Combination of thresholding and fitting methods for measuring nanoparticle sizes and size distributions in (S)TEM

The practical need for a simple and reliable tool for routine size analysis of nanoparticles with diameters down to a few nm embedded in a polymer matrix motivated the development of a new approach. The idea underlying the method proposed in this work is to combine intensity thresholding and contrast fitting procedures in the same software for particle recognition and measurements of sizes and size distributions of nanoparticles in transmission and scanning transmission electron microscopy images. Particle recognition in images is performed in an interactive process of manual setting the numerical threshold level after image preprocessing. We show that fitting the calculated gray level distribution to the real images is able to provide a maximum accuracy in measurements of the particle diameters in contrast to thresholding approaches. The fitting procedure is applied in the vicinity of nanoparticle images with the mass-thickness, diffraction, and chemical contrast. The grayscale function associated to the nanoparticle thickness is described using polynomial gt=g0+g1t+g2t2+g3t3… with degree ⩾ 2 and undetermined coefficients. The program for particle detection and size measurement-Analyzer of Nanoparticles (AnNa)-has been written and is described here. It was successfully tested on systems containing Ag nanoparticles grown and stabilized in aqueous solutions of different polymers for biomedical use and is available from the authors.