Ultrasound-enhanced microvortex transport for rapid, label-free DNA detection on a tapered fiber Mach−Zehnder interferometer biosensor

Optical fiber biosensors offer exceptional physical sensitivity for label-free bio-molecular detection, yet their practical performance in ultra-low concentration regimes is frequently stifled by diffusion-limited mass transport. To bridge the gap between intrinsic optical sensitivity and actual biosensing capability, an ultrasound-enhanced tapered fiber Mach−Zehnder interferometer (MZI) biosensor has been introduced, which leveraged acoustofluidic microvortices to overcome thermodynamic and kinetic barriers. A sector-shaped tip microstructure was rationally designed, which enables the generation of stable tip-coupled microbubbles within the tip chamber adjacent to the fiber surface under the actuation of a piezoelectric transducer. These bubbles can induce powerful and stable acoustic streaming vortices that actively transport analytes from the bulk solution to the sensor surface. Using ssDNA detection as the model, this active-transport mechanism lowered the limit of detection (LOD) by one order of magnitude—from 100 fM under static conditions to 10 fM under ultrasonic excitation—while boosting the overall sensitivity by 22.5%. Furthermore, the hydrodynamic shear forces generated by the microvortices provided a real-time “acoustic wash” effect, which thermodynamically destabilized non-specific binding events, thereby significantly amplifying the specificity. This work presented a generic, non-contact strategy to unlock the full potential of fiber-optic sensors, offering a robust pathway for rapid, high-fidelity clinical diagnostics.