Inexpensive hardware and software for photon statistics and correlation spectroscopy

Single-molecule sensitive microscopies and spectroscopies are transforming biophysics and materials science laboratories. Techniques such as fluorescence correlation spectroscopy (FCS) and single-molecule sensitive fluorescence resonance energy transfer (FRET) are now commonly available in research laboratories but are as yet infrequently available in teaching laboratories. We describe inexpensive electronics and open-source software that bridges this gap, making state-of-the-art measurement research capabilities accessible to undergraduates interested in biophysics. We include a pedagogical discussion of the intensity correlation function relevant to FCS and its calculation directly from photon arrival times. We demonstrate the system with a measurement of the hydrodynamic radius of a protein using FCS that is suitable for an undergraduate teaching laboratory. The FPGA-based electronics, which are easy to construct, are suitable for more advanced measurements as well, and several applications are demonstrated. As implemented, the system has 8 ns timing resolution, outputs to control up to four laser sources, and inputs for as many as four photon-counting detectors.