Assessing the localization, stability, and recovery of inorganic nanoparticles in tree bark using advanced degradation techniques

This study presents methodological advancements for characterizing atmospheric nanoparticles (NPs) accumulated in tree bark, a passive bioindicator of urban nanoparticle pollution. Accurate assessment requires efficient degradation methods that remove organic matter while preserving inorganic NP properties. Preliminary imaging (SEM, X-ray microtomography) and chemical mapping revealed heterogeneous NP distributions, with preferential accumulation at the bark surface and partial penetration into internal porous structures. Two degradation strategies - tetramethylammonium hydroxide (TMAH) digestion and cold O₂ plasma treatment - were evaluated using bark doped with enginereed NPs (Au, Fe₂O₃, MnO₂, TiO₂, Al₂O₃, CuO) at environmentally relevant concentrations. TMAH removed 27 % of bark dry weight, while O₂ plasma achieved 89 %, confirmed by FTIR analysis showing disappearance of lignin and cellulose peaks. spICP-MS quantification demonstrated that O₂ plasma preserved TiO₂ and Al₂O₃ (∼100 % recovery), partially degraded Fe₂O₃ (-43 % mass loss), and fully degraded CuO and MnO₂, likely due to ionization energy thresholds and oxidation state transitions. TMAH induced partial Fe₂O₃ dissolution and CuO aggregation via complexation and electrostatic interactions. These results highlight the critical influence of degradation protocols on NP stability and underscore the need for optimized methods that efficiently remove organic matter while maintaining NP integrity. This work provides a robust analytical framework for environmentally relevant studies of nanoparticle behavior, persistence, and potential hazard in complex biological matrices.