Multi-sensing function integrated nitrogen-doped fluorescent carbon dots as the platform toward multi-mode detection and bioimaging

A smartphone-assisted 2D Cd-MOF-based mixed-matrix membrane exhibiting visual and on-site quantitative sensing of antibiotics and pesticides for food safety

Food contamination is a current global concern, thus rapid and accurate quantitative detection of contaminants is essential for ensuring food safety. Herein, a MOF-based mixed-matrix membrane (1@PMMA) was fabricated by incorporating a stable 2D luminescent Cd-MOF, {[Cd2(L)2(DMSO)2]·2DMSO}n (1) (H2L = 5-(4-(pyridin-4-yl)benzamido)benzene-1,3-dioic acid), into a flexible poly(methyl methacrylate) (PMMA) matrix. The resulting 1@PMMA exhibited sensitive, strong anti-interference, recyclable, and visual detection of nitrofurazone (NFZ) and 2,6-dichloro-4-nitroaniline (DCN). Furthermore, a portable smartphone-assisted sensing platform was developed by coupling the luminescent 1@PMMA with a smartphone, to realize visual and on-site quantitative detection of NFZ and DCN in real food samples. This work provides a portable and intelligent sensing platform for the visual and on-site quantitative detection of antibiotic and pesticide residues in food samples, demonstrating significant potential for food safety monitoring and quality control.

Molecularly imprinted polymer sensors for biomarker detection in cardiovascular diseases

Cardiovascular diseases (CVDs) are recognized as a significant threat to global health. The rapid, sensitive, and precise measurement of relevant biomarkers is essential for the timely diagnosis of CVDs. Molecularly imprinted polymers (MIPs), which act as artificial receptor recognition materials, have been extensively utilized in the detection of CVD biomarkers. Their widespread application is due to their cost-effectiveness, physical and chemical stability, straightforward preparation processes, and excellent compatibility with various sensor types. This review introduces the principles of MIP sensors in combination with electrochemical, optical, thermal transfer, and acoustic detection techniques for detecting CVD-related biomarkers. It then discusses methods developed over the past decade for detecting biomarkers of three major CVDs-coronary artery disease (CAD), acute myocardial infarction (AMI), and heart failure (HF)-using MIP sensors. Finally, the review summarizes the potential of MIP sensors in CVD biomarker detection and provides an outlook on future research directions.

Lateral flow assemblies and allied application of carbon quantum dots derived from cigarette tobacco in biosensing, anticounterfeiting and fluorescent films: Theoretical and experimental overview

The bio-sensing activity of fluorescence based nanoprobes is one of the most significant aspects to scrutinize the analytical pursuance in modern security and lateral flow assays. Herein, potent transmogrification of waste cigarette tobacco into fluorescent carbon quantum dots (CQDs) has been achieved by calcination approach. The waste transformation to CQDs holds diverse benefits, comprising high quantum yield, low toxicity and scale up synthesis. The developed CQDs were able to identify tetracycline with phenomenal selectivity and sensitivity through fluorescence based method. The sensing mechanism was fully explored using Density Functional Theory (DFT) and Molecular docking studies. Governing features comprising tetracycline concentration, interfering studies, and real water analysis on the identification of tetracycline were also investigated. Along with, the prepared CQDs act as colorimetric probe, facilitating the detection of tetracycline with the naked eye. The lateral flow device was constructed for the on-site detection of tetracycline in real water samples. To the best of our knowledge, the present work represents a novel approach to designing CQDs and demonstrates their significant potential for application in anticounterfeiting measures and lateral flow devices. This work holds significant prospective as the prepared CQDs was fully utilized to its maximum usage in developing films and fluorescent anti-counterfeiting applications. Concisely, current work opens up distinctive opportunities for rapid on-site, real-time and visualized surveillance of tetracycline using CQDs prepared with a quite simple green approach.

Conformational dynamics of trypsin in the presence of caffeic acid: a spectroscopic and computational investigation

Caffeic acid is one of the widely distributed phenolic compounds in nature and can be found in planet products. On the other hand, trypsin is a vital digestive enzyme in the intestine that plays an essential role in the immune response, blood coagulation, apoptosis and protein maturation like protein digestion. Several studies have revealed the inhibitory effects of the phenolic compound on the digestive enzyme. The present study reports functional and conformational alteration of trypsin after caffeic acid addition using multiple experimental and computational techniques for the first time. The intrinsic fluorescence of trypsin is quenched in the presence of caffeic acid via a static mechanism. The percent of secondary structures (α-helix and β-sheet) of trypsin alter after caffeic acid addition. In the kinetic study, a reduction in the trypsin function is obtained with a lower Vmax and Kcat upon interaction with caffeic acid. The thermal study reveals an unstable structure of trypsin upon complex formation with this phenolic compound. Also, the binding sites and conformational changes of trypsin are elucidated through molecular docking and molecular dynamic simulation.Communicated by Ramaswamy H. Sarma.

Strategy to synthesize dual-emission carbon dots and their application for pH variation and hydrogen sulfide sensing and bioimaging

In this work, dual emission nitrogen and sulfur co-doped fluorescent carbon dots (DE-CDs) were designed for pH variation and hydrogen sulfide (H₂S) sensing and bioimaging through fluorescence enhancement. The DE-CDs with green-orange emission were facilely prepared by one-pot hydrothermal strategy using neutral red and sodium 1,4-dinitrobenzene sulfonate as precursors, manifesting intriguing dual-emission behavior at 502 and 562 nm. As the pH increases from 2.0 to 10.2, the fluorescence of DE-CDs gradually increases. The linear ranges are 2.0-3.0 and 5.4-9.6, respectively, which are attributed to the abundant amino groups on the surface of the DE-CDs. Meanwhile, H₂S can be employed as an enhancer to increase the fluorescence of DE-CDs. The linear range is 25-500 μM, and the LOD is calculated to be 9.7 μM. Besides, the DE-CDs can be used as imaging agents for pH variation and H₂S sensing in living cells and zebrafish due to their low toxicity and good biocompatibility. All of the results demonstrated that the DE-CDs can monitor pH fluctuations and H₂S in aqueous and biological environments, and have promising applications in the fields of fluorescence sensing, disease detection, and bioimaging.

Strategy to synthesize long-wavelength emission carbon dots and their multifunctional application for pH variation and arginine sensing and bioimaging

In this work, we designed N and S co-doped carbon dots (N,S-CDs) with long-wavelength emission and their multifunctional application in pH variation, arginine (Arg) sensing, bioimaging in living cells and zebrafish, and fluorescent materials. The N,S-CDs with excitation wavelength-dependent properties were prepared using neutral red (NR) and dl-methionine (DL-Met) as raw materials by one-pot hydrothermal strategy. The N,S-CDs exhibited a unique pH-sensitive luminescence trait within pH range of 3.2-11.0 and have great linear relationship of 4.8-8.0, which indicating their potential application as an imaging reagent in physiological environments. Arg can quench the PL of N,S-CDs due to static quenching. (SQ). The linear range is 2.5-62.5 μM and the LOD is calculated as 0.68 μM. Furthermore, the as-proposed N,S-CDs can be applied as imaging reagents for monitoring of pH and Arg in vivo and vitro owing to outstanding biocompatibility and low cytotoxicity. Interestingly, the N,S-CDs were also used in fluorescent composite films and phosphors owing to exceptional optical properties. All these results indicate that the N,S-CDs have huge potentiality in the areas of fluorescence sensing, bioimaging and fluorescent materials.

Construction of Effective Nanosensor by Combining Semiconducting Polymer Dots with Diphenylcarbazide for Specific Recognition of Trace Cr (VI) Ion in Water and Vitro

Hexavalent chromium (Cr (VI)) ion, as highly toxic environmental pollution, severely endangers the ecological environment and public health. Herein, a fluorescent nanosensor (PFO-DPC) was constructed by combining semiconducting polymer dots with diphenylcarbazide (DPC) for sensing Cr (VI) ion in aqueous solution and living cells. DPC and poly (styrene-co-maleic anhydride) (PSMA) polymer mixed with polyfluorene (PFO) were utilized for selectively indicating Cr (VI) ion and improving the efficiency of detection, respectively. The presence of Cr (VI) ion effectively turned off the blue and green fluorescence of PFO-DPC in the aqueous environment, and the fluorescence quenching efficiency exhibited a good linear relationship between the range of 0.0 to 2.31 nM (R2 = 0.983) with a limit of detection (LOD) of 0.16 nM. The mechanism of fluorescence quenching could possibly be attributed to the internal filtration effect (IFE). Additionally, PFO-DPC showed a satisfactory performance in monitoring intracellular Cr (VI) ion. Our results indicate that the sensor is promising in various applications.

Mechanisms for carbon dots-based chemosensing, biosensing, and bioimaging: A review

Due to the favorable biocompatibility, photostability and fluorescence emissions, carbon dots (CDs) are being widely investigated as fluorescent probes. Current CD-based fluorescent probe designs depend largely on conventional fluorescence sensing mechanisms, for e.g. the inner filter effect, photoinduced electron transfer, and Förster resonance energy transfer. Although these mechanisms have been successful, it is still desirable to introduce new sensing mechanisms. In recent years, emerging mechanisms such as aggregation-induced emission, hydrogen-bond induced emission, and intramolecular charge transfer have been developed for CD-based probes. This review summarizes both conventional and emerging mechanisms, and discuss CDs in the context of chemosensing, biosensing, and bioimaging. We provide an outlook for several other mechanisms such as CN isomerization, the short-wavelength inner filter technique, excited-state intramolecular proton transfer, and twisted intramolecular charge transfer, which have been applied to organic fluorescent probes design but not as much in CD-based sensing systems. We envision that this review will provide insights that inspire further development of CD-based fluorescent probes as for biological applications.

Nanoparticulate Photoluminescent Probes for Bioimaging: Small Molecules and Polymers

Recent interest in research on photoluminescent molecules due to their unique properties has played an important role in advancing the bioimaging field. In particular, small molecules and organic dots as probes have great potential for the achievement of bioimaging because of their desirable properties. In this review, we provide an introduction of probes consisting of fluorescent small molecules and polymers that emit light across the ultraviolet and near-infrared wavelength ranges, along with a brief summary of the most recent techniques for bioimaging. Since photoluminescence probes emitting light in different ranges have different goals and targets, their respective strategies also differ. Diverse and novel strategies using photoluminescence probes against targets have gradually been introduced in the related literature. Among recent papers (published within the last 5 years) on the topic, we here concentrate on the photophysical properties and strategies for the design of molecular probes, with key examples of in vivo photoluminescence research for practical applications. More in-depth studies on these probes will provide key insights into how to control the molecular structure and size/shape of organic probes for expanded bioimaging research and applications.

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 Quantum Dots-Based Fluorescent Hydrogel Hybrid Platform for Sensitive Detection of Iron Ions

In this study, we prepared novel fluorescent carbon quantum dots/hydrogel nanocomposite material (CQDsHG) with good adsorption and stable fluorescence detection of Fe3+. The materials were subsequently characterized according to their morphological features, chemical composition, adsorption, and optical properties. The carbon quantum dots (CQDs) were prepared using a microwave-assisted hydrothermal method in no more than 15 min, and the as-prepared CQDs exhibited excellent water solubility, as well as emitted strong bright blue fluorescence with an ultrahigh quantum yield of 93.60%. The CQDs were then loaded into a hydrogel (HG) using the sol-gel method to obtain a functional CQDsHG. The CQDsHG exhibited high adsorption amounts (31.94 mg/g) and a good quenching response for Fe3+, thus, it could be used as a sensor to selectively detect Fe3+ in the linear range of 0–150 μM with a detection limit of 0.24 μM. We observed minimal difference in the fluorescence lifetimes between the CQDsHG with and without a quencher (Fe3+), with values of 5.816 ns and 5.824 ns, respectively, confirming that Fe3+ was statically quenched on CQDsHG. The results indicated that the innovative combination of CQDs and HG can improve the synergistic performance of each component for the adsorption and quantitative detection of heavy metal ions in the aqueous environment.

Nitrogen-doped carbon dots as a fluorescent probe for folic acid detection and live cell imaging

The folic acid (FA) level in human body can be used as an indicator for body's normal physiological activities and offer insight into the growth and reproduction of the body's cells. But the abnormal level of FA can cause some diseases. Herein, we designed a simple and convenient approach to prepare fluorescent N-doped carbon dots (N-CDs) for the FA detection. These N-CDs have excellent hydrophilicity, high photostability, and outstanding biocompatibility, as well as excitation-independent emission behavior with typical excitation/emission peaks at 295 nm/412 nm. Upon the existence of FA, the fluorescence emission spectrum of N-CDs was significantly quenched through the synergy of static quenching mechanism and internal filtering effect (IFE). Under optimal conditions, the limit of detection was 28.0 nM (S/N = 3) within the FA concentration range of 0-200.0 μM. In addition, N-CDs were successfully employed to detect FA in real samples such as urine and fetal bovine serum (FBS), with a recovery rate of 99.6%-100.7% for quantitative addition. Furthermore, cell experiments confirmed the low toxicity and the cell imaging performance of these N-CDs, indicating that the obtained N-CDs could be served as a credible quantitative probe for FA analysis in the field of biosensing.

A sensitive fluorescence-visualized sensor based on an InP/ZnS quantum dots-sodium rhodizonate system for monitoring fish freshness

A fluorescence-visualized sensor based on 3-mercaptopropionic acid (MPA)-capped indium phosphide/ zinc sulfide quantum dots (InP/ZnS QDs) and sodium rhodizonate (SR) was designed to sensitively monitor fish freshness. MPA-InP/ZnS QDs, which exhibit orange-red fluorescence, were synthesized by a solvothermal method. In the MPA-InP/ZnS QDs-SR system, the fluorescence of MPA-InP/ZnS QDs was quenched by SR due to the combined function of the inner filter effect (IFE) and static quenching effect (SQE) at pH = 3. When ammonia was added, the fluorescence was recovered, and the color changed from colorless to bright orange-red under UV light (365 nm). The sensing performance for volatile amine gas was studied, and the sensor demonstrated good linearity between the fluorescence intensity, the total volatile basic nitrogen (TVB-N) and the total color change (ΔE) of bighead carp stored at room temperature (25 °C) and refrigerated temperature (4 °C). The proposed sensor has potential applications in monitoring fish freshness.

A Continuously Tunable Full-Color Emission Nitrogen-Doped Carbon Dots and for Ultrasensitive and Highly Selective Detection of Ascorbic Acid

Nitrogen-doped carbon dots exhibiting excitation-dependent full-color emissions (F-NCDs) were prepared via the one-step hydrothermal method with citric acid and phenylenediamine. Specifically, the emission wavelength of the F-NCDs tuned from 452 nm to 602 nm due to the introduction of new energy levels by C=O and C=N functional groups. We exploited its stability in illumination, ionic strength, and pH, as well as its specificity, sensitivity, especially in ascorbic acid (AA) detection. F-NCDs could measure the AA concentration in the linear ranges of 0~0.1 and 0.1~1 mmol/L with the detection limit (LOD, S/N = 3) as low as 2.6 nmol/L. Additionally, we successfully detected AA in bovine serum with our F-NCDs and obtained the result within 1 min. Because of full-color emission features, we believe our F-NCDs have a great potential in fluorescent sensor detection.

Facile synthesis of multifunctional carbon dots with 54.4% orange emission for label-free detection of morin and endogenous/exogenous hypochlorite

Carbon dots with long-wavelength emission (orange to red), high quantum yield (QY) and good biocompatibility are of great significance for biomedical applications, but achieving this is still a highly challenging task. In this work, multifunctional carbon dots with 54.4% orange emission (O-CDs) were prepared through one-pot solvothermal treatment of nileblueasulphate and citric acid as precursor for label-free recognition of morin and endogenous/exogenous hypochlorite (ClO^-) and bioimaging in cellular and zebrafish. Morin can quench the luminescence of O-CDs by static quenching (SQ). The linear range is 5-125 μM and LOD is 0.84 μM. ClO^- reduce the photoluminescence intensity of O-CDs via SQ. The linear range is 2.5-90 μM and LOD was 0.46 μM. In addition, The obtained O-CDs have successfully realized the monitoring of morin and endogenous/ exogenous ClO^- in living cells and zebrafish owing to its superior biocompatibility, exceptional photostability and lower toxicity. This work opens up a novel opportunity for the development of long-wavelength emission multifunctional nanomaterial with high quantum yield based on CDs for biosensing, biolabeling and biomedical optical imaging.

Cuttlefish ink-based N and S co-doped carbon quantum dots as a fluorescent sensor for highly sensitive and selective para-nitrophenol detection

Para-nitrophenol (PNP) is an important raw material for organic synthesis and its extensive use has produced a series of environmental problems. Here, we develop a highly sensitive and selective fluorescent detection method for PNP with cuttlefish ink-based carbon quantum dots (CQDs). The cuttlefish ink, which is rich in eumelanin, is utilized as the only precursor to synthesize the CQDs via a one-step hydrothermal method. The resultant CQDs were co-doped with nitrogen and sulfur and exhibited excellent fluorescence properties. Two optimal emissions can be observed at the excitation/emission wavelengths of 320/385 nm and 390/465 nm, respectively. In the presence of PNP, the two emissions are remarkably quenched. PNP can be measured in the linear detection concentration range of 1.25-50 μM (Em = 385 nm and R² = 0.9884) or 1.25-27.5 μM (Em = 465 nm and R² = 0.9818) with a detection limit of 0.05 μM. Significantly, it is found that a much wider linear detection range of 0.05-125 μM with a lower detection limit of 0.039 μM (3σ/k) can be achieved when log(I_{385 nm} + I_{465 nm}) was utilized to quantify PNP. The investigations of the sensing mechanism suggested that the inner filter effect and photoinduced electron transfer of PNP and N,S-CQDs leads to fluorescence quenching. The sensing method is successfully applied for PNP detection in real water samples with satisfactory recoveries (91.18-103.14%). A new sustainable waste-prevention strategy of cuttlefish ink and a feasible alternative to PNP detection methods is provided in this article.

A Highly Sensitive and Selective Fluorescent Probe Using MPA-InP/ZnS QDs for Detection of Trace Amounts of Cu2+ in Water

Detection of copper (II) ions (Cu²⁺) in water is important for preventing them from entering the human body to preserve human health. Here, a highly sensitive and selective fluorescence probe that uses mercaptopropionic acid (MPA)-capped InP/ZnS quantum dots (MPA-InP/ZnS QDs) was proposed for the detection of trace amounts of Cu²⁺ in water. The fluorescence of MPA-InP/ZnS QDs can be quenched significantly in the presence of Cu²⁺, and the fluorescence intensity shows excellent linearity when the concentration of Cu²⁺ varies from 0–1000 nM; this probe also exhibits an extremely low limit of detection of 0.22 nM. Furthermore, a possible fluorescence-quenching mechanism was proposed. The MPA-InP/ZnS QDs probes were further applied to the detection of trace Cu²⁺ in real water samples and drink samples, showing good feasibility.

Quantitative Structure-Activity Relationship Enables the Rational Design of Lipid Droplet-Targeting Carbon Dots for Visualizing Bisphenol A-Induced Nonalcoholic Fatty Liver Disease-like Changes

Lipid droplets (LDs) play indispensable roles in numerous physiological processes; hence, the visualization of the dynamic behavior of LDs in living cells is of great importance in physiological and pathological research. In this article, the quantitative structure-activity relationship (QSAR) theory was employed as an effective design strategy for the development of organelle-targeting carbon dots (CDs). The lipid-water partition coefficient (Log P) of the QSAR was adopted as a key parameter to predict the cellular uptake and subcellular localization of CDs in live cells. By carefully adjusting the molecular structure and lipophilicity of the precursors, p-phenylenediamine-derivatized nucleolus-targeting hydrophilic CDs were converted to lipophilic CDs [4-piperidinoaniline (PA) CDs] with inherent LD-targeting performance. The PA CDs were able to indicate the dynamic behavior of LDs and visualize the changes of bisphenol A-induced nonalcoholic fatty liver disease-like changes in a cellular model. The QSAR strategy of CDs demonstrated here is expected to be increasingly exploited as a powerful design tool for developing various organelle-targeting CDs.

Lysosome targeting, Cr(vi) and l-AA sensing, and cell imaging based on N-doped blue-fluorescence carbon dots

N-doped blue-fluorescence carbon dots (N-CDs) were fabricated via a one-pot hydrothermal method using folic acid and p-phenylenediamine. The obtained N-CDs exhibited strong fluorescence (FL) with a considerable quantum yield (QY) of 21.8% and exceptional optical stability under different conditions. Upon introducing Cr(vi), blue FL of N-CDs was distinctly quenched. On subsequent addition of l-AA, the FL of N-CDs could be partially recovered. The fluorescence changes of N-CDs have been utilized to detect Cr(vi) and l-AA in aqueous solutions with linear ranges of 0.10-150 μM and 0.75-2.25 mM, respectively, as well as limit of detection values of 9.4 nM and 25 μM, respectively. Furthermore, as-obtained N-CDs can be extended to monitor the fluctuation of intracellular Cr(vi) and l-AA. More intriguingly, N-CDs can target lysosomes with a satisfactory Pearson correction coefficient of 0.87, which indicates a promising application prospect in the biomedical field.

Carbon dots for ratiometric fluorescence detection of morin

The B,N dual-doped carbon dots (B,N-CDs) for ratiometric fluorescence detection the morin were prepared from sodium tetraborate and polyethyleneimine through the single-step hydrothermal method. The B,N-CDs exhibited the optimum excitation and emission wavelength at 340 nm and 467 nm, respectively. Interestingly, the intensities of emission peak at 467 nm of B,N-CDs reduced meanwhile a new peak emerged at 560 nm with the continuous addition of morin, which revealed the ratio fluorescence characteristic between F_560nm/F_467nm and morin concentration with the linearity range and detection limit of 14.5-32.5 μmol/L and 0.3 μmol/L (S/N = 3), respectively. The interference of common antibiotics and remedies could be ignored when the concentration of morin was detected by the B,N-CDs, which demonstrating the outstanding selectivity. Furthermore, the proposed fluorescence method is used to detect morin in urine with recoveries are 99.8-104.5%. The results of this research indicate the feasibility and practicality of B,N-CDs as an effective fluorescent probe for the determination of morin.

Chitosan-derived N-doped carbon dots for fluorescent determination of nitrite and bacteria imaging

N-doped carbon dots (N-CDs) were successfully synthesized via simple one-step hydrothermal carbonization using chitosan as carbon and nitrogen sources. The obtained N-CDs contained a variety of functional groups on the NCDs surface, and exhibited excitation-independent behavior and strong blue fluorescence with a relatively higher fluorescence quantum yield (QY = 35%). It also presented excellent water solubility, resistance to pH change, high ion strength and UV irradiation. Since the fluorescence of the N-CDs could be selectively quenched by NO₂^-, they could act as a fluorescent sensor for the determination of NO₂^- in real tap water and lake water samples with a wide linear range (1-500 μM) and low detection limit (0.1 μM). They could also be used for bacterial imaging as multicolor fluorescent probes. The results indicated that N-CDs could be a promising candidate material for biomedical applications.

One-step synthesis of red emission multifunctional carbon dots for label-free detection of berberine and curcumin and cell imaging

In this work, the red emission multifunctional carbon dots (R-CDs) were prepared via one-pot hydrothermal strategy of neutral red (NR) and ethylenediamine (EDA) for the label-free detection of berberine and curcumin, cell imaging, and fluorescent flexible film. The as-fabricated R-CDs not only possess good water dispersibility and excellent fluorescence stability, but also were successfully employed as a photoluminescent nanoprobe for label-free monitoring of berberine (BRH) and curcumin (Cur) based on dynamic quenching and internal filter effect (IFE), respectively. More importantly, as-proposed R-CDs displayed outstanding cellular permeability and lower cytotoxicity for cellular applications, which was consistent with the results of confocal fluorescence imaging and cell viability measurement of SMMC7721 cells. Thus, the multifunctional R-CDs may provide a rich tool library for biosensing and cellular imaging reagent applications. Interestingly, R-CDs were also used to manufacture R-CDs/PVA composites as fluorescent flexible films. To the best of our knowledge, this is the first demonstration of a label-free multifunctional fluorescent nanoprobe for berberine and curcumin based on red emission CDs.

A facile synthesis of long-wavelength emission nitrogen-doped carbon dots for intracellular pH variation and hypochlorite sensing

Intracellular pH and hypochlorite (ClO^-) concentration play an important role in life activities, so there is an urgent need to develop a valid strategy to monitor pH and ClO^- in biological systems with high sensitivity and specificity. In this study, we report long-wavelength emission nitrogen-doped carbon dots (N-CDs) and their potential applications in intracellular pH variation, ClO^- sensing and cell imaging. The N-CDs were prepared via a facile one-pot hydrothermal method of neutral red (NR) and glutamine (Gln). N-CDs exhibited a pH-sensitive response in the range of 4.0-9.0 and a good linear relationship in the range of 5.6-7.4, which indicated that N-CDs are an ideal agent for monitoring pH fluctuations in living cells. In addition, ClO^- was capable of reducing the photoluminescence of N-CDs based on static quenching. The linear range is 1.5-112.5 μM and 112.5-187.5 μM, and the LOD is 0.27 μM. Besides, the as-fabricated N-CDs have been smoothly achieved to monitor pH and ClO^- in PC-12 living cells due to their great biocompatibility and lower cytotoxicity, demonstrating their promising applications in the biomedical field. Compared with other CD-based methods, the as-proposed N-CDs have a longer fluorescence emission, which makes them potentially valuable in biological systems. The results pave a way towards the construction of long-wavelength carbon-based nanomaterials for fluorescence sensing and cell imaging.

Carbon dots derived from Fusobacterium nucleatum for intracellular determination of Fe3+ and bioimaging both in vitro and in vivo

Intracellular Fe3+ amount is one of the critical determinants of human health. The development of simple and effective probes for the quantitative detection of Fe3+in vivo is of great significance for the early diagnosis of disease or disorder associated with iron deficiency or overload. In this study, remarkable carbon dots, which can serve as a biosensor for efficient intracellular Fe3+ detection, were synthesized by hydrothermal carbonization of Fusobacterium nucleatum, an anaerobic bacterium. The achieved F. nucleatum-carbon dots (Fn-CDs) possessed the features of strong fluorescence, high stability and excellent biocompatibility. The obtained Fn-CDs could easily internalize into both plant cells and human cells with excellent ability for cell tracking and biomedical labeling. The fluorescence of Fn-CDs could still remain for another 24 hours after penetrating into cells. Furthermore, the fluorescent Fn-CDs were very sensitive to the presence of Fe3+ ions even in cells, exhibiting great promising applications in in vivo detection of Fe3+ ions. In addition, the Fn-CDs posed no harm to the mice, being circulated and excreted within a short time, making the Fn-CDs an excellent candidate for bioimaging and biosensing in vivo.

You Don't Learn That in School: An Updated Practical Guide to Carbon Quantum Dots

Carbon quantum dots (CQDs) have started to emerge as candidates for application in cell imaging, biosensing, and targeted drug delivery, amongst other research fields, due to their unique properties. Those applications are possible as the CQDs exhibit tunable fluorescence, biocompatibility, and a versatile surface. This review aims to summarize the recent development in the field of CQDs research, namely the latest synthesis progress concerning materials/methods, surface modifications, characterization methods, and purification techniques. Furthermore, this work will systematically explore the several applications CQDs have been subjected to, such as bioimaging, fluorescence sensing, and cancer/gene therapy. Finally, we will briefly discuss in the concluding section the present and future challenges, as well as future perspectives and views regarding the emerging paradigm that is the CQDs research field.

Surface modifications of carbon nanodots reveal the chemical source of their bright fluorescence

Fluorescent carbon nanodots (CNDs) have drawn increasing attention in recent years. These cost-effective and eco-friendly nanomaterials with bright fluorescence have been investigated as promising materials for electrooptic and bioimaging applications. However, the chemical source stimulating their strong fluorescence has not been completely identified to date. Depending on the chemical composition, two absorption peaks are observed in the visible range. In this study, we applied selected chemical modifications to CNDs in order to elucidate the correlation between the chemical structure and optical behavior of CNDs. Varying the amount of acetic acid in the synthesis process resulted in different effects on the absorbance and fluorescence photo-spectra. Specifically, at a low concentration (10%), the fluorescence is dramatically red shifted from 340 to 405 nm. Comprehensive characterization of the chemical modification by FTIR and XPS allows identification of the role of acetic acid in the reaction mechanism leading to the modified photoactivity. The functional group responsible for the 405 nm peak was identified as HPPT. We describe a chemical mechanism involving acetic acid that leads to an increased concentration of HPPT groups on the surface of the CNDs. Applying two additional independent chemical and consequently optical modifications namely solution pH and annealing on the nanodots further supports our proposed explanation. Understanding the molecular origin of CND fluorescence may promote the design and control of effective CND fluorescence in optical applications.

An efficient fluorescent nano-sensor of N-doped carbon dots for the determination of 2,4,6-trinitrophenol and other applications

N-Doped carbon dots (CDs) had been simply produced by a one-pot synthesis process using amygdalic acid and threonine. The resulting product was water-soluble and exhibited strong luminescence emission with a fluorescence quantum yield of 19.25%. The emission of CDs was obviously and selectively decreased upon adding 2,4,6-trinitrophenol (TNP). It was proved that the fluorescence resonance energy transfer was the main mechanism for quenching. An efficient fluorescence probe with satisfied sensitivity for TNP determination was found. The range of the linear response for TNP detection was 0.5-40.0 μmol L^-1, and the limit of detection was 20 nmol L^-1. The content of trace TNP in water samples was successfully detected with this method. The CDs were also applied in HepG2 cell imaging and the fabrication of fluorescent films by dispersing the solid freeze-drying CD (SCD) powder into PMMA, which exhibited some application value in biology and photovoltaics.

A Cu2+-assisted fluorescence switch biosensor for detecting of coenzyme A employing nitrogen-doped carbon dots

Herein, a simple and sensitive Cu²⁺-assisted fluorescence switch biosensor for the detection of coenzyme A (CoA) was proposed by employing nitrogen-doped carbon dots (N-CDs). N-CDs were successfully synthesized by sodium alginate and melatonin via pyrolysis. The as-prepared N-CDs were spherical with an average diameter of 2.8 nm and exhibited blue emission (λ_em = 480 nm, λ_ex = 360 nm) with a high fluorescence quantum yield of 50.2%. The intense blue emission of the N-CDs could be effectively quenched by copper ions through the formation of the N-CDs/Cu²⁺ complex. With the introduction of CoA, a more stable CoA/Cu²⁺ complex formed, leading to the fluorescence recovery of N-CDs. Based on this strategy, CoA could be sensitively and selectively detected with a good linear relationship in the range of 0.02-5.00 μM and with a detection limit of 12 nM. In addition, this sensor was applied for CoA detection in human serum samples with satisfactory recovery. The results showed great potential towards advancing applications in CoA-dependent bioresearch.

A ratiometric fluorescent biosensor for the sensitive determination of α-glucosidase activity and acarbose based on N-doped carbon dots

In this work, a novel ratiometric fluorescent platform for α-glucosidase (α-glu) and its inhibitor was constructed based on N-doped carbon dots (N-CDs). The α-glucosidase present can catalyze the release of hydroquinone (HQ) from α-arbutin. Then, the generated HQ can be oxidized and copolymerized with polyethyleneimine (PEI) to form a yellowish green fluorescence copolymer (PHQ-PEI) with intense fluorescence emission at 510 nm. When the PHQ-PEI was formed, blue fluorescence of N-CDs at 425 nm was decreased, whereas the fluorescence of PHQ-PEI at 510 nm increased sharply as a result of the fluorescence resonance energy transfer (FRET) effect between N-CDs and PHQ-PEI. However, in the presence of acarbose, the activity of α-glucosidase is inhibited, and α-arbutin cannot be hydrolyzed to hydroquinone, leading to the fluorescence recovery of N-CDs at 425 nm and the fluorescence decrease of PHQ-PEI at 510 nm. The linear range from 0.2 to 1.6 mU mL-1 and 25-150 μmol L-1 was obtained for α-glucosidase and acarbose detection, respectively, and the detection limit (LOD) for α-glucosidase and acarbose was as low as 0.082 mU mL-1 and 14.5 μmol L-1. Thus, a ratiometric fluorescent sensor with good sensitivity and high specificity was established for α-glucosidase assay and satisfactory results were acquired in real sample determination.

Selective determination of 2,4,6-trinitrophenol by using a novel carbon nanoparticles as a fluorescent probe in real sample

2,4,6-Trinitrophenol (TNP) is widely used in our daily life; however, excessive use of TNP can lead to a large number of diseases. Therefore, it is necessary to find an effective method to detect TNP. Herein, the rapid fluorescence quenching by TNP was developed for the fluorometric determination of TNP in aqueous medium based on the internal filter effect. Nitrogen-sulfur-codoped carbon nanoparticles (N,S-CNPs), synthesized by a one-pot solvothermal method with the precursors of L-cysteine and citric acid, were applied for the determination of TNP as a fluorescent probe. The excitation peak center of N,S-CNPs and the emission peak center are 340 nm and 423 nm, respectively. The probe can be used in a variety of conditions to detect TNP due to its relatively stable properties. Meanwhile, it has a fast response time (< 1 min), wide linear response range (0.1-40 μM), and low detection limit (43.0 nM). This probe still has excellent selectivity and high sensitivity. The method was also used to detect standard water samples with a satisfactory recovery rate, and it will be used in the application of pollutants and clinical diseases. Graphical abstract.

Hg2+ detection, pH sensing and cell imaging based on bright blue-fluorescent N-doped carbon dots

A multifunctional sensing platform based on bright blue-fluorescent nitrogen-doped carbon dots (N-CDs) has been ingeniously designed for the sensitive determination of Hg²⁺ and pH.