Mitochondrial Imaging with Combined Fluorescence and Stimulated Raman Scattering Microscopy Using a Probe of the Aggregation-Induced Emission Characteristic

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.

Modulation of Thermally Activated Delayed Fluorescence in Waterborne Polyurethanes via Charge-Transfer Effect

Here, we designed several waterborne polyurethanes (WPUs) with efficient thermally activated delayed fluorescence (TADF) via serving charge-transfer (CT) states as a mediate bridge between singlet and triplet states to boost reverse intersystem crossing (RISC). By tuning substituents of diphenyl sulfone (DS), we found that O,O'- and S,S'-substituted DS covalently incorporated in WPUs solely show typical fluorescence emission with lifetimes in the nanosecond range. Interestingly, TADF appears by replacing the substituent with the nitrogen atom, of which lifetimes are up to ≈10 microseconds and ≈1 millisecond in air and vacuum, respectively, even though the energy gap between singlet and triplet states (ΔE_ST ) is still large for generating TADF. To explain this phenomenon, an energy level mode based on CT states and an ³ (n-π*) receiver state was proposed. By the rational modulation of CT states, it is possible to tune the ΔE_ST to render TADF-based materials suitable for versatile applications.

Dietary fatty acids promote lipid droplet diversity through seipin enrichment in an ER subdomain

Exogenous metabolites from microbial and dietary origins have profound effects on host metabolism. Here, we report that a sub-population of lipid droplets (LDs), which are conserved organelles for fat storage, is defined by metabolite-modulated targeting of the C. elegans seipin ortholog, SEIP-1. Loss of SEIP-1 function reduces the size of a subset of LDs while over-expression of SEIP-1 has the opposite effect. Ultrastructural analysis reveals SEIP-1 enrichment in an endoplasmic reticulum (ER) subdomain, which co-purifies with LDs. Analyses of C. elegans and bacterial genetic mutants indicate a requirement of polyunsaturated fatty acids (PUFAs) and microbial cyclopropane fatty acids (CFAs) for SEIP-1 enrichment, as confirmed by dietary supplementation experiments. In mammalian cells, heterologously expressed SEIP-1 engages nascent lipid droplets and promotes their subsequent expansion in a conserved manner. Our results suggest that microbial and polyunsaturated fatty acids serve unexpected roles in regulating cellular fat storage by promoting LD diversity.

Lipid droplets (LDs) are fat storage organelles that are initiated and expanded by seipins at ER contact sites. Here the authors show that the C. elegans seipin ortholog SEIP-1 is recruited to these sites by certain dietary fatty acids to support the expansion of a subset of LDs.

Raman Imaging of Small Biomolecules

Imaging techniques greatly facilitate the comprehensive knowledge of biological systems. Although imaging methodology for biomacromolecules such as protein and nucleic acids has been long established, microscopic techniques and contrast mechanisms are relatively limited for small biomolecules, which are equally important participants in biological processes. Recent developments in Raman imaging, including both microscopy and tailored vibrational tags, have created exciting opportunities for noninvasive imaging of small biomolecules in living cells, tissues, and organisms. Here, we summarize the principle and workflow of small-biomolecule imaging by Raman microscopy. Then, we review recent efforts in imaging, for example, lipids, metabolites, and drugs. The unique advantage of Raman imaging has been manifested in a variety of applications that have provided novel biological insights.

A Freezing-Induced Turn-On Imaging Modality for Real-Time Monitoring of Cancer Cells in Cryosurgery

Cryosurgery has attracted much attention for the treatment of tumors owing to its clear advantages. However, determining the volume of frozen tissues in real-time remains a challenge, which greatly lowers the therapeutic efficacy of cryosurgery and hinders its broad application for the treatment of cancers. Herein, we report a freezing-induced turn-on strategy for the selective real-time imaging of frozen cancer cells. As a type of aggregation-induced emission (AIE) fluorogen, TABD-Py molecules interact specifically with ice crystals and form aggregates at the ice/water interface. Consequently, bright fluorescent emission appears upon freezing. TABD-Py molecules are enriched mostly in the cancer cells and exhibit high biocompatibility as well as low cytotoxicity; therefore, a freezing-induced turn-on imaging modality for cryosurgery is developed, which will certainly maximize the therapeutic efficacy of cryosurgery in treating tumors.

Tandem fluorescence and Raman (fluoRaman) characterisation of a novel photosensitiser in colorectal cancer cell line SW480

A novel photosensitiser, DC473, designed with solvatochromatic fluorescence and distinct Raman signal, is detected with tandem fluoRaman in SW480 cells.

The development of new imaging tools, molecules and modalities is crucial to understanding biological processes and the localised cellular impact of bioactive compounds. A small molecule photosensitiser, DC473, has been designed to be both highly fluorescent and to exhibit a strong Raman signal in the cell-silent region of the Raman spectrum due to a diphenylacetylene structure. DC473 has been utilised to perform a range of novel tandem fluorescence and Raman (fluoRaman) imaging experiments, enabling a thorough examination of the compound's cellular localisation, exemplified in colorectal cancer cells (SW480). This multifunctional fluoRaman imaging modality revealed the presence of the compound in lipid droplets and only a weak signal in the cytosol, by both Raman and fluorescence imaging. In addition, Raman microscopy detected the compound in a cell compartment we labelled as the nucleolus, whereas fluorescence microscopy did not detect the fluoRaman probe due to solvatochromatic effects in a local polar environment. This last finding was only possible with the use of tandem confocal Raman and fluorescence methods. By following the approach detailed herein, incorporation of strong Raman functional groups into fluorophores can enable a plethora of fluoRaman experiments, shedding further light on potential drug compound's cellular behaviour and biological activity.

Quantitative Imaging of Lipid Synthesis and Lipolysis Dynamics in Caenorhabditis elegans by Stimulated Raman Scattering Microscopy

Quantitative methods to precisely measure cellular states in vivo have become increasingly important and desirable in modern biology. Recently, stimulated Raman scattering (SRS) microscopy has emerged as a powerful tool to visualize small biological molecules tagged with alkyne (C≡C) or carbon-deuterium (C-D) bonds in the cell-silent region. In this study, we developed a technique based on SRS microscopy of vibrational tags for quantitative imaging of lipid synthesis and lipolysis in live animals. The technique aims to overcome the major limitations of conventional fluorescent staining and lipid extraction methods that do not provide the capability of in vivo quantitative analysis. Specifically, we used three bioorthogonal lipid molecules (the alkyne-tagged fatty acid 17-ODYA, deuterium-labeled saturated fatty acid PA-D₃₁, and unsaturated fatty acid OA-D₃₄) to investigate the metabolic dynamics of lipid droplets (LDs) in live Caenorhabditis elegans ( C. elegans). Using a hyperspectral SRS (hsSRS) microscope and subtraction method, the interfering non-Raman background was eliminated to improve the accuracy of lipid quantification. A linear relationship between SRS signals and fatty acid molar concentrations was accurately established. With this quantitative analysis tool, we imaged and determined the changes in concentration of the three fatty acids in LDs of fed or starved adult C. elegans. Using the hsSRS imaging mode, we also observed the desaturation of fatty acids in adult C. elegans via spectral analysis on the SRS signals from LDs. The results demonstrated the unique capability of hsSRS microscopy in quantitative analysis of lipid metabolism in vivo.

Polymerization of phenylacetylenes by binuclear rhodium catalysts with different para-binucleating phenoxyiminato linkages

A binuclear rhodium catalyst 3b with a 2,5-phenyloxydiiminato linkage and an nbd ligand exhibits cooperative effects in terms of enhanced catalytic activity in the polymerization of phenylacetylene and its functional derivatives.

A red mitochondria-targeted AIEgen for visualizing H2S in living cells and tumours

A red mitochondria-targeted AIEgen with greater conjugate and more positive charges for visualizing H₂S in cells and tumours.

Aggregation-induced emission based PET probe for liver function imaging

A novel aggregation-induced emission based PET probe for liver function imaging was developed.

A tripodal supramolecular sensor to successively detect picric acid and CN− through guest competitive controlled AIE

Herein, we report a simple and efficient method for the selective and sensitive detection of picric acid (PA) and CN⁻via a novel guest competitive controlled aggregation-induced emission (AIE) mechanism based on a tris-naphthalimide derivative TG.

Simultaneous enhancement of transition dipole strength and vibrational lifetime of an alkyne IR probe via π-d backbonding and vibrational decoupling

Alkyne IR probes 1–6 with Si and S (or Se) atoms incorporated into the CC bond were synthesized, and the vibrational properties of their CC stretch mode were studied using FTIR and femtosecond IR PP spectroscopies and quantum chemical calculations.

Direct observation of selective autophagy induction in cells and tissues by self-assembled chiral nanodevice

The interactions between chiral nanomaterials and organisms are still challenging and mysterious. Here, a chiral nanodevice made of yolk-shell nanoparticles tetrahedron (UYTe), centralized with upconversion nanoparticles (UCNPs), was fabricated to induce autophagy in vivo. The proposed chiral nanodevice displayed a tunable circular dichroism (CD) signal when modified with different enantiomers of glutathione (GSH). Notably, UYTe showed significant chirality-dependent autophagy-inducing ability after D-GSH-modification because the enhanced oxidative stress and accumulation in living cell. The activation of autophagy resulted in the reduced intracellular CD intensity from the disassembly of the structure. The intracellular ATP concentration was simultaneously enhanced in response to autophagy activity, which was quantitatively bio-imaged with the upconversion luminescence (UCL) signal of the UCNP that escaped from UYTe. The autophagy effect induced in vivo by the chiral UYTe was also visualized with UCL imaging, demonstrating the great potential utility of the chiral nanostructure for cellular biological applications.

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.

Super-Resolution Tracking of Mitochondrial Dynamics with An Iridium(III) Luminophore

Combining luminescent transition metal complex with super-resolution microscopy is an excellent strategy for the long-term visualization of the dynamics of subcellular structures in living cells. However, it remains unclear whether iridium(III) complexes are applicable for a particular type of super-resolution technique, structured illumination microscopy (SIM), to image subcellular structures. Herein, an iridium(III) dye, to track mitochondrial dynamics in living cells under SIM is described. The dye demonstrates excellent specificity and photostability and satisfactory cell permeability. While using SIM to image mitochondria, an ≈80 nm resolution is achieved that allows the clear observation of the structure of mitochondrial cristae. The dye is used to monitor and quantify mitochondrial dynamics relative to lysosomes, including fusion involved in mitophagy, and newly discovered mitochondria-lysosome contact (MLC) under different conditions. The MLC remains intact and fusion vanishes when five receptors, p62, NDP52, OPTN, NBR1, and TAX1BP1, are knocked out, suggesting that these two processes are independent.

Rational Design for Multicolor Flavone-Based Fluorophores with Aggregation-Induced Emission Enhancement Characteristics and Applications in Mitochondria-Imaging

Fluorophores with aggregation-induced emission enhancement (AIEE) properties have attracted more attention in recent years. In order to realise more valuable applications, the different kinds of AIEE molecules are in serious need of further development. Therefore, a novel flavone-based AIEE system derived from restriction of intramolecular rotation (RIR) was designed and synthesized in this work. The results revealed that six of the compounds showed typical AIEE characteristics, with fluorescence emissions from purple, blue, cyan to green, tunable by changing substituent groups. This flavone-based AIEE system has never been reported before. The AIEE characteristics were investigated by optical spectroscopy, fluorescence photographs, scanning electron microscopy (SEM), fluorescence quantum yields (Ф_F) and fluorescence lifetime in the CH₃OH/H₂O mixed solution. Moreover, benefiting from the simple structures and small molecular weight, they could permeate cells faster than current high-molecular-weight AIEE molecules. Furthermore, to examine possible biomedical applications, fluorescence imaging in living A549 lung cells and cell viabilities were examined, and the results displayed that these fluorophores showed good cellular uptake and low cytotoxicity within the experimental concentration range. In addition, these AIEE compounds possessed excellent specificity for mitochondrial targeting and mitochondrial morphological change tracking, besides, they displayed superior photostability, which indicated they are potential candidates for mitochondrial imaging.

Engineering Organelle-Specific Molecular Viscosimeters Using Aggregation-Induced Emission Luminogens for Live Cell Imaging

Subcellular viscosity is essential for cell functions and may indicate its physiological status. We screen two fluorescent probes by engineering tetraphenylethene (TPE) for measuring viscosity in mitochondria and lysosomes, respectively. These two probes are only weakly emissive in nonviscous medium and the emission signals are greatly enhanced in viscous medium due to the restriction of intramolecular motion. The presence of pyridium has endowed one probe with mitochondrial specificity, while the presence of indole ring has granted the other probe with lysosome-targeting ability. Their optical properties are characterized in vitro and their applications in imaging viscosity variations in mitochondria and lysosomes are also demonstrated in living cells under different stimulated processes. In addition, an increase in both mitochondrial and lysosomal viscosity during mitophagy was revealed for the first time with our probes. To our knowledge, this is the first time that TPE is engineered to be fluorescent molecular viscosimeters that possess desirable aqueous solubility, red-shifted emission, and organelle specificity.

Cyanotriphenylamine-based polyimidothioethers as multifunctional materials for ambipolar electrochromic and electrofluorochromic devices, and fluorescent electrospun fibers

New luminescent and electrochromic polyimidothioethers were synthesized and fabricated as ambipolar electrochromic and electrofluorochromic devices, and fluorescent electrospun fibers.

Cross-linked AIE supramolecular polymer gels with multiple stimuli-responsive behaviours constructed by hierarchical self-assembly

Cross-linked AIE supramolecular polymer gels were successfully constructed by hierarchical self-assembly.