The decrease in size could be attributed to the sum of several contributions towards the formation of the nanoconjugates made by the ZnS ‘core’ and chitosan ‘shell’. At a relatively lower pH (pH = 4), most of the amine groups of chitosan are protonated (pH < < pKa of chitosan); thereby, positively charged transition metal has to compete with hydrogen ion for complexation with amine electron pair (metal-ligand interactions), as represented in Equations 5 and 6 [50]: (5) (6) However, as the pH increases (pH = 6), more amine groups become available in the chitosan chain for dative bonding (electron donor) with zinc divalent cations, thus reducing the electrostatic repulsion

(Zn2+ ↔ NH3 +) and favouring the stabilisation of the ZnS nanocrystals at smaller dimensions due to the increase of the number of nucleation sites. It is also Selleckchem RG-7388 interesting to note that the shift of the secondary alcohol vibration in FTIR spectra of conjugates BAY 63-2521 mouse was inversely proportional to the extent of protonation. Both the amine/protonated Adavosertib mouse amine and the C3-OH group are at the same side of the chitosan chain. The presence of a higher number of -NH3 + charged groups may affect the

interaction of -OH groups with metal cations (Zn2+) during the nucleation, growth and stabilisation of QDs. Additionally, sulphide anions (S2-) may have electrostatically interacted with -NH3 + groups of chitosan during the synthesis Acesulfame Potassium of ZnS QDs at lower pH, which could also affect the sizes of the nanocrystals formed. In addition, photoluminescence properties were also affected by pH. The PL relative efficiency of the CHI-ZnS bioconjugates was higher under more acidic synthesis conditions (pH = 4.0). PL quenching may be attributed to several features. In this case, at relatively higher pH levels (pH = 5.0 and pH = 6.0), the smaller sizes of the nanoparticles were observed, and most of the amine groups were deprotonated

(pH closer to pKa). As the nanoparticle size decreases, surface disorder and dangling bonds may dominate the luminescence properties, thus creating non-radiative pathways that dissipate quantum dot emission, which resulted in the decreased PL intensity [56, 57]. Considering spherical quantum dots, as the nanoparticle size reduces (radius, R), the relative surface (S) to volume (V) ratio (S/V = 4πR 2 / (4/3)πR 3) = 3/R) is significantly increased leading to more surface defects. Additionally, amine groups can act as hole scavengers, which quench the photoluminescence [58]. Conclusions In the present work, ZnS QDs directly biofunctionalised by chitosan were synthesised using a single-step colloidal process in aqueous medium at room temperature. The results demonstrated that varying the pH from 4.0 to 6.0 of the chitosan solutions significantly affected the average size of ZnS nanocrystals produced ranging from 3.8 to 4.7 nm.