Determine the values of Stern-Volmer constant: Quenching of fluorescence can be described by the Stern-Volmer equation: [1]

= + (1) Here F0 and F are the fluorescence intensities in the absence and presence of quencher, respectively, [Q] is the concentration of quencher, and KSV is the Stern-Volmer constant which is related to the quenching efficiency. It’s useful to note that KSV equal to the reciprocal of quencher’s concentration at which F0/F=2, or 50 % of the intensity is quenched. If the quenching is dynamic (collisional) quenching, KSV will be represented by KD, and 0D qK k τ = (2) Here KD is the dynamic quenching constant, kq is the bimolecular quenching constant, 0τ is the lifetime of QDs in the absence of quencher. In Figure 1b, a 50 % quenching efficiency was observed at the concentration of 0.66 mM 4-AP. Thereby a calculated Stern-Volmer quenching constant KSV of 1.51×103 M-1 would be obtained. The measured average fluorescence lifetime of the QDs used was 15.7 ns, and thus a calculated bimolecular quenching constant kq of 9.62×1010 M-1s-1 for the quenching was obtained. A bimolecular quenching constant near 1×1010 M-1s-1 may be considered as the largest possible value in aqueous solution, [2] so the calculated value was obviously larger than that possible for a diffusion-controlled bimolecular quenching, and the static quenching as a result of the binding interaction between TGA-CdTe QDs and 4-AP would be expected. In the control experiment, a concentration of 33 mM 4-HP was needed to quench half of the fluorescence of the QDs. Thus a calculated Stern-Volmer quenching constant KSV of 30.3 M-1 was obtained.