Single-cell analysis, or cytometry, is a ubiquitous tool in the biomedical sciences. Whereas most cytometers use fluorescent probes to ascertain the presence or absence of targeted molecules, biophysical parameters such as the cell density, refractive index, and water content are difficult to obtain. We present quantitative phase imaging as an effective technique to quantify the absolute intracellular water content in single cells at video rate, using an assumption of a spherical cellular geometry. Our study demonstrates the utility of QPI for rapid intracellular water quantification and shows a path forward for identifying biophysical mechanisms using label-free imaging. We further demonstrate the use of two complementary techniques - quantitative phase imaging and Brillouin spectroscopy—as a label-free image cytometry platform capable of measuring more than a dozen biophysical properties of individual cells simultaneously. Our system will unlock new avenues of research in biophysics, cell biology, and medicine.
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