Measuring Nanoparticle Polarizability Using Fluorescence Microscopy

Abstract

Using a novel method developed to quantify the polarizability of photoluminescent nanoparticles in water, we present experimental observation of the extraordinary polarizability exhibited by nanoparticles of commensurate size to the Debye screening length, confirming previously reported theory. Semiconductor quantum dots (QDs) are ideal model nanoparticles to demonstrate this assay, due to their tunable size and bright photoluminescence. This assay is based upon microfluidic chambers with microelectrodes that generate trapping potentials that are weaker than thermal energy. By comparing the local electric field strength and variations in QD concentration, their polarizability was computed and found to agree with estimates based upon the hydrodynamic diameter found using light scattering. Strikingly, the polarizability of the nanoparticles increased 30-fold in low salt conditions compared to high salt conditions due to the increased thickness of the Debye layer relative to the particle radius. In addition to providing evidence that corroborates theoretical work studying direct solutions to the Poisson-Nernst-Planck equations, these observations provide an explanation for previously observed conductivity dependence of biomolecule polarizability. As the polarizability of nanoparticles is of high importance to the electrically-directed assembly of particles, as well as their interactions with other materials in complex environments, we anticipate that these results will be highly relevant to ongoing efforts in materials by design and nanomedicine.

Bead based facile assay for sensitive quantification of native state green fluorescent protein

“…Then, the uorescence signal intensity was monitored on the beads using uorescence microscopy. 26 As shown in Fig. 1, the uorescence intensity of ht-GFP gradually decreased with decrease in the amount of ht-GFP until the case of 50 pg incubation.…”

Dielectrophoretically Modulated Optical Spectroscopy of Colloidal Nanoparticle Solutions in Microfluidic Channels

“…This is in line with recent findings obtained using DEP and fluorescence microscopy for 20 nm diameter quantum dots in water at low medium conductivity. [ 40 ]…”

Theory for hierarchical assembly with dielectrophoresis and the role of particle anisotropy

“…(), it is easy to predict whether a given particle will assemble in specific electric field. For example, there are numerous examples in the literature of interdigitated electrodes that are used to direct micro‐ and nanoparticle assembly by increasing the electric field until the DEP energy associated with the particle being in the high field region exceeds thermal energy [5,7,12]. Critically, if a second particulate species that had a substantially smaller volume was introduced, it would not be expected to assemble due to the particle's being correspondingly smaller.…”

“…Electric fields have become useful tools for directing the assembly of materials through the confluence of sophisticated processes for fabricating microelectrodes [1,2] and the growing understanding of myriad interactions between electric fields and matter [3]. Of these interactions, dielectrophoresis (DEP) [4], or the motion of induced dipoles in nonuniform electric fields, has been widely used for numerous tasks including the manipulation of cells [5], nanoparticles [6,7], biomolecules [8], and even fluids [9,10]. An especially interesting feature of DEP‐based assembly is that materials can be assembled that are significantly smaller than the electrodes, effectively offering a reduction in feature size that has been leveraged to position dielectric and metallic micro‐ and nanoparticles [11].…”