Biofunctionalized CdS Quantum Dots: A Case Study on Nanomaterial Toxicity in the Photocatalytic Wastewater Treatment Process

Kavitha Shivaji; Kishore Sridharan; D David Kirubakaran; Jayaramakrishnan Velusamy; Seyedeh Sadrieh Emadian; Satheesh Krishnamurthy; Anitha Devadoss; Sanjay Nagarajan; Santanu Das; Sudhagar Pitchaimuthu

doi:10.1021/acsomega.3c00496

Biofunctionalized CdS Quantum Dots: A Case Study on Nanomaterial Toxicity in the Photocatalytic Wastewater Treatment Process

ACS Omega. 2023 May 22;8(22):19413-19424. doi: 10.1021/acsomega.3c00496. eCollection 2023 Jun 6.

Affiliations

¹ Department of Biotechnology, K. S. Rangasamy College of Technology, Tiruchengode 637215, India.
² Department of Nanoscience and Technology, School of Physical Sciences, University of Calicut, Thenhipalam 673635, India.
³ Department of Physics, K. S. R College of Arts and Science for Women, Tiruchengode 637215, India.
⁴ Department of Chemical Engineering and Biotechnology, University of Cambridge, Philippa Fawcett Drive, Cambridge CB3 0AS, U.K.
⁵ School of Engineering and Innovation, The Open University, Milton Keynes MK7 6AA, U.K.
⁶ Institute of Biological Chemistry, Biophysics and Bioengineering (IB3), School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, U.K.
⁷ Department of Chemical Engineering, University of Bath, Bath BA2 7AY, U.K.
⁸ Department of Ceramic Engineering, Indian Institute of Technology (BHU), Varanasi 221005, India.
⁹ Research Centre for Carbon Solutions, Institute of Mechanical, Processing and Energy Engineering, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, U.K.

Abstract

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.