Typically, hazardous, expensive solvents and high temperatures are used in industrial settings to create quantum dots; this method is neither cost-effective nor ecologically benign. However, a research team in a new study was able to carry out the procedure in the lab using water as a solvent, producing an end product that was stable at ambient temperature.
Michael Hecht, professor of chemistry in collaboration with his research group at Princeton University, has discovered first known de novo protein that catalyzes, or drives, the synthesis of quantum dots. By demonstrating that protein sequences not found in nature may be used to create functional materials, their work opens the door to producing nanomaterials in a more environmentally friendly manner.
By demonstrating that protein sequences not found in nature may be used to create functional materials, their work opens the door to producing nanomaterials in a more environmentally friendly manner.
Michael Hecht, who led the research with Greg Scholes, the William S. Tod Professor of Chemistry and department chair, said, “We’re interested in making life molecules, proteins, that did not arise in life. In some ways, we’re asking, are there alternatives to life as we know it? All life on earth arose from common ancestry. But if we make lifelike molecules that did not arise from common ancestry, can they do cool stuff?”
“So here, we’re making novel proteins that never arose in life doing things that don’t exist.”
The method developed by the scientists allows for fine-tuning of nanoparticle size, which affects the color that quantum dots fluoresce. This opens up opportunities for labeling chemicals present in biological systems, such as in vivo staining of cancer cells.
Because of their sizes, quantum dots have very interesting optical properties: They’re very good at absorbing light and converting it to chemical energy. These properties make them useful for solar panels and photo sensors. Plus, as they can efficiently emit light at a certain desired wavelength, they can be used in making LED screens.
Hecht said, “And because they’re small — composed of only about 100 atoms and maybe 2 nanometers across — they’re able to penetrate some biological barriers, making their utility in medicines and biological imaging especially promising.”
Leah Spangler, lead author on the research and a former postdoc in the Scholes Lab, said, “I think using de novo proteins opens up a way for designability. A key word for me is ‘engineering.’ I want to be able to engineer proteins to do something specific, and this is a type of protein you can do that with.”
“The quantum dots we’re making aren’t great quality yet, but that can be improved by tuning the synthesis. We can improve quality by engineering the protein to influence quantum dot formation differently.”
The scientists employed a de novo protein it created called ConK to catalyze the process based on work done by corresponding author Sarangan Chari, a senior chemist in Hecht’s group. In 2016, scientists extracted ConK for the first time from a sizable combinatorial library of proteins. It still contains natural amino acids, but because its sequence differs significantly from a natural protein, it is said to be “de novo.”
Scientists found that ConK enabled the survival of E. coli in otherwise toxic concentrations of copper, suggesting it might be useful for metal binding and sequestration. The quantum dots used in this research are made out of cadmium sulfide. Cadmium is a metal, so researchers wondered if ConK could be used to synthesize quantum dots.
Spangler said, “The hunch paid off. ConK breaks down cysteine, one of the 20 amino acids, into several products, including hydrogen sulfide. That acts as the active sulfur source that will then react with the metal cadmium. The result is CdS quantum dots.”
“To make a cadmium sulfide quantum dot, you need the cadmium source and the sulfur source to react in solution. What the protein does is make the sulfur source slowly over time. So, we add the cadmium initially, but the protein generates the sulfur, which then reacts to make distinct sizes of quantum dots.”