Biosynthesis of fluorescent CdS nanocrystals with semiconductor properties: Comparison of microbial and plant production systems

• Fission yeast and hairy roots of tomato were compared as in vitro production systems for biological synthesis of CdS quantum dots.
• The rate of production and yield of CdS nanoparticles was lower in the hairy root cultures compared with yeast.
• Yeast-produced CdS nanocrystallites were smaller than those produced by hairy roots.
• Hairy roots produced quantum dots of higher fluorescence quantum efficiency and greater photostability than those generated by yeast.
• Choice of cellular host had a significant effect on the yield, production rate and functional properties of biologically produced quantum dots.

This study investigated fission yeast (Schizosaccharomyces pombe) and hairy roots of tomato (Solanum lycopersicum) as in vitro production vehicles for biological synthesis of CdS quantum dots. Cd added during the mid-growth phase of the cultures was detoxified within the biomass into inorganic sulphide-containing complexes with the quantum confinement properties of semiconductor nanocrystals. Significant differences were found between the two host systems in terms of nanoparticle production kinetics, yield and quality. The much slower growth rate of hairy roots compared with yeast is a disadvantage for commercial scaled-up production. Nanoparticle extraction from the biomass was less effective for the roots: 19% of the Cd present in the hairy roots was recovered after extraction compared with 34% for the yeast. The overall yield of CdS quantum dots was also lower for the roots: relative to the amount of Cd taken up into the biomass, 8.5% was recovered in yeast gel filtration fractions exhibiting quantum dot properties whereas the result for hairy roots was only 0.99%. Yeast-produced CdS crystallites were somewhat smaller with diameters of approximately 2–6 nm compared with those of 4–10 nm obtained from the roots. The average ratio of inorganic sulphide to Cd for the purified and size-fractionated particles was 0.44 for the yeast and 1.6 for the hairy roots. Despite the limitations associated with hairy roots in terms of culture kinetics and product yield, this system produced CdS nanoparticles with enhanced photostability and 3.7–13-fold higher fluorescence quantum efficiency compared with those generated by yeast. This work demonstrates that the choice of cellular host can have a significant effect on nanoparticle functional properties as well as on the bioprocessing aspects of biological quantum dot synthesis.