Abstract View

Objectives: Carbon-14 radiolabeling enables quantitative in vivo tracing of plastic nanoparticles, provided the label remains associated with the particle during biological exposure. In this study, ¹⁴C was incorporated into the polymer backbone during synthesis to generate stable radiotracers of irregular polystyrene ([¹⁴C]iPS), spherical polystyrene ([¹⁴C]PS), and poly(methyl methacrylate) ([¹⁴C]PMMA), avoiding detachable dye or surface labels that may leach or alter particle behavior. PS and PMMA were selected as common consumer polymers, while spherical and irregular PS enabled morphology comparison. These radiotracers were applied in a repeated intranasal dosing model, selected to mimic an inhalation-relevant route of airborne nanoplastic exposure, to compare material- and morphology-dependent tissue retention.

Methods: [¹⁴C]iPS, [¹⁴C]PS, and [¹⁴C]PMMA nanoparticles were synthesized from ¹⁴C-labeled monomers by emulsion polymerization and characterized by dynamic light scattering. Particle morphology was confirmed by transmission electron microscopy using matched nonradioactive analogs. Hydrodynamic diameters were 178 ± 44 nm for [¹⁴C]iPS, 134 ± 35 nm for [¹⁴C]PS, and 165 ± 41 nm for [¹⁴C]PMMA, with PDIs of 0.062, 0.069, and 0.061. Female C57BL/6J mice (7-10 weeks) received 0.1 mg of nanoplastic in 20 µL via intranasal administration every other day for 30 days (15 doses total), with n = 5 per group. Specific activities were 136, 100, and 300 nCi/mg for [¹⁴C]iPS, [¹⁴C]PS, and [¹⁴C]PMMA, respectively. Tissues were collected 24 h after the final dose, digested, and analyzed by liquid scintillation counting; biodistribution was reported as %ID/g. Statistics used corrected multiple unpaired Welch tests.

Results: All materials showed dominant pulmonary retention. Lung accumulation was highest for [¹⁴C]PMMA at 59.95 %ID/g, compared with 13.74 %ID/g for [¹⁴C]iPS and 14.06 %ID/g for [¹⁴C]PS. [¹⁴C]PMMA showed significantly higher lung retention than [¹⁴C]PS, adjusted p = 0.03985. Activity was also detected in gastrointestinal contents and feces, consistent with mucociliary clearance and swallowing after intranasal administration. Values in stomach contents were 0.195, 0.348, and 0.287 %ID/g; small-intestinal contents were 0.129, 0.222, and 0.104 %ID/g; large-intestinal contents were 0.220, 0.368, and 0.321 %ID/g; and feces were 0.229, 0.868, and 1.084 %ID/g for [¹⁴C]iPS, [¹⁴C]PS, and [¹⁴C]PMMA, respectively. Low but measurable activity was observed in selected peripheral tissues, including liver, spleen, ovaries, pancreas, and fat. Fat accumulation differed significantly between [¹⁴C]iPS and [¹⁴C]PS, adjusted p =0.00809, and between [¹⁴C]PS and [¹⁴C]PMMA, adjusted p = 0.000632.

Conclusions: Repeated intranasal exposure to [¹⁴C]-labeled nanoplastics resulted in persistent pulmonary retention, gastrointestinal-associated activity, and low-level peripheral distribution. The significantly higher lung retention of [¹⁴C]PMMA compared with [¹⁴C]PS demonstrates that polymer composition can strongly influence tissue retention after inhalation-relevant exposure, while differences between irregular and spherical [¹⁴C]PS suggest morphology-dependent effects in selected tissues. These findings support backbone-incorporated carbon-14 labeling as a stable, sensitive radiochemical strategy for quantitative in vivo tracking of nanoplastics after repeated exposure.

Acknowledgments: This work was supported by the National Institutes of Health (R00ES034053).

References: [1] Al-Sid-Cheikh M, et al. Communications Materials. 2020;1:97.