SEM images of (left) Control tomato root tip. (right) Tomato root tip after 14 days exposure to 100µm CdSO4. Note reduction in length of root hairs and damage to root cap. Dimension markers indicate 200µm.

New applications of quantum nanoparticles are being developed in areas such as medical imaging, diagnostics, targeted therapies and microelectronics. Commercial expansion of semiconductor quantum nanocrystal technology is limited by difficulties with mass production. The functional properties of quantum dots depend directly on quantum confinement effects, therefore, for the optimal development of applications close control must be exercised over the particle size.

Quantum dots are most commonly produced using chemical methods involving crystal nucleation in aqueous or organic solvents. However, using this approach it is difficult to produce monodisperse nanocrystals cheaply, reproducibly and in large quantities while preventing particle instability and aggregation. A potential solution to these problems is the biological production of quantum dots. As microscopic reaction chambers, cells provide the space confinement conditions that permit nanoparticles of restricted size to be generated.
Our multidisciplinary research has found that hairy root cultures of tomato produce II-VI semiconductor cadmium sulphide (CdS) nanocrystals in response to exposure to CdSO4. When faced with an excess of heavy metals in the environment plants may either attempt to avoid the stress by restricting the uptake of metals through the root or attempt to minimize the toxic effects of the metal ions within the cell. The treated tomato hairy roots complex the toxic cadmium with sulphur and then cap (i.e. biofunctionalise) the resultant nanocrystals with chelating peptides. Exposure to CdSO4 has also been found to have a significant and detrimental effect on the ultrastructure of the hairy root culture, changing root morphology, damaging cell structures and disturbing water balance and mineral nutrition, etc. Work is continuing to characterise the nanocrystals using advanced microanalytical techniques and optimize their production of the nanocrystals to control their useful properties.

Katie Levick and Marion Stevens-Kalceff (Physics)
Hongqing Zhao and Pauline Doran (BABS)