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• Transmembrane ion transportation is essential for the physiological function of living cells and is regulated by ion transporters [1,2]. The coordination of various ion transporters creates ion concentration gradients between intracellular and extracellular environments, influencing a multitude of vital physiological processes, including cell proliferation, signal transduction, and transcellular transport [3–5]. Among these ions, chloride is the most abundant anion in living organisms, with natural chloride transporters widely distributed across various cell membranes [6,7]. Dysfunction of these chloride transporters can lead to numerous related diseases, including cystic fibrosis, congenital myotonia, hereditary renal lithiasis, and Dent's disease [8–10]. Therefore, the development of high-quality artificial chloride transporters is of significant importance for studying diseases related to natural chloride transporters, screening anticancer drugs, and advancing biosensing technologies [11–14].

  To date, a variety of artificial chloride transporters based on supramolecular chemistry have been developed, including macrocyclic transporters [15,16], nanotube-based artificial transporters [17,18], and supramolecular cage-based transporters [10,19]. Supramolecular metallocages with nanocavity molecular structures are formed through coordination-driven self-assembly using pre-designed ligands and metal ions [20–25]. Firstly, metallocages possess a rigid skeletal structure that provides stability, enhancing their efficacy as ion transporters [26]. Furthermore, the three-dimensional structures and properties of supramolecular metallocages can be readily tuned through ligand modification, improving their permeability with phospholipid bilayers and optimizing ion transport efficiency and selectivity [27–29]. These advantages suggest that metallocages hold significant potential as artificial chloride transporters. Currently, there are only two reports of metallocages functioning as chloride transporters, demonstrating a similar transportation preference for halide anions: Cl⁻ < Br⁻ < I⁻ [10,19]. Thus, further in-depth and extensive research is needed on metallocages as selective chloride transporters.

  In this study, we designed a series of benzo[c][1,2,5]thiadiazole (BTZ)-based metallohelicates (C1 and C2), an important class of metallocages, as anion transporters, specifically prioritizing the transport of chloride [30]. Longer alkyl chains (-C₈H₁₇) were introduced to the exterior of the metallohelicate framework to enhance structural compatibility with the hydrophobic tails of phospholipid molecules, thereby promoting their integration into the phospholipid bilayer. Zinc(Ⅱ) bis(trifluoromethane)sulfonimide (Zn(NTf₂)₂) was selected as the coordination node due to its biocompatibility. Additionally, the BTZ-based metallohelicates feature a rigid backbone that maintains fixed internal cavity structures, which remain unaffected by compression from phospholipid molecules. Furthermore, these metallohelicates exhibit high fluorescence quantum yields, while the phospholipids used are non-fluorescent. This luminescence provides significant advantages for characterizing the incorporation of metallohelicates into phospholipid environments [31]. The BTZ-based metallohelicates also demonstrate electrostatic interactions and binding capabilities with various anions, facilitating selective anion transport. Notably, C1 shows the highest selective transport activity for chloride, likely due to its strong affinity for chloride ions, along with potential contributions from the smaller ionic size.

  The self-assembly process of the BTZ-based metallohelicates C1 and C2 is illustrated in Fig. 1a. It is reasonable to expect that the effective integration of these metallohelicates into the phospholipid bilayer is crucial for their performance as molecular transporters in cellular environments. Notably, the hydrophobic alkyl chains play a vital role in this integration process, influencing both the permeability and stability of the metallohelicates within the lipid bilayer. To modulate this property, subcomponents L1 and L2 (Figs. S1–S8 in Supporting information), which feature alkyl chains of varying lengths, were selected for the preparation of metallohelicates C1 and C2, respectively.

  Figure 1

  

  Figure 1.  (a) Subcomponent self-assembly of metallohelicates C1 and C2. (b) ¹H NMR spectrum of C1 (CD₃CN, 500 MHz, 298 K). (c) Experimental and theoretical ESI-MS spectra of metallohelicate C1. (d) Optimized structure of metallohelicate C1 obtained using Molecular Orbital PACkage (MOPAC) software.

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  As illustrated in Schemes S1–S4 (Supporting information), metallohelicates C1 and C2 were synthesized through the self-assembly of subcomponents L1 and L2 (3 equiv.), bipyridine formaldehyde L3 (2 equiv.), and Zn(NTf₂)₂ (2 equiv.) in acetonitrile, under reflux conditions for 12 h at 70 ℃ (Fig. 1a). Multinuclear nuclear magnetic resonance (NMR) analysis, including ¹H, ¹³C, two-dimensional diffusion-ordered spectroscopy (2D DOSY), and ¹H–¹H correlation spectroscopy (COSY), of assemblies C1 and C2 confirmed the formation of discrete metallohelicates. For example, in the ¹H NMR spectra of assembly C1 (Figs. S9–S11 in Supporting information), the characteristic aldehyde peak at 10.15 ppm disappeared, replaced by a peak at approximately 8.99 ppm, indicative of imidyl hydrogen (Fig. 1b). The sharp signals observed in both the ¹H NMR and DOSY spectra of C1 suggest the formation of a highly symmetric, single-component species, ruling out the presence of oligomers, with a diffusion coefficient of 9.0 × 10⁻¹⁰ m²/s (Fig. S12 in Supporting information). Electrospray ionization mass spectrometry (ESI-MS) further confirmed the formation of discrete metallohelicates. Additionally, electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) revealed major peaks at m/z 1551.3403, 940.9253, and 635.7163, corresponding to various charge states resulting from the loss of bis(trifluoromethylsulfonyl)imide counterions. These peaks can be assigned to the self-assembled [3 + 2] species [M − 2NTf₂⁻]²⁺, [M − 3NTf₂⁻]³⁺, and [M − 4NTf₂⁻]⁴⁺, respectively, where M represents the intact assembly (Fig. 1c). The isotopic distribution of each peak matched simulated patterns, confirming the formation of M₂L₃-type discrete supramolecular metallohelicates. Similar ¹H NMR and ESI-MS spectra were observed for metallohelicate C2 (Figs. S12–S16 in Supporting information).

  To definitively ascertain the structures of the metallohelicates C1 and C2, attempts were made to obtain single crystals of both compounds. Unfortunately, these efforts were unsuccessful, likely due to the multiple conformationally disordered states introduced by the long side chains, which hinder crystallization. As a result, a theoretical optimization of C1 was performed using the PM6 semiempirical method [32]. The computational results indicate that C1 adopts a helical structure (Fig. 1d), with dimensions of 26.73 Å in length, 22.35 Å in width, and 35.36 Å in height. It also features an inner volume radiusof 4.36 Å, providing sufficient space for the transport of halogen ions, particularly considering that the chloride ion has a radius of 1.81 Å [33]. Notably, all the semiempirical optimization were performed using the MOPAC software [34,35].

  The ultraviolet-visible absorption spectroscopy (UV–vis) absorption and fluorescence spectra of the subcomponents L1 and L2 are shown in Fig. 2a. The BTZ-based subcomponents L1 and L2 exhibited maximum absorption peaks at 588 and 586 nm, respectively, while their fluorescence was completely quenched. In comparison, the metallohelicates C1 and C2 displayed a shift in absorption maxima toward shorter wavelengths (Δλ = 41–54 nm, Fig. 2a). Furthermore, the fluorescence of the BTZ moiety was activated during self-assembly, yielding quantum yields of Φ_F = 29.1% and 27.3% for C1 and C2, respectively, making them well-suited for high-resolution fluorescence imaging (Fig. 2b). The changes in optical properties of the subcomponents following cage formation can be attributed to the disruption of ICT/exciplex states during imine formation [36].

  Figure 2

  

  Figure 2.  UV–vis absorption spectra (a) and fluorescence spectra (b) of C1, C2, L1, and L2 in DMSO solution with quantum yield (QY) under conditions of concentration (c) = 10⁻⁶ mol/L, λ_ex = 590 nm. Fluorescence image from mixed solution of GUVs and C1 (c), blank group without C1 (d).

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  Metallohelicate C1 is highly fluorescent, while the phospholipid membranes used in this experiment are non-fluorescent. Both C1 and the hydrophobic layer of the phospholipid membranes exhibit hydrophobicity, which promotes the incorporation of C1 into the membranes due to hydrophobic interactions. To characterize the incorporation of luminescent C1 into phospholipid membranes, a confocal laser scanning microscope (CLSM) was employed. We successfully prepared giant unilamellar vesicles (GUVs) using the electroforming method in the presence of an alternating current electric field [37,38]. The CLSM images revealed a ring of clear fluorescence signals around the GUVs in the C1 and GUVs mixed solution (Fig. 2c), whereas no fluorescence was observed in the blank group without C1 (Fig. 2d) [39]. These results demonstrate that C1 can insert into the hydrophobic region of the phospholipid bilayer without damaging its structural integrity.

  To investigate the ion transport activity of metallohelicates, we used 8‑hydroxy-1,3,6-trisulphonic acid trisodium salt (HPTS), a pH-sensitive fluorescent indicator, to monitor pH changes in vesicles (Fig. 3a). HPTS was encapsulated in large unilamellar vesicles (LUVs) [38,40], and a pH gradient of ΔpH = 0.4 was established between the intravesicular and extravesicular buffers. After adding metallohelicate (0.2 mmol/L in DMSO) to the LUVs suspension, the HPTS fluorescence intensity decreased continuously over a 600-s observation period. These results indicate that the pH within the LUVs was decreasing, likely due to H⁺ influx or OH⁻ efflux. Subsequently, at the end of the 600 s, Triton X-100 was added to lyse the LUVs, exposing all HPTS to the external buffer. The lowest fluorescence intensity was recorded, and the fluorescence intensity data were normalized. Firstly, we examined the effects of side chain length and internal cavity of the metallohelicates on ion transport efficiency. The ion transport activities of metallohelicates C1 and C2, which have the same helical structure, along with their ligands L1 and L2, were explored separately. As shown in Fig. 3b, at the identical concentration of 1.50 mol%, the transport efficiency of C1 reached 49%, compared to only 22% for C2, indicating that C1 exhibited more than twice the activity of C2 during the 600-s monitoring period. Furthermore, the transport activities of L1 and L2 were only 23% and 16%, respectively. In comparison, C1 demonstrated significantly higher ion transport activity. Thus, we speculate that the long alkyl side chains potencially enhance the permeability and hydrophobicity of C1, facilitating its insertion into the phospholipid bilayer. Meanwhile, the results also suggest that a cavity structure is necessary for enabling ion transport, especially when compared to the ligands.

  Figure 3

  

  Figure 3.  (a) Schematic representation of the ion transport experiment using LUVs loaded with the pH-sensitive fluorophore HPTS and exposed to a pH gradient (internal pH 7.2; external pH 6.8). (b) Ion transportation activities of C1, C2, L1, and L2 at a concentration of 1.50 mol%. (c) Cl⁻ transportation activity of C1 at varying concentrations. (d) Normalized calcein fluorescence traces over time in the presence of different concentrations of C1. FL, fluorescence intensity.

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  To further investigate the transportation capability of C1 as transporter, we designed concentration dependent experiments. As shown in Fig. 3c, the ion transportation activity was up to 65% when the concentration of C1 (mol% of C1 relative to total lipid concentration) was 3.75 mol% during the monitoring process of 600 s. Moreover, the ions transportation activitygradually decreased with the decline of C1 concentration, exhibiting strong concentration dependence. Meanwhile, the ions transportation efficiency could still reach to 37% even the concentration of C1 was only 0.15 mol%, which was significantly higher than that of the blank group (6%). Such transportation activity of C1 is comparable with those of reported metallocages as transporters, including naphthalene diimide-based metallocages, in our previous report [10]. These experimental results suggest that C1 indeed mediates ion transportation and is a promising ion transporter.

  To further confirm that C1 mediates the ion transport process through its internal cavity structure, we conducted additional studies on transportation efficiency using calcein, a concentration-sensitive fluorescent indicator. The internal cavity diameter of C1 is 8.72 Å, significantly smaller than calcein's hydration diameter of approximately 16 Å, preventing calcein from being transported through a single C1 molecule [41]. Calcein experiments were performed by encapsulating calcein (40 mmol/L) in LUVs containing 10 mmol/L HEPES, 100 mmol/L NaCl, and pH 7.4 buffer [42]. As shown in Fig. 3d, the addition of different concentrations of C1 resulted in minimal change in fluorescence intensity (approximately 16%) even at 7.50 mol%, similar to the blank group (~7%), indicating no leakage of calcein from the intravesicular to the extravesicular buffer through C1 transporters. Combined with the HPTS experiments described above, we propose that the transporters formed by C1 embedded in the phospholipid bilayer may be mediated by internal cavities [26].

  Next, we investigated the differences in the transportation activities of C1 for various cations and anions. The selectivity of C1 for cation transport was assessed using HPTS experiments. The anion (Cl⁻) concentrations remained unchanged in the external buffer of LUVs, while alkali metal cations were sequentially replaced with Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺. As shown in Fig. 4a, fluorescence intensity changes were approximately 50% across all five different cationic buffers at a C1 concentration of 1.50 mol%. These results indicate that C1 exhibits no selectivity for different cations. This lack of selectivity may be attributed to the helical structure of C1, which features two positively charged zinc ions as coordination centers and three electron-deficient organic ligands, resulting in a deficiency of structural units that can effectively bind to cations to modulate their transport [3].

  Figure 4

  

  Figure 4.  (a) Normalized fluorescence changes of HPTS associated with the transport of alkali metal cations (10 mmol/L HEPES, 100 mmol/L NaCl, M = Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺) by C1 (1.50 mol%) in LUVs suspension as a function of time. (b) Normalized emission intensity of the chloride-sensitive lucigenin dye (λ_ex = 455 nm, λ_em = 506 nm) at various concentrations of C1. (c) Normalized fluorescence changes of HPTS associated with the transport of anions (10 mmol/L HEPES, 100 mmol/L NaX, X = Cl⁻, Br⁻, I⁻, NO₃⁻, or SO₄²⁻) by C1 (1.50 mol%) in LUVs suspension as a function of time. (d) Sequence of binding energies between the anions Cl⁻, Br⁻, I⁻, NO₃⁻, SO₄²⁻, and C1.

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  To elucidate the involvement of chloride in the anion transport processes of C1, we employed lucigenin, a fluorescent indicator sensitive to chloride concentration, to investigate whether chloride was involved in the anion transport processes of C1 [43]. A chloride concentration gradient was established across the inner and outer sides of the LUVs membrane. The influx behavior of chloride was monitored by observing the fluorescence quenching of lucigenin. As shown in Fig. 4b, the fluorescence intensity of lucigenin decreased by 31% at a C1 concentration of 0.15 mol%. As the concentration of C1 increased, the quenching of lucigenin's fluorescence became more pronounced. At a C1 concentration of 2.25 mol%, lucigenin exhibited the greatest decrease in fluorescence intensity, reaching a 65% reduction within 600 s. These experimental results confirm that C1 indeed has the capacity to transport Cl⁻.

  Finally, we investigated the anion transport selectivity by monitoring the transport activities of different anions mediated by C1 through the aforementioned HPTS experiments. The alkali metal cation (Na⁺) in both the intravesicular and extravesicular buffer solutions remained unchanged, while the anions were sequentially replaced with Cl⁻, Br⁻, I⁻, NO₃⁻, or SO₄²⁻. As shown in Fig. 4c, C1 exhibited the highest transport activity for Cl⁻, reaching up to 49%. In comparison, the transport efficiencies for Br⁻ and I⁻ were 35% and 25%, respectively, while the transport activities for NO₃⁻ and SO₄²⁻ were only 16% and 13%. Thus, C1 demonstrated a preference for halogen anions, with the highest activity for chloride, and the lowest for the oxygenated anions NO₃⁻ and SO₄²⁻. We observed a negative correlation between anion size and C1’s transport activity (1.81 Å for Cl⁻ < 1.96 Å for Br⁻ < 2.2 Å for I⁻ < 2.7 Å for NO₃⁻ < 2.9 Å for SO₄²⁻) [33]. Smaller ionic sizes facilitate easier transport through C1, favoring halogen ions over larger NO₃⁻ and SO₄²⁻, which exhibit negligible transport efficiency [19]. Additionally, binding energy calculations indicate that C1’s transport activity strongly depends on the binding energies of anions, ranked as Cl⁻ (2.43 kcal/mol) > Br⁻ (1.69 kcal/mol) > NO₃⁻ (1.35 kcal/mol) > SO₄²⁻ (0.51 kcal/mol) > I⁻ (0.11 kcal/mol) (Fig. 4d, the detailed calculation methods can be found in Supporting information). Chloride, with the highest binding energy and optimal size, is transported most efficiently. This indicates that the combination of ion size and binding energy cooperatively regulates anion transport activity, leading to the highest transport efficiency for chloride.

  In conclusion, we have reported a novel chloride-selective transporter based on metallohelicates. We initially constructed a series of BTZ-based metallohelicates designed for incorporation into phospholipid bilayers. As ion transporters, metallohelicate C1 exhibits strong fluorescence properties, which enhance our ability to characterize the integration of metallohelicates into phospholipid bilayers and to conduct subsequent experiments in living cells using fluorescence-based methods. Metallohelicate C1 is composed of positively charged zinc ions and three electron-deficient organic ligands, resulting in an overall electron-deficient structure that facilitates the binding and transmembrane transport of anions. Notably, metallohelicate C1 demonstrates effective transport capacity for halogen anions, showing the highest selective transportation efficiency for chloride. This selectivity arises from the small ionic size of chloride and the strong affinity between metallohelicate C1 and chloride ions. This work holds significant implications for addressing chloride-related transporter diseases, drug delivery, and other applications.

  Declaration of competing interest

  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.

  CRediT authorship contribution statement

  Xinyu Hu: Writing – original draft, Conceptualization. Bo Song: Writing – original draft, Investigation, Conceptualization. Shukai Song: Investigation. Qinghui Ling: Methodology. Bangkun Yue: Writing – original draft, Investigation, Conceptualization. Lianrui Hu: Writing – original draft, Methodology. Feifei Wang: Methodology. Li He: Methodology. Lin Xu: Writing – review & editing, Conceptualization.

  Acknowledgments

  The authors acknowledge the financial support by the Zhejiang Provincial Natural Science Foundation (No. LQ21B020001), the Wenzhou Science and Technology Association (No. kjfw04), the National Natural Science Foundation of China (Nos. 22103062 and 52105479), the Shanghai Pujiang Program (No. 22PJ1402800), the Shanghai Frontiers Science Center for Molecular Intelligent Syntheses, and the Fundamental Research Funds for the Central Universities.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.110918.

  
  
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  Figure 1  (a) Subcomponent self-assembly of metallohelicates C1 and C2. (b) ¹H NMR spectrum of C1 (CD₃CN, 500 MHz, 298 K). (c) Experimental and theoretical ESI-MS spectra of metallohelicate C1. (d) Optimized structure of metallohelicate C1 obtained using Molecular Orbital PACkage (MOPAC) software.

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  Figure 2  UV–vis absorption spectra (a) and fluorescence spectra (b) of C1, C2, L1, and L2 in DMSO solution with quantum yield (QY) under conditions of concentration (c) = 10⁻⁶ mol/L, λ_ex = 590 nm. Fluorescence image from mixed solution of GUVs and C1 (c), blank group without C1 (d).

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  Figure 3  (a) Schematic representation of the ion transport experiment using LUVs loaded with the pH-sensitive fluorophore HPTS and exposed to a pH gradient (internal pH 7.2; external pH 6.8). (b) Ion transportation activities of C1, C2, L1, and L2 at a concentration of 1.50 mol%. (c) Cl⁻ transportation activity of C1 at varying concentrations. (d) Normalized calcein fluorescence traces over time in the presence of different concentrations of C1. FL, fluorescence intensity.

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  Figure 4  (a) Normalized fluorescence changes of HPTS associated with the transport of alkali metal cations (10 mmol/L HEPES, 100 mmol/L NaCl, M = Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺) by C1 (1.50 mol%) in LUVs suspension as a function of time. (b) Normalized emission intensity of the chloride-sensitive lucigenin dye (λ_ex = 455 nm, λ_em = 506 nm) at various concentrations of C1. (c) Normalized fluorescence changes of HPTS associated with the transport of anions (10 mmol/L HEPES, 100 mmol/L NaX, X = Cl⁻, Br⁻, I⁻, NO₃⁻, or SO₄²⁻) by C1 (1.50 mol%) in LUVs suspension as a function of time. (d) Sequence of binding energies between the anions Cl⁻, Br⁻, I⁻, NO₃⁻, SO₄²⁻, and C1.

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