Ammu Mathew

A fifteen atom silver cluster confined in bovine serum albumin

Luminescent Ag15 clusters confined in bovine serum albumin (BSA) have been prepared by a simple wet chemical route. The luminescence, exhibiting a maximum at 685 nm, is observable to the naked eye. The chemical composition of these clusters was analyzed using matrix assisted laser desorption ionization mass spectrometry (MALDI MS), X-ray photoelectron spectroscopy (XPS), and energy dispersive analysis of X-rays (EDAX). Intact Ag15@BSA is observed by MALDI MS. Multiple charge states of the cluster are observed confirming the mass assignment. The clusters showed a quantum yield of 10.71% in water and the luminescence was stable in a pH range of 1–12. Stability of the clusters was enhanced by the addition of polyvinylpyrrolidone (PVP). The clusters showed luminescence in the solid state as well. Evolution of clusters with variation in the amount of reducing agent added shows that the cluster formation goes through an intermediate state of bound silver, formed instantaneously after the addition of Ag+, which transforms to the cluster. High yield synthesis and exciting photophysical properties make our new material interesting for various applications such as biolabeling and imaging.

Selective Visual Detection of TNT at the Sub-Zeptomole Level†

Realizing the limits of sensitivity, while maintaining selectivity, is an ongoing quest. Among the multitude of requirements, national security, early detection of diseases, safety of public utilities, and radiation prevention are some of the areas in need of ultralow detection. Structural, functional, and electronic features of nanomaterials are used to develop reliable analytical methods. Several kinds of surfaceenhanced spectroscopy, surface-enhanced Raman in particular, can be used for such applications; the technique may be further enhanced by spatially separating the analyte and the active plasmonic nanostructure with an insulator, a method known as shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). Creating uniform anisotropic structures with nanoscale attributes by simple solution chemistry and combining analyte-selective chemistry on such surfaces enables ultrasensitive and selective detection methods. Noble metal quantum clusters (QCs), a new family of atomically precise nanomolecules with intense luminescence, along with their protein protected analogues, are highly sensitive and selective for specific analytes. Anchoring such QCs on mesoscale (100 nm to a few mm) particles leads to surface-enhancement of their luminescence and can create a new platform for ultrasensitive detection, especially when combined with the use of optical microscopy. Gold mesoflowers (MFs) are anisotropic materials with unique five-fold symmetric stems containing surface-enhancing nanoscale features. An entire MF is only a few micrometers in size, and its distinct shape allows for unique identification by optical microscopy; thus, changes in the properties of an MF can be used for the immediate and efficient detection of analytes. Herein, we demonstrate the selective detection of 2,4,6trinitrotoluene (TNT) at the sub-zeptomole level (10 21 moles) through a combination of these strategies on a mesostructure. Our method involves anchoring silver clusters, which are comprised of fifteen atoms and embedded in bovine serum albumin (BSA), on silica-coated Au MFs, termed Au@SiO2@Ag15 MFs, and using this system for analyte detection. Syntheses of the various components are described in the experimental section. The Au@SiO2 MFs have a tip-totip length of ca. 4 mm (Supporting Information, Figure S1a). The BSA-protected silver cluster (Ag15), is a red luminescent water-soluble QC prepared by a previously reported procedure (see Figure S2 for essential characterization data). Apart from a high quantum yield (10.7%) in water, it is stable over a wide pH range and exhibits emission in the solid state. We exposed varying concentrations of TNT to Au@SiO2@Ag15 MFs and found that even a concentration of less than one zeptomole of TNT per mesoflower quenches the luminescence of the composite mesoflowers within 1 min. The simultaneous disappearance of the luminescence of Ag15 on theMFand the appearance of the luminescence of another embedded fluorophore allows for easy identification of the analyte. Characterization data for the various composite MFs used in this study are presented in the Supporting Information. The hybrid structures, Au@SiO2@Ag15 MFs, with unique structural attributes are observable under an optical microscope (see Figure S3 for a schematic of the setup used). Dark field microscopic images of theseMFs show their well-defined features; they are star-shaped in a two dimensional projection (Figure 1A). The fluorescence image of the same MF (ca. 490 nm excitation, emitted light was passed through a triplepass filter and imaged) shows a characteristic red emission owing to the QCs anchored on its surface (Figure 1A). Unlike with other spherical single particle sensors, which are difficult to locate and distinguish by light-based microscopy, the welldefined shapes of the MFs ensure that the desired particles alone are analyzed. Furthermore, the analyte adsorption capacity of theMFs is enhanced by the thin inert layer of silica employed as a base. Au core/silica shell structures of this type can provide enhanced fluorescence and Raman scattering. The better stability of the QCs on the silica layer, along with a reduction in the luminescence quenching of the QCs on the MF surface and ease of functionalization are among the added advantages of this material (see the Supporting Information). Exposure of the Au@SiO2@Ag15 MFs to TNT (2.5 mL) at a concentration of one part per trillion (ppt) decreases the luminescence intensity slightly without affecting the optical image (Figure 1B), whereas at one part per billion (ppb) of TNT the luminescence feature disappears completely (Figure 1C; note that the MFs shown in Figure 1A–C are different in each case). For spectral intensity data collected from the surface of these MFs, see the Supporting Information, Figure S4. The quenching of cluster luminescence is due to the formation of a Meisenheimer complex by the [*] A. Mathew, Dr. P. R. Sajanlal, Prof. T. Pradeep DST Unit of Nanoscience (DST UNS), Department of Chemistry, Indian Institute of Technology Madras Chennai 600036 (India) E-mail: pradeep@iitm.ac.in [] Current address: Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology Atlanta, GA 30332-0400 (USA)

Supramolecular Functionalization and Concomitant Enhancement in Properties of Au25 Clusters

We present a versatile approach for tuning the surface functionality of an atomically precise 25 atom gold cluster using specific host-guest interactions between β-cyclodextrin (CD) and the ligand anchored on the cluster. The supramolecular interaction between the Au25 cluster protected by 4-(t-butyl)benzyl mercaptan, labeled Au25SBB18, and CD yielding Au25SBB18∩CDn (n = 1, 2, 3, and 4) has been probed experimentally using various spectroscopic techniques and was further analyzed by density functional theory calculations and molecular modeling. The viability of our method in modifying the properties of differently functionalized Au25 clusters is demonstrated. Besides modifying their optoelectronic properties, the CD moieties present on the cluster surface provide enhanced stability and optical responses which are crucial in view of the potential applications of these systems. Here, the CD molecules act as an umbrella which protects the fragile cluster core from the direct interaction with many destabilizing agents such as metal ions, ligands, and so on. Apart from the inherent biocompatibility of the CD-protected Au clusters, additional capabilities acquired by the supramolecular functionalization make such modified clusters preferred materials for applications, including those in biology.

Nanoscience in India: a perspective

India has emerged as a leading player in the field of nanoscience and nanotechnology over the last decade. The Indian nano-endeavor got its initial push through the Nano Science and Nano Technology (NS&NT) initiative (now the Nano Mission) of the Department of Science and Technology (DST), Government of India in 2002 and has accelerated very fast since then. This article is intended to sketch a brief picture of the recent nanoscience and technology activities in India with special emphasis on synthesis of nanomaterials and emergence of new properties in them. Application of nanomateials into the very basic needs of India like water purification and energy creation along with the recent developments at the bio-nano interface will be discussed. State of nanoscience education at educational institutions in India and nanoscience based industrial initiatives will be touched upon.