The author discusses the laser interference method to measure the size of small bio-particles: extracellular vesicles (EVs), exosomes and viruses of nanometer scale in flow cytometry. By introducing a new laser configuration in place of conventional optical system of the flow cytometry, the interference fringe (periodic intensity modulation) is created inside the flow which provides a calibrated scale for the size measurement. The fringe pitch is precisely determined by the crossing angle of laser beams and the wavelength. The transverse size of the beam spot can be much larger than wavelength, for example, 30 micro-meters, where the flow cell design in conventional system can be used with moderated dimensional tolerance. The interaction length of sample with laser becomes longer: ~30 micro-meter and thus the scattering light and fluorescent light in case of labeled particle becomes much higher. Importantly, those signals are modulated in MHz frequency range associated with the periodic intensity modulation of the interference fringe. By measuring modulation depth, we can determine the particle size. We may utilize a bandpass filter or the digital signal processing of FFT, and thus we can drastically improve S/N ratio. This is because of the narrow spectrum of signal and thus effectively eliminates noise from various debris contained in the sample, and also low frequency shot noise in the photo detector signal. Thanks to the above-mentioned merits, it will be possible to measure the size of EVs and exosomes in particle-by-particle basis in reliable dimensional scale at nano-meter resolution. It will be also possible to realize the fast virus detection system for screening passengers in the airport at the current pandemic of COVID-19 and in future.
Proposal of Absolute Nanometer Size Measurement in Flow Cytometry Based on Laser Interferometry