Detailed correlations between SERS enhancement and plasmon resonances in subwavelength closely spaced Au nanorod arrays

Single-Cell Analysis with Silver-Coated Pipette by Combined SERS and SICM

The study of individual cell processes that occur both on their surface and inside is highly interesting for the development of new medical drugs, cytology and cell technologies. This work presents an original technique for fabricating the silver-coated pipette and its use for the cell analysis by combination with surface-enhanced Raman spectroscopy (SERS) and scanning ion-conducting microscopy (SICM). Unlike the majority of other designs, the pipette opening in our case remains uncovered, which is important for SICM. SERS-active Ag nanoparticles on the pipette surface are formed by vacuum-thermal evaporation followed by annealing. An array of nanoparticles had a diameter on the order of 36 nm and spacing of 12 nm. A two-particle model based on Laplace equations is used to calculate a theoretical enhancement factor (EF). The surface morphology of the samples is investigated by scanning electron microscopy while SICM is used to reveal the surface topography, to evaluate Young's modulus of living cells and to control an injection of the SERS-active pipettes into them. A Raman microscope-spectrometer was used to collect characteristic SERS spectra of cells and cell components. Local Raman spectra were obtained from the cytoplasm and nucleus of the same HEK-293 cancer cell. The EF of the SERS-active pipette was 7 × 105. As a result, we demonstrate utilizing the silver-coated pipette for both the SICM study and the molecular composition analysis of cytoplasm and the nucleus of living cells by SERS. The probe localization in cells is successfully achieved.

Toward microfluidic SERS and EC-SERS applications via tunable gold films over nanospheres

Many promising applications of surface-enhanced Raman scattering (SERS), such as microfluidic SERS and electrochemical (EC)-SERS, require immersion of plasmonic nanostructured films in aqueous media. Correlational investigations of the optical response and SERS efficiency of solid SERS substrates immersed in water are absent in the literature. This work presents an approach for tuning the efficiency of gold films over nanospheres (AuFoN) as SERS substrates for applications in aqueous environment. AuFoN are fabricated by convective self-assembly of colloidal polystyrene nanospheres of various diameters (300-800 nm), followed by magnetron sputtering of gold films. The optical reflectance of the AuFoN and Finite-Difference Time-Domain simulations in both water and air reveal the dependence of the surface plasmon band on nanospheres' diameter and environment. SERS enhancement of a common Raman reporter on AuFoN immersed in water is analyzed under 785 nm laser excitation, but also using the 633 nm line for the films in air. The provided correlations between the SERS efficiency and optical response in both air and water indicate the best structural parameters for high SERS efficiency and highlight a route for predicting and optimizing the SERS response of AuFoN in water based on the behavior in air, which is more practical. Finally, the AuFoN are successfully tested as electrodes for EC-SERS detection of the thiabendazole pesticide and as SERS substrates integrated in a flow-through microchannel format. The obtained results represent an important step toward the development of microfluidic EC-SERS devices for sensing applications.

Cellulose Acetate-Based Plasmonic Crystals for Surface-Enhanced Raman and Fluorescence Spectroscopy

In order to meet environmental concerns, there is an increasing demand for biodegradable and sustainable materials in many areas, including photonics. Cellulose and its derivatives are potentially eco-friendly alternatives to conventional plastics, because of their abundance and lower environmental impact. Here, we report the fabrication of plasmonic structures by molding cellulose acetate into submicrometric periodic lattices, using soft lithography. The fabricated platforms can be used for the enhancement of Raman and fluorescence signals of a range of analytes including a model immunoassay utilizing a streptavidin-conjugated dye, which is characterized by a 23-fold enhancement in fluorescence signal intensity, which shows the potential of the platform to be further used for the assay-based development of diagnostic tools.

Plasmonic Pollen Grain Nanostructures: A Three-Dimensional Surface-Enhanced Raman Scattering (SERS)-Active Substrate

A new route has been developed to design plasmonic pollen grain-like nanostructures (PGNSs) as surface-enhanced Raman scattering (SERS)-active substrate. The nanostructures consisting of silver (Ag) and gold (Au) nanoparticles along with zinc oxide (ZnO) nanoclusters as spacers were found highly SERS-active. The morphology of PGNSs and those obtained in the intermediate stage along with each elemental evolution has been investigated by a high-resolution field emission scanning electron microscopy. The optical band gaps and crystal structure have been identified by UV-vis absorption and X-ray powder diffraction (XRD) measurements, respectively. For PGNSs specimen, three distinct absorption bands related to constituent elements Ag, Au, and ZnO were observed, whereas XRD peaks confirmed the existence of Ag, Au, and ZnO within the composition of PGNSs. SERS-activity of PGNSs was confirmed using Rhodamine 6G (R6G) as Raman-active dyes. Air-cooled solid-state laser kits of 532 nm were used as excitation sources in SERS measurements. SERS enhancement factor was estimated for PGNSs specimen and was found as high as 3.5×10⁶ . Finite difference time domain analysis was carried out to correlate the electromagnetic (EM) near-field distributions with the experiment results achieved under this investigation. EM near-field distributions at different planes were extracted for s-, p- and 45° of incident polarizations. EM near-field distributions for such nanostructures as well as current density distributions under different circumstances were demonstrated and plausible scenarios were elucidated given SERS enhancements. Such generic fabrication route as well as correlated investigation is not only indispensable to realize the potential of SERS applications but also unveil the underneath plasmonic characteristics of complex SERS-active nanostructures.

Concurrent Imaging of Surface-Enhanced Raman and Mie Scattering from Built-in Nanogap Plasmonic Particles

We report a bimodal imaging method that can spatially resolve and concurrently correlate SERS and background-free Mie scattering signals. By examining two types of nanoparticle assemblies with different types of plasmonic junctions, namely raspberry-like metamolecules (raspberry-MMs) containing intraparticle nanogaps and groups of Au nanocubes forming interparticle gaps, we were able to rapidly screen SERS-active particles among the entire population of nanoparticles. Ratiometric analysis of SERS/Mie scattering revealed distinct behaviors for these intra- and interparticle nanogaps. In particular, raspberry-MMs showed a high fraction of SERS-active particles with the SERS intensity essentially insensitive to the nanoparticle aggregation state and a predictable environmental dependence. In comparison, nanocube clusters exhibited highly heterogeneous SERS/Mie scattering ratios and unpredictable intensity fluctuations but higher maximum SERS intensity. This dual-imaging approach constitutes an in situ visualization tool that enables simultaneous and stoichiometric analysis of dual signals consisting of elastic and inelastic scattering, which can significantly improve the reliability of SERS measurements.

Detection of organic dyes by surface-enhanced Raman spectroscopy using plasmonic NiAg nanocavity films

This work presents the NiAg nanocavity film for the detection of organic dyes by surface-enhanced Raman spectroscopy (SERS). Nanocavity films were prepared by colloidal lithography using 518-nm polystyrene spheres combined with the electrochemical deposition of Ni supporting layer and Ag nanoparticles homogeneous SERS-active layer. The theoretical study was modelled by finite-difference time-domain (FDTD) simulation of electromagnetic field enhancement near the nanostructured surface and experimentally proven by SERS measurement of selected organic dyes (rhodamine 6G, crystal violet, methylene blue, and malachite green oxalate) in micromolar concentration. Furthermore, the concentration dependence was investigated to prove the suitability of NiAg nanocavity films to detect ultra-low concentrations of samples. The detection limit was 1.3 × 10^-12, 1.5 × 10^-10, 1.4 × 10^-10, 7.5 × 10^-11 mol·dm^-3, and the standard deviation was 20.1%, 13.8%, 16.7%, and 19.3% for R6G, CV, MB, and MGO, respectively. The analytical enhancement factor was 3.4 × 10⁵ using R6G as a probe molecule. The principal component analysis (PCA) was performed to extract the differences in complex spectra of the dyes where the first and second PCs carry 42.43% and 31.39% of the sample variation, respectively. The achieved results demonstrated the suitability of AgNi nanocavity films for the SERS-based detection of organic dyes, with a potential in other sensing applications.

Abnormal dewetting of Ag layer on three-dimensional ITO branches to form spatial plasmonic nanoparticles for organic solar cells

Three-dimensional (3D) plasmonic structures have attracted great attention because abnormal wetting behavior of plasmonic nanoparticles (NPs) on 3D nanostructure can enhance the localized surface plasmons (LSPs). However, previous 3D plasmonic nanostructures inherently had weak plasmonic light absorption, low electrical conductivity, and optical transmittance. Here, we fabricated a novel 3D plasmonic nanostructure composed of Ag NPs as the metal for strong LSPs and 3D nano-branched indium tin oxide (ITO BRs) as a transparent and conductive framework. The Ag NPs formed on the ITO BRs have a more dewetted behavior than those formed on the ITO films. We experimentally investigated the reasons for the dewetting behavior of Ag NPs concerning the geometry of ITO BRs. The spherical Ag NPs are spatially separated and have high density, thereby resulting in strong LSPs. Finite-domain time-difference simulation evidenced that spatially-separated, high-density and spherical Ag NPs formed on ITO BRs dramatically boost the localized electric field in the active layer of organic solar cells (OSCs). Photocurrent of PTB7:PCBM OSCs with the ITO BRs/Ag NPs increased by 14%.

Reducing the loss of electric field enhancement for plasmonic core-shell nanoparticle dimers by high refractive index dielectric coating

Plasmonic nanoparticle (NP) dimers, generating highly intense areas of electric field enhancement named hot spots, have been playing a vital role in various applications like surface enhanced Raman scattering. For stabilization and functionalization, such metallic NPs are often coated with dielectric shells, yet suffer from a rapid degeneration of the hot spot with the increase of the shell thickness. Herein, it is demonstrated that the use of appropriately high refractive dielectric coatings can greatly reduce the loss of local electric field enhancement, maintaining usable hot spots. Two mechianisms work synergistically. Firstly, the high refractive index dielectric coating enables a great leap of the local electric fields reaching the gap, which follows the boundary conditions at the interface within electrodynamics. Secondly, owing to its strong Mie resonances that can participate in the plasmon hybridization, the high refractive index dielectric coating contributes to a strong light coupling effect in terms of improving the light absorption. Taking advantage of the proposed physical process decomposition, both the resonance shift and local electric field enhancement can be elaborated. These findings should be of significant importance in extended applications of surface enhanced spectroscopies and related plasmonic devices based on hot spots.

Standing wave type localized surface plasmon resonance of multifold Ag nanorods

Multifold Ag nanorods (AgNRs) have demonstrated great potentials in applications such as surface-enhanced Raman scattering technique due to their specially organized nanostructures. However, there is so far no systematic understanding of their localized surface plasmon resonance (LSPR) behaviors. This work comprehensively studied the plasmonic behaviors of AgNRs with 1, 2 and 3 folded arms. LSPR modes with charge oscillations resembling standing waves were excited in all nanostructures. As arm length increases, there were linear relationships between resonance wavelength and arm length, which applied to all LSPR modes studied. In addition, directly proportional relationships between the slopes of the linear functions and arm number were found for same order LSPR modes of AgNRs. For different modes of a specific AgNR, inversely proportional relationships between the slope and the resonance order N were discovered. These findings evidenced AgNR's standing wave type LSPR characteristics.

Unexpected large nanoparticle size of single dimer hotspot systems for broadband SERS enhancement

We have numerically demonstrated the feasibility and possibility to achieve broadband surface-enhanced Raman scattering (SERS) enhancement in the visible and near-infrared wavelength range using single nanoparticle (NP) dimer hotspot systems. Instead of the conventionally reported sub-100 nm, we find that the optimal NP size is as large as 200 nm in diameter for both Ag and Au. The key lies in the continuous arising of the bonding dipole plasmon mode and higher-order resonances at shorter wavelengths. Further, it is revealed that the near- and far-field optical responses of these hotspot systems correlate well with each other, despite the intrinsic enormous near- to far-field redshift for individual large NPs. The physical principles demonstrated here benefit significantly the fundamental understanding and engineering optimization of broadband SERS substrates.