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.