Use of acidophilic bacteria of the genus Acidithiobacillus tobiosynthesize CdS fluorescent nanoparticles (quantum dots) withhigh tolerance to acidic pH

Abstract

The use of bacterial cells to produce fluorescent semiconductor nanoparticles (quantum dots, QDs) rep-resents a green alternative with promising economic potential. In the present work, we report for thefirst time the biosynthesis of CdS QDs by acidophilic bacteria of the Acidithiobacillus genus. CdS QDs wereobtained by exposing A. ferrooxidans, A. thiooxidans and A. caldus cells to sublethal Cd2+concentrations inthe presence of cysteine and glutathione. The fluorescence of cadmium-exposed cells moves from greento red with incubation time, a characteristic property of QDs associated with nanocrystals growth. Biosyn-thesized nanoparticles (NPs) display an absorption peak at 360 nm and a broad emission spectra between450 and 650 nm when excited at 370 nm, both characteristic of CdS QDs. Average sizes of 6 and 10 nmwere determined for green and red NPs, respectively. The importance of cysteine and glutathione on QDsbiosynthesis in Acidithiobacillus was related with the generation of H2S. Interestingly, QDs produced byacidophilic bacteria display high tolerance to acidic pH. Absorbance and fluorescence properties of QDswas not affected at pH 2.0, a condition that totally inhibits the fluorescence of QDs produced chemicallyor biosynthesized by mesophilic bacteria (stable until pH 4.5–5.0). Results presented here constitute thefirst report of the generation of QDs with improved properties by using extremophile microorganisms.