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: [email protected] [] Current address: Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology Atlanta, GA 30332-0400 (USA)