Simion Astilean

Light transmission through metallic channels much smaller than the wavelength

Abstract A resonant cavity-enhanced light transmission mechanism in metallic gratings with subwavelength apertures is theoretically interpreted for operation with visible light. It is shown that under appropriate boundary conditions, the apertures behave as open Fabry–Perot resonant cavities delivering a high photon flux, and that the coupling between the incident light and the fundamental mode supported by the aperture is strongly controlled by surface waves. Compared to the well-known perfect metallic case, we show that the effective index of the fundamental mode strongly depends on the aperture dimension, especially when aperture widths much smaller than the wavelength are considered. Consequently, it is predicted that small variations of aperture shape or dimensions have a huge effect on the transmission properties of real metallic gratings.

Chitosan-coated triangular silver nanoparticles as a novel class of biocompatible, highly effective photothermal transducers for in vitro cancer cell therapy

One of the relevant directions that nanotechnology is taking nowadays is connected with nanomedicine and specifically related to the use of light and nanoparticles in early diagnosis and effective therapeutics of cancer. Noble-metal nanoparticles can act under laser irradiation as effective photothermal transducers for triggering localized hyperthermia of tumors. In this work we report the performance of newly synthesized chitosan-coated silver nanotriangles (Chit-AgNTs) with strong resonances in near-infrared (NIR) to operate as photothermal agents against a line of human non-small lung cancer cells (NCI-H460). The hyperthermia experiments were conducted by excitation of nanoparticles-loaded cells at 800 nm wavelength from a Ti:Sapphire laser. We found that the rate of cell mortality in the presence of Chit-AgNTs is higher than in the presence of thiolated poly(ethylene) glycol capped gold nanorods (PEG-AuNRs) - a common hyperthermia agent used as reference-, while no destructive effects were noticed on the control sample (cells without nanoparticles) under identical irradiation conditions. Additionally, we conducted cytotoxicity assays and found Chit-AgNTs to be efficiently uptaken by the cells while exhibiting good biocompatibility for healthy human embryonic cells (HEK), which is essential for any in vivo applications. Our results reveal a novel class of biocompatible plasmonic nanoparticles with high potential to be implemented as effective phototherapeutic agents in the battle against cancer.

One-mode model and Airy-like formulae for one-dimensional metallic gratings

We present a semi-analytical model that quantitatively predicts the transmission, the absorption and the resonance locations of one-dimensional lamellar metallic gratings with subwavelength slit apertures. The model relies on the fact that subwavelength apertures perforated in metallic films behave like monomode waveguides and provide Airy-like formulae for the transmission and reflection coefficients. Limitations are outlined for too shallow grating depths.

Controlling gold nanoparticle assemblies for efficient surface-enhanced Raman scattering and localized surface plasmon resonance sensors

Gold nanoparticle assemblies with different particle densities were immobilized upon functionalized glass substrates and their morphology and optical properties are analysed with TEM and UV?vis absorption spectroscopy. The efficiency of self-assembled gold colloidal nanoparticles as SERS-active substrates at 633?nm excitation line is evaluated by employing p-aminothiophenol. The average enhancements in the range of 105?107 are consistent with signals being dominated by molecules adsorbed at junctions inside nanoparticle assemblies. We employ an annealing treatment to induce changes in the initial morphology of gold assemblies and produce sensitive LSPR substrate. The sensitivity of the as-prepared substrate to transduce changes of the surrounding medium refractive index is further assessed.

Flower-shaped gold nanoparticles: synthesis, characterization and their application as SERS-active tags inside living cells

The detection of Raman signals inside living cells is a topic of great interest in the study of cell biology mechanisms and for diagnostic and therapeutic applications. This work presents the synthesis and characterization of flower-shaped gold nanoparticles and demonstrates their applicability as SERS-active tags for cellular spectral detection. The particles were synthesized by a facile, rapid new route that uses ascorbic acid as a reducing agent of gold salt. Two triarylmethane dyes which are widely used as biological stains, namely malachite green oxalate and basic fuchsin, were used as Raman-active molecules and the polymer mPEG-SH as capping material. The as-prepared SERS-active nanoparticles were tested on a human retinal pigment epithelial cell line and found to present a low level of cytotoxicity and high chemical stability together with SERS sensitivity down to picomolar particle concentrations.

Detoxification of gold nanorods by conjugation with thiolated poly(ethylene glycol) and their assessment as SERS-active carriers of Raman tags

We present an effective, low cost protocol to reduce the toxicity of gold nanorods induced by the presence of cetyltrimethylammonium bromide (CTAB) on their lateral surface as a result of the synthesis process. Here, we use thiolated methoxy-poly(ethylene) glycol (mPEG-SH) polymer to displace most of the CTAB bilayer cap from the particle surface. The detoxification process, chemical and structural stability of as-prepared mPEG-SH-conjugated gold nanorods were characterized using a number of techniques including localized surface plasmon resonance (LSPR), transmission electron microscopy (TEM) and surface-enhanced Raman spectroscopy (SERS). In view of future applications as near-infrared (NIR) nanoheaters in localized photothermal therapy of cancer, we investigated the thermal behaviour of mPEG-SH-conjugated gold nanorods above room temperature. We found a critical temperature at around 40 degrees C at which the adsorbed polymer layer is susceptible to undergo conformational changes. Additionally, we believe that such plasmonic nanoprobes could act as SERS-active carriers of Raman tags for application in cellular imaging. In this sense we successfully tested them as effective SERS substrates at 785 nm laser line with p-aminothiophenol (pATP) as a tag molecule.

Solution-phase, dual LSPR-SERS plasmonic sensors of high sensitivity and stability based on chitosan-coated anisotropic silver nanoparticles

There is a need to design highly sensitive plasmonic sensors which impart a good biocompatibility and optical stability to detect low levels of analytes in biological media. In this study we report the formation of chitosan-coated silver nanoparticles of triangular shape in solution by synergistic action of chitosan and trisodium citrate in the presence of silver seeds and ascorbic acid. It has been revealed that these anisotropic silver nanoparticles entrapped in biopolymeric shells are particularly stable and can be successfully used as versatile plasmonic substrates for molecular sensing in solution. In particular, the binding of the probe molecule monolayer (para-aminothiophenol, p-ATP) at the surface of individual chitosan-coated silver nanoparticles was demonstrated both by localized surface plasmon resonance (LSPR) shifts and surface-enhanced Raman scattering (SERS) spectra. While the LSPR-shift assay is operational for signaling molecular binding events, the SERS allows identifying the probe molecules and elucidating its orientation on the metal surface. The proof of concept for biosensing applications and dual functionality of plasmonic platform are evaluated through the combined LSPR-SERS detection of significant biological molecules, adenine. The potential of chitosan–silver nanostructures to extend the standard approach of LSPR sensing by integrating SERS measurements and operate as dual plasmonic sensors would be very attractive for investigation of analytes in biological fluids.

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.

A new green, ascorbic acid-assisted method for versatile synthesis of Au–graphene hybrids as efficient surface-enhanced Raman scattering platforms

A new green method for the synthesis of reduced graphene oxide–gold nanoparticle (rGO–AuNP) hybrids in aqueous solution that exploits the ability of ascorbic acid (AA) to operate as an effective dual agent for both graphene oxide (GO) and gold ion reduction is reported. Through careful investigation of the production of rGO–AuNP hybrids stabilized with polyvinylpyrrolidone (PVP), several versatile routes were devised with the aim of controlling the size, shape and distribution of AuNPs anchored onto the graphene sheets as well as the GO reduction. Particularly, when rGO is used as a platform for Au ion nucleation, a relative sparse distribution of AuNPs of size ranging from 20 nm to 50 nm is noticed. In contrast, when gold ions are added to the solution prior to any GO reduction, the density of large AuNPs is rather low relative to the uniformly packed small sized AuNPs (3–12 nm). The progress of GO reduction is explained by considering the contribution of the catalytic activity of AuNPs, besides the reducing activity of AA. Finally, a plausible mechanism for the nucleation and distribution of AuNPs onto the graphenic surface is assumed, highlighting the significance of oxygen moieties. The green method developed here is promising for the fabrication of gold–graphene nanocomposites with tunable surface “decoration”, suitable for surface-enhanced Raman spectroscopy (SERS).

The synthesis of biocompatible and SERS-active gold nanoparticles using chitosan

In this study we present a clean, nontoxic, environmentally friendly synthesis procedure to generate a large variety of gold nanoparticles (GNPs) by using chitosan, a biocompatible, biodegradable, natural polymer, as reducing and stabilizing agent. The formation of gold-chitosan nanocomposites was characterized by UV-vis absorption spectroscopy, transmission electron microscopy (TEM), x-ray diffraction (XRD) and Raman spectroscopy. The results show that the reaction temperature plays a crucial role in controlling the size, shape and crystalline structure of GNPs. In addition, it is demonstrated that chitosan can perform as a scaffold for the assembly of GNPs, which were successfully applied as substrate for surface-enhanced Raman scattering (SERS). To test the SERS activity, a relevant biological molecule--tryptophan--was adopted as the analyte.