When materials occur commonly in the environment, such as TiO2, it can be difficult to track the nanoform in order to achieve understanding of fate and exposure. To enable this in NanoFASE experiments, chemically-labelled ENM have been produced and used. The labelled ENM can be traced throughout an experimental procedure and its fate and transformations tracked by detecting the ratio of the core and labelling (or so-called dopant) elements.
Chemical labelling involves the intentional introduction of another element into a metal or metal oxide ENM core in order to modulate its optical, structural, or toxicological properties without changing its functionalities too much. This has been achieved by ‘doping’ the material of interest with an element which is less common and ideally fits within the crystal lattice. Where this approach has not been possible NanoFASE has produced particles which have a core of the rarer, traceable element surrounded by a shell of the material of interest. In both approaches the environmental concentration of the rarer element can be detected, and utilised to quantify the presence of the engineered form as distinct from the common element present as a background concentration.
Within NanoFASE, chemical labelling of titanium oxide (TiO2) ENMs is used to distinguish the NanoFASE engineered TiO2 particles from titanium arising from naturally occurring mineral forms such as rutile (TiO2) and the abundant mineral ilmenite (FeTiO3). Synthesis of Holmium-labelled TiO2 ENMs was performed by partner University of Birmingham as a means to track TiO2 ENMs at low concentrations in the natural environment, such as in river water, or following waste water treatment. In a similar manner, Dysprosium labelling of Cu2O ENMs was used to support the nanopesticides case study.
The elements used as dopants or chemical labels for ENMs in NanoFASE include:
Dysprosium (Dy)A rare earth element, dysprosium is never found in nature as a free element but only in various minerals. In NanoFASE, Dy has been used to chemically label Cu(OH)2 nanowires utilised as nanopesticides. Here, the goal was to investigate the bio-accumulation of copper (Cu)-containing ENMs. The Dy label was used to discriminate Cu cations that had been taken up as part of the ENMs (and as such co-exist with Dy) versus Cu ions that were released and taken up as ions, where there would be no corresponding Dy signal in ICP-MS measurements. Transmission electron microscopy (TEM) and High-resolution (HR) TEM confirmed the nanowire structure of Cu(OH)2.
Zirconium (Zr)In NanoFASE Zr-doped CeO2 ENM, developed in the earlier project FP7 NanoMILE to adjust the band-gap of CeO2 ENM making the particles less redox active, were investigated in terms of their fate in WWTPs and in terms of their physical and chemical transformations in soil pore water.
In NanoFASE Co-doped Fe3O4 ENMs, developed in NanoMILE to enhance the redox activity and hence their effectiveness for water purification, were assessed in terms of their behaviour in the environment.
Holmium (Ho)In NanoFASE, holmium has been used to chemically label TiO2 NMs. Due to the larger atomic radius of holmium compared with titanium it is difficult to introduce into the TiO2 lattice without altering the properties of the TiO2 particles. We thus used an alternative approach whereby a core of the tracer element (Ho) surrounded by a shell of the material of interest (TiO2) was developed. This core-shell approach is preferred for toxicological and environmental fate studies, since the material that comes into contact with the environment or living organisms will be the surface material and should be a close analogue of the undoped material, assuming factors such as ENM density are not significantly altered, and appropriate crystal phase / morphology of the shell can be obtained.
The resulting Ho core and TiO2 shell ENMs (NaHoF4@TiO2) were used in studies assessing:
- the fate and transformation of ENMs in water, sludge (WWTP) and soil, and for detection of ENM against a high background of naturally occurring Ti in river water using spICP-MS.
- use of spICP-MS for characterisation of bi-metallic oxides and whether it was possible to determine the structure of the particles (doped versus core-shell) and quality (homogeneous or heterogeneous distribution of dopant) based on their spICP-MS signals.
Cui X, Fryer B, Zhou D, Lodge R, Khlobystov A, Valsami-Jones E, Lynch I (2019) Core-shell NaHoF4@TiO2 NPs: A labelling method to trace engineered nanomaterials of ubiquitous elements in the environment. ACS Applied Materials & Interfaces 11, 21, 19452-19461. https://doi.org/10.1021/acsami.9b03062
Articles explaining synthesis routes for some of the ENMs used in NanoFASE: