A Greener Way to Create Quantum Dots

A method for manufacturing quantum dots (QDs) from a single enzyme may provide a simple, cost-effective and environmentally friendly way to produce these semiconducting nanocrystals, prized for their optical and electronic properties. Current chemical manufacturing methods for QDs are time-consuming and often require toxic solvents.

The discovery of a biological method to manufacture QDs using a single enzyme began when Lehigh University professors Bryan Berger, Chris Kiely and Steven McIntosh were asked to investigate a bacteria that was growing on metal surfaces at a nearby hospital. The team found that the bacteria seemed to be taking in electrical charges from the metal surfaces and expelling clusters of metal particles. They further found that the bacteria could be altered to produce cadmium sulphide QDs, and that a single enzyme produced by the bacteria was responsible for the QD generation.

Tubes filled with quantum dots produced in the Lehigh University lab using a single enzyme. Courtesy of Christa Neu/Lehigh University Communications & Public Affairs.

The team engineered the enzyme to not only make the crystal structure of the QDs, but to also control their size, thus controlling the uniformity of the QDs that are produced. The team has also demonstrated that the cadmium sulphide QDs can be generated with the same enzyme synthesized from other easily engineered bacteria such as E. coli. This dual-function, single-enzyme route to functional material synthesis shows potential for engineered functional material biomineralization, leading to the scalable and green biomanufacturing of functional nanomaterials.

Industrial processes for growing QD nanocrystals take many hours, and are then followed by additional processing and purifying steps. Biosynthesis, on the other hand, takes minutes to a few hours maximum to make the full range of QD sizes (about two to three nanometers) in a continuous, environmentally friendly process at ambient conditions in water that needs no post-processing steps to harvest the final, water-soluble product.

Perfecting the methodology to structurally analyze individual nanoparticles in the QD required a highly sophisticated scanning transmission electron microscope (STEM). Lehigh's Electron Microscopy and Nanofabrication Facility was able to provide a $4.5 million state-of-the-art instrument that allowed the researchers to examine the structure and composition of each QD, which is composed of tens to hundreds of atoms.

The instrument scans an ultra-fine electron beam across a field of QDs. The atoms scatter the electrons in the beam, producing a kind of shadow image on a fluorescent screen, akin to the way an object blocking light produces a shadow on the wall. A digital camera records the highly magnified atomic resolution image of the nanocrystal for analysis.

The team is poised to scale-up its laboratory success into a manufacturing enterprise making inexpensive QDs in an eco-friendly manner. Conventional chemical manufacturing costs $1,000 to $10,000 per gram. A biomanufacturing technique could potentially slash the price by at least a factor of 10. Taking a long view, the engineers hope that their method will lead to a plethora of future QD applications, such as greener manufacturing of methanol, an eco-friendly fuel that could be used for transportation, heating and lighting.

Ultimately, the team would like to make large-scale functional materials as well as individual quantum dots. They envision developing a process by which individual quantum dots arrange themselves into macrostructures, such as a solar cell, similar to the way nature grows a mollusk shell out of individual inorganic nanoparticles or humans grow artificial tissue in a lab.

The research was published in Proceedings of the National Academy of Sciences.