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 Microchannel
<ol>
	<li>Fabrication of the microchannel mold by the photolithograph process
	<ol>
		<li>Completely remove foreign substances on the 4-inch Si wafer surface with piranha cleaning (Figure 1a). Mix sulfuric acid (H2SO4) and hydrogen peroxide (H2O2) at a ratio of 3:1 to make the piranha solution in the dish. Mix by gradually adding small amounts of the strong acid (H2O2) to the weak acid (H2SO4); reversing this order may cause an explosion because of the highly reactive strong acid.</li>
		<li>Immerse the wafer in the piranha solution for 10 min. Subsequently, immerse the wafer in deionized (DI) water for 3 min to remove the remaining piranha solution. Rinse the wafer with flowing DI water for 10 s. Repeat the rinsing procedure 3 times and dry with N2 gas to remove the remaining DI.</li>
		<li>Place the wafer on a hot plate for 20 min at 180 &#176;C to further dehydrate the wafer.</li>
		<li>Pour 5 mL of the negative photoresist on top of the wafer and spin coat for 45 s at 1,500 rpm (Figure 1b); after spin coating, a photoresist bead is created at the wafer edge because of the relatively high viscosity of the photoresist.</li>
		<li>Balance the photoresist-coated wafer by planarization on a leveling stand for 5 h.</li>
		<li>Place the photoresist-coated wafer on a hot plate for 12 min at 65 &#176;C, 35 min at 95 &#176;C, and 12 min at 65 &#176;C (soft baking).</li>
		<li>Fix the film mask on the mask holder and the soft-baked wafer on the substrate stage of the aligner. Expose to ultraviolet (UV) light for 43 s at 650 mJ/cm2 to solidify the photoresist.</li>
		<li>Place the wafer on the hot plate for 5 min at 65 &#176;C, 15 min at 95 &#176;C, and 5 min at 65 &#176;C (post-exposure baking).</li>
		<li>Immerse the wafer in the photoresist developer for 30 min to remove unsolidified photoresist.</li>
		<li>Rinse the wafer with isopropyl alcohol (IPA) and dry with N2 gas to remove the remaining IPA.</li>
	</ol>
	</li>
	<li>Manufacture of the PDMS microchannel
	<ol>
		<li>Treat the surface of the wafer and the photoresist mold for 1 min at a power of 200 W using an atmospheric plasma machine25; the gas flows of CH4 and He should be 6 and 30 sccm, respectively. Perform this hydrophobic treatment to easily detach the polydimethylsiloxane (PDMS) microchannel from the surface of the wafer and photoresist mold (Figure 1c).</li>
		<li>Prepare the PDMS solution by mixing the PDMS base and curing agent at a ratio of 10:1. Stir the mixture for 2 min.</li>
		<li>Place the wafer inside a Petri dish (150 mm x 15 mm) and add 100 mL of the PDMS solution. Remove the bubbles that were created from stirring using a desiccator.</li>
		<li>Place the Petri dish in the oven for 2 h at 80 &#176;C to solidify the PDMS solution (Figure 1d and h).</li>
		<li>Cut along the contours of the PDMS microchannel with a razor blade and detach it from the wafer; the fabricated PDMS microchannel should have the following dimensions: 13 mm long, 300 &#181;m wide, and 150 &#181;m high (Figures 1e, f, and i). 
		NOTE: Two types of holes are produced by a micropuncture to insert the single-mode fiber (SMF) cable and the tubes (inlet and outlet) on the PDMS microchannel (Figure 1g). The SMF cable is used to emit the laser beam to the nanohole milled on the gold plate. The tube is used to insert/extract the particle solution to/from the PDMS microchannel.</li>
		<li>Puncture 1.5-mm inlet and outlet holes at each end of the PDMS microchannel. Puncture a 0.3-mm SMF cable hole at the center of the PDMS microchannel.</li>
	</ol>
	</li>
</ol>
2. Etching Process of the Gold Plate
<ol>
	<li>Prepare a commercially available gold plate with the dimensions of 25 x 6.25 mm2 (Figure 2a).</li>
	<li>Remove any foreign substances on the gold plate with the following cleaning procedures. Clean in the following order by immersing in acetone, methanol, and DI water for 5 min each.</li>
	<li>Rinse the gold plate 3 times with DI water for 10 s and dry the plate with N2 gas to remove the remaining DI water.</li>
	<li>Place the gold plate on a hot plate for 20 min at 180 &#176;C to completely remove any remaining moisture.</li>
	<li>Pour 0.5 mL of hexamethyldisilazane (HMDS) on the gold plate and spin coat for 40 s at 3,000 rpm.</li>
	<li>Pour 0.5 mL of positive photoresist on top of the spin-coated HMDS and spin coat for 40 s at 3,000 rpm (Figure 2b).</li>
	<li>Place the photoresist-coated gold plate on the hot plate for 90 s at 110 &#176;C (soft baking).</li>
	<li>Fix the film mask on the glass wafer and place the soft-baked gold plate on the substrate stage. Expose to UV light for 4.5 s at 64 mJ/cm2 to dissolve the photoresist.</li>
	<li>Immerse the gold plate in the photoresist developer for 1 min to remove the dissolved photoresist (Figure 2c). Rinse the gold plate with DI water and dry with N2 gas.</li>
	<li>Immerse the gold plate in the Au etchant for 45 s at an etching rate of 28 &#197;/s to remove the exposed Au (Figure 2d). Rinse the gold plate with DI water and dry with N2 gas.</li>
	<li>Immerse the gold plate in the Ti etchant for 5 s at an etching rate of 25 &#197;/s to remove the exposed Ti (Figure 2e). Rinse the gold plate with DI water and dry with N2 gas.</li>
	<li>Remove the remaining photoresist on the gold plate by immersing it in acetone, methanol, and DI water for 3 min each (Figure 2f); immerse the plate in the written order.</li>
	<li>Rinse the gold plate 3 times with DI water for 10 s. Dry with N2 gas to remove the DI water.</li>
	<li>Place the gold plate on the hot plate for 3 min at 120 &#176;C to completely remove the moisture; the produced gold block should be 400 x 150 &#181;m2 (Figure 2h).</li>
	<li>Mill a 400-nm nanohole using a focused ion beam (FIB) at the center of the gold block that was fabricated after etching (Figures 2g and i). Create a 370-nm circle pattern to focus on the gold block with an ion accelerating voltage of 30 kV at 28 pA for 3 s.</li>
</ol>
3. Assembly of the Microchip
<ol>
	<li>Treat the two surfaces of the PDMS microchannel and gold plate for 1 min with O2 plasma to attach them together with a plasma system at a power of 80 W and a pressure of 825 mTorr25. 
	NOTE: It is notably difficult to attach them with precision because the gold block and PDMS microchannel are on the micrometer level. Hence, use an aligner with a camera and a manual stage.</li>
	<li>Fix the glass wafer that is used to attach the film mask to the mask holder of the aligner (Figure 3a).</li>
	<li>Attach the O2-plasma-treated PDMS microchannel to the glass wafer; because the PDMS is hydrophilic, it will easily attach to the glass wafer without any adhesion solution. Fix the gold plate on the substrate stage of the aligner (Figure 3a).</li>
	<li>Locate the centers of the SMF cable hole and gold block, which are aligned on the same axis, using the camera on the aligner. Lift the manual stage to combine the two parts (Figures 3b and c).</li>
</ol>
4. Improvement of the Microchip Side Surface Roughness by PDMS Coating
NOTE: The gold plate with fixed dimensions of 400 x 150 &#181;m2 is relatively more difficult to cut out than the PDMS material. Therefore, to detach the PDMS microchannel from the wafer, a razor blade is used to cut out a larger piece than the gold plate. After combining the two parts, the excess parts of the PDMS relative to the gold plate must then be cut so that the inside of the channel can be observed from the side using a microscope (Figure 4a). However, the cut surface, which is used as a window, has a high surface roughness and consequently produces cloudy images of the particles that flow in the channel (Figure 4b). Coating with the PDMS solution is performed again to resolve this problem.
<ol>
	<li>Prepare the PDMS solution by mixing the PDMS base and curing agent at a 10:1 ratio and stir for 2 min.</li>
	<li>Pour 2 mL of the PDMS solution into the Petri dish and perform the spin coating for 30 s at 1,000 rpm (Figure 4c).</li>
	<li>Place the microchip surface that is going to be located on the microscope on the Petri dish (Figure 4d). Place the Petri dish in the oven for 1 h at 80 &#176;C to solidify the PDMS solution.</li>
	<li>Cut the border of the microchip and PDMS using a razor blade and subsequently detach it from the Petri dish (Figures 4e, f).</li>
</ol>
5. Laser Coupling to Insert the SMF Cable to the Microchip
NOTE: For the plasmonic tweezer system, an optical fiber incident laser with a 1,064-nm wavelength is used. The SMF cable is used because the diameter of the incident laser (5 mm) is too immense to emit the laser beam at the nanohole milled on the gold block (400 x 150 &#181;m2) in the microchip. The cladding diameter of the SMF cable is 125 &#181;m. Thus, the incident laser and SMF cable must be coupled.
<ol>
	<li>Connect a 40X objective lens to the microscope objective mount on the SMF coupler. Fix the SMF cable on the fiber clamp of the SMF coupler. Align the incident laser beam to fill in the back aperture of the objective lens.</li>
	<li>Focus the laser beam to the core of the SMF cable by adjusting the three-axis manual stage equipped on the SMF coupler.</li>
	<li>Insert the opposite end of the SMF cable into the SMF cable hole of the microchip. 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.</li>
	<li>Seal the SMF cable hole using epoxy glue to block the leakage of the flowing particle solution from the gap between the SMF cable hole (300 &#181;m) and the cladding of the SMF cable (125 &#181;m); the end of the inserted fiber cable should not enter the microchannel to avoid the fluid flow. Manually align the fiber cable using visual feedback so that it is perpendicular to the gold block that hosts the nanohole.</li>
</ol>
6. Plasmonic Trapping of Single Fluorescent Polystyrene Particle in the Microchip
<ol>
	<li>Attach the syringe, which is filled with the particle solution, to a syringe micropump. Place the cover glass on the sample stage of the fluorescent microscope. Connect tubes to the inlet/outlet holes of the microchip. Place the PDMS-coated microchip surface on top of the cover glass.</li>
	<li>Position the microchip orthogonally to the 60X water immersion objective lens by observing the inside of the channel with the camera installed on the fluorescent microscope. Use transparent tape to fix the microchip in place. Connect the inlet tube of the microchip with the syringe needle.</li>
	<li>Insert the particle solution to the microchip by controlling the micropump at 20 &#181;m/s. At this moment, confirm that the fluorescent particle can be observed well in the channel when the fluorescent lamp is turned on.</li>
	<li>Wait until the particle solution exits from the outlet of the microchip. Set the speed to 3.4 &#181;m/s.</li>
	<li>Turn the laser source device so that it emits the laser into the nanohole; the fluorescent particle will be trapped at the rim of the nanohole.</li>
	<li>Ramp the fluid speed in increments of 0.4 &#181;m/s by controlling the micropump until the trapped particle escapes. Measure the fluid speed when the trapped particles escape. Obtain the maximal trapping force for each laser intensity using this measured fluid speed.</li>
</ol>

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The authors have nothing to disclose.

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 tolerated because the laser beam diverges once it is emitted from the end of the 0.14 NA SMF cable edge, and the beam affects a much larger region. Because the microchip was configured to be perpendicular to the optical axis of the microscope, we could not directly observe the location of the nanohole. The location of the nanohole can only be indirectly determined by observing the location of the plasmonically trapped particle at the nanohole. A solution can be provided by installing a camera at the fiber cable and using it to monitor the gold block.
The distinctive feature of the microchip is its ability to monitor particle motion near the plasmonic nanohole in real time. The motion of the particle follows the scenario described below. When the fluid streams the particles forward, some particles move toward the gold block. In some cases, a particle gets notably close to the rim of the nanohole due to attraction to the nanohole and eventually becomes immobilized. At this moment, the optical force exerted on the particle exceeds the fluid force. Subsequently, the immobilized particle escapes from the nanohole rim when the fluid velocity increases; thus, the fluid force becomes stronger than the optical force. The maximal trapping force can be measured from this terminal fluid velocity. However, the conventional drag force equation cannot be used because the particle is in physical contact with the gold wall at the nanohole. To consider the surface effect of the gold wall, we used the finite-element method, which considers the fluid motion near the surface, and obtained the fluid force.
We have introduced a new plasmonic tweezer setup that enables the monitoring of particle dynamics along the laser beam axis. In contrast, previous studies have only introduced particle movement in the plane perpendicular to the laser beam axis, such as with the nanoblock12, nanodisk13,14,19,21, nanostick20, and nanopyramid18. Furthermore, in the case of nanohole types, trapping can only be witnessed by monitoring the scattering signal, and not by visual monitoring10,11,23. However, we could not precisely measure the particle position because of the limited capabilities of current imaging techniques. The imaging quality should be further improved to confirm the exact dislocation measurements. This technique can be applied in the characterization and biosensing of a single molecule.

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><tbody><tr><td>Negative photoresist&amp;nbsp;</td><td>MicroChem</td><td>SU-8 2075</td><td></td></tr><tr><td>Developer</td><td>MicroChem</td><td>SU-8 Developer</td><td></td></tr><tr><td>Positive photoresist&amp;nbsp;</td><td>Merck Ltd.</td><td>AZ GXR-601</td><td></td></tr><tr><td>AZ Photoresist Developers</td><td>Merck Ltd.</td><td>AZ 300 MIF</td><td></td></tr><tr><td>HMDS</td><td>Merck Ltd.</td><td>AZ Adhesion Promoter</td><td></td></tr><tr><td>Aligner</td><td>Midas System</td><td>MDA 400M</td><td></td></tr><tr><td>Atmospheric plasma machine&amp;nbsp;</td><td>Atmospheric Process&lt;br/&gt; Plasma Co.</td><td>IDP-1000</td><td></td></tr><tr><td>Polydimethylsiloxane (PDMS)</td><td>Dow Corning</td><td>Sylgard 184 A/B</td><td></td></tr><tr><td>Gold coated test slides</td><td>EMF Co.</td><td>TA124(Ti/Au)</td><td></td></tr><tr><td>Au etchant&amp;nbsp;</td><td>Transene Inc.</td><td>TFA</td><td></td></tr><tr><td>Ti etchant&amp;nbsp;</td><td>Transene Inc.</td><td>TFT</td><td></td></tr><tr><td>40X objective lens&amp;nbsp;</td><td>Edmund Optics</td><td>40X DIN</td><td></td></tr><tr><td>60X water immersion&lt;br/&gt; objective lens&amp;nbsp;</td><td>Olympus</td><td>LUMPLFLN 60XW</td><td></td></tr><tr><td>Optical fiber incident laser&amp;nbsp;</td><td>IPG Photonic</td><td>YLR 10</td><td></td></tr><tr><td>SMF coupler</td><td>Thorlabs</td><td>MBT612D/M</td><td></td></tr><tr><td>Syringe micropump</td><td>Harvard</td><td>PC2 70-4501</td><td></td></tr><tr><td>Fluorescent microscope&amp;nbsp;</td><td>Olympus</td><td>IX-51</td><td></td></tr><tr><td>Plasma system</td><td>Femto Science Inc</td><td>CUTE-MPR</td><td></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 method to solve this problem, as shown in Figure 4.
We observed 5-&#181;m, flowing polystyrene particles in the microchip to confirm the effect of the PDMS coating. Figure 5 shows the actual fabricated microchip and particles observed in the microchip using the microscope. Figure 5a and c are the before and after appearances of the microchip. Figure 5b and d are the magnified surfaces of each. Figure 5e shows blurred particles flowing, whereas Figure 5f shows that the edges of the particles are notably clear and that movements can be monitored. As above, the PDMS coating of the microchip surface is essential to the monitoring of trapped particles.
Figure 6 shows the 100-nm polystyrene particle undergoing plasmonic optical trapping by the plasmonic tweezer system. An SMF cable with a 0.14 numerical aperture (NA) was used. A tube was inserted at the inlet/outlet holes of the microchip channel. A micropump was used to insert and collect the 100-nm fluorescent polystyrene particle solution. To emphasize the interior appearance of the trapped particle by the plasmonic phenomenon, the dotted parts of Figure 6a have been enlarged as an inset, Figure 6b. The laser delivered by the SMF cable inserted in the microchip was emitted to the nanohole of the gold block, and the particle flowed from left to right. Movement was detected at the nanohole rim, where the field enhancement was the strongest.
Figure 7 shows consecutive images where a 100-nm fluorescent polystyrene particle that flowed in the microchannel was trapped and released at the nanohole at the intensity of 0.42 mW/&#181;m2. The particles flowed at a constant speed of 3.4 &#181;m/s in the fluid direction, as shown in Figure 7a. After the laser was turned on, one of the particles was trapped at the nanohole, as shown in Figure 7b. On the contrary, another particle flowed into the stream, as shown in Figure 7c. Then, the flow speed was increased until the trapped particle escaped. Figure 7d shows the particle escaping from the trap. At this moment, we can estimate the trapping force with direct observation by measuring the fluid velocity when the particle escaped. We also worked in the opposite direction. Instead of increasing the fluid velocity, we gradually decreased the laser power in decrements of 1 mW and recorded the intensity when the particle escaped. This laser intensity is defined as the minimum trapping laser intensity and was measured to be 0.24 mW/&#181;m2.
<img alt="Figure 1" src="/files/ftp_upload/55258/55258fig1.jpg" /> 
Figure 1. Fabrication of the PDMS microchannel. (a) Preparation of the Si wafer. (b) Photoresist spin coating of the wafer. (c) Fabricated microchannel mold by the photolithography process. (d) PDMS solidification using an oven after pouring the PDMS solution on the wafer. (e) PDMS microchannel cutting. (f) PDMS microchannel detachment from the wafer. (g) Inlet/outlet and SMF cable holes punctured on the PDMS microchannel. (h) Actual solidified PDMS on the wafer. (i) Actual detached PDMS microchannel. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/55258/55258fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/55258/55258fig2.jpg" /> 
Figure 2. Fabrication of the nanohole on the gold plate after the etching process. (a) Deposition of Au and Ti on the glass. (b) Photoresist spin coating of the gold plate. (c) Dissolved photoresist removal after UV light exposure. (d) Au etching. (e) Ti etching. (f) Remaining photoresist removal. (g) Nanohole milling by a focused ion beam on the gold block. (h) Actual fabricated gold block. (i) Actual milled nanohole on the gold block. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/55258/55258fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/55258/55258fig3.jpg" /> 
Figure 3. Assembly process of the microchip. (a) Fix the PDMS microchannel and gold plate on the mask holder and substrate stage, respectively, equipped on the aligner. (b) Combination of the PDMS microchannel part and the gold plate after surface treatment with O2 plasma. (c, d) Assembled microchip after combination. (e) Removal of excess amount of the PDMS microchannel. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/55258/55258fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" src="/files/ftp_upload/55258/55258fig4.jpg" /> 
Figure 4. Process of surface roughness improvement by PDMS coating. (a) Remove excess amount using a razor blade after combining the two parts. (b) High surface roughness of the microchip after cutting. (c) PDMS solution spin coating in a Petri dish. (d) Dipping the window surface of the microchip into the spin-coated PDMS solution. (e) PDMS-coated microchip detachment from the Petri dish. (f) Improvement of surface roughness by PDMS coating. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/55258/55258fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" src="/files/ftp_upload/55258/55258fig5.jpg" /> 
Figure 5. Assembled microchip and observation of 5-&#181;m polystyrene particles in the microchannel before and after PDMS coating. (a, b) Microchip before PDMS coating and magnified view. (c, d) Microchip after PDMS coating and magnified view. (e, f) Observation of particles in the microchannel before and after PDMS coating. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/55258/55258fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" src="/files/ftp_upload/55258/55258fig6.jpg" /> 
Figure 6. Designed plasmonic tweezer system. (a) Schematic of the plasmonic tweezer system. (b) Trapping of a 100-nm fluorescent polystyrene particle at the rim of the nanohole in the microchip. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/55258/55258fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" src="/files/ftp_upload/55258/55258fig7.jpg" /> 
Figure 7. Trapping and releasing of a 100-nm fluorescent polystyrene particle in the microchannel. (a) The microchannel with a particle flowing into the stream. (b, c) Trapped particle at the nanohole compared to another particle. (d) Particle that escaped from the trap due to the increased fluid force. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/55258/55258fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

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 ...

plasmonic-trapping-release-nanoparticles-monitoring

Nanyang Technological University

Research

JoVE Journal

Engineering

7.6K Views.  Korea Basic Science Institute (KBSI).  Een microchip fabricageproces dat plasmonische pincet opgenomen wordt hier gepresenteerd. De microchip maakt het afbeelden van een gevangen deeltje trapping maximale krachten meten. 

Video: Plasmonic Trapping and Release of Nanoparticles in a Monitoring Environment

Polycrystalline Silicon Thin-film Solar cells with Plasmonic-enhanced Light-trapping

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To compensate for lower light absorption such physically thin devices have to incorporate light-trapping which increases their optical thickness. Light scattering by textured surfaces is a common technique but it cannot be universally applied to all solar cell technologies. Some cells, for example those made of evaporated silicon, are planar as produced and they require an alternative light-trapping means suitable for planar devices. Metal nanoparticles formed on planar silicon cell surface and capable of light scattering due to surface plasmon resonance is an effective approach.
The paper presents a fabrication procedure of evaporated polycrystalline silicon solar cells with plasmonic light-trapping and demonstrates how the cell quantum efficiency improves due to presence of metal nanoparticles.
To fabricate the cells a film consisting of alternative boron and phosphorous doped silicon layers is deposited on glass substrate by electron beam evaporation. An Initially amorphous film is crystallised and electronic defects are mitigated by annealing and hydrogen passivation. Metal grid contacts are applied to the layers of opposite polarity to extract electricity generated by the cell. Typically, such a ~2 &mu;m thick cell has a short-circuit current density (Jsc) of 14-16 mA/cm2, which can be increased up to 17-18 mA/cm2 (~25% higher) after application of a simple diffuse back reflector made of a white paint.
To implement plasmonic light-trapping a silver nanoparticle array is formed on the metallised cell silicon surface. A precursor silver film is deposited on the cell by thermal evaporation and annealed at 23&deg;C to form silver nanoparticles. Nanoparticle size and coverage, which affect plasmonic light-scattering, can be tuned for enhanced cell performance by varying the precursor film thickness and its annealing conditions. An optimised nanoparticle array alone results in cell Jsc enhancement of about 28%, similar to the effect of the diffuse reflector. The photocurrent can be further increased by coating the nanoparticles by a low refractive index dielectric, like MgF2, and applying the diffused reflector. The complete plasmonic cell structure comprises the polycrystalline silicon film, a silver nanoparticle array, a layer of MgF2, and a diffuse reflector. The Jsc for such cell is 21-23 mA/cm2, up to 45% higher than Jsc of the original cell without light-trapping or ~25% higher than Jsc for the cell with the diffuse reflector only.
Introduction
Light-trapping in silicon solar cells is commonly achieved via light scattering at textured interfaces. Scattered light travels through a cell at oblique angles for a longer distance and when such angles exceed the critical angle at the cell interfaces the light is permanently trapped in the cell by total internal reflection (Animation 1: Light-trapping). Although this scheme works well for most solar cells, there are developing technologies where ultra-thin Si layers are produced planar (e.g. layer-transfer technologies and epitaxial c-Si layers) 1 and or when such layers are not compatible with textures substrates (e.g. evaporated silicon) 2. For such originally planar Si layer alternative light trapping approaches, such as diffuse white paint reflector 3, silicon plasma texturing 4 or high refractive index nanoparticle reflector 5 have been suggested.
Metal nanoparticles can effectively scatter incident light into a higher refractive index material, like silicon, due to the surface plasmon resonance effect 6. They also can be easily formed on the planar silicon cell surface thus offering a light-trapping approach alternative to texturing. For a nanoparticle located at the air-silicon interface the scattered light fraction coupled into silicon exceeds 95% and a large faction of that light is scattered at angles above critical providing nearly ideal light-trapping condition (Animation 2: Plasmons on NP). The resonance can be tuned to the wavelength region, which is most important for a particular cell material and design, by varying the nanoparticle average size, surface coverage and local dielectric environment 6,7. Theoretical design principles of plasmonic nanoparticle solar cells have been suggested 8. In practice, Ag nanoparticle array is an ideal light-trapping partner for poly-Si thin-film solar cells because most of these design principle are naturally met. The simplest way of forming nanoparticles by thermal annealing of a thin precursor Ag film results in a random array with a relatively wide size and shape distribution, which is particularly suitable for light-trapping because such an array has a wide resonance peak, covering the wavelength range of 700-900 nm, important for poly-Si solar cell performance. The nanoparticle array can only be located on the rear poly-Si cell surface thus avoiding destructive interference between incident and scattered light which occurs for front-located nanoparticles 9. Moreover, poly-Si thin-film cells do not requires a passivating layer and the flat base-shaped nanoparticles (that naturally result from thermal annealing of a metal film) can be directly placed on silicon further increases plasmonic scattering efficiency due to surface plasmon-polariton resonance 10.
The cell with the plasmonic nanoparticle array as described above can have a photocurrent about 28% higher than the original cell. However, the array still transmits a significant amount of light which escapes through the rear of the cell and does not contribute into the current. This loss can be mitigated by adding a rear reflector to allow catching transmitted light and re-directing it back to the cell. Providing sufficient distance between the reflector and the nanoparticles (a few hundred nanometers) the reflected light will then experience one more plasmonic scattering event while passing through the nanoparticle array on re-entering the cell and the reflector itself can be made diffuse - both effects further facilitating light scattering and hence light-trapping. Importantly, the Ag nanoparticles have to be encapsulated with an inert and low refractive index dielectric, like MgF2 or SiO2, from the rear reflector to avoid mechanical and chemical damage 7. Low refractive index for this cladding layer is required to maintain a high coupling fraction into silicon and larger scattering angles, which are ensured by the high optical contrast between the media on both sides of the nanoparticle, silicon and dielectric 6. The photocurrent of the plasmonic cell with the diffuse rear reflector can be up to 45% higher than the current of the original cell or up to 25% higher than the current of an equivalent cell with the diffuse reflector only.

Polycrystalline silicon thin-film solar cells on glass are fabricated by deposition of boron and phosphorous doped silicon layers followed by crystallisation, defect passivation and metallisation. Plasmonic light-trapping is introduced by forming Ag nanoparticles on the silicon cell surface capped with a diffused reflector resulting in ~45% photocurrent enhancement.

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To ...

Optical Trapping of Nanoparticles

Optical trapping is a technique for immobilizing and manipulating small objects in a gentle way using light, and it has been widely applied in trapping and manipulating small biological particles. Ashkin and co-workers first demonstrated optical tweezers using a single focused beam1. The single beam trap can be described accurately using the perturbative gradient force formulation in the case of small Rayleigh regime particles1. In the perturbative regime, the optical power required for trapping a particle scales as the inverse fourth power of the particle size. High optical powers can damage dielectric particles and cause heating. For instance, trapped latex spheres of 109 nm in diameter were destroyed by a 15 mW beam in 25 sec1, which has serious implications for biological matter2,3.
A self-induced back-action (SIBA) optical trapping was proposed to trap 50 nm polystyrene spheres in the non-perturbative regime4. In a non-perturbative regime, even a small particle with little permittivity contrast to the background can influence significantly the ambient electromagnetic field and induce a large optical force. As a particle enters an illuminated aperture, light transmission increases dramatically because of dielectric loading. If the particle attempts to leave the aperture, decreased transmission causes a change in momentum outwards from the hole and, by Newton's Third Law, results in a force on the particle inwards into the hole, trapping the particle. The light transmission can be monitored; hence, the trap can become a sensor. The SIBA trapping technique can be further improved by using a double-nanohole structure.
The double-nanohole structure has been shown to give a strong local field enhancement5,6. Between the two sharp tips of the double-nanohole, a small particle can cause a large change in optical transmission, thereby inducing a large optical force. As a result, smaller nanoparticles can be trapped, such as 12 nm silicate spheres7 and 3.4 nm hydrodynamic radius bovine serum albumin proteins8. In this work, the experimental configuration used for nanoparticle trapping is outlined. First, we detail the assembly of the trapping setup which is based on a Thorlabs Optical Tweezer Kit. Next, we explain the nanofabrication procedure of the double-nanohole in a metal film, the fabrication of the microfluidic chamber and the sample preparation. Finally, we detail the data acquisition procedure and provide typical results for trapping 20 nm polystyrene nanospheres.

The following setup approach details low power optical trapping of dielectric nanoparticles using a double-nanohole in metal film.

Optical trapping is a technique for immobilizing and manipulating small objects in a gentle way using light, and it has been widely applied in trapping ...

Construction of a High Resolution Microscope with Conventional and Holographic Optical Trapping Capabilities

High resolution microscope systems with optical traps allow for precise manipulation of various refractive objects, such as dielectric beads 1 or cellular organelles 2,3, as well as for high spatial and temporal resolution readout of their position relative to the center of the trap. The system described herein has one such "traditional" trap operating at 980 nm. It additionally provides a second optical trapping system that uses a commercially available holographic package to simultaneously create and manipulate complex trapping patterns in the field of view of the microscope 4,5 at a wavelength of 1,064 nm. The combination of the two systems allows for the manipulation of multiple refractive objects at the same time while simultaneously conducting high speed and high resolution measurements of motion and force production at nanometer and piconewton scale.

The system described herein employs a traditional optical trap as well as an independent holographic optical trapping line, capable of creating and manipulating multiple traps. This allows for the creation of complex geometric arrangements of refractive particles while also permitting simultaneous high-speed, high-resolution measurements of the activity of biological enzymes.

High resolution microscope systems with optical traps allow for precise manipulation of various refractive objects, such as dielectric beads 1 or cellular ...

Utilization of Plasmonic and Photonic Crystal Nanostructures for Enhanced Micro- and Nanoparticle Manipulation

A method to manipulate the position and orientation of submicron particles nondestructively would be an incredibly useful tool for basic biological research. Perhaps the most widely used physical force to achieve noninvasive manipulation of small particles has been dielectrophoresis(DEP).1 However, DEP on its own lacks the versatility and precision that are desired when manipulating cells since it is traditionally done with stationary electrodes. Optical tweezers, which utilize a three dimensional electromagnetic field gradient to exert forces on small particles, achieve this desired versatility and precision.2 However, a major drawback of this approach is the high radiation intensity required to achieve the necessary force to trap a particle which can damage biological samples.3 A solution that allows trapping and sorting with lower optical intensities are optoelectronic tweezers (OET) but OET's have limitations with fine manipulation of small particles; being DEP-based technology also puts constraint on the property of the solution.4,5
This video article will describe two methods that decrease the intensity of the radiation needed for optical manipulation of living cells and also describe a method for orientation control. The first method is plasmonic tweezers which use a random gold nanoparticle (AuNP) array as a substrate for the sample as shown in Figure 1. The AuNP array converts the incident photons into localized surface plasmons (LSP) which consist of resonant dipole moments that radiate and generate a patterned radiation field with a large gradient in the cell solution. Initial work on surface plasmon enhanced trapping by Righini et al and our own modeling have shown the fields generated by the plasmonic substrate reduce the initial intensity required by enhancing the gradient field that traps the particle.6,7,8 The plasmonic approach allows for fine orientation control of ellipsoidal particles and cells with low optical intensities because of more efficient optical energy conversion into mechanical energy and a dipole-dependent radiation field. These fields are shown in figure 2 and the low trapping intensities are detailed in figures 4 and 5. The main problems with plasmonic tweezers are that the LSP's generate a considerable amount of heat and the trapping is only two dimensional. This heat generates convective flows and thermophoresis which can be powerful enough to expel submicron particles from the trap.9,10 The second approach that we will describe is utilizing periodic dielectric nanostructures to scatter incident light very efficiently into diffraction modes, as shown in figure 6.11 Ideally, one would make this structure out of a dielectric material to avoid the same heating problems experienced with the plasmonic tweezers but in our approach an aluminum-coated diffraction grating is used as a one-dimensional periodic dielectric nanostructure. Although it is not a semiconductor, it did not experience significant heating and effectively trapped small particles with low trapping intensities, as shown in figure 7. Alignment of particles with the grating substrate conceptually validates the proposition that a 2-D photonic crystal could allow precise rotation of non-spherical micron sized particles.10 The efficiencies of these optical traps are increased due to the enhanced fields produced by the nanostructures described in this paper.

Plasmonic tweezers and photonic crystal nanostructures are shown to produce useful enhancements in the efficiency and orientation control of optically trapping micro- and nano-particles.

A method to manipulate the position and orientation of submicron particles nondestructively would be an incredibly useful tool for basic biological ...

Bioengineering

Analyzing the Movement of the Nauplius 'Artemia salina' by Optical Tracking of Plasmonic Nanoparticles

We demonstrate how optical tweezers may provide a sensitive tool to analyze the fluidic vibrations generated by the movement of small aquatic organisms. A single gold nanoparticle held by an optical tweezer is used as a sensor to quantify the rhythmic motion of a Nauplius larva (Artemia salina) in a water sample. This is achieved by monitoring the time dependent displacement of the trapped nanoparticle as a consequence of the Nauplius activity. A Fourier analysis of the nanoparticle&#39;s position then yields a frequency spectrum that is characteristic to the motion of the observed species. This experiment demonstrates the capability of this method to measure and characterize the activity of small aquatic larvae without the requirement to observe them directly and to gain information about the position of the larvae with respect to the trapped particle. Overall, this approach could give an insight on the vitality of certain species found in an aquatic ecosystem and could expand the range of conventional methods for analyzing water samples.

Analyzing the Movement of the Nauplius 'Artemia salina' by Optical Tracking of Plasmonic Nanoparticles

We use optical tracking of plasmonic nanoparticles to probe and characterize the frequency movements of aquatic organisms.

We demonstrate how optical tweezers may provide a sensitive tool to analyze the fluidic vibrations generated by the movement of small aquatic organisms. A ...

Fabrication and Operation of a Nano-Optical Conveyor Belt

The technique of using focused laser beams to trap and exert forces on small particles has enabled many pivotal discoveries in the nanoscale biological and physical sciences over the past few decades. The progress made in this field invites further study of even smaller systems and at a larger scale, with tools that could be distributed more easily and made more widely available. Unfortunately, the fundamental laws of diffraction limit the minimum size of the focal spot of a laser beam, which makes particles smaller than a half-wavelength in diameter hard to trap and generally prevents an operator from discriminating between particles which are closer together than one half-wavelength. This precludes the optical manipulation of many closely-spaced nanoparticles and&#160;limits the&#160;resolution of optical-mechanical systems. Furthermore, manipulation using focused beams requires beam-forming or steering optics, which can be very bulky and expensive. To address these limitations in the system scalability of conventional optical trapping our lab has devised an alternative technique which utilizes near-field optics to move particles across a chip. Instead of focusing laser beams in the far-field, the optical near field of plasmonic resonators produces the necessary local optical intensity enhancement to overcome the restrictions of diffraction and manipulate particles at higher resolution. Closely-spaced resonators produce strong optical traps which can be addressed to mediate the hand-off of particles from one to the next in a conveyor-belt-like fashion. Here, we describe how to design and produce a conveyor belt using a gold surface patterned with plasmonic C-shaped resonators and how to operate it with polarized laser light to achieve super-resolution nanoparticle manipulation and transport. The nano-optical conveyor belt chip can be produced using lithography techniques and easily packaged and distributed.

The scalability and resolution of conventional optical manipulation techniques are limited by diffraction. We circumvent the diffraction limit and describe a method of optically transporting nanoparticles across a chip using a gold surface patterned with a path of closely spaced C-shaped plasmonic resonators.

The technique of using focused laser beams to trap and exert forces on small particles has enabled many pivotal discoveries in the nanoscale biological ...

Construction and Operation of a Light-driven Gold Nanorod Rotary Motor System

The possibility to generate and measure rotation and torque at the nanoscale is of fundamental interest to the study and application of biological and artificial nanomotors and may provide new routes towards single cell analysis, studies of non-equilibrium thermodynamics, and mechanical actuation of nanoscale systems. A facile way to drive rotation is to use focused circularly polarized laser light in optical tweezers. Using this approach, metallic nanoparticles can be operated as highly efficient scattering-driven rotary motors spinning at unprecedented rotation frequencies in water.
In this protocol, we outline the construction and operation of circularly-polarized optical tweezers for nanoparticle rotation and describe the instrumentation needed for recording the Brownian dynamics and Rayleigh scattering of the trapped particle. The rotational motion and the scattering spectra provides independent information on the properties of the nanoparticle and its immediate environment. The experimental platform has proven useful as a nanoscopic gauge of viscosity and local temperature, for tracking morphological changes of nanorods and molecular coatings, and as a transducer and probe of photothermal and thermodynamic processes.

Plasmonic gold nanorods can be trapped in liquids and rotated at kHz frequencies using circularly-polarized optical tweezers. Introducing tools for Brownian dynamics analysis and light scatteringspectroscopy leads to a powerful system for research and application in numerous fields of science.

The possibility to generate and measure rotation and torque at the nanoscale is of fundamental interest to the study and application of biological and ...

Trapping of Micro Particles in Nanoplasmonic Optical Lattice

The plasmonic optical tweezer has been developed to overcome the diffraction limits of the conventional far field optical tweezer. Plasmonic optical lattice consists of an array of nanostructures, which exhibit a variety of trapping and transport behaviors. We report the experimental procedures to trap micro-particles in a simple square nanoplasmonic optical lattice. We also describe the optical setup and the nanofabrication of a nanoplasmonic array. The optical potential is created by illuminating an array of gold nanodiscs with a Gaussian beam of 980 nm wavelength, and exciting plasmon resonance. The motion of particles is monitored by fluorescence imaging. A scheme to suppress photothermal convection is also described to increase usable optical power for optimal trapping. Suppression of convection is achieved by cooling the sample to a low temperature, and utilizing the near-zero thermal expansion coefficient of a water medium. Both single particle transport and multiple particle trapping are reported here.

We describe a procedure to optically trap micro-particles in nanoplasmonic optical lattice.

The plasmonic optical tweezer has been developed to overcome the diffraction limits of the conventional far field optical tweezer. Plasmonic optical ...

Monitoring Conformational Dynamics of Single Unmodified Proteins using Plasmonic Nanotweezers

Current single-molecule techniques to characterize proteins typically require labels, tethers, or the use of non-native solution conditions. Such changes can alter protein biophysics and reduce the usefulness of the data acquired. Plasmonic nanotweezers is a technique that uses localized surface plasmon resonance (LSPR) on gold nanostructures to enhance the electric field within a confined hotspot region. This field enhancement allows for the use of low laser powers to trap single nanoparticles far smaller than conventional optical tweezers, down to only a few nanometers in diameter, such as single proteins. Trapping of single protein molecules within the hotspot region induces a shift in the local refractive index (nprotein &#62; nwater), altering light scattering as a product of the molecule&#39;s polarisability, which is affected by its volume, shape anisotropy, and refractive index. An avalanche photodiode (APD) collects the subsequent changes in light scattering. These alterations can then be analyzed to determine changes in the trapped molecule, including its size, global conformation, and dynamics of conformational change over time. The incorporation of microfluidics within the system allows for controlled environmental changes and real-time monitoring of their subsequent effects on the molecule. In this protocol, we demonstrate the steps to trap single protein molecules, alter their environmental solution conditions, and monitor their corresponding conformational changes using a plasmonic nanotweezers system.

Plasmonic nanotweezers use localized surface plasmon resonance in gold nanostructures to trap single nanoparticles, including proteins, within a nanometer-scale optical field. Changes in the scattered signal reveal protein presence and conformational dynamics, enabling monitoring without fluorophore modifications or surface tethering.

Current single-molecule techniques to characterize proteins typically require labels, tethers, or the use of non-native solution conditions. Such changes ...

Optical Trapping of Plasmonic Nanoparticles for In Situ Surface-Enhanced Raman Spectroscopy Characterizations

Surface-enhanced Raman spectroscopy (SERS) enables the ultrasensitive detection of analyte molecules in various applications due to the enhanced electric field of metallic nanostructures. Salt-induced silver nanoparticle aggregation is the most popular method for generating SERS-active substrates; however, it is limited by poor reproducibility, stability, and biocompatibility. The present protocol integrates optical manipulation and SERS detection to develop an efficient analytical platform to address this. A 1064 nm trapping laser and a 532 nm Raman probe laser are combined in a microscope to assemble silver nanoparticles, which generate plasmonic hotspots for in situ SERS measurements in aqueous environments. Without aggregating agents, this dynamic plasmonic silver nanoparticle assembly enables an approximately 50-fold enhancement of the analyte molecule signal. Moreover, it provides spatial and temporal control to form the SERS-active assembly in as low as 0.05 nM analyte-coated silver nanoparticle solution, which minimizes the potential perturbation for in vivo analysis. Hence, this optical trapping-integrated SERS platform holds great potential for efficient, reproducible, and stable molecular analyses in liquids, especially in aqueous physiological environments.

Optical Trapping of Plasmonic Nanoparticles for In Situ Surface-Enhanced Raman Spectroscopy Characterizations

The present protocol describes a convenient approach to integrating optical trapping and surface-enhanced Raman spectroscopy (SERS) to manipulate plasmonic nanoparticles for sensitive molecular detection. Without aggregating agents, the trapping laser assembles plasmonic nanoparticles to enhance the SERS signals of target analytes for in situ spectroscopic measurements.

Surface-enhanced Raman spectroscopy (SERS) enables the ultrasensitive detection of analyte molecules in various applications due to the enhanced electric ...

Plasmonic Trapping and Release of Nanoparticles in a Monitoring Environment

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