Yongkuk Lee

Rapid and efficient sonochemical formation of gold nanoparticles under ambient conditions using functional alkoxysilane

Gold nanoparticles (NPs) are rapidly and efficiently formed under ambient conditions with a novel and highly-efficient sonochemical promoter. Despite of the presence of free oxygen, 3-glycidoxypropyltrimethoxysilane (GPTMS) showed remarkable efficiency in promoting the reduction rate of Au (III) than that of conventional promoters (primary alcohols). This is likely attributed to the formation of a variety of radical scavengers, which are alcoholic products from sonochemical hydrolysis of the epoxide group and methoxysilane moieties of GPTMS under weakly acidic conditions. Interestingly, the promotion is quenched by amine- or thiol-functionalized alkoxysilane, thereby producing marginal amounts of gold NPs. Furthermore, products of hydrolyzed GPTMS were confirmed to attach on the surface of gold NPs by attenuated total reflectance-Fourier transform infrared spectroscopy. However, according to transmission electron microscopy images, gold NPs that were produced in the presence of GPTMS tend to fuse with each other as condensation of silanols occurs, forming worm- or nugget-like gold nanostructures. The use of long chain surfactants (i.e. polyethylene glycol terminated with hydroxyl or carboxyl) inhibited the fusion, leading to mono-dispersed gold NPs. Additionally, the fact that this approach requires neither an ultrasound source with high frequency nor anaerobic conditions provides a huge advantage. These findings could potentially open an avenue for rapid and large-scale green-synthesis of gold NPs in future work.

Directional transport by nonprocessive motor proteins on fascin-cross-linked actin arrays.

In this study, the unidirectional transport of heavy meromyosin (HMM)-coated beads is demonstrated on fascin-cross-linked actin arrays. The streptavidin-coated surface was properly blocked to prevent nonspecific binding of F-actin and, as a result, a high population of long gelsolin-actin complexes was suspended in the medium for subsequent processes. A flow field was utilized to lay down F-actin aligned along the direction of the flow and fascin cross-linked laid F-actin to prevent F-actin resuspension. When HMM-coated beads came into contact with the fascin-cross-linked actin arrays, they started to move in the same direction over long distances. Because of the nonprocessive nature of myosin II motor protein, the bead size limited the number of HMM heads on the area in contact with F-actin arrays, which resulted in beads traveling at different velocities according to their sizes. Furthermore, this study demonstrates the patterning of actin arrays, which could serve as a basis for the development of applications.

A visualized observation of calcium-dependent gelsolin activity upon the surface coverage of fluorescent-tagged actin filaments

Gelsolin regulates the dynamics of F-actin by binding to F-actin to sever and cap. In the present study, a novel approach is introduced to observe gelsolin activity through the coverage of surface-bound F-actin. Gelsolin was immobilized on streptavidin coated surface using biotinylation and, as a result, the interaction between gelsolin and F-actin was visualized. Consequently, the coverage of F-actin reflects the activity of gelsolin as a function of free Ca(2+) concentrations. In order to prevent non-specific binding of F-actin, the combinations of BSA and Tween-20 as blocking agents were investigated. Moreover, the measurement of the length of F-actin with actin-gelsolin mixtures at various ratios provided the verification of gelsolin activity after biotinylation. The data shows the increase in Ca(2+) concentration leads to a proportional increase in F-actin coverage, giving to half-maximal coverage at ~2.9 μM. Furthermore, the length of bound F-actin was found to decrease along with increasing Ca(2+) concentration, and full-length F-actin was rarely observed. This may suggest that severing and capping activities of gelsolin occur without more additional Ca(2+) for subsequent activation after full-length gelsolin binds to a side of F-actin. This finding may provide a key to understand gelsolin activity.

The movement of actin-myosin biomolecular linear motor under AC electric fields: an experimental study

The role of actin-myosin as a biomolecular linear motor is considered a transport system at nanoscale because of their size, efficiency and functionality. To utilize the ability to transport, it is essential to control the random movement of actin filaments (F-actin) on myosin coated substrate. In the presence of an alternating current (AC) electric field, the direction of F-actin movement is regulated by electro-orientation torque and, as a result, its movement is perpendicularly toward the electrode edges. Our data confirm such aligned movement is proportional to the strength of applied electric field. Interestingly, the aligned movement is found frequency-dependent and the electrothermal effect is observed by means of the velocity measurement of aligned F-actin movement. The findings in this study may provide constructive information for manipulating actin-myosin nanotransport system to build functional nanodevices in future work.

Carbon nanotube - actin hybrid assemblies

Carbon Nanotubes (CNTs) have unique electrical and chemical properties that make them viable candidates for diverse applications ranging from biology and medicine, to electronics and identification. Also, CNTs can be integrated with biomolecules and used as scaffolds for biosensors. However, these biosensors generally have lower detection capabilities due to their limited flexibility and placement in specific locations. In this paper we investigate the immobilization of actin cytoskeletal protein onto CNTs. Actin is a eukaryotic globular protein that serves as scaffold for molecular motors involved in cell motility. Our results indicate that actin monomers attach to CNTs. Moreover, the attached protein is still functional and able to polymerize into actin filaments. Further integration of CNT-actin filament assemblies with molecular motors hold promise for developing “mobile” biosensors to be used for selective placement, sensing and detection.

Unidirectional transport of HMM coated particles on fascin crosslinked F-actin arrays

Actin-myosin system plays an important role in biological transport and it can be utilized for various applications: delivery, sorting, and powering microdevices in in vitro. A key of harnessing this powerful and efficient system is directional control. To perform directional control, unipolar F-actin arrays were created on a substrate. Biotinylated gelsolin was used as a tool to anchor F-actin on the streptavidin coated surface. To prevent nonspecific binding of F-actin on the surface, blocking agents were employed. In presence of a flow field, anchored Factin was aligned and fascin helped forming F-actin arrays on the surface. When unipolar F-actin arrays were created, HMM coated particles were successfully moved along Factin arrays at approx. 1.3 μm/s. This technique will provide constructive information for the future applications of transport and actuation systems using actinmyosin at nanoscale.

Isopolar actin filament pathways using AC electrokinetic pump

To demonstrate unidirectional transport using actomyosin system in vitro, polarized F-actin pathway is essential. Polarized F-actin pathway whose ends are immobilized using biotinylated actin-capping proteins on a surface can provide directional movement of myosin coated microbeads. Creating polarized F-actin pathway requires the generation of proper flows in which flow speed and direction in high ionic solution is critical. In the present work, various AC electrokinetic pump designs are investigated to create a flow field in a microchannel. Moreover, the polarity of F-actin is determined using SEM. Once these two techniques are established, it will help the development of a device utilizing actomyosin system. This investigation will lay foundation for the future applications of autonomous transport and actuation systems, whether biological or synthetic in nature at nanoscale.

Selective photoimmobilization of actin filaments for developing an intelligent nanodevice

In the present study we present a method of selective immobilization of actin filaments based on the bioaffinity reaction of streptavidin/biotion on an aminopropyltriethoxysilane (APTES)-functionalized glass surfaces. Atomic force microscopy was employed to verify APTES film with higher roughness compared with bare glass. Patterning was achieved since photobiotin was covalently attached to the surface with masks of varying size and geometry under ultraviolent light. The patterning was characterized by the fluorescence of dye-tagged streptavidin. After blocked, dye-tagged biotinylated actin patterning was obtained, implying the feasibility of a biomolecular transportation system based upon the selective immobilization of these motor proteins.

Biomolecular Shuttles Under Dielectrophoretic Forces

The concept of biomolecular shuttles using interactions of myosin (the motor) and actin filament (F- actin, the shuttle) is that they can be used to transport micro or nano-sized cargos to a desired destination at nanoscale in synthesized environment. To utilize their ability to transport, dielectrophoretic forces which are generated by induced polarization under a nonuniform electrical field are introduced as a viable candidate to control the direction of their random movement. Under an AC electric field, actin filaments which slide on a heavy meromyosin coated surface show perpendicularly bidirectional movement between embedded electrodes and the aligned movement depends on the strength of dielectrophoretic forces. This paper explores the behaviors of F-actin movement under dielectrophoretic forces. Furthermore, the velocity and orientation of F-actin movement under the AC electric field are measured with various AC voltages, frequencies and distances between electrodes.