Green synthesis of Ce 3+ rich CeO 2 nanoparticles and its antimicrobial studies K. Mohan Kumar a , M. Mahendhiran b , M. Casales Diaz a , N. Hernandez-Como c , A. Hernandez-Eligio d , Gilberto Torres-Torres e , S. Godavarthi f,⇑ , L. Martinez Gomez a a Instituto de Ciencias Fisicas, Universidad Nacional Autonoma de Mexico, C.P 62210 Cuernavaca, Morelos, Mexico b Centro de Ciencias Genomicas, Universidad Nacional Autonoma de Mexico, Cuernavaca, Morelos C.P 62210, Mexico c Centro de Nanociencias y Micro y Nanotecnologías, Instituto Politécnico Nacional, Mexico d CONACyT – Instituto de Biotecnologia, Universidad Nacional Autonoma de Mexico, Cuernavaca, Morelos C.P 62210, Mexico e Universidad Juárez Autónoma de Tabasco, Centro de Investigación de Ciencia y Tecnología Aplicada de Tabasco (CICTAT), C.P. 86690 Cunduacán, Tabasco, Mexico f CONACyT – Universidad Juárez Autónoma de Tabasco, Centro de Investigación de Ciencia y Tecnología Aplicada de Tabasco (CICTAT), C.P. 86690 Cunduacán, Tabasco, Mexico article info Article history: Received 7 September 2017 Received in revised form 14 November 2017 Accepted 21 November 2017 Available online 22 November 2017 Keywords: Cerium oxide Tannic acid Oxygen vacancies Antibacterial Toxicity ROS abstract Cerium oxide nanoparticles (CeO 2 -np) with Ce 3+ rich surfaces were synthesized by ecofriendly route using tannic acid. XRD and SAED patterns confirm the cubic structure of CeO 2 -np. A semi-quantitative XPS analysis revealed the presence of 34 % Ce 3+ ions. Tannic acid not only mediates the CeO 2 -np synthe- sis, but also helps in reduction of surface states from Ce 4+ to Ce 3+ . Oxygen vacancies were evidenced from XPS and photoluminescence studies. The synthesized nanoparticles showed good antimicrobial activity towards both gram positive (Bacillus subtilis) and gram negative (Escherichia coli) bacteria. The toxicity of CeO 2 -np towards tested bacteria is due to production of Reactive oxygen species (ROS). The presence of Ce 3+ ions and rich surface oxygen vacancies may lead to excellent production of ROS and simultane- ously causing cell wall damage. Present findings show that tannic acid assisted CeO 2 -np with obtained properties acts as good antibacterial agent. Ó 2017 Elsevier B.V. All rights reserved. 1. Introduction Among rare earths, cerium the first element of lanthanide group with 4f electrons has tremendous attention in all research fields including physics, chemistry and biology [1,2]. Upon its combina- tion with oxygen during the formulation of nanomaterial, cerium oxide takes on crystalline fluorite structure that comes into sight as an interesting material with wide range of applications. The general synthesis strategies in preparing CeO 2 -np includes chemical and physical methods which are complex, time consum- ing and uses expensive and hazardous chemicals. Hence, green chemistry approaches for the production of CeO 2 -np was adopted successfully using natural resources such as Gloriosa superba L. leaf extract [3] and honey [4]. There are difficulties to bring out the mechanism of formation of CeO 2 -np when using crude extracts and other natural resources. Only few reports are available on the antimicrobial activity of CeO 2 -np. Kannan et al. [5] reported the antibacterial effect of CeO 2 -np synthesized using Acalypha indica leaf extract against both gram positive and negative bacteria. Kuang et al. [6] provided the comparative bactericidal effect between bulk ceria and CeO 2 -np. In addition to the above, reports are available with polymer coated CeO 2 -np on bactericidal activity [7]. A contradictory exists for CeO 2 -np toxicity due to discrepancy of data on its bio-activity and has no clear consent about property of nanoparticle that is responsible for biological effects. In this present work, CeO 2 -np was synthesized using the sec- ondary plant metabolite ‘Tannic acid’ and investigated the struc- tural and optical properties. To best of our knowledge, there are no previous reports to synthesize Ce 3+ rich CeO 2 -np using tannic acid, and attempted to explain how high levels of Ce 3+ and oxygen vacancies helped in achieving good antibacterial activity. 2. Materials and methods 2.1. Synthesis of CeO 2 -np using tannic acid Tannic (0.5 g) acid was dissolved in 25 mL of deionized water using ultrasonicator. The pH of the solution was brought down slightly towards basic (pH = 7.8 ± 0.2) in order to initiate the https://doi.org/10.1016/j.matlet.2017.11.097 0167-577X/Ó 2017 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail addresses: sgodavarthi@conacyt.mx, srinivas.godavarthi@ujat.mx (S. Godavarthi). Materials Letters 214 (2018) 15–19 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/mlblue