M. Patrini

Nanoscale chemical mapping using three-dimensional adiabatic compression of surface plasmon polaritons

The fields of plasmonics, Raman spectroscopy and atomic force microscopy have recently undergone considerable development, but independently of one another. By combining these techniques, a range of complementary information could be simultaneously obtained at a single molecule level. Here, we report the design, fabrication and application of a photonic-plasmonic device that is fully compatible with atomic force microscopy and Raman spectroscopy. Our approach relies on the generation and localization of surface plasmon polaritons by means of adiabatic compression through a metallic tapered waveguide to create strongly enhanced Raman excitation in a region just a few nanometres across. The tapered waveguide can also be used as an atomic force microscope tip. Using the device, topographic, chemical and structural information about silicon nanocrystals may be obtained with a spatial resolution of 7 nm.

A hybrid plasmonic-photonic nanodevice for label-free detection of a few molecules.

Noble metal nanowaveguides supporting plasmon polariton modes are able to localize the optical fields at nanometer level for high sensitivity biochemical sensing devices. Here we report on the design and fabrication of a novel photonic-plasmonic device which demonstrates label-free detection capabilities on single inorganic nanoparticles and on monolayers of organic compounds. In any case, we determine the Raman scattering signal enhancement and the device detection limits that reach a number of molecules between 10 and 250. The device can be straightforwardly integrated in a scanning probe apparatus with the possibility to match topographic and label-free spectroscopic information in a wide range of geometries.

Antibacterial Activity of Glutathione-Coated Silver Nanoparticles against Gram Positive and Gram Negative Bacteria

In the present paper, we study the mechanism of antibacterial activity of glutathione (GSH) coated silver nanoparticles (Ag NPs) on model Gram negative and Gram positive bacterial strains. Interference in bacterial cell replication is observed for both cellular strains when exposed to GSH stabilized colloidal silver in solution, and microbicidal activity was studied when GSH coated Ag NPs are (i) dispersed in colloidal suspensions or (ii) grafted on thiol-functionalized glass surfaces. The obtained results confirm that the effect of dispersed GSH capped Ag NPs (GSH Ag NPs) on Escherichia coli is more intense because it can be associated with the penetration of the colloid into the cytoplasm, with the subsequent local interaction of silver with cell components causing damages to the cells. Conversely, for Staphylococcus aureus, since the thick peptidoglycan layer of the cell wall prevents the penetration of the NPs inside the cytoplasm, the antimicrobial effect is limited and seems related to the interaction with the bacterial surfaces. Experiments on GSH Ag NPs grafted on glass allowed us to elucidate more precisely the antibacterial mechanism, showing that the action is reduced because of GSH coating and the limitation of the translational freedom of NPs.

Self-assembled monolayers of silver nanoparticles firmly grafted on glass surfaces: Low Ag + release for an efficient antibacterial activity

A two-step, easy synthetic strategy in solution has been optimized to prepare authentic monolayers of silver nanoparticles (NP) on MPTS-modified glass surfaces, that were investigated by AFM imaging and by quantitative silver determination techniques. NP in the monolayers remain firmly grafted (i.e. not released) when the surfaces are exposed to air, water or in the physiological conditions mimicked by phosphate saline buffer, as UV-Vis spectroscopy and AFM studies demonstrate. About 15% silver release as Ag(+) ions has been found after 15days when the surfaces are exposed to water. The released silver cations are responsible of an efficient local microbicidal activity against Escherichia coli and Staphylococcus aureus bacterial strains.

3D Hollow Nanostructures as Building Blocks for Multifunctional Plasmonics

We present an advanced and robust technology to realize 3D hollow plasmonic nanostructures which are tunable in size, shape, and layout. The presented architectures offer new and unconventional properties such as the realization of 3D plasmonic hollow nanocavities with high electric field confinement and enhancement, finely structured extinction profiles, and broad band optical absorption. The 3D nature of the devices can overcome intrinsic difficulties related to conventional architectures in a wide range of multidisciplinary applications.

Quantum dot strain engineering of InAs/InGaAs nanostructures

We present a complete study both by experiments and by model calculations of quantum dot strain engineering, by which a few optical properties of quantum dot nanostructures can be tailored using the strain of quantum dots as a parameter. This approach can be used to redshift beyond 1.31μm and, possibly, towards 1.55μm the room-temperature light emission of InAs quantum dots embedded in InGaAs confining layers grown on GaAs substrates. We show that by controlling simultaneously the lower confining layer thickness and the confining layers’ composition, the energy gap of the quantum dot material and the band discontinuities in the quantum dot nanostructure can be predetermined and then the light emission can be tuned in the spectral region of interest. The availability of two degrees of freedom allows for the control of two parameters, which are the emission energy and the emission efficiency at room temperature. The InAs∕InGaAs structures were grown by the combined use of molecular beam epitaxy and atomic l...

Synthesis and characterization of cluster-assembled carbon thin films

Nanostructured carbon thin films have been produced by deposition of supersonic cluster beams. The clusters are generated by a pulsed arc cluster ion source modified in order to achieve high fluxes and stability. Scanning electron microscopy, Raman, and optical spectroscopy show that the films are a low density network of nanometer-size particles. The nature of the films is essentially graphite-like with a large number of distorted bonds. The formation of structures based on sp3 bondings is not observed. The use of cluster beam deposition for the synthesis of nanocrystalline thin films is discussed.

Direct evidence of light confinement and emission enhancement in active silicon-on-insulator slot waveguides

The authors experimentally demonstrate strong light confinement and enhancement of emission at 1.54μm in planar silicon-on-insulator waveguides containing a thin layer (slot) of SiO2 with Er3+ doped Si nanoclusters. Angle-resolved attenuated total reflectance is used to excite the slab guided modes, giving a direct evidence of the strong confinement of the electric field in the low-index active material for the fundamental transverse-magnetic mode. By measuring the guided photoluminescence from the cleaved-edge of the sample, the authors observe a more than fivefold enhancement of emission for the transverse-magnetic mode over the transverse-electric one. These results show that Si-based slot waveguides could be important as starting templates for the realization of Si-compatible active optical devices.

P-type macroporous silicon for two-dimensional photonic crystals

Macroporous silicon with two-dimensional periodicity has been produced by electrochemical etching, using a p-type doped silicon substrate. The structure shows photonic energy gaps in the infrared region, as demonstrated by variable angle reflectance measurements. The agreement between measurement and band calculations confirms the high quality of the samples. The use of an optimized electrolyte allows the fabrication of very high quality samples, with high aspect ratio and low roughness both at the surface and on the pore walls. The best results are obtained with aprotic and protophilic solvents.

Chitosan-associated SLN: in vitro and ex vivo characterization of cyclosporine A loaded ophthalmic systems

The aim of this study was to develop cyclosporine A (CsA) loaded solid lipid nanoparticles (SLN) associated with chitosan (CS), to improve interaction and internalization in corneal cells. The SLN were prepared using high shear homogenization and ultrasound methods with CS in the aqueous phase. The lipid phase was based on Compritol or Precirol. The SLN were characterized for particle size, polydispersity index, morphology, zeta potential and encapsulation efficiency. The systems were freeze-dried to increase physical stability and trehalose was used as a cryo/lyo-protector to stabilize the SLN. The penetration and permeation properties of the SLN were assessed in vitro (cell culture) and ex vivo (excised pig cornea). The cell uptake of SLN was studied by means of confocal laser scanning microscopy. CS-associated SLN based on Compritol were biocompatible and enhanced the permeation/penetration of CsA along with a possible mechanism of internalization/uptake of the nanoparticles both in vitro and ex vivo.