Phosphate Favors the Biosynthesis of CdS Quantum Dots in Acidithiobacillus thiooxidans ATCC 19703 by Improving Metal Uptake and Tolerance

Abstract

Recently, we reported the production of Cadmium sulfide (CdS) fluorescent semiconductor nanoparticles (quantum dots, QDs) by acidophilic bacteria of the Acidithiobacillus genus. Here, we report that the addition of inorganic phosphate to Acidithiobacillus thiooxidans ATCC 19703 cultures favors the biosynthesis of CdS QDs at acidic conditions (pH 3.5). The effect of pH, phosphate and cadmium concentrations on QDs biosynthesis was studied by using Response Surface Methodology (RSM), a multivariate technique for analytical optimization scarcely used in microbiological studies to date. To address how phosphate affects intracellular biosynthesis of CdS QDs, the effect of inorganic phosphate on bacterial cadmium-uptake was evaluated. By measuring intracellular levels of cadmium we determined that phosphate influences the capacity of cells to incorporate this metal. A relation between cadmium tolerance and phosphate concentrations was also determined, suggesting that phosphate participates in the adaptation of bacteria to toxic levels of this metal. In addition, QDs-biosynthesis was also favored by the degradation of intracellular polyphosphates. Altogether, our results indicate that phosphate contributes to A. thiooxidans CdS QDs biosynthesis by influencing cadmium uptake and cadmium tolerance. These QDs may also be acting as a nucleation point for QDs formation at acidic pH. This is the first study reporting the effect of phosphates on QDs biosynthesis and describes a new cadmium-response pathway present in A. thiooxidans and most probably in other bacterial species.

Biological Synthesis of CdS/CdSe Core/Shell Nanoparticles and Its Application in Quantum Dot Sensitized Solar Cells

“…The size of biosynthesized QDs was analyzed by DLS. Average hydrodynamic sizes of 12.7 and 16.7 nm were determined for CdS and CdS/CdSe QDs (Figure 4A), which is in agreement with the size previously determined for CdS QDs biosynthesized by different microorganisms (Kang et al, 2008; Gallardo et al, 2014; Ulloa et al, 2016, 2018). As expected, Core/Shell QDs are bigger than QDs composed only by the CdS core.…”

“…Consequently, in order to develop simpler an economical protocols for production with lower environmental impact, the use of microorganisms to biosynthesize these nanoparticles has gained attention (Li et al, 2011; Hulkoti and Taranath, 2014). In addition, it has been reported that biosynthesized QDs display unique properties such as increased acid and salt stability (Ulloa et al, 2016, 2018; Bruna et al, 2019). Until now, the microorganism most used for QDs biosynthesis is Escherichia coli , capable of producing CdS, CdSe, and CdTe QDs (Sweeney et al, 2004; Park et al, 2010; Monras et al, 2012; Yan et al, 2014).…”

Synthesis of salt-stable fluorescent nanoparticles (quantum dots) by polyextremophile halophilic bacteria

“…We mainly focus in extracellular biosynthesis since this method facilitates NPs purification and favors future applications. The capacity of all obtained isolates to biosynthesize QDs was evaluated following the protocol described before 4,16,26,27,36 . Only three cadmium-resistant isolates were able to biosynthesize cadmium-based QDs in the presence of NaCl up to 8%, as determined by fluorescence of cell supernatants (Fig.…”

“…To date, the biological production of QDs with novel or improved properties has been scarcely studied. Our group recently reported the biosynthesis of QDs with increased tolerance to acidic conditions by using acidophilic extremophile bacteria 35,36 . We also determined that metal resistant psychrotolerant bacteria isolated from Antarctica could synthesize Cd-QDs at low temperatures 26,27 .…”

“…Accordingly, we reported the biological synthesis of QDs at low temperatures using novel psychrotolerant bacterial strains isolated from Antarctica 26,27 . In addition, our group recently reported the production of fluorescent Cd-nanoparticles by acidophilic bacteria used in biomining operations 35,36 . Bacteria of the Acidithiobacillus gender ( A. ferrooxidans , A. thiooxidans and A. caldus ) were used to biosynthesize Cd-QDs under acidic conditions (pH 3,0).…”