Plasmonic Biosensing via Quantum-Tunneling Current Readout

Many modern sensors detect tiny changes in materials by analyzing how light interacts with them. These methods often require complex optical equipment, such as spectrometers and precise alignment of light sources, which can make sensors expensive, bulky, and difficult to use outside laboratory settings. This project develops a new type of sensor that replaces optical measurements with simple electrical signals, enabling more robust and compact devices. The technology uses nanoscale structures that concentrate light into extremely small regions. When the surrounding environment changes, such as when biomolecules are present, the local light field changes as well. Instead of measuring these changes with optical instruments, the system converts them into an electrical current using a quantum tunneling process. This allows the sensor to operate with a fixed light source and without the need for complex optical detection. This approach has the potential to enable smaller, more reliable sensors for applications such as medical diagnostics, environmental monitoring, and chemical detection. The project supports student training in advanced materials and device engineering and will engage K–12 students through outreach activities, helping to inspire interest in science and engineering careers. This project develops a current-mode plasmonic sensing platform based on optical rectification in tunnel junctions, enabling direct electrical detection of refractive-index changes without optical spectral interrogation. Conventional plasmonic sensors rely on wavelength-resolved measurements to track resonance shifts, requiring spectrometers, wavelength tuning, and precise optical alignment. In contrast, the proposed approach uses fixed-wavelength excitation and converts plasmonic near-field variations into measurable direct current through nonlinear tunneling transport. The sensing mechanism is based on plasmonic nanostructures integrated as electrodes in ultrastable tunnel junctions. Changes in the surrounding dielectric environment shift the plasmonic resonance, modulating the local electromagnetic field within the tunneling gap. This variation alters the optically induced voltage across the barrier, generating a rectified tunneling current that serves as the sensing signal. The magnitude and responsivity of the signal are determined by both plasmonic field enhancement and the nonlinear current–voltage characteristics of the junction. The research combines electromagnetic simulations, tunneling transport modeling, nanofabrication, and electrical measurements to (i) establish the underlying sensing mechanism, (ii) optimize plasmonic geometry and tunnel-barrier parameters for enhanced responsivity and stability, and (iii) demonstrate refractometric sensing in controlled and biologically relevant environments. The project advances fundamental understanding of coupled optical and electronic transport at the nanoscale and provides design principles for electrically addressable plasmonic devices. The resulting sensing architecture offers a scalable and integration-friendly alternative to conventional optical biosensors. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.