Macrocycle-based supramolecular polymers have attracted more interest in recent years and found many practical applications with controlled shape and unique physical and chemical properties. In this work, a binary supramolecular polymer is prepared in water by the strong host-guest interactions between a cationic pillar[5]arene and a symmetric sulfonate with dual binding sites. Isothermal titration calorimetry measurements show that the binding process of host-guest complexation is governed in a thermodynamically favorable way with a high stability constant of up to 10⁷ m⁻¹ order of magnitude. Moreover, spectroscopic and microscopic investigations jointly demonstrate that the resulting stable polymer is multistimuli responsive towards temperature and competitive metal coordination. We anticipate that our obtained multistimuli-responsive supramolecular polymer may offer a new way in the construction of more functionalized pillararene-based nanoassemblies.

A supramolecular assembly was constructed with a cationic cyclodextrin (EICD) and native hyaluronan (HA). The cationic carboxylic ester pendants on HA support hyaluronidase (HAase)-responsive sites and the EICD supports artificial carboxylic esterase responsive sites. Substrate-binding models were investigated by using environment-sensitive fluorescence probes 2-p-toluidino-6-naphthalenesulfoniate sodium (2,6-TNS) and thioflavin T (ThT). On a HA/EICD assembly, EICD was able to bind an anionic substrate and HA and EICD constructed the cationic substrate binding site together. This assembly could be used as a sequential dual-substrate carrier.

Enzyme-responsive assembly represents one of the increasingly significant topics in biomaterials research and finds feasible applications to the controlled release of therapeutic agents at specific sites at which the target enzymes are located. In this work, based on the concept of host–guest chemistry, a trypsin-responsive supramolecular vesicle using p-sulfonatocalix[4]arene as the macrocyclic host and natural serine protease trypsin-cleavable cationic protein protamine as the guest molecule, is reported. The complexation of p-sulfonatocalix[4]arene with protamine directs the formation of a supramolecular binary vesicle, which is dissipated by trypsin with high selectivity. Therefore, the present system represents a principle-of-concept to build a controlled-release carrier at trypsin-overexpressed sites.

Cyclodextrins (CDs), a class of cyclic oligosaccharides, are water-soluble, nontoxic, and commercial available with a low price, and their well-defined hydrophobic cavity can bind various organic/biological substrates. Through their molecular assembly mediated by organic, inorganic, or polymeric molecules as templates, CDs and their functional derivatives can be assembled to 1D supramolecular strands, wherein the functional groups of the CDs are closely located in a highly ordered manner. This structural feature greatly favors the cooperative effect of numerous functional groups in the supramolecular strand, as well as the interactions of the supramolecular strands with the multiple binding sites of substrates, especially biological substrates. Therefore, CD-based 1D supramolecular strands exhibit many material, biological, and catalytic functions, and these properties can be further improved through their secondary assembly. An overview of recent advances in the development of the construction and functions of CD-based 1D supramolecular strands and their secondary assemblies is given here. It is expected that the representative contributions described can inspire future investigations and lead to discoveries that promote the research of CD-based functional materials.

A photochemically interconvertible supramolecular nanotube–nanoparticle system was constructed through secondary assembling of self-aggregates of amphiphilic porphyrin derivatives mediated by trans- and cis-azobenzene-bridged bis(permethyl-β-cyclodextrin). Significantly, these nanotubes and nanoparticles were able to interconvert upon irradiation at different wavelengths, and this photocontrolled morphological conversion is reversible and recyclable for tens of times, which will provide a feasible and convenient way to construct the ordered nanostructure with various morphologies that can be smartly controlled by the environmentally benign external stimulus.

A photochemically interconvertible supramolecular nanotube–nanoparticle system was constructed through secondary assembling of self-aggregates of amphiphilic porphyrin derivatives mediated by trans- and cis-azobenzene-bridged bis(permethyl-β-cyclodextrin). Significantly, these nanotubes and nanoparticles were able to interconvert upon irradiation at different wavelengths, and this photocontrolled morphological conversion is reversible and recyclable for tens of times, which will provide a feasible and convenient way to construct the ordered nanostructure with various morphologies that can be smartly controlled by the environmentally benign external stimulus.

An amphiphilic compound, 5-(4′-dodecyloxyphenyl)-10,15,20-tri(permethyl-β-CD)-modified Zn^II–porphyrin (1; β-CD=β-cyclodextrin), was synthesized by means of the click reaction of an alkylated Zn–porphyrin derivative with 6-deoxy-6-azidopermethyl-β-CD. The complexation between 1 and tetrasodium tetraphenylporphyrintetrasulfonate (5) with different molar ratios led to the formation of two distinctly different nanoarchitectures, which were proven to be vesicle and network aggregates, respectively, by using dynamic light scattering, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. On the basis of the results of the time-dependent TEM studies, fluorescence, and NMR spectroscopic measurements, we have determined that the mechanism of the morphology transition from vesicles to networks is controlled by the stepwise complexation of 1 with 5. Furthermore, these supramolecular nanoarchitectures show the controlled- release property of doxorubicin as potential candidates for drug delivery.

A hyperbranched supramolecular polymer (HSP) is constructed based on the intermolecular inclusion complexation of bridged tris(permethyl-β-cyclodextrin) with Mn^III-porphyrin bearing poly(ethylene glycol) (PEG) side chains (Mn^III-TPP), and characterized by UV/vis absorption spectroscopy, isothermal titration calorimetry (ITC), NMR, dynamic light scattering (DLS), atomic force microscopy (AFM) and transmission electron microscopy (TEM), respectively. The presence of permethyl-β-cyclodextrin can stabilize the low-valent Mn^II-TPP. In vitro cell experiment indicates that this HSP has no remarkable cellular toxicity against NIH 3T3 cells. In vitro MR imaging experiment indicates a slightly increased T₁ relaxivity for this HSP compared to that for the linear supramolecular polymer.

A water-soluble supramolecular polymer with a high degree of polymerization and viscosity has been constructed based on the strong host–guest interaction between p-sulfonatocalix[4]arenes (SC4As) and viologen. A homoditopic doubly ethyl-bridged bis(p-sulfonatocalix[4]arene) (d-SC4A) was prepared and its binding behavior towards methyl viologen compared with the singly ethyl-bridged bis(p-sulfonatocalix[4]arene) (s-SC4A) by NMR spectroscopy and isothermal titration calorimetry. By employing a viologen dimer (bisMV⁴⁺) as the homoditopic guest, two linear AA/BB-type supramolecular polymers, d-SC4A⊃bisMV⁴⁺ and s-SC4A⊃bisMV⁴⁺, were successfully constructed. Compared with s-SC4A⊃bisMV⁴⁺, d-SC4A⊃bisMV⁴⁺ shows much higher solubility and viscosity, and has also been characterized by viscosity, diffusion-ordered NMR spectroscopy, dynamic light scattering, and atomic force microscopy measurements. Furthermore, the polymer is responsive to electrostimulus as viologen is electroactive, which was studied by cyclic voltammetry. This study represents a proof-of-principle as the polymer can potentially be applied as a self-healing and degradable polymeric material.

A linear supramolecular architecture was successfully constructed by the inclusion complexation of α-cyclodextrin with azobenzene and the host-stabilized charge-transfer interaction of naphthalene and a bispyridinium guest with cucurbit[8]uril in water, which was comprehensively characterized by ¹H NMR spectroscopy, UV/Vis absorption, fluorescence, circular dichroism spectroscopy, dynamic laser scattering, and microscopic observations. Significantly, because it benefits from the photoinduced isomerization of the azophenyl group and the chemical reduction of bispyridinium moiety with noncovalent connections, the assembly/disassembly process of this supramolecular nanostructure can be efficiently modulated by external stimuli, including temperature, UV and visible-light irradiation, and chemical redox.

The condensation of DNA in a controlled manner is one of the key steps in gene delivery and gene therapy. For this purpose, a water-soluble supramolecular nanostructure is constructed by coating 14 β-cyclodextrins onto the surface of a gold nanoparticle, followed by the noncovalent association of different amounts of anthryl-modified adamantanes with coated β-cyclodextrins. The strong binding of β-cyclodextrins with anthryl adamantanes (K_S=8.61×10⁴ M⁻¹) efficiently stabilizes the supramolecular nanostructure. Spectrophotometric fluorescence spectra and microscopic studies demonstrated that, with many anthryl grafts that can intercalate in the outer space of the DNA double helix, this supramolecular nanostructure showed good condensation abilities to calf thymus DNA. Significantly, the condensation efficiency of supramolecular nanostructure towards DNA could be conveniently controlled by adjusting the ratio between gold nanoparticles and anthryl adamantane grafts, leading to the formation of DNA condensates of a size that are suitable for the endocytosis of hepatoma cells, which will make it potentially applicable in many fields of medicinal science and biotechnology.

The extremely strong noncovalent complexation between the rigid host of phthalocyanine-bridged β-cyclodextrins and the amphiphilic guest carboxylated porphyrin is employed to construct a hollow tubular structure as a supramolecular nanoreactor. A representative coupling reaction occurs in the hydrophobic interlayers of the tubular walls in pure water at room temperature, leading to an enhancement of ten times higher reaction rate without any adverse effect on catalytic activity and conversion.

A π-conjugated phenylaza-15-crown-5-triazol-substituted coumarin fluoroionophore 1 was synthesized by copper(I)-catalyzed Huisgen alkyne-azide 1,3-dipolar cycloaddition (CuAAC "click" reaction). 1 can display selective fluorescence enhancement toward Fe³⁺ over Hg²⁺, Cr³⁺ and the other metal ions in aqueous solution. In sharp contrast, the fluorescence behavior between Fe³⁺ and Hg²⁺ is completely reversed in EtOH. That is, Hg²⁺ gives the largest fluorescence enhancement over Cr³⁺, Fe³⁺ and the other metal ions.

The social self-sorting supramolecular assembly is described by non-covalent interactions among four organic components. Toward this goal, a series of self-sorting systems have been investigated by mixing two or three different compounds; naphthyl-bridged bis(α-cyclodextrin), N,N′-dioctyl-4,4′-bipyridinium, 2,6-dihydroxynaphthalene, and cucurbit[8]uril. The influence of alkyl chains of viologen derivatives and the binding abilities of these systems have also been studied. Their integrative self-sorting led to the exclusive formation of the purple supramolecular heterowheel polypseudorotaxane. The heterowheel polypseudorotaxane is a thermodynamically stable self-sorted product, and consists of two different macrocycles with three sorts of different non-covalent interactions. Its structure was established by NMR spectroscopy and UV/Vis absorption spectroscopy, transmission electron microscopy (TEM), atomic force microscopy (AFM), dynamic light-scattering (DLS), diffusion-ordered spectroscopy (DOSY), and viscosity measurements.

The binding geometries, abilities and thermodynamic parameters for the intermolecular complexation of three water-soluble p-sulfonatocalix[n]arenes (SCnAs), p-sulfonatocalix[4]arene (SC4A), p-sulfonatocalix[5]arene (SC5A), andp-sulfonatothiacalix[4]arene (STC4A), with five local anesthetics (LA), procaine, tetracaine, lidocaine, dibucaine, and procainamide, have been determined by means of ¹H NMR spectroscopy, X-ray crystallography, and isothermal titration calorimetry (ITC). The obtained results show that the complexation of SCnAs with LA guests shows unappreciable guest selectivity, but regular host selectivity: SC4A > SC5A > STC4A for each guest. A large difference in the enthalpy term is responsible for the decrease in the stability constants. The intrinsic relationship between binding structures and host selectivities were comprehensively analyzed and discussed from the viewpoint of thermodynamics. Furthermore, the complexation of SC4A towards LA guests were compared with other macrocyclic hosts, β-cyclodextrin and cucurbit[7]uril.

A fluorescent cyclodextrin/carbon nanotube assembly was easily constructed through the non-covalent attachment of adamantanylpyrene on carbon nanotube and the following association of cyclodextrin derivative bearing fluorescent substituent, and its structure was fully characterized by UV/Vis/NIR spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy and atomic force microscopy. Fluorescence spectroscopic and fluorescence microscopic studies showed that the resultant non-covalently functionalized fluorescent nanotube could be used as a highly selective fluorescent probe for Zn²⁺ in both water and living cells. Without carbon nanotube, the fluorescence probe was unable to enter the cell but only anchored on the cell membrane. This approach will overcome the disadvantage of many spectral sensors that are unable to enter living cells and greatly improve the application of naotube-related supramolecular architecture in nanoscience and technology.

The complexation-induced critical aggregation concentrations of 1-pyrenemethylaminium by mono-p-sulfonatocalix[n]arenes and bis-p-sulfonatocalix[n]arenes (n=4, 5) were systemically measured by fluorescence spectroscopy. In all cases, the complexation-induced critical aggregation concentration decreases by about 3 times upon addition of p-sulfonatocalix[n]arenes. However, the optimal molar ratios for the aggregation of 1-pyrenemethylaminium by mono-p-sulfonatocalix[n]arenes and bis-p-sulfonatocalix[n]arenes are distinctly different: For mono-p-sulfonatocalix[n]arenes, the optimum mixing ratio for the aggregation of 1-pyrenemethylaminium is 1:4 mono-p-sulfonatocalix[n]arenes/1-pyrenemethylaminium, whereas only 2.5 molecules of 1-pyrenemethylaminium can be bound by one cavity of bis-p-sulfonatocalix[n]arenes. The intermolecular complexation of mono-p-sulfonatocalix[n]arenes and bis-p-sulfonatocalix[n]arenes with 1-pyrenemethylaminium led to the formation of two distinctly different nanoarchitectures, which were shown to be nanoscale vesicle and rod aggregates, respectively, by using dynamic laser scattering, TEM, and SEM. This behavior is also different from the fiber-like aggregates with lengths of several micrometers that were formed by 1-pyrenemethylaminium itself above its critical aggregation concentration. Furthermore, the obtained nanoaggregates exhibit benign water solubility, self-labeled fluorescence, and, more importantly, temperature responsiveness.

A multiple supramolecular assembly, in which a folic acid-modified β-cyclodextrin (1) acted as a target unit, an adamantanyl porphyrin (2) acted as a linker unit, and graphene oxide acted as a carrier unit, was successfully fabricated through non-covalent interactions and comprehensively investigated by means of UV/Vis, fluorescence, and X-ray photoelectron spectroscopies, and electron microscopy. Significantly, the graphene oxide unit could associate with the anticancer drug doxorubicin through π–π interactions, and the folic acid-modified β-cyclodextrin unit could recognize the folic acid receptors in cancer cells. Owing to the cooperative contribution of these three units, the resulting multiple supramolecular assembly, after association with doxorubicin, exhibited better drug activity and much lower toxicity than free doxorubicin in vivo.

Employing bis(p-sulfonatocalix[4]arenes) (bisSC4A) and N′,N′′hexamethylenebis(1-methyl-4,4′-bipyridinium) (HBV⁴⁺) as monomer building blocks, the assembly morphologies can be modulated by cucurbit[n]uril (CB[n]) (n=7, 8), achieving the interesting topological conversion from cyclic oligomers to linear polymers. The binary supramolecular assembly fabricated by HBV⁴⁺ and bisSC4A units, forms an oligomeric structure, which was characterized by NMR spectroscopy, atomic force microscopy (AFM), transmission electron microscopy (TEM), dynamic light scattering (DLS), isothermal titration calorimetry (ITC), and gel permeation chromatography (GPC) experiments. The ternary supramolecular polymer participated by CB[8] is constructed on the basis of host–guest interactions by bisSC4A and the [2]pseudorotaxane HBV⁴⁺@CB[8], which is characterized by means of AFM, DLS, NMR spectroscopy, thermogravimetric analysis (TGA), UV/Vis spectroscopy, and elemental analysis. CB[n] plays vital roles in rigidifying the conformation of HBV⁴⁺, and reinforcing the host–guest inclusion of bisSC4A with HBV⁴⁺, which prompts the formation of a linear polymer. Moreover, the CB[8]-participated ternary assembly could disassemble into the molecular loop HBV²⁺@CB[8] and free bisSC4A after reduction of HBV⁴⁺ to HBV²⁺, whereas the CB[7]-based assembly remained unchanged after the reduction. CB[8] not only controlled the topological conversion of the supramolecular assemblies, but also improved the redox-responsive assembly/disassembly property practically.

A [2]catenane and a pretzelane that consist of one aromatic dialkylammonium spacer, one dibenzo-24-crown-8 (DB24C8) unit, and one Sn–porphyrin dihydroxide component were synthesized. In the [2]catenane, dibenzo-24-crown-8 and the Sn–porphyrin dihydroxide are two independent components, whereas in the pretzelane, dibenzo-24-crown-8 and the Sn–porphyrin dihydroxide are linked covalently. The interlocked molecules were obtained by the axial ligation of Sn–porphyrin dihydroxide with the resorcinol moieties of both termini in the aromatic dialkylammonium spacer.

Cucurbiturils (CBs) and cyclodextrins (CDs) are two important classes of macrocyclic molecules, both of which have the capability of forming stable complexes with various molecular or ionic substrates. These capabilities consequently make them attractive as both receptors for molecular recognition and building blocks for the construction of nano-scaled supramolecular systems. This review mainly deals with representative contributions in comparative studies of selective binding and molecular assembly behaviors of cucurbiturils and cyclodextrins possessing different types of hydrophobic cavities. It also gives a description of the construction of nanometer-scaled supramolecular architectures through the cooperative assembly employing cucurbiturils and cyclodextrins, as well as their functions. This review will be addressed to students and researchers interested in the supramolecular chemistry of cucurbiturils and cyclodextrins, especially that involving these two classes of macrocyclic receptors in a single system.

The complex stability constants (K_S) and thermodynamic parameters (ΔH° and TΔS°) for the 1:1 complexation of two water-soluble calixarenes, p-sulfonatocalix[4]arene (SC4A) and 5,11,17,23-tetrasulfonato-25,26,27,28-tetrakis(n-butyl)calix[4]arene (SC4A-Bu), with organic ammonium cations and neutral spherical organic molecules, have been determined by means of isothermal titration calorimetry (ITC) in aqueous solutions at 298.15 K. The obtained results indicate that, upon complexation with these guests by SC4A-Bu, the enthalpy changes become less favorable, whereas the entropy changes become more favorable relative to SC4A complexation. These differences can be attributed to differential degrees of desolvation and removal of high-energy water as well as the change in conformation or conformational degrees of freedom upon complexation. The calorimetric investigations, accompanied by ¹H NMR and UV/Vis spectroscopy and X-ray crystallography provide a thermodynamic explanation for the different complexation behavior of SC4A and SC4A-Bu towards charged and neutral organic guests. Binding ability and molecular selectivity are discussed from the viewpoint of the conformational geometry and electronic properties of hosts and guests.

Two supramolecular structures were obtained by inclusion complexation of p-sulfonatocalix[4]arene with 1,2-bis(pyridinium)ethane and 1,2-bis(4,4′-dipyridinium)ethane in single-crystal form, which have been examined from the aspects of binding mode, stoichiometry and extended structure. Effects of guest molecules, especially charges and terminal groups, on host conformation, binding and extended structures were carefully compared and summarized.

A supramolecular hybrid is prepared by the supramolecular surface modification of single-walled carbon nanotube (SWCNT) with cationic β-cyclodextrin-tethered ruthenium complexes through a spacer molecule that contains both an adamantane and a pyrene moiety. By employing the supramolecular hybrid, spatially controllable DNA condensation along the SWCNT skeleton is achieved by anchoring cationic ruthenium complexes on the surface. Furthermore, because of the unique physiological properties of SWCNTs, the cationic supramolecular hybrid can be used as a nonviral gene delivery system with the ruthenium complexes as a fluorescent probe to monitor uptake of DNA by cells.

A nanoscopic supramolecular aggregate is constructed from perylene bisimide-bridged bis-(permethyl-β-cyclodextrins) 1 via π–π stacking interactions. Its self-assembly behavior in organic and aqueous solutions is investigated by UV–Vis, fluorescence, and ¹H NMR spectroscopy. Transmission electron microscopy and scanning electron microscopy images show the 1D nanorod aggregation of 1, which is birefringent under crossed polarizer conditions and strongly fluorescent as depicted in the fluorescence microscopy image. X-ray powder diffraction measurements indicate that 1 forms a well-ordered crystalline arrangement with a π–π stacking distance of 4.02 Å. Furthermore, the solid-state fluorescence sensing is explored by utilizing the poly(vinylidene fluoride) membrane-embedded 1, giving that 1, as a novel vapor detecting material, can probe several kinds of volatile organic compounds and, especially, exhibits high sensitivity to organic amines.

The binding behavior of permethylated β-cyclodextrin (PM-β-CD) upon complexation with 4-hydroxyazobenzene (4-HAB) and 4-aminoazobenzene (4-AAB) was investigated by X-ray crystallography, circular dichroism, 2D NMR spectra, and isothermal titration calorimetry. In comparison to the reported β-cyclodextrin (β-CD) complexes with 4-HAB and 4-AAB, PM-β-CD complexes 3 and 4 present not only unique binding regioselectivity with azobenzene guests but also have distinct spatial arrangements, that is, β-CDs arrange in a head-to-head manner, whereas PM-β-CDs arrange in a head-to-tail manner. The results obtained from circular dichroism and 2D NMR spectra indicate that the binding modes of complexes 3 and 4 in solution are in accordance with those in the solid state. Furthermore, the binding abilities and thermodynamic parameters of β-CD and PM-β-CD upon complexation with azobenzene derivatives were discussed from the viewpoints of the flexibilities of the frameworks of the hosts and driving forces. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

The Hirsch–Bingel reaction of bis{4-methyl[1,2,3]triazolyl}malonic ester-bridged bis(permethyl-β-cyclodextrin) 1 with C₆₀ has led to the formation of a new fullerene-bridged bis(permethyl-β-cyclodextrin) 2, which has been comprehensively characterized by NMR spectroscopy, MALDI-MS, and elemental analysis. Taking advantage of the high affinity between 2 and 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (3) or [5,10,15,20-tetrakis(4-sulfonatophenyl)porphinato]zinc(II) (4), linear supramolecular architectures with a width of about 2 nm and a length ranging from hundreds of nanometers to micron dimension were conveniently constructed and fully investigated by transmission electron microscopy (TEM), atomic force microscopy (AFM), and scanning electron microscopy (SEM). Significantly, the photoinduced electron-transfer (PET) process between porphyrin and C₆₀ moieties takes place within the 2⋅3 and 2⋅4 supramolecular architectures under light irradiation, leading to the highly efficient quenching of the porphyrin fluorescence. The PET process and the charge-separated state were investigated by means of fluorescence spectroscopy, fluorescence decay, cyclic voltammetry, and nanosecond transient absorption measurements.

Two robust divalent complexes have been successfully constructed by using complementary rigid spacers (anthracene vs. 1,4,5,8-naphthalenediimide (NDI)) and two pairs of [24]crown-8 ethers and secondary dialkylammonium functionalities as binding motifs. It was demonstrated that properly selected, rigid spacers are more efficient than flexible ones for achieving strong multivalent association. This is presumably due to the preorganization of the rigid spacers, the cooperation between charge-transfer interactions of rigid spacers, and the complexation of the binding motifs. Furthermore, the intermolecular photoinduced electron transfer (PET) between rigid spacers in these robust complexes could be switched on and off by modulating their complexation through acid–base reactions, which is reminiscent of a plug–socket system capable of electron transfer. In addition, the self-sensitized photooxidation of the divalent host with anthracene as a spacer can be completely inhibited after complexation with the divalent guests that contain NDI as spacers. This process could also be understood by invoking intermolecular PET and could be turned on and off through acid–base reactions. The photophysical and photochemical properties of these robust complexes have been interpreted as molecular keypad locks with alarm systems. Thus, a double plug–socket system and molecular keypad locks were successfully integrated inside robust multivalent systems and then the normal molecular devices were endowed with logic functions.

A water-soluble benzenesulfonamidoquinolino-β-cyclodextrin has been successfully synthesized in 30 % yield by incorporating a N-(8-quinolyl)-p-aminobenzenesulfonamide (HQAS) group to β-cyclodextrin through a flexible linker. This compound exhibits a good fluorescence response in the presence of Zn²⁺ in water but gives poor fluorescence responses with other metal ions commonly present in a physiological environment under similar conditions. Fluorescence microscopic and two-dimensional NMR experiments showed that benzenesulfonamidoquinolino-β-cyclodextrin could bind to the loose bilayer membranes. As a result, benzenesulfonamidoquinolino-β-cyclodextrin was found to act as an efficient cell-impermeable Zn²⁺ probe, showing a specific fluorescent sensing ability to Zn²⁺-containing damaged cells whilst exhibiting no response in the presence of healthy cells.