In parallel to planar optical sensing solutions, also miniaturized, robust, portable, but most importantly highly sensitive approaches based on optical fibers have been proposed . Such devices show several notable advantages over the planar ones. First, they are inexpensive thanks to the well-established fiber manufacturing technology. Moreover, they can easily couple light with microfluidic channels due to their similar size. Also, their immunity to electromagnetic interference or possibility of remote measurements are worth mentioning. In general, optical fibers may be integrated with microfluidics in three different ways. First, thanks to the microstructure of some fibers, they can serve as both light transmission medium and microfluidic channel, as it is in the case of, e.g., a hollow core fiber . Second, by introduction of the microstructuring processes such as those based on femtosecond (fs) laser micromachining or ion beam lithography creating lab-in-fiber systems . Third, the most common one, due to the flexibility in application of various types of sensors, they are achieved by the integration of fiber-based sensors with extrinsic microfluidic channels. Interferometers , various gratings , micro/nano fibers , and nano-coated fibers (based on surface plasmon resonance or lossy-mode resonance effects) have been shown following this approach. However, integration of the optical fiber sensor with microfluidic system plays crucial role during the sensing process. The way of introducing the analyte to the microchannels and efficiency of this process are especially important. Furthermore, the sensitivity of the sensor to external changes needs to be high enough to enable measurements of the analyte present in the microchannel. Volume of the analyte needed for the detection process is also important. It must be noted that some of the sensors, e.g., gratings , or lossy-mode resonance , due to their size/length and inaccurately defined sensing area need hundreds of microliters of samples per single analysis. Such volumes may be too large in the case of limited in availability biological and chemical materials. Hence, the structures based on in-fiber microcavities deserve particular attention. Microcavity inline Mach-Zehnder Interferometer (µIMZI) is an open cavity micromachined in a fiber cladding and partly in a fiber core. In this configuration, the microcavity splits incoming light into two parts, where one remains in the fiber core (reference beam), while the other one propagates through the cavity (sensing beam). The two beams interfere at the far sidewall of the cavity . Due to the precise, one-step fabrication process, the platform is highly reproducible and provides a well-defined sensing area. The µIMZI with a diameter of tens of micrometers allows investigating sub-nanoliter volumes of liquids. The sensitivity of the µIMZI reach even over 20,000 nm/RIU what makes it a perfect choice for bio-and chemo-sensing, especially when small volumes of analytes are considered. Although the μIMZI has been reported for different applications, such as bacteria detection and real-time monitoring of isothermal DNA amplification , its integration with the extrinsic microfluidic channel has never been reported. Incorporation of the µIMZI in a microfluidic system may also be advantageous because it limits evaporation of the analyte and allows a more accurate way of handling a solution inside the microcavity as it can be easily exchanged. Moreover, due to limited exposure of the sensor to changes in environmental conditions, such as drying during the procedure, can be mitigated some additional effects possibly modifying the sensor surface and disturbing the measurements .