Optimization Based on the Surface Plasmon Optical Properties of Adjustable Metal Nano-Microcavity System for Biosensing

This paper mainly studies the plasma optical properties of the silver nanorod and gold film system with gap structure. During the experiment, the finite element analysis method and COMSOL Multiphysics are used for modeling and simulation. The study changes the thickness of the PE spacer layer between the silver nanorod and the gold film, the conditions of the incident light and the surrounding environment medium. Due to the anisotropic characteristics of silver nanorod, the microcavity system is extremely sensitive to the changes of internal and external conditions, and the system exhibits strong performance along the long axis of the nanorod. By analyzing the extinction spectrum of the nanoparticle and the electric field section diagrams at resonance peak, it is found that the plasma optical properties of the system greatly depend on the gap distance, and the surrounding electric field of the silver nanorod is confined in the gap. Both ends of the nanorod and the gap are distributed with high concentrations of hot spots, which reflects the strong hybridization of multiple resonance modes. Under certain excitation conditions, the plasma hybridization behavior will produce a multi-pole mode, and the surface electric field distribution of the nanorod reflects the spatial directionality. In addition, the system is also highly sensitive to the environmental media, which will cause significant changes in its optical properties. The plasma microcavity system with silver nanorod and gold film studied in this paper can be used to develop high-sensitivity biosensors, which has great value in the field of biomedical detection.

Au@Ag Dendritic Nanoforests for Surface-Enhanced Raman Scattering Sensing

The effects of Au cores in Ag shells in enhancing surface-enhanced Raman scattering (SERS) were evaluated with samples of various Au/Ag ratios. High-density Ag shell/Au core dendritic nanoforests (Au@Ag-DNFs) on silicon (Au@Ag-DNFs/Si) were synthesized using the fluoride-assisted Galvanic replacement reaction method. The synthesized Au@Ag-DNFs/Si samples were characterized using scanning electron microscopy, energy-dispersive X-ray spectroscopy, reflection spectroscopy, X-ray diffraction, and Raman spectroscopy. The ultraviolet-visible extinction spectrum exhibited increased extinction induced by the addition of Ag when creating the metal DNFs layer. The pure Ag DNFs exhibited high optical extinction of visible light, but low SERS response compared with Au@Ag DNFs. The Au core (with high refractive index real part) in Au@Ag DNFs maintained a long-leaf structure that focused the illumination light, resulting in the apparent SERS enhancement of the Ag coverage.

High-resolution optical constants of crystalline ammonium nitrate for infrared remote sensing of the Asian Tropopause Aerosol Layer

Infrared spectroscopic observations have shown that crystalline ammonium nitrate (AN) particles are an abundant constituent of the upper tropospheric aerosol layer which is formed during the Asian summer monsoon period, the so-called Asian Tropopause Aerosol Layer (ATAL). At upper tropospheric temperatures, the thermodynamically stable phase of AN is different from that at 298 K, meaning that presently available room-temperature optical constants of AN, that is, the real and imaginary parts of the complex refractive index, cannot be applied for the quantitative analysis of these infrared measurements. In this work, we have retrieved the first low-temperature data set of optical constants for crystalline AN in the 800–6000 cm−1 wavenumber range with a spectral resolution of 0.5 cm−1. The optical constants were iteratively derived from an infrared extinction spectrum of 1 µm sized AN particles suspended in a cloud chamber at 223 K. The uncertainties of the new data set were carefully assessed in a comprehensive sensitivity analysis. We show that our data accurately fit aircraft-borne infrared measurements of ammonium nitrate particles in the ATAL.

Оптическое поглощение гибридных молекулярных кластеров С-=SUB=-32-=/SUB=-Н-=SUB=-24-=/SUB=-, С-=SUB=-32-=/SUB=-Н-=SUB=-36-=/SUB=- на основе фрагментов даймондена и графена

Using the density functional theory (DFT), the geometry optimised for molecular clusters C32H24 and C32H36, constructed with a hybrid of fragments of diamondene and graphene with the dangling bonds passivated with hydrogen. The time-dependent DFT allowed calculating molar extinction spectra of the clusters. The comparison of the calculated spectra with the results of astrophysical observations evidence on a possible contribution of the clusters to the extinction spectrum of light by the carbon-based constituent of the interstellar medium.

Plasmon extinguishment by bandedge shift identified as a second-order spectroscopic differentiation

Optical excitation of metallic nanostructures induces strong intraband transitions, leaving transient depletion below the Fermi level, which allows transient interband transition to this depletion band. This is equivalent to the lowering of the threshold for interband transitions and pushes the plasmonic band to the red. As a result, localized surface plasmon resonance (LSPR) is “extinguished” or “quenched” around the bandedge, in contrast, the interband optical absorption becomes enhanced and redshifted. The corresponding transient absorption (TA) signals have equal lifetimes and opposite signs. Moreover, the TA spectrum is found to be a second-order differential of the steady-state optical extinction spectrum over the studied band. This is a commonly existing mechanism for metallic nanostructures and verified with gold in this work. Such a discovery is completely different from the optical-excitation-induced redshift of LSPR through enhanced electronic scattering and is important for understanding the ultrafast spectroscopic response of plasmonic nanostructures with clear photophysical insights, supplying solid basis for exploring optical logic device and optical data processing techniques.

Colorimetric Test for Fast Detection of SARS-CoV-2 in Nasal and Throat Swabs

Mass testing is fundamental to face the pandemic caused by the coronavirus SARS-CoV-2 discovered at the end of 2019. To this aim, it is necessary to establish reliable, fast and cheap tools to detect viral particles in biological material so to identify the people capable to spread the infection. We demonstrate that a colorimetric biosensor based on gold nanoparticle (AuNP) interaction induced by SARS-CoV-2 lends itself as an outstanding tool for detecting viral particles in nasal and throat swabs. The extinction spectrum of a colloidal solution of multiple viral-target gold nanoparticles - AuNPs functionalized with antibodies targeting three surface proteins of SARS-CoV-2 (spike, envelope and membrane) - is redshifted in few minutes when mixed to a solution containing the viral particle. The optical density of the mixed solution measured at 560 nm was compared to the threshold cycle (Ct) of a Real Time-PCR (gold standard for detecting the presence of viruses) finding that the colorimetric method is able to detect very low viral load with a detection limit approaching that of RT-PCR. Since the method is sensitive to the infecting viral particle rather than to its RNA, the achievements reported here open new perspective not only in the context of the current and possible future pandemics, but also in microbiology as the biosensor proves itself to be a powerful though simple tool for measuring the viral particle concentration.

High-resolution optical constants of crystalline ammonium nitrate for infrared remote sensing of the Asian Tropopause Aerosol Layer

Infrared spectroscopic observations have shown that crystalline ammonium nitrate (AN) particles are an abundant constituent of the upper tropospheric aerosol layer which is formed during the Asian summer monsoon period, the so-called Asian Tropopause Aerosol Layer (ATAL). At upper tropospheric temperatures, the thermodynamically stable phase of AN is different from that at 298 K, meaning that presently available room-temperature optical constants of AN, that is, the real and imaginary parts of the complex refractive index, cannot be applied for the quantitative analysis of these infrared measurements. In this work, we have retrieved the first low-temperature data set of optical constants for crystalline AN in the 800–6000 cm−1 wavenumber range with a spectral resolution of 0.5 cm−1. The optical constants were iteratively derived from an infrared extinction spectrum of 1 micrometer-sized AN particles suspended in a cloud chamber at 223 K. The uncertainties of the new data set were carefully assessed in a comprehensive sensitivity analysis. We show that our data accurately fit aircraft-borne infrared measurements of ammonium nitrate particles in the ATAL.

Combined extinction and absorption UV–vis spectroscopy reveals shape imperfections of metallic nanoparticles

Copyright © 2019 American Chemical Society. Metallic nanoparticle solutions are routinely characterized by measuring their extinction spectrum (with UV-vis spectroscopy). Theoretical predictions such as Mie theory for spheres can then be used to infer important properties, such as particle size and concentration. Here we highlight the benefits of measuring not only the extinction (the sum of absorption and scattering) but also the absorption spectrum (which excludes scattering) for routine characterization of metallic nanoparticles. We use an integrating sphere-based method to measure the combined extinction-absorption spectra of silver nanospheres and nanocubes. Using a suite of electromagnetic modeling tools (Mie theory, T-matrix, surface integral equation methods), we show that the absorption spectrum, in contrast to extinction, is particularly sensitive to shape imperfections such as roughness, faceting, or edge rounding. We study in detail the canonical case of silver nanospheres, where small discrepancies between experimental and calculated extinction spectra are still common and often overlooked. We show that this mismatch between theory and experiment becomes much more important when considering the absorption spectrum and can no longer be dismissed as experimental imperfections. We focus in particular on the quadrupolar localized plasmon resonance of silver nanospheres, which is predicted to be very prominent in the absorption spectrum but is not observed in our experiments. We consider and discuss a number of possible explanations to account for this discrepancy, including changes in the dielectric function of Ag, size polydispersity, and shape imperfections such as elongation, faceting, and roughness. We are able to pinpoint faceting and roughness as the likely causes for the observed discrepancy. A similar analysis is carried out on silver nanocubes to demonstrate the generality of this conclusion. We conclude that the absorption spectrum is in general much more sensitive to the fine details of a nanoparticle geometry, compared to the extinction spectrum. The ratio of extinction to absorption also provides a sensitive indicator of size for many types of nanoparticles, much more reliably than any observed plasmon resonance shifts. Overall, this work demonstrates that combined absorption-extinction measurements provide a much richer characterization tool for metallic nanoparticles.

Combined extinction and absorption UV–vis spectroscopy reveals shape imperfections of metallic nanoparticles

Copyright © 2019 American Chemical Society. Metallic nanoparticle solutions are routinely characterized by measuring their extinction spectrum (with UV-vis spectroscopy). Theoretical predictions such as Mie theory for spheres can then be used to infer important properties, such as particle size and concentration. Here we highlight the benefits of measuring not only the extinction (the sum of absorption and scattering) but also the absorption spectrum (which excludes scattering) for routine characterization of metallic nanoparticles. We use an integrating sphere-based method to measure the combined extinction-absorption spectra of silver nanospheres and nanocubes. Using a suite of electromagnetic modeling tools (Mie theory, T-matrix, surface integral equation methods), we show that the absorption spectrum, in contrast to extinction, is particularly sensitive to shape imperfections such as roughness, faceting, or edge rounding. We study in detail the canonical case of silver nanospheres, where small discrepancies between experimental and calculated extinction spectra are still common and often overlooked. We show that this mismatch between theory and experiment becomes much more important when considering the absorption spectrum and can no longer be dismissed as experimental imperfections. We focus in particular on the quadrupolar localized plasmon resonance of silver nanospheres, which is predicted to be very prominent in the absorption spectrum but is not observed in our experiments. We consider and discuss a number of possible explanations to account for this discrepancy, including changes in the dielectric function of Ag, size polydispersity, and shape imperfections such as elongation, faceting, and roughness. We are able to pinpoint faceting and roughness as the likely causes for the observed discrepancy. A similar analysis is carried out on silver nanocubes to demonstrate the generality of this conclusion. We conclude that the absorption spectrum is in general much more sensitive to the fine details of a nanoparticle geometry, compared to the extinction spectrum. The ratio of extinction to absorption also provides a sensitive indicator of size for many types of nanoparticles, much more reliably than any observed plasmon resonance shifts. Overall, this work demonstrates that combined absorption-extinction measurements provide a much richer characterization tool for metallic nanoparticles.