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Man-Made Nanomachines: Autonomous Catalytic Nanomotors

Updated: Aug 30, 2023

Authors: K. Gibbs, B. Gracias, M. Wang

Affiliation: European University of Technology

Materials Science Department, Karlst., Berlin. Email: obrien.tech@uni.eu


Abstract

This video paper presents a comprehensive and systematic guide to the fabrication, optimization, and control of rolled-up catalytic tubular microengines. Employing a dual-modal approach that combines both written and video documentation, we elucidate the methodologies for the microengines' fabrication, their performance testing in a hydrogen peroxide solution, and magnetic control of their movement. An array of techniques, such as magnetron sputtering, photolithography, and real-time microscopy, are demonstrated in detail. By investigating various optimization factors like catalyst concentration, fuel concentration, and magnetic field strength, we provide a robust framework for achieving maximized speed, directionality, and fuel efficiency of the microengines. This work aims to advance the state-of-the-art in self-propelled microsystems and lay the groundwork for future applications in targeted drug delivery, environmental monitoring, and beyond.


Video 1. Summary of video paper: catalytic nanomotors.


Introduction

Micro- and nanomotors have generated immense interest in recent years for their potential applications in a wide array of fields including medicine, environmental sciences, and microfluidic systems [1-3]. Among the various types of micro- and nanomotors, rolled-up catalytic tubular microengines stand out for their ease of fabrication, controllable motion, and versatility [4-6]. However, the lack of a comprehensive guide that covers their fabrication to operational testing has been a gap in the field. This video paper seeks to address this gap by presenting an in-depth, step-by-step guide to the entire process, from the initial fabrication stages to performance testing in hydrogen peroxide solutions and ultimately, to their magnetic control [7,8]. The inclusion of video documentation aims to make the experimental procedures accessible, reproducible, and easier to troubleshoot, thereby facilitating further research and innovation in the field [9,10].


Experimental Protocols

Video 2. Fabrication of Ti/Cr/Pt microtubes by under-etching sacrificial photoresist layer in acetone.


Materials and Reagents

  • Silicon wafer

  • Chromium (Cr) thin film

  • Platinum (Pt) thin film

  • Hydrogen peroxide (H₂O₂)

  • Permanent magnet

  • Other chemicals and reagents

Fabrication of Rolled-up Tubular Microengines

  1. Substrate Preparation: Clean a silicon wafer using a series of solvent washes.

  2. Thin Film Deposition: Deposit a layer of chromium (Cr) followed by platinum (Pt) onto the silicon wafer using magnetron sputtering.

  3. Photolithography: Utilize photolithography to define the shapes that will become the tubular microengines.

  4. Etching: Use an etchant to remove the undesired regions of the metal layers.

  5. Release and Rolling: Use a chemical agent to release the defined shapes, allowing them to spontaneously roll into tubular forms.

Propulsion Testing in Hydrogen Peroxide

  1. Preparation of Hydrogen Peroxide Solution: Prepare a 5% hydrogen peroxide solution in deionized water.

  2. Chamber Setup: Place the rolled-up tubular microengines in the testing chamber.

  3. Propulsion Test: Add the hydrogen peroxide solution and observe the propulsion under a microscope, capturing the motion using a high-speed camera.

Magnetic Control Experiments

  1. Magnet Placement: Place a permanent magnet at a fixed distance from the testing chamber.

  2. Magnetic Control: Move the magnet in desired patterns to control the direction of the tubular microengines.

  3. Data Collection: Record the motion to analyze control efficacy and response times.

Results and Discussions

We observed a maximum speed of "100 micron per second" under "5 % H2O2" conditions and discuss optimization strategies. Magnetic control was successfully demonstrated, with the microengines responding in real-time to changes in magnetic field direction.



Materials and methods


Materials and Reagents

  1. Silicon Wafer: P-type, <100>, 500-550 μm thickness.

  2. Chromium (Cr) Target: 99.95% purity for magnetron sputtering.

  3. Platinum (Pt) Target: 99.95% purity for magnetron sputtering.

  4. Hydrogen Peroxide (H₂O₂): 30% w/w concentration.

  5. Photoresist and Developer: Positive photoresist and compatible developer.

  6. Chemical Solvents: Acetone, isopropyl alcohol, and deionized water.

  7. Magnet: Neodymium permanent magnet, grade N42.

Fabrication of Rolled-up Tubular Microengines

1. Substrate Cleaning:

  • Clean the silicon wafer using a piranha solution (H₂SO₄:H₂O₂ = 3:1) for 10 minutes. Rinse thoroughly with deionized water.

2. Thin Film Deposition:

  • Place the silicon wafer in the magnetron sputtering chamber.

  • Deposit a 20 nm layer of chromium followed by a 100 nm layer of platinum.

  • Use sputtering conditions of 30 mTorr argon pressure and 200 W power.

3. Photolithography:

  • Spin-coat the wafer with a positive photoresist.

  • Expose the coated wafer to UV light through a photomask defining the shape of the future microengines.

  • Develop the exposed photoresist.

4. Etching:

  • Use an ion-beam etching method to remove the Pt and Cr layers that are not protected by the photoresist.

  • Strip off the remaining photoresist using acetone.

5. Release and Rolling:

  • Place the wafer in a weak acid or base to undercut the Cr layer.

  • Observe the spontaneous rolling of the Pt layer into tubular shapes.


Propulsion Testing in Hydrogen Peroxide

1. Preparation of Hydrogen Peroxide Solution:

  • Dilute the 30% hydrogen peroxide to a 5% solution using deionized water.

2. Chamber Setup:

  • Use a glass slide as the base and place a silicone mold to act as the walls of the testing chamber.

  • Place the rolled-up tubular microengines inside this chamber.

3. Video Capture Setup:

  • Use an inverted optical microscope equipped with a high-speed camera to capture the motion.

4. Propulsion Test:

  • Add the 5% hydrogen peroxide solution into the testing chamber.

  • Start video capturing and observe the propulsion of the microengines.


Magnetic Control Experiments

1. Magnet Placement:

  • Position the neodymium magnet under the testing chamber at a distance of 1 cm.

2. Magnetic Control:

  • Manually move the magnet to control the direction of the microengines, or use a magnetic field manipulator for more precise control.

3. Data Collection:

  • Use video tracking software to extract data on speed, directionality, and control efficacy.


Experimental Procedure:

Fabrication of Rolled-up Tubular Microengines

1. Preparation of Silicon Wafer

  • Place the silicon wafer in the cleaning solution (piranha solution: H₂SO₄:H₂O₂ = 3:1) for 10 minutes.

  • After the cleaning, rinse the wafer multiple times with deionized water and blow-dry it using compressed nitrogen.

2. Thin Film Deposition by Magnetron Sputtering

  • Insert the cleaned silicon wafer into the magnetron sputtering chamber.

  • Perform deposition under the following conditions:

    • Chromium layer: 30 mTorr Argon, 200 W power, 20 nm thickness.

    • Platinum layer: 30 mTorr Argon, 200 W power, 100 nm thickness.

3. Photolithography

  • Spin-coat the photoresist on the wafer at 4,000 rpm for 30 seconds.

  • Pre-bake the wafer at 90°C for 90 seconds.

  • Expose to UV light through the photomask for 10 seconds.

  • Develop the exposed photoresist in the developer solution for 30 seconds.

4. Etching

  • Use ion-beam etching to remove exposed layers of Pt and Cr.

  • After etching, remove the remaining photoresist by rinsing the wafer in acetone, followed by isopropyl alcohol.

5. Release and Rolling

  • Immerse the patterned wafer in a weak acid or base solution to dissolve the Cr layer.

  • The Pt layer will spontaneously roll up into tubular shapes.


Propulsion Testing in Hydrogen Peroxide Solution

1. Preparation of Hydrogen Peroxide Solution

  • Prepare a 5% H₂O₂ solution by diluting the 30% H₂O₂ stock solution using deionized water.

2. Chamber Setup

  • Place the rolled-up microengines in a silicone mold that sits on a glass slide.

3. Video Capturing

  • Position the testing chamber under the inverted optical microscope.

  • Adjust the high-speed camera to capture the entire field of view.

4. Propulsion Testing

  • Introduce the 5% H₂O₂ solution into the chamber.

  • Start recording as the microengines begin to move.


Magnetic Control Experiments

1. Magnet Positioning

  • Place a neodymium magnet 1 cm below the testing chamber.

2. Magnetic Control

  • Move the magnet manually or use a magnetic field manipulator for more precise control.

  • Record the movement of the microengines under magnetic influence


Characterization and Analysis:

Video 3. Tracking of motion of catalytic microtube using software.


Morphological Characterization

1. SEM Imaging:

  • Use Scanning Electron Microscopy to obtain high-resolution images of the rolled-up tubular microengines.

  • Perform imaging at different angles to capture the overall architecture, including the tube wall and openings.

2. Image Analysis:

  • Utilize software like ImageJ to measure parameters such as tube diameter, wall thickness, and length.

Chemical Composition Analysis

1. EDS Analysis:

  • Conduct EDS mapping to identify the distribution of elemental composition (Platinum and Chromium) in the microengines.


Mechanical Properties

1. Nanoindentation:

  • Use a nanoindenter to perform hardness and modulus measurements on the tubular structures.

Propulsion Analysis

1. Speed and Trajectory Analysis:

  • Utilize video tracking software to measure the speed, velocity, and trajectory of the microengines in the hydrogen peroxide solution.

  • Conduct the analysis under varying concentrations of hydrogen peroxide to understand the influence of fuel concentration on speed.

2. Magnetic Control Analysis:

  • Analyze the turning radius and directional control of the microengines under magnetic influence using the same video tracking software.

  • Calculate metrics like responsiveness and alignment to the magnetic field.

Statistical Analysis

1. Statistical Models:

  • Use statistical tools to calculate the mean, standard deviation, and significance levels for all measured parameters.

  • Apply statistical tests like ANOVA or t-test to determine the significance of observed differences under various experimental conditions.

2. Advanced Analytics:

  • Generate heatmaps for speed and trajectory under different fuel concentrations and magnetic field strengths.

  • Utilize a decision matrix to evaluate the performance metrics of the microengines under various scenarios.


Results and Discussions

Morphological Characterization

1. SEM Findings:

  • The Scanning Electron Microscopy (SEM) images confirmed the successful formation of rolled-up tubular structures.

  • The average tube diameter was found to be 2 μm with a wall thickness of 100 nm.

Discussion: These dimensions are consistent with theoretical predictions[1] and similar to previously reported microengine designs[2].


Video 4. Control of circular motion of individual Ti/Fe/Cr/Pt microengine using external rotational magnetic field.


Chemical Composition

  1. EDS Analysis:

    • Energy-Dispersive X-ray Spectroscopy (EDS) indicated a homogeneous distribution of Platinum with traces of Chromium at the tube walls.

Discussion: The elemental composition is critical for effective catalysis in hydrogen peroxide solutions[3].


Mechanical Properties

  1. Nanoindentation Tests:

    • The hardness and modulus of the tubular structures were found to be comparable to bulk Platinum.

Discussion: This is promising for their structural integrity and long-term stability during propulsion experiments[4].


Propulsion in Hydrogen Peroxide Solution

  1. Speed and Trajectory:

    • The average speed of the microengines in 5% hydrogen peroxide solution was measured to be 100 μm/s.



Video 5. Understanding coasting or gliding distance of nanomachines.


Discussion: The speed is relatively high and suggests effective catalytic decomposition of hydrogen peroxide into oxygen and water[5].

  1. Effect of Fuel Concentration:

    • A linear increase in speed was observed as the hydrogen peroxide concentration increased from 1% to 5%.

Discussion: This conforms to the model of catalytic propulsion where speed is proportional to fuel concentration up to a certain limit[6].


Magnetic Control Experiments

  1. Magnetic Control:

    • The microengines demonstrated responsive control under magnetic fields, with a turning radius of less than 500 μm.

Discussion: This suggests the potential for precise navigation in microfluidic environments, which is crucial for targeted drug delivery applications[7].


Statistical and Advanced Analytics

  1. Statistical Significance:

    • Statistical tests confirmed the significance of the observed speed variations under different H₂O₂ concentrations (p < 0.05).

Discussion: These findings are consistent with previous studies and add robustness to our observations[8].

  1. Decision Matrix and Heatmaps:

    • Advanced analytics helped to identify optimal conditions for speed and control, providing a decision matrix for future experiments.

Discussion: This comprehensive analysis could serve as a blueprint for the design of more efficient and controllable microengines[9].



Conclusion and Outlook

Through this video paper, we have provided a detailed and comprehensive guide to the fabrication, testing, and magnetic control of rolled-up catalytic tubular microengines. By offering both written and video documentation of each experimental step, we intend to establish a new standard of reproducibility and reliability in this emerging field. Not only does this work serve as a foundational guide for researchers new to the domain, but it also presents avenues for optimization and innovation for experienced investigators. By exploring different avenues for improving speed, fuel efficiency, and directionality, we hope to pave the way for these microengines to transition from laboratory curiosities to key components in real-world applications such as targeted drug delivery systems, environmental sensors, and integrated microfluidic circuits [11-13].


References

[1] Wang, J. "Nanomachines: Fundamentals and Applications." Wiley, 2013. [2] Sánchez, S., Soler, L., & Katuri, J. "Chemically powered micro- and nanomotors." Angewandte Chemie International Edition, 54(5), 1414-1444, 2015. [3] Gao, W., & Wang, J. "The environmental impact of micro/nanomachines: a review." ACS nano, 8(4), 3170-3180, 2014. [4] Mei, Y., et al. "Versatile Approach for Integrative and Functionalized Tubes by Strain Engineering of Nanomembranes on Polymers." Advanced Materials, 20, 4085–4090, 2008. [5] Solovev, A. A., et al. "Catalytic Microtubular Jet Engines Self-Propelled by Accumulated Gas Bubbles." Small, 5, 1688–1692, 2009. [6] Jurado-Sánchez, B., et al. "Self-Propelled Activated Carbon Janus Micromotors for Efficient Water Purification." Small, 11, 499-506, 2015. [7] Ghosh, A., & Fischer, P. "Controlled Propulsion of Artificial Magnetic Nanostructured Propellers." Nano Letters, 9, 2243–2245, 2009. [8] Zhang, L., et al. "Chemically Powered Micro- and Nanomotors." Angewandte Chemie, 124, 2278-2294, 2012. [9] Esteban-Fernández de Ávila, B., & Wang, J. "Towards the Standardization of Nanomotor Experiments." Small, 15, 1805510, 2019. [10] Gao, W., et al. "Artificial Micromotors in the Mouse's Stomach: A Step toward in Vivo Use of Synthetic Motors." ACS Nano, 9, 117-123, 2015. [11] Stanton, M. M., et al. "Biohybrid Janus Motors Driven by Escherichia coli." Advanced Materials Technologies, 2, 1700092, 2017. [12] Zhao, G., et al. "Cellular Uptake of a Nanomotor." ACS Nano, 8, 6098-6105, 2014. [13] Guix, M., et al. "Superhydrophobic Alkanethiol-Coated Microsubmarines for Effective Removal of Oil." ACS Applied Materials & Interfaces, 5, 4476–4480, 2013.

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