Iron oxide nanoparticles are having been extensively investigated for several biomedical applications such as hyperthermia and magnetic resonance imaging. However, one of the biggest problems of these nanoparticles is their aggregation.

Taking this into account, in this study the influence of three different surfactants (oleic acid, sodium citrate and Triton X-100) each one with various concentrations in the colloidal solutions stability was analyzed by using a rapid and facile method, the variation in the optical absorbance along time.

The synthesized nanoparticles through chemical precipitation showed an average size of 9 nm and a narrow size distribution. X-ray diffraction pattern and Fourier Transform Infrared analysis confirmed the presence of pure magnetite. SQUID measurements showed superparamagnetic properties with a blocking temperature around 155 K. In addition it was observed that neither sodium citrate nor Triton X-100 influences the magnetic properties of the nanoparticles. On the other hand, oleic acid in a concentration of 64 mM decreases the saturation magnetization from 67 to 45 emu/g. Oleic acid exhibits a good performance as stabilizer of the iron oxide nanoparticles in an aqueous solution for 24 h, for concentrations that lead to the formation of the double layer.

The eutrophication of surface waters caused by cyanobacteria is a worldwide problem, leading to expensive
water treatment costs [1]. In addition, the production of microcystins by these microalgae may cause many
health problems to humans and animals (e.g. liver cancer) and even death [2]. Therefore, a variety of
methods have been developed to control cyanobacteria blooms, including physical and chemical treatments.
However, they have negative impacts on other species of (micro) algae and on other aquatic biota. As a
consequence, ultrasonic algae treatment has been proposed as a clean approach to controlling the blooms of
some algae species and microcystins degradation [3]. Still, the specific effects of ultra-sonication on
cyanobacteria are not well known. The present work aimed to study the effects of ultra-sonication on the
cyanobacteria structure under different ultrasound conditions (changing frequency and power) by using
conventional histology and electron microscopy methods.
Microcystis spp. were harvested in a lake from Azores (Portugal) and stored in the cool and dark until
transported to the laboratory. Cyanobacteria were cultured in liquid BG-11 axenic medium at 22ºC in an
incubator chamber, under continuous illumination (fluorescent cold white light).
Samples were collected and suspensions of cells (1ml each) were subjected to ultrasonic irradiation using
diverse ultrasonic equipment (UP100H; UP200S, sonoreactor UTR 200 and ultrasonic bath) and testing
different exposure times. All the experimental algal suspensions were exposed for 5 min to ultrasonication
(on ice for periods of 10s to avoid heating). After ultrasonication cyanobacteria growth was assessed for a
period of 14 days and structural changes in cells were evaluated by light (LM) and scanning electron
microscopy (SEM) examination. The results show growth inhibition of the cyanobacteria according to
intensity and power used in each ultrasonic device. The use of the most powerful devices (sonoreactor and
UP200S) resulted in a massive disrupting of cell walls with consequent cell death (Fig. 1e,f). Similar results
were obtained by Ahan et al. [1] and Nakano et al. [4] and showing cell wall disruption. However, even
after exposure to the most powerful instrumentation it was possible to detect some viable cells and after 14
days colonies were already visible. The results from light and electron microscopy showed noticeable
changes at the structural level such as disruption of cell gas vacuoles (arrowhead), colony disaggregation and
damage of cell walls of cells (Fig. 1c-f).
As a consequence, the use of ultrasounds to improve water quality from eutrophic waters must be considered
with careful in terms of efficiency and other complementary methods should be considered to assure good
water quality criteria. In addition, the effects of ultrasonication in other aquatic organisms require further
studies before using this technology to control algae blooms.