Cosmin Farcau

High-Sensitivity Strain Gauge Based on a Single Wire of Gold Nanoparticles Fabricated by Stop-and-Go Convective Self-Assembly

High-sensitivity strain gauges based on single wires of close-packed 14 nm colloidal gold nanoparticles are obtained by a novel variant of convective self-assembly (CSA). This CSA mode named stop-and-go CSA enables the fabrication of nanoparticle wires only a few micrometers wide, separated by distances that can be easily tuned over tens to hundreds of micrometers. Nanoparticle wires are obtained in a single step by direct deposition of nanoparticles from suspensions onto flexible polyethylene terephthalate films, without any lithographic prepatterning. When connected between two electrodes, such single nanoparticle wires function as miniature resistive strain gauges. The high sensitivity, repeatability, and robustness demonstrated by these single-wire strain gauges make them extremely promising for integration into micro-electromechanical systems or for high-resolution strain mapping.

Chitosan-coated anisotropic silver nanoparticles as a SERS substrate for single-molecule detection

Surface-enhanced Raman spectroscopy (SERS) is a technique that has become widely used for identifying and providing structural information about molecular species in low concentration. There is an ongoing interest in finding optimum particle size, shape and spatial distribution for optimizing the SERS substrates and pushing the sensitivity toward the single-molecule detection limit. This work reports the design of a novel, biocompatible SERS substrate based on small clusters of anisotropic silver nanoparticles embedded in a film of chitosan biopolymer. The SERS efficiency of the biocompatible film is assessed by employing Raman imaging and spectroscopy of adenine, a significant biological molecule. By combining atomic force microscopy with SERS imaging we find that the chitosan matrix enables the formation of small clusters of silver nanoparticles, with junctions and gaps that greatly enhance the Raman intensities of the adsorbed molecules. The study demonstrates that chitosan-coated anisotropic silver nanoparticle clusters are sensitive enough to be implemented as effective plasmonic substrates for SERS detection of nonresonant analytes at the single-molecule level.

Tunable Conductive Nanoparticle Wire Arrays Fabricated by Convective Self-Assembly on Nonpatterned Substrates

Ordered arrays of centimeter-long nanoparticle wires are fabricated by convective self-assembly from aqueous suspensions of 18 nm gold colloids, on flat SiO(2)/Si substrates without any prepatterning. The orientation of the wires can be switched from parallel to perpendicular to the substrate-liquid-air contact line by controlling the substrate temperature. While the wires parallel to the meniscus are obtained by a stick-slip process, a mechanism based on critical density-triggered particle pinning is proposed to explain the formation of wires perpendicular to the meniscus. The geometry of the wire arrays is tuned by simply controlling the meniscus translation speed. Wires are typically characterized by widths of a few micrometers (1.8-8.2 µm), thicknesses of mono- to multilayers (18-70 nm), and spacings of few tens of micrometers. The fabricated nanoparticle wires are conductive, exhibiting a metallic resistive behavior in ambient conditions. Resistivity values of 5 × 10(-6) and 5 × 10(-2) Ωm are obtained on multilayer and monolayer nanoparticle wires, respectively. Such conductive nanoparticle wire arrays, fabricated by a simple and low-cost bottom-up strategy, offer opportunities for developing nanoparticle-based functional devices.

Stability of Ag nanocrystals synthesized by ultra-low energy ion implantation in SiO2 matrices

Ultra low energy ion implantation is a promising technique for the wafer-scale fabrication of Silver nanoparticle planar arrays embedded in thermal silica on silicon substrate. The stability versus time of these nanoparticles is studied at ambient conditions on a time scale of months. The plasmonic signature of Ag NPs vanishes several months after implantation for as-implanted samples, while samples annealed at intermediate temperature under N2 remain stable. XPS and HREM analysis evidence the presence of Silver oxide nanoparticles on aged samples and pure Silver nanoparticles on the annealed ones. This thermal treatment does not modify the size-distribution or position of the particles but is very efficient in stabilizing the metallic particles and to prevent any form of oxidation.

Evidence of a surface plasmon-mediated mechanism in the generation of the SERS background

Herein, we correlate the SERS images recorded by Scanning Confocal Raman Microscopy with the plasmonic response of a Ag nanostructured film of controlled morphology to put forward direct evidence for the involvement of a plasmonic mechanism in the generation of the SERS background.

Covalent conjugation of carbon dots with Rhodamine B and assessment of their photophysical properties

The unique photoluminescent properties of carbon dots (CDs) continue to encourage a great interest in their development for a wide range of applications in energy conversion, optoelectronics or sensing. Engaging carbon dots in resonance energy transfer processes with organic dyes could enable the design of functional materials to greatly enhance the performance of solar cells and other optoelectronic devices, or to create new types of sensors. In this work, CDs were functionalized with Rhodamine B (RhB) isothiocyanate, (CD–PEG1500N–Rh) via a simple procedure after surface modification of bare carbon nanoparticles with poly(ethylene glycol) bis(3-aminopropyl) (PEG1500N). The morphology of CD–PEG1500N was ascertained using HR-TEM while the covalent linkage of Rhodamine B at the surface of PEG1500N capped CDs was proved by spectroscopic analysis. The overlap between the emission spectra of CDs and the absorption spectrum of RhB molecules favoured fluorescent (Forster) resonance energy transfer (FRET) from the CDs to the dye molecules. The FRET mechanism was firstly demonstrated by steady-state fluorescence measurements and its efficiency was estimated by photoluminescence lifetime measurements, using the time correlated single photon counting (TCSPC) method with the excitation of picosecond pulse lasers. The synthetic accessibility and the transfer efficiency of these conjugates make them reliable candidates for fluorescent materials to be later used in FRET based sensing platforms and photovoltaic devices.

Controlled synthesis of buried delta-layers of Ag nanocrystals for near-field plasmonic effects on free surfaces

We report on the shallow synthesis by low energy ion implantation of delta-layers of Ag nanocrystals in SiO2 at few nanometers under its free surface. Transmission electron microscopy observations, ballistic simulations, and reflectance measurements are coupled to define the conditions for which the synthesis is fully controlled and when, on the contrary, this control is lost. We show that low dose implantation leads to the formation of a well-defined single plane of nanocrystals, while for larger doses, sputtering and diffusion effects limit the control of the size, position, and volume amount of these nanocrystals. This paper provides the experimental evidence of the incorporated dose saturation predicted in the literature when implanting metal ions at high doses in glass matrices. Its consequences on the particle population and the plasmonic optical response of the composite layers are carefully analyzed. We show here that this saturation phenomenon is underestimated in standard simulation predictions ...

Metanephrine neuroendocrine tumor marker detection by SERS using Au nanoparticle/Au film sandwich architecture

Neuroendocrine tumors, such as pheochromocytoma or paraganglioma, are dangerous tumors that constitute a potential threat for a large number of patients. Currently, the biochemical diagnosis of neuroendocrine tumors is based on measurement of the direct secretory products of the adrenomedullary-sympathetic system or of their metabolites, such as catecholamines or their metanephrine derivatives, from plasma or urine. The techniques used for analysis of plasma free metanephrines, i.e. high-performance liquid chromatography or high-performance liquid chromatography coupled with mass-spectrometry are technically-demanding and time consuming, which limit their availability. Here we demonstrate a simple, fast and low-cost method for detecting metanephrine by Surface Enhanced Raman Scattering (SERS). The protocol consists in using evaporation-induced self-assembly of gold (Au) nanoparticles incubated with the analyte, on planar gold films. The assembly process produces regions with a dense distribution of both inter-particle gaps and particle-film gaps. Finite-difference time-domain simulations confirm that both kinds of gaps are locations of enhanced electromagnetic fields resulting from inter-particle and particle-film plasmonic coupling, useful for SERS amplification. Metanephrine vibrational bands assignment was performed according to density functional theory calculations. Metanephrine metabolite was detected in liquid at concentration levels lower than previously reported for other similar metabolites. The obtained results demonstrate that the Au nanoparticle/Au film exhibits noticeable SERS amplification of the adsorbed metabolite and can be used in the design of efficient, stable SERS-active substrates for the detection and identification of specific tumor markers.

Surface-enhanced spectroscopy on plasmonic oligomers assembled by AFM nanoxerography

Surface-enhanced Raman scattering (SERS) and surface-enhanced fluorescence (SEF) from individual plasmonic oligomers are investigated by confocal Raman micro-spectroscopy and time-resolved fluorescence microscopy coupled to steady state micro-spectroscopy. The nanoparticle (NP) oligomers are made of either ligand protected Au or Au@SiO2 core-shell colloidal NPs, which were assembled into ordered arrays by atomic force microscopy (AFM) nanoxerography. A strong dependence of the SERS emission on the polarization of incident light relative to the specific geometry of the plasmonic oligomer was observed. The SEF studies, performed on a large collection of NP oligomers of various known configurations showed interesting fluorophore decay rate modification and red-shift of the emission spectra. The experimental results are analyzed theoretically by employing finite-difference time-domain (FDTD) simulations on equivalent realistic structures, within the local density of optical states (LDOS) framework. The presented results, together with the proven potential of the LDOS approach as a useful common tool for analyzing both SERS and SEF effects further the general understanding of plasmon-related phenomena in nanoparticle oligomers.

Combining elastic and resonant inelastic optical spectroscopies for multiscale probing of embedded nanoparticle architectures

Composite materials consisting of metal nanoparticles (NPs) embedded in a dielectric matrix have a great potential for photonic and plasmonic applications. A set of expensive, time-consuming, and destructive methods (like electron microscopy, electron energy loss, or secondary ion mass spectroscopy) are extensively being used for the structural characterization of such buried NP assemblies. Here, we show the power of combining complementary, noninvasive optical techniques to characterize planar arrays of Ag NPs embedded in a silica film. We use UV-Vis optical reflectivity and resonant Brillouin–Raman scattering, sustained by simulations, to show the sensitivity of these methods to the presence, density, size distribution, and spatial localization of NPs. The accuracy of the results is validated by transmission electron microscopy investigations. Finally the method is applied to obtain images of embedded plasmonic structures from reflectivity and Raman scanning microscopy.