F 4600 fluorescence spectrofluorometer

The surface morphologies of the composite fibers were characterized using a S-4800 scanning electron microscope (SEM) (Hitachi, East Coast Harbor, Honshu Island, Japan). Crystal structures were analyzed with X-ray powder diffraction (XRD), using a Bruker D8ADVANCE diffractometer (Bruker, Karlsruhe, Germany) with Cu-Kα radiation (λ = 1.54178 Å). The crystal distributions of the composite fibers were characterized using a Joel-2100F transmission electron microscope (TEM) (JEOL Ltd., Tokyo, Japan). The ultra-violet-near infra-red (UV-Vis-NIR) diffused reflectance spectrum (DRS) was recorded on a HITACHI U-4100 UV/Vis-NIR spectrophotometer (Hitachi, East Coast Harbor, Honshu Island, Japan). The photoluminescence (PL) spectra were recorded on a HITACHI F-4600 fluorescence spectrofluorometer (Hitachi, East Coast Harbor, Honshu Island, Japan). Thermogravimetric analysis (TGA) was performed using a PerkinElmer STA-6000 thermogravimetric analyzer (PerkinElmer, Waltham, MA, USA).

The fluorescence spectroscopy was obtained using a F-4600 fluorescence spectrofluorometer from Hitachi Corporation (Tokyo, Japan) with a fixed concentration of pancreatic lipase (2.0 mL of 5 × 10⁻⁶ mol/L) in the presence of different volumes of anthocyanins. Briefly, the final concentration rates of the anthocyanins and pancreatic lipase were 0, 0.5:1, 1:1, 2:1, 4:1, 5:1, 10:1, and 20:1, respectively. Then, the solutions were mixed at 20 or 30 °C. The excitation wavelength was set at 280 nm and the emission spectra were scanned in the range of 300 to 400 nm. Finally, fluorescence quenching is usually described by the linear Stern–Volmer equation:
where F₀/F is the intensity ratio in the absence or presence of quencher (anthocyanins), [Q] is the concentration of anthocyanins, τ₀ is the average fluorescence lifetime (about 10⁻⁸ s), K_q is the bimolecular quenching constant, and K_sv is the Stern-Volmer quenching constant calculated by K_qτ₀.
For the fluorescence quenching mechanism, the apparent binding constant (K_a) between anthocyanins and pancreatic lipase and the number of binding sites (n) were calculated from the static quenching formula: