Selenolate protected, stable and atomically precise, hollow silver cluster was synthesized using solid state as well as solution state routes. The optical absorption spectrum shows multiple and sharp features similar to the thiolated Ag44 cluster, Ag44(SR)30 whose experimental structure was reported recently. High-resolution electrospray ionization mass spectrometry (HRESI MS) shows well-defined molecular ion features with two, three, and four ions with isotopic resolution, due to Ag44(SePh) 30. Additional characterization with diverse tools confirmed the composition. The closed-shell 18 electron superatom electronic structure, analogous to Ag44(SR)30 stabilizes the dodecahedral cage with a large HOMO-LUMO gap of 0.71 eV. The time-dependent density functional theory (TDDFT) prediction of the optical absorption spectrum, assuming the Ag44(SR)30 structure, matches the experimental data, confirming the structure. © 2013 American Chemical Society.

Pradeep Thalappil

Department Of Chemistry

ESI MS

MALDI-MS

NANO-MOLECULES

Selenolate

SILVER CLUSTER

SUPERATOMS

TDDFT

Electronic structure

Mass spectrometry

Optical materials

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Journal of Physical Chemistry Letters

Nanoparticles rival pharmaceuticals in synthetic inefficiency, in particular as measured by process mass intensity, with by far the largest contribution to waste being from solvents. We have therefore developed a greener method of synthesizing silver nanoparticles using a paste format instead of a metal salt solution. The paste-based synthesis requires 87% less solvent yet still produces exclusively Na4Ag44(p-MBA)30 nanoparticles, without size sorting, with 89% yield. By using a stoichiometric silver-thiolate polymer as a precursor to intimately mix the metal atoms and ligands, and by using a small amount of liquid to form a paste to promote mass transport, the heterogeneity and kinetics problems that are associated with entirely solid-state syntheses were avoided. Because the nanoparticle product was also a paste, solvent use for postprocessing was minimized. Use of the silver-thiolate polymer can also reduce health risks associated with hazardous free thiols in conventional solution-phase syntheses. Using this strategy, the process mass intensity was reduced from ∼1800 for the solution-phase reaction to ∼200 and ∼100 for the paste-based reaction with and without protonation of the ligands, respectively. © 2017 American Chemical Society.

Journal of Physical Chemistry C

High-Yield Paste-Based Synthesis of Thiolate-Protected Silver Nanoparticles

Atomically precise gold and silver clusters are a new class of sensitizers which can be used as substitutes for dyes in the classical dye-sensitized solar cells (DSCs). Here noble metal clusters protected by proteins and thiols (Au30@BSA, Au25SBB18, and Ag44MBA30) have been used for photovoltaic studies. These metal clusters were used as sensitizers for the photoanodes fabricated using TiO2 nanotubes (NTs) and the commercial P25 TiO2 nanoparticles. The TiO2, clusters and the solar cells were characterized by spectroscopy, microscopy, current-voltage (I-V) and incident photon-to-current conversion efficiency (IPCE) measurements. A systematic I-V study revealed a conversion efficiency of 0.35 % for the Au30@BSA sensitized solar cell made from TiO2 NTs which showed an IPCE maximum of 3 % at ∼ 400 nm. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ChemistrySelect

Atomically Precise Noble Metal Clusters Harvest Visible Light to Produce Energy

Highly organized crossed bilayer assemblies of nanowires (NWs) are made using directed hydrogen bonding between the protecting ligand shells of atomically precise cluster molecules anchored on NWs. Layers of quantum clusters remain sandwiched between two neighboring NWs at a defined distance, dictated by the core-size of the cluster, while the orientation of the ligands in space dictates the interlayer geometry. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA.

Advanced Materials

Cluster-Mediated Crossed Bilayer Precision Assemblies of 1D Nanowires

The first atomically precise and monolayer protected iridium cluster in solution, Ir9(PET)6 (PET-2-phenyethanethiol) was synthesized via a solid state method. The absence of a plasmonic band at ∼350 nm, expected in the UV/Vis spectra for spherical Ir particles of 10 nm size indicated that the synthesized cluster is smaller than this dimension. Small angle X-ray scattering (SAXS) showed that the cluster has a particle size of ∼2 nm in solution which was confirmed by transmission electron microscopy (TEM). The blue emission of the cluster is much weaker than many noble metal clusters investigated so far. X-ray photoelectron spectroscopy (XPS) measurements showed that all Ir atoms of the cluster are close to the zero oxidation state. The characteristic S-H vibrational peak of PET at 2560 cm-1 was absent in the FT-IR spectrum of the cluster indicating RS-Ir bond formation. The molecular formula of the cluster, Ir9(PET)6 was assigned based on the most significant peak at m/z 2553 in the matrix assisted laser desorption ionization mass spectrum (MALDI MS), measured at the threshold laser intensity. Density functional theory calculations on small Ir@SCH3 and Ir@PET clusters and comparison of the predictions with the IR and 1H-NMR spectra of Ir9(PET)6 suggested that the PET ligands have two distinct structural arrangements and are likely to be present as bridging thiolates -(Ir-SR-Ir)- and singly attached thiolates -(Ir-SR). © 2016 The Royal Society of Chemistry.

RSC Advances

Atomically precise and monolayer protected iridium clusters in solution

The cluster Ag44SePh30, originally prepared from silver selenolate, upon oxidative decomposition by H2O2 gives the same cluster back, in an apparently reversible synthesis. Such an unusual phenomenon was not seen for the corresponding thiolate analogues. From several characterization studies such as mass spectrometry, Raman spectroscopy, etc., it has been confirmed that the degraded and as-synthesized selenolates are the same in nature, which leads to the reversible process. The possibility of making clusters from the degraded material makes cluster synthesis economical. This observation makes one to consider cluster synthesis to be a reversible chemical process, at least for selenolates. © The Royal Society of Chemistry 2014.

Nanoscale

Reversible formation of Ag44 from selenolates

An atomically precise silver cluster, Ag152 protected with thiolate ligands, was used as a surface-enhanced Raman scattering (SERS) substrate. The cluster shows intense enhancement of Raman signals of crystal violet with an enhancement factor of 1.58 × 109. Adaptability of the substrate for a wide range of systems starting from dyes to biomolecules is demonstrated. Solid-state drop casting method was used here, and SERS signals were localized on the Ag152 crystallites, confirmed from Raman images. Excellent periodicity of clusters, their plasmonic nature, and absence of visible luminescence are the main reasons for this kind of large enhancement. SERS was compared with smaller clusters and larger nanoparticles, and the size regime of Ag152 was found to be optimum. Several control experiments were done to understand the SERS activity in detail. The method has wide adaptability as the cluster can be easily drop-casted on any surface like paper, cotton, and so forth to produce effective SERS media. The work suggests that atomically precise clusters, in general, can show SERS activity. © 2013 American Chemical Society.

Fulltext

Atomically precise silver clusters as new SERS substrates

A cluster obtained in high yield from the reduction of a silver-thiolate precursor, Ag-SCH2CH2Ph, exhibited a single sharp peak near 25 kDa in the matrix-assisted laser desorption mass spectrum (MALDI MS) and a well-defined metal core of ∼2 nm measured with transmission electron microscopy (TEM). The cluster yields a single fraction in high-performance liquid chromatography (HPLC). Increased laser fluence fragments the cluster until a new peak near 19 kDa predominates, suggesting that the parent cluster-Ag152(SCH2CH2Ph)60-evolves into a stable inorganic core-Ag152S60. Exploiting combined insights from investigations of clusters and surface science, a core-shell structure model was developed, with a 92-atom silver core having icosahedral-dodecahedral symmetry and an encapsulating protective shell containing 60 Ag atoms and 60 thiolates arranged in a network of six-membered rings resembling the geometry found in self-assembled monolayers on Ag(111). The structure is in agreement with small-angle X-ray scattering (SAXS) data. The protective layer encapsulating this silver cluster may be the smallest known three-dimensional self-assembled monolayer. First-principles electronic structure calculations show, for the geometry-optimized structure, the development of a ∼0.4 eV energy gap between the highest-occupied and lowest-unoccupied states, originating from a superatom 90-electron shell-closure and conferring stability to the cluster. The optical absorption spectrum of the cluster resembles that of plasmonic silver nanoparticles with a broad single feature peaking at 460 nm, but the luminescence spectrum shows two maxima with one attributed to the ligated shell and the other to the core. © 2012 American Chemical Society.

Nano Letters

The superstable 25 kDa monolayer protected silver nanoparticle: Measurements and interpretation as an icosahedral Ag152(SCH 2CH2Ph)60 cluster

We describe the application of a recently discovered family of materials called quantum clusters, which are sub-nanometer particles composed of a few atoms with well-defined molecular formulae, exhibiting intense absorption and emission in the visible region in metal ion sensing, taking Ag 25 as an example. The changes in the optical properties of the cluster, in both absorption and emission upon exposure to various metal ions in aqueous medium are explored. The cluster can detect Hg 2+ down to ppb levels. It can also detect 5d block ions (Pt 2+, Au 3+ and Hg 2+) down to ppm limits. Hg 2+ interacts with the metal core as well as the functional groups of the capping agents and the interaction is concentration-dependent. To understand the mechanism behind this type of specific interaction, we have used spectroscopic and microscopic techniques such as UV-vis spectroscopy, luminescence spectroscopy, Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD). Specific reasons responsible for the interaction of Hg 2+ have been proposed. © 2011 Elsevier B.V.

Journal of Hazardous Materials

Luminescent sub-nanometer clusters for metal ion sensing: A new direction in nanosensors

We report the first high temperature solution state synthesis of glutathione (-SG) protected atomically precise silver clusters. Noble metal cluster synthesis from metal ions generally requires ice cold temperatures as they are extremely sensitive and high temperature routes are used only for core reduction methods, starting from nanoparticles. The clusters formed by the new route have distinct features in their absorption profile and they exhibit red luminescence. They are characterised by other spectroscopic and microscopic techniques and a tentative formula of Ag 75(SG) 40 has been assigned. © The Royal Society of Chemistry 2012.

Chemical Communications

High temperature nucleation and growth of glutathione protected ∼ag 75 clusters

We show that the time-dependent biomineralization of Au3+ by native lactoferrin (NLf) and bovine serum albumin (BSA) resulting in near-infrared (NIR) luminescent gold quantum clusters (QCs) occurs through a protein-bound Au1+ intermediate and subsequent emergence of free protein. The evolution was probed by diverse tools, principally, using matrix-assisted laser desorption ionization mass spectrometry (MALDI MS), X-ray photoelectron spectroscopy (XPS), and photoluminescence spectroscopy (PL). The importance of alkaline pH in the formation of clusters was probed. At neutral pH, a Au1+-protein complex was formed (starting from Au3+) with the binding of 13-14 gold atoms per protein. When the pH was increased above 12, these bound gold ions were further reduced to Au(0) and nucleation and growth of cluster commenced, which was corroborated by the beginning of emission; at this point, the number of gold atoms per protein was ∼25, suggesting the formation of Au25. During the cluster evolution, at certain time intervals, for specific molar ratios of gold and protein, occurrence of free protein was noticed in the mass spectra, suggesting a mixture of products and gold ion redistribution. By providing gold ions at specific time of the reaction, monodispersed clusters with enhanced luminescence could be obtained, and the available quantity of free protein could be utilized efficiently. Monodispersed clusters would be useful in obtaining single crystals of protein-protected noble metal quantum clusters where homogeneity of the system is of primary concern. © 2011 American Chemical Society.

Journal	Journal of Physical Chemistry Letters
ISSN	19487185
Open Access	No