Core–Shell NaHoF4@TiO2 NPs: A Labeling Method to Trace Engineered Nanomaterials of Ubiquitous Elements in the Environment

Understanding the fate and behavior of nanoparticles (NPs) in the natural environment is important to assess their potential risk. Single particle inductively coupled plasma mass spectrometry (spICP-MS) allows for the detection of NPs at extremely low concentrations, but the high natural background of the constituents of many of the most widely utilized nanoscale materials makes accurate quantification of engineered particles challenging. Chemical doping, with a less naturally abundant element, is one approach to address this; however, certain materials with high natural abundance, such as TiO 2 NPs, are notoriously difficult to label and differentiate from natural NPs. Using the low abundance rare earth element Ho as a marker, Ho-bearing core -TiO 2 shell (NaHoF 4 @TiO 2 ) NPs were designed to enable the quantification of engineered TiO 2 NPs in real environmental samples. The NaHoF 4 @TiO 2 NPs were synthesized on a large scale (gram), at relatively low temperatures, using a sacrificial Al(OH) 3 template that confines the hydrolysis of TiF 4 within the space surrounding the NaHoF 4 NPs. The resulting NPs consist of a 60 nm NaHoF 4 core and a 5 nm anatase TiO 2 shell, as determined by TEM, STEM-EDX mapping, and spICP-MS. The NPs exhibit excellent detectability by spICP-MS at extremely low concentrations (down to 1 × 10 -3 ng/L) even in complex natural environments with high Ti background.