Surfactants Control Optical Trapping near a Glass Wall

Abstract

Beyond their original capability to grab and hold tiny objects, optical tweezers have emerged as a powerful tool to investigate fundamental physics at microscopic scales. A precise characterization of the optical trap is one of the key requirements in such applications. A typical trapping system often involves a colloidal particle, stabilized in a fluid as an optical probe. Surfactants are commonly added to provide colloidal stability, but their incidental effects on the tweezer–particle interactions have been overlooked despite their prevalent use. Here, we study the interplay among the tweezer, the particle, and the surfactants adsorbed on the interfaces, including a nearby glass wall. In trapped particles’ motion analysis, we find that the surfactants can affect the motion of the particle through the interactions between them. We discuss the effect of the surfactants’ assembly structures on the particles’ statistical behaviors. In particular, we investigate the thermal effect on the particle surroundings induced by the optically heated particle by analyzing the difference between metallic and dielectric probes. Our results explain how, under nanoscale confinement, the adsorbed surfactants can affect the particle behavior in an optical trap and propose a possible strategy of using an optically heated particle for localized surface modulation.

Probing Surfactant Bilayer Interactions by Tracking Optically Trapped Single Nanoparticles

“…In addition, the absence of any changes in $${\bar{r}_t}$$ for the bare glass case indicates that some long‐term effects of the optical tweezer, such as optical heating, are still negligible in our experimental conditions. (To be more precise, the optical heating itself is not negligible based on our previous study comparing thermal and non‐thermal probes, [ 56 ] and in fact, it can accelerate the interactions by providing thermal energy. However, the thermal equilibrium is reached almost instantaneously compared to the timescale discussed in the present study, which we confirmed by optical heating simulations (Section , Supporting Information).…”

Probing surfactant bilayer interactions by tracking optically trapped single nanoparticles

“…Its trajectory was traced using a python toolkit, Trackpy [49], which is based on a particle-tracking algorithm developed by Crocker and Grier [50]. More detailed information about the optical trapping setup and methods can be found in ref [51]. A dataset for all the particle recordings and the corresponding trajectories is available in our Zenodo data repository [52].…”

“…We found that the CMC of our particle-CTAC mixture is 1.4 mM (Supplementary Figure ??a), which is higher than the values reported in the literature (1.0 to 1.1 mM) due to the existence of the gold colloids in the solution. We refer to our recent publication for more details [51].…”

“…The distribution of r30 for bare glass is not shown in Figure 3b for clarity as it remains similar to (i), i.e., no time-dependence in rt for bare glass as indicated in Figure 2d. We also refer the readers to our previous work [51] for more details about the particle interactions with spherical admicelles on bare glass surfaces. Here we focus on the difference between bare and activated surfaces, and especially, the particles' behaviors on bilayer membranes.…”

Optothermal Trapping of Fluorescent Nanodiamonds using a Drop-casted Gold Nanoparticle

“…Although useful, this design of using separate laser beams for trapping and spectroscopy can lead to constraints due to charge state perturbations when an IR excitation is involved 9,13 and may not be conducive in sensitive environments such as a living cell, where exposure to high-power density lasers can potentially lead to denaturing of biomolecules and damage of the cell. This clearly creates an imperative to design and develop nano-optical and optothermophoretic trapping methods [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] in which a single low-power laser can facilitate the capability of trapping and spectroscopic probing and imaging. 29 Recently, we have shown 30 that a drop-casted single gold nanoparticle can drive thermoplasmonic fields, facilitating the trapping of single metallic nanostructures and creating a large assembly at a very low power density (0.01 mW/µm 2 ).…”