Creation of quantum biosensor inspired by QLED TVs

Many of the most powerful quantum sensors can be created in bits of diamond, but that creates a separate issue, which is that it’s difficult to stick a diamond in a cell and get it to operate.

“All kinds of those processes that you really need to probe on a molecular level, you cannot use something very big. You have to go inside the cell. For that, we need nanoparticles,” said Uri Zvi, PhD candidate, University of Chicago Pritzker School of Molecular Engineering (UChicago PME). “People have used diamond nanocrystals as biosensors before, but they discovered that they perform worse than what we would expect. Significantly worse.”

Together with researchers from UChicago PME, Zvi combined learnings from cellular biology, quantum computing, semiconductors and high-definition TVs to create a new quantum biosensor.

By encasing a diamond nanoparticle with a specially engineered shell - a technique inspired by QLED televisions - the team created a quantum biosensor ideal for a living cell and also uncovered insights into how the material’s surface can be modified.

“It's already one of the most sensitive things on earth, and now they've figured out a way to enhance that further in a number of different environments,” said Zvi’s principal investigator, UChicago PME Prof. Aaron Esser-Kahn, a co-author of the paper.

Qubits hosted in diamond nanocrystals maintain quantum coherence even when the particles are small enough to be taken up by a living cell. But the smaller the diamond particles, the weaker the quantum signal.

“It excited people for a while that these quantum sensors can be brought into living cells and, in principle, be useful as a sensor,” said Peter Maurer, Assistant Professor, UChicago PME, and a co-author of the paper. “However, while these kinds of quantum sensors inside of a big piece of bulk diamond have really good quantum properties, when they are in nano diamonds, the coherent properties, the quantum properties, are actually significantly reduced.”

Zvi turned to quantum dot LED televisions for inspiration. QLED TVs use vibrant fluorescent quantum dots to broadcast in rich, full colours. In the early days of their creation, the colours were bright but unstable.

“Researchers found that surrounding the quantum dots with carefully designed shells suppresses detrimental surface effects and increases their emission,” said Zvi. “And today you can use a previously unstable quantum dot as part of your TV.”

Working collaboratively with Professor Dmitri Talapin, quantum dot expert at UChicago PME and Chemistry Department, Zvi reasoned that both sets of challenges - the quantum dots’ fluorescence and the nanodiamond weakened signal - originated with the surface state, and a similar approach might work.

But because the sensor is meant to go into a living body, not every shell would work. An immunoengineering expert Esser-Khan helped to develop a silicon-oxygen shell that would both enhance the quantum properties and not tip off the immune system that something is off.

“The surface properties of most of these materials are sticky and disordered in a way that the immune cells can tell it’s not supposed to be there. They look like a foreign object to an immune cell,” explained Esser-Kahn. “Siloxane-coated things look like a big, smooth blob of water. And so the body is much more happy to engulf and then chew on a particle like that.”

Previous efforts to improve the quantum properties of diamond nanocrystals using surface engineering had had limited success. The team only expected modest improvements but saw up to fourfold improvements in spin coherence.

That increase, alongside a 1.8 fold increase in fluorescence and separate significant increases to charge stability, was a riddle.

“I would try to go to bed at night but stay up thinking ‘What’s happening there? The spin coherence is getting better—but why?” said Denis Candido, Assistant Professor University of Iowa, and second author of the new paper. “I’d think ‘What if we do this experiment? What if we do this calculation?’ It was very, very exciting, and in the end, we found the underlying reason for the improvement of the coherence.”

The interdisciplinary team—bioengineer-turned-quantum-scientist Zvi, immunoengineer Esser-Kahn and quantum engineers Maurer and Talapin—brought Candido and University of Iowa Physics and Astronomy Professor Michael Flatté in to provide some of the theoretical framework for the research.

“What I found really exciting about this is that some old ideas that were critical for semiconductor electronic technology turned out to be really important for these new quantum systems,” said Flatté.

Adding the silica shell didn’t only protect the diamond surface but fundamentally altered the quantum behaviour inside. The material interface was driving electron transfer from the diamond into the shell. Depleting electrons from the atoms and molecules that normally reduce quantum coherence made a more sensitive and stable way to read signals from living cells.

This allowed the team to identify the specific surface sites that degrade coherence and make quantum devices less effective; solving a long-standing mystery in the field of quantum sensing and opening new doors for engineering innovation and research.

“The end impact is not just a better sensor, but a new, quantitative framework for engineering coherence and charge stability in quantum nanomaterials,” concluded Zvi.