Picomolar detection of thrombin with fiber-optic ball resonator sensor using optical backscatter reflectometry

A ball resonator, positioned on the tip of an optical fiber, has been developed as a biosensor for the prototypic detection of thrombin. The device was fabricated with a fast and repeatable CO₂ laser splicing method, followed by gold-sputtering and functionalization for the measurement of various protein concentrations. The ball resonator acts as a weak interferometer with a return loss below − 50 dB, and it is interrogated with an optical backscatter reflectometer measuring the reflection spectrum. We report here a sample presenting high sensitivity (1273.74 nm/RIU, RIU = refractive index units), which allows protein detection in the range 0.4–100 pM, with a limit of detection of 1.56 pM in logarithmic response.