The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

Core/Shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Characterization, and Applications

12.2.2. Four-Wave Mixing (FWM) 4849 12.2.3. Dye Aggregation 4850 12.2.4. Optoelectronic Nanodevices 4850 12.3. Sensor 4851 12.3.1. Chemical Sensor 4851 12.3.2. Biological Sensor 4851 12.4. Catalysis 4852 13. Conclusion and Perspectives 4852 14. Abbreviations 4853 15. Acknowledgements 4854 16. References 4854 * Corresponding author E-mail: tpal@chem.iitkgp.ernet.in. † Raidighi College. § Indian Institute of Technology. 4797 Chem. Rev. 2007, 107, 4797−4862

Interparticle Coupling Effect on the Surface Plasmon Resonance of Gold Nanoparticles: From Theory to Applications

Chemical sensors respond to the presence of a specific analyte in a variety of ways. One of the most convenient is a change in optical properties, and in particular a visually perceptible colour change. Here we report the preparation of a material that changes colour in response to a chemical signal by means of a change in diffraction (rather than absorption) properties. Our material is a crystalline colloidal array of polymer spheres (roughly 100 nm diameter) polymerized within a hydrogel that swells and shrinks reversibly in the presence of certain analytes (here metal ions and glucose). The crystalline colloidal array diffracts light at (visible) wavelengths determined by the lattice spacing, which gives rise to an intense colour. The hydrogel contains either a molecular-recognition group that binds the analyte selectively (crown ethers for metal ions), or a molecular-recognition agent that reacts with the analyte selectively. These recognition events cause the gel to swell owing to an increased osmotic pressure, which increases the mean separation between the colloidal spheres and so shifts the Bragg peak of the diffracted light to longer wavelengths. We anticipate that this strategy can be used to prepare 'intelligent' materials responsive to a wide range of analytes, including viruses.

Polymerized colloidal crystal hydrogel films as intelligent chemical sensing materials

Gold colloids have been homogeneously coated with silica using the silane coupling agent (3-aminopropyl)trimethoxysilane as a primer to render the gold surface vitreophilic. After the formation of a thin silica layer in aqueous solution, the particles can be transferred into ethanol for further growth using the Stober method. The thickness of the silica layer can be completely controlled, and (after surface modification) the particles can be transferred into practically any solvent. Varying the silica shell thickness and the refractive index of the solvent allows control over the optical properties of the dispersions. The optical spectra of the coated particles are in good agreement with calculations using Mie's theory for core−shell particles.

http://nathan.instras.com/documentDB/paper-29.pdf

Synthesis of Nanosized Gold−Silica Core−Shell Particles

We report the development of a method for creating new submicron periodic materials. These materials will have numerous applications in various areas of technology. This periodic material is composed of a body-centered cubic (BCC), face-centered cubic (FCC), or random hexagonal stacked array of spherical particles in which the periodicity is locked into a hydrogel polyacrylamide network.’ The lattice constant of this array can be varied between

Self-Assembly Motif for Creating Submicron Periodic Materials. Polymerized Crystalline Colloidal Arrays

Preparation and Properties of Tailored Morphology, Monodisperse Colloidal Silica-Cadmium Sulfide Nanocomposites

The first one-pot enantioselective oxidative coupling of cyclic benzylic ethers with aldehydes has been developed. A variety of benzylic ethers were transformed into the corresponding oxygen heterocycles with high enantioselectivity. Mechanistic experiments were conducted to determine the nature of the reaction intermediates. The application of this strategy to coupling reactions with other nucleophiles besides aldehydes was also explored.

Catalytic enantioselective oxidative cross-coupling of benzylic ethers with aldehydes.

Highly enantioselective catalytic cross-dehydrogenative coupling of N-carbamoyl tetrahydroisoquinolines and terminal alkynes.

Selectively activating chemical bonds that are generally considered to be inert is an attractive strategy for introducing functionality into and enhancing the structural complexity of easily-prepared substrates, particularly when bond activation ultimately leads to carbon–carbon bond formation. We have reported several examples in which oxidative carbon– carbon bond activation can be used to initiate cyclization reactions through carbon–carbon bond formation. Reaction initiation through single-electron oxidation alleviates chemoselectivity problems that can arise from conventional Lewis acid initiated methods for electrophile formation. To facilitate substrate synthesis and improve reaction atom economy we have initiated a program that is directed toward promoting oxidative electrophile formation by carbon–hydrogen bond activation. Toward this objective we initially chose to exploit the propensity of 2,3-dichloro-5,6dicyano-1,4-benzoquinone (DDQ) to form aryl-substituted oxocarbenium ions from benzylic ethers. This process has been utilized for bimolecular carbon–carbon bond formation, but these reactions proceed efficiently only with electron-rich arenes, and either require high temperatures with ketone nucleophiles or dicarbonyl/Lewis acid mixtures, or the addition of pregenerated nucleophiles such as enolsilanes after cation formation. Successful and broad application of DDQ-mediated carbon–hydrogen bond activation and subsequent carbon– carbon bond formation to annulation reactions requires that the nucleophiles be stable toward DDQ, that the reaction products not be subject to additional oxidation, and that a wide range of ethers serve as substrates. Herein we report that DDQ promotes the formation of stabilized carbocations by benzylic carbon–hydrogen bond activation under ambient conditions in the presence of appended nucleophilic groups and leads to diastereoselective carbon–carbon bond formation. Particularly important is the observation that, relative to bimolecular addition reactions, appending nucleophilic groups to the ether enhances the range of the benzylic groups that can serve as cation precursors. We also demonstrate that the scope of the process can be dramatically expanded by applying the protocol in an efficient approach to ring formation through allylic carbon–hydrogen bond activation. The incorporation of oxygen-containing groups into the substrates and the impact of arene or alkene substitution on the reaction rate is also discussed. Our initial studies focused on the conversion of paramethoxybenzyl (PMB) ether 1 into tetrahydropyrone 2 (Scheme 1) by DDQ-mediated oxocarbenium ion formation.

Lei Liu

Papers

Self-Assembly Motif for Creating Submicron Periodic Materials. Polymerized Crystalline Colloidal Arrays

Preparation and Properties of Tailored Morphology, Monodisperse Colloidal Silica-Cadmium Sulfide Nanocomposites

Catalytic enantioselective oxidative cross-coupling of benzylic ethers with aldehydes.

Highly enantioselective catalytic cross-dehydrogenative coupling of N-carbamoyl tetrahydroisoquinolines and terminal alkynes.

Diastereoselective Tetrahydropyrone Synthesis through Transition‐Metal‐Free Oxidative Carbon–Hydrogen Bond Activation