Electromagnetic Hot Spots Research Articles

Fabrication of hybrid Pd@Ag core-shell and fully alloyed bi-metallic AgPd NPs and SERS enhancement of Rhodamine 6G by a unique mixture approach with graphene quantum dots

In this work, an innovative and simple growth approach of two-step solid state dewetting (SSD) has been introduced for the fabrication of bimetallic and core-shell nanoparticle (NP) configurations. A significant surface-enhanced Raman spectroscopy (SERS) enhancement on the hybrid Pd@Ag core-shell NP substrate is systematically demonstrated by a unique hybrid approach of mixing the graphene quantum dots (GQDs) and Rhodamine 6G (R6G). Due to the substantially increased electromagnetic hotspots in the hybrid Pd@Ag core-shell NP configuration and also the charge transfer enhancement by the addition of GQDs through the chemical enhancement mechanism, over 100 times of SERS signal enhancement is observed for the detection of R6G. The hybrid Pd@Ag core-shell NPs with the background Ag NPs and fully alloyed bi-metallic AgPd NPs without the background Ag NPs are demonstrated on sapphire (0001). The bimetallic and core-shell NPs demonstrate much enhanced localized surface plasmon resonance (LSPR) and thus significantly improved SERS responses. In specific, the hybrid Pd@Ag core-shell NPs demonstrate a gradual blue-shift and narrowing of LSPR peaks while the fully alloyed bi-metallic AgPd NPs show a continuing redshift and broadening of LSPR peaks.

Highly Sensitive W18O49 Mesocrystal Raman Scattering Substrate with Large-Area Signal Uniformity.

Although mesocrystals with ordered building units have great potential in many fields because of the large amount of organic molecules used as structure-directing agents during their synthesis process, severe matrix interference makes their surface-enhanced Raman scattering (SERS) properties rarely understood. Herein, a rapid (20 s) and green microwave synthetic route is developed for the 100 g scale preparation of plasmonic W18O49 mesocrystals with no additives. The ultrathin (1.5 nm) and oxygen vacancy-rich W18O49 nanowires as a building unit greatly improve the interface charge transfer, while the periodic mesocrystal structure significantly enhances the localized surface plasmon resonance effect and creates high-density electromagnetic hot spots among the nanowires. An outstanding enhancement factor of 1.2 × 107 and an ultralow detectable limit of 10-11 M are achieved, and the single-molecule imaging is also realized on this mesocrystal-based SERS substrate. Moreover, the relative standard deviation of the fabricated large-area (2.2 cm2) SERS substrate is only 6.8%.

Plasmon coupling nanorice trimer for ultrahigh enhancement of hyper-Raman scattering

A new tactic that using Ag nanorice trimer as surface-enhanced hyper Raman scattering substrate is proposed for realizing maximum signal enhancement. In this paper, we numerically simulate and theoretically analyze the optical properties of the nanorice trimer consisting of two short nanorices and a long nanorice. The Ag nanorice trimer can excite Fano resonance at optical frequencies based on the strong interaction between the bright and the dark mode. The bright mode is attributed to the first longitudinal resonance of the short nanorice pair, while the dark mode originates from the third longitudinal mode resonance of the long nanorice. The electric field distributions demonstrate that the two resonances with the largest field strength correspond to the first-order resonance of the long nanorice and the Fano resonance of the trimer, respectively. Two plasmon resonances with maximum electromagnetic field enhancements and same spatial hot spot regions can match spectrally with the pump and second-order Stokes beams of hyper Raman scattering, respectively, through reasonable design of the trimer structure parameters. The estimated enhancement factor of surface-enhanced hyper Raman scattering can achieve as high as 5.32 × 1013.

Enhancing Nonfouling and Sensitivity of Surface-Enhanced Raman Scattering Substrates for Potent Drug Analysis in Blood Plasma via Fabrication of a Flexible Plasmonic Patch.

Surface-enhanced Raman scattering (SERS) is an ultrasensitive analytical technique, which is capable of providing high specificity; thus, it can be used for toxicological drug assay (detection and quantification). However, SERS-based drug analysis directly in human biofluids requires mitigation of fouling and nonspecificity effects that commonly appeared from unwanted adsorption of endogenous biomolecules present in biofluids (e.g., blood plasma and serum) onto the SERS substrate. Here, we report a bottom-up fabrication strategy to prepare ultrasensitive SERS substrates, first, by functionalizing chemically synthesized gold triangular nanoprisms (Au TNPs) with poly(ethylene glycol)-thiolate in the solid state to avoid protein fouling and second, by generating flexible plasmonic patches to enhance SERS sensitivity via the formation of high-intensity electromagnetic hot spots. Poly(ethylene glycol)-thiolate-functionalized Au TNPs in the form of flexible plasmonic patches show a twofold-improved signal-to-noise ratio in comparison to triethylamine (TEA)-passivated Au TNPs. Furthermore, the plasmonic patch displays a SERS enhancement factor of 4.5 ×107. Utilizing the Langmuir adsorption model, we determine the adsorption constant of drugs for two different surface ligands and observe that the drug molecules display stronger affinity for poly(ethylene glycol) ligands than TEA. Our density functional theory calculations unequivocally support the interaction between drug molecules and poly(ethylene glycol) moieties. Furthermore, the universality of the plasmonic patch for SERS-based drug detection is demonstrated for cocaine, JWH-018, and opioids (fentanyl, despropionyl fentanyl, and heroin) and binary mixture (trace amount of fentanyl in heroin) analyses. We demonstrate the applicability of flexible plasmonic patches for the selective assay of fentanyl at picogram/milliliter concentration levels from drug-of-abuse patients' blood plasma. The fentanyl concentration calculated in the patients' blood plasma from SERS analysis is in excellent agreement with the values determined using the paper spray ionization mass spectrometry technique. We believe that the flexible plasmonic patch fabrication strategy would be widely applicable to any plasmonic nanostructure for SERS-based chemical sensing for clinical toxicology and therapeutic drug monitoring.

Helically Grooved Gold Nanoarrows: Controlled Fabrication, Superhelix, and Transcribed Chiroptical Switching

Plasmonic chiroptical materials, coupled by chirality and surface plasmons, have been attracting great interest due to their potential applications, such as photonics, chiral recognition, asymmetri...

Coaxial hole array fabricated by ultrafast femtosecond-laser processing with spatially multiplexed vortex beams for surface enhanced infrared absorption

Here, we combined shaping and multiplexing of the laser beam to achieve ultrafast femtosecond-laser patterning of Au films with coaxial and circular hole arrays at a printing rate of 106 elements per second. Fabrication quality of the developed parallel laser-printing approach was found to be enough to replicate coaxial hole arrays with a resonant transmission over 90% in the mid-IR spectral range resulted from coupling between localized electromagnetic mode supported by coaxial unit cell and the lattice-type SPPs. Coupling of the surface plasmons localized within the coaxial holes to the vibration modes of the deposited analyte, widely used antihistamine drag Diphendramine, was found to provide around 800-fold enhancement of the amplitude of characteristic IR bands allowing reliable SEIRA-based fingerprint identification at trace concentrations. Our findings are consistent with the calculated amplitude of the electromagnetic hot spots suggesting their predominant contribution to the observed SEIRA effect. The ability to tune in a facile way the hole geometry and arrangement in the process of fabrication allows to tailor the spectral position of the resonance spanning practically relevant spectral range from 4 to 10 μm. This makes the coaxial microhole arrays produced with ultrafast direct laser printing promising for IR filtering and sensing.

Core–Shell Bimetallic Nanoparticle Trimers for Efficient Light-to-Chemical Energy Conversion

Incorporation of catalytically active materials into plasmonic metal nanostructures can efficiently merge the reactivity and energy harvesting abilities of both types of materials for visible light photocatalysis. Here we explore the influence of electromagnetic hotspots in the ability of plasmonic core-shell colloidal structures to induce chemical transformations. For this study, we developed a synthetic strategy for the fabrication of Au nanoparticle (NP) trimers in aqueous solution through fine controlled galvanic replacement between Ag nanoprisms and Au precursors. Core-shell Au@M NP trimers with catalytically active metals (M = Pd, Pt) were subsequently synthesized using Au NP trimers as templates. Our experimental and computational results highlight the synergy of geometry and composition in plasmonic catalysts for plasmon-driven chemical reactions.

Raman enhancement properties of a high uniformity PS microsphere-Ag nanoparticle substrate

A structured SERS substrate with high uniformity and sensitivity was fabricated orderly by hydrophilizing the monocrystalline silicon substrate, self-assembling a layer of dense polystyrene (PS) microsphere array on the silicon, and depositing a layer (5 nm, 10 nm, 20 nm, 30 nm or 50 nm) of silver film. Rhodamine 6G (R6G) was used as a probe to characterize the performance of Raman enhancement. Experimental results show that our sample with deposition of a 5 nm silver film (PS-Ag-5) can obtain a large number of spatially distributed local electromagnetic field hot spots, with an enhancement factor of over 108, which is several orders of magnitude higher than that of other samples. The sensitivity of PS-Ag-5 to the concentration of probe molecules and the optical enhancement effect of PS microspheres were also analyzed. This novel structured substrate can achieve considerable uniformity, and the calculated relative standard deviation (RSD) of the characteristic peak at 1650 cm-1 is approximately 8%.

Capture of Phenylalanine and Phenylalanine-Terminated Peptides Using a Supramolecular Macrocycle for Surface-Enhanced Raman Scattering Detection.

The cucurbit[n]uril (CB[n]) family of macrocycles are known to bind a variety of small molecules with high affinity. These motifs thus have promise in an ever-growing list of trace detection methods. Surface-enhanced Raman scattering (SERS) detection schemes employing CB[n] motifs exhibit increased sensitivity due to selective concentration of the analyte at the nanoparticle surface, coupled with the ability of CB[n] to facilitate the formation of well-defined electromagnetic hot spots. Herein, we report a CB[7] SERS assay for quantification of phenylalanine (Phe) and further demonstrate its utility for detecting peptides with an N-terminal Phe. The CB[7]-guest interaction improves the sensitivity 5-25-fold over direct detection of Phe using citrate-capped silver nanoparticle aggregates, enabling use of a portable Raman system. We further illustrate detection of insulin via binding of CB[7] to the N-terminal Phe residue on its B-chain, suggesting a general strategy for detecting Phe-terminated peptides of clinically relevant biomolecules.

Dynamics of deterministically positioned single‐bond surface‐enhanced Raman scattering from DNA origami assembled in plasmonic nanogaps

AbstractWe study the dynamics of single bonds through the surface‐enhanced Raman scattering (SERS) from single SERS‐marker molecules containing a distinctive single alkyl bond. Assembly of the nanogaps and positioning of single molecules inside the electromagnetic hotspot are precisely controlled using DNA origami constructs. The observed SERS intensities and their spectral wandering, together with electromagnetic simulations, all confirm the role of picocavities in this nanogap geometry in allowing observation of SERS signatures from individual vibrating bonds. The strong electromagnetic field around each picocavity and the transient binding of the SERS‐marker molecule reveal significant modifications to bond vibrations and selection rules over time.

Beetle wing inspired fabrication of nanojunction based biomimetic SERS substrates for sensitive detection of analytes

ABSTRACT Surface architecture belonging to flora and fauna is preferred because of their unique three dimensional micro/nanostructures, which can be utilised as novel support in SERS based chemical detection. In this work, we have fabricated biomimetic SERS substrates by replicating the wings of two different beetles with interesting surface morphology by employing soft lithography technique. Micro-bowls and nanogaps between globular microstructures on the replicated artificial wings surface have been utilised for aggregation of nanoparticles to create electromagnetic hot spots and enhanced interaction of nanoparticles with analyte molecules. The surface of these substrates has been engineered to form a homo-nanojunction between gold thin film and gold nanoparticles, which makes it more SERS active. The remarkable performance of these nano-junction based substrates was examined for SERS based sensitive detection of an organic dye, methylene blue, and a fungicide, carbendazim. This work paves way for the design and development of substrates using biomimetic approaches.

Plasmonic Particle-on-Film Nanocavity in Tightly Focused Vector Beam: a Full-Wave Theoretical Analysis from Near-Field Enhancement to Far-Field Radiation

Based on the local electric field enhancement of gap-mode surface plasmon resonance, the plasmonic particle-on-film nanocavity (PPoFN) can realize Raman signal enhancement of multiple orders of magnitude, even single molecule detection. The tightly focused radially polarized (Bessel-Gaussian) beam (TFRPB), containing a cylindrical symmetry longitudinal component, is a superior excitation source for PPoFN to generate a more intriguing electromagnetic hot spot in the gap. For the optical response of PPoFN in TFRPB, some theoretical and experimental studies at a certain wavelength have been reported, but a full-wave analysis about the near- and far-field optical properties is still lacking because it is still difficult to generate a full-wave radially polarized beam and achieve a dispersion-free confocal light field in the experiment. In this paper, we analyze the full-wave near- and far-field optical properties of PPoFN in TFRPB by using numerical simulation with the finite element method (FEM). For comparison, a full-wave analysis of PPoFN in tightly focused linearly polarized (Gaussian) beam (TFLPB) is also implemented. Based on the comprehensive results of local field enhancement, nanofocusing property, and far-field radiation, it is found that the PPoFN in TFRPB with the strongest toroidal dipole (TD) mode at a certain wavelength can achieve good performance in Raman signal excitation and detection. Importantly, this result will provide a theoretical reference for the application of PPoFN in tightly focused vector beam.

Distinguishing Among All Possible Activation Mechanisms of a Plasmon-Driven Chemical Reaction

Localized surface plasmon resonances (LSPRs) in metal nanoparticles can drive chemical reactions at their surface, but it is often challenging to disentangle the exact activation mechanism. The decay of LSPRs can lead to photothermal heating, electromagnetic hot spots, and the ejection of nonthermalized charge carriers, but all of these processes typically occur simultaneously and on ultrafast time scales. Here, we develop a plasmon-assisted Au@Ag core@shell nanorod synthesis in which each plasmon-decay mechanism can be independently assessed. Using different illumination wavelengths combined with extinction spectroscopy, transmission electron microscopy, thermal characterization, and finite-difference time-domain simulations, we unequivocally identify the transfer of interband holes to ascorbic acid as the rate-limiting step in the silver shell growth reaction. Our conclusion is corroborated by single-particle studies of gold nanospheres that display isotropic reactivity, consistent with interband hole-driven nanoparticle syntheses. Our strategy for distinguishing among plasmon-activation mechanisms can be extended to a variety of light-driven processes, including photocatalysis, nanoparticle syntheses, and drug delivery.

Photoinduced Temperature Gradients in Sub‐Wavelength Plasmonic Structures: The Thermoplasmonics of Nanocones

AbstractPlasmonic structures are renowned for their capability to efficiently convert light into heat at the nanoscale. However, despite the possibility to generate deep sub‐wavelength electromagnetic hot spots, the formation of extremely localized thermal hot spots is an open challenge of research, simply because of the diffusive spread of heat along the whole metallic nanostructure. Here this challenge is tackled by exploiting single gold nanocones. It is theoretically shown how these structures can indeed realize extremely high temperature gradients within the metal, leading to deep sub‐wavelength thermal hot spots, owing to their capability of concentrating light at the apex under resonant conditions even under continuous wave illumination. A 3D finite element method model is employed to study the electromagnetic field in the structure and subsequent thermoplasmonic behavior, in terms of the 3D temperature distribution. How the latter is affected by nanocone size, shape, and composition of the surrounding environment is shown. Finally, the use of photoinduced temperature gradients in nanocones is anticipated for applications in optofluidics and thermoelectrics or for thermally induced nanofabrication.

Plasmonic Antennas with Electric, Magnetic, and Electromagnetic Hot Spots Based on Babinet’s Principle

We theoretically study plasmonic antennas featuring areas of extremely concentrated electric or magnetic field, known as hot spots. We combine two types of electric-magnetic complementarity to increase the degree of freedom for the design of the antennas: bow-tie and diabolo duality and Babinet's principle. We evaluate the figures of merit for different plasmon-enhanced optical spectroscopy methods: field enhancement, decay rate enhancement, and quality factor of the plasmon resonances. The role of Babinet's principle in interchanging electric and magnetic field hot spots and its consequences for practical antenna design are discussed. In particular, diabolo antennas exhibit slightly better performance than bow-ties in terms of larger field enhancement and larger Q factor. For specific resonance frequency, diabolo antennas are considerably smaller than bow-ties which makes them favourable for the integration into more complex devices but also makes their fabrication more demanding in terms of spatial resolution. Finally, we propose Babinet-type dimer antenna featuring electromagnetic hot spot with both the electric and magnetic field components treated on equal footing.

Bioinspired Disordered Flexible Metasurfaces for Human Tear Analysis Using Broadband Surface-Enhanced Raman Scattering.

Flexible surface-enhanced Raman scattering (SERS) has received attention as a means to move SERS-based broadband biosensing from bench to bedside. However, traditional flexible periodic nano-arrangements with sharp plasmonic resonances or their random counterparts with spatially varying uncontrollable enhancements are not reliable for practical broadband biosensing. Here, we report bioinspired quasi-(dis)ordered nanostructures presenting a broadband yet tunable application-specific SERS enhancement profile. Using simple, scalable biomimetic fabrication, we create a flexible metasurface (flex-MS) of quasi-(dis)ordered metal–insulator–metal (MIM) nanostructures with spectrally variable, yet spatially controlled electromagnetic hotspots. The MIM is designed to simultaneously localize the electromagnetic signal and block background Raman signals from the underlying polymeric substrate—an inherent problem of flexible SERS. We elucidate the effect of quasi-(dis)ordering on broadband tunable SERS enhancement and employ the flex-MS in a practical broadband SERS demonstration to detect human tear uric acid within its physiological concentration range (25–150 μM). The performance of the flex-MS toward noninvasively detecting whole human tear uric acid levels ex vivo is in good agreement with a commercial enzyme-based assay.

Petal-like Gap-Enhanced Raman Tags with Controllable Structures for High-Speed Raman Imaging.

Surface-enhanced Raman scattering (SERS) is widely used for in vitro and in vivo bioimaging applications. However, reproducible and controllable fabrication of SERS tags with high density of electromagnetic hot-spots is still challenging. We report an improved strategy for the synthesis of core/shell Raman tags with high density of hot-spots and high immobilization of reporter molecules. The strategy is based on simultaneous growth and functionalization of an Au shell around Au nanospheres coated with 4-nitrobenzenethiol (NBT). The amount of added 4-NBT is key factor to control the structure SERS response of the resulting particles. Specifically, we demonstrate the formation of gap-enhanced Raman tags (GERTs) with a smooth solid shell (sGERTs), petal-like GERTs (pGERTs), and mesoporous Au particles (mGERTs) filled with Raman molecules. In contrast to NBT molecules, similar thiols such as 1,4-benzenedithiol (BDT) and 2-naphtalenethiol (NT) do not support the formation of pGERTs and mGERTs. To explain this finding, we proposed a growth mechanism based on the unique chemical structure of NBT. The SERS response of optimized pGERTs is 50 times higher than that from usual sGERTs, which makes pGERTs suitable for single-particle spectroscopy. We demonstrate successful application of pGERTs for high-speed cell imaging using 10 ms accumulation time per pixel and a total imaging time of about 1 min. Because of the high SERS response and unique porous structure, these nanoparticles have great potential for bioimaging and other applications.

Effect of deposition angle on fabrication of plasmonic gold nanocones and nanodiscs

Metal nanocones can exhibit several strong plasmonic resonances, which are associated with intense and accessible electromagnetic hot spots. They can thus be used to enhance light–matter interactions or to facilitate location-specific sensing while enabling separation of some non-specific contributions towards the sensing signal. Nanocones and similar 3D structures are often fabricated with the use of the so-called self-shading effect, which occurs during the evaporation of a metal film into circular nanowells. Unfortunately, a full description of a successful deposition process with all the essential details is currently missing in literature. Here we present a detailed view of the fabrication of ordered arrays of conical gold nanostructures using electron beam lithography and gold electron beam evaporation. We show that the symmetry of the fabricated nanostructures is influenced by the lateral position of the substrate on the sample holder during the deposition. Off-axis deposition or tilt of the sample leads to asymmetric nanostructures. When the deposited film is thick enough, or the nanowells narrow enough, the entrance aperture is clogged, and nanocones with sharp tips are formed. In contrast, flat-top truncated cones are produced for thinner films or wider nanowells. All these findings help to identify inherent limits for the production of wafer-scale arrays of such non-planar nanostructures. On the other hand, they also suggest new fabrication possibilities for more complicated structures such as mutually connected nanocones for electrically addressable chips.

Nanoplasmonic On-Chip PCR for Rapid Precision Molecular Diagnostics.

Emerging molecular diagnosis requires ultrafast polymerase chain reaction (PCR) on chip for rapid precise detection of infectious diseases in the point-of-care test. Here, we report nanoplasmonic on-chip PCR for rapid precision molecular diagnostics. The nanoplasmonic pillar arrays (NPA) comprise gold nanoislands on the top and sidewall of large-scale glass nanopillar arrays. The nanoplasmonic pillars enhance light absorption of a white light-emitting diode (LED) over the whole visible range due to strong electromagnetic hotspots between the nanoislands. As a result, they effectively induce photothermal heating for ultrafast PCR thermal cycling. The temperature profile of NPA exhibits 30 cycles between 98 and 60 °C for a total of 3 min and 30 s during the cyclic excitation of white LED light. The experimental results also demonstrate the rapid DNA amplification of both 0.1 ng μL-1 of λ-DNA in 20 thermal cycles and 0.1 ng μL-1 of complementary DNA of Middle East respiratory syndrome coronavirus in 30 thermal cycles using a conventional PCR volume of 15 μL. This nanoplasmonic PCR technique provides a new opportunity for rapid precision molecular diagnostics.

CoFe2O4@HNTs/AuNPs Substrate for Rapid Magnetic Solid-Phase Extraction and Efficient SERS Detection of Complex Samples All-in-One.

Fast and accurate practical sample detection is a great challenge in on-site detection. Herein, we developed a CoFe2O4@HNTs/AuNPs substrate for rapid and efficient magnetic solid-phase extraction (MSPE) surface-enhanced Raman scattering (SERS) detection of aromatic amines and nitrofuran in real samples all-in-one. Magnetic CoFe2O4 beads filled inside halloysite nanotubes (HNTs) can avoid aggregation of particles, endow the substrate with the rapid magnetic separation ability to simplify the pretreatment procedure, and reduce complex matrix interference. Meanwhile, outer surface AuNPs can generate electromagnetic enhancement and hot spots to amplify Raman signals of target molecules enriched/concentrated by HNTs. The CoFe2O4@HNTs/AuNPs substrate exhibited excellent SERS activity (high sensitivity, good reproducibility, and repeatability), pH stability (3.0-11.0), and good MSPE ability (fast magnetic enrichment/separation ability within 5 min). The CoFe2O4@HNTs/AuNPs MSPE SERS substrate can be applied for the determination of 4,4'-thioaniline and nitrofurantoin with a linear range of 0.054-21.7 mg/L and 0.05-1.0 mg/L, and the limits of detection were down to 0.026 mg/L and 0.014 mg/L, respectively. Furthermore, the enhancement factor (EF) of the substrate to 4,4'-thioaniline is up to 2.7 × 107. Besides, the substrate can realize practical SERS determination of trace 4,4-thioaniline in cosmetics and nitrofurantoin in fish feed and aquatic samples. The recoveries were varied from 71.6% to 103.6% for 4,4-thioaniline in hair dyes and 81.9% to 116.3% for nitrofurantoin in fish feed and aquatic samples, respectively. Such a robust and efficient MSPE SERS substrate possesses great potential in rapid detection (within 15 min) for a practical sample, and it also provides a methodology for the preparation of other HNTs-based composites.