SignificanceMembranes can adopt distinct phases. The endoplasmic reticulum (ER) is the largest membrane system inside cells and also harbors the richest metabolic activity including lipid synthesis. Unlike plasma membrane where separated “lipid raft” domains have been predicted and observed, ER membrane is thought to be uniformly fluidic. However, such understanding is based on biophysical studies of model membrane under thermodynamic equilibrium. It remains unclear whether and how lipid synthesis activity perturbs the equilibrium and promotes phase segregation in ER membrane. Herein, we utilized coherent Raman imaging technique to track lipid synthesis and surprisingly revealed solid-like domains emerging from liquid ER membrane. Interestingly, this phenomenon can be tuned by the incoming nutrient source, demonstrating the susceptibility of ER membrane to nonequilibrium modulation.

Powerful optical tools have revolutionized science and technology. The prevalent fluorescence detection offers superb sensitivity down to single molecules but lacks sufficient chemical information [1][2][3] . In contrast, Raman-based vibrational spectroscopy provides exquisite chemical specificity about molecular structure, dynamics and coupling, but is notoriously insensitive [3][4][5] . Here we report a hybrid technique of Stimulated Raman Excited Fluorescence (SREF) that integrates superb detection sensitivity and fine chemical specificity. Through stimulated Raman pumping to an intermediate vibrational eigenstate followed by an upconversion to an electronic fluorescent state, SREF encodes vibrational resonance into the excitation spectrum of fluorescence emission. By harnessing narrow vibrational linewidth, we demonstrated multiplexed SREF imaging in cells, breaking the "color barrier" of fluorescence. By leveraging superb sensitivity of SREF, we achieved all-far-field single-molecule Raman spectroscopy and imaging without plasmonic enhancement, a long-sought-after goal in photonics. Thus, through merging Raman and fluorescence spectroscopy, SERF would be a valuable tool for chemistry and biology.

Three-dimensional visualization of tissue structures using optical microscopy facilitates the understanding of biological functions. However, optical microscopy is limited in tissue penetration due to severe light scattering. Recently, a series of tissue-clearing techniques have emerged to allow significant depth-extension for fluorescence imaging. Inspired by these advances, we develop a volumetric chemical imaging technique that couples Raman-tailored tissue-clearing with stimulated Raman scattering (SRS) microscopy. Compared with the standard SRS, the clearing-enhanced SRS achieves greater than 10-times depth increase. Based on the extracted spatial distribution of proteins and lipids, our method reveals intricate 3D organizations of tumor spheroids, mouse brain tissues, and tumor xenografts. We further develop volumetric phasor analysis of multispectral SRS images for chemically specific clustering and segmentation in 3D. Moreover, going beyond the conventional label-free paradigm, we demonstrate metabolic volumetric chemical imaging, which allows us to simultaneously map out metabolic activities of protein and lipid synthesis in glioblastoma. Together, these results support volumetric chemical imaging as a valuable tool for elucidating comprehensive 3D structures, compositions, and functions in diverse biological contexts, complementing the prevailing volumetric fluorescence microscopy.

As a superb tool to visualize and study the spatial-temporal distribution of chemicals, Raman microscopy has made big impacts to many disciplines of science. While the label-free imaging has been the prevailing strategy in Raman microscopy, recent development and applications of vibrational/Raman tags, particularly when coupled with stimulated Raman Scattering (SRS) microscopy, have generated intense excitement in biomedical imaging. SRS imaging of vibrational tags has enabled researchers to study a wide range of small biomolecules with high specificity, sensitivity and multiplex capability, at single live cell level, tissue level or even in vivo. As reviewed in the article, this platform has facilitated imaging distribution and dynamics of small molecules such as glucose, lipids, amino acids, nucleic acids, and drugs that are otherwise difficult to do with other means. As both the vibrational tags and Raman instrumental development progress rapidly and synergistically, we anticipate that the technique will shed light onto an even broader spectrum of biomedical problems.

Imaging the spatial distribution of biomolecules is at the core of modern biology. The development of fluorescence techniques has enabled researchers to investigate subcellular structures with nanometer precision. However, multiplexed imaging, i.e. observing complex biological networks and interactions, is mainly limited by the fundamental ‘spectral crowding’ of fluorescent materials. Raman spectroscopy-based methods, on the other hand, have a much greater spectral resolution, but often lack the required sensitivity for practical imaging of biomarkers. Addressing the pressing need for new Raman probes, herein we present a series of Raman-active  nanoparticles (Rdots) that exhibit the combined advantages of ultra-brightness and compact sizes (~20 nm). When coupled with the emerging stimulated Raman scattering (SRS) microscopy, these Rdots are brighter than previously reported Raman-active organic probes by two to three orders of magnitude. We further obtain evidence supporting for SRS imaging of Rdots at single particle level. The compact size and ultra-brightness of Rdots allows immunostaining of specific protein targets (including cytoskeleton and low-abundant surface proteins) in mammalian cells and tissue slices with high imaging contrast. These Rdots thus offer a promising tool for a large range of studies on complex biological networks.

Single-cell multiparameter measurement has been increasingly recognized as a key technology toward systematic understandings of complex molecular and cellular functions in biological systems. Despite extensive efforts in analytical techniques, it is still generally challenging for existing methods to decipher a large number of phenotypes in a single living cell. Herein we devise a multiplexed Raman probe panel with sharp and mutually resolvable Raman peaks to simultaneously quantify cell surface proteins, endocytosis activities, and metabolic dynamics of an individual live cell. When coupling it to whole-cell spontaneous Raman micro-spectroscopy, we demonstrate the utility of this technique in 14-plexed live-cell profiling and phenotyping under various drug perturbations. In particular, single-cell multiparameter measurement enables powerful clustering, correlation, and network analysis with biological insights. This profiling platform is compatible with live-cell cytometry, of low instrument complexity and capable of highly multiplexed measurement in a robust and straightforward manner, thereby contributing a valuable tool for both basic single-cell biology and translation applications such as high-content cell sorting and drug discovery.

Inspired by the revolutionary impact of super-resolution fluorescence microscopy, super-resolution Raman imaging has been long pursued because of its much higher chemical specificity than the fluorescence counterpart. However, vibrational contrasts are intrinsically less sensitive compared with fluorescence, resulting in only mild resolution enhancement beyond the diffraction limit even with strong laser excitation power. As such, it is still a great challenge to achieve biocompatible super-resolution vibrational imaging in the optical far-field. In 2019 Stimulated Raman Excited Fluorescence (SREF) was discovered as an ultrasensitive vibrational spectroscopy that combines the high chemical specificity of Raman scattering and the superb sensitivity of fluorescence detection. Herein we developed a novel super-resolution vibrational imaging method by harnessing SREF as the contrast mechanism. We first identified the undesired role of anti-Stokes fluorescence background in preventing direct adoption of super-resolution fluorescence technique. We then devised a frequency-modulation (FM) strategy to remove the broadband backgrounds and achieved high-contrast SREF imaging. Assisted by newly synthesized SREF dyes, we realized multicolor FM-SREF imaging with nanometer spectral resolution. Finally, by integrating stimulated emission depletion (STED) with background-free FM-SREF, we accomplished high-contrast super-resolution vibrational imaging with STED-FM-SREF whose spatial resolution is only determined by the signal-to-noise ratio. In our proof-of-principle demonstration, more than two times of resolution improvement is achieved in biological systems with moderate laser excitation power, which shall be further refined with optimized instrumentation and imaging probes. With its super resolution, high sensitivity, vibrational contrast, and mild laser excitation power, STED-FM-SREF microscopy is envisioned to aid a wide variety of applications.

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) as a new hybrid spectroscopy that combines the vibrational specificity of Raman spectroscopy with the superb sensitivity of fluorescence spectroscopy (down to 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 pre-resonance condition is lacking. Herein 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 of 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 pre-resonance 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 super-multiplex vibrational imaging.