Plasmonic Hot Spots Research Articles

Surface potential modulation as a tool for mitigating challenges in SERS-based microneedle sensors

Raman spectroscopic-based biosensing strategies are often complicated by low signal and the presence of multiple chemical species. While surface-enhanced Raman spectroscopy (SERS) nanostructured platforms are able to deliver high quality signals by focusing the electromagnetic field into a tight plasmonic hot-spot, it is not a generally applicable strategy as it often depends on the specific adsorption of the analyte of interest onto the SERS platform. This paper describes a strategy to address this challenge by using surface potential as a physical binding agent in the context of microneedle sensors. We show that the potential-dependent adsorption of different chemical species allows scrutinization of the contributions of different chemical species to the final spectrum, and that the ability to cyclically adsorb and desorb molecules from the surface enables efficient application of multivariate analysis methods. We demonstrate how the strategy can be used to mitigate potentially confounding phenomena, such as surface reactions, competitive adsorption and the presence of molecules with similar structures. In addition, this decomposition helps evaluate criteria to maximize the signal of one molecule with respect to others, offering new opportunities to enhance the measurement of analytes in the presence of interferants.

Nanoscale Imaging of Plasmonic Hot Spots in Au Nanocapsule Dimers by Ultrafast Electron Microscopy

Nanoscale Imaging of Plasmonic Hot Spots in Au Nanocapsule Dimers by Ultrafast Electron Microscopy

Plasmonic hot spots reveal local conformational transitions induced by DNA double-strand breaks

DNA double-strand breaks (DSBs) are typical DNA lesions that can lead to cell death, translocations, and cancer-driving mutations. The repair process of DSBs is crucial to the maintenance of genomic integrity in all forms of life. However, the limitations of sensitivity and special resolution of analytical techniques make it difficult to investigate the local effects of chemotherapeutic drugs on DNA molecular structure. In this work, we exposed DNA to the anticancer antibiotic bleomycin (BLM), a damaging factor known to induce DSBs. We applied a multimodal approach combining (i) atomic force microscopy (AFM) for direct visualization of DSBs, (ii) surface-enhanced Raman spectroscopy (SERS) to monitor local conformational transitions induced by DSBs, and (iii) multivariate statistical analysis to correlate the AFM and SERS results. On the basis of SERS results, we identified that bands at 1050 cm−1 and 730 cm−1 associated with backbone and nucleobase vibrations shifted and changed their intensities, indicating conformational modifications and strand ruptures. Based on averaged SERS spectra, the PLS regressions for the number of DSBs caused by corresponding molar concentrations of bleomycin were calculated. The strong correlation (R2 = 0.92 for LV = 2) between the predicted and observed number of DSBs indicates, that the model can not only predict the number of DSBs from the spectra but also detect the spectroscopic markers of DNA damage and the associated conformational changes.

Generation and Detection of Strain-Localized Excitons in WS2 Monolayer by Plasmonic Metal Nanocrystals.

Excitons in a transition-metal dichalcogenide (TMDC) monolayer can be modulated through strain with spatial and spectral control, which offers opportunities for constructing quantum emitters for applications in on-chip quantum communication and information processing. Strain-localized excitons in TMDC monolayers have so far mainly been observed under cryogenic conditions because of their subwavelength emission area, low quantum yield, and thermal-fluctuation-induced delocalization. Herein, we demonstrate both generation and detection of strain-localized excitons in WS2 monolayer through a simple plasmonic structure design, where WS2 monolayer covers individual Au nanodisks or nanorods. Enhanced emission from the strain-localized excitons of the deformed WS2 monolayer near the plasmonic hotspots is observed at room temperature with a photoluminescence energy redshift up to 200 meV. The emission intensity and peak energy of the strain-localized excitons can be adjusted by the nanodisk size. Furthermore, the excitation and emission polarization of the strain-localized excitons are modulated by anisotropic Au nanorods. Our results provide a promising strategy for constructing nonclassical integrated light sources, high-sensitivity strain sensors, or tunable nanolasers for future dense nanophotonic integrated circuits.

Uncovering strong π-metal interactions on Ag and Au nanosurfaces under ambient conditions via in-situ surface-enhanced Raman spectroscopy

Uncovering strong π-metal interactions on Ag and Au nanosurfaces under ambient conditions via in-situ surface-enhanced Raman spectroscopy

Maximizing the Accessibility in DNA Origami Nanoantenna Plasmonic Hotspots

AbstractDNA nanotechnology has conquered the challenge of positioning quantum emitters in the hotspot of optical antenna structures for fluorescence enhancement. Therefore, DNA origami serves as the scaffold to arrange nanoparticles and emitters, such as fluorescent dyes. For the next challenge of optimizing the applicability of plasmonic hotspots for molecular assays, a Trident DNA origami structure that increases the accessibility of the hotspot is introduced, thereby improving the kinetics of target molecule binding. This Trident NanoAntenna with Cleared HOtSpot (NACHOS) is compared with previous DNA origami nanoantennas and improved hotspot accessibility is demonstrated without compromising fluorescence enhancement. The approach taps into the potential of Trident NACHOS for single‐molecule‐based plasmonic biosensing.

Optical Trapping of Plasmonic Nanoparticles for In Situ Surface-Enhanced Raman Spectroscopy Characterizations.

Surface-enhanced Raman spectroscopy (SERS) enables the ultrasensitive detection of analyte molecules in various applications due to the enhanced electric field of metallic nanostructures. Salt-induced silver nanoparticle aggregation is the most popular method for generating SERS-active substrates; however, it is limited by poor reproducibility, stability, and biocompatibility. The present protocol integrates optical manipulation and SERS detection to develop an efficient analytical platform to address this. A 1064 nm trapping laser and a 532 nm Raman probe laser are combined in a microscope to assemble silver nanoparticles, which generate plasmonic hotspots for in situ SERS measurements in aqueous environments. Without aggregating agents, this dynamic plasmonic silver nanoparticle assembly enables an approximately 50-fold enhancement of the analyte molecule signal. Moreover, it provides spatial and temporal control to form the SERS-active assembly in as low as 0.05 nM analyte-coated silver nanoparticle solution, which minimizes the potential perturbation for in vivo analysis. Hence, this optical trapping-integrated SERS platform holds great potential for efficient, reproducible, and stable molecular analyses in liquids, especially in aqueous physiological environments.

Optical Trapping of Plasmonic Nanoparticles for <em>In Situ</em> Surface-Enhanced Raman Spectroscopy Characterizations

Surface-enhanced Raman spectroscopy (SERS) enables the ultrasensitive detection of analyte molecules in various applications due to the enhanced electric field of metallic nanostructures. Salt-induced silver nanoparticle aggregation is the most popular method for generating SERS-active substrates; however, it is limited by poor reproducibility, stability, and biocompatibility. The present protocol integrates optical manipulation and SERS detection to develop an efficient analytical platform to address this. A 1064 nm trapping laser and a 532 nm Raman probe laser are combined in a microscope to assemble silver nanoparticles, which generate plasmonic hotspots for in situ SERS measurements in aqueous environments. Without aggregating agents, this dynamic plasmonic silver nanoparticle assembly enables an approximately 50-fold enhancement of the analyte molecule signal. Moreover, it provides spatial and temporal control to form the SERS-active assembly in as low as 0.05 nM analyte-coated silver nanoparticle solution, which minimizes the potential perturbation for in vivo analysis. Hence, this optical trapping-integrated SERS platform holds great potential for efficient, reproducible, and stable molecular analyses in liquids, especially in aqueous physiological environments.

Engineering Graphene Grain Boundaries for Plasmonic Multi-Excitation and Hotspots.

Surface plasmons, merging photonics and electronics in nanoscale dimensions, have been the cornerstones in integrated informatics, precision detection, high-resolution imaging, and energy conversion. Arising from the exceptional Fermi-Dirac tunability, ultrafast carrier mobility, and high-field confinement, graphene offers excellent advantages for plasmon technologies and enables a variety of state-of-the-art optoelectronic applications ranging from tight-field-enhanced light sources, modulators, and photodetectors to biochemical sensors. However, it is challenging to co-excite multiple graphene plasmons on one single graphene sheet with high density, a key step toward plasmonic wavelength-division multiplexing and next-generation dynamical optoelectronics. Here, we report the heteroepitaxial growth of a polycrystalline graphene monolayer with patterned gradient grain boundary density, which is synthesized by creating diverse nanosized local growth environments on a centimeter-scale substrate with a polycrystalline graphene ring seed in chemical vapor deposition. Such geometry enables plasmonic co-excitation with varied wavelength diversification in the nanoscale. Via using high-resolution scanning near-field optical microscopy, we demonstrate rich plasmon standing waves, even bright plasmonic hotspots with a size up to 3 μm. Moreover, by changing the grain boundary density and annealing, we find the local plasmonic wavelengths are widely tunable, from 70 to 300 nm. Theoretical modeling supports that such plasmonic versatility is due to the grain boundary-induced plasmon-phonon interactions through random phase approximation. The seed-induced heteroepitaxial growth provides a promising way for the grain boundary engineering of two-dimensional materials, and the controllable grain boundary-based plasmon co-generation and manipulation in one single graphene monolayer will facilitate the applications of graphene for plasmonics and nanophotonics.

Plasmonic enhancement of exciton and trion photoluminescence in 2D MoS2 decorated with Au nanorods: Impact of nonspherical shape

Two-dimensional (2D) MoS2 is emerging as a promising candidate for novel optoelectronics and flexible devices, but the photoluminescence (PL) efficiency is still limited for actual photonic applications. Plasmonic enhancement is the best way to increase the PL emission of 2D materials. In this study, the PL enhancement of 2D MoS2 decorated with Au nanorods (NRs) and nanospheres (NSs) at room temperature is investigated, focusing in particular on the effect of the nonspherical shape of NRs. To have a proper comparison, both nanoparticles have very similar diameters of 13–14 nm, while the NR average length is 81 nm. After decorating with Au NRs and NSs, the average PL enhancement of few-layer MoS2 is about 4.3 and 2 times, respectively. The Raman spectra are similarly enhanced, 3.8 and 2.4 times (A1g mode), respectively. The maximal enhancement is observed for the samples with Au NRs: this feature can be related to highly localized plasmonic hot spots in vicinity of the NR tips. A strong electric field within the hot spots causes the generation of additional free electron-hole pairs and, in turn, the enhancement of exciton excitation and the increase of the PL intensity. Simultaneously, the injection of hot electrons from the metal increases the probability of trions generation, thus contributing to the PL enhancement.

Ultrasensitive Mid-Infrared Biosensing in Aqueous Solutions with Graphene Plasmons.

Identifying nanoscale biomolecules in aqueous solutions by Fourier transform infrared spectroscopy (FTIR) provides an in situ and noninvasive method for exploring the structure, reactions, and transport of biologically active molecules. However, this remains a challenge due to the strong and broad IR absorption of water which overwhelms the respective vibrational fingerprints of the biomolecules. In this work, a tunable IR transparent microfluidic system with graphene plasmons is exploited to identify ≈2nm-thick proteins in physiological conditions. The acquired in situ tunability makes it possible to eliminate the IR absorption of water outside the graphene plasmonic hotspots by background subtraction. Most importantly, the ultrahigh confinement of graphene plasmons (confined to ≈15nm) permits the implementation of nanoscale sensitivity. Then, the deuterium effects on monolayer proteins are characterized within an aqueous solution. The tunable graphene-plasmon-enhanced FTIR technology provides a novel platform for studying biological processes in an aqueous solution at the nanoscale.

Interfacial engineering of plasmonic nanoparticle metasurfaces

SignificanceMolecules interacting with metallic nanostructures can show tunable exciton-plasmon coupling, ranging from weak to strong. One factor that influences the interactions is the spatial organization of the molecules relative to the localized plasmon-enhanced electromagnetic fields. In this work, we show that the arrangement of aromatic dye molecules can be tuned within plasmonic hotspots by interfacial engineering of nanoparticle surfaces. By controlling the local chemical and physical interactions, we could modulate lasing thresholds. Surface-functionalized plasmonic metasurfaces open prospects for programmable light-matter interactions at the nanoscale.

Broadband Nanoscale Surface‐Enhanced Raman Spectroscopy by Multiresonant Nanolaminate Plasmonic Nanocavities on Vertical Nanopillars

AbstractSurface‐enhanced Raman spectroscopy (SERS) has become a sensitive detection technique for biochemical analysis. Despite significant research efforts, most SERS substrates consisting of single‐resonant plasmonic nanostructures on the planar surface suffer from limitations of narrowband SERS operation and unoptimized nano‐bio interface with living cells. Here, it is reported that nanolaminate plasmonic nanocavities on 3D vertical nanopillar arrays can support a broadband SERS operation with large enhancement factors (>106) under laser excitations at 532, 633, and 785 nm. The multi‐band Raman mapping measurements show that nanolaminate plasmonic nanocavities on vertical nanopillar arrays exhibit broadband uniform SERS performance with diffraction‐limited resolution at a single nanopillar footprint. By selective exposure of embedded plasmonic hotspots in individual metal–insulator–metal (MIM) nanogaps, nanoscale broadband SERS operation at the single MIM nanocavity level with visible and near‐infrared (vis–NIR) excitations is demonstrated. Numerical studies reveal that nanolaminate plasmonic nanocavities on vertical nanopillars can support multiple hybridized plasmonic modes to concentrate optical fields across a broadband wavelength range from 500 to 900 nm at the nanoscale.

Resonance Photoluminescence Enhancement of Monolayer MoS2 via a Plasmonic Nanowire Dimer Optical Antenna.

Two-dimensional transition-metal dichalcogenides (TMDs) such as monolayer MoS2 exhibit remarkable optical properties. However, the intrinsic absorption and emission rates of MoS2 are very low, thus severely hindering its application in electronics and photonics. Combining MoS2 with a plasmonic optical antenna is an alternative solution to enhance the emission rates of the 2D semiconductor, and this can drastically increase the photoresponsivity of the corresponding photodetector. Herein, we have constructed a plasmonic gap cavity of a nanowire dimer (NWD) system as an optical antenna to brighten the emission of MoS2 off the hot spot. Different from the conventional enhancement concept which occurred in the plasmonic hot spot, the light emission off the nanogap hot spot was thoroughly investigated. We demonstrate that this new plasmonic optical nanostructure leads to a strong enhancement due to the Purcell effect. The NWD optical antenna can trap light to the near field through a high-efficiency plasmonic gap mode (PGM); then the PL emission was enhanced drastically up to 14.5-fold due to the resonance of the plasmonic gap mode (PGM) in the NWD with the excitonic band of monolayer MoS2. Theoretical simulations reveal that this NWD can alter the efficiency of convergence and excitation, which was consistent with our experimental results. This study can provide a pathway toward enhancing and controlling PGM-enhanced light emission of TMD materials beyond the plasmonic hot spot.

Two-Photon-Excited Single-Molecule Fluorescence Enhanced by Gold Nanorod Dimers.

We demonstrate two-photon-excited single-molecule fluorescence enhancement by single end-to-end self-assembled gold nanorod dimers. We employed biotinylated streptavidin as the molecular linker, which connected two gold nanorods in end-to-end fashion. The typical size of streptavidin of around 5 nm separates the gold nanorods with gaps suitable for the access of fresh dyes in aqueous solution, yet small enough to give very high two-photon fluorescence enhancement. Simulations show that enhancements of more than 7 orders of magnitude can be achieved for two-photon-excited fluorescence in the plasmonic hot spots. With such high enhancements, we successfully detect two-photon-excited fluorescence for a common organic dye (ATTO 610) at the single-molecule, single-nanoparticle level.

Enhanced third-harmonic generation via gold nanospheres localized surface plasmonic resonance

In the progress of nanoengineering of noble metals, the extreme nonlinear optics within nanometric volumes is of great research interest in recent years. In particular, plasmonic nanostructures offer unique route for enhancing high-order harmonic generation and other nonlinear optical progress. Here, we report that strong third harmonic emission in zinc oxide (ZnO) microbelts (MBs) via a strong localization of electromagnetic field caused by localized surface plasmon resonances. In MB/Au-nanoparticles composite, it was demonstrated that the intensity of third-harmonic generation (THG) was enhanced by a factor of five compared to bare MB. Additionally, the polarization characteristics of THG in the same system were illustrated in detail. With respect to the bare MB, the polarization degree of THG was improved significantly. The nonlinear polarization features can be attributed to the crystal structure and the ensemble of anisotropic plasmonic hot spots on the surface of Au nanoparticles. Finally, intrinsic enhancement process of THG was explored comprehensively by finite difference time domain method. The simulation results are consistent with our experimental observation.

High Uniformity and Enhancement Au@AgNS 3D Substrates for the Diagnosis of Breast Cancer.

Breast cancer appears to be one of the leading causes of cancer-related morbidity and mortality for women worldwide. The accurate and rapid diagnosis of breast cancer is hence critical for the treatment and prognosis of patients. With the vibrational fingerprint information and high detection sensitivity, surface-enhanced Raman spectroscopy (SERS) has been extensively applied in biomedicine. Here, an optimized bimetallic nanosphere (Au@Ag NS) 3D substrate was fabricated for the aim of the diagnosis of breast cancer based on the SERS analysis of the extracellular metabolites. The unique stacking mode of 3D Au@Ag NSs provided multiple plasmonic hot spots according to the theoretical calculations of the electromagnetic field distribution. The low relative standard deviation (RSD = 2.7%) and high enhancement factor (EF = 1.42 × 105) proved the uniformity and high sensitivity. More importantly, the normal breast cells and breast cancer cells could be readily distinguished from the corresponding SERS spectra based on the extracellular metabolites. Furthermore, the clear clusters of SERS spectra from MCF-7 and MDA-MB-231 extracellular metabolites in the orthogonal partial least-squares discriminant analysis plot indicate the distinct metabolic fingerprint between breast cancer cells, which imply their potential clinical application in the diagnosis of breast cancer.

Effective Enrichment of Plasmonic Hotspots for SERS by Spinning Droplets on a Slippery Concave Dome Array.

Surface-enhanced Raman scattering (SERS) detection requires dense hotspots and a uniform distribution of analytes to obtain a stable signal with good repeatability. However, due to the coffee-ring effect on the hydrophilic substrate, and the difficulty of droplet manipulation on the superhydrophobic substrate, few substrates can ensure that the analytes are evenly distributed. In this work, we develop a method that can efficiently enrich plasmonic hotspots for SERS measurement on the superhydrophobic concave dome array (SCDA). The SCDA is formed by spraying hydrophobic silica nanoparticles onto a polydimethylsiloxane (PDMS) slab with a concave dome array that can physically confine the droplets and overcome the coffee-ring effect. During droplet evaporation, the SCDA is driven by a horizontal spinner, and the droplets spin on the SCDA, enabling the plasmonic nanoparticles to become closely packed to form the SERS hotspots. The limit of detection (LOD) of the dynamic-enriched SERS hotspots for crystal violet and methylene blue can reach up to 10−11 M. Moreover, the LOD for melamine in milk can reach 5 × 10−7 M, which is lower than the safety threshold defined by the Food and Drug Administration (FDA). Based on this SERS platform, an effective, low-cost, and simple method for SERS detection in analytical chemistry and food safety is highly expected.

Spatially Controlled Fabrication of Surface-Enhanced Raman Scattering Hot Spots through Photoinduced Dewetting of Silver Thin Films.

A well-designed narrow gap between noble metal nanostructures plays a prominent role in surface-enhanced Raman scattering (SERS) to concentrate electromagnetic fields at the local point, called a "hot spot". However, SERS-active substrate fabrication remains a substantial hurdle due to the high process cost and the difficulty of engineering efficient plasmonic hot spots at the target area. In this study, we demonstrate a simple photolithographic method for generating ultrasensitive SERS hot spots at desired positions. The solid-state dewetting of a Ag thin film (thickness of ∼10 nm) using a continuous-wave laser (∼1 MW/cm2) generates a closely packed assembly of hemispherical Ag nanoislands. Some of these nanoislands provide substantial plasmonic-field enhancement that is sufficient for single-molecule detection and plasmon-catalyzed chemical reaction. Such hot spot structures can be patterned on the substrate with a spatial resolution of better than 1 μm. In integrated analytical devices, the patterned SERS hot spots can be used as position-specific chemical-sensing elements.

Microscopic Understanding of Reaction Rates Observed in Plasmon Chemistry of Nanoparticle-Ligand Systems.

Surface-enhanced Raman scattering (SERS) is an effective and widely used technique to study chemical reactions induced or catalyzed by plasmonic substrates, since the experimental setup allows us to trigger and track the reaction simultaneously and identify the products. However, on substrates with plasmonic hotspots, the total signal mainly originates from these nanoscopic volumes with high reactivity and the information about the overall consumption remains obscure in SERS measurements. This has important implications; for example, the apparent reaction order in SERS measurements does not correlate with the real reaction order, whereas the apparent reaction rates are proportional to the real reaction rates as demonstrated by finite-difference time-domain (FDTD) simulations. We determined the electric field enhancement distribution of a gold nanoparticle (AuNP) monolayer and calculated the SERS intensities in light-driven reactions in an adsorbed self-assembled molecular monolayer on the AuNP surface. Accordingly, even if a high conversion is observed in SERS due to the high reactivity in the hotspots, most of the adsorbed molecules on the AuNP surface remain unreacted. The theoretical findings are compared with the hot-electron-induced dehalogenation of 4-bromothiophenol, indicating a time dependency of the hot-carrier concentration in plasmon-mediated reactions. To fit the kinetics of plasmon-mediated reactions in plasmonic hotspots, fractal-like kinetics are well suited to account for the inhomogeneity of reactive sites on the substrates, whereas also modified standard kinetics model allows equally well fits. The outcomes of this study are on the one hand essential to derive a mechanistic understanding of reactions on plasmonic substrates by SERS measurements and on the other hand to drive plasmonic reactions with high local precision and facilitate the engineering of chemistry on a nanoscale.