Time-resolved fluorescence (TRF) eliminates autofluorescence interference by exploiting the long-lived microsecond-to-millisecond emission lifetimes of lanthanide complexes. While most background fluorescence from biological samples decays within nanoseconds, trFluor™ probes continue emitting for much longer durations, enabling time-gated detection that dramatically improves signal-to-background ratios in complex matrices. TR-FRET combines this sensitivity with proximity-based FRET specificity for homogeneous ‘mix-and-read’ assay formats commonly used in drug discovery and high-throughput screening.

AAT Bioquest's trFluor™ europium (Eu) and terbium (Tb) probes offer large Stokes shifts (>200 nm), minimal self-quenching, and high conjugation efficiency. Unlike classic lanthanide chelates, trFluor™ dyes require no enhancing solutions or fluoride additions, simplifying assay workflows.

Donor selection: trFluor™ Eu (Em 617 nm) pairs with red/far-red acceptors (iFluor® 647, APC, Cy5®); trFluor™ Tb (Em 544 nm) pairs with green acceptors (iFluor® 488, FITC). See FRET Building Blocks for additional acceptor options.

AAT Bioquest offers kits optimized for microscale labeling. These kits contain fast protocols with no purification steps required. ReadiLink™ kits label antibodies in two steps under one hour. The Buccutite™ D2 kit prepares TR-FRET acceptor conjugates that pair with Eu-labeled donors for sandwich assays.

See also: ReadiLink™ Protein Labeling Kits | Buccutite™ Protein Crosslinking Kits

In addition to ready-to-use labeling kits, reactive labeling dyes are available for custom conjugation to proteins, peptides, or small molecules. Each dye contains a single reactive group to ensure defined stoichiometry. NHS esters react with amines (pH 7.5–8.5); maleimides react with thiols (pH 6.5–7.5); DBCO enables copper-free click chemistry with azides.

See also: DBCO Building Blocks | Small Fluorescent Labeling Dyes

XL665 is a modified allophycocyanin optimized for pairing with europium donors. It serves as a drop-in replacement for the D2 acceptor with improved brightness and stability. The streptavidin conjugate enables rapid assay development with any biotinylated probe.