Biophysics and Beyond

Green biosynthesis of nanoparticles using microorganisms has emerged as a promising and environmentally sustainable alternative to conventional physicochemical synthesis methods, which frequently require hazardous chemicals, organic solvents, high temperatures and substantial energy inputs. Among microbial systems, extremophilic bacteria represent an important yet relatively under explored resource for nanoparticle production because of their ability to survive and remain metabolically active under extreme environmental conditions, including high temperature, salinity, acidity, alkalinity and pressure. Their unique physiological adaptations, enzymes, proteins, metabolites and metal-resistance mechanisms can facilitate metal-ion reduction, nucleation, nanoparticle growth, stabilization and surface functionalization. This review provides an overview of the green biosynthesis of nanoparticles using extremophilic bacteria, with particular emphasis on thermophilic and haloalkaliphilic microorganisms. The intracellular and extracellular mechanisms involved in microbial nanoparticle formation are discussed, highlighting the contribution of cellular biomolecules to nanoparticle synthesis and stabilization. Examples of extremophile-mediated production of silver nanoparticles (AgNPs) and selenium nanoparticles (SeNPs) are examined, together with their physicochemical properties and antimicrobial potential. In addition, magnetite (Fe₃O₄) nanoparticles are discussed in terms of their structural, physical, thermal, magnetic and optical properties, as well as conventional synthesis approaches including co-precipitation, solvothermal, wet-chemical reduction and micro-/nano-emulsion methods. Particular emphasis is placed on the potential advantages of using extremophilic bacteria for the sustainable synthesis and biological surface functionalization of Fe₃O₄ nanoparticles. Important characterization techniques, including UV-Vis spectroscopy, Fourier-transform infrared spectroscopy, transmission electron microscopy, dynamic light scattering and zeta-potential analysis, are also considered. Finally, challenges associated with reproducibility, nanoparticle size and morphology control, stability, purification, scalability and mechanistic understanding are addressed. Overall, extremophilic bacteria offer significant potential as green nanofactories for producing biologically functional nanoparticles with applications in biomedicine, environmental remediation, agriculture and other emerging fields.