A Review on the Microbial Synthesis of Cadmium-Based Quantum Dots and Their Applications

Cadmium (Cd)-based quantum dots (QDs), noted for their size-tunable optoelectronic properties and remarkable photoluminescence, have attracted considerable interest and found widespread commercial applications. Preparation methods for Cd-based QDs typically involve harsh, environmentally hazardous chemical and/or physical processes, whereas microbial biosynthesis offers a sustainable, green alternative. This review provides an overview of microbial production of Cd-based QDs (cadmium sulfide (CdS), cadmium selenide (CdSe), and cadmium telluride (CdTe)) using wild-type and genetically engineered bacteria, fungi, and viruses, with particular attention to the underlying biosynthesis mechanisms. Key proteins and peptides implicated in catalyzing or promoting QD biosynthesis include cystathionine γ-lyase, threonine dehydratase, sulfide-quinone reductase, phytochelatin (oligoglutathione peptides), and other metal-binding proteins such as metallothionein. The roles of capping agents (e.g., phosphates, L-cysteine, and glutathione), Cd (II) efflux genes, and extracellular polymeric substances are discussed. The application of this process in hazardous waste bioremediation, whereby Cd- or sulfur-containing waste is converted into valuable Cd-based QDs, is also addressed. Diverse applications are summarized, including QD-sensitized solar cells, fluorescent labeling, photocatalytic degradation of organic pollutants, and CdS-bacterial hybrid systems enabling photosynthetic production of organic compounds. This review contributes to a better understanding of Cd-based QD biosynthesis, although further efforts are needed to control synthesis and address biosafety concerns.