The development of semiconductor nanoparticles such as cadmium selenide (CdSe) and zinc sulfide (ZnS) has gained significant attention due to their exceptional optoelectronic properties, making them suitable for applications in light-emitting diodes, solar cells, and bioimaging. Traditional synthesis methods often involve hazardous chemicals, high temperatures, and toxic byproducts, raising environmental and safety concerns. In contrast, bio-assisted synthesis leverages sulfur- and selenium-rich biomolecules derived from natural sources, offering a sustainable and eco-friendly alternative. This approach not only reduces reliance on harmful precursors but also introduces inherent capping agents that passivate nanoparticle surfaces, improving stability and mitigating toxicity.

Microorganisms, plant extracts, and biomolecular precursors play a crucial role in the bio-assisted synthesis of CdSe and ZnS nanoparticles. Sulfur-containing biomolecules such as cysteine, glutathione, and methionine serve as effective sulfur donors for ZnS formation, while selenium-rich compounds like selenocysteine or selenomethionine facilitate CdSe nanoparticle synthesis. These biomolecules act as both reducing and stabilizing agents, eliminating the need for additional surfactants. For instance, bacterial strains such as *Escherichia coli* and *Pseudomonas aeruginosa* have been employed to synthesize CdSe quantum dots intracellularly, where cellular enzymes mediate the reduction of selenite ions to form selenium precursors. Similarly, plant extracts containing phytochemicals with thiol or selenol groups enable room-temperature synthesis of nanoparticles with controlled size distributions.

A key advantage of bio-assisted synthesis is the inherent surface functionalization of nanoparticles by biomolecules, which reduces toxicity and enhances biocompatibility. The organic ligands derived from biological precursors form a protective layer around the nanoparticles, preventing the release of toxic metal ions such as Cd²⁺. Studies have demonstrated that CdSe quantum dots synthesized using glutathione exhibit significantly lower cytotoxicity compared to those synthesized via conventional chemical routes. Additionally, the biomolecular capping layer can be further modified to improve solubility in aqueous or organic solvents, broadening their applicability in biomedical and optoelectronic devices.

Tuning the optoelectronic properties of bio-synthesized CdSe and ZnS nanoparticles is achievable through careful selection of biomolecular precursors and reaction conditions. The bandgap of these semiconductor nanoparticles, which dictates their optical absorption and emission characteristics, can be modulated by controlling particle size and surface chemistry. For example, altering the concentration of cysteine during ZnS synthesis influences nucleation rates, leading to variations in crystallite size and photoluminescence efficiency. Similarly, the use of selenium-rich yeast extracts in CdSe synthesis allows for precise control over nanoparticle size, enabling emission wavelength tuning across the visible spectrum. The presence of biomolecular defects or dopants can also introduce mid-gap states, affecting charge carrier recombination dynamics and enhancing photocatalytic performance.

Despite these advantages, challenges remain in achieving uniform size distribution and high crystallinity in bio-assisted synthesis. Biological precursors often introduce variability due to differences in composition across batches, necessitating stringent quality control. Optimization of parameters such as pH, temperature, and incubation time is critical to ensure reproducibility. For instance, mildly acidic conditions favor the formation of smaller CdSe nanoparticles with narrow size distributions, while neutral to alkaline conditions promote larger crystallites. Advanced characterization techniques such as high-resolution transmission electron microscopy and X-ray diffraction are essential for verifying structural properties and ensuring batch-to-batch consistency.

Environmental sustainability is a driving force behind the adoption of bio-assisted synthesis methods. By utilizing renewable biological resources, this approach minimizes waste generation and energy consumption compared to conventional high-temperature syntheses. The use of non-toxic precursors also aligns with green chemistry principles, reducing the environmental footprint of nanoparticle production. Furthermore, the biodegradability of biomolecular capping agents addresses concerns regarding nanoparticle persistence in ecosystems, making them more suitable for large-scale industrial applications.

In optoelectronic applications, bio-synthesized CdSe and ZnS nanoparticles exhibit promising performance metrics. CdSe quantum dots produced via yeast-mediated synthesis have demonstrated photoluminescence quantum yields exceeding 40%, comparable to those obtained through chemical methods. Their narrow emission spectra make them attractive for display technologies and fluorescence tagging in bioimaging. Similarly, ZnS nanoparticles synthesized using plant-derived thiols exhibit efficient UV emission, suitable for light-emitting devices and sensors. The biocompatibility of these nanoparticles further enables their integration into hybrid bio-electronic systems, where they can interface with biological components without inducing adverse effects.

Future research directions include the exploration of novel biomolecular precursors with higher reducing capacities and improved stabilizing properties. The integration of metabolic engineering techniques could enhance the efficiency of microbial synthesis, enabling higher yields and better control over nanoparticle properties. Additionally, the development of hybrid bio-chemical approaches may bridge the gap between eco-friendly synthesis and industrial scalability, ensuring that performance requirements are met without compromising environmental safety.

In summary, bio-assisted synthesis represents a viable and sustainable pathway for producing CdSe and ZnS nanoparticles with tailored optoelectronic properties. By leveraging sulfur- and selenium-rich biomolecules, this method mitigates toxicity concerns while maintaining high performance in applications ranging from renewable energy to biomedical imaging. Continued advancements in biological precursor selection and process optimization will further solidify its role in the future of nanomaterial manufacturing.