Stimulated Raman Scattering: From Bulk to Nano

Stimulated Raman Excited Fluorescence Spectroscopy of Visible Dyes

Fluorescence spectroscopy and Raman spectroscopy are two major classes of spectroscopy methods in physical chemistry. Very recently, stimulated Raman excited fluorescence (SREF) has been demonstrated ( Xiong, H.; et al. Nature Photonics , 2019 , 13 , 412 - 417 ) as a new hybrid spectroscopy that combines the vibrational specificity of Raman spectroscopy with the superb sensitivity of fluorescence spectroscopy (down to the single-molecule level). However, this proof-of-concept study was limited by both the tunability of the commercial laser source and the availability of the excitable molecules in the near-infrared. As a result, the generality of SREF spectroscopy remains unaddressed, and the understanding of the critical electronic preresonance condition is lacking. In this work, we built a modified excitation source to explore SREF spectroscopy in the visible region. Harnessing a large palette of red dyes, we have systematically studied SREF spectroscopy on a dozen different cases with a fine spectral interval of several nanometers. The results not only establish the generality of SREF spectroscopy for a wide range of molecules but also reveal a tight window of proper electronic preresonance for the stimulated Raman pumping process. Our theoretical modeling and further experiments on newly synthesized dyes also support the obtained insights, which would be valuable in designing and optimizing future SREF experiments for single-molecule vibrational spectroscopy and supermultiplex vibrational imaging.

Far-Field Super-Resolution Vibrational Spectroscopy

Potential label-free alternatives to super-resolution fluorescence techniques have been the focus of considerable research due to the challenges intrinsic in the reliance on fluorescent tags. In this Feature, we discuss efforts to develop super-resolution techniques based on vibrational spectroscopies and address possible sample applications as well as future potential resolution enhancements.

A Review on Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) has become a powerful tool in chemical, material and life sciences, owing to its intrinsic features (i.e., fingerprint recognition capabilities and high sensitivity) and to the technological advancements that have lowered the cost of the instruments and improved their sensitivity and user-friendliness. We provide an overview of the most significant aspects of SERS. First, the phenomena at the basis of the SERS amplification are described. Then, the measurement of the enhancement and the key factors that determine it (the materials, the hot spots, and the analyte-surface distance) are discussed. A section is dedicated to the analysis of the relevant factors for the choice of the excitation wavelength in a SERS experiment. Several types of substrates and fabrication methods are illustrated, along with some examples of the coupling of SERS with separation and capturing techniques. Finally, a representative selection of applications in the biomedical field, with direct and indirect protocols, is provided. We intentionally avoided using a highly technical language and, whenever possible, intuitive explanations of the involved phenomena are provided, in order to make this review suitable to scientists with different degrees of specialization in this field.

Microscopy and Cell Biology: New Methods and New Questions

Understanding the cellular basis of human health and disease requires the spatial resolution of microscopy and the molecular-level details provided by spectroscopy. This review highlights imaging methods at the intersection of microscopy and spectroscopy with applications in cell biology. Imaging methods are divided into three broad categories: fluorescence microscopy, label-free approaches, and imaging tools that can be applied to multiple imaging modalities. Just as these imaging methods allow researchers to address new biological questions, progress in biological sciences will drive the development of new imaging methods. We highlight four topics in cell biology that illustrate the need for new imaging tools: nanoparticle-cell interactions, intracellular redox chemistry, neuroscience, and the increasing use of spheroids and organoids. Overall, our goal is to provide a brief overview of individual imaging methods and highlight recent advances in the use of microscopy for cell biology.

Multicolor Raman Beads for Multiplexed Tumor Cell and Tissue Imaging and in Vivo Tumor Spectral Detection

Developing new nanomaterials with strong and distinctive Raman vibrations in the biological Raman-silent region (1800-2800 cm^-1) were highly desirable for Raman hyperspectral detection and imaging in living cells and animals. Herein, polymeric nanoparticles with monomers containing alkyne, cyanide, azide, and carbon-deuterate were prepared as Raman-active nanomaterials (Raman beads) for bioimaging applications. Intense Raman signals were obtained due to the high density of alkyne, cyanide, azide, and carbon-deuterate in single nanoparticles, in absence of metal (such as Au or Ag) as Raman enhancers. We have developed a library of Raman beads for frequency multiplexing through the end-capping substitutions of monomers and demonstrated five-color SRS imaging of mixed nanoparticles with distinct Raman frequencies. In addition, with further surface functionalization of targeting moieties (such as nucleic acid aptamers and targeting peptides), targetable Raman beads were successfully used as probes for tumor targeting and Raman spectroscopic detection, including multicolor SRS imaging in living tumor cells and tissues with high specificity. Further in vivo studies indicated that Raman beads anchored with targeting moieties were successfully employed to target tumors in living mice after tail intravenous injection, and Raman spectral detection of tumor in live mice was achieved only through spontaneous Raman signal at the biological Raman-silent region without any signal enhancement due to a high density of Raman reporters in Raman beads. With further copolymerization of these monomers, Raman beads with supermultiplex barcoding could be readily achieved.

Windows into Microbial Seascapes: Advances in Nanoscale Imaging and Application to Marine Sciences

Geochemical cycles of all nonconservative elements are mediated by microorganisms over nanometer spatial scales. The pelagic seascape is known to possess microstructure imposed by heterogeneous distributions of particles, polymeric gels, biologically important chemicals, and microbes. While indispensable, most traditional oceanographic observational approaches overlook this heterogeneity and ignore subtleties, such as activity hot spots, symbioses, niche partitioning, and intrapopulation phenotypic variations, that can provide a deeper mechanistic understanding of planktonic ecosystem function. As part of the movement toward cultivation-independent tools in microbial oceanography, techniques to examine the ecophysiology of individual populations and their role in chemical transformations at spatial scales relevant to microorganisms have been developed. This review presents technologies that enable geochemical and microbiological interrogations at spatial scales ranging from 0.02 to a few hundred micrometers, particularly focusing on atomic force microscopy, nanoscale secondary ion mass spectrometry, and confocal Raman microspectroscopy and introducing promising approaches for future applications in marine sciences.

Raman spectroscopy of a near infrared absorbing proteorhodopsin: Similarities to the bacteriorhodopsin O photointermediate

Microbial rhodopsins have become an important tool in the field of optogenetics. However, effective in vivo optogenetics is in many cases severely limited due to the strong absorption and scattering of visible light by biological tissues. Recently, a combination of opsin site-directed mutagenesis and analog retinal substitution has produced variants of proteorhodopsin which absorb maximally in the near-infrared (NIR). In this study, UV-Visible-NIR absorption and resonance Raman spectroscopy were used to study the double mutant, D212N/F234S, of green absorbing proteorhodopsin (GPR) regenerated with MMAR, a retinal analog containing a methylamino modified β-ionone ring. Four distinct subcomponent absorption bands with peak maxima near 560, 620, 710 and 780 nm are detected with the NIR bands dominant at pH <7.3, and the visible bands dominant at pH 9.5. FT-Raman using 1064-nm excitation reveal two strong ethylenic bands at 1482 and 1498 cm-1 corresponding to the NIR subcomponent absorption bands based on an extended linear correlation between λmax and γC = C. This spectrum exhibits two intense bands in the fingerprint and HOOP mode regions that are highly characteristic of the O640 photointermediate from the light-adapted bacteriorhodopsin photocycle. In contrast, 532-nm excitation enhances the 560-nm component, which exhibits bands very similar to light-adapted bacteriorhodopsin and/or the acid-purple form of bacteriorhodopsin. Native GPR and its mutant D97N when regenerated with MMAR also exhibit similar absorption and Raman bands but with weaker contributions from the NIR absorbing components. Based on these results it is proposed that the NIR absorption in GPR-D212N/F234S with MMAR arises from an O-like chromophore, where the Schiff base counterion D97 is protonated and the MMAR adopts an all-trans configuration with a non-planar geometry due to twists in the conjugated polyene segment. This configuration is characterized by extensive charge delocalization, most likely involving nitrogens atoms in the MMAR chromophore.

Substructure imaging of heterogeneous nanomaterials with enhanced refractive index contrast by using a functionalized tip in photoinduced force microscopy

The opto-mechanical force response from light-illuminated nanoscale materials has been exploited in many tip-based imaging applications to characterize various heterogeneous nanostructures. Such a force can have two origins: thermal expansion and induced dipoles. The thermal expansion reflects the absorption of the material, which enables one to chemically characterize a material at the absorption resonance. The induced dipole interaction reflects the local refractive indices of the material underneath the tip, which is useful to characterize a material in the spectral region where no absorption resonance occurs, as in the infrared (IR)-inactive region. Unfortunately, the dipole force is relatively small, and the contrast is rarely discernible for most organic materials and biomaterials, which only show a small difference in refractive indices for their components. In this letter, we demonstrate that refractive index contrast can be greatly enhanced with the assistance of a functionalized tip. With the enhanced contrast, we can visualize the substructure of heterogeneous biomaterials, such as a polyacrylonitrile-nanocrystalline cellulose (PAN-NCC) nanofiber. From substructural visualization, we address the issue of the tensile strength of PAN-NCC fibers fabricated by several different mixing methods. Our understanding from the present study will open up a new opportunity to provide enhanced sensitivity for substructure mapping of nanobiomaterials, as well as local field mapping of photonic devices, such as surface polaritons on semiconductors, metals and van der Waals materials.

A ratiometric Raman probe for live-cell imaging of hydrogen sulfide in mitochondria by stimulated Raman scattering

Stimulated Raman Scattering (SRS) coupled with alkyne tags has been an emerging imaging technique to visualize small-molecule species with high sensitivity and specificity. Here we describe the development of a ratiometric Raman probe for visualizing hydrogen sulfide (H2S) species in living cells as the first alkyne-based sensor for SRS microscopy. This probe uses an azide unit as a selective reactive site, and it targets mitochondria with high specificity. The SRS ratiometric images show a strong response to H2S level changes in living cells.

Controlling and exploiting cell-to-cell variation in metabolic engineering

Individual cells within a population can display diverse phenotypes due to differences in their local environment, genetic variation, and stochastic expression of genes. Understanding this cell-to-cell variation is important for metabolic engineering applications because variability can impact production. For instance, recent studies have shown that production can be highly heterogeneous among engineered cells, and strategies that manage this diversity improve yields of biosynthetic products. These results suggest the potential of controlling variation as a novel approach towards improving performance of engineered cells. In this review, we focus on identifying the origins of cell-to-cell variation in metabolic engineering applications and discuss recent developments on strategies that can be employed to diminish, accept, or even exploit cell-to-cell variation.

Multiorder Stimulated Raman Scattering in Colliding Droplets

Nonlinear Raman spectroscopy is a versatile method to enhance the intensities of Raman scattering. It requires an intense light field that can be provided by a liquid droplet acting as a high-quality optical cavity. Here, colliding droplets were used as a novel optical cavity to enhance the intensity of Raman scattering. Specifically, multiorder stimulated Raman-scattered light was generated with significant intensity from colliding droplets of carbon tetrachloride (CCl₄). The intensities of the Raman bands were analyzed with a simple theory that roughly reproduced the experimental spectrum. Overall, the method facilitates Raman spectroscopy of molecules in liquids because of its high sensitivity and resolution.

Electronic Preresonance Stimulated Raman Scattering Microscopy

Optical microscopy has generated great impact for modern research. While fluorescence microscopy provides the ultimate sensitivity, it generally lacks chemical information. Complementarily, vibrational imaging methods provide rich chemical-bond-specific contrasts. Nonetheless, they usually suffer from unsatisfying sensitivity or compromised biocompatibility. Recently, electronic preresonance stimulated Raman scattering (EPR-SRS) microscopy was reported, achieving simultaneous high detection sensitivity and superb vibrational specificity of chromophores. With newly synthesized Raman-active dyes, this method readily breaks the optical color barrier of fluorescence microscopy and is well-suited for supermultiplex imaging in biological samples. In this Perspective, we first review previous utilizations of electronic resonance in various Raman spectroscopy and microscopy. We then discuss the physical origin and uniqueness of the electronic preresonance region, followed by quantitative analysis of the enhancement factors involved in EPR-SRS microscopy. On this basis, we provide an outlook for future development as well as the broad applications in biophotonics.

Operando and three-dimensional visualization of anion depletion and lithium growth by stimulated Raman scattering microscopy

Visualization of ion transport in electrolytes provides fundamental understandings of electrolyte dynamics and electrolyte-electrode interactions. However, this is challenging because existing techniques are hard to capture low ionic concentrations and fast electrolyte dynamics. Here we show that stimulated Raman scattering microscopy offers required resolutions to address a long-lasting question: how does the lithium-ion concentration correlate to uneven lithium deposition? In this study, anions are used to represent lithium ions since their concentrations should not deviate for more than 0.1 mM, even near nanoelectrodes. A three-stage lithium deposition process is uncovered, corresponding to no depletion, partial depletion, and full depletion of lithium ions. Further analysis reveals a feedback mechanism between the lithium dendrite growth and heterogeneity of local ionic concentration, which can be suppressed by artificial solid electrolyte interphase. This study shows that stimulated Raman scattering microscopy is a powerful tool for the materials and energy field.

The relationship between Li-ion concentration and Li deposition remains an issue to be addressed. Here the authors show that stimulated Raman scattering microscopy offers insight into the concentration evolution and its impact on the dendrite growth, which is not possible by existing techniques.

X-Ray Sum Frequency Diffraction for Direct Imaging of Ultrafast Electron Dynamics

X-ray diffraction from molecules in the ground state produces an image of their charge density, and time-resolved x-ray diffraction can thus monitor the motion of the nuclei. However, the density change of excited valence electrons upon optical excitation can barely be monitored with regular diffraction techniques due to the overwhelming background contribution of the core electrons. We present a nonlinear x-ray technique made possible by novel free electron laser sources, which provides a spatial electron density image of valence electron excitations. The technique, sum frequency generation carried out with a visible pump and a broadband x-ray diffraction pulse, yields snapshots of the transition charge densities, which represent the electron density variations upon optical excitation. The technique is illustrated by ab initio simulations of transition charge density imaging for the optically induced electronic dynamics in a donor or acceptor substituted stilbene.

Three-beam double stimulated Raman scatterings: Cascading configuration

Two-beam stimulated Raman scattering (SRS) has been used in diverse label-free spectroscopy and imaging applications of live cells, biological tissues, and functional materials. Recently, we developed a theoretical framework for the three-beam double SRS processes that involve pump, Stokes, and depletion beams, where the pump-Stokes and pump-depletion SRS processes compete with each other. It was shown that the net Stokes gain signal can be suppressed by increasing the depletion beam intensity. The theoretical prediction has been experimentally confirmed recently. In the previous scheme for a selective suppression of one SRS by making it compete with another SRS, the two SRS processes occur in a parallel manner. However, there is another possibility of three-beam double SRS scheme that can be of use to suppress either Raman gain of the Stokes beam or Raman loss of the pump beam by depleting the Stokes photons with yet another SRS process induced by the pair of Stokes and another (second) Stokes beam. This three-beam double SRS process resembles a cascading energy transfer process from the pump beam to the first Stokes beam (SRS-1) and subsequently from the first Stokes beam to the second Stokes beam (SRS-2). Here, the two stimulated Raman gain-loss processes are associated with two different Raman-active vibrational modes of solute molecule. In the present theory, both the radiation and the molecules are treated quantum mechanically. We then show that the cascading-type three-beam double SRS can be described by coupled differential equations for the photon numbers of the pump and Stokes beams. From the approximate solutions as well as exact numerical calculation results for the coupled differential equations, a possibility of efficiently suppressing the stimulated Raman loss of the pump beam by increasing the second Stokes beam intensity is shown and discussed. To further prove a potential use of this scheme for developing a super-resolution SRS microscopy, we present a theoretical expression and numerical simulation results for the full-width-at-half-maximum of SRS imaging point spread function, assuming that the pump and Stokes beam profiles are Gaussian and the second Stokes beam has a doughnut-shaped spatial profile. It is clear that the spatial resolution with the present 3-beam cascading SRS method can be enhanced well beyond the diffraction limit. We anticipate that the present work will provide a theoretical framework for a super-resolution stimulated Raman scattering microscopy that is currently under investigation.

Stimulated Raman scattering of polymer nanoparticles for multiplexed live-cell imaging

A novel nanoparticle-based imaging strategy is introduced that couples biocompatible organic nanoparticles and stimulated Raman scattering (SRS) microscopy. Polymer nanoparticles with vibrational labels incorporated were readily prepared for multi-color SRS imaging with excellent photo-stability. The Raman-active polymer dots are nontoxic, rapidly enter various cell types, and are applied in multiplexed cell-type sorting.

Size-dependent Raman shift of semiconductor nanomaterials determined using bond number and strength

Significant variations in Raman shifts with decreasing material size, D, have been detected in Raman spectroscopy. In this study, we propose a simple and unified model to determine and explain the size-dependent Raman shift, ω(D), of low-dimensional semiconductor nanomaterials. ω(D) was found to be a function of bond number in a system, with an obvious decline in Raman shift observed when size dropped to the nanoscale. This arose from a decrease in coordination number, Z(D), and increase in single bond strength, ε(D). The predicted results show good agreement with experimental data for a series of semiconductor nanomaterials, showing that bond number can be used to calculate Raman shifts of nanomaterials. Moreover, this theoretical model was successfully applied to both single crystals and some binary semiconductor nanomaterials. Furthermore, bond number, which is directly related to the nanomaterial shape and size, becomes the only parameter required to determine ω(D) in this model, as both Z(D) and ε(D) can be determined from the bond number. This indicates that the established model has the potential to determine Raman shifts of nanomaterials with different shapes and sizes.

Selective suppression of CARS signal with three-beam competing stimulated Raman scattering processes

A switching-off of the CARS signal is achieved by using a three-beam double SRS scheme.

Two-color vibrational imaging of glucose metabolism using stimulated Raman scattering

A two-color vibrational imaging technique for simultaneously mapping glucose uptake and incorporation activity inside single living cells is reported. Heterogeneous patterns of glucose metabolism are directly visualized from the ratiometric two-color images for various cell types, cells undergoing epithelia-to-mesenchymal transitions and live mouse brain tissues. The two-color imaging of glucose metabolism here demonstrates the development of multi-functional vibrational probes for multicolor imaging of cellular metabolism.

Imaging chemistry inside living cells by stimulated Raman scattering microscopy

Stimulated Raman scattering (SRS) microscopy is a vibrational imaging platform developed to visualize chemical content of a biological sample based on molecular vibrational fingerprints. With high-speed, high-sensitivity, and three-dimensional sectioning capability, SRS microscopy has been used to study chemical distribution, molecular transport, and metabolic conversion in living cells in a label-free manner. Moreover, aided with bio-orthogonal small-volume Raman probes, SRS microscopy allows direct imaging of metabolic activities of small molecules in living cells.

Characterizing Slow Photochemical Reaction Kinetics by Enhanced Sampling of Rare Events with Capillary Optical Fibers and Kramers' Theory

Characterization of slow chemical reactions is essential for assessing catalytic efficiency in chemistry and biology. Traditionally, chemical reaction rates are obtained from population relaxation kinetics measurements and the Arrhenius equation. Unfortunately, it is difficult to use this approach to characterize reactions wherein concentrations change slowly. Thus, it is interesting to see whether a dynamical view of chemical reactions may be used to obtain the reaction rates of slow processes. In the present work, we perform Brownian dynamics simulations of an asymmetric double-well potential to investigate how enhanced sampling of barrier crossing at transition states improves the characterization of reaction rate constants. We then present the design of a liquid-filled capillary optical fiber-based fluorescence spectrometer, which, like rare events, is also based on Poissonian statistics. We use the instrument to characterize the slow photochemical degradation kinetics of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) in o-dichlorobenzene. We have employed in situ optical microscopy measurements and electrodynamics simulations to characterize the excitation beam profile inside a liquid-filled capillary fiber. We compare the cuvette and capillary fiber sample holders and show that the MEH-PPV fluorescence line shape is independent of the sample holder, as expected. We characterize the photochemical degradation kinetics of MEH-PPV in o-dichlorobenzene solutions placed in the cuvette versus that in the capillary fiber. We observe small and slow changes in the time-dependent fluorescence spectra when the degradation reaction is performed in the cuvette. On the other hand, we are able to characterize reactant-concentration decay and product-concentration buildup from the time-dependent fluorescence spectra recorded during photochemical degradation of MEH-PPV performed inside the capillary optical fiber. Ultrafast optically heterodyne-detected optical Kerr effect spectroscopy and multimode Brownian oscillator analysis provide further insights into the role of bath oscillator modes of friction in the mechanism of MEH-PPV photochemical degradation. Overall, the work presented herein shows that slow photochemical degradation kinetics of MEH-PPV can be successfully and efficiently assessed in the capillary fiber fluorescence spectrometer.

Recent Advances in Nonlinear Optical Analyses of Pharmaceutical Materials in the Solid State

The past decade has seen an increase in the use of nonlinear optical (NLO) techniques such as second harmonic generation, coherent antistokes Raman scattering, stimulated Raman scattering, and two-photon fluorescence for the solid-state characterization of pharmaceutical materials. These combined techniques offer several advantages (e.g., speed, selectivity, quantitation) of potential interest to the pharmaceutical community, as decreased characterization times in formulation development and testing could help decrease the time required to bring new, higher quality drugs to market. The large body of literature recently published in this field merits a review. Literature will be discussed in order of drug development, starting with applications in initial therapeutic molecule crystallization and polymorphic analysis, followed by final dosage form characterization, and ending with drug product performance testing.