The Optical Absorption Force Allows Controlling Colloidal Assembly Morphology at an Interface

Abstract

three-dimensionally manipulate micrometer-sized objects in solution in a nondestructive way, [1] optical tweezers have been used in research fields ranging from material science to biology. [2][3][4][5][6][7][8][9][10][11][12][13][14][15] Inside bulk solution, stable trapping is only achieved when the gradient force is larger than the scattering force and overcomes Brownian motion and gravity. [1] However, at an interface, the picture is completely different: both optical forces (gradient and scattering) assist in trapping and gathering of particles. The particles trapped inside the focus scatter and propagate the trapping laser, which expands the optical potential, in this way assisting in collecting particles outside the focus. Thus, so-called dynamically evolving assemblies of different materials (e.g., metallic and polymeric particles, proteins, small weight molecules) have been prepared by optical trapping at an interface. [16][17][18][19][20][21] In general, the morphology and the behavior of these trapped particles are dependent on the material properties; however, all these assemblies have in common that they expand far-beyond the irradiated area and that they are dynamic in owning to bespoke light-particle and particle-particle interactions.Gaining control on particle-particle interactions and selfassembled structures is one of the key objectives for colloidal Gaining control on particle-particle interactions and in this way on their (self )-assembled structures is essentialfor colloidal and material sciences. Currently, different strategies are described to achieve such control, however, all of them lack the spatiotemporal resolution required at the microscale. In this work, the potential of combining optical trapping and resonant photo excitation for modifying particle-particle interactions and subsequent assembling of dye-doped particles at the solution interface is demonstrated. The particle assemblies prepared by nonresonant 1064 nm optical trapping undergo morphology changes after resonant photoexcitation of the embedded dye molecules. Depending on the physicochemical properties of interface, quick hexagonal close packing (HCP)-rearrangement or explosive dispersion of assemblies is observed at air/solution (A/S) and glass/solution interfaces, respectively. By contrast, by resonant photoexcitation only, the dispersed dye-doped particles are pushed toward the A/S interface, followed by association to yield HCP-structured assemblies. The results are rationalized by considering the optical absorption force coupled with other nonoptical forces (e.g., capillary force, dipole-dipole or electrostatic repulsion) at the solution interface. Due to the inherent spatiotemporal properties of light and electronic transition of materials, absorption force is a unique element to control and modify the structural order of particle assemblies at interfaces.

Gaining control on optical force by the stimulated-emission resonance effect

“…This ORE-based methodology could have the potential to become an alternative to other (nonlight involved) trapping techniques such as Anti-Brownian ELectrokinetic (ABEL) or dielectrophoretic traps. 49,50 Even more, the role of ORE forces has been recently expanded from a single particle system to the ensemble particle level, 51 where the ORE force triggers the gathering and assembling of dyedoped particles at an interface as well as modifying the assembly morphology.…”

Plasmonic Dipole and Quadrupole Scattering Modes Determine Optical Trapping, Optical Binding, and Swarming of Gold Nanoparticles

“…Laser trapping at an interface offers a unique platform for assembling nanomaterials, where the optical potential and induced assembly cooperatively extend beyond the focal spot . For example, optical-trapping-induced crystallization occurs solely at the solution surface, where the concentration of so-called molecular clusters of amino acids or proteins around the trapping laser focus increases and leads to their crystallization or aggregation. , Instead, when nanoparticles (NPs) are trapped at the interface, a periodically aligned structure is generated. − In particular, gold (Au) NPs form a dynamically fluctuating swarming assembly that expands a few micrometers beyond the focal trapping spot . In the initial stage, periodic structures resembling Yagi–Uda antennas are assembled inside the focal spot perpendicular to the linear laser polarization, demonstrating the critical role of the dipole scattering mode of Au NPs on this phenomenon .…”

Optically Controlled Development of a Waveguide from a Reservoir of Microparticles

“…However, recent interest in another type of microparticle manipulation has begun growing. It is focused on laser-induced assembly of nano-and micro-particles at interfaces between water and air or water and a solid, [17][18][19] where assembly of periodic arrangements [20] and swarming [21] were observed. For example, the group of Masuhara has observed assembly of MPs with extended horns consisting of linearly aligned particles [22] and "pistollike ejections" resulting in a few linearly aligned MPs being ejected from a metastable assembly of particles with a concentric circle-like symmetry.…”