Described herein is a time-gated, two-step FRET relay effective to provide temporal transference of a prompt FRET pathway, or provide spectro-temporal encoding analytical signals and other information. A FRET relay assembly includes a long lifetime FRET donor (for example, a lanthanide complex), a semiconductor quantum dot (QD) configured as an intermediate acceptor/donor in FRET, and a fluorescent dye configured as a terminal FRET acceptor, wherein the long lifetime FRET donor has an excited state lifetime of at least one microsecond and the QD and fluorescent dye each have excited state lifetimes of less than 100 nanoseconds.