Optimal geometry for plasmonic sensing with non-interacting Au nanodisk arrays.

Combining finite elements method electrodynamic simulations and cost-effective and scalable nanofabrication techniques, we carried out a systematic investigation and optimization of the sensing properties of non-interacting gold nanodisk arrays. Such plasmonic nanoarchitectures offer a very effective platform for fast and simple, label-free, optical bio- and chemical-sensing. We varied their main geometrical parameters (diameter and height) to monitor the plasmonic resonance position and to find the configurations that maximize the sensitivity to small layers of an analyte (local sensitivity) or to the variation of the refractive index of an embedding medium (bulk sensitivity). The spectral position of the plasmonic resonance can be tuned over a wide range from the visible to the near-IR region (500-1300 nm) and state-of-the-art performances can be obtained using the optimized nanodisks; we obtained local and bulk sensitivities of S 0 = 11.9 RIU-1 and S bulk = 662 nm RIU-1, respectively. Moreover, the results of the simulations are compared with the performances of experimentally synthesized non-interacting Au nanodisk arrays fabricated by combining sparse colloidal lithography and hollow mask lithography, with the parameters obtained by the sensitivity numerical optimization. An excellent agreement between the experimental and the simulated results is demonstrated, confirming that the optimization performed with the simulations is directly applicable to nanosensors realized with cost-effective methods, due to the quite large stability basin around the maximum sensitivities.

3D Nanofabrication of SiOC Ceramic Structures.

Shaping ceramic materials at the nanoscale in 3D is a phenomenal engineering challenge, that can offer new opportunities in a number of industrial applications, including metamaterials, nano-electromechanical systems, photonic crystals, and damage-tolerant lightweight materials. 3D fabrication of sub-micrometer ceramic structures can be performed by two-photon laser writing of a preceramic polymer. However, polymer conversion to a fully ceramic material has proven so far unfeasible, due to lack of suitable precursors, printing complexity, and high shrinkage during ceramic conversion. Here, it is shown that this goal can be achieved through an appropriate engineering of both the material and the printing process, enabling the fabrication of preceramic 3D shapes and their transformation into dense and crack-free SiOC ceramic components with highly complex, 3D sub-micrometer architectures. This method allows for the manufacturing of components with any 3D specific geometry with fine details down to 450 nm, rapidly printing structures up to 100 µm in height that can be converted into ceramic objects possessing sub-micrometer features, offering unprecedented opportunities in different application fields.

Nanoroughness, Surface Chemistry, and Drug Delivery Control by Atmospheric Plasma Jet on Implantable Devices.

Implantable devices need specific tailored surface morphologies and chemistries to interact with the living systems or to actively induce a biological response also by the release of drugs or proteins. These customized requirements foster technologies that can be implemented in additive manufacturing systems. Here, we present a novel approach based on spraying processes that allow to control separately topographic features in the submicron range (∼60 nm to 2 µm), ammine or carboxylic chemistry, and fluorophore release even on temperature-sensitive biodegradable polymers such as polycaprolactone (PCL). We developed a two-steps process with a first deposition of 220 nm silica and poly(lactic- co-glycolide) (PLGA) fluorescent nanoparticles by aerosol followed by the deposition of a fixing layer by an atmospheric pressure plasma jet (APPJ). The nanoparticles can be used to create the nanoroughness and to include active molecule release, while the capping layer ensures stability and the chemical functionalities. The process is enabled by a novel APPJ which allows deposition rates of 10-20 nm·s-1 at temperatures lower than 50 °C using argon as the process gas. This approach was assessed on titanium alloys for dental implants and on PCL films. The surfaces were characterized by Fourier transform infrared, atomic force microscopy, and scanning electron microscopy (SEM). Titanium alloys were tested with the preosteoblast murine cells line, while the PCL film was tested with fibroblasts. Cell behavior was evaluated by viability and adhesion assays, protein adsorption, cell proliferation, focal adhesion formation, and SEM. The release of a fluorophore molecule was assessed in the cell growing media, simulating a drug release. Osteoblast adhesion on the plasma-treated materials increased by 20% with respect to commercial titanium alloy implants. Fibroblast adhesion increased by a 100% compared to smooth PCL substrates. The release of the fluorophore by the dissolution of the PLGA nanoparticles was verified, and the integrity of the encapsulated drug model was confirmed.

Rare-earth fluorescence thermometry of laser-induced plasmon heating in silver nanoparticles arrays.

The laser-induced plasmon heating of an ordered array of silver nanoparticles, under continuous illumination with an Ar laser, was probed by rare-earth fluorescence thermometry. The rise in temperature in the samples was monitored by measuring the temperature-sensitive photoluminescent emission of a europium complex (EuTTA) embedded in PMMA thin-films, deposited onto the nanoparticles array. A maximum temperature increase of 19 °C was determined upon resonant illumination with the surface plasmon resonance of the nanoarray at the highest pump Ar laser power (173 mW). The experimental results were supported by finite elements method electrodynamic simulations, which provided also information on the temporal dynamics of the heating process. This method proved to be a facile and accurate approach to probe the actual temperature increase due to photo-induced plasmon heating in plasmonic nanosystems.

Ultra-fast dynamics in the nonlinear optical response of silver nanoprism ordered arrays.

In this work we present the study of the ultra-fast dynamics of the nonlinear optical response of a honeycomb array of silver triangular nanoprisms, performed using a femtosecond pulsed laser tuned with the dipolar surface plasmon resonance of the nanoarray. Nonlinear absorption and refraction, and their time-dependence, were explored using the z-scan and time-resolved excite-probe techniques. Nonlinear absorption is shown to change sign with the input irradiance and the behavior was explained on the basis of a three-level model. The response time was determined to be in the picosecond regime. A technique based on a variable frequency chopper was also used in order to discriminate the thermal and electronic contributions to the nonlinearity, which were found to have opposite signs. All these findings propel the investigated nanoprism arrays as good candidates for applications in advanced ultra-fast nonlinear nanophotonic devices.

Gold-silver alloy semi-nanoshell arrays for label-free plasmonic biosensors.

Nanosphere lithography coupled with reactive ion etching has been used to synthesize hexagonal ordered arrays of Au-Ag bimetallic semi-nanoshells to be used as plasmonic biosensors. The degree of lateral interaction between adjacent semi-nanoshells can be controlled by tailoring the reactive ion etching time in order to boost the global plasmonic properties through the formation of near-field hot-spots, which in turn can improve the sensitivity of the biosensors. To test the efficiency of the proposed system as a biosensor, we used an established protocol for the detection of biomolecules (local sensitivity), based on the receptor-ligand approach and using the biotin-streptavidin model system. We also tested the sensitivity to a homogeneous change in the refractive index of the buffer over the sensor (bulk sensitivity). Comparing the obtained results to those of an array of nanoprisms, chosen as a benchmark, significantly higher performances both in local and bulk sensitivities have been found, in agreement with electrodynamics simulations based on finite-element methods.

Spectral dependence of nonlinear absorption in ordered silver metallic nanoprism arrays.

Ordered metallic nanoprism arrays have been proposed as novel and versatile systems for the observation of nonlinear effects such as nonlinear absorption. The study of the effect of the local field reinforcement on the fast optical third order nonlinear response around the Surface Plasmon Resonance is of great interest for many plasmonic applications. In this work, silver nanoprism arrays have been synthesized by the nanosphere lithography method. A low repetition rate tunable picosecond laser source was used to study the irradiance and wavelength dependence of the nonlinear absorption properties around the dipolar and quadrupolar resonances of the nanoarray with the use of the z-scan technique. The irradiance dependence of the on-resonance nonlinearity was studied, and a spectral region where nonlinear absorption is negligible was identified. This is important for the possible application of these materials in optical information processing devices.

Degenerately Doped Metal Oxide Nanocrystals as Plasmonic and Chemoresistive Gas Sensors.

Highly doped wide band gap metal oxide nanocrystals have recently been proposed as building blocks for applications as transparent electrodes, electrochromics, plasmonics, and optoelectronics in general. Here we demonstrate the application of gallium-doped zinc oxide (GZO) nanocrystals as novel plasmonic and chemiresistive sensors for the detection of hazardous gases including hydrogen (H2) and nitrogen dioxide (NO2). GZO nanocrystals with a tunable surface plasmon resonance in the near-infrared are obtained using a colloidal heat-up synthesis. Thanks to the strong sensitivity of the plasmon resonances to chemical and electrical changes occurring at the surface of the nanocrystals, such optical features can be used to detect the presence of toxic gases. By monitoring the changes in the dopant-induced plasmon resonance in the near-infrared, we demonstrate that GZO thin films prepared depositing an assembly of highly doped GZO colloids are able to optically detect both oxidizing and reducing gases at mild (<100 °C) operating temperatures. Combined optical and electrical measurements show that trivalent dopants within ZnO nanocrystals enhance the gas sensing response compared to undoped ZnO. Moreover, improved sub-ppm of NO2 gas sensitivity is achieved by activating the sensors response through combined purple-blue (λ = 430 nm) light irradiation and mild heating at 75 °C. In addition, these thin films based on degenerately doped semiconductors are highly transparent in the visible range, enabling the fabrication of "invisible" gas sensors. The use of highly doped semiconductive nanocrystals for both IR plasmonic and chemiresistive sensors represent a marked advancement toward the development of highly sensitive and selective devices.

Nonlinear absorption tuning by composition control in bimetallic plasmonic nanoprism arrays.

The nonlinear absorption properties of bidimensional arrays of Au-Ag bilayered nanoprisms have been investigated by z-scan measurements as a function of the bimetallic nanoprism composition. A tunable ps laser system was used to excite the ultrafast, electronic nonlinear response matching the laser wavelength with the quadrupolar surface plasmon resonances, in the visible range, of each nanoprism array. Due to the strong electromagnetic field confinement effects at the nanoprism tips, demonstrated by finite element method simulations, these nanosystems proved to have enhanced nonlinear optical properties. Moreover, a tunable changeover from reverse saturable absorption (RSA) to saturable absorption (SA) can be obtained by properly controlling the bimetallic composition of the nanoprisms, without modifying the overall morphology of the nanosystems. This capability makes these nanosystems extremely interesting for the realization of solid-state nanophotonic devices with enhanced ultrafast nonlinear optical properties.

Au-Ag nanoalloy molecule-like clusters for enhanced quantum efficiency emission of Er³âº ions in silica.

The occurrence of a very efficient non-resonant energy transfer process forming ultrasmall Au-Ag nanoalloy clusters and Er(3+) ions is investigated in silica. The enhancement of the room temperature Er(3+) emission efficiency by an order of magnitude is achieved by coupling rare-earth ions to molecule-like (Au(x)Ag(1-x))N alloy nanoclusters with N = 10-15 atoms and x = 0.6 obtained by optimized sequential ion implantation on Er-implanted silica. For comparison, AuN nanoclusters obtained by the same approach and with the same size and numerical density showed an enhancement by only a factor of 2 with respect to pure Er emission, demonstrating the beneficial effect of using nanoalloyed clusters. The temperature evolution of the energy transfer process is investigated by photoluminescence and exhibits a maximum efficiency at about 600 °C, where the clusters reach the optimal size and the silica matrix completely recovers the implantation damage. The nanoalloy cluster composition and size have been studied by EXAFS analysis, which indicated a stronger Ag-O interaction with respect to the Au-O one and a preferential location of the Ag atoms at the nanoalloy cluster surface.

Core-shell-like Au sub-nanometer clusters in Er-implanted silica.

The very early steps of Au metal cluster formation in Er-doped silica have been investigated by high-energy resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS). A combined analysis of the near-edge and extended part of the experimental spectra shows that Au cluster nucleation starts from a few Au and O atoms covalently interconnected, likely in the presence of embryonic Au-Au correlation. The first Au clusters, characterized by a well defined Au-Au coordination distance, form upon 400 °C inert annealing. The estimated upper limit of the Gibbs free energy for the associated heterogeneous nucleation is 0.06 eV per atom, suggesting that the Au nucleation is assisted by matrix defects, most likely non-bridging oxygen atoms. The experimental results indicate that the formed subnanometer Au clusters can be applied as effective core-shell systems in which the Au atoms of the 'core' develop a metallic character, whereas the Au atoms in the 'shell' can retain a partially covalent bond with O atoms of the silica matrix. High structural disorder at the Au site is found upon neutral annealing at a moderate temperature (600 °C), likely driven by the configurational disorder of the defective silica matrix. A suitable choice of the Au concentration and annealing temperature allows tailoring of the Au cluster size in the sub-nanometer range. The interaction of the Au cluster surface with the surrounding silica matrix is likely responsible for the infrared luminescence previously reported on the same systems.

Optimal geometric parameters of ordered arrays of nanoprisms for enhanced sensitivity in localized plasmon based sensors.

Plasmonic sensors based on ordered arrays of nanoprisms are optimized in terms of their geometric parameters like size, height, aspect ratio for Au, Ag or Au0.5-Ag0.5 alloy to be used in the visible or near IR spectral range. The two figures of merit used for the optimization are the bulk and the surface sensitivity: the first is important for optimizing the sensing to large volume analytes whereas the latter is more important when dealing with small bio-molecules immobilized in close proximity to the nanoparticle surface. A comparison is made between experimentally obtained nanoprisms arrays and simulated ones by using Finite Elements Methods (FEM) techniques.

Energy-transfer from ultra-small Au nanoclusters to Er³âº ions: a short-range mechanism.

Sub-nanometric Au nanoclusters are known to act as very efficient sensitizers for the luminescent emission of Er(3+) ions in silica through a non-resonant broad-band energy-transfer mechanism. In the present work the energy-transfer process is investigated in detail by room temperature photoluminescence characterization of Er and Au co-implanted silica systems in which a different degree of coupling between Er(3+) ions and Au nanoclusters is obtained. The results allow us to definitely demonstrate the short-range nature of the interaction in agreement with non-radiative energy-transfer mechanisms. Moreover, an upper limit to the interaction length is also set by the Au-Au intercluster semi-distance which is smaller than 2.4 nm in the present case.

Silver nanoprism arrays coupled to functional hybrid films for localized surface plasmon resonance-based detection of aromatic hydrocarbons.

We report the achievement of sensitive gas detection using periodic silver nanoprisms fabricated by a simple and low-cost lithographic technique. The presence of sharp tips combined with the periodic arrangement of the nanoprisms allowed the excitement of isolated and interacting localized surface plasmon resonances. Specific sensing capabilities with respect to aromatic hydrocarbons were achieved when the metal nanoprism arrays were coupled in the near field with functional hybrid films, providing a real-time, label-free, and reversible methodology. Ultra-high-vacuum temperature-programmed desorption measurements demonstrated an interaction energy between the sensitive film and analytes in the range of 55-71 kJ/mol. The far-field optical properties and the detection sensitivity of the sensors, modeled using a finite element method, were correlated to experimental data from gas sensing tests. An absorbance variation of 1.2% could be observed and associated with a theoretical increase in the functional film refractive index of ∼0.001, as a consequence to the interaction with 30 ppm xylene. The possibility of detecting such a small variation in the refractive index suggests the highly promising sensing capabilities of the presented technique.