L-1) used to neutralise a solution of m CHI (g) of chitosan in 0.1 mol.L-1 HCl. V 2 (L) is click here the volume of NaOH added until neutralisation of the ammonium ions from chitosan, and V 1 (L) is the volume of NaOH added to cause the neutralisation of HCl in excess. MMCHI is the molecular mass of glucosamine units (161 g.mol-1). The extent of protonation (EPpH) of chitosan can be calculated
from Equation 2: (2) where% NH2 is the amount of non-protonated amine groups estimated from Equation 1 considering that V 2 is equal to the added volume of base to neutralise the ammonium ions from chitosan at the pH of interest (4.0, 5.0 and 6.0). Zeta potential analyses were performed using a Brookhaven ZetaPALS instrument with a laser light wavelength of 660 nm (35-mW
red diode laser, Holtsville, NY, USA). Standard square acrylic cells with a volume of 4.5 mL were used. The zeta potential measurements were performed at (25.0°C ± 2°C) under the Smoluchowski approximation , and 100 runs (five measurements of 20 cycles) were chosen for a good reproducibility. Results Characterisation of ZnS quantum dots capped by chitosan UV–vis spectroscopy The UV–vis absorption spectra of the ZnS nanoparticles produced using chitosan as the stabilising ligand (ZnS-chitosan nanoconjugates) are shown learn more in Figure 1A. The curves exhibit a broad absorption band between 250 and 360 nm associated with the first excitonic transition indicating that ZnS nanocrystals were synthesised within the ‘quantum confinement regime’  at different pH to form colloidal suspensions capped by carbohydrate-based ligands (after 24 h). The band gap of quantum dots may be assessed Sclareol by theoretical, semi-empirical and empirical models. In this study, the optical band gap energy (E QD) was assessed from absorption coefficient data as a function of wavelength using the ‘Tauc relation’ . This procedure allows to estimate the dimensions of nanoparticles in diluted colloidal suspensions in situ once the average
size of the ZnS nanocrystals can be estimated using the empirical model published in the literature [33, 34], which relates the nanoparticle size (r) to the E QD from a UV–vis spectrum (Equation 3): (3) Figure 1 UV–vis spectroscopy analysis. (A) Spectra of ZnS-chitosan conjugates synthesised at different pH. (B) Optical band gap using the Tauc relation of ZnS-chitosan conjugates synthesised at different pH. (a) pH = 4.0, (b) pH = 5.0, (c) pH = 6.0. Inset: analysis of the effect of pH during the synthesis on the average ZnS quantum dot size (2r) and respective band gap energy (E QD). The E QD values extracted from the curves using the Tauc relation (Figure 1B) were equal to 3.74 ± 0.02, 3.79 ± 0.02 and 3.92 ± 0.02 eV for pH = 4.0, 5.0 and 6.0, respectively. These band gap values are higher than the reference bulk value of 3.54 to 3.