Keywords: continuous flow synthesis, ZnO quantum dots, anti-cancer activity
Summary:Zinc oxide quantum dots (ZnO QDs) with rich-defect chemistry significantly impact in the areas of energy, agriculture, and health care. The most dominant of all the surface defects in ZnO is the singly ionized oxygen vacancies (VO•). The anomalous nature of ZnO QDs is attributed to these vacancies. Enriching the concentration with controlled size lends numerous physiochemical, optical, and biological characteristics in QD’s. Fabrication methods and the process parameters allow tuning of size and the performance of QD’s. Of the conventional synthesis routes, the batch process is the most predominant. However, with rapidly evolving technologies over the last 20 years, the synthesis strategies have undergone modifications to meet the increased demand for high-quality novel ZnO QDs. Several attempts to bridge the gap between lab-scale synthesis and pilot-scale production have culminated in the rising popularity of the design and implementation of environmentally-benign continuous flow synthesis platforms. In this work, a unique, sustainable, and facile continuous flow synthesis approach for defect-engineered ZnO QDs (E-ZnO) is reported. Nucleation-growth kinetics of E-ZnO formation served as the central pillar for the effective and rational design of the reactor assembly. The reactor’s transport properties and hydrodynamic performance were revealed by the estimated Reynolds number (Re) of 57.3. The geometry and aspect ratio (17.25) of the reactor effectuated the centrifugal and viscous forces arising from the hydrodynamic flow. The disparity of these two resulted in the formation of Dean vortices that is responsible for improved mixing efficiency. The magnitude of the Dean number for the optimized flow rate and its corresponding Re is 13.7, thus affirming an aspect ratio-dependent behaviour. A systematic analysis was performed to gain an understanding of the inter link between the structure, surface chemistry, reaction conditions inside the flow reactor, and intrinsic properties of the fabricated E-ZnO. Using this scalable “green” synthesis strategy, monodispersed E-ZnO was obtained that exhibited a remarkably narrow stable hydrodynamic size of 7.6 nm over 6 months, superior photoluminescence quantum yield (PLQY) of 0.89, and photostability, with or without the exposure of UV radiations over a time frame of 72 h and 6 months respectively. The photostability was mechanistically evaluated from the photoluminescence (PL) studies, and time-dependent real-time measurements of VO• were carried out by electron paramagnetic resonance spectroscopy (EPR) to observe whether quenching of VO• occurred with time or not. Additionally, E-ZnO manifested significantly enhanced bioactivity in contrast to the commercial-grade ZnO NPs, precisely their anti-cancer characteristics as understood from the in vitro studies performed against a human breast cancer cell line (MDA-MB-231) using MTT assay for varying concentrations (10-500 μg/mL) of E-ZnO and ZnO NPs. It was observed that the survival rate of the cells substantially decreased with increasing concentration of E-ZnO, and the percentage cell viability had reached almost zero for 500 μg/mL. Thus, these E-ZnO with enriched bioactivity can aptly serve as a potential agent for cancer therapy and other appropriate fields in healthcare.