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Nanoradiators allow precise optical manipulation of nanoparticles (w/video)

(Nanowerk Spotlight) Manipulation of nanoscale objects is important in both nanofabrication and biological study. For example, precise control of biological communications such as receptor-ligand and DNA-DNA interactions is essential for life science investigation.
Conventional dynamic manipulation techniques involve the application of mechanical force such as micropipettes on nanoparticles or cells. However, these bulky and complex methods are not desirable and may cause mechanical damage to the objects that are to be investigated or manipulated.
Optical manipulation has been established as a versatile and efficient tool to achieve contact-free manipulation with high throughput. However, current optical manipulation methods typically require high optical power to achieve stable trapping for small objects. This can lead to optical damage of biological samples.
The enhanced optical heating effect will also greatly reduce the trapping stability. In addition, the trapping range in case of plasmonic tweezers is limited by the decay length of the surface plasmon (sub-100nm).
Why not turn the heat from losses to gains? A team, led by Yuebing Zheng from Mechanical Engineering and Materials Science & Engineering at the University of Texas at Austin, has developed a new type of optical manipulation method to achieve versatile manipulation of objects with different sizes and types using optical heating (ACS Nano, "Nanoradiator-Mediated Deterministic Opto-Thermoelectric Manipulation").
"Heat is always considered a hindrance in optical trapping. However, in our opto-thermoelectrical tweezer, we use the optical heating to facilitate optical trapping," Yaoran Liu, the first author of the paper, tells Nanowerk. "Nanoparticles get trapped at temperature hot spots instead of electrical hot spots, which demonstrates a different working mechanism and approach from the traditional optical manipulation techniques. Due to this unique working principle, we achieve stable trapping of large metallic nanoparticles and miniscule quantum dots on single nanoantennas with extremely low optical power (0.08mW/µm2). Our new technique also shows 100 times improvement in trapping stability and 10 times enhancement in trapping range over the traditional optical trapping method."
"The understanding of controlling the nanoscale heat transfer allows us to achieve dynamic manipulation of nano-objects within the beam spot and stable trapping of ultra-small quantum dots with tunable optical response, whose size is as small as 30nm," he adds.
Opto-Thermoelectric Trapping of Single Particles on Single Nanoantennas
Opto-Thermoelectric Trapping of Single Particles on Single Nanoantennas. a) Dispersion of a positively charged nanoparticle and multiple ions in solvent surrounding the Au nanoantenna when the laser is off. b) Thermophoresis-induced redistribution of the solutes in the solvent when the laser is on. The temperature gradient from the optical heating of the nanoantenna generates thermoelectric force (F) that traps the nanoparticle at the center of the nanoantenna. c) Three-dimensional view of opto-thermoelectric trapping of a nanoparticle (NP) on the AuNR. The incident laser beam is normal to the substrate with its polarization along y-direction. (Reprinted with permission by American Chemical Society) (click on image to enlarge)
The opto-thermoelectric tweezer can be widely used in reconfigurable and active photonic devices. The long-range and rapid trapping behavior with high sensitivity can also be applied to detect molecules with ultralow concentration, which is essential for future investigations of molecular diseases and in immunology.
The potential applications of this new optical trapping are:
Tunable light emitters realized by precisely trapping nanoemitters such as quantum dots on plasmonic nanoantennas, and simultaneously tuning the light emission in the materials.
Plasmonic biosensors: low-power and long-range trapping of different bio-molecules on plasmonic antennas can enable highly sensitive and efficient label-free biosensors.
All-optical based sorting achieved by selectively trapping different sizes or types of objective by engineering the heating transfer at the nanoscale.
Directed Nanoparticle Transport via Control of Light Polarization.
"Our all optically based thermoelectric trapping shows many advantages over current optical methods such as significantly reduced power and trapping range," Zheng, tells Nanowerk. "The rigid structures of current nanophotonic devices, fabricated by conventional lithographic techniques such as electron beam lithography, limited the tunability. Our novel technique is an powerful method to achieve dynamic plasmonic structures and tunable plasmon-exciton coupling for active photonic devices."
The team will conduct further studies for sorting, preconcentration, plasmonic biosensing, nanolaser and so on. Potential challenges lie in controlling the heat transfer at the nanoscale and achieving 3D optical manipulation at low power.
Provided by the University of Texas at Austin as a Nanowerk exclusive.

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