,000-fold lower diffusivity in water. This leads to
a significant variation in the fuel concentration within the motor droplet at short times. This is con
-
sistent with the results from our mathematical model (see SI) where the transport of hydrazine within
the motor droplet is treated as a diffusion process within the hemisphere. We assume radial symmetry,
which inherently captures the no-flux of hydrazine through the flat substrate, thus allowing us to solve
the problem in spherical coordinates. The simulated hydrazine concentration profiles within the motor
droplet, shown in Fig. 4(B,C) and SI Video 5, illustrate such decrease in the local hydrazine concentration
from the droplet edges to the center. As time increases, the hydrazine concentration increases within the
Figure 3.
Spatio-temporal speed dependence of vapor-powered micromotors.
(
A
) Dependence of the
motor speed upon time and fuel concentration at a fixed separation distance of 0.5
cm. (a–c) Track lines
of the micromotor motion, over a 2
sec period, taken from SI Video 3, 5
min after placing droplets of 10%,
20%, and 30% hydrazine, respectively, 0.5
cm apart. (
B
) Dependence of the motor speed upon the time and
separation distance using a source droplet containing 20% hydrazine. (a–c) Track lines of the micromotor
motion, over 2
second periods, taken from SI Video 4, 5
min after placing the fuel droplet containing 20%
hydrazine at separation distances of 1, 2, and 3
cm, respectively. (d) Corresponding 3D plots. Scale bar,
10
μ
m. Droplet diameter and volume: 2.5
mm and 1
μ
L, respectively.
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domain and asymptotically reaches the boundary concentration
c
m
. Accordingly, motors closer to the
motor-droplet boundary (
ρ
=
r/R
close to 1) are expected to travel faster than those at the center. Overall,
the data of Fig. 4A clearly illustrate that the partition of the fuel into the motor droplet, leads to distinct
position-concentration dependent speed variations. In agreement with the theoretical predictions, we
observed noticeably faster average motor speeds around the droplet edges compared to the center of
the droplet (Fig. 4A). In addition, we found that motors at equivalent radial distances moved at similar
speeds over the range of experimental separation distances considered here. These experimental data are
in good agreement with our model, where the hydrazine concentration immediately surrounding the
small 2.5
mm droplet is assumed to be uniform on the scale of the droplet. This also serves as confir
-
mation of the minimal effects of potentially disruptive convective streams outside the sample droplet, as
such streams would disturb the surrounding hydrazine concentration and contribute to different speeds
at similar radial distances, counter to our experimental observations. Furthermore, the significant spa
-
tial dependence of the micromotor speed indicates the negligible effects of convective streams within
the sample droplet, such as those due to evaporation. If such flows were present and strong, this would
lead to mixing and enhancement of the hydrazine transport inside the motor droplet, which could be
Figure 4.
Profiles of the hydrazine level and the motor speed within the motor droplet.
(
A
) Motor
speeds at several radial distances from the center within the sample droplet after a one min exposure to the
20% hydrazine source droplet (the negative distance being closest to the source, and positive being furthest).
Droplets (2.5
mm diameter, 1
μ
L) separated by 0.5
cm. (
B
) 3D simulated plot of the normalized hydrazine
concentration within the sample droplet 1
min after exposure. (
C
) Normalized hydrazine concentration
profile as a function of the radial position (
ρ
=
r/R
) within the sample droplet for different times after
placing the hydrazine source: 0, 0.5, 1, 2.5, and 5
min (a–e).
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modeled as an effective diffusivity (at long times). The timescale for hydrazine transport within the motor
droplet - calculated from pure diffusion - matches our experiments, however indicating that the potential
effect of convective flow-induced transport in our study is at best weak. Overall, the agreement of the
observed dependence of the motor speed on the source-sample separation distance, hydrazine source
concentration, and radial position within the motor droplet with the theoretical predictions validates our
modeling approach and theoretical assumptions
In conclusion, we have demonstrated a new type of catalytic micromotors that are responsive to their
surrounding environment. The movement of these motors is triggered remotely by a fuel vapor parti-
tioned from the surrounding atmosphere into the micromotor solution. This new concept was illustrated
using vapor-propelled iridium-based micromotors that display efficient motion of over 30
μ
m/s in the
presence of hydrazine vapor without adding the hydrazine fuel directly to the sample solution. Factors
affecting such vapor-powered micromotor motion have been investigated using a theoretical model for
hydrazine transport. Such vapor-powered micromotors have the capability to act as highly selective sen-
sors for toxic gases, with a distinct ‘Off-On’ response to low atmospheric concentrations of the target
vapor. Similarly, ‘On-Off ’ gas sensing schemes may be developed by pollutants inhibiting the catalytic
activity of the motors, analogous to the water toxicity testing based on the inhibition of catalase-based
micromotors by solubilized toxins
38
. Pollutants affecting the motor speed may also be explored, analo-
gous to the strong effect of trace silver ions upon the propulsion of Au-Pt nanowire motors, as well as
for the enhanced detection of Ag-nanoparticle tagged DNA
39–41
. These motion-based sensing studies
indicate the strong potential of the new vapor-powered motors to selectively detect airborne toxins.
Future efforts will aim at exploiting new volatile fuels and new catalytic motors for expanding the con-
cept of remotely-triggered movement towards the detection of different air pollutants. Such ability of
micromotors to swim in response to the presence of a remote fuel source offers considerable promise
for designing future environmentally-responsive micromachines for a wide range of important future
defense and environmental applications.
Methods
Synthesis of Janus micromotors. The Janus micromotors were prepared using gold microparticles
(1.15
μ
m mean diameter, Alfa Aesar, Ward Hill, MA, USA) as the base particles. 10
μ
g of gold particles
were first dispersed into isopropyl alcohol (A451-4, Fisher, Pittsburgh, PA, USA) and centrifuged. Then,
the gold particles were redispersed in 150
μ
L isopropyl alcohol. The sample was then spread onto glass
slides and dried uniformly to form particle monolayers. The particles were sputter coated with a thin
Ir layer using an Emitech K575X Sputter Coater for 3 cycles with 10
s per cycle. The Ir layer thickness
was found to be 20
nm, as measured by the Veeco DEKTAK 150 Profilometer. The micromotors were
subsequently released from the glass slides via pipette pumping and dispersed into double distilled water.
Speed calibration experiments. To determine the relationship between the Ir-Au motor speed and
fuel concentration, aqueous hydrazine solutions (Sigma #309400) ranging from 0.0000002% to 20% were
prepared and directly mixed with the motor droplets. The propulsion calibration experiments were per
-
formed by mixing 1
μ
L of the motor and hydrazine solutions each.
Vapor experiments. Aqueous hydrazine solutions, ranging from 5% to 30%, were prepared for the
vapor experiments. A 1
μ
L droplet containing the micromotors was placed first on the glass slide within
a containment enclosure atop the microscope stage, in order to minimize the potential effects of atmos-
pheric convective streams. The diameter of the motor droplet was found to be 2.5
mm. After the motors
settled into focus of the microscope, a 1
μ
L droplet of hydrazine fuel was placed on the slide at a fixed
separation distance from the motor droplet.
Equipment. Videos were captured by an inverted optical microscope (Nikon Instrument Inc. Ti-S/
L100), coupled with 40x objectives, and a Hamamatsu digital camera C11440 using the NIS-Elements
AR 3.2 software.
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Acknowledgements
This project received support from the Defense Threat Reduction Agency-Joint Science and Technology
Office for Chemical and Biological Defense (Grant no. HDTRA1-13-1-0002), UCSD Calit2 Strategic
Research Opportunities (CSRO) program (to J.W.) and from NSF Grant No. CBET-1151590 (to D.S.).
R.D. and T.X. acknowledges the China Scholarship Council (CSC) for financial support. The authors
thank M. Kang and J. Uy for their help.
Author Contributions
R.D., I.R. and J.L. performed the experiments. J.L., T.X., I.R., B.E., B.R., C.C. and W.G. analysed the data.
B.E., D.S. performed the modelling. J.L., R.D., I.R., B.E. and J.W. wrote the manuscript. All the authors
discussed the results and commented on the manuscript.
Additional Information
Supplementary information
accompanies this paper at http://www.nature.com/srep
Competing financial interests:
The authors declare no competing financial interests.
How to cite this article
: Dong, R.
et al.
Vapor-Driven Propulsion of Catalytic Micromotors.
Sci. Rep.
5
,
13226; doi: 10.1038/srep13226 (2015).
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