Measurements are most easily performed using the provided Python program. The software, "SensorInterface.py", was written in Python 3.8.8, and requires installation of PyQt5, pyqtgraph, and numpy. A screenshot of the interface is shown in Fig. . After connecting the sensor to the PC via powered USB, and supplying 20-24 V DC power to the unit, connect to the device by typing in the correct serial address (COM4 in Fig. ) and click "Connect". The blank/background sample should then be loaded into the sample holder, and the desired number of 0.1-s readings to average over should be specified. To acquire a background measurement, click "Background". The value of the background measurement and the standard deviation are displayed in the software's graphical user interface (GUI). Load a test sample and click "Measure once" to acquire a single measurement; a checkbox determines whether or not background subtraction is carried out. Clicking "Start Continuous" causes the data stream to be continuously plotted within the GUI over a user-defined time range. Checking the "Record" checkbox causes the data stream to be saved to a comma separated variable (CSV) file; data is recorded both with and without a background subtraction. The continuous measurement can be interrupted at any time for a single measurement or a new background acquisition. The detector was further applied to the detection of cholesterol, using a fluorimetric assay based on the widely used probe Amplex Red (see Fig. ). The enzyme cholesterol oxidase catalyses the oxidation of cholesterol to cholestenone, with the concurrent reduction of oxygen to hydrogen peroxide (H2O2). In the presence of horseradish peroxidase, the generated H2O2 converts Amplex Red to the highly fluorescent dye molecule resorufin, with the measured fluorescence intensity being directly proportional to the initial cholesterol concentration. A stock solution of 4 U/ml cholesterol oxidase, 4 U/ml horseradish peroxidase (HRP), and 300 µM Amplex red was made up in PBS with 1% (v/v) TritonX-100. Six 1-ml aliquots were taken from the stock solution and combined with 1-ml cholesterol solutions with concentrations ranging from 780 nM to 25 µM. A negative control sample of 1% TritonX-100 in PBS (without cholesterol) was also prepared, and was used as the "blank" sample. The cholesterol solutions were prepared in PBS and contained 1% (v/v) TritonX-100. Hence seven 2-ml samples -each having a concentration of 2 U/ml HRP, 2 U/ml cholesterol oxidase and 150 µM Amplex red -were obtained with cholesterol concentrations ranging from 0 to 12.5 µM cholesterol. The samples were left for 30 minutes at room temperature for the reaction to complete, and were then loaded sequentially into the flow-cell to determine the fluorescence intensity. For each sample the lock-in amplitude 𝑅 2 was recorded for 10 seconds after stabilisation.