  Plastic nanoparticles present technological opportunities and environmental
concerns, but measurement challenges impede product development and hazard
assessment. To meet these challenges, we advance a lateral nanoflow assay that
integrates complex nanofluidic replicas, optical localization microscopy, and
novel statistical analyses. We apply our sample-in-answer-out system to measure
polystyrene nanoparticles that sorb and carry hydrophobic fluorophores. An
elegant scaling of surface forces automates advection and dominates diffusion
to drive the analytical separation of colloidal nanoparticles by their steric
diameters. Reference nanoparticles, with a mean of 99 nm and a standard
deviation of 8.4 nm, test the unknown limits of silicone replicas to function
as separation matrices. New calibrations correct aberrations from microscope
and device, improving the accuracy of reducing single micrographs to joint
histograms of steric diameter and fluorescence intensity. A dimensional model
approaches the information limit of the system to discriminate size exclusion
from surface adsorption, yielding errors of the mean ranging from 0.2 nm to 2.3
nm and errors of the standard deviation ranging from 2.2 nm to 4.2 nm. A
hierarchical model accounts for metrological, optical, and dimensional
variability to reveal a fundamental structure-property relationship. Intensity
scales with diameter to the power of 3.6 +/- 0.5 at 95 % coverage, confounding
basic concepts of surface adsorption or volume absorption. Distributions of
fluorescivity - the product of the number density, absorption cross section,
and quantum yield of an ensemble of fluorophores - are ultrabroad and
asymmetric, limiting any inference from fluorescence intensity. This surprising
characterization of common nanoplastics resets expectations for optimizing
products, applying standards, and understanding byproducts.
