In this study, the impact of alpha-iron oxide (α-Fe 2 O 3 , 20-40 nm) and gamma iron oxide (γ-Fe 2 O 3 , 20-40 nm) nanoparticles (NPs) on phytoplankton species Selenastrum capricornutum and Nannochloropsis oculata was investigated Characterizations of the NPs were systematically carried out by TEM, dynamic light scattering, zeta potential, X-ray diffraction, SEM, and Fourier transformation infrared spectroscopy. Acute toxicity was tested between 0.2 and 50 mg/L for each NP for a period of 72 hours exposure. γ-Fe 2 O 3 NP inhibited development of N oculata at the rate of 54% in 0.2 mg/L group with a high mortality rate of up to 82%. α-Fe 2 O 3 NPs were less toxic that induced 97% mortality on N oculata at 10 mg/L suspensions. In contrast, α-Fe 2 O 3 NP inhibited growth of S capricornutum strongly (73%) in 0.2 mg/L group. γ-Fe 2 O 3 NPs showed similar growth inhibition (72%) on S capricornutum in 10 mg/L suspensions.Despite the differential effects, the results indicated acute toxicity of α-Fe 2 O 3 and γ-Fe 2 O 3 NPs on N oculata and S capricornutum. K E Y W O R D S alfa-iron oxide, gamma-iron oxide, nanoparticle, phytoplankton, toxicity 1 | INTRODUCTION Iron (Fe) and iron oxide (Fe n O m ) nanoparticles (NPs) exhibit different properties, including low fusion and sintering temperature, superplastic behavior, high ductility and toughness, high thermal expansion coefficient, different thermo-physical properties, and high selfdiffusion coefficient. Iron is the most abundant transition metal in Earth's crust 1 in the form of Fe 2 O 3 or rust; there are around 16 identified Fe 2 O 3 in the nature. Due to its abundance, Fe 2 O 3 is commonly used as catalyzer, additive in durable pigments (coatings, paints, and colored concrete) and includes in hemoglobin molecules 2 besides its important roles in many biological and geological processes. The most common forms of iron oxides in the nature are magnetite (Fe 3 O 4 ), maghemite (γ-Fe 2 O 3 ), and hematite (α-Fe 2 O 3 ). These oxides are important materials in technological applications. 3 Small Fe 2 O 3 particles have been used in in-vitro examinations over 40 years. In order to synthesize magnetic NPs with appropriate surface chemistry, various physical, chemical, and biological methods have been applied. 4 Ferromagnetic NPs (smaller than 20 nm) exhibits unique magnetic properties (eg, superparamagnetism). Ferromagnetic materials cover elemental metals, alloys, oxides, and other chemical compounds magnetized by an exterior magnetic field. 5 Based on their unique mesoscopic, physical, chemical, thermal and mechanical properties, such as low toxicity, high surface area-to-volume ratio, superparamagnetic α-Fe 3 O 4 , α-Fe 2 O 3 , γ-Fe 3 O 4 , and γ-Fe 2 O 3 NPs have received much attention. They have been used in cellular therapy, cell labelling and targeting and in biomedical applications, including tissue regeneration, drug delivery and release, magnetic resonance imaging (MRI), hyperthermia, and magnetofection. 6,7
