Genetic engineering, the deliberate resequencing of an organism's DNA, has many useful purposes. It has been used to impart pesticide and drought resistance in crops, to produce plant-based vaccines, and to enable animals to produce pharmaceutical or other molecules in their milk. It has yielded and continues to yield very useful results, in the laboratory as well as for the public benefit.
A genetically modified organism is typically rigged to produce synthetic molecules, or an unnatural amount of a protein. Now, far more complex and valuable materials can be produced, unlike anything observed in nature. Scientists at the University of California (Riverside) have genetically rejiggered bacteria to synthesize quantum dots.
What are quantum dots?
Quantum dots are very useful synthetic fluorescent metal nanoparticles. They enable simultaneous tracking of multiple processes in living cells, simultaneous disease diagnosis using only one sample, and other useful applications.
Unfortunately, many current methods to synthesize quantum dots are very toxic, dangerous, and/or expensive. The UC scientists have addressed this problem by turning bacteria into nanoparticle production factories.
Rigging the bacteria.
Escherichia coli (E. coli) bacteria were genetically engineered to produce peptide molecules known as phytochelatins. These peptides trap cadmium molecules and shuttle them to regions of the cell known as vacuoles.
Once there, the cadmium molecules react with sulfur molecules, producing cadmium sulfide quantum dots. The phytochelatin peptide template serves to control the eventual size of the quantum dots.
The scientists added cadmium chloride to a suspension of E. coli bacteria during their exponential growth stage, upon which time they took up the cadmium molecules. After three hours, sodium sulfide was added, and the bacteria were then able to synthesize the quantum dots. After one hour, the cells were torn apart, and the quantum dots were isolated from the ruptured cells by anion-exchange chromatography.
Quality of the quantum dots.
It is important that the diameter of the quantum dots be as uniform as possible. This is because the fluorescence properties of quantum dots strongly depend on their diameter. A small diameter change can, for example, change fluorescence from red to blue.
High-resolution transmission electron microscopy demonstrated that the quantum dots were 2-6 nm in diameter, a large diameter range due to the heterogeneous population of phytochelatin peptides used by the bacteria for synthesis. However, decreasing the amount of cadmium chloride added and switching to another bacterial strain (that was genetically engineered to produce primarily only one type of phytochelatin peptide) enabled production of more uniform quantum dots, 3-4 nm diameter.
Unfortunately, the quantum yield of the quantum dots was low, 0.007%. This means that much more light is used to turn on fluorescence than is emitted. However, this is not a huge limitation, because shining a lot of light on quantum dots does not destroy them, as is the case with small synthetic fluorescent molecules.
Outlook.
These UC scientists have rejiggered bacteria to produce quantum dots of a relatively uniform size distribution. There is no risk of these bacteria "escaping from the laboratory" and suffocating the world in quantum dots, because they can only synthesize the nanoparticles if the appropriate concentration of cadmium and sulfur are present, and cadmium is not a common molecule. This is an exciting example of reworking the innards of bacteria to safely produce highly valued and otherwise difficult to prepare materials for physicists, chemists, and biologists.
for more information:
Kang, S. H.; Bozhilov, K. N.; Myung, N. V.; Mulchandani, A.; Chen, W.
Microbial synthesis of CdS nanocrystals in genetically
engineered E. coli.
Angew. Chem. Int. Ed. 2008, 47, 5186-5189.