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Friday, February 3, 2023

Designer proteins drive the growth of quantum dots

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In 2016 researchers introduced collections of artificial DNA sequences into Escherichia coli and tested how the transformed cells survived in a stressful environment with toxic copper levels. The genetic codes led E. coli to produce novel proteins that were designed by researchers rather than evolved naturally. (For more on designer proteins, see Physics Today, June 2020, page 17.) One such de novo protein, a bundle of four helices known as Construct K (ConK), helped the cells withstand copper levels twice those that are fatal to an unaltered cell. The reason why was unclear, but the observation suggested that the protein transforms metal ions into stable complexes.

Now Michael Hecht of Princeton University and his colleagues have shown that ConK catalyzes the growth of cadmium sulfide quantum dots (QDs). Those nanocrystals are efficient light absorbers and emitters, and they’re promising for applications such as LEDs, in addition to displaying interesting physics because of their effective zero-dimensionality.

Three vials filled with blue, yellow, and orange liquid, respectively from left to right, which shows increasing growth time
Credit: L. C. Spangler et al., Proc. Natl. Acad. Sci. USA 119, e2204050119 (2022)

QDs are usually grown at high temperatures and with toxic, expensive solvents. The researchers’ new technique instead happens at room temperature and uses water as a solvent. After sitting for 10–48 hours, depending on the ingredient concentrations, a mixture that includes ConK, cysteine, and cadmium chloride produces distinct absorbance peaks associated with CdS QDs. The process relies on ConK facilitating the breakdown of cysteine into several parts, including hydrogen sulfide, which then reacts with Cd.

The incubation time tunes the size of the QDs. The fluorescence wavelength depends on that size, so the growth is easy to track by the solution’s color over time, as shown in the photo. The process can be stopped at the desired nanocrystal size. Eventually the QDs stop growing on their own and stabilize at around 3 nm in diameter.

A similar process using a natural protein has also been shown to catalyze the growth of QDs and to do so much faster, at less than 30 minutes. Although the de novo–protein process is slower, the gradual growth prevents the dots from clumping together and losing their desirable properties, as happens in other growth techniques.

The quality of the protein-grown QDs still has room for improvement; for example, they emit light with less than a hundredth the efficiency of those produced through conventional industrial methods. In the future, researchers can tinker further with the protein’s genetic sequence to improve the performance. (L. C. Spangler et al., Proc. Natl. Acad. Sci. USA 119, e2204050119, 2022.)

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