ENMs can be complex structures. For example, an engineered nanoparticle may be composed of a core of one material and a shell of another. This composition can extend and enhance the materials’ applications. Particles may also be coated to increase their stability in suspension or ease of processing. The chemistry of the ENM core is the main driver of the NanoFASE studies of the fate and behaviour of the ENM, as it determines the physico-chemical characteristics of the ENM such as solubility and dissolution rate or the environmental persistence of the ENM. Some transformations that may be undergone in e.g. waste treatment or the environment—specifically those that alter the chemical structure of the particle, such as sulfidation—will change the core physico-chemical properties.
ENMs used in NanoFASE include:
- Titanium dioxide (TiO2)
- Cerium dioxide (CeO2)
- Iron (Fe) Zero valent iron
- Gold (Au)
- Silver (Ag) and Silver sulphide (Ag2S)
- Copper (Cu), Cupric oxide (CuO), Cuprous oxide (Cu2O) and Copper hydroxide (Cu(OH2))
- Zinc Oxide (ZnO)
- Silicon dioxide (SiO2)
Navigate this page using the links to read about the particular utility of these particles for NanoFASE study aims as well as some notes on their synthesis and physico-chemical characteristics.
Titanium Dioxide (TiO2) ENMs
TiO2 exists as three common crystal forms, with engineered nano-TiO2 typically taking the rutile or anatase form or a mixture of the two. The largest use of non-nano TiO2 is as a white pigment because it efficiently scatters visible light. As the size decreases so does this efficiency, making TiO2 nanoparticles transparent. TiO2 is a good absorber of UV radiation so a common application of TiO2 ENM is in sunscreen and cosmetics where particles are often coated with silica or aluminium oxide. TiO2 is also used as a photocatalyst with the anatase form being the most catalytically active.
Within NanoFASE, TiO2 ENMs were used in a case study of a commercial grade of anatase nano-TiO2 applied in a photocatalytic coating for roads. Additionally, chemically-labelled TiO2 ENMs were produced within NanoFASE to allow detection of the ENM in real environmental samples where there are high background levels of titanium, as well as assessment of ENMs’ environmental transformations in waste water treatment plants for example.
TiO2 ENMs were created with a core of holmium and a shell of TiO2 to mimic TiO2 ENM behaviour in the environment whilst the holmium enabled tracking in the environment and determine fate in spite of the high natural level of TiO2.
Promethean provided test TiO2 ENMs to NanoFASE
|Partner Promethean Particles produced a series of TiO2 ENMs for NanoFASE, utilising their patented hydrothermal continuous-flow reactor systems. This solution-based synthesis process is clean and scalable, and as the particles are always in liquid state (rather than in dry powder form) the human occupational exposure and health risks are low. Using this method, TiO2 (anatase) ENMs with a primary particle size of approximately 13 nm were produced. This was the uncapped variant. The role of polymer capping on ENM stability and environmental fate and behaviour was assessed as well, using bespoke PVP-capped TiO2 ENMs. Capping agents (polymers such as PVP, Pluronic F127 and Dispex A4040) were pumped into the system downstream of the reactor, allowing the particles to be coated in situ, as part of the synthesis process. The capping agents were added at a concentration of 50 wt% relative to the mass of the TiO2 ENM product. Read more about Promethean’s activities in NanoFASE Main Outcomes.|
Cerium Dioxide (CeO2) ENMs
Cerium dioxide (CeO2) ENMs are used in the automotive industry in catalytic converters to convert harmful carbon monoxide to less harmful carbon dioxide. Cerium oxide is also used in the automotive industry to catalyse the oxidation of soot (carbon) particles in the combustion chamber (engine) and as collected on the diesel particulate filter. The semi-conductor industry uses CeO2 ENM as a fine polishing agent in the manufacturing of computer chips. Cerium ions can exist in two valence states (Ce3+/Ce4+) and their reactivity depends on the Ce3+/Ce4+ ratio at the ENM surface – this ratio can change as the ENMs interact with their surroundings. CeO2 ENM have been shown to be act as cell-protective agents, reducing oxidative stress through their ability to scavenge reactive oxygen species.
In NanoFASE, uncoated and coated CeO2 ENMs have been assessed for their fate in aquatic environments A series of zirconium (Zr)-doped CeO2 ENMs, previously tested for their toxicity in the FP7 project NanoMILE, were among the materials assessed for their transformations in soil pore water (where they reacted with phosphorus to form sea-urchin shaped CePO4 particles) and during waste water treatment.
Promethean provided test CeO2 ENMs to NanoFASE
Partner Promethean Particles produced CeO2 ENMs, as well as the series of Zr-doped CeO2 ENMs for NanoFASE, utilising their patented hydrothermal continuous-flow reactor. Using this method, CeO2 ENMs and Ce1-xZrxO2 ENM with primary particle sizes less than 10 nm were produced. PVP capped CeO2 ENMs were also produced to allow assessment of the role of polymer capping on ENM stability and environmental fate and behaviour. Capping agents (polymers) were pumped into the system downstream of the reactor, allowing the particles to be coated in situ, as part of the synthesis process. The capping agents were added at a concentration of 50 wt% relative to the mass of the CeO2 ENM product.
Iron (Fe) ENM
Within NanoFASE zero valent iron nanoparticles developed by Czech company NanoIron for clean-up of environmental pollution have been utilised as a demonstration test case for NanoSight Nanoparticle Tracking Analysis (NTA) to characterise and quantify the presence of the EMNs in complex environmental samples such as groundwater, where they can be intentionally added as part of a pollution remediation strategy.
In the presence of water and oxygen, iron ENMs are easily oxidized to iron oxide, a process known as rusting.
Gold (Au) ENMs
Gold ENMs are used for method development in NanoFASE since: (1) they come monodispersed in a wide range of sizes; (2) they are insoluble in essentially all environmental media and compartments, allowing time-resolved studies; and (3) they are detected by a wide range of methodologies with excellent resolution and low limits of detection so can be tracked in environmental samples. Among the methods utilised in NanoFASE to detect Au ENMs are UV-VIS spectroscopy, spICP-MS, DLS, NanoSight Nanoparticle Tracking Analysis, and TEM.
Within NanoFASE, commercially available BBI Au ENMs of sizes 20nm and 80 nm were utilised in the development of the soil column and batch tests, and for assessment of the suitability of spICP-MS for detection and quantification of ENMs in complex matrices.
NanoFASE partner University of Wageningen requested Au or Au@Ag NPs of specific sizes. UoB designed an approach to synthesis of Au and Au@Ag core-shell nanoparticles with a size of 25/50 nm, in which 13 nm Au is synthesised initially as a crystal seed, and then grows a further Au layer and/or an Ag layer to reach the specific target size. These particles have been used in the NanoFASE bioaccumulation studies using earthworms.
Silver (Ag) ENM and Silver sulphide (Ag2S) ENMs
Silver is known for its antimicrobial properties related to the release of Ag+ ions. Alongside their antimicrobial properties, silver nanoparticles’ low mammalian cell toxicity makes them a common addition to consumer and medical products.
Consumer products containing silver ENMs include textiles. NanoFASE collaborated with partner InoTEX on a small-scale production test of an antibacterial textile using Ag nanoparticles, Ag nanowires of approximately 3 µm and 30 µm in length. In the consumer use phase of similar products, washing of Ag ENM impregnated textiles leads to the release of ENM into the waste water management system. Here, sulfidation is the most rapid reaction occurring, leading to formation of Ag2S ENM. To mimic this, direct synthesis of Ag2S ENMs (see Protocols and procedures) was also performed in NanoFASE to allow comparison of the toxicity of Ag and Ag2S ENM to organisms, both individually and as part of a food web (experimental studies of the living environment).
Two approaches were used to produce Ag2S NPs to mimic the major Functional Fate transformation undergone by Ag NPs in waste water treatment plants (WWTP). The first takes the existing Ag NPs and exposes them to high concentrations of sulphate at 37 degrees C (to mimic the WWTP process) for different durations, while the second is a direct synthesis of Ag2S ENMs to have more control over the particle size and size distribution and the chemical composition.
Sulfidation of existing Ag NPs was performed by University of Birmingham, and direct synthesis of Ag2S ENMs was undertaken by partner Applied Nanoparticles.
Partner Amepox collaborated in a case study of their silver nanoinks which are used in printed and flexible electronics.
Copper (Cu), Copper Oxide (Cu2O) and Copper hydroxide (Cu(OH)2) ENMs
Copper and copper oxide ENMs are used as active ingredients in biocides for e.g. organic farming and in anti-fouling coating for water-going vessels.
NanoFASE partners HEMPEL and LEITAT tested development of antifouling paints, designing and preparing five different variants, all of them containing copper-based compounds as a biocide ingredient: 1) Cu2O microparticles from Nordox AS; 2) Cu2O nanoparticles; 3) Cu0 microparticles; 4) Cu0 nanoparticles from Hongwu International Group LTD; 5) Cu0 nanoparticles from Promethean Particles.
Cu(OH)2 nanowires were utilised in the nanopesticides case study and a chemically labelled version, doped with dysprosium, was also produced to facilitate tracking of the dissolution in situ in soil and uptake by organisms.
Zinc Oxide (ZnO) ENM
ZnO ENMs have antimicrobial properties, and as such are widely used in textiles and coatings and personal care products. As a result of their UV-A and UV-B absorption (similar to TiO2 ENMs) ZnO ENMs are widely used in sunscreen.
Partner Promethean Particles produced needle-shaped ZnO ENMs utilising their patented hydrothermal continuous-flow reactor systems. From electron microscopy analysis, the particles were observed as having diameters of 30-50 nm and lengths up to 200 nm. The stability and transformations of the ZnO ENMs in aquatic media were assessed in NanoFASE.
Silicon dioxide (SiO2) ENMs
Within NanoFASE, SiO2 ENMs were utilised to test applicability of the various methods for a wider range of ENMs and to provide further parameterisation of the environmental fate models Si is challenging to measure by ICP-MS due to the presence of 14N2+ and 12C16O+, which form in the plasma and have the same mass to charge ratio (m/z) as the most abundant Si isotope (28Si≈ 92% abundance). As a result, the background at m/z 28 is very high, which inhibits low-level Si determination and can make detection of SiO2 ENMs difficult. NanoFASE partner PerkinElmer had previously demonstrated the ability of the NexION 350 ICP-MS single particle analyzer to measure SiO2 ENMs in Standard mode by spICP-MS, utilising the speed of analysis and the short dwell times which reduce the background on Si, allowing measurement of 100 nm SiO2 ENMs. Within NanoFASE the work has focused on characterizing smaller SiO2 ENMs using Reaction mode to further reduce the background signal.
NanoFASE Main Outcomes.