Briefly, the droplets were generated using Mitos Dropix (Dolomite, UK). A custom sequence of droplets (150 nl aqueous phase, 300 nl oil spacing) was generated using negative pressure (microfluidic pump). The droplets were guided through a polythene tubing to the incubation chamber. Within the incubation chamber, the halogenated substrate was delivered to the droplets via a combination of microdialysis and partitioning between the oil (FC 40) and the aqueous phase. The reaction solution consisted of a weak buffer (1 mM HEPES, 20 mM Na2SO4, pH 8.2) and a complementary fluorescent indicator 8-hydroxypyrene-1,3,6-trisulfonic acid (50 μM HPTS). The fluorescence signal was obtained using an optical setup with an excitation laser (450 nm), a dichroic mirror with a cut-off at 490 nm filtering the excitation light, and a Si-detector. By employing a pH-based fluorescence assay, small changes in the pH were observed and enabling monitoring of the enzymatic activity. Reaction progress was analyzed as an end-point measurement recorded after the passing of 10 droplets/sample through the incubation chamber. The reaction time was 4 min. The raw signal of every single measurement was first processed by the in-house LabView-based (National Instruments, USA) software MicroPEX Data Analyzer 1.0. The peaks were assigned to the particular sample, and the mean signal was calculated for them. The output XLS file gathering mean signal values for every sample type (calibration, enzyme activity, buffer, blank 3 buffer, and blank enzyme) for the dataset served as an input for the MatLab (Mathworks, USA) script to calculate specific activities using the same principle as for previous measurements . The activities were classified as "not determined" whenever the measured product concentration was below the limit of detection (LOD -3 times the standard deviation of the noise signal). Each substrate had a different calibration curve, so the LOD product concentration was in the range of 10-100 μM).