Trapping of Micro Particles in Nanoplasmonic Optical Lattice
September 5th, 2017
We describe a procedure to optically trap micro-particles in nanoplasmonic optical lattice.
The overall goal of this experiment is to demonstrate how to trap microparticles with plasmonic optical lattices with a technique that suppresses photothermal convection. The main feature of this technique is that we use the array of plasmonic nanostructures to enhance the trapping efficiency. Also we use a unique brewing property of water, 0%thermal expansion coefficient at low temperature to suppress the photothermal convection.
Demonstrating this procedure will be Dinesh Bhalothia, a grad student from my laboratory. The setup for the experiment builds on a optical tweezer kit. This modified optical tweezer kit with a florescence module is ready on a breadboard.
An LED and a diode laser provide light for florescence and manipulation. Mirrors direct light through an objective lens. The objective focuses the light on a sample stage which also serves as a heat sink.
A CCD camera captures images from the sample. These elements are more evident in this schematic. A 470 nanometer blue LED is the light source for the florescence module.
A 980 nanometer laser diode provides laser light in loose focus for manipulation. The lens is a long working distance microscope objective. Activity on the sample stage is recorded by a CCD camera.
Turn on the power supply and current for the 980 nanometer laser diode. Use the CCD camera to check the laser beam alignment. If the beam is well aligned, the camera image will be a Gaussian spot.
Turn off the laser for the next steps. An important aspect of the setup is the cooling system. The sample stage is a heat sink designed to accommodate a thermoelectric cooler.
Work with the system electronics to prepare to add the thermoelectric cooler. There is a custom driver circuit for this experiment. Make the connections between the driver circuit and the electronic control board.
Next, get a thermoelectric cooling element that will fit in the sample stage and with a hole in it to allow the laser beam through. Connect the output of the driver circuit to the thermoelectric cooling element. Move the cooling element to the sample stage.
Before continuing, connect the driver circuit to a five volt power supply. To monitor the temperature, use a forward-looking infrared camera, and check that the system is cooling properly before proceeding. For this, use a resistance thermometer unit and its sensor.
Begin with the sensor on a glass cover slip. Transfer the sensor and cover slip to the sample stage. Apply a small amount of thermal paste to ensure thermal contact.
There, put the the assembly in contact with the stage. At the controls, adjust the power to the thermoelectric cooling element. After three minutes, read the temperature using the resistance temperature detector thermometer.
In addition, record the temperature with the forward-looking infrared camera. Repeat these two measurements at various output power settings to obtain a temperature calibration curve similar to this one. Calibration is essential.
Before proceeding, shut off the power to the thermoelectric cooler. The final element of the setup is the nanoplasmonic array. The slide cover has a custom fabricated array and is ready to mount in the experiment.
This scanning electron microscope image of the array provides more detail. It is an array of about 16 micrometers square of 22 by 22 gold nanodiscs. Each nanodisc has a thickness of 40 nanometers and a diameter of 550 nanometers.
The center to center distance between discs is 750 nanometers. Put the slide cover with the nanoplasmonic array on the sample stage. It should be in contact with the thermoelectric cooler.
Next, set up the light source. Turn on the florescent light source, and set the power to five milliwatts for bright field imaging. Monitor the CCD image while manipulating the slide.
Use the marker on the slide to locate and align the array. Make sure the array is in the center of the region of interest on the computer screen. Now, turn to the sample for the experiment.
It is a mixture of two micrometer diameter polystyrene particles in deionized water. Use a micropipette to dispense 10 microliters onto the nanoarray slide. Move to the current supply of the 980 nanometer laser diode.
Turn it on to excite the plasmonic resonance of the array. Then work with the power supply for the cooling system to achieve a temperature of four degrees Celsius. Finally, start recording video of the microparticles with the CCD camera.
This is an example of the video recorded during an experiment with two micrometer polystyrene spheres. The optical power of the 980 nanometer wavelength laser used for plasmonic resonance excitation is five milliwatts. Note the particles cluster in a hexagonal, close-packed structure.
These still images are of the accumulated trapped microparticles over time. Again, the hexagonal close-pack structure is clear. Images such as these can provide data to produce a plot of the number of trapped microparticles as a function of time.
These five colored curves are examples of microparticle trajectories that can be extracted from the recorded video using image processing techniques, and the centroid algothrithm. The scale bar represents two micrometers. The procedure reported here enables a researcher to reproduce trapping on a daily basis.
While attempting this procedure, it's important to remember to use the infrared camera to monitor the sample temperature to avoid the sample breakage. After its development, this technique pave the way for researchers in the field of optical trapping to study a larger class of transport phenomena in plasmonic optical lattice. After watching this video, you should have a good understanding of how to do the optical trapping with the plasmonic optical lattice,
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Overview
This article describes a procedure to optically trap micro-particles using plasmonic optical lattices. The technique aims to suppress photothermal convection while enhancing trapping efficiency.
Key Study Components
Area of Science
- Optical trapping
- Nanoplasmonics
- Micro-particle manipulation
Background
- Optical tweezers are commonly used for trapping small particles.
- Plasmonic nanostructures can enhance optical trapping efficiency.
- Photothermal convection can interfere with trapping stability.
- Water's unique properties can be utilized to mitigate thermal effects.
Purpose of Study
- To demonstrate a method for trapping micro-particles effectively.
- To utilize plasmonic optical lattices for improved trapping.
- To suppress photothermal convection during the trapping process.
Methods Used
- Modified optical tweezer kit with fluorescence module.
- Array of plasmonic nanostructures for enhanced trapping.
- Utilization of water's 0% thermal expansion coefficient at low temperatures.
- Experimental setup demonstrated by a graduate student.
Main Results
- Successful trapping of micro-particles using the described method.
- Demonstrated suppression of photothermal convection.
- Enhanced trapping efficiency observed with plasmonic structures.
- Experimental setup validated for future studies.
Conclusions
- The technique provides a reliable method for trapping micro-particles.
- Plasmonic optical lattices can significantly improve trapping efficiency.
- Future applications may explore further enhancements in optical trapping.
Frequently Asked Questions
Executive Industry Relevance
This plasmonic optical lattice technique addresses a key challenge in biopharma R&D: achieving precise, stable manipulation of micro-scale biological entities without thermal artifacts. By suppressing photothermal convection through low-temperature operation and leveraging water's near-zero thermal expansion coefficient, the method enhances trapping reliability for downstream applications. This supports early-stage target validation and assay development by enabling controlled particle positioning in aqueous environments critical for biomolecular interaction studies.
Strategic Applications in Biopharma R&D
Early Discovery & Target Validation
- Scientific Value: Enables interrogation of therapeutic hypotheses through precise spatial control of microparticles in plasmonic potentials.
- Operational Value: Provides a reproducible platform for functional target validation by stabilizing particle interactions under suppressed convection conditions.
Screening & Assay Development
- Scientific Value: Prepares validated biological systems for downstream workflows by demonstrating controlled transport and clustering of polystyrene microparticles as model systems.
- Operational Value: Addresses assay standardization and reproducibility through quantifiable particle trapping metrics and temperature-stabilized optical potentials.
Translational & Preclinical Research
- Scientific Value: Supports mechanistic de-risking by enabling observation of particle dynamics in disease-relevant aqueous environments without thermal confounding.
- Operational Value: Enhances predictive confidence in preclinical models by providing a stable platform for studying transport phenomena in plasmonic lattices.
Pipeline & Workflow Integration
The method integrates into the discovery continuum from hypothesis testing through lead identification by providing a tunable, reproducible trapping mechanism for microparticle-based assays.
- Discovery Biology: Supports hypothesis testing and pathway clarification via controlled particle manipulation in nanoplasmonic potentials.
- Screening: Enables assay readiness through quantifiable trapping efficiency and particle trajectory analysis using centroid algorithms.
- Analytics: Provides quantitative outputs such as trapped particle counts over time and microparticle trajectories for comparative condition analysis.
- Translational Research: Connects to preclinical continuity by enabling stable observation of particle behavior in temperature-controlled aqueous systems.
- Enterprise Reuse: Establishes a reusable capability for daily trapping reproduction across multiple experimental runs.
Operational & Enterprise Impact
- Scientific Value: Predictive confidence through reduced mechanistic ambiguity in particle trapping studies.
- Operational Value: Standardization and scalability via temperature-controlled suppression of photothermal convection.
- Strategic Value: Better go/no-go decisions by minimizing false positives from thermal artifacts in trapping data.
- Portfolio Impact: Risk-adjusted prioritization through reliable, reproducible particle manipulation data.
Implementation Considerations
- Requires expertise in optical setup alignment, nanofabrication handling, and thermal management systems.
- Needs instrumentation including 980 nm laser diode, CCD camera, thermoelectric cooler, and resistance thermometer for temperature monitoring.
- Demands cross-team standardization of cooling protocols and temperature calibration curves for consistent convection suppression.
- Involves adaptation considerations for different particle types and aqueous media while maintaining near-zero thermal expansion conditions.
- Includes practical limitations such as the need for precise temperature control to avoid sample breakage and maintain optical stability.
Biopharma FAQs
Chapters in this video
0:05
Title
0:44
The Optical Setup
2:04
The Sample Cooling System and Temperature Calibration
3:51
The Nanoplasmonic Array and Trapping Microparticles
5:36
Results: Observation of Clustering and Determination of Single Particle Trajectories for 2 μm Micro-spheres in Deionized Water Using a Nanoplasmonic Optical Lattice Trap
6:32
Conclusion
