The binding between WBT and spermine was first examined by
1H NMR spectroscopic experiments (
Fig. 3a). Upon addition of 1.0 equiv. of WBT, all of the protons of spermine showed remarkable upfield shifts (Δ
δ = −0.65, −0.85, −0.98, −0.44 and −0.85 ppm for H
1, H
3, H
4, H
2 and H
5, respectively). These protons also displayed significant broadening effects. These results suggested that protons of spermine were encapsulated by WBT's cavity. 2D NOESY NMR of an aqueous solution of WBT (5 mmol/L) and spermine (5 mmol/L) showed distinct NOE correlation signals (such as H
5 of spermine and H
d, H
e, H
f of WBT), which further confirmed the formation of an inclusion complex (Fig. S15 in Supporting information). It was found that in the presence of approximately 1.0 equiv. WBT, proton signals from other biogenic amines also show upfield shifts, but the NMR changes are smaller than that of spermine (Figs. S16-S22 in Supporting information). To determine the binding affinity of WBT towards different types of biogenic amines, UV–vis titration experiments were performed in sodium phosphate buffer (pH 7.4) (
Fig. 3b). Job plot showed the binding stoichiometry of the water-soluble host WBT with these biogenic amine guests were 1:1 (Fig. S23 in Supporting information). The association constant (
Ka) determined
via nonlinear fitting for spermine is up to 2.9 × 10
5 L/mol which is higher than other aliphatic amines (
Fig. 3c,
Table 1 and Figs. S24-S27 in Supporting information). For aromatic and heterocyclic amines, their
Ka values are in the order of magnitude of 10
2–10
3 L/mol (Figs. S28-S30 in Supporting information). Obviously, aliphatic spermine exhibited stronger binding affinities than others. The highest binding affinity of spermine may be due to its polycationic nature, which contributes to tighter recognition by multiple electrostatic interactions between carboxylate and amino groups. Combined with AIE effect of the water-soluble macrocycle, it should be feasible to achieve selective detection of spermine by means of these different affinities.