3.1. Nanoparticles Suspension Stability

Nanoparticle suspension stability tests were performed by using visual observation and turbidity scanning. For each type of nanoparticle (FNP, FNP-HCl, FNP-MD and ANP), samples with five different concentrations (0.1, 0.2, 0.3, 0.4 and 0.5 wt. %) were prepared and put into test tubes, which were then placed into a heating cabinet at 70 °C. Photographs of all test tubes were taken after certain period of time to show the stability of samples over time. A Turbiscan LAB colloidal stability analyzer (Formulaction Inc., Toulouse, France) was used to quantify nanoparticle suspension stability. Samples of nanoparticle suspension were scanned at 60 °C with an 880 nm near-infrared light-emitting diode (LED) source and the transmission and backscattered signals were registered by detectors. Since nanoparticle size can affect these signals, delta transmission and backscattering were used to determine nanoparticle stability. The dimensionless turbidity scan index (TSI) defined by the Turbiscan software (TurbiSoft Lab, 2.2.0.82-2, Toulouse, France) was used to quantify nanoparticles suspension stability. Some samples need longer scanning time. They were scanned continuously for the first 10 days then were taken out of the instrument and put into the heating cabinet with a temperature of 60 °C. They were placed back for a single scan every second day. Calculation of TSI is based on comparing each scan to the previous one for the selected height and dividing the result by the total selected height in order to obtain a result which does not depend on the quantity of product in the measuring cell. The lower the TSI value, the better is the stability of the sample. The following equation was used to calculate the TSI [31].