Recent Developments of Fluorescence Sensors Constructed from Pillar[n]arene-based Supramolecular Architectures Containing Metal Coordination Sites

1. Introduction

Supramolecular chemistry synergizes elements with diverse structures and functions via non-covalent interactions [1,2,3,4,5,6,7,8], enabling the construction of dynamically reversible smart composites [9,10,11,12]. Increasing attention has been given to intelligent supramolecular structures formed by host–guest interactions, particularly for their stimuli-responsive properties and expansive application potential. As a vital segment of supramolecular chemistry, macrocyclic hosts have found extensive use across various domains. Presently, there are five prominent macrocyclic hosts, including crown ethers, cyclodextrins, calix[n]arenes, cucurbit[n]urils, and pillar[n]arenes. Each host can specifically accommodate different guest molecules by offering suitable cavities. Pillar[n]arenes, in particular, stand out due to their rigid and electron-rich cavities, symmetrical frame structure, and exceptional host–guest properties [13,14,15,16,17,18,19,20,21]. The positional modification of pillar[n]arenes endows them with distinct characteristics [22,23,24,25,26,27,28]. This review focuses on pillar[n]arene-based supramolecular architectures (PSAs) that are functionalized with chelating metal ions [29,30].

The coordination metal ions discussed in this review include Ag⁺, Hg⁺, Cu²⁺, Zn²⁺, Pt²⁺, Fe³⁺, Al³⁺, Eu³⁺, and Tb³⁺. Host–guest complexes with metal coordination sites offer considerable scientific value, featuring a pillar-like framework and various self-assembly driving forces such as hydrophobic/hydrophilic, π−π, C−H−π, and cation−π interactions, among others. We categorize two types of supramolecular architectures with metal ion coordination sites: (i) functional pillar[n]arenes with metal coordination sites (e.g., N, O, S atoms) are initially designed and synthesized. These pillar[n]arenes can form supramolecular structures through intermolecular π−π stacking and cation−π interactions after coordinating with metal ions; (ii) guest molecules with metal coordination sites can undergo host–guest interactions with specific pillar[n]arenes to form complexes. These complexes then coordinate with metal ions to create supramolecular structures.

Environmental pollutants, such as hydrazine hydrate, nitrobenzene, nitroaromatics, cyanide perchlorate, halides, and various metal ions, has been recognized as a critical global issue, posing significant threats to public health and natural ecosystems. Consequently, there is an urgent demand for the development of analytical methods to detect these toxic substances. However, challenges persist across multiple fronts. Analytical methods that offer low limit of detection (LOD), excellent sensitivity, outstanding selectivity, and satisfactory removal efficiency are essential, requiring appropriate strategies. Among the methods reported, fluorescence sensing based on supramolecular systems stands out for its accuracy, convenience, and stability, offering superior temporal and spatial resolution and enabling real-time analysis [31,32,33,34,35]. Aggregation-induced emission (AIE) has emerged as a promising sensing mechanism in recent years. This approach involves materials that exhibit negligible emission in dilute solutions and significantly enhanced fluorescence in aggregated states [36,37]. The unique host–guest interactions of pillar[n]arenes can be harnessed to manipulate AIEgens (aggregation-induced emission generators, indicating smart molecules with AIE properties), leading to supramolecular assembly-induced enhanced emission (SAIEE). This offers the potential to develop new materials with photoluminescent properties. Furthermore, materials based on pillar[n]arene-based supramolecular architectures (PSAs) containing metal coordination sites have shown promising results in detecting and absorbing heavy metal ion contamination and other pollutants.

Above all, to our knowledge, the recent advancements in fluorescence sensing using PSAs containing metal coordination sites have not been systematically reviewed. This review discusses recent research results, from molecular structure design and luminescence studies to practical application demonstrations in fluorescence sensing. This discussion should inspire further research into self-assembly systems based on PSAs in fluorescence sensing.

2. Fluorescence Sensors

Stimulation-responsive fluorescence materials represent a significant category of smart materials. In recent years, these materials have garnered attention due to their tunable luminescence, which can be influenced by external stimuli such as light, temperature, pressure, guest compounds, etc. In this section, PSAs are systematically categorized and described based on the range of external stimulus conditions they can response.

2.1. Single-stimulus responsive sensors

Typically, most materials respond to a single stimulus with high sensitivity (Table 1). The molecular structures of pillar[n]arenes and guest molecules mentioned in this section are listed in Scheme 1.

A prime example is the widespread concern over environmental pollution caused by heavy metal ions. Silver(I) (Ag⁺) ions are known for their excellent electrical and thermal conductivity, ductility, and stability. However, excessive silver can disrupt active enzymes by binding to sulfhydryl groups in proteins, leading to their accumulation in the food chain. Therefore, developing economical, simple, environmentally friendly, and efficient materials for detecting and removing Ag⁺ ions is crucial.

Scheme 1. Molecular structures of pillar[n]arenes and guest molecules in the section of single-stimulus responsive fluorescence sensors.

Scheme 1. Molecular structures of pillar[n]arenes and guest molecules in the section of single-stimulus responsive fluorescence sensors.

Table 1. Summary of fluorescence sensors described in the section of single-stimulus responsiveness.

Pillar[n]arenes	Guests	Coordinated metal ions	Sensing properties	Ref.
P1	G1	Ag+	Analyte: Ag+. Detection type: turn−on. λex/λem: 310/470 nm. LOD: 1.50 × 10−7 mol/L. Liner range: 0−8.00 × 10−6 mol/L. Solvent: DMSO/H2O = 1/99.	[38]
P2	−	Ag+	Analyte: hydrazine hydrate. Detection type: turn−off. λex/λem: 310/450 nm. LOD: 2.68 × 10−8 mol/L. Solvent: DMSO/H2O = 4/1.	[39]
P3	−	Hg2+	Analyte: Hg2+. Detection type: turn−off. λex/λem: 332/486 nm. LOD: 4.30 × 10−8 mol/L. Solvent: DMSO/H2O = 1/1.	[40]
P4	G2	Hg2+	Analyte: Hg2+. Detection type: turn−on. λex/λem: 312/388 nm. LOD: 3.00 × 10−7 mol/L. Liner range: 0−1.50 × 10−5 mol/L. Solvent: CHCl3/acetone/H2O = 1/4/495.	[41]
P5	G3	Hg2+	Analyte: Hg2+. Detection type: turn−off. λex/λem: 510/565 nm. LOD: 7.17 × 10−7 mol/L. Solvent: H2O.	[42]
P6	−	Cu2+	Analyte: Cu2+. Detection type: ratiometric. λex/λem: 333/384 nm. LOD: 1.85 × 10−7 mol/L. Solvent: CH2Cl2/CH3CN = 1/1.	[43]
P7	−	Cu2+	Analyte: Cu2+. Detection type: turn−off. λex/λem = 310/411 nm. Solvent: chloroform	[44]
P8	G4	Zn2+	Analyte: nitrobenzene. Detection type: turn−off. λex/λem: 295/460 nm. LOD: 1.66 × 10−4 mol/L. Liner range: 1.00−5.00 × 10−5 mol/L. Solvent: CH3CN/CHCl3 = 1/1.	[45]
P9	−	Fe3+	Analyte: Fe3+, L-Cys. Detection type: turn−off/on. λex/λem: 375/535 nm. LOD: 6.06 × 10−8 mol/L (Fe3+); 1.00 × 10−8 mol/L (L-Cys). Solvent: cyclohexanol.	[46]
P9, P10	P10	Fe3+	Analyte: Fe3+, H2PO4−. Detection type: turn−off/on. λex/λem: 375/530 nm. LOD: 7.54 × 10−9 mol/L. Solvent: cyclohexanol/H2O = 1/20.	[47]

Table 1. Summary of fluorescence sensors described in the section of single-stimulus responsiveness.

Pillar[n]arenes	Guests	Coordinated metal ions	Sensing properties	Ref.
P1	G1	Ag+	Analyte: Ag+. Detection type: turn−on. λex/λem: 310/470 nm. LOD: 1.50 × 10−7 mol/L. Liner range: 0−8.00 × 10−6 mol/L. Solvent: DMSO/H2O = 1/99.	[38]
P2	−	Ag+	Analyte: hydrazine hydrate. Detection type: turn−off. λex/λem: 310/450 nm. LOD: 2.68 × 10−8 mol/L. Solvent: DMSO/H2O = 4/1.	[39]
P3	−	Hg2+	Analyte: Hg2+. Detection type: turn−off. λex/λem: 332/486 nm. LOD: 4.30 × 10−8 mol/L. Solvent: DMSO/H2O = 1/1.	[40]
P4	G2	Hg2+	Analyte: Hg2+. Detection type: turn−on. λex/λem: 312/388 nm. LOD: 3.00 × 10−7 mol/L. Liner range: 0−1.50 × 10−5 mol/L. Solvent: CHCl3/acetone/H2O = 1/4/495.	[41]
P5	G3	Hg2+	Analyte: Hg2+. Detection type: turn−off. λex/λem: 510/565 nm. LOD: 7.17 × 10−7 mol/L. Solvent: H2O.	[42]
P6	−	Cu2+	Analyte: Cu2+. Detection type: ratiometric. λex/λem: 333/384 nm. LOD: 1.85 × 10−7 mol/L. Solvent: CH2Cl2/CH3CN = 1/1.	[43]
P7	−	Cu2+	Analyte: Cu2+. Detection type: turn−off. λex/λem = 310/411 nm. Solvent: chloroform	[44]
P8	G4	Zn2+	Analyte: nitrobenzene. Detection type: turn−off. λex/λem: 295/460 nm. LOD: 1.66 × 10−4 mol/L. Liner range: 1.00−5.00 × 10−5 mol/L. Solvent: CH3CN/CHCl3 = 1/1.	[45]
P9	−	Fe3+	Analyte: Fe3+, L-Cys. Detection type: turn−off/on. λex/λem: 375/535 nm. LOD: 6.06 × 10−8 mol/L (Fe3+); 1.00 × 10−8 mol/L (L-Cys). Solvent: cyclohexanol.	[46]
P9, P10	P10	Fe3+	Analyte: Fe3+, H2PO4−. Detection type: turn−off/on. λex/λem: 375/530 nm. LOD: 7.54 × 10−9 mol/L. Solvent: cyclohexanol/H2O = 1/20.	[47]

In 2022, Wang W.M. and Yang Y.W. designed a stable supramolecular system utilizing host–guest interactions and coordination effects [38]. They employed pillar [5]arene with two adenine binding sites (P1) and tetraphenylvinyl (TPE, G1) functionalized with cyano groups, which possess AIE properties (Figure 1). Spherical supramolecular aggregates formed via 1:1 coordination between P1’s adenines and Ag⁺ ions. The synergistic effect of coordination between P1 and Ag⁺, coupled with the host–guest interaction between P1 and G1, resulted in a crosslinked P1⊂G1@Ag⁺ assembly. This assembly triggered restriction of intramolecular rotation (RIR) and SAIEE mechanisms. The fluorescence lifetime and fluorescence quantum yield of P1⊂G1@Ag⁺ were 4.60 ns and 58.38%, respectively, with the non-radiative decay rate constant being half that of P1⊂G1. Under 365 nm UV lamp irradiation, the assembly displayed bright blue-green fluorescence upon Ag⁺ ion addition, allowing for selective detection and efficient adsorption of Ag⁺ ions. Moreover, the supramolecular assembly could be easily processed without activity loss, offering a valuable tool for practical applications.

The need for effective devices for sensitive toxic gas detection is also paramount. For instance, hydrazine hydrate (DH) is extensively used in synthesizing pesticides and medicines, fuels for satellites and rockets, and as a preservative in nuclear and power plants. However, DH is highly toxic, causing severe damage to the skin and central organs upon human absorption. Consequently, developing flexible, fast, and sensitive materials and methods for DH detection is critically important.

A year latter, Lin Q.’s group developed a metallic gel by coordinating the nitrogen atom of 5-hydroxyquinoline functionalized pillar[5]arene (P2) with Ag⁺ ions [39]. P2 formed a one-dimensional coordination polymer through interaction with Ag⁺ ions via the N atoms on the quinoline groups (Figure 2). Adjacent coordination polymers were interconnected through π–π interactions between the P2 groups, facilitating the metallogel’s formation. The mean squared displacement of the gel P2 was lower than that of the P2-Ag gel, indicating that the addition of Ag⁺ enhanced the elasticity of the P2-Ag gel. The supramolecular assembly was further analyzed from a microscopic morphology perspective. P2 alone exhibited a lamellar structure. After adding Ag⁺ ions, the resultant metallogel P2-Ag transformed into a folded membrane structure, which is attributable to the coordination bond between Ag⁺ ions and P2. When exposed to DH vapor, the folded membrane structure of P2-Ag altered into a microspherical structure, suggesting the disruption of Ag⁺ ion coordination and the formation of a microspherical structure by P2 based on the hydrophobic effect. This gel enabled multi-channel sensitive detection of DH through visual, fluorescence, and electrochemical means. DH disrupted the coordination by reducing Ag⁺ ions under DMSO/H₂O conditions (f_w = 20%), leading to the collapse and fluorescence quenching of the metallogel. The lowest critical gelation concentration (CGC) was 8% (w/v, 10 mg/mL = 1%). The gel−sol transition temperature was 62–63 °C. Multi-channel detection of DH could be conveniently and efficiently realized in both water and air through sound and light alarms. The LOD reached 0.10 mg/m³ in air and 2.68 × 10⁻⁸ mol/L in water, below the US Environmental Protection Agency’s standard for drinking water.

Mercury (Hg²⁺), one of the most toxic heavy metals, poses significant health and environmental risks. Exposure to Hg²⁺ ions, even in minute concentrations, presents a potential hazard to humans. For instance, in 2018, the group led by Lin Q. designed and synthesized another sensor based on artificial intelligence, which utilized bi-pillar[5]arene-based assemblies incorporating advanced AIEgens (P3) [40]. The assembly was driven by intermolecular hydrogen bonding (such as −N−H···C=O− and −C−H···N=C−), π–π stacking interactions, and hydrophobic effects. The Tyndall effect was observed in a 30% aqueous solution, with the critical aggregation fraction of water being 24% for P3, which exhibited a fluorescence quantum yield of 21%. P3 formed a sharp rod-shaped structure in DMSO/H₂O (f_w = 50%). The aggregated P3 was disassembled at low concentrations or high temperatures. A 1:2 complex was formed by binding P3 with Hg²⁺ ions (binding constant: 2.50 × 10³ L²/mol²). The sensing mechanism was based on the coordination between the acylhydrazone group of P3 and Hg²⁺ ions. To explore practical applications, a film was created by immersing a glass sheet in a high concentration of P3. This film enabled the convenient detection of Hg²⁺ ions in water, effectively separating and sensitively detecting them with a LOD of 4.30 × 10⁻⁸ mol/L. This innovative bi-pillar[5]arene AIEgen could pave the way for new designs and developments in pillar[n]arene AIEgens.

The pillar[n]arenes discussed in this paper include two types: pillar[5]arene and pillar[6]arene [41]. The latter was developed by Yang Y.W.’s group. They utilized the strong covalent bonds between thymine (T) and Hg²⁺ ions. Building on this, Dai D.H. and Yang Y.W. constructed a crosslinked supramolecular polymer through host–guest interactions, using a newly designed [2]biphenyl-extended pillar[6]arene with two thymine sites as arms (P4) and a TPE-bridged bis(quaternary ammonium) guest (G2) with AIE properties (Figure 3). The tight T-Hg²⁺-T pairing between the thymine groups and Hg²⁺ ions led to the formation of spherical assemblies with an average diameter of 164 nm. Fluorescent emission occurred immediately upon the addition of Hg²⁺ ions. The introduction of Hg²⁺ ions into the supramolecular system initiated supramolecular SAIEE. With its integrated mode of operation, this supramolecular polymer successfully achieved real-time detection and removal of Hg²⁺ ions from water with high selectivity and a rapid adsorption rate (removal efficiency: 90%). The assembly of P4 and G2 demonstrated excellent recyclability, maintaining effectiveness over more than five cycles of the removal process.

The Hg²⁺ ions, as one of the most toxic heavy metal ions in the environment and water food sources, represent a significant pollution problem worldwide. Numerous research groups are dedicated to designing and synthesizing novel materials for efficient detection and removal of Hg²⁺ ions. Beyond the contributions of the previously mentioned research groups, Jiang X.M. and Cao D.R. reported a self-assembly composed of carboxylatopillar[5]arene sodium salts (P5) and a diketopyrrole-bridged bis-quaternary ammonium guest (G3) via host–guest interaction [42]. Adding P5 to a G3 aqueous solution resulted in the morphological transformation of multilayer nanostructures (Figure 4). This host–guest complex exhibited multiple forces, including electrostatic and hydrophobic interactions and π–π stacking interactions in aqueous solutions. The developed supramolecular system effectively detected and removed Hg²⁺ in real environmental water samples. The Hg²⁺ ions demonstrated synergistic interactions, including coordination with G3 and P5 and the Hg²⁺-cavity, forming a crosslinked network of P5⊂G3@Hg²⁺. The method exhibited good selectivity with a low LOD of 7.17 × 10⁻⁷ mol/L. Furthermore, the quenched fluorescence could be recovered post-treatment with Na₂S, exhibiting a reversible process.

Copper ions (Cu²⁺) are also critical in various industries and can lead to bioaccumulation, soil pollution, and reduced agricultural productivity. Therefore, developing a selective and sensitive Cu²⁺ ions fluorescent chemical sensor, preferably a proportional chemical sensor, remains crucial in ion sensing research.

Pillar[5]arene was modified by Chang R. and Chang K.C. with five neighboring naphthalimide groups to form a new ligand, P6, for metal ion coordination, effectively serving as a ratiometric fluorescence sensor for Cu²⁺ ions in a CH₂Cl₂/CH₃CN = 1/1 system [43]. This sensitivity was also observed in 10% aqueous methanol solutions. The introduction of naphthalimide groups enhanced intramolecular π–π stacking. P6 exhibited dual emission, comprising both the monomer and excimer emissions of the naphthalimide moieties. A synergistic interaction occurred between Cu²⁺ ions and the triazole groups on P6. P6 rapidly bound with Cu²⁺, maintaining stable fluorescence intensity. Upon complexing with Cu²⁺ ions, the excimer emission of ligand P6 was weakened, while the monomer emission intensified. The binding complexation ratio of P6 with Cu²⁺ ions was 1:1, with a binding constant of (3.39 ± 0.40) × 10⁵ L²/mol² and a LOD of 1.85 × 10⁻⁶ mol/L. Particles with diameters ranging from (192 ± 65) to (206 ± 67) nm were produced. The relative fluorescence quantum yields of P1 and P1-Cu²⁺ were 0.13 and 0.11, respectively. Future research may further functionalize the pillar[5]arene framework to enhance its selectivity for specific metal ions or adapt it for various sensing platforms.

Liu S.Y. and Han J. developed another sensor for Cu²⁺ ions [44]. They designed a pillar[5]arene framework by functionalizing it with a 1,3,4-oxadiazole subunit and a cyanobutoxy moiety (P7). This structure facilitated host–guest interactions between the electron-rich pillar[5]arene cavities and appropriately-sized neutral cyanobutoxy moieties, resulting in brush supramolecular polymers. Notably, the larger electron-deficient 1,3,4-oxadiazole groups remained outside the pillar[n]arene (P7) cavity after the formation of the host–guest inclusions, acting as a “brush” and enhancing the ability of the self-assembled supramolecular materials to interact with metal ions, such as Cu²⁺ ions. The critical aggregation concentration of P7 was 6.00 × 10⁻² mol/L. These supramolecular brush-polymer architectures displayed distinct structural changes in response to fluorescence quenching after adding Cu²⁺ ions, suggesting a potential transformation into a crosslinked supramolecular network. The specific recognition of Cu²⁺ might be explained by the Irving-Williams order of stability. Therefore, this supramolecular brush polymer holds potential for application in metal cationic fluorescent chemical sensors.

In 2021, Chong H. et al. prepared a “three-component” supramolecular assembly by combining terpyridine attached pillar[5]arene (P8), cyano and triazole bearing alkyl chain (G4), and Zn²⁺ ions in a CHCl₃ and CH₃CN solvent system [45]. Terpyridine is a versatile building block due to its high binding affinity for a wide range of transition metal ions. The resulting composition exhibited a closely crosslinked porous morphology with nanoscale pore size. The P8 segment was incorporated into the cyano and triazole segment upon encapsulation. An organogel formed at a concentration of 1.00 mol/L. The polymerization was driven by host–guest interaction and metal-chelate cooperative forces. The P8 and Zn²⁺ assembly had a flake-like morphology. Nitrobenzene (picric acid, o-nitrobenzene, and phenol) was used as a sample. The assembly was most sensitive to picric acid among the three analytes, with a LOD of 1.66 × 10⁻⁴ mol/L. The quenching mechanism was believed to involve mixed processes of photo-induced electron transfer (PET) and fluorescence resonance energy transfer (FRET). This study represents a useful example of creating functional “multi-component” supramolecular systems with capabilities for explosive sensing.

The role of Fe³⁺ ions as essential metal elements in human physiology is well-recognized. However, both deficiency and excess of Fe³⁺ ions beyond permissible limits can lead to severe health issues, such as anemia, tumorigenesis, organ dysfunction, and neurodegenerative diseases like Alzheimer’s and Parkinson’s. Consequently, sensitive detection of Fe³⁺ ions is a critical concern in environmental and health monitoring.

In 2018, Zhang Y.M. and Qi L. discovered that the pillar[5]arene structure (P9) could self-assemble into high-molecular-weight supramolecular π-gels with AIE properties [46]. Notably, the simplicity of the supramolecular system components enhanced its practical application value. P9 was self-assembled into one-dimensional linear supramolecular π-gel chains through π–π stacking interactions. These π–gel chains formed a two-dimensional supramolecular network via π–π interactions between the naphthalimide and pillar[5]arene. Subsequently, hydrophobic interactions compressed the two-dimensional network into microspheres. The CGC was 5% (w/v, 10 mg/mL = 1%). The gel−sol transition temperature was 43–45 ℃. The LOD was 6.06 × 10⁻⁸ mol/L for Fe³⁺ ions. The gel demonstrated excellent recyclability and a 99.80% removal rate for Fe³⁺ ions in aqueous solutions. Additionally, the Fe³⁺ ion-coordinated supramolecular gel selectively sensed L-Cys with a LOD of 1.00 × 10⁻⁸ mol/L. This gel could be utilized in logic gates, offering significant advantages over previous logic devices in terms of reversibility, sensitivity, and potential applications for continuous detection of transition metals and amino acids.

In recent years, the development of ultrasensitive materials and responses has garnered extensive attention due to their critical roles in chemical, biological, and environmental applications. In light of this, the above-mentioned group continued to enhance the sensitivity and stability of sensor materials based on their prior findings. They achieved a LOD for Fe³⁺ ions detection reduced from 10⁻⁸ to 10⁻⁹ magnitude [47]. Specifically, they developed a novel supramolecular organic framework (SOF) based on a bis-naphthalimide functionalized pillar[5]arene host (P9) and a bis-ammonium functionalized pillar[5]arene guest (P10). This SOF formed a stable supramolecular gel in cyclohexanol, capable of ultra-sensitive response to Fe³⁺ ions through the competition of cation–π and π–π interactions. Upon adding 0.50 equivalent of Fe³⁺ ions, the yellow fluorescence of the gel was quenched with a LOD of 7.54 × 10⁻⁹ mol/L. The Fe³⁺ ion-coordinated gel could sense H₂PO₄⁻ ions with a fluorescence “turn−on” response and a LOD of 4.21 × 10⁻⁹ mol/L. Response films were prepared using these soft gels, which could also serve as ultra-sensitive ion sensors, fluorescent display materials, and sensitive logic gates.

2.2. Dual-stimuli responsive sensors

The development of dual-response materials (Table 2) for complex systems is of great significance and challenge. The molecular structures of pillar[n]arenes and guest molecules associated with these smart materials are listed in Scheme 2.

In 2018, the group of Zhang Y.M. and Wei T.B. constructed an advanced bifunctional supramolecular pseudorotaxane chemosensor using host pillar[5]arene P11 and guest G5 [48]. Adding Ag⁺ and Hg²⁺ ions to this system resulted in fluorescence quenching and significant changes of fluorescence color, respectively. This method demonstrated high sensitivity and selectivity, with LODs of 1.20 × 10⁻⁸ and 5.00 × 10⁻⁷ mol/L for Ag⁺ and Hg²⁺ ions, respectively.

The group led by Wei T.B. [49] developed a novel metallosupramolecular polypseudorotaxane by integrating thioacetylhydrazine functionalized pillar[5]arene (P12) and bis-butyltrimethyl ammonium functionalized pillar[5]arene (P13) with Zn²⁺ ions (Figure 5). The hydrazyl groups served as coordination and hydrogen bond sites for self-assembly with Zn²⁺ ions. This gel acted as an ultrasensitive fluorescence sensor for detecting Fe³⁺ and Cu²⁺ ions, with LODs of 8.93 × 10⁻¹⁰ and 4.57 × 10⁻⁸ mol/L, respectively. Thin films based on this gel could be used as convenient test kits. Furthermore, this study presented a simple and effective strategy for achieving multiple stimulus responses by introducing multiple non-covalent interactions in metal supramolecular gels.

In the context of escalating environmental pollution and its threats to human health and safety, nitroaromatic compounds (NACs) have been identified as major pollutants. NACs are extensively used in industries such as dyes, explosives, pesticides, and pharmaceuticals. For instance, p-nitrophenol can cause serious health issues, including fever, liver, kidney damage, and methemoglobinemia. In 2023, Yu D.Y., Deng W.T., and Wei X.Q. synthesized a blue-green fluorescent supramolecular aggregate capable of self-assembling from pillar[5]arene-based Cu²⁺ ions coordination complexes [50]. The Cu²⁺ ions were octahedrally coordinated with two carboxyl O atoms from one P14 molecule and four O atoms from four water molecules (Figure 6). These coordination complexes were self-assembled into spherical aggregates through multiple non-covalent forces, including hydrophobic effects, van der Waals forces, and CH−π interactions.The aggregate exhibited high sensitivity and selectivity to nitroaromatic compounds and metal ions. The fluorescence emission of the aggregate was specifically and significantly quenched by p-nitrophenol and Fe³⁺ ions, with low LOD values of 3.90 × 10⁻⁷ and 4.90 × 10⁻⁶ mol/L, respectively. Additionally, due to their exceptional performances, these coordinated supramolecular aggregates based on pillar[n]arenes hold promise for various applications in adsorption, sensing, cargo transport, and catalysis.

Pillar[5]arene, decorated with sulfur substituents (P15), demonstrated control over binding affinities towards organic guest encapsulations within its cavity [51]. The exterior cavity of P15 tended to form metal-ion coordination bonds (Figure 7). Adding a dinitrile guest (G6) containing a bis-triazole benzene spacer led to the formation of pseudorotaxane host–guest complexes. The fluorescence emission signals were significantly reduced in the presence of Hg²⁺ and Cu²⁺ ions. These macrocycles facilitated the transfer of Cu²⁺ ions from the aqueous to the organic phase, enhancing efficiency compared to extraction processes without pillar[5]arene.

Zhang Y.M. and Lin Q. created a unique SOF gel using pillar[5]arene host (P2) and a bilateral bromohexyl functionalized pillar[5]arene guest (P16) [52]. The SOF gel was constructed through CH−π, CH−O, and “exo−wall π–π” interactions between P2 and P16. The introduction of Fe³⁺ ions into the gel resulted in quenched fluorescence. The gel exhibited a LOD of 1.02 × 10⁻¹⁰ mol/L for Fe³⁺ ions. With the competitive coordination of F⁻ ions, a fluorescent “turn−on” response of the Fe³⁺ ions coordinated metallogel was achieved, having a LOD of 9.79 × 10⁻⁹ mol/L. The xerogel of this SOF gel demonstrated an adsorption rate of 99.90% for removing Fe³⁺ ions. This SOF gel and the Fe³⁺ coordinated metallogel served as an ultrasensitive and reversible fluorescence sensor and test kit for Fe³⁺ ions and F⁻ anions.

The detection of toxic pollutant ions is always a significant problem. Many anions are harmful to human health. Perchlorates (ClO₄⁻ anions) interfere with the synthesis and secretion of thyroxine and, therefore, affect the normal metabolism and metabolism of the human body. Cyanide (CN⁻ anion) is a highly toxic compound. The CN⁻ anions were widely used in industry and occur in nature. Therefore, detecting and removing these toxic ions is an important task. Zhang Q. and Lin Q. have been actively working to address these challenges [53]. The powder of P17 displayed a rod-like structure, while G7 formed a blocky structure. P17 and G7 assembled via π–π interactions to create a xerogel structure with a crosslinked network. This xerogel exhibited strong yellow AIE properties. It showed a coordination ability for rare earth metal ions Eu³⁺ and Tb³⁺, enabling it to detect ClO₄⁻ and CN⁻ anions through a fluorescence “turn−on” mode. Additionally, the xerogel demonstrated efficient removal capabilities for these two anions.

2.3. Multi-stimuli responsive sensors

To accomplish multi-stimuli-responsive detection in complex environments, it is crucial to continuously explore various units to construct novel multi-stimuli-responsive fluorescence materials. The sensing capacities of these materials are presented in Table 3. And the molecular structures of pillar[n]arenes and guest molecules newly mentioned in this section are listed in Scheme 3.

Yang H.B.’s group synthesized a dipyridyl donor (P18) containing a TPE scaffold and two pillar[5]arene units [54]. P18 exhibited broad absorption bands at a wavelength of 336 nm with molar absorption coefficients of 2.10 × 10⁴ L¹∙mol⁻¹∙cm⁻¹. Utilizing P18, they constructed a rhomboidal metallacycle with four pillar[5]arene units and a hexagonal metallacycle with six pillar[5]arene units (Figure 8). These structures led to a new family of crosslinked AIE supramolecular polymer gels, driven by hierarchical self-assembly involving coordination between the nitrogen and platinum atoms and host–guest interactions. These supramolecular polymer gels demonstrated gel−sol transitions responsive to various stimuli, including temperature, competing guest molecules, halides, and reversible “on−off” fluorescence. With their multifaceted response characteristics, these crosslinked AIE supramolecular polymer gels are expected to significantly contribute to diverse fields, such as molecular sensors, biological imaging, and drug delivery.

The pillar[5]arene (P2) was able to coordinate with Al³⁺ ions, forming a supramolecular polymer metallogel that exhibited light blue AIE in the DMSO/H₂O binary solution [55]. This gel showed responsiveness to multiple external stimuli, including temperature, guest compounds, ions (Fe³⁺ ions, F⁻ anions), acids (trifluoroacetic acid, TFA), and bases (triethylamine, TEA). The LODs ranged from 4.39 × 10⁻⁹ to 1.82 × 10⁻⁷ mol/L for Fe³⁺ ions, F⁻ anions, TFA, and TEA. The gel could detect Fe³⁺ ions, and Fe³⁺ ions coordinated metallogel could detect F⁻ anions through cation–π interactions and competitive coordination. The LOD for Fe³⁺ ions in the gel was 4.39 × 10⁻⁹ mol/L, while the LOD for F⁻ anions in the Fe³⁺ ion-coordinated metallogel was approximately 2.75 × 10⁻⁸ mol/L. Furthermore, the gel could detect TFA and TEA sequentially through protonation and deprotonation processes. Protonation enhanced the π–π stacking interaction between P2 molecules. This material could be employed for multiple “write−erase−write” cycles, showcasing its versatility in detecting various stimuli.

Functional supramolecular polymer networks (SPNs) exhibit tunable luminescence properties [56]. These characteristics enhance the repertoire of functional building blocks for macrocyclic arene-based supramolecular gels, differentiating them from traditional pillar[n]arenes. Developing soft materials based on pillar[n]arenes has captured the interest of many researchers. In 2019, Liu Z.J. and Yang Y.W. synthesized a monofunctionalized pillar[6]arene (P19) bearing a single terpyridine arm. They constructed a fluorescent SPN from P19, a TPE aggregation-induced emission generator G8, and Zn²⁺ ions as a coordinating linkage. The disruption of metal coordination and/or host–guest interactions by competitive binders like TFA or pillar[5]arene, as well as heating, could weaken non-covalent interactions in the supramolecular polymer gels, inducing a gel−to−sol transition. These novel architectures offer promising avenues for the development of fluorescent SPN materials.

Another SPN was developed through the hierarchical self-assembly of a double-armed pillar[5]arene host (P20), a ditopic guest (G9), and Zn²⁺ ions [57]. This assembly exhibited sol–gel transition properties responsive to various external stimuli, including temperature, redox, pH changes, and competitive guests (Figure 9). Additionally, this assembly served as a convenient and feasible test kit for sensing OH⁻ anions. Given the advantageous properties of host–guest molecular recognition and the induction of metal-ligand coordination interactions, this work laid the groundwork for creating multi-stimuli responsive metal supramolecular gels based on pillar[n]arene. These developments are significant for advancing intelligent supramolecular materials with novel structures and diverse functionalities.

3. Conclusions and Outlooks

This review summarizes the recent advancements in PSAs for developing fluorescence sensors capable of detecting environmental pollutants. Over the past five years, significant progress has been made in delineating the molecular structures, self-assembly mechanisms, photophysical properties, and sensing applications of these compounds. Recent PSAs-based sensors demonstrate enhanced capabilities in detecting various pollutants, offering superior solvent compatibility, porosity, versatility, and diverse detection capacities compared to traditional fluorescence sensors.

As highlighted above, fluorescence sensors have achieved notable success in designing and synthesizing innovative molecular structures and applying stimulus-responsive smart materials. However, the field faces new challenges and opportunities for future exploration and innovation. Firstly, the low concentration of analytes in real samples necessitates improvements in sensitivity and selectivity. Secondly, the fluorescence modulation mechanism under supramolecular regulation remains unclear, warranting further investigation into the luminescence mechanism, particularly concerning charge distribution and molecular architecture accumulation. Thirdly, the complexity of the constructed supramolecular systems often results in poor stability, posing challenges in practical applications. Simplifying these assemblies while maintaining or enhancing detection performance is critical for future research. Fourthly, developing supramolecular architectures that combine detection and adsorption functions requires further study, particularly in creating stable, reusable materials. Fifthly, there is a need for more research into PSAs capable of detecting multiple analytes simultaneously. Finally, while significant research has been conducted in developing supramolecular fluorescence sensors, their reliance on organic solvents hinders their applicability in biological and environmental systems.

In summary, fluorescence sensors based on PSAs containing metal coordination sites have been rapidly and continuously developed. We hope this review can help to promote design strategies for these sensors to better detect and adsorb environmental pollutants.