Usually, hazardous, expensive solvents and high temperatures are used in industrial settings to create quantum dots; this method is neither cost effective nor environmentally benign. However, a research team in a new study was able to perform the procedure in the lab using water as the solvent, creating a final product that was stable at room temperature.

Michael Hecht, a professor of chemistry in collaboration with his research group at Princeton University, has discovered the first known de novo protein that catalyzes or directs the synthesis of quantum dots. By showing that protein sequences that do not occur in nature can be used to make functional materials, their work opens the door to producing nanomaterials in a more environmentally friendly way.

By showing that protein sequences that do not occur in nature can be used to make functional materials, their work opens the door to producing nanomaterials in a more environmentally friendly way.

Michael Hecht, who led the research with Greg Scholes, the William S. Tod Professor of Chemistry and Department Chair, said: “We are interested in making life molecules, proteins, that did not originate in life. In some ways we wonder: Are there alternatives to life as we know it? All life on Earth originated from common ancestors. But if we make lifelike molecules that don’t come from common ancestors, can they do cool things?”

“So here we’re making new proteins that never existed in life, while doing things that don’t exist.”

The method developed by the scientists makes it possible to fine-tune the size of nanoparticles, which influences the color that quantum dots fluoresce. This opens possibilities for labeling chemicals present in biological systems, such as in vivo staining of cancer cells.

Due to their size, quantum dots have very interesting optical properties: they are very good at absorbing light and converting it into chemical energy. These properties make them useful for solar panels and photo sensors. In addition, since they can efficiently emit light of 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 wide — they can break through some biological barriers, making their utility in medicine and biological imaging particularly promising.”

Leah Spangler, lead author of the study and 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 make proteins to do something specific, and this is a kind of protein that allows you to do that.”

“The quantum dots we make are not yet of great quality, but that can be improved by fine-tuning the synthesis. We can improve the quality by developing the protein in such a way that it influences the formation of quantum dots in a different way.”

The scientists used a de novo protein it created, called ConK, to catalyze the process based on work by corresponding author Sarangan Chari, a senior chemist in Hecht’s group. In 2016, scientists extracted ConK from a large combinatorial library of proteins for the first time. It still contains natural amino acids, but because the sequence differs significantly from a natural protein, it is called “de novo”.

Scientists found that ConK enabled E. coli survival in otherwise toxic copper concentrations, suggesting it could be useful for metal binding and sequestration. The quantum dots used in this research are made 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. This acts as the active sulfur source which will then react with the metal cadmium. The result is CdS quantum dots.”

“To make a quantum dot of cadmium sulfide, 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 first, but the protein generates the sulfur, which then reacts to make quantum dots of different sizes.

Magazine reference:

  1. Leah C. Spangler, Yueyu Yao et al. A de novo protein catalyzes the synthesis of semiconductor quantum dots. Proceedings of the National Academy of Sciences. DOI: 10.1073/pnas.2204050119