Characterization of biogenic selenium nanoparticles in hypersaline media by single particle inductively coupled plasma mass spectrometry: Haloferax mediterranei case

Background: Single particle inductively coupled plasma mass spectrometry (spICP-MS) is extensively employed for the characterization of biogenic selenium nanoparticles (SeNPs) produced by mesophilic microorganisms. Nevertheless, because halophilic microorganisms are also well-known to produce SeNPs, further research efforts are required to investigate spICP-MS applicability for characterizing such nanomaterials in hypersaline media. The goal of this work is to develop a methodology for characterizing SeNPs in hypersaline media by spICP-MS. To this end, plasma operating conditions, non-spectral interferences and calibration strategies were investigated. The proposed method was employed to investigate the capabilities of the halophilic archaea Haloferax mediterranei to produce SeNPs.

Results: By the appropriate selection of experimental conditions, SeNPs can be accurately analyzed in hypersaline media by spICP-MS. Unlike previous works in the literature, no differences in ionic signal were observed between SeNPs and dissolved Se and, hence, there is no need to apply any empirical corrector factor for obtaining accurate particle size distributions. Non-spectral interferences are mitigated by diluting the sample at least 1:10³ which allows the use of water standards. Size (30 nm) and particle (7 × 10⁵ particles mL^-1) detection limits were low enough to characterize biogenic SeNPs produced by halophilic microorganisms. The use of the optimized methodology reveals that Haloferax mediterranei can produce SeNPs when it is exposed to selenite up to 1 mM, but no formation is produced for selenate exposure. Depending on incubation parameters (selenite concentration and time), the particle median diameter ranged from 80 to 100 nm, whereas particle concentration varied from 0.8 to 1.9 × 10¹³ particles mL^-1.

Significance: This represents the first methodology for characterizing biogenic SeNPs in hypersaline media by spICP-MS with accuracy and precision using non-matrix matched standards. It opens the opportunity to investigate the capabilities of halophilic microorganisms (e.g., H. mediterranei) to produce Se-based nanomaterials.