(Nanowerk Spotlight) The ability to precisely position microscopic particles into useful three-dimensional configurations has been a long-standing goal in fields ranging from materials science and photonics to biological sensing and drug delivery. Colloidal particles, which are small particles suspended in a medium, can serve as building blocks for advanced engineered materials and devices if they can be manipulated and assembled with sufficient control.
The Quest for Precise 3D Particle Control
Over the years, scientists have developed various techniques to tackle this challenge. Self-assembly methods rely on the particles’ natural tendencies to organize into ordered structures but offer limited tunability. Top-down fabrication approaches like lithography allow precise patterning but are often limited to planar geometries. And while optical tweezers, which use focused laser beams to trap and move particles, provide dynamic control in three dimensions, the particles are only held in place by the laser light – as soon as it’s turned off, Brownian motion causes them to drift apart.
Despite decades of progress, a flexible method for assembling particles into stable, reconfigurable 3D structures has remained elusive. But now, a research team from the University of Texas at Austin, may have found a solution. By combining optical trapping with a cleverly designed phase-change material, they’ve developed a platform for manipulating colloidal particles with unprecedented versatility, which they term optothermal manipulation using phase change (OMPC).
The core innovation of OMPC lies in the medium surrounding the particles. Rather than a simple liquid, the researchers employ a special gel composed of surfactant molecules that self-assemble into a solid but reconfigurable scaffold. This matrix has a remarkable property – when heated by a focused laser, it locally transitions from a rigid “coagel” state to a softer “gel” phase in the beam path. By moving the laser focus, this phase transition can be induced at arbitrary points to dynamically control the medium’s rigidity in three dimensions.
Working mechanism of optothermal manipulation using phase change (OMPC): (a) A laser beam is imposed on a particle embedded in the surfactant matrix. XP and ZP are the distances between the particle center and the laser beam center (intersection of beam axis and focal plane) along the X and Z directions, respectively. (b) Initially, the surfactant matrix around the particle exists in its coagel phase. When the laser is imposed on the particle, the temperature rises, and the increase in both the energy of the bilayers and the water content between the bilayers alters the interaction potential between the particle and the surfactant. As the temperature increases beyond a transition temperature (TTR), the matrix reversibly changes its phase into a gel state, allowing the particle to be manipulated. (Reprinted with permission by American Chemical Society)
“When the beam is focused on a light-absorbing particle, the particle heats up, melting the surrounding gel and allowing the optical forces to move it to a new position,” Professor Yuebing Zheng, principal investigator of this work, explains to Nanowerk. “Then, when the laser is switched off, the gel re-solidifies and locks the particle in place. By iteratively trapping, moving, and immobilizing particles, complex configurations can be built up piece by piece.”
Zheng and his team demonstrate several modes of particle manipulation using OMPC, including pushing particles along the beam path, pulling them towards the focus, and ‘nudging’ them laterally. Compared to existing techniques like optical tweezers in liquid media, this new method offers the unique advantage of being able to stably hold particles in place after the laser is turned off. The gel medium prevents Brownian motion from disrupting the assembled structures, allowing for more complex and durable configurations.
They showcase the assembly of 3D cubic lattices, reconfigurable patterns, and even particles riding along the surface of a larger sphere. Importantly, the gel medium retains the crafted structures for long time periods, preventing the disruption that plagues liquid-based assembly. The team reports that particles remained in their assembled positions for over 120 days, demonstrating the long-term stability of this approach.
Beyond spatial patterning, the team also explores the use of their OMPC technique to control the interaction of nanostructures with 2D materials. By raising and lowering a silicon particle above a monolayer of tungsten diselenide, they can tune the strength and spectral profile of light emission from the 2D material. This capability could be harnessed for optical sensing, light emission, and other nanophotonic applications.
A Wide Range of Potential Applications
The implications of this work extend to a wide range of potential applications. In photonics, the ability to assemble 3D lattices of dielectric particles could enable the creation of photonic crystals with tunable bandgaps and optical metamaterials with exotic properties. Reconfigurable nanostructures could also find use in adaptive optics and tunable lenses. In the field of biological sensing, the precise positioning of colloidal particles functionalized with biomolecules like antibodies or enzymes could lead to advanced biosensors with improved sensitivity and specificity. And in the context of drug delivery, the optical manipulation of drug-loaded nanocarriers could provide a means of targeted therapeutics.
While the current work establishes an impressive proof of concept, the scientists note that there is still room for improvement. “The flexibility of the OMPC technique is currently limited by the manual positioning of a single laser beam, which could be augmented in the future with algorithms for automated control and multi-beam manipulation,” notes Zheng. “Exploring a broader palette of gel materials could also expand the achievable temperature ranges and particle compatibilities.”
Versatility of OMPC: (a) Schematic and sequential microscopic images of a 3D array of 1.5 µm Si particles precisely arranged via OMPC. Imaging plane is indicated by the particle highlighted by the dashed circle. All other particles are on different planes but form a cube (dashed lines; also see Supporting Information). (b) Schematic and final optical microscopic images of particles arranged in (i) a two-dimensional “2” pattern, which is reconfigured into (ii) a three-dimensional “3” pattern. Particles of the “3” pattern are at varying heights on a tilted virtual plane. The two microscopic images show the same “3” pattern at two different imaging planes where the foci are on the particles highlighted by the dashed circles. (c) Schematic and optical images of a 1.5 µm Si particle manipulated on the surface of a 10 µm PS particle. Position of the imaging plane is highlighted in the upper right corner (unit: µm). The Si particle is manipulated from the left bottom (first panel) to the right bottom (last panel) of the PS particle. The schematics on the bottom right corner show the relative position of the Si particle with respect to the PS particle. Scale bar: 5 µm. (Reprinted with permission by American Chemical Society)
The researchers also acknowledge challenges that need to be addressed for practical applications. The micron-scale undulations in the gel medium and the laser positioning accuracy currently limit the manipulation of sub-500 nm particles. Optimizing the gel synthesis and implementing closed-loop feedback control could help overcome these limitations. The team also notes that further work is needed to enable the simultaneous translation and rotation of high-aspect-ratio structures like nanowires.
Looking ahead, the team is exploring several avenues to advance the OMPC platform. Efforts are underway to automate the particle assembly process using image-based feedback algorithms, which could greatly enhance the speed and complexity of achievable structures. They are also investigating the use of multiple laser beams for parallel manipulation and the incorporation of spatial light modulators for dynamic beam shaping. Expanding the library of compatible materials, including stimuli-responsive polymers and liquid crystals, is another area of active research.
“In the broader context of nanotechnology, our work represents an exciting step towards the goal of programmable matter – the idea that materials can be designed to dynamically reconfigure themselves in response to external stimuli,” Zheng concludes. “By providing a means to reversibly assemble and disassemble microscopic building blocks on demand, this optothermal manipulation technique brings us closer to realizing this vision.”
By combining the precision of optical trapping with the stability of phase-change media, this work adds an important new tool to the toolkit of nanotechnology. While there is still work to be done to fully harness the capabilities of OMPC, it marks an exciting stride towards our ability to fabricate functionalized micro- and nanostructures on demand. As research in this area progresses, we can anticipate a future where reconfigurable 3D nanostructures are routinely used to push the boundaries of materials science, photonics, biology, and beyond.
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