Silver Cluster Assembled Materials: A Model‐Driven Perspective on Recent Progress, with a Spotlight on Ag12 Cluster Assembly

The exploration of individual nanoclusters is rapidly advancing, despite stability concerns. To address this challenge, the assembly of cluster nodes through linker molecules has been successfully implemented. However, the linking of the cluster nodes itself introduces a multitude of possibilities, especially when additional factors come into play. While this method proves effective in enhancing material stability, the specific reasons behind its success remain elusive. In our laboratory, we have undertaken extensive studies on Ag cluster‐assembled materials. So, here our goal is to establish a model system that allows for the discernment of various factors, eliminating unnecessary complexities during the linking approach. So, we hope that the systematic discourse presented in here will contribute significantly to future endeavors, helping to set clear priorities, and provide solutions to concerns that arise when working with a model system.


Introduction
In recent years, the field of nanoscience has undergone a notable evolution marked by the emergence of innovative materials and structures that demonstrate distinctive and enhanced properties when compared to their bulk counterparts.Within this realm of progress, the synthesis and exploration of metal nanoclusters (NCs) have attracted considerable attention, primarily owing to their atomically precise structural architectures. [1,2]The discerned relationship between the structural configuration and inherent properties of these NCs has underscored their significance in nanoscience. [3,4][17][18] While surfaceprotecting groups initially confer stability to these NCs, the ultimate application potential is contingent upon their longterm durability, a quality not universally present in most structures of this nature. [19]Despite the extraordinary properties exhibited by NCs, their long-term durability is subject to temporal constraints, which ultimately restrict their widespread applicability.This limitation is especially evident and concerning in silver (Ag) nanoclusters (NCs) when compared to gold (Au) NCs.Thus, while these nanomaterials showcase unparalleled properties, their full realization in practical applications is hampered by challenges associated with their stability issues.
In response to the stability challenges posed by Ag NCs, researchers have been actively pursuing potential solutions through various strategies. [20,21]24] However, this improvement is limited when exclusively targeting labile solvents coordinated with the structures.[27] While this avenue offers promise, it encounters limitations related to the reduced solubility of materials.[30] However, it has also been discovered that in certain cases, this method significantly alters the properties of individual NCs, resulting in a complete transformation of their overall characteristics.[33][34][35][36] Despite these advancements, challenges persist in terms of solubility and chemical reactivity.Furthermore, it is noteworthy that each of these approaches necessitates an extensive post-synthetic process.This procedural demand not only complicates the overall synthesis but also hinders the broader applicability of these strategies.Consequently, the quest for an optimal solution to enhance the stability of metal NCs remains a multifaceted challenge that requires further exploration and refinement of existing methodologies.
Later the utilization of individual Ag NCs as building blocks in the construction of extended structures has emerged as a promising solution. [37]However, the arrangement of these building blocks is contingent upon various factors that significantly influence the ultimate stabilities of the resulting structures.
[40][41] However, the interactions among the ligands of individual nanoclusters (NCs) are constrained, impeding the broadening of this domain as a comprehensive research avenue.Literature also indicates that employing the counter anion can serve as a linkage to create extended three-dimensional frameworks, resulting in substantially improved stabilities. [42]However, such examples are exceedingly rare.Nonetheless, extensive research has revealed the remarkable capabilities of organic linker molecules in influencing the assembly process.The introduction of organic linker molecules during the synthesis of NCs expands the possibilities for assembling individual cluster nodes. [43]The structural architecture of the linker molecules plays a pivotal role in controlling the assembly process, with the dimensionality of the resulting structures being influenced accordingly.Notably, N-based organic linker molecules have gained prominence due to their coordination specificities in facilitating the assembly process. [44]Achieving precise control over the size of the linker molecules and their chelating ability is paramount in governing the overall structures formed.It has been observed that the utilization of N-based organic linker molecules allows for fine-tuning the assembly process, leading to the creation of well-defined and stable extended structures.
These frameworks, known as Ag CAMs, belong to a captivating category of nanomaterials that offer remarkable possibilities across diverse applications. [20]The synthesis of Ag CAMs has experienced significant advancements driven by innovative methodologies and state-of-the-art techniques.This progress has resulted in an enhanced ability to precisely control the size, shape, and composition of these materials.The evolving synthesis techniques not only contribute to a deeper understanding of NC assembly but also unveil a multitude of superior attributes and functionalities that set Ag CAMs apart from individual NCs.Consequently, this field has experienced rapid advancements in various directions (change in structural architecture of the cluster nodes, change in the linker molecules, change in the dimensionality of the final structures), yet without a comprehensive understanding of the previously reported structure-property correlations. [20]So, the extensive global research activities in this domain present a challenge in grasping the fundamental structural aspects crucial for achieving specific applications.As a result, there is an urgent demand to consolidate and summarize the diverse approaches employed worldwide.The aim is to establish a model system that can serve as a basis for promoting systematic studies on the applications of different Ag CAMs.In this scientific narrative, our objective is to systematically explore a model system, elucidating the rationale behind its selection and delineating the distinct advantages inherent in adopting such a model.Furthermore, we aim to delve into the cuttingedge applications of these materials in recent years, elucidating their correlated structural architecture within the selected model system.This comprehensive examination seeks to enhance our understanding of the intricate structural nuances associated with Ag CAMs, ultimately paving the way for more precise, targeted, and efficacious applications across a spectrum of scientific and technological domains.

Unrevealing the Facts Behind the Choice of Ag 12 Cluster Assembly as a Model System
Wang et al. demonstrated a straightforward linking approach to connect silver (Ag) cluster nodes using organic linker molecules, representing a noteworthy advancement in materials synthesis. [45]lthough previous studies exist, the notation of the metal atom geometry as the cluster node was not fully established at that time. [46,47]Anyways, the approach described by Wang et al. involved the assembly of Ag 14 cluster nodes by replacing labile solvent molecules with a bi-dentate pyrazine linker Sourav Biswas presently serves as a postdoctoral research fellow in Professor Yuichi Negishi's laboratory within the Department of Applied Chemistry at the Tokyo University of Science (TUS).Having obtained his Ph.D. in Chemistry in 2020 from the National Institute of Technology Durgapur, India, his current research pursuits revolve around synthesizing and exploring potential applications for novel copper nanoclusters and silver clusterassembled materials.molecule.This innovative technique resulted in the formation of a one-dimensional right-handed helical structure, which exhibited enhanced thermal stability, maintaining its crystal structure even at 150 °C.Notably, the structural dimensionality could be modified by simply substituting the linker molecules, underscoring the versatility of this approach.Importantly, this alteration in dimensionality also exerted an influence on the thermal stability of the resulting structures.Thus, the incorporation of organic linker molecules not only enhanced the stability of the cluster nodes but also allowed for a tailored design of their structural architecture based on the chosen linking approaches.
The linking of cluster nodes was achieved through an insitu synthetic approach, where cluster nodes were simultaneously formed, and their interconnection occurred without the need for additional synthesis steps.So, the precise control over the reaction is very crucial for designing specific type of the cluster node connected geometrical architecture.This control is contingent upon several factors, such as the coordinating sites and the geometry of the linker molecules.The dimensionality of the resulting structure is also influenced by the geometry of the cluster nodes and their inherent specificities towards coordinating sites.Introducing another crucial parameter, the geometry of the cluster nodes itself is determined a most important parameter, including the structure of the ligands, reaction conditions, and the number of metal atoms involved.Therefore, it is essential to grasp the intricacies of these multifaceted factors becomes pivotal in tailoring the interconnected geometrical architecture of cluster nodes.
Consider an example where 4,4'-azopyridine linker molecules were employed to connect two distinct cluster nodes, namely Ag 14 and Ag 11 . [48,49]Surprisingly, despite the identical planar geometry of the linker molecules, their structures exhibited different dimensionality.This underscores the crucial role played by the intrinsic geometry of the cluster node in orchestrating the organized connection with linker molecules.Intriguingly, even a minor adjustment in the coordinating solvent molecules resulted in alterations in the optical properties of the final organized structure, showcasing the sensitivity of the outcome to changes in coordinating sites. [49]Any modification in these coordinating sites introduces complexity in the precise determination of material properties.The onepot reaction protocol for the formation of cluster nodes and their linking necessitates meticulous control over several parameters to achieve the desired assembly with specific properties.In the initial report on such assembly, 1,2-dithiolo-carborane was utilized as a thiolate ligand to form the Ag 14 cluster node connected through the linker. [45]Recognizing that the stability and properties of assembled frameworks are significantly influenced by the cluster nodes, there arises a critical need to explore simpler ligand architectures.Addition-ally, it has been firmly established that alterations in thiolate ligands have a direct and significant influence on the geometric configuration of the cluster node.This underscores the complexity inherent in customizing assemblies to achieve specific properties.Consequently, there is a need to impose constraints on the geometry of the cluster node by employing straightforward ligand structures.These specified cluster nodes can serve as a foundation for constructing a system wherein they connect through linker molecules, forming a model.This model can then be employed to explore different arrangements by incorporating various linker molecules, thereby expanding the versatility of the system.
In the quest for a straightforward ligand structure, the tertbutyl thiolate ligand emerges as a promising candidate.This ligand has been extensively employed in various studies for constructing Ag cluster nodes in addition with simple trifluoroacetate as an auxiliary ligand.Intriguingly, despite the use of different in-situ linking approaches with diverse linker molecules, the tert-butyl thiolate ligand consistently forms Ag 12 cluster nodes. [20]This observation underscores the importance of understanding the impact of ligands on the stability and properties of cluster-assembled materials, emphasizing the need for further exploration and study with simpler ligand architectures.While there are reports of Ag 10 , [50] Ag 11 , [48] Ag 12 , [51] Ag 14 , [49] Ag 16 , [52] Ag 18 , [53] and Ag 27 [54] CAMs where different types of thiols and linker molecules were utilized separately in the literature.So, creating generalized facts becomes challenging when the cluster nodes undergo continuous changes along with other factors connected to linker molecules which we have discuused earlier.The ongoing challenge lies in deciphering the structural variations in cluster nodes induced by diverse linker molecules, leading to the quest for a consistent cluster node.Therefore, our current focus is directed towards the exploration of the Ag 12 cluster node as a model system.This choice is driven by the intention to establish a systematic framework for investigating how different linker structures influence the structural properties of the cluster.This approach aims to provide insights into the effects of various linker molecules, contributing to a more comprehensive understanding of cluster-assembled materials.

General Synthesis Methods and Structural Architecture
The synthesis of the Ag CAM typically follows a one-pot reaction protocol.It commences with the interaction of surface-protecting ligands and the Ag(I)-thiolate complex, followed by the connection through organic linker molecules. [55]Since our emphasis is on the Ag 12 cluster node as a model system, owing to its uncomplicated ligand architecture, the initial complexation entails Ag(I) and S t Bu À ligands in the presence of a basic solution medium.It is worth noting that altering the thiolate ligands during this step can result in different Ag(I)-thiolate complexes, ultimately leading to the formation of distinct cluster nodes.Subsequently, the synthesized Ag(I)-thiolate complex is combined with AgCF 3 COO in different solvent media, taking into account the solubility of the linker molecules.The linker is then introduced either in solid form or as a solution.Thus, the initial reaction takes place between the Ag(I)-thiolate complex and AgCF 3 COO, resulting in the creation of the cluster node.Exploring its structural architecture in detail, as depicted in Figure 1, we observe that 12 Ag(I) atoms arrange themselves in a specific hollow cuboctahedral geometry through argentophilic interactions. [55]The S t Bu À ligands are affixed to each isosceles trapezoid facet, while CF 3 COO À ligands are attached to Ag atoms within the equilateral triangular planes, forming the Ag 12 cluster node.Subsequently, these cluster nodes are interconnected by incorporating N-coordinated linkers.Nevertheless, the linking strategy is largely contingent on the structural characteristics of the linker molecules.However, researchers have employed specific techniques to influence the linking process to achieve a desired growth pattern.One such approach involves the incremental addition of various solvent media to decrease the solubility of the cluster nodes, thereby slowing down the attachment process.Moreover, the linking process is occasionally carried out at the interface of two distinct solvent media, ensuring the favored growth of a singlecrystal structure.

Investigating the Correlation Between Ag 12 Cluster Assemblies and Their Versatile Applications
In the early stages of Ag CAM research, Prof. T. C. W. Mak and his research group extensively utilized Ag 12 cluster node as a model system for the first time.They synthesized various Ag 12 CAMs by altering the linker architecture, thereby inducing changes in their overall structural configuration and dimensionality and tried to understand their different comparative properties.Initially, Huang et al. introduced 4,4'bipyridine as a a linear bidentate linker to coordinate with the Ag 12 cluster nodes. [56]It was observed that the attachment of these linkers onto the cluster nodes replaced acetonitrile solvent molecules within the [(Ag 12 (S t Bu) 6 (CF 3 COO) 6 (CH 3 CN) 6 ] cluster node.This linking process actively transformed the crystal system from triclinic to tetragonal, accompanied by a specific change in the space group.The result was a three-dimensional connectivity of the cluster node facilitated by the linking of these linker molecules.Interestingly, they noted a higher stability of this CAM compared to the individual cluster nodes and observed a change in their photoluminescence (PL) emission properties.While the Ag 12 cluster node exhibited weak, short-lived red emission, the designed CAM displayed an intense, bright-green PL emission in a vacuum.The intensity of the PL emission in the CAM showed a remarkable 60-fold enhancement when compared with the individual cluster nodes.However, they discovered that the sensitivity of emission intensities was influenced by the presence of oxygen molecules.An emission quenching effect was observed with an increase in oxygen pressure.Capitalizing on this phenomenon, they designed a high-performing solid-state O 2 sensor using this CAM.Expanding their investigation, they explored the vapochromism effect of the CAM when exposed to different protic and aprotic volatile organic compounds (VOCs).They identified a bathochromic shift in the emission band, dependent on the polarity of the VOCs.Subsequently, Wu et al. presented

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another Ag 12 CAM, employing a tetradentate linker molecule, namely 1,1,2,2-tetrakis(4-(pyridin-4-yl)phenyl)-ethene. [57]In this configuration, each Ag 12 cluster node was connected to six individual linker molecules, as illustrated in Figure 2a.However, due to the non-planar nature of the linker, it resulted in connecting different cluster nodes in various planes.Consequently, this linker attachment led to the formation of a mesoporous three-dimensional architecture (Figure 2b).They identified a few dimethylacetamide molecules remaining as residual coordinating solvents in the structural framework.They emphasized its exceptional thermal and photostability as noteworthy characteristics.In addition to its stability, they also investigated the PL emission properties of this CAM, noting emission at the 454 nm region upon excitation at 365 nm.
Expanding the application of this material, they utilized its PL emission for sensor design, adopting a host-guest approach.This approach involved tailoring the emission properties through the encapsulation of guest molecules within the host matrix, showcasing the versatility of this Ag 12 CAM in sensor development.
In a subsequent study, Cao et al. introduced 5,10,15,20tetra(4-pyridyl)porphyrin as a linker molecule to establish connections between the Ag 12 cluster nodes. [58]It has been observed that each cluster node connected with four individual cluster nodes as depicted in Figure 3a.The linear architecture of the linker facilitated a two-dimensional linkage of cluster nodes (Figure 3b), further stacking in a three-dimensional arrangement through an AB stacking mode.Notably, they observed exceptional chemical and structural stability of this construct over time in an open-air environment.The porous network of this CAM exhibited a high surface area of 234.02 m 2 g À 1 , showcasing its remarkable porosity.The photoactivity of this CAM was examined through ultraviolet-visible

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diffuse reflectance spectroscopy, revealing a bandgap value of 1.756 eV, indicative of its favorable light-harvesting capability.Furthermore, the O 2 sorption isotherms demonstrated type-I behavior, with a substantial sorption capacity of 89.47 cm 3 g À 1 .
These characteristic positions this CAM as a promising candidate for generating singlet oxygen (  '-bipyridine linker molecule with 3-amino-4,4'-bipyridine. [59] The inclusion of amino groups in this modification serves dual pivotal roles.Firstly, it results in the production of a distinct blue sub-nanosecond fluorescence component peaking at around 456 nm and, a yellow phosphorescence component emerges at 556 nm region. Secondly, the introduction of amino groups significantly triggers spin-orbit coupling, leading to an increased rate of intersystem crossing (ISC).This heightened ISC rate contributes to the augmentation of triplet excitons, consequently extending the lifetime of the phosphorescence component by an impressive factor of approximately 15,000-fold.The initial submicrosecond duration of 0.2 μs is extended to milliseconds with a simultaneous increase in the quantum yield (QY) from 12.1 % to 14.6 % under oxygen isolated environment.Now, these distinctive combinations equips this CAM to function effectively as a singular Fluorescence-Phosphorescence ratiometric sensor for molecular oxygen.The sensor demonstrates rapid responses to trace amounts of oxygen gas, achieving an outstanding response time of 0.3 s.Furthermore, the limit of detection (LOD) is exceptionally low, reaching 0.1 ppm.The sensitivity is also visually evident through color variations, with responses intricately correlated to the concentration of O 2 , particularly noticeable at levels below 20 ppm.Wang et al. identified a different conformational flexibility when linking Ag 12 cluster nodes with two separate linker molecules. [60]The structural variance between the linker molecules played a crucial role in the optical properties of the assembly.
Specifically, when employing 1,2-bis(4-pyridyl)ethane as the linker, the overall assembly showed no PL emission.However, substituting the linker with 1,2-bis(4-pyridyl)ethylene induced the desired PL emission properties.So, the structural architecture with the designed linker molecules widens up its application possibilities in various approaches.Das et al. conducted a noteworthy study on a novel Ag 12 CAM employing 4,4'-azopyridine linker molecules. [61]The structural uniqueness of the linker, characterized by its bidentate and linear configuration, facilitated the connection of each cluster node to six distinct linker molecules (Figure 4a).This led to the expansion of growth in a two-dimensional layer (Figure 4b), where the separation between cluster nodes was precisely 13.58 Å.The resulting two-dimensional layer structure, however, exhibited an additional intricacy: the layers were interconnected with a separation distance of 7.28 Å through interlayer noncovalent interactions.Despite the absence of a direct link between the layers, the researchers

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sought to isolate individual layers using a mechanical exfoliation technique, successfully obtaining a single layer of Ag CAM.Its thickness, determined through Atomic Force Microscopy (AFM), was measured at a mere 0.76 nm.This breakthrough holds particular significance as it addresses the quest for materials with graphitic resemblances.Thicknessdependent electrical property investigations were conducted via conducting AFM studies, revealing a noteworthy trend: as the thickness increased from a single layer, conductivity diminished.This suggests an in-plane conductivity phenomenon, emphasizing the lack of direct connections across the plane.To further explore the potential applications of the single-layer Ag CAM, the researchers designed a field-effect transistor (FET) device.Their study uncovered a p-type channel behavior with an impressive ON/OFF current ratio of 4500 and a hole mobility of 1.215 cm 2 V À 1 s À 1 .This outcome underscores the promising prospects of Ag CAM materials as transistor channel materials, marking a significant advancement in the field.Dar et al. reported two distinct Ag 12 assemblies through the alteration of linker molecules. [62]The use of 4,4'-bipyridine led to a two-dimensional architecture, whereas pyrazine linker molecules yielded a three-dimensional framework.Moreover, the three-dimensional structure was employed as a luminescent sensor for nitro compounds, exploiting its interaction with heterocyclic amines.
In our recent research endeavors, we developed several CAMs, employing a strategy that involves the manipulation of linker molecules on Ag 12 cluster node.Our exploration commences with an Ag 12 CAM incorporating a bidentate 1,4bis(pyridin-4-ylethynyl)benzene linker. [63]In this configuration, each cluster node is intricately linked by six such molecules (Figure 5a), resulting in the observation of a planar layer growth pattern (Figure 5b).Interestingly, the threedimensional architecture of the CAM emerges due to noncovalent interactions between layers.The distinctive interlayer connectivity plays a vital role in enhancing both the structural and chemical stability of this material, with the achieved thermal stability extending up to 150 °C.Furthermore, the versatility of this CAM goes beyond its structural attributes.We have effectively employed this CAM in detecting label-free DNA during hybridization.The detection mechanism hinges on the changes in emission intensities caused by strain quenching as DNA molecules aggregate through electrostatic interactions on the surface of the CAM.This innovative approach showcases the versatility of our Ag 12 CAMs and their potential applications in the realm of biosensing.In a subsequent phase of our research, we engineered two distinct binodal 4,6-coordinated three-dimensionally linked Ag 12 CAMs. [55]This unique structural architecture was achieved through the utilization of two tetradentate linker molecules: 1,1,2,2-tetrakis(4-(pyridin-4-ylethynyl)phenyl)ethene and 1,1,2,2-tetrakis(4-((E)-2-(pyridin-4-yl)vinyl)phenyl)ethene.Despite a certain alteration in the linker structure, both crystal formations exhibited identical crystalline characteristics, adopting a trigonal crystal system with an R3 ̄c (No. 167) space group.In both instances, the resulting three-dimensional frameworks exhibited a stacking arrangement characterized by an ABA pattern.Within the layers, each cluster node was separated by 26.84 Å, while across the layers, the separation was measured at 17.67 Å. Notably, both structures displayed exceptional thermal and chemical stabilities.Despite their structural similarities, a notable discrepancy was identified in their emission properties.In our investigation, we meticulously assigned unique labels, namely TUS 1 and TUS 2, to the structures under consideration.TUS 1 was identified by an emission maximum at 530 nm, while TUS 2 exhibited a distinct emission maximum at 561 nm.These identifications were established by observing distinctive variations in the emission characteristics of individual linkers, as the majority of emission properties originate from the linker molecules and their coordination with the cluster nodes.Beyond the variation in emission maximum positions, a significant contrast was observed in the quantum yields of TUS 1 and TUS 2. TUS 1 displayed an approximately threefold lower quantum yield compared to TUS 2. This observation underscored the linear dependence of the structural rigidity on the emission quantum yield, providing valuable insights into the intricacies of these structures.Leveraging these unique emission properties, we explored their effectiveness in metal ion sensing, particularly focusing on their stability in water media.Given the high stability of these CAMs in water, we developed a highly sensitive and selective assay for detecting Fe 3 + in aqueous environments, operating under the assumption that the metal ions selectively adhere to the pores within the framework structure.The limits of detection (LOD) were determined to be 0.05 nM L À 1 for TUS 1 and 0.86 nM L À 1 for TUS 2.
Importantly, these values fall well below the maximum permissible concentration of Fe 3 + in drinking water (3.57μmol L À 1 ) mandated by the European Union.Real water sampling further validated the practical applicability and efficacy of these structures in real-time scenarios.
Later in our research, we synthesized another Ag 12 CAM employing a tridentate benzene-1,3,5-tricarboxylic acid trispyridin-4-ylamide as the linker molecule. [64]Intriguingly, our observations revealed that each Ag 12 cluster node interconnected with six linker molecules (Figure 6a), in such a way that each linker molecules attached to the other three cluster nodes in a similar plane forming a two-dimensional network (Figure 6b).The non-covalent interactions among these linkers meticulously arranged the layers at a specific distance of 17.2 Å, culminating in the development of a three-dimensional architectural framework.The crystal structure exhibited a trigonal crystal system, falling under the R3 ̄(No.148) space group.Brunauer-Emmett-Teller analysis indicated a noteworthy specific surface area of 394 m 2 g À 1 and a pore size of 7.03 nm.Stability assessments in various solvent media, including water with and without NaBH 4 , underscored the robust nature of the material.Thermogravimetric analysis confirmed its thermal stability up to 100 °C.Further exploration revealed PL emission of this structure peaking at 445 nm upon excitation at 330 nm, exhibiting an absolute QY of 0.55 based on its linker emission properties.Notably, the material displayed a featureless UV-vis absorbance peak spanning from 300 nm to 800 nm.Leveraging its exceptional stability and high surface area, we harnessed this porous material as a catalyst for the reduction of Fe(CN) 6 3À .While different sized nanoparticles have traditionally been studied for such catalytic applications, our pioneering investigation to understand the efficacy of the atom precisely designed well organised Ag 12 CAM efficacy through UV-vis absorbance studies yielded remarkable results.The reaction exhibited a half-life of 14 seconds, attributed to several favorable factors, with the liberation of a continuous electron flow in the presence of an electron acceptor being a key contributor, as evidenced by the framework structure.This underscores the structural architecture of the material as a catalyst surface, facilitating a continuous flow of electrons through adsorbed reagents within its pores.In essence, our findings showcase the Ag 12 CAM has a unique role as catalyst surface, shedding new light on its potential applications in electron transfer processes.
Subsequently, we engineered an additional Ag 12 CAM incorporating a tetra-coordinated 2,2',7,7'-tetra(pyridin-4-yl)-9,9'-spirobi(fluorene) linker molecule. [65]The resulting Ag 12 CAM forms crystals in a cubic crystal system, exhibiting a space group of Pn3 ̄n (No. 222).Within this structure, each cluster node is connected to six distinct linker molecules (Figure 7a), while each linker is attached to four other cluster nodes in different planes (Figure 7b).This unique arrangement gives rise to a three-dimensional architecture with layers stacked using an AA stacking approach.The precise separation between cluster nodes within a layer measures 1.97 nm, closely resembling the measured pore size of 1.8 nm.This assembly demonstrated significant stability in a water medium, retaining its structural integrity even at temperatures as high as 120 °C.
Leveraging the distinctive emission properties of the linker molecules, this CAM exhibits emission behavior upon excitation, with a noteworthy QY of 0.47 %.Exploiting the porous framework and exceptional chemical stability of this CAM, we employed it for the detection of Hg 2 + ion through surface-enhanced Raman spectroscopy (SERS).Remarkably, we observed an overall peak enhancement in SERS measure-ments when Hg 2 + ions interacted with the Ag CAM, with enhancement levels correlating to the concentration of Hg 2 + .We assessed the selectivity of the CAM for Hg 2 + against other mono-, di-, and trivalent metal ions.The identified limit of detection for Hg 2 + was determined to be 0.07 pg mL À 1 , representing the lowest limit achieved through this method, owing to the precise structural architecture.A crucial role of NaBH 4 in the detection process was revealed, as it is preadsorbed on the porous network of the CAM, facilitating the formation of AgÀ Hg amalgam from Ag(I) and Hg(II).The atomically precise structure, ordered arrangement, precise porous network, and improved stability collectively enable the Ag CAM to be effectively utilized for detecting Hg 2 + with an unprecedentedly low detection limit.

Summary and Prospects
In summary, the preceding article comprehensively expounds upon the essential factors and associated parameters undergoing alterations during the linking of cluster nodes.The elucidation extends to a demonstration of the requisite adjustments essential for attaining a precise structural architecture conducive to the generation of a model system.A focal point of our discourse revolves around Ag 12 , elucidated as a paradigmatic cluster node, intricately linked with a myriad of linker molecules.Our scrutiny reveals a discernible shift in various properties concomitant with alterations in the dimensionality of the structures.Noteworthy is the recognition that these properties exhibit dependence on both the coordinating environment and the inherent characteristics of the linkers employed.The amalgamation of these identified factors imparts versatility to the resulting assembled materials, rendering them suitable for a wide range of applications.Consequently, we anticipate that, in forthcoming investigations, a deeper exploration of these linker molecules utilized in model system generation for specific applications will transpire.The aim is to discern the superiority of certain cluster nodes and their linking applicability.Furthermore, it becomes imperative to expand the existing model table to include additional linker molecules recently implemented in Ag CAM design.This expansion seeks to identify the actual variations in their properties, thereby contributing to a more comprehensive understanding of their efficacy.In light of these considerations, we anticipate a promising trajectory for future research endeavors, characterized by enhanced optimization of properties and an expanded realm of application possibilities.(grant no.22H04562), the Yazaki Memorial Foundation for Science and Technology, and the Ogasawara Foundation for the Promotion of Science and Engineering.

Figure 1 .
Figure 1.Geometrical architecture of 12 Ag(I) atoms which surface is protected by thiolate and trifluoroacetate ligands to form a Ag 12 cluster node and finally the attachment of the linker molecules on the Ag 12 cluster node.

Figure 4 .
Figure 4. (a) Attachment of 4,4'-azopyridine linker molecules on each Ag 12 cluster node and (b) corresponding two-dimensional architecture of the assembly.