Synthesis of Reduced Graphene Oxide-Silver Nanocomposites and Assessing Their Toxicity on the Green Microalga Chlorella vulgaris Fatemeh Nazari 1 & Ali Movafeghi 1 & Saeed Jafarirad 2 & Morteza Kosari-Nasab 3 & Baharak Divband 4 # Springer Science+Business Media, LLC, part of Springer Nature 2018 Abstract The growing demands for nanotechnology in the recent years have resulted in environmental release of nanomaterials. In the current study, reduced graphene oxide-silver nanocomposites (Ag-rGO) were synthesized by an easy method and their charac- teristics were determined using X-ray diffraction (XRD), ultraviolet-visible spectroscopy (UV-Vis), energy dispersive X-ray spectroscopy (EDX), and scanning electron microscopy (SEM) techniques. Subsequently, toxicity of Ag-rGO was examined on the marine microalga Chlorella vulgaris. After treatment of algal cells with different concentrations of Ag-rGO for 24 h, growth parameters have been significantly decreased. In addition, a considerable reduction in viability of the treated cells was designated. Further considerable effects of Ag-rGO treatments have been revealed by increments in the activities of a number of antioxidant enzymes and reductions in the photosynthetic pigment contents. Our results showed that the main toxic effects of Ag-rGO are associated with the presence of Ag nanoparticles in the structure of these nanocomposites. Keywords Ag-rGO nanocomposites . Chlorella vulgaris . Green microalga . Toxicity . Oxidative stress 1 Introduction Graphene is a two-dimensional and one-atom-thick sheet that is formed of monolayers of carbon atoms organized in a honeycombed network containing six-membered rings [1]. Because of its exceptional electronic and mechanical proper- ties, excessive mobility of charge carriers, specific magnetism, and noteworthy thermal conductivity, graphene has attracted a lot of attention in various scientific and industrial areas [2]. Graphene oxide (GO) is an oxidized derivative of graphene, which has oxygen-containing groups including epoxy, hy- droxyl, and carboxyl. As one of the most major graphene- derived materials, GO exhibits outstanding electrochemical properties and has promising applications in numerous fields such as biotechnology and electronics [3]. GO can act as the platform for growing and immobilization of metal nano- particles. Accordingly, GO and its composites have a wide range of potential utilizations on transistors, polymer rein- forcement transparent conductors, biomaterials, and bioen- gineering fields [4]. However, harmful effects of GO on organisms have been indicated by both in vitro and in vivo analyses. Investigations of cell lines and animal species have confirmed the distribution and translocation of GO in living organisms [5]. High catalytic activity of silver (Ag) for quite a lot of chem- ical reactions, including dissociation of ozone, reduction of NO x , ethylene and propylene epoxidation, and reduction of nitrophenols, has attracted increasing attention. High surface energy of nanoparticles leads to their thermodynamic instabil- ity and aggregation, which resulted in a gradual elimination of the catalytic efficiency of the nanoparticles. GO is one of the suitable support substrates that is used for the development and stabilization of Ag nanoparticles in order to generate high-performance Ag nanoparticle-based catalysts such as re- duced graphene oxide-silver nanocomposites (Ag-rGO) [6]. Algae as primary producers play a main role in the aquatic ecosystems by providing oxygen and organics for other organisms [7]. Microalgae are widespread in lakes * Ali Movafeghi movafeghi@tabrizu.ac.ir 1 Department of Plant Biology, Faculty of Natural Sciences, University of Tabriz, Tabriz, Iran 2 Research Institute for Fundamental Sciences (RIFS), University of Tabriz, Tabriz, Iran 3 Drug Applied Research Center, Tabriz University of Medical Sciences, Tabriz, Iran 4 Dental and Periodontal Research Center, Tabriz University of Medical Sciences, Tabriz, Iran BioNanoScience https://doi.org/10.1007/s12668-018-0561-0