Putting hypersensitive quantum sensors into a living cell holds high promise for tracking cell growth and diagnosing diseases, as well as cancers, in their early stages. Although many of the best, most powerful quantum sensors can be created in small bits of diamond, it is difficult to place a diamond in a cell and get it to work. “People have used diamond nanocrystals as biosensors before, but they discovered that they perform worse than what we would expect. Significantly worse,” said Uri Zvi, a Ph.D. candidate at the University of Chicago. Researchers from the University of Chicago Pritzker School of Molecular Engineering and the University of Iowa have united insights from cellular biology, quantum computing, semiconductors, and high-definition TVs to create a new quantum biosensor. In doing so, the collaborators have shed light on a longstanding mystery in quantum materials. University of Chicago Pritzker School of Molecular Engineering assistant professor Peter Maurer (left) and first author and Ph.D. candidate Uri Zvi. The researchers’ work has led to a more efficient quantum nanoparticle that can be used to track and monitor living cells. Courtesy of University of Chicago/Jason Smith. “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 first author Zvi. 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, as opposed to a bigger diamond particle which exhibits more ideal coherent and quantum properties. To get around this, the researchers took inspiration from the early days of quantum dot LED (QD-LED) TVs, in which colors were bright, but unstable — prone to suddenly blinking off. “Researchers found that surrounding the quantum dots with carefully designed shells suppresses detrimental surface effects and increase their emission,” Zvi said. The researchers developed a silicon-oxygen (siloxane) shell that would enhance the quantum properties without activating the immune system's defense response. The approach saw a 4× improvement in spin coherence, as well as a 1.8× increase in fluorescence and separate significant increases to charge stability. Further testing traced these increases to the siloxane, which both protected the diamond surface but fundamentally altered the quantum behavior inside. According to the researchers, the material interface was driving electron transfer from the diamond into the shell. Depleting electrons from the atoms and molecules that normally reduce the quantum coherence made a more sensitive and stable way to read signals from living cells. “The end [effect] is not just a better sensor, but a new, quantitative framework for engineering coherence and charge stability in quantum nanomaterials,” Zvi said. The discovery enabled the team to identify the specific surface sites that degrade coherence and make quantum devices less effective, opening new doors for both engineering innovation and fundamental research in the quantum sensing field. The research was published in Applied Physical Sciences (www.doi.org/10.1073/pnas.2422542122).