2.2 Preparation of nanofibers
Cellulose acetate solution was prepared using 9 ml acetone, 3 ml dimethylformamide, 1.44 g cellulose acetate, and 1% wt annatto extract, as previously described [26]. The solution was mixed with an IKA HS 7 magnetic stirrer for 2 h at RT. When utilized, annatto extract was added after mixing all the other components to mitigate degradation.
The cellulose acetate nanofibers (CA) and cellulose acetate nanofibers with annatto extract (CA@A) were obtained by electrospinning, as previously described [26]. Briefly, the cellulose acetate and cellulose acetate loaded with annatto extract were dissolved in acetone-dimethylformamide (3:1 v/v) to obtain 12 wt% (w/v) solution. The polymer solution was fed into a 10 mL standard syringe attached to a 0.8 mm diameter steel needle and spun on an NB-EN1 (NanoBond, China) electrospinning device with electric voltage of 16 kV, 14 cm working distance, collector rotation at 400 rpm, and solution gravity-fed at room temperature.
Physicochemical characterization
UV-vis spectroscopy of annatto extract was performed in a Perkin Elmer Lambda 1050 spectrometer (Waltham, USA) with wavelength range of 250–800 nm and scanning speed of 267 nm/min. The annatto extract used in this analysis was diluted in acetone 1:50 (v/v), and measured immediately after the extract was prepared. Samples were evaluated by Fourier transform infrared spectroscopy (FTIR) using a Shimadzu IRPrestige-21 device in the range between 4000 and 400 cm-1.
Morphology was evaluated with a Phenom XL (Phenom-World, Eindhoven, Netherlands) scanning electron microscope (SEM) using medium vacuum (60 Pa) and auto focus on an accelerating voltage of 5 kV. Samples were sputtered with gold for 20 minutes using a sputter coater (Cressington 108 model; Cressington Scientific Instruments, Watford, England). Average fiber diameter and size distribution were obtained with ImageJ software based on the SEM images. From three SEM images of each sample, 200 fibers were randomly selected and their diameter measured manually using the Line tool.
Surface wettability of the nanofibers was measured using deionized water at room temperature with a contact angle analyzer (KRÜSS model DSA-100; KRÜSS Scientific, Hamburg, Germany). Deionized water was automatically dripped onto the samples and five contact angle measurements were averaged to obtain a reliable value.
Thermogravimetric analysis (TGA) of the nanofibers was conducted with a TA Instruments Q50 device (TA Instruments, New Castle, USA), heating the samples from room temperature to 900 °C at 20 ºC/min in N2 atmosphere.