Preparation of Gold-Carbon Dots and Ratiometric Fluorescence Cellular Imaging

Smartphone-assisted detection of methylene blue using leaf-derived, dual-emissive carbon dots as ratiometric fluorescence probes

Specific assay for methylene blue (MB) in aquaculture is critically required to assess the safety of ecosystems and aquatic products. In this work, a facile method is demonstrated for ratiometric fluorescence determination of MB based on blue/red dual emissive carbon dots (b/r-CDs). b/r-CDs, derived from the leaves of Impatiens balsamina, show weak and strong emission peaks at 432 and 672 nm, respectively, and possess a high photoluminescence quantum yield of 35.7%. The red fluorescence of b/r-CDs at 672 nm, serving as the response signal, can be significantly quenched by MB due to the mechanism of inner filter effect, while the blue fluorescence at 432 nm is insensitive to MB, thus acting as the background reference. With addition of an increased amount of MB, the fluorescence color of b/r-CDs gradually changes from red to blue, which realizes the visual detection. The linear range and limit of detection are measured to be 0.05-10 μM and 15 nM, respectively. Notably, a portable smartphone sensing platform is built for quantitative detection of MB by analyzing the RGB values. Moreover, the practical applications of b/r-CDs for monitoring MB in real water and aquatic products using both fluorescence spectrophotometer and smartphone achieve satisfactory results.

Fruit waste-derived carbon dots with rhodamine B for the ratiometric detection of Fe3+ and Cu2

A green and eco-friendly solvothermal approach is proposed for the synthesis of carbon quantum dots (CQDs) from watermelon rind. The as-prepared CQDs exhibited superior teal fluorescence in aqueous solutions, with a quantum yield of 13.9%. The CQDs and rhodamine B (RhB) were demonstrated to selectively react with Fe3+ and Cu2+, leading to a fluorescence (FL) quenching effect, which was successfully used for constructing "double-response-off" type ratiometric FL probes. A comparative study was conducted to assess the sensitivity and accuracy of ratiometric fluorescent probes, specifically those based on CQDs alone and in combination with RhB, for the selective detection of Fe3+ and Cu2+. By plotting the ratio of the differential fluorescence (ΔF) signals of CQDs to that of RhB against the practical application analyte concentration, the detection limits for Fe3+ (1.75 μM) and Cu2+ (0.43 μM) were markedly improved. The quenching mechanism was further explored, and the detection of Fe3+ and Cu2+ in surface water was demonstrated, showcasing the potential of efficient and effective nanosensors based on a static quenching effect. Futhermore, the addition of ascorbic acid can restore the fluorescence quenched by Fe3+. Therefore, in the presence of copper and iron, the ratiometric probe can demonstrate the ability to identify two different metals.

Carbon Dots Employed for the Detection of Ranitidine and Elaborating the Detecting Mechanism

Carbon dots (CDs) has been widely utilized in multiple fields, especially towards kinds of drug analyses, owing to its superior optical properties and satisfactory stability. Herein, we rapidly synthesized one kind of soluble bright-blue fluorescent CDs through a facile microwave method, while disodium ethylenediaminetetraacetic acid and phosphoric acid served as the raw materials. Importantly, introducing ranitidine into these CDs resulted in its decreased fluorescence, and thus an innovative method of detecting ranitidine was successfully established, which showed the favorable selectivity and anti-interference ability. With the optimal conditions, the standard curve diagram of F0/F against concentration of ranitidine was linear in the range of 6-2000 µM with a correlation coefficient of 0.9833, and the limit of detection (LOD) was calculated to be 4.2 µM. Meanwhile, we also explored the detecting mechanism of ranitidine by CDs, and elaborated that as the internal filtration effect. Consequently, we may broaden the avenues of detecting ranitidine on the basis of CDs.

Light-Controlled Regulation of Dual-Enzyme Properties in YbGd-Carbon Quantum Dots Nano-Hybrid for Advanced Biosensing

Compared to nanozymes with single enzyme activity, those with multiple enzyme activities possess broader application potential due to their diversified enzymatic functionalities. However, the multienzyme nanozymes currently face challenges of interference among different enzymatic activities during practical applications. In this study, we report the synthesis of a light-responsive YbGd-carbon quantum dots nano-hybrid, termed YbGd-CDs, which exhibits controllable enzyme-mimicking activities. This light-responsive behavior enables selective control of the enzymatic activities. Under visible light irradiation, YbGd-CDs demonstrate robust oxidase-like activity. Conversely, under dark conditions, they primarily exhibit peroxidase-like activity. Leveraging the dual-enzyme-mimicking capabilities of YbGd-CDs, we developed colorimetric assays for sensitive detection of total antioxidant capacity (TAC) in both normal and cancer cells as well as d-amino acids in human saliva. This study not only advances the synthesis of carbon-based nanozymes but also highlights their potential in biosensing applications.

Water-soluble silver nanoclusters with multicolor fluorescence generated by dialdehyde nanofibrillated cellulose for biological imaging

Water-soluble silver nanoclusters (AgNCs) as a new type of fluorescent material have attracted much attention for their remarkable optical properties and excellent cytocompatibility. However, it is still challenging to synthesize water-soluble AgNCs with good cytocompatibility and excellent fluorescence. Herein, the dialdehyde nanofibrillated cellulose (DANFC)- reduced water-soluble AgNCs capped by glutathione (GSH) with tunable fluorescence emissions were first reported. The DANFC provides a mild reduction environment and crystal growth system for the coordination between silver ions and GSH compared to conventional methods using strong reducing agents. The AgNCs with intense red fluorescence (R-AgNCs@GSH, size ∼2.24 nm) and green fluorescence (G-AgNCs@GSH, size ∼1.93 nm) were produced by varying the ratios of silver sources and ligands, and could maintain stable fluorescence intensity over 6 months. Moreover, the CCK-8 study demonstrated that the R-AgNCs@GSH and G-AgNCs@GSH reduced by DANFC of excellent cytocompatibility (cell viability >90 %) and enable precise multicolor intracellular imaging of Hela cells in 1 h. This work proposes a novel method to synthesize water-soluble AgNCs with tunable fluorescence emission at room temperature based on the classical silver- mirror reaction (SMR) using DANFC as reducing agent, and the synthesized fluorescent AgNCs have great potential as novel luminescent nanomaterials in biological research.

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.

An insight into recent developments of copper, silver and gold carbon dots: cancer diagnostics and treatment

Cancer is one of the most fatal diseases globally, however, advancement in the field of nanoscience specifically novel nanomaterials with nano-targeting of cancer cell lines has revolutionized cancer diagnosis and therapy and has thus attracted the attention of researchers of related fields. Carbon Dots (CDs)-C-based nanomaterials-have emerged as highly favorable candidates for simultaneous bioimaging and therapy during cancer nano-theranostics due to their exclusive innate FL and theranostic characteristics exhibited in different preclinical results. Recently, different transition metal-doped CDs have enhanced the effectiveness of CDs manifold in biomedical applications with minimum toxicity. The use of group-11 (Cu, Ag and Au) with CDs in this direction have recently gained the attention of researchers because of their encouraging results. This review summarizes the current developments of group-11 (Cu, Ag and Au) CDs for early diagnosis and therapy of cancer including their nanocomposites, nanohybrids and heterostructures etc. All The manuscript highlights imaging applications (FL, photoacoustic, MRI etc.) and therapeutic applications (phototherapy, photodynamic, multimodal etc.) of Cu-, Ag- and Au-doped CDs reported as nanotheranostic agents for cancer treatment. Sources of CDs and metals alogwith applications to give a comparative analysis have been given in the tabulated form at the end of manuscript. Further, future prospects and challenges have also been discussed.

Multifunctional Gold-Silver-Carbon Quantum Dots Nano-Hybrid Composite: Advancing Antibacterial Wound Healing and Cell Proliferation

The urgent need for innovative materials that effectively eliminate bacteria while promoting cell growth to accelerate wound healing has led to the exploration of new options, as current antimicrobial nanoparticles often exhibit high cytotoxicity, which hinders wound closure. In this study, a nano-hybrid composite, named gold-silver-carbon quantum dots (AuAg-CDs), was prepared by embedding gold and silver nanoclusters into carbon dots. The AuAg-CDs nano-hybrid composite demonstrates remarkable biocompatibility, displays potent antibacterial activity, and possesses a unique capability to promote cell proliferation. By physically disrupting bacterial membranes and promoting mammalian cell proliferation, this composite emerges as a highly promising material for wound healing applications. The underlying mechanism of the multifunctional AuAg-CDs was investigated through comprehensive analyses encompassing cell morphology, bacterial membrane potential, levels of reactive oxygen species (ROS), and adenosine triphosphate (ATP) production in both bacterial and mammalian cells. Additionally, AuAg-CDs were incorporated into alginate to create a hydrogel wound dressing, which underwent evaluation using animal models. The results underscore the remarkable potential of the AuAg-CDs wound dressing in facilitating the proliferation of wound fibroblasts and combating bacterial infections. The significance of designing multifunctional nanomaterials to address the challenges associated with pathogenic bacterial infections and regenerative medicine is highlighted by this study, paving the way for future advancements in these fields.

Encapsulating functionalized graphene quantum dot into metal-organic framework as a ratiometric fluorescent nanoprobe for doxycycline sensing

A distinctive fluorescent nanoprobe with the function of doxycycline identification was designed by encapsulating histidine and serine-functionalized graphene quantum dots (His-GQDs-Ser) into the luminescent metal-organic frameworks (MOF). The synthesized nanoprobe displayed the merits of prominent selectivity, wide detection range, and high sensitivity. The interaction of doxycycline and the fabricated fluorescent nanoprobe contributed to the phenomenon of the suppression of the fluorescence of the His-GQDs-Ser and enhancement of the MOF fluorescence. Linear relation between the concentration of doxycycline and the ratio fluorescence intensity of the nanoprobe was observed, which evidenced the brilliant capability in the ranges 0.003-6.25 μM and 6.25-25 μM with a detection limit of 1.8 nM. Additionally, the practicability of the probe was verified in analysis of spiked milk sample, and the satisfactory recoveries of doxycycline varied from 97.39 to 103.61%, with relative standard deviations in the range 0.62-1.42%. A proportional fluorescence sensor for doxycycline detection in standard solution was constructed, which provides a potential for the development of other fluorescence detection systems.

Development and application of naturally derived, cost-effective CQDs with cancer targeting potential

Carbon quantum dots (CQDs) derived from natural sources have obtained potential interest in biomedical imaging and therapy because of their excellent biocompatibility properties, which include water solubility, simple synthesis and low cytotoxicity. Here the cytotoxicity of ethylene-diamine doped carbon quantum dots (N-CQDs) delivered to breast cancer MCF-7 cells was investigated. Folic acid was used to raise folate recognition and increase FA-NCQD accumulation in the cells, then apoptosis was assayed using nuclear fragmentation, acridine orange labeling, fluorescence imaging, flow cytometry, and caspase 3 expression. The data show that functionalization of these CQDs, derived from a natural source, have potential application in eliminating cancer cells, as shown here for the invasive breast cancer cells, MCF-7. This nano-delivery system provides a novel target therapy possibility therapeutic approach for cancer cells.

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.

Fighting Non-Small Lung Cancer Cells Using Optimal Functionalization of Targeted Carbon Quantum Dots Derived from Natural Sources Might Provide Potential Therapeutic and Cancer Bio Image Strategies

Non-small cell lung cancer (NSCLC) is an important sub-type of lung cancer associated with poor diagnosis and therapy. Innovative multi-functional systems are urgently needed to overcome the invasiveness of NSCLC. Carbon quantum dots (CQDs) derived from natural sources have received interest for their potential in medical bio-imaging due to their unique properties, which are characterized by their water solubility, biocompatibility, simple synthesis, and low cytotoxicity. In the current study, ethylene-diamine doped CQDs enhanced their cytotoxicity (98 ± 0.4%, 97 ± 0.38%, 95.8 ± 0.15%, 86 ± 0.15%, 12.5 ± 0.14%) compared to CQDs alone (99 ± 0.2%, 98 ± 1.7%, 96 ± 0.8%, 93 ± 0.38%, 91 ± 1.3%) at serial concentrations (0.1, 1, 10, 100, 1000 μg/mL). In order to increase their location in a specific tumor site, folic acid was used to raise their functional folate recognition. The apoptotic feature of A549 lung cells exposed to N-CQDs and FA-NCQDs was characterized by a light orange-red color under fluorescence microscopy. Additionally, much nuclear fragmentation and condensation were seen. Flow cytometry results showed that the percentage of cells in late apoptosis and necrosis increased significantly in treated cells to (19.7 ± 0.03%), (27.6 ± 0.06%) compared to untreated cells (4.6 ± 0.02%), (3.5 ± 0.02%), respectively. Additionally, cell cycle arrest showed a strong reduction in cell numbers in the S phase (14 ± 0.9%) compared to untreated cells (29 ± 0.5%). Caspase-3 levels were increased significantly in A549 exposed to N-CQDs (2.67 ± 0.2 ng/mL) and FA-NCQDs (3.43 ± 0.05 ng/mL) compared to untreated cells (0.34 ± 0.04 ng/mL). The functionalization of CQDs derived from natural sources has proven their potential application to fight off non-small lung cancer.

Fabrication of avidin-stabilized gold nanoclusters with dual emissions and their application in biosensing

Protein-stabilized gold nanoclusters (Prot-Au NCs) have been widely used in biosensing and cell imaging owing to their excellent optical properties and low biotoxicity. However, several Prot-Au NCs reported in the literature do not retain the biological role of the protein, which greatly limits their ability to directly detect biomarkers. This study demonstrated for the first time the successful synthesis of dual-function avidin-stabilized gold nanoclusters (Av–Au NCs) using a one-pot method. The resulting Av–Au NCs exhibited intense blue and red emissions under 374 nm excitation. Furthermore, the Av–Au NCs retained the native functionality of avidin to bind to biotin. When DNA strands modified with biotin at both ends (i.e., linker chains) were mixed with Av–Au NCs, large polymers were formed, indicating that Av–Au NCs could achieve fluorescence signal amplification by interacting with biotin. Taking advantage of the aforementioned properties, we constructed a novel enzyme-free fluorescent biosensor based on the Av–Au NCs-biotin system to detect DNA. The designed fluorescent biosensor could detect target DNA down to 0.043 nM, with a wide line range from 0.2 nM to 20 µM. Thus, these dual-functional Av–Au NCs were shown to be an excellent fluorescent material for biosensing.

Avidin-stabilized gold nanoclusters (Av–Au NCs) were synthesized for the first time by a water-bath method.

The synthesized Av–Au NCs not only exhibited intense blue and red emissions under 374 nm excitation, but also retained the native functionality of avidin to bind to biotin.

The fluorescent signal amplification system constructed by the interaction of Av–Au NCs with biotin was successfully applied to detect target DNA in vitro.

Solid-Phase Synthesis of Red Fluorescent Carbon Dots for the Dual-Mode Detection of Hexavalent Chromium and Cell Imaging

The excellent optical properties and biocompatibility of red fluorescence carbon dots (R-CDs) provide a new approach for the effective analysis of hexavalent chromium Cr(VI) in environmental and biological samples. However, the application of R-CDs is still limited by low yield and unfriendly synthesis route. In this study, we developed a new type of R-CDs based on a simple and green solid-phase preparation strategy. The synthesized R-CDs can emit bright red fluorescence with an emission wavelength of 625 nm and also have an obvious visible light absorption capacity. Furthermore, the absorption and fluorescence signals of the R-CDs aqueous solution are sensitive to Cr(VI), which is reflected in color change and fluorescence quenching. Based on that, a scanometric and fluorescent dual-mode analysis system for the rapid and accurate detection of Cr(VI) was established well within the limit of detection at 80 nM and 9.1 nM, respectively. The proposed methods also possess high specificity and were applied for the detection of Cr(VI) in real water samples. More importantly, the synthesized R-CDs with good biocompatibility were further successfully applied for visualizing intracellular Cr(VI) in Hela cells.

Combining metal nanoclusters and carbon nanomaterials: Opportunities and challenges in advanced nanohybrids

The development of functional materials with uniquely advanced properties lies at the core of nanoscience and nanotechnology. From the myriad possible combinations of organic and/or inorganic blocks, hybrids combining metal nanoclusters and carbon nanomaterials have emerged as highly attractive colloidal materials for imaging, sensing (optical and electrochemical) and catalysis, among other applications. While the metal nanoclusters provide extraordinary luminescent and electronic properties, the carbon nanomaterials (of zero, one or two dimensions) convey versatility, as well as unique interfacial, electronic, thermal, optical, and mechanical properties, which altogether can be put to use for the desired application. Herein, we present an overview of the field, for experts and non-experts, encompassing the basic properties of the building blocks, a systematic view of the chemical preparation routes and physicochemical properties of the hybrids, and a critical analysis of their ongoing and emerging applications. Challenges and opportunities, including directions towards green chemistry approaches, are also discussed.

Intelligently design primary aromatic amines derived carbon dots for optical dual-mode and smartphone imaging detection of nitrite based on specific diazo coupling

Primary aromatic amines derived carbon dots (PAA-CDs) with the protonated amino groups and high quantum yield of 46% were favorably obtained by one-step solvothermal treatment of m-phenylenediamine (m-PDA) in acidic environment. The interaction between the PAA-CDs and nitrite (NO₂^-) was inherited the characteristic reaction of m-PDA (a primary aromatic amine) and NO₂^-, resulting in strong fluorescence quenching and obvious absorption variation of the PAA-CDs. Meanwhile, a chromogenic reaction of diazo coupling can cause significant color changes. Hence, the PAA-CDs were developed for an optical dual-mode and smartphone imaging sensor for NO₂^- detection in the range of 3.0 ~ 40.0 μM with high selectivity, good sensitivity, and excellent anti-interference capability. A limit of detection (LOD) of 0.024 μM and 0.16 μM was implemented by fluorometry and colorimetry, respectively. For smartphone imaging colorimetry, the LODs of 0.46 μM (visible color) and 0.99 μM (fluorescence color) were acquired. More importantly, the established sensor has been successfully applied for the dynamic detection of NO₂^- in various food samples with the satisfying results. A smartphone imaging colorimetry method based on the CDs was firstly proposed to visually and quantitatively detect NO₂^-, which will broaden the application range of the CDs in food safety inspection.

A label-free electrochemical impedimetric DNA biosensor for genetically modified soybean detection based on gold carbon dots

A label-free electrochemical impedimetric biosensor was constructed based on gold carbon dots (GCDs) modified screen-printed carbon electrode for the detection of genetic modified (GM) soybean. The structure and property of GCDs were investigated. The GCDs can directly bind to single-stranded DNA probes through Au-thiol interaction and boost electric conductivity for the DNA sensor construction. The quantification of target DNA was monitored by the change of electron-transfer resistance (Ret) upon the DNA hybridization on sensor surface. Under the optimal conditions, the Ret response (vs. Ag reference electrode) increased with the logarithm of target DNA concentrations in a wide linear range of 1.0 × 10-7 - 1.0 × 10-13 M with a detection limit of 3.1 × 10-14 M (S/N = 3). It was also demonstrated that the proposed DNA sensor possessed high specificity for discriminating target DNA from mismatched sequences. Moreover, the developed biosensor was applied to detect SHZD32-1 in actual samples, and the results showed a good consistency with those obtained from the gel electrophoresis method. Compared with the previous reports for DNA detection, the label-free biosensor showed a comparatively simple platform due to elimination of complicated DNA labeling. Therefore, the proposed method showed great potential to be an alternative device for simple, sensitive, specific, and portable DNA sensor.

Broadened optical absorption, enhanced photoelectric conversion and ultrafast carrier dynamics of N, P co-doped carbon dots

Carbon dots (CDs) have attracted extensive attention for their unique properties and promising applications in many fields. Many efforts have been made to improve the optical and physicochemical properties of CDs using an atomic doping strategy; however, the photoelectric properties of CD-based devices have been less studied and the photocurrent density is far from satisfactory for practical operation. Deep understanding of the doping effects on the electronic structure and photophysical properties of CDs is fundamental and essential for effectively improving the optical and photoelectrical performance of CD-based devices. Here, we have synthesized nitrogen (N) and phosphorus (P) co-doped CDs (N, P-CDs) through a one-step hydrothermal approach, and systematically investigated the effects of P-dopants on the improved optical and photoelectric properties of N, P-CDs. The introduction of P atoms into N-CDs significantly changes the electronic structure and extends the absorption spectral region, enhancing the light-harvesting ability of N, P-CDs. Meanwhile, the regulated carrier dynamics have been investigated using time-resolved fluorescence and transient absorption spectroscopy. We found that the carrier recombination was decreased with introducing P atoms, and the photogenerated electrons in the higher excited states could be efficiently transferred to the lowest excited state. Moreover, the photocurrent density of N, P-CDs was increased by twelve times compared with that of N-CDs. Therefore, the effective doping of P atoms can significantly regulate the electronic structure, optical properties, carrier dynamics and photoelectric conversion of N, P-CDs. The achieved broadband light-harvesting, good photoelectric properties and photostability of the as-prepared N, P-CDs demonstrate an important example of P-doping to improve the optical and photoelectrical properties of CD-based devices.

Carbon dot composites for bioapplications: a review

Carbon dots (CDs) have received extensive attention in the last decade for their excellent optical, chemical and biological properties. In recent years, CD composites have also received significant attention due to their ability to improve the intrinsic properties and expand the application scope of CDs. In this article, the synthesis processes of four types of CD composites (metal-CD, nonmetallic inorganics-CD, and organics-CD as well as multi-components-CD composites) are systematically summarized first. Then the recent advancements in the bioapplications (bioimaging, drug delivery and biosensing) of these composites are also highlighted and discussed. Last, the current challenges and future trends of CD composites in biomedical fields are discussed.

Nanomaterial-Based Dual-Emission Ratiometric Fluorescent Sensors for Biosensing and Cell Imaging

Owing to the unique optophysical properties of nanomaterials and their self-calibration characteristics, nanomaterial-based (e.g., polymer dots (Pdots) quantum dots (QDs), silicon nanorods (SiNRs), and gold nanoparticle (AuNPs), etc.) ratiometric fluorescent sensors play an essential role in numerous biosensing and cell imaging applications. The dual-emission ratiometric fluorescence technique has the function of effective internal referencing, thereby avoiding the influence of various analyte-independent confounding factors. The sensitivity and precision of the detection can therefore be greatly improved. In this review, the recent progress in nanomaterial-based dual-emission ratiometric fluorescent biosensors is systematically summarized. First, we introduce two general design approaches for dual-emission ratiometric fluorescent sensors, involving ratiometric fluorescence with changes of one response signal and two reversible signals. Then, some recent typical examples of nanomaterial-based dual-emission ratiometric fluorescent biosensors are illustrated in detail. Finally, probable challenges and future outlooks for dual-emission ratiometric fluorescent nanosensors for biosensing and cell imaging are rationally discussed.

Logic-Gated Cell-Derived Nanovesicles via DNA-Based Smart Recognition Module

Engineering cell-derived nanovesicles with active-targeting ligands is an important strategy to enhance the targeting efficiency. However, the enhanced binding capability to targeting cells also leads to the binding with nontarget cells that share the same biomarkers. DNA-based logic gate is a kind of molecular system that responds to chemical inputs by generating output signals, and the relationship between the input and the output is based on a certain logic. Thus, the DNA-based logic gate could provide a new approach to improve the delivery efficiency of the nanovesicle. In this work, we developed a DNA logic-gated module that coupled two tumor cell-targeting factors (e.g., low pH and a tumor cell biomarker) in a Boolean manner. Immobilization of this module on the surface of the nanovesicle enables the nanovesicle to sense tumor cell-targeting factors and regard these cues as inputs AND logic gate. With the guide of DNA-based logic gate, gold carbon dots (GCDs) encapsulated within nanovesicles were delivered into target cells, and then the intracellular redox status variation was reflected by fluorescence change of GCDs. Overall, we developed DNA logic-gated nanovesicles that contract different targeting factors into a unique tag for target cells. This facile functionalization strategy can pave the way for constructing smart nanovesicles and would broaden their application in the field of precision medicine and personalized treatment.

Synthesis of Yellow-Fluorescent Carbon Nano-dots by Microplasma for Imaging and Photocatalytic Inactivation of Cancer Cells

In recent years, multifunctional nanoparticles with combined diagnostic and therapeutic functions show great promise in nanomedicine. In this study, we report the environmentally friendly synthesis of fluorescent carbon nano-dots such as carbon quantum dots (CQDs) by microplasma using o-phenylenediamine. The produced CQDs exhibited a wide absorption peaks at 380-500 nm and emitted bright yellow fluorescence with a peak at 550 nm. The CQDs were rapidly taken up by HeLa cancer cells. When excited under blue light, a bright yellow fluorescence signal and intense reactive oxygen species (ROS) were efficiently produced, enabling simultaneous fluorescent cancer cell imaging and photodynamic inactivation, with a 40% decrease in relative cell viability. Furthermore, about 98% cells were active after the incubation with 400 μg mL^-1 CQDs in the dark, which revealed the excellent biocompatibility of CQDs. Hence, the newly prepared CQDs are thus demonstrated to be materials which might be effective and safe to use for in vivo bioimaging and imaging-guided cancer therapy.

Synthesis of Yellow-Fluorescent Carbon Nano-dots by Microplasma for Imaging and Photocatalytic Inactivation of Cancer Cells

In recent years, multifunctional nanoparticles with combined diagnostic and therapeutic functions show great promise in nanomedicine. In this study, we report the environmentally friendly synthesis of fluorescent carbon nano-dots such as carbon quantum dots (CQDs) by microplasma using o-phenylenediamine. The produced CQDs exhibited a wide absorption peaks at 380-500 nm and emitted bright yellow fluorescence with a peak at 550 nm. The CQDs were rapidly taken up by HeLa cancer cells. When excited under blue light, a bright yellow fluorescence signal and intense reactive oxygen species (ROS) were efficiently produced, enabling simultaneous fluorescent cancer cell imaging and photodynamic inactivation, with a 40% decrease in relative cell viability. Furthermore, about 98% cells were active after the incubation with 400 μg mL^-1 CQDs in the dark, which revealed the excellent biocompatibility of CQDs. Hence, the newly prepared CQDs are thus demonstrated to be materials which might be effective and safe to use for in vivo bioimaging and imaging-guided cancer therapy.

Real-Time Imaging Redox Status in Biothiols and Ferric Metabolism of Cancer Cells in Ferroptosis Based on Switched Fluorescence Response of Gold Carbon Dots

Ferroptosis is an iron-dependent form of regulated cell death. In this study, a ratiometric fluorescent probe, gold carbon dots (GCDs) consisting of carbon skeleton and gold nanoclusters, was used for in situ imaging to monitor redox status in biothiols (glutathione and cysteine) and ferric metabolism of cancer cells in ferroptosis. The as-prepared GCDs can selectively respond to biothiols, interestingly, the fluorescence may be switched to sense ferric ions without interference by biothiols under proper conditions. The robust GCDs-probe exhibits excellent photobleaching resistance and can reversibly respond to intracellular biothiols/ferric ion with high temporal resolution. The 8 h real-time imaging of living cells was employed to track the fluctuation of biothiols, showing the change of redox status in ferroptosis. In addition, release of ferric ions in cells was monitored. The real-time imaging of depletion of biothiols and release of ferric ion in cells indicates the GCDs-probe can monitor how the ferroptosis regulates redox status in biothiols and ferric metabolism.

Sensitively detecting mTBI biomarker S100B by using peptide-modified ratiometric fluorescent C/AuNCs nanoprobe

Mild traumatic brain injury (mTBI) has become a tough nut in forensic science because of its minor damages but serious consequences. Utilizing biomarkers to diagnose mTBI has become a promising approach due to various shortcomings of traditional diagnostic methods. In this work, we developed a peptide-modified ratiometric fluorescent nanoprobe based on carbon dots (CDs) and gold nanoclusters (AuNCs) for the measurements of a pivotal biomarker S100B protein in the early diagnosis of mTBI. It has been found that florescence intensity of AuNCs at 580 nm was decreased as report signal while the florescence intensity of CDs was unchanged as reference signal in this sensing system when the surface modified peptide bind tightly with calcium-activated S100B. Under the optimized conditions, S100B concentration ranging from 0.03 to 1 μg/mL was successfully determined within 30 min, and the detection limit of 0.01 μg/mL was acquired through the standard rule (S/N = 3). Moreover, the detection of S100B in spiked blood samples were conducted with satisfactory recoveries. The as-prepared ratiometric fluorescent nanoprobe was proved to be a time-saving, convenient, and sensitive strategy, and it showed great prospects in the early diagnosis of mTBI in forensic practice.

Dual-emission carbon dots for ratiometric detection of Fe3+ ions and acid phosphatase

We have developed a simple and convenient route to prepare fluorescent carbon dots with dual emission peaks respectively at 470 and 570 nm. The prepared dual-emission carbon dots can be used for ratiometric detection of Fe3+ ions in the range from 0 to 50 μmol·L-1 with 0.8 μmol·L-1 detection limit based on the fluorescence quenching at 570 nm. The quenched fluorescence induced by Fe3+ ions could be recovered by pyrophosphate. We further used the carbon dots-Fe3+ ions-pyrophosphate mixed system for ratiometric detection of acid phosphatase in the range from 0.08 to 6.75 μg·mL-1 with 0.01 μg·mL-1 detection limit.

One-step synthesis of carbon dots for selective bacterial inactivation and bacterial differentiation

Novel carbon dots (CDs) were synthesized by a one-pot hydrothermal approach using ampicillin as a precursor, and the as-prepared CDs exhibited a high quantum yield (23%). The CDs were found to possess abundant surface functional groups, thus providing good permeability to the cell, and the antibacterial activity of CDs was evaluated. S. aureus and L. monocytogenes were selected as model bacteria, and our results showed that the CDs exhibited antibacterial activity against S. aureus and L. monocytogenes under visible light illumination, even at low concentrations. The antibacterial mechanism is believed to be the production of reactive oxygen species (ROS) from CDs under visible light irradiation, which attacked the bacterial cell membranes, resulting in the death of the bacteria. In addition, because of the multicolor fluorescence properties of CDs, staining of S. aureus and L. monocytogenes obtained multicolor fluorescence images at different excitation wavelengths. Based on these results, CDs are a promising candidate material for biological applications. Graphical abstract.

On-off-on fluorescent carbon dots from waste tea: Their properties, antioxidant and selective detection of CrO42-, Fe3+, ascorbic acid and L-cysteine in real samples

In this work, we reported an economical plant-based hydrothermal method for one-pot green synthesis of water-soluble carbon dots (Tea-CDs) by using waste tea extract as a carbon source. The synthesized Tea-CDs were characterized by UV-visible, fluorescence, FT-IR, TEM, XPS and XRD. The Tea-CDs were found to remove hydroxyl and superoxide anion radical in vitro. In addition, the Tea-CDs exhibited bright blue fluorescence under UV-light (λ_ex = 365 nm), and the fluorescence could be effectively quenched by CrO₄^2- and Fe³⁺ ions. Meanwhile, the fluorescence of Tea-CDs-CrO₄^2- and Tea-CDs-Fe³⁺ systems could be again easily recovered by ascorbic acid (AA) and L-cysteine (L-Cys). As an on-off-on fluorescent nano-sensor of the Tea-CDs, the sensitive detection of CrO₄^2-, Fe³⁺, AA and L-Cys were all performed, showing that the good linear relationships between fluorescence intensity of Tea-CDs and concentration of all testing samples. Finally, the sensors successfully detected CrO₄^2-, Fe³⁺, AA and L-Cys in commercially available real samples with satisfactory recovery ranges. The prepared sensors offer distinct advantages including low cost, simple handling, good sensitivity and high selectivity.

Yellow-emissive carbon dots as a fluorescent probe for chromium(VI)

The authors describe a one-step method for the preparation of yellow fluorescent carbon dots (CDs) starting from 4-aminoacetanilide hydrochloride and 4-acetamidobenzaldehyde. The CDs have excitation/emission peaks at 470/550 nm, good water solubility, salt-tolerance and photostability. Their fluorescence is quenched by hexavalent chromium [Cr(VI)] via static quenching. Fluorescence intensity drops linearly in the 1 to 400 μM Cr(VI) concentration range, and the limit of detection is 0.13 μM. This method is selective for Cr(VI) over potential metal ion interferences and was successfully applied to the detection of Cr(VI) in spiked water and biological tissue samples. Recoveries from spiked samples ranged from 97.7% to 103.8%. Graphical abstract Schematic presentation of (a) the preparation of the CD fluorescent probe and (b), the principle of Cr(VI) determination.

A Facile Synthesis of Highly Nitrogen-Doped Carbon Dots for Imaging and Detection in Biological Samples

A facile, green, and high-output hydrothermal synthesis was proposed for the fabrication of highly fluorescent nitrogen-doped carbon quantum dots (N-doped CDs). The nitrogen content in N-doped CDs reached 19.2% and demonstrated strong blue fluorescence emission was obtained with fluorescence quantum yield (QY) of up to 32.9%, which exhibit high fluorescence quantum yield, high photostability, and excellent biocompatibility. The N-doped CDs possess high photostability, low toxicity, and excellent biocompatibility, based on which the N-doped CDs were successfully applied as a fluorescence probe for cell imaging. Moreover, it was then successfully demonstrated for sensitive and selective detection of Fe3+ in serum.

Transition Metal Ion (Mn2+, Fe2+, Co2+, and Ni2+)-Doped Carbon Dots Synthesized via Microwave-Assisted Pyrolysis: A Potential Nanoprobe for Magneto-fluorescent Dual-Modality Bioimaging

Heteroatom-doped carbon dots (C-dots) have captured widespread research interest owing to high fluorescence and biocompatibility for multimodal bioimaging applications. Here, we exemplify a rapid, facile synthesis of ethylenediamine (EDA)-functionalized transition metal ion (Mn²⁺, Fe²⁺, Co²⁺, and Ni²⁺)-doped C-dots via one-pot microwave (MW)-assisted pyrolysis at 800 W within 6 min using Citrus limon (lemon) extract as a carbon source. During MW pyrolysis, the precursor extract undergoes simultaneous carbonization and doping of metal ions onto C-dot surfaces in the presence of EDA. The EDA-functionalized transition metal ion-doped C-dots (i.e., Mn/C, Fe/C, Co/C, and Ni/C-dots) are collectively termed as TMCDs. The water-soluble TMCDs exhibited a size of 3.2 ± 0.485 nm and were enriched with amino and oxo functionalities and corresponding metal-oxide traces on the surfaces, as revealed from Fourier transfer infrared and X-ray photoelectron spectroscopy analyses. Interestingly, TMCDs demonstrated excitation-wavelength-dependent emission with brighter photoluminescence (PL) at 460 nm. Compared to pristine C-dots with a PL quantum yield (QY) of 48.31% and a fluorescence lifetime of 3.6 ns, the synthesized Mn/C, Fe/C, Co/C, and Ni/C-dots exhibited PL QY values of 35.71, 41.72, 75.07, and 50.84% as well as enhanced fluorescence lifetimes (τ_av) of 9.4, 8.6, 9.2, and 8.9 ns, respectively. The TMCDs significantly exhibited enhanced biocompatibility in human colon cancer cells (SW480) for fluorescence bioimaging and showed ferromagnetic and superparamagnetic behavior with vibrant T₁-contrast ability. Interestingly, the maximum longitudinal (r₁) relaxivity of 0.341 mM^-1 s^-1 was observed for Mn/C-dots in comparison to that of 3.1-3.5 mM^-1 s^-1 of clinically used Gd-DTPA magnetic resonance (MR)-contrast agent in vitro (1.5 T). Similarly, the maximum longitudinal relaxivity (r₁) of 0.356 mM^-1 s^-1 was observed for Ni/C-dots (1.5 T) with respect to 4.16 ± 0.02 mM^-1 s^-1 attained for Gd-DTPA in vivo (8.45 T). Thus, the rapid, energy-efficient MW-assisted pyrolysis presents lemon extract derived, EDA-functionalized TMCDs with enhanced PL and efficient T₁ contrast as potential magneto-fluorescent nanoprobes for dual-modality bioimaging applications.

Gold-carbon dots for the intracellular imaging of cancer-derived exosomes

As a novel fluorescent nanomaterial, gold-carbon quantum dots (GCDs) possess high biocompatibility and can be easily synthesized by a microwave-assisted method. Owing to their small sizes and unique optical properties, GCDs can be applied to imaging of biological targets, such as cells, exosomes and other organelles. In this study, GCDs were used for fluorescence imaging of exosomes. Tumor-specific antibodies are attached to the GCDs, forming exosome specific nanoprobes. The nanoprobes can label exosomes via immuno-reactions and thus facilitate fluorescent imaging of exosomes. When incubated with live cells, exosomes labeled with the nanoprobes can be taken up by the cells. The intracellular experiments confirmed that the majority of exosomes were endocytosed by cells and transported to lysosomes. The manner by which exosomes were taken up and the intracellular distribution of exosomes are unaffected by the GCDs. The experimental results successfully demonstrated that the presented nanoprobe can be used to study the intrinsic intracellular behavior of tumor derived exosomes. We believe that the GCDs based nanoprobe holds a great promise in the study of exosome related cellular events, such as cancer metastasis.

Ratiometric optical nanoprobes enable accurate molecular detection and imaging

Exploring and understanding biological and pathological changes are of great significance for early diagnosis and therapy of diseases. Optical sensing and imaging approaches have experienced major progress in this field. Particularly, an emergence of various functional optical nanoprobes has provided enhanced sensitivity, specificity, targeting ability, as well as multiplexing and multimodal capabilities due to improvements in their intrinsic physicochemical and optical properties. However, one of the biggest challenges of conventional optical nanoprobes is their absolute intensity-dependent signal readout, which causes inaccurate sensing and imaging results due to the presence of various analyte-independent factors that can cause fluctuations in their absolute signal intensity. Ratiometric measurements provide built-in self-calibration for signal correction, enabling more sensitive and reliable detection. Optimizing nanoprobe designs with ratiometric strategies can surmount many of the limitations encountered by traditional optical nanoprobes. This review first elaborates upon existing optical nanoprobes that exploit ratiometric measurements for improved sensing and imaging, including fluorescence, surface enhanced Raman scattering (SERS), and photoacoustic nanoprobes. Next, a thorough discussion is provided on design strategies for these nanoprobes, and their potential biomedical applications for targeting specific biomolecule populations (e.g. cancer biomarkers and small molecules with physiological relevance), for imaging the tumor microenvironment (e.g. pH, reactive oxygen species, hypoxia, enzyme and metal ions), as well as for intraoperative image guidance of tumor-resection procedures.

Fluorescent hydrogel waveguide for on-site detection of heavy metal ions

Hydrogels have great applications in tissue engineering and drug delivery. Recently, there have been intense interests in developments and applications of nanocomposite hydrogels by incorporating nanomaterials into polymer matrix, which endows the hydrogels with new functionalities. Here, we report on the first carbon dots (CDs) doped hydrogel waveguide for selective, on-site detection of heavy metal ions in aqueous solutions. The CDs-doped hydrogel waveguide exhibits efficient light confinement in water due to the refractive index contrast. The smooth waveguide surfaces lead to low light scattering loss. Real-time spectra measurement of the CDs-doped hydrogel waveguide with a compact interrogation setup demonstrates that the novel design can be used as a portable, robust sensing platform for on-site analysis and assessment of heavy metal ions.

Terbium ion-coordinated carbon dots for fluorescent aptasensing of adenosine 5'-triphosphate with unmodified gold nanoparticles

This work reports on a novel time-resolved fluorescent aptasensing platform for the quantitative monitoring of adenosine 5'-triphosphate (ATP) by interaction of dispersive/agglomerate gold nanoparticles (AuNPs) with terbium ion-coordinated carbon dots (Tb-CDs). To construct such a fluorescent nanoprobe, Tb-CDs with high-efficient fluorescent intensity are first synthesized by the microwave method with terbium ions (Tb(3+)). The aptasensing system consists of ATP aptamer, AuNP and Tb-CD. The dispersive/agglomerate gold nanoparticles are acquired through the reaction of the aptamer with target ATP. Upon target ATP introduction, the aptamers bind with the analytes to form new aptamer-ATP complexes and coat on the surface of AuNPs to inhibit their aggregation in the high salt solution. In this case, the fluorescent signal of Tb-CDs is quenched by the dispersive AuNPs on the basis of the fluorescence resonance energy transfer (FRET). At the absence of target analyte, gold nanoparticles tend to aggregate in the high salt state even if the aptamers are present. Thus, the added Tb-CDs maintain their intrinsic fluorescent intensity. Experimental results indicated that the aptasensing system exhibited good fluorescent responses toward ATP in the dynamic range from 40nM to 4.0μM with a detection limit of 8.5nM at 3sblank criterion. The repeatability and intermediate precision is less than 9.5% at three concentrations including 0.04, 0.4 and 2.0μM ATP. The selectivity was acceptable toward guanosine 5'-triphosphate, uridine 5'-triphosphate and cytidine 5'-triphosphate. The methodology was applied to evaluate the blank human serum spiked with target ATP, and the recoveries (at 3 concentration levels) ranged between 97.0% and 103.7%. Importantly, this detection scheme is rapid, simple, cost-effective, and does not require extensive sample preparation or separation.

One-pot microwave-assisted in situ reduction of Ag+and Au3+ions by Citrus limon extract and their carbon-dots based nanohybrids: a potential nano-bioprobe for cancer cellular imaging

In situreduction and microwave-assisted synthesis of Ag/C-dot and Au/C-dot nanohybrids using natural lemon extract for cancer cellular imaging applications.