Ruby nanoflakes (rubyene) for efficient 2D Förster resonance energy transfer: implications for engineered emitters in multiplexed imaging

Förster resonance energy transfer (FRET) provides a unique means to probe processes occurring on the nanoscale. Efficient point-to-plane FRET between acceptors arrayed in two-dimensional (2D) sheets and point-dipole donors is known to exhibit an energy transfer proportional to d^-4, where d is the distance from the donor to the acceptor-plane. We developed a 2D nanomaterial with surface-adjacent acceptors based on chromium-doped aluminum oxide (ruby) that supports 2D-FRET architecture. Ruby exhibits bright, narrow-band photoluminescence with a long lifetime and excellent photostability, and it was synthesized and processed to yield 2D ruby nanoflakes of 5 nm thickness, termed “rubyene”. Rubyenes exhibit FRET with a remarkable efficiency of 96% as the donor paired with the acceptor indocyanine green (ICG) dye, exhibiting a distance dependence of d^-n, where n varies from 4 to 6, as determined by the acceptor surface density. The dependence approaches d^-4 in the limit of the acceptors’ continuum distribution. The developed model of the random distribution of donor Cr³⁺ ions inside rubyenes and a discrete planar array of the acceptor ICG molecules in combination with high-density surface traps acting as nonresonant acceptors provides a nearly perfect fit to the experimental results and explains the emission lifetime variation from 5.8 to 0.7 versus the ICG surface density. We envisage potential applications of the demonstrated rubyenes for environmental sensing, optical labeling, FRET-scaled precision measurement, and engineered emission lifetime for multiplexed imaging.