Functionalized luminescent nanocrystals on patterned surfaces obtained by radio frequency glow discharges.

In this work a genuine combination of a bottom-up approach, which is based on synthesis and functionalization of emitting nanocrystals (NCs), with a top-down strategy, which relies on a flexible and versatile cold plasma process, is shown. Luminescent semiconducting colloidal NCs consisting of a CdSe core coated with a ZnS shell (CdSe@ZnS) are directly assembled onto micro-patterned substrates previously functionalized by means of glow discharges performed through physical masks. The NC assembly is driven by electrostatic interactions that led to their successful organization into spatially resolved domains. Two distinct protocols are tested, the former using a plasma deposition process combined with an electrostatic layer-by-layer procedure, the latter based on a two-step plasma deposition/treatment process. The procedures are thoroughly monitored with fluorescence microscopy, atomic force microscopy, x-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, transmission electron microscopy and scanning electron microscopy. The two-step plasma protocol is demonstrated to be more efficient in directing a uniform and specific assembly of luminescent NCs with respect to the hybrid procedure. The presented 'mix and match' approach offers great potential for integrating NCs, with their unique size-dependent properties, into microstructures, providing a universal platform for the fabrication of sensors, biochips, displays and switches.

Nanocomposites based on highly luminescent nanocrystals and semiconducting conjugated polymer for inkjet printing.

In this work nanocomposites based on organic-capped semiconductor nanocrystals formed of a core of CdSe coated with a shell of ZnS (CdSe@ZnS), with different sizes, and a semiconducting conjugated polymer, namely poly[(9,9-dihexylfluoren-2,7-diyl)-alt- (2,5-dimethyl-1,4-phenylene)] (PF-DMB) have been investigated. The nanocomposites are prepared by mixing the pre-synthesized components in organic solvents, thereby assisting the dispersion of the organic-coated nano-objects in the polymer host. UV-vis steady state and time-resolved spectroscopy along with (photo)electrochemical techniques have been performed to characterize the obtained materials. The study shows that the embedded nanocrystals increase the PF-DMB stability against oxidation and, at the same time, extend the light harvesting capability to the visible spectral region, thus resulting in detectable photocurrent signals. The nanocomposites have been dispensed by means of a piezo-actuated inkjet system. Such inks present viscosity and surface tension properties well suited for stable and reliable drop-on-demand printing using an inkjet printer. The fabrication of arrays of single-color pixels made of the nanocomposites and micrometers in size has been performed. Confocal and atomic force microscopy have confirmed that inkjet-printed microstructures present the intrinsic emission properties of both the embedded nanocrystals and PF-DMB, resulting in a combined luminescence. Finally, the morphology of the printed pixels is influenced by the embedded nanofillers.

Emerging methods for fabricating functional structures by patterning and assembling engineered nanocrystals.

Inorganic nanocrystals and nanoparticles have aroused increasing attention in the last years due to their original optoelectronic, thermodynamic, mechanical and catalytic properties, which are extremely attractive for fundamental understanding as well as for their huge potential in applications. The ability to strongly exploit the original potential of such nano-objects and access their properties relies on the ability to bridge the gap between the nanoscopic and mesoscopic scale. Indeed, to integrate nanoparticles in structures, materials and finally devices, their incorporation in processable systems, and their organization in morphologically controlled assembly and/or ordered arrays is crucial. The fabrication of 2/3 D patterned micro- and nanostructure is a promising strategy for integrating the nanoparticles in macroscopic entities in order to properly exploit their unprecedented functionality for biomedical, electronic, catalytic materials and devices. In this paper, different and complementary strategies able to engineer inorganic colloidal nanocrystals due to their organization in original functional materials and structures will be described.

Interplay between amplified spontaneous emission, Forster resonant energy transfer, and self-absorption in hybrid poly(9,9-dioctylfluorene)-CdSe/ZnS nanocrystal thin films.

We investigated the excitation density dependence of the photoluminescence spectra of hybrid poly(9,9-dioctylfluorene)-CdSe/ZnS nanocrystals (PF8-NCs) thin films. We demonstrate that this experiment allows the determination of the efficiency of all the CdSe/ZnS NCs excitation processes and that the presence of amplified spontaneous emission (ASE) from the PF8 leads to a strong dependence of the NC excitation processes from the laser excitation density. Below the PF8 ASE threshold only about 6% of the excitons in the NCs are due to pump laser absorption, while about 94% of the NC excitation is due to the interaction with the PF8, and it is due for about 58% to PF8-->NC Forster resonant energy transfer (FRET) and for about 37% to reabsorption by the NCs of the PF8 luminescence. The presence of PF8 ASE significantly modifies this scenario by strongly decreasing the FRET importance and strongly increasing the reabsorption one. The interplay between reduced FRET and increased reabsorption overall decreases the NC excitation due to PF8 indicating that ASE from the donors should be avoided if efficient NCs excitation under strong pumping is wished.

Hybrid nanocomposites based on luminescent colloidal nanocrystals in poly(methyl methacrylate): spectroscopical and morphological studies.

We report on preparation process and optical characterization of a nanocomposite material obtained dispersing colloidal semiconductor nanocrystals (NCs), namely CdS and CdSe@ZnS core-shell system in poly(methyl methacrylate) (PMMA). Such method allows a large flexibility on nanocrystal materials and on the choice of the polymer characteristics. Nanocomposite thin films were extensively investigated by means optical and morphological techniques. The effects on NC composition, concentration, size, and surface chemistry on the spectroscopical and structural behaviour of the nanocomposite properties were studied. The NC size dependent optical properties of the nanocomposites are mainly accounted by the NC composition and size, while the morphology of the films is explained on the base of the NC surface characteristics and their concentration in the nanocomposites.

Modification of spontaneous emission of (CdSe)ZnS nanocrystals embedded in nanoimprinted photonic crystals.

Highly luminescent (CdSe)ZnS nanocrystals, with band edge emission in the red region of the visible spectrum, were successfully synthesized and incorporated in a resist, namely mr-NIL 6000. The nanocomposite material was imprinted by using conventional nanoimprint lithography (NIL) process. We report on the fabrication and characterization of nanoimprinted photonic crystals in this new functional material. Experiments showed good imprint properties of the NC/polymer based material and that the surface nanostructuration improves the light extraction efficiency by over 2 compared to a nanoimprinted unpatterned surface.

Lipid-based systems loaded with PbS nanocrystals: near infrared emitting trackable nanovectors.

Hydrophobic PbS nanocrystals (NCs) emitting in the near infrared spectral region were encapsulated in the core of micelles and in the bilayer of liposomes, respectively, to form polyethylene glycol (PEG)-grafted phospholipids. The phospholipid-based functionalization process of PbS NCs required the replacement of the pristine capping ligand at the NC surface with thiol molecules. The procedures carried out for two systems, micelles and liposomes, using PEG-modified phospholipids were carefully monitored by optical, morphological and structural investigations. The hydrodynamic diameter and the colloidal stability of both micelles and liposomes loaded with PbS NCs were evaluated using Dynamic Light Scattering (DLS) and ζ-potential experiments, and both were satisfactorily stable in physiological media. The cytotoxicity of the resulting PbS NC-loaded nanovectors was assessed by the in vitro investigation on Saos-2 cells, indicating that the toxicity of the PbS NC loaded liposomes was lower than that of the micelles with the same NC cargo, which is reasonable due to the different overall composition of the two prepared nanocarriers. Finally, the cellular uptake in the Saos-2 cells of both the NC containing systems was evaluated by means of confocal microscopy studies by exploiting a visible fluorescent phospholipid and demonstrating the ability of both luminescent nanovectors to be internalized. The obtained results show the great potential of the prepared emitting nanoprobes for imaging applications in the second biological window.

NIR Emitting Nanoprobes Based on Cyclic RGD Motif Conjugated PbS Quantum Dots for Integrin-Targeted Optical Bioimaging.

Here, silica-coated PbS quantum dots (QDs) with photoluminescence emission properties in the near-infrared (NIR) region are proposed as potential effective single particle optical nanoprobes for future in vivo imaging of tumors. The dispersibility in aqueous medium of hydrophobic PbS QDs was accomplished by growing a silica shell on their surface by exploiting a base assisted water-in-oil microemulsion method. The silica-coated PbS QDs were then conjugated with a specifically designed cyclic arginine-glycine-aspartic acid (cRGD) peptide that is able to specifically recognize αvß3 integrins, which are overexpressed in angiogenic tumor-induced vasculatures and on some solid tumors, to achieve tumor-specific targeting. The cRGD peptide PbS silica-coated QDs were systematically characterized, at each step of their preparation, by means of complementary optical and structural techniques, demonstrating appropriate colloidal stability and the maintenance of their optical futures in aqueous solutions. The cellular uptake of cRGD peptide functionalized luminescent nanostructures in human melanoma cells, where overexpression of αvß3 was observed, was assessed by means of confocal microscopy analysis and cytometric study. The selectivity of the cRGD peptide PbS silica-coated QDs for the αvß3 integrin was established, consequently highlighting the significant potential of the developed NIR emitting nanostructures as optically traceable nanoprobes for future αvß3 integrin receptor in vivo targeting in the NIR region.

Highly selective luminescent nanostructures for mitochondrial imaging and targeting.

Here a luminescent hybrid nanostructure based on functionalized quantum dots (QDs) is used as a fluorescent imaging agent able to target selectively mitochondria thanks to the molecular recognition of the translocator protein (TSPO). The selective targeting of such an 18 kDa protein mainly located in the outer mitochondrial membrane and overexpressed in several pathological states including neurodegenerative diseases and cancers may provide valuable information for the early diagnosis and therapy of human disorders. In particular, the rational design of amino functionalized luminescent silica coated QD nanoparticles (QD@SiO2 NPs) provides a versatile nanoplatform to anchor a potent and selective TSPO ligand, characterized by a 2-phenyl-imidazo[1,2-a]pyridine acetamide structure along with a derivatizable carboxylic end group, useful to conjugate the TSPO ligand and achieve TSPO-QD@SiO2 NPs by means of a covalent amide bond. The colloidal stability and optical properties of the proposed nanomaterials are comprehensively investigated and their potential as mitochondrial imaging agents is fully assessed. Sub-cellular fractionation, together with confocal laser scanning fluorescence microscopy and co-localization analysis of targeted TSPO-QD@SiO2 NPs in C6 glioma cells overexpressing the TSPO, proves the great potential of these multifunctional nanosystems as in vitro selective mitochondrial imaging agents.

Photoelectrochemical study on photosynthetic pigments-sensitized nanocrystalline ZnO films.

Hetero-structures formed by quantum-sized ZnO nanocrystals and photosynthetic pigments were prepared by adsorbing either chlorophyll a, carotenoids or their mixture onto a film of organic-capped ZnO nanoparticles. Photoelectrochemical measurements were comparatively performed on both bulk and nanocrystalline ZnO films after dye-covering in order to probe the photosensitization process occurring at the hetero-junction. The photoconversion process was found to be greatly enhanced at the nanocrystalline electrodes upon sensitization with a dye mixture. The sensitization process is discussed on the basis of the aggregation state of chlorophyll a, and of the specific photoprotective action played by carotenoids.

Biotin-decorated silica coated PbS nanocrystals emitting in the second biological near infrared window for bioimaging.

Nanoparticles (NPs) emitting in the second biological near infrared (NIR) window of the electromagnetic spectrum have been successfully synthesized by growing a silica shell on the hydrophobic surface of OLEA/TOP PbS nanocrystals (NCs), by means of a reverse microemulsion approach, and subsequently decorated with biotin molecules. The fabrication of very uniform and monodisperse NPs, formed of SiO2 shell coated single core PbS NCs, has been demonstrated by means of a set of complementary optical and structural techniques (Vis-NIR absorption and photoluminescence spectroscopy, transmission electron microscopy) that have highlighted how experimental parameters, such as PbS NC and silica precursor concentration, are crucial to direct the morphology and optical properties of silica coated PbS NPs. Subsequently, the silica surface of the core-shell NPs has been grafted with amino groups, in order to achieve covalent binding of biotin to NIR emitting silica coated NPs. Finally the successful reaction with a green-fluorescent labelled streptavidin has verified the molecular recognition response of the biotin molecules decorating the PbS@SiO2 NP surface. Dynamic light scattering (DLS) and ζ-potential techniques have been used to monitor the hydrodynamic diameter and colloidal stability of both PbS@SiO2 and biotin decorated NPs, showing their high colloidal stability in physiological media, as needed for biomedical applications. Remarkably the obtained biotinylated PbS@SiO2 NPs have been found to retain emission properties in the 'second optical window' of the NIR region of the electromagnetic spectrum, thus representing attractive receptor-targeted NIR fluorescent probes for in vivo tumour imaging.

Hybrid charge transfer complexes based on archaeal glycolipids wrapping single walled carbon nanotubes.

SWNTs have been functionalized with an archaeal glycolipid which wraps around the nano-objects in a single layer or bilayer, as a function of the nanotube diameter. Hydrogen bonds between the lipid glucose rings and the aromatic SWNT walls are involved in the formation of hybrid complexes resulting in electron transfer from the glycolipid to the nanotubes.

Metallic nanoparticles enhanced the spontaneous emission of semiconductor nanocrystals embedded in nanoimprinted photonic crystals.

We report on a method to enhance the light-emission efficiency of printable thin films of a polymer doped with luminescent (CdSe)ZnS nanocrystals via metallic nanoparticles and nanoimprinted photonic crystals. We experimentally show a strong fluorescence enhancement of nanocrystals by coupling exciton-plasmon with the localized surface plasmon of metallic nanoparticles. The emitted light is efficiently diffracted by photonic crystals structures directly imprinted in the nanocomposite polymer. By combining the field susceptibility technique with optical Bloch equations, we examine the interaction of the quantum and plasmonic entities at small distances.

TiO(2) nanorods/PMMA copolymer-based nanocomposites: highly homogeneous linear and nonlinear optical material.

Original nanocomposites have been obtained by direct incorporation of pre-synthesized oleic acid capped TiO(2) nanorods into properly functionalized poly(methyl methacrylate) copolymers, carrying carboxylic acid groups on the repeating polymer unit. The presence of carboxylic groups on the alkyl chain of the host functionalized copolymer allows an highly homogeneous dispersion of the nanorods in the organic matrix. The prepared TiO(2)/PMMA-co-MA nanocomposites show high optical transparency in the visible region, even at high TiO(2) nanorod content, and tunable linear refractive index depending on the nanoparticle concentration. Finally measurements of nonlinear optical properties of TiO(2) polymer nanocomposites demonstrate a negligible two-photon absorption and a negative value of nonlinear refractive index, highlighting the potential of the nanocomposite for efficient optical devices operating in the visible region.