  We report a method to measure the size of single dielectric nanoparticles
with high accuracy and precision using quantitative differential interference
contrast (DIC) microscopy. Dielectric nanoparticles are detected optically by
the conversion of the optical phase change into an intensity change using DIC.
Phase images of individual nanoparticles were retrieved from DIC by Wiener
filtering, and a quantitative methodology to extract nanoparticle sizes was
developed. Using polystyrene beads of 100 nm radius as size standard, we show
that the method determines this radius within a few nm accuracy. The smallest
detectable polystyrene bead is limited by background and shot-noise, which
depend on acquisition and analysis parameters, including the objective
numerical aperture, the DIC phase offset, and the refractive index contrast
between particles and their surrounding. A sensitivity limit potentially
reaching down to 1.8 nm radius was inferred. As application example, individual
nanodiamonds with nominal sizes below 50 nm were measured, and were found to
have a nearly exponential size distribution with 28 nm mean value. Considering
the importance of dielectric nanoparticles in many fields, from naturally
occurring virions to polluting nanoplastics, the proposed method could offer a
powerful quantitative tool for nanoparticle analysis, combining accuracy,
sensitivity and high-throughput with widely available and easy-to-use DIC
microscopy
