Quantitative high-throughput optical sizing of individual colloidal nanoparticles by wide-field imaging extinction microscopy

We present a wide-field imaging technique recently developed by us to measure quantitatively the optical extinction cross section σ_ext of individual nanoparticles. The technique is simple, high speed, and enables the simultaneous acquisition of hundreds of nanoparticles in the wide-field image for statistical analysis, with a sensitivity corresponding to the detection of a single gold nanoparticle down to 2nm diameter. Notably, the method is applicable to any nanoparticle (dielectric, semiconducting, metallic), and can be easily and cost-effectively implemented on a conventional wide-field microscope. Of specific significance for accurate quantification, we show that σ_ext depends on the numerical aperture of the microscope illumination due to the oblique incidence, even for spherical particles in an isotropic environment. This "long shadow" effect needs to be taken into account when comparing σ_ext to theoretical values calculated under plane wave illumination at normal incidence. Owing to the accurate experimental quantification of σ_ext, one can then use it to determine the nanoparticle size, as demonstrated here on gold nanoparticles of 30nm nominal diameter. This technique thus has the potential to become a simple and cost-effective new tool for accurate size characterization of single small nanoparticles, complementing time consuming and expensive methods such as electron microscopy.