*Article* **One-Minute Synthesis of Size-Controlled Fucoidan-Gold Nanosystems: Antitumoral Activity and Dark Field Imaging**

**Ricardo J. B. Pinto 1,\*, Daniela Bispo 1, Carla Vilela 1, Alexandre M. P. Botas 2, Rute A. S. Ferreira 2, Ana C. Menezes 3, Fábio Campos 3, Helena Oliveira 3, Maria H. Abreu 4, Sónia A. O. Santos 1 and Carmen S. R. Freire 1,\***


Received: 6 February 2020; Accepted: 24 February 2020; Published: 28 February 2020

**Abstract:** Gold nanoparticles (AuNPs) are one of the most studied nanosystems with grea<sup>t</sup> potential for biomedical applications, including cancer therapy. Although some gold-based systems have been described, the use of green and faster methods that allow the control of their properties is of prime importance. Thus, the present study reports a one-minute microwave-assisted synthesis of fucoidan-coated AuNPs with controllable size and high antitumoral activity. The NPs were synthesized using a fucoidan-enriched fraction extracted from *Fucus vesiculosus*, as the reducing and capping agent. The ensuing monodispersed and spherical NPs exhibit tiny diameters between 5.8 and 13.4 nm for concentrations of fucoidan between 0.5 and 0.05% (w/v), respectively, as excellent colloidal stability in distinct solutions and culture media. Furthermore, the NPs present antitumoral activity against three human tumor cell lines (MNT-1, HepG2, and MG-63), and flow cytometry in combination with dark-field imaging confirmed the cellular uptake of NPs by MG-63 cell line.

**Keywords:** gold nanoparticles; fucoidan; microwave irradiation; antitumoral activity; darkfield imaging
