Structural and pH-Responsive Optical Properties of RGD-Functionalized CIS/ZnS Core/Shell Nanoparticles for Biomedical Applications

Copper Indium Sulphide-based quantum dots (CIS QDs) are cadmium-free materials owing to their tunable optical properties with potential for biomedical applications. In this work, CIS/ZnS nanoparticles (NPs) are synthesized by hydrothermal synthesis of CIS QDs, followed by the growth of ZnS shell, and further functionalized by RGD-peptide to study the effect of shell formation, peptide functionalization, and their pH-dependent optical response. The effect of pH and peptide functionalization on the optical properties of CIS/ZnS core/shell NPs has remained largely unexplored. Detailed characterizations were carried out using HRTEM, FT-IR, UV-Vis, PL, DLS, and zeta potential measurements. HRTEM micrographs revealed near-spherical NPs with size distributions indicative of morphological changes as a result of shell formation and peptide functionalization, while FT-IR data showed RGD-peptide related amide bands in the functionalized NPs, providing supportive evidence for possible amide-coupling process. The PL intensity measurements indicated an increased emission after the growth of a ZnS shell. The calculated relative PL QY compared to Rhodamine 6G as a standard were 18.3%, 2.93%, and 4.51% for CIS/ZnS, CP, and CSP NPs, respectively. The increased emission due to shell formation was attributed to the decreased non-radiative recombination on the surface, while RGD functionalization led to the possibility of PL quenching. pH responsive properties of these novel RGD-functionalized nanomaterials were further confirmed through pH dependent UV-Vis and PL studies indicating maximal emission in the near infra-red range at pH 6. These findings demonstrate that RGD-functionalized core/shell NPs possess favourable pH-responsive optical properties, posing their potential for targeted biomedical applications.