Direct imaging-based gradient metasurface sensor enabling spectrometer-free ultrasensitive biomolecule detection.

Biosensors play a crucial role in modern diagnostics, where simple, portable systems can enable rapid on-site testing. Here, we introduce a direct imaging-based biosensor capable of reconstructing spectral shifts from transmission images without the need for a spectrometer. By introducing a continuous geometry gradient in a dielectric metasurface, we spatially encode distinct resonance wavelengths across the device, enabling quasi-continuous spectral readout from a single camera image. Our platform achieves a high-quality factor and fine 0.1 nm spectral step size across a broad 30 nm spectral window within a 300 µm footprint, without spectroscopic instrumentation. This direct imaging-based approach exhibits high sensitivity with a figure of merit of 67.2 per refractive index unit and enables label-free detection of both proteins and DNA, reaching a limit of detection down to 388 picomolar. This spectrometer-free biosensor presents a compact sensing platform with a clear pathway toward a potentially more cost-effective implementation than conventional optical refractometric sensors, enabled by a simplified optical architecture facilitating molecular testing beyond specialized laboratory settings.