Advances in fluorescence labeling strategies for dynamic cellular imaging

Dual-Organelle-Targeted Fluorescent Probe for Lipid Droplets and Endoplasmic Reticulum Using Bay-Region Substituted Perylene Diimides

In the complex cellular microenvironment, lipid droplets (LDs) and the endoplasmic reticulum (ER) are essential for maintaining cellular homeostasis. Their interactions are crucial for regulating processes such as lipid metabolism. Nevertheless, simultaneously monitoring these two organelles remains a challenging task. In this study, a dual-organelle-targeted perylene diimides (PDIs) based probe, LD-ER-PDI, by incorporating a 2-azetidinone substituent in the bay-region and two distinct targeting groups for LDs and ER at opposite ends of the imide positions was developed, enabling discrimination between LDs and ER. Moreover, it can effectively visualize the dynamic behaviors of LDs and ER under starvation and oleic acid-treated conditions, and also allows real-time monitoring of LDs formation from the ER. This work not only provides a valuable tool for studying the dynamic interactions between LDs and ER but also highlights a strategy for designing dual-organelle-targeted fluorescent probes.

Methionine-Specific Bioconjugation for Single-Molecule Force Spectroscopy of Cell Surface Proteins

Cell surface proteins play crucial roles in various cellular processes, including intercellular communication, adhesion, and immune responses. However, investigating these proteins using single-molecule force spectroscopy (SMFS) has been hindered by challenges in site-specific protein modification while preserving their native state. Here, we introduce a methionine-specific bioconjugation strategy utilizing a bespoke hypervalent iodine reagent for highly selective, rapid, and robust methionine labeling. Since methionine is often the first amino acid incorporated into proteins via initiator tRNA, this approach enables precise N-terminal labeling and attachment, facilitating more reliable SMFS studies. The resulting covalent linkage remains intact during mechanical unfolding or conformational changes of proteins, with a mechanical stability exceeding 600 pN, allowing accurate measurements before detachment from AFM cantilever tips or cell surfaces. Additionally, this method improves sampling rates and data quality. We successfully applied this technique to light-induced protein printing and natural surface protein studies, demonstrating its potential for advancing protein mechanics research in living cells. This strategy provides significant advantages for SMFS in the study of complex cellular systems.

Selective Addressing of Versatile Nanodiamonds via Physically-Enabled Classifier in Complex Biosystems

Nitrogen-vacancy (NV) centers show great potential for nanoscale biosensing and bioimaging. Nevertheless, their envisioned bioapplications suffer from intrinsic background noise due to unavoidable light scattering and autofluorescence in cells and tissues. Herein, we develop a unique all-optical modulated imaging method via a physically enabled classifier, for on-demand and direct access to NV fluorescence at pixel resolution while effectively filtering out background noise. Specifically, NV fluorescence can be modulated optically to exhibit sinusoid-like variations, providing a basis for classification. We validate our method in various complex biological scenarios with fluorescence interference, ranging from cells to organisms. Notably, our classification-based approach achieves an enhancement of signal-to-background ratio from 1.92 to 60.39 dB for fluorescent nanodiamonds in neural protein imaging. We also demonstrate a 4-fold contrast improvement in optically detected magnetic resonance measurements inside stained cells. Our technique offers a generic, explainable, and robust solution, applicable for realistic high-fidelity imaging and sensing in challenging noise-laden scenarios.

Merging Vibrational Spectroscopy with Fluorescence Microscopy: Combining the Best of Two Worlds

Vibrational spectroscopy and fluorescence spectroscopy have historically been two established but separate fields of molecular spectroscopy. While vibrational spectroscopy provides exquisite chemical information, fluorescence spectroscopy often offers orders of magnitude higher detection sensitivity. However, they each lack the advantages of each other. In recent years, a series of novel nonlinear optical spectroscopy studies have been developed that merge both spectroscopies into a single double-resonance process. These techniques combine the chemical specificity of Raman or infrared (IR) spectroscopy with the superb detection sensitivity and spatial resolution of fluorescence microscopy. Many facets have been explored, including Raman transition versus IR transition, time domain versus frequency domain, and spectroscopy versus microscopy. Notably, single-molecule vibrational spectroscopy has been achieved at room temperature without the need for plasmonics. Even superresolution vibrational imaging beyond the diffraction limit was demonstrated. This review summarizes the growing field of vibrational-encoded fluorescence microscopy, including key technical developments, emerging applications, and future prospects.

Resolving Conformational Plasticity in Mammalian Cells with High-Resolution Fluorescence Tools

Investigating protein dynamic structural changes is fundamental for understanding protein function, drug discovery, and disease mechanisms. Traditional studies of protein dynamics often rely on investigations of purified systems, which fail to capture the complexity of the cellular environment. The intracellular milieu imposes distinct physicochemical constraints that affect macromolecular interactions and dynamics in ways not easily replicated in isolated experimental setups. We discuss the use of fluorescence resonance energy transfer, fluorescence anisotropy, and minimal photon flux imaging technologies to address these challenges and directly investigate protein conformational dynamics in mammalian cells. Key findings from the application of these techniques demonstrate their potential to reveal intricate details of protein conformational plasticity. By overcoming the limitations of traditional in vitro methods, these approaches offer a more accurate and comprehensive understanding of protein function and behavior within the complex environment of mammalian cells.

A conformational switch-controlled RNA sensor based on orthogonal dCas12a for RNA imaging in live cells

RNA imaging technology is essential for understanding the complex RNA regulatory mechanisms and serves as a powerful tool for disease diagnosis. However, conventional RNA imaging methods often require multiple fluorescent tags for the specific labeling of individual targets, complicating both the imaging process and subsequent analysis. Herein, we develop an RNA sensor that integrates a blocked CRISPR RNA (crRNA)-based conformational switch with a controllable CRISPR activation (CRISPRa) system and apply for RNA imaging. By leveraging nuclease-inactive Cas12a (dCas12a)-mediated processing of precursor crRNA (pre-crRNA) and the orthogonality of dCas12a from different bacteria, our sensor establishes an artificial link between two unrelated RNA targets, enabling cells to sense one RNA target and image another with a single fluorescent signal. By visualizing a single target for dual-target analysis, our method significantly reduces the reliance on multiple fluorescent tags. Our sensor provides a new platform for RNA imaging, enhancing both biomedical research and the development of advanced molecular diagnostics.

Fluorescence correlation spectroscopy for particle sizing: A notorious challenge

In many quantitative investigations of biological systems, including, e.g., the study of biomolecular interactions, assembly and disassembly, aggregation, micelle and vesicle formation, or drug encapsulation, accurate determination of particle sizes is of key interest. Fluorescence correlation spectroscopy (FCS), with its exceptional sensitivity for molecular diffusion properties, has long been proposed as a valuable method to size small, freely diffusible particles with superior precision. It is conceptually related to the more widespread particle sizing technique dynamic light scattering (DLS) but offers greater selectivity and sensitivity due to the use of fluorescence rather than scattered light. However, in spite of these apparent advantages, FCS has never become established as a biophysical routine for particle sizing. This is due to the fact that sensitivity can, under certain conditions, indeed be disadvantageous, as it renders the technique error prone and overly susceptible to signal disturbances. Here, we discuss the systematic challenges, as well as the advances made over the past decades, to employing FCS in polydisperse samples. The problematic role of large particles, a common issue in DLS and FCS, and the effect of fluorescent labeling are discussed in detail, along with strategies for respective error mitigation in experiments and data analysis. We expect this overview to guide future users in successfully applying FCS to their particle sizing problems in the hope of fostering a more widespread and routine use of FCS-based technology.

Advances in Optical Contrast Agents for Medical Imaging: Fluorescent Probes and Molecular Imaging

Optical imaging is an excellent non-invasive method for viewing visceral organs. Most importantly, it is safer as compared to ionizing radiation-based methods like X-rays. By making use of the properties of photons, this technique generates high-resolution images of cells, molecules, organs, and tissues using visible, ultraviolet, and infrared light. Moreover, optical imaging enables real-time evaluation of soft tissue properties, metabolic alterations, and early disease markers in real time by utilizing a variety of techniques, including fluorescence and bioluminescence. Innovative biocompatible fluorescent probes that may provide disease-specific optical signals are being used to improve diagnostic capabilities in a variety of clinical applications. However, despite these promising advancements, several challenges remain unresolved. The primary obstacle includes the difficulty of developing efficient fluorescent probes, and the tissue autofluorescence, which complicates signal detection. Furthermore, the depth penetration restrictions of several imaging modalities limit their use in imaging of deeper tissues. Additionally, enhancing biocompatibility, boosting fluorescent probe signal-to-noise ratios, and utilizing cutting-edge imaging technologies like machine learning for better image processing should be the main goals of future research. Overcoming these challenges and establishing optical imaging as a fundamental component of modern medical diagnoses and therapeutic treatments would require cooperation between scientists, physicians, and regulatory bodies.

Fluorescent labeling strategies for molecular bioimaging

Super-resolution microscopy (SRM) has transformed biological imaging by circumventing the diffraction limit of light and enabling the visualization of cellular structures and processes at the molecular level. Central to the capabilities of SRM is fluorescent labeling, which ensures the precise attachment of fluorophores to biomolecules and has direct impact on the accuracy and resolution of imaging. Continuous innovation and optimization in fluorescent labeling are essential for the successful application of SRM in cutting-edge biological research. In this review, we discuss recent advances in fluorescent labeling strategies for molecular bioimaging, with a special focus on protein labeling. We compare different approaches, highlight technological breakthroughs, and address challenges such as linkage error and labeling density. By evaluating both established and emerging methods, we aim to guide researchers through all aspects that should be considered before opting for any labeling technique.

Bright and Stable Cyan Fluorescent RNA Enables Multicolor RNA Imaging in Live Escherichia coli

Fluorescent RNAs (FRs), which are RNA aptamers that bind and activate their cognate small fluorogenic dyes, have provided a particularly useful approach for imaging RNAs in live cells. Although the color palette of FRs is greatly expanded, a bright and stable cyan FR with good biocompatibility and biorthogonality with currently available FRs remains desirable but is not yet developed. Herein, the development of Myosotis is described, an RNA aptamer that emits bright cyan fluorescence upon binding a novel GFP chromophore-like fluorophore called DBT. Myosotis has a nanomolar affinity for DBT and shows a weak dependence on magnesium for folding. Further studies reveal that the Myosotis-DBT complex has a long fluorescence lifetime, good photostability, and enhance cellular brightness. It is further shown that Myosotis-DBT is biorthogonal to Pepper and Clivia FRs, allowing multiplex fluorescence imaging of RNA in live bacteria. Myosotis can also use to image mRNA in live bacteria, revealing potential coupling between mRNA translation and stability. It is believed that this cyan FR will be a useful tool for studying the functionality and mechanism of RNA underlying diverse biological processes.

Advancements in fluorescent labeling in assessing the probiotic adhesion capacity - A review

Adhesion capacity of probiotics is closely related to their intestinal-protective effects. The conventional techniques used to evaluate probiotic adhesion capacity have limitations in terms of imaging resolution and quantitative analysis. Fluorescent labelling technology has shown immense potential in recent years owing to its high specificity and sensitivity for resolving probiotic adhesion mechanisms. Although there are still problems with the fluorescence signal intensity and hysteresis effect, this technology has significantly advanced the accurate detection and evaluation of probiotic adhesion capacity. This review examines the critical role of probiotic adhesion and its detection methods, with a special focus on the application of fluorescent-labeling technology. Our objective was to identify more accurate and efficient approaches for evaluating the adhesion capacity of probiotic bacteria while promoting in-depth research into the underlying mechanisms that govern probiotic adhesion.

Unveiling the promise of peptide nucleic acids as functional linkers for an RNA imaging platform

Linkers in chemical biology provide more than just connectivity between molecules; their intrinsic properties can be harnessed to enhance the stability and functionality of chemical probes. In this study, we explored the incorporation of a peptide nucleic acid (PNA)-based linker into RNA-targeting probes to improve their affinity and specificity. By integrating a PNA linker into a small molecule probe of the Riboglow platform, we enabled dual binding events: cobalamin (Cbl)-RNA structure-based recognition and sequence-specific PNA-RNA interaction. We show that incorporating a six-nucleotide PNA sequence complementary to a region of wild type RNA aptamer (env8) results in a 30-fold improvement in binding affinity compared to the probe with a nonfunctional PEG linker. Even greater improvements are observed when the PNA probe was tested against truncated versions of the RNA aptamer, with affinity increasing by up to 280-fold. Additionally, the PNA linker is able to rescue the Cbl-RNA interaction even when the cobalamin binding pocket is compromised. We demonstrate that PNA probes effectively bind RNA both in vitro and in live cells, enhancing visualization of RNA in stress granules and U-bodies at low concentrations. The modular nature of the Riboglow platform allows for flexible modifications of the PNA linker, fluorophore, and RNA tag, while maintaining high specificity and affinity. This work establishes a new approach for enhancing RNA imaging platforms through the use of PNA linkers, highlighting the potential of combining short oligonucleotides with small molecules to improve the affinity and specificity of RNA-targeting probes. Furthermore, this dual-binding approach presents a promising strategy for driving advancements in RNA-targeted drug development.

Advances in posterity of visualization in paradigm of nano‐level ultra‐structures for nano–bio interaction studies

The progression in contemporary scientific field is facilitated by a multitude of sophisticated and cutting‐edge methodologies that are employed for various research purposes. Among these methodologies, microscopy stands out as a fundamental and essential technique utilized in scientific investigations. Moreover, due to the continuous evolution and enhancement of microscopic methodologies, nanotechnology has reached a highly developed stage within modern scientific realm, particularly renowned for its wide‐ranging applications in the fields of biomedicine and environmental science. When it comes to conducting comprehensive and in‐depth experimental analyses to explore the nanotechnological aspects relevant to biological applications, the concept of nano–biological interaction emerges as the focal point of any research initiative. Nonetheless, this particular study necessitates a meticulous approach toward imaging and visualization at diverse magnification levels to ensure accurate observations and interpretations. It is widely acknowledged that modern microscopy has emerged as a sophisticated and invaluable instrument in this regard. This review aims to provide a comprehensive discussion on the progress made in microscopic techniques specifically tailored for visualizing the interactions between nanostructures and biological entities, thereby facilitating the exploration of the practical applications of nanotechnology in the realm of biological sciences.

Visualizing Endoplasmic Reticulum Stress and Autophagy in Alzheimer's Model Cells by a Peroxynitrite-Responsive AIEgen Fluorescent Probe

Endoplasmic reticulum (ER) stress and autophagy (ER-phagy) occurring in nerve cells are crucial physiological processes closely associated with Alzheimer's disease (AD). Visualizing the two processes is paramount to advance our understanding of AD pathologies. Among the biomarkers identified, peroxynitrite (ONOO-) emerges as a key molecule in the initiation and aggravation of ER stress and ER-phagy, highlighting its significance in the underlying mechanisms of the two processes. In this work, we designed and synthesized an innovative ONOO--responsive AIEgen-based fluorescent probe (DHQM) with the ability to monitor ER stress and ER-phagy in AD model cells. DHQM demonstrated excellent aggregation-induced emission (AIE) properties, endowing it with outstanding ability for washing-free intracellular imaging. Meanwhile, it exhibited high sensitivity, remarkable selectivity to ONOO-, and exceptional ER-targeting ability. The probe was successfully applied for fluorescence imaging of ER ONOO- fluctuations to assess the ER stress status in aluminum-induced AD model cells. Our findings revealed that aluminum-induced ferroptosis, a regulated cell death process, was pivotal in the excessive ONOO- production, which in turn activated and exacerbated ER stress. Furthermore, the aluminum-stimulated ER-phagy was observed utilizing DHQM, which might be crucial in inhibiting ferroptosis and mitigating aberrant ER stress. Overall, this study not only offers valuable insights into the pathological mechanisms of AD at the ER level but also opens new potential therapeutic avenues targeting these pathways.

Full-color peptide-based fluorescent nanomaterials assembled under the control of amino acid doping

Peptide-based biofluorescents are of great interest due to their controllability and biocompatibility, as well as their potential applications in biomedical imaging and biosensing. Here, we present a simple approach to synthesizing full-color fluorescent nanomaterials with broad-spectrum fluorescence emissions, high optical stability, and long fluorescence lifetimes. By doping amino acids during the enzyme-catalyzed oxidative self-assembly of tyrosine-based peptides, we can precisely control the intermolecular interactions to obtain nanoparticles with fluorescence emission at different wavelengths. The synthesized peptide-based fluorescent nanomaterials with excellent biocompatibility and stable near-infrared fluorescence emission were shown to have potential for bioimaging applications. This research provides new ideas for the development of new bioluminescent materials that are cost-effective, environmentally friendly, and safe for biomedical use.

Automated discovery of experimental designs in super-resolution microscopy with XLuminA

Driven by human ingenuity and creativity, the discovery of super-resolution techniques, which circumvent the classical diffraction limit of light, represent a leap in optical microscopy. However, the vast space encompassing all possible experimental configurations suggests that some powerful concepts and techniques might have not been discovered yet, and might never be with a human-driven direct design approach. Thus, AI-based exploration techniques could provide enormous benefit, by exploring this space in a fast, unbiased way. We introduce XLuminA, an open-source computational framework developed using JAX, a high-performance computing library in Python. XLuminA offers enhanced computational speed enabled by JAX's accelerated linear algebra compiler (XLA), just-in-time compilation, and its seamlessly integrated automatic vectorization, automatic differentiation capabilities and GPU compatibility. XLuminA demonstrates a speed-up of 4 orders of magnitude compared to well-established numerical optimization methods. We showcase XLuminA's potential by re-discovering three foundational experiments in advanced microscopy, and identifying an unseen experimental blueprint featuring sub-diffraction imaging capabilities. This work constitutes an important step in AI-driven scientific discovery of new concepts in optics and advanced microscopy.

Water-Soluble Bimodal Magnetic-Fluorescent Radical Dendrimers as Potential MRI-FI Imaging Probes

Dual or multimodal imaging probes have become potent tools for enhancing detection sensitivity and accuracy in disease diagnosis. In this context, we present a bimodal imaging dendrimer-based structure that integrates magnetic and fluorescent imaging probes for potential applications in magnetic resonance imaging and fluorescence imaging. It stands out as one of the rare examples where bimodal imaging probes use organic radicals as the magnetic source, despite their tendency to entirely quench fluorophore fluorescence. Opting for organic radicals over metal-based contrast agents like gadolinium (Gd3+)-chelates is crucial to mitigate associated toxicity concerns. We utilized an amino-terminated polyamide dendrimer containing a 1,8-naphthalimide (Naft) fluorescent group, amino acid derivatives as linkers to enhance water solubility, and TEMPO organic radicals as terminal groups. The same dendrimer structure, featuring an equivalent number of branches but lacking the fluorophore group, was also functionalized with amino acid and terminal radicals to serve as a reference. Remarkably, we achieved a fully water-soluble dendrimer-based structure exhibiting both magnetic and fluorescent properties simultaneously. The fluorescence of the Naft group in the final structure is somewhat quenched by the organic radicals, likely due to photoinduced electron transfer with the nitroxyl radical acting as an electron acceptor, which has been supported by density functional theory calculations. Molecular dynamics simulations are employed to investigate how the dendrimers' structure influences the electron paramagnetic resonance characteristics, relaxivity, and fluorescence. In summary, despite the influence of the radicals-fluorophore interactions on fluorescence, this bimodal dendrimer demonstrates significant fluorescent properties and effective r1 relaxivity of 1.3 mM-1 s-1. These properties have proven effective in staining the live mesenchymal stem cells without affecting the cell nucleus.

Microbial single-cell applications under anoxic conditions

The field of microbiology traditionally focuses on studying microorganisms at the population level. Nevertheless, the application of single-cell level methods, including microfluidics and imaging techniques, has revealed heterogeneity within populations, making these methods essential to understand cellular activities and interactions at a higher resolution. Moreover, single-cell sorting has opened new avenues for isolating cells of interest from microbial populations or complex microbial communities. These isolated cells can be further interrogated in downstream single-cell "omics" analyses, providing physiological and functional information. However, applying these methods to study anaerobic microorganisms under in situ conditions remains challenging due to their sensitivity to oxygen. Here, we review the existing methodologies for the analysis of viable anaerobic microorganisms at the single-cell level, including live-imaging, cell sorting, and microfluidics (lab-on-chip) applications, and we address the challenges involved in their anoxic operation. Additionally, we discuss the development of non-destructive imaging techniques tailored for anaerobes, such as oxygen-independent fluorescent probes and alternative approaches.

Molecular probes for tracking lipid droplet membrane dynamics

Lipid droplets (LDs) feature a unique monolayer lipid membrane that has not been extensively studied due to the lack of suitable molecular probes that are able to distinguish this membrane from the LD lipid core. In this work, we present a three-pronged molecular probe design strategy that combines lipophilicity-based organelle targeting with microenvironment-dependent activation and design an LD membrane labeling pro-probe called LDM. Upon activation by the HClO/ClO- microenvironment that surrounds LDs, LDM pro-probe releases LDM-OH probe that binds to LD membrane proteins thus enabling visualization of the ring-like LD membrane. By utilizing LDM, we identify the dynamic mechanism of LD membrane contacts and their protein accumulation parameters. Taken together, LDM represents the first molecular probe for imaging LD membranes in live cells to the best of our knowledge, and represents an attractive tool for further investigations into the specific regulatory mechanisms with LD-related metabolism diseases and drug screening.

Rapid Detection of Single Bacteria Using Filter-Array-Based Hyperspectral Imaging Technology

Rapid and accurate detection of bacterial pathogens is crucial for preventing widespread public health crises, particularly in the food industry. Traditional methods are often slow and require extensive labeling, which hampers timely responses to potential threats. In response, we introduce a groundbreaking approach using filter-array-based hyperspectral imaging technology, enhanced by a super-resolution demosaicking technique. This innovative technology streamlines the detection process and significantly enhances the resolution of mosaic hyperspectral imaging. By utilizing a snapshot hyperspectral camera with a 15 ms integration time, it facilitates the identification of bacteria at the single-cell level without requiring chemical labels. The integration of a 3D convolutional neural network optimizes the recognition of pathogenic bacteria, achieving an impressive accuracy of 91.7%. Our approach dramatically improves the efficiency and effectiveness of bacterial detection, providing a promising solution for critical applications in public health and the food industry.

Novel Vibrational Proteins

Genetically encoded green fluorescent protein (GFP) and its brighter and redder variants have tremendously revolutionized modern molecular biology and life science by enabling direct visualization of gene regulated protein functions on microscopic and nanoscopic scales. However, the current fluorescent proteins (FPs) only emit a few colors with an emission width of about 30-50 nm. Here, we engineer novel vibrational proteins (VPs) that undergo much finer vibrational transitions and emit rather narrow vibrational spectra (0.1-0.3 nm, roughly 3-10 cm-1). In response to an amber stop codon (UAG), a terminal alkyne bearing an unnatural amino acid (UAA, pEtF) is directly incorporated in place of Tyr64 in the chromophore of pr-Kaede by genetic code expansion. Essentially, the UAA64 further conjugates into a large π system with the contiguous two editable amino acid residues (His63 and Gly65), resulting in a programmable Raman resonance shift of the embedded alkyne. In the proof-of-concept experiment, we constructed a series of novel pEtF-VP mutants and observed fine Raman shifts of the alkynyl group in different chromophores. The genetically encoded novel VPs, could potentially label tens of proteins in the future.

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.

Multicolor Tuning of Perylene Diimides Dyes for Targeted Organelle Imaging In Vivo

The adjustment of the emission wavelengths and cell permeability of the perylene diimides (PDI) for multicolor cell imaging is a great challenge. Herein, based on a bay-region substituent engineering strategy, multicolor perylene diimides (MCPDI) were rationally designed and synthesized by introducing azetidine substituents on the bay region of PDIs. With the fine-tuned electron-donating ability of the azetidine substituents, these MCPDI showed high brightness, orange, red, and near infrared (NIR) fluorescence along with Stokes shifts increasing from 35 to 110 nm. Interestingly, azetidine substituents distorted to the plane of the MCPDI dyes, and the twist angle of monosubstituted MCPDI was larger than that of disubstituted MCPDI, which might efficiently decrease their π-π stacking. Moreover, all of these MCPDI dyes were cell-permeable and selectively stained various organelles for multicolor imaging of multiple organelles in living cells. Two-color imaging of lipid droplets (LDs) and other organelles stained with MCPDI dyes was performed to reveal the interaction between the LDs and other organelles in living cells. Furthermore, a NIR-emitting MCPDI dye with a mitochondria-targeted characteristic was successfully applied for tumor-specific imaging. The facile synthesis, excellent stability, high brightness, tunable fluorescence emission, and Stokes shifts make these MCPDI promising fluorescent probes for biological applications.

Site-specific protein labeling strategies for super-resolution microscopy

Super-resolution microscopy (SRM) has transformed our understanding of proteins' subcellular organization and revealed cellular details down to nanometers, far beyond conventional microscopy. While localization precision is independent of the number of fluorophores attached to a biomolecule, labeling density is a decisive factor for resolving complex biological structures. The average distance between adjacent fluorophores should be less than half the desired spatial resolution for optimal clarity. While this was not a major limitation in recent decades, the success of modern microscopy approaching molecular resolution down to the single-digit nanometer range will depend heavily on advancements in fluorescence labeling. This review highlights recent advances and challenges in labeling strategies for SRM, focusing on site-specific labeling technologies. These advancements are crucial for improving SRM precision and expanding our understanding of molecular interactions.

Bioisostere-conjugated fluorescent probes for live-cell protein imaging without non-specific organelle accumulation

Specific labeling of proteins using membrane-permeable fluorescent probes is a powerful technique for bioimaging. Cationic fluorescent dyes with high fluorescence quantum yield, photostability, and water solubility provide highly useful scaffolds for protein-labeling probes. However, cationic probes generally show undesired accumulation in organelles, which causes a false-positive signal in localization analysis. Herein, we report a design strategy for probes that suppress undesired organelle accumulation using a bioisostere for intracellular protein imaging in living cells. Our design allows the protein labeling probes to possess both membrane permeability and suppress non-specific accumulation and has been shown to use several protein labeling systems, such as PYP-tag and Halo tag systems. We further developed a fluorogenic PYP-tag labeling probe for intracellular proteins and used it to visualize multiple localizations of target proteins in the intracellular system. Our strategy offers a versatile design for undesired accumulation-suppressed probes with cationic dye scaffolds and provides a valuable tool for intracellular protein imaging.

A NIR-Fluorochrome for Live Cell Dual Emission and Lifetime Tracking from the First Plasma Membrane Interaction to Subcellular and Extracellular Locales

Molecular probes with the ability to differentiate between subcellular variations in acidity levels remain important for the investigation of dynamic cellular processes and functions. In this context, a series of cyclic peptide and PEG bio-conjugated dual near-infrared emissive BF2-azadipyrromethene fluorophores with maxima emissions at 720 nm (at pH > 6) and 790 nm (at pH < 5) have been developed and their aqueous solution photophysical properties determined. Their inter-converting emissions and fluorescence lifetime characteristics were exploited to track their spatial and temporal progression from first contact with the plasma membrane to subcellular locales to their release within extracellular vesicles. A pH-dependent reversible phenolate/phenol interconversion on the fluorophore controlled the dynamic changes in dual emission responses and corresponding lifetime changes. Live-cell confocal microscopy experiments in the metastatic breast cancer cell line MDA-MB-231 confirmed the usability of the dual emissive properties for imaging over prolonged periods. All three derivatives performed as probes capable of real-time continuous imaging of fundamental cellular processes such as plasma membrane interaction, tracking endocytosis, lysosomal/large acidic vesicle accumulation, and efflux within extracellular vesicles without perturbing cellular function. Furthermore, fluorescence lifetime imaging microscopy provided valuable insights regarding fluorophore progression through intracellular microenvironments over time. Overall, the unique photophysical properties of these fluorophores show excellent potential for their use as information-rich probes.

Acoustofluidic scanning fluorescence nanoscopy with a large field of view

Large-field nanoscale fluorescence imaging is invaluable for many applications, such as imaging subcellular structures, visualizing protein interactions, and high-resolution tissue imaging. Unfortunately, conventional fluorescence microscopy requires a trade-off between resolution and field of view due to the nature of the optics used to form the image. To overcome this barrier, we developed an acoustofluidic scanning fluorescence nanoscope that simultaneously achieves superior resolution, a large field of view, and strong fluorescent signals. The acoustofluidic scanning fluorescence nanoscope utilizes the superresolution capabilities of microspheres that are controlled by a programmable acoustofluidic device for rapid fluorescence enhancement and imaging. The acoustofluidic scanning fluorescence nanoscope resolves structures that cannot be resolved with conventional fluorescence microscopes with the same objective lens and enhances the fluorescent signal by a factor of ~5 without altering the field of view of the image. The improved resolution realized with enhanced fluorescent signals and the large field of view achieved via acoustofluidic scanning fluorescence nanoscopy provides a powerful tool for versatile nanoscale fluorescence imaging for researchers in the fields of medicine, biology, biophysics, and biomedical engineering.

Chemical Approaches to Optimize the Properties of Organic Fluorophores for Imaging and Sensing

Organic fluorophores are indispensable tools in cells, tissue and in vivo imaging, and have enabled much progress in the wide range of biological and biomedical fields. However, many available dyes suffer from insufficient performances, such as short absorption and emission wavelength, low brightness, poor stability, small Stokes shift, and unsuitable permeability, restricting their application in advanced imaging technology and complex imaging. Over the past two decades, many efforts have been made to improve these performances of fluorophores. Starting with the luminescence principle of fluorophores, this review clarifies the mechanisms of the insufficient performance for traditional fluorophores to a certain extent, systematically summarizes the modified approaches of optimizing properties, highlights the typical applications of the improved fluorophores in imaging and sensing, and indicates existing problems and challenges in this area. This progress not only proves the significance of improving fluorophores properties, but also provide a theoretical guidance for the development of high-performance fluorophores.

Capturing Doublet Intermediate Emitters by Chemically Crosslinking Confinement towards Spatiotemporal Encryption

Photoluminescence is one of the most meticulous ways to manipulate light energy. Typical photoluminescent emitters are mostly stable substances with a pure photophysical process of spontaneous photon-emission from their excited states. Intermediate emitters are elusive attributing to their synchronous energy transfer process including photophysical and incomplete photochemical pathways. An intermediate emitter containing radicals is more difficult to be observed due to its inherent chemical reactivity. Here, these challenges are overcome by spontaneously formed space limitations in polymer crosslinking networks meanwhile chemically active intermediates are captured. These doublet intermediates exhibit unique long-wavelength emissions under chemically crosslinking confinement conditions, and their luminous mechanism provides a novel perspective for designing intermediate emitters with liquid-crystal character and photoresponsive features towards spatiotemporal encryption, promising for the detection of photochemical reactions and the development of fascinating luminescent systems.

Strategic design of an NIR probe for viscosity imaging in inflammatory and non-alcoholic steatohepatitis mice

Two novel near-infrared (NIR) fluorescent probes Cy-Vis1 and Cy-Vis2 with large Stokes shifts (>100 nm) were constructed using a "symmetry collapse" strategy. Notably, Cy-Vis2 was significantly more sensitive to viscosity than Cy-Vis1 through an enhanced intramolecular interaction strategy. The fluorescence intensities of Cy-Vis1 and Cy-Vis2 exhibited increases, by 7.6- and 19.9-fold, respectively, across the viscosity range from 0.8 cp to 359.9 cp. Cy-Vis2 was successfully used to visualize viscosity abnormalities in lipopolysaccharide (LPS)-induced inflammatory and NASH model mice.

Organic fluorescent probes for live-cell super-resolution imaging

The development of super-resolution technology has made it possible to investigate the ultrastructure of intracellular organelles by fluorescence microscopy, which has greatly facilitated the development of life sciences and biomedicine. To realize super-resolution imaging of living cells, both advanced imaging systems and excellent fluorescent probes are required. Traditional fluorescent probes have good availability, but that is not the case for probes for live-cell super-resolution imaging. In this review, we first introduce the principles of various super-resolution technologies and their probe requirements, then summarize the existing designs and delivery strategies of super-resolution probes for live-cell imaging, and finally provide a brief conclusion and overview of the future.

Giant Magnetoresistance Based Biosensors for Cancer Screening and Detection

Early-stage screening of cancer is critical in preventing its development and therefore can improve the prognosis of the disease. One accurate and effective method of cancer screening is using high sensitivity biosensors to detect optically, chemically, or magnetically labeled cancer biomarkers. Among a wide range of biosensors, giant magnetoresistance (GMR) based devices offer high sensitivity, low background noise, robustness, and low cost. With state-of-the-art micro- and nanofabrication techniques, tens to hundreds of independently working GMR biosensors can be integrated into fingernail-sized chips for the simultaneous detection of multiple cancer biomarkers (i.e., multiplexed assay). Meanwhile, the miniaturization of GMR chips makes them able to be integrated into point-of-care (POC) devices. In this review, we first introduce three types of GMR biosensors in terms of their structures and physics, followed by a discussion on fabrication techniques for those sensors. In order to achieve target cancer biomarker detection, the GMR biosensor surface needs to be subjected to biological decoration. Thus, commonly used methods for surface functionalization are also reviewed. The robustness of GMR-based biosensors in cancer detection has been demonstrated by multiple research groups worldwide and we review some representative examples. At the end of this review, the challenges and future development prospects of GMR biosensor platforms are commented on. With all their benefits and opportunities, it can be foreseen that GMR biosensor platforms will transition from a promising candidate to a robust product for cancer screening in the near future.

Xanthene, cyanine, oxazine and BODIPY: the four pillars of the fluorophore empire for super-resolution bioimaging

In the realm of biological research, the invention of super-resolution microscopy (SRM) has enabled the visualization of ultrafine sub-cellular structures and their functions in live cells at the nano-scale level, beyond the diffraction limit, which has opened up a new window for advanced biomedical studies to unravel the complex unknown details of physiological disorders at the sub-cellular level with unprecedented resolution and clarity. However, most of the SRM techniques are highly reliant on the personalized special photophysical features of the fluorophores. In recent times, there has been an unprecedented surge in the development of robust new fluorophore systems with personalized features for various super-resolution imaging techniques. To date, xanthene, cyanine, oxazine and BODIPY cores have been authoritatively utilized as the basic fluorophore units in most of the small-molecule-based organic fluorescent probe designing strategies for SRM owing to their excellent photophysical characteristics and easy synthetic acquiescence. Since the future of next-generation SRM studies will be decided by the availability of advanced fluorescent probes and these four fluorescent building blocks will play an important role in progressive new fluorophore design, there is an urgent need to review the recent advancements in designing fluorophores for different SRM methods based on these fluorescent dye cores. This review article not only includes a comprehensive discussion about the recent developments in designing fluorescent probes for various SRM techniques based on these four important fluorophore building blocks with special emphasis on their effective integration into live cell super-resolution bio-imaging applications but also critically evaluates the background of each of the fluorescent dye cores to highlight their merits and demerits towards developing newer fluorescent probes for SRM.

Bond-selective fluorescence imaging with single-molecule sensitivity

Bioimaging harnessing optical contrasts and chemical specificity is of vital importance in probing complex biology. Vibrational spectroscopy based on mid-infrared (mid-IR) excitation can reveal rich chemical information about molecular distributions. However, its full potential for bioimaging is hindered by the achievable sensitivity. Here, we report bond selective fluorescence-detected infrared-excited (BonFIRE) spectral microscopy. BonFIRE employs two-photon excitation in the mid-IR and near-IR to upconvert vibrational excitations to electronic states for fluorescence detection, thus encoding vibrational information into fluorescence. The system utilizes tuneable narrowband picosecond pulses to ensure high sensitivity, biocompatibility, and robustness for bond-selective biological interrogations over a wide spectrum of reporter molecules. We demonstrate BonFIRE spectral imaging in both fingerprint and cell-silent spectroscopic windows with single-molecule sensitivity for common fluorescent dyes. We then demonstrate BonFIRE imaging on various intracellular targets in fixed and live cells, neurons, and tissues, with promises for further vibrational multiplexing. For dynamic bioanalysis in living systems, we implement a high-frequency modulation scheme and demonstrate time-lapse BonFIRE microscopy of live HeLa cells. We expect BonFIRE to expand the bioimaging toolbox by providing a new level of bond-specific vibrational information and facilitate functional imaging and sensing for biological investigations.

Internally tagged Vps10p-domain receptors reveal uptake of the neurotrophin BDNF

The Vps10p-domain (Vps10p-D) receptor family consists of Sortilin, SorLA, SorCS1, SorCS2, and SorCS3. They mediate internalization and intracellular sorting of specific cargo in various cell types, but underlying molecular determinants are incompletely understood. Deciphering the dynamic intracellular itineraries of Vps10p-D receptors is crucial for understanding their role in physiological and cytopathological processes. However, studying their spatial and temporal dynamics by live imaging has been challenging so far, as terminal tagging with fluorophores presumably impedes several of their protein interactions and thus functions. Here, we addressed the lack of appropriate tools and developed functional versions of all family members internally tagged in their ectodomains. We predict folding of the newly designed receptors by bioinformatics and show their exit from the endoplasmic reticulum. We examined their subcellular localization in immortalized cells and primary cultured neurons by immunocytochemistry and live imaging. This was, as far as known, identical to that of wt counterparts. We observed homodimerization of fluorophore-tagged SorCS2 by coimmunoprecipitation and fluorescence lifetime imaging, suggesting functional leucine-rich domains. Through ligand uptake experiments, live imaging and fluorescence lifetime imaging, we show for the first time that all Vps10p-D receptors interact with the neurotrophin brain-derived neurotrophic factor and mediate its uptake, indicating functionality of the Vps10p-Ds. In summary, we developed versions of all Vps10p-D receptors, with internal fluorophore tags that preserve several functions of the cytoplasmic and extracellular domains. These newly developed fluorophore-tagged receptors are likely to serve as powerful functional tools for accurate live studies of the individual cellular functions of Vps10p-D receptors.

Fluorescent Investigation of Proteins Using DNA-Synthetic Ligand Conjugates

The unfathomable role that fluorescence detection plays in the life sciences has prompted the development of countless fluorescent labels, sensors, and analytical techniques that can be used to detect and image proteins or investigate their properties. Motivated by the demand for simple-to-produce, modular, and versatile fluorescent tools to study proteins, many research groups have harnessed the advantages of oligodeoxynucleotides (ODNs) for scaffolding such probes. Tight control over the valency and position of protein binders and fluorescent dyes decorating the polynucleotide chain and the ability to predict molecular architectures through self-assembly, inherent solubility, and stability are, in a nutshell, the important properties of DNA probes. This paper reviews the progress in developing DNA-based, fluorescent sensors or labels that navigate toward their protein targets through small-molecule (SM) or peptide ligands. By describing the design, operating principles, and applications of such systems, we aim to highlight the versatility and modularity of this approach and the ability to use ODN-SM or ODN-peptide conjugates for various applications such as protein modification, labeling, and imaging, as well as for biomarker detection, protein surface characterization, and the investigation of multivalency.

Live cell single-molecule imaging to study DNA repair in human cells

The genetic material in human cells is continuously exposed to a wide variety of insults that can induce different DNA lesions. To maintain genomic stability and prevent potentially deleterious genetic changes caused by DNA damage, mammalian cells have evolved a number of pathways that repair specific types of DNA damage. These DNA repair pathways vary in their accuracy, some providing high-fidelity repair while others are error-prone and are only activated as a last resort. Adding additional complexity to cellular mechanisms of DNA repair is the DNA damage response which is a sophisticated a signaling network that coordinates repair outcomes, cell-cycle checkpoint activation, and cell fate decisions. As a result of the sheer complexity of the various DNA repair pathways and the DNA damage response there are large gaps in our understanding of the molecular mechanisms underlying DNA damage repair in human cells. A key unaddressed question is how the dynamic recruitment of DNA repair factors contributes to repair kinetics and repair pathway choice in human cells. Methodological advances in live cell single-molecule imaging over the last decade now allow researchers to directly observe and analyze the dynamics of DNA repair proteins in living cells with high spatiotemporal resolution. Live cell single-molecule imaging combined with single-particle tracking can provide direct insight into the biochemical reactions that control DNA repair and has the power to identify previously unobservable processes in living cells. This review summarizes the main considerations for experimental design and execution for live cell single-molecule imaging experiments and describes how they can be used to define the molecular mechanisms of DNA damage repair in mammalian cells.

Electronic Preresonance Stimulated Raman Scattering Spectromicroscopy Using Multiple-Plate Continuum

Stimulated Raman scattering (SRS) spectromicroscopy is a powerful technique that enables label-free detection of chemical bonds with high specificity. However, the low Raman cross section due to typical far-electronic resonance excitation seriously restricts the sensitivity and undermines its application to bio-imaging. To address this bottleneck, the electronic preresonance (EPR) SRS technique has been developed to enhance the Raman signals by shifting the excitation frequency toward the molecular absorption. A fundamental weakness of the previous demonstration is the lack of dual-wavelength tunability, making EPR-SRS only applicable to a limited number of species in the proof-of-concept experiment. Here, we demonstrate the EPR-SRS spectromicroscopy using a multiple-plate continuum (MPC) light source able to examine a single vibration mode with independently adjustable pump and Stokes wavelengths. In our experiments, the C═C vibration mode of Alexa 635 is interrogated by continuously scanning the pump-to-absorption frequency detuning throughout the entire EPR region enabled by MPC. The results exhibit 150-fold SRS signal enhancement and good agreement with the Albrecht A-term preresonance model. Signal enhancement is also observed in EPR-SRS images of the whole Drosophila brain stained with Alexa 635. With the improved sensitivity and potential to implement hyperspectral measurement, we envision that MPC-EPR-SRS spectromicroscopy can bring the Raman techniques closer to a routine in bio-imaging.

Non-Invasive Detection, Precise Localization, and Perioperative Navigation of In Vivo Deep Lesions Using Transmission Raman Spectroscopy

Non-invasive detection and precise localization of deep lesions have attracted significant attention for both fundamental and clinical studies. Optical modality techniques are promising with high sensitivity and molecular specificity, but are limited by shallow tissue penetration and the failure to accurately determine lesion depth. Here the authors report in vivo ratiometric surface-enhanced transmission Raman spectroscopy (SETRS) for non-invasive localization and perioperative surgery navigation of deep sentinel lymph nodes in live rats. The SETRS system uses ultrabright surface-enhanced Raman spectroscopy (SERS) nanoparticles with a low detection limit of 10 pM and a home-built photosafe transmission Raman spectroscopy setup. The ratiometric SETRS strategy is proposed based on the ratio of multiple Raman spectral peaks for obtaining lesion depth. Via this strategy, the depth of the phantom lesions in ex vivo rat tissues is precisely determined with a mean-absolute-percentage-error of 11.8%, and the accurate localization of a 6-mm-deep rat popliteal lymph node is achieved. The feasibility of ratiometric SETRS allows the successful perioperative navigation of in vivo lymph node biopsy surgery in live rats under clinically safe laser irradiance. This study represents a significant step toward the clinical translation of TRS techniques, providing new insights for the design and implementation of in vivo SERS applications.

Manipulating fluorescence by photo-switched spin-state conversions in an iron(ii)-based SCO-MOF

Manipulating fluorescence by photo-switched spin-state conversions is an attractive prospect for applications in smart magneto-optical materials and devices. The challenge is how to modulate the energy transfer paths of the singlet excited state by light-induced spin-state conversions. In this work, a spin crossover (SCO) Fe^II-based fluorophore was embedded into a metal-organic framework (MOF) to tune the energy transfer paths. Compound 1 {Fe(TPA-diPy)[Ag(CN)₂]₂}·2EtOH (1) has an interpenetrated Hofmann-type structure, wherein the Fe^II ion is coordinated by a bidentate fluorophore ligand (TPA-diPy) and four cyanide nitrogen atoms and acts as the fluorescent-SCO unit. Magnetic susceptibility measurements revealed that 1 underwent an incomplete and gradual spin crossover with T_1/2 = 161 K. Photomagnetic studies confirmed photo-induced spin state conversions between the low-spin (LS) and high-spin (HS) states, where the irradiation of 532 and 808 nm laser lights converted the LS and HS states to the HS and LS states, respectively. Variable-temperature fluorescence spectra study revealed an anomalous decrease in emission intensity upon the HS → LS transition, confirming the synergetic coupling between the fluorophore and SCO units. Alternating irradiation of 532 and 808 nm laser lights resulted in reversible fluorescence intensity changes, confirming spin state-controlled fluorescence in the SCO-MOF. Photo-monitored structural analyses and UV-vis spectroscopic studies demonstrated that the photo-induced spin state conversions changed energy transfer paths from the TPA fluorophore to the metal-centered charge transfer bands, ultimately leading to the switching of fluorescence intensities. This work represents a new prototype compound showing bidirectional photo-switched fluorescence by manipulating the spin states of iron(ii).

Acoustofluidic scanning fluorescence nanoscopy with large field of view

Nanoscale fluorescence imaging with a large-field view is invaluable for many applications such as imaging of subcellular structures, visualizing protein interaction, and high-resolution tissue imaging. Unfortunately, conventional fluorescence microscopy has to make a trade-off between resolution and field of view due to the nature of the optics used to form an image. To overcome this barrier, we have developed an acoustofluidic scanning fluorescence nanoscope that can simultaneously achieve superior resolution, a large field of view, and enhanced fluorescent signal. The acoustofluidic scanning fluorescence nanoscope utilizes the super-resolution capability of microspheres that are controlled by a programable acoustofluidic device for rapid fluorescent enhancement and imaging. The acoustofluidic scanning fluorescence nanoscope can resolve structures that cannot be achieved with a conventional fluorescent microscope with the same objective lens and enhances the fluorescent signal by a factor of ~5 without altering the field of view of the image. The improved resolution with enhanced fluorescent signal and large field of view via the acoustofluidic scanning fluorescence nanoscope provides a powerful tool for versatile nanoscale fluorescence imaging for researchers in the fields of medicine, biology, biophysics, and biomedical engineering.

Design of a palette of SNAP-tag mimics of fluorescent proteins and their use as cell reporters

Naturally occurring fluorescent proteins (FPs) are the most widely used tools for tracking cellular proteins and sensing cellular events. Here, we chemically evolved the self-labeling SNAP-tag into a palette of SNAP-tag mimics of fluorescent proteins (SmFPs) that possess bright, rapidly inducible fluorescence ranging from cyan to infrared. SmFPs are integral chemical-genetic entities based on the same fluorogenic principle as FPs, i.e., induction of fluorescence of non-emitting molecular rotors by conformational locking. We demonstrate the usefulness of these SmFPs in real-time tracking of protein expression, degradation, binding interactions, trafficking, and assembly, and show that these optimally designed SmFPs outperform FPs like GFP in many important ways. We further show that the fluorescence of circularly permuted SmFPs is sensitive to the conformational changes of their fusion partners, and that these fusion partners can be used for the development of single SmFP-based genetically encoded calcium sensors for live cell imaging.

Induced-volatolomics for the design of tumour activated therapy

The discovery of tumour-associated markers is of major interest for the development of selective cancer chemotherapy. Within this framework, we introduced the concept of induced-volatolomics enabling to monitor simultaneously the dysregulation of several tumour-associated enzymes in living mice or biopsies. This approach relies on the use of a cocktail of volatile organic compound (VOC)-based probes that are activated enzymatically for releasing the corresponding VOCs. Exogenous VOCs can then be detected in the breath of mice or in the headspace above solid biopsies as specific tracers of enzyme activities. Our induced-volatolomics modality highlighted that the up-regulation of N-acetylglucosaminidase was a hallmark of several solid tumours. Having identified this glycosidase as a potential target for cancer therapy, we designed an enzyme-responsive albumin-binding prodrug of the potent monomethyl auristatin E programmed for the selective release of the drug in the tumour microenvironment. This tumour activated therapy produced a remarkable therapeutic efficacy on orthotopic triple-negative mammary xenografts in mice, leading to the disappearance of tumours in 66% of treated animals. Thus, this study shows the potential of induced-volatolomics for the exploration of biological processes as well as the discovery of novel therapeutic strategies.

Visible-light-induced protein labeling in live cells with aryl azides

Chemical labeling of proteins in live cells helps to probe their native functions in biological systems. Aryl azides are chemically inert under physiological conditions, but they are activated by certain external stimuli. Recently, photocatalytic live-cell applications of aryl azides by visible light irradiation have become a burgeoning new field in chemical biology. In this Feature Article, we focus on the recent progress of protein labeling in live cells with aryl azides induced by visible-light irradiation. Light irradiation activates aryl azides to generate highly reactive intermediates, which enables protein labeling for protein functionalization, crosslinking, and profiling. The activation mechanism of aryl azides by light irradiation is categorized as photolysis, energy-transfer, and electron-transfer. The extracellular and intracellular protein labeling applications in live cells with aryl azides induced by visible light are discussed, including recent advances in red-light-induced extracellular protein labeling.

Bone-Targeting Exosome Mimetics Engineered by Bioorthogonal Surface Functionalization for Bone Tissue Engineering

Extracellular vesicles have received a great interest as safe biocarriers in biomedical engineering. There is a need to develop more efficient delivery strategies to improve localized therapeutic efficacy and minimize off-target adverse effects. Here, exosome mimetics (EMs) are reported for bone targeting involving the introduction of hydroxyapatite-binding moieties through bioorthogonal functionalization. Bone-binding ability of the engineered EMs is verified with hydroxyapatite-coated scaffolds and an ex vivo bone-binding assay. The EM-bound construct provided a biocompatible substrate for cell adhesion, proliferation, and osteogenic differentiation. Particularly, the incorporation of Smoothened agonist (SAG) into EMs greatly increased the osteogenic capacity through the activation of hedgehog signaling. Furthermore, the scaffold integrated with EM/SAG significantly improved in vivo reossification. Lastly, biodistribution studies confirmed the accumulation of systemically administered EMs in bone tissue. This facile engineering strategy could be a versatile tool to promote bone regeneration, offering a promising nanomedicine approach to the sophisticated treatment of bone diseases.

Ultrasound contrast agents from microbubbles to biogenic gas vesicles

Microbubbles have been the earliest and most widely used ultrasound contrast agents by virtue of their unique features: such as non-toxicity, intravenous injectability, ability to cross the pulmonary capillary bed, and significant enhancement of echo signals for the duration of the examination, resulting in essential preclinical and clinical applications. The use of microbubbles functionalized with targeting ligands to bind to specific targets in the bloodstream has further enabled ultrasound molecular imaging. Nevertheless, it is very challenging to utilize targeted microbubbles for molecular imaging of extravascular targets due to their size. A series of acoustic nanomaterials have been developed for breaking free from this constraint. Especially, biogenic gas vesicles, gas-filled protein nanostructures from microorganisms, were engineered as the first biomolecular ultrasound contrast agents, opening the door for more direct visualization of cellular and molecular function by ultrasound imaging. The ordered protein shell structure and unique gas filling mechanism of biogenic gas vesicles endow them with excellent stability and attractive acoustic responses. What's more, their genetic encodability enables them to act as acoustic reporter genes. This article reviews the upgrading progresses of ultrasound contrast agents from microbubbles to biogenic gas vesicles, and the opportunities and challenges for the commercial and clinical translation of the nascent field of biomolecular ultrasound.