Size-Tunable Assembly of Gold Nanoparticles Using Competitive AC Electrokinetics

Abstract

Alternating current (AC) electrokinetics is a facile way of patterning colloidal particles into advanced structures. We demonstrate the combined use of AC dielectrophoresis (AC-DEP) and AC electrohydrodynamics (AC-EHD) in a microwell electrode geometry for size-tunable assembly of gold nanoparticles (AuNPs) into one-dimensional microwires and two-dimensional films. The AC-DEP force scales with both particle size and field frequency, whereas the AC-EHD force depends only on the field frequency. So, a critical particle diameter (d c ) exists, below which the EHD phenomenon becomes more important and beyond which the DEP force is dominating. We performed theoretical and experimental studies to determine "d c " and how it gets affected by operating parameters like field frequency, voltage, particle number, electrolyte concentration, electrode size, and geometry. Our results show that the morphologies of the colloidal structures transition from films to microwires as the NP diameters vary from nanometers (<d c ) to microns (>d c ), and no assembly takes place at intermediate sizes (∼d c ). While the film formation is governed purely by surface EHD flows, microwire synthesis is a result of EHD-assisted DEP phenomenon. Also, a minimum particle number, a low salt concentration, and an optimum frequency range is required to initiate assembly.

Low-Voltage Nongassing Electroosmotic Pump and Infusion Device with Polyoxometalate-Encapsulated Carbon Nanotubes

“…The solution has been proposed with AC mode electrokinetic devices. 24,25 Luft, Kuhl, and Richter as early as 1977 reported an alternate solution for the gassing EOPs using consumable Ag/AgCl electrodes. They proposed a subcutaneous insulin delivery system with Ag/AgCl electrodes and ion-exchange membranes with the concept of indirect pumping to avoid the fouling of the ion-exchange membrane, 26 but very small electroosmotic flux limited the intended applications.…”

“…The device fabricated from thin metal electrodes integrated within microfluidic channels at the substrate and cover plate interface show linear dependence of flow velocity with the applied DC voltage, but they have serious concern of gas bubbles generated from electrolysis products. , These generated gas bubbles must be removed or recombined from the system. The solution has been proposed with AC mode electrokinetic devices. , Luft, Kuhl, and Richter as early as 1977 reported an alternate solution for the gassing EOPs using consumable Ag/AgCl electrodes. They proposed a subcutaneous insulin delivery system with Ag/AgCl electrodes and ion-exchange membranes with the concept of indirect pumping to avoid the fouling of the ion-exchange membrane, but very small electroosmotic flux limited the intended applications.…”

Directed Assembly of Nanomaterials for Making Nanoscale Devices and Structures: Mechanisms and Applications

“…However, systematic investigation in these aspects is currently lacking. Although more than one electrokinetic phenomenon is considered in some articles, ,,,, systematically addressing the effect of all the electrokinetic phenomena on the particle assembly is generally lacking. In addition, local Joule heating caused by the electric field gives rise to an additional electrokinetic phenomenon in the form of electrothermal flow acting on the particles. , Establishing equations combining all the electrokinetic phenomena or setting up criteria for judging which electrokinetic phenomenon is dominant is a fertile field worthy of in-depth studies.…”

“…In reality, each effect mentioned above contributes to the motion of the nanoparticles. Although more than one effect is employed or considered in some articles, ,,, systematically addressing the influence of all the effects on the particle assembly is generally lacking. The significance of each force under various assembly parameters, such as electric field strength, frequency, particle size, dielectric constant and conductivity of the nanoparticles and the medium, solvent type, pH, electrolyte type and concentration, and pattern geometry, is a fertile field worthy of in-depth study.…”

“…Therefore, a DC pulse of 12 V was applied for 2 s to fix the assembled TBPe particles on the ITO electrode. 234 Besides causing nonuniformity in current density, Goel et al 235 showed that the electrode patterning can also lead to nonuniformity of the electric field. This arises due to the concentrated field effect and manifests as a tangential electric field component and a localized high electric field at the edge of the patterns.…”