The present invention provides methods for determining fluorescence lifetime of a fluorophor or an optical sensor using frequency domain gated detection. The method comprises the steps of exciting the fluorophor with frequency modulated light from a pulsed light source, gating a photodetector during excitation of the fluorophore, detecting light emitted from the fluorophor with the photodetector, where the emission light exhibits a phase shift in frequency from that of the excitation light, converting the detected excitation light to an amplified electric signal, and evaluating the amplified electric signal as a measure of fluorescence lifetime of the fluorophor. Also provided is a device with which to use the methods disclosed herein.