Light-induced access to a fluorescent Cd₇Ag₂₁ nanocluster from a halide-supported Cd₈X nanocluster directed by a face-capping macrocyclic metalloligand

Abstract

Metalloligands with multiple preorganized binding sites provide unique advantages for modulating structural patterns and enhancing the surface stability of metal nanoclusters. However, the precise structure-directing effect of metalloligands in nanocluster transformation remains a missing puzzle piece. Herein, three nested concentric bitetrahedral Cd₈X nanoclusters were synthesized through a stepwise evolutionary pathway. These nano-sized architectures comprise tetrahedrally arranged tripodal Cd^II macrocyclic metalloligands encapsulating a halide-supported Cd₄ tetrahedron, and the template effect of halide ions on the stabilization of Cd₈X was dependent on their ionic size. Initiated by Ag^I, the released metalloligands from Cd₈Cl precisely direct the light-activated fabrication of a yellow-emissive 8e⁻ superatomic Cd₇Ag₂₁ nanocluster. Evidently, different carboxylate O-donor triads and tetrads from the metalloligands geometrically adaptively cap the surface Ag₃ triangles of the convex polyhedral kernel in Cd₇Ag₂₁, thus shaping and protecting the multi-silver kernel in the post-transformed nanocluster. Due to the labile coordination between the carboxylate-rich metalloligand and surface atoms of the Ag₂₀ kernel, the selective fluorescence quenching response of Cd₇Ag₂₁ towards cysteine (Cys) can be rationalized by a ligand replacement-cluster aggregation process. This study demonstrates the distinctive utility of the preformed tripodal metalloligand as a passivating agent, facilitating shape- and size-matching with the surface motifs of metal nanoclusters to induce structural transformation, thereby enabling the synthesis of novel nanocluster architectures.

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Introduction

As the key nexus between discrete silver atoms and silver nanoparticles (Ag NPs), atomically precise silver nanoclusters (Ag NCs) smaller than sub-3 nm exhibit discontinuous energy levels and molecular-like physicochemical properties. They (Ag NCs) have recently attracted considerable attention in the field of metal nanomaterials owing to extensive applications in optical devices, bio-imaging and catalysis.^1–11 Based on soft-hard acid–base theory, some typical soft-base ligands, including thiolates, phosphines and alkynyls, have been intentionally selected as the stabilizer of soft-acid silver atoms in a general bottom-up route to Ag NCs.^12–21 As the hard–soft mismatch between N/O donors and silver atoms makes it more challenging to stabilize Ag NCs using ligands containing only hard N/O donors, only a handful of groundbreaking studies in recent years have successfully prepared Ag NCs from hard-base ligands, including all-nitrogen-donor ligands, carboxylates and inorganic polyoxometalates.^22–29 With respect to simple N/O-donor ligands, preorganized metalloligands with multiple uncoordinated N/O donors impart higher stabilization and stronger structure-directing control for the growth of Ag NCs.^30–32 For instance, by using the trifurcate TiL₃ metalloligands as capping vertices, the Zhang group constructed a family of progressively larger Ag–Ti NCs where TiL₃ units induce a tetrahedral geometry to the obtained NCs.³³ The Sun group reported that some thiacalix[4]arene-modified polyoxometalates integrate dual functionalities as passivating ligands and anion templates to create Ag NCs with complicated geometric architectures.³⁴ Nevertheless, a broader exploration is needed for expanding the list of N/O-based metalloligands with enhanced robustness and structure-directing effect for the assembly of Ag NCs.

The ligand- or metal-exchange-induced structural transformation has been found to be a reliable post-synthesis tactic to acquire new metal NCs with some structural features inherited from the precursor NCs. Usually, the presence of these foreign species causes the transformation products to retain the original core size, but it may occasionally lead to the formation of new NCs with distinct sizes and structures.^35–43 Hitherto, stimuli-induced structural transformation of metalloligand-stabilized metal NCs has hardly been explored, and the structure-directing and/or size-manipulating effects of metalloligands for the transformation products remain an unexplored territory. Recently, we have successfully achieved the modular assembly of a trigonal-bipyramidal Ni^II-organic cage starting from a tripodal Ni^II-based macrocyclic metalloligand, whose facial ligating mode resembles that of the aforementioned TiL₃ moiety in Ag NCs. Inspired by this, we can consider whether this type of metalloligand is competent as a structure-directing and stabilizing agent to achieve the Ag^I-triggered conversion of the metalloligand-based nanocluster precursor to heterometallic Ag NCs.^33,44

In this work, common halide ions except F⁻ can serve as a central template to assist the formation of a series of Cd₈X (X = Cl⁻, Br⁻ and I⁻) NCs, where each triangular face of the central halide-supported Cd₄ tetrahedron is capped by a [Cd(tacntp)]⁻ metalloligand. Time-resolved ESI-MS analysis explicitly mapped out a stepwise evolution from the mononuclear Cd^II metalloligand to the intact cluster skeleton of a Cd₈I NC. Competitive binding together with halide substitution assays mutually confirmed that the templating effect of halide ions is enhanced with an increase in the ion size. Owing to the moderate stability of Cd₈Cl in CH₃OH, Ag^I drives its disassembly to liberate the Cd^II metalloligand, which integrates multiple functions of Ag^I immobilization, kernel shaping and surface passivation to exercise a determining impact on the light-induced production of a yellow-emissive Cd₇Ag₂₁ NC. Extraneous Cys can substitute for surface-anchored metalloligands of Cd₇Ag₂₁ followed by the formation of non-luminous Ag NPs. Therefore, the selective recognition of Cys can be achieved via a fluorescence switch-off response.

Results and discussion

Three halide-supported Cd₈X NCs were obtained as colorless block-like crystals by slowly evaporating an acidic aqueous solution containing Cd(NO₃)₂·4H₂O, 1,4,7-triazacyclononane-1,4,7-tripropionic acid (tacntpH₃), and the corresponding potassium halide (KCl, KBr, KI) in a molar ratio of 4 : 2 : 1 for one week. The detailed preparation procedures are included in the SI. However, the attempts to acquire an octanuclear Cd^II NC containing an interstitial F⁻ ion failed when KF was used as a halide source.

The structural elucidation revealed that the three Cd₈X NCs adopt different counter anions—OH⁻ in Cd₈Cl and CdX₄²⁻ in both Cd₈Br and Cd₈I—but share the same Cd₈ cationic skeleton except its entrapped halide ion (Fig. 1a and S1). The crystallographic data and structure refinement parameters for these NCs are listed in Table S1, and the selected bond lengths and angles are included in Table S2. The structural details of the cationic skeleton are presented with Cd₈Cl as a representative, where the central Cl₁ atom in the cationic moiety is tetrahedrally bonded to four Cd^II atoms (denoted as Cd_i) to generate an inner Cd₄ tetrahedron (Cd^inner₄) (Fig. 1b and S2). Benefiting from the preorganized state of the [Cd(tacntp)]⁻ metalloligand with hydrophobic macrocyclic backbone and hydrophilic carboxylate groups residing at the opposite sides of the encapsulated Cd^II center, each triangular face of Cd^inner₄ is facially capped by the three uncoordinated carbonyl groups of a metalloligand to afford a tetracapped tetrahedron. Especially, four Cd^II atoms in the metalloligands (denoted as Cd_o) constitute an outer Cd₄ tetrahedron (Cd^outer₄), which is also centered by the Cl₁ atom. Therefore, the cationic motif may be described as a nested concentric bitetrahedron with a radius of 7.719 Å (Fig. S3). Either of two crystallographically inequivalent Cd_o atoms presents a distorted octahedral geometry completed by three N donors from the backbone of tacntp³⁻ and three O atoms provided by different carboxylate side arms, while each Cd_i atom in Cd^inner₄ is hepta-coordinated to six carboxylate O atoms stemming from three distinct metalloligands along with the central Cl atom, achieving a highly distorted pentagonal bipyramidal configuration (Fig. 1c and d). The Cd⋯Cd edge lengths of Cd^inner₄ range between 4.382(6) and 4.546(7) Å, and the distances between Cd_o and Cd_i atoms connected by a bridging-chelating carboxylate group are 4.070(6)–4.209(7) Å. The Cd_i–Cl distances (2.733(10) and 2.742(10) Å) are much shorter than the sum of the van der Waals radii for Cd and Cl atoms of 3.42 Å, signifying the strong binding between Cd^II and Cl⁻. Compared to Cd₈Cl, with increasing size of the templating halide ion, the radius of the cationic motif varies from 7.719 Å of Cd₈Cl to 7.771 Å of Cd₈Br and finally to 7.796 Å of Cd₈I. The augmentation of the cluster size is partly attributed to the increase in Cd_i–X distances (Table S3). The variation in the type of counter anion of the Cd₈X NCs is associated with a change in the space group from I4₁/a to R3̄c, as evidenced by the distinctly different PXRD profiles of Cd₈Cl compared with those of the other two NCs (Fig. S4). As for the isomorphic Cd₈Br and Cd₈I, a majority of the PXRD peaks of Cd₈I are shifted towards smaller diffraction angles due to the larger sizes of both Cd₈I cation and CdI₄²⁻ anion. Additionally, the high air stability of these Cd₈X NCs was confirmed, as no perceptible differences were observed between their PXRD patterns before and after exposure to ambient air for 24 h (Fig. S5). The EDX elemental analysis for Cd₈X samples showed that the element content ratios of Cd to halogen align with the results obtained from single-crystal X-ray diffraction studies (Fig. S6–S8).

Fig. 1 (a) Structural details of the cationic skeleton of Cd₈Cl (30% probability ellipsoids). Symmetry code: #1 – x + 1, −y + 1/2, z. Hydrogen atoms are omitted for clarity. (b) The nested concentric bitetrahedral arrangement of Cd^II atoms in Cd₈Cl, with Cd^outer₄ shown in green and Cd^inner₄ shown in light orange. (c and d) The coordination polyhedra of Cd_o and Cd_i.

Once the crystals of corresponding Cd₈X NCs began to precipitate, their mother liquors were immediately analyzed by ESI-MS to investigate the water robustness of the intact Cd₈ skeleton. Unfortunately, no MS signals due to the whole cationic skeleton of Cd₈Cl and Cd₈Br were detected, and the highest m/z peak for these two NCs was found at 1170.91 and 1214.86, respectively, corresponding to the tetranuclear species [Cd₄(tacntp)₂X]⁺ (X = Cl or Br) (Fig. S9 and Table S4). By contrast, the intact octanuclear motif of Cd₈I could be preserved in aqueous solution, signified by the occurrence of the MS peak of [Cd₈(tacntp)₄(OH)I]²⁺ at m/z = 1206.40 besides the identified segments ranging from mononuclear to tetranuclear species. On account of the higher water stability of Cd₈I, the time-resolved ESI-MS data were recorded over seven consecutive days to track its assembly process (Fig. 2a). On mixing the starting reagents, an ion peak was immediately observed at m/z = 502.05 corresponding to a sodium ion adduct of Cd^II metalloligand, [Na₂Cd(tacntp)]⁺; meanwhile, the major peak appearing at m/z = 679.98 was assigned as the dinuclear Cd^II species [Na₂Cd₂(tacntp)(OH)₂(CH₃OH)]⁺. As expected, tetranuclear Cd^II species [Cd₄(tacntp)₂]²⁺ at m/z = 566.97 was assembled through the fusion of the established dinuclear Cd^II species. At the higher m/z region of 1100–1300, the signals with z = 1 charge state were present at m/z = 1150.95, 1195.93, 1222.97 and 1260.85, attributable to the tetranuclear Cd^II segments [Cd₄(tacntp)₂(OH)]⁺, [Cd₄(tacntp)₂(NO₃)]⁺, [NaCd₄(tacntp)₂(OH)₂(CH₃OH)]⁺ and [Cd₄(tacntp)₂I]⁺, respectively. At this time, no cluster fragments with higher nuclearity were found and the I⁻-containing species [Cd₄(tacntp)₂I]⁺ can be regarded as the crucial intermediate for the further evolution into Cd₈I (Fig. 2b, S10 and Table S5). With a prolonged reaction time, the intensities of the MS signals arising from the tetranuclear Cd^II intermediates gradually increased, and [Cd₄(tacntp)₂I]⁺ became the dominant species on the sixth day, accompanied by the suppressed MS peaks of mononuclear and dinuclear species. Apparently, tetranuclear Cd^II species were afforded at the expense of the depletion of those lower nuclearity intermediates. On the seventh day, some transparent crystals of Cd₈I were formed, and the dinuclear Cd^II species became the preferred species again. The drastic decrease in the peak intensities of tetranuclear segments might be attributed to the combination of iodide-free tetranuclear species with [Cd₄(tacntp)₂I]⁺ into the intact cluster skeleton, which precipitated immediately due to the poor aqueous solubility. Fortunately, an MS peak attributed to the intact octanuclear dication [Cd₈(tacntp)₄I(OH)]²⁺ could be detected at m/z = 1206.40 after zooming in the MS spectrum in the m/z range of 1140 and 1280. The ESI-MS analysis explicitly mapped out a stepwise evolution pathway for Cd₈I: Cd₁ metalloligand → Cd₂ intermediate → Cd₄ and I@Cd₄ intermediates → Cd₈I skeleton (Fig. 2c). In CH₃OH, Cd₈Cl exhibited enhanced solution stability, and the Cl⁻-stabilized octanuclear cation was monitored as the predominant species in the MS spectrum (Fig. S11 and Table S6). Nevertheless, MS signals from Cd₈Br and Cd₈I could not be detected due to their insolubility in CH₃OH.

Fig. 2 (a) The time-resolved ESI-MS spectra of the reaction solution during the assembly of Cd₈I. (b) The local magnified ESI-MS images in the m/z range of 1100–1300. (c) The proposed assembly mechanism of Cd₈I.

Such a halide-supported Cd₈X library prompts us to explore the templating effect of different halide ions through a competitive halide binding assay. When the nanocluster was prepared following the aforementioned procedure—except that the pure potassium halide was replaced by a mixture of equimolar amounts of KCl, KBr and KI—colorless crystals with irregular shape were deposited and collected. The EDX measurement for the as-synthesized crystalline samples verified the uniform distribution of Cl, Br and I elements on their surface. However, the content of I is greater than that of the other two halogen elements, with the I : Br : Cl molar content ratio of 2.51 : 0.92 : 0.66 (Fig. S12). The I⁻ selectivity is noticeable, tentatively ascribed to the consequence of soft–soft acid–base interaction and size-matching between the intra-cavity of the Cd^inner₄ tetrahedron and the encapsulated anion. The size-dependent templating effect of halide ions on Cd₈X NC stabilization was further probed via template ion exchange experiments. After dispersing 0.1 mmol ground powder of Cd₈Cl in CH₃OH solution containing three equivalents of KBr or KI, the mixture was then kept under ambient conditions with stirring for one week. The resulting solids were collected and evaluated by PXRD analysis. The complete disappearance of the characteristic diffraction peaks for Cd₈Cl and the good agreement of the experimental PXRD pattern with that of either Cd₈Br or Cd₈I undoubtedly confirm the successful conversion of Cd₈Cl to the other two Cd₈X NCs via halide ion exchange. Likewise, I⁻ can substitute for Br⁻ wrapped in the cavity of Cd₈Br. On the contrary, Cd₈I dispersed in CH₃OH solution, including three equivalents of KCl or KBr, could not afford the corresponding halide-exchanged product (Fig. S13). Obviously, the halide encapsulation preference by the bitetrahedral NCs follows the size order of halide ions: I⁻ > Br⁻ > Cl⁻.

It is conceivable that the Cd^II metalloligand present in Cd₈X may be an appropriate surface-protecting ligand for Ag NCs for the following two reasons: (1) the tripodally arranged pendant arms with abundant carboxylate O sites are available for Ag^0/I binding, and (2) the flexible orientation of the pendant arms relative to the macrocyclic backbone favors the formation of geometrically matching O-donor triads for passivating the surface Ag₃ trigons of Ag NCs with convex polyhedral skeletons. Meanwhile, the appropriate solution stability of Cd₈Cl suggests that it can be used as a precursor to access heterometallic Ag NCs through structural transformation. First, it could be hypothesized that such a precursor nanocluster can release free metalloligands through the abstraction of halide ions by Ag^I. Subsequently, the released metalloligand can adopt the robust face-capping modes to steer the construction and protection of the polyhedral Ag kernel. To realize the above concept, 2.85 × 10⁻³ mmol of Cd₈Cl was dissolved in 5 mL of CH₃OH by the addition of 50 equivalents of AgNO₃, and then the mixed solution was exposed to the ambient environment. After three days, a small amount of brown solid was precipitated from the solution (Fig. S14). PXRD analysis revealed that the precipitate (referred to as Cd₈Cl_AgNO₃) is a mixture of AgCl and elemental Ag, along with some unidentified substances (Fig. S15), indicating that the Cl⁻ ion was abstracted from Cd₈Cl by AgNO₃. Then, ESI-MS analysis was performed on the reaction supernatant to uncover the skeleton robustness of the octanuclear Cd^II cluster free of Cl⁻. Unexpectedly, no MS signals due to Cd₈ species could be detected in the positive-ion mode, and mono-charged peaks corresponding to several Ag^I-bound oligonuclear species, such as [Ag₂Cd(tacntp)]⁺, [Ag₂Cd(tacntpH)(NO₃)(CH₃OH)]⁺, [Ag₃Cd(tacntp)Cl]⁺ and [Ag₃Cd(tacntp)(NO₃)(CH₃OH)]⁺, were detected at m/z = 669.87, 764.92, 813.75, and 872.82, respectively (Fig. S16a and Table S7). Apparently, AgNO₃ triggers the disintegration of Cd₈Cl, and then the released [Cd(tacntp)]⁻ metalloligand binds to Ag^I to produce some polynuclear Cd–Ag species. The destruction of the Cd₈Cl skeleton was also confirmed by the dominant peak (m/z = 456.07) ascribed to the solo metalloligand found in the negative-ion mode MS spectrum (Fig. S16b). More interestingly, the supernatant is yellow-emissive under 302 nm UV irradiation. In addition, the steady-state fluorescence measurement revealed that the supernatant shows a broad emission peak centered around 556 nm, which is essentially independent of the excitation wavelength (Fig. S17). This confirmed the homogeneity of the photoluminescent components within the supernatant. TEM investigation showed the presence of small, well-dispersed particles in the supernatant with an average size of approximately 2.31 nm (Fig. S18). On the slow diffusion of ethyl acetate into the supernatant under ambient conditions, rod-like brown-red single crystals of Cd₇Ag₂₁ NC were obtained and structurally elucidated (Fig. S19).

The overall chemical composition of Cd₇Ag₂₁ inferred by single-crystal X-ray diffraction analysis was formulated as [Ag₂₁Cd₇(tacntp)₇(NO₃)₂](NO₃)₂·Cl₂·14H₂O. The crystallographic data and structure refinement parameters for Cd₇Ag₂₁ are listed in Table S8, and the selected bond lengths and angles are included in Table S9. The nanocluster comprises an Ag₂₀ kernel shielded by seven face-capping Cd^II metalloligands with Ag–O bond lengths in the range of 2.21(2)–2.590(15) Å. An extra Ag(NO₃)₂⁻ unit is left at the periphery of the nanocluster, which is tethered to the inner Ag₂₀ core via one pendant carboxylate group of a metalloligand (Fig. 3a). As expected, the diameter of the intact cationic unit of Cd₇Ag₂₁ is about 2.2 nm, which is comparable to the size of the luminescent particles present in the supernatant during the conversion process. The Ag₂₀ core features a centered Ag₁₃ icosahedron with the remaining seven Ag atoms constituting a three-petal-like receptacle to accommodate the icosahedron (Fig. 3b). Argentophilic interactions are pivotal to keep the twenty Ag atoms together corroborated by the fact that the radial and peripheral Ag–Ag separations in the Ag₁₃ icosahedron vary between 2.723(2)–2.894(2) Å and 2.766(2)–3.220(2) Å, respectively, and those in the receptacle lie in the range of 2.728(3)–3.002(2) Å. Six metalloligands with a staggered six-membered ring arrangement act as fasteners to join the icosahedron and its receptacle, besides the Ag–Ag interactions (2.728(3)–3.025(2)) Å between icosahedral and receptacle Ag atoms). A quasi-C₃ axis passes through the central Ag₁₈ atom of the receptacle and the Ag₁ center within the Ag₁₃ icosahedron, and each petal of the receptacle consists of an Ag₂ pair being disposed clockwise about the axis; therefore, the Ag₂₀ core is essentially chiral (Fig. 3c and d). As each metalloligand possesses six O donors (three of them orient inwards to coordinate with the Cd^II center and the other three orient outwards as free donors) for potential Ag binding, the moderate flexibility of its propionate pendant arms provides the chance to adjust their orientation and thus the separations between O donors. This is conducive for the metalloligand to shield more Ag₃ triangles on the surface of the Ag₂₀ core. Notably, four different binding motifs are exhibited by seven passivating Cd^II metalloligands (denoted as ML_Cd) based on their connectivity to Ag atoms. ML_Cd1, ML_Cd2 and ML_Cd5 assume a (λλλ) conformation on their macrocyclic backbone, and one propionate pendant arm is oriented anticlockwise with respect to the backbone, but the other two are oriented clockwise. The six O donors of each these metalloligands chelate five Ag atoms, by which three Ag₃ triangles are generated via edge-sharing. Among these trigons, two are capped by three inward and one outward O atoms, while the other is mounted by one inward and two outward O atoms (motif I). ML_Cd3, ML_Cd6 and ML_Cd4 feature the same backbone conformation and similar arrangement of the pendant arms to the above metalloligands. Differently, ML_Cd3 and ML_Cd6 bridge six Ag atoms stemming from four edge-shared Ag₃ triangles. Two of four Ag₃ trigons are protected by one inward and two outward O atoms, the third trigon is only connected to three inward O atoms, and the remaining one is covered by two inward and one outward O atoms (motif II). The coordination pattern of ML_Cd4 is analogous to motif II, and the only discrepancy is that one outward O-donor functions as a μ₂ bridge, linking an additional Ag21 atom (motif III). The backbone of ML_Cd7 adopts a (δδδ) conformation, and three pendant arms are all oriented anticlockwise. It serves as a capping shell for four Ag atoms in an Ag₄ triangular pyramid with Ag–Ag contacts between the apex and base atoms, revealing a coordination mode quite different from that of motifs I–III. Each of the three Ag₃ lateral surfaces (viewed as the pseudo-surface Ag₃ trigon) of the triangular pyramid is connected to two inward and two outward O atoms of ML_Cd7 (motif IV) (Fig. 4 and S20). Thanks to the perfect passivation of the macrocyclic metalloligands for the Ag₃ triangular face, the Ag₂₀ core with 34 edge-shared surface Ag₃ trigons can be stabilized. Because of the chirality of the metallic core and the asymmetric anchoring of chiral metalloligands on its surface, an individual cationic cluster unit of Cd₇Ag₂₁ is chiral. However, equivalent enantiomers are packed in a single crystal and therefore, Cd₇Ag₂₁ is a racemate, crystallizing in a centrosymmetric triclinic space group, P1̄. Moreover, the homogeneous phase of the as-prepared Cd₇Ag₂₁ during the nanocluster transformation process was determined by the PXRD analysis. After being exposed to air for 24 h, the observed line broadening of the PXRD patterns of Cd₇Ag₂₁ seems to originate from a partial loss of crystallinity (Fig. S21).

Fig. 3 Surface arrangement of Cd^II metalloligands (a) and the inner Ag₂₀ core featuring a centered Ag₁₃ icosahedron (b) in Cd₇Ag₂₁. Side view (inset: Ag₁₃ icosahedron and its receptacle) (c) and top view (d) of the Ag₂₀ core in Cd₇Ag₂₁ with the quasi-C₃ axis passing through Ag₁ and Ag₁₈ atoms. Color codes: purple, yellow and light-green spheres, Ag; green sphere, Cd; red sphere, O; lilac sphere, N; gray sphere, C.

Fig. 4 Schematic representation of the binding modes of Cd^II metalloligands to surface Ag₃ trigons in Cd₇Ag₂₁.

The XPS survey spectrum suggested that Cd₇Ag₂₁ consists of C, N, O, Cl, Ag and Cd elements (Fig. S22a). The narrow-scan Ag 3d spectrum showed two peaks at binding energies of 373.76 eV and 367.79 eV, which could be deconvoluted into four sub-bands (Fig. S22b). These sub-bands indicate the presence of metallic Ag⁰ and Ag^I in the nanocluster (373.62 and 367.70 eV for Ag^I 3d_3/2 and 3d_5/2, 374.25 and 368.22 eV for Ag⁰ 3d_3/2 and 3d_5/2);^24,45,46 hence, Cd₇Ag₂₁ was proven to be a superatomic species. The free valence electron number was calculated as 8 {n = 21 − 7 − 2 − 4},⁴⁷ which fills a closed electronic shell of 1S²1P⁶ based on a jellium model. The photophysical properties of Cd₇Ag₂₁ in CH₃OH were further investigated. The UV-visible absorption spectrum of the nanocluster showed two absorption peaks at 342 and 391 nm, and the latter is close to its optimal excitation wavelength (Fig. S23). Considering the relatively weak affinity of Cd^II metalloligands to the Ag₂₀ core, the 556 nm emission of Cd₇Ag₂₁ under 396 nm excitation might be tentatively ascribed to cluster core-centered charge transfer. The skeleton rigidity of the Ag₂₀ kernel imposed by the peripheral metalloligands contributes to the intense yellow photoluminescence of Cd₇Ag₂₁. The short lifetime of 1.52 ns is indicative of fluorescence decay characteristics, and the total quantum yield is 1.53% (Fig. S24). Moreover, the remarkable stability of Cd₇Ag₂₁ in CH₃OH was indicated by the unchanged fluorescence intensity of this solution after storing under ambient conditions for one week (Fig. S25). Conversely, the excellent photophysical stability of the methanolic solution of Cd₇Ag₂₁ means that it has great potential in fluorescence sensing.

A series of control experiments was conducted and fluorescently monitored to determine the key factors that drive the generation of fluorescent Cd₇Ag₂₁. First, it should be noted that neither starting reagents used to prepare Cd₈Cl nor Cd₈Cl itself exhibited yellow fluorescence under a 302 nm UV lamp (Fig. S26a). Upon treatment with AgNO₃, the Cd₈Cl solution became luminous after 2 hours of exposure to natural light, and the emission gradually enhanced in the next three days (Fig. S26b and S27). Moreover, the production of Cd₇Ag₂₁ could be accelerated by UV light, and the emission brightness of the reaction supernatant reached a maximum within 30 min of irradiation at 302 nm (Video in SI). For comparison, a mixed solution of Cd₈Cl and AgNO₃ kept in the dark for three days only showed insignificant fluorescence (Fig. S28), which indicated that Cd₇Ag₂₁ can only be accessed via a photochemical process. Second, the critical role of the choice of Cd₈X precursor for the preparation of the target Ag NC was evaluated. When substituting Cd₈Br for Cd₈Cl as the precursor, while the other reaction conditions were kept constant, the supernatant of the reaction mixture emitted only a faint yellow fluorescence under a 302 nm lamp. The large amount of brown precipitate (referred to as Cd₈Br_AgNO₃) at the bottom of the container was identified as AgBr via PXRD analysis, as well as some unidentified amorphous products (Fig. S29a). If Cd₈I was applied as the precursor, no luminescent nanostructures could be obtained during the whole reaction period, and the as-formed pale-yellow precipitate (referred to as Cd₈I_AgNO₃) was recognized as Ag₂INO₃ (i.e., a mixed phase of AgNO₃ and AgI) (Fig. S29b). These observations demonstrated that Cd₈Br and Cd₈I tend to form AgX precipitates rather than Cd₇Ag₂₁ when treated with AgNO₃. Third, a mixture of tacntpH₃, AgNO₃ and KCl dissolved in CH₃OH with a molar ratio of 4 : 50 : 1 subjected to 30 min of UV irradiation was non-fluorescent (Fig. S30). The inability of tacntpH₃ to produce fluorescent NCs might arise from the random orientation of carboxylate pendant arms, preventing the free macrocyclic ligand from effectively mediating the formation of the Ag core and the passivation of its surface. If tacntpH₃ was replaced by the preformed metalloligand (a mixed solution of equimolar tacntpH₃ and Cd^II in CH₃OH), the irradiated mixture exhibited a faint blue emission (Fig. S31). The TEM analysis for the collected nanostructures showed the particles with an average size (1.71 nm) smaller than that of Cd₇Ag₂₁ (Fig. S32). Evidently, Cd₈Cl as a carrier of Cd^II metalloligand plays a determining role in the directed assembly of Cd₇Ag₂₁. Fourth, the unique regulatory effect of Cd₈Cl on the light-induced synthesis of Cd₇Ag₂₁ was further evaluated by deliberately varying the molar ratio of AgNO₃ and Cd₈Cl from 1 : 1 to 300 : 1 (Fig. S33–S36). After UV illumination, the resulting reaction supernatants were sampled and analyzed through TEM. It is worth noting that a few non-fluorescent Ag NPs larger than 10 nm were fabricated at the lowest molar ratio of 1 : 1. When the molar ratio of AgNO₃ to Cd₈Cl was either 10 : 1 or 150 : 1, the supernatant emitted a weak green/yellowish green fluorescence. The particles were polydisperse in size, consisting of a mixture of non-fluorescent Ag NPs and fluorescent Cd₇Ag₂₁. When the molar ratio of AgNO₃ to Cd₈Cl was increased to 300 : 1, the supernatant became non-luminous again, and the dense Ag nanostructures in this supernatant had a wide size distribution ranging from 2 to 8 nm. Only at a molar ratio of 50 : 1 for AgNO₃ to Cd₈Cl was the highest emission brightness observed for the supernatant, in which uniform-sized Cd₇Ag₂₁ NCs were stabilized. Fifth, additional comparative experiments revealed that yellow-emissive Cd₇Ag₂₁ NCs can only be prepared through the light-induced reaction in CH₃OH or a mixture of CH₃OH/CH₃CH₂OH or CH₃OH/DMF (v/v = 1/1) (Fig. S37), evidencing that CH₃OH is indispensable for the conversion of Cd₈Cl to Cd₇Ag₂₁. These findings raise the question as to whether CH₃OH functions as a reducing agent for Ag^I in this conversion. However, after the light-induced conversion of Cd₈Cl in CH₃OH, corresponding oxidation products (e.g. HCHO and HCOOH) of CH₃OH could not be detected in the reaction mixture by ion chromatography and ¹³C-NMR, and EPR measurement using PBN as a spin trap failed to observe the resonance signal produced from the reductive hydroxymethyl radical (Fig. S38).^48,49 Consequently, there was no direct evidence to suggest that CH₃OH acts as a reducing agent during the nanocluster conversion process. Finally, the impact of reaction temperature on the light-induced conversion of Cd₈Cl was investigated under natural light at the optimal molar ratio of AgNO₃ and Cd₈Cl. At temperatures of 25 and 40 °C, the luminous intensity of the reaction supernatants was independent of temperature, and the formation yield of Cd₇Ag₂₁ remained constant. As the temperature was raised above 50 °C, the yellow emission intensity of the supernatants dropped with the increasing temperature, accompanied by a decreasing yield of Cd₇Ag₂₁ as well as an increasing amount of black precipitate (suspected to be silver particles) (Fig. S39 and S40). Based on the above investigations, the photochemical synthesis mechanism of Cd₇Ag₂₁ was supposed as follows: (1) Cd₈Cl experiences an Ag^I-triggered cluster disassembly to provide Ag^I-anchored metalloligands. (2) Light-induced partial reduction of Ag^I ions bound to the metalloligand and the subsequent fusion of the metalloligand-bound silver species via Ag–carboxylate coordination and argentophilic interaction to shape the Ag kernel. (3) Full surface coverage of metalloligands on the established Ag kernel to restrict its further growth or aggregation (Fig. 5). Notably, Cd₈Cl as a slow-releasing carrier of [Cd(tacntp)]⁻ provides a unique environment for light-induced reduction of Ag^I together with a preferable concentration ratio of metalloligand to Ag^I, which is beneficial for the creation of fluorescent Ag NCs with excellent monodispersity in an otherwise unattainable chemical microenvironment.

Fig. 5 Schematic diagram of the transformation process from Cd₈Cl to Cd₇Ag₂₁.

A few photoluminescent Ag NCs have been developed as sensitive and selective sensors for thiol-containing amino acids such as Cys due to the strong affinity of thiol groups with the Ag NC surface.^50–53 On the basis of soft and hard acid–base theory, as a hard base, the tripodal metalloligand with all carboxylate binding sites may exhibit relatively weaker interaction with soft-acid Ag ion/atom of Ag NCs than thiol-containing compounds. Thus, the as-prepared Cd₇Ag₂₁ was expected to be applicable for detecting thiol-containing substrates through ligand-exchange-induced fluorescence change. Upon adding 80 μM of Cys to a methanolic solution of Cd₇Ag₂₁ at a concentration of 55 μM, the kernel-based emission was quenched to about 15% of the original intensity, accompanied by a blue shift of the emission maximum from 556 to 534 nm (Fig. 6a and b). Under a UV lamp, the complete disappearance of the yellow photoluminescence of Cd₇Ag₂₁ when Cys was present further evidenced the quenching ability of Cd₇Ag₂₁ by Cys. The quenching efficiency of Cys on Cd₇Ag₂₁ was linearly related to the concentration of Cys in the range of 60–80 μM (R² = 0.9941), and the quenching constant K_SV was calculated to be 1.51 × 10⁵ M⁻¹, with a detection limit as low as 0.16 μM (Fig. S41). The detection sensitivity of Cd₇Ag₂₁ for Cys is comparable to that of some existing coinage metal NCs (Table S10).

Fig. 6 (a) Emission spectra (λ_ex = 396 nm) of Cd₇Ag₂₁ in the presence of different concentrations of Cys (inset showing images of the fluorescence change of Cd₇Ag₂₁ before and after addition of 80 μM of Cys). (b) Fluorescence response of Cd₇Ag₂₁ dissolved in CH₃OH to 100 μM of different interfering substances in the absence and presence of 80 μM Cys. (c) Optical images of the Tyndall effect of a methanol solution of Cd₇Ag₂₁ before and after the addition of 80 μM of Cys. (d) XPS survey spectra of Cd₇Ag₂₁ and Cd₇Ag₂₁_Cys. (e) XPS high-resolution spectra of Ag 3d for Cd₇Ag₂₁ and Cd₇Ag₂₁_Cys. (f) XPS high-resolution spectra of Cd 3d for Cd₇Ag₂₁ and Cd₇Ag₂₁_Cys.

Some analyses were conducted for the Cd₇Ag₂₁ sample after Cys treatment (referred to as Cd₇Ag₂₁_Cys) to make clear the quenching mechanism of Cys for the emission of Cd₇Ag₂₁. After sensing Cys, the Tyndall effect of the sample solution became more obvious (Fig. 6c), which tentatively proved the larger size of Cd₇Ag₂₁_Cys. Subsequent TEM results further confirmed that the average diameter of Cd₇Ag₂₁_Cys particles approaches 20 nm, larger than that of Cd₇Ag₂₁ (Fig. S42). The EDX mapping of Ag showed that this element is homogeneously present on the surface of the obtained nanoparticles, as is the S element from the exogenous Cys (Fig. S43). In addition, the full XPS spectrum clearly showed that Cd₇Ag₂₁_Cys is composed of C, N, O, S, Cl, Cd, and Ag elements (Fig. 6d). The binding energies of Ag doublets were positively shifted to 373.92 and 367.92 eV, indicating that Ag might interact with the sulfhydryl group of Cys (Fig. 6e). The peak at 162.08 eV in the high-resolution spectrum was attributed to S 2p, reconfirming the presence of Ag–S interactions (Fig. S44).^54–56 Furthermore, the positions of the Cd 3d_3/2 and Cd 3d_5/2 signals were significantly shifted to higher binding energies, so Cys might also abstract Cd^II from the metalloligand (Fig. 6f).^57–59 Therefore, a possible interpretation for the Cys-induced fluorescence quenching is that Cys may destabilize Cd₇Ag₂₁ by partially/completely replacing the metalloligands anchored on its surface. Then the inferior coverage of Cys for the Ag kernel leads to the agglomeration of these Cys-protected Ag NCs into non-luminous Ag NPs with a larger size. The disappearance of the corresponding –SH stretching vibration band of Cys at 2550 cm⁻¹ in the FT-IR spectrum of Cd₇Ag₂₁_Cys provided another support that Cys was anchored to silver NPs via Ag–S bonds (Fig. S45).^60–62 ESI-MS analysis of a methanol solution of Cd₇Ag₂₁ after Cys treatment was conducted to gain direct information for Cys replacement. Peaks corresponding to the complete skeleton of Cd₇Ag₂₁, as well as the fragments containing metalloligand could not be identified in the mass spectrum, but instead a peak found at m/z 346.20 could be attributed to the mono-protonated macrocyclic ligand, which arose from the Cys-induced destruction of the metalloligand (Fig. S46). The presence of the free ligand in the solution indirectly proved that the aggregation of Ag NCs into non-fluorescent NPs occurs after surface ligand exchange. The proposed mechanism was further corroborated by the quenching effects of some alkyl thiols (EtSH, 1-PrSH and t-BuSH) towards the yellow fluorescence of Cd₇Ag₂₁ (Fig. S47). The quenching efficiency of these quenchers decreased in the order t-BuSH > 1-PrSH > EtSH, which coincides with their Ag-binding affinity depending on the electron-donating ability of alkyl groups of thiols.

Particularly, the assay is specific for the detection of Cys among seventeen selected amino acids, including two other S-containing amino acids, methionine (Met) and taurine (Tau), as well as glutathione (GSH, a tripeptide containing Cys moiety). All other analytes barely showed turn-off effects on the fluorescence of Cd₇Ag₂₁ even at a concentration equal to 100 μM. Moreover, none of them could apparently disturb the quenching ability of Cys for Cd₇Ag₂₁ (Fig. 6b). The inability of GSH, Met or Tau to turn off the fluorescence of Cd₇Ag₂₁ might be ascribed to the following aspects: the less exposed thiol group of GSH, the bulky methyl group attached to the sulfur atom of Met, and the oxygen atoms surrounding the sulfur atom of Tau preclude these substances from replacing the metalloligands anchored on the surface of Cd₇Ag₂₁ through Ag–S bonds, and thus the destabilization and agglomeration of small-sized Ag NCs into non-luminous NPs could not occur.

Conclusion

In summary, Ag^I triggers the light-induced structural transformation of a chloride-centered bitetrahedral Cd₈Cl nanocluster into a yellow-emissive heterometallic Cd₇Ag₂₁ nanocluster featuring an icosahedron-based Ag₂₀ kernel. The Cd^II macrocyclic metalloligand released from Cd₈Cl behaves as a powerful structure-directing agent by face-capping on the surface Ag₃ trigons to achieve the shaping and protection of the metallic kernel. Meanwhile, the precursor nanocluster as a carrier of metalloligands supplies an appropriate chemical microenvironment for the nanocluster conversion. The labile coordination between the metalloligands and surface Ag ions/atoms of Cd₇Ag₂₁ provides the fluorescent nanocluster with the inherent capability to selectively recognize Cys through switch-off response deriving from ligand replacement-cluster aggregation. Further efforts are focused on metalloligand tailoring to improve the controllability of metalloligand-mediated cluster transformation to target heterometallic NCs with designed structures and attractive properties suitable for versatile applications.

Author contributions

Biliu Lan: investigation, methodology, data curation, writing – original draft, review & editing. Ziling Li: investigation, data curation. Yanfang Feng: investigation. Tao Deng: data curation. Min Pan: investigation. Zhong Zhang: supervision, conceptualization, writing – review & editing, funding acquisition.

Conflicts of 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.

Data availability

CCDC 2464348–2464351 contain the supplementary crystallographic data for this paper.^63a–d

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: materials and instruments, experimental section and additional figures and tables; FT-IR spectra, PXRD profiles, TEM images, ESI-MS spectra, UV-vis absorption spectra, emission spectra, photoluminescence lifetime decay curve, EPR spectra, XPS spectra, crystal structure images, crystal data tables, and performance comparison table for Cys detection; a video showing the fluorescence change of the reaction supernatant during 30 min UV irradiation. See DOI: https://doi.org/10.1039/d5sc04963f.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21861003).

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