Abstract A new and compact fluorescence biosensor is constructed. The distinguished feature of this setup is the use of an ellipsoidal reflector as an optical collector of the fluorescence light. A traditional collecting lens with numerical aperture of 0.5 can only collect a few percent of the available signal, while the incorporation of the ellipsoidal reflector enables the apparatus to provide a signal increase of at least 66 times. A very good linear relationship between the concentration of fluorescent dye and the fluorescence intensity is achieved with correlation coefficient R = 0.99979 . The 3 σ detection limit is 5 nM for microliter volume sample. Meanwhile, it is found that laser light scattering by dirts and scratches on the sample cell's windows and by turbidity in the sample solutions is the dominant limitation to increase the detection sensitivity. Approach to solve this problem is suggested.