Tomoyuki Yasukawa

ABSTRACT We developed a novel electrorotation (ROT) device featuring a microwell array with three electrodes. This device allows to monitor the increase in membrane capacitance of cells subjected to chemical stimulation. The microwell array is integrated into the bottom of a fluidic channel and holds rotating cells during stimulation with a solution containing a chemical agent. Positive dielectrophoresis (p‐DEP) effectively traps cells in microwells, whereas negative DEP (n‐DEP) facilitates the rapid formation of single‐cell presence. Alternating current (AC) voltages with a 120° phase shift applied across the three electrodes enable vertical and simultaneous rotation of cells. We observed a peak in rotation rate as a function of applied frequency, with the frequency spectrum shifting to lower frequencies as membrane capacitance increased. A positive correlation was identified between rotation rate and membrane capacitance, so monitoring in the low‐frequency range is advantageous. Although n‐DEP at lower frequencies risks removing cells from microwells, the continuous monitoring of the ROT rate during chemical stimulation was achieved by regulating the height of the ROT center of cells. We demonstrated the monitoring of membrane capacitance increase induced by Ca²⁺ influx from ionomycin. This simple configuration facilitates statistical analysis of ROT rates without fluorescent labeling, making it suitable for label‐free assessments of white blood cells’ responses to stimuli.

Surface plasmon resonance is an optical phenomenon that can be applied for label-free, real-time sensing to directly measure biomolecular interactions and detect biomarkers in solutions. Previous studies using plasmonic nanohole arrays have monitored and detected various biomolecules owing to the propagating surface plasmon polaritons (SPPs). Extraordinary optical transmission (EOT) that occurs in the near-infrared (NIR) and infrared (IR) regions is usually used for detection. Although these plasmonic nanohole arrays improve the sensitivity and throughput for biomolecular detection, these arrays have the following disadvantages: (1) molecular diffusion in the solution (making the detection of biomolecules difficult), (2) the device fabrication's complexities, and (3) expensive equipments for detection in the NIR or IR regions. Therefore, there is a need to fabricate plasmonic nanohole arrays as biomolecular detection platforms using a simple and highly reproducible procedure based on other SPP modes in the visible region instead of the EOT in the NIR or IR regions while suppressing molecular diffusion in the solution. In this paper, we propose the combination of a polymer-based gold nanohole array (Au NHA) obtained through an easy process as a simple platform and dielectrophoresis (DEP) as a biomolecule manipulation method. This approach was experimentally demonstrated using SPP and LSPR modes (not EOT) in the visible region and simple, label-free, rapid, cost-effective trapping and enrichment of nanoparticles (trapping time: <50 s) and bovine serum albumin (trapping time: <1000 s) was realized. These results prove that the Au NHA-based DEP devices have great potential for real-time digital and Raman bioimaging, in addition to biomarker detection.

Simultaneous electrorotation in microwells during chemical stimulation label-free monitoring effect of chemicals in single-cell manner.