The development of a compact, self-illuminating plasmonic sensor could make high-performing optical biosensors more accessible for rapid diagnostics and environmental monitoring and in point-of-care settings. The plasmonic biosensor can focus light waves down to a scale small enough to detect proteins and amino acids, without needing a bulky, expensive external light source. By exploiting a quantum phenomenon called inelastic electron tunneling, researchers at the École Polytechnique Fédérale de Lausanne (EPFL), aided by colleagues at ETH Zurich, ICFO, and Yonsei University, created a biosensor that requires only a steady flow of electrons, in the form of an applied electrical voltage, to illuminate and detect molecules. A metasurface of gold nanowires drives quantum light emission and concentrates the resulting light waves to detect molecules. Courtesy of Ella Maru Studio/BIOS EPFL CCC BY SA 4.0. As an electron passes through a multilayer (metal-insulator-metal) film in the sensor structure, it transfers some of its energy to a plasmon, which then emits a photon. The intensity and spectrum of the light changes in response to contact with a biomolecule. “If you think of an electron as a wave, rather than a particle, that wave has a certain low probability of ‘tunneling’ to the other side of an extremely thin insulating barrier while emitting a photon of light,” researcher Mikhail Masharin said. “What we have done is create a nanostructure that both forms part of this insulating barrier and increases the probability that light emission will take place.” The multilayer structure has an aluminum electrode as the bottom layer, with a thin isolating layer of alumina, formed by thermal oxidation of the film, acting as a tunneling barrier. The upper electrode consists of a doubly periodic metasurface made of resonant gold nanowire antennas. The plasmonic metasurface serves a dual purpose. It both creates the conditions for quantum tunneling and controls the resulting light emission, simultaneously providing enhanced electron-to-light conversion and far-field light emission. This dual capability is due to the arrangement of the gold nanowires, which act as nanoantennas to concentrate the light at the nm volumes required to detect biomolecules efficiently. The optically resonant, doubly periodic nanowire metasurface provides uniform emission over large areas, amplified by the nanoantennas that simultaneously enhance the spectral and refractive index sensitivity. (Left): Comparison of light emission from the metasurface when coated with a polymer (orange box) versus no polymer (green box). (Right): Comparison of light emission from the metasurface when partially covered with the amino acid alanine (black box) versus no alanine (green box). Courtesy of EPFL. “Inelastic electron tunneling is a very low-probability process, but if you have a low-probability process occurring uniformly over a very large area, you can still collect enough photons,” researcher Jihye Lee said. “This is where we have focused our optimization, and it turns out to be a very promising new strategy for biosensing.” The researchers tested the biosensor with various analytes including thin layers of polymer and biomolecules. They observed that both the intensity and the spectral profile of the emitted light were modulated by the local refractive index changes produced by the presence of the analyte. “Tests showed that our self-illuminating biosensor can detect amino acids and polymers at picogram concentrations — that’s one-trillionth of a gram — rivaling the most advanced sensors available today,” researcher Hatice Altug said. The biosensor provides an integrated, nanoscale light source without requiring any labels. With plasmonic antennas serving both as a sensing element and a light source, the sensor has a considerably smaller device footprint compared with designs involving the integration of plasmonic structures on top of LEDs or photodetectors. In addition to being compact and sensitive, the quantum platform is scalable and compatible with sensor manufacturing methods. Less than one square millimeter of active area is required for sensing, demonstrating the potential for its use in handheld biosensors. Because it removes the need for an external light source, the on-chip, optical biosensor could be appropriate for various point-of-care applications. “Our work delivers a fully integrated sensor that combines light generation and detection on a single chip,” researcher Ivan Sinev said. “With potential applications ranging from point-of-care diagnostics to detecting environmental contaminants, this technology represents a new frontier in high-performance sensing systems.” The research was published in Nature Photonics (www.doi.org/10.1038/s41566-025-01708-y).