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Small particles in natural sources are a subject of interest for their potential role in intercellular, inter-organism, and inter-species interactions, but their harvesting and assessment present a challenge due to their small size and transient identity. We applied a recently developed interferometric light microscopy (ILM) to assess the number density and hydrodynamic radius (R h ) of isolated small cellular particles (SCPs) from blood preparations (plasma and washed erythrocytes) (B), spruce needle homogenate (S), suspension of flagellae of microalgae Tetraselmis chuii (T), conditioned culture media of microalgae Phaeodactylum tricornutum (P), and liposomes (L). The aliquots were also assessed by flow cytometry (FCM), dynamic light scattering (DLS), ultraviolet-visible spectrometry (UV-vis), and imaging by cryogenic transmission electron microscopy (cryo-TEM). In R h , ILM showed agreement with DLS within the measurement error in 10 out of 13 samples and was the only method used here that yielded particle density. Cryo-TEM revealed that representative SCPs from Tetraselmis chuii flagella (T) did not have a globular shape, so the interpretation by R h of the batch methods was biased. Cryo-TEM showed the presence of thin filaments in isolates from Phaeodactylum tricornutum conditioned culture media (P), which provides an explanation for the considerably larger R h obtained by batch methods than the sizes of particles observed by cryo-TEM images. ILM proved convenient for assessment of number density and R h of SCPs in blood preparations (e.g., plasma); therefore, its use in population and clinical studies is indicated.

Vesna Spasovski,1,2,* Anna Romolo,1,3,* Urška Zagorc,4 Vesna Arrigler,4 Matic Kisovec,5 Apolonija Bedina Zavec,5 Matevž Arko,1 Adrienn Molnár,6,7 Gitta Schlosser,7 Aleš Iglič,3,8 Ksenija Kogej,4 Veronika Kralj-Iglič1 1University of Ljubljana, Faculty of Health Sciences, Laboratory of Clinical Biophysics, Ljubljana, Slovenia; 2Institute of Molecular Genetics and Genetic Engineering, University of Belgrade, Belgrade, Serbia; 3University of Ljubljana, Faculty of Electrical Engineering, Laboratory of Physics, Ljubljana, Slovenia; 4University of Ljubljana, Faculty of Chemistry and Chemical Technology, Ljubljana, Slovenia; 5National Institute of Chemistry, Department of Molecular Biology and Nanobiotechnology, Ljubljana, Slovenia; 6Hevesy György PhD School of Chemistry, ELTE Eötvös Loránd University, Budapest, Hungary; 7MTA-ELTE Lendület Ion Mobility Mass Spectrometry Research Group, Faculty of Science, Institute of Chemistry, ELTE Eötvös Loránd University, Budapest, Hungary; 8University of Ljubljana, Faculty of Medicine, Laboratory of Clinical Biophysics, Ljubljana, Slovenia*These authors contributed equally to this workCorrespondence: Veronika Kralj-Iglič, Email veronika.kralj-iglic@zf.uni-lj.siIntroduction: Lipid nanovesicles associated with bioactive phytochemicals from spruce needle homogenate (here called nano-sized hybridosomes or nanohybridosomes, NSHs) were considered.Methods: We formed NSHs by mixing appropriate amounts of lecithin, glycerol and supernatant of isolation of extracellular vesicles from spruce needle homogenate. We visualized NSHs by light microscopy and cryogenic transmission electron microscopy and assessed them by flow cytometry, dynamic light scattering, ultraviolet–visual spectroscopy, interferometric light microscopy and liquid chromatography–mass spectrometry.Results: We found that the particles consisted of a bilayer membrane and a fluid-like interior. Flow cytometry and interferometric light microscopy measurements showed that the majority of the particles were nano-sized. Dynamic light scattering and interferometric light microscopy measurements agreed well on the average hydrodynamic radius of the particles Rh (between 140 and 180 nm), while the concentrations of the particles were in the range between 1013 and 1014/mL indicating that NSHs present a considerable (more than 25%) of the sample which is much more than the yield of natural extracellular vesicles (EVs) from spruce needle homogenate (estimated less than 1%). Spruce specific lipids and proteins were found in hybridosomes.Discussion: Simple and low-cost preparation method, non-demanding saving process and efficient formation procedure suggest that large-scale production of NSHs from lipids and spruce needle homogenate is feasible.Plain Language Summary: Cells shed into their exterior nanoparticles (here referred to as extracellular vesicles – EVs) that are free to move, reach distant cells and are taken up by them. As they carry bioactive constituents, EVs may have important impact on the recipient cells. The mechanisms of EV formation and mediation can be employed in designing therapeutic, prophylactic and diagnostic methods for various medical issues. EVs can be harvested from biological samples; however, their yield is small,12 and there are potential side effects. Artificial vesicles – liposomes – have high yield; however, in vivo, they can be degraded before reaching the target and their reproducibility is yet insufficient. In order to combine advantages of both types of nanoparticles, we have composed nanohybridosomes (NSHs) from soya lecithin, water and supernatant of isolation of EVs from spruce needle homogenate, visualized them by cryogenic electron microscopy and characterized them with respect to their size, concentration and protein/nucleic acid content. We have applied a recently developed interferometric light microscopy to determine the hydrodynamic radius and the concentration of EVs. We found that the majority of composed particles are nano-sized and that they enclose more than 25% of the incoming volume of liquid, which is considerably more than about 1% that can be harvested by isolation of EVs from spruce needle homogenate by (ultra)centrifugation.Keywords: hybridosomes, liposomes, nanovesicles, extracellular particles, small cellular particles, drug delivery