## 3 Results and Discussion
### AFB1 and CdSe/ZnS QDs FRET  
To study the energy transfer between AFB1 and CdSe/ZnS QDs, a series of AFB1 and CdSe/ZnS QD samples were prepared with the volume ratio as showed in Table 1. Figure 2a and b show the absorption and fluorescence spectra under 405 nm excitation of the AFB1 and CdSe/ZnS QD mixtures. From Figure 2b, it was observed that the fluorescence spectra of AFB1 and CdSe/ZnS QDs mixtures are characterized by emission peaks at 435 and 545 nm of AFB1 and CdSe/ ZnS, respectively. The fluorescence intensity of CdSe/ZnS QDs without AFB1 (M6) is higher than the fluorescence intensity of pure AFB1 (M0). This may be caused by the excitation wavelength in the weak absorption region of AFB1 and in the strong absorption region of CdSe/ZnS (Figure 2a) aside from the greater quantum yield of CdSe/ZnS QDs compared with AFB1. The fluorescence intensity of AFB1 decreases with respect to pure AFB1 in the mixed samples, while the fluorescence intensity of CdSe/ZnS increases. This is mainly due to the transfer of energy from AFB1 molecule to CdSe/ZnS QDs via FRET. Results show that the absorption peak and emission spectra of AFB1 depends on the solvent solution. The emission peak can also change from 398, 418, 438 nm, etc. depending on the solvents used such as benzene, lipids, or phosphate buffer solution (PBS) [20]. The FRET efficiency can be calculated from the variation of the fluorescence intensity according to Equation (5). However, Figure 2b shows that the fluorescence spectra of AFB1 and CdSe/ZnS QDs are noisy, and the maximum value of the emission peak fluctuates. Moreover, to confirm that the interaction between AFB1 and CdSe/ZnS QDs in solution is FRET, the fluorescence lifetime of the AFB1 and CdSe/ZnS QDs mixtures were performed. Figure 3 shows the time-resolved fluorescence spectra for all samples at room temperature. The spectra have been measured at the peak wavelength of 435 nm, observed in steady-state photoluminescence spectra, under 405 nm excitation. In the case of a sample with only one recombination process (single molecule case), it is possible to fit the fluorescence attenuation path according to the single exponential function. When there are recombinant processes occurring in the sample, it is necessary to fit the curve according to the biexponential, triple exponential, or the average of the stretch-exponent. Figure 2: (**a**) Absorption spectra and (**b**) fluorescence spectra (\(\lambda_{\text{exc. }}=405\) nm) of AFB1 and CdSe/ZnS QDs mixtures with different volume ratios in methanol. In our case, the decay curves can be well-fitted to a biexponential function described by the equation:  
\[I(t)\ =\ A_{1}.\exp(-\frac{t}{\tau_{1}})+\ A_{2}.\exp\!\left(-\frac{t}{\tau_{2}}\right) \tag{6}\]  
where \(\tau_{1}\) and \(\tau_{2}\) represent the fluorescence lifetimes for each exponential decay term and \(A_{1}\) and \(A_{2}\) represent their respective amplitudes. For samples with a high concentration of AFB1 (M6, M1, and M2), the fluorescence decay curves decrease with a single exponential function. On the other hand, for samples with a small concentration of AFB1, their fluorescence intensities are weak, and the fluorescence decay curves decrease quickly and then decrease slowly. Therefore, the fluorescence decay curves were fitted to the biexponential function, where the first fluorescence lifetime (<100 ps) is due to instrumentation response function (IRF), and second fluorescence lifetime is AFB1 fluorescence lifetime in the excited state. The fluorescence lifetime of CdSe/ZnS QDs \(\tau_{1}\) and \(\tau_{2}\) can be attributed to the band edge and the deep trap recombination [21]. The fluorescence lifetime \(\tau_{1}\) almost does not change, while \(\tau_{2}\) increases with increasing AFB1 concentration. Fluorescence lifetime values of AFB1 at 435 nm and CdSe/ZnS QDs at 545 nm were shown in Table 2. Figure 4 shows the variation of AFB1 concentration as a polynomial second-order function of the fluorescence lifetime.