ORIGINAL CONTRIBUTION Photothermal property assessment of gold nanoparticle assemblies obtained by hydroxylamine reduction István Sz. Tódor 1 & Oana T. Marişca 1 & Dumitrița Rugină 2 & Zorița Diaconeasa 3 & Loredana F. Leopold 3 & Cristina Coman 3 & Elisabeta Antonescu 4 & László Szabó 1,5 & Stefania D. Iancu 1,5 & Zoltán Bálint 1,5 & Nicolae Leopold 1,5 Received: 14 February 2020 /Revised: 3 August 2020 /Accepted: 5 August 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract The ideal photothermal therapy agent should exhibit a high energy conversion capacity, but it should not induce any cytotoxic effects unless it is exposed to electromagnetic radiation. We present data on biocompatibility and photothermal conversion capacity of colloidal gold nanoparticle assemblies (NPAs), obtained by hydroxylamine hydrochloride reduction at room tem- perature. The cytotoxic profile of NPAs with a mean diameter of 20 and 120 nm, respectively, was assessed using 3-[4,5- dimethylthiazole-2-yl]-2,5-diphenyltetrazolium bromide (MTT) assay, demonstrating tolerable cytotoxic effect on A549 human lung adenocarcinoma cell line at gold concentrations up to 150 μg gold/mL. Insights regarding the oxidative stress were gained using peroxide-sensitive fluorescent probes, which showed no additional oxidative stress due to NPA exposure. Using infrared thermal imaging, the photothermal conversion was evidenced, the effect being more prominent for the 120-nm diameter NPAs irradiated with the 785-nm laser. The photothermal capabilities of NPAs were highlighted also by Raman imaging studies, showing intracellular carbon by-products due to the thermal damage. Intracellular carbon deposits overlapping with the nanoparticle-rich sites were identified. These results highlight that NPAs are a versatile platform with possible applications in nanoparticle-based photothermal therapy. Keywords Gold nanoparticles . Nanoparticle assemblies . Photothermal effect . Raman mapping . Phototherapeutic agent Introduction Hyperthermia is an emerging anti-cancer strategy [1] that involves heat generation (temperatures between 41 and 46 °C) within the tumor site, causing intracellular stress and other changes in the cell physiology that eventually lead to cell death. Hyperthermia induces protein misfolding and aggregation, alteration in signal transduction, changes in pH, reduction of oxygen transfer, and ultimately apo- ptosis. Nanoparticle-based photothermal therapy is thus a promising technique for localized cancer treatment [2]. Metal nanoparticles such as silver or gold are able to convert electromagnetic radiation from the visible and near-infrared domain into heat, a process which involves light absorption by metal surface plasmons at resonant conditions. Such plasmonic effects confer metal nanopar- ticle versatile properties that have been exploited in sev- eral fields, including medicine [3]. Among the various types of nanoparticles developed so far, gold nanoparticles (AuNPs) have been extensively studied due to their facile synthesis, easily controllable size and shape, good biocompatibility, and unique optical properties [4]. AuNPs have been considered for several biomedical applica- tions, including imaging, photothermal therapy, drug and gene delivery, and biocatalysis [5]. István Sz. Tódor and Oana T. Marişca contributed equally to this work. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00396-020-04721-5) contains supplementary material, which is available to authorized users. * Nicolae Leopold nicolae.leopold@phys.ubbcluj.ro 1 Faculty of Physics, Babeş-Bolyai University, Cluj-Napoca, Romania 2 Faculty of Veterinary Medicine, University of Agricultural Sciences and Veterinary Medicine, Cluj-Napoca, Romania 3 Faculty of Food Science and Technology, University of Agricultural Sciences and Veterinary Medicine, Cluj-Napoca, Romania 4 Faculty of Medicine, Lucian Blaga University, Sibiu, Romania 5 IMOGEN Research Institute, County Clinical Emergency Hospital, Cluj-Napoca, Romania https://doi.org/10.1007/s00396-020-04721-5 / Published online: 17 August 2020 Colloid and Polymer Science (2020) 298:1369–1377