Visualization of label-free titanium dioxide nanoparticle deposition on surfaces with nanoscale roughness

Understanding the transport of engineered nanoparticles (ENPs) and their interaction with environmental surfaces is critical to predicting their fate and environmental implications. Here, we report an innovative technique to visualize and quantify the deposition of titanium dioxide nanoparticles (TiO ₂ NP) on engineered surfaces with nanoscale roughness, i.e., slanted columnar thin films (SCTFs). The attachment of TiO ₂ NP on the surface of SCTFs changed the optical property of SCTFs, which was detected using a generalized ellipsometry (GE)-based instrument, an anisotropy contrast optical microscope (ACOM). An anisotropic effective medium model was applied to quantitatively analyze ACOM images of SCTF surfaces, which provided the mass distribution of TiO ₂ NP. The TiO ₂ NP mass measured by ACOM was in good agreement with the known amount of TiO ₂ NP mass dispersed by controlled piezoelectric plotting. The detection of a few picograms of nanoparticle mass by an individual pixel measuring 7 × 7-micrometer squared was demonstrated. Further, a glass microfluidic channel with SCTF embedded was developed. The areal mass density of attached TiO ₂ NP on SCTF surfaces as they flow through the channel under various flow rates was quantitatively measured in situ. At the end of the experiment, the distribution of the attached TiO ₂ NP on the SCTF surface was visualized. The averaged mass density estimated by integrating the distribution map was in close agreement with the estimation from dynamic measurements and between repeating experiments. The capability of this novel technique to sense, quantify and visualize the mass distribution of TiO ₂ NP provides a valuable approach to investigate the behavior of nanoparticles at the interface of flow and rough surfaces.