A lateral nanoflow assay reveals nanoplastic fluorescence heterogeneity

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.