The development of multicolor carbon dots and their applications in the field of anti-counterfeiting

The diversity of photoluminescence emission of multicolor carbon dots (mCDs) greatly broadens their application field. Counterfeiting is a problem that has serious consequences for individuals and society. The fluorescent ink based on mCDs has been developed due to their low toxicity and good resistance to photobleaching in the field of anti-counterfeiting. In the background, the preparation strategies of mCDs were discussed in detail, including top-down method and bottom-up method. The common carbon sources for the preparation of mCDs were further summarized, such as phenylenediamine class, amino-substituted naphthalene class, dihydroxyhenzene and benzoquinone class, biomass, and other common carbon sources. Furthermore, in order to understand the structure of CDs, this paper focused on the classification and common characterization techniques of CDs. In addition, the application of mCDs as fluorescent ink in the field of anti-counterfeiting was classified by simple anti-counterfeiting and good multiple anti-counterfeiting. Finally, the paper offered insight into the future development prospects of CDs-based fluorescent ink based on the current development trends.

Gadolinium Functionalized Carbon Dot Complexes for Dual-Modal Imaging: Structure, Performance, and Applications

Gadolinium functionalized carbon dot complexes (Gd-CDs) have both the fluorescent properties of carbon dots and the magnetic characteristics of gadolinium ions, exhibiting excellent biocompatibility, high spatial resolution, high sensitivity, and deep tissue penetration in bioimaging. As fluorescence (FL) and magnetic resonance imaging (MRI) probes, Gd-CDs have attracted significant attention in dual-modal biological imaging. This review summarizes recent advances in Gd-CDs, focusing on their structure, optical and magnetic properties, and applications in dual-modal imaging. First, according to the different existing forms of gadolinium in carbon dots, the structures of Gd-CDs are categorized into chelation, electrostatic interaction, and encapsulation. Second, the mechanisms and performances of Gd-CDs in dual-modal imaging are introduced in detail. The reported Gd-CDs have a maximum quantum yield of 69.86%, with a fluorescence emission wavelength reaching up to 625 nm, and the optimum longitudinal and transverse relaxivity rates are 35.39 and 115.6 mM-1 s-1, respectively, showing excellent FL/MRI capacities. Subsequently, the progress in their applications in dual-modal cellular imaging, in vivo imaging, and integrated cancer diagnosis and therapy is reviewed. Finally, the challenges and issues faced by Gd-CDs in their development are summarized, providing new insights for their controlled synthesis and widespread application in the biomedical field of dual-modal imaging.

Metal-doped carbon dots for biomedical applications: From design to implementation

Carbon dots (CDs), as a new kind of fluorescent nanomaterials, show great potential for application in several fields due to their unique nano-size effect, easy surface functionalization, controllable photoluminescence, and excellent biocompatibility. Conventional preparation methods for CDs typically involve top-down and bottom-up approaches. Doping is a major step forward in CDs design methodology. Chemical doping includes both non-metal and metal doping, in which non-metal doping is an effective strategy for modulating the fluorescence properties of CDs and improving photocatalytic performance in several areas. In recent years, Metal-doped CDs have aroused the interest of academics as a promising nano-doping technique. This approach has led to improvements in the physicochemical and optical properties of CDs by altering their electron density distribution and bandgap capacity. Additionally, the issues of metal toxicity and utilization have been addressed to a large extent. In this review, we categorize metals into two major groups: transition group metals and rare-earth group metals, and an overview of recent advances in biomedical applications of these two categories, respectively. Meanwhile, the prospects and the challenges of metal-doped CDs for biomedical applications are reviewed and concluded. The aim of this paper is to break through the existing deficiencies of metal-doped CDs and fully exploit their potential. I believe that this review will broaden the insight into the synthesis and biomedical applications of metal-doped CDs.

Carbon Quantum Dots: Properties, Preparation, and Applications

Carbon quantum dots are a novel form of carbon material. They offer numerous benefits including particle size adjustability, light resistance, ease of functionalization, low toxicity, excellent biocompatibility, and high-water solubility, as well as their easy accessibility of raw materials. Carbon quantum dots have been widely used in various fields. The preparation methods employed are predominantly top-down methods such as arc discharge, laser ablation, electrochemical and chemical oxidation, as well as bottom-up methods such as templates, microwave, and hydrothermal techniques. This article provides an overview of the properties, preparation methods, raw materials for preparation, and the heteroatom doping of carbon quantum dots, and it summarizes the applications in related fields, such as optoelectronics, bioimaging, drug delivery, cancer therapy, sensors, and environmental remediation. Finally, currently encountered issues of carbon quantum dots are presented. The latest research progress in synthesis and application, as well as the challenges outlined in this review, can help and encourage future research on carbon quantum dots.

Targeted Delivery of Sucrose-Coated Nanocarriers with Chemical Cargoes to the Plant Vasculature Enhances Long-Distance Translocation

Current practices for delivering agrochemicals are inefficient, with only a fraction reaching the intended targets in plants. The surfaces of nanocarriers are functionalized with sucrose, enabling rapid and efficient foliar delivery into the plant phloem, a vascular tissue that transports sugars, signaling molecules, and agrochemicals through the whole plant. The chemical affinity of sucrose molecules to sugar membrane transporters on the phloem cells enhances the uptake of sucrose-coated quantum dots (sucQD) and biocompatible carbon dots with β-cyclodextrin molecular baskets (suc-β-CD) that can carry a wide range of agrochemicals. The QD and CD fluorescence emission properties allowed detection and monitoring of rapid translocation (<40 min) in the vasculature of wheat leaves by confocal and epifluorescence microscopy. The suc-β-CDs more than doubled the delivery of chemical cargoes into the leaf vascular tissue. Inductively coupled plasma mass spectrometry (ICP-MS) analysis showed that the fraction of sucQDs loaded into the phloem and transported to roots is over 6.8 times higher than unmodified QDs. The sucrose coating of nanoparticles approach enables unprecedented targeted delivery to roots with ≈70% of phloem-loaded nanoparticles delivered to roots. The use of plant biorecognition molecules mediated delivery provides an efficient approach for guiding nanocarriers containing agrochemicals to the plant vasculature and whole plants.

Interfacing manganese-based carbonaceous nanocomposites with plasma components: insights on protein interaction, structure and opsonization

Metal encapsulation delivers a straightforward strategy to improve miscellaneous nanoparticle properties and qualifies the resulting nanocomposite for exceptional application, including bioimaging, drug release, and theranostic development. Besides crucial applications, investigations associated with the nanocomposite impact on the biological media are highly relevant from a pharmacological viewpoint. Such studies can be conducted by exploring nanocomposite attributes and all aspects of their interaction with proteins existing in biofluids. Based on these aspects, the present work examines manganese-encapsulated carbonaceous nanocomposite (MnCQD) and their interaction with plasma proteins. On one side, the obtained nanocomposite has almost spherical shapes (≈12 nm in size), an appropriate composition and interesting optical properties for bioimaging applications. On another side, MnCQD quenches the fluorescence of two plasma proteins (BSA and HTF) following a static mechanism, confirming the formation of the MnCQD-BSA and MnCQD-HTF complexes. Although hydrophobic forces guide the stability of both formed complexes, MnCQD binds preferentially to BSA compared to HTF, with affinity constants differing by almost an order of magnitude. Furthermore, HTF and BSA underwent modifications in their secondary structure provoked due to contact with the nanocomposite, which also presented neglectable opsonization levels when exposed to appropriate biological media. These results highlight the MnCQD outstanding potential to be employed in diverse bioapplications.Communicated by Ramaswamy H. Sarma.

A smartphone-based fluorospectrophotometer and ratiometric fluorescence nanoprobe for on-site quantitation of pesticide residue

Cost-effective and user-friendly quantitation at points-of-need plays an important role in food safety inspection, environmental monitoring, and biomedical analysis. This study reports a stand-alone smartphone-based fluorospectrophotometer (the SBS) installed with a custom-designed application (the SBS-App) for on-site quantitation of pesticide using a ratiometric sensing scheme. The SBS can collect fluorescence emission spectra in the wavelength range of 380-760 nm within 5 s. A ratiometric fluorescence probe is facilely prepared by directly mixing the blue-emissive carbon nanodots (the Fe3+-specific fluorometric indicator) and red-emissive quantum dots (the internal standard) at a ratio of 11.6 (w/w). Based on the acetylcholinesterase/choline oxidase dual enzyme-mediated cascade catalytic reactions of Fe2+/Fe3+ transformation, a ratiometric fluorescence sensing scheme is developed. The practicability of the SBS is validated by on-site quantitation of chlorpyrifos in apple and cabbage with a comparable accuracy to the GC-MS method, offering a scalable solution to establish a cost-effective surveillance system for pesticide pollution.

BSA-templated synthesis of Ir/Gd bimetallic oxide nanotheranostics for MR/CT imaging-guided photothermal and photodynamic synergistic therapy

Precision medicine urges the development of theranostics which can efficiently integrate precise diagnosis and effective therapy. In this study, a facile synthesis of Ir/Gd bimetallic oxide nanotheranostics (termed BSA@Gd₂O₃/IrO₂ NPs) with good biocompatibility was demonstrated using a biomineralization method where bovine serum albumin (BSA) served as a versatile template. BSA@Gd₂O₃/IrO₂ NPs exhibited high longitudinal relaxivity (5.2 mM^-1 s^-1) and X-ray absorption capability (14.5 Hu mM^-1), illustrating them to be a good contrast agent for magnetic resonance (MR) and computed tomography (CT) dual-modal imaging. Moreover, BSA@Gd₂O₃/IrO₂ NPs can act as not only a photothermal conversion agent with ultrahigh efficiency (66.7%) as well as a good photosensitizer, but also an effective catalase to decompose endogenous H₂O₂ to produce O₂, thus relieving hypoxia and enhancing the phototherapeutic effect. Both in vitro and in vivo experiments demonstrated the high effectiveness of BSA@Gd₂O₃/IrO₂ NPs in MR/CT dual-modal imaging and photothermal and photodynamic synergistic tumor treatments. This work sheds new light on the development of versatile nanotheranostic systems using mild and robust biomineralization methods.

Carbon dot-based nanomaterials: a promising future nano-platform for targeting tumor-associated macrophages

The tumor microenvironment (TME) is the internal environment that tumors depend on for survival and development. Tumor-associated macrophages (TAMs), as an important part of the tumor microenvironment, which plays a crucial role in the occurrence, development, invasion and metastasis of various malignant tumors and has immunosuppressant ability. With the development of immunotherapy, eradicating cancer cells by activating the innate immune system has yielded encouraging results, however only a minority of patients show a lasting response. Therefore, in vivo imaging of dynamic TAMs is crucial in patient-tailored immunotherapy to identify patients who will benefit from immunotherapy, monitor efficacy after treatment, and identify alternative strategies for non-responders. Meanwhile, developing nanomedicines based on TAMs-related antitumor mechanisms to effectively inhibit tumor growth is expected to become a promising research field. Carbon dots (CDs), as an emerging member of the carbon material family, exhibit unexpected superiority in fluorescence imaging/sensing, such as near infrared imaging, photostability, biocompatibility and low toxicity. Their characteristics naturally integrate therapy and diagnosis, and when CDs are combined with targeted chemical/genetic/photodynamic/photothermal therapeutic moieties, they are good candidates for targeting TAMs. We concentrate our discussion on the current learn of TAMs and describe recent examples of macrophage modulation based on carbon dot-associated nanoparticles, emphasizing the advantages of their multifunctional platform and their potential for TAMs theranostics.

Structure-Property-Activity Relationships in Carbon Dots

Carbon dots (CDs) are one of the most versatile nanomaterials discovered in the 21st century. They possess many properties and thus hold potentials in diverse applications. While an increasing amount of attention has been given to these novel nanoparticles, the broad scientific community is actively engaged in exploring their limits. Recent studies on the fractionalization and assembly of CDs further push the limits beyond just CDs and demonstrate that CDs are both a mixture of heterogeneous fractions and promising building blocks for assembly of large carbon-based materials. With CDs moving forward toward both microscopic and macroscopic levels, a good understanding of the structure-property-activity relationships is essential to forecasting the future of CDs. Hence, in this Perspective, structure-property-activity relationships are highlighted based on the repeatedly verified findings in CDs. In addition, studies on CD fractionalization and assembly are briefly summarized in this Perspective. Eventually, these structure-property-activity relationships and controllability are essential for the development of CDs with desired properties for various applications especially in photochemistry, electrochemistry, nanomedicine, and surface chemistry. In summary, in our opinion, since 2004 until the present, history has witnessed a great development of CDs although there is still some room for more studies. Also, considering many attractive properties, structure-property-activity relationships, and the building block nature of CDs, a variety of carbon-based materials of interest can be constructed from CDs with control. They can help reduce blind trials in the development of carbon-based materials, which is of great significance in materials science, chemistry, and any fields related to the applications.

Emerging metal doped carbon dots for promising theranostic applications

As a bridge between organic fluorophores and inorganic quantum dots, carbon dots (CDs) have been recognized as emerging nanotheranostics for biomedical applications owing to their distinctive merits such as superior optical properties, flexible modification, adjustable functionalities, and remarkable photoactive therapeutic outcome, etc. Compared to metal free CDs, the introduction of metal ion in CDs endowed metal-doped CDs (MCDs) with tunable optical properties and new intrinsic properties, thereby illustrating its different capabilities from metal-free CDs for bioimaging and therapy. This review aims to summarize the recent progress of photonic MCDs as emerging nanoagent for theranostic application such as disease-related diagnostic (involving biosensing and bioimaging) and cancer therapy. The challenges and potential development of MCDs in nanotheranostic fields are also discussed.

Recent Developments in Heteroatom/Metal-Doped Carbon Dot-Based Image-Guided Photodynamic Therapy for Cancer

Carbon nanodots (CNDs) are advanced nanomaterials with a size of 2-10 nm and are considered zero-dimensional carbonaceous materials. CNDs have received great attention in the area of cancer theranostics. The majority of review articles have shown the improvement of CNDs for use in cancer therapy and bioimaging applications. However, there is a minimal number of consolidated studies on the currently developed doped CNDs that are used in various ways in cancer therapies. Hence, in this review, we discuss the current developments in different types of heteroatom elements/metal ion-doped CNDs along with their preparations, physicochemical and biological properties, multimodal-imaging, and emerging applications in image-guided photodynamic therapies for cancer.

Fluorescent Carbon Dot-Supported Imaging-Based Biomedicine: A Comprehensive Review

Carbon dots (CDs) provide distinctive advantages of strong fluorescence, good photostability, high water solubility, and outstanding biocompatibility, and thus are widely exploited as potential imaging agents for in vitro and in vivo bioimaging. Imaging is absolutely necessary when discovering the structure and function of cells, detecting biomarkers in diagnosis, tracking the progress of ongoing disease, treating various tumors, and monitoring therapeutic efficacy, making it an important approach in modern biomedicine. Numerous investigations of CDs have been intensively studied for utilization in bioimaging-supported medical sciences. However, there is still no article highlighting the potential importance of CD-based bioimaging to support various biomedical applications. Herein, we summarize the development of CDs as fluorescence (FL) nanoprobes with different FL colors for potential bioimaging-based applications in living cells, tissue, and organisms, including the bioimaging of various cell types and targets, bioimaging-supported sensing of metal ions and biomolecules, and FL imaging-guided tumor therapy. Current CD-based microscopic techniques and their advantages are also highlighted. This review discusses the significance of advanced CD-supported imaging-based in vitro and in vivo investigations, suggests the potential of CD-based imaging for biomedicine, and encourages the effective selection and development of superior probes and platforms for further biomedical applications.

Gadolinium-porphyrin based polymer nanotheranostics for fluorescence/magnetic resonance imaging guided photodynamic therapy

Nanotheranostics for fluorescence/magnetic resonance (FL/MR) dual-modal imaging guided photodynamic therapy (PDT) are highly desirable in precision and personalized medicine. In this study, a facile non-covalent electrostatic interaction induced self-assembly strategy is developed to effectively encapsulate gadolinium porphyrin (Gd-TCPP) into homogeneous supramolecular nanoparticles (referred to as Gd-PNPs). Gd-PNPs exhibit the following advantages: (1) excellent FL imaging property, high longitudinal relaxivity (16.157 mM^-1 s^-1), and good singlet oxygen (¹O₂) production property; (2) excellent long-term colloidal stability, dispersity and biocompatibility; and (3) enhanced in vivo FL/MR imaging guided tumor growth inhibition efficiency for CT 26 tumor-bearing mice. This study provides a new strategy to design and synthesize metalloporphyrin-based nanotheranostics for imaging-guided cancer therapy with enhanced theranostic properties.

Red Fluorescent Emissive Gd-Phenolic Nanoparticles for In Vivo Fluorescence and Magnetic Resonance Bimodal Imaging

Due to the combination of the high resolution of fluorescence imaging and the no limitation in penetration depth of magnetic resonance imaging, dual-mode imaging of magnetic resonance and fluorescence (MR/FI) have attracted extensive research in recent years. Herein, a novel MR/FI bimodal imaging probe is facile fabricated by attaching the rhodamine fluorophore covalently to the surface of the Gd-phenolic coordination polymer nanoparticles. The contents of Gd³⁺ and RB of the as prepared probe are calculated to be 8.2% and 12.5%. The quantum yield of the probe is about 8.84% as well as red fluorescent emissive. The longitudinal r1 value is 6.94 mM^-1 s^-1 and the ratio r2/r1 is very low and about 1.22. Subsequently, the and MR imaging and fluorescence both in vitro and In Vivo are performed. The metabolic pathways In Vivo are inferred by studying the bio-distribution of the probe in major organs. The as-prepared probe exhibits excellent imaging performance and biocompatibility, which is conducive to its further application.

Gadolinium-doped carbon dots with high-performance in dual-modal molecular imaging

Carbon dots (CDs), possessing unexpected advantages of photostability, biocompatibility and low toxicity, are regarded as novel nanomaterials in fluorescence (FL) imaging. Doping Gd element in CDs makes them have the ability to be used for magnetic resonance (MR) and FL imaging simultaneously. However, CDs reported before exhibit obvious defects like low photoluminescence (PL) quantum yield (QY) or biotoxicity. In this work, we use gadolinium meglumine, a material with relatively low biotoxicity, along with citric acid and diethylenetriamine to synthesize Gd-doped CDs (Gd-CDs) by a one-step hydrothermal method. The prepared Gd-CDs exhibit excitation-independent emission with a PL QY of 78.05% and a longitudinal relaxivity of 7.37 mM^-1 S^-1, which endows the composite with high-performance in MR/FL imaging. Meanwhile, the FL intensity of Gd-CDs remains stable in the presence of multiple amino acids, which indicates that the FL imaging effect should not be impacted significantly in microenvironments in vivo. In addition to the inconspicuous cytotoxicity, Gd-CDs could be used efficiently for dual-modal molecular imaging to detect diseases such as tumors in the early stages.

Microfluidic synthesis of quantum dots and their applications in bio-sensing and bio-imaging

Bio-sensing and bio-imaging of organisms or molecules can provide key information for the study of physiological processes or the diagnosis of diseases. Quantum dots (QDs) stand out to be promising optical detectors because of their excellent optical properties such as high brightness, stability, and multiplexing ability. Diverse approaches have been developed to generate QDs, while microfluidic technology is one promising path for their industrial production. In fact, microfluidic devices provide a controllable, rapid and effective route to produce high-quality QDs, while serving as an effective in situ platform to understand the synthetic mechanism or optimize reaction parameters for QD production. In this review, the recent research progress in microfluidic synthesis and bio-detection applications of QDs is discussed. The definitions of different QDs are first introduced, and the advances in microfluidic-based fabrication of quantum dots are summarized with a focus on perovskite QDs and carbon QDs. In addition, QD-based bio-sensing and bio-imaging technologies for organisms of different scales are described in detail. Finally, perspectives for future development of microfluidic synthesis and applications of QDs are presented.

Single-Atom Gadolinium Anchored on Graphene Quantum Dots as a Magnetic Resonance Signal Amplifier

A single-atom metal doped on carbonaceous nanomaterials has attracted increasing attention due to its potential applications as high-performance catalysts. However, few studies focus on the applications of such nanomaterials as nanotheranostics for simultaneous bioimaging and cancer therapy. Herein, it is pioneeringly demonstrated that the single-atom Gd anchored onto graphene quantum dots (SAGd-GQDs), with dendrite-like morphology, was successfully prepared. More importantly, the as-fabricated SAGd-GQDs exhibits a robustly enhanced longitudinal relaxivity (r₁ = 86.08 mM^-1 s^-1) at a low Gd³⁺ concentration of 2 μmol kg^-1, which is 25 times higher than the commercial Gd-DTPA (r₁ = 3.44 mM^-1 s^-1). In vitro and in vivo studies suggest that the obtained SAGd-GQDs is a highly potent and contrast agent to obtain high-definition MRI, thereby opening up more opportunities for future precise clinical theranostics.

Synthesis of fluorescent nanoprobe with simultaneous response to intracellular pH and Zn2+ for tumor cell distinguishment

A novel dual-functional nanoprobe was designed and synthesized by facile assembly of quinoline derivative (PEIQ) and meso-tetra (4-carboxyphenyl) porphine (TCPP) via electrostatic interaction for simultaneous sensing of fluorescence of Zn²⁺ and pH. Under the single-wavelength excitation at 400 nm, this nanoprobe not only exhibits "OFF-ON" green fluorescence at 512 nm by specific PEIQ-Zn²⁺ chelation, but also presents red fluorescence enhancement at 654 nm by H⁺-triggered TCPP release. The nanoprobe demonstrated excellent sensing performance with a good linear range (Zn²⁺, 1-40 μM; pH, 5.0-8.0), low detection limit (Zn²⁺, 0.88 μM), and simultaneous response towards Zn²⁺ and pH in pure aqueous solution within 2 min. More importantly, this dual-functional nanoprobe demonstrates the capability of discerning cancerous cells from normal cells, as evidenced by the fact that cancerous HepG2 cells in tumor microenvironment exhibit substantially higher red fluorescence and significantly lower green fluorescence than normal HL-7702 cells. The simultaneous, real-time fluorescence imaging of multiple analytes in a living system could be significant for cell analysis and tracking, cancer diagnosis, and even fluorescence-guided surgery of tumors.

Metal-doped and hybrid carbon dots: A comprehensive review on their synthesis and biomedical applications

Carbon dots (CDs) are the most promising candidates of the carbon family with superior properties like ultra-small size, high aqueous solubility, low cytotoxicity, and inherent photoluminescence which makes them suitable for diverse biomedical applications. Methods have been developed to enhance their applications. Doping/surface passivation of CDs improves their physicochemical properties, visible light absorption probability, and quantum yield by controlling their size, morphology, structure, and band-gap energy. Recently, metal-doped CDs have emerged as an important class of nanomaterials with numerous biomedical applications. Additionally, the conjugation of CDs with semiconductor metal-oxide nanoparticles (NPs) enhances their free radical production rates under visible light irradiation. Conjugation of fluorescent CDs with magnetic NPs leads to the development of multimodal imaging platforms. Similarly, ternary conjugates composed of fluorescent CDs, near-infrared (NIR) responsive, and magnetic NPs are useful for multi-modal imaging-guided, and NIR-responsive synergistic chemo-phototherapy. However, no comprehensive review is published yet which covers metal-doped and hybrid CDs. Therefore, herein we provide detailed information about their synthesis and important biomedical applications. Firstly, we have covered various synthesis methods for CD conjugation including the critical analysis of the effects of the reaction conditions and doping/conjugation on the structure and properties of the CDs. Then we have extensively reviewed their biomedical applications as antimicrobial, antioxidant, and bioimaging agents, and in the field of cancer phototherapy with special emphasis on their mechanisms of actions. Finally, the future directions of research and the applications of the metal-doped and hybrid CDs have been discussed. We believe that this review article will enrich the understanding of different synthetic routes of CD-nanocomposites and their biomedical applications.

When rare earth meets carbon nanodots: mechanisms, applications and outlook

Rare earth (RE) elements are widely used in the luminescence and magnetic fields by virtue of their abundant 4f electron configurations. However, the overall performance and aqueous stability of single-component RE materials need to be urgently improved to satisfy the requirements for multifunctional applications. Carbon nanodots (CNDs) are excellent nanocarriers with abundant functional surface groups, excellent hydrophilicity, unique photoluminescence (PL) and tunable features. Accordingly, RE-CND hybrids combine the merits of both RE and CNDs, which dramatically enhance their overall properties such as luminescent and magnetic-optical imaging performances, leading to highly promising practical applications in the future. Nevertheless, a comprehensive review focusing on the introduction and in-depth understanding of RE-CND hybrid materials has not been reported to date. This review endeavors to summarize the recent advances of RE-CNDs, including their interaction mechanisms, general synthetic strategies and applications in fluorescence, biosensing and multi-modal biomedical imaging. Finally, we present the current challenges and the possible application perspectives of newly developed RE-CND materials. We hope this review will inspire new design ideas and valuable references in this promising field in the future.

Carbon quantum dots as fluorescence sensors for label-free detection of folic acid in biological samples

Carbon quantum dots (CQDs) have been fabricated by a facile single-step pyrolysis method from citric acid and ethylene imine polymer. When excited at 359 nm, CQDs show intense blue fluorescence at 434 nm. The fluorescence can be effectively quenched by folic acid (FA), which is attributed to the combination of static quenching and inner filter effect. Thus, the CQDs are developed as an efficient fluorescent sensing platform for label-free sensitive and selective detection of FA. Key parameters influencing the detection were investigated, such as incubation time, salt concentration, selectivity and potential interferences. Under the optimal conditions, a good linearity was observed for the emission intensity against 1.14-47.57 μM with a correlation coefficient of 0.99. The limit of detection was found to be 0.38 μM. The practical application of the sensing system was demonstrated by analyzing FA in human urine samples. The sample recoveries fell in the range of 82.0%-113.1% with RSDs ≤ 10.9%, which presented its reliable and feasible application in real samples.

Gadolinium-doped carbon quantum dots loaded magnetite nanoparticles as a bimodal nanoprobe for both fluorescence and magnetic resonance imaging

Nowadays, it is highly desired to develop dual-modal fluorescence and magnetic resonance imaging (FI/MRI) probes in medical imaging because it unites the respective advantages of each imaging modality: high sensitivity of FI and superior spatial resolution of MRI. In this study, a facile strategy to fabricate a new bimodal imaging nanoprobe (Gd-CQDs@N-Fe₃O₄) was reported by integrating the fluorescence ability of carbon quantum dots (CQDs) and T₁ and T₂ contrast-enhancing functionality of Gd(III) ions and Fe₃O₄ nanoparticles into a single hybrid nanostructure. The hybrid composites were investigated by FT-IR, XRD, TEM, XPS, VSM, and so on, which confirmed that Gd-CQDs@N-Fe₃O₄ nanoparticles were successfully obtained and exhibited superparamagnetic property at room temperature. The derived nanoprobes presented an excitation wavelength-independent emission behavior. In addition, r₁ and r₂ relaxivities of the synthesized imaging nanoprobes were measured to be 5.16 and 115.6 mM^-1 s^-1, which nominated Gd-CQDs@N-Fe₃O₄ nanocomposites as a suitable T₁-T₂ contrast agent. The Gd-CQDs@N-Fe₃O₄ nanoparticles combining two synergetic imaging modalities showed great potential in FI/MRI dual-modal imaging for a more complementary and accurate detection.

A carbon dot based theranostic platform for dual-modal imaging and free radical scavenging

Magnetofluorescent carbon dots (Cdots) doped with both P³⁺ and Mn²⁺ (abbreviated as PMn@Cdots) have been synthesized in an aqueous solution via a microwave-assisted pyrolysis method. In this system, a P³⁺ dopant was introduced to enhance the emission efficiency of the Cdots, while the presence of a Mn²⁺ dopant granted magnetic resonance imaging (MRI) capability. To the best of our knowledge, the present work is the first attempt to regulate red-emission and free radical scavenging of PMn@Cdots to serve as a dual-modal imaging nanoprobe and an antioxidant agent. Unlike most red-emitting Cdots, the as-prepared PMn@Cdots can be readily purified from unreacted precursors through antisolvent precipitation instead of by time-consuming purification methods. The whole synthetic procedure is rapid, facile, efficiently reproducible, and scalable. More importantly, further conjugation of the PMn@Cdots with hyaluronic acid (termed PMn@Cdots/HA) gives them good in vivo and in vitro biocompatibility as well as the capability to selectively target CD44-overexpressing cancer cells, as investigated by flow cytometry, fluorescence, and MRI. Meanwhile, PMn@Cdots exhibit antioxidant activity against multiple DPPH, hydroxyl, and superoxide radicals, which is comparable to that for ascorbic acid. Favorably, PMn@Cdots/HA showed a dose-dependent cytoprotective capability against H₂O₂-induced oxidative stress in B16F1, HeLa, and HEL cells. Therefore, the Cdot based theranostic platform can simultaneously function as a potential therapeutic candidate and as a dual-modal probe for enabling accurate diagnosis in future clinical applications.

Fluorescent carbon dots functionalization

Carbon dots (CDs), as a new type of luminescent zero-dimensional carbon nanomaterial, have been applied in a variety of fields. Currently, functionalization of CDs is an extremely useful method for effectively tuning their intrinsic structure and surface state. Heteroatom doping and surface modification are two functionalization strategies for improving the photophysical performance and broadening the range of applications for fluorescent CDs. Heteroatom doping in CDs can be used to tune their intrinsic properties, which has received significant research interests because of its simplicity. Surface modification can be applied for varying active sites and the functional groups on the CDs surface, which can endow fluorescent CDs with the unique properties resulting from functional ligand. In this review, we summarize the structural and physicochemical properties of functional CDs. We focused our review on the latest developments in functionalization strategies for CDs and discuss the detailed characteristics of different functionalization methods. Ultimately, we hope to inform researchers on the latest progress in functionalization of CDs and provide perspectives on future developments for functionalization of CDs and their potential applications.

Magnetic resonance imaging and photothermal conversion properties of Gd-C nanocomposites for interstitial lymphography

Dual-functional agents for magnetic resonance imaging (MRI) guided photothermal therapy (PTT) of lymph cancer are highly desired. Signal enhancement, selectivity between lymphatic nodes/vessels and blood vessels, and photothermal conversion property are the criteria for such dual-functional agent. In the current work, we demonstrated the potential of Gd-C nanocomposites as dual-functional agents for the MRI and PTT of lymph node cancer. Gd-C nanocomposites were synthesized via a hydrothermal carbonization approach with gadolinium chloride as Gd source and citric acid (CA) as C source. The particle size of the nanocomposites ranges from 40 to 100 nm which is smaller than the intercellular space of lymphatic vessels but much larger than that of the blood vessels. The nanocomposites were successfully applied to the MRI of cervical lymph nodes of rabbits. The signal enhancement of the lymph nodes reached the maximum value of 434% at 10 min after injection, without displaying any blood vessel. The Gd-C nanocomposites also exhibited strong photothermal conversion effect. Under the illumination of an 808 nm laser, the aqueous suspension containing 1.0 wt % Gd-C nanocomposites gave a maximum temperature rise of 28.2 °C and a light utilization efficiency of 30.4%. The results indicate that Gd-C nanocomposites have significant potential in MRI guided PTT of lymph cancer.

A critical comparison of lanthanide based upconversion nanoparticles to fluorescent proteins, semiconductor quantum dots, and carbon dots for use in optical sensing and imaging

The right choice of a fluorescent probe is essential for successful luminescence imaging and sensing and especially concerning in vivo and in vitro applications, the development of new classes have gained more and more attention in the last years. One of the most promising class are upconversion nanoparticles (UCNPs)-inorganic nanocrystals capable to convert near-infrared light in high energy radiation. In this review we will compare UCNPs with other fluorescent probes in terms of (a) the optical properties of the probes, such as their brightness, photostability and excitation wavelength; (b) their chemical properties such as the dispersibility, stability under experimental or physiological conditions, availability of chemical modification strategies for labelling; and (c) the potential toxicity and biocompatibility of the probe. Thereby we want to provide a better understanding of the advantages and drawbacks of UCNPs and address future challenges in the design of the nanocrystals.

Facile synthesis of Silicon quantum dot-Gadolinium: A potential fluorescent/T1-T2 multimodal imaging agent

Highly stable and multifunctional fluorescent quantum dots are particularly attractive in practical applications. Here, a new kind of ultra-small-sized silicon quantum dot-gadolinium (SiQD-Gd) was successfully fabricated by a newly-designed facile hydrothermal growth and chelating method. The obtained SiQD-Gd exhibited outstanding water dispersibility, stability and good fluorescent property with the quantum yield of 11.6%. SiQD-Gd displayed a low cytotoxicity in normal cell lines (HELF, HEK293F) and tumor cell lines (H1299, A549). Meanwhile, SiQD-Gd showed excellent magnetic resonance response with r₁ relaxation rate of 10.5 mmol L^-1·s^-1 and r₂ relaxation rate of 47.5 mmol L^-1·s^-1, which are 2.5 and 7.4 times enhanced comparing to that of the commercial MR agent Magnevist. In vivo studies showed significant contrast enhancement effect of its T₁- and T₂-weighted MR imaging. In addition, in vivo fluorescent imaging for mice and zebrafish indicated its potential applications in fluorescent tracking. Thus, the excellent multimodal imaging capacity and biocompatibility of SiQD-Gd make it a potential imaging agent for clinic applications.

Linear PVA–DTPA–Gd conjugate for magnetic resonance imaging

In this study, we report the preparation and characterization of the PVA–DTPA–Gd conjugate as a potential MRI contrast agent (CA). The r₁ value and the r₂/r₁ ratio were about 5.6 mM⁻¹ s⁻¹ and 1.31, respectively. In vitro toxicity studies not only demonstrated that the polymeric system possessed good biocompatibility, but also proved that the conjugate could be an attractive candidate for CA.

In this study, we report the preparation and characterization of the PVA–DTPA–Gd conjugate as a potential MRI contrast agent (CA).

A review on nanostructured carbon quantum dots and their applications in biotechnology, sensors, and chemiluminescence

Carbon quantum dots (CQDs) are a member of carbon nanostructures family which have received increasing attention for their photoluminescence (PL), physical and chemical stability and low toxicity. The classical semiconductor quantum dots (QDs) are semiconductor particles that are able to emit fluorescence by excitation. The CQDs is mainly referred to photoluminescent carbon nanoparticles less than 10 nm, with surface modification or functionalization. Contrary to other carbon nanostructures, CQDs can be synthesized and functionalized fast and easily. The fluorescence origin of the CQDs is a controversial issue which depends on carbon source, experimental conditions, and functional groups. However, PL emissions originated from conjugated π-domains and surface defects have been proposed for the PL emission mechanisms of the CQDs. These nanostructures have been used as nontoxic alternatives to the classical heavy metals containing semiconductor QDs in some applications such as in-vivo and in-vitro bio-imaging, drug delivery, photosensors, chemiluminescence (CL), and etc. This paper will introduce CQDs, their structure, and PL characteristics. Recent advances of the application of CQDs in biotechnology, sensors, and CL is comprehensively discussed.

Mechanochemical Green Synthesis of Exfoliated Edge-Functionalized Boron Nitride Quantum Dots: Application to Vitamin C Sensing through Hybridization with Gold Electrodes

Two-dimensional boron nitride quantum dots (2D BNQDs) with excellent chemical stability, high photoluminescence efficiency, and low toxicity are a new class of advanced materials for biosensing and bioimaging applications. To overcome the current challenge about the lack of facile, scalable, and reproducible synthesis approach of BNQDs, we introduce a green and facile approach based on mechanochemical exfoliation of bulk h-BN particles in ethanol. Few-layered hydroxylated-functionalized QDs with a thickness of 1-2 nm and a lateral dimension of 2-6 nm have been prepared. The synthesized nanocrystals exhibit a strong fluorescence emission at 407 and 425 nm with a quantum efficiency of ∼6.2%. Spectroscopic analyses determine that interactions between oxygen groups of the solvent with boron sites occur, which along with the mechanical forces, lead to efficient exfoliation of the hexagonal structure and surface functionalization with -OH groups. We also demonstrate that the orbital interaction between BNQDs and the gold surface results in a profound electrochemical catalytic activity toward oxidation of vitamin C. It is shown that the BNQD-modified screen-printed gold electrode exhibits a decreased onset oxidation potential for about 0.37 V/AgCl. In addition to high catalytic activity, electrochemical studies also reveal that this electrode allows selective and sensitive detection of vitamin C with a good response over a wide range from 0.80 μM to 5.0 mM with a detection limit of 0.45 μM (S/N = 3) and a sensitivity of 1.3 μA μM^-1 cm^-2. Finally, the potential application of the hybrid sensor for detecting vitamin C in commercial drinks is demonstrated.

Gadolinium-Encapsulated Graphene Carbon Nanotheranostics for Imaging-Guided Photodynamic Therapy

Photosensitizers (PS) are an essential component of photodynamic therapy (PDT). Conventional PSs are often porphyrin derivatives, which are associated with high hydrophobicity, low quantum yield in aqueous solutions, and suboptimal tumor-to-normal-tissue (T/N) selectivity. There have been extensive efforts to load PSs into nanoparticle carriers to improve pharmacokinetics. The approach, however, is often limited by PS self-quenching, pre-mature release, and nanoparticle accumulation in the reticuloendothelial system organs. Herein, a novel, nanoparticle-based PS made of gadolinium-encapsulated graphene carbon nanoparticles (Gd@GCNs), which feature a high 1 O2 quantum yield, is reported. Meanwhile, Gd@GCNs afford strong fluorescence and high T1 relaxivity (16.0 × 10-3 m-1 s-1 , 7 T), making them an intrinsically dual-modal imaging probe. Having a size of approximately 5 nm, Gd@GCNs can accumulate in tumors through the enhanced permeability and retention effect. The unbound Gd@GCNs cause little toxicity because Gd is safely encapsulated within an inert carbon shell and because the particles are efficiently excreted from the host through renal clearance. Studies with rodent tumor models demonstrate the potential of the Gd@GCNs to mediate image-guided PDT for cancer treatment. Overall, the present study shows that Gd@GCNs possess unique physical, pharmaceutical, and toxicological properties and are an all-in-one nanotheranostic tool with substantial clinical translation potential.