Aggregation Kinetics of TiO2 Nanoparticles in Human and Artificial Sweat Solutions: Effects of Particle Properties and Sweat Constituents

Dermal penetration potentials of titanium dioxide nanoparticles (TiO2 NPs) may be affected by aggregation upon contact with sweat. This study investigated the aggregation kinetics of three TiO2 NPs in thirty human sweat samples and four artificial sweat standards. Effects of particle concentration, sweat type, and inorganic (sodium chloride, disodium hydrogen phosphate, and sodium dihydrogen phosphate) and organic (l-histidine, lactic acid, and urea) constituents were examined. Three TiO2 NPs remained colloidally stable in >20/30 human sweat samples and showed significant negative correlations (P < 0.01) between aggregation rates and |zeta potentials|. They aggregated rapidly over 20 min to >750 nm in three artificial sweat standards, while remained more stable in the International-Standard-Organization-pH-5.5 standard. Aggregation behaviors of three TiO2 NPs mostly followed the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, allowing for determining their critical coagulation concentrations in inorganic constituents (15–491 mM) and Hamaker constants (3.3–7.9 × 10–21 J). Higher concentrations of particles, inorganic constituents, and l-histidine destabilized three TiO2 NPs, whereas urea inhibited aggregation. Three TiO2 NPs adsorbed organic sweat constituents via complexation with amino or carboxyl groups, with isotherms following the Langmuir model. Correlation analyses further suggested that the adsorbed organic constituents may stabilize three TiO2 NPs against aggregation in sweat by steric hindrance.