Article Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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Structure−Function Dependence on Template-Based Micromotors Yong Wang,† Carmen C. Mayorga-Martinez,‡ James Guo Sheng Moo,† and Martin Pumera*,‡ †
Division of Chemistry & Biological Chemistry, School of Physical Mathematical Science, Nanyang Technological University, Singapore 637371, Singapore ‡ Center for Advanced Functional Nanorobots, Department of Inorganic Chemistry, Faculty of Chemical Technology, University of Chemistry and Technology Prague, Technická 5, 166 28 Prague 6, Czech Republic
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S Supporting Information *
ABSTRACT: Catalytically bubble propelled micromotors are one of the most frequently used self-propelled micromachines in the literature. Typically, the structure−function relationship is not investigated. We present detailed study of how the structural geometry influences the propulsion of these micromachines. We prepared different micromotors with membrane template-assisted electrodeposition by applying varied electroplating time. We obtained a variety of differently sized micromotors with a range of propulsion dynamics. We used varied electroplating time to tailor the length of the micromotor, but also the wall thickness, microtube opening diameter, and even the structural integrity of the micromotor. The study of propulsion dynamics of micromotors in different H2O2 concentrations indicated that both geometric parameters and the chemical environment affect the velocity of micromotors. Microtubular motors prepared by short electrodeposition time (e.g., 300 s) require higher fuel concentration to activate and move. The bubble ejection frequency and diameter were measured and demonstrated that micromotors with shorter length produce smaller bubbles and relatively lower velocities. This fundamental study provides experimental insights into the length-related propulsion dynamics of a microtube, and it has profound implications for the design of micromotors. KEYWORDS: microtube, microengine, electrodeposition time, structural geometry, propulsion dynamic rolled-up micromotors.21 Besides that, Zhao et al. investigated the effect of micromotor geometry on mass transport.10 They found that O2 flux, bubble generation frequency, and average velocity of tubular micromotor are closely related to the geometric parameters and fuel concentration. Furthermore, a model used to describe the propulsion mechanism of a rolledup microtube was also established.11 Interestingly, it is worthy to note that all of these models are based on the rolled-up micromotors. Notably, these kinds of micromotors are considerably larger (about 50 μm long) than their opponents, the template-based micromotors. Our group found that the geometric parameters and chemical environment affect the propulsion dynamic of micromotors in nano- and microscale.22 Following this line of thinking, we studied first the effect of the electroplating time during micromotor fabrication using membrane at fixed diameter on the geometry parameters (length, wall thickness, and diameter). Afterward, the geometric parameters associated with their influence on the propulsion dynamic of a tubular micromotor are also investigated in this work.
1. INTRODUCTION Development of nanotechnology has enabled us to manufacture and manipulate miniaturized devices in nano-/microscale. A micromotor is one such fascinating miniaturized machine which has been given wide attention.1−6 The tubular microengine with conical configuration and platinum inner surface, prepared using an electroplating template-based method, has received particular tremendous concern because of their superior performance and efficient movement even in highly viscous fluid and biofluid.7−9 The conical shape of these micromotors is suitable for bubble trap and growth when Pt catalyzes the H2O2 decomposition to oxygen and water; the generated bubbles sequentially eject from one opening of the microtube, and these detached bubbles result in an instantaneous thrust for propulsion.10,11 This membrane template-assisted electrodeposition method uses the membrane with a huge number of uniform double-conical pores and electrodeposition of Pt.12,13 This is a greatly simplified and cost efficient method for preparation of microtubes in the miniaturized scale. The applications of this tubular micromotor have been widely explored and developed.14−20 Nevertheless, insight into the influence of deposition time upon structure still requires careful study. Previously, Mei and co-workers proposed a theoretical and experimental model to elucidate the influence of the geometric parameters such as length and diameters on the dynamic of the © XXXX American Chemical Society
Received: April 16, 2018 Accepted: June 19, 2018
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DOI: 10.1021/acsaem.8b00605 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Energy Materials
2. EXPERIMENTAL SECTION
Afterward, the effects on the dynamic propulsion of the varying geometry results are evaluated upon mobility performance by recording and tracking the velocity of the micromotors in different fuel concentrations. Figure 1 displays the SEM micrographs of four types of microtubes prepared by different electrodeposition time (1800,
Materials. The cyclopore polycarbonate membranes with pores of 3 μm in diameter were purchased from Whatman, USA. Ethanol and methylene chloride were obtained from Tedia, USA. Hydrogen peroxide (H2O2, 35 wt %) were purchased from Alfa Aesar, Singapore. The platinum plating solution was obtained from Technic Inc., USA and sodium dodecyl sulfate (SDS) were purchased from SigmaAldrich. Ag/AgCl reference electrode and platinum counter electrode were obtained from CH instruments, USA. Ultrapure water (18.2 MΩ cm) was used to prepare the solutions. Apparatus. Electrodeposition of platinum microtube was conducted by using an Autolab PGSTAT 101 electrochemical analyzer (Eco Chemie, Utrecht, The Netherlands) controlled by NOVA v 1.8 software (Eco Chemie). This deposition utilized a three electrodes setup at room temperature with a platinum electrode as counter electrode and an Ag/AgCl electrode as the reference electrode. The sputtering of the cyclopore polycarbonate membranes was conducted by using a JEOL JFC-1600 auto fine coater. In addition, a Fisherbrand FB 11203 ultrasonicator was used for sonication and a Hermle Z233m-2 microcentrifuge was used for centrifugation during the experimental process. Scanning electron microscopy (SEM) was performed with a JEOL 7600F field emission scanning electron microscope (JEOL, Japan). Optical microscope videos and images of the movement of the micromotor were taken by using a Nikon Eclipse 50i microscope, and these video sequences were processed with Nikon NIS-Elements software. Preparation of the Platinum Micromotor with the Different Electrodeposition Times. The preparation of a Pt micromotor was conducted by using a modified electrodeposition procedure with a cyclopore polycarbonate membrane. Silver nanoparticles (100 nm) were sputtered onto one side of the membrane, and this membrane was attached to the copper tape which served as the working electrode for the electrodeposition of the Pt micromotor. This membrane was then assembled into a customized electrochemical deposition cell, where platinum worked as a counter electrode and Ag/AgCl as a reference electrode. Electrodeposition of the Pt micromotor was then conducted by using an Autolab PGSTAT 101 electrochemical analyzer (Eco Chemie). The membrane was rinsed with 3 mL of ultrapure water for 4 times, and the Pt micromotror was deposited galvanostatically from the commercial plating solution at −20 mA for 1800, 1200, 600, 300, and 200 s. When the deposition of microtubes was finished, the membrane was rinsed 5 times with ultrapure water after removing the plating solution. Then the membrane disassembled from the electrochemical cell was polished with 5 μm alumina powder in order to remove Ag nanoparticles. Following this, the membrane was washed with ultrapure water for 5 times by sonication. Afterward, the membrane was placed into an Eppendorf tube with about 2 mL of methylene chloride. This Eppendorf tube was untrasonciated for 5 min in order to dissolve the membrane. The Pt micromotors were then retrieved by centrifugation at 8000 rpm for 3 min and washed with methylene chloride for 3 times. Following, these micromotors were washed with ethanol and ultrapure water each for 5 times and collected by centrifugation for 5 min. The Pt microtubes were stored in ultrapure water at room temperature. Operation of the Platinum Microtube. In order to study the motion of the micromotors, aqueous solutions containing 1−10 wt % hydrogen peroxide and constant surfactant SDS concentrations (1 wt % SDS) were prepared. Then 10 μL of the mixed micromotor solution was put on the clean glass slide for the movement study. Optical images and videos of the movement of the micromotors were taken by a Nikon Eclipse 50i microscope and processed with Nikon NIS-Elements software.
Figure 1. SEM images of four types of tubular micromotors prepared by template-assisted electrodeposition method at different deposition times:( A) 1800, (B) 1200, (C) 600 s, and (D) 300 s. In all the cases, polycarbonate membranes with 3 μm pores are used and a current of −20 mA is applied. Left panels correspond to lateral view at 1000× magnification, and right panels correspond to top view at 25000× magnification.
1200, 600, and 300 s) using a galvanostatic current of −20 mA and conical polycarbonate membranes with 3 μm pores. In this study we used membranes with 3 μm pores as an optimal compromise between typical sizes of 2 and 5 μm.22 The lateral views of the micromotors are shown in left panels, while the right panels show the top views of the micromotors. Supporting Information Table S1 summarizes the length, wall thickness, larger opening, and inner diameters of the four types of micromotors prepared for these studies. The micromotors prepared using 1800 s of electrodeposition time give an average length of 9.38 ± 0.51 μm as well as the aspect ratio of length to average diameter of 3.37. The length to average diameter aspect ratio was calculated using eq 1:
3. RESULTS AND DISCUSSION The micromotors are synthesized from 3 μm membrane template-assisted electrodeposition using different electrodeposition times (1800, 1200, 600, and 300 s). SEM is performed to evaluate the geometry parameters (length, wall thickness, and diameters) of the obtained micromotors. B
DOI: 10.1021/acsaem.8b00605 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Energy Materials
δ = L /D
(1)
time. If the electrodeposition time is reduced to 300 s and even 200 s, the quality of micromotor becomes poor and structural defects from the incomplete openings at the ends are observable (see Figures 1 and S1). Moreover, the average wall thickness of four types of micromotors (1800, 1200, 600, and 300 s) were also measured and obtained from the SEM images. As expected, when the electrodeposition time increases, the wall thickness of the micromotors increases (see Figure 2B and Table S1) with good linearity (r = 0.990) and consequently the diameter of the inner opening of the microtube decreases (see Table S1). The main reason for thinner wall thickness observed in the micromotors at short electrodeposition time was due to a lower amount of platinum being deposited. Moreover, a rough surface of inner wall was clearly visible from SEM images in all synthesized micromotors. Previous literature has reported that microtubes with rough/porous inner metallic surface increases the contact area with fuel, and it results in a higher speed of the micromotor.23−25 Here it is important to note that at least 30 micromotors of each type were taken into account for collecting the geometry parameters data. Thus, it is safe to say that the desired length and wall thickness of the micromotor can be achieved by applying suitable plating time. Moreover, the quality of the micromotors has a vital impact on the motion of the micromotor and should be carefully considered when involving the short deposition time such as 300 s. Subsequently, the effects of the deposition time on micromotor motion are evaluated. The motion mechanism of these micromotors is based on the oxygen bubble recoil mechanism, the bubbles are generated inside of a microtube by decomposition of H2O2 to oxygen and water through Pt. This process involves first the bubble nucleation, followed by bubble growth, and bubble ejection from the tube, which gives a thrust on the tubular micromotors to propel it step by step.10,11 The absolute velocity is measured as an indicator to correlate the H2O2 fuel concentration with the geometric configuration of the microtubes. Figure 3A shows that the velocities of the micromotors across different fuel concentrations are influenced by the electrodeposition time used during their manufacture. The micromotors prepared at higher electrodeposition time show enhanced performance in terms of their velocities. Previous micromotor modeling results indicate that the velocity of a micromotor is linearly dependent on the H2O2 concentration.21 In our case, we studied the linear dependency using the slope−intercept form of the linear equation, from the plot tabulated between the velocity (μm/s (Figure 3A) and body length/s (Figure 3B)) and H2O2 concentration for the micromotors prepared using different electroplating times; a general trend is found in which the absolute average velocity increases when the H2O2 fuel concentration increases, which is consistent with the reported literature. Nevertheless, the linear dependence of the velocity vs H2O2 concentration is affected by the electroplating time. For instance, a tubular micromotor with the plating time of 1800 s shows a linear dependence of r = 0.996 followed by the micromotors produced at 1200 s (r = 0.966) and 600 s (r = 0.966). As expected, a poor linear dependence is found in micromotors made at 300 s with r = 0.9014. Similar results were observed when the velocity is expressed in body length/s. Moreover, the electroplating time influences the H2O2 concentration where micromotors show go/no go behavior. The micromotors prepared by shorter electrodeposition time
where δ is the length to average diameter aspect ratio, L is the length, and D is the average diameter of the two openings of the micromotors. When we reduced the electrodeposition time to 1200 s, the micromotors showed an average length of 7.37 ± 0.68 μm and the length to average diameter aspect ratio obtained by calculation was 2.71. The micromotors with further shorter electrodeposition time of 600 s display a length of 5.65 ± 0.71 μm and length to average diameter aspect ratio of 2.12. In order to prepare the micromotors with even smaller length and length to average diameter aspect ratio, we decided to tune the plating time to 300 and 200 s. As we expected, reduced deposition time results in shorter length as well as smaller length to average diameter aspect ratio. The length of the micromotors with electrodeposition time of 300 s is 4.25 ± 0.52 μm, giving the length to average diameter aspect ratio of 1.63. SEM images of microtubes prepared using the electrodeposition time of 200 s are shown in Figure S1; the length and length to average diameter aspect ratio are measured to be 3.28 ± 0.47 μm and 1.27, respectively. As can be seen, when the electrodeposition time increases, the length of the micromotors grows in a proportional way with good linearity (r = 0.997) as can be seen in Figure 2A. In order to validate the effect of the electrodeposition time in the length of the micromotors, three different batches are prepared for each deposition time; no statistically significant differences are observed (see Figure S2). In addition, we observed that the integrity of the micromotors is also affected by the deposition
Figure 2. (A) Dependence of the length of micromotors on the different electrodeposition time. (B) Variation of wall thickness of micromotors with the electrodeposition times of 300, 600, 1200, and 1800 s. C
DOI: 10.1021/acsaem.8b00605 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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ACS Applied Energy Materials
growth, and bubble ejection, the velocity of micromotor is correlated with the bubble ejection frequency and diameter. Here the bubble ejection frequency and diameter were further investigated associated with the geometry of micromotors. Table 1 shows the bubble ejection frequency and bubble Table 1. Dynamic Parameters of Four Types of Micromotors with Different Deposition Times, 1800, 1200, 600, and 300 s, in 3 wt % H2O2 Concentration average bubble diameter (μm)
estimate average velocity (μm/s)
estimate O2 production (μm3/s)
measured velocity (μm/s)
300 600 1200 1800
18.69 35.41 46.19 36.63
3.45 4.76 5.47 7.35
32.24 84.28 126.33 134.62
401.65 1998.60 3956.29 7611.62
37.41 92.59 121.114 144.17
diameter of four types of micromotors in 3 wt % H2O2 concentration. It is important to note that the concentration of SDS is fixed at 1 wt % in order to decrease the surface tension of fluid. The micromotors with longer length prepared by longer plating time generate bigger bubbles. When the electrodeposition time is reduced, the corresponding bubble diameter shrinks. The reason for this bubble diameter change upon electrodeposition time is due to the larger inner surface area, which contributes to higher oxygen generation at the same fuel concentration. We calculated the amount of oxygen production from H2O2 decomposition of four types of micromotors by simply summation of oxygen bubble volume shown in Table 1. The results demonstrate our expectation that longer micromotors with larger inside surface area produce more oxygen at the same fuel concentration. The bubble diameter is dominated by the Laplace pressure difference ΔP, which can be expressed as eq 3:
Figure 3. Velocity of four types of micromotors in various H2O2 concentration environments (expressed in μm/s) (A) and body length/s (B). Conditions in all experiments: 23 °C and 1.0 wt % SDS. The error bars represent the standard deviations of 20 micromotors.
(300 s) did not exhibit any motion below the fuel concentration of 3 wt % (Figure 3), this is because this kind of micromotor shows less catalytic area (shorter length) and imperfect tubular structure needing higher H2O2 concentration to produce oxygen bubbles and start locomotion. This effect also results in slower velocity (in μm/s and body length/s) in higher fuel concentration. For the other types of micromotors when the electroplating time increases, the absolute velocity (μm/s) increases, but for the velocity in body length/s the performances of the micromotors prepared at 600, 1200, and 1800 s are almost the same (Figure 3B). Since body length/s is a dimensionless quantity for describing the performance of micromotor regardless of their length differences, we can assume that after 600 s of electroplating time the performances of micromotors are not compromised. Peclet numbers of four types of micromotors are calculated by using eq 2: Pe = VL /D
deposition time (s)
bubble freq (Hz)
ΔP = 2γ /R
(3)
where γ is surface tension of fluid and R is bubble radius. Among four types of micromotors, bigger bubbles inside the longer micromotors indicate lower pressure inside the microtubes despite the higher oxygen production. The velocity of the micromotor is closely related to the bubbles, and researchers have demonstrated micromotor velocity is a function of bubble radius and bubble ejection frequency.27 From Table 1, the estimated velocities of four types of micromotors calculated from the bubble radius and ejection frequency are similar to the velocities obtained by tracking the movement. Oxygen generation frequency increased with the size of the micromotors in consequence with the electrodeposition time; this has possible implication for application in fuel cell research.29 As previously reported, a micromotor can exhibit straight, circular, and spiral movement, which is a result of asymmetric configuration of the micromotor and deviation of drag force from moving direction.9,28 In this study, we found that most of the trajectories of the micromotors are straight and circular; we show the tracking trajectories of the micromotors in Figure S3. Figure 4 displays the consecutive four video frames of four types of micromotor with different deposition times of 1800, 1200, 600, and 300 s in 3 wt % H2O2 fuel environment. The oxygen bubbles generated by the inside of the micromotor come out one by one and propel the micromotor step by step. Once the bubble detaches from the microtube, it will expand
(2)
where V is the mean average velocity, L is the linear length, and D is the diffusion coefficient in order to figure out the role of Brownian motion in the movement of micromotors. The diffusion coefficient D in 3 wt % H2O2 can be obtained from the method built by Tirado et al.26 The calculated Peclet numbers of four types of micromotor (1800, 1200, 600, and 300 s) are 1.55 × 104, 8.02 × 103, 3.60 × 103, and 824, respectively, which indicate negligible Brownian motion in the movement of micromotors. Besides, smaller dimension micromotors tend to be affected by the effect of Brownian motion, which is in agreement with a previous report that nanosized tubes possess a smaller Peclet number and strong diffusion.22 Since the movement of a micromotor is based on the recoil mechanism of oxygen production, oxygen bubble nucleation− D
DOI: 10.1021/acsaem.8b00605 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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Figure 4. Consecutive video frames of four types of micromotors in 3 wt % H2O2 for bubble diameter study. (A) Microtubes electrodeposited at −20 mA for 1800 s. (B) Microtubes with shorter deposition time of 1200 s at −20 mA. (C) Microtubes prepared by electrodeposition at −20 mA for 600 s. (D) Microtubes electrodeposited at −20 mA for 300 s. Scale bars are 50 μm.
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due to the change of Laplace pressure which can be easily observed in video frames. A visible tail of a bubble is clearly observed at one end of a microtube. The video frames clearly demonstrate that longer lengths of micromotors produce bubbles with larger diameters in a faster manner.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00605. SEM images of tubular micromotors prepared by electrodeposition time of 200 s, histogram of length comparison of three different batches of micromotors, trajectories of the motion of four types of micromotors, and geometric parameters of four types of micromotors (PDF)
4. CONCLUSION Four types of micromotors are prepared by membrane template-assisted electrodeposition with different plating times. The geometric parameters associated with their influence on the dynamics of the micromotor are studied. It is found that plating time not only decides the length of the microtube but also alters the thickness of the wall of the microtube. The length of the micromotor is linearly proportional to plating time, which means shorter electrodeposition time results in shorter micromotor length. The propulsion dynamics of the micromotor is influenced by both geometric parameters and chemical environment. A shorter micromotor with smaller catalytic area requires a higher fuel environment to move, which is demonstrated by the absence of movement of a micromotor for a plating time 300 s in a 1 wt % H2O2 environment. Bubble ejection frequency and bubble diameter are further explored with relation to the velocity of the micromotor. Longer micromotors generate a greater amount of oxygen because of higher Pt inner surface area. Micromotors with longer lengths produced bigger bubbles, which contribute to their higher average velocity. This fundamental study shows the concern about the dimension of a microtube and its influence upon the propulsion dynamic of a microtube, which will be helpful for practical applications in the future. Moreover, this kind of study can be done in electrodeposition membrane template-assisted multilayer micromotors as well as in micromotors made by different catalytic materials such as Ag, Pd, and MnO2 to see how the electrodeposition time affects their performance.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Martin Pumera: 0000-0001-5846-2951 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the project Advanced Functional Nanorobots (Registration No. CZ.02.1.01/0.0/0.0/15_003/ 0000444 financed by the EFRR). Funding from A*Star, Singapore (Grant No. SERC A1783c0005) is appreciated.
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DOI: 10.1021/acsaem.8b00605 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
