Carbon dots with tunable dual emissions: from the mechanism to the specific imaging of endoplasmic reticulum polarity

Selective delivery of imaging probes and therapeutics to the endoplasmic reticulum or Golgi apparatus: Current strategies and beyond

To maximize therapeutic effects and minimize unwanted effects, the interest in drug targeting to the endoplasmic reticulum (ER) or Golgi apparatus (GA) has been recently growing because two organelles are distributing hubs of cellular building/signaling components (e.g., proteins, lipids, Ca2+) to other organelles and the plasma membrane. Their structural or functional damages induce organelle stress (i.e., ER or GA stress), and their aggravation is strongly related to diseases (e.g., cancers, liver diseases, brain diseases). Many efforts have been developed to image (patho)physiological functions (e.g., oxidative stress, protein/lipid-related processing) and characteristics (e.g., pH, temperature, biothiols, reactive oxygen species) in the target organelles and to deliver drugs for organelle disruption using organelle-targeting moieties. Therefore, this review will overview the structure, (patho)physiological functions/characteristics, and related diseases of the organelles of interest. Future direction on ER or GA targeting will be discussed by understanding current strategies and investigations on targeting, imaging/sensing, and therapeutic systems.

Deuteration-Induced Energy Level Structure Reconstruction of Carbon Dots for Enhancing Photoluminescence

Constrained by a limited understanding of the structure and luminescence mechanisms of carbon dots (CDs), achieving precise enhancement of their photoluminescence (PL) performance without altering the emission wavelength and color remains a challenge. In this work, a deuterated CD is first achieved by simply replacing the reaction solvent from H2O to D2O. The substitution of D atoms for H atoms is not limited on the surface but also within the internal structure of CDs. Deuteration affects the formation of the π-conjugated network structure by altering the content of sp2 carbon and sp3 carbon, ultimately inducing a reconstruction for energy level structure of CDs. Both the intrinsic state and surface state emission, including quantum yield, emission intensity and lifetime, are significantly enhanced after deuteration. It benefits from the reduction in non-radiative transitions, since the lowered vibrational frequencies of D atoms and optimized local energy level distribution in CDs structure. The deuterated CDs are applied in the fabrication of white-light-emitting diodes to show their application potential. This work provides a highly versatile route for improving and controlling photoluminescence performance of CDs and has opportunities to guide the development of CDs for practical applications.

Carbon Dots in Photodynamic/Photothermal Antimicrobial Therapy

Antimicrobial resistance (AMR) presents an escalating global challenge as conventional antibiotic treatments become less effective. In response, photodynamic therapy (PDT) and photothermal therapy (PTT) have emerged as promising alternatives. While rooted in ancient practices, these methods have evolved with modern innovations, particularly through the integration of lasers, refining their efficacy. PDT harnesses photosensitizers to generate reactive oxygen species (ROS), which are detrimental to microbial cells, whereas PTT relies on heat to induce cellular damage. The key to their effectiveness lies in the utilization of photosensitizers, especially when integrated into nano- or micron-scale supports, which amplify ROS production and enhance antimicrobial activity. Over the last decade, carbon dots (CDs) have emerged as a highly promising nanomaterial, attracting increasing attention owing to their distinctive properties and versatile applications, including PDT and PTT. They can not only function as photosensitizers, but also synergistically combine with other photosensitizers to enhance overall efficacy. This review explores the recent advancements in CDs, underscoring their significance and potential in reshaping advanced antimicrobial therapeutics.

The construction of dual-emissive ratiometric fluorescent probes based on fluorescent nanoparticles for the detection of metal ions and small molecules

With the rapid development of fluorescent nanoparticles (FNPs), such as CDs, QDs, and MOFs, the construction of FNP-based probes has played a key role in improving chemical sensors. Ratiometric fluorescent probes exhibit distinct advantages, such as resistance to environmental interference and achieving visualization. Thus, FNP-based dual-emission ratiometric fluorescent probes (DRFPs) have rapidly developed in the field of metal ion and small molecule detection in the past few years. In this review, firstly we introduce the fluorescence sensing mechanisms; then, we focus on the strategies for the fabrication of DRFPs, including hybrid FNPs, single FNPs with intrinsic dual emission and target-induced new emission, and DRFPs based on auxiliary nanoparticles. In the section on hybrid FNPs, methods to assemble two types of FNPs, such as chemical bonding, electrostatic interaction, core satellite or core-shell structures, coordination, and encapsulation, are introduced. In the section on single FNPs with intrinsic dual emission, methods for the design of dual-emission CDs, QDs, and MOFs are discussed. Regarding target-induced new emission, sensitization, coordination, hydrogen bonding, and chemical reaction induced new emissions are discussed. Furthermore, in the section on DRFPs based on auxiliary nanoparticles, auxiliary nanomaterials with the inner filter effect and enzyme mimicking activity are discussed. Finally, the existing challenges and an outlook on the future of DRFP are presented. We sincerely hope that this review will contribute to the quick understanding and exploration of DRFPs by researchers.

A Review of Dual-Emission Carbon Dots and Their Applications

Carbon dots (CDs), as a rising star among fluorescent nanomaterials with excellent optical properties and fascinating dual-emission characteristics, have attracted increasing attention in sensing, bio-imaging, drug delivery, and so on. The synthesis of dual-emission CDs (DE-CDs) and the establishment of ratiometric fluorescence sensors can effectively diminish background interference and provide more accurate results than single-emission CDs. Although DE-CDs have generated increased attention in many fields, the review articles about DE-CDs are still insufficient. Therefore, we summarized the latest results and prepared this review. This review first provides an overview of the primary synthesis route and commonly used precursors in DE-CDs synthesis. Then, the photoluminescence mechanism behind the dual-emission phenomenon was discussed. Thirdly, the application of DE-CDs in metal cation detection, food safety analysis, biosensing, cell imaging, and optoelectronic devices has been extensively discussed. Finally, the main challenges and prospects for further development are presented. This review presents the latest research progress of DE-CDs synthesis and its application in ratiometric sensing; hopefully, it can help and encourage researchers to overcome existing challenges and broaden the area of DE-CDs research.

Synthesis Strategies, Optical Mechanisms, and Applications of Dual-Emissive Carbon Dots

Tuning the optical properties of carbon dots (CDs) and figuring out the mechanisms underneath the emissive phenomena have been one of the most cutting-edge topics in the development of carbon-based nanomaterials. Dual-emissive CDs possess the intrinsic dual-emission character upon single-wavelength excitation, which significantly benefits their multi-purpose applications. Explosive exploitations of dual-emissive CDs have been reported during the past five years. Nevertheless, there is a lack of a systematic summary of the rising star nanomaterial. In this review, we summarize the synthesis strategies and optical mechanisms of the dual-emissive CDs. The applications in the areas of biosensing, bioimaging, as well as photoelectronic devices are also outlined. The last section presents the main challenges and perspectives in further promoting the development of dual-emissive CDs. By covering the most vital publications, we anticipate that the review is of referential significance for researchers in the synthesis, characterization, and application of dual-emissive CDs.

Optical Properties of Carbon Dots Synthesized by the Hydrothermal Method

In this study, the optical and structural properties of carbon dots (CDs) synthesized using a hydrothermal method were investigated. CDs were prepared from various precursors such as citric acid (CA), glucose, and birch bark soot. The SEM and AFM results show that the CDs are disc-shaped nanoparticles with dimensions of ~7 nm × 2 nm for CDs from CA, ~11 nm × 4 nm for CDs from glucose, and ~16 nm × 6 nm for CDs from soot. The TEM images of CDs from CA showed stripes with a distance of 0.34 nm between them. We assumed that the CDs synthesized from CA and glucose consisted of graphene nanoplates located perpendicular to the disc plane. The synthesized CDs contain oxygen (hydroxyl, carboxyl, carbonyl) and nitrogen (amino, nitro) functional groups. CDs have strong absorption in the ultraviolet region in the range of 200-300 nm. All CDs synthesized from different precursors displayed bright luminescence in the blue-green region of the spectrum (420-565 nm). We found that the luminescence of CDs depended on the synthesis time and type of precursors. The results show that the radiative transitions of electrons occur from two levels with energies ~3.0 eV and ~2.6 eV, which are due to the presence of functional groups.

Multifunctional carbon dots for glutathione detection and Golgi imaging

Glutathione (GSH) is present in almost every cell in the body and plays various integral roles in many biological processes. The Golgi apparatus is a eukaryotic organelle for the biosynthesis, intracellular distribution, and secretion of various macromolecules; however, the mechanism of GSH in the Golgi apparatus has not been fully elucidated. Here, specific and sensitive sulfur-nitrogen co-doped carbon dots (SNCDs) with orange-red fluorescence was synthesized for the detection of GSH in the Golgi apparatus. The SNCDs have a Stokes shift of 147 nm and excellent fluorescence stability, and they exhibited excellent selectivity and high sensitivity to GSH. The linear response of the SNCDs to GSH was in the range of 10-460 μM (LOD = 0.25 μΜ). More importantly, we used SNCDs with excellent optical properties and low cytotoxicity as probes, and successfully realized golgi imaging in HeLa cells and GSH detection at the same time.

Preparations of antibacterial yellow-green-fluorescent carbon dots and carbon dots-lysozyme complex and their applications in bacterial imaging and bacteria/biofilm inhibition/clearance

The preparation of functional long-wavelength-emitting nanomaterials and the researches on their applications in antibacterial and antibiofilm fields have important significance. This paper reports the preparation of yellow-green-fluorescent and high- quantum yield carbon dots (4-ACDs) with 4-aminosalicylic acid and polyethylene imine as raw materials through one-step route, and the impacts of raw material structure and the reaction conditions upon the optical properties of the products have been investigated. 4-ACDs exhibit excellent broad-spectrum antibacterial activity, and their good biocompatibility ensures them as ideal fluorescent nano-probe for cell imaging. However, 4-ACDs could not effectively eliminate the biofilm of Staphylococcus aureus (S. aureus). CDs-LZM complex was prepared through the coupling between 4-ACDs and lysozyme (LZM) and the complex showed strong antibacterial activity against Gram-positive bacteria, particularly with MIC against S. aureus at 5 μg mL^-1. Besides, CDs-LZM showed excellent ability against the biofilm of S. aureus. At the concentration of 60 μg mL^-1, its inhibition rate against the growth of biofilm was 86 %, and elimination rate against biofilm reached 76 %. CDs-LZM exhibited obvious antibiofilm ability through removing extracellular matrix of biofilm, greatly reducing the thickness of biofilm under confocal microscopy. The application of novel long-wavelength-emitting nanomaterial in eliminating pathogenic bacteria is of great significance.

Post-synthetic regulation of the fluorescence of CDs: insights into the fluorescence mechanism

Exploring the origin of emission is fundamental in the field of carbon dots (CDs). Due to the lack of suitable in situ probing techniques, it is necessary to explore effective alternative methods that can accurately reflect the relationship between the emission and the composition of the functional groups of CDs. Herein, we propose a new method of post-synthetic treatment of CDs by photo-oxidation to investigate the origin of emission for CDs. After the addition of a photo-oxidant into pre-prepared CDs under UV irradiation, the fluorescence of CDs can be regulated from the original orange emission to the final green emission due to the damage of original functional groups and the formation of new functional groups on CDs during the post-treatment process. The abundant dynamic information about the functional groups and emissions of CDs during the visible and ready-to-monitor post-treatment process makes it possible to quantitatively analyze the origin of the emission of CDs. Our results suggest that the emission sub-peaks at 560 nm and 600 nm relate to the CD surface-state-associated -NH₃⁺ groups, while the emission sub-peak at 537 nm or 494 nm is associated with the CD surface-state-associated -OH groups or the CD surface-state-associated carbonyl groups (CO). Under UV irradiation, the CD surface-state-associated -NH₃⁺ groups can be continuously converted into the CD surface-state-associated -OH groups and the CD surface-state-associated carbonyl groups (CO), leading to the changed emission color of CDs.

Photoluminescence Mechanism of Carbon Dots: Triggering Multiple Color Emissions through Controlling the Degree of Protonation

Carbon dots (CDs) have excellent optical properties, low toxicity and easy preparation, which have led to them being widely used in biomedicine, sensing and optical devices. However, although great progress has been made in the preparation of CDs, the detailed exploration of their photoluminescence (PL) mechanism is still under debate due to their complex structures and surface functionalities. Here, we proposed a single change in the pH of the synthesis condition, which had no effect on the CDs intrinsic core states and avoided the mutual influence of multiple PL origins. The m-phenylenediamine (m-PD) served as a carbon source, whose protonation degree determined the surface state of the resulting CDs and the accompanying fluorescence characteristics. The as-obtained CDs materials can be applied in the chemical sensor and anti-counterfeiting fields in a targeted manner. Therefore, our work not only contributes to the explanation of the CDs PL mechanism, but also obtains a series of CDs materials with controllable PL properties.

Recent Progress in Fluorescent Probes for Diabetes Visualization and Drug Therapy

Diabetes has become one of the most prevalent endocrine and metabolic diseases that threaten human health, and it is accompanied by serious complications. Therefore, it is vital and pressing to develop novel strategies or tools for prewarning and therapy of diabetes and its complications. Fluorescent probes have been widely applied in the detection of diabetes due to the fact of their attractive advantages. In this report, we comprehensively summarize the recent progress and development of fluorescent probes in detecting the changes in the various biomolecules in diabetes and its complications. We also discuss the design of fluorescent probes for monitoring diabetes in detail. We expect this review will provide new ideas for the development of fluorescent probes suitable for the prewarning and therapy of diabetes in future clinical transformation and application.

Development of a carbon dot and methylene blue NIR-emitting FLIM-FRET pair in niosomes for controlled ROS generation

Non-ionic surfactant vesicular systems (niosomes) are structurally similar to lipid vesicles, differing only in the bilayer composition. Herein we report a unique method to generate reactive oxygen species (ROS) utilizing a FLIM-FRET technique involving niosome-trapped yellow emissive carbon dots (YCDs) and methylene blue (MB) in aqueous medium under neutral conditions. Niosomes are biologically important because of their good stability and extremely low toxicity. Fluorescent CDs, emitting in the higher wavelengths on visible light excitation, are of incredible importance in bio-imaging and optoelectronics. Hence, we prepared nitrogen-containing YCDs from a single precursor, o-phenylenediamine, and explained their detailed photophysics upon incorporation into the niosomal bilayer. The YCDs are polarity sensitive, and are rotationally restricted in niosomes, which increases their fluorescence quantum yield from 29% (in water) to 91%. These YCDs are tactically employed to develop a near infrared (NIR) FRET pair with methylene blue (MB), which is a very well-known type-I and type-II photosensitizer. This FRET pair, which emits in the NIR region, is found to be an ideal system to generate ROS by excitation in the lower visible wavelengths. Interestingly, the ROS production by MB from the dissolved oxygen is enhanced inside the niosomes. The donor and the acceptor moieties in this unique NIR-emitting FRET pair display an unprecedented 300 nm Stokes shift. The findings could be influential in bio-imaging in the NIR region evading cellular autofluorescence and the controllably generated ROS can be further applied as a potential photodynamic therapeutic agent.

Accelerated antibacterial red-carbon dots with photodynamic therapy against multidrug-resistant Acinetobacter baumannii

The emergence of antibiotic resistance in bacteria is a major public-health issue. Synthesis of efficient antibiotic-free material is very important for fighting bacterial infection-related diseases. Herein, red-carbon dots (R-CDs) with a broad range of spectral absorption (350-700 nm) from organic bactericides or intermediates were synthesized through a solvothermal route. The prepared R-CDs not only had intrinsic antibacterial activities, but also could kill multidrug-resistant bacteria (multidrug-resistant Acinetobacter baumannii (MRAB) and multidrug-resistant Staphylococcus aureus (MRSA)) effectively by generating reactive oxygen species. Furthermore, R-CDs could eliminate and inhibit the formation of MRAB biofilms, while conferring few side effects on normal cells. A unique property of R-CDs was demonstrated upon in vivo treatment of antibiotic-sensitive MRAB-induced infected wounds. These data suggested that this novel R-CDs-based strategy might enable the design of next-generation agents to fight drug-resistant bacteria.

ELECTRONIC SUPPLEMENTARY MATERIAL

Supplementary material is available for this article at 10.1007/s40843-021-1770-0 and is accessible for authorized users.

Spying on the Polarity Dynamics during Wound Healing of Zebrafish by Using Rationally Designed Carbon Dots

Wound healing is an essential and complex biological process. Research into its mechanism and factors that influence its effectiveness has led to better treatments. Changes in the microenvironment are demonstrated to affect wound healing. Cell polarity is a significant microenvironment-related parameter that is associated with many physiological and pathological activities. However, dynamic changes in polarity during wound healing have not been investigated. Monitoring cell polarity during wound healing may open up a new avenue for developing better treatments. Here, a method is developed to monitor cell polarity that involved taking advantage of the fascinating optical properties and biocompatibility of carbon dots (CDs). Specifically, near-infrared (NIR) polarity-sensitive N-phenyl-p-phenylenediamine (PPh-CDs) are successfully prepared, which exhibit high sensitivity to polarity, with 509-fold stronger fluorescence in dioxane than in water. The PPh-CDs are successfully applied to monitor the changes of lysosomal polarity during starvation conditions. Using this method, dynamic changes of polarity during wound healing of zebrafish are monitored for the first time. Upon an amputation performed at the zebrafish tail, stronger PPh-CDs fluorescence appeared at the wound sites, and the intensity increased for 25 min and then gradually decreased. This report provides an important experimental basis for investigating wound healing by employing polarity-sensitive CDs.

Tuning optical properties of nitrogen-doped carbon dots through fluorescence resonance energy transfer using Rhodamine B for the ratiometric sensing of mercury ions

Carbon dots (CDs) that exhibit fluorescence properties are generally derived from carbonaceous materials, and possess ultrasmall sizes with various exciting physical, chemical and photo-properties, which have been used in many different fields in recent time. Here, we have focused on the preparation of nitrogen-doped CDs (N-CDs) that emit a bright blue fluorescence upon exposure to UV excitation. Furthermore, by employing Rhodamine B (RhB) as a donor molecule, the emission color of N-CDs is altered from blue to red. Interestingly, the optical tuning based upon emission from one particular color to various other colors has been achieved by varying the doping ratio of the donor molecule, RhB. The reason is mainly attributed to the non-radiative energy transfer of the exciton energy from an excited donor to an acceptor through fluorescence resonance energy transfer (FRET). Furthermore, this emission behavior is explored for the ratiometric sensing of mercury ion (Hg2+) in aqueous medium. Among different color emissions, we chose one particular emission color, namely violet, for the detection of the Hg2+ ion. The photoluminescence properties of N-CDs are effectively and systematically quenched with the addition of different mercury ion concentrations, leading to efficient energy transformation arising from the synergetic effect of the electrostatic interaction and metal - ligand coordination between the surface functional groups of N-CDs and Hg2+ ion. On the other hand, RhB has no interaction with Hg2+ ions. These findings provide a way for developing a cheap, selective and suitable sensing matrix for the detection of toxic metal ions, such as mercury (Hg2+) at a low concentration level.

Carbon Dots: An Emerging Smart Material for Analytical Applications

Carbon dots (CDs) are optically active carbon-based nanomaterials. These nanomaterials can change their light emission properties in response to various external stimuli such as pH, temperature, pressure, and light. The CD's remarkable stimuli-responsive smart material properties have recently stimulated massive research interest for their exploitation to develop various sensor platforms. Herein, an effort has been made to review the major advances made on CDs, focusing mainly on its smart material attributes and linked applications. Since the CD's material properties are largely linked to their synthesis approaches, various synthesis methods, including surface passivation and functionalization of CDs and the mechanisms reported so far in their photophysical properties, are also delineated in this review. Finally, the challenges of using CDs and the scope for their further improvement as an optical signal transducer to expand their application horizon for developing analytical platforms have been discussed.