To fully understand the quenching behavior of
NKU-130 in the presence of PA, the quenching mechanism needs to be well corroborated. Common fluorescence quenching mechanisms include structural collapse, energy resonance theory (RET) and photo-induced electron transfer (PET) [
31,
40,
41]. Firstly, PXRD pattern of the powder sample after recycling test showed that the framework structure was intact during the detection process, which excludes the decomposition induced fluorescence quenching mechanism [
42]. Secondly, the fluorescence resonance energy transferred from the fluorophore to the analyte might be one reason for the quenching process [
43]. However, the UV–vis adsorption of nitro explosives and
NKU-130 shows negligible overlap, which rules out this possibility (Fig. S20 in Supporting information). The observed overlap between the absorption band of the nitro explosives and the excitation band of the sensing
MOF-130 indicates there is a competitive adsorption mechanism (Fig. S21 in Supporting information). With an excitation wavelength of 327 nm, PA has a much stronger absorption over other explosives. This electronic characteristic of PA results in lower absorbance of the ligand and lower energy transfer from the ligand to Tb
3+ ion, hence ultimately leading to the fluorescence quenching. Thirdly, photo-induced electron transfer (PET) can also contribute to the fluorescence quenching process. The lower the lowest unoccupied orbit (LUMO), the more likely the process of photogenic electron transfer will occur. Since the LUMOs of PA are lower compared to those from other nitro compounds (PA < 1, 4-DNB < 1, 3-DNB < 2, 6-DNT < NB), and electron transfer from
NKU-130 to PA is more likely to process to result in the sudden extinction (
Figs. 4a and
b). Considering the aforementioned two latter factors, it can be concluded that the competitive absorption of excitation light energy and electronic interactions between PA and ligands largely affect the ligand-to-metal energy transfer process, thus bursting the characteristic emission of Tb
3+ ions. The non-linear SV curves also indicate the existence of both static and dynamic bursting mechanisms in the process (
Fig. 4c).