2.1. Preparation and Characterization of Nanocolloids

The colloid of platinum nanoparticles was purchased from Nano-koloid (Warsaw, Poland). This material is produced by a patented electric non-explosive method (Polish patent 380649) from high-purity metal (99.9999%) and high-purity demineralized Milli-Q water. Graphene oxide powder (GO) (purity 99.99%) was purchased from CEZEMAT (Warsaw, Poland) and dispersed in ultrapure water to prepare a 1.0 mg/mL solution.

After 30 minutes of sonification, the hydrocolloids of GO-NP-Pt, NP-Pt, and GO were diluted to different concentrations with 1× Dulbecco’s modified Eagle’s culture medium (Sigma-Aldrich, St. Louis, MO, USA) immediately prior to exposure to cells.

Suspensions of GO (200 μg/mL) and NP-Pt (100 μg/mL) were prepared in ultrapure water and used without additional purification and filtration. Ultrasonic coating of GO nanosheets for 30 min took place in a 50-mL glass flask.

The shape and size of the NP-Pt GO and GO-NP-Pt complexes were inspected using a scanning transmission electron microscope. Electron microscope images were taken with the use of a Hitachi SU8230 ultra-high-resolution field emission scanning electron microscope (Hitachi High-Technologies Corporation, Tokyo, Japan), using the transmission mode at 30.0 kV accelerating voltage. Images were taken with the use of gold transmission electron microscopy (TEM) grids coated with Lacey carbon film, which were immersed in GO samples and dried prior to observation.

The FTIR spectra of dry samples of NP-Pt, GO, and GO-NP-Pt were recorded using a Nicolet 6700 FTIR spectroscope with diamond ATR pickup (Thermo Scientific, Waltham, MA, USA). All samples were prepared by dropping 500 µL of sample suspension on a microscope glass and drying the suspension.

The ζ-potentials of NP-Pt, GO, and GO-NP-Pt were measured by the laser dynamic scattering electrophoretic method, using the Smoluchowski approximation with a Zetasizer Nano ZS90 (Malvern Instruments, Malvern, UK). Each sample was measured after stabilization at 25 °C for 120 s. All measurements were performed in triplicate.