Automated Dielectrophoretic Tweezers-Based Force Spectroscopy System in a Microfluidic Device.

We reported an automated dielectrophoretic (DEP) tweezers-based force spectroscopy system to examine intermolecular weak binding interactions, which consists of three components: (1) interdigitated electrodes and micro-sized polystyrene particles used as DEP tweezers and probes inside a microfluidic device, along with an arbitrary function generator connected to a high voltage amplifier; (2) microscopy hooked up to a high-speed charge coupled device (CCD) camera with an image acquisition device; and (3) a computer aid control system based on the LabVIEW program. Using this automated system, we verified the measurement reliability by measuring intermolecular weak binding interactions, such as hydrogen bonds and Van der Waals interactions. In addition, we also observed the linearity of the force loading rates, which is applied to the probes by the DEP tweezers, by varying the number of voltage increment steps and thus affecting the linearity of the force loading rates. This system provides a simple and low-cost platform to investigate intermolecular weak binding interactions.

Detection of Silver Ions Using Dielectrophoretic Tweezers-Based Force Spectroscopy.

Understanding of the interactions of silver ions (Ag+) with polynucleotides is important not only to detect Ag+ over a wide range of concentrations in a simple, robust, and high-throughput manner but also to investigate the intermolecular interactions of hydrogen and coordinate interactions that are generated due to the interplay of Ag+, hydrogen ions (H+), and polynucleotides since it is critical to prevent adverse environmental effects that may cause DNA damage and develop strategies to treat this damage. Here, we demonstrate a novel approach to simultaneously detect Ag+ satisfying the above requirements and examine the combined intermolecular interactions of Ag+-polycytosine and H+-polycytosine DNA complexes using dielectrophoretic tweezers-based force spectroscopy. For this investigation, we detected Ag+ over a range of concentrations (1 nM to 100 µM) by quantifying the rupture force of the combined interactions and examined the interplay between the three factors (Ag+, H+, and polycytosine) using the same assay for the detection of Ag+. Our study provides a new avenue not only for the detection of heavy metal ions but also for the investigation of heavy metal ions-polynucleotide DNA complexes using the same assay.

Characterization of the Stiffness of Multiple Particles Trapped by Dielectrophoretic Tweezers in a Microfluidic Device.

Characterization of the stiffness of multiple particles trapped by tweezers-based force spectroscopy is a key step in building simple, high-throughput, and robust systems that can investigate the molecular interactions in a biological process, but the technology to characterize it in a given environment simultaneously is still lacking. We first characterized the stiffness of multiple particles trapped by dielectrophoretic (DEP) tweezers inside a microfluidic device. In this characterization, we developed a method to measure the thermal fluctuations of the trapped multiple particles with DEP tweezers by varying the heights of the particles in the given environment at the same time. Using the data measured in this controlled environment, we extracted the stiffness of the trapped particles and calculated their force. This study not only provides a simple and high-throughput method to measure the trap stiffness of multiple particles inside a microfluidic device using DEP tweezers but also inspires the application of the trapped multiple particles to investigate the dynamics in molecular interactions.

Biaxial Dielectrophoresis Force Spectroscopy: A Stoichiometric Approach for Examining Intermolecular Weak Binding Interactions.

The direct quantification of weak intermolecular binding interactions is very important for many applications in biology and medicine. Techniques that can be used to investigate such interactions under a controlled environment, while varying different parameters such as loading rate, pulling direction, rupture event measurements, and the use of different functionalized probes, are still lacking. Herein, we demonstrate a biaxial dielectrophoresis force spectroscopy (BDFS) method that can be used to investigate weak unbinding events in a high-throughput manner under controlled environments and by varying the pulling direction (i.e., transverse and/or vertical axes) as well as the loading rate. With the BDFS system, we can quantitatively analyze binding interactions related to hydrogen bonding or ionic attractions between functionalized microbeads and a surface within a microfluidic device. Our BDFS system allowed for the characterization of the number of bonds involved in an interaction, bond affinity, kinetic rates, and energy barrier heights and widths from different regimes of the energy landscape.

Characterization of anomalous movements of spherical living cells on a silicon dioxide glassy substrate.

The random walk of spherical living cells on a silicon dioxide glassy substrate was studied experimentally and numerically. This random walk trajectory exhibited erratic dancing, which seemingly obeyed anomalous diffusion (i.e., Lévy-like walk) rather than normal diffusion. Moreover, the angular distribution (-π to π) of the cells' trajectory followed a "U-shaped pattern" in comparison to the uniform distribution seen in the movements of negatively charged polystyrene microspheres. These effects could be attributable to the homeostasis-driven structural resilient character of cells and physical interactions derived from temporarily retained nonspecific binding due to weak forces between the cells and substrates. Our results provide new insights into the stochastic behavior of mesoscopic biological particles with respect to structural properties and physical interactions.