Citation: Velgosova, O.; Maˇ cák, L.; ˇ Cižmárová, E.; Mára, V. Influence of Reagents on the Synthesis Process and Shape of Silver Nanoparticles. Materials 2022, 15, 6829. https:// doi.org/10.3390/ma15196829 Academic Editor: Fabien Delpech Received: 9 September 2022 Accepted: 28 September 2022 Published: 1 October 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article Influence of Reagents on the Synthesis Process and Shape of Silver Nanoparticles Oksana Velgosova 1, * ,Lívia Maˇ cák 1 , Elena ˇ Cižmárová 2 and VladimírMára 2 1 Institute of Materials and Quality Engineering, Faculty of Materials Metallurgy and Recycling, Technical University of Košice, Letná 9/A, 042 00 Košice, Slovakia 2 Department of Materials Engineering, Faculty of Mechanical Engineering, Czech Technical University in Prague, Karlovo nám. 13, 121 32 Prague 2, Czech Republic * Correspondence: oksana.velgosova@tuke.sk; Tel.: +421-55-602-2533; Fax: +421-55-602-2770 Abstract: The aim of this study was to prepare the silver nanoparticles (AgNPs) via chemical reduction and analyze the impact of used reduction agents: sodium borohydride (NaBH 4 ), trisodium citrate (TSC), polyvinylpyrrolidone (PVP), and hydrogen peroxide (H 2 O 2 ) on the reduction rate of Ag + ions to Ag 0 , and on nanoparticles shape. It was proven that combinations of reduction agents dramatically influence the synthesis rate of AgNPs and the color of solutions, which depends on the shape and size of nanoparticles. NaBH 4 , TSC, and PVP showed good reduction power. In particular, TSC proved to be a key factor influencing the shape of AgNPs. The shape of nanoparticles influences the color of colloidal solutions. Yellow solutions, where UV-vis absorbance maxima (ABS max ) are in the wavelength interval 380–420 nm, contain spherical particles with a mean size of 25 nm, whereas the blue shift of ABS max to wavelengths higher than 750 nm indicate the presence of triangular nanoparticles (size interval 18–150 nm). A mixture of spherical, triangular, irregular, and hexagonal nanoparticles give different color, e.g., green. The formation and stability of AgNPs was tracked by UV-vis spectroscopy, size and shape by TEM techniques, and particle size distribution was studied by particle size analyzer. Keywords: silver; nanoparticles; chemical reduction; TEM; nanoparticles shape 1. Introduction Nanoparticles are defined as a particle of size 1–100 nm. The oldest known example, where the nanoparticles were used/produced is the Lycurgus Cup from ancient Rome. The glass cup contains gold and silver nano-powders, which are able to change color, the so-called Dichroic effect. The paradox is that, despite the current technical achievements, it is not clear how they made the cups with such optical properties. Silver has accompanied humanity for centuries, and in recent decades, due to current technical progress, its use has been even significantly expanded. The development of science and technology made it possible to produce silver on the nanoscale purposefully. With the reduction of the size of silver particles, unique properties appeared, which silver on the macroscale does not offer. Nano size considerably changed optical, physical, chemical, electrical, thermal, and biological properties due to their surface-to-volume ratio. Thanks to them, nanosilver finds application in various areas of science and technology, e.g., surface enhancer Raman spectroscopy, sensors, and AgNPs are also widely used in medicine as antimicrobial agents, biomedical device coatings, drug delivery carriers, imaging probes, and diagnostic and optoelectronic platforms since they have discrete physical and optical properties [1–4]. The effect of concentration, shape, and size of the nanoparticles on antibacterial properties has also been documented [5,6]. For instance, Martınez-Castanon et al. studied the effect of shape and size of the nanoparticles on anti-bacterial properties and proved that size has a significant impact [7]. Materials 2022, 15, 6829. https://doi.org/10.3390/ma15196829 https://www.mdpi.com/journal/materials