Ultrasensitive Refractometry via Supercritical Angle Fluorescence

Background: refractive index (RI), provides information about various sample properties such as concentrations and molecular structure. We have developed a method for label-free sensing of the refractive index: a device composed of a glass coverslip coated with a thin, dense layer of fluorophores inside a microfluidics channel.

Methods: The device is imaged using a fluorescence microscope, where the detector is placed in a conjugate back focal plane of a high-NA objective lens. The RI is then calculated using a robust, custom algorithm for determining the transition between the under-critical and super-critical fluorescence.

Results and Conclusion: We validate the methodology over a broad range of indices with ultra-sensitivity. Our microfluidic- based device and analysis technique for refractometry, enable label-free measurements of tiny amounts of liquid sample (picolitres of imaged volume) with high throughput and an order of magnitude higher precision than was previously reported. The sensitivity of our device enables detection of RI changes smaller than 4⋅10^(-5) RI units, corresponding to a change of less than a 0.1% glycerol concentration in a water-glycerol solution. We furthermore demonstrate our system’s applicability for biosensing, by measuring E. coli bacteria growth in the chamber and scanning over a sparse sample for single bacteria detection.

Ferdman, B., Weiss, L. E., Alalouf, O., Haimovich, Y., & Shechtman, Y. (2018). Ultrasensitive Refractometry via Supercritical Angle Fluorescence. ACS nano , 12(12), 11892-11898.‏