Plasmonic Hot Spots Research Articles

UV-Laser Interference Lithography for Local Functionalization of Plasmonic Nanostructures with Responsive Hydrogel.

A novel approach to local functionalization of plasmonic hotspots at gold nanoparticles with biofunctional moieties is reported. It relies on photocrosslinking and attachment of a responsive hydrogel binding matrix by the use of a UV interference field. A thermoresponsive poly(N-isopropylacrylamide)-based (pNIPAAm) hydrogel with photocrosslinkable benzophenone groups and carboxylic groups for its postmodification was employed. UV-laser interference lithography with a phase mask configuration allowed for the generation of a high-contrast interference field that was used for the recording of periodic arrays of pNIPAAm-based hydrogel features with the size as small as 170 nm. These hydrogel arrays were overlaid and attached on the top of periodic arrays of gold nanoparticles, exhibiting a diameter of 130 nm and employed as a three-dimensional binding matrix in a plasmonic biosensor. Such a hybrid material was postmodified with ligand biomolecules and utilized for plasmon-enhanced fluorescence readout of an immunoassay. Additional enhancement of the fluorescence sensor signal by the collapse of the responsive hydrogel binding matrix that compacts the target analyte at the plasmonic hotspot is demonstrated.

How Peptides Dissociate in Plasmonic Hot Spots.

Plasmon-induced hot carriers enable dissociation of strong chemical bonds by visible light. This unusual chemistry has been demonstrated for several diatomic and small organic molecules. Here, the scope of plasmon-driven photochemistry is extended to biomolecules and the reactivity of proteins and peptides in plasmonic hot spots is described. Tip-enhanced Raman spectroscopy (TERS) is used to both drive the reactions and to monitor their products. Peptide backbone bonds are found to dissociate in the hot spot, which is reflected in the disappearance of the amide I band in the TER spectra. The observed fragmentation pathway involves nonthermal activation, presumably by dissociative capture of a plasmon-induced hot electron. This fragmentation pathway is known from electron transfer dissociation (ETD) of peptides in gas-phase mass spectrometry (MS), which suggests a general similarity between plasmon-induced photochemistry and nonergodic reactions triggered by electron capture. This analogy may serve as a design principle for plasmon-induced reactions of biomolecules.

In situ modulation of gold nanorod's surface charge drives the growth of end-to-end assemblies from dimers to large networks that enhance single-molecule fluorescence by 10 000-fold.

End-to-end assemblies of anisotropic plasmonic nanostructures with small nanogaps are of great interest as they create strong hot spots for enhancing weak fluorescence and/or scattering of molecules. Here we report the growth of dithiol-linked end-to-end assemblies of gold nanorods from dimers to large networks containing thousands of individual nanorods, directed by in situ tuning of nanorod's surface charge. Surface charge was lowered to initiate the aggregation process but was subsequently increased to achieve slow tip-specific growth over seven days to form end-to-end networks of nanorods, which were stable in solution for over one month. Furthermore, we showed that these assemblies contained strong plasmonic hot spots which enhanced the fluorescence signal of a weak emitter by 104-fold. This enhancement is approximately 10-fold larger than that obtained using a single gold nanorod and is comparable to the largest enhancement obtained using more expensive lithographically made in-plane antenna arrays.

Ultrasensitive melanoma biomarker detection using a microchip SERS immunoassay with anisotropic Au-Ag alloy nanoboxes.

The detection of circulating biomarkers in liquid biopsies has the potential to provide a non-invasive route for earlier cancer diagnosis and treatment management. Melanoma chondroitin sulfate proteoglycan (MCSP) is a membrane protein characteristic for melanoma cell migration and tissue invasion with its soluble form (sMCSP) serving as a potential promising diagnostic surrogate. However, at the initial disease stage, the detection of sMCSP is challenging because of its low abundance and the required high specificity to analyze sMCSP in complex bodily fluids. Herein, we report a highly sensitive and high-throughput microchip that enables Surface Enhanced Raman Spectroscopy (SERS) immunoassay for parallel detection of up to 28 samples. Key to assay speed and sensitivity is the stimulation of an alternating current-induced nanofluidic mixing that improves target-sensor collision and displacement of non-specific molecules. Anisotropic Au–Ag alloy nanoboxes (NB's) with strong plasmonic hot spots provide single SERS particle sensitivity that enables ultrasensitive sMCSP detection of as low as 0.79 pM (200 pg ml−1). As a proof of concept study, we investigate the assay performance in simulated melanoma patient samples.

Quantitative insights into non-uniform plasmonic hotspots due to symmetry breaking induced by oblique incidence.

Localized surface plasmon resonance draws great attentions mainly due to its enhanced near electric field, i.e., plasmonic hotspots. The symmetry breaking via oblique incidence of light is predicted to influence the intensity of plasmonic hotspots. However, relevant experimental investigation in quantitative comparison with theory is still lacking. Here, we visualize the polarization-dependent plasmonic hotspots of a triangular Ag nanoplate through oblique-incidence photoemission electron microscopy (PEEM), revealing a non-uniform near-field enhancement. Under oblique incidence, two bright spots and one dark spot were identified in the polarization-averaged PEEM image, different from that for normal illumination where bright spots with equal intensity are anticipated. In polarization-dependent PEEM images, plasmonic hotspots appeared at specific corners of a triangular Ag nanoplate, and rotated in a manner consistent with the rotation of polarization angle. The experimental intensity maps of the photoelectron were well reproduced by simulation on a quantitative level. This work provides a quantitative understanding of how the orientation of incidence light relative to a plasmonic antenna influences the near-field enhancement.

Addressing the plasmonic hotspot region by site-specific functionalization of nanostructures.

Strong electromagnetic fields emerge around resonant plasmonic nanostructures, focusing the light in tiny volumes, usually referred to as hotspots. These hotspots are the key regions governing plasmonic applications since they strongly enhance properties, signals or energies arising from the interaction with light. For a maximum efficiency, target molecules or objects would be exclusively placed within hotspot regions. Here, we propose a reliable, universal and high-throughput method for the site-specific functionalization of hotspot regions over macroscopic areas. We demonstrate the feasibility of the approach using crescent-shaped nanostructures, which can be fabricated using colloidal lithography. These structures feature polarization-dependent resonances and near-field enhancement at their tips, which we use as target regions for the site-selective functionalization. We modify the fabrication process and introduce a defined passivation layer covering the central parts of the gold nanocrescent, which, in turn, selectively uncovers the tips and thus enables a localized functionalization with thiol molecules. We demonstrate and visualize a successful targeting of the hotspot regions by binding small gold nanoparticles and show a targeting efficiency of 90%. Finally, we demonstrate the versatility of the method exemplarily by translating the principle to different nanostructure geometries and architectures.

A Paper-Fiber-Supported 3D SERS Substrate

Surface-enhanced Raman scattering (SERS) spectroscopy is an effective approach for trace-level detection of molecular substance. Plasmonic metallic nanostructures with high distribution densities and small gap widths are always expected for constructing SERS substrates. We report here a paper-based SERS substrate, where the three-dimensional (3D) network of paper fibers was used as the platform for supporting the gold nanoparticle clusters. Such a 3D arrangement of plasmonic porous clusters supplies high-density hotspots with large total volume and large surface area for the interaction with molecules. Comparison between different papers found that the filter papers commonly available in labs are the best choice. An enhancement factor higher than 104 has been achieved in the detection of R6G molecules. The preparation of such SERS substrates is very simple and convenient, implying low-cost, disposable, and environment-friendly SERS techniques. Furthermore, the paper-based flexible SERS substrates can be easily tailored into different shapes and sizes for fitting different applications.

Ultrasensitive amyloid β-protein quantification with high dynamic range using a hybrid graphene-gold surface-enhanced Raman spectroscopy platform.

Surface enhanced Raman spectroscopy (SERS) holds great promise in biosensing because of its single-molecule, label-free sensitivity. We describe here the use of a graphene-gold hybrid plasmonic platform that enables quantitative SERS measurement. Quantification is enabled by normalizing analyte peak intensities to that of the graphene G peak. We show that two complementary quantification modes are intrinsic features of the platform, and that through their combined use, the platform enables accurate determination of analyte concentration over a concentration range spanning seven orders of magnitude. We demonstrate, using a biologically relevant test analyte, the amyloid β-protein (Aβ), a seminal pathologic agent of Alzheimer's disease (AD), that linear relationships exist between (a) peak intensity and concentration at a single plasmonic hot spot smaller than 100 nm, and (b) frequency of hot spots with observable protein signals, i.e. the co-location of an Aβ protein and a hot spot. We demonstrate the detection of Aβ at a concentration as low as 10-18 M after a single 20 μl aliquot of the analyte onto the hybrid platform. This detection sensitivity can be improved further through multiple applications of analyte to the platform and by rastering the laser beam with smaller step sizes.

Enhanced subwavelength coupling and nano-focusing with optical fiber-plasmonic hybrid probe.

Metallic nanowires supporting surface plasmon polaritons can localize optical fields at nanoscale tapered ends for near-field imaging. Radially polarized light through the optical fiber or free space efficiently excites the converging radial plasmons at the apex of a sharp tip. However, such radial vector mode excitation through optical fiber requires precise polarization control and strongly polarization maintaining state in optical fiber, thus inducing a complexity for optical applications. In this paper, we propose a photonic-plasmonic probe that uses the linearly polarized source to excite the nanoscale plasmonic hotspot. The linearly polarized fiber mode is converted to radial surface plasmon polaritons (SPPs) through asymmetric coupling at the base of the metallic nano-tip. The radial SPP's then propagate along the half ellipsoid tip and are focused at the tip apex giving rise to the nanoscale concentration of optical energy. The probe can be implemented with near-field imaging techniques such as tip-enhanced Raman microscopy to obtain topographic and chemical spectroscopic information in atomic resolution for studying light-matter interaction at the nanoscale.

Monitoring the growth dynamics of colloidal gold-silver core-shell nanoparticles using in situ second harmonic generation and extinction spectroscopy.

The growth dynamics of gold-silver core-shell (Au@Ag) nanoparticles are studied using in situ time-dependent second harmonic generation (SHG) and extinction spectroscopy to investigate the nanoparticle shell formation. The silver shell is grown by reduction of silver cations onto a 14 nm gold core using ascorbic acid in colloidal aqueous solution under varying reaction concentrations producing Au@Ag nanoparticles of final sizes ranging from 51 to 78 nm in diameter. The in situ extinction spectra show a rapid increase in intensity on the timescale of 5-6 s with blue shifting and narrowing of the plasmonic peak during the silver shell formation. The in situ SHG signals show an abrupt rise at early times of the reaction, followed by a time-dependent biexponential decrease, where the faster SHG lifetime corresponds to the timescale of the shell growth, and where the slower SHG lifetime is attributed to changes in the nanoparticle surface charge density. A large enhancement in the SHG signal at early stages of the reaction is caused by plasmonic hot spots due to the nanoparticle surface morphology, which becomes smoother as the reaction proceeds. The final extinction spectra are compared to finite-difference time-domain (FDTD) calculations, showing general agreement with experiment, where the plasmon peak red shifts and increases in spectral width as the silver shell thickness increases. These in situ SHG and extinction spectroscopy results, combined with FDTD calculations, help characterize the complicated processes involved in colloidal nanoparticle shell formation in real time for developing potential plasmon-enhanced nanomaterial applications.

SERS discrimination of single DNA bases in single oligonucleotides by electro-plasmonic trapping

Surface-enhanced Raman spectroscopy (SERS) sensing of DNA bases by plasmonic nanopores could pave a way to novel methods for DNA analyses and new generation single-molecule sequencing platforms. The SERS discrimination of single DNA bases depends critically on the time that a DNA strand resides within the plasmonic hot spot. In fact, DNA molecules flow through the nanopores so rapidly that the SERS signals collected are not sufficient for single-molecule analysis. Here, we report an approach to control the residence time of molecules in the hot spot by an electro-plasmonic trapping effect. By directly adsorbing molecules onto a gold nanoparticle and then trapping the single nanoparticle in a plasmonic nanohole up to several minutes, we demonstrate single-molecule SERS detection of all four DNA bases as well as discrimination of single nucleobases in a single oligonucleotide. Our method can be extended easily to label-free sensing of single-molecule amino acids and proteins.

Ag Ion Soldering: An Emerging Tool for Sub-nanomeric Plasmon Coupling and Beyond.

Self-assembly represents probably the most flexible way to construct metastructured materials and devices from a wealth of colloidal building blocks with synthetically controllable sizes, shapes, and elemental compositions. In principle, surface capping is unavoidable during the synthesis of nanomaterials with well-defined geometry and stability. The ligand layer also endows inorganic building blocks with molecular recognition ability responsible for their assembly into desired structures. In the case of plasmonic nanounits, precise positioning of them in a nanomolecule or an ordered nanoarray provides a chance to shape their electrodynamic behaviors and thereby assists experimental demonstration of modern nanoplasmonics toward practical uses. Despite previous achievements in bottom-up nanofabrication, a big challenge exists toward strong coupling and facile charge transfer between adjacent nanounits in an assembly. This difficulty has impeded a functional development of plasmonic nanoassemblies. The weakened interparticle coupling originates from the electrostatic and steric barriers of ionic/molecular adsorbates to guarantee a good colloidal stability. Such a dilemma is rooted in fundamental colloidal science, which lacks an effective solution. During the past several years, a chemical tool termed Ag ion soldering (AIS) has been developed to overcome the above situation toward functional colloidal nanotechnology. In particular, a dimeric assembly of plasmonic nanoparticles has been taken as an ideal model to study plasmonic coupling and interparticle charge transfer. This Account starts with a demonstration of the chemical mechanism of AIS, followed by a verification of its workability in various self-assembly systems. A further use of AIS to realize postsynthetic coupling of DNA-directed nanoparticle clusters evidences its compatibility with DNA nanotechnology. Benefiting from the sub-nanometer interparticle gap achieved by AIS, a conductive pathway is established between two nanoparticles in an assembly. Accordingly, light-driven charge transfer between the conductively bridged plasmonic units is realized with highly tunable resonance frequencies. These situations have been demonstrated by thermal/photothermal sintering of silica-isolated nanoparticle dimers as well as gap-specific electroless gold/silver deposition. The regioselective silver deposition is then combined with galvanic replacement to obtain catalytically active nanofoci (plasmonic nanogaps). The resulting structures are useful for real time and on-site Raman spectroscopic tracking of chemical reactions in the plasmonic hotspots (nanogaps) as well as for study of plasmon-mediated/field-enhanced catalysis. The Account is concluded by a deeper insight into the chemical mechanism of AIS and its adaption to conformation-rich structures. Finally, AIS-enabled functional pursuits are suggested for self-assembled materials with strongly coupled and easily reshapable physicochemical properties.

Plasmonic Oligomers with Tunable Conductive Nanojunctions.

Engineering plasmonic hot spots is essential for applications of plasmonic nanoparticles. A particularly appealing route is to weld plasmonic nanoparticles together to form more complex structures sustaining plasmons with symmetries targeted to given applications. However, control of the welding and subsequent hot spot characteristics is still challenging. Herein, we demonstrate an original method that connects gold particles to their neighbors by another metal of choice. We first assemble gold bipyramids in a tip-to-tip configuration, yielding short chains of variable length, and grow metallic junctions in a second step. We follow the chain formation and the deposition of the second metal (i.e., silver or palladium) via UV/vis spectroscopy, and we map the plasmonic properties using electron energy loss spectroscopy. The formation of silver bridges leads to a huge red shift of the longitudinal plasmon modes into the mid-infrared region, while the addition of palladium results in a red shift accompanied by significant plasmon damping.

Plasmonic hotspot in toroidal metamaterial

Toroidal dipole resonance under a normal incident excitation can be realized by designing a metallic circular groove with a depth asymmetry. Due to its specific radial charge current configuration, charges gather together on the central circular platform and form a plasmonic hotspot, covering a large circular region with a diameter of 200 nm, where E-field magnitude attains 34-time enhancement than that of the incident light, providing more potentials for energy harvesting or nanoparticle trapping, manipulating of light–matter interaction, and optical data storage.

Near-Field-Induced Femtosecond Breakdown of Plasmonic Nanoparticles

We studied the evolution of femtosecond breakdown in lithographically produced plasmonic nanoparticles with increasing laser intensity. Localized plasmons were generated with 40-fs laser pulses with up to 1.4 × 1012 W/cm2 peak intensity. The damage morphology shows substantial variation with intensity, starting with the detachment of hot spots and stochastic nanoparticle removal. For higher intensities, we observe precise nanolithographic mapping of near-field distributions via ablation. The common feature of these phenomena is the central role played by the single plasmonic hot spot of the triangular nanoparticles used. We also derive a damage threshold value from stochastic damage trends on the arrays fostering the optimization of novel nanoarchitectures for nonlinear plasmonics.

Nanoscale Surface Redox Chemistry Triggered by Plasmon-Generated Hot Carriers.

Direct photoexcitation of charges at a plasmonic metal hotspot produces energetic carriers that are capable of performing photocatalysis in the visible spectrum. However, the mechanisms of generation and transport of hot carriers are still not fully understood and under intense investigation because of their potential technological importance. Here, spectroscopic evidence proves that the reduction of dye molecules tethered to a Au(111) surface can be triggered by plasmonic carriers via a tunneling mechanism, which results in anomalous Raman intensity fluctuations. Tip-enhanced Raman spectroscopy (TERS) helps to correlate Raman intensity fluctuations with temperature and with properties of the molecular spacer. In combination with electrochemical surface-enhanced Raman spectroscopy, TERS results show that plasmon-induced energetic carriers can directly tunnel to the dye through the spacer. This organic spacer chemically isolates the adsorbate from the metal but does not block photo-induced redox reactions, which offers new possibilities for optimizing plasmon-induced photocatalytic systems.

Probing protein-nanorod aggregates

Plasmonics The interaction of proteins with nanoparticles can enhance circular dichroism signals and provide a route for sensitive biodetection. However, ensemble measurements are insufficient for resolving the origin of plasmon-coupled circular dichroism (PCCD). Zhang et al. used single-particle circular differential scattering spectroscopy along with correlated tomographic reconstruction and electromagnetic simulations to study individual aggregates of bovine serum albumin and gold nanorods (see the Perspective by Kim and Kotov). Aggregates contribute to PCCD activity through formation of plasmonic hotspots, but single nanoparticles do not contribute. The protein does not function simply as a chiral chromophore but enables assembly of the chiral nanorod-protein complexes. Science , this issue p. [1475][1]; see also p. [1378][2] [1]: /lookup/doi/10.1126/science.aax5415 [2]: /lookup/doi/10.1126/science.aay7776

Unraveling the origin of chirality from plasmonic nanoparticle-protein complexes.

Plasmon-coupled circular dichroism has emerged as a promising approach for ultrasensitive detection of biomolecular conformations through coupling between molecular chirality and surface plasmons. Chiral nanoparticle assemblies without chiral molecules present also have large optical activities. We apply single-particle circular differential scattering spectroscopy coupled with electron imaging and simulations to identify both structural chirality of plasmonic aggregates and plasmon-coupled circular dichroism induced by chiral proteins. We establish that both chiral aggregates and just a few proteins in interparticle gaps of achiral assemblies are responsible for the ensemble signal, but single nanoparticles do not contribute. We furthermore find that the protein plays two roles: It transfers chirality to both chiral and achiral plasmonic substrates, and it is also responsible for the chiral three-dimensional assembly of nanorods. Understanding these underlying factors paves the way toward sensing the chirality of single biomolecules.

High-speed imaging of surface-enhanced Raman scattering fluctuations from individual nanoparticles.

The concept of plasmonic hotspots is central to the interpretation of the surface-enhanced Raman scattering (SERS) effect. Although plasmonic hotspots are generally portrayed as static features, single-molecule SERS (SM-SERS) is marked by characteristic time-dependent fluctuations in signal intensity. The origin of those fluctuations can be assigned to a variety of dynamic and complex processes, including molecular adsorption or desorption, surface diffusion, molecular reorientation and metal surface reconstruction. Since each of these mechanisms simultaneously contributes to a fluctuating SERS signal, probing their relative impact in SM-SERS remains an experimental challenge. Here, we introduce a super-resolution imaging technique with an acquisition rate of 800,000 frames per second to probe the spatial and temporal features of the SM-SERS fluctuations from single silver nanoshells. The technique has a spatial resolution of ~7 nm. The images reveal short ~10 µs scattering events localized in various regions on a single nanoparticle. Remarkably, even a fully functionalized nanoparticle was 'dark' more than 98% of the time. The sporadic SERS emission suggests a transient hotspot formation mechanism driven by a random reconstruction of the metallic surface, an effect that dominates over any plasmonic resonance of the particle itself. Our results provide the SERS community with a high-speed experimental approach to study the fast dynamic properties of SM-SERS hotspots in typical room-temperature experimental conditions, with possible implications in catalysis and sensing.

3D Hotspots Platform for Plasmon Enhanced Raman and Second Harmonic Generation Spectroscopies and Quantitative Analysis

AbstractConstructing plasmonic hotspots in metal nanostructures is essential for plasmon‐enhanced spectroscopies (PESs), but their application in quantitative analysis is a long‐standing challenge. Herein, highly uniform, reproducible, stable, and sensitive 3D hotspots platform substrates are assembled on hydrophobic silicon wafers using a drop of solution containing silica coated surface plasmon resonance (SPR) active nanoparticles (NPs). Plasmon‐enhanced second harmonic generation (PESHG) and surface‐enhanced Raman scattering (SERS) experimental signals increase sharply with the increase of droplet evaporation time, which is attributed to the rapid increase of 3D hotspots generated by gradual decrease of distance between adjacent NPs. Compared with 3D bare metal NP substrate, 3D metal shell‐isolated nanoparticles (SHINs) substrate demonstrates much more stable PESHG and SERS signals attributing to the protecting SiO2 shells. Besides that, uniform distribution of 3D “hotspots” on the hydrophobic substrate can facilitate the quantitative analysis of an analyte using SERS. Experimental and theoretical results demonstrate that the 3D hotspots platform has the potential to be developed as a plasmon‐enhanced substrate for linear and nonlinear spectroscopies with practical sensing applications.