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

Illuminating life processes by vibrational probes

Vibration of chemical bonds can serve as imaging contrast. Vibrational probes, synergized with major advances in chemical bond imaging instruments, have recently flourished and proven valuable in illuminating life processes. Here, we review how the development of vibrational probes with optimal biocompatibility, enhanced sensitivity, multichromatic colors and diverse functionality has extended chemical bond imaging beyond the prevalent label-free paradigm into various novel applications such as imaging metabolites, metabolic imaging, drug imaging, super-multiplex imaging, vibrational profiling and vibrational sensing. These advancements in vibrational probes have greatly facilitated understanding living systems, a new field of vibrational chemical biology.

Analysis of Drug Molecules in Living Cells

Cells are the fundamental units of life, comprising a highly concentrated and complex assembly of biomolecules that interact dynamic ally across spatial and temporal scales. Living cells are constantly undergoing dynamic processes, therefore, to understand the interactions between drug molecules and living cells is of paramount importance in the biomedical sciences and pharmaceutical development. Compared with traditional end-point assays and fixed cell analysis, analysis of drug molecules in living cells can provide more insight into the effects of drugs on cells in real-time and allowing for a better understanding of drug mechanisms and effects, which will contribute to the development of drug developing and testing and personalize medicine. However, the high demands of living cell analysis, including high costs, technical complexity, and throughput limitations, hinder the widespread application of this technology. In recent years, the rapid development of analytical methods such as Raman spectroscopy and fluorescence has made the in situ and real-time detection possible, allowing the analysis of single cell or cell populations at various conditions. In this review, we summarize the advanced analytical methods and technologies from last few years for drug detection in living cells.

The evolution of immune profiling: will there be a role for nanoparticles?

Immune profiling provides insights into the functioning of the immune system, including the distribution, abundance, and activity of immune cells. This understanding is essential for deciphering how the immune system responds to pathogens, vaccines, tumors, and other stimuli. Analyzing diverse immune cell types facilitates the development of personalized medicine approaches by characterizing individual variations in immune responses. With detailed immune profiles, clinicians can tailor treatment strategies to the specific immune status and needs of each patient, maximizing therapeutic efficacy while minimizing adverse effects. In this review, we discuss the evolution of immune profiling, from interrogating bulk cell samples in solution to evaluating the spatially-rich molecular profiles across intact preserved tissue sections. We also review various multiplexed imaging platforms recently developed, based on immunofluorescence and imaging mass spectrometry, and their impact on the field of immune profiling. Identifying and localizing various immune cell types across a patient's sample has already provided important insights into understanding disease progression, the development of novel targeted therapies, and predicting treatment response. We also offer a new perspective by highlighting the unprecedented potential of nanoparticles (NPs) that can open new horizons in immune profiling. NPs are known to provide enhanced detection sensitivity, targeting specificity, biocompatibility, stability, multimodal imaging features, and multiplexing capabilities. Therefore, we summarize the recent developments and advantages of NPs, which can contribute to advancing our understanding of immune function to facilitate precision medicine. Overall, NPs have the potential to offer a versatile and robust approach to profile the immune system with improved efficiency and multiplexed imaging power.

Viewing 3D spatial biology with highly-multiplexed Raman imaging: from spectroscopy to biotechnology

Understansding complex biological systems requires the simultaneous characterization of a large number of interacting components in their native 3D environment with high spatial resolution. Highly-multiplexed Raman imaging is an emerging general strategy for detecting biomarkers with scalable multiplexity and ultra-sensitivity based on a series of stimulated Raman scattering (SRS) techniques. Here we review recent advances in highly-multiplexed Raman imaging and how they contribute to the technological revolution in 3D spatial biology, focusing on the developmental pathway from spectroscopy study to biotechnology invention. We envision highly-multiplexed Raman imaging is taking off, which will greatly facilitate our understanding in biological and medical research fields.

Biomedical applications, perspectives and tag design concepts in the cell - silent Raman window

Spectroscopic studies increasingly employ Raman tags exhibiting a signal in the cell - silent region of the Raman spectrum (1800-2800 cm-1), where bands arising from biological molecules are inherently absent. Raman tags bearing functional groups which contain a triple bond, such as alkyne and nitrile or a carbon-deuterium bond, have a distinct vibrational frequency in this region. Due to the lack of spectral background and cell-associated bands in the specific area, the implementation of those tags can help overcome the inherently poor signal-to-noise ratio and presence of overlapping Raman bands in measurements of biological samples. The cell - silent Raman tags allow for bioorthogonal imaging of biomolecules with improved chemical contrast and they have found application in analyte detection and monitoring, biomarker profiling and live cell imaging. This review focuses on the potential of the cell - silent Raman region, reporting on the tags employed for biomedical applications using variants of Raman spectroscopy.

Stimulated Raman Scattering Microscopy Reveals Bioaccumulation of Small Microplastics in Protozoa from Natural Waters

Microplastics (MPs) are pollutants of global concern, and bioaccumulation determines their biological effects. Although microorganisms form a large fraction of our ecosystem's biomass and are important in biogeochemical cycling, their accumulation of MPs has never been confirmed in natural waters because current tools for field biological samples can detect only MPs > 10 μm. Here, we show that stimulated Raman scattering microscopy (SRS) can image and quantify the bioaccumulation of small MPs (<10 μm) in protozoa. Our label-free method, which differentiates MPs by their SRS spectra, detects individual and mixtures of different MPs (e.g., polyethylene, polypropylene, polyvinyl chloride, polyethylene terephthalate, polystyrene, and poly(methyl methacrylate)) in protozoa. The ability of SRS to quantify cellular MP accumulation is similar to that of flow cytometry, a fluorescence-based method commonly used to determine cellular MP accumulation. Moreover, we discovered that protozoa in water samples from Yangtze River, Xianlin Wastewater Treatment Plant, Lake Taihu and the Pearl River Estuary accumulated MPs < 10 μm, but the proportion of MP-containing cells was low (∼2-5%). Our findings suggest that small MPs could potentially enter the food chain and transfer to organisms at higher trophic levels, posing environmental and health risks that deserve closer scrutiny.

Single-particle imaging of nanomedicine entering the brain

Nanomedicine has emerged as a revolutionary strategy of drug delivery. However, fundamentals of the nano-neuro interaction are elusive. In particular, whether nanocarriers can cross the blood-brain barrier (BBB) and release the drug cargo inside the brain, a basic process depicted in numerous books and reviews, remains controversial. Here, we develop an optical method, based on stimulated Raman scattering, for imaging nanocarriers in tissues. Our method achieves a suite of capabilities-single-particle sensitivity, chemical specificity, and particle counting capability. With this method, we visualize individual intact nanocarriers crossing the BBB of mouse brains and quantify the absolute number by particle counting. The fate of nanocarriers after crossing the BBB shows remarkable heterogeneity across multiple scales. With a mouse model of aging, we find that blood-brain transport of nanocarriers decreases with age substantially. This technology would facilitate development of effective therapeutics for brain diseases and clinical translation of nanocarrier-based treatment in general.

Strategies and Progress of Raman Technologies for Cellular Uptake Analysis of the Drug Delivery Systems

Nanoparticle (NP)-based drug delivery systems have the potential to significantly enhance the pharmacological and therapeutic properties of drugs. These systems enhance the bioavailability and biocompatibility of pharmaceutical agents via enabling targeted delivery to specific tissues or organs. However, the efficacy and safety of these systems are largely dependent on the cellular uptake and intracellular transport of NPs. Thus, it is crucial to monitor the intracellular behavior of NPs within a single cell. Yet, it is challenging due to the complexity and size of the cell. Recently, the development of the Raman instrumentation offers a versatile tool to allow noninvasive cellular measurements. The primary objective of this review is to highlight the most recent advancements in Raman techniques (spontaneous Raman scattering, bioorthogonal Raman scattering, coherence Raman scattering, and surface-enhanced Raman scattering) when it comes to assessing the internalization of NP-based drug delivery systems and their subsequent movement within cells.

Reviews of bio-orthogonal probes in bioscience by stimulated Raman scattering microscopy

Stimulated Raman scattering (SRS) microscopy, is a nonlinear optical imaging method for visualizing chemical content based on molecular vibrational bonds, with high sensitivity, resolution, speed, and specificity. In the current review, we provided a comprehensive and critical review of the most recent developments in the field of SRS in combination with bio-orthogonal Raman tags or labels in bioscience. Firstly, we introduced the fundamentals of SRS microscopy and the theory principle of bio-orthogonal Raman tags. In particular, present the applications of each kind of bio-orthogonal Raman tags, including heavy water (D2O), stable isotope probes (SIP), and triple-bonds tags. And shared our vision for the remaining challenges, research needs, and potential future breakthroughs for SRS technology lastly. We envision that the advanced SRS imaging and analysis will be a major force in future biological discovery.

The Emerging Role of Raman Spectroscopy as an Omics Approach for Metabolic Profiling and Biomarker Detection toward Precision Medicine

Omics technologies have rapidly evolved with the unprecedented potential to shape precision medicine. Novel omics approaches are imperative toallow rapid and accurate data collection and integration with clinical information and enable a new era of healthcare. In this comprehensive review, we highlight the utility of Raman spectroscopy (RS) as an emerging omics technology for clinically relevant applications using clinically significant samples and models. We discuss the use of RS both as a label-free approach for probing the intrinsic metabolites of biological materials, and as a labeled approach where signal from Raman reporters conjugated to nanoparticles (NPs) serve as an indirect measure for tracking protein biomarkers in vivo and for high throughout proteomics. We summarize the use of machine learning algorithms for processing RS data to allow accurate detection and evaluation of treatment response specifically focusing on cancer, cardiac, gastrointestinal, and neurodegenerative diseases. We also highlight the integration of RS with established omics approaches for holistic diagnostic information. Further, we elaborate on metal-free NPs that leverage the biological Raman-silent region overcoming the challenges of traditional metal NPs. We conclude the review with an outlook on future directions that will ultimately allow the adaptation of RS as a clinical approach and revolutionize precision medicine.

Bringing Vibrational Imaging to Chemical Biology with Molecular Probes

As an emerging optical imaging modality, stimulated Raman scattering (SRS) microscopy provides invaluable opportunities for chemical biology studies using its rich chemical information. Through rapid progress over the past decade, the development of Raman probes harnessing the chemical biology toolbox has proven to play a key role in advancing SRS microscopy and expanding biological applications. In this perspective, we first discuss the development of biorthogonal SRS imaging using small tagging of triple bonds or isotopes and highlight their unique advantages for metabolic pathway analysis and microbiology investigations. Potential opportunities for chemical biology studies integrating small tagging with SRS imaging are also proposed. We next summarize the current designs of highly sensitive and super-multiplexed SRS probes, as well as provide future directions and considerations for next-generation functional probe design. These rationally designed SRS probes are envisioned to bridge the gap between SRS microscopy and chemical biology research and should benefit their mutual development.

Imaging approaches for monitoring three-dimensional cell and tissue culture systems

The past decade has seen an increasing demand for more complex, reproducible and physiologically relevant tissue cultures that can mimic the structural and biological features of living tissues. Monitoring the viability, development and responses of such tissues in real-time are challenging due to the complexities of cell culture physical characteristics and the environments in which these cultures need to be maintained in. Significant developments in optics, such as optical manipulation, improved detection and data analysis, have made optical imaging a preferred choice for many three-dimensional (3D) cell culture monitoring applications. The aim of this review is to discuss the challenges associated with imaging and monitoring 3D tissues and cell culture, and highlight topical label-free imaging tools that enable bioengineers and biophysicists to non-invasively characterise engineered living tissues.

Super-multiplexed vibrational probes: Being colorful makes a difference

Biological systems with intrinsic complexity require multiplexing techniques to comprehensively describe the phenotype, interaction, and heterogeneity. Recent years have witnessed the development of super-multiplexed vibrational microscopy, overcoming the 'color barrier' of fluorescence-based optical techniques. Here, we will review the recent progress in the design and applications of super-multiplexed vibrational probes. We hope to illustrate how rainbow-like vibrational colors can be generated from systematic studies on structure-spectroscopy relationships and how being colorful makes a difference to various biomedical applications.

Preparation, characterization, evaluation and mechanistic study of organic polymer nano-photocatalysts for solar fuel production

This review provides the guidelines and knowledge gained so far on current strategies used to prepare, optimize and investigate polymer nanoparticles for fuel production, highlighting the future directions of polymer nano-photocatalyst development.

Multiplex Raman imaging of organelles in endothelial cells

Raman imaging using molecular reporters is a relatively new approach in subcellular investigations. It enables the visualization of organelles in cells with better selectivity and sensitivity compared to the label-free approach. Essentially Raman reporters possess in their structure an alkyne molecular group that can be selectively identified in the spectral region silent for biomolecules, hence facilitate the localization of individual organelles. The aim of this work is to visualize the main cell organelles in endothelial cells (HMEC-1) using established reporters (EdU and MitoBADY), but also to test a new one, namely falcarinol, which exhibits lipophilic properties. Moreover, we tested the possibility to use Raman reporters as a probe to detect changes in distribution of certain organelles after induced endothelial dysfunction (ED) in in vitro models. In both cases, induced ED is characterized by the formation of lipid droplets in the cells, which is why a good tool for the detection of lipid-rich organelles is so important in these studies. Two-dimensional Raman images were obtained, visualizing the distribution of selected organic compounds in the cell, such as proteins, lipids, and nucleic acids. Additionally, the distribution of EdU, MitoBADY and falcarinol in endothelial cells (ECs) was determined. Moreover, we highlight some drawback of established Raman reporter and the need for testing them in various physiological state of the cell.

Multiplexed live-cell profiling with Raman probes

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.

Toward Raman Subcellular Imaging of Endothelial Dysfunction

Multiple diseases are at some point associated with altered endothelial function, and endothelial dysfunction (ED) contributes to their pathophysiology. Biochemical changes of the dysfunctional endothelium are linked to various cellular organelles, including the mitochondria, endoplasmic reticulum, and nucleus, so organelle-specific insight is needed for better understanding of endothelial pathobiology. Raman imaging, which combines chemical specificity with microscopic resolution, has proved to be useful in detecting biochemical changes in ED at the cellular level. However, the detection of spectroscopic markers associated with specific cell organelles, while desirable, cannot easily be achieved by Raman imaging without labeling. This critical review summarizes the current advances in Raman-based analysis of ED, with a focus on a new approach involving molecular Raman reporters that could facilitate the study of biochemical changes in cellular organelles. Finally, imaging techniques based on both conventional spontaneous Raman scattering and the emerging technique of stimulated Raman scattering are discussed.

Advances in stimulated Raman scattering imaging for tissues and animals

Stimulated Raman scattering (SRS) microscopy has emerged in the last decade as a powerful optical imaging technology with high chemical selectivity, speed, and subcellular resolution. Since the invention of SRS microscopy, it has been extensively employed in life science to study composition, structure, metabolism, development, and disease in biological systems. Applications of SRS in research and the clinic have generated new insights in many fields including neurobiology, tumor biology, developmental biology, metabolomics, pharmacokinetics, and more. Herein we review the advances and applications of SRS microscopy imaging in tissues and animals, as well as envision future applications and development of SRS imaging in life science and medicine.

Ultra-bright Raman dots for multiplexed optical imaging

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.

Raman-based imaging of biomarkers is often challenging due to low sensitivity. Here, the authors use a swelling-diffusion approach to develop a series of Raman probes that are both ultra-bright and compact in size, and demonstrate multiplexed imaging of specific protein targets in cells and tissue slices.

Biocompliant Composite Au/pHEMA Plasmonic Scaffolds for 3D Cell Culture and Noninvasive Sensing of Cellular Metabolites

The field of 3D printing is an area of active research, with a substantial focus given to the design and construction of customized tools for applications in technology. There exists a particular need in these developing areas of opportunity for new multi-functional soft materials that are biologically compatible for the growth and directed culturing of cells. Herein, a composite material consisting of gold nanoparticles with useful plasmonic properties embedded within a highly hydrophilic poly-2-hydroxyethylmethacrylate matrix is described and characterized. This composite material serves dual functions as both host framework scaffold for cell lines such as pre-osteoblasts as well as a plasmonic biosensor for in situ measurements of living cells. The plasmonic properties of this system are characterized as a function of the material properties and related to compositional features of the material through a proposed light-directed mechanism. This chemistry provides a tunable, 3D printable plasmonic composite material of encapsulated gold nanoparticles in a biologically-compliant, acrylate-based hydrogel matrix. Surface-enhanced Raman scattering studies of 3D-microcultures supported by the scaffolds are carried out and the strong influence of perm-selective molecular diffusion in its analytical responses is established. Most notably, specific, largely hydrophilic, cellular metabolites are detected within the supported live cultures.

Review of Stimulated Raman Scattering Microscopy Techniques and Applications in the Biosciences

Stimulated Raman scattering (SRS) microscopy is a nonlinear optical imaging method for visualizing chemical content based on molecular vibrational bonds. Featuring high speed, high resolution, high sensitivity, high accuracy, and 3D sectioning, SRS microscopy has made tremendous progress toward biochemical information acquisition, cellular function investigation, and label-free medical diagnosis in the biosciences. In this review, the principle of SRS, system design, and data analysis are introduced, and the current innovations of the SRS system are reviewed. In particular, combined with various bio-orthogonal Raman tags, the applications of SRS microscopy in cell metabolism, tumor diagnosis, neuroscience, drug tracking, and microbial detection are briefly examined. The future prospects for SRS microscopy are also shared.

In vitro Label Free Raman Microspectroscopic Analysis to Monitor the Uptake, Fate and Impacts of Nanoparticle Based Materials

The continued emergence of nanoscale materials for nanoparticle-based therapy, sensing and imaging, as well as their more general adoption in a broad range of industrial applications, has placed increasing demands on the ability to assess their interactions and impacts at a cellular and subcellular level, both in terms of potentially beneficial and detrimental effects. Notably, however, many such materials have been shown to interfere with conventional in vitro cellular assays that record only a single colorimetric end-point, challenging the ability to rapidly screen cytological responses. As an alternative, Raman microspectroscopy can spatially profile the biochemical content of cells, and any changes to it as a result of exogenous agents, such as toxicants or therapeutic agents, in a label free manner. In the confocal mode, analysis can be performed at a subcellular level. The technique has been employed to confirm the cellular uptake and subcellular localization of polystyrene nanoparticles (PSNPs), graphene and molybdenum disulfide micro/nano plates (MoS₂), based on their respective characteristic spectroscopic signatures. In the case of PSNPs it was further employed to identify their local subcellular environment in endosomes, lysosomes and endoplasmic reticulum, while for MoS₂ particles, it was employed to monitor subcellular degradation as a function of time. For amine functionalized PSNPs, the potential of Raman microspectroscopy to quantitatively characterize the dose and time dependent toxic responses has been explored, in a number of cell lines. Comparing the responses to those of poly (amidoamine) nanoscale polymeric dendrimers, differentiation of apoptotic and necrotic pathways based on the cellular spectroscopic responses was demonstrated. Drawing in particular from the experience of the authors, this paper details the progress to date in the development of applications of Raman microspectroscopy for in vitro, label free analysis of the uptake, fate and impacts of nanoparticle based materials, in vitro, and the prospects for the development of a routine, label free high content spectroscopic analysis technique.

Alkyne-Tagged PLGA Allows Direct Visualization of Nanoparticles In Vitro and Ex Vivo by Stimulated Raman Scattering Microscopy

Polymeric nanoparticles (NPs) are attractive candidates for the controlled and targeted delivery of therapeutics in vitro and in vivo. However, detailed understanding of the uptake, location, and ultimate cellular fate of the NPs is necessary to satisfy safety concerns, which is difficult because of the nanoscale size of these carriers. In this work, we show how small chemical labels can be appended to poly(lactic acid-co-glycolic acid) (PLGA) to synthesize NPs that can then be imaged by stimulated Raman scattering microscopy, a vibrational imaging technique that can elucidate bond-specific information in biological environments, such as the identification of alkyne signatures in modified PLGA terpolymers. We show that both deuterium and alkyne labeled NPs can be imaged within primary rat microglia, and the alkyne NPs can also be imaged in ex vivo cortical mouse brain tissue. Immunohistochemical analysis confirms that the NPs localize in microglia in the mouse brain tissue, demonstrating that these NPs have the potential to deliver therapeutics selectively to microglia.

Multicolor Cocktail for Breast Cancer Multiplex Phenotype Targeting and Diagnosis Using Bioorthogonal Surface-Enhanced Raman Scattering Nanoprobes

Early precise diagnosis of cancers is crucial to realize more effective therapeutic interventions with minimal toxic effects. Cancer phenotypes may also alter greatly among patients and within individuals over the therapeutic process. The identification and characterization of specific biomarkers expressed on tumor cells are in high demand for diagnosis and treatment, but they are still a challenge. Herein, we designed three new bioorthogonal surface-enhanced Raman scattering (SERS) nanoprobes and successfully applied the cocktail of them for MDA-MB-231 and MCF-7 breast cancer multiplex phenotype detection. The SERS nanoprobes containing Raman reporters with diynl, azide, or cyano moieties demonstrated apparent Raman shift peaks in 2205, 2120, and 2230 cm-1, respectively, in the biologically Raman-silent region. Three target ligands, including oligonucleotide aptamer (AS1411), arginine-glycine-aspatic acid (RGD) peptide, and homing cell adhesion molecule antibody (anti-CD44), were separately conjugated to the nanoprobes for active recognition capability. The cocktail of the nanoprobes manifested minimal cytotoxicity and simultaneously multiplex phenotype imaging of MDA-MB-231 and MCF-7 cells. Quantitative measurement of cellular uptake by inductively coupled plasma mass spectrometry (ICPMS) verified that MDA-MB-231 cells harbored a much higher expression level of CD44 receptor than MCF-7 cells. For in vivo SERS detection, Raman shift peaks of 2120, 2205, and 2230 cm-1 in the micro-tumor were clearly observed, representing the existence of three specific biomarkers of nucleolin, integrin αvβ3, and CD44 reporter, which could be used for early cancer phenotype identification. The biodistribution results indicated that target ligand modified nanoprobes exhibited much more accumulation in tumors than those nanoprobes without target ligands. The multicolor cocktail of bioorthogonal SERS nanoprobes offers an attractive and insightful strategy for early cancer multiplex phenotype targeting and diagnosis in vivo that is noninvasive and has low cross-talk, unique spectral-molecular signature, high sensitivity, and negligible background interference.

Biological imaging of chemical bonds by stimulated Raman scattering microscopy

All molecules consist of chemical bonds, and much can be learned from mapping the spatiotemporal dynamics of these bonds. Since its invention a decade ago, stimulated Raman scattering (SRS) microscopy has become a powerful modality for imaging chemical bonds with high sensitivity, resolution, speed and specificity. We introduce the fundamentals of SRS microscopy and review innovations in SRS microscopes and imaging probes. We highlight examples of exciting biological applications, and share our vision for potential future breakthroughs for this technology.

Cellular Imaging Using Stimulated Raman Scattering Microscopy

Cellular imaging is an active area of research that enables researchers to monitor cellular dynamics, as well as responses to various external stimuli (physiological stress, exogenous compounds, etc.). Stimulated Raman scattering (SRS) microscopy is one popular experimental tool used to image cells, largely because of its chemical specificity, high spatial resolution, and high image acquisition speed. In this Perspective, the theoretical background and experimental implementation of SRS microscopy are discussed and recent developments in the field of cellular imaging with SRS are highlighted and summarized.

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.

Probe design for super-multiplexed vibrational imaging

Optical microscopy has served biomedical research for decades due to its high temporal and spatial resolutions. Among various optical imaging techniques, fluorescence imaging offers superb sensitivity down to single molecule level but its multiplexing capacity is limited by intrinsically broad bandwidth. To simultaneously capture a vast number of targets, the newly emerging vibrational microscopy technique draws increasing attention as vibration spectroscopy features narrow transition linewidth. Nonetheless, unlike fluorophores that have been studied for centuries, a systematic investigation on vibrational probes is underemphasized. Herein, we reviewed some of the recent developments of vibrational probes for multiplex imaging applications, particularly those serving stimulated Raman scattering (SRS) microscopy, which is one of the most promising vibrational imaging techniques. We wish to summarize the general guidelines for developing bioorthogonal vibrational probes with high sensitivity, chemical specificity and most importantly, tunability to fulfill super-multiplexed optical imaging. Future directions to significantly improve the performance are also discussed.

Raman Imaging of Nanocarriers for Drug Delivery

The efficacy of pharmaceutical agents can be greatly improved through nanocarrier delivery. Encapsulation of pharmaceutical agents into a nanocarrier can enhance their bioavailability and biocompatibility, whilst also facilitating targeted drug delivery to specific locations within the body. However, detailed understanding of the in vivo activity of the nanocarrier-drug conjugate is required prior to regulatory approval as a safe and effective treatment strategy. A comprehensive understanding of how nanocarriers travel to, and interact with, the intended target is required in order to optimize the dosing strategy, reduce potential off-target effects, and unwanted toxic effects. Raman spectroscopy has received much interest as a mechanism for label-free, non-invasive imaging of nanocarrier modes of action in vivo. Advanced Raman imaging techniques, including coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS), are paving the way for rigorous evaluation of nanocarrier activity at the single-cell level. This review focuses on the development of Raman imaging techniques to study organic nanocarrier delivery in cells and tissues.

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.

Synthesis and Postpolymerization Modification of Fluorine-End-Labeled Poly(Pentafluorophenyl Methacrylate) Obtained via RAFT Polymerization

Chain-end-labeled polymers are interesting for a range of applications. In polymer nanomedicine, chain-end-labeled polymers are useful to study and help understand cellular internalization and intracellular trafficking processes. The recent advent of fluorescent label-free techniques, such as nanoscale secondary ion mass spectrometry (NanoSIMS), provides access to high-resolution intracellular mapping that can complement information obtained using fluorescent-labeled materials and confocal microscopy and flow cytometry. Using poly(N-(2-hydroxypropyl)methacrylamide) (PHPMA) as a prototypical polymer nanomedicine, this paper presents a synthetic strategy to polymers that contain trace element labels, such as fluorine, which can be used for NanoSIMS analysis. The strategy presented in this paper is based on reversible addition fragmentation chain transfer (RAFT) polymerization of pentafluorophenyl methacrylate (PFMA) mediated by two novel chain-transfer agents (CTAs), which contain either one (α) or two (α,ω) fluorine labels. In the first part of this study, via a number of polymerization experiments, the polymerization properties of the fluorinated RAFT CTAs were established. 19F NMR spectroscopy revealed that these fluorinated RAFT agents possess unique spectral signatures, which allow to directly monitor RAFT agent conversion and measure end-group fidelity. Comparison with 4-cyanopentanoic acid dithiobenzoate, which is a standard CTA for the RAFT polymerization of PFMA, revealed that the introduction of one or two fluorine labels does not significantly affect the polymerization properties of the CTA. In the last part of this paper, a proof-of-concept study is presented that demonstrates the feasibility of the fluorine-labeled poly(pentafluorophenyl methacrylate) polymers as platforms for the postpolymerization modification to generate PHPMA-based polymer nanomedicines.

Label-Free Imaging of Nanoparticle Uptake Competition in Single Cells by Hyperspectral Stimulated Raman Scattering

Imaging and quantification of nanoparticles in single cells in their most natural condition are expected to facilitate the biotechnological applications of nanoparticles and allow for better assessment of their biosafety risks. However, current imaging modalities either require tedious sample preparation or only apply to nanoparticles with specific physicochemical characteristics. Here, the emerging hyperspectral stimulated Raman scattering (SRS) microscopy, as a label-free and nondestructive imaging method, is used for the first time to investigate the subcellular distribution of nanoparticles in the protozoan Tetrahymena thermophila. The two frequently studied nanoparticles, polyacrylate-coated α-Fe₂ O₃ and TiO₂ , are found to have different subcellular distribution pattern as a result of their dissimilar uptake routes. Significant uptake competition between these two types of nanoparticles is further discovered, which should be paid attention to in future bioapplications of nanoparticles. Overall, this study illustrates the great promise of hyperspectral SRS as an analytical imaging tool in nanobiotechnology and nanotoxicology.