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Posted: Sep 21, 2016

Plasmon-enhanced thermophoresis for the reversible assembly of plasmonic nanoparticles

(Nanowerk Spotlight) The optical manipulation of plasmonic nanoparticles – metal nanoparticles that are highly efficient at absorbing and scattering light – has advantages for applications such as nanofabrication, drug delivery and biosensing. To that end, researchers have been developing techniques for the reversible assembly of plasmonic nanoparticles that can be used to modulate their structural, electrical and optical properties.
The latest such technique is a low-power assembly that is enabled by thermophoretic migration of nanoparticles due to the plasmon-enhanced photothermal effect and the associated enhanced local electric field over a plasmonic substrate.
An international research team, led by Yuebing Zheng, Assistant Professor of Mechanical Engineering and Materials Science & Engineering at the University of Texas at Austin, has developed a new optical assembly technique known as plasmon-enhanced thermophoresis to assemble plasmonic nanoparticles reversibly by optically controlling a temperature field.
This plasmon-enhanced thermophoresis can be exploited to confine plasmonic nanoparticles in a higher-temperature regime under a thermoelectric field.
The researchers reported their findings in the September 17, 2016 online edition of ACS Nano ("Light-Directed Reversible Assembly of Plasmonic Nanoparticles Using Plasmon-Enhanced Thermophoresis").
Light-directed reversible assembly of plasmonic nanoparticles based on plasmon-enhanced thermophoresis
Light-directed reversible assembly of plasmonic nanoparticles based on plasmon-enhanced thermophoresis. (a) Schematic representation of the light-directed reversible assembly of positively charged gold nanotriangles (AuNTs) functionalized with CTAC (cetyltrimethylammonium chloride). (b) Scanning electron micrograph of a single AuNT on the gold nanoislands (AuNIs) substrate. (c) Successive optical images during light-directed assembly of AuNTs. (d) Successive optical images showing the disassembly of an AuNT aggregate after the laser is turned off. The red and blue dot circles indicate that the laser is off and on, respectively. Scale bars: 10 µm. (Reprinted with permission by American Chemical Society) (click on image to enlarge)
"It is well known that plasmonic nanoparticles and their assemblies can manipulate light at the subwavelength scale where the intense localized electromagnetic field strongly couples with nanoscale objects, leading to various light-matter interactions and applications," Dr. Linhan Lin, the paper's first author, explains to Nanowerk. "However, the manipulation and assembly of nanoparticles using traditional optical tweezers requires high optical power and strict optical alignment. The technique we developed here overcomes the high-power requirement, with the operation power at least 3 orders of magnitude lower than that used in optical tweezers."
Moreover, as he points out, the team achieved dynamical and parallel manipulation of the assembly arrays and applied them to improve SERS of molecules in native liquid environments.
"The capability to assemble plasmonic nanoparticles at a low optical power paves the way towards biomolecular analyses in their native environment, smart drug delivery with plasmonic nanoparticles, and bottom-up assembly of metamaterials," notes Lin.
Previous works, which exploit thermophoresis for particle trapping, couldn't achieve the stable confinement of nanoparticles in the hot region where the laser beam is. The migration of nanoparticles towards the cold region limits the trapping stability and versatile manipulation of the nanoparticles. To overcome this limitation, the team exploits a cationic surfactant to modify the surface charge of the nanoparticles and design the thermoelectric field on the particles to trap it in the hot region, which enables the light-driven versatile manipulation of the nanoparticles.
This work proposes a general strategy to modify the surface charge of plasmonic nanoparticles and achieve trapping and assembly of the nanoparticles, which will advance the multiple fields of optical manipulations, nanofabrication, and plasmofluidics.
The novel method can solve current challenges in assembling various nanoparticles with different size, shape and components for applications. Due to the generality of the method, it can be extended to trap and manipulate dielectric nanoparticles, quantum dots and biological cells, paving the way towards bottom-up assembly of functional devices and point-of-care medical research and diagnostics.
For example, this manipulation of plasmonic nanoparticles will open up a new window of opportunity in drug screening and cellular biology. The plasmonic nanoparticle assembly can be widely applied to biological sensing of live cells by enhancing the Raman signal of molecules on the cellular membrane, as well as drug delivery in live cells.
This video shows the reversible assembly process of the gold nanotriangles at the laser spot.
Going forward, the team aims to 1) establish prediction models for the optical assembly of nanoparticles and test the ultimate limits and 2) apply the plasmonic nanoparticle assemblies to cellular biology and disease diagnostics.
"Our work at the interface of thermophoresis, colloid and surface chemistry, and plasmonics will advance multiple fields of materials science and engineering," concludes Zheng. "Our future work will be focused on developing prediction models for various types of nanoparticles, testing the ultimate limits of plasmon-enhanced opto-thermo-fluidics in nanoparticle manipulation, and applying assemblies of nanoparticles as functional structures and metamaterials to life sciences, point-of-care diagnosis, and national security."
The challenges for the scientists will be to achieve three-dimensional control of nanoparticle assemblies together with functional molecules at the single-nanoparticle and single-molecule level and to do these without the use of plasmonic substrates.
In a previous Nanowerk Spotlight we reported on the Zheng group's use of plasmon-enhanced photothermal effects to develop a novel ithographic technique: bubble-pen lithography.
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