Spermine is a bioactive polyamine that is abundant in living cells [1,2]. However, the concentration of spermine is elevated in patients with lung cancer, prostate cancer and liver tumor [3]. In addition, excessive intake of spermine can lead to respiratory distress, headache, hypertension and vomiting [4]. Therefore, the detection of biogenic amines involves public health problems [5,6], and their determination deserves attention. Conventional methods for determining spermine include HPLC, thin layer chromatography, capillary electrophoresis and so on. In the present work, our aim was to construct an indicator displacement assay (IDA) for the fluorescence sensing of spermine. various host–guest interactions have been exploited to design supramolecular-based molecular sensors, known as indicator displacement assays (IDAs) [7-9]. IDAs are based on the competitive binding of the analyte and a fluorescent indicator to a supramolecular host [10]. Therefore, simple, sensitive and preferably in vivo probes for trace spermine are greatly desirable for not only biomedical research but also environmental monitoring.

Simulated natural light-harvesting systems have good potential applications in the fields of biological imaging and optoelectronic devices [11-13]. Inspired by nature, there are two key conditions to realize efficacious energy transfer from a donor to an acceptor through the Föster resonance energy transfer (FRET) process [14,15]. Firstly, the donor should not only be densely packed, but also avoid self-quenching of intramolecular fluorescence caused by the aggregation-caused quenching (ACQ) effect. Secondly, the ratio of the donor to acceptor should be relatively high to ensure that multiple donors correspond to one acceptor. In addition, natural light-harvesting systems are constructed through the non-covalent interactions between chlorophyll and protein. However, supramolecular light-harvesting systems are constructed by non-covalent interactions and without multiple syntheses [16,17].

Macrocyclic hosts with double cavities are excellent materials for constructing supramolecular assembly. In the family of cucurbit[n]urils (n = 5–8, 10, 13–15) [18-24], nor-seco-cucurbit[10]uril (ns-Q[10]), as a special double cavity compound with rigid structure, which was synthesized and discovered by Isaacs in 2006 [25]. After almost twenty years of research and exploration, ns-Q[10] has been applied to molecular recognition [26-30], supramolecular hydrogel [31], supramolecular polymers [32] and so on. In the study of supramolecular polymers based on ns-Q[10], Isaacs designed and functionalized poly(N-hydroxyethylacrylamide) assembly with viologen, followed by addition of ns-Q[10] through specific and targeted bonding to form a ternary host-guest polymer [33]. Zhang fabricated ns-Q[10] based supramolecular assembly via self-sorting [34]. Recently, Yang and Li constructed a linear supramolecular dynamic rotaxane assembly by ns-Q[10], Q[7] and long liner derivatives of 4,4′-bipyridinium [35]. The aforementioned results jointly make clear that ns-Q[10]-based ternary complexation is a versatile platform for constructing various supramolecular polymers.

Herein, we designed and constructed an aqueous PDI-nm/ns-Q[10] assembly, which was based on the non-covalent host-guest interactions of a macrocyclic host nor-seco-cucurbit[10]uril (ns-Q[10]), a perylene bis(diimide) derivative (PDI-nm) as guest. There are two main purposes of introducing ns-Q[10] as the macrocyclic host: (1) the PDI-based chromophores are generally hydrophobic and show strong aggregation-caused quenching (ACQ) effect driven by π-π stacking in aqueous solution [36,37], so the chromophore needs to be sufficiently separated to minimize self-quenching. By assembling ns-Q[10] and PDI-nm, PDI-nm can be prevented from self-aggregating and a PDI-nm/ns-Q[10] assembly with high emission can be obtained. (2) the ns-Q[10] host has an inherent double cavity structure, which can form host-guest inclusion complexes PDI-nm/ns-Q[10] with methyl imidazole groups of PDI-based derivatives, and further assemble to form microspheres. Therefore, according to the above research, we have constructed a multifunctional supramolecular assembly based on ns-Q[10] (Scheme 1). On the one hand, PDI-nm/ns-Q[10] assembly was applied to the sensing of spermine in aqueous solution and in plasma solution based on the principe of IDAs. The detection limit of assembly PDI-nm/ns-Q[10] towards the spermine could reach about 7.84 × 10⁻⁷ mol/L and 3.69 × 10⁻⁷ mol/L, respectively. On the other hand, an artificial light-harvesting system was constructed by using this assembly as donor and Nile red (NiR) as acceptor in aqueous solution. The PDI-nm/ns-Q[10]-NiR system showed a high energy transfer efficiency. This was of great significance for better understanding and simulating the natural light-harvesting antenna systems.

A comprehensive account of the synthesis, characterization, and analysis of the perylene diimide derivative, PDI-nm, was available in Scheme S1, Figs. S1 and S2 (Supporting information). Initially, supramolecular interaction of ns-Q[10] and PDI-nm was investigated by UV–vis and fluorescence spectra (Figs. 1a and b), the PDI-nm (1 × 10⁻⁵ mol/L) exhibited two adsorption peaks at 500 nm and 537 nm in aqueous solution. With the gradual increase of ns-Q[10], the Abs₅₀₀ gradually decreases, and Abs₅₃₇ gradually increases, accompanied by an equal absorption point appears at 514 nm, which was attributed to the shielding interaction of ns-Q[10]. Meanwhile, fluorescence intensity increased sharply, which was attributed to ns-Q[10] can form a host-guest complex with PDI-nm, inhibiting the π-π stacking aggregation of PDI-nm and thus boosting the fluorescence intensity remarkably. The molar titration plots confirmed that the binding stoichiometry of the host ns-Q[10] with the guest PDI-nm was 1:1 in aqueous solution (Figs. S3 and S4 in Supporting information). The Job’s plots was further demonstrated to form a 1:1 host-guest inclusion complex through UV–Vis and fluorescence spectra (Fig. S5 in Supporting information). Moreover, as shown in Fig. S6 (Supporting information), the HR-MS of PDI-nm/ns-Q[10] also showed the 1:1 stochiometric ratio. After complexation with ns-Q[10] in aqueous solution at a molar ratio of 1:1 (PDI-nm/ns-Q[10]), the maximum emission wavelength of PDI-nm/ns-Q[10] was at 500 nm with a strong emission band, and the fluorescence quantum yield was 12.91% (Fig. 1c), and the maximum emission wavelength exhibited the lifetime was 5.09 ns (Fig. S7 in Supporting information).

Further analysis of the modes of action using ¹H NMR spectroscopic technique, Fig. S8 (Supporting information) showed the corresponding ¹H NMR titration spectra. When 1.0 equiv. of ns-Q[10] was added, the signals corresponding to the 1-methylimidazole cation (H_e,i,j,h) protons of the guest PDI-nm shifted upfiled, while the remainder protons (H_a-d, H_f-g and H_k-l) of the PDI-nm shifted downfield. This clearly indicated that 1-methylimidazole cation moieties were buried inside the hydrophobic cavity of the ns-Q[10], while the remainder moieties of the guest PDI-nm reside outside of the ns-Q[10] portals. Meanwhile, the signals in the aromatic region were weak and noisy, which indicated that the PDI-nm aromatic rings were in the aggregated state. In contrast, the ¹H NMR spectrum for the PDI-nm/ns-Q[10] assembly became clearer and the peaks were separated from each other, which indicated that the ns-Q[10] inhibited the close π-π stacking of the PDI-nm chromophores in the assembly [38-40].

Dynamic light scattering (DLS), Tyndall effect, transmission electron microscope (TEM) and fluorescence microscopy images were used to investigated the induced aggregation behavior of PDI-nm/ns-Q[10] assembly. DLS (Fig. S9 in Supporting information) revealed that the average hydrodynamic diameter of the nanospheres was 229.9 nm upon equimolar mixing of ns-Q[10] and PDI-nm, indicating the formation of highly polymerised supramolecular assembly. Then, the simple mixing of ns-Q[10] and PDI-nm exhibited obvious Tyndall effect in aqueous solution, which also indicated that a large supramolecular assembly was formed. But under the same conditions, only PDI-nm did not exhibit Tyndall effect, which indicated that PDI-nm did not form large self-aggregates. The high affinity between ns-Q[10] and PDI-nm resulted in the formation of supramolecular nanospheres with a ball structure, as shown by transmission electron microscopy (TEM, Fig. 1d). The PDI-nm/ns-Q[10] solution was dropped onto a glass slide to prepare a thin film. Under fluorescence microscope, the film emits intense red light, while the thin film of PDI-nm was scarcely emissive (Fig. S10 in Supporting information).

To demonstrate the sensing of PDI-nm/ns-Q[10], we investigated the response of PDI-nm/ns-Q[10] to spermine in aqueous solution. Spermine can preferably bind to ns-Q[10] due to the existence of multiple positive charges and long alkyl chains [41,42]. The experimental results suggested that spermine exhibits a binding affinity with ns-Q[10], one molecular. The binding constant of spermine and ns-Q[10] was 5.411 × 10⁵ L/mol, significantly higher than that of PDI-nm and ns-Q[10] (Figs. S11-S13 in Supporting information). This leads to the dissociation of the supra-amphiphiles and concurrent quenching of the fluorescence. The free Perylene diimide (PDI) derivative, PDI-nm, tended to form non-fluorescent π-π stacks in aqueous. Therefore, the fluorescence of PDI-nm/ns-Q[10] was quenched. As shown in Figs. 2a and c, after spermine was added, the absorption intensity of PDI-nm/ns-Q[10] solution gradually weakened, and the color changed from pink to transparent under natural light (Fig. S14 in Supporting information). A notable thing was that a new absorption band was observed in the wavelength region around 577−620 nm upon stepwise addition of spermine, accompanied by the appearance of one isosbestic points and bathochromic shift. The phenomena confirmed the intermolecular charge transfer interaction arising from the π-π stacking of the PDI-nm chromophores on accounts of the competitive complexation of spermine with ns-Q[10] [43]. Subsequently, the fluorescence changes of PDI-nm/ns-Q[10] during the increase of spermine content were tested. With the addition of spermine, the fluorescence emission intensity around 554 nm gradually decreased (Fig. 2b). The fluorescence intensity at 554 nm was almost quenched when the concentration of spermine reached 4 × 10⁻⁵ mol/L (illustration of Fig. 2b). As shown by Fig. S15 (Supporting information), the fluorescence intensity of PDI-nm/ns-Q[10] no longer decreased when the concentration of spermine reached 4 × 10⁻⁵ mol/L, and 4 × 10⁻⁵ mol/L spermine was selected in the following experiment. There was a good linear relationship between the decrease of fluorescence intensity and the increase of spermine concentration (3.6 × 10⁻⁵ – 6.3 × 10⁻⁵ mol/L, Fig. 2d). According to the formula LOD = 3σ/k (σ is the standard deviation of 10 blank samples, k is the slope of fluorescence intensity (I_554nm) against spermine concentration) recommended by IUPAC, the detection limit (LOD) was as low as 7.84 × 10⁻⁷ mol/L. The binding kinetics of PDI-nm/ns-Q[10] and spermine were investigated, which reached the plateau within 35 s (Figs. 3a and b). Binding kinetics were faster than covalent reaction rate. When PDI-nm/ns-Q[10] reached the plateau, the quenching effect kept stable, and this results suggested the fluorescence of PDI-nm/ns-Q[10]-spermine complex was stable. Consequently, PDI-nm/ns-Q[10] has great potential to detect spermine as a highly sensitive fluorescent probe.

In the process of detecting spermine by PDI-nm/ns-Q[10] assembly, the thermal stability, pH tolerance and specific selectivity of the assembly were investigated. As various biologically related species exist, including some amino acids (L-tryptophan (L-Trp), L-methionine (L-Met), L-threonine (L-Thr), L-valine (L-Val), L-lysine (L-Lys), L-leucine (L-Leu), L-isoleucine (L-Iso), L-phenylalanine (L-Phe), tyramine (TY), trimethylamine (TMA), dopamine (DA), 4 × 10⁻⁵ mol/L, respectively) and cation/anion (Cu²⁺, Mn²⁺, Fe³⁺, CO₃²⁻, Br⁻, SO₃²⁻, NO₃⁻, HPO₄²⁻, F⁻, 4 × 10⁻⁵ mol/L, respectively), under the excitation of 500 nm, the fluorescence change of PDI-nm/ns-Q[10] was measured. PDI-nm/ns-Q[10] had negligible response to these amino acids and anions/cations (Fig. 3c). Conversely, when PDI-nm/ns-Q[10] was mixed with spermine, it showed obvious fluorescence quenching. The influence of temperature on fluorescence was investigated. To imitate the environmental temperature, the temperature values of 10 ℃, 20 ℃, 30 ℃, 40 ℃ and 50 ℃ were selected, and the assembly itself showed good temperature stability, and the mixed solution of the assembly with spermine was almost unaffected by temperature (Fig. 3d). Besides, when the pH values were 2, 3, 4, 5, 6, 7, 8, 9 and 10, the fluorescence test showed that the assembly showed excellent pH stability (Fig. S16 in Supporting information).

The satisfactory LOD and high selectivity of PDI-nm/ns-Q[10] for spermine, proved that the performance of the fluorescence sensor can be further investigated in real samples. As shown in Fig. 4a, the maximum emission wavelength of PDI-nm/ns-Q[10] assembly shifted from 554 nm to 545 nm, it showed that some components in plasma can change the maximum fluorescence emission wavelength of PDI-nm/ns-Q[10] assembly. Besides, with the increase of spermine concentration, the fluorescence image of PDI-nm/ns-Q[10] assembly can be clearly observed by naked eyes from bright yellow to quenched color. The ratio of emission intensity at 545 nm exhibited a good linear relationship in the with spermine concentration range of 12 × 10⁻⁶ mol/L to 28 × 10⁻⁴ mol/L (Fig. 4b), which conformed to the following equation: ΔI = 32.52C-320.97, R² = 0.9909. And LOD in serum was 3.69 × 10⁻⁷ mol/L based on LOD = 3σ/k. Table S1 (Supporting information) showed the recovery rates for human plasma ranged from 96.54% and 101.93%. The above results indicated that the fast responses and excellent linear relationship provided a real-time quantitative detection method for spermine in biological samples.

The excellent photoluminescence performance and orderly, compact spatial organization of the PDI-nm/ns-Q[10] assembly were conducived to its potential application prospect in the preparation of high energy transfer efficiency artificial light-harvesting systems. Subsequently, the hydrophobic fluorescent dye NiR, serving as the energy acceptor, was loaded into the hydrophobic environment of the PDI-nm/ns-Q[10] assembly to avoid self-quenching and shorten the distance with donor chromophores, thereby achieving an effective Föster resonance energy transfer (FRET) process. The emission of PDI-nm/ns-Q[10] overlaps well with the absorption of NiR (Fig. 5a), promising the feasibility of energy transfer. As depicted in Fig. 5b, a gradual addition of NiR to the PDI-nm/ns-Q[10] assembly resulted in a decrease in the fluorescence intensity of the PDI-nm/ns-Q[10] assembly (donor) at 554 nm, while the fluorescence emission of NiR (acceptor) at 653 nm increased when excited at 500 nm, indicating an obvious antenna effect at a donor/acceptor ratio of up to 10:1. These results suggested successful loading of NiR into the interior of the PDI-nm/ns-Q[10] assembly, with efficient energy transfer from the donor PDI-nm/ns-Q[10] to the acceptor NiR in aqueous solution, despite their low concentrations (PDI-nm/ns-Q[10] = 1 × 10⁻⁵ mol/L and NiR = 1 × 10⁻⁶ mol/L), resulting an efficient light-harvesting system in aqueous. The energy transfer efficiency and antenna effect were calculated. Consequently, the energy transfer efficiency was determined to be 87% (Fig. S17 in Supporting information), and the antenna effect was calculated as 6.9 (Fig. S18 in Supporting information) at a donor/acceptor ratio of 10:1. These parameters were crucial for evaluating the light-harvesting ability of an artificial system [44].

In summary, the supramolecular assembly PDI-nm/ns-Q[10] acts as a fluorescence indicator displacement assay for the biomolecule spermine. The PDI-nm/ns-Q[10] assembly has great application potential in detecting spermine in aqueous solution and in plasma solution. Additionally, in the constructed artificial light-harvesting system experiment, the hydrophobic fluorescent dye NiR was successfully incorporated into the PDI-nm/ns-Q[10] supramolecular assembly. This led to a highly efficient energy transfer process from PDI-nm/ns-Q[10] to NiR, achieving an energy transfer efficiency of up to 87%. These findings were important for understanding and simulating natural light-harvesting antenna systems. The unique structures and versatile properties of the ns-Q[10]-based supramolecular assembly suggested promising future applications in various fields.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

We acknowledge the support of the National Natural Science Foundation of China (No. 22361011).

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.109744.