“…UCNPs are conjugated with energy acceptors, whose excitation spectra has to overlap with the emission spectra of the NPs. The acceptor can be organic fluorophores [111,144,344,345], fluorescent proteins [346], metallic NPs [341], graphene oxide (GO) [338,339,347,348], carbon NPs [342] and semiconductor QDs [345,349,350]. The working principle of upconversion LRET [depicted in Figure 21(i)] has demonstrated its huge potential through the development of sensing systems for the determination of 17β-estradiol [336], enzymatic activity [351], matrix metalloproteinase [342], mycotoxins [338], ATP [339], IgG [352], glucose [347] and pesticides [353], among others.…”