Raman-active Molecules Research Articles

High-power femtosecond molecular broadening and the effects of ro-vibrational coupling

Scaling spectral broadening to higher pulse energies and average powers is a critical step in ultrafast science, especially for narrowband Yb-based solid state lasers, which have become the new state-of-the-art. Despite their high nonlinearity, molecular gases as a broadening medium inside hollow-core fibers have been limited to 25 W, at best. We demonstrate spectral broadening in nitrogen at ten-fold average powers up to 250 W with repetition rates from 25 to 200 kHz. The observed ten-fold spectral broadening is stronger compared to the more expensive krypton gas and enables pulse compression from 1.3 ps to 120 fs. We identified an intuitive explanation for the observed average power scaling based on the density of molecular ro-vibrational states of Raman active molecules. To verify this ansatz, the spectral broadening limitations in O2 and N2O are experimentally measured and agree well. On these grounds, we propose a perspective on the role, suitability, and limits of stimulated Raman scattering at high average and peak powers. Finally, high-harmonic generation is demonstrated at 200 kHz. These findings can have strong implications for intense, high-repetition-rate, pulsed ps laser propagation in the atmosphere where the dominant species are N2 and O2.

Distinctive Electrochemical Surface-Enhanced Raman Spectroscopy and Its Application for DNA-Sensor

Raman scattering is used in basic research on the physical chemistry of molecules and practical analysis applications for molecular detection. To obtain sufficient intensity of Raman scattering, Surface-enhanced Raman Scattering (SERS) is adopted, in which signals are amplified by the localized surface plasmons of metallic nanomaterials. Electrochemical surface-enhanced Raman spectroscopy (EC-SERS) is employed to further enhance SERS signals. Conventional EC-SERS further enhances signals by simply biasing the SERS substrate and forming a resonance condition in which a charge transfer between the SERS substrate and Raman-active molecules can occur.We amplified Raman signals using an electrochemical reaction in a different way from conventional EC-SERS. We provided a potential to the SERS substrate in the presence of a plasmonic precursor to generate secondary plasmonic layer and to induce additional chemical enhancement, resulting in the significant amplification of SERS signal. To deepen our understanding of the phenomena occurring in the EC-SERS process, we fabricated a nanoporous Au substrate and systematically varied its characteristics (e.g., pore size, thickness of a porous layer) and observed the dynamic response of the EC-SERS signal. The condition of the SERS substrate that maximizes the EC-SERS signal was explored.An application experiment was conducted to detect DNA with high sensitivity under these experimental conditions. DNA capture was achieved via a sandwich hybridization procedure: capture DNA was immobilized onto the SERS substrate and bound to a portion of the target DNA, while probe DNA with a tethered Raman-active probe bound to the remaining portion. Raman-active probes were positioned near SERS substrate only in the presence of target DNA. EC-SERS signals were measured by inducing a secondary plasmonic surface through an electrochemical reaction. We confirmed that these EC-SERS could serve as effective DNA-sensor, providing higher detection sensitivity than SERS.

(Invited) Bringing Electrochemistry to Raman Spectroscopy Again with Unconventional Approach

Since the landmark discovery of enhanced Raman scattering at electrochemically roughened metal surfaces in 1974 (Chem. Phys. Lett. 1974, 26, 163), electrochemistry has ushered in a new era of Raman spectroscopy. Although the Raman signal is highly specific to molecular species and thus has great potential as a practical tool for molecular identification, its low cross-section has presented a significant challenge. This challenge has been met by the employment of plasmonic nanomaterials, the origins of which can be found in the study reported in 1974. Metallic surface with nanostructure, such as the electrochemically roughened metal surface, has surface plasmon that can greatly amplify both the incident electromagnetic wave and the scattered electromagnetic wave, which is termed electromagnetic (EM) enhancement, resulting in enhanced Raman scattering. This surface-enhanced Raman scattering (SERS) has emerged as one of the most popular analytical tools, thanks to the extensive development of new plasmonic nanomaterials. In addition to this material-wise signal enhancement, an electrochemical method can enhance Raman scattering by inducing a resonance condition: a charge-transfer resonance by the electron transfer between metal and Raman-active molecule. For achieving this chemical (CM) enhancement, it is essential to ensure proper alignment of the Fermi level of metal, the energy level of molecular orbitals, and the energy of incident light, and the application of electrochemical potential to the metal can adjust the Fermi level to meet the resonance condition.Here, we present a unique application of electrochemical processes that can induce both EM and CM enhancements. In the presence of a metal cation (i.e., a precursor of a plasmonic substance), a suitable electrode potential can generate a new plasmonic surface on the electrode. This process can enhance the Raman signal of the molecule adsorbed on the electrode prior to the potential application, via the EM enhancement mechanism. Since the newly formed plasmonic surface is retained even after the potential application is terminated, the continuation of EM enhancement is expected; however, the Raman signal is significantly reduced. In addition, the Raman enhancement observed during the potential application is mode-selective, which is one of the characteristics of CM enhancement. In combination, the electrochemical process involving a plasmonic metal precursor harnesses both EM and CM enhancements, yielding a great enhancement of the Raman signal. The mechanistic study of this platform and its applications will be presented in this talk.

Plasma treated bimetallic nanofibers as sensitive SERS platform and deep learning model for detection and classification of antibiotics

Plasma treated bimetallic nanofibers as sensitive SERS platform and deep learning model for detection and classification of antibiotics

Hybrid lipid-AuNP clusters as highly efficient SERS substrates for biomedical applications

Although Surface Enhanced Raman Scattering (SERS) is widely applied for ultrasensitive diagnostics and imaging, its potential is largely limited by the difficult preparation of SERS tags, typically metallic nanoparticles (NPs) functionalized with Raman-active molecules (RRs), whose production often involves complex synthetic approaches, low colloidal stability and poor reproducibility. Here, we introduce LipoGold Tags, a simple platform where gold NPs (AuNPs) clusters form via self-assembly on lipid vesicle. RRs embedded in the lipid bilayer experience enhanced electromagnetic field, significantly increasing their Raman signals. We modulate RRs and lipid vesicle concentrations to achieve optimal SERS enhancement and we provide robust structural characterization. We further demonstrate the versatility of LipoGold Tags by functionalizing them with biomolecular probes, including antibodies. As proof of concept, we successfully detect intracellular GM1 alterations, distinguishing healthy donors from patients with infantile GM1 gangliosidosis, showcasing LipoGold Tags as advancement in SERS probes production.

Metal-Organic Framework-Enabled Trapping of Volatile Organic Compounds into Plasmonic Nanogaps for Surface-Enhanced Raman Scattering Detection.

Utilizing electromagnetic hotspots within plasmonic nanogaps is a promising approach to create ultrasensitive surface-enhanced Raman scattering (SERS) substrates. However, it is difficult for many molecules to get positioned in such nanogaps. Metal-organic frameworks (MOFs) are commonly used to absorb and concentrate diverse molecules. Herein, we combine these two strategies by introducing MOFs into plasmon-coupled nanogaps, which has so far remained experimentally challenging. Ultrasensitive SERS substrates are fabricated through the construction of nanoparticle-on-mirror structures, where Au nanocrystals are encapsulated with a zeolitic imidazolate framework-8 (ZIF-8) shell and then coupled to a gold film. The ZIF-8 shell, as a spacer that separates the Au nanocrystal and the Au film, can be adjusted in thickness over a wide range, which allows the electric field enhancement and plasmon resonance wavelength to be varied. By trapping Raman-active molecules within the ZIF-8 shell, we show that our plasmon-coupled structures exhibit a superior SERS detection performance. A range of volatile organic compounds at the concentrations of 10-2 mg m-3 can be detected sensitively and reliably. Our study therefore offers an attractive route for synergistically combining plasmonic electric field enhancement and MOF-enabled molecular enrichment to design and create SERS substrates for ultrasensitive detection.

Exploring the interplay of LSPR and CT: Hierarchical shell thickness optimization for advanced SERS sensing using AgNW@ZIF-8 Core@Shell substrates

Exploring the interplay of LSPR and CT: Hierarchical shell thickness optimization for advanced SERS sensing using AgNW@ZIF-8 Core@Shell substrates

Enhanced Total Vibrational Excitation Yield in a Slow Narrow-Pulsed Hydrogen Molecular Beam.

High-efficiency excitation of a molecular beam is critical for investigating state-selected chemistry. However, achieving vibrational excitation of the entire beam for Raman-active molecules such as H2 proves extremely challenging, primarily because laser pulses are much shorter than the molecular beam. In this study, we achieve a total excitation efficiency of over 20% by employing stimulated Raman pumping (SRP) in a slow, narrow-pulsed molecular beam. Through optimizing the intensity and spot shape of the SRP lasers, we attain saturated excitation within the laser crossing region. Furthermore, by reducing the beam velocity and narrowing the beam pulse using a cold valve and a fast chopper, we significantly enhance the total excitation yield. COMSOL simulation and a newly developed model reveal that a critical velocity allows the chopper to block unexcited molecules and reserve most of the excited ones from the beam, resulting in the highest overall excitation yield. This innovative setup opens new possibilities for state-selected experiments in surface science and ion-molecule reaction dynamics, particularly involving weak transitions and pulsed lasers.

Exploring the potential of branched submicron channels-based structures in ultra-thin Ag films for SERS molecular sensing

Exploring the potential of branched submicron channels-based structures in ultra-thin Ag films for SERS molecular sensing

An amplification-free CRISPR-SERS biosensor for specific, sensitive and rapid detection of Salmonella Typhimurium in poultry

An amplification-free CRISPR-SERS biosensor for specific, sensitive and rapid detection of Salmonella Typhimurium in poultry

Construction of High-Active SERS Cavities in a TiO2 Nanochannels-Based Membrane: A Selective Device for Identifying Volatile Aldehyde Biomarkers.

The accurate, sensitive, and selective on-site screening of volatile aldehyde biomarkers for lung cancer is of utmost significance for preclinical cancer diagnosis and treatment. Applying surface-enhanced Raman scattering (SERS) for gas sensing remains difficult due to the small Raman cross section of most gaseous molecules and interference from other components in exhaled breath. Using an Au asymmetrically coated TiO2 nanochannel membrane (Au/TiO2 NM) as the substrate, a ZIF-8-covered Au/TiO2 NM SERS sensing substrate is designed for the detection of exhaled volatile organic compounds (VOCs). Au/TiO2 NM provides uniformly amplified Raman signals for trace measurements in this design. Importantly, the interfacial nanocavities between Au nanoparticles (NPs) and metal-organic frameworks (MOFs) served as gaseous confinement cavities, which is the key to enhancing the capture and adsorption ability toward gaseous analytes. Both ends of the membrane are left open, allowing gas molecules to pass through. This facilitates the diffusion of gaseous molecules and efficient capture of the target analyte. Using benzaldehyde as a typical gas marker model of lung cancer, the Schiff base reaction with a Raman-active probe molecule 4-aminothiophene (4-ATP) pregrafted on Au NPs enabled trace and multicomponent detection. Moreover, the combination of machine learning (ML) and Raman spectroscopy eliminates subjective assessments of gaseous aldehyde species with the use of a single feature peak, allowing for more accurate identification. This membrane sensing device offers a promising design for the development of a desktop SERS analysis system for lung cancer point-of-care testing (POCT).

Culture-Independent Multiplexed Detection of Drug-Resistant Bacteria Using Surface-Enhanced Raman Scattering.

The rapid and accurate detection of bacteria resistance to β-lactam antibiotics is critical to inform optimal treatment and prevent overprescription of potent antibiotics. Here, we present a fast, culture-independent method for the detection of extended-spectrum β-lactamases (ESBLs) using surface-enhanced Raman scattering (SERS). The method uses Raman probes that release sulfur-based Raman active molecules in the presence of β-lactamases. The released thiol molecules can be captured by gold nanoparticles, leading to amplified Raman signals. A broad-spectrum cephalosporin probe R1G and an ESBL-specific probe R3G are designed to enable duplex detection of bacteria expressing broad-spectrum β-lactamases or ESBLs with a detection limit of 103 cfu/mL in 1 h incubation. Combined with a portable Raman microscope, our culturing-free SERS assay has reduced screening time to 1.5 h without compromising sensitivity and specificity.

Developing highly reliable SERS substrates based on Ag grown on alumina nanomeshes anodized under 1 V for efficiently sensing Raman-active molecules

Developing highly reliable SERS substrates based on Ag grown on alumina nanomeshes anodized under 1 V for efficiently sensing Raman-active molecules

Design and Synthesis of SERS Materials for In Vivo Molecular Imaging and Biosensing.

Surface-enhanced Raman scattering (SERS) is a feasible and ultra-sensitive method for biomedical imaging and disease diagnosis. SERS is widely applied to in vivo imaging due to the development of functional nanoparticles encoded by Raman active molecules (SERS nanoprobes) and improvements in instruments. Herein, the recent developments in SERS active materials and their in vivo imaging and biosensing applications are overviewed. Various SERS substrates that have been successfully used for in vivo imaging are described. Then, the applications of SERS imaging in cancer detection and in vivo intraoperative guidance are summarized. The role of highly sensitive SERS biosensors in guiding the detection and prevention of diseases is discussed in detail. Moreover, its role in the identification and resection of microtumors and as a diagnostic and therapeutic platform is also reviewed. Finally, the progress and challenges associated with SERS active materials, equipment, and clinical translation are described. The present evidence suggests that SERS could be applied in clinical practice in the future.

Self-assembled PVP-gold nanostar films as plasmonic substrates for surface-enhanced spectroscopies: influence of the polymeric coating on the enhancement efficiency.

Colloidal nanoparticles exhibiting anisotropic morphologies are preferred in the structural design of spectroscopically active substrates due to the remarkable optical properties of this type of nano-object. In the particular case of star-like nanoparticles, their sharp tips can act as antennae for capturing and amplifying the incident light, as well as for enhancing the light emitted by nearby fluorophores or the scattering efficiency of Raman active molecules. In the current work, we aimed to implement such star-shaped nanoparticles in the fabrication of nanoparticle films and explore their use as solid plasmonic substrates for surface-enhanced optical spectroscopies. High-density, compact and robust self-assembled gold nanostar films were prepared by directly depositing them from aqueous colloidal suspension on polystyrene plates through convective self-assembly. We investigated the role of the polymeric coating, herein polyvinylpyrrolidone (PVP), in the particle assembly process, the resulting morphology and consequently, the plasmonic response of the obtained films. The efficacy of the plasmonic films as dual-mode surface-enhanced fluorescence (SEF) and surface-enhanced Raman scattering (SERS) substrates was evidenced by testing Nile Blue A (NB) and Rhodamine 800 (Rh800) molecular chromophores under visible (633 nm) versus NIR (785 nm) laser excitation. Steady-state and time-resolved fluorescence investigations highlight the fluorescence intensity and fluorescence lifetime modification effects. The experimental results were corroborated with theoretical modelling by finite-difference time-domain (FDTD) simulations. Furthermore, to prove the extended applicability of the proposed substrates in the detection of biologically relevant molecules, we tested their SERS efficiency for sensing metanephrine, a metabolite currently used for the biochemical diagnosis of neuroendocrine tumors, at concentration levels similar to other catecholamine metabolites.

Dual Biomimetic Recognition-Driven Plasmonic Nanogap-Enhanced Raman Scattering for Ultrasensitive Protein Fingerprinting and Quantitation.

Protein assays with fingerprints and high sensitivity are essential for biomedical research and applications. However, the prevailing methods mainly rely on indirect or labeled immunoassays, failing to provide fingerprint information. Herein, we report a dual biomimetic recognition-driven plasmonic nanogap-enhanced Raman scattering (DBR-PNERS) strategy for ultrasensitive protein fingerprinting and quantitation. A pair of molecularly imprinted nanoantennas were rationally engineered for specifically trapping a target protein into well-defined plasmonic nanogaps through dual-terminal recognition for ultrahigh Raman signal amplification. Meanwhile, a Raman-active small molecule was embedded into the nanoantenna as an internal standard to provide a ratiometric assay for robust quantitation. DBR-PNERS exhibited several significant merits over existing approaches, including fingerprinting, ultrahigh sensitivity, quantitation robustness, speed, sample consumption, and so on. Therefore, it can be a promising tool for a protein assay and holds a great perspective in important applications.

Coherent Raman spectroscopy on hydrogen with in-situ generation, in-situ use, and in-situ referencing of the ultrabroadband excitation.

Time-resolved spectroscopy can provide valuable insights in hydrogen chemistry, with applications ranging from fundamental physics to the use of hydrogen as a commercial fuel. This work represents the first-ever demonstration of in-situ femtosecond laser-induced filamentation to generate a compressed supercontinuum behind a thick optical window, and its in-situ use to perform femtosecond/picosecond coherent Raman spectroscopy (CRS) on molecular hydrogen (H2). The ultrabroadband coherent excitation of Raman active molecules in measurement scenarios within an enclosed space has been hindered thus far by the window material imparting temporal stretch to the pulse. We overcome this challenge and present the simultaneous single-shot detection of the rotational H2 and the non-resonant CRS spectra in a laminar H2/air diffusion flame. Implementing an in-situ referencing protocol, the non-resonant spectrum measures the spectral phase of the supercontinuum pulse and maps the efficiency of the ultrabroadband coherent excitation achieved behind the window. This approach provides a straightforward path for the implementation of ultrabroadband H2 CRS in enclosed environment such as next-generation hydrogen combustors and reforming reactors.

Strong and Elastic Membranes via Hydrogen Bonding Directed Self-Assembly of Atomically Precise Nanoclusters.

2D nanomaterials have provided an extraordinary palette of mechanical, electrical, optical, and catalytic properties. Ultrathin 2D nanomaterials are classically produced via exfoliation, delamination, deposition, or advanced synthesis methods using a handful of starting materials. Thus, there is a need to explore more generic avenues to expand the feasibility to the next generation 2D materials beyond atomic and molecular-level covalent networks. In this context, self-assembly of atomically precise noble nanoclusters can, in principle, suggest modular approaches for new generation 2D materials, provided that the ligand engineering allows symmetry breaking and directional internanoparticle interactions. Here the self-assembly of silver nanoclusters (NCs) capped with p-mercaptobenzoic acid ligands (Na4 Ag44 -pMBA30 ) into large-area freestanding membranes by trapping the NCs in a transient solvent layer at air-solvent interfaces is demonstrated. The patchy distribution of ligand bundles facilitates symmetry breaking and preferential intralayer hydrogen bondings resulting in strong and elastic membranes. The membranes with Young's modulus of 14.5 ± 0.2GPa can readily be transferred to different substrates. The assemblies allow detection of Raman active antibiotic molecules with high reproducibility without any need for substrate pretreatment.

Heralded single-photon source based on an ensemble of Raman-active molecules

Light with high mutual correlations at different frequencies can be used to create heralded single-photon sources, which may serve as the basic elements of existing quantum cryptography and quantum teleportation schemes. One of the important examples in natural systems of light with high mutual correlations is the light produced by spontaneous Raman scattering on an ensemble of molecules. In this paper, we investigate the possibility of using Raman light to create a heralded single-photon source. We show that when using Stokes scattered light for postselection of anti-Stokes scattered light, the latter may possess single-photon properties. We analyze the influence of various negative factors on the characteristics of such a heralded single-photon source, which include a time delay between Stokes and anti-Stokes photons, the finiteness of the correlation radius of an external source, and background radiation. We show that the low value of the second-order autocorrelation function of the single-photon source is preserved even when the flow of uncorrelated photons exceeds the flow of correlated photons in scattered Raman light by an order of magnitude.

Electric-Field-Induced Coherent Anti-Stokes Raman Scattering of Hydrogen Molecules in Visible Region for Sensitive Field Measurement.

Nonintrusive optical measuring of electric fields in space is crucial for various sciences and technologies. In this study, a simple and highly sensitive optical electric field measurement is demonstrated in high-pressure hydrogen by performing electric-field-induced coherent anti-Stokes Raman scattering (E-CARS) in the visible region. The minimum detectable electric field is 0.5 V/mm at atmospheric pressure. This method does not require excited or atomic species and shows a considerably higher sensitivity than those demonstrated by the commonly applied E-CARS method in the infrared region and electric-field-induced second-harmonic generation. This method can be applied to various Raman-active molecules.