Sketch of the multiple analyte−emitter complexes randomly distributed surrounding the hemisphere dimer. The emitter label inside the gap center is strongly coupled with plasmonic resonance, while the others remain weakly coupled.

© A*STAR’s Institute of High Performance Computing (IHPC)

A quantum leap for biosensors

3 Aug 2020

A biosensor designed with quantum properties is 15 times more sensitive than classical sensors.

Easing COVID-19 lockdown restrictions and returning to a semblance of normal life will require numerous safety precautions to be put in place. Antigen testing, in particular, has been touted as a key measure for easing lockdowns, with businesses and governments even mulling over the use of ‘immunity certificates.’

Quantifying the levels of antigens in a sample is currently carried out using a method called an immunoassay, where a target molecule is captured by an antibody specific for it. One variant of this method, plasmonic immunoassay, detects altered plasmon oscillation and changes in reflected light when a target molecule is captured onto a metal surface.

At infinitesimal quantities of a target molecule approaching the detection limit, however, frequency shifts become subtler and harder to detect. Now, scientists led by Lin Wu from A*STAR’s Institute of High Performance Computing (IHPC) and Ortwin Hess from Imperial College London have found a way to enhance the sensitivity of classical plasmonic immunoassays by applying principles based on quantum mechanics.

In their simulation of a quantum plasmonic immunoassay, a metal surface displays a pair of gold nanohemispheres. Each pair—or nanodimer—is separated by a one-nanometer space where target molecule binding occurs. This space functions as a nanoscopic lens that focuses light on a nano-hotspot.

“At such a hotspot, light-matter interaction is enhanced so drastically that the system enters the strong-coupling regime, where the coupling exceeds dissipative rates,” explained Wu.

Detection in their assay relies on a phenomenon called Rabi splitting, which is much more sensitive than classical sensing and no longer dependent on the concentration of the captured target molecule, Wu said. Through computational experiments, the researchers found a massive sensitivity boost of nearly 15 times compared to conventional plasmonic sensors.

“To the best of our knowledge, our paper describes the first proposal of a quantum plasmonic biosensor that is sensitive enough to detect single molecules,” Wu said.

This method of strong coupling could also be used in applications, such as monitoring air quality and food safety, as it would be possible to detect single molecules and conduct quantum sensing at room temperature. Wu acknowledged that their assay design remains difficult to achieve in reality at this point, but she hoped that their work would inspire advances in plasmonic immunoassay sensors.

“Moving forward, our research team at IHPC will continue collaborating with our colleagues at A*STAR and at the local universities to find innovative solutions for plasmonic applications, such as next-generation electronics and ultra-high sensitivity biosensors,” she said.

The A*STAR-affiliated researcher contributing to this research is from the A*STAR Institute of High Performance Computing (IHPC).

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Kongsuwan, N., Xiong, X., Bai, P., You, J., Png, C.E., et al. Quantum Plasmonic Immunoassay Sensing. Nano Letters 19: 5853-5861 (2019) | article

About the Researcher

Lin Wu received a degree in electrical and electronic engineering (first class honors) in 2005, and a PhD degree in microelectronics in 2009, from Nanyang Technological University, Singapore. In 2009, she joined A*STAR’s Institute of High Performance Computing (IHPC) as a computational scientist. Her research interests include nanoplasmonics and nanophotonics and their emerging applications in quantum technology and sensing. Wu has authored or co-authored two book chapters and published more than 50 refereed journal papers. She is also a co-author on three US patents.

This article was made for A*STAR Research by Wildtype Media Group