(471b) Biosensor Combining the Principles of Metal Enhanced Fluorescence and Surface Acoustic Waves (MEF-SAW)

We propose a novel biosensor that integrates the two principles of metal enhanced fluorescence (MEF) and surface acoustic waves (SAW) on a single chip. MEF enhances the optical sensing signal intensity, whereas, Rayleigh SAWs have previously been shown to remove non-specifically bound (NSB) proteins and enhance fluid mixing by the acoustic streaming phenomenon. Silver island films were sputtered on the delay path of split-finger IDTs on a LiNbO₃ chip. AFM images revealed the formation of varying size silver islands. By optimizing the sputtering conditions, some silver islands were bigger than 15 nm, resulting in greater fluorescence enhancement.

Though fluoro-immunoassay is a widely used tool in medical diagnostics, it suffers from certain photophysical constraints like low quantum yields, autofluorescence, and low photostability. However, these constraints can be addressed by modifying the spectral properties of the fluorophore using MEF, a physical phenomenon in which the fluorescence signal of an emitting fluorophore is considerably enhanced by the presence of vicinal metal nanoparticles. MEF immunoassay suffers from certain limitations like low signal-to-noise ratio due to spurious signals due to interference from non-specific binding proteins and longer detection times. However, these limitations of the MEF immunoassay technique are easily overcome by combining it with a Rayleigh wave SAW device, in which Rayleigh waves help in efficient droplet mixing of the fluid, thereby considerably reducing the detection time. Additionally, the SAW-based Rayleigh waves help in selective removal of NSB proteins, which is useful for renewing the surfaces of the biosensors and in eliminating biofouling.

Experiments were conducted to demonstrate the concepts of MEF, the removal of NSB proteins, and the efficient micro-mixing on the MEF-SAW device. The results demonstrate enhanced sensing due to the combined effects of MEF and SAW. This novel MEF-SAW device provides a unique platform for both detection and removal of biological antigens on the same substrate, with shorter detection times than standard fluoro-immunoassays, enhanced signal-to-noise ratio, and increased sensitivity.