Abstract
The toxic nature of inorganic nanostructured materialsas photocatalystsis often not accounted for in traditional wastewater treatment reactions.Particularly, some inorganic nanomaterials employed as photocatalystsmay release secondary pollutants in the form of ionic species thatleach out due to photocorrosion. In this context, this work is a proof-of-conceptstudy for exploring the environmental toxicity effect of extremelysmall-sized nanoparticles (<10 nm) like quantum dots (QDs) thatare employed as photocatalysts, and in this study, cadmium sulfide(CdS) QDs are chosen. Typically, CdS is an excellent semiconductorwith suitable bandgap and band-edge positions that is attractive forapplications in solar cells, photocatalysis, and bioimaging. However,the leaching of toxic cadmium (Cd2+) metal ions due tothe poor photocorrosion stability of CdS is a matter of serious concern.Therefore, in this report, a cost-effective strategy is devised forbiofunctionalizing the active surface of CdS QDs by employing tealeaf extract, which is expected to hinder photocorrosion and preventthe leaching of toxic Cd2+ ions. The coating of tea leafmoieties (chlorophyll and polyphenol) over the CdS QDs (referred tohereafter as G-CdS QDs) was confirmed through structural, morphological,and chemical analysis. Moreover, the enhanced visible-light absorptionand emission intensity of G-CdS QDs in comparison to that of C-CdSQDs synthesized through a conventional chemical synthesis approachconfirmed the presence of chlorophyll/polyphenol coating. Interestingly,the polyphenol/chlorophyll molecules formed a heterojunction withCdS QDs and enabled the G-CdS QDs to exhibit enhanced photocatalyticactivity in the degradation of methylene blue dye molecules over C-CdSQDs while effectively preventing photocorrosion as confirmed fromcyclic photodegradation studies. Furthermore, detailed toxicity studieswere conducted by exposing zebrafish embryos to the as-synthesizedCdS QDs for 72 h. Surprisingly, the survival rate of the zebrafishembryos exposed to G-CdS QDs was equal to that of the control, indicatinga significant reduction in the leaching of Cd2+ ions fromG-CdS QDs in comparison to C-CdS QDs. The chemical environment ofC-CdS and G-CdS before and after the photocatalysis reaction was examinedby X-ray photoelectron spectroscopy. These experimental findings provethat biocompatibility and toxicity could be controlled by simply addingtea leaf extract during the synthesis of nanostructured materials,and revisiting green synthesis techniques can be beneficial. Furthermore,repurposing the discarded tea leaves may not only facilitate the controlof toxicity of inorganic nanostructured materials but can also helpin enhancing global environmental sustainability.