Devices and methods for performing frequency domain (FD) fluorescence lifetime spectroscopy are provided. The devices include a modulated light source, a focusing optical fiber, a detecting optical fiber, and a detector. The methods include focusing sinusoidal modulated incident light from a light source on a biological sample containing a protein, detecting a range of wavelengths of sinusoidal modulated fluorescent light emitted from the protein, determining a phase shift for the modulated fluorescent light, determining an amplitude modulation of the modulated fluorescent light, and determining a fluorescence lifetime of the protein.
