Hecht lab proposes a novel method of producing quantum dots

For the first time in history, the production of semiconductor quantum dots has been achieved at room temperature using lab-designed proteins, Princeton chemists reported in a paper published in the Proceedings of the National Academy of Sciences. The work opens the door to more economical and environmentally friendly production of quantum dots.

Quantum dots are fluorescent particles that are a few nanometers in size with optical and electronic properties different from those of larger particles. They are used in a variety of electronic applications, such as LED screens and solar panels.

In general, color originates from a particular pigment, which has color because it absorbs specific wavelengths of light while reflecting others. The color of a pigment is the same regardless of the size of a sample.

Quantum dots are different in that the color does not depend on the pigment itself but rather on the size of the particle. Namely, due to atomic-level effects, a larger quantum dot will emit red light while a smaller one emits blue light. If we shine light on two crystals with the same chemical composition but particles of different sizes, they will fluoresce in different colors.

Quantum dots are usually produced at high temperatures and with toxic, expensive solvents. In this study, however, Princeton chemists have achieved stable quantum dots at room temperature using water as a solvent. The key to their work is the use of newly created, or de novo, proteins as the catalyst to speed up the reaction that yields the quantum dots.

“Nature evolved proteins using natural amino acid sequences. We then went a step further and asked, can we make novel amino acid sequences? And then can we make them do things that are life-sustaining? Okay, we did that,” said Michael Hecht, Professor of Chemistry in an email to The Daily Princetonian.

“But this recent work with the quantum dots is about whether we can make novel proteins that don’t exist in nature and ask them to perform functions that don’t exist in biology because living systems don’t make quantum dots,” Hecht added .

The protein the team used is Construct K (ConK), which is a de novo protein that enables the survival of E. Coli in otherwise toxic concentrated copper environments. ConK was first isolated at Hecht’s lab in 2015 from a large combinatorial library of proteins.

ConK’s ability to rescue E. Coli in toxic concentrations of copper suggested to the researchers that it might be useful for metal binding. The quantum dots that the chemists worked with were made of an inorganic compound called cadmium sulfide, which can be obtained by a reaction that binds metallic cadmium with sulfide ions. The researchers wondered if ConK might help produce cadmium sulfide quantum dots.

Their hypothesis was correct.

ConK catalyzes the production of hydrogen sulfide from cysteine, one of the 20 naturally occurring amino acids. Hydrogen sulfide is a reactive sulfur source that can react with metallic cadmium. The end product is quantum dots made of cadmium sulfide.

One of the main challenges that the research team faced in this project was to figure out the mechanism through which ConK functions.

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“We chose [ConK] because we knew it can enable E. Coli to grow in a medium that has a very high concentration of toxic metals. Therefore, we thought it might be producing some counter ions to precipitate the metal and make it not be fully toxic to E. Coli,” Yueyu Yao, co-author on the paper and a graduate student in Hecht’s lab, said in an interview with the ‘Prince.’

“However, what reaction it actually does, or how it does so, is totally unknown. So we knew that from the beginning [ConK] can help make quantum dots, but we also needed to figure out why and how,” she added.

“I think what is cool about this type of work is that it is really interdisciplinary, and it happens when you bring people with different backgrounds together,” Leah Spangler, the paper’s lead author, told the ‘Prince.’

Spangler was a postdoctoral fellow at the laboratory of Gregory Scholes, the William S. Tod Professor of Chemistry, at the time of this research, and is now an assistant professor at the Virginia Commonwealth University, where she is continuing the interdisciplinary work she started at Princeton.

Both Spangler and Hecht said there are exciting potential future applications of their research.

“A way to extend this work in the future is by starting to look for ways to enhance the quality of the quantum dots, maybe using de novo proteins as binding to the quantum dots and enhancing energy transfer out of the quantum dots for applications,” Spangler said.

“Also, we can look at different types of materials. So can we go beyond sulfides? Or are there other proteins that we can use? So those are the most directly related extensions of the current work,” she added.

Hecht said that the reaction conditions could allow for the method to be applied in an industrial setting.

“Here we have a biological method that works in aqueous solutions at room temperature, which is very different from the standard commercial methods,” Hecht said. “We now have an environmentally friendly way of making quantum dots. That is a nicer way to do it than current industrial methods,” he added.

However, scaling up the process remains difficult.

“It’ll be a while until we’re able to scale up [the process] to anything that would actually be useful on an industrial scale,” Hecht said. “But conceptually, we could do that.”

Mahya Fazel-Zarandi is a staff News writer for the ‘Prince.’

Please direct any correction requests to corrections[at]dailyprincetonian.com.

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