A method to prepare catalytically active Janus colloids that can "swim" in fluids and determine their 3D trajectories is presented.
The overall goal of this experiment is to measure the 3D trajectory for a catalytic swimming device. This technique can help explain a variety of phenomena for swimming devices, such as how they respond to chemical gradients and gravitational fields while they're moving in volt solutions. A key advantage for this technique is that it can be applied using any conventional fluorescence microscope.
This technique will be demonstrated by my PhD student, Richard Archer. For this protocol, prepare glass slides as described in the text. Next, prepare the colloidal dispersion for deposition on the slide.
First, pipette ten microliters of aqueous 10%by weight fluorescent colloid solution into 990 microliters of ethanol for a one milliliter, 0.1%by weight colloidal suspension. Then, vortex the mix for ten seconds. Next, spin coat the colloidal dispersion onto the prepared glass slide substrate.
Prepare to load 100 microliters of the diluted colloidal solution onto the slide. Once at 2000 rpm, gradually deposit the suspension onto the center of the slide. Spin for 30 seconds from the start of the deposition.
Transfer the coated glass slide to an optical microscope and verify that an even dispersion of mostly non-touching separate colloids covers the central region of the slide. Next, vacuum evaporate platinum metal onto the glass slide in a metal evaporator. Make certain to load the slide with the colloids facing the evaporation source.
Use a platinum metal evaporation source and deposit 15 nanometers of platinum on the slide. After the metal application, store the slide under an inert atmosphere. This completes the fabrication of stock Janus particles.
The first step is to suspend the Janus particles in solution. To do this, prepare a one by one centimeter square of lens tissue and dampen the end of it with ten microliters of DI water. Then, holding the paper with tweezers, gently rub the wetted part along the surface of the platinum-coated colloid decorated glass slide.
Next, submerge the lens tissue into a tube with 1.5 milliliters of DI water. Cap the tube and manually shake it for 30 seconds. Then, remove the lens tissue and pipette one milliliter of the water, now containing colloids, into a small tube with one milliliter of 30%weight by volume hydrogen peroxide solution.
Gently mix the solutions. Then, transfer the tube to an ultrasonic bath at room temperature. The container should not be sealed, as oxygen may need to escape.
After five minutes of sonication, let the mixture incubate for 25 minutes at room temperature without any agitation. Meanwhile, dry out 100 microliters of the remaining aqueous colloidal solution and document it with a scanning electron microscope to verify the Janus colloid structure. Next, add one milliliter of DI water to the two milliliters of solution to reduce the hydrogen peroxide concentration to ten percent, which is a suitable fuel strength for fast propulsion of the Janus colloids.
Then, fill a low volume rectangular quartz glass cuvette with the prepared solution, and loosely attach a push-in cap so that the solution can breathe. Now, load the cuvette into a fluorescence microscope as outlined in the text protocol. Before starting a video capture, quickly focus the microscope so that the particle of interest produces a concentric ring with the particle under the focus position.
Do not move the focus plane during video capture. Once the particle of interest is found, record it with 30 second videos at 30 frames per second. About 20 videos from one experiment will provide enough detail for trajectory reconstruction which is described in the text protocol.
Colloids were deposited on a clean glass slide. Prior to depositing the platinum, the dispersion of polystyrene microspheres on the surface of the slide was observed using an optical microscope. The scale bar is 40 microns.
After platinum was added, an SEM image was taken to confirm the desired hemispherical platinum layer. The scale bar is at two microns. Fluorescent Janus swimmers were clearly visible when fixed in Gellan gum a symmetrical ring features under optimally defocused conditions.
The radius of the ring was used to determine the relative Z position of the colloid. The centers of the colloid were calculated by extracting a series of vertical and horizontal lines and finding the mean midpoint between the bright peaks. Then, the ring radii were calculated from the peak intensity of the spline fitted to the average pixel wave values radiating out from the ring's center.
A calibration curve was then made using a fixed colloidal sample and calibrated microscope to relate apparent colloidal size and distance from the focused position. Thus, the three-dimensional trajectory for a fluorescent Janus particle swimmer was arrived at from the data. After watching this experiment, you should now be able to track swimming devices in three dimensions using a conventional fluorescence microscope.
This method has been used by researchers to explore phenomena such as gravitaxis. This experiment involves use of hydrogen peroxide which is a hazardous chemical and this is particularly dangerous when it's combined with the catalytic swimming devices due to the evolution of lots of oxygen gas. So during these stages, it's important that the container is not securely sealed.
