Preparation of Yellow-Green-Emissive Carbon Dots and Their Application in Constructing a Fluorescent Turn-On Nanoprobe for Imaging of Selenol in Living Cells

Design of Fluorescence Enhancing Sensor for Mercury Detection via Bamboo Cellulose-Derived Carbon Dots

Mercury contamination of the environment is extremely hazardous to human health because of its significant toxicity, especially in water. Biomass-derived fluorophores such as carbon dots (CDs) have emerged as eco-friendly and cost-effective alternative sensors that provide comparable efficacy while mitigating the environmental and economic drawbacks of conventional methods. In this work, we report the fabrication of a selective fluorescence-enhancing sensor based on sulfur-doped carbon dots (SCDs) using waste bamboo-derived cellulose and sodium thiosulfate as the soft base dopant, which actively complexes with mercury ions for detection. SCDs with an average size of 4 nm were synthesized hydrothermally, and the sulfur doping was confirmed quantitatively with an atomic percentage of 6.5%. Optical studies reveal an abnormal fluorescence enhancement of SCD in the presence of mercury due to the aggregation of carbon dots via sulfur-containing functional groups. The fabricated sensor exhibits a low detection limit of 5.16 nM, suggesting its application potential as a reliable mercury sensor. Real-time analyses carried out using tap water samples spiked with mercury and industry samples showed high efficiency for Hg(II) detection. The sensing performance was also demonstrated by using SCD-coated filter paper strips.

Luminescent Carbon Dots From Biomass Waste for the Sensitive Determination of Ascorbic Acid in Tablet Dosage Forms

Affordable and eco-friendly green spectrofluorometric (FL) methods can enhance the safety and cost-effectiveness of quality assurance and control in ascorbic acid (ASA) formulations. However, most current techniques for ASA analysis have faced challenges like complexity, delayed response times, low throughput, time-consuming procedures, and requirements for expensive equipment and hazardous chemicals for analyte modification. The study is aimed at producing natural carbon quantum dots (NACQDs) from pumpkin seed peels (PSPs), a natural waste material, using a rapid microwave-assisted method. A variety of techniques, including transmission electron microscopy, scanning electron microscopy, energy-dispersive x-ray spectroscopy, and Fourier-transform infrared spectroscopy, were employed to characterize the PSP-based NACQDs. The NACQDs were used as probes for FL analysis of ASA, where the addition of ASA caused fluorescence quenching of the NACQDs. The developed method demonstrated good linearity (r = 0.996), sensitivity, accuracy (percentage recovery ranging from 99.36% to 100.35%), and precision (%RSD less than 0.21%) in the quantification of ASA in the range of 0.3-15 μg/mL. The method's LOD and LOQ values were 0.1 and 0.3 μg/mL, respectively. The successful analysis of ASA in tablet formulations demonstrated the practicality of the proposed method. Greenness assessment tools highlighted its superior eco-friendliness compared to reference techniques.

Multifunctional Fluorescent Material Based Red-emitting Carbon Dots for Cell Imaging and Photodynamic Sterilization

In this paper, a new carbon dot (R1-CDs) was prepared by one-pot hydrothermal method by using 1,8-diaminonaphthalene and o-phthalic acid (o-PA) as precursors. Due to the high purity of R1-CDs, NMR analysis was performed to identify the types of H and C atoms in their graphene sheets. From our research findings, three important information was disclosed such as (1) five types H atoms are presented in R1-CDs; (2) 18 kinds of C atoms in the graphene sheets are observed, and 8 kinds of them are quaternary atoms, and 10 kinds of carbon atoms as tertiary one; (3) functional groups of -COOH and -NH2 from precursors cannot be inherited into the edges or defect sites of graphene sheet. Obviously, our research findings for the first time revealed the more details of chemical structures of CDs. We believe that our works can supply a general concept to fabricate CDs by selecting specific precursors, also can encourage CDs' development in a more "chemistry" way by employing NMR as a powerful method.

A photo-controlled fluorescent switching based on carbon dots and photochromic diarylethene for bioimaging

Carbon dots (CDs) as luminescent zero-dimensional carbon nanomaterials have good aqueous dissolution, photostability, high quantum yield, and tunability of emission color. It has great application potential in many fields, including bioimaging, labeling of biological species, drug delivery, and sensing in biomedical. However, controlling the fluorescence emission of carbon dots remains a formidable challenge. Herein, we designed and exploited a photo-controlled fluorescent switching based on photochromic diarylethene (DT) and CDs for bioimaging. It could be modulated reversibly between "ON" and "OFF" under UV/vis light exposure. The fluorescent modulation efficiency was as high as 95.3%. The fluorescent switching could be used to the bioimaging in HeLa cells with low cell toxicity. Therefore, this fluorescent switching could be a promising candidate in many potential application areas, especially in bioimaging.

Rational N,P-Codoped pH-Activatable Red Carbon Dot for In Vitro and In Vivo Tumor Imaging

Tumor detection and imaging via tumor microenvironmental indicators can have practical value. Here, a low-pH-responsive red carbon dot (CD) was prepared via a hydrothermal reaction for specific tumor imaging in vitro and in vivo. The probe responded to the acidic tumor microenvironment. The CDs are codoped by nitrogen and phosphorene and contain anilines on the surface. These anilines are efficient electron donors and modulate the pH response: Fluorescence is undetectable at common physical pH (>7.0), but red fluorescence (600-720 nm) increases with decreasing pH. The inactivation of fluorescence is due to three aspects: photoinduced electron transfer from anilines, deprotonation-induced energy states changing, and particle aggregation-induced quenching. It is believed that this pH-responsive character of CD is better than other reported CDs. Thus, in vitro images of HeLa cells show strong fluorescence that is 4-fold higher than normal cells. Subsequently, the CDs are used for in vivo imaging of tumors in mice. Tumors can be clearly observed within 1 h, and clearance of CDs will be finished within 24 h due to the small size of the CDs. The CDs offer excellent tumor-to-normal tissue (T/N) ratios and have great potential for biomedical research and disease diagnosis.

Recent progress in the development of small-molecule fluorescent probes for detection and imaging of selenocysteine and application in thyroid disease diagnosis

Selenocysteine (SeCys) is the 21st genetically encoded amino acid present in proteins and is involved in various biological functions. Inappropriate levels of SeCys can be considered as a sign of various diseases. Therefore, small molecular fluorescent probes for the detection and imaging of SeCys in vivo in biological systems are considered to be of significant interest for understanding the physiological role of SeCys. Thus, this article mainly provides a critical evaluation of recent advances made in SeCys detection along with the biomedical applications based on small molecular fluorescent probes published in the literature during the past half a dozen years. Therefore, the article primarily deals with the rational design of fluorescent probes, wherein these were selective towards SeCys over other biologically abundant molecules, in particular the thiol-based ones. The detection has been monitored by different spectral techniques, such as fluorescence and absorption spectroscopy and in some cases even visual color changes. Further, the detection mechanism and the utility of fluorescent probes for in vitro and in vivo cell imaging applications are addressed. For clarity, the main features have been conveniently divided into four categories based on the chemical reactions of the probe, viz., in terms of the cleavage of the responsive group by the SeCys nucleophile: (i) 2,4-dinitrobene sulphonamide group, (ii) 2,4-dinitrobenesulfonate ester group, (iii) 2,4-dinitrobenzeneoxy group and (iv) miscellaneous types. Overall this article deals with the analysis of more than two dozen fluorescent probes demonstrated for selective detection of SeCys along with their applications towards disease diagnosis.

Ratiometric Sensing of Intracellular pH Based on Dual Emissive Carbon Dots

Accurate monitoring of intracellular pH in living cells is critical for developing a better understanding of cellular activities. In the current study, label-free carbon dots (p-CDs), which were fabricated using a straightforward one-pot solvothermal treatment of p-phenylenediamine and urea, were employed to create a new ratiometric pH nanosensor. Under single-wavelength excitation (λ_ex = 500 nm), the p-CDs gave dual emission bands at 525 and 623 nm. The fluorescent intensity ratio (I₅₂₅/I₆₂₃) was linearly related to pH over the range 4.0 to 8.8 in buffer solutions, indicating that the ratiometric fluorescence nanoprobe may be useful for pH sensing. In pH measurements, the p-CDs also demonstrated outstanding selectivity, reversibility, and photostability. Owing to the advantages outlined above, the nanoprobe was used to monitor the pH of HeLa cells effectively. The label-free CD-based ratiometric nanoprobe features comparatively easy manufacturing and longer excitation and emission wavelengths than the majority of previously reported CD-based ratiometric pH sensors, which is ultimately beneficial for applications in biological imaging.

Highly efficient red-emitting carbon dots as a "turn-on" temperature probe in living cells

Nanothermometers, which can precisely detect the intracellular temperature changes, have great potential to solve questions concerning the cellular processes. Thus, the temperature sensors that provide fluorescent "turn-on" signals in the biological transparency window are of highly desirable. To meet these criteria, this work reported a new "turn-on" carbon dot (CD)-based fluorescent nanothermometry device for sensing temperature in living cells. The CDs that emit bright red fluorescence (R-CDs; λ_max = 610 nm in water) were synthesized with o-phenylenediamine as carbon precursor via a facile solvothermal method. The R-CDs in water were almost nonfluorescent at 15 °C. As the temperature increased, the fluorescence intensity of R-CDs exhibited a gradual increase and the final enhancement factor was greater than 21-fold. The fluorescence intensity exhibited a linear response to temperature and a high-sensitive variation of ≈13.3 % °C^-1 was detected within a broad temperature range of 28-60 °C. Moreover, the R-CD thermal sensors also exhibited high storage stability, excellent response reversibility and superior photo- and thermo-stability. Due to its good biocompatibility and "intelligent" response to external temperature, the nanothermometer could be applied for sensing temperature changes in biological media.

Synthesis of N-doped carbon dots for highly selective and sensitive detection of metronidazole in real samples and its cytotoxicity studies

The current investigation reports the synthesis of N-CDs using glucosamine, ascorbic acid, and ethylenediamine precursors by a simple hydrothermal technique. The formation of N-CDs was proved by various characterisation techniques such as X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), and Fourier-Transform Infrared Spectrophotometer (FT-IR). The optical properties were investigated by fluorescence and UV-vis spectrophotometer. Also, N-CDs showed high selectivity in detecting the MTZ compared to several other analytes. However, the metronidazole serves as an antibiotic against several microbial diseases but also a genotoxic, carcinogenic to the human when used in excessive dosage. The synthesised N-CDs showed high selectivity in detecting the MTZ compared to several other analytes. Besides, the cytotoxicity of the N-CDs was studied to evaluate its toxicity against the HeLa cancer cells. It showed 65.6% cell viability and 34.3% toxicity against the cancerous cells, and similarly 71% of cells viability against H9C2 cells. Thus, the current investigation explores the promising selective sensing of N-CDs against MTZ, along with that, it proved its cytotoxicity against HeLa cancerous cells and non-toxicity against H9C2 cells. The synthesised CDs can be better MTZ sensors and anti-cancer agents on further development at the industrial scale.

Synthesis and application of novel N, Si-carbon dots for the ratiometric fluorescent monitoring of the antibiotic balofloxacin in tablets and serum

A ratiometric fluorescent probe with blue-emission fluorescence based on N, Si-doped carbon dots (N, Si-CDs) for the detection of balofloxacin (BLFX) was synthesized by simple one-pot hydrothermal carbonization using methotrexate and 3-aminopropyltriethoxysilane (APTES) as carbon materials. The obtained N, Si-CDs showed dual-emission band fluorescence characterization at 374 nm and 466 nm. Furthermore, the synthesized N, Si-CD probe exhibited evidence of ratiometric fluorescence emission characteristics (F 466/F 374) toward BLFX along with a decrease in fluorescence intensity at 374 nm and an increase in fluorescence intensity at 466 nm. Based on this probe, a highly sensitive and fast detection method for the analysis of BLFX has been established with a linear range of 1-60 μM and a low detection limit of 0.1874 μM, as well as a rapid response time of 5.0 s. The developed assay has also been successfully applied for the detection of BLFX in tablets and rat serum.

Oxidase-like ZnCoFe Three-Atom Nanozyme as a Colorimetric Platform for Ascorbic Acid Sensing

Great enthusiasm in single-atom catalysts for various catalytic reactions continues to heat up. However, the poor activity of the existing single/dual-metal-atom catalysts does not meet the actual requirement. In this scenario, the precise design of triple-metal-atom catalysts is vital but still challenging. Here, a triple-atom site catalyst of FeCoZn catalyst coordinated with S and N, which is doped in the carbon matrix (named FeCoZn-TAC/SNC), is designed. The FeCoZn catalyst can mimic the activity of oxidase by activating O₂ into ^•O₂^- radicals by virtue of its atomically dispersed metal active sites. Employing this characteristic, triple-atom catalysts can become a great driving force for the development of novel biosensors featuring adequate sensitivity. First, the property of FeCoZn catalyst as an oxidase-like nanozyme was explored. The obtained FeCoZn-TAC/SNC shows remarkably enhanced catalytic performance than that of FeCoZn-TAC/NC and single/dual-atom site catalysts (FeZn, CoZn, FeCo-DAC/NC and Fe, Zn, Co-SAC/NC) because of trimetallic sites, demonstrating the synergistic effect. Further, the utility of the oxidase-like FeCoZn-TAC/SNC in biosensor field is evaluated by the colorimetric sensing of ascorbic acid. The nanozyme sensor shows a wide concentration range from 0.01 to 90 μM and an excellent detection limit of 6.24 nM. The applicability of the nanozyme sensor in biologically relevant detection was further proved in serum. The implementation of TAC in colorimetric detection holds vast promise for further development of biomedical research and clinical diagnosis.

Construction of a photo-controlled fluorescent switching with diarylethene modified carbon dots

Photo-controlled fluorescent switching is of great utility in fluorescence sensors, reversible data storage, and logic circuit, based on their modifiable emission intensity and spectra. In this work, a novel photo-controlled reversible fluorescent switching system was constructed based on photochromic diarylethene (DT) molecular modified fluorescent carbon dots (CDs). The fluorescent CDs acted as fluorescent donors and the photochromic diarylethene molecular functioned as acceptors in this fluorescent switching system. The fluorescence modulation efficiency of the fluorescent switching was determined to be 97.1%. The result was attributable to Förster resonance energy transfer between the CDs and the diarylethene molecular. The fluorescent switching could undergo 20 cycles without significant decay.

ZIF-based carbon dots with lysosome-Golgi transport property as visualization platform for deep tumour therapy via hierarchical size/charge dual-transform and transcytosis

The poor penetration of nanomaterials in solid tumours and difficulty in monitoring their penetration depth are major obstacles in their application for the treatment of solid tumours. Herein, pH-responsive carbon dots (ZCD) based on a zeolitic imidazolate framework (ZIF-8) were fabricated to achieve the deep delivery of the chemotherapeutic doxorubicin (DOX) via a hierarchical size/charge dual-transformation and transcytosis. The as-prepared ZCD accumulated in the solid tumour and the acidic tumour microenvironment further triggered its decomposition. Firstly, ZCD was decomposed by the weakly acidic extracellular microenvironment of the solid tumour, enabling it to transform into small and neutrally charged particles. Subsequently, these particles were endocytosed by lysosomes, and further disintegrated into smaller and positively charged particles, which could target the Golgi apparatus. Consequently, ZCD delivered DOX deep into the solid tumour via a size-shrinking strategy and Golgi-mediated transcytosis, thus significantly improving its antitumour efficacy. In addition, carbonization endowed ZCD with superior fluorescence property, which was enhanced in the acidic microenvironment, thus improving the sensitivity and accuracy of ex vivo monitoring of the penetration depth of the nanomedicine in real time. Collectively, our results confirmed that the carbon dots obtained via the direct carbonization of ZIF-8 simultaneously exhibited enhanced deep penetration into solid tumours and fluorescence, which could be monitored, and that the carbonization of functional materials is effective to enhance their fluorescence, and further broaden their applications.

Rational construction of a new water soluble turn-on colorimetric and NIR fluorescent sensor for high selective Sec detection in Se-enriched foods and biosystems

As a naturally occurring amino acid, selenocysteine (Sec) plays a key role in a variety of cellular functions and Se-enriched foods. In this work, a robust water soluble fluorescence turn-on near-infrared (NIR) sensor NIR-Sec was constructed for Sec detection over biothiols in Se-enriched foods. Specifically, NIR-Sec contains a readily prepared water soluble NIR dicyanoisophorone fluorophore and a well-known response-site 2,4-dinitrobenzenesulfonyl moiety with strong intramolecular charge transfer (ICT) effect to quench the fluorescence intensity of NIR fluorophore. Upon addition of Sec, the NIR dicyanoisophorone fluorophore was released and a bright red emission at 663 nm was observed. Moreover, NIR-Sec toward Sec exhibited rapid response time (∼1 min), a large stoke shift (183 nm), and high selectivity and sensitivity (LOD: 52 nM). Impressively, NIR-Sec was successfully employed to detect and image Sec in Se-enriched foods and shrimp, indicating NIR-Sec could provide a robust tool for investigating the role of Sec in complex real-food samples.

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.

Carbon Dots: An Excellent Fluorescent Probe for Contaminant Sensing and Remediation

Pollution-induced degradation of the environment is a serious problem for both developing and developed countries. Existing remediation methods are restricted, necessitating the development of novel remediation technologies. Nanomaterials with unique characteristics have recently been developed for remediation. Quantum dots (QDs) are semiconductor nanoparticles (1-10 nm) with optical and electrical characteristics that differ from bigger particles owing to quantum mechanics, making them intriguing for sensing and remediation applications. Carbon dots (CDs) offer better characteristics than typical QDs, such as, CdSe QDs in terms of contaminant sensing and remediation. Non-toxicity, chemical inertness, photo-induced electron transfer, good biocompatibility, and adjustable photoluminescence behavior are all characteristics of CDs. CDs are frequently made from sustainable raw materials as they are cost-effective, environmentally compactable, and excellent in reducing waste generation. The goal of this review article is to briefly describe CDs fabrication methods, to deeply investigate the criteria and properties of CDs that make them suitable for sensing and remediation of contaminants, and also to highlight recent advances in their use in sensing and remediation of contaminants.

Facile one-step fabrication of Cu-doped carbon dots as a dual-selective biosensor for detection of pyrophosphate ions and measurement of pH

High-performance determination of pyrophosphate ions (PPi) and pH is an important goal in biological systems. In this work, Cu-doped carbon dots (Cu-CDs) were synthesized rapidly and simply via a one-pot hydrothermal method. The as-obtained Cu-CDs, with an average size of 2.55 nm, exhibit an excitation-independent fluorescence emission and possess desirable functional groups of carboxyl and amine, which can be served as fluorescence nanoprobes for detection of PPi based on surface passivation. Under the optimal condition, the linear range for detection of PPi was 0.05-20 µM, and the corresponding limit of detection (LOD) was 0.013 µM, indicative of a promising assay for the PPi. Moreover, the fluorescent intensity of the Cu-CDs is linear against pH value from 6 to 8.7 in buffer solution, suggesting the feasibility as a pH sensor. The synthesized Cu-CDs coated fluorescent paper indeed can monitor pH in urine with satisfaction by naked eyes through ultraviolet irradiation. The successful detection of PPi and the visual detection of pH value suggest a highly promising application of Cu-CDs in the field of biosensing.

Two-photon-excited tumor cell fluorescence targeted imaging based on transferrin-functionalized silicon nanoparticles

Transferrin-functionalized silicon nanoparticles (Trf-SiNPs) were fabricated and utilized for targeted fluorescence imaging in tumor cells. Silicon nanoparticles (SiNPs) was firstly synthesized by microwave irradiation method, and then coupled with transferrin in the presence of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS). The structural informations of Trf-SiNPs were measured by transmission electron microscope and Fourier transform infrared spectrometer. The optical properties of Trf-SiNPs were characterized by ultraviolet absorption spectrum, fluorescence emission spectrum, fluorescence quantum yield, fluorescence lifetime, photo-stability, and so on. MTT assay evidenced the low toxicity of Trf-SiNPs. Finally, Trf-SiNPs were successfully applied in HeLa cells and HepG2 cells for targeted fluorescence imaging under single-photon excitation and two-photon excitation.

Preparation of yellow emissive nitrogen-doped carbon dots from o-phenylenediamine and their application in curcumin sensing

In this work, o-phenylenediamine (o-PD) and ethanol are used as raw materials, and a simple solvothermal method is used to prepare yellow emissive nitrogen-doped CDs (YNCDs) (with yellow emission λex/λem = 410/555 nm).

A two-photon "turn-on" fluorescent probe for both exogenous and endogenous selenocysteine detection and imaging in living cells and zebrafish

Selenocysteine (Sec) is recognized as the 21st amino acid employing as an essential building block for selenoproteins (SePs), which plays a significant role in various physiological processes. Therefore, there is an urgent need to reasonably develop some reliable and rapid methods for Sec detection in biological systems. In this work, we reported a new two-photon (TP) fluorescent probe BNT-Sec for Sec detection and imaging in living cells and zebrafish with two part: (1) a D-π-A-structured naphthalene derivative as a TP fluorophore; (2) a well-know Sec responsive site with strong intromolecular charge transfer effect (ICT) to selectively detect endogenous and exogenous. In the presence of Sec, probe BNT-Sec can initiate a Se-dependent specific aromatic nucleophilic substitution reaction, which exhibited BNT-Sec had a large fluorescence intensity enhancement with ~18.9-fold at 510 nm, a high sensitivity low LOD value' 10.6 nM, good light stability, strong specificity, pH stability and low cytotoxicity. In addition, BNT-Sec can be conveniently used to detect Sec in living cells and zebrafish for TP imaging due to the great TP measurement properties of fluorophore, exhibiting it has the potential to reveal the role of selenocysteine in physiological and pathological processes in further biological applications.

Fluorescent nitrogen-doped carbon dots for high selective detecting p-nitrophenol through FRET mechanism

A facile, friendly and one-step hydrothermal protocol was used to synthesize nitrogen-doped carbon dots (N-CDs) by utilizing hexamethylenetetramine and ethanediamine as the carbon and nitrogen sources. It demonstrated good water solubility and fluorescence properties were stable, whether in acidic or alkaline. Quantum yield (QY) of N-CDs was 8.3% at an excitation wavelength of 325 nm with maximum emission at 425 nm. The fluorescence of N-CDs achieved very high fluorescence quenching of 60% in the detection of p-nitrophenol (p-NP) in aqueous medium via fluorescence resonance energy transfer (FRET) mechanisms. Under optimum conditions, fluorescence probs of N-CDs had strong selectivity to p-NP, and the fluorescence intensity was linearly proportional to p-NP concentration from 0.5 to 70.0 μM with a detection limit of 0.201 μM. The corresponding cell experiments were also performed, indicating that the prepared N-CDs possessed low cytotoxicity and good biocompatibility. Meanwhile, the N-CDs can be used for the determination of p-NP in river water and industrial wastewater.

Ratiometric detection of p-nitrophenol and its derivatives using a dual-emissive neuron cell-like carbonized probe based on a ππ stacking quenching mechanism

p-Nitrophenol and its derivatives can cause serious harm to the health of mankind and the earth's ecosystem. Therefore, it is necessary to develop a novel and rapid detection technology for p-nitrophenol and its derivative. Herein, excellent water-soluble, large-size and dual-emissive neuron cell-analogous carbon-based probes (NCNPs) have been prepared via a solvothermal approach, using o-phenylenediamine as the only precursor, which exhibit two distinctive fluorescence (FL) peaks at 420 and 555 nm under 345 nm excitation. The NCNPs show a neuron cell-like branched structure, are cross-connected, and are in the range of 10-20 nm in skeleton diameter. Interestingly, their blue-green dual-colour fluorescence is quenched by p-nitrophenol or its derivative due to the specific mechanism of the ππ stacking interactions or internal filtration effect. Accordingly, a simple, rapid, direct and free-label ratiometric FL detection of p-nitrophenol is proposed. An excellent linear relationship shows linear regions over the range of 0.1-50 μM between the ratio of the FL intensity (FL555 nm/FL420 nm) and the concentrations of p-nitrophenol. The detection limit is as low as 43 nM (3σ). Importantly, the NCNP-based probe also shows acceptable repeatability and reproducibility for the detection of p-nitrophenol and its derivatives, and the recovery results for p-nitrophenol in real wastewater samples are favourable.

Design and synthesis of a ratiometric photoacoustic imaging probe activated by selenol for visual monitoring of pathological progression of autoimmune hepatitis

Photoacoustic (PA) imaging with both the high contrast of optical imaging and the high spatial resolution of ultrasound imaging has been regarded as a robust biomedical imaging technique. Autoimmune hepatitis (AIH) is the second largest liver inflammatory disease after viral hepatitis, but its pathogenesis is not fully understood probably due to the lack of an effective in vivo monitoring approach. In this work, an innovative selenol-activated ratiometric PA imaging probe APSel was developed for visual monitoring of pathological progress of AIH. Selenols including selenocysteine (Sec, the major form of Se-containing species in vivo) have been demonstrated to have an effective antioxidant role in inflammation. The reaction of APSel with selenol results in a blue shift of the PA spectrum peak from 860 nm to 690 nm, which enables the ratiometric PA imaging. The APSel probe displays high sensitivity and selectivity to Sec and other selenols. The APSel probe was then employed for ratiometric PA imaging of selenol in cells, and for monitoring the development of AIH in a murine model by tracking the changes of selenol level. The results revealed that the level of selenol was closely correlated with the development of AIH. The proposed APSel, as the first example of a selenol-responsive PA imaging probe, provides a new tool and approach to study and diagnose AIH diseases.

Fluorescent probes based on nucleophilic aromatic substitution reactions for reactive sulfur and selenium species: Recent progress, applications, and design strategies

Reactive sulfur species (RSS) and reactive selenium species (RSeS) are important substances for the maintenance of physiological balance. Imbalance of RSS and RSeS is closely related to a series of human diseases, so it is considered to be an important biomarker in early diagnosis, treatment, and stage monitoring. Fast and accurate quantitative analysis of different RSS and RSeS in complex biological systems may promote the development of personalized diagnosis and treatment in the future. One way to explore the physiological function of various types of RSS and RSeS in vivo is to detect them at the molecular level, and one of the most effective methods for this is to use fluorescent probes. Nucleophilic aromatic substitution (SNAr) reactions are commonly exploited as a detection mechanism for RSS and RSeS in fluorescent probes. In this review, we cover recent progress in fluorescent probes for RSS and RSeS based on SNAr reactions, and discuss their response mechanisms, properties, and applications. Benzenesulfonate, phenyl-O ether, phenyl-S ether, phenyl-Se ether, 7-nitro-2,1,3-benzoxadiazole (NBD), benzoate, and selenium-nitrogen bonds are all good detection groups. Moreover, based on an integration of different reports, we propose the design and synthesis of RSS- and RSeS-selective probes based on SNAr reactions, current challenges, and future research directions, considering the selection of active sites, the effect of substituents on the benzene ring, and the introduction of other functional groups.

Nitrogen-sulfur co-doped pH-insensitive fluorescent carbon dots for high sensitive and selective hypochlorite detection

Carbon dots (CDs) are novel fluorescent carbon nanomaterial with exceptional properties and have drawn great attention in recent years. However, the preparation and applications of high-quality carbon dots remain challenging. Here, we describe a simple hydrothermal synthesis route using citric acid as a carbon source for stable fluorescent CDs. The CDs are modified with glutathione and exhibit high fluorescent quantum yields (30.2%) and excellent photo-stability. In addition, the fluorescence intensity of CDs remains stable over a wide range of pH values (3-12). Hypochlorite (ClO^-) can effectively quench the fluorescence of the CDs by destroying the pyrrolic ring and conjugate structure of the CDs. Thus, the CDs can be used to detect ClO^-. Under optimized conditions, the fluorescence intensity changes of CDs correspond selectively to ClO^- in the range of 100-800 nmol/L with a LOD of 16 nmol/L. Practical applications of the proposed method for free chlorine detection in tap water show similar results and recovery compared to the standard DPD-based method. These results suggest that the pH-insensitive CDs prepared via this facile procedure are a promising chemosensor for free chlorine and have great potential in analytical applications.

A "Polymer Template" Strategy for Carbonized Polymer Dots with Controllable Properties

Limited avenues are available for property control of carbonized polymer dots (PDs) owing to the unsatisfactory understanding of PDs" formation. Herein, a de novo "polymer template" strategy is presented for PDs with customizable functional surface groups (FSG), size, and underlying fluorescence, with a detailed mechanism. The strategy relies on novel di-active site polymers (DASPs) prepared from alkenyl azides via [3+2] cycloaddition and guanidino hydrolysis. Benefiting from these specific reactions, the DASPs were convenient for mass production and stable for storage, and could be transformed to PDs upon addition of nucleophilic agents through nucleophilic addition and substitution at 70 °C. By regulating the types of alkenyl azides, nucleophilic agents, and reaction conditions, the as-prepare PDs could be tailored with controlled types of core, FSG, and particle size, as well as fluorescence properties of quantum yield from 8.2-55.6 %, and emission maximum from 380-500 nm. These specialties make this "polymer template" strategy a promising start for building PDs-based sensor platforms. Moreover, the strategy could further our understanding towards PDs' formation, and open up a new way to customize PDs for specific needs in the fields of analysis, catalysis, images, etc.

One-Step and One-Precursor Hydrothermal Synthesis of Carbon Dots with Superior Antibacterial Activity

Discovering efficient antibacterial materials is crucial in the area of increasing drug resistance. Herein, we synthesized carbon dots (C-dots) with superior antibacterial activity through a simple one-step hydrothermal method. In this method, p-phenylenediamine serves as not only the carbon source but also the origin for the functional group anchored on the obtained C-dots. The antibacterial activity of the obtained C-dots was tested against Staphylococcus aureus and Escherichia coli. The minimum bactericidal concentrations of the synthesized C-dots against S. aureus and E. coli were 2 and 30 μg/mL, respectively, which are lower than that of previously reported C-dots. The antibacterial mechanism was investigated, and the results indicated that a large number of -NH₃⁺ groups on the C-dots' surface enhanced their antibacterial activity. Besides, the C-dots exhibited negligible cytotoxicity.

Fe3+ and intracellular pH determination based on orange fluorescence carbon dots co-doped with boron, nitrogen and sulfur

The fluorescent boron, nitrogen and sulfur co-doped carbon dots (BNSCDs) were prepared by simple hydrothermal reaction of 4-carboxyphenylboronic acid and 2,5-diaminobenzenesulfonic acid at 200 °C for 8 h. The fluorescence of the BNSCDs could be quenched by Fe³⁺ based on the electron transfer between Fe³⁺ and BNSCDs, so a label-free, good selectivity and high sensitivity method for Fe³⁺determination was established with linear range and LOD of 1.5-692 μmol/L and 87 nmol/L, respectively. And then the fluorescent probe was employed for detection of Fe³⁺ in tap water, coal gangue, fly ash and food samples successfully. Moreover, the as-prepared BNSCDs could serve as a novel pH fluorescent probe in the range of pH 1.60-7.00, which could be attributed to the proton transfer of carboxyl groups on the surface of BNSCDs. More importantly, the pH fluorescent probe possesses fast, real-time and low toxicity, applying for intracellular pH fluorescence imaging in HIC, HIEC, LO2 and SMMC7721 cells. In view of its simplicity, timely response and outstanding compatibility, the as-fabricated BNSCDs show the potential applications in water quality and solid waste monitoring, food detection, real-time measuring of intracellular pH change in vitro.

Determination of melamine in milk based on β-cyclodextrin modified carbon nanoparticles via host-guest recognition

Illegal addition of melamine (MEL) to milk has caused serious food safety accident. It is urgent to develop a highly sensitive method for detecting MEL in milk. β-Cyclodextrin with inner hydrophobic and outer hydrophilic cavities have been widely used in smart sensors design. In this study, an "ON-OFF-ON" sensor for MEL detection was constructed based on β-cyclodextrin modified carbon nanoparticles (β-CD-CNPs). The sensor is switched "OFF" when Fe³⁺ interacts with β-CD-CNPs and switched "ON" when MEL replaces Fe³⁺. Fluorescence recovery of β-CD-CNPs exhibits good linear correlations with MEL concentration ranging from 10.00 ng/mL ~ 180.00 ng/mL and 180.00 ~ 1000.00 ng/mL, the detection limit is 6.82 ng/mL. The sensor was applied to analysis melamine in milk samples with recovery between 94.80% ~ 102.05%, and RSD bellow 12.61%. The results show that this method can meet the requirements of real sample analysis.

Synthesis of Lanthanide-Functionalized Carbon Quantum Dots for Chemical Sensing and Photocatalytic Application

Tunable photoluminescent-functionalized carbon quantum dots CQDs@Ln (TFA)3 (Ln = Eu, Tb; TFA: trifluoroacetylacetone) were designed and synthesized by introducing lanthanide complexes into the modified CQDs surface through the carboxyl group. The as-prepared CQDs@Ln (TFA)3 emit strong blue–green light with the peak at 435 nm and simultaneously show the characteristic emission of Ln3+ under irradiation of 365 nm light in aqueous solution. Moreover, these functionalized CQDs exhibit excellent photoluminescence properties. In addition, a white luminescent solution CQDs@Eu/Tb (TFA)3 was obtained by adjusting the ratio of Eu3+/Tb3+ and the excitation wavelengths. Moreover, CQDs@Tb (TFA)3 can be utilized as a fluorescent probe for the sensitive and selective detection of MnO4− without interference from other ions in aqueous solution. These results provide the meaningful data for the multicomponent assembly and the photoluminescent-functionalized materials based on the modified CQDs and lanthanide, which can be expected to have potential application in photocatalytic or sensors.

One-step synthesis of mitochondrion-targeted fluorescent carbon dots and fluorescence detection of silver ions

Silver ions (Ag+) are the most representative harmful ions found in polluted water and widely used in many industries; excessive ingestion of Ag+ in the human body may result in interaction with different metabolites in the human body and in aquatic microorganisms, leading to many diseases. Therefore, there is a great desire to develop good fluorescent probes for Ag+. Herein, a kind of mitochondrion-targeted fluorescent carbon dot was developed. These carbon dots exhibit 29.5% fluorescence quantum yield in water, good photostability and thermal stability. The as-fabricated carbon dots can quickly detect Ag+ in 100% water solution with good selectivity and anti-interference ability. Further, the carbon dots have been successfully applied to monitor Ag+ in living cells via the dual-channel method.

Application of Zero-Dimensional Nanomaterials in Biosensing

Zero-dimensional (0D) nanomaterials, including graphene quantum dots (GQDs), carbon quantum dots (CQDs), fullerenes, inorganic quantum dots (QDs), magnetic nanoparticles (MNPs), noble metal nanoparticles, upconversion nanoparticles (UCNPs) and polymer dots (Pdots), have attracted extensive research interest in the field of biosensing in recent years. Benefiting from the ultra-small size, quantum confinement effect, excellent physical and chemical properties and good biocompatibility, 0D nanomaterials have shown great potential in ion detection, biomolecular recognition, disease diagnosis and pathogen detection. Here we first introduce the structures and properties of different 0D nanomaterials. On this basis, recent progress and application examples of 0D nanomaterials in the field of biosensing are discussed. In the last part, we summarize the research status of 0D nanomaterials in the field of biosensing and anticipate the development prospects and future challenges in this field.

A review of bioselenol-specific fluorescent probes: Synthesis, properties, and imaging applications

Bioselenols are important substances for the maintenance of physiological balance and offer anticancer properties; however, their causal mechanisms and effectiveness have not been assessed. One way to explore their physiological functions is the in vivo detection of bioselenols at the molecular level, and one of the most efficient ways to do so is to use fluorescent probes. Various types of bioselenol-specific fluorescent probes have been synthesized and optimized using chemical simulations and by improving biothiol fluorescent probes. Here, we review recent advances in bioselenol-specific fluorescent probes for selenocysteine (Sec), thioredoxin reductase (TrxR), and hydrogen selenide (H₂Se). In particular, the molecular design principles of different types of bioselenols, their corresponding sensing mechanisms, and imaging applications are summarized.

One-step synthesis of yellow-emissive carbon dots with a large Stokes shift and their application in fluorimetric imaging of intracellular pH

A new nanoprobe based on yellow-emissive carbon dots (Y-CDs) was developed for sensing full-range intracellular pH values. By using o-phenylenediamine as the raw material, Y-CDs with a quantum yield of 31% were prepared through a one-pot solvothermal carbonization method. The Y-CDs exhibited a distinctive fluorescence emission peak at 570 nm with excitation at 450 nm, showing a very large Stokes shift (120 nm). Notably, the nanoprobe revealed a linear relationship between fluorescence intensity and pH value within the range of pH 4.0 to 8.2, exhibiting the ability of this probe to monitor full-range intracellular pH variations. In addition, the nanosensor possessed excellent photostability and fluorescence reversibility in pH measurements and showed excellent selective detection of the influences of other biological species. The CD-based nanoprobe was successfully used to perform quantitative fluorescence imaging of intracellular pH variation, demonstrating its promise for application in cellular systems.

A TP-FRET-based fluorescent sensor for ratiometric visualization of selenocysteine derivatives in living cells, tissues and zebrafish

Selenium is a biologically essential micronutrient element serving as an essential building block for selenoproteins (SePs), which is playing a key role in various cellular functions. Hence, it is of great significance to developing a reliable and rapid method for detection of Sec in biosystems. Compared with the previously reported probes that have been developed for selective detection of Sec, two-photon (TP) ratiometric Sec-specific probes would be advantageous for the NIR excitation and built-in correction of the dual emission bands. To quantitatively and selectively detect Sec over biothiols with rapid and sensitive response, we for the first time report a new fluorescence resonance energy transfer (FRET)-based TP ratiometric fluorescence probe CmNp-Sec, which was constructed by conjugating a TP fluorophore 6 (coumarin derivative with a D-π-A-structure) with a naphthalimide fluorophore 9 via a non-conjugated linker, and employed a 4-dinitrobenzene-ether (DNB) with a strong ICT effect as Sec responsive moiety. It exhibits quantitatively detect Sec in a wide range (0-50 μM) with a limit of detection of 7.88 nM within 10 min. More impressively, this probe can be conveniently used to detect Sec in living cells, tissues and zebrafish, demonstrating it has the latent capability in further biological applications.

Bioluminescence Imaging of Selenocysteine in Vivo with a Highly Sensitive Probe

Selenocysteine (Sec), a vital member of reactive selenium species, is closely implicated in diverse pathophysiological states, including cancer, cardiovascular diseases, diabetes, neurodegenerative diseases, and male infertility. Monitoring Sec in vivo is of significant interest for understanding the physiological roles of Sec and the mechanisms of human diseases associated with abnormal levels of Sec. However, no bioluminescence probe for real-time monitoring of Sec in vivo has been reported. Herein, we present a novel bioluminescent probe BF-1 as an effective tool for the determination of Sec in living cells and in vivo for the first time. BF-1 has advantages of high sensitivity (a detection limit of 8 nM), remarkable bioluminescence enhancement (580-fold), reasonable selectivity, low cytotoxicity, and high signal-to-noise ratio imaging feasibility of Sec in living cells and mice. More importantly, BF-1 affords high sensitivity for monitoring Sec stimulated by Na₂SeO₃ in tumor-bearing mice. These results demonstrate that our new probe could serve as a powerful tool to selectively monitor Sec in vivo, thus providing a valuable approach for exploring the physiological and pathological functions and anticancer mechanisms of selenium.

Luminescent carbon nanoparticles separation and purification

Nowadays luminescent carbon-based nanoparticles can be synthesized by a wide range of physical and chemical methods from a large variety of carbon-based sources. However, in most of the cases the product of synthesis is a complex mixture of compounds, which results in significant challenges in understanding the structure and optical properties of the reaction products. Consequently, a number of separation and purification methodologies have been developed to alleviate these challenges. In this review, we provide a detailed analysis of the current state of the art for methods of luminescent carbon nanoparticles separation and purification. We specifically target such methods as sucrose density gradient centrifugation, chromatography techniques, and electrophoresis because of their ability for fine separation of the reaction products with into a number of fractions. The aim of our comparative analysis is to help development of future strategies for reaction product separation and purification leading to a better understanding of carbon nanoparticles structure and luminescent mechanism as well as to underpin their applications.

Fluorescence Solvatochromism of Carbon Dot Dispersions Prepared from Phenylenediamine and Optimization of Red Emission

Fluorescent carbon dots (CDs) are of interest as a promising alternative to quantum dots, partly because they do not include heavy metals. However, most CDs exhibit blue or green emission, while red-emitting CDs are required for a variety of applications. In the present work, CDs were synthesized by refluxing three phenylenediamine (PD) isomers with amino groups at different positions (o-PD, m-PD, and p-PD) in diphenyl ether at 250 °C for 4 h. Upon dispersing the resulting CDs in eight solvents with different polarities, emission colors ranging from green to red were observed. Among these CDs, p-PD-derived CDs exhibited both the longest emission wavelength range, from 538 to 635 nm, and the highest absolute red photoluminescence quantum yield (PLQY) of 15%. Herein the results are discussed based on a comparison of the polymerization processes of o-PD, m-PD, and p-PD. This work demonstrated that the optimum reaction time was 2 h, which yields a p-PD-derived CD dispersion in methanol with red emission and an absolute PLQY as high as 18%. Additionally, the use of 1-decanol and deuterated methanol in place of methanol improved the maximum absolute PLQY to 25% and 36%, respectively. These improved values are attributed to reduced concentration quenching by suppression of π-π stacking interactions and inhibition of the nonradiative relaxation process through the vibration of OH groups, respectively.

Synthesis, applications and potential photoluminescence mechanism of spectrally tunable carbon dots

Due to the prominent characteristics of carbon-based luminescent nanostructures (known colloquially as carbon dots), such as inexpensive precursors, excellent hydrophilicity, low toxicity, and intrinsic fluorescence, these nanomaterials are regarded as potential candidates to replace traditional quantum dots in some applications. As such, research in the field of carbon dots has been increasing in recent years. In this mini-review, we summarize recent progress in studies of multicolor carbon dots focusing on potential photoluminescence (PL) mechanisms, strategies for effective syntheses, and applications in ion/molecule and temperature sensing, light emitting diodes and high-resolution bioimaging techniques.