We present an example of host–guest complexes of atomically precise noble metal nanoparticles with cucurbit7uril (CB) in water, specifically concentrating on Ag29(LA)12 (where LA is α-lipoic acid), ...a well-known red luminescent silver cluster. Such host–guest interactions resulted in enhanced luminescence of about 1.25 times for the modified system, compared to the parent cluster. We extended our study to cyclodextrins (CDs), where about 1.5 times enhanced luminescence was estimated compared to the parent cluster. The formation of supramolecular complexes was confirmed using high-resolution electrospray ionization mass spectrometry (HRESI MS) and nuclear magnetic resonance spectroscopy. Molecular docking and density functional theory calculations supported our experimental results and showed that while CB formed inclusion complexes by encapsulation of one of the LA ligands of the cluster, CD formed supramolecular adducts by interaction with the cavity built by the ligands on the cluster surface. The complexation was favored by geometrical compatibility. Consequently, these superstructures are labeled as Ag29LA12∩CB n and Ag29LA12@CD n (n = 1–3), where ∩ and @ indicate the inclusion complex and supramolecular adduct, respectively. Solution-phase Ag29LA12@CD n complexes were employed to detect dopamine (10 nM). Luminescent Ag29LA12@CD n and Ag29LA12∩CB n complexes in water could be potential candidates for organic pollutant sensing and biomedical applications.
Two ligand‐protected nanoscale silver moieties, Ag46(SPhMe2)24(PPh3)8(NO3)2 and Ag40(SPhMe2)24(PPh3)8(NO3)2 (abbreviated as Ag46 and Ag40, respectively) with almost the same shell but different cores ...were synthesized simultaneously. As their external structures are identical, the clusters were not distinguishable and become co‐crystallized. The occupancy of each cluster was 50 %. The outer shell of both is composed of Ag32S24P8, which is reminiscent of fullerenes, and it encapsulates a well‐studied core, Ag14 and a completely new core, Ag8, which correspond to a face‐centered cube and a simple cube, respectively, resulting in the Ag46 and Ag40 clusters. The presence of two entities (Ag40 and Ag46 clusters) in a single crystal and their molecular formulae were confirmed by detailed electrospray ionization mass spectrometry. The optical spectrum of the mixture showed unique features which were in good agreement with the results from time‐dependent density functional theory (TD‐DFT).
Silver lining: Two ligand‐protected silver clusters, Ag46 and Ag40, with identical shells but different cores have been crystallized together. These clusters are the first examples of a new class of cluster solids.
We report the formation of supramolecular adducts between monolayer-protected noble metal nanoclusters and fullerenes, specifically focusing on a well-known silver cluster, Ag29(BDT)123–, where BDT ...is 1,3-benzenedithiol. We demonstrate that C60 molecules link with the cluster at specific locations and protect the fragile cluster core, enhancing the stability of the cluster. A combination of studies including UV–vis, high-resolution electrospray ionization mass spectrometry, collision-induced dissociation, and nuclear magnetic resonance spectroscopy revealed structural details of the fullerene-functionalized clusters, Ag29(BDT)12(C60) n 3– (n = 1–9). Density functional theory (DFT) calculations and molecular docking simulations affirm compatibility between the cluster and C60, resulting in its attachment at specific positions on the surface of the cluster, stabilized mainly by π–π and van der Waals interactions. The structures have also been confirmed from ion mobility mass spectrometry by comparing the experimental collision cross sections (CCSs) with the theoretical CCSs of the DFT-optimized structures. The gradual evolution of the structures with an increase in the number of fullerene attachments to the cluster has been investigated. Whereas the structure for n = 4 is tetrahedral, that of n = 8 is a distorted cube with a cluster at the center and fullerenes at the vertices. Another fullerene, C70, also exhibited similar behavior. Modified clusters are expected to show interesting properties.
Collision cross sections (CCSs) of ligand-protected metal clusters were evaluated using ion mobility mass spectrometry. The targets used in this study were phosphine-protected clusters PdAu8(PPh3)82+ ...and Au9(PPh3)83+, for which the total structures have been resolved by single-crystal X-ray analysis. The arrival time distributions of PdAu8(PPh3)82+ as a function of the He flow rate in a cell located just in front of a traveling wave ion mobility cell filled with N2 buffer gas demonstrated that it got converted to another structural isomer having a smaller CCS, with the increase in the nominal collision energy. A similar phenomenon was observed for Au9(PPh3)83+. These results were explained by the collisional excitation and cooling with the buffer gas inducing the conversion of the packing arrangement of the ligands rather than the atomic structure of the metallic core: the ligand layer was converted from disordered to the closely packed arrangement found in a single crystal during this process. This study showed that the ligand layer with a disordered arrangement in solution was retained during desolvation upon electrospray ionization and was annealed into the most stable closely packed arrangement by collisions.
Au25(SR)18− nanoclusters have been tested as a model catalyst in multiple oxidation reactions involving CO, alkenes, cyclohexane, and alcohols. Oxygen is used as an oxidizing agent in most of the ...reactions. Hence, O2 activation is of great interest in catalysis. The mechanism of these reactions, the role of intact nanoclusters as catalytically active species, and the utility of such nanoclusters as homogeneous catalysts are not completely clear. Herein, we investigate the interaction of Au25(SR)18− in solution with O2 using electrospray ionization mass spectrometry and density functional theory (DFT) calculations. Up to three O2 molecules attach to an Au25(SR)18− in dichloromethane (DCM) when O2 gas is passed through a solution of the former. Oxygen addition to the nanocluster leads to its decomposition. The nanocluster is most stable in toluene and least stable in tetrahydrofuran when kept under a continuous flow of O2, where no O2 adduct peaks are observed. It shows intermediate stability in DCM in the presence of O2, and the decomposition products, in this case, are of a different type compared to the former solvents. The appearance of O2 adducts and the variation in the stability of the nanocluster in different solvents is assumed to be due to the difference in oxygen solubility in these solvents. DFT calculations suggest that the first two O2 molecules interact with the surface Au atoms through the cavities formed by the ligands and staples in the nanocluster and the third O2 interacts only with 2-phenylethane thiol ligands.
Conspectus Supramolecular chemistry is a major area of chemistry that utilizes weaker non-covalent interactions between molecules, including hydrogen bonding, van der Waals, electrostatic, π···π, and ...C–H···π interactions. Such forces have been the basis of several molecular self-assemblies and host–guest complexes in organic, inorganic, and biological systems. Atomically precise nanoclusters (NCs) are materials of growing interest that display interesting structure–property correlations. The evolving science of such systems reaffirms their molecular behavior. This gives a possibility of exploring their supramolecular chemistry, leading to assemblies with similar or dissimilar cluster molecules. Such assemblies with compositional, structural, and conformational precision may ultimately result in cluster-assembled hybrid materials. In this Account, we present recent advancements on different possibilities of supramolecular interactions in atomically precise cluster systems that can occur at different length scales. We first present a brief discussion of the aspicule model of clusters, considering Au25(SR)18 as an example, that can explain various aspects of its atomic precision and distinguish the similar or dissimilar interacting sites in their structures. The supramolecular interaction of 4-tert-butylbenzyl mercaptan (BBSH)-protected Au25(SBB)18− NCs with cyclodextrins (CD) to form Au25SBB18∩CD n (n = 1–4) and that of Ag29(BDT)123– with fullerenes to form Ag29(BDT)12(C60) n 3– (n = 1–9) (BDT = 1,3-benzenedithiolate) are discussed subsequently. The formation of these adducts was studied by electrospray ionization mass spectrometry (ESI MS), optical absorption and NMR spectroscopy. In the subsequent sections, we discuss how variation in intercluster interactions can lead to polymorphic crystals, which are observable in single-crystal X-ray diffraction. Taking Ag29(BDT)12(TPP)43– (TPP = triphenylphosphine) clusters as an example, we discuss how the different patterns of C–H···π and π···π interactions between the secondary ligands can alter the packing of the NCs into cubic and trigonal lattices. Finally, we discuss how the supramolecular interactions of atomically precise clusters can result in their hybrid assemblies with plasmonic nanostructures. The interaction of p-mercaptobenzoic acid (p-MBA)-protected Ag44(p-MBA)30 NCs with tellurium nanowires (Te NWs) can form crossed-bilayer precision assemblies with a woven-fabric-like structure with an angle of 81° between the layers. Similar crossed-bilayer assemblies show an angle of 77° when Au102(p-MBA)44 clusters are used to form the structure. Such assemblies were studied by transmission electron microscopy (TEM). Precision in these hybrid assemblies of Te NWs was highly controlled by the geometry of the ligands on the NC surface. Moreover, we also present how Ag44(p-MBA)30 clusters can encapsulate gold nanorods to form cage-like nanostructures. Such studies involved TEM, scanning transmission electron microscopy (STEM), and three-dimensional tomographic reconstructions of the nanostructures. The hydrogen bonding interactions of the −COOH groups of the p-MBA ligands were the major driving force in both of these cases. An important aspect that is central to the advancement of the area is the close interplay of molecular tools such as MS with structural tools such as TEM along with detailed computational modeling. We finally conclude this Account with a future perspective on the supramolecular chemistry of clusters. Advancements in this field will help in developing new materials with potential optical, electrical, and mechanical properties.
Rapid solution-state exchange dynamics in nanoscale pieces of matter is revealed, taking isotopically pure atomically precise clusters as examples. As two isotopically pure silver clusters made of
Ag ...and
Ag are mixed, an isotopically mixed cluster of the same entity results, similar to the formation of HDO, from H
O and D
O. This spontaneous process is driven by the entropy of mixing and involves events at multiple time scales.
The current studies show that by altering the shape of the matrix a cell response (genomic profile) similar to that achieved
with chemical stimulation can be elicited. Here, we show that bone ...formation can be achieved with efficiency similar to that
of dexamethasone with the added benefit that endothelial cell development is not inhibited. We further show that the mechanism
of action of the topographies and dexamethasone differs. This could have an implication for tissue engineering in which a
simultaneous, targeted, development of a tissue, such as bone, without the suppression of angiogenesis to supply nutrients
to the new tissue is required. The results further demonstrate that perhaps the shape of the extracellular matrix is critical
to tissue development., The understanding of cellular response to the shape of their environment would be of benefit in the development of artificial
extracellular environments for potential use in the production of biomimetic surfaces. Specifically, the understanding of
how cues from the extracellular environment can be used to understand stem cell differentiation would be of special interest
in regenerative medicine., In this paper, the genetic profile of mesenchymal stem cells cultured on two osteogenic nanoscale topographies (pitted surface
versus raised islands) are compared with cells treated with dexamethasone, a corticosteroid routinely used to stimulate bone
formation in culture from mesenchymal stem cells, using 19k gene microarrays as well as 101 gene arrays specific for osteoblast
and endothelial biology.
Cloud computing gained much popularity in the recent past due to its many internet-based services related to data, application, operating system, and eliminating the need for central hardware access. ...Many of the challenges associated with cloud computing can be specified as network load, security intrusion, authentication, biometric identification, and information leakage. Numerous algorithms have been proposed and evaluated to solve those challenges. Among those, bio-inspired algorithms such as Evolutionary, Swarm, Immune, and Neural algorithms are the most prominent ones which are developed based on nature's ecosystems. Bio-inspired algorithms' adaptability allows many researchers and practitioners to utilize them to solve many security-related cloud computing issues. This paper aims to explore previous research, recent studies, challenges, and scope for further analysis of cloud security. Therefore, this study provides an overview of bio-inspired algorithms application and evaluations, taking into account cloud security challenges, such as Identity and Authentication, Access Control Systems, Protocol and Network Security, Trust Management, Intrusion Detection, Virtualization, and Forensic.
•Define authentication factors.•Evaluate trustworthy values of different authentication factors.•Evaluate trustworthy values of different sets of authentication factors.•Design multi-objective ...optimization strategies for adaptive multi-factor authentication.•Conducting experiments for checking the efficiency and effectiveness of the proposed approach.
Authentication is the fundamental safeguard against any illegitimate access to a computing device and other sensitive online applications. Because of recent security threats, authentication through a single factor is not reliable to provide adequate protection of these devices and applications. Hence, to facilitate continuous protection of computing devices and other critical online services from unauthorized access, multi-factor authentication can provide a viable option. Many authentication mechanisms with varying degrees of accuracy and portability are available for different types of computing devices. As a consequence, several existing and well-known multi-factor authentication strategies have already been utilized to enhance the security of various applications. Keeping this in mind, we developed a framework for authenticating a user efficiently through a subset of available authentication modalities along with their several features (authentication factors) in a time-varying operating environment (devices, media, and surrounding conditions, like light, noise, motion, etc.) on a regular basis. The present work is divided into two parts, namely, a formulation for calculating trustworthy values of different authentication factors and then the development of a novel adaptive strategy for selecting different available authentication factors based on their calculated trustworthy values, performance, selection of devices, media, and surroundings. Here, adaptive strategy ensures the incorporation of the existing environmental conditions on the selection of authentication factors and provides significant diversity in the selection process. Simulation results show the proposed selection approach performs better than other existing and widely used selection strategies, mainly, random and optimal cost selections in different settings of operating environments. The detailed implementation of the proposed multi-factor authentication strategy, along with performance evaluation and user study, has been accomplished to establish its superiority over the existing frameworks.
PDF
