Spectroscopy Enhancement Factor Research Articles

Au nanodisks-based 2D photonic crystals fabricated by single-beam laser interference lithography: A simple and reliable alternative for highly efficient large-scale SERS molecular sensors

Noble metal-based Photonic Crystals (PCs) have emerged as outstanding candidates for precise light management, projecting applications in strategic areas for society like high-sensitivity and fast molecular (inorganic/organic/bio) sensing by Surface-Enhanced Raman Spectroscopy (SERS). In this work, we report an exhaustive study on the potential of large-scale (active area >1 [cm2]) Au nanodisks-based 2D PCs fabricated by single-beam Laser Interference Lithography (LIL) for high-performance SERS molecular sensing. This technique was used to fabricate periodic nanoarrays (period of 470 [nm]) of Au nanodisks with thicknesses from 50 up to 125 [nm]. The period was chosen following Finite-Difference Time-Domain (FDTD) simulations that suggested the best electric-near field enhancement for this condition. Confocal Raman microscopy and Methylene Blue (MB) as active Raman marker, were used to assess the samples' performance for molecular sensing. SERS studies have shown that the nanodisks' thickness can be a considerable size parameter for the Raman signal amplification, observing higher signal enhancements for higher thicknesses. The observed thickness effects on the Raman signal enhancement were consistent with FDTD simulations, which predicted higher electric-near field amplifications for higher thickness within the red/near-infrared range. Results show that our PCs enable to measure the characteristic Raman footprint of the analyte with good spectral resolution using relatively low powers (0.04–1 [mW]) and short acquisition times (1–30 [s]), considering an MB surface mass density as low as 2.6 [ng/cm2]. SERS enhancement factors as high as 2 x 107 were achieved for PCs with the highest thickness, representing a competitive performance concerning typically reported values (104–107) for current noble metal-based PCs technologies and a new record concerning PCs fabricated by LIL (104–105). This research demonstrates the high competitivity of these simple Au nanodisks-based 2D PCs, fabricated using an efficient large-scale and low-cost lithography technique, for fast, high spectral resolution and highly reproducible SERS-based molecular sensing.

In situ electrochemical regeneration of nanogap hotspots for continuously reusable ultrathin SERS sensors

Surface-enhanced Raman spectroscopy (SERS) harnesses the confinement of light into metallic nanoscale hotspots to achieve highly sensitive label-free molecular detection that can be applied for a broad range of sensing applications. However, challenges related to irreversible analyte binding, substrate reproducibility, fouling, and degradation hinder its widespread adoption. Here we show how in-situ electrochemical regeneration can rapidly and precisely reform the nanogap hotspots to enable the continuous reuse of gold nanoparticle monolayers for SERS. Applying an oxidising potential of +1.5 V (vs Ag/AgCl) for 10 s strips a broad range of adsorbates from the nanogaps and forms a metastable oxide layer of few-monolayer thickness. Subsequent application of a reducing potential of −0.80 V for 5 s in the presence of a nanogap-stabilising molecular scaffold, cucurbit[5]uril, reproducibly regenerates the optimal plasmonic properties with SERS enhancement factors ≈106. The regeneration of the nanogap hotspots allows these SERS substrates to be reused over multiple cycles, demonstrating ≈5% relative standard deviation over at least 30 cycles of analyte detection and regeneration. Such continuous and reliable SERS-based flow analysis accesses diverse applications from environmental monitoring to medical diagnostics.

Synthesis of SiO2@Ag Nanocomposite for Investigating Metal-Enhanced Fluorescence and Surface-Enhanced Raman Spectroscopy.

SiO2@Ag nanocomposite (NC) has been synthesized by the chemical reduction and Stӧber method for Metal-enhanced fluorescence (MEF) of Rhodmine 6G (R6G) and Surface-enhanced Raman spectroscopy (SERS) of Malachite green (MG). As-synthesized SiO2@Ag NC indicated SiO2 nanosphere (NS) and Ag nanoparticle (NP) morphologies. The SiO2@Ag NC was high quality with a well-defined crystallite phase with average sizes of 24nm and 132nm for Ag NP and SiO2 NC, respectively. By using SiO2@Ag NC, the photoluminescence (PL) intensity of the R6G (at 59.17ppm) was increased approximately 133 times. The SERS of the MG (at 1.0ppm) with SiO2@Ag NC as substrate clearly observed vibrational modes in MG dye at 798, 916, 1172, 1394, and 1616cm-1. As a result, the SERS enhancement factor (EFSERS) at 1172cm-1 obtained 6.3 × 106. This initial study points to the potential of SiO2@Ag NC as a promising material for MEF and SERS substrates to detect dyes at low concentrations.

Gold-coated nanoripples produced by UV-Femtosecond lasers for surface enhanced Raman spectroscopy

Femtosecond (fs) lasers have been recognized as a powerful tool for micro/nanofabrication, due to their advantages of high flexibility, high repeatability, and minimal introduction of heat. Many studies have been made to employ fs laser processing in fabrication of nanostructures on substrates for surface-enhanced Raman spectroscopy (SERS). However, there have been few reports of ultraviolet (UV)-fs-laser-induced nanoripples for SERS experiments. In this study, we developed simple, easy-to-prepare, and large-active-area SERS substrates using UV-fs laser irradiation. Nanoripples induced in open air by the UV-fs laser on Silicon (Si) substrates with gold coatings were used as SERS-active substrates. Nanoripple evolution by the UV-fs laser was explored to establish the processing windows for nanoripple formation as functions of laser fluence and hatching distances. Moreover, SERS enhancement factors (EFs) were correlated with the laser induced nanoripple patterns. SERS EFs up to 2.4 × 107 were achieved for Rhodamine 6G (R6G) molecules on the laser-textured Si substrates with Au coatings of 34 nm thick. The average period of nanoripples for the highest SERS EF is 253 nm, fabricated by a laser fluence of 0.48 J/cm2 and a hatching distance of 38 µm. R6G concentration down to 10−9 mol/L was detected on the laser-textured Si substrate. Besides Si substrates, three other materials, stainless steel, glass, and polystyrene, were also investigated for nanoripple formation and SERS measurements. Nanoripple structures were formed on stainless-steel surfaces and the highest SERS EF of 1.5 × 107 was achieved. For glass and polystyrene substrates, no nanoripple structures were found. As a result, the SERS EFs (∼106) of glass and polystyrene are one order of magnitude lower than those of the Si and stainless-steel substrates. The SERS demonstration using these materials further supports the mechanisms of nanoripple formation and provides a foundation to make UV-fs laser processing an effective technique for SERS applications using various materials. Applications for Sudan I, small extracellular vesicles (sEVs), and glucose were explored and demonstrated the capability of the SERS substrates.

Ultra-small gold particles via Citrus × sinensis: Theoretical and experimental SERS study

In this study, we have addressed the green synthesis of sub-nanostructured gold species using Citrus × sinensis. Absorption bands in the ultraviolet region (338 nm) were observed. Analysis by atomic resolution microscopy revealed the identification of sub-nanostructured Au species with particle sizes of about 0.9 nm. With favorable results, these systems were tested as substrates for surface-enhanced Raman spectroscopy (SERS) using pyridine. The chemical enhancement mechanism was studied as the source of the SERS effect. Complementarily, the interaction between gold clusters (Au2n, with n = 1–5) and pyridine was modeled using Density Functional at the Becke-3-parameter-Lee-Yang-Parr level of approximation in combination with the LANL2DZ basis set. A correlation between the adsorption energy and the SERS enhancement factor was analyzed as a function of the Au-N interaction distance.

SERS detection of shikonin dye by metal-coordinated tethering on gold nanofilm

We developed a spectroscopic detection method of dye molecules by combining surface-enhanced Raman spectroscopy (SERS) and a metal-coordinated tethering method. In order to mimic the surface of cellulose fiber, a gold nanofilm (AuNF) surface was modified with hydroxyl groups, and then treated following a tethering process to form a metal ion-mediated dye ligand complex (Al3+-shikonin dye complex) on the AuNF. Then, the colloidal gold nanoparticles (AuNPs) were dropped and dried to form SERS hot-spots, where shikonin locates in the interstitial gap between AuNP and AuNF. The SERS enhancement factor (EF) of AuNP-on-AuNF was calculated to be 4.4 × 107 via SERS measurement of 4-mercaptobenzoic acid (4-MBA). The SERS heatmap of shikonin from the ‘dyed AuNP-on-AuNF’ substrate showed that shikonin was uniformly tethered on AuNF surface in the same way as the dyeing of dye molecules on cellulose fibers. Furthermore, the spectral differences were observed between ‘dyed AuNP-on-AuNF’ and salt-induced AuNP aggregates due to the difference in vibrational modes between dyed shikonin and free shikonin. This method can apply to various natural dyes owing to the versatility of the dyeing method with a metal ion. Moreover, SERS libraries obtained using this method can directly compare with dyes tethered on fibers.

Effect of the Gas Temperature on Agglomeration of Au Nanoparticles Synthesized by Spark Discharge and Their Application in Surface-Enhanced Raman Spectroscopy

In this work, we have, for the first time, experimentally verified the hypothesis of reducing the agglomeration rate of aerosol nanoparticles produced by spark discharge upon decreasing the carrier gas temperature in the range of 24 °C to –183 °C. The synthesis of nanoparticles was implemented as a result of spark ablation of electrodes manufactured from Au with a purity of 99.998% installed in a specially designed gas chamber dipped into liquid nitrogen (−196 °C) to cool down the carrier gas supplied through one of hollow electrodes. It follows from the analysis of transmission electron microscopy images that both the average size of primary nanoparticles and the degree of their sintering become lower if the gas is cooled. For example, in the case of using nitrogen as a carrier gas, the average size of primary nanoparticles decreases from 9.4 nm to 6.6 nm as the gas temperature decreases from 24 °C to –183 °C. This also causes the aggregates to become more branched, manifested by the reduction in their solidity from 92% to 76%. The agglomeration model of Feng based on Smoluchowski theory was employed to calculate particle size distributions that were found to be consistent with the experimental data. The gold nanoparticles synthesized at room and cryogenic temperatures of the carrier gas (N₂, Ar + H₂, He) were used to pattern plasmonic nanostructures on ceramic alumina substrates by using aerosol jet printing technology for the purpose of demonstrating the possibility of their application in surface-enhanced Raman spectroscopy (SERS). The SERS enhancement factor was estimated at 2 × 106 from the analysis of SERS and normal Raman spectra of 1,2-bis(4-pyridyl)ethylene used as an analyte.

Highly Sensitive, Robust, and Recyclable TiO2/AgNP Substrate for SERS Detection.

Label-free biosensors provide an important platform for detecting chemical and biological substances without needing extra labeling agents. Unlike surface-based techniques such as surface plasmon resonance (SPR), interference, and ellipsometry, surface-enhanced Raman spectroscopy (SERS) possesses the advantage of monitoring analytes both on surfaces and in solutions. Increasing the SERS enhancement is crucial to preparing high-quality substrates without quickly losing their stability, sensitivity, and repeatability. However, fabrication methods based on wet chemistry, nanoimprint lithography, spark discharge, and laser ablation have drawbacks of waste of time, complicated processes, or nonreproducibility in surface topography. This study reports the preparation of recyclable TiO2/Ag nanoparticle (AgNP) substrates by using simple arc ion plating and direct-current (dc) magnetron sputtering technologies. The deposited anatase-phased TiO2 ensured the photocatalytic degradation of analytes. By measuring the Raman spectra of rhodamine 6G (R6G) in titrated concentrations, a limit of detection (LOD) of 10−8 M and a SERS enhancement factor (EF) of 1.01 × 109 were attained. Self-cleaning was performed via UV irradiation, and recyclability was achieved after at least five cycles of detection and degradation. The proposed TiO2/AgNP substrates have the potential to serve as eco-friendly SERS enhancers for label-free detection of various chemical and biological substances.

Hydrogel-Assisted 3D Volumetric Hotspot for Sensitive Detection by Surface-Enhanced Raman Spectroscopy.

Effective hotspot engineering with facile and cost-effective fabrication procedures is critical for the practical application of surface-enhanced Raman spectroscopy (SERS). We propose a SERS substrate composed of a metal film over polyimide nanopillars (MFPNs) with three-dimensional (3D) volumetric hotspots for this purpose. The 3D MFPNs were fabricated through a two-step process of maskless plasma etching and hydrogel encapsulation. The probe molecules dispersed in solution were highly concentrated in the 3D hydrogel networks, which provided a further enhancement of the SERS signals. SERS performance parameters such as the SERS enhancement factor, limit-of-detection, and signal reproducibility were investigated with Cyanine5 (Cy5) acid Raman dye solutions and were compared with those of hydrogel-free MFPNs with two-dimensional hotspots. The hydrogel-coated MFPNs enabled the reliable detection of Cy5 acid, even when the Cy5 concentration was as low as 100 pM. We believe that the 3D volumetric hotspots created by introducing a hydrogel layer onto plasmonic nanostructures demonstrate excellent potential for the sensitive and reproducible detection of toxic and hazardous molecules.

Facial Fabrication of Large-Scale SERS-Active Substrate Based on Self-Assembled Monolayer of Silver Nanoparticles on CTAB-Modified Silicon for Analytical Applications.

Surface-enhanced Raman spectroscopy (SERS) has been proven to be a promising analytical technique with sensitivity at the single-molecule level. However, one of the key problems preventing its real-world application lies in the great challenges that are encountered in the preparation of large-scale, reproducible, and highly sensitive SERS-active substrates. In this work, a new strategy is developed to fabricate an Ag collide SERS substrate by using cetyltrimethylammonium bromide (CTAB) as a connection agent. The developed SERS substrate can be developed on a large scale and is highly efficient, and it has high-density “hot spots” that enhance the yield enormously. We employed 4-methylbenzenethiol(4-MBT) as the SERS probe due to the strong Ag–S linkage. The SERS enhancement factor (EF) was calculated to be ~2.6 × 106. The efficacy of the proposed substrate is demonstrated for the detection of malachite green (MG) as an example. The limit of detection (LOD) for the MG assay is brought down to 1.0 × 10−11 M, and the relative standard deviation (RSD) for the intensity of the main Raman vibration modes (1620, 1038 cm−1) is less than 20%.

Plasmonic hotspot engineering of Ag-coated polymer substrates with high reproducibility and photothermal stability

Plasmonic silver nanoparticles (AgNPs) arranged on polyimide (PI) nanopillars (AgNP/PI) were developed as a stable photothermal surface-enhanced Raman spectroscopy (SERS) platform. The AgNP/PI substrates were prepared via maskless plasma etching and metal deposition using an in situ vacuum sputtering system. The AgNP/PI with high periodicity and high areal density led to high sensitivity, with a SERS enhancement factor (EF) of 2.2 × 108, and excellent reproducibility (relative standard deviation of 7.6%), as evaluated using the high-precision Raman mapping technique. To support the high SERS EF and sensitivity, the near-field enhancements of the dense AgNP array were calculated using a finite-difference time domain method. Notably, compared with AgNPs on polyethylene terephthalate (PET) nanopillar substrates, the AgNPs formed on the thermally stable PI nanopillar substrates exhibited excellent photothermal stability under illumination with a high-powered laser. The PI and PET surface changes induced by photoinduced local heating were investigated in terms of the glass-transition temperature, coefficient of thermal expansion, and thermal conductivity. The new plasmonic SERS platform has strong potential for reliable molecular detection in practical sensing applications with high-powered laser illuminations.

Au/SiO2-Nanolaminated Plasmonic Nanoantennas as Refractive-Index-Insensitive and Transparent Surface-Enhanced Raman Spectroscopy Substrates

Surface-enhanced Raman spectroscopy (SERS) has become a powerful technique for ultrasensitive biochemical detection providing molecular fingerprint information. Due to the strong dependence of surface plasmon resonance wavelength on plasmonic nanostructures’ surrounding refractive index (RI), SERS enhancement factors (EFs) at hotspots are sensitive to changes in background RI, which is detrimental to quantitative SERS biochemical analysis in real-world applications with spatially and temporally varying RI matrices. This work reports on a tapered-shape nanolaminate plasmonic nanoantenna (TNLNA) platform that supports multiple, spatially overlapped, hybridized magnetoelectric localized surface plasmon modes revealing high SERS EFs (>107) and an insensitivity to background RI variations (between 1.30 and 1.60). Furthermore, we demonstrate that the uniform arrays of TNLNAs can be manufactured on flexible transparent polymer films to achieve backside-excitable and reversible SERS measurements for in situ label-free glucose monitoring on a skin phantom.

SERS and the scientific career of Richard P. Van Duyne (1945–2019)

AbstractThis article describes the scientific career of our dear friend Richard P. Van Duyne. Rick was the discoverer of surface‐enhanced Raman spectroscopy (SERS), and over the period from the mid‐1970s until his death in 2019, he served as both the intellectual leader of the field, and the technical genius who pushed the field forward through his numerous inventions and discoveries. Rick's work went through many phases, and even involved a significant period of inactivity that Rick was able to break out of, but especially after 2000, there was a period of incredible creativity that ultimately led to a detailed understanding of what the SERS enhancement factor is all about and also led to fabrication techniques for making optimum SERS substrates such that applications of SERS to a wide variety of real‐world problems in sensing became possible. In addition, Rick's SERS work spawned numerous related technologies, including index of refraction sensing based on localized surface plasmon resonance (LSPR) measurements, and the extension of SERS to its tip‐enhanced cousin TERS, to electrochemical versions of SERS and TERS, and to ultrafast nonlinear techniques. The practical applications he pursued included diagnostics for diseases such as cancer, hand‐held glucose sensing, and SERS studies related to art conservation. However, Rick's legacy will probably be best known through the numerous students, postdocs, and collaborators that passed through his lab and who are now active researchers in areas related to SERS at universities and industrial labs around the world.

Partial Leidenfrost Evaporation-Assisted Ultrasensitive Surface-Enhanced Raman Spectroscopy in a Janus Water Droplet on Hierarchical Plasmonic Micro-/Nanostructures.

The conventional methods of creating superhydrophobic surface-enhanced Raman spectroscopy (SERS) devices are by conformally coating a nanolayer of hydrophobic materials on micro-/nanostructured plasmonic substrates. However, the hydrophobic coating may partially block hot spots and therefore compromise Raman signals of analytes. In this paper, we report a partial Leidenfrost evaporation-assisted approach for ultrasensitive SERS detection of low-concentration analytes in water droplets on hierarchical plasmonic micro-/nanostructures, which are fabricated by integrating nanolaminated metal nanoantennas on carbon nanotube (CNT)-decorated Si micropillar arrays. In comparison with natural evaporation, partial Leidenfrost-assisted evaporation on the hierarchical surfaces can provide a levitating force to maintain the water-based analyte droplet in the Cassie-Wenzel hybrid state, i.e., a Janus droplet. By overcoming the diffusion limit in SERS measurements, the continuous shrinking circumferential rim of the droplet, which is in the Cassie state, toward the pinned central region of the droplet, which is in the Wenzel state, results in a fast concentration of dilute analyte molecules on a significantly reduced footprint within several minutes. Here, we demonstrate that a partial Leidenfrost droplet on the hierarchical plasmonic surfaces can reduce the final deposition footprint of analytes by 3-4 orders of magnitude and enable SERS detection of nanomolar analytes (10-9 M) in an aqueous solution. In particular, this type of hierarchical plasmonic surface has densely packed plasmonic hot spots with SERS enhancement factors (EFs) exceeding 107. Partial Leidenfrost evaporation-assisted SERS sensing on hierarchical plasmonic micro-/nanostructures provides a fast and ultrasensitive biochemical detection strategy without the need for additional surface modifications and chemical treatments.

Effective SERS substrate obtained by Au deposition at silica surfaces through a top down method

Background: In the last years some of us developed methods for preparing, through simple synthesis protocols, gold nanoparticles supported on silica surfaces: in that cases, bottom-up approaches has been adopted (i.e. starting from HAuCl4 precursor) and high surface area silica (Aerosil 300) was chosen as support (together with controlled pH conditions), to facilitate Au dispersion. The obtained substrates showed the ability to enhance Raman signals of dosed molecules pyridine and bi-pyridine and/or of silanols species populating the silica surface. Following this idea, in this paper we will present results concerning surface-enhanced Raman spectroscopy (SERS) activity of substrates obtained by a top-down technique (i.e. gold sputtering) which allowed depositing gold nanoparticles at surfaces of silica nanoparticles. Pyridine molecule has been then used as a probe molecule to estimate the SERS activity of the obtained substrates. Objectives: The purpose of the work was to study the SERS activity of the prepared substrates through the estimation of limit of detection (LoD) of pyridine molecule dosed from solutions (benzene was used as solvent) with decreasing pyridine concentration. Materials and methods: For the preparation of the samples AOX50 silica (Evonik Industries, surface area of 50 m2/g) in form of pressed disk, a gold target and a K575X Turbo Sputter Coater (Quorum Technologies) were used. Methods applied: UV-Vis-NIR spectroscopy; HR-TEM microscopy; Raman spectroscopy. Results: Prepared Au/AOX50 substrates with nominal Au thickness 10 nm were characterized by the UV-Vis spectroscopy and HR-TEM microscopy. They showed a relatively complex absorption profile extending to the NIR region. The substrates contained gold nanoparticles with diameter in the range of 2.0–3.5 nm. By using the 785 nm exciting laser line (suggested by the results coming from UV-Vis spectroscopy) for recording Raman spectra, it was possible to observe characteristic pyridine Raman signals by contacting Au/AOX50 substrates with vapour phase of benzene solutions with pyridine concentration as low as 10-7 M comprising LoD. This allowed us to estimate roughly an enhancement factor of 108, as compared with 12.4 M pyridine dosed at naked AOX50 silica surface. Conclusion: Through a top-down technique it was possible to prepare effective SERS substrates composed by Au nanoparticles dispersed at AOX50 silica surfaces. LoD for pyridine dosed from the vapour of its benzene solution appeared to be 10-7 M and the SERS enhancement factor, using as a reference Raman spectra of 12.4 M pyridine interacting with naked AOX50 silica, has been estimated to be approximately 108.

Integrated “Hot Spots”: Tunable Sub‐10 nm Crescent Nanogap Arrays

AbstractPlasmonic crescent nanogap arrays (CNGAs) integrated by nanoscale gaps and nanotips exhibit strong capability of light confinement and thus lead to extremely electric field enhancement. The CNGAs with tunable gap‐width are fabricated via a low‐cost and simple process combining colloidal lithography and nanoskiving techniques. Incident light is captured in the nanogaps and at the two tips of nanocrescent, where extremely strong electric fields are excited. The strong electric fields lead to greatly enhanced surface‐enhanced Raman spectroscopy (SERS) performance with typical enhancement factor of 8 and 360 times in comparison with the crescent gold nanoparticles and linear gold nanowire, and the SERS enhancement factor at CNGAs with G = 1 nm is up to about 6.04 × 108. The CNGAs with strong electric fields and remarkable SERS signals are promising to advance the applications of optical filters, surface‐enhancement spectroscopies, nonlinear optics, single‐molecule analysis, and nanolasing.

Refractive-Index-Insensitive Nanolaminated SERS Substrates for Label-Free Raman Profiling and Classification of Living Cancer Cells.

Surface-enhanced Raman spectroscopy (SERS) has emerged as an ultrasensitive molecular-fingerprint-based technique for label-free biochemical analysis of biological systems. However, for conventional SERS substrates, SERS enhancement factors (EFs) strongly depend on background refractive index (RI), which prevents reliable spatiotemporal SERS analysis of living cells consisting of different extra-/intracellular organelles with a heterogeneous distribution of local RI values between 1.30 and 1.60. Here, we demonstrate that nanolaminated SERS substrates can support uniform arrays of vertically oriented nanogap hot spots with large SERS EFs (>107) insensitive to background RI variations. Experimental and numerical studies reveal that the observed RI-insensitive SERS response is due to the broadband multiresonant optical properties of nanolaminated plasmonic nanostructures. As a proof-of-concept demonstration, we use RI-insensitive nanolaminated SERS substrates to achieve label-free Raman profiling and classification of living cancer cells with a high prediction accuracy of 96%. We envision that RI-insensitive high-performance nanolaminated SERS substrates can potentially enable label-free spatiotemporal biochemical analysis of living biological systems.

Photocatalytical decoration of thin titania coatings with silver nanostructures provides a robust and reproducible SERS signal

AbstractCost‐effective photocatalytic synthesis was used to fabricate silver nanostructures (AgNSs) on TiO2 coatings (AgNSs/TiO2) with plasmonic properties. In this study, the AgNSs growth controlled by variation of the concentration of Ag+ ions, hole scavengers, and ultraviolet illumination was systematically studied in terms of morphological characterization, extinction, and surface‐enhanced Raman spectroscopy (SERS). These structures with well‐defined and high photocatalytic properties show a plethora of shapes (from spherical to polygonal objects), diameter (8–108 nm), and surface coverage (12–62%). Morphological properties strongly affect nanoplasmonic features as revealed by electronic and SERS spectra. Surface enhancement of Raman signal was probed by using 4‐mercaptobenzoic acid to elucidate the stability and uniformity of the designed substrates. A detailed examination and correlation of geometrical parameters and SERS performance indicate synergistic contribution of the formation of hot spots between AgNSs. Some of the fabricated AgNSs present a very good homogeneity and reproducibility as well as long‐time stability. The highest enhancement factor in SERS was obtained for the Ag–TiO2 hybrids with a diameter and surface coverage of AgNSs less than 30 nm and larger than 50%, respectively.

Fiber-optic plasmonic probe with nanogap-rich Au nanoislands for on-site surface-enhanced Raman spectroscopy using repeated solid-state dewetting.

.We report a fiber-optic plasmonic probe with nanogap-rich gold nanoislands for on-site surface-enhanced Raman spectroscopy (SERS). The plasmonic probe features nanogap-rich Au nanoislands on the top surface of a single multimode fiber. Au nanoislands were monolithically fabricated using repeated solid-state dewetting of thermally evaporated Au thin film. The plasmonic probe shows in SERS enhancement factor and 100 nM in limit-of-detection for crystal violet under both the excitation of laser light and the collection of SERS signals through the optical fiber. The fiber-through measurement also demonstrates the label-free SERS detection of folic acid at micromolar level. The plasmonic probe can provide a tool for on-site and in vivo SERS applications.

Probing Electrolyte Solvents at Solid/Liquid Interface Using Gap-Mode Surface-Enhanced Raman Spectroscopy

Understanding the aprotic solution structures at the immediate vicinity of solid/liquid interface (SLI) is critically important for next generation lithium ion battery development. Yet, it is still challenging to investigate the carbonate chemical profiles close to the diffuse layer (about 10 nm) of the electrical double layer at SLI due to the lack of a ultrahigh surface sensitive tool. In this work, we demonstrate the structures of commonly used carbonate solvents (ethylene carbonate (EC) and diethyl carbonate (DEC)) and a carbonate additive (fluoroethylene carbonate (FEC)) in a commercial Li-ion battery electrolyte can be determined at ∼17 nm above the electrode surface. This is only enabled by a nanogap surface-enhanced Raman spectroscopy (SERS) technique based on a monolayer gold nanoparticle (Au NP) ensemble. The SERS enhancement factor (EF) of those carbonates was found to depend on the molecular polarizability, with the maximum EF at ∼105 found for EC and FEC. Despite their alike chemical structures, this monolayer Au NP SERS substrate is fully capable of discrimiating the different Raman finger prints of EC and FEC. Compared to EC, several vibration modes in FEC, such as C-C skeletal deformation, ring breathing band and C=O stretching band, shift to higher frequencies because of the displacement of a hydrogen atom by a much heavier fluorine atom in a methylene bridge. This counterintuitive observation against the commonly used “ball and spring” model in vibrational spectroscopy is mostly due to the increased bond strength in the FEC ring versus that of EC. A second order empirical polynomial best describes the correlation between the SERS band integration of EC or DEC molar concentration. Our findings open up new opportunities for in-depth understanding of the electrolyte molecular vibrational behaviors at direct solid/liquid interface and developing advanced electrolytes for next generation lithium-ion batteries.