A novel and sensitive fluorescence sensor for glutathione detection by controlling the surface passivation degree of carbon quantum dots

Polyethyleneimine capped silver nanoclusters based turn-off-on fluorescence sensor for the determination of glutathione

A polyethyleneimine capped silver nanoclusters (PEI-AgNCs) based turn-off-on fluorescence sensor has been developed to determine glutathione (GSH) effectively. The fluorescence intensity of silver nanoclusters (AgNCs) has been quenched by Cu(II) and recovered by adding GSH. The quenching of fluorescence intensity of PEI-AgNCs by Cu(II) and recovery of the emission intensity of PEI-AgNCs after the addition of GSH is supposed to be ground state adduct formation. Due to the greater affinity of Cu(II) towards GSH compared to that to PEI-AgNCs, the defragmentation of PEI-AgNCs-Cu(II) adduct occurs after the addition of GSH to the solution, resulting in the recovery of emission intensity of PEI-AgNCs. Characterisation studies of the probe have been done using FT-IR spectroscopy, XPS analysis, XRD analysis, UV-visible and Fluorescence spectrophotometry, EDX spectroscopy and TEM analysis. Different experimental parameters were optimised. Under optimised analytical conditions, the sensor showed a wide linear range for the quantification of GSH from 1.00 × 10-4 M to 3.00 × 10-6 M with a detection limit (LOD) of 8.00 × 10-7 M. Selectivity and interference studies were done in the presence of different structurally similar and coexisting species of GSH in blood. The practical utility of the proposed sensor has been validated in artificial blood serum.

Unveiling thiol biomarkers: Glutathione and cysteamine

The physiological and clinical importance of Glutathione and Cysteamine is emphasized by their participation in a range of conditions, such as diabetes, cancer, renal failure, Parkinson's disease, and hypothyroidism. This necessitates the requirement for accessible, expedited, and cost-efficient testing that can facilitate clinical diagnosis and treatment options. This article examines numerous techniques used to detect both glutathione and cysteamine. The discussed methods include electroanalytical techniques such as voltammetry and amperometry, which are examined for their sensitivity and ability to provide real-time analysis. Furthermore, this study investigates the accuracy of gas chromatography-mass spectrometry (GC-MS) and high-performance liquid chromatography (HPLC) in measuring the concentrations of glutathione and cysteamine. Additionally, the potential of new nanotechnology-based methods, such as plasmonic nanoparticles and quantum dots, to improve the sensitivity of detecting glutathione and cysteamine is emphasized.

Lysozyme functionalized silver nanoclusters as a dual channel optical sensor for the effective determination of glutathione

This article describes the development of a facile and efficient fluorescence sensor for the determination of glutathione (GSH). Presence of the antioxidant glutathione in blood serum is considered as a biomarker for catastrophe like colorectal cancer. Silver nanoclusters with strong fluorescence and good water solubility synthesized from relatively cheaper precursors are one of the species very much explored in fluorescence sensors and bioimaging. Here, Chicken egg derived-lysozyme functionalized silver nanoclusters (Lyz AgNCs) with red fluorescence emission has been synthesized and developed to a turn-off fluorescence sensor for GSH through which colorimetric determination is also possible. Due to the ground state 'Ag-S' interaction between Lyz AgNCs and GSH, the determination of the analyte is possible from 1.00 × 10-5 M to 1.00 × 10-6 M via fluorimetric and from 9.00 × 10-6 to 8.00 × 10-7 M via spectrophotometric techniques with a limit of detection 2.86 × 10-7 M and 4.76 × 10-7 M, respectively. Selectivity of the sensor has been studied and applicability of the sensor in artificial blood serum samples has been demonstrated.

The utilization of quantum dot labeling as a burgeoning technique in the field of biological imaging

Quantum dots (QDs), with their unique optical and physical properties, have revolutionized the field of biological imaging, providing researchers with tools to explore cellular processes and molecular interactions in unprecedented detail. This review explores the diverse properties of QDs, emphasizing their application in biological imaging and addressing both their advantages and challenges. We discuss the developments in QD technology that have facilitated their integration into bioimaging, highlighting the role of surface modifications in enhancing their biocompatibility and functionality. The varied applications of QDs in both in vitro and in vivo imaging settings are examined, showcasing their capacity to deliver brighter, more stable, and multiplexed imaging solutions compared to traditional fluorescent dyes. Furthermore, we delve into the challenges associated with QD use, particularly concerns regarding their potential toxicity and long-term effects on biological systems, and explore ongoing research aimed at mitigating these issues. Finally, we discuss future directions in QD technology, anticipating advancements that will further solidify their role in biological imaging and open up new avenues for scientific exploration.

Physico-Chemical Properties of CdTe/Glutathione Quantum Dots Obtained by Microwave Irradiation for Use in Monoclonal Antibody and Biomarker Testing

In this report, we present the results on the physicochemical characterization of cadmium telluride quantum dots (QDs) stabilized with glutathione and prepared by optimizing the synthesis conditions. An excellent control of emissions and the composition of the nanocrystal surface for its potential application in monoclonal antibody and biomarker testing was achieved. Two samples (QDYellow, QDOrange, corresponding to their emission colors) were analyzed by dynamic light scattering (DLS), and their hydrodynamic sizes were 6.7 nm and 19.4 nm, respectively. Optical characterization by UV-vis absorbance spectroscopy showed excitonic peaks at 517 nm and 554 nm. Photoluminescence spectroscopy indicated that the samples have a maximum intensity emission at 570 and 606 nm, respectively, within the visible range from yellow to orange. Infrared spectroscopy showed vibrational modes corresponding to the functional groups OH-C-H, C-N, C=C, C-O, C-OH, and COOH, which allows for the formation of functionalized QDs for the manufacture of biomarkers. In addition, the hydrodynamic radius, zeta potential, and approximate molecular weight were determined by dynamic light scattering (DLS), electrophoretic light scattering (ELS), and static light scattering (SLS) techniques. Size dispersion and the structure of nanoparticles was obtained by Transmission Electron Microscopy (TEM) and by X-ray diffraction. In the same way, we calculated the concentration of Cd2+ ions expressed in mg/L by using the Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-OES). In addition to the characterization of the nanoparticles, the labeling of murine myeloid cells was carried out with both samples of quantum dots, where it was demonstrated that quantum dots can diffuse into these cells and connect mostly with the cell nucleus.

Application of surface nitrogen-doped graphene quantum dots in the sensing of ferric ions and glutathione: Spectroscopic investigations and DFT calculations

Developing a sensing platform that can quickly and accurately measure glutathione (GSH) is crucial for the early detection of various human diseases. GQDs have shown great potential in many technological and biological applications. This study focused on synthesizing nitrogen-doped GQDs (NGQDs) with stable blue fluorescence using a simple and easy hydrothermal method in one step. The bamboo fiber was used as the green source for this synthesis. The NGQDs had a tiny particle size of 4.7 nm and emitted light at 405 nm when excited. They displayed a remarkable quantum yield of 40.36 % and were effectively used as fluorescent probe to specifically detect Fe3+. The energy transfer mechanism led to the NGQDs' fluorescence being deactivated by Fe3+ ions (turn- "off"). However, with the addition of GSH to the system, the fluorescence intensity of NGQDs was reactivated (turn- "on"). Thus, a fluorescence turn "off-on" system was developed for the sensitive detection of Fe3+ and GSH. Using density functional theory (DFT), it was theoretically calculated that the surface of the fabricated NGQDs possess lone pairs of electrons on oxygens and doped nitrogen causing a photo-induced electron transfer (PET) process to occur. This PET process was suppressed previously owing to complex formation between oxygen atoms of modeled structure and ferric ions. The sensing platform displayed a sensitive response to Fe3+ in the 1-1000 μM range with LOD of 34 nM and GSH in the range of 1-50 μM, with a detection limit of 45 nM. Furthermore, the NGQDs exhibited high selectivity towards Fe3+ and GSH over other electrolytes and biomolecules. Additionally, the probe exhibited non-cytotoxicity and was practically applicable for the detection of GSH in HeLa cells.

Highly Sensitive Fluorescence Detection of Three Organophosphorus Pesticides Based on Highly Bright DNA-Templated Silver Nanoclusters

It is still challenging to achieve simultaneous and sensitive detection of multiple organophosphorus pesticides (OPs). Herein, we optimized the ssDNA templates for the synthesis of silver nanoclusters (Ag NCs). For the first time, we found that the fluorescence intensity of T base-extended DNA-templated Ag NCs was over three times higher than the original C-riched DNA-templated Ag NCs. Moreover, a "turn-off" fluorescence sensor based on the brightest DNA-Ag NCs was constructed for the sensitive detection of dimethoate, ethion and phorate. Under strong alkaline conditions, the P-S bonds in three pesticides were broken, and the corresponding hydrolysates were obtained. The sulfhydryl groups in the hydrolyzed products formed Ag-S bonds with the silver atoms on the surface of Ag NCs, which resulted in the aggregation of Ag NCs, following the fluorescence quenching. The fluorescence sensor showed that the linear ranges were 0.1-4 ng/mL for dimethoate with a limit of detection (LOD) of 0.05 ng/mL, 0.3-2 µg/mL for ethion with a LOD of 30 ng/mL, and 0.03-0.25 µg/mL for phorate with a LOD of 3 ng/mL. Moreover, the developed method was successfully applied to the detection of dimethoate, ethion and phorate in lake water samples, indicating a potential application in OP detection.

N-doped carbon dots as the multifunctional fluorescent probe for mercury ion, glutathione and pH detection

Recently, carbon dots (CDs) have exhibited promising applications in the fluorescence detection of various ions and biomolecules. In this work, one kind of nitrogen-doped CDs (N-CDs) with high fluorescence intensity was synthesized, characterized by transmission electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, Fourier-transform infrared, UV-vis absorption spectra, and fluorescence spectra. The results show that the spherical and uniform N-CDs (quantum yield: 60.2%) have remarkable fluorescence properties and photostability, which makes N-CDs can be utilized as an 'on-off-on' sensor for Hg²⁺and glutathione (GSH). In addition, the pH-sensitive behavior of N-CDs makes it also applicable to H⁺detection under acid conditions (pKa = 3.53). The linear range of the 'turn-off' sensor detecting Hg²⁺was 0.014-50μM, with a 0.014μM limit of detection (LOD). GSH was detected by the fluorescence 'turn-on' method with a linear range of 0.125-60μM and a LOD of 0.125μM. The outstanding performance of N-CDs makes it potential applications in ecological pollution and biomolecule visualization monitoring.

Nano-Nutraceuticals for Health: Principles and Applications

The use of nanotechnological products is increasing steadily. In this scenario, the application of nanotechnology in food science and as a technological platform is a reality. Among the several applications, the main use of this technology is for the development of foods and nutraceuticals with higher bioavailability, lower toxicity, and better sustainability. In the health field, nano-nutraceuticals are being used as supplementary products to treat an increasing number of diseases. This review summarizes the main concepts and applications of nano-nutraceuticals for health, with special focus on treating cancer and inflammation.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s43450-022-00338-7.

Influence of particle size of flame-synthesized carbon nanoparticles on band gap and photoluminescence properties

Band gap and photoluminescence (PL) properties were investigated for flame-synthesized carbon nanoparticles (CNPs) of four different particle size distributions. The particles were collected by varying the flow rates of fuel and oxidizer in a controllable co-flow diffusion flame. The results indicated the possibility of both direct and indirect band gaps in the synthesized CNPs. The band gaps were found to increase with a decrease in the particle size, thereby exhibiting the quantum confinement effect. The PL properties observed with the pure powdered sample showed strong emission in the green region. However, for samples dispersed in isopropyl alcohol, the emission wavelength shifts towards the blue region, showing strong solvent dependence. The particle size of CNPs, however, only affects the emission peak intensities.

Establishment of fumonisin B1 detection method for catalytic fluorescence detection of aptamer-regulated carbon dots

Mycotoxin, common in agricultural products, is a small secondary metabolite with strong toxicity. Fumonisin B₁ (FB₁) is the most common and the most toxic. Establishing a rapid detection method is important for preventing and controlling FB₁ pollution. This study prepared carbon dots (CDs) from 2,2'-dithiosalicylic acid (DTSA). Tetramethylbenzidine (TMB) can be catalyzed to produce fluorescence by CDs, while FB₁ can adhere to the surface of CDs, decreasing fluorescence. Aptamer F10 of FB₁ combines with FB₁ attached to the surface of CDs to restore the catalytic ability of CDs and increase the fluorescence value. This method has good linearity in the FB₁ concentration range from 0 to 1.0 μg mL^-1. The standard curve was Y = -0.2512x + 661.4, R² = 0.9903, the limit of detection (LOD) was 17.67 ng mL^-1 and limit of quantitation (LOQ) was 53.55 ng mL^-1. The recovery of the corn sample was 89.83-98.62%, and the detection time was 30 min.

Carbon dots-based nanomaterials for fluorescent sensing of toxic elements in environmental samples: Strategies for enhanced performance

Rapid industrialization and manufacturing expansion have caused heavy metal pollution, which is a critical environmental issue faced by global population. In addition, the disadvantages presented by conventional detection methods such as the requirement of sophisticated instruments and qualified personnel have led to the development of novel nanosensors. Recently, carbon dots (CDs) have been presented as a multifunctional nanomaterial alternative for the accurate detection of heavy metal ions in water systems. The capacity of CDs to detect contaminants in wastewater -including heavy metals- can be found in the literature; however, to the best of our knowledge, none of them discusses the most recent strategies to enhance their performance. Therefore, in this review, beyond presenting successful examples of the use of CDs for the detection of metal ions, we further discuss the strategies to enhance their photoluminescence properties and their performance for environmental monitoring. In this manner, strategies such as heteroatom-doping and surface passivation are reviewed in detail, as well as describing the mechanisms and the effect of precursors and synthesis methods. Finally, the current challenges are described in detail to propose some recommendations for further research.

Synergistic effects of the hybridization between boron-doped carbon quantum dots and n/n-type g-C3N4 homojunction for boosted visible-light photocatalytic activity

Dye wastewater has raised a prevalent environmental concern due to its ability to prevent the penetration of sunlight through water, thereby causing a disruption to the aquatic ecosystem. Carbon quantum dots (CQDs) are particularly sought after for their highly tailorable photoelectrochemical and optical properties. Simultaneously, graphitic carbon nitride (g-C₃N₄) has gained widespread attention due to its suitable band gap energy as well as excellent chemical and thermal stabilities. Herein, a novel boron-doped CQD (BCQD)-hybridized g-C₃N₄ homojunction (CN) nanocomposite was fabricated via a facile hydrothermal route. The optimal photocatalyst sample, 1-BCQD/CN (with a 1:3 mass ratio of boron to CQD) accomplished a Rhodamine B (RhB, 10 mg/L) degradation efficiency of 96.8% within 4 h under an 18 W LED light irradiation. The kinetic rate constant of 1.39 × 10^-2 min^-1 achieved by the optimum sample was found to be 3.6- and 2.8-folds higher than that of pristine CN and un-doped CQD/CN, respectively. The surface morphology, crystalline structure, chemical composition and optical properties of photocatalyst samples were characterized via TEM, FESEM-EDX, XRD, FTIR, UV-Vis DRS and FL spectrometer. Based on the scavenging tests, it was revealed that the photogenerated holes (h⁺), superoxide anions (∙O₂^-) and hydroxyl radicals (∙OH) were the primary reactive species responsible for the photodegradation process. Overall, the highly efficient 1-BCQD/CN composite with excellent photocatalytic activity could provide a cost-effective and robust means to address the increasing concerns over global environmental pollution.

Green-emitting carbon quantum dots as a dual-mode fluorescent and colorimetric sensor for hypochlorite

In this work, green-emitting carbon quantum dots were successfully prepared through a facile one-step solid-state reaction method. The obtained green-emitting carbon dots (G-CDs) showed good fluorescence stability in NaCl aqueous solution and different pH values. Moreover, the G-CDs showed high sensitivity and selectivity for detecting hypochlorite by both fluorometry and colorimetry. Under the optimized condition, a highly sensitive detection of hypochlorite was established in the range of 0.2-100 μM and 10-150 μM for fluorescent and colorimetric methods, respectively. The corresponding limits of detection (LOD) were 0.0781 μM and 1.82 μM, respectively. Therefore, the G-CDs were successfully applied to determinate hypochlorite in actual water samples. In addition, a paper-based sensor loading with the G-CDs was also developed for rapid visual detection of hypochlorite. The results suggested that the G-CDs could be a promising candidate to detect hypochlorite.

Self-doping synthesis of iodine–carbon quantum dots for sensitive detection of Fe(iii) and cellular imaging

Iodine-doped carbon quantum dots (I-CQDs) were synthesized via p-iodobenzoic acid self-doping for the detection of ferric ions (Fe3+) and cell imaging.

Rapid FRET-based homogeneous immunoassay of procalcitonin using matched carbon dots labels

A novel method for the detection of procalcitonin in a homogeneous system by matched carbon dots (CDs) labeled immunoprobes was proposed based on the principle of FRET and double antibody sandwich method. Blue-emitting carbon dots with a strong fluorescence emission range of 400-550 nm and red-emitting carbon dots with the best excitation range of 410-550 nm were prepared before they reacted with procalcitonin protoclone antibody pairs to form immunoprobes. According to the principles of FRET, blue-emitting carbon dots were selected as the energy donor and red-emitting carbon dots as the energy receptor. The external light source excitation (310 nm) could only cause weak luminescence of CDs. However, once procalcitonin was added, procalcitonin and antibodies would be combined with each other quickly (≤20 min). Here, blue-emitting carbon dots acquired energy could be transferred to red-emitting carbon dots efficiently, causing the emitted fluorescence enhancement of red-emitting carbon dots. The fluorescence detection results in PBS buffer solution and diluted rabbit blood serum showed that the fluorescence intensity variation was linear with the concentration of procalcitonin. There was a good linear relationship betweenF/F0 and procalcitonin concentrations in PBS buffer solution that ranged from 0 to 100 ng ml^-1, and the linear equation wasF/F0 = 0.004 *C_pct + 0.98359. Detection in the diluted rabbit serum led to the results that were linear in two concentration ranges, including 0-40 ng ml^-1and 40-100 ng ml^-1, and the detection limit based on 3σK^-1was 0.52 ng ml^-1. It is likely that this matched CDs labeled immunoprobes system can provide a new mode for rapid homogeneous detection of disease markers.

Morphology-Dependent Performance of MnO2 Nanostructure-Carbon Dot-Based Biosensors for the Detection of Glutathione

Herein, we develop a facile, sensitive, and selective fluorescent nanosensor for the detection of glutathione (GSH). In this protocol, carbon dots (Cdots) with a fairly high quantum yield were synthesized by a microwave-assisted pyrolysis technique. Moreover, different shapes of the MnO₂ nanostructure were also prepared by the hydrothermal technique. A comparative photophysical study of different morphology-dependent Cdots@MnO₂ nanostructure-based biosensors was explored, which showed different results for the quenching values of ("turn-off") fluorescence intensity, quantum yields, electron transfer rate, and average lifetime. The structure, property, and performance of nanomaterials are interdependent. Therefore, the different shapes of MnO₂, that is, nanoflowers (NFs), nanorods (NRs), and a mixture of NFs/NRs was prepared by the hydrothermal method owing to different specific surface areas (23-69 m² g^-1) which put the impact on their sensing activity. It was observed that the variation in the different photophysical parameters of fluorescent Cdots such as quantum yield (Φ), average lifetime values [τ_av (ns)], radiative (k_r) rate constant, nonradiative (k_nr) rate constant, rate of electron transfer (k_ET), the efficiency of electron transfer (Φ_EET), FRET efficiency (E), and Förster distance (R₀) were dependent on the different shapes of the MnO₂ nanostructure. These results indicate that the transfer of energy occurs between the Cdots and different shapes of MnO₂ nanostructures based on fluorescence resonance energy transfer at different charge-transfer rates. The recovery rate ("turn-on") of fluorescence of Cdots with the addition of GSH was obtained best for the NF structure by conversion of MnO₂ to Mn²⁺, and the limit of detection was obtained as ∼19 μM for GSH. The developed sensing probes were rapid, easy, cheap, and eco-friendly for the determination of GSH.

Graphene Quantum Dots and Their Applications in Bioimaging, Biosensing, and Therapy

Graphene quantum dots (GQDs) are carbon-based, nanoscale particles that exhibit excellent chemical, physical, and biological properties that allow them to excel in a wide range of applications in nanomedicine. The unique electronic structure of GQDs confers functional attributes onto these nanomaterials such as strong and tunable photoluminescence for use in fluorescence bioimaging and biosensing, a high loading capacity of aromatic compounds for small-molecule drug delivery, and the ability to absorb incident radiation for use in the cancer-killing techniques of photothermal and photodynamic therapy. Recent advances in the development of GQDs as novel, multifunctional biomaterials are presented with a focus on their physicochemical, electronic, magnetic, and biological properties, along with a discussion of technical progress in the synthesis of GQDs. Progress toward the application of GQDs in bioimaging, biosensing, and therapy is reviewed, along with a discussion of the current limitations and future directions of this exciting material.

Fluorescence assay for the determination of glutathione based on a ring-fused 2-pyridone derivative in dietary supplements

Herein, a novel fluorescent method for the determination of GSH levels in aqueous solutions involving the utilization of citric acid as a derivatization reagent was developed. Therefore, the crucial parameters of the derivatization process were established from what has resulted in the development of a sensitive, reproducible, and accurate GSH assay. The method was validated, and its applicability in the characterization of the GSH concentration in dietary supplements concerning the selectivity in the determination of GSH over GSSG was both confirmed. The chemical structure of the new fluorophore 3-[(carboxymethyl)carbamoyl]-5-oxo-2,3-dihydro-5H-[1,3]thiazolo[3,2-a]pyridine-7-carboxylic acid - CTPC was elucidated using detailed NMR: one-dimensional (¹H, ¹³C), as well as two-dimensional NMR spectra (¹H-¹H COSY, ¹H-¹³C HSQC, ¹H-¹³C HMBC, ¹H-¹⁵N HSQC, ¹H-¹⁵N HMBC) experiments. Besides, the essential optical, biological and antioxidative properties of CTPC were investigated.

A simple and convenient dry-state SEIRS method for glutathione detection based on citrate functionalized silver nanoparticles in human biological fluids

Graphical representation for determination of glutathione using citrate functionalized AgNPs enriched dry-state SEIRS method.

Ratiometric fluorescence sensing of glutathione by using the oxidase-mimicking activity of MnO2 nanosheet

In this work, a facile ratiometric fluorescence sensor for GSH measurement was designed based on MnO₂ nanosheet (NS), carbon dots (CDs), as well as a simple substrate o-phenylenediamine (OPD). Herein, MnO₂ NS played triple essential roles in the sensing system. First, it could be reduced by GSH through a special reaction, and therefore served as GSH recognizer. Second, it played as a fluorescence nanoquencher to strongly quench the fluorescence of CDs. Third, it could directly oxidize OPD to yield a luminescent product 2, 3-diaminophenazine (DAP) via the intrinsic oxidase-like activity. It revealed that MnO₂ NS could be reduced to Mn²⁺ in the presence of GSH. Thus its oxidase-like activity and fluorescence quenching abilities were inhibited, which then restricted the generation of DAP and recovered the fluorescence of CDs. Based on this phenomenon, a novel ratiometric fluorescence sensor for GSH determination was fabricated by measuring the ratio of fluorescent intensity of DAP to that of CDs. Besides, the constructed ratiometric fluorescent sensor, which could be facilely operated with single-wavelength excitation, exhibited high sensitivity and selectivity with a wider linear range and a lower detection limit.

A Review of Carbon Dots Produced from Biomass Wastes

The fluorescent carbon dot is a novel type of carbon nanomaterial. In comparison with semiconductor quantum dots and fluorescence organic agents, it possesses significant advantages such as excellent photostability and biocompatibility, low cytotoxicity and easy surface functionalization, which endow it a wide application prospect in fields of bioimaging, chemical sensing, environmental monitoring, disease diagnosis and photocatalysis as well. Biomass waste is a good choice for the production of carbon dots owing to its abundance, wide availability, eco-friendly nature and a source of low cost renewable raw materials such as cellulose, hemicellulose, lignin, carbohydrates and proteins, etc. This paper reviews the main sources of biomass waste, the feasibility and superiority of adopting biomass waste as a carbon source for the synthesis of carbon dots, the synthetic approaches of carbon dots from biomass waste and their applications. The advantages and deficiencies of carbon dots from biomass waste and the major influencing factors on their photoluminescence characteristics are summarized and discussed. The challenges and perspectives in the synthesis of carbon dots from biomass wastes are also briefly outlined.

An "on-off-on" selective fluorescent probe based on nitrogen and sulfur co-doped carbon dots for detecting Cu2+ and GSH in living cells

The abnormal level of Cu²⁺ or GSH can cause variety of neurodegenerative diseases in humans. Thus, the selective and sensitive detection of Cu²⁺ and GSH has inspired intensive research efforts in biological sample analysis fields. Herein, an "on-off-on" fluorescent probe based on nitrogen and sulfur co-doped carbon dots (N,S-CDs) has been successfully prepared for the detection of Cu²⁺ and GSH. The "turn-off" process of fluorescence in the presence of Cu²⁺ ions was induced by forming a non-luminescent ground state complex due to the interaction between surface groups of the probe and Cu²⁺ ions. Moreover, the strong coordination between GSH and Cu²⁺ could destroy the structure of the complex and restore the fluorescence to "turn-on". This fluorescent probe had excellent selectivity and high sensitivity toward Cu²⁺ and GSH with the limits of detection (LODs) of 38 nM and 41 nM. More importantly, the as-prepared N,S-CDs served as an efficient fluorescent probe for not only detecting Cu²⁺ ions in lake water and tap water, and GSH in BSA solution, but also sensing Cu²⁺ and GSH in living cells. Therefore, these N,S-CDs could be considered as a promising fluorescence probe candidate for environmental monitoring and biological imaging.

Smart, Tunable CQDs with Antioxidant Properties for Biomedical Applications-Ecofriendly Synthesis and Characterization

Carbon quantum dots (CQDs) are nanoobjects of a size below 10 nm. Due to their favorable features, such as tunable luminescence, unique optical properties, water solubility, and lack of cytotoxicity, they are willingly applied in biomedicine. They can be obtained via bottom-up and top-down methods. However, to increase their quantum yield they must undergo post-processing. The aim of the following research was to obtain a new type of CQDs modified with a rhodamine b derivative to enhance their fluorescence performance without biocompability deterioration. For their preparation glucose was used as a precursor and four different carbonizing agents which affected semi- and final products luminescence properties. The ready nanomaterials were investigated over their chemical structure by FTIR and NMR, whereas morphology was investigated by the TEM method. Their optical properties were determined by UV–VIS spectroscopy. Fluorescence behavior, photo- and pH-stability, as well as solvatochromism showed their applicability in various biomedical applications due to the controlled properties. The samples exhibited excellent antioxidant activity and lack of cytotoxicity on L929 mouse fibroblasts. The results showed that proposed strategy enables preparation of the superior nanomaterials with outstanding luminescence properties such as quantum yield up to 17% which can be successfully applied in cell labelling, bioimaging, and theranostics.

A novel optical fiber glucose biosensor based on carbon quantum dots-glucose oxidase/cellulose acetate complex sensitive film

A novel optical fiber glucose biosensor based on fluorescent carbon quantum dots (CQDs)-glucose oxidase (GOD)/cellulose acetate (CA) complex sensitive film was fabricated, in which the dip-coating method was adopted to immobilize the CQDs-GOD/CA complex sensitive film onto the end face of the optical fiber. The surface morphology, microstructure and optical performances of the sensitive film were characterized by field emission scanning electron microscope (FESEM), atomic force microscope (AFM), Zeiss Axiovert 25 inverted microscope, Fourier transform infrared spectroscopy (FTIR), Ultraviolet-visible spectrophotometer and fluorescence spectrophotometer, respectively. The developed fiber-optic biosensor exhibits high sensitivity and repeatability for continuous online detection of low concentration glucose, allowing visualization of real-time glucose fluctuations over a period of time. The change ratios in fluorescence intensity of the biosensor are linear with glucose concentration in various ranges including micromole and nanomole levels, and the relationship between relative fluorescence intensity ratio and glucose concentration complies well with the modified Stern-Volmer equation in the range of 10-200 μmol/L with the detection limit of 6.43 μM, and in the range of 10-100 nmol/L with the detection limit of 25.79 nM, respectively.

An efficient turn-on fluorescence biosensor for the detection of glutathione based on FRET between N,S dual-doped carbon dots and gold nanoparticles

Fluorescence resonance energy transfer (FRET) is a kind of energy transfer mechanism depending on the distance between donor and acceptor, which exhibited potential application in biosensors. In this study, an efficient fluorescence "turn-on" strategy for the detection of glutathione (GSH) has been established based on FRET between nitrogen and sulfur dual-doped carbon dots (N,S-CDs) and gold nanoparticles (Au NPs). A novel N,S-CDs was synthesized by a one-pot hydrothermal treatment of 3-aminothiophenol, which possessed excellent fluorescence property with the maximum emission wavelength of 530 nm. Then, the as-prepared N,S-CDs served as energy donor to transfer energy to Au NPs via FRET process, resulting in fluorescence quenching of N,S-CDs. However, the fluorescence of N,S-CDs was recovered efficiently by adding GSH into the mixture solution of N,S-CDs and Au NPs. Therefore, the FRET assembly of N,S-CDs and Au NPs was used as a fluorescence probe for the "turn-on" sensing GSH with the linear range from 3.8 to 415.1 μM and the limit detection of 0.21 μM. This nanosensor platform was employed to monitor GSH in serum samples with satisfying results. Graphical abstract.

Review of Carbon and Graphene Quantum Dots for Sensing

Carbon and graphene quantum dots (CQDs and GQDs), known as zero-dimensional (0D) nanomaterials, have been attracting increasing attention in sensing and bioimaging. Their unique electronic, fluorescent, photoluminescent, chemiluminescent, and electrochemiluminescent properties are what gives them potential in sensing. In this Review, we summarize the basic knowledge on CQDs and GQDs before focusing on their application to sensing thus far followed by a discussion of future directions for research into CQDs- and GQD-based nanomaterials in sensing. With regard to the latter, the authors suggest that with the potential of these nanomaterials in sensing more research is needed on understanding their optical properties and why the synthetic methods influence their properties so much, into methods of surface functionalization that provide greater selectivity in sensing and into new sensing concepts that utilize the virtues of these nanomaterials to give us new or better sensors that could not be achieved in other ways.

Thin film nanocomposite hollow fiber membranes comprising Na+-functionalized carbon quantum dots for brackish water desalination

We have incorporated Na⁺-functionalized carbon quantum dots (Na-CQDs) into the polyamide layer via interfacial polymerization reaction and developed novel thin film nanocomposite (TFN) hollow fiber membranes for brackish water desalination. Comparing with the conventional thin film composite (TFC) membranes, the TFN membranes comprising Na-CQDs have a larger effective surface area, thinner polyamide layer and more hydrophilic oxygen-containing groups in the polyamide layer. Besides, the interstitial space among the polyamide chains becomes larger due to the presence of Na-CQDs. As a result, the incorporation of 1 wt% Na-CQDs into the polyamide layer could improve the pure water permeability (PWP) of the membranes from 1.74 LMH/bar to 2.56 LMH/bar by 47.1% without compromising their NaCl rejection of 97.7%. Interestingly, stabilization of the TFN hollow fiber membranes containing 1 wt% Na-CQDs at 23 bar could further promote the PWP to 4.27 LMH/bar and the salt rejection to 98.6% under the same testing conditions due to the deformation of the membranes under a high hydraulic pressure. When using a 2000 ppm NaCl aqueous solution as the feed, the optimal water flux and rejection of the newly developed TFN membranes at 15 bar are 57.65 ± 3.26 LMH and 98.6% ± 0.35% respectively. The Na-CQDs incorporated TFN hollow fiber membranes show promising applications in the field of brackish water desalination.

Dual role of BSA for synthesis of MnO2 nanoparticles and their mediated fluorescent turn-on probe for glutathione determination and cancer cell recognition

A MnO₂ nanoparticle (MnO₂ NP)-mediated fluorescent turn-on probe for sensitively and selectively detecting glutathione (GSH) and recognizing cancer cells was established in this work. MnO₂ NPs were synthesized simply and quickly through an in situ redox reaction by mixing bovine serum albumin (BSA) and KMnO₄, in which BSA served the dual roles of template and reductant. It was found that the MnO₂ NPs served as an effective energy acceptor and quenched the fluorescence intensity of carbon dots (CDs), owing to the fluorescence resonance energy transfer (FRET) process. Further, the addition of GSH triggered the decomposition of MnO₂, breaking the FRET between MnO₂ NPs and CDs and thus restoring the fluorescence intensity of CDs. Based on this mechanism, quantitative determination of GSH was performed. Under optimal conditions, a satisfactory linear range of 0.05-90 μM and limit of detection of 39 nM were obtained, and GSH content in human serum samples was detected. Moreover, taking advantage of the higher levels of GSH in cancer cells than in normal cells, the MnO₂ NP-CD probe was applied to distinguish SMMC-7721 cancer cells from L02 normal cells. The FRET was interrupted by GSH in cancer cells, and strong fluorescence was observed. This work provides a facile approach for synthesizing MnO₂ NPs, and this rapid, low-cost method with no need for reductants makes synthesis green and convenient. The MnO₂ NP-mediated fluorescent turn-on response to GSH could improve the MnO₂ nanomaterial-based biochemical analysis applications.

A Fluorescence Inner-Filter Effect Based Sensing Platform for Turn-On Detection of Glutathione in Human Serum

A novel turn-on fluorescence assay was developed for the rapid detection of glutathione (GSH) based on the inner-filter effect (IFE) and redox reaction. Molybdenum disulfide quantum dots (MoS₂ QDs), which have stable fluorescent properties, were synthesized with hydrothermal method. Manganese dioxide nanosheets (MnO₂ NSs) were prepared by exfoliating the bulk δ-MnO₂ material in bovine serum albumin (BSA) aqueous solution. The morphology structures of the prepared nanoparticles were characterized by transmission electron microscope (TEM). Studies have shown that the fluorescence of MoS₂ QDs could be quenched in the presence of MnO₂ NSs as a result of the IFE, and is recovered after the addition of GSH to dissolve the MnO₂ NSs. The fluorescence intensity showed a good linear relationship with the GSH concentration in the range 20⁻2500 μM, the limit of detection was 1.0 μM. The detection method was applied to the analysis of GSH in human serum samples. This simple, rapid, and cost-effective method has great potential in analyzing GSH and in disease diagnosis.

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

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

Bright carbon dots via inner filter effect for the sensitive determination of the purine metabolic disorder in human fluids

In this paper, the high performance fluorescent carbon dots were synthesized with maleic acid, tris and benzoic acid as raw materials by one-step hydrothermal method. The obtained carbon dots with uniform size emitted strong blue fluorescence, which the maximum excitation and emission wavelengths at 250 nm and 415 nm, respectively. Under the optimum condition, it was meaningfully founded that the reaction between the carbon dots and uric acid resulting in the fluorescence quenching of the carbon dots at the emission spectrum of 415 nm. The reason was that they had a synergistic effect between the fluorescence internal filtering effect and the static quenching effect. The fluorescence internal filter effect sensing system was constructed by using uric acid as the absorbable material and carbon dots as the luminophore. Hence, a fluorescence quenching method for the determination of uric acid was established in the concentration range from 5.0 to 400 μM with the detection limit (3σ/S) of 2.26 μM. Thus, a fluorescent sensing assay for the determination of uric acid was founded and confirmed in human fluids.

Peroxidase mimetic activity of fluorescent NS-carbon quantum dots and their application in colorimetric detection of H2O2 and glutathione in human blood serum

Interest is growing in the development of artificial enzymes to overcome the drawbacks of natural enzymes. Herein, we have synthesized nitrogen-sulphur dual-doped carbon quantum dots (NS-CQDs) via a one-step hydrothermal method; the NS-CQDs possess excellent optical properties and a high fluorescent quantum yield (46%). Significantly, the NS-CQDs exhibited peroxidase mimetic enzyme activity without support from metals or polymeric materials and efficiently catalyzed the oxidation of peroxidase substrate 3,3,5,5-tetramethylbenzidine (TMB) in the presence of H₂O₂ to produce a blue solution with an absorption maximum at 652 nm. Mechanistic studies suggest that the small size and high electron density of NS-CQDs play vital roles and accelerate the reduction of H₂O₂ to generate ˙OH radical, which facilitates the oxidation of TMB. The catalytic activity is based on Michaelis-Menten kinetic behavior, and steady state kinetic analysis suggests that the NS-CQDs exhibit a higher affinity for H₂O₂ than TMB, similar to the natural enzyme horseradish peroxidase (HRP). Moreover, the catalytic pathway follows a ping-pong mechanism. Therefore, these findings offer a worthy platform for colorimetric detection of H₂O₂ in a linear range of 0.02 mM to 0.1 mM with a limit of detection of 0.004 mM. Interestingly, the blue colour of oxidized TMB showed excellent selectivity over non-thiolate biological molecules, especially amino acids, and glutathione can be detected up to 0.07 μM via colorimetric and fluorimetric assays. Additionally, this system showed excellent recovery (96.0-108.3%) of GSH from human blood serum. Thus, the proposed sensing system is simple, convenient, and straightforward and can be potentially applied for real time monitoring of H₂O₂ and glutathione in biological samples.

Nitrogen-Doped Carbon Nanoparticles Derived from Silkworm Excrement as On⁻Off⁻On Fluorescent Sensors to Detect Fe(III) and Biothiols

On⁻off⁻on fluorescent sensors based on emerging carbon nanoparticles (CNPs) or carbon dots (CDs) have attracted extensive attention for their convenience and efficiency. In this study, dumped silkworm excrement was used as a novel precursor to prepare fluorescent nitrogen-doped CNPs (N-CNPs) through hydrothermal treatment. The obtained N-CNPs showed good photoluminescent properties and excellent water dispersibility. Thus, they were applied as fluorescence “on⁻off⁻on” probes for the detection of Fe(III) and biothiols. The “on⁻off” process was achieved by adding Fe(III) into N-CNP solution, which resulted in the selective fluorescence quenching, with the detection limit of 0.20 μM in the linear range of 1⁻500 μM. Following this, the introduction of biothiols could recover the fluorescence efficiently, in order to realize the “off⁻on” process. By using glutathione (GSH) as the representative, the linear range was in the range of 1⁻1000 μM, and the limit of detection was 0.13 μM. Moreover, this useful strategy was successfully applied for the determination of amounts of GSH in fetal calf serum samples.