Sensing Biomolecular Interactions by the Luminescence of a Planar Gold Film

We describe the operating principle and performance of a recently developed surface plasmon-enhanced optical sensor that utilizes two-photon excited luminescence of a planar gold film as the reporter signal. The sensor enables direct visualization of nanoscopic binding events near a sensing surface. Light is coupled to the Au/sample interface in an objective-based Kretschmann configuration to excite surface plasmon polariton (SPP) modes at a metal-dielectric interface. The gold luminescence induced by the confined optical field between the particle and the film is detected in the epi-direction by a far-field camera where individual binding events show up as diffraction limited bright spots against a dark background. We study the sensor's emission spectrum and the distance dependence between the target and substrate, which both suggest that the optical signal of the sensor originates from electron-hole pair excitations in the planar Au film. In addition, we show that the well-behaved pointspread function of the sensor enables a straightforward implementation of super-resolution techniques. Finally, we demonstrate the utility of the sensor for detecting DNA binding events, underlining the sensor's usefulness for label-free imaging of nanoscopic particles and biomolecular interactions.