AC Electrokinetics of Polarizable Tri-Axial Ellipsoidal Nano-Antennas and Quantum Dot Manipulation.

Touvia Miloh

doi:10.3390/mi10020083

Abstract

By realizing the advantages of using a tri-axial ellipsoidal nano-antenna (NA) surrounded by a solute for enhancing light emission of near-by dye molecules, we analyze the possibility of controlling and manipulating the location of quantum dots (similar to optical tweezers) placed near NA stagnation points, by means of prevalent AC electric forcing techniques. First, we consider the nonlinear electrokinetic problem of a freely suspended, uncharged, polarized ellipsoidal nanoparticle immersed in a symmetric unbounded electrolyte which is subjected to a uniform AC ambient electric field. Under the assumption of small Peclet and Reynolds numbers, thin Debye layer and ‘weak-field’, we solve the corresponding electrostatic and hydrodynamic problems. Explicit expressions for the induced velocity, pressure, and vorticity fields in the solute are then found in terms of the Lamé functions by solving the non-homogeneous Stokes equation forced by the Coulombic density term. The particular axisymmetric quadrupole-type flow for a conducting sphere is also found as a limiting case. It is finally demonstrated that stable or equilibrium (saddle-like) positions of a single molecule can indeed be achieved near stagnation points, depending on the directions of the electric forcing and the induced hydrodynamic (electroosmotic) and dielectrophoretic dynamical effects. The precise position of a fluorophore next to an ellipsoidal NA, can thus be simply controlled by adjusting the frequency of the ambient AC electric field.

Highlights

The subject of plasmonic luminescence enhancement and quenching of dye molecules or quantum dots (QD) placed near metallic nanoparticles (NP) or nano-antennas (NA), has recently attracted much interest because of its immense potential applications in the field of nano-photonics and nano-plasmonics
For example [14,15], for typical fluorophores or QDs located next to gold (Au) nano-spheres of diameters ranging from 20–100 nm that are illuminated at laser wave-length excitation of 400–650 nm, the optimal QD/NP spacing is rather small and typically lies in the range of 8–15 nm
Single molecule fluorescence imaging and spectroscopy are very powerful present-day techniques often used in various branches of physics, chemistry, biology and material sciences that are based on controlling and positioning of free fluorophores next to nano-antennas (NA) of non-spherical shapes

Summary

Introduction

The subject of plasmonic luminescence enhancement and quenching of dye molecules or quantum dots (QD) placed near metallic nanoparticles (NP) or nano-antennas (NA), has recently attracted much interest because of its immense potential applications in the field of nano-photonics and nano-plasmonics (for example [1,2,3,4,5,6,7,8,9,10]). This is in direct contrast to the more often studied case of metallic (conducting) nano-spheres, which have only a single plasmonic frequency determined by the corresponding Frohlich resonance condition (depending on its material) regardless of its size In this context, most fluorophores are characterized by two distinct emission and absorption frequency bands which are separated by the Stokes drift [33] (e.g., typical wave lengths of 346 nm and 445 nm for Alexa 532-type fluorophore) and for this reason ellipsoidal NAs possessing at least two resonant frequencies, are considered more effective NAs compared to the commonly used spherical NAs. It is worth mentioning that non-spherical laser-heated conducting nanoparticles immersed in electrolyte (unlike spherical NPs), are generally subjected to a non-vanishing self-induced thermoosmotic (quadrupole-type) flow driven by the Seebeck effect [32].

Theoretical Background

The Electrostatic Problem of a Conducting Tri-Axial Ellipsoid

Hydrodynamic Problem

Spheres and Spheroids as Limiting ACEO Cases

Summary and Discussions