Quantifying the Role of the Surfactant and the Thermophoretic Force in Plasmonic Nano-optical Trapping

Abstract

Plasmonic nano-tweezers use intense electric field gradients to generate optical forces able to trap nano-objects in liquids. However, part of the incident light is absorbed into the metal, and a supplementary thermophoretic force acting on the nano-object arises from the resulting temperature gradient. Plasmonic nano-tweezers thus face the challenge of disentangling the intricate contributions of the optical and thermophoretic forces. Here, we show that commonly added surfactants can unexpectedly impact the trap performance by acting on the thermophilic or thermophobic response of the nano-object. Using different surfactants in double nanohole plasmonic trapping experiments, we measure and compare the contributions of the thermophoretic and the optical forces, evidencing a trap stiffness 20× higher using sodium dodecyl sulfate (SDS) as compared to Triton X-100. This work uncovers an important mechanism in plasmonic nano-tweezers and provides guidelines to control and optimize the trap performance for different plasmonic designs.

Opto-refrigerative tweezers

“…However, note that negative Soret coefficients can be observed in certain circumstances, where ORT is not applicable. For instance, the presence of surfactants in the solution can alter the thermophoretic response of colloidal particles ( 39 ). Currently, the substrate, i.e., Yb:YLF nanoparticle layer, is prepared by a simple drop-cast method.…”

Hydrodynamic manipulation of nano-objects by optically induced thermo-osmotic flows

“…They can be controlled by the light intensity heating laser and are quickly switched due to the extremely fast heat conduction at these length scales. Moreover the finding of the vdW dominated thermo-osmotic flows suggest that such contributions must be present in any plasmonic trapping experiment with extended gold structures 12,16,17,27 . Using F TF x = 6πηR γ ∥ vx and F TF z = 6πηR γ ⊥ vz, where γ ∥ and γ ⊥ are the correction factors for the friction coefficient of a sphere close to a surface we are able to extract the hydrodynamic forces that are exerted on the AuNP tracers (see equation (2) and Supplementary Information for details).…”

“…The latter is achieved with optical 3 and plasmonic tweezers 11,12 , magnetic fields 13 , or using electrokinetic 14 or opto-electronic 15 effects. Especially in the field of plasmonic tweezers and nanoantennas where light is used to excite collective electron motion in noble-metals, the Joule losses lead to the unavoidable generation of heat at boundaries as an unwanted side effect 16,17 . Yet, such optically generated temperature fields seem also suitable for the manipulation of nano-objects in liquids, for example, for the trapping of nanoparticles 18 and single molecules 19 or protein aggregates 20 as well as for manufacturing active particles [21][22][23][24] .…”

Plasmonic Nanotweezers and Nanosensors for Point‐of‐Care Applications

“…Recently, Wegner group illustrated the contribution of thermophoretic force in LSPR plasmonic tweezers and demonstrated how surfactant can influence the trapping behavior of nanoparticles by modifying their thermophoretic coefficient. [ 80 ] For example, trapping of nanoparticle dispersed in sodium dodecyl sulfate showed higher trap stiffness as opposed to those dispersed in Triton X‐100.…”