Nuclear waste containing radionuclides including selenium isotopes, Se⁷⁹, will be disposed of in future deep geological repositories. Due to the long lifetime of these radioisotopes, studies on the impact of microbial processes on their chemical speciation would contribute significantly to understanding the risks associated with these repositories. Here we report a green method for biogenic reduction of Se(IV), production of amorphous Se(0) (a-Se) nanospheres and their subsequent transformation to one-dimensional (1D) trigonal selenium (t-Se) nanostructures using a combination of methods (XRD, STEM/HAADF, HRTEM/EDX, ESEM, etc.). The bacterial strain used, Stenotrophomonas bentonitica, was isolated from Spanish bentonites considered as artificial barriers for future Spanish repositories. After 24 h of incubation, 30–200 nm sized biogenic individual a-Se nanospheres were synthesized and then started to coalesce, forming aggregates after 48 and 72 h of incubation. The 144 h sample presented a mixture of single crystal and polycrystalline 1D t-Se nanostructures with different shapes (e.g. nanowires, hexagonal, polygonal, etc.) and diameters of 30–400 nm, in addition to a-Se nanospheres. The HRTEM analysis showed that the 1D nanostructures presented different lattice spacings corresponding to the (100), (101) and (111) planes of t-Se. Thus, a-Se nanospheres were initially synthesized and then would transform into t-Se nanostructures. STEM/HAADF and ESEM revealed that the cells and their extracellular proteins play an important role in this transformation process. Due to the low solubility of t-Se nanostructures compared to that of a-Se nanospheres and Se(IV), the mobility of selenium in future repositories may be significantly reduced.

In this work, we obtained carbon dots from glucose or saccharose as the nucleation source and passivated them with branched polyethylenimines for developing dsRNA nanocomposites. The CDs were fully characterized using hydrodynamic analyses, transmission electron microscopy, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. The ζ potential determined that the CDs had positive charges, good electrophoretic mobility and conductivity, and were suitable for obtaining dsRNA nanocomposites. DsRNA naked or coated with the CDs were delivered to leaves of cucumber plants by spraying. Quantitation of the dsRNA that entered the leaves showed that when coated with the CDs, 50-fold more dsRNA was detected than when naked dsRNA. Moreover, specific siRNAs derived from the sprayed dsRNAs were 13 times more abundant when the dsRNA was coated with the CDs. Systemic dsRNAs were determined in distal leaves and showed a dramatic increase in concentration when delivered as a nanocomposite. Similarly, systemic siRNAs were significantly more abundant in distal leaves when spraying with the CD-dsRNA nanocomposite. Furthermore, FITC-labeled dsRNA was shown to accumulate in the apoplast and increase its entry into the plant when coated with CDs. These results indicate that CDs obtained by hydrothermal synthesis are suitable for dsRNA foliar delivery in RNAi plant applications.

The environmental conditions for the planned geological disposal of radioactive waste —including hyper-alkaline pH, radiation or anoxia—are expected to be extremely harsh for microbial activity. However, it is thought that microbial communities will develop in these repositories, and this would have implications for geodisposal integrity and the control of radionuclide migration through the surrounding environment. Nuclear waste contains radioactive isotopes of selenium (Se) such as ⁷⁹Se, which has been identified as one of the main radionuclides in a geodisposal system. Here, we use the bacterial species Stenotrophomonas bentonitica, isolated from bentonites serving as an artificial barrier reference material in repositories, to study the reduction of selenite (Se^IV) under simulated geodisposal conditions. This bacterium is able to reduce toxic Se^IV anaerobically from a neutral to alkaline initial pH (up to pH 10), thereby producing elemental selenium (Se⁰) nanospheres and nanowires. A transformation process from amorphous Se (a-Se) nanospheres to trigonal Se (t-Se) nanowires, through the formation of monoclinic Se (m-Se) aggregates as an intermediate step, is proposed. The lesser solubility of Se⁰ and t-Se makes S. bentonitica a potential candidate to positively influence the security of a geodisposal system, most probably with lower efficiency rates than those obtained aerobically.

Carbon dots (CDs) have been proposed for nucleic acid delivery in many biological applications. In this work we have obtained carbon dots by a hydrothermal synthesis method for developing nanocomposites with dsRNA. These CDs were produced using glucose or saccharose as the nucleation source and passivated with branched polyethyleneimines for conferring positive charges. Hydrodynamic analyses and transmission electron microscopy TEM showed that they sized on average 4 and 5 nm, depending on the sugar. The CDs were fluorescent and showed a peak at 468 nm when excited with UV light. Physicochemical characteristics of their surfaces were revealed by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR). The ζ potential determined that both types of CDs had positive charges as well as good electrophoretic mobility and conductivity. Coating of the CDs to dsRNA was efficient but did not protect from nucleases. DsRNA naked or coated with the CDs were delivered to leaves of cucumber plants by spraying at 2.5 bar. Quantitation of the dsRNA that entered the leaves showed that when coated with the CDs, 50-fold more dsRNA was detected than when naked dsRNA was applied. Moreover, specific siRNAs derived from the sprayed dsRNAs were 130 times more abundant when the dsRNA was coated with the CDs. Systemic dsRNAs were determined in distal leaves showing a dramatic increase in concentration when delivered as a nanocomposite. Similarly, systemic siRNAs were significantly more abundant in distal leaves when spraying with the CD-dsRNA nanocomposite. Furthermore, FITC-labeled dsRNA was shown to accumulate in the apoplast and increase its entry in the plant when coated with CDs. These results indicate that CDs obtained by hydrothermal synthesis are suitable for dsRNA foliar delivery in RNAi plant applications and provide insight into CD-dsRNA translocations.