Optical characterization of aqueous colloidal CdSeS/ZnS nanocrystals for biosensor application

A comprehensive study of the optical properties of aqueous and non-polar solvent colloidal CdSeS/ZnS nanocrystals (NCs) is reported that includes the absorption and photoluminescence (PL). The different ligands have been change from oleic acid to carboxyl (COOH) and amine (NH3) to enhance capability of aqueous NCs in biosensor application. The absorption shows typically 2 peak appear as shoulder where main peak centered at 460, 512 and 530 nm respectively. The photoluminescence spectra are found to be single Gaussian in nature, displaying inhomogeneous broadening due to well distribution of NCs size. PL peak centered at 545 nm for oleic acid ligand, red shifted to 547 nm (NH3) and 554 nm (COOH) due to clustering effect from ligand and solvent exchange. The Stokes shift is calculated from absorption peak and PL peak show reduction from 0.4 eV to 0.1 eV and 0.15 eV indicating the phonon production in NCs is reduce once ligand and solvent changes. Thus, this will affect the PL efficiency due to less phonon-photon assisting emission process and resulting less PL intensity for both aqueous NCs. Aqueous solvent helping dispersing the NCs for clustering when ligand is exchange, however due to present of oxygen’s atom in aqueous medium the NCs oxidisation process may occur much faster.