Plasmonic Trapping and Release of Nanoparticles in a Monitoring Environment

The overall goal of this experiment is to monitor particles in two dimensions, both parallel and orthogonal to the the symmetric axis of a plasmonic nanohole structure and to quantitatively estimate the maximum trapping force of the system. This method can help answer questions in the life sciences relating to rapid immunoassay development, cell and bacterial trapping, liposome analysis, and nanoparticle drug release studies. The main advantage of this technique is that it allows the monitoring of the particle motion in two dimensions, one in the laser beam axis, and the other in the direction orthogonal to it.

To begin, fabricate the microchannel mold using standard photolithographic techniques and place the wafer into a 150 millimeter diameter Petri dish. Then, mix 10 parts PDMS base with one part curing agent and stir the mixture for two minutes. Once combined, pour 100 milliliters of the PDMS mixture over the wafer.

Place the Petri dish into a vacuum chamber and pull a vacuum to remove any bubbles from the PDMS. Next, place the Petri dish in an oven for two hours at 80 degrees Celsius to solidify the PDMS solution. Once cool, remove the dish from the oven, and cut along the contours of the PDMS microchannel with a razor blade, and detach it from the wafer.

Then, use a 1.5 millimeter biopsy punch to form 1.5 millimeter inlet and outlet holes at each end of the PDMS microchannel. Use a 0.3 millimeter punch to form a hole for the single-mode optical fiber cable at the center of the micro channel. Start by cleaning a commercially available gold plate as described in the accompanying text protocol.

Then, place the plate onto spin coater, and pour 0.5 milliliters of hexamethyldisilazane onto the gold plate. Spin-coat the plate for 40 seconds at 3000 RPMs. Next, pour 0.5 milliliters of a positive photoresist on top of the spin-coated hexamethyldisilazane, and spin-coat the plate for an additional 40 seconds at 3000 RPMs.

Remove the plate from the spin-coater, and place it on hot plate for 90 seconds at 110 degrees Celsius to soft-bake the photoresist. Then, use scotch tape to fix the film mask on the glass wafer, and place the soft-baked gold plate on the substrate stage. Expose the sample to UV light for 4 and a half seconds to dissolve the photoresist and create a gold rectangle that is 400 microns long by 150 microns wide.

Next, immerse the gold plate in the photoresist developer for one minute to remove the dissolved photoresist. Then, rinse the gold plate with deionized water and dry it using nitrogen gas. Immerse the gold plate in the gold etchant for 45 seconds to remove the exposed gold layer.

Then, rinse the gold plate with deionized water and dry it again using nitrogen gas. Next, immerse the plate in the titanium etchant for five seconds to remove the exposed titanium, then once more, rinse the gold plate with deionized water and dry it with nitrogen gas. Remove the remaining photoresist on the gold plate by immersing it sequentially in acetone, methanol, and finally deionized water for three minutes each.

Rinse the plate three times with deionized water for 10 seconds each, and then dry the plate once more with nitrogen gas. Next, place the plate on hot plate for three minutes at 120 degrees Celsius to completely remove any remaining moisture. Using a focused ion beam, mill a 400 nanometer nanohole at the center of the fabricated gold block as described in the accompanying text protocol.

Place both the PDMS microchannel and the gold plate into a plasma chamber. Treat the contacting surfaces for one minute using oxygen plasma. Fix the gold plate on the substrate stage of the aligner.

Then, use the camera on the aligner to locate the centers of the SMF cable hole and the hole in the gold block so that they are aligned on the same axis. Lift the manual stage to combine the two parts. Next, pour two milliliters of a mixed PDMS solution into a Petri dish, and spin-coat the dish for 30 seconds at 1000 RPMs.

Then, take the cut-out microchip and place the surface that is going to be located on the microscope into the PDMS that is on the Petri dish. Place the Petri dish in the oven for one hour at 80 degrees Celsius to solidify the PDMS solution. Once cooled, cut the border of the microchip and PDMS using a razor blade, and subsequently detach it from the Petri dish.

First, connect a 40x objective lens to the microscope objective mount on the single mode optical fiber cable coupler. Next, fix the single mode optical fiber cable on the fiber clamp of the cable coupler. Align the incident laser beam to fill in the back aperture of the objective lens.

Then, focus the laser beam to the core of the cable by adjusting the three axis manual stage equipped on the coupler. Measure the laser power prior to the insertion at the edge of the fiber cable, because the fixed fiber cable at the microchip cannot be detached. Then, insert the opposite end of the cable into the cable hole of the microchip.

Manually align the fiber cable using visual feedback so that it is perpendicular to the gold block that hosts the nanohole. The end of the inserted fiber cable should not enter the microchannel so that it does not block the fluid flow. Finally, seal the cable hole using epoxy glue to block the leakage of the flowing particle solution from the 87.5 micrometer gap between the cable hole and the cladding of the optical fiber cable.

Attach a syringe containing a solution of 100 nanometer diameter polystyrene particles to a syringe micropump. Next, connect tubes to the inlet and outlet holes of the microchip. Once connected, set the syringe pump to 20 micrometers per second and inject the particle solution into the microchip.

Next, turn on the fluorescent lamp and confirm that the fluorescent particle can be observed in the channel. When the particle solution exits the outlet of the microchip, reduce the pump speed to 3.4 micrometers per second. Then, turn on the laser source so that it emits the laser into the nanohole.

This will trap the fluorescent particle at the rim of the nanohole. Finally, ramp the fluid speed in increments of 0.4 micrometers per second by controlling the micropump until the trapped particle escapes. Use this information to obtain the maximal trapping force for each laser intensity.

Shown here is an enlarged version of the microchannel setup showing the alignment of the parts and the location of the nanohole. As 100 nanometer fluorescent polystyrene particles flow through the microchannel, they become trapped by the laser at the nanohole. Untrapped particles are also shown as they flow past the nanohole, while the trapped particle stays in position.

Once the flow speed was increased, the trapped particle was able to escape. After several practices, the procedures can be done within about 14 hours, if it is performed properly. While attempting this procedure, it is important to improve the microchip's size surface roughness of the PDMS coating.

After watching this video, you should have a good understanding of how to monitor the particles in both parallel and orthogonal directions to the symmetric axis of a plasmonic nanostructure.