Fig. 1. The excitation schemes and mechanism of UC-LRET between Yb/Er donor NPs and Rose Bengal acceptors.

a Three different excitation schemes were used, i.e., (TTL980) 4 ms pulse from CW 980 nm laser diode controlled with TTL, (OPO980) 10 ns pulse of 980 nm from OPO, and (OPO520) 10 ns pulse of 520 nm from OPO. b Energy scheme and transfer in Yb³⁺/Er³⁺—RB system (energy level for RB taken from ref. ⁴⁹); ETU energy transfer upconversion, EM energy migration, BET back energy transfer, ISC inter system crossing, CR cross-relaxation, Q quenching, LRET luminescence resonance energy transfer, schematic presentation of (c) YbEr core-only (gray area) UCNP donor nanoparticle compared with (d) core (reddish area)-shell (gray area) Yb@YbEr donor NPs, both with RB on the surface. Arrow thickness is qualitatively proportional to the intensity; – Er³⁺ ions, – Yb³⁺ ions