Progress toward Light-Harvesting SelfElectrophoretic Motors: Highly Efficient Bimetallic Nanomotors and Micropumps in Halogen Media Flory Wong and Ayusman Sen* Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *
ABSTRACT: We have developed a highly efficient, bubble-free autonomous nanomotor based on a nanobattery. Bimetallic silver−platinum nanorods are powered by self-electrophoresis and show speeds much higher than those of other electrophoretic motors at similar fuel concentrations. The fuel (I2) can be regenerated by exposure to ambient light, leading to renewed motion of the motor. This versatile system can also be made into a micropump that transports fluid and particles.
KEYWORDS: self-electrophoresis, light sensitive, nanomotor, micropump
W
allic motor consisting of copper and platinum (Cu−Pt). The nanomotor acts as a nanobattery in dilute aqueous bromine (Br2) or iodine (I2) solutions. This bimetallic motor exhibits directed motion with speeds of about 10−20 μm/s.17 We have continued to seek more efficient motor designs. There have been examples in the literature regarding lightdriven or light-assisted motors, including Janus TiO2 microparticles,18−20 silver−salt microparticles,21−23 and DNA walker molecules.24 These, however, use light as its primary or secondary fuel, meaning discontinued use of light will cause the motor function to cease. A particularly intriguing possibility involves using light to reverse the redox reaction that causes electrophoretic propulsion. This will lead to the regeneration of the fuel; in essence, the energy for propulsion will be derived ultimately from light, with the bipolar redox reaction merely acting as the conduit for the conversion of light energy to mechanical motion (Scheme 1). Here, we report on a highly efficient, light-modulated, self-electrophoretic bimetallic Ag−Pt motor based on the reversible reaction of silver with iodine (I2) that involves the following steps: (a) bipolar redox reaction leading to the conversion of Ag to AgI, resulting in propulsion,
hile we have become proficient at fabricating and manipulating materials on the micro- and nanoscale, the ability to control the motion of the materials themselves is still an underdeveloped field. In an effort to emulate and understand motion on a nanoscale, various types of synthetic nanomotors have been fabricated and characterized over the past decade.1−5 Among the different types of nanomotors, self-electrophoretic motors are of particular interest due to the electric field generated by the motors that lead to directed motion. The first self-electrophoretic motor synthesized was a bimetallic nanorod composed of gold and platinum segments (Au−Pt) in hydrogen peroxide (H2O2), synthesized by Mallouk and Sen in 2004.6−8 The bimetallic nanorod exhibits directed motion, moving at 5−20 μm/s. The asymmetric decomposition of H2O2 occurring at the opposite end of the motor leads to the self-electrophoresis.8−11 Using H2O2 as the fuel source, a wide variety of metallic motors have been synthesized and shown to move by a similar mechanism.12−16 However, H2O2 is far from the perfect fuel source. Aside from the asymmetric electrochemical decomposition as a result of the short circuit between platinum and gold, there is a second decomposition reaction that occurs solely on platinum.6 This leads to loss of fuel through routes that do not lead to propulsion. Peroxide decomposition also releases oxygen, leading to bubble formation. Liu and Sen developed an efficient, bubble-free self-electrophoretic bimet© 2016 American Chemical Society
Received: May 25, 2016 Accepted: June 23, 2016 Published: June 23, 2016 7172
DOI: 10.1021/acsnano.6b03474 ACS Nano 2016, 10, 7172−7179
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and (b) photochemical decomposition of AgI back to elemental silver (Ag) and I2.25,26
RESULTS Autonomous Movement. In aqueous iodine solutions, the motion of the Ag−Pt nanorods is visibly non-Brownian. They move along their long axis with the silver end leading (Supporting Video SV1; for better visualization, Au was used in place of Pt). We can characterize the motion of the nanorods by tracking the motors’ position over time after iodine exposure. Figure 1A−C shows the trace of a single Ag−Pt nanomotor at different time stamps in 28 μM iodine taken from Supporting Video SV2.
Figure 2. Ag−Pt nanomotor speeds at different iodine concentrations during initial exposures (speeds recorded over 5 s and averaged) and the corresponding current density.
Scheme 2. Detailed Diagram of the Conversion Cycle and How the Motion of the Silver-Based Nanorod Occurs
double layer on the surface of the negatively charged nanorod causes electro-osmotic fluid flow along the rod in the direction of the electric field (Ag to Pt), thereby causing the nanomotor to move the silver side first. Supporting Video SV1 shows a real-time video of gold−silver segmented nanorods, which exhibit the same chemistry, in iodine solution moving the silver side first. A major piece of evidence for a self-electrophoretic mechanism is the absence of motion with a nonconductive polymer tail in place of conductive platinum. Nanorods composed of a poly(3,4-ethylenedioxythiophene) segment in contact with silver do not show motion when exposed to iodine fuel (Supporting Video SV3). To further confirm the reaction mechanism, electrochemical potentials and the open-circuit current were measured using bulk silver and platinum electrodes. The redox reactions that occur with Ag and Pt in an I2 solution are as follow: Silver is oxidized to silver ion (eq 1). In an iodine solution, there are two possible reduction reactions (eqs 2 and 3):
Figure 1. Cropped frames of a single moving Ag−Pt nanomotor at different time stamps in 28 μM I2 solution. The nanomotor is moving at 6.34 μm/s.
It was found that varying the iodine concentration significantly impacted the nanomotor speeds, with the nanomotors moving 2−3 μm/s at low iodine concentrations of 14.1 μM I2 while moving 20 μm/s at 113 μM I2. Figure 2 shows nanomotor speeds at different iodine concentrations with their corresponding current density, measured as described in the Methods section. Both motor speed and current density of the system have a strong linear correlation to iodine concentration. Mechanism of Directed Linear Movement. Scheme 2 shows the Ag/AgI conversion process and how it powers the motion of the rods. Conversion of the Ag segment of a bimetallic nanorod to AgI occurs from exposure to an aqueous iodine (I2) solution. Oxidation occurs at the silver segment, where electrons lost travel along the length of the rod to the platinum, and reduces iodine into iodide (I−) ions. I− ions subsequently flow along the length of the rod and combine with the oxidized silver (Ag+) to form AgI. Positive ions in the
Ag + I− → AgI + e− , E 0 = 0.152 V +
−
−
0
(1)
HIO + H + 2e → I + H 2O, E = 0.987 V
(2)
I 2 + 2e− → 2I−, E 0 = 0.535 V
(3)
Open-circuit potential between Ag and Pt in a saturated I2 solution was measured to be 0.66 V, suggesting that I2 rather than HIO was being reduced. Nanomotor Efficiency. In comparison to analogous systems, Ag−Pt motors in I2 move at comparable speeds at orders of magnitude lower fuel consumption. Table 1 shows a comparison of nanomotors and their fuel concentrations when 7173
DOI: 10.1021/acsnano.6b03474 ACS Nano 2016, 10, 7172−7179
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ACS Nano
material, and this property can be used to further enhance motor speeds without the need for additional iodine. This is explored in a later section. Nanomotor Characterization. To visualize the difference between normal Ag−Pt nanorods and the converted AgI−Pt rods, scanning electron microscopy (SEM) and energydispersive spectroscopy (EDS) were used to characterize the surface of the rods. Figure 3 compares pure Ag−Pt rods (a) to
Table 1. List of Nanomotor Systems and Their Fuel Concentration Required for Motion on the Order of 7 μm/ s17,27 nanomotor system
fuel
[fuel] (mM)
motor speeds (μm/s)
Au−Pt Cu−Pt Ag−Pt
H2O2 Br2/I2 I2
180 0.3 0.028
7−10 7 7
running at about 7 μm/s in ambient lighting conditions. Ag−Pt motors use 10 times less fuel to achieve the same speeds in comparison to Cu−Pt motors, which provide a direct comparison due to their similar mechanisms. The Ag−Pt nanomotor moves at about 7 μm/s using 28 μM I2, whereas the closest analogue, the Cu−Pt/halide system developed by Liu and Sen,17 will have an estimated speed of
