Copyright © 2015 American Scientific Publishers All rights reserved Printed in the United States of America Nanoscience and Nanotechnology Letters Vol. 7, 920–929, 2015 Formation of Non-Toxic Au Nanoparticles with Bimodal Size Distribution by a Modular Redesign of Ultrasonic Spray Pyrolysis Peter Majeriˇ c 1 ∗ , Darja Jenko 2 , Bojan Budiˇ c 3 , Sergej Tomi´ c 4 , Miodrag ˇ Coli´ c 5 , Bernd Friedrich 6 , and Rebeka Rudolf 1 7 1 University of Maribor, Faculty of Mechanical Engineering, Smetanova ulica 17, 2000 Maribor, Slovenia 2 Institute of Metals and Technology, IMT, Lepi pot 11, 1000 Ljubljana, Slovenia 3 National Institute of Chemistry, Hajdrihova 19, 1001 Ljubljana, Slovenia 4 Institute of Medical Research, Military Medical Academy, Crnotravska 17, 11000 Beograd, Serbia 5 Medical Faculty, University of Nis, Bulevar Dr Zorana D – ind – i´ ca 81, 18000 Niš, Serbia 6 IME Institute of Process Metallurgy and Metal Recycling, RWTH Aachen, Intzestraße 3, 52056 Aachen, Germany 7 Zlatarna Celje d.d., Kersnikova 19, 3000 Celje, Slovenia This article reports about a new synthesis approach by the Ultrasonic Spray Pyrolysis of Au nanoparticles which consists of a separate heating zone for evaporation and subsequent connec- tion of reduction gas by entering directly into the reaction zone. This redesign was made in order to control each step of the USP to find those influential parameters which dictate final gold nanopar- ticles’ morphology and size. For the precursor a starting solution of HAuCl 4 and water was used with various concentrations of Au (0.625 g/l, 1.25 g/l and 2.5 g/l). Other variable parameters were evaporation temperatures (75  C, 80  C and 85  C) and time of synthesis (3 h, 6 h, 9 h), while the temperature in the heating zone was constant (350  C) during performing all the experiments. Char- acterization of synthesized Au nanoparticles was carried out by Transmission Electron Microscopy (TEM). It was found that Au nanoparticles have bimodal size distribution. An investigation of the Au nanoparticles’ electron diffractions enabled us to find the possible growth of Au nanocrystal types which was finally the base for setting up the synthesis mechanisms of Au nanoparticles. The cyto- compatibility investigations of the Au nanoparticles suggested that they were not cytotoxic for L929 cells in vitro, but they can exhibit anti-proliferative properties, depending on their size distribution. Keywords: Ultrasonic Spray Pyrolysis (USP), Gold Nanoparticles, Transmission Electron Microscopy (TEM), Bimodal Particle Size Distribution, Nanoparticle Characterization, Cytocompatibility. 1. INTRODUCTION The potential benefits of Au nanoparticles (AuNPs) used in medicine attract considerable attention in the scientific community. 1–6 The exploitation of surface plasmon reso- nance, ease of surface functionalization and biocompat- ibility that AuNPs inherit can aid the current methods for diagnosis and treatment substantially. 7–11 Pre-clinical proofs of the concept of treatments using nanoparticles have already been established. 12 Currently there is a need for a scaled up production of nanoparticles, able to take up generation of the quantities of nanoparticles required for clinical testing. Technologies for generating nanoparticle ∗ Author to whom correspondence should be addressed. powders and suspensions have been existent for several decades 13 and are continuing to being improved upon, while novel approaches are also being studied, 14 15 mostly because the former method produces large batch-to-batch variations in the AuNPs’ properties. One such method for the synthesis of AuNPs is the Ultrasonic Spray Pyroly- sis (USP). 16 This process is considered to be relatively cost-effective and easily scalable from the laboratory to an industrial level. Several different types of nanoparticles can be produced with this process, as we showed for Au, 16 Ag, 17 TiO 2 , 18 etc. Furthermore, by the modifications to the process, several different structure types can also be pro- duced, such as solid or hollow nanoparticles, core–shell and ball-in-ball structures, etc. 19–23 920 Nanosci. Nanotechnol. Lett. 2015, Vol. 7, No. 11 1941-4900/2015/7/920/010 doi:10.1166/nnl.2015.2046