Bacteria play a pivotal role in the green synthesis of nanoparticles, offering a sustainable and eco-friendly alternative to conventional chemical and physical methods. The process leverages bacterial metabolic pathways to reduce metal ions into nanoparticles, either intracellularly or extracellularly, with precise control over size, morphology, and composition. This approach minimizes toxic byproducts and energy consumption while enabling the production of nanoparticles with tailored properties for applications in biomedicine, sensing, and environmental remediation.

Intracellular synthesis occurs within bacterial cells, where metal ions are transported across the cell membrane and reduced by enzymes or other biomolecules. For example, Shewanella oneidensis, a metal-reducing bacterium, employs cytochromes and reductases to convert metal ions such as gold (Au³⁺) or silver (Ag⁺) into nanoparticles. The nanoparticles form in the periplasmic space or cytoplasm, with sizes typically ranging from 5 to 50 nm, depending on bacterial strain and growth conditions. The intracellular environment influences nanoparticle morphology, often yielding spherical or quasi-spherical shapes due to the stabilizing effects of cellular proteins and peptides.

Extracellular synthesis involves the secretion of reducing agents and stabilizing molecules into the surrounding medium, where metal ions are reduced to nanoparticles. Bacillus species, including Bacillus subtilis and Bacillus licheniformis, are widely studied for extracellular synthesis. These bacteria secrete enzymes like nitrate reductases and peptides that reduce metal ions extracellularly, producing nanoparticles with sizes between 10 and 100 nm. The extracellular route simplifies downstream processing since nanoparticles do not require cell disruption for extraction. The morphology of extracellularly synthesized nanoparticles can vary from spherical to triangular or hexagonal, influenced by pH, temperature, and the concentration of metal ions.

Species-specific pathways highlight the diversity of bacterial nanoparticle synthesis. For instance, Pseudomonas aeruginosa produces silver nanoparticles through the action of pyocyanin, a redox-active secondary metabolite, resulting in particles with potent antimicrobial properties. Lactobacillus strains, on the other hand, synthesize gold nanoparticles using cell wall-associated enzymes, yielding particles with uniform dispersity and sizes below 20 nm. These variations underscore the importance of selecting bacterial strains based on desired nanoparticle characteristics.

Metabolic involvement is central to the synthesis process. Aerobic bacteria often rely on NADH-dependent reductases to transfer electrons to metal ions, while anaerobic bacteria like Clostridium species use fermentative pathways to generate reducing equivalents. The presence of specific electron donors, such as glycerol or glucose, can enhance reduction rates and nanoparticle yield. For example, Escherichia coli cultured in media supplemented with glycerol produces silver nanoparticles with higher monodispersity compared to glucose-based media. The metabolic state of bacteria, whether in log or stationary phase, also affects nanoparticle properties, with stationary-phase cultures often yielding smaller particles due to slower reduction kinetics.

Nanoparticle properties are finely tuned by synthesis conditions. Gold nanoparticles synthesized by Rhodopseudomonas capsulata exhibit size variations from 10 to 50 nm when the pH is adjusted from 7 to 9, while silver nanoparticles produced by Klebsiella pneumoniae show a shift from spherical to rod-like structures at higher temperatures. The presence of biomolecules like proteins and polysaccharides acts as capping agents, preventing aggregation and stabilizing nanoparticles in colloidal suspensions. These biomolecular coatings also confer biocompatibility, making the nanoparticles suitable for biomedical applications.

Scalability remains a challenge in bacterial nanoparticle synthesis. Batch-to-batch variability, dependence on bacterial growth conditions, and the need for sterile environments complicate large-scale production. Continuous bioreactor systems have been explored to improve yield and consistency, with some success in extracellular synthesis routes. However, intracellular synthesis faces hurdles in scaling due to the additional steps required for cell lysis and nanoparticle purification. Optimizing bacterial growth media and metal ion concentrations can enhance productivity, but economic viability must be balanced against the cost of traditional synthesis methods.

In biomedicine, bacterially synthesized nanoparticles show promise as antimicrobial agents, drug carriers, and imaging probes. Silver nanoparticles produced by Bacillus species exhibit broad-spectrum antimicrobial activity against pathogens like Staphylococcus aureus and Escherichia coli, with minimal cytotoxicity to human cells. Gold nanoparticles from Shewanella oneidensis are employed in photothermal therapy due to their strong plasmonic absorption in the near-infrared region. The biocompatibility of these nanoparticles, coupled with their functionalizable surfaces, enables targeted drug delivery and diagnostic applications.

Sensing applications leverage the unique optical and electronic properties of bacterially synthesized nanoparticles. Quantum dots produced by Pseudomonas aeruginosa display tunable fluorescence for heavy metal detection in environmental samples. Similarly, silver nanoparticles synthesized by Lactobacillus strains serve as substrates for surface-enhanced Raman spectroscopy (SERS), enhancing the detection of trace analytes. The integration of these nanoparticles into portable sensors offers rapid and sensitive monitoring tools for healthcare and environmental safety.

The green synthesis of nanoparticles using bacteria represents a convergence of microbiology, nanotechnology, and materials science. By harnessing bacterial metabolism, researchers can produce nanoparticles with controlled properties while adhering to principles of sustainability. Future advancements in genetic engineering and process optimization will address scalability challenges, expanding the industrial and commercial potential of this approach. The intersection of bacterial synthesis with emerging technologies like AI-driven strain selection and automated bioreactors promises to further refine nanoparticle production, unlocking new possibilities in nanotechnology.