Name: plasmonic trapping and release of nanoparticles in a monitoring environment
Uploaded: 2017-04-04T11:00:00
Description: Watch this Scientific Journal Video about Plasmonic Trapping and Release of Nanoparticles in a Monitoring Environment at JoVE.com

This work was supported&#160;by the ICT R&#38;D program of MSIP/IITP (R0190-15-2040, Development of a contents configuration management system and a simulator for 3D printing using smart materials).

Caution: Please refer to all relevant material safety regulations before use. Several of the chemicals used in microchip fabrication are acutely toxic and carcinogenic. Please use all appropriate safety practices when performing the photolithography and etching processes, including the use of engineering controls (fume hood, hot plate, and aligner) and personal protective equipment (safety glasses, gloves, lab coat, full-length pants, and closed-toe shoes).
1. Fabrication of the PDMS Microchanne...

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The SMF cable was inserted in the SMF cable hole on the microchip, as shown in the rectangular dot of Figure 6a. Because the SMF cable hole is larger than the cable diameter, epoxy glue was used to seal the gap to block the leakage of the flowing particle solution. Before the application of epoxy glue, the gold block and cable edge should be coaxially aligned by hand using a microscope. Although it is ideal for the inserted cable edge and the nanohole to be coaxially aligned, a slight misalignment can be...

The ability to manipulate microscale objects is an indispensable feature for many micro/nano experiments. Direct contact manipulations can damage the manipulated objects. Releasing the previously held objects is also challenging because of stiction problems. To overcome these issues, several indirect methods using fluidic1, electric2, magnetic3, or photonic forces4,5,6,7,8 have been proposed. Plasmonic tweezers that use photonic forces are based on the physics of extraordinary field enhancement several orders larger than the incident intensity9. This extremely strong field enhancement enables the trapping of extremely small nanoparticles. For example, it has been shown to immobilize and manipulate nanoscale objects, such as polystyrene particles7,10,11,12,13,14, polymer chains15, proteins16, quantum dots17, and DNA molecules8,18. Without plasmonic tweezers, it is difficult to trap nanoparticles because they quickly disappear before they are effectively examined or because they are damaged due to the high intensity of the laser.
Many plasmonic studies have used various nanoscale gold structures. We can categorize the gold structures as protruding nanodisk types12,13,14,15,19,20,21 or suppressed nanohole types7,8,10,11,22,23. In terms of imaging convenience, the nanodisk types are more suitable than the nanohole types because, for the latter, the gold substrates can obstruct the observation view. Moreover, the plasmonic trapping occurs near the plasmonic structure and makes observation even more challenging. To the best of our knowledge, plasmonic trapping on nanohole types was only verified using indirect scattering signals. However, no successful direct observations, such as microscopic images, have been reported. Few studies have described the position of trapped particles. One such result was presented by Wang et al. They created a gold pillar on a gold substrate and observed the particle motion using a fluorescent microscope24. However, this is only effective for monitoring lateral movements not in the direction parallel to the beam axis.
In this paper, we introduce new fluidic microchip design and fabrication procedures. Using this chip, we demonstrate the monitoring of plasmonically trapped particles, both in directions parallel and orthogonal to the plasmonic nanostructure. Furthermore, we measure the maximal force of the immobilized particle by increasing the fluid velocity to find the tipping velocity in the microchip. This study is unique because most studies on plasmonic tweezers cannot quantitatively show the maximal trapping forces used in their experimental setups.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Negative photoresist&#160;</td>
			<td>MicroChem</td>
			<td>SU-8 2075</td>
		</tr>
		<tr>
			<td>Developer</td>
			<td>MicroChem</td>
			<td>SU-8 Developer</td>
		</tr>
		<tr>
			<td>Positive photoresist&#160;</td>
			<td>Merck Ltd.</td>
			<td>AZ GXR-601</td>
		</tr>
		<tr>
			<td>AZ Photoresist Developers</td>
			<td>Merck Ltd.</td>
			<td>AZ 300 MIF</td>
		</tr>
		<tr>
			<td>HMDS</td>
			<td>Merck Ltd.</td>
			<td>AZ Adhesion Promoter</td>
		</tr>
		<tr>
			<td>Aligner</td>
			<td>Midas System</td>
			<td>MDA 400M</td>
		</tr>
		<tr>
			<td>Atmospheric plasma machine&#160;</td>
			<td>Atmospheric Process 
			Plasma Co.</td>
			<td>IDP-1000</td>
		</tr>
		<tr>
			<td>Polydimethylsiloxane (PDMS)</td>
			<td>Dow Corning</td>
			<td>Sylgard 184 A/B</td>
		</tr>
		<tr>
			<td>Gold coated test slides</td>
			<td>EMF Co.</td>
			<td>TA124(Ti/Au)</td>
		</tr>
		<tr>
			<td>Au etchant&#160;</td>
			<td>Transene Inc.</td>
			<td>TFA</td>
		</tr>
		<tr>
			<td>Ti etchant&#160;</td>
			<td>Transene Inc.</td>
			<td>TFT</td>
		</tr>
		<tr>
			<td>40X objective lens&#160;</td>
			<td>Edmund Optics</td>
			<td>40X DIN</td>
		</tr>
		<tr>
			<td>60X water immersion 
			objective lens&#160;</td>
			<td>Olympus</td>
			<td>LUMPLFLN 60XW</td>
		</tr>
		<tr>
			<td>Optical fiber incident laser&#160;</td>
			<td>IPG Photonic</td>
			<td>YLR 10</td>
		</tr>
		<tr>
			<td>SMF coupler</td>
			<td>Thorlabs</td>
			<td>MBT612D/M</td>
		</tr>
		<tr>
			<td>Syringe micropump</td>
			<td>Harvard</td>
			<td>PC2 70-4501</td>
		</tr>
		<tr>
			<td>Fluorescent microscope&#160;</td>
			<td>Olympus</td>
			<td>IX-51</td>
		</tr>
		<tr>
			<td>Plasma system</td>
			<td>Femto Science Inc</td>
			<td>CUTE-MPR</td>
		</tr>
	</tbody>
</table>

plasmonic trapping and release of nanoparticles in a monitoring environment

Plasmonic tweezers use surface plasmon polaritons to confine polarizable nanoscale objects. Among the various designs of plasmonic tweezers, only a few can observe immobilized particles. Moreover, a limited number of studies have experimentally measured the exertable forces on the particles. The designs can be classified as the protruding nanodisk type or the suppressed nanohole type. For the latter, microscopic observation is extremely challenging. In this paper, a new plasmonic tweezer system is introduced to monitor particles, both in directions parallel and orthogonal to the symmetric axis of a plasmonic nanohole structure. This feature enables us to observe the movement of each particle near the rim of the nanohole. Furthermore, we can quantitatively estimate the maximal trapping forces using a new fluidic channel.

The fabrication process of the PDMS microchannel and nanohole gold plate is shown in Figures 1 and 2. The method to combine the two parts and the actual microchip is shown in Figure 3. The PDMS was cut to reveal the inside of the channel from the side of the microchip. However, it was difficult to observe the particles flowing in the channel because of the surface roughness of the cutting plane. Therefore, we introduced the PDMS coating m...

Watch this Scientific Journal Video about Plasmonic Trapping and Release of Nanoparticles in a Monitoring Environment at JoVE.com

Plasmonic Trapping and Release of Nanoparticles in a Monitoring Environment

A microchip fabrication process that incorporates plasmonic tweezers is presented here. The microchip enables the imaging of a trapped particle to measure maximal trapping forces.

Plasmonic tweezers use surface plasmon polaritons to confine polarizable nanoscale objects. Among the various designs of plasmonic tweezers, only a few ...

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.

Research

JoVE Journal

Engineering

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One of the most widely used methods for manufacturing colloidal gold nanospherical particles involves the reduction of chloroauric acid (HAuCl4) to ...