The toxic nature of inorganic nanostructured materials
as photocatalysts
is often not accounted for in traditional wastewater treatment reactions.
Particularly, some inorganic nanomaterials employed as photocatalysts
may release secondary pollutants in the form of ionic species that
leach out due to photocorrosion. In this context, this work is a proof-of-concept
study for exploring the environmental toxicity effect of extremely
small-sized nanoparticles (<10 nm) like quantum dots (QDs) that
are employed as photocatalysts, and in this study, cadmium sulfide
(CdS) QDs are chosen. Typically, CdS is an excellent semiconductor
with suitable bandgap and band-edge positions that is attractive for
applications in solar cells, photocatalysis, and bioimaging. However,
the leaching of toxic cadmium (Cd2+) metal ions due to
the poor photocorrosion stability of CdS is a matter of serious concern.
Therefore, in this report, a cost-effective strategy is devised for
biofunctionalizing the active surface of CdS QDs by employing tea
leaf extract, which is expected to hinder photocorrosion and prevent
the leaching of toxic Cd2+ions. The coating of tea leaf
moieties (chlorophyll and polyphenol) over the CdS QDs (referred to
hereafter as G-CdS QDs) was confirmed through structural, morphological,
and chemical analysis. Moreover, the enhanced visible-light absorption
and emission intensity of G-CdS QDs in comparison to that of C-CdS
QDs synthesized through a conventional chemical synthesis approach
confirmed the presence of chlorophyll/polyphenol coating. Interestingly,
the polyphenol/chlorophyll molecules formed a heterojunction with
CdS QDs and enabled the G-CdS QDs to exhibit enhanced photocatalytic
activity in the degradation of methylene blue dye molecules over C-CdS
QDs while effectively preventing photocorrosion as confirmed from
cyclic photodegradation studies. Furthermore, detailed toxicity studies
were conducted by exposing zebrafish embryos to the as-synthesized
CdS QDs for 72 h. Surprisingly, the survival rate of the zebrafish
embryos exposed to G-CdS QDs was equal to that of the control, indicating
a significant reduction in the leaching of Cd2+ions from
G-CdS QDs in comparison to C-CdS QDs. The chemical environment of
C-CdS and G-CdS before and after the photocatalysis reaction was examined
by X-ray photoelectron spectroscopy. These experimental findings prove
that biocompatibility and toxicity could be controlled by simply adding
tea 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 control
of toxicity of inorganic nanostructured materials but can also help
in enhancing global environmental sustainability.