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The author discusses the laser interference method to measure the size ofsmall bio-particles: extracellular vesicles (EVs), exosomes and viruses ofnanometer scale in flow cytometry. By introducing a new laser configuration inplace of conventional optical system of the flow cytometry, the interferencefringe (periodic intensity modulation) is created inside the flow whichprovides a calibrated scale for the size measurement. The fringe pitch isprecisely determined by the crossing angle of laser beams and the wavelength.The transverse size of the beam spot can be much larger than wavelength, forexample, 30 micro-meters, where the flow cell design in conventional system canbe used with moderated dimensional tolerance. The interaction length of samplewith laser becomes longer: ~30 micro-meter and thus the scattering light andfluorescent light in case of labeled particle becomes much higher. Importantly,those signals are modulated in MHz frequency range associated with the periodicintensity modulation of the interference fringe. By measuring modulation depth,we can determine the particle size. We may utilize a bandpass filter or thedigital signal processing of FFT, and thus we can drastically improve S/Nratio. This is because of the narrow spectrum of signal and thus effectivelyeliminates noise from various debris contained in the sample, and also lowfrequency shot noise in the photo detector signal. Thanks to theabove-mentioned merits, it will be possible to measure the size of EVs andexosomes in particle-by-particle basis in reliable dimensional scale atnano-meter resolution. It will be also possible to realize the fast virusdetection system for screening passengers in the airport at the currentpandemic of COVID-19 and in future.

Proposal of Absolute Nanometer Size Measurement in Flow Cytometry Based on Laser Interferometry