**Boris Chelobanov** †**, Julia Poletaeva** †**, Anna Epanchintseva, Anastasiya Tupitsyna, Inna Pyshnaya and Elena Ryabchikova \***

Institute of Chemical Biology and Fundamental Medicine, Siberian Branch of Russian Academy of Science, Lavrent'ev av., 8, 630090 Novosibirsk, Russia; boris.p.chelobanov@gmail.com (B.C.); fabaceae@yandex.ru (J.P.); annaepanch@gmail.com (A.E.); aysa@ngs.ru (A.T.); pyshnaya@niboch.nsc.ru (I.P.)

**\*** Correspondence: lenryab@yandex.ru; Tel.: +7-383-363-51-63

† These authors contributed equally to this work.

Received: 27 August 2020; Accepted: 13 October 2020; Published: 16 October 2020

**Abstract:** Use of multicellular spheroids in studies of nanoparticles (NPs) has increased in the last decade, however details of NPs interaction with spheroids are poorly known. We synthesized AuNPs (12.0 ± 0.1 nm in diameter, transmission electron microscopy (TEM data) and covered them with bovine serum albumin (BSA) and polyethyleneimine (PEI). Values of hydrodynamic diameter were 17.4 ± 0.4; 35.9 ± 0.5 and ±125.9 ± 2.8 nm for AuNPs, AuBSA-NPs and AuPEI-NPs, and Z-potential (net charge) values were −33.6 ± 2.0; −35.7 ± 1.8 and 39.9 ± 1.3 mV, respectively. Spheroids of human hepatocarcinoma (HepG2) and human embryo kidney (HEK293) cells (Corning ®spheroid microplates CLS4515-5EA), and monolayers of these cell lines were incubated with all NPs for 15 min–4 h, and fixed in 4% paraformaldehyde solution. Samples were examined using transmission and scanning electron microscopy. HepG2 and HEK2893 spheroids showed tissue-specific features and contacted with culture medium by basal plasma membrane of the cells. HepG2 cells both in monolayer and spheroids did not uptake of the AuNPs, while AuBSA-NPs and AuPEI-NPs readily penetrated these cells. All studied NPs penetrated HEK293 cells in both monolayer and spheroids. Thus, two different cell cultures maintained a type of the interaction with NPs in monolayer and spheroid forms, which not depended on NPs Z-potential and size.

**Keywords:** AuNPs; AuPEI-NPs; AuBSA-NPs; electron microscopy; ultrastructure of HepG2 cells and spheroids; ultrastructure of HEK293 cells and spheroids; penetration of NPs into monolayer and spheroids
