Present and Future of Surface-Enhanced Raman Scattering

Abstract

The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.

Keywords: SEIRA; SERS tags; TERS; biosensing; catalysis; charge transfer; chemosensors; hot electrons; nanomedicine; surface-enhanced Raman scattering.

Figure 1

SERS involves inelastic light scattering by molecules adsorbed onto corrugated metal surfaces such as silver or gold nanoparticles.

Figure 2

Predicted SERS enhancement as a function of surface coverage, for monolayers of gold NPs with different shapes (a). Solid curves obtained for excitation at 785 nm light wavelength (resonant with 65 × 21 nm nanorods); dashed curves obtained for 633 nm (resonant with 51 nm nanospheres) and 900 nm (resonant with nanostars: 20 nm core, 10 nm branches). Schemes in (b) illustrate low- and high-density NP surface coverage. Reproduced from ref (19). Copyright 2017 American Chemical Society.

Figure 3

Comparison of experiment (top) and theory (bottom) for the CO tip-enhanced Raman scattering intensity associated with an Ag-CO tip structure near a gold surface, as a function of potential bias. (inset) The tip model. Adapted from ref (43). Copyright 2018 American Chemical Society.

Figure 4

Representative anion events for both BPY-h₈ and BPY-d₈. (A) Top waterfall plot depicts time-dependent SERS data as a function of optical pump time. Anion modes appear starting at 148 s, indicated by white dotted line. Bottom plot shows the neutral spectrum for a BPY-h₈ + BPY-d₈ nanosphere assembly (black); the midspectrum depicts the contribution from neutral molecules plus BPY-h₈ anion modes that appear (red); the bottom spectrum shows open-shell DFT (CAM-B3LYP) calculation for the radical BPY-h₈ anion. (B) Waterfall plot from a different nanosphere assembly that shows a preference for BPY-d₈ anionic behavior, with anion activity appearing at 209 s of pump time. The top spectrum in the lower plot consists of neutral molecules only (black); the midspectrum is neutral plus BPY-d₈ anion modes; the bottom spectrum is DFT (CAM-B3LYP) calculated BPY-d₈ anion modes. Reproduced from ref (49). Copyright 2017 American Chemical Society.

Figure 5

(a, b) Measured differential absorbance spectra of CV and R6G adsorbed on 60 nm silver nanospheres at low surface coverage, compared with the reference absorption spectra in water. (c) Extinction and absorbance spectra of the silver nanospheres (at 8 pM) compared with the bare R6G spectrum in water. (d) Concentration dependence for the modified absorbance spectra of R6G on 60 nm silver nanospheres. See ref (58) for further experimental details.

Figure 6

Artist’s view of a plasmonic gap formed by metallic nanoparticles hosting a set of organic molecules. A picocavity is formed due to the protrusion of a single atom, localizing the electromagnetic field at the atomic scale, thus producing sub-nanometer resolution and close-to-strong optomechanical coupling. Theoretical methods to address this extreme interaction regime in surface-enhanced Raman scattering are outlined on the sides, including electrodynamics, cavity QED, quantum chemistry, and TDDFT.

Figure 7

Structures ranging from nanogap geometry (NPoM) to nanoparticle sensing using CB:AuNP aggregates enable surface-enhanced Raman scattering applications from single-molecule detection to personalized medicine.

Figure 8

Total interaction pair potentials between two (···) Au@TA, (- - -) Au@MHA, and (—) Au@MUA nanoparticles predicted using xDLVO theory. Reproduced from ref (137). Copyright 2015 American Chemical Society.

Figure 9

Gold nanostar morphologies vary substantially depending on the synthetic protocol used. (A) PVP-capped gold nanostars synthesized via seed-mediated approach. Adapted with permission from ref (190). Copyright 2012 Elsevier B. V. (B) A Good’s buffer-mediated seedless method yields nanostars with few rounded branches. Adapted from ref (182). Copyright 2016 American Chemical Society. (C) Multipod nanostars with pyramidal branches can be synthesized with high control and reproducibility employing PVP in the presence of dimethylamine. Adapted from ref (180). Copyright 2015 American Chemical Society. (D) Gold nanostars with few, long spikes can be obtained with high reproducibility employing Triton X 100 as surfactant. Adapted with permission from ref (189). Copyright 2019 Royal Society of Chemistry. (E) Tunable spike length and number can be achieved in gold nanostars employing ascorbic acid as the capping agent. Adapted from ref (200). Copyright 2018 American Chemical Society.

Figure 10

Quantitative SERS enhancement using 6T molecules encapsulated in CNT. (a) Encapsulation process, adapted from ref (227). Copyright 2016 American Chemical Society. (b) SEM image of a gold dimer with 6T@CNT deposited in the gap. (c) Scanning near-field optical microscopy image of the plasmonic hotspot. (d) SERS (red) and Raman (black) spectra of 6T measured under plasmon resonance conditions (638 nm). The total enhancement on several dimers varied between 3 × 10⁴ and 9 × 10⁵, corresponding to 1 × 10⁶ to 4 × 10⁷ for the enhancement of the Raman cross section. Reproduced with permission from ref (224). Copyright 2017 Royal Society of Chemistry.

Figure 11

(A) Schematic illustration of “promoter” induced NP interfacial self-assembly. (B) SEM characterization of SENS showing short-range hexagonal particle packing. Insets 1 and 2 show optical images of a Au MeLLF and a Au SENS, respectively. Inset 3 is a tilted SEM image of SENS showing the NPs anchored in the polymer. (C) Schematic illustration of SENS used for solvent-free SERS analysis. Adapted with permission from refs ( and 239), copyrights 2016 American Chemical Society and 2018 Elsevier B. V., respectively.

Figure 12

Self-organized gold nanoparticle layers as surface-enhanced Raman scattering substrates. (a) TEM images of hexagonally packed gold nanoparticles in a monolayer (left), bilayer (middle), and trilayer (right). BM–bright mode, DM–dark mode. (b) Absorption spectra and plasmon eigenmodes of the gold layers shown in (a). (c) Raman spectrum of polystyrene measured on the monolayer (black) and the bilayer (red). The 5–10 increase in total scattering intensity corresponds to ∼10⁴ enhancement of the Raman cross section. Adapted with permission from refs ( and 244). Copyrights 2018 American Chemical Society and 2019 Royal Society of Chemistry, respectively.

Figure 13

(a) SEM image of a vertically aligned AuNR array. (b) SERS spectra of CBN on AuNR arrays. Adapted with permission from ref (248). Copyright 2018 Elsevier B. V.

Figure 14

SEM (a, b) and TEM (c) images of HP modified with two bilayers of gold nanoparticles. (d) Photographs of dispersions of LbL-modified HPs with gold layer thickness increasing from left to right, in 1 M NaCl. (e, f) Confocal microscopy images of Au-coated HPs (e) and polystyrene beads (f), in 1 M NaCl. Adapted from ref (272). Copyright 2018 American Chemical Society.

Figure 15

(a) Background-corrected Raman scattering intensity of various AuNP-modified HPs for detection of 1 μM methylene blue with increasing loading of AuNPs; one bilayer (blue), one bilayer with salt (red), two bilayers with salt (black). (b) Raman scattering spectra obtained with HP (black) and polystyrene beads (red) modified with two bilayers of gold nanoparticles of 1 μM MB and 1 μM R6G in TSB. Adapted from ref (272). Copyright 2018 American Chemical Society.

Figure 16

Resonant nanostructures fabricated through HCL. (a) Side view of SERS substrate consisting of gold nanodisk dimers fabricated on top of a Au mirror with a SiO₂ spacer layer in between; (b) top view of a Au dimer-on-mirror SERS substrate illustrating the uniform dimensions and orientations of the individual nanostructures. (c) Si nanopillars and (d) chiral Si colloids fabricated by a variant of HCL; (e) illustration of near-field amplification and Raman scattering enhancement in Si nanodisks supporting an anapole resonance. Reproduced with permission from refs (− and 286). Copyrights 2015 Royal Society of Chemistry, 2017 American Chemical Society, 2017 Wiley-VCH, and 2018 American Chemical Society, respectively.

Figure 17

Concept of leaning nanopillar substrate. (a–c) Scheme of the leaning mechanism. (d) Scheme of the enhancement mechanism. When solvent evaporates, surface tension pulls the silicon nanopillars together, trapping analytes at the hotspot, giving rise to a large Raman signal. (e) SEM image of a cluster of leaning silver-coated silicon nanopillars. (f) Tilted SEM image of the outer perimeter of the evaporated solvent droplet area. The nanopillars to the right have leaned to form hotspots, while the nanopillars to the left remain vertical and free-standing. (g) SEM image of a nanopillar substrate, seen perpendicular to the surface. The line indicates the outer perimeter of the evaporated solvent droplet. (bottom) Individual free-standing nanopillars. (top) Clusters of nanopillars. Reproduced with permission from ref (288). Copyright 2012 Wiley-VCH.

Figure 18

Comparison of spectral emission profiles: fluorescence from Cy5 and SERS from 4-NTB on Au nanoparticles excited with 632.8 nm laser radiation. Reproduced with permission from ref (332). Copyright 2014 Royal Society of Chemistry.

Figure 19

Sketch of an SERS nanotag comprising a hollow Au/Ag nanoshell coated with an SAM of an aromatic thiol as the Raman reporter molecule. The particle is protected by a silica shell. Rerproduced with permission from ref (328). Copyright 2009 Wiley-VCH.

Figure 20

(A, B) Resonant Raman configuration; multiple-dye conjugates capable of producing signals over a wide range of common excitation wavelengths (A) and dye conjugates with similar optical properties for multiplexing (B). Adapted with permission from refs ( and 360). Copyrights 2014 American Chemical Society and 2015 Wiley-VCH, respectively. (C–F) Configuration for protection and stabilization; thiolated polystyrene (C), poly(N-isopropylacrylamide) shell (D), PSS–PAA block copolymer encapsulation (E) and phospholipid-coated surface-enhanced Raman scattering nanotags (F). Adapted with permission from refs^{, , , and} . Copyrights 2007 Springer-Nature, 2015 Wiley-VCH, 2018 Royal Society of Chemistry, and 2010 American Chemical Society, respectively.

Figure 21

SERS nanotag-antibody conjugate with a heterobifunctional linker molecule. Reproduced with permission from ref (333). Copyright 2009 Wiley-VCH.

Figure 22

Schematic view of the fabrication process of a chiral discrimination system based on SERS. Reprinted from ref (429). Copyright 2016 American Chemical Society.

Figure 23

(A, D) Normalized SERS spectra of the MPY–Ag complex separately immersed in different chiral alcohols (2-butanol and MOP, respectively) in their optical pure and racemic forms. (B, E) and (C, F) Magnifications of the 1175–1250 cm^–1 and 1540–1650 cm^–1 spectral regions of the SERS spectra shown in (A) and (D), respectively. Reproduced with permission from ref (428). Copyright 2014 Wiley-VCH.

Figure 24

Prospects and challenges of quantitative SERS (top) and representative plasmonic nanogap structures for quantitaitve SERS (bottom).

Figure 25

Whereas in conventional SERS the molecules to be probed are hard to trap and to control, a Raman probe based on 2D materials can be used to build a robust sub-nanometer gap. It enables a perfect orientation match between the vibrational modes and the local plasmonic fields. Reprinted with permission from ref (404). Copyright 2018 Springer Nature.

Figure 26

(a) (i) SEM, (ii) scattering, and (iii) Raman images of a nanoparticle-coupled nanowire plasmonic waveguide for remote SERS. The green cross in (iii) marks the illumination position. Reprinted from ref (454). Copyright 2009 American Chemical Society. (b) (i) Transmission and (ii) remote SERS images of a live HeLa cell with the nanoparticle-coupled nanowire endoscopy. (iii) The SERS spectrum from the nucleus region of the cell. Reprinted with permission from ref (457). Copyright 2014 Wiley-VCH.

Figure 27

Schematic diagram of the principle of SHINERS. Reprinted with permission from ref (465). Copyright 2010 Springer Nature.

Figure 28

SERS intensity of 10 nM R6G adsorbed on silver colloids recorded vs time. The spectral data set is shown in the inset. Excitation was at 633 nm, and acquisition time was 1 s. Adapted with permission from ref (500). Copyright 2013 D. P. dos Santos.

Figure 29

(A) Analytical calibration curve using the average value of the SERS intensities (represented as non-negative matrix factorization resolution method scores). The error bars are related to the magnitude of the fluctuations; (B) digital SERS mapping for CIPRO (100 molecules/μm²) adsorbed on immobilized gold NPs; (C) revised calibration curve obtained using the digital SERS procedure; (D) digital SERS mapping for CIPRO (10 molecules/μm²) adsorbed on immobilized gold NPs. Adapted from ref (503). Copyright 2018 American Chemical Society.

Figure 30

(A) SEM images of a typical nanorod supercrystal island film and SERS spectra of (a) natural and (b) spiked human blood; (c) natural and (d) spiked human plasma. (e) SERS spectra spiked human plasma after spectral subtraction of the matrix (human plasma). (f) SERS spectra of the scrambled prion. Adapted with permission from ref (249). Copyright 2011 National Academy of Sciences. (B) SERS of 141-nucleobase ssDNA fragment of the wild-type K-Ras gene and with different single-point mutations and its classification by using partial least-squares discriminant analysis. Adapted with permission from ref (516). Copyright 2017 Wiley-VCH. (C) SERS detection of the oncoprotein c-MYC. The sensor includes a specific peptide (H1) for c-MYC chemically attached to an optical molecular spring (mercapto-N-methylbenzamide, MMB), which is bound to a silver nanoparticle. Theoretical and experimental Raman spectrum of MMB and SERS spectra of MMB, MB-H1, and MB-H1 in the presence of c-MYC, on SiO₂@Ag. Magnification of the spectral windows between 730–800 and 990–1050 cm^–1 are also shown. (C–B) Model used in the estimation of the molecular orientation. Absolute orientation of the molecule on the surface and relative orientation of the ring over the surface are represented by XYZ and xyz axes, respectively. Adapted from ref (526). Copyright 2016 American Chemical Society.

Figure 31

(A). Operating principle of the iMS detection approach (“Off-to-On” scheme). The “stem-loop” DNA probe of the iMS, having a Raman label at one end of the stem, is immobilized onto a metallic nanoparticle or nanostar via a metal–thiol bond. In the absence of the target, the probe is “open” with very low SERS signal (“Off” state). Upon exposure to a target sequence, the target first binds to the toehold region (intermediate I) and starts displacing the DNA probe from the placeholder via branch migration (intermediate II), finally releasing the placeholder from the nanoparticle system. This enables the stem-loop to “close” and brings the Raman label closer to the plasmonic metal surface, producing a strong SERS signal (“On” state). Adapted from ref (533). Copyright 2016 American Chemical Society. (B) Nanowave platform consisting of nanosphere arrays coated with a silver film. Adapted from ref (534). Copyright 1984 American Chemical Society. The inset represents the unit cell used as a 3D model for finite element modeling calculations. Adapted from ref (535). Copyright 2012 American Chemical Society. (C) AFM image of a bimetallic Nanowave chip used for detection of Dengue nucleic acid biotargets. Adapted with permission from ref (539). Copyright 2014 Royal Society of Chemistry. (D) Synthesis of cubic nanorattles to be used in an integrated “lab-in-a-stick” device. TEM images of (E) AuNP; (F) AuNP@AgCube; (G) reporter-loaded AuNP@CubeCage; (H) cube nanorattles. Adapted with permission from ref (540). Copyright 2018 Springer Nature.

Figure 32

(a) Schematic illustration of an integrated SERS-based microfluidic channel composed of six microdroplet compartments: (i) droplet generation from the shear force at the interface between the aqueous and oil phases, (ii) droplet mixing for the first immunoreaction, (iii) droplet merging for the formation of magnetic immunocomplexes, (iv) droplet mixing for the second immunoreaction, (v) droplet splitting for the wash-free immunoassay, and (vi) Raman detection of unbound SERS nanotags in supernatant solution droplets. (b) Extended images for (i) droplet generation, (iii) droplet merging, and (vi) droplet splitting. Adapted from ref (547). Copyright 2017 American Chemical Society.

Figure 33

(A) Multiple detection of biomarkers (prostrate-specific antigen, thrombin, and mucin-1), using SERS-encoded Ag-pyramids based on three Ag NPs modified by different Raman reporters. Adapted with permission from ref (635). Copyright 2015 Wiley-VCH. (B) GO–Au structures used for simultaneous detection of EpCAM and miR-21 using CD and SERS signals. Adapted with permission from ref (634). Copyright 2017 John Wiley & Sons, Inc. (C) Au nanorod dimer–upconverting NP core–satellite nanostructures for miR-21 SERS detection and telomerase detection by luminescence. Adapted with permission from ref (636). Copyright 2017 American Chemical Society.

Figure 34

In situ detection and imaging of pyocyanin produced by P. aeruginosa PA14 grown on micropatterned SiO₂-coated Au nanorod supercrystal substrates. (a) SEM images; scale bar: 5 μm. (b) Representative SERRS spectra measured at 0, 1, 3, and 20 h of bacteria growth. (c) Relative SERRS intensities (1600 cm^–1), recorded at 0, 1, 3, and 20 h. (d) Optical image of the substrate and SERRS mapping of pyocyanin (1600 cm^–1) at 20 h of growth. Scale bar: 5 μm. (e, f) SEM images of supercrystals colonized by P. aeruginosa (20 h) at different magnifications. Scale bar: 5 μm. Reproduced with permission from ref (655). Copyright 2016 Springer Nature.

Figure 35

Conceptual view of a microorganism optical detection system and its relevant components. Silver NPs are separately labeled with different Raman-active molecules and functionalized with bacteria-selective antibodies (1). A NP dispersion is mixed in a vessel (3 mL) with the possibly infected sample fluid (2). Several types of bacteria are targeted using NPs prepared with specific combinations of Raman molecules and antibodies. The presence of one of these microorganisms induces aggregation of antibody-matching NPs on its membrane, rapidly evolving toward full random coverage (3). The mixture circulates through a millifluidic channel (4) and passes through the focus of a 785 nm laser (5), which is in turn spectrally analyzed to record the SERS signal generated by the Raman-active molecules (6). Targeted bacteria produce a large increase in the SERS signal, whose spectral fingerprints enable the identification of the pathogen. Correlation between a time series of spectra and the SERS reference of labeled NPs. The analyzed serum samples contain either one pathogen (1–4, see labels) or no pathogen (5, blank). Series 6 shows the result for a blood sample spiked with a combination of three different bacteria (S. aureus, E. coli, and S. agalactiae). Large correlation values reveal the passage of an individual bacteria or CFU. Bacterial cultures (24–48 h) for the microorganism inoculated in the blood samples (series 6). White spots correspond to the CFUs. Adapted with permission from ref (662). Copyright 2016 Springer Nature.

Figure 36

SERS tags can be employed to (A) aid in the identification of tumor margins intraoperatively. Adapted from ref (683). Copyright 2014 American Chemical Society. (B) identify and localize tumor tissues endoscopically. Adapted with permission from ref (684). Copyright 2015 Optical Society of America. (C) aid in the multiplex detection of circulating tumor cells, by identifying overexpressed membrane biomarkers. Adapted with permission from ref (692). Copyright 2014 Springer Nature.

Figure 37

(A) SERS active gold nanostar dimer for mercury ion detection (top) and gold nanoparticle–nanorod heteroassemblies for bisphenol A detection (bottom). Adapted with permission from refs ( and 704). Copyrights 2013 Royal Society of Chemistry and 2016, Elsevier B. V., respectively. (B) SERS active gold nanorod assembly for toxin detection. Adapted with permission from ref (559). Copyright 2012 Royal Society of Chemistry. (C) Plasmonic nanoparticle heterochains and SERS enhancement properties. Adapted from ref (710). Copyright 2013 American Chemical Society.

Figure 38

Schematic view of various approaches used to localize a Raman target within the enhancing electromagnetic field. Reproduced from ref (723). Copyright 2018 American Chemical Society.

Figure 39

(a) SERS detection of 1-naphtol using core–shell Au@poly(N-isopropylacryamide) colloids. Reproduced with permission from ref (742). Copyright 2009 Wiley-VCH. (b) Plasmonic thin films fabricated through LbL assemby of Au nanoparticles and ammonium pillar[5]arene (AP[5]A) for (multiplexed) SERS sensing of PAHs in gas or liquid phase. Reproduced from ref (750). Copyright 2017 American Chemical Society. (c) ZIF8-coated silver film over nanospheres for the detection of benzene, toluene, nitrobenzene, or 2,6-di-tert-butylpyridine in gas phase. Reproduced with permission from ref (757). Copyright 2014 Royal Society of Chemistry.

Figure 40

SESORRS false-color two-dimensional (2D) heat maps of the peak intensity at (a) 1178 cm^–1 (dye 823), (b) 1181 cm^–1 (dye 813), (c) 1185 cm^–1 (dye 810) and (d) 1181 cm^–1 (triplex). Measurements were performed using an xy translational stage in step sizes of 3 mm to create an image of 8 × 8 pixels. 2D heat maps were generated and show the tracking of each of the four MTS models through 10 mm of tissue. Clear discrimination is seen between spectra collected at the point of maximum intensity, where the nanotags were spotted, and that collected where the nanotags were not present. The corresponding maximum and minimum collected 8 mm offset spectra also confirm the presence of the nanotags in regions where the MTS were spotted (a–d). Reproduced with permission from ref (763). Copyright 2018 Royal Society of Chemistry.

Figure 41

(a) Intensity–electrochemical potential profile for the 1020 cm^–1 line of piperidine on a silver electrode. The dots show the fit of theoretical analysis with Γ = 0.3 eV. (b) Electrochemical potential maximum of the SERS of PATP signal as a function of excitation light energy. Reproduced with permission from ref (769). Copyright 1986 American Institute of Physics. (c) Schematic presentation of the electrochemical tuning of the strong coupling strength between dye excitons and plasmons. Reproduced from refs ( and 789). Copyrights 2008 and 2018, respectively, American Chemical Society.

Figure 42

Schematic representation of vibrational transitions: linear Stokes Raman scattering (A), Stokes hyper Raman scattering (B), anti-Stokes hyper Raman scattering (C), Stokes resonant Raman scattering (D), and Stokes resonant hyper Raman scattering (E). Molecular systems undergo vibrational transitions from the initial state (ν = 0 for Stokes, ν = 1 for anti-Stokes) to the final state (ν = 1 for Stokes, ν = 0 for anti-Stokes), linked to a normal mode k with frequency nk. One possible resonance condition is depicted in (E). Reproduced with permission from ref (796). Copyright 2017 Royal Society of Chemistry.

Figure 43

Schematic illustration of STM- (a) and AFM- (b) based TERS setups. (c) Experimental TERS images obtained with ultrahigh-vacuum TERS, which enables visualization of different normal modes of a single molecule. (d) Simulation results. (e) Assigned vibrational normal modes. (c−e) Reproduced with permission from ref (834). Copyright 2019 Springer Nature.

Figure 44

(a) Cross sections: σ_ext for extinction, σ_abs for absorption, and σ_sca for scattering of light. These cross sections have been calculated for gold antennas at the fundamental resonance by FDTD simulations and are shown as quantities normalized to the geometric cross section. Reproduced from ref (889). Copyright 2015 American Chemical Society. The maximum near-field enhancement is expected when σ_abs = σ_sca at the plasmon resonance. In (b) the relative transmittance (at normal incidence of light) of a 10 nm thick layer of a tetrafluorinated zinc phthalocyanine complex (C₃₂H₁₂F₄N₈Zn, short name: F4ZnPc) on a CaF₂ wafer is shown on top. Spectra of 10 nm F4ZnPc on 50 nm high nanoantennas produced on CaF₂ substrates, from various metals, are shown in the middle (as transmittance at normal incidence of light with polarization along the antennas divided by the bare substrate’s transmittance) and in the bottom panel as baseline-corrected vibrational spectra. The width of the nanoantennas has been adjusted to the maximum SEIRA enhancement, which is achieved if σ_abs = σ_sca. SEIRA is directly obvious, and, furthermore, it should be noticed that the SEIRA enhancement comes only from the small sample regions with near-field enhancement. Bigger enhancement with Cu, Ag, and Au antennas is attained because of the lower electronic damping. Reproduced from ref (895). Copyright 2018 American Chemical Society.

Figure 45

(a) Near normal relative reflectance (normalized to the reflectance of the flat gold layer, polarization as indicated in the inset) of one bowtie aperture in a 50 nm thick gold layer (on 2.5 nm Cr as adhesion layer) on CaF₂. The SiO₂ nanosphere (85 nm in diameter) in the gap of the bowtie aperture gives rise to an anti-absorption-like feature on the broader plasmonic resonance spectrum. The geometry of the nanostructure can be recognized in the SEM image shown as inset. (b) Baseline-corrected spectrum of the nanosphere. The peak is related to a localized phonon-polariton excitation of the sphere in the Si–O–Si stretching vibration band and is thus material-specific. The inset shows an SEM image from (a) enlarged to the gap region with the sphere colored in red (scale bar: 400 nm). Reproduced with permission from ref (905). Copyright 2019 American Physical Society.

Figure 46

Schematic representation of PEMS and PMCR.

Figure 47

Potential-dependent spatial origin of SERS from surface-tethered Nile blue molecules (colored points) overlaid on an SEM image of the underlying AuNP aggregate. The applied potential range is indicated at the top of each panel. The data show that molecules located near junction regions are the most difficult to reduce/easiest to oxidize. Potentials are reported relative to a Ag|AgCl electrode. Reproduced from ref (954). Copyright 2015 American Chemical Society.

Figure 48

Emerging strategies to monitor liquid-based (A) and gas-based (B) reactions using SERS. (A) Particle-assembled microdroplets, such as plasmonic liquid marble and plasmonic colloidosome, have been designed to track (i) immiscible liquid–liquid reactions; adapted from ref (958), Copyright 201, American Chemical Society; and (ii) electrochemical reactions; adapted with permission from ref (958), copyright 2016 Wiley-VCH. (B) Integrating MOFs with SERS to read-out molecular events during (i) gas–solid interaction; adapted from ref (961), copyright 2017 American Chemical Society; (ii) gas–liquid reaction; adapted with permission from ref (962), copyright 2018, Wiley-VCH.