Nanotechnology is one of the fastest-growing technologies in the world. Silver
nanomaterials (Ag NMs) and multi walled carbon nanotubes (MWCNTs) are the most
commonly used NMs in consumer products for their antimicrobial properties and for
their unique mechanical and electrical properties. Their use can lead to contamination
of the aquatic environment. In this research study, the freshwater microalga
Raphidocelis subcapitata was used to identify the impacts of NMs on the aquatic
environment. The aim of this study was to carry out an evaluation of the toxicity of
Ag NMs and MWCNTs via acute exposure to R. subcapitata. Moreover, two types of
sonication – bath and probe – were used for comparison, to evaluate how suspension
methods may affect NMs. DLS and TEM were used for NM characterisation. The
probe-sonication dispersion method was a more effective process to prepare Ag NM
suspensions since they were dispersed better, compared to bath sonication. However,
the probe sonication was not an appropriate way for MWCNTs preparation.
The OECD 201 test guideline was used to evaluate the toxicity of representative NM
at 24, 48 and 72 h. Effects were assessed by using the following four different
methods: chlorophyll extraction; optical density; protein content, and photosynthetic
activity. The impact of the sonication method (bath or probe) used to prepare NM
dispersions on the toxicity to algae was also evaluated. Overall outputs, based on
chlorophyll extraction, demonstrated a high sensitivity of R. subcapitata exposed to
Ag NMs and MWCNTs, which led to an increase in growth inhibition with increasing
concentration of the tested materials. The chlorophyll extraction method was the best
method for evaluating the algal biomass with a clear concentration response and no
interference of the NMs with the method.
Evidence for internalisation of Ag NMs into the R. subcapitata cells was shown for
Ag NM dispersed using the probe-sonication methods, lower levels of Ag NM uptake
were observed for bath sonicated Ag NMs. Finally, the role of oxidative stress in NM
toxicity to algae was investigated. This highlighted an enhanced anti-oxidant defence
system of R. subcapitata when subjected to NM suspensions prepared by probe
sonication, as compared to bath sonication. In summary, this research has shown that
Ag NMs are more toxic to R. subcapitata than MWCNTs and that the sonication
method used to suspend the NMs affects toxicity, as Ag NMs show higher toxicity
when dispersed by bath sonication, whereas MWCNTs are more toxic when dispersed via probe sonication. It is of the utmost importance to consider carefully NM
dispersion approaches, particularly when devising standard methods. It is also very
important to understand how dispersant NMs may be affected by different dispersion
methods
