Adhesion forces between AFM tips and TiO₂ nanoparticles to investigate the formation of natural coatings

When nanoparticles enter aquatic environments, they acquire natural surface coatings (eco-coronas) that govern their fate, transport, and toxicity. This study employs atomic force microscopy (AFM) adhesion force mapping to characterize these coatings following in situ formation under natural conditions. Titanium dioxide nanoparticles (n-TiO$_2$) and flat single-crystal TiO$_2$ substrates (absence of curvature and roughness effects) were exposed to three chemically distinct surface waters, revealing pronounced variability in adhesion force strength, range (min–max differences), and spatial patchiness, nanoscale heterogeneity metrics inaccessible to bulk techniques.
Moran’s I analysis of crystal-TiO$_2$ confirmed positive spatial autocorrelation across samples, indicating patchy eco-coronas. Terrestrially dominated water (high DOC, humic-rich) produced the widest adhesion ranges, reflecting polydisperse humic adsorption. Microbial-enriched waters exhibited high adhesion forces comparable to terrestrial waters but lower variability ranges, while calcareous water (high Ca$^{2+}$, ionic strength) yielded the lowest forces with less ordered spatial patterns. ... mehrMoreover, sessile drop contact angle measurements on crystals further revealed macroscopic hydrophilicity changes.
In addition, complementary analyses of particle size, morphology, surface charge, and functional groups of the exposed nanoparticles to the surface waters provided multi-technique validation of surface modifications. These results establish AFM adhesion mapping as a powerful approach for resolving eco-corona heterogeneity under environmentally relevant conditions.