Pillar[5]arene-based supramolecular pseudorotaxane polymer material for ultra-sensitive detection of Fe³⁺ and F⁻

Abstract

Recent advancements in ultra-sensitive detection, particularly the Aggregation Induced Emission (AIE) materials, have demonstrated a promising detection method due to their low cost, real-time detection, and simplicity of operation. Here, coumarin functionalized pillar[5]arene (P5C) and bis-bromohexyl pillar[5]arene (DP5) were successfully combined to create a linear AIE supramolecular pseudorotaxane polymer (PCDP-G). The use of PCDP-G as a supramolecular AIE polymer material for recyclable ultra-sensitive Fe³⁺ and F⁻ detection is an interesting application of the materials. According to measurements, the low detection limits of PCDP-G for Fe³⁺ and F⁻ are 4.16 × 10⁻¹⁰ M and 6.8 × 10⁻¹⁰ M, respectively. The PCDP-G is also a very effective logic gate and a material for luminous displays.

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Introduction

The significance of ultrasensitive response in biological, chemical, and environmental applications has recently attracted a lot of attention.^1–5 Many techniques have been employed up to this point for the crucial guest detection. Liu et al.⁶ show an ultra-sensitive photodetector based on a graphene/monolayer MoS₂ vertical heterostructure operating at room temperature, while Liao and colleagues⁷ present an orthogonal framework for cfDNA cancer monitoring via genome-wide mutational integration, enabling ultra-sensitive detection, overcoming the limitation of cfDNA abundance, and empowering treatment optimization in low-disease-burden oncology care. The development of an effective material for the ultrasensitive response to special guests is still a fascinating task, despite the abundance of approaches that have been described.

Pseudorotaxanes, a type of typical molecular machine built by interlocked molecules,⁸ have been greatly implied in drug delivery,^9–11 conducting materials,^12,13 artificial molecular machines,¹⁴ gene delivery,^15,16 functional bioimaging,¹⁷ supra-amphiphiles,^18,19 as well as functional supramolecular systems.^20–23 The supramolecular polymers created from pseudorotaxanes can sense, process, and actuate responses to external change on their own because of the dynamic and reversible nature of noncovalent interactions.^24,25 Due to this characteristic, supramolecular polymers have a considerable advantage over other materials when used as ultrasensitive response material.

Pillar[5]arenes, first reported by Ogoshi,²⁶ have been the main body of the new-generation macrocyclic ring due to its sophisticated pillar structure with electron-rich cavities, ease of functionalization,²⁷ and aggregation-inducing properties.²⁸ Moreover, the pillar[5]arene group provides supramolecular systems with a variety of assembly-driving forces including C–H⋯π, π⋯π and cation⋯π interactions, which can be employed as the primary building block for creating stimuli-responsive supramolecular assembly systems.^29–33 Nowadays, nonporous adaptive crystals,^34–36 chemical sensors,^37–40 catalysis,⁴¹ pseudorotaxanes,^42,43 and supramolecular materials^44,45 have all been made using new supramolecular systems based on functionalized pillar[5]arenes. The development of a novel supramolecular system based on pillar[5]arene for the ultra-sensitive detection of ions is thus of tremendous significance.

In this work, we rationally designed and synthesized a novel pillar[5]arene host P5C by joining a pillar[5]arene group and coumarin group via a hydrazide group in light of the aforementioned factors and our long-standing interest in supramolecular systems. A novel AIE bi-component supramolecular polypseudorotaxane polymer PCDP-G that was created from P5C and DP5 has the potential to be used as an ultra-sensitive luminous material. Our strategies are as follows. Firstly, pseudorotaxanes are formed by one side of the alkyl chain of DP5 being threaded into the pillar[5]arene cavities of P5C and the other being threaded into the pillar[5]arene cavity of DP5. Secondly, both the coumarin and pillar[5]arene moieties of P5C and DP5 could not only act as π–π interaction sites but also serve as fluorophores. As expected, P5C and DP5 could self-assemble into the stable supramolecular polymer PCDP-G. Interestingly, the polymer PCDP-G shows strong AIE properties and could ultra-sensitively detect Fe³⁺ and F⁻. In addition, the PCDP-G could serve as a fluorescent display material as well as an effective logical gateway.

Results and discussion

The synthetic details of pillar[5]arene P5C are shown in Scheme S1.† P5C and their intermediates have been characterized by ¹H NMR, ¹³C NMR and ESI mass spectrometry (Fig. S1–S4, ESI†).

At first, the P5C and DP5 could self-assemble into a supramolecular pseudorotaxanes polymer in cyclohexanol solution (Table S1, ESI†). The lowest critical gelation concentration (CGC) is 10% (w/v, 10 mg mL⁻¹ = 1%), and the higher gel–sol transition temperature (T_gel) is 58–60 °C. Interestingly, the mixture solution of P5C and DP5 in cyclohexanol showed negligible fluorescence (T > T_gel); with the temperature of the hot cyclohexanol solution cooling below the T_gel, the supramolecular pseudorotaxanes polymer PCDP-G showed strong yellow fluorescence at 365 nm and reached a steady state within three minutes (Fig. S5, ESI†). Meanwhile, the sol–gel transition process exhibits excellent circularity (Fig. S6, ESI†). These results indicated that the strong yellow fluorescence of PCDP-G was induced by aggregation-induced emission (AIE).⁴⁶

The self-assembly mechanism of PCDP-G was carefully investigated by ¹H NMR, 2D NOESY NMR and scanning electron microscopy (SEM). As shown in ¹H NMR (Fig. 1 and S7, ESI†), the addition of 1 equiv. DP5 caused the proton signal peaks H_a, H_b, H_c on P5C and H₁, H₂, H₃ on DP5 to show distinct down-field shifts, respectively, while the proton signal peak H₄ on DP5 showed an up-field shifts, which implied that one side of the alkyl chain of DP5 partially threaded into the pillar[5]arene cavity of P5C and the other side threaded into the pillar[5]arene cavity of DP5. As shown in the 2D NOESY spectra (Fig. S8, ESI†), the correlation peaks A and B between the signals of proton H₄ on DP5 with H_a,c on P5C and H_1,2 on DP5, respectively, also evidenced the pseudorotaxanes formed. At the same time, the correlation peaks C of proton H₃ with H_1,2 on DP5 and D of proton H_b with H_a,c on P5C indicated that the pillar[5]arene groups of P5C and DP5 are self-assembled through C–H⋯π interactions, respectively. Moreover, the proton signal peaks H_i, H_j, H_k, H_l, H_m on P5C shifted to downfield also supported that the π⋯π interactions of coumarin moieties were present. The π⋯π interactions mechanism is also supported by the peak E correlations of H_f, H_m with H_i, H_l on P5C. SEM morphological features of PCDP showed a regular spherical morphology, which also supported the self-assemble process (Fig. S9, ESI†). Therefore, there are firstly formed the supramolecular pseudorotaxanes, and then self-assembly via C–H⋯π interactions of the pillar[5]arene groups on P5C and DP5, respectively, and π⋯π interactions of the coumarin group in the formation of supramolecular pseudorotaxanes in the system (Scheme 1).

Fig. 1 Partial concentration-dependent ¹H NMR spectra (600 MHz, 298 K) in DMSO-d₆: (a) free P5C 10 mg mL⁻¹; (b) P5C 10 mg mL⁻¹ and DP5 10.0 mg mL⁻¹; (c) DP5 10.0 mg mL⁻¹.

Scheme 1 The chemical structures, and the self-assemble process of P5C and DP5 for the formation of the supramolecular polypseudorotaxane polymer PCDP-G as well as the mechanism of PCDP-G for fluorescence responses of Fe³⁺ and F⁻.

The fluorescence response abilities of PCDP-G toward cations were studied by diffusion with various cations: Cu²⁺, Co²⁺, Cd²⁺, Ni²⁺, Pb²⁺, Zn²⁺, Cr³⁺, Ca²⁺, Fe³⁺, Mg²⁺, Ba²⁺, Tb³⁺, Ag⁺, Hg²⁺ and La³⁺ (using their solid perchlorate salts as sources). After addition of above cations (water solution, 0.1 M), only Fe³⁺ caused the fluorescence of PCDP-G to quench, and other cations did not show a similar response (Fig. 2), which implied that PCDP-G could selectively detect Fe³⁺. The fluorescence titration of PCDP-G for Fe³⁺ was also carefully carried out. Only 0.009 equiv. Fe³⁺ (water solution, 0.1 M) can induce the fluorescence of PCDP-G to be quenched (Fig. S10, ESI†). Moreover, according to the fluorescence titration of PCDP-G for Fe³⁺ and calculations based on the 3σ/m method,⁴⁷ the lowest detection of the fluorescence spectra change (LOD) of the PCDP-G for Fe³⁺ is 4.16 × 10⁻¹⁰ M, which indicated the PCDP-G could ultra-sensitively detect Fe³⁺ (Fig. S10 and S11, ESI†).

Fig. 2 Fluorescence response of the PCDP-G upon addition of various mental aqueous solution (λ_ex = 375 nm) in cyclohexanol system.

Furthermore, the successive response properties of the supramolecular metal polymer PCDP-GFe (PCDP-G containing 0.01 equiv. Fe³⁺) towards various anions were further discussed by adding aqueous solutions (C = 0.1 M) of various anions, including F⁻, Cl⁻, Br⁻, I⁻, ClO₄⁻, AcO⁻, HSO₄⁻, SCN⁻, CN⁻ and N₃⁻ (for F⁻, Cl⁻, Br⁻, I⁻, HSO₄⁻, AcO⁻ and ClO₄⁻, using their tetrabutylammonium (TBA) salts; for CN⁻, OH⁻, N₃⁻, SCN⁻ and S₂⁻, using their sodium salts). As shown in Fig. 3, only F⁻ induced the quenching of the fluorescence of PCDP-GFe changed to yellow, and other anions could not induce any change. These results indicated that PCDP-GFe could selectivity detect F⁻ in water.

Fig. 3 Fluorescence response of the PCDP-GFe upon addition of various anions aqueous solution (λ_ex = 375 nm) in cyclohexanol system.

Then, in order to investigate the fluorescent response efficiency of the PCDP-GFe towards F⁻, we carried out fluorescence emission titration experiments (Fig. S12, ESI†). After the increasing concentration of F⁻ was gradually added into the PCDP-GFe, the emission intensity of PCDP-GFe recovered. The limits of the lowest detection of the fluorescence spectra change calculated on the basis of 3δ/s method are 6.8 × 10⁻¹⁰ M (Fig. S13, ESI†), which indicated that the PCDP-GFe could be used for ultra-sensitive detection of F⁻. Based on the successive response properties of PCDP-G for Fe³⁺ and F⁻, we can see that the cyclic fluorescence detection was recyclable at least 3 times (Fig. S14, ESI†). These results illustrate that PCDP-G has higher sensitivity when compared with other fluorescence sensors for Fe³⁺ and F⁻ (Tables S3 and S4, ESI†). Therefore, the supramolecular pseudorotaxanes polymer PCDP-G can be used for recyclable ultrasensitive detections of Fe³⁺ and F⁻.

The fluorescence response mechanisms of PCDP-G were carefully investigated via FT-IR and SEM. As shown in the FT-IR spectrum (Fig. 4), the stretching vibration peaks of –NH on PCDP-G appeared at 3419 cm⁻¹. After addition of Fe³⁺, the stretching vibration peaks of –NH shifted to 3455 and 3293 cm⁻¹, and the lactone carbonyl stretching vibration on coumarin moieties at 1722 cm⁻¹ got shifted to a lower wave number at 1717 cm⁻¹ upon complexation. The results are attributed to the oxygen atom and nitrogen atom in the acylhydrazone-based derivatives having strong coordination abilities with Fe³⁺ and the Fe³⁺ trigger amide tautomerization probably.^48–50 The further addition of F⁻ induced the stretching vibration peaks of –NH to shift to 3450 cm⁻¹ and 3298 cm⁻¹, and the stretching vibration peaks of C=O on coumarin returned to 1720 cm⁻¹. These were attributed to the strong combination of F⁻ with Fe³⁺.⁵¹ According to the SEM spectra, the regular spherical morphology of PCDP changed to the cross-linked spherical morphology by adding Fe³⁺ and subsequently changed to the dumbbell pattern. These results also support the above supposition. Therefore, according to these results, the proposed reversible Fe³⁺ and F⁻ response mechanism is shown in Scheme 1, which is based on coordination interactions and competitive binding processes.

Fig. 4 FT-IR spectra of CPDP-G, CPDP-GFe and CPDP-GFe + F⁻ complex powdered in KBr disks.

As the PCDP-G could act as an Fe³⁺ and F⁻ controlled “OFF-On-OFF” fluorescence response switch, this inspired us to apply it as a logic gate strategy for the detection of ions. Herein, the sensitization of PCDP-G luminescence enabled the design of an “IMPLICATION” logic gate. In this logic gate (Fig. 5a and b), the two inputs are Fe³⁺ and F⁻; their absence and presence are defined as “0” and “1”, respectively. The change in fluorescence intensity of PCDP-G acted as an output. The defined “1” and “0” represent the strong fluorescence intensity and weak fluorescence intensity, respectively. When there is no input (0, 0), the output is “1”. Similarly, only the Fe³⁺ input (1, 0) caused the fluorescence intensity to be extremely weak, and the output was “0”. With only F⁻ input, there was a strong fluorescence intensity, and the output was “1”. When the system was input with Fe³⁺ and F⁻ together (1, 0), the fluorescence intensity increased and gave an output signal of “1”. Therefore, this simple supramolecular polymer material was a potential candidate for the development of new generations of digital devices.

Fig. 5 Implementation and truth table for IMP logic function using the supramolecular polymer PCDP-G.

Conclusions

In summary, a novel coumarin-functionalized pillar[5]arene derivative (P5C) was successfully synthesized. P5C and bis-bromohexyl pillar[5]arene (DP5) could form a linear AIE supramolecular poly-pseudorotaxane material (PCDP-G) by C–H⋯π and π⋯π stacking interactions, and the PCDP-G exhibits strong yellow aggregation-induced emission. Interestingly, PCDP-G can be used as an AIE supramolecular polymer material for recyclable ultra-sensitive detection of Fe³⁺ and F⁻. The low detection limits of PCDP-G for Fe³⁺ and F⁻ were measured to be 4.16 × 10⁻¹⁰ M and 6.8 × 10⁻¹⁰ M, respectively. Moreover, the fluorescence changes of PCDP-G upon the addition of Fe³⁺ and F⁻ were utilized as an “IMPLICATION” logic gate. The new strategy of preparing supramolecular polymer material with the yellow AIE effect for sensitive detection of Fe³⁺ and F⁻ may stimulate the development of new approaches to fabricating functional supramolecular materials.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Young Doctor Fund Projects of Gansu, China (No 2021QB-119), the Doctoral Start-up Founds from Long Dong University (No XYBY202013), the Youth Science Foundation of Gansu province (No 21JR7RM194) and the Natural Science Foundation of Qingyang City (No QY2021A-F010).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, synthesis of P5C and DP5, ¹H NMR, ¹³C NMR and MS spectra, and other materials. See DOI: https://doi.org/10.1039/d3ra00997a

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