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Nanoparticles of various compositions and structures and with diameters ranging from a few nanometers to 150 nm are the subjects of intensive investigation for the treatment,prevention,and diagnosis of diseases,and for bio- and chemosensing.Due to the intrinsic heterogeneity in these nanoentities,multiparameter characterization of their physical and chemical properties on a particle by particle basis is highly desirable.Employing hydrodynamic focusing in a microflow channel,we built an apparatus so called high-sensitivity flow cytometer (HSFCM) for the simultaneous detection of both the light scattering and fluorescence signals emitted by single nanoparticles or viruses as they transverse through the laser beam individually.By reducing the probe volume to tens of femtoliters,the apparatus is capable of detecting single gold nanoparticles of 10 nm in diameter and silica nanoparticles with a diameter of 40 nm by light scattering measurement.High-resolution particle size distribution can be acquired in 1~2 minutes by analyzing thousands of nanoparticles individually.We demonstrate the discrimination of individual virions from empty viral capsids and free nucleic acids on the HSFCM.Moreover,we have developed an efficient approach to determine the copy number distribution of antibodies immobilized on the nanoparticles for specific targeting.HSFCM provides a versatile and powerful platform for the multiparameter characterization of individual nanoparticles and viruses.A wide application in nanomedicine and biosensing can be envisioned.