Initial Growth Kinetics of Luminescent Quantum Clusters of Silver within Albumin Family Protein Templates

We probed the initial growth kinetics of luminescent quantum clusters of silver (AgQCs) within two albumin family proteins, bovine serum albumin (BSA) and ovalbumin (Ova). Shorter time scale (seconds to minutes) growth of AgQCs monitored using real time photoluminescence spectroscopy has shown that, at lower concentrations of Ag⁺, only unstable QCs were formed. The major role of basic pH in the synthesis was not only to facilitate Ag⁺-BSA conjugation but also to provide well dispersed medium for controlled nucleation of QCs. Increase in the concentration of NaBH₄ affects growth kinetics greatly and leads to increase in the growth rate of AgQCs; but for NaBH₄ concentrations higher than the optimum value, growth rate becomes constant. Precise measurements have shown that excitation and emission of AgQCs exhibit linear red-shift with the increasing concentration of NaBH₄ whereas protein excitation remains constant. Similar results were observed for both the proteins, Ova and BSA. We believe that various insights provided by this study will be helpful for further improvements in the synthetic methodology and applications of protein protected AgQCs

Quantum Clusters in Cavities: Trapped Au₁₅ in Cyclodextrins

We have prepared Au15 quantum clusters anchored to α-, β-, and γ-cyclodextrin (CD) cavities. The synthesis process involves the core etching of larger clusters and the simultaneous trapping of the clusters formed inside the CD cavities. The clusters were characterized by various tools, such as optical absorption and luminescence spectroscopies, electrospray ionization−mass spectrometry (ESI-MS), X-ray photoelectron spectroscopy (XPS), circular dichroism spectroscopy, and two-dimensional nuclear magnetic resonance (2D NMR) spectroscopy. Trapping of the cluster in the CD cavity was proven by circular dichroism and also by rotational Overhauser effect spectroscopy (ROESY), in terms of the distinct cross peak between proton “e” of the glutathione (−SG) ligand and the “H3” proton of CD. Dynamic light scattering (DLS) studies showed a hydrodynamic diameter of ∼3−4 nm, indicating one CD molecule per cluster with an extension of one water of hydration. The clusters are intensely luminescent, with major lifetime components of 28, 71, and 24 ps for Au15@αCD, Au15@βCD, and Au15@γCD, respectively. The clusters also are strongly luminescent in the solid state. Both in the solution and in the solid state, the luminescence is sensitive to solvents/vapors. The clusters adhere to glass plates, and the solvent dependency of luminescence was used to create patterns that are erased upon gradual evaporation of the solvent. This self-erasing property was further demonstrated with clusters supported on a thin layer chromatography (TLC) plate. Selective detection of metal ions using the luminescence of the clusters is reported. Evaporation of the cluster solutions leads to luminescent gel-like materials

Quantum Clusters in Cavities: Trapped Au₁₅ in Cyclodextrins

We have prepared Au15 quantum clusters anchored to α-, β-, and γ-cyclodextrin (CD) cavities. The synthesis process involves the core etching of larger clusters and the simultaneous trapping of the clusters formed inside the CD cavities. The clusters were characterized by various tools, such as optical absorption and luminescence spectroscopies, electrospray ionization−mass spectrometry (ESI-MS), X-ray photoelectron spectroscopy (XPS), circular dichroism spectroscopy, and two-dimensional nuclear magnetic resonance (2D NMR) spectroscopy. Trapping of the cluster in the CD cavity was proven by circular dichroism and also by rotational Overhauser effect spectroscopy (ROESY), in terms of the distinct cross peak between proton “e” of the glutathione (−SG) ligand and the “H3” proton of CD. Dynamic light scattering (DLS) studies showed a hydrodynamic diameter of ∼3−4 nm, indicating one CD molecule per cluster with an extension of one water of hydration. The clusters are intensely luminescent, with major lifetime components of 28, 71, and 24 ps for Au15@αCD, Au15@βCD, and Au15@γCD, respectively. The clusters also are strongly luminescent in the solid state. Both in the solution and in the solid state, the luminescence is sensitive to solvents/vapors. The clusters adhere to glass plates, and the solvent dependency of luminescence was used to create patterns that are erased upon gradual evaporation of the solvent. This self-erasing property was further demonstrated with clusters supported on a thin layer chromatography (TLC) plate. Selective detection of metal ions using the luminescence of the clusters is reported. Evaporation of the cluster solutions leads to luminescent gel-like materials

One-Step Route to Luminescent Au₁₈SG₁₄ in the Condensed Phase and Its Closed Shell Molecular Ions in the Gas Phase

We report a one-step route for the synthesis of highly luminescent and water-soluble Au₁₈SG₁₄ (SG- glutathione in thiolate form) in nearly pure form using a slow reduction process. The cluster shows step-like behavior in its absorption profile. It emits red light in both aqueous and solid state under UV illumination. Quantum yield of the cluster is 0.053, nearly 25-fold higher than that of Au₂₅SG₁₈. The cluster exhibits distinct features corresponding to multiply charged ions in electrospray ionization mass spectrometry. This composition is also confirmed from MALDI MS along with other quantitative analyses. The cluster makes closed shell molecular ions in the gas phase. The possibility of making clusters of different core sizes is also demonstrated. The simplicity of this method and identification of the cluster with exact composition may facilitate the exploration of experimental and theoretical research on this material

Coalescence of Atomically Precise Clusters on Graphenic Surfaces

The interaction of ultrasmall metal clusters with surfaces of graphene is important for developing promising future applications of graphenic materials. In the experiment, chemically synthesized reduced graphene oxide (RGO) in water was mixed with Au₂₅SR₁₈ (where SR, SCH₂CH₂Ph, is a ligand protecting the cluster core) in tetrahydrofuran, and a completely new cluster, larger in mass, was formed at the liquid–liquid interface. Matrix assisted laser desorption ionization mass spectrometry of the product attached to RGO show that the peak due to Au₂₅SR₁₈ disappears gradually upon reaction and a single sharp peak referred to here as “135 ± 1 kDa cluster” appears. The composition of the new cluster is very close to the well-known magic cluster, Au₁₄₄SR₆₀ while the peak maximum is at Au₁₃₅SR₅₇. The formation of 35 ± 1 kDa cluster from the parent Au₂₅ is proposed to be governed by the trapping of smaller clusters in a deep potential well generated at the graphene surface. We theoretically model the active role of the surface in stabilizing the large clusters. Our studies indicate a general mechanism of stabilization of clusters of precise size via the competition between the interfacial fluctuations and the energy scales of interaction of the clusters with the surface. The chemical transformation occurs at deformable surfaces at reduced particle densities which is in good agreement with the theoretical model. Transformations of this kind are important in controlled tuning of particles at graphenic surfaces

Ag₉ Quantum Cluster through a Solid-State Route

A silver cluster having the composition Ag9(H2MSA)7 (H2MSA = mercaptosuccinic acid) was synthesized in macroscopic quantities using a solid-state route. The clusters were purified by PAGE and characterized by UV−vis, FTIR, luminescence, and NMR spectroscopy, TEM, XPS, XRD, TG, SEM/EDAX, elemental analysis, and ESI MS. The solid-state route provides nearly pure Ag9 clusters, and nanoparticle contamination was insignificant for routine studies. Formation of various clusters was observed by modifying the conditions. The effect of ligands on the synthesis was checked. The cluster decomposed slowly in water, and the decomposition followed first-order kinetics. However, it could be stabilized in solvent mixtures and in the solid state. Such materials may be important in cluster research because of their characteristic absorption profiles, which are similar to those of Au25 and Au38. The cluster showed luminescence with a quantum yield of 8 × 10−3 at 5 °C

Understanding the Evolution of Luminescent Gold Quantum Clusters in Protein Templates

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

Low-Temperature Thermal Dissociation of Ag Quantum Clusters in Solution and Formation of Monodisperse Ag₂S Nanoparticles

We report the effect of temperature on the stability of glutathione-protected Ag₂₅ clusters. The clusters are stable up to 50 °C. Interestingly, above this temperature, they decompose to yield Ag₂S nanoparticles with an average diameter of 3 ± 1 nm, crystallizing in monoclinic acanthite polymorph. Unlike conventional methods of syntheses of Ag₂S, where a temperature of ∼200 °C is needed, our study shows the possibility of synthesis of Ag₂S nanoparticles at much lower temperatures. This is in contrast with silver nanoparticles protected with thiolates, which typically give silver and alkyl/aryl disulfide upon thermal activation. The mechanism of cluster decomposition and formation of silver sulphide nanoparticles was investigated using various analytical techniques such as ultraviolet–visible spectroscopy, X-ray diffraction scanning electron microscopy, energy-dispersive analysis of X-rays, transmission electron microscopy, and electrospray ionization mass spectrometry. The monolayer of the cluster undergoes S–C bond cleavage, as revealed by mass spectrometry. This is somewhat unusual because Ag–S cleavage is expected in view of its lower bond energy

Metal-Ion-Induced Luminescence <i>Enhancement</i> in Protein Protected Gold Clusters

We probed the interaction between Au38@BSA and various heavy metal ions using luminescence spectroscopy. Interestingly, Au38@BSA showed luminescence enhancement upon interaction with Cd2+ and Pb2+ at concentrations higher than 1 ppm, due to the formation of cluster aggregates. Such aggregates were detected by dynamic light scattering (DLS) and high resolution electron microscopy (HRTEM) studies. Luminescence enhancement of Au38@BSA in the presence of Cd2+ was due to the interaction of Cd2+ with the cluster core, while Pb2+-induced luminescence enhancement was due to BSA-Pb2+ interaction. Observations were further supported by X-ray photoelectron spectroscopy (XPS) studies. This kind of phenomenon has been observed in protein protected clusters for the first time. We believe that such metal-ion-induced luminescence enhancement can be used to synthesize cluster systems with enhanced optical properties and different ion–cluster interactions can be used to develop metal ion sensors using Au38@BSA