Pradeep Thalappil

The performance of semiconductor devices can be fine-tuned through chemical transformation of their nanostructured components. Such transformations are often carried out in controlled conditions. Herein, the use of electrospray deposition of metal ions from solutions in air is reported, to bring about chemical transformations across mm2-sized areas of nanostructures. This is illustrated with monolayer assemblies of ultrathin tellurium nanowires (NWs). The process does not require any reducing agent and can transform the NWs chemically, in the solid state itself, under ambient conditions. By using suitable masks, the beam of ions can be patterned to localize such transformations with nanometer precision to obtain aligned multiphasic NWs, containing atomically precise phase boundaries. By controlling the time of exposure of the spray, the scope of the process is further expanded to produce tellurium-metal telluride core–shell NWs. The method described here represents a crucial step for ambient processing of nanostructured components, useful for applications such as semiconductor device fabrication.

A blue luminescent 11-atom platinum cluster showing step-like optical features and the absence of plasmon absorption was synthesized. The cluster was purified using high performance liquid chromatography (HPLC). Electrospray ionization (ESI) and matrix assisted laser desorption ionization (MALDI) mass spectrometry (MS) suggest a composition, Pt11(BBS)8, which was confirmed by a range of other experimental tools. The cluster is highly stable and compatible with many organic solvents. This journal is © the Partner Organisations 2014.

Arsenic (As) toxicity is a significant threat to global public health. Moreover, the lacks of social awareness and understanding of the impact of As in affected communities are also of concern. Therefore, subppm level detection of As in environmental waters and associated public awareness are crucial for remediation programs. We developed a sustainable As sensing methodology by merging the fundamental concepts of As cytotoxicity with an alternative approach for selectivity. A cellular platform was prepared on an electrospun scaffold using As-sensitive keratinocyte cells. Arsenic-induced reactive oxygen species (ROS) were quantified using a fluorimetric probe, 2′,7′-dichlorofluorescin diacetate, commonly used to detect oxidative stress within cells. Experiments were conducted with a mixture of arsenite and arsenate, the predominant forms of As present in natural conditions, in a 1:1 ratio. We also quantified unknown As concentrations in real water samples. The selectivity to As was achieved by exposing the contaminated water composed of several ions to an As adsorbing material, namely, confined metastable 2-line ferrihydrite (CM2LF). An adsorption-desorption protocol enabled As extraction in field conditions. The ROS resulting from cells' responses to the As extract were used as the signature of As concentration. The sensor could precisely quantify even 5 ppb of As in tap water, and the theoretical limit of detection (LOD) was 2.7 ppb. A sustainable device using the cellular platform is proposed for As detection in field conditions that can also be used for social awareness, demonstrating the impact of As on human biology in affected regions.

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.

Noble metal nanoclusters protected with carboranes, a 12-vertex, nearly icosahedral boron-carbon framework system, have received immense attention due to their different physicochemical properties. We have synthesized ortho-carborane-1,2-dithiol (CBDT) and triphenylphosphine (TPP) coprotected [Ag42(CBDT)15(TPP)4]2- (shortly Ag42) using a ligand-exchange induced structural transformation reaction starting from [Ag18H16(TPP)10]2+ (shortly Ag18). The formation of Ag42 was confirmed using UV-vis absorption spectroscopy, mass spectrometry, transmission electron microscopy, X-ray photoelectron spectroscopy, infrared spectroscopy, and multinuclear magnetic resonance spectroscopy. Multiple UV-vis optical absorption features, which exhibit characteristic patterns, confirmed its molecular nature. Ag42 is the highest nuclearity silver nanocluster protected with carboranes reported so far. Although these clusters are thermally stable up to 200 °C in their solid state, light-irradiation of its solutions in dichloromethane results in its structural conversion to [Ag14(CBDT)6(TPP)6] (shortly Ag14). Single crystal X-ray diffraction of Ag14 exhibits Ag8-Ag6 core-shell structure of this nanocluster. Other spectroscopic and microscopic studies also confirm the formation of Ag14. Time-dependent mass spectrometry revealed that this light-activated intercluster conversion went through two sets of intermediate clusters. The first set of intermediates, [Ag37(CBDT)12(TPP)4]3- and [Ag35(CBDT)8(TPP)4]2- were formed after 8 h of light irradiation, and the second set comprised of [Ag30(CBDT)8(TPP)4]2-, [Ag26(CBDT)11(TPP)4]2-, and [Ag26(CBDT)7(TPP)7]2- were formed after 16 h of irradiation. After 24 h, the conversion to Ag14 was complete. Density functional theory calculations reveal that the kernel-centered excited state molecular orbitals of Ag42 are responsible for light-activated transformation. Interestingly, Ag42 showed near-infrared emission at 980 nm (1.26 eV) with a lifetime of >1.5 μs, indicating phosphorescence, while Ag14 shows red luminescence at 626 nm (1.98 eV) with a lifetime of 550 ps, indicating fluorescence. Femtosecond and nanosecond transient absorption showed the transitions between their electronic energy levels and associated carrier dynamics. Formation of the stable excited states of Ag42 is shown to be responsible for the core transformation.

New metal-carbon clusters, M4C9+ (M =Ti, V), generated using a combined thermal arc discharge evaporation set-up, have been studied with quadrupole mass spectrometry. Reactivities of these clusters have been investigated by means of association reactions with H2O. Metal-carbon clusters of other compositions have also been studied. We speculate on the mechanism of formation of larger metal-carbon clusters.

Imprinted charged aqueous droplets of micrometer dimensions containing spherical gold and silver nanoparticles, gold nanorods, proteins and simple molecules were visualized using dark-field and transmission electron microscopies. With such studies, we hoped to understand the unusual chemistry exhibited by microdroplets. These droplets with sizes in the range of 1-100 μm were formed using a home-built electrospray source with nitrogen as the nebulization gas. Several remarkable features such as mass/size-selective segregation and spatial localization of solutes in nanometer-thin regions of microdroplets were visualized, along with the formation of micro-nano vacuoles. Electrospray parameters such as distance between the spray tip and surface, voltage and nebulization gas pressure influenced particle distribution within the droplets. We relate these features to unusual phenomena such as the enhancement of rates of chemical reactions in microdroplets.

ConspectusSupramolecular 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 Au 25 (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 [Au 25 (SBB) 18 ] - NCs with cyclodextrins (CD) to form Au 25 SBB 18 ∩CD n (n = 1-4) and that of [Ag 29 (BDT) 12 ] 3- with fullerenes to form [Ag 29 (BDT) 12 (C 60 ) 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 [Ag 29 (BDT) 12 (TPP) 4 ] 3- (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 Ag 44 (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 Au 102 (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 Ag 44 (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. ©

Intensive farming, rapid industrialization, and increasingly sophisticated lifestyles have added artificial chemicals into many water bodies. Although pesticide residues in groundwater were unexpected years ago as soil was thought to act as a filter, it is an established fact that water derived from groundwater sources is contaminated with them in many parts of the world. Even though these levels are significant vis-à-vis the permissible limits, the concentrations are low in comparison to those of other commonly encountered chemicals, and purification technologies have to be efficient for them to be removed at affordable cost. In addition, the process kinetics has to be reasonably fast so that the amount of adsorbent required is minimal. For such a solution to be useful for all strata of society, the solution should be economically attractive, requiring zero electricity and minimum maintenance. It is imperative to understand that any novel technology should solve drinking water contamination problems in their entirety and not result in toxic by-products or residuals. These offer numerous challenges to chemistry and engineering, some of which are discussed in this chapter with selected examples. © 2009 William Andrew Inc. Published by Elsevier Inc. All rights reserved.