Luminescent semiconductor quantum dots (QDs) are nanometer-sized particles with size-dependent optical and electronic properties that have been under intensive research and development for a broad range of applications, such as energy-efficient displays and lighting, photovoltaic devices, and biological markers. [1][2][3][4][5][6] The estimated market size for QD-based products is $721 million by 2013. 2 However, the intrinsic toxicity and potential environmental hazards associated with many of these materials represent considerable challenges to their practical applications. The emerging light-emitting, quantum-sized carbon dots (CDs) 7 appear to be a promising alternative to semiconductor QDs in many of applications because of their advantages in low toxicity and cheaper cost. 6 Following the laser ablation method, 7 various methods have been established to produce quantum-sized carbon nanoparticles, 8-12 like electrochemical release or exfoliation from a graphitic source, separation of combusted carbon soot, 13,14 carbonizing polymerized resols on silica spheres, 15 thermal oxidation of suitable molecular precursors, 16-18 and dehydration of carbohydrates using concentrated sulfuric acid. 19 However, most of these synthesis methods need several steps. 7-14 In addition, the photoluminescent quantum yields (QY) of these resultant carbon dots are very low, 7-14,16-18 usually below 6%. The highest quantum yield 6 reported until now reached 20%, where the laser ablated carbon dots were conjugated with PEG 1500N in neat SOCl 2 . In view of the significant potential of this zero-dimension carbon nanomaterial in various fields, a facile and scalable synthetic approach for luminescent carbon dots is highly desired.Here, we report a novel strategy to synthesize highly luminescent oil-soluble carbon dots (OCDs) by carbonizing carbon precursors, e.g., citric acid, in hot noncoordinating solvent. This one-step method is inspired by the synthesis of various monodispersed and size-controlled semiconductor and magnetic nanocrystals. 20-22 The obtained OCDs possess a maximum QY at room temperature up to 53% (excited at 360 nm), which is the best result reported so far. Furthermore, as a natural extension of this facile method, we also synthesized water-soluble CDs (WCDs) via changing the reaction solvent and capping agent. The QY of the WCDs is lower than the OCDs, yet it still can reach 17% (excited at 360 nm). In this communication, we focus on the synthesis of OCDs, whereas the details of WCDs are presented in the Supporting Information.The OCDs was prepared using octadecene (ODE) as the noncoordinating solvent, 1-hexadecylamine (HDA) as the surface passivation agent, and citric acid anhydrous as the carbon precursor. Typically, a mixture of 15 mL ODE and 1.5 g of HDA loaded in a three-neck flask was heated to 300°C under argon flow, and then 1 g of citric acid was quickly injected into the reaction flask. Aliquots were taken at different time intervals for the optical and Fromer, N. A.; Geier, M. L.; Alivisatos, A. P. Scie...

Noble-metal nanoparticles exhibit unique plasmon resonances compared to bulk metal that depend on the nanoparticle size, [1] shape, [2,3] and local dielectric environment. [4] At resonance they may be regarded as antennas because they are able to increase the electrical field of an incident plane wave by orders of magnitude in a small volume. This leads, at the same time, to an increase of the emission rate of a radiating molecular dipole that is placed in the volume of enhanced coupling. [5] This antenna property is the basis for applications of subwavelength metal structures in surface-enhanced Raman spectroscopy (SERS), [6] plasmon-enhanced fluorescence spectroscopy, [7] chemical and biological sensing, [8,9] and nearfield microscopy. [10,11] Furthermore, the plasmon resonances of complex metal structures may exhibit far-field responses that cannot be realized by classical molecular resonators. For example, the formation of strong magnetic dipoles in irregularly shaped metal nanoparticles may lead to an especially intriguing application as a material with a negative refractive index [12,13] and highly unusual optical properties. An experimental signature for such an effect has been demonstrated recently at infrared wavelengths (k = 3 lm) for split-ring structures.[14]An essential component for all of these applications is the ability to tailor the particle plasmon resonances according to the desired application. A strong dependence of the strength and wavelength of the plasmon resonances on the geometrical shape of a nanometer-sized metal object is well established for spheroids.[15] More recent calculations [16,17] proved that for increasingly complex structures several distinct and strong resonances may exist and the resonance wavelengths can be tuned by varying the nanoparticle geometry. The same calculations have shown that at resonance the field enhancements may be highly localized and dramatically enhanced at features with small dimensions, such as thin gaps [17] or at the corners [16,18] of a nanoparticle structure. These results imply that for maximized enhancement effects in nano-optical experi-COMMUNICATIONS

Abstract.The geometry of crescent-shaped noble-metal nanoparticles is systematically varied in terms of shape and size. The resulting changes in the plasmonic resonances of these structures are investigated by extinction spectroscopy revealing a rich polarization-dependent response in the nearinfrared region of the electromagnetic spectrum. A first approach towards the understanding of this behaviour, in analogy to previous models on confined modes in nanometric metal slabs, is presented and discussed. Variations in several geometrical parameters lead to changes in the optical response that can be understood within this model. Qualitative changes in the response are seen at the transition of the structures from an open 'crescent' to a fully connected ring, pointing to a high field localization between the two tips of the structure.

Surface modification of citrate-reduced gold nanoparticles with 2-mercaptosuccinic acid (MSA) was carried out in the aqueous phase. This provides a way to obtain carboxylic acid functionalized gold nanoparticles with diameter above 10 nm. The influence of the protecting MSA layer on the behavior of the modified gold nanoparticles in comparison with that of the traditional citrate-reduced gold colloid was evaluated by HCl titration, cyanide etching, and seeded growth tests. The modified gold nanoparticles show an improved stability against pH changes and cyanide etching. They do show further growth, which appears to be more homogeneous than for the unmodified particles.

We demonstrate that fluorescence of single molecules in the nanometric vicinity of a thin gold film can be effectively excited and detected through the film with an epi-illumination scanning confocal microscope. A full theoretical treatment of the fluorescence signal indicates that both excitation and emission are surface-plasmon mediated. Remarkably, the number of photons detectable from chromophores perpendicular to the interface is enhanced by the presence of the metal.

Microcontact printing biomolecules from elastomeric micropatterned stamps onto surfaces is a versatile method to prepare surfaces for diagnostic applications. We show how to create patterns of proteins having a lengthscale lower than 100 nm using high-resolution microcontact printing. The elastomeric stamps used have meshes composed of 100-and 40-nm-wide lines, arrays of 100 × 400 nm 2 features, and arrays of 100-nm-wide posts. The spherical geometry of the posts on the stamps contributes to reduce the printed areas below the effective size of the molded features. Proteins adsorb onto the hydrophobic surface of the stamp during the inking step, and by varying the concentration of the protein solutions, it is possible to adsorb a single or a few protein molecules, such as antibodies (fluorescently labeled) or green fluorescence proteins, on each of the elements forming the high-resolution pattern of the stamp. The transfer of the proteins from the stamp to a hydrophilic glass surface occurs during the printing step. Characterization of the printed patterns using atomic force microscopy and fluorescence confocal microscopy reveals sites unoccupied or occupied by one or more protein molecules that are located within 50 nm of the expected printed locations. The placement of a small number of protein molecules on a surface at precise locations is the key to localizing and identifying single proteins and might constitute a method of choice to study single protein molecules on surfaces.

We apply colloidal lithography to construct stacked nanocrescent dimer structures with an exact vertical alignment and a separation distance of approximately 10 nm. Highly ordered, large arrays of these nanostructures are accessible using nonclose-packed colloidal monolayers as masks. Spatially separated nanocrescent dimers are obtained by application of spatially distributed colloids. The polarization dependent optical properties of the nanostructures are investigated in detail and compared to single crescents. The close proximity of the nanocrescents leads to a coupling process that gives rise to new optical resonances which can be described as linear superpositions of the individual crescents' plasmonic modes. We apply a plasmon hybridization model to explain the spectral differences of all polarization dependent resonances and use geometric arguments to explain the respective shifts of the resonances. Theoretical calculations are performed to support the hybridization model and extend it to higher order resonances not resolved experimentally.

A facile chemical method has been developed to synthesise highly efficient functionalized carbon dots; when illuminated with 407 nm light, both the solution and film emitted white-light directly.