A lateral nanoflow assay for the dimensional and optical metrology of nanoplastics*

Polystyrene nanoparticles sorbing and carrying hydrophobic fluorophores are important standards and model nanoplastics with many uses. However, even after decades of development, the variable fluorescence of such nanoparticles is mysterious. To elucidate this variability, we advance a lateral nanoflow assay, integrating complex nanofluidic replicas, super-resolution microscopy, and novel statistical analyses to characterize these standards. An elegant scaling of surface forces hydrodynamically automates advection and dominates diffusion of nanoparticles. Steric interaction with the replica structure separates nanoparticles by size, enabling dimensional and optical metrology of single nanoparticles with high throughput. A comprehensive statistical model approaches the information limit of the system, discriminates size exclusion from surface adsorption, and reduces non-ideal data to return the diameter distribution to within a root-mean-square error of 2 nm. A Bayesian statistical analysis of the diameter and intensity of single nanoparticles reveals a power-law exponent of nearly four and attributes fluorescivity, an intrinsic property, as the dominant source of variability. These surprising results reset expectations for optimizing and applying nanoplastic standards.