Megalamane Siddaramappa Bootharaju

Copper Quantum Clusters in Protein Matrix: Potential Sensor of Pb2+ Ion

A one-pot synthesis of extremely stable, water-soluble Cu quantum clusters (QCs) capped with a model protein, bovine serum albumin (BSA), is reported. From matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry, we assign the clusters to be composed of Cu(5) and Cu(13) cores. The QCs also show luminescence properties having excitation and emission maxima at 325 and 410 nm, respectively, with a quantum yield of 0.15, which are found to be different from that of protein alone in similar experimental conditions. The quenching of luminescence of the protein-capped Cu QCs in the presence of very low hydrogen peroxide concentration (approximately nanomolar, or less than part-per-billion) reflects the efficacy of the QCs as a potential sensing material in biological environments. Moreover, as-prepared Cu QCs can detect highly toxic Pb(2+) ions in water, even at the part-per-million level, without suffering any interference from other metal ions.

A practical silver nanoparticle-based adsorbent for the removal of Hg2+ from water.

In this work, we describe the use of silver nanoparticles of 9 ± 2 and 20 ± 5 nm core diameter, protected by mercaptosuccinic acid (MSA) and supported on activated alumina for the removal of mercuric ions present in contaminated waters, at room temperature (28 ± 1 °C). These two nanoparticle samples were prepared by using two Ag:MSA ratios 1:6 and 1:3, respectively, during synthesis and were loaded on alumina at 0.5 and 0.3% by weight. The mechanism of interaction of silver nanoparticles with Hg(2+) ions was studied using various analytical techniques such as ultraviolet-visible spectroscopy (UV-vis), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), dynamic light scattering (DLS), inductively coupled plasma-optical emission spectrometry (ICP-OES), energy dispersive analysis of X-rays (EDAX), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Interactions of the metal ion with the metal core, the surface head group and the monolayer functionality were investigated. A high removal ability of 0.8 g of mercury per gram of Ag@MSA was achieved in the case of 1:6 Ag@MSA. These two materials show better uptake capacity of Hg(2+) in the pH range of 5-6. The ease of synthesis of the nanomaterial by wet chemistry, capability to load on suitable substrates to create stable materials and affordable cost will make it possible to use this approach in field applications, especially for the treatment of Hg(2+) contaminated waters.

[Ag25(SR)18]−: The “Golden” Silver Nanoparticle

Silver nanoparticles with an atomically precise molecular formula [Ag25(SR)18](-) (-SR: thiolate) are synthesized, and their single-crystal structure is determined. This synthesized nanocluster is the only silver nanoparticle that has a virtually identical analogue in gold, i.e., [Au25(SR)18](-), in terms of number of metal atoms, ligand count, superatom electronic configuration, and atomic arrangement. Furthermore, both [Ag25(SR)18](-) and its gold analogue share a number of features in their optical absorption spectra. This unprecedented molecular analogue in silver to mimic gold offers the first model nanoparticle platform to investigate the centuries-old problem of understanding the fundamental differences between silver and gold in terms of nobility, catalytic activity, and optical property.

Ag29(BDT)12(TPP)4: A Tetravalent Nanocluster

The bottom-up assembly of nanoparticles into diverse ordered solids is a challenge because it requires nanoparticles, which are often quasi-spherical, to have interaction anisotropy akin to atoms and molecules. Typically, anisotropy has been introduced by changing the shape of the inorganic nanoparticle core. Here, we present the design, self-assembly, optical properties, and total structural determination of Ag29(BDT)12(TPP)4, an atomically precise tetravalent nanocluster (NC) (BDT, 1,3-benzenedithiol; TPP, triphenylphosphine). It features four unique tetrahedrally symmetrical binding surface sites facilitated by the supramolecular assembly of 12 BDT (wide footprint bidentate thiols) in the ligand shell. When each of these sites was selectively functionalized by a single phosphine ligand, particle stability, synthetic yield, and the propensity to self-assemble into macroscopic crystals increased. The solid crystallized NCs have a substantially narrowed optical band gap compared to that of the solution state, suggesting strong interparticle electronic coupling occurs in the solid state.

Templated Atom-Precise Galvanic Synthesis and Structure Elucidation of a [Ag24Au(SR)18](-) Nanocluster.

Synthesis of atom-precise alloy nanoclusters with uniform composition is challenging when the alloying atoms are similar in size (for example, Ag and Au). A galvanic exchange strategy has been devised to produce a compositionally uniform [Ag24Au(SR)18](-) cluster (SR: thiolate) using a pure [Ag25(SR)18](-) cluster as a template. Conversely, the direct synthesis of Ag24Au cluster leads to a mixture of [Ag(25-x)Au(x)(SR)18](-), x=1-8. Mass spectrometry and crystallography of [Ag24Au(SR)18](-) reveal the presence of the Au heteroatom at the Ag25 center, forming Ag24Au. The successful exchange of the central Ag of Ag25 with Au causes perturbations in the Ag25 crystal structure, which are reflected in the absorption, luminescence, and ambient stability of the particle. These properties are compared with those of Ag25 and Ag24Pd clusters with same ligand and structural framework, providing new insights into the modulation of cluster properties with dopants at the single-atom level.

Tuning Properties in Silver Clusters

The properties of Ag nanoclusters are not as well understood as those of their more precious Au cousins. However, a recent surge in the exploration of strategies to tune the physicochemical characteristics of Ag clusters addresses this imbalance, leading to new insights into their optical, luminescence, crystal habit, metal-core, ligand-shell, and environmental properties. In this Perspective, we provide an overview of the latest strategies along with a brief introduction of the theoretical framework necessary to understand the properties of silver nanoclusters and the basis for their tuning. The advances in cluster research and the future prospects presented in this Perspective will eventually guide the next large systematic study of nanoclusters, resulting in a single collection of data similar to the periodic table of elements.

Understanding the degradation pathway of the pesticide, chlorpyrifos by noble metal nanoparticles.

Application of nanoparticles (NPs) in environmental remediation such as water purification requires a detailed understanding of the mechanistic aspects of the interaction between the species involved. Here, an attempt was made to understand the chemistry of noble metal nanoparticle-pesticide interaction, as these nanosystems are being used extensively for water purification. Our model pesticide, chlorpyrifos (CP), belonging to the organophosphorothioate group, is shown to decompose to 3,5,6-trichloro-2-pyridinol (TCP) and diethyl thiophosphate at room temperature over Ag and Au NPs, in supported and unsupported forms. The degradation products were characterized by absorption spectroscopy and electrospray ionization mass spectrometry (ESI MS). These were further confirmed by ESI tandem mass spectrometry. The interaction of CP with NP surfaces was investigated using transmission electron microscopy, energy dispersive analysis of X-rays, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). XPS reveals no change in the oxidation state of silver after the degradation of CP. It is proposed that the degradation of CP proceeds through the formation of AgNP-S surface complex, which is confirmed by Raman spectroscopy. In this complex, the P-O bond cleaves to yield a stable aromatic species, TCP. The rate of degradation of CP increases with increase of temperature and pH. Complete degradation of 10 mL of 2 ppm CP solution is achieved in 3 h using 100 mg of supported Ag@citrate NPs on neutral alumina at room temperature at a loading of ∼0.5 wt %. The effect of alumina and monolayer protection of NPs on the degradation of CP is also investigated. The rate of degradation of CP by Ag NPs is greater than that of Au NPs. The results have implications to the application of noble metal NPs for drinking water purification, as pesticide contamination is prevalent in many parts of the world. Study shows that supported Ag and Au NPs may be employed in sustainable environmental remediation, as they can be used at room temperature in aqueous solutions without the use of additional stimulus such as UV light.

Protein-directed synthesis of NIR-emitting, tunable HgS quantum dots and their applications in metal-ion sensing.

The development of luminescent mercury sulfide quantum dots (HgS QDs) through the bio-mineralization process has remained unexplored. Herein, a simple, two-step route for the synthesis of HgS quantum dots in bovine serum albumin (BSA) is reported. The QDs are characterized by UV-vis spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, luminescence, Raman spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), circular dichroism (CD), energy dispersive X-ray analysis (EDX), and picosecond-resolved optical spectroscopy. Formation of various sizes of QDs is observed by modifying the conditions suitably. The QDs also show tunable luminescence over the 680-800 nm spectral regions, with a quantum yield of 4-5%. The as-prepared QDs can serve as selective sensor materials for Hg(II) and Cu(II), based on selective luminescence quenching. The quenching mechanism is found to be based on Dexter energy transfer and photoinduced electron transfer for Hg(II) and Cu(II), respectively. The simple synthesis route of protein-capped HgS QDs would provide additional impetus to explore applications for these materials.

Gold Doping of Silver Nanoclusters: A 26‐Fold Enhancement in the Luminescence Quantum Yield

A high quantum yield (QY) of photoluminescence (PL) in nanomaterials is necessary for a wide range of applications. Unfortunately, the weak PL and moderate stability of atomically precise silver nanoclusters (NCs) suppress their utility. Herein, we accomplished a ≥26-fold PL QY enhancement of the Ag29 (BDT)12 (TPP)4 cluster (BDT: 1,3-benzenedithiol; TPP: triphenylphosphine) by doping with a discrete number of Au atoms, producing Ag29-x Aux (BDT)12 (TPP)4 , x=1-5. The Au-doped clusters exhibit an enhanced stability and an intense red emission around 660 nm. Single-crystal XRD, mass spectrometry, optical, and NMR spectroscopy shed light on the PL enhancement mechanism and the probable locations of the Au dopants within the cluster.

Investigation into the reactivity of unsupported and supported Ag7 and Ag8 clusters with toxic metal ions.

We report the chemical interactions of unsupported and alumina-supported Ag(7) and Ag(8) clusters protected with MSA (mercaptosuccinic acid) with heavy metal ions Hg(II), Cd(II), and Pb(II) in water at different concentrations. The investigation was carried out to determine the feasibility of this interesting new class of materials called quantum clusters for water purification. These systems were studied using various spectroscopic and microscopic techniques such as ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, dynamic light scattering, zeta potential measurements, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and photoluminescence spectroscopy and in detail by X-ray photoelectron spectroscopy. We observed that the metal ions interact with both the silver atoms of the clusters and the functional groups of the capping agent (MSA). The mercuric ions were reduced to metallic mercury by both supported and unsupported clusters, due to the feasibility of the redox reaction, whereas no reduction was observed for Cd(II) and Pb(II). As a result of the interaction, the luminescence of the cluster is lost which can be used to sense Hg(II). At lower concentrations, the metal ions were chemically bonded to the carboxylate groups of MSA. Absence of reduction of Hg(II) at lower concentration is due to the chemical affinity of the ligands and the lower silver content per cluster compared to the number of carboxylate groups.