Facile synthesis of ternary quantum dot bio-conjugates for the sensing of vascular endothelial growth factor

Abstract

In nanotechnology, the synthesis of semiconductor nanoparticles known as quantum dots (QDs) has gained much interest due to their excellent optical and electronic properties compared to traditionally used organic dyes. In practice, quantum dots are usually prepared from toxic elements of group II-VI and group II-VI on the periodic table, including compounds such as PbTe, PbS, CdSe and CdTe. Their inherent toxicity and reported leakage of Cd2+ / Pb2+ ions from the core of QDs into their surrounding environment have limited the biological applications of these QDs. Modern research has therefore focused on developing less toxic QDs such as CuInS and AgInS/ZnS QDs. However, challenges with these QDs are their synthetic methods which have been mainly reported to be time-consuming, costly and, use harsh conditions such as high temperature and organic solvents, thus limiting their biological applications. Bio-conjugation, the chemical modification of biomolecules, has been reported to improve quantum dots' biocompatibility and biological targeting. Although bio-conjugation of Cd-free QDs has been reported, only some researchers have focused on their electrochemical characterization of ternary QDs. Electrochemical techniques play an essential role in studying electrons that may undergo quantum confinement effects, which are reflected in their electrochemical behaviour. This study synthesized water-soluble glutathione-capped AgInS core QDs and AgInS/ZnS core/shell QDs via the reflux, heat-up and mono-wave 50 synthesis methods. Various synthetic conditions such as reaction time, pH, Ag: In ratios, ZnS shell layers, capping agent, GSH concentration and stabilizing agent on the fluorescence properties of the ternary AgInS/ZnS based QDs were examined. Optimal conditions were obtained using the reflux method for AgInS core QDs synthesis at pH 7.58, Ag: In molar ratio of 1:4, and glutathione (GSH) concentration of 0.145 mmol after 45 min. FTIR analysis confirmed the formation of GSH capping on the QDs the via S-metal bond. The successful passivation of AgInS core by ZnS shell resulted in a PLQY increase from 31.6 to 35.4% for AgInS core and AgInS/ZnS core/shell QDs, respectively. The as-synthesized QDs were conjugated to Bovine serum albumin (BSA) to form AgInS/ZnS-BSA bioconjugate. The fluorescence intensity was enhanced after the conjugation of QDs to BSA, indicating improved quality of the QDs. Electrochemical properties of AgInS core, AgInS/ZnS core/shell, QDs and AgInS/ZnS-BSA bioconjugate were evaluated using cyclic voltammetry (CV), differential pulse voltammetry (DPV), square wave voltammetry (SWV) and electrochemical impedance spectroscopy (EIS). A comparative study of the electrochemical properties of AgInS QDs and AgInS/ZnS QDs showed that AgInS core QDs and AgInS/ZnS core/shell QDs exhibited chemical and electrochemical composition-dependent properties enabling the use of the materials in both electronics and bio-applications. Studies of the electrochemical properties of AgInS-BSA and AgInS/ZnS-BSA bioconjugates showed enhanced peak currents suggesting higher conductivity and electrical properties. EIS confirmed improved charge transfer resistance for AgInS/ZnS-BSA core/shell bioconjugate. Developing an immunosensor using AgInS/ZnS-BSA bioconjugate for electrochemical sensing of vascular endothelial growth factor (VEGF) was done using DPV. The immunosensor was tested against VEGF at different concentrations and had a satisfactory response. The method’s linear range was from 0.003 to 0.017 μg/ml. The LOD and LOQ were determined to be 1.5 x 10-3 μg/ml and 5 x 10-3 μg/ml, respectively. The immunosensor showed improvement in stability over a period of five weeks.