A sensitive and compact optical detector based on digital lock-in amplification

We report a sensitive, fixed-wavelength, lock-in-based optical detector built from a light-emitting diode, two colour filters, a photodetector, a small number of discrete analogue components, and a low-cost microcontroller development board. We describe the construction, operating principle, use and performance of the optical detector, which may be used for both absorption and fluorescence measurements in either a 10-mm pathlength cuvette or a low-volume (<100 μl) flow-cell. For illustrative purposes the detector is applied here to a cholesterol assay based on the enzyme-mediated conversion of (non-emissive) Amplex Red into the fluorescent dye resorufin, providing a detection limit of $$ 200 nM - some four orders of magnitude lower than the typical concentration of cholesterol in human serum. (The resorufin molecule itself is detectable down to concentrations of $$ 20 nM). The system may be readily adapted to other biomolecules through a simple change of enzyme.