Fluorescent Lead(IV) Sulfide Nanoparticles Synthesized by Idiomarina sp. Strain PR58-8 for Bioimaging Applications

The fabrication of nanoparticles by microorganisms presents a "green" method for generating biocompatible nanomaterials. We discovered the intracellular biosynthesis of fluorescent lead(IV) sulfide nanoparticles by the moderate halophile, Idiomarina sp. strain PR58-8. The bacterium tolerated up to 8 mM Pb(NO₃)₂ during growth. Non-protein thiols dose-dependently increased in response to metal exposure, which suggests they are involved in the growth of PbS₂ crystals and lead detoxification. Using X-ray diffraction, transmission electron microscopy (TEM), high-resolution TEM, and energy dispersive analysis of X-rays, the nanoparticles were characterized as spherical β-PbS₂ nanoparticles (PbS₂NPs) with a tetragonal crystal lattice, a crystallite domain size of 2.38 nm, and an interplanar distance of 0.318 nm. A narrow symmetric emission spectrum with a Gaussian distribution and an emission maximum at 386 nm was obtained when the particles were excited at 570 nm. The PbS₂NPs exhibited a large Stokes' shift (8,362 cm^-1) and a relatively high quantum yield (67%). These properties, along with fluorescence that was maintained in various microenvironments and their biocompatibility, make these nanoparticles excellent candidates for bioimaging. The particles were internalized by HeLa cells and evenly distributed within the cytoplasm, exhibiting their potential for in situ bioimaging applications. The "as-synthesized" lead(IV) sulfide nanoparticles may provide expanded opportunities for targeted bioimaging via modifying the surface of the particles.IMPORTANCE This article reports the intracellular synthesis of fluorescent lead(IV) sulfide nanoparticles (PbS₂NPs) by a microorganism. All previous reports on the microbial synthesis of lead-based nanoparticles are on lead(II) sulfide that exhibits near-infrared fluorescence, requiring expensive instrumentation for bioimaging. Bioimaging using PbS₂NPs can be achieved using routine epifluorescence microscopes, as it fluoresces in the visible range. The research on PbS₂ nanoparticles to date is on their chemical synthesis employing toxic precursors, extreme pH, pressure, and temperature, resulting in cytotoxic products. In this context, the synthesis of PbS₂ nanoparticles by Idiomarina sp. strain PR58-8, described in this work, occurs at ambient temperature and pressure and results in the generation of biocompatible nanoparticles with no hazardous by-products. The excellent fluorescence properties that these particles exhibit, as well as their abilities to easily penetrate the cells and evenly distribute within the cytoplasm, make them exceptional candidates for bioimaging applications. This study demonstrated the synthesis and fluorescence bioimaging application of microbially synthesized PbS₂ nanoparticles.