A one-step route for the conversion of Cd waste into CdS quantum dots by <i>Acidithiobacillus</i> sp. <i>via</i> unique biosynthesis pathways

Microorganisms serve as biological factories for the synthesis of nanomaterials such as CdS quantum dots. Based on the uniqueness of <i>Acidithiobacillus</i> sp., a one-step route was explored to directly convert cadmium waste into CdS QDs using these bacteria. First, an exhaustive study was conducted to reveal the specific pathways involved in the biosynthesis of CdS QDs. The widely known homologous enzyme, cysteine desulfhydrase, which catalyzes the synthesis of CdS QDs from a cysteine substrate, is also present in <i>Acidithiobacillus</i> sp. and is referred to as the OSH enzyme. The structure of the OSH enzyme was determined through X-ray crystallography. Moreover, we identified two new pathways. One involved the SQR enzyme in <i>Acidithiobacillus</i> sp., which catalyzed the formation of sulfur globules and subsequently catalyzed further reactions with GSH to release H₂S; subsequently, a CdS QD biosynthesis pathway was successfully constructed. The other pathway involved extracellular polyphosphate, a bacterial metabolic product, which with the addition of GSH and Cd²⁺, resulted in the formation of water-soluble fluorescent CdS QDs in the supernatant. Based on the above-described mechanism, after the bioleaching of Cd²⁺ from cadmium waste by <i>Acidithiobacillus</i> sp., CdS QDs were directly obtained from the bacterial culture supernatants. This work provides important insights into cleaner production and cadmium bioremediation with potential industrial applications.