Forbidden light detection from single molecules

We present a new concept for ultrasensitive detection of surface-generated fluorescence which is made possible by a new optical module. The detection method leads to an enhancement in fluorescence collection efficiency to more than 65% of the total of emitted light, whereas high-aperture microscope objectives are able to collect 44% at best. Moreover, by employing this new optical module, the detection volume can be restricted to approximately 10(-17) L. This allows for an exceptional discrimination of bulk-generated against surface-generated fluorescence, which may be of great value when surface-binding processes are monitored. We demonstrate the performance of the new detection system by detecting single fluorescent molecules and by determining antigen concentrations down to 5 fmol.

Highly efficient optical detection of surface-generated fluorescence.

We present a theoretical study of a new highly efficient system for optical light collection, designed for ultrasensitive fluorescence detection of surface-bound molecules. The main core of the system is a paraboloid glass segment acting as a mirror for collecting the fluorescence. A special feature of the system is its ability to sample not only fluorescence that is emitted below the angle of total internal reflection (the critical angle) but also particularly the light above the critical angle. As shown, this is especially advantageous for collecting the fluorescence of surface-bound molecules. A comparison is made with conventional high-aperture microscope objectives. Furthermore, it is shown that the system allows not only for highly efficient light collection but also for confocal imaging of the detection region, which is of great importance for rejecting scattered light in potential applications such as the detection of only a few molecules.