All work performed under the support of the National Institute of Standards and Technology. Certain commercial equipment, instruments, or materials are identified to foster understanding of this protocol. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

Caution: Please consult all relevant safety programs before the experiment. All the experimental procedures described in this protocol are performed in accordance with the NIST LASER safety program as well as other applicable regulations. Please be sure to select and wear proper personal protective equipment (PPE) such as laser protection glasses designed for the specific wavelength and power. Handling dry nano/microparticles may require additional respiratory protection.
1. Design and Fabrication of a PZT Holder and a Sample Enclosure
<ol>
	<li>Design a PZT holder and a sample enclosure 
	NOTE: Particular design values vary depending on the selection of a PZT.
	<ol>
		<li>Open the computer-aided design (CAD) software package. Draw a two-dimensional (2D) sketch of a holder for a given PZT dimension. Develop the 2D sketch to volumetric features using combinations of Extrude/Extrude-cut.</li>
		<li>Click Sketch, draw a rectangle and extrude it to make a rectangular cube.</li>
		<li>Sketch a disk on the top surface of the cube to define a circularly recessed feature to cover and hold the ring-type PZT.</li>
		<li>Define a central hole to have an optical access for both real-time imaging and trapping.</li>
		<li>Define a circular guide along the rim of the central hole to insert a flat metallic (copper) ring to concentrate the ultrasonic power toward the center area as shown in Figure 2a.</li>
		<li>Create two bore holes for M6 screws on the PZT holder to be assembled with a bottom plate (purchased, 4 mm thick bottom aluminum plate with a hole in the center), as shown in Figure 2c and 2d.</li>
		<li>In a similar manner, design a rectangular frame of the sample enclosure. Click Sketch, and draw a rectangle, extrude the rectangle to make it a rectangular box.</li>
		<li>Draw a smaller rectangle on the top surface of the rectangular box and extrude-cut the rectangle to make it as a rectangular tube.</li>
		<li>Draw a smaller rectangle on the side wall of the tube and Extrude-cut to transform it into the frame of sample enclosure box.</li>
		<li>Convert these three-dimensional (3D) models into a stereolithography (STL) file format for a 3D printing process (Figure 2b).</li>
	</ol>
	</li>
	<li>3D printing of the designed objects
	<ol>
		<li>Open the design file (&#34;-.STL&#34;) from the 3D printer operating software. Lay the object flat 0/.and center the object on (0, 0, 0) by clicking the object to select it and using the alignment functions: &#34;Move&#34;, &#34;On Platform&#34;, and &#34;Center&#34;. Orient the PZT holder to face the delicate features upward. The recessed surface will be faced upward.</li>
		<li>In the menu go to the &#34;Settings&#34; and the &#34;Quality&#34; tab. Set the printing values as following, Infill: 100%, Number of shells: 2, and Layer height: 0.2 mm.</li>
		<li>Preview the objects to check the total print time and make sure the layered objects will be printed as desired. Export the 3D print file in a &#34;.x3g&#34; format and save it to use in the 3D printer.</li>
		<li>Turn on the 3D printer and warm it up until the temperature of the extrusion nozzle reaches an operating temperature, 230 &#176;C. Load the design file from a memory card or network drive.</li>
		<li>During the warm up, place the Build platform with blue painter&#39;s tape to help objects adhere securely. As a thermoplastic material for the printing job, use a polylactic acid (PLA) filament for both objects.</li>
		<li>Print the designed objects. Once the printing job is finished, turn off the printer after it has cooled down.</li>
		<li>Detach the printed object from the platform using a chisel. Straighten up the printed objects. If the orientation is appropriately chosen, the PZT holder can be directly used without further post-processing.</li>
		<li>For the sample enclosure, prepare one pair of indium tin oxide (ITO) coated coverslips and three glass coverslips to cover the frame. Use a diamond cutter to fit the coverslip to the enclosure.</li>
		<li>Wire the two parallel conducting plates using a fast drying silver paint to supply voltage across two plates. Glue these five windows onto the sample enclosure using an instant adhesive glue. 
		NOTE: The one pair of ITO coated coverslips are installed on the sample enclosure in parallel (facing each other) to provide uniform electric field and to generate ballistic motion of the naturally charged particle along the electric field. The three conventional coverslip cover the rest of sample enclosure surfaces (top and two other sides) to protect the trapped particle from the external flow of air</li>
	</ol>
	</li>
</ol>
2. Optical Trap Loading of a Selected Microparticle
<ol>
	<li>Sample preparation
	<ol>
		<li>Store the microparticles in an evacuated desiccator to reduce contact with moisture in the air before the experiment.</li>
		<li>Pour out a small portion of microparticles onto a glass slide and immediately put the manufacture&#39;s bottle back in the desiccator.</li>
		<li>Pick up some of the microparticles with a glass capillary tube. Scatter the particles over the substrate by gently tapping on the capillary while holding the capillary over the coverslip.</li>
		<li>Verify the quantity and distribution of deposited particles on the substrate using a dark-field microscope. 
		Note: In the sample preparation step, the particle is just scattered on a coverslip and imaged with an optical microscope to verify overall arrangement before inserting them (a coverslip with scattered microparticles) between the PZT and PZT holder. Since the surface adhesion is strong enough to hold individual microparticles on the substrate, the adhered particles are firmly fixed unless significant external force is applied.</li>
	</ol>
	</li>
	<li>Piezoelectric launcher assembly
	<ol>
		<li>Obtain all the components of the piezoelectric launcher: the flat bottom plate, insulating film, the PZT, the glass coverslip, a copper ring, the PZT holder, two M6 screws, and the sample enclosure.</li>
		<li>Apply a thin film (or tape) on the bottom plate to insulate the PZT. The glass coverslip isolates the top of the stack.</li>
		<li>Assemble the stack by centering the PZT on top of the flat plate now insulated with tape, followed by the coverslip, the copper ring, and the PZT holder. Screw the stack together maintaining the centering of the PZT to avoid shorting the PZT to the holder if the holder is conducting as shown in Figure 2c and 2d. The copper ring provides an evenly distributed mechanical preload on the stack for plastic PZT holders.</li>
		<li>Finally, glue the sample enclosure onto the stack and mount the assembly on an XYZ translational stage in the microscope.</li>
	</ol>
	</li>
	<li>Configuration of the PZT launcher 
	NOTE: Driving the PZT with a high voltage signal has potential electrical hazards. Please consult with safety personnel before the experiment. All the electrical connections should be secured before the experiment. Turn off the amplifier and disconnect PZT leads whenever possible.
	<ol>
		<li>Connect the PZT leads to the voltage amplifier and connect the function generator to an input port of the voltage amplifier.</li>
		<li>Turn on the function generator and configure it to generate continuous square waves with an output voltage of 1 V. Do not generate the voltage signal until all the connections are verified and secured.</li>
		<li>Turn on the voltage amplifier and generate the square wave of output voltage 1 V by enabling the output.</li>
		<li>Connect the monitoring output port (output voltage 200 V) of the amplifier to an oscilloscope. Configure the amplifier to have gain of 200 V/V by turning the gain knob on the front panel. Verify that the monitoring output voltage has an amplitude of 1 V as measured by the oscilloscope.</li>
		<li>Once the function generator and the amplifier are configured, find the resonant frequency of the PZT launcher by scanning the modulation frequency of the driving signal while the real-time video microscope images adhered particles. Repeat the scanning until the microparticle motion is a maximum. Use this frequency (64 kHz here) to release particles. 
		NOTE: The modulation frequency is manually changed (scanned) from zero to 150 kHz to find the resonant frequency.</li>
		<li>Configure the function generator to generate a square wave with a specified number of cycles in burst mode. Press the &#34;Burst&#34; button on the front panel and select &#34;N Cycle Burst&#34;.</li>
		<li>Choose the burst count by pressing &#34;# Cycles&#34; soft key and set the count to 10 or 20.</li>
		<li>Configure the square waveform to generate voltage signals with an amplitude of 600 V (three times the voltage used for continuous excitation) at the resonant frequency of 64 kHz which has found from the previous step. Verify that the pulsing signal releases the target particle in a repeatable manner by ensuring particles move after each pulse.</li>
	</ol>
	</li>
	<li>Selective optical trap loading 
	NOTE: The PZT launcher assembly is installed on a manual linear translation xy stage. The particles can be translated relative to the fixed beam focus by moving the translational stage.
	<ol>
		<li>Remove the laser line filter to identify the focus of the trapping beam by rotating the microscope turret (Figure 3a). Move the motorized focusing block back and forth vertically around the best focus of the visible image to optimize focus.</li>
		<li>Once the focus position is verified, put the filter back to give a clear real-time video without interference from the trapping beam.</li>
		<li>Translate the sample to place a selected particle at the focus position of the trapping laser. Focus on the particle to image the center of a selected particle, which places the nominal trapping position below the particle center by about one half radius while leaving the levitation position above the particle.</li>
		<li>Adjust the power supply connected to the electro-optic modulator (EOM) driver to set the optical trapping power. The optimal power depends on particle size and material. The optical power was found through repeated trials to determine the power sufficient to levitate the particle without ejecting it from the beam. Here, use an optical power of 140 mW at the back focal plane of the objective to trap the 20 &#181;m diameter polystyrene (PS) particles.</li>
		<li>After the center of the selected particle is aligned, actuate the piezoelectric launcher with several pulses. The change of the particle image from a static focused image to a moving blurred image indicates successful loading to the levitation position.</li>
		<li>Translate the levitated particle vertically about a millimeter above the substrate by moving the objective lens up to prevent possible surface interactions. Then reduce the optical power to transition the levitated particle (Figure 3b) into the nominal trapping position (Figure 3c) which is more stable. 
		NOTE: The optical power of trapping laser can be modulated by an electro-optic modulator (EOM). The EOM regulates the output power with a bias voltage supplied through a digital power supply. One can observe the transition from the levitation to trapping position through the CCD while slowly reduces the optical power.</li>
		<li>For the position measurement, as depicted in Figure 3c to 3d, carefully move the center of the PZT holder to the optical axis and then move the objective lens up (vertically) to translate the particle into the middle of sample enclosure (9 mm above the substrate) where the fringe electric field is minimized.</li>
		<li>After performing the measurement as described below, place the particle on the substrate by moving the objective down until the particle touches the substrate. Since most of the particles are applied near the corners, the trapped particle can be easily recognized and re-trapped when it is placed in the central area. This enables reversible trap loading to measure changes occurring beyond a single trapping event such as contact interactions of the particle and substrate.</li>
	</ol>
	</li>
</ol>
3. Data Acquisition
<ol>
	<li>Align the condenser and the focusing lens to maximize the QPD &#34;SUM&#34; signal with a particle in the trap.</li>
	<li>Align the focusing lens to nominally zero the X and Y channels of the QPD, as shown in Figure 4c.</li>
	<li>Repeat the adjustment of the condenser and the focusing lens until the Fourier transformed position signals (or power spectrum density (PSD) plots) of the X and Y channels superimpose to show balanced sensitivity. Properly aligned QPD signals (X and Y) show almost identical behavior, as shown in Figure 4b.</li>
	<li>Once the QPD alignment is verified, connect the voltage amplifier to the two ITO plates. Connect the voltage monitoring output signal of the amplifier to the DAQ system to record the step excitation signal and the induced particle trajectory synchronously.</li>
	<li>Supply a continuous square wave of 400 V to generate an electric field (Figure 4d) that moves the particle transversely to the optical axis by about 500 nm (Figure 4e). Measure the step response of the trapped particle using the QPD.</li>
	<li>Average multiple periods as necessary to reduce the effects of Brownian motion. The induced motion can be used to measure the optical force over a wider range of motion than that of thermal fluctuations.12,17 Figure 4d and 4e shows averaged signals of applied voltage and the induced particle trajectory over 50 iterations of step excitation.</li>
</ol>

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The authors declare no competing financial interest.

The piezoelectric launcher is designed to optimize the dynamic performance of a selected PZT. Proper selection of PZT materials and management of ultrasonic vibrations are the key steps to yield a successful experiment. PZTs have different characteristics depending on the type of transducer (bulk or stacked) and component materials (hard or soft). A bulk type PZT made of a hard piezoelectric material is chosen for the following reasons. First, hard piezoelectric materials have lower dielectric losses and higher mechanica...

Ashkin reported the acceleration and trapping of microparticles by radiation pressure in 1970.1 His novel achievement promoted the development of optical trapping techniques as a primary tool for fundamental studies of physics and biophysics.2,3,4,5 To date, the application of optical trapping has focused mainly on liquid environments, and been used to study a very wide range of systems, from the behavior of colloids to the mechanical properties of single biomolecules.6,7,8 Application of optical trapping to gaseous media, however, requires resolving several new technical issues.
Recently, optical trapping in air/vacuum has been increasingly applied in fundamental research. Since optical levitation potentially provides nearly-complete isolation of a system from the surrounding environment, the optically levitated particle becomes an ideal laboratory for studying quantum ground states in small objects,4 measuring high-frequency gravitational waves,9 and searching for fractional charge.10 Moreover, the low viscosity of air/vacuum allows one to use inertia to measure the instantaneous velocity of a Brownian particle11 and to create ballistic motion over a wide range of motion beyond the linear spring-like regime.12 Therefore, detailed technical information and practices for optical traps in gaseous media have become more valuable to the broader research community.
New experimental techniques are required to load nano/microparticles into optical traps in gaseous media. A piezoelectric transducer (PZT), a device that converts electric energy into mechano-acoustic energy, has been used to deliver small particles into optical traps in air/vacuum5,12 since the first demonstration of optical levitation.1 Since then, several loading techniques have been proposed to load smaller particles using volatile aerosols generated by a commercial nebulizer13 or an acoustic wave generator.14 The floating aerosols with solid inclusions (particles) randomly pass near the focus and are trapped by chance. Once the aerosol is trapped, the solvent evaporates out and the particle remains in the optical trap. However, these methods are not well suited to identify desired particles from within a sample, load a selected particle and to track its changes if released from the trap. This protocol is intended to provide details to new practitioners on selective optical trap loading in air, including the experimental setup, fabrication of a PZT holder and sample enclosure, trap loading, and data acquisition associated with the analysis of particle motion in both the frequency and time domains. Protocols for trapping in liquid media have also been published.15,16
The overall experimental setup is developed on a commercial inverted optical microscope. Figure 1 shows a schematic diagram of the setup used to demonstrate steps of the selective optical trap loading: freeing the resting microparticles, lifting the chosen particle with the focused beam, measuring its motion, and placing it onto the substrate again. First, translational stages (transverse and vertical) are used to bring a selected microparticle on the substrate to the focus of a trapping laser (wavelength 1064 nm) focused by an objective lens (near-infrared corrected long-working distance objective: NA 0.4, magnification 20X, working distance 20 mm) through the transparent substrate. Then, a piezoelectric launcher (a mechanically pre-loaded ring-type PZT) generates ultrasonic vibrations to break the adhesion between microparticles and a substrate. Thus, any freed particle can be lifted by the single-beam gradient laser trap focused on the selected particle. Once the particle is trapped, it is translated to the center of the sample enclosure containing two parallel conducting plates for electrostatic excitation. Finally, a data acquisition (DAQ) system simultaneously records the particle motion, captured by a quadrant-cell photodetector (QPD), and the applied electric field. After finishing the measurement, the particle is controllably placed onto the substrate so that it can be trapped again in a reversible manner. This overall process can be repeated hundreds of times without particle loss to measure changes such as contact electrification occurring over several trapping cycles. Please refer to our recent article for details.12

<table><tbody><tr><td>ScotchBlue Painter&#039;s Tape Original</td><td>3M</td><td>3M2090</td><td></td></tr><tr><td>Scotch 810 Magic Tape</td><td>3M</td><td>3M810</td><td></td></tr><tr><td>Function/Arbitrary Waveform generator</td><td>Agilent</td><td>HP33250A</td><td></td></tr><tr><td>Power supply/Digital voltage supplier</td><td>Agilent</td><td>E3634A</td><td></td></tr><tr><td>Ring-type piezoelectric transducer</td><td>American Piezo Company</td><td>item91</td><td></td></tr><tr><td>Electro-optic modulator</td><td>Con-Optics</td><td>350&amp;minus;80-LA</td><td></td></tr><tr><td>Amplifier for Electro-optic modulator</td><td>Con-Optics</td><td>302RM</td><td></td></tr><tr><td>Mitutoyo NIR infinity Corrected Objective</td><td>Edmund optics</td><td>46-404</td><td>Manufactured by Mitutoyo and Distributed by Edmund optics</td></tr><tr><td>LOCTITE SUPER GLUE LONGNECK BOTTLE</td><td>Loctite</td><td>230992</td><td></td></tr><tr><td>3D printer</td><td>MakerBot</td><td>Replicator 2</td><td></td></tr><tr><td>Polylactic acid (PLA) filament</td><td>MakerBot</td><td>True Red PLA Small Spool</td><td></td></tr><tr><td>Data Acquisition system</td><td>National Instruments</td><td>780114-01</td><td></td></tr><tr><td>Quadrant-cell photodetector</td><td>Newport</td><td>2031</td><td></td></tr><tr><td>Translational stage</td><td>Newport</td><td>562-XYZ</td><td></td></tr><tr><td>Inverted optical microscope</td><td>Nikon Instruments</td><td>EclipsTE2000</td><td></td></tr><tr><td>Fluorescence filter (green)</td><td>Nikon Instruments</td><td>G-2B</td><td></td></tr><tr><td>Flea3/CCD camera</td><td>Point Grey</td><td>FL3-U3-13S2M-CS</td><td>Trapping laser</td></tr><tr><td>Diode pumped neodymium yttrium vanadate(Nd:YVO&lt;sub&gt;4&lt;/sub&gt;)</td><td>Spectra Physics</td><td>J20I-8S-12K/ BL-106C</td><td></td></tr><tr><td>Indium tin oxide (ITO) Coated coverslips</td><td>SPI supplies</td><td>06463B-AB</td><td>Polystyrene microparticles</td></tr><tr><td>Fast Drying Silver Paint</td><td>Tedpella</td><td>16040-30</td><td></td></tr><tr><td>Dri-Cal size standards</td><td>Thermo Scientific</td><td>DC-20</td><td></td></tr><tr><td>Optical Fiber</td><td>Thorlabs</td><td>P1&amp;minus;1064PM-FC-5</td><td>bottom plate</td></tr><tr><td>Aluminium plate&amp;nbsp;</td><td>Thorlabs</td><td>CP4S</td><td></td></tr><tr><td>High voltage power amplifier</td><td>TREK</td><td>PZD700A M/S</td><td></td></tr></tbody></table>

optical trap loading of dielectric microparticles in air

We demonstrate a method to trap a selected dielectric microparticle in air using radiation pressure from a single-beam gradient optical trap. Randomly scattered dielectric microparticles adhered to a glass substrate are momentarily detached using ultrasonic vibrations generated by a piezoelectric transducer (PZT). Then, the optical beam focused on a selected particle lifts it up to the optical trap while the vibrationally excited microparticles fall back to the substrate. A particle may be trapped at the nominal focus of the trapping beam or at a position above the focus (referred to here as the levitation position) where gravity provides the restoring force. After the measurement, the trapped particle can be placed at a desired position on the substrate in a controlled manner. 
In this protocol, an experimental procedure for selective optical trap loading in air is outlined. First, the experimental setup is briefly introduced. Second, the design and fabrication of a PZT holder and a sample enclosure are illustrated in detail. The optical trap loading of a selected microparticle is then demonstrated with step-by-step instructions including sample preparation, launching into the trap, and use of electrostatic force to excite particle motion in the trap and measure charge. Finally, we present recorded particle trajectories of Brownian and ballistic motions of a trapped microparticle in air. These trajectories can be used to measure stiffness or to verify optical alignment through time domain and frequency domain analysis. Selective trap loading enables optical tweezers to track a particle and its changes over repeated trap loadings in a reversible manner, thereby enabling studies of particle-surface interaction.

The PZT launcher is designed using a CAD software package. Here, we use a simple sandwich structure for the preloading (a PZT clamped with two plates), as shown in Figure 2. The PZT holder and the sample enclosure can be fabricated from a variety of materials and methods. For a quick demonstration, we choose 3D printing with thermoplastic as illustrated in Figure 2d. Based on the fabricated components, optical trap loading is shown in Figure 3</st...

Watch this Scientific Journal Video about Optical Trap Loading of Dielectric Microparticles In Air at JoVE.com

Optical Trap Loading of Dielectric Microparticles In Air

A protocol for launching and stably trapping selected dielectric microparticles in air is presented.

We demonstrate a method to trap a selected dielectric microparticle in air using radiation pressure from a single-beam gradient optical trap. Randomly ...

optical-trap-loading-of-dielectric-microparticles-in-air

Research

JoVE Journal

Engineering

9.0K Views.  National Institute of Standards and Technology. A protocol for launching and stably trapping selected dielectric microparticles in air is presented.

Video: Optical Trap Loading of Dielectric Microparticles In Air

A Microfluidic-based Hydrodynamic Trap for Single Particles

The ability to confine and manipulate single particles in free solution is a key enabling technology for fundamental and applied science. Methods for particle trapping based on optical, magnetic, electrokinetic, and acoustic techniques have led to major advancements in physics and biology ranging from the molecular to cellular level. In this article, we introduce a new microfluidic-based technique for particle trapping and manipulation based solely on hydrodynamic fluid flow. Using this method, we demonstrate trapping of micro- and nano-scale particles in aqueous solutions for long time scales. The hydrodynamic trap consists of an integrated microfluidic device with a cross-slot channel geometry where two opposing laminar streams converge, thereby generating a planar extensional flow with a fluid stagnation point (zero-velocity point). In this device, particles are confined at the trap center by active control of the flow field to maintain particle position at the fluid stagnation point. In this manner, particles are effectively trapped in free solution using a feedback control algorithm implemented with a custom-built LabVIEW code. The control algorithm consists of image acquisition for a particle in the microfluidic device, followed by particle tracking, determination of particle centroid position, and active adjustment of fluid flow by regulating the pressure applied to an on-chip pneumatic valve using a pressure regulator. In this way, the on-chip dynamic metering valve functions to regulate the relative flow rates in the outlet channels, thereby enabling fine-scale control of stagnation point position and particle trapping. The microfluidic-based hydrodynamic trap exhibits several advantages as a method for particle trapping. Hydrodynamic trapping is possible for any arbitrary particle without specific requirements on the physical or chemical properties of the trapped object. In addition, hydrodynamic trapping enables confinement of a "single" target object in concentrated or crowded particle suspensions, which is difficult using alternative force field-based trapping methods. The hydrodynamic trap is user-friendly, straightforward to implement and may be added to existing microfluidic devices to facilitate trapping and long-time analysis of particles. Overall, the hydrodynamic trap is a new platform for confinement, micromanipulation, and observation of particles without surface immobilization and eliminates the need for potentially perturbative optical, magnetic, and electric fields in the free-solution trapping of small particles.

In this article, we present a microfluidic-based method for particle confinement based on hydrodynamic flow. We demonstrate stable particle trapping at a fluid stagnation point using a feedback control mechanism, thereby enabling confinement and micromanipulation of arbitrary particles in an integrated microdevice.

The ability to confine and manipulate single particles in free solution is a key enabling technology for fundamental and applied science.  Methods for ...

Bioengineering

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 ...

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 ...

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 ...

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.

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 ...

Experimental Methods for Trapping Ions Using Microfabricated Surface Ion Traps

Ions trapped in a quadrupole Paul trap have been considered one of the strong physical candidates to implement quantum information processing. This is due to their long coherence time and their capability to manipulate and detect individual quantum bits (qubits). In more recent years, microfabricated surface ion traps have received more attention for large-scale integrated qubit platforms. This paper presents a microfabrication methodology for ion traps using micro-electro-mechanical system (MEMS) technology, including the fabrication method for a 14 &#181;m-thick dielectric layer and metal overhang structures atop the dielectric layer. In addition, an experimental procedure for trapping ytterbium (Yb) ions of isotope 174 (174Yb+) using 369.5 nm, 399 nm, and 935 nm diode lasers is described. These methodologies and procedures involve many scientific and engineering disciplines, and this paper first presents the detailed experimental procedures. The methods discussed in this paper can easily be extended to the trapping of Yb ions of isotope 171 (171Yb+) and to the manipulation of qubits.

This paper presents a microfabrication methodology for surface ion traps, as well as a detailed experimental procedure for trapping ytterbium ions in a room-temperature environment.

Ions trapped in a quadrupole Paul trap have been considered one of the strong physical candidates to implement quantum information processing. This is due ...

Use of Dual Optical Tweezers and Microfluidics for Single-Molecule Studies

Visual biochemistry is a powerful technique for observing the stochastic properties of single enzymes or enzyme complexes that are obscured in the averaging that takes place in bulk-phase studies. To achieve visualization, dual optical tweezers, where one trap is fixed and the other is mobile, are focused into one channel of a multi-stream microfluidic chamber positioned on the stage of an inverted fluorescence microscope. The optical tweezers trap single molecules of fluorescently labeled DNA and fluid flow through the chamber and past the trapped beads, stretches the DNA to B-form (under minimal force, i.e., 0 pN) with the nucleic acid being observed as a white string against a black background. DNA molecules are moved from one stream to the next, by translating the stage perpendicular to the flow to enable the initiation of reactions in a controlled manner. To achieve success, microfluidic devices with optically clear channels are mated to glass syringes held in place in a syringe pump. Optimal results use connectors permanently bonded to the flow cell, tubing that is mechanically rigid and chemically resistant, and which is connected to switching valves that eliminate bubbles that prohibit laminar flow.

Visual, single-molecule biochemistry studied through microfluidic chambers is greatly facilitated using glass barrel, gas-tight syringes, stable connections of tubing to flow cells, and elimination of bubbles by placing switching valves between the syringes and tubing. The protocol describes dual optical traps that enable visualization of DNA transactions and intermolecular interactions.

Visual biochemistry is a powerful technique for observing the stochastic properties of single enzymes or enzyme complexes that are obscured in the ...

Biochemistry

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 ...

Direct Force Measurements of Subcellular Mechanics in Confinement using Optical Tweezers

During the development of a multicellular organism, a single fertilized cell divides and gives rise to multiple tissues with diverse functions. Tissue morphogenesis goes in hand with molecular and structural changes at the single cell level that result in variations of subcellular mechanical properties. As a consequence, even within the same cell, different organelles and compartments resist differently to mechanical stresses; and mechanotransduction pathways can actively regulate their mechanical properties. The ability of a cell to adapt to the microenvironment of the tissue niche thus is in part due to the ability to sense and respond to mechanical stresses. We recently proposed a new mechanosensation paradigm in which nuclear deformation and positioning enables a cell to gauge the physical 3D environment and endows the cell with a sense of proprioception to decode changes in cell shape. In this article, we describe a new method to measure the forces and material properties that shape the cell nucleus inside living cells, exemplified on adherent cells and mechanically confined cells. The measurements can be performed non-invasively with optical traps inside cells, and the forces are directly accessible through calibration-free detection of light momentum. This allows measuring the mechanics of the nucleus independently from cell surface deformations and allowing dissection of exteroceptive and interoceptive mechanotransduction pathways. Importantly, the trapping experiment can be combined with optical microscopy to investigate the cellular response and subcellular dynamics using fluorescence imaging of the cytoskeleton, calcium ions, or nuclear morphology. The presented method is straightforward to apply, compatible with commercial solutions for force measurements, and can easily be extended to investigate the mechanics of other subcellular compartments, e.g., mitochondria, stress-fibers, and endosomes.

Here, we present a protocol to investigate the intracellular mechanical properties of isolated embryonic zebrafish cells in three-dimensional confinement with direct force measurement by an optical trap.

During the development of a multicellular organism, a single fertilized cell divides and gives rise to multiple tissues with diverse functions. Tissue ...

Developmental Biology

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 ...

Optical Trap Loading of Dielectric Microparticles In Air

Summary

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Chapters in this video

A Microfluidic-based Hydrodynamic Trap for Single Particles

Optical Trapping of Nanoparticles

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

Fabrication and Operation of a Nano-Optical Conveyor Belt

Plasmonic Trapping and Release of Nanoparticles in a Monitoring Environment

Experimental Methods for Trapping Ions Using Microfabricated Surface Ion Traps

Use of Dual Optical Tweezers and Microfluidics for Single-Molecule Studies

Trapping of Micro Particles in Nanoplasmonic Optical Lattice

Direct Force Measurements of Subcellular Mechanics in Confinement using Optical Tweezers

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