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Furthermore the donor state cannot be extremely short lived and 1st order time-dependent perturbation theory needs to be applicable, i.e., back energy transfer from acceptor to donor should be minimal. Regardless of the nature of the energy transfer process, the timescale(s) of luminescence rise and decay change when energy transfer is occurring. Obviously the donor lifetime is quenched while the acceptor lifetime is enhanced; this is generally considered the best evidence selleck compound of FRET. Studies have shown that QDs are effective energy donors to organic dye acceptors in a wide range of FRET-based processes [37,38,39] due to their high quantum yields and extinction coefficients, narrow and symmetrical emissions, and photochemical stability. This is somewhat surprising as the large size of nanocrystals would seemingly force the donor-acceptor distance to be so far (>several nm) as to preclude any significant efficiency. The mechanism of the energy transfer process is due to a dipole-dipole interaction [40] and follows the standard r6-dependent FRET efficiency. Regardless, there are some alterations to the standard model that need to be implemented. For example, it is not feasible to construct a single QD donor-single dye acceptor system. In this case, the FRET efficiency where a single QD donor interacts with n acceptors simultaneously and all the acceptors click here are located at the same distance can be expressed by [41]: E(n,?r)=nR06nR06+r6 (2) This is further complicated by the fact that there should be a heterogeneous distribution of the n acceptors per QD that follows Poissonian statistics [42], where the probability of a QD having n acceptors is: P(n,��)=��ne?��n! (3) where �� is the average dye:QD ratio. There are a few other issues to note, such as the odd fact that the FRET efficiency calculated from time-resolved data is rarely equivalent to that measured by total donor emission quenching (Equation (1)). This is likely due to the complex nature of QD donor lifetimes which are generally multi-exponential in nature. Last, QDs are not good FRET acceptors from organic dye donors [43]. This was conjectured to result from the longer excitation lifetime of the QD (>10 ns) compared to organic dyes (NK cell in its derivation as discussed above. However, the use of the QDs as acceptors with longer lifetime donors such as lanthanidesis efficient [44,45]. Furthermore, new and interesting variations of energy transfer called bioluminescence- and chemiluminescence- resonance energy transfer have been recently demonstrated. In these systems, biologically emissive proteins conjugated to QDs create ��self-lighting�� dot systems that require chemical, rather than physical, excitation. The use of QDs as donors in FRET systems offers several advantages over conventional fluorophores [46,47,48].