Combination of Porous Silk Fibroin Substrate and Gold Nanocracks as a Novel SERS Platform for a High-Sensitivity Biosensor

Novel concepts for developing a surface-enhanced Raman scattering (SERS) sensor based on biocompatible materials offer great potential in versatile applications, including wearable and in vivo monitoring of target analytes. Here, we report a highly sensitive SERS sensor consisting of a biocompatible silk fibroin substrate with a high porosity and gold nanocracks. Our silk-based SERS detection takes advantage of strong local field enhancement in the nanoscale crack regions induced by gold nanostructures evaporated on a porous silk substrate. The SERS performance of the proposed sensor is evaluated in terms of detection limit, sensitivity, and linearity. Compared to the performance of a counterpart SERS sensor with a thin gold film, SERS results using 4-ABT analytes present that a significant improvement in the detection limit and sensitivity by more than 4 times, and a good linearity and a wide dynamic range is achieved. More interestingly, overlap is integral, and a quantitative measure of the local field enhancement is highly consistent with the experimental SERS enhancement.

Plasmon-Modulated Excitation-Dependent Fluorescence and Antimicrobial Activities of Silver and Gold Nanoparticles Prepared with Eugenia unifloraL. extracts

Green synthesis using plant extract is a sustainable method to obtain silver and gold nanoparticles (Ag and AuNPs) and was employed in this work. The Eugenia uniflora L. fruits and leaves extracts were used in nanoparticles synthesis. The photoreduction process with a xenon lamp and pH control improved optical properties and nanoparticles stability. The UV-vis, TEM, FTIR, and Zeta potential of the prepared solutions were obtained. The fluorescence spectra of Ag and AuNPs were investigated at different excitation wavelengths, which showed two kinds of fluorescence peaks. The shorter wavelength peaks red-shift with the increasing excitation wavelength, which results from the electron interband transitions, and the longer fixed wavelength peaks due to the local field enhancement. Finally, the antimicrobial tests were performed with Gram-negative and Gram-positive bacteria and Candida albicans. The best results were obtained with EuAgNPs prepared with fruits extract, photoreduction, and pH 7.0 (with a mean of 95.12% ± 10.29% of inhibition).

Ponderomotive Forces in the System of Two Nanoparticles

Mechanical consequences of electromagnetic interaction of two nanoparticles have been studied theoretically. It has been shown that local field enhancement effect, which causes appearance of the local field gradients, can lead to ponderomotive forces acting on the nanoparticles. Distribution of the local field in the system has been calculated and ponderomotive forces directions and values were evaluated. It has been shown that in the system of two different-sized nanoparticles the forces act mainly on the surface of bigger nanoparticle and for some systems its value may acquire up to several tens of nanonewtons. Possible application of the results to study of biological systems has been also discussed.

Comparative Simulations of Conductive Nitrides as Alternative Plasmonic Nanostructures for Solar Cells

Particle layers employing conductive transition metal nitrides have been proposed as possible alternative plasmonic materials for photovoltaic applications due to their reduced losses compared to metal nanostructures. We critically compare the potential photocurrent gain from an additional layer made of nanopillars of nitrides with other material classes obtained in an optimized c-Si baseline solar cell, considering an experimental doping profile. A relative photocurrent gain enhancement of on average 5% to 10% is observed, achieving for a few scenarios around 30% gain. The local field enhancement is moderate around the resonances for nitrides which spread over the whole ultraviolet and visible range. We can characterize two types of nitrides: nitrides for which the shading effect remains a problem similar to for metals, and others which behave like dielectric scatterers with high photocurrent gain.

Nanofocusing: reaching out

Nanofocusing, the term coined by Mark Stockman, has been observed in many different tapered waveguide configurations, demonstrating the possibility for optical modes to be efficiently delivered to and concentrated into nanoscale regions far beyond the diffraction limit in dielectric media. Strong and broadband local field enhancement and slowdown effects associated with the nanofocusing have been exploited for enhancing linear and nonlinear optical phenomena and reaching out to a broad spectrum of diverse applications, from electron generation to water vaporization. Starting with the historical background, we carefully elaborate on the basic concepts and mechanisms involved. We then provide examples of the latest developments in this exciting quest for bringing the fundamental physical phenomenon of nanofocusing into the realm of practical applications in modern nanotechnology.

Computational and Experimental Analysis of Gold Nanorods in Terms of Their Morphology: Spectral Absorption and Local Field Enhancement

A nanoparticle’s shape and size determine its optical properties. Nanorods are nanoparticles that have double absorption bands associated to surface plasmon oscillations along their two main axes. In this work, we analize the optical response of gold nanorods with numerical simulations and spectral absorption measurements to evaluate their local field enhancement—which is key for surface-enhanced Raman spectroscopic (SERS) applications. Our experimental results are in good agreement with finite element method (FEM) simulations for the spectral optical absorption of the nanoparticles. We also observed a strong dependence of the optical properties of gold nanorods on their geometrical dimension and shape. Our numerical simulations helped us reveal the importance of the nanorods’ morphology generated during the synthesis stage in the evaluation of absorption and local field enhancement. The application of these gold nanorods in surface-enhancement Raman spectroscopy is analyzed numerically, and results in a 5.8×104 amplification factor when comparing the values obtained for the nanorod deposited on a dielectric substrate compared to the nanorod immersed in water.

Time-resolved and Temperature-dependent Broadband Emission of Plasmon-coupled Quantum Dots

The broadband photoluminescence (PL) emissions from CdSe QDs and plasmon-coupled QDs were characterized with time-resolved and temperature-dependent spectroscopy for the application of solid-state white light. The origin of broad spectral emission includes the transitions from bandedge and surface-trapped states. The emission intensity enhancement of plasmon-coupled QDs with respect to that of bare QDs is attributable to the reduction of nonradiative decay and the local field enhancement with plasmon-exciton coupling through the Coulomb interaction. The temperature-dependent and time-resolved PL spectroscopy revealed the existence of selective contribution strength of both the local field enhancement and the reduction of nonradiative decay with plasmon-exciton coupling at different spectral regions.