One-step synthesis of ultrabright amphiphilic carbon dots for rapid and precise tracking lipid droplets dynamics in biosystems

Amphiphilic Carbon Dots for Ultrafast and Wash-Free Mitochondria-Targeted Imaging

Carbon dots (CDs) exhibit exceptional biocompatibility and programmable amphiphilicity, establishing them as transformative nanomaterials for subcellular visualization with exceptional resolution. However, existing CD-based probes lack the spatiotemporal precision required for real-time organelle tracking, particularly in mitochondrial-targeted imaging via ultrafast, wash-free protocols. To overcome these limitations, this study describes the solvent-free, high-temperature (280 °C) and short-time (2 h) preparation of green-emitting CDs (GCDs) with distinctive amphiphilic architectures utilizing benzoylurea and citric acid in a sealed high-pressure reactor. GCDs may form micelle-like structures driven to hydrophobic interactions, producing long-wavelength emission in contrast to blue emission in low-polar solvents. They also simultaneously activate the synergy of numerous endocytotic modes, achieving ultrafast (<5 s) and wash-free imaging. GCDs can also effectively target the mitochondria, more significantly, in both normal and cancer cells (Person's value ≈ 0.90/0.91), which is explained by the minor adjustment of mitochondrial membrane potential. This work describes assembly mechanisms of amphiphilic CDs while establishing potential design principles for mitochondria-targeted nanostructures with wash-free, ultrafast tracking.

Light-up lipid droplets dynamic behaviors using rationally designed carbon dots

Lipid droplets (LDs) are essential organelles used to store lipids and participate in cellular lipid metabolism. Imaging LDs is an intuitive approach to comprehend their biological functions. Herein, the LDs-targeted CDs (LD-CDs) featuring robust solvatochromic emission were elaborately designed by a Schiff base reaction using 1, 2-diamino-4-fluorobenzene, 3-dimethylaminophenol, and thiourea as precursors. The LD-CDs exhibited a remarkable sensitivity to polarity changes over a broad linear range (0.0205-0.3213), owing to its unique intramolecular charge transfer effect (ICT). Moreover, the favorable lipophilicity of LD-CDs endows it with the capability to light up LDs with high specificity. Due to their good lipophilicity and biocompatibility, the LD-CDs were able to differentiate cancer cells from normal cells and keep real-time track of LDs dynamic behaviors, including dissociation, migration, and fusion. Leveraging the multifunctionality of LD-CDs, we have also observed the dynamic changes of lysosomes and LDs during lipophagy. Additionally, the LD-CDs were employed to reveal the polarity change of LDs in living cells and zebrafish under oleic acid stimulation and to visualize lipid metabolism in zebrafish. We deem that this work will expand the applications of CDs in biological imaging and make further contributions to the field of LDs-associated metabolism and diseases.

Lipid droplet-targeting carbonized polymer dots for intracellular polarity visualization under wash-free conditions

BACKGROUND

For an extended period, lipid droplets (LDs) have been regarded to be the storage depots for metabolic lipids. As a key to energy storage and regulation, lipid droplets (LDs) have been found to perform crucial functions in various physiological processes such as oxidative stress and cell migration. Abnormal polarity variations of LDs are associated with various pathological conditions, including lipid metabolism disorders, the development of atherosclerosis and cancer. Therefore, developing LD-targeting polarity-sensitive probes and effectively monitoring the variations in the LDs microenvironment is highly meaningful.

RESULTS

A highly specific LD-targeting carbonized polymer dot (LD-CPD)-based fluorescent probe with amphiphilicity was designed and synthesized under mild conditions. The new probe exhibits high polarity-sensitivity, low viscosity crosstalk, strong solvatochromic and high photobleaching resistance. The solvatochromic responses of LD-CPD are attributed to hydrogen bonding and solvent dipole-dipole interactions, with hydrogen bonding exerting a stronger influence. In addition, it demonstrates a turn-off response to water, which makes it effective for LDs washing-free imaging. Moreover, as a solvatochromic probe, the proposed CPD can well achieve ratiometric imaging of subtle polarity changes within LDs during starvation and oxidative stress, indicating its capability to observe physiological dynamic changes.

SIGNIFICANCE

All the features remind that the developed LD-CPD probe has great potential for diagnosis in LDs monitoring and analysis. It will serve as a robust monitoring platform for identifying diseases associated with abnormal LDs polarity. It also expands the application of CPDs in bioanalysis.

A review on green synthesis, biological applications of carbon dots in the field of drug delivery, biosensors, and bioimaging

Since the beginning of nanoscience and nanotechnology, carbon dots (CDs) have been the foundational idea and have dominated the growth of the nano-field. CDs are an intriguing platform for utilization in biology, technology, catalysis, and other fields thanks to their numerous distinctive structural, physicochemical, and photochemical characteristics. Since several carbon dots have already been created, they have been assessed based on their synthesis process, and luminescence characteristics. Due to their biocompatibility, less toxic effects, and most significantly their fluorescent features in contrast to other carbon nanostructures, CDs have several benefits. This review focuses on the most recent advancements in the characterization, applications, and synthesis techniques used for CDs made from natural sources. It will also direct scientists in the creation of a synthesis technique for adjustable carbon dots that is more practical, effective, and environmentally benign. With low toxicity and low cost, CDs are meeting the new era's requirements for more selectivity and sensitivity in the detection and sensing of various things, such as biomaterial sensing, enzymes, chemical contamination, and temperature sensing. Its variety of properties, such as optical properties, chemiluminescence, and morphological analysis, make it a good option to use in bioimaging, drug delivery, biosensors, and cancer diagnosis.

Comprehensive Overview of Controlled Fabrication of Multifunctional Fluorescent Carbon Quantum Dots and Exploring Applications

In recent years, carbon dots (CDs) have garnered increasing attention due to their simple preparation methods, versatile performances, and wide-ranging applications. CDs can manifest various optical, physical, and chemical properties including quantum yield (QY), emission wavelength (Em), solid-state fluorescence (SSF), room-temperature phosphorescence (RTP), material-specific responsivity, pH sensitivity, anti-oxidation and oxidation, and biocompatibility. These properties can be effectively regulated through precise control of the CD preparation process, rendering them suitable for diverse applications. However, the lack of consideration given to the precise control of each feature of CDs during the preparation process poses a challenge in obtaining the requisite features for various applications. This paper is to analyze existing research and present novel concepts and ideas for creating CDs with different distinct features and applications. The synthesis methods of CDs are discussed in the first section, followed by a comprehensive overview of the important properties of CDs and the modification strategy. Subsequently, the application of CDs and their requisite properties are reviewed. Finally, the paper outlines the current challenges in controlling CDs properties and their applications, discusses potential solutions, and offers suggestions for future research.

Polymeric microenvironment enhancing polarity response sensitivity for discriminating lipid droplets in cancer cells

Cellular micro-environment analysis via fluorescence probe has become a powerful method to explore the early-stage cancer diagnosis and pathophysiological process of relevant diseases. The polarity change of intracellular lipid droplets (LDs) is closely linked with disorders or diseases, which result in various physiological and pathological processes. However, the efficient design strategy for lipid droplet polarity probes with high sensitivity is lacking. To overcome this difficulty, two kinds of LDs-targeting and polarity-sensitive fluorescent probes containing carbazole and siloxane groups were rationally designed and synthesized. With the carbazole-based rotor and bridge-like siloxanes, two probes (P1 and P2) behave high sensitivity to polarity changes and show different fluorescent intensity in normal and cancer cells. Notably, polysiloxanes groups promoted the response sensitivity of the probes dramatically for the polymeric microenvironment. In addition, due to the polarity changes of LDs in cancer cells, the distinct fluorescent intensities in different channels of laser scanning confocal microscope were observed between NHA cell and U87 cells. This work could offer an opportunity to monitor the dynamic behaviors of LDs and further provide a powerful tool to be potentially applied in the early-stage diagnosis of cancer.

Recent developments in lignin-based fluorescent materials

Biomass-based fluorescent materials are an alternative to plastic-based materials for their multifunctional applications. Lignin, an inexpensive and easily available raw material, demonstrates outstanding environment-responsive properties such as pH, metal ions, dyes sensing, bioimaging and so on. To date, only a little work has been reported on the synthesis of lignin-based fluorescent materials. In this review report, synthetic approaches and light-responsive applications of lignin-based fluorescent carbon dots and other materials are summarized. The results reveal that lignin-based fluorescent carbon dots are prepared by hydrothermal method, exhibit small size <10 nm, reveal significant quantum yield, biocompatibility, non-toxicity, photostability and display substantial tunable emission and can be efficiently employed for sensing, bioimaging and energy storage applications. Finally, the forthcoming challenges, investigations, and options open for the chemical and/or physical modification of lignin into fluorescent materials for future applications are well-addressed. To our knowledge, this is the first comprehensive review report on lignin-based fluorescent materials and their light-responsive applications. In addition, this review will attract remarkable consideration and thrust for the researchers and biochemical technologists working with the preparation of lignin-based fluorescent materials for broad applications.

Mild synthesis of ultra-bright carbon dots with solvatochromism for rapid lipid droplet monitoring in varied physiological processes

Lipid droplets (LDs) participating in various cellular activities and are increasingly being emphasized. Fluorescence imaging provides powerful tool for dynamic tracking of LDs, however, most current LDs probes remain inconsistent performance such as low Photoluminescence Quantum Yield (PLQY), poor photostability and tedious washing procedures. Herein, a novel yellow-emissive carbon dot (OT-CD) has been synthesized conveniently with high PLQY up to 90%. Besides, OT-CD exhibits remarkable amphiphilicity and solvatochromic property with lipid-water partition coefficient higher than 2, which is much higher than most LDs probes. These characters enable OT-CD high brightness, stable and wash-free LDs probing, and feasible for in vivo imaging. Then, detailed observation of LDs morphological and polarity variation dynamically in different cellular states were recorded, including ferroptosis and other diseases processes. Furthermore, fast whole imaging of zebrafish and identified LD enrichment in injured liver indicate its further feasibility for in vivo application. In contrast to the reported studies to date, this approach provides a versatile conventional synthesis system for high-performance LDs targeting probes, combing the advantages of easy and high-yield production, as well as robust brightness and stability for long-term imaging, facilitating investigations into organelle interactions and LD-associated diseases.

Ultrabright organic fluorescent probe for quantifying the dynamics of cytosolic/nuclear lipid droplets

Lipid droplets (LDs) are extremely active organelles that play a crucial role in energy metabolism, membrane formation, and the production of lipid-derived signaling molecules by regulating lipid storage and release. Nevertheless, directly limited by the lack of superior fluorescent probes, studies of LDs dynamic motion velocity have been rarely reported, especially for nuclear LDs. Herein, a novel organic fluorescent probe Lipi-Bright has been rationally developed based on bridged cyclization of distyrylbenzene. The fully ring-fused molecule structure endows the probe with high photostability. Moreover, this new fluorescent probe displays the features of excellent LDs staining specificity as well as ultrahigh fluorescence brightness. Lipi-Bright labeled LDs was dozens of times brighter than representative probes BODIPY 493/503 or Nile Red. Consequently, by in-situ time-lapse fluorescence imaging, the dynamics of LDs have been quantitatively studied. For instance, the velocities of cytosolic LDs (37 ± 15 nm/s) are found to be obviously faster than those of nuclear LDs (24 ± 4 nm/s), and both the cytosolic LDs and the nuclear LDs would be moved faster or slower depend on the various stimulations. Overall, this work providing plentiful information on LDs dynamics will greatly facilitate the in-depth investigation of lipid metabolism.

Polarity-Sensitive Probe for Two-Photon Fluorescence Lifetime Imaging of Lipid Droplets In Vitro and In Vivo

Lipid droplets (LDs) are crucial organelles used to store lipids and participate in lipid metabolism in cells. The abnormal aggregation and polarity change of LDs are associated with the occurrence of diseases, such as steatosis. Herein, the polarity-sensitive probe TBPCPP with a donor-acceptor-π-acceptor (D-A-π-A) structure was designed and synthesized. The TBPCPP has a large Stokes shift (∼220 nm), excellent photostability, high LD targeting, and considerable two-photon absorption (TPA) cross-section (∼226 GM), enabling deep two-photon imaging (∼360 μm). In addition, the fluorescence lifetime of TBPCPP decreases linearly with increasing solvent polarity. Therefore, with the assistance of two-photon fluorescence lifetime imaging microscopy (TP-FLIM), TBPCPP has successfully achieved not only the visualization of polarity changes caused by LD accumulation in HepG-2 cells but also lipid-specific imaging and visualization of different polarities in lipid-rich regions in zebrafish for the first time. Furthermore, TP-FLIM revealed that the polarity gradually decreases during steatosis in HepG-2 cells, which provided new insights into the diagnosis of steatosis.

Zero-Dimensional Carbon Nanomaterials for Fluorescent Sensing and Imaging

Advances in nanotechnology and nanomaterials have attracted considerable interest and play key roles in scientific innovations in diverse fields. In particular, increased attention has been focused on carbon-based nanomaterials exhibiting diverse extended structures and unique properties. Among these materials, zero-dimensional structures, including fullerenes, carbon nano-onions, carbon nanodiamonds, and carbon dots, possess excellent bioaffinities and superior fluorescence properties that make these structures suitable for application to environmental and biological sensing, imaging, and therapeutics. This review provides a systematic overview of the classification and structural properties, design principles and preparation methods, and optical properties and sensing applications of zero-dimensional carbon nanomaterials. Recent interesting breakthroughs in the sensitive and selective sensing and imaging of heavy metal pollutants, hazardous substances, and bioactive molecules as well as applications in information encryption, super-resolution and photoacoustic imaging, and phototherapy and nanomedicine delivery are the main focus of this review. Finally, future challenges and prospects of these materials are highlighted and envisaged. This review presents a comprehensive basis and directions for designing, developing, and applying fascinating fluorescent sensors fabricated based on zero-dimensional carbon nanomaterials for specific requirements in numerous research fields.

"Three-in-one" platform based on Fe-CDs nanozyme for dual-mode/dual-target detection and NIR-assisted bacterial killing

As nanozymes, carbon dots (CDs) have attracted increasing attention due to their remarkable properties. Besides general enzyme activity, their photoluminescence and photothermal properties have been explored rarely, whereas their synergistic effects might produce CDs-based nanozymes of high performance. Here, iron-doped CDs (Fe-CDs) with tunable fluorescence and enhanced peroxidase-like activity were designed to develop a novel "three-in-one" multifunctional platform to provide dual-mode/dual-target detection and near infrared (NIR)-assisted antibacterial ability. This proposed strategy for a H₂O₂ test exhibited a wide linear relationship with a low limit of detection (LOD) of 0.16 μM (colorimetric) and 0.14 μM (ratiometric fluorescent). Furthermore, due to the nature of cholesterol being oxidized to H₂O₂ by cholesterol oxidase, sensitive and selective detection of cholesterol was realized, and the LOD was 0.42 μM (colorimetric) and 0.27 μM (ratiometric fluorescent), surpassing that reported previously. This result suggested that Fe-CDs could be used for dual-mode quantification of large family of H₂O₂-producing metabolites, thereby paving the way for developing multi-mode sensing strategies based on nanozymes. Moreover, this platform showed synergistic effects for antibacterial application, indicating great prospects for bacterial killing as well as wound disinfection and healing. Hence, this platform could contribute to the construction of multifunctional CDs with high performance.

Carbonized Polymer Dot Probe for Two-Photon Fluorescence Imaging of Lipid Droplets in Living Cells and Tissues

As a dynamic and multifunctional organelle, lipid droplets (LDs) are essential in maintaining lipid balance and transducing biological signals. LD accumulation and catabolism are closely associated with energy metabolism and cell signaling. In order to easily trace LDs in living cells, a novel carbonized polymer dot (CPD)-based fluorescent nanoprobe is reported to serve the needs of LD-targeting imaging. This probe exhibits the advantages of excellent biocompatibility, simple preparation, good lipophilicity, and high compatibility with commercial dyes. Transient absorption spectroscopy was employed to discuss the luminescence mechanism of CPDs, and the results indicate that the excellent fluorescence property and the environment-responsive feature of our CPDs are derived from the intramolecular charge transfer (ICT) characteristics and the D-π-A structure that possibly formed in CPD. This nanoprobe is available for one-photon fluorescence (OPF) and two-photon fluorescence (TPF) imaging and is also practicable for staining LDs in living/fixed cells and lipids in tissue sections. The staining process is completed within several seconds, with no washing step. The intracellular LDs involving the intranuclear LDs (nLDs) can be selectively lit up. This probe is feasible for visualizing dynamic interactions among LDs, which suggests its great potential in revealing the secret of LD metabolism. The in situ TPF spectra were analyzed to determine surrounding microenvironment according to the polarity-responsive feature of our CPDs. This work expands the applications of CPDs in biological imaging, helps design new LD-selective fluorescent probes, and has implications for studying LD-related metabolism and diseases.

Orange-Emissive Carbon Dots with High Photostability for Mitochondrial Dynamics Tracking in Living Cells

Mitochondria play significant roles in maintaining a stable internal environment for cell metabolism. Hence, real-time monitoring of the dynamics of mitochondria is essential for further understanding mitochondria-related diseases. Fluorescent probes provide powerful tools for visualizing dynamic processes. However, most mitochondria-targeted probes are derived from organic molecules with poor photostability, making long-term dynamic monitoring challenging. Herein, we design a novel mitochondria-targeted probe based on carbon dots with high performance for long-term tracking. Considering that the targeting ability of CDs is related to surface functional groups, which are generally determined by the reaction precursors, we successfully constructed mitochondria-targeted O-CDs with emission at 565 nm through solvothermal treatment of m-diethylaminophenol. The O-CDs are bright with a high quantum yield of 12.61%, high mitochondria-targeting ability, and good stability. The O-CDs possess a high quantum yield (12.61%), specific mitochondria-targeting ability, and outstanding optical stability. Owing to the abundant hydroxyl and ammonium cations on the surface, O-CDs showed obvious accumulation in mitochondria with a high colocalization coefficient of up to 0.90 and remained steady even after fixation. Besides, O-CDs showed outstanding compatibility and photostability under various interruptions or long-time irradiation. Therefore, O-CDs are preferable for the long-term tracking of dynamic mitochondrial behavior in live cells. We first observed the mitochondrial fission and fusion behaviors in HeLa cells, and then, the size, morphology, and distribution of mitochondria in physiological or pathological conditions were clearly recorded. More importantly, we observed different dynamics interactions between mitochondria and lipid droplets during the apoptosis and mitophagy processes. This study provides a potential tool for exploring interactions between mitochondria and other organelles, further promoting the research on mitochondria-related diseases.

High biocompatible nitrogen and sulfur Co-doped carbon dots for Hg(II) detection and their long-term biological stability in living cells

Fluorescent carbon dots have been highly reported nanomaterials in recent times because of their excellent physio-chemical properties and various field of applications. Herein, a one-step hydrothermal approach was used to synthesize high biocompatible nitrogen and sulfur co-doped carbon dots, and examined their chemical sensing (Hg²⁺) and biological imaging properties. The N,S-CDs exhibited blue light, demonstrating a high quantum yield of up to 44.5% and excitation-independent fluorescent characteristics. Cytotoxicity was observed by CCK-8 assay using T-ca cells as a target source. Cell viability was recorded over 80% even after 7 days of treatment with a concentration up to 400 μg/mL, indicating low-toxicity of N,S-CDs. Notably, the bright blue fluorescence of N,S-CDs was quenched by introducing toxic Hg²⁺ ions into the solution. The detection limit was calculated to be about ∼3.5 nM, which is quite impressive compared to previous reports. Because of their low-toxicity, nano-size, and environment friendly properties, N,S-CDs could be excellent fluorescent agents for bio-imaging applications. The biological stability of fluorescent N,S-CDs was tested over time, and the findings were significant even after 8 days of incubation with T-ca cells. Because of good biocompatibility and bright fluorescence, N,S-CDs were suitable for in vivo imaging.

A thermostable terbium(III) complex with high fluorescence quantum yields

A novel complex (C6H16N)3[Tb2(Hsal)3(NO3)6] (1) was synthesised and characterized by X-ray crystallography. The PXRD indicate that the complex 1 is stable after heating at 180°C for half an hour. More...