Carbon dot cluster as an efficient "off-on" fluorescent probe to detect Au(III) and glutathione

Cu-BTC Derived Mesoporous CuS Nanomaterial as Nanozyme for Colorimetric Detection of Glutathione

In this paper, Cu-BTC derived mesoporous CuS nanomaterial (m-CuS) was synthesized via a two-step process involving carbonization and sulfidation of Cu-BTC for colorimetric glutathione detection. The Cu-BTC was constructed by 1,3,5-benzenetri-carboxylic acid (H3BTC) and Cu2+ ions. The obtained m-CuS showed a large specific surface area (55.751 m2/g), pore volume (0.153 cm3/g), and pore diameter (15.380 nm). In addition, the synthesized m-CuS exhibited high peroxidase-like activity and could catalyze oxidation of the colorless substrate 3,3',5,5'-tetramethylbenzidine to a blue product. Peroxidase-like activity mechanism studies using terephthalic acid as a fluorescent probe proved that m-CuS assists H2O2 decomposition to reactive oxygen species, which are responsible for TMB oxidation. However, the catalytic activity of m-CuS for the oxidation of TMB by H2O2 could be potently inhibited in the presence of glutathione. Based on this phenomenon, the colorimetric detection of glutathione was demonstrated with good selectivity and high sensitivity. The linear range was 1-20 μM and 20-300 μM with a detection limit of 0.1 μM. The m-CuS showing good stability and robust peroxidase catalytic activity was applied for the detection of glutathione in human urine samples.

The Application of Biomedicine in Chemodynamic Therapy: From Material Design to Improved Strategies

Chemodynamic therapy (CDT) has garnered significant interest as an innovative approach for cancer treatment, owing to its notable tumor specificity and selectivity, minimal systemic toxicity and side effects, and absence of the requirement for field stimulation during treatment. This treatment utilizes nanocatalytic medicines containing transitional metals to release metal ions within tumor cells, subsequently initiating Fenton and Fenton-like reactions. These reactions convert hydrogen peroxide (H2O2) into hydroxyl radical (•OH) specifically within the acidic tumor microenvironment (TME), thereby inducing apoptosis in tumor cells. However, insufficient endogenous H2O2, the overexpressed reducing substances in the TME, and the weak acidity of solid tumors limit the performance of CDT and restrict its application in vivo. Therefore, a variety of nanozymes and strategies have been designed and developed in order to potentiate CDT against tumors, including the application of various nanozymes and different strategies to remodel TME for enhanced CDT (e.g., increasing the H2O2 level in situ, depleting reductive substances, and lowering the pH value). This review presents an overview of the design and development of various nanocatalysts and the corresponding strategies employed to enhance catalytic drug targeting in recent years. Additionally, it delves into the prospects and obstacles that lie ahead for the future advancement of CDT.

Preparation of carbon quantum dots from ionic liquid modified biomass for the detection of Fe3+ and Pd2+ in environmental water

A new type of green carbon quantum dots (ILB-CQDs) was prepared by hydrothermal method using ionic liquid as a modifier and grape skin as carbon source, and was obtained from hydrogen-bonded lattice structure ionic liquid preparation, which makes the CQDs in a ring-like stable structure with a stability period of more than 90 day. There is also the catalytic effect of the ionic liquid on cellulose, which makes the prepared CQDs show good advantages, such as uniform particle size, high quantum yield (26.7%), and very good fluorescence performance. This is a smart material for the selective detection of Fe³⁺ and Pd²⁺. It has a detection limit of 0.001 nM for Fe³⁺ and 0.23 µM for Pd²⁺ in pure water. It has a detection limit of 3.2 nmol/L for Fe³⁺ and 0.36 µmol/L for Pd²⁺ in actual water, both of which meet the requirements of WHO drinking water standards. And there is to achieve more than 90% of water restoration effect.

Ratiometric fluorescent probe based on non-conjugated polymer dots for pH measurements in ordinary Portland cement-based materials

An organic fluorescent probe, citric acid-1,3-Propanediamine-Rhodamine B (CPR) was synthesized to determine pH of ordinary Portland cement requiring only a small sample size (less than 500 μL cement leachate). The SEM, XRD, and FTIR investigations demonstrate that citric acid-1,3-Propanediamine are polymer dots with a fusiform structure. Ratio pH probe is constructed by rhodamine B with polymer dots, which exhibits a linear response in high alkaline range. A 6-fold increase in fluorescence intensity (455 nm) is achieved at pH from 12.00 to 13.25. Combined with measurements of the isothermal calorimeter, mineral composition, and microscopic morphology, variation of pH is used to evaluate the changes of components during hydration. Furthermore, CPR can be applied to measure pH of high-dose pulverized fuel ash blending systems, the non-pure cement with slightly lower alkalinity.

Review on Carbon Dot-Based Fluorescent Detection of Biothiols

Biothiols, such as cysteine (Cys), homocysteine (Hcy), and glutathione (GSH), play a vital role in gene expression, maintaining redox homeostasis, reducing damages caused by free radicals/toxins, etc. Likewise, abnormal levels of biothiols can lead to severe diseases, such as Alzheimer's disease (AD), neurotoxicity, hair depigmentation, liver/skin damage, etc. To quantify the biothiols in a biological system, numerous low-toxic probes, such as fluorescent quantum dots, emissive organic probes, composited nanomaterials, etc., have been reported with real-time applications. Among these fluorescent probes, carbon-dots (CDs) have become attractive for biothiols quantification because of advantages of easy synthesis, nano-size, crystalline properties, low-toxicity, and real-time applicability. A CDs-based biothiols assay can be achieved by fluorescent "Turn-On" and "Turn-Off" responses via direct binding, metal complex-mediated detection, composite enhanced interaction, reaction-based reports, and so forth. To date, the availability of a review focused on fluorescent CDs-based biothiols detection with information on recent trends, mechanistic aspects, linear ranges, LODs, and real applications is lacking, which allows us to deliver this comprehensive review. This review delivers valuable information on reported carbon-dots-based biothiols assays, the underlying mechanism, their applications, probe/CDs selection, sensory requirement, merits, limitations, and future scopes.

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.

Selective Determination of Glutathione Using a Highly Emissive Fluorescent Probe Based on a Pyrrolidine-Fused Chlorin

We report the use of a carboxylated pyrrolidine-fused chlorin (TCPC) as a fluorescent probe for the determination of glutathione (GSH) in 7.4 pH phosphate buffer. TCPC is a very stable, highly emissive molecule that has been easily obtained from meso-tetrakis(4-methoxycarbonylphenyl) porphyrin (TCPP) through a 1,3-dipolar cycloaddition approach. First, we describe the coordination of TCPC with Hg(II) ions and the corresponding spectral changes, mainly characterized by a strong quenching of the chlorin emission band. Then, the TCPC-Hg²⁺ complex exhibits a significant fluorescence turn-on in the presence of low concentrations of the target analyte GSH. The efficacy of the sensing molecule was tested by using different TCPC:Hg²⁺ concentration ratios (1:2, 1:5 and 1:10) that gave rise to sigmoidal response curves in all cases with modulating detection limits, being the lowest 40 nM. The experiments were carried out under physiological conditions and the selectivity of the system was demonstrated against a number of potential interferents, including cysteine. Furthermore, the TCPC macrocycle did not showed a significant fluorescent quenching in the presence of other metal ions.

A comprehensive review on multi-colored emissive carbon dots as fluorescent probes for the detection of pharmaceutical drugs in water

Exposure to constituent hazardous chemicals in medical products has become a threat to environmental health across the globe. Excessive medication and the mishandling of pharmaceutical drugs can lead to the increased presence of chemicals in the aquatic environment, causing water pollution. Only a few nanomaterials exist for the detection of these chemicals and they are limited in use due to their adverse toxicity, instability, cost, and low aqueous solubility. In contrast, carbon dots (C-dots), a member of the family of carbon-based nanomaterials, have various beneficial properties including excellent biocompatibility, strong photoluminescence, low photobleaching, tunable fluorescence, and easy surface modification. Herein, we summarize recent advancements in various synthetic strategies for high-quality tunable fluorescent C-dots. The root of fluorescence has been briefly explained via the quantum confinement effect, surface defects, and molecular fluorescence. The surface functional moieties of C-dots have been investigated in depth to recognize the various types of pharmaceutical drugs that are used for the treatment of patients. The modulation of C-dot fluorescence in the course of their interactions with these drugs has been carefully explained. Different types of interaction mechanisms behind the C-dot fluorescence alteration have been discussed. Finally, the challenges and future perspectives of C-dots have been proposed for the vibrant field development of C-dot-based drug sensors.

Green synthesis of carbon quantum dots and their environmental applications

Green synthesis of scalable, high-quality, fluorescent carbon quantum dots (CQDs) from natural biomass remains attractive due to their outstanding environmental application. CQDs are an emerging class of zero-dimensional carbon nanomaterials (<10 nm) that have recently attracted much attention due to their strong optical properties, biocompatibility, nontoxicity, uniform particle size, high photostability, low-cost synthesis, and highly tunable photoluminescence. The unique properties of CQDs possess a broad range of prospective applications in a number of fields such as metal ions detection, photocatalysis, sensing, medical diagnosis, bioimaging, and drug delivery. CQD nanostructures are synthesized using various techniques such as hydrothermal method, laser ablation, microwave irradiation, electrochemical oxidation, reflux method, and ultrasonication. However, this type of fabrication approach requires several chemical reactions including oxidation, carbonization, and pyrolysis. Green synthesis of CQDs has several advantages such as the use of low-cost and non-toxic raw materials, renewable resources, simple operations, and being environment-friendly. This review article will discuss the physicochemical properties of CQDs techniques used in the production of CQDs, and the stability of CQDs along with their applications in wastewater treatment and biomedical fields.

Optical Detection of Acetone Using "Turn-Off" Fluorescent Rice Straw Based Cellulose Carbon Dots Imprinted onto Paper Dipstick for Diabetes Monitoring

Persistent bad breath has been reported as a sign of serious diabetes health conditions. If an individual's breath has a strong odor of acetone, it may indicate high levels of ketones in the blood owing to diabetic ketoacidosis. Thus, acetone gas in the breath of patients with diabetes can be detected using the current easy-to-use fluorescent test dipstick. In another vein, rice straw waste is the most well-known solid pollutant worldwide. Thus, finding a simple technique to change rice straw into a valuable material is highly important. A straightforward and environmentally friendly approach for reprocessing rice straw as a starting material for the creation of fluorescent nitrogen-doped carbon dots (NCDs) has been established. The preparation process of NCDs was carried out via one-pot hydrothermal carbonization using NH₄OH as a passivation substance. A testing strip was developed on the basis of cellulose CD nanoparticles (NPs) immobilized onto cellulose paper assay. The NCDs demonstrated a quantum yield of 23.76%. A fluorescence wavelength was detected at 443 nm upon applying an excitation wavelength of 354 nm. NCDs demonstrated remarkable selectivity for acetone gas as their fluorescence was definitely exposed to quenching by acetone as a consequence of the inner filter effect. A linear correlation was observed across the concentration range of 0.5-150 mM. To detect and measure acetone gas, the present cellulose paper strip has a "switch off" fluorescent signal. A readout limit was accomplished for an aqueous solution of acetone as low as 0.5 mM under ambient conditions. The chromogenic fluorescence of the cellulose assay responsiveness depends on the fluorescence quenching characteristic of the cellulose carbon dots in acetone. A thin fluorescent cellulose carbon dot layer was deposited onto the surface of cellulose strips by a simple impregnation process. CDs were made using NP morphology and analyzed using infrared spectroscopy and transmission electron microscopy. The carbon dot distribution on the paper strip was evaluated by scanning electron microscope and energy-dispersive X-ray analysis. The absorption and fluorescence spectral analyses were investigated. The paper sheets' mechanical qualities were also examined.

Multifunctional carbon quantum dots as a theranostic nanomedicine for fluorescence imaging-guided glutathione depletion to improve chemodynamic therapy

To minimize unwanted reactions with high concentrations of reduced glutathione (GSH) in the tumor microenvironment (TME) during chemodynamic therapy (CDT), a simple and effective strategy was developed to fabricate a TME stimuli-responsive theranostic nanomedicine (Fe-CD) for fluorescence imaging-guided GSH depletion and cancer therapy by combining fluorescent imaging carbon dots (CD) and Fe(III). Introducing Fe(III) into Fe-CD not only quenched the fluorescence of CD while reacting with and consuming intracellular GSH for fluorescence imaging of the depletion of GSH but also provided a source of metal ions to generate more abundant hydroxyl radicals (•OH) with hydrogen peroxide (H₂O₂) through the Fenton reaction to improve CDT. Fe-CD showed promising •OH generation under H₂O₂ to effectively degrade methylene blue in vitro and obviously activate the green fluorescence of the reactive oxygen species (ROS) probe in cells. Benefiting from the fluorescence enhancement in response to TME stimulation, Fe-CD greatly enhanced CDT cytotoxicity while monitoring successful GSH depletion by fluorescence imaging. Fe-CD has the potential to act as a theranostic nanomedicine for fluorescence imaging-guided GSH depletion to amplify CDT.

Feasibility Study on Facile and One-step Colorimetric Determination of Glutathione by Exploiting Oxidase-like Activity of Fe3O4-MnO2 Nanocomposites

A facile and one-step colorimetric assay is described for the determination of glutathione (GSH). It is based on the use of manganese dioxide-decorated magnetic (Fe₃O₄@MnO₂) nanocomposite that was prepared by an in-situ redox reaction. It exhibits oxidase-mimicking activity and can catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) without H₂O₂to form a blue colored product (oxTMB) with an absorption maximum at 651 nm. Once GSH is introduced, the component of MnO₂ can be rapidly reduced to Mn²⁺ ions, which leads to inhibit the formation of oxTMB. Based on these findings, a one-step colorimetric assay was developed for the detection GSH in the range of 0.2 to 25 μM with a low detection limit of 0.2 μM without using any procedures of separation and washing. Importantly, the proposed approach is also used to accurately evaluate the intracellular GSH levels. In our perception, the assay is rapid, sensitive and specific.

Ratiometric fluorescence and smartphone dual-mode detection of glutathione using carbon dots coupled with Ag+-triggered oxidation of o-phenylenediamine

Developing ratiometric fluorescence and smartphone dual-mode bioanalysis methods is important but challenging. A ratiometric fluorescence method for determining glutathione (GSH) using carbon dots (CDs) and Ag⁺-triggered o-phenylenediamine (OPD) oxidation is described here. Ag⁺oxidizes OPD to give 2,3-diaminophenazine (oxOPD), which effectively quenches CD fluorescence at 436 nm through the inner filter effect and causes a new emission peak at 561 nm. GSH chelates with Ag⁺and prevents the Ag⁺oxidizing OPD and therefore effectively preserves CD emission at 436 nm (blue) and allows only weak oxOPD fluorescence at 561 nm (orange) to occur. The oxOPD to CD fluorescence intensity ratio decreased linearly as the GSH concentration increased in the range 0-150 nM, and the detection limit was 15 nM. The ratiometric fluorescence probe lit with an ultraviolet lamp clearly changed color from orange to blue as the GSH concentration increased. An image was acquired using a smartphone camera and converted into digital values. The blue and red channel ratio was calculated and used to quantify GSH. The method therefore allows dual-mode detection of GSH.

Furfural and organic acid targeted carbon dot sensor array for the accurate identification of Chinese baijiu

Baijiu quality control has always been a major challenge for researchers. In this paper, taking furfural which is closely related to baijiu brewing process and organic acids related to baijiu fermentation process and microorganism types as the main discriminating factors, a carbon dot (CDs) sensor array targeting furfural and organic acids was constructed to identify 41 kinds of baijiu. Through the fluorescence response investigation of CDs synthesized by isomers of benzenediol, aminophenol, and phenylenediamine to different baijiu, two CDs synthesized by meta-benzene substitutions containing -NH₂ were selected to build a fluorescence sensor array. Due to the aggregation-induced enhancement effect between furfural and the CDs, and the protonation of organic acid and the CDs, different fluorescence changes were observed, the sensor array combined with partial least squares regression could quantitatively analyze furfural and organic acids. What is more, semi-quantitative analysis of furfural and lactic acid in baijiu was performed. Owing to the interaction of the two CDs with furfural and organic acids in baijiu, the sensor array could accurately identify different baijiu through linear discriminant analysis. This sensor array has potential applications in the quantitative analysis of flavor substances in other alcoholic beverages, moreover, this method could provide a quick response and practical tool for real-time quality control monitoring in the baijiu industry.

A Label-Free Fluorometric Glutathione Assay Based on a Conformational Switch of G-quadruplex

In this paper, a label-free fluorescent method for glutathione (GSH) detection based on a thioflavin T/G-quadruplex conformational switch is developed. The sensing assay is fabricated depending on the virtue of mercury ions to form a thymine–thymine mismatch, which collapses the distance between two ssDNA and directs the guanine-rich part to form an intra-strand asymmetric split G-quadruplex. The newly formed G-quadruplex efficiently reacts with thioflavin T and enhances the fluorescent intensity. In the presence of GSH, Hg²⁺ is absorbed, destroying the G-quadruplex formation with a significant decrease in fluorescence emission. The proposed fluorescent assay exhibits a linear range between 0.03–5 μM of GSH with a detection limit of 9.8 nM. Furthermore, the efficacy of this method is examined using human serum samples to detect GSH. Besides GSH, other amino acids are also investigated in standard samples, which display satisfactory sensitivity and selectivity. Above all, we develop a method with features including potentiality, facility, sensitivity, and selectivity for analyzing GSH for clinical diagnostics.

Turn-on fluorescent glutathione detection based on lucigenin and MnO2 nanosheets

In this work, a glutathione (GSH) sensing nano-platform using lucigenin as a fluorescent probe in the presence of MnO2 nanosheets was reported for the first time. Unlike the earlier fluorescent detection systems based on MnO2 nanosheets, which depend on Förster resonance energy transfer (FRET) or the dynamic quenching effect (DQE), the mechanism of the quenching process of MnO2 nanosheets on lucigenin fluorescence was attributed mainly to a static quenching effect (SQE) with a minor contribution of the inner filter effect (IFE). A double exponential fluorescence decay of lucigenin was obtained in various MnO2 nanosheet concentrations as a result of their SQE and IFE. Based on this phenomenon and taking advantage of the redox reaction between GSH and MnO2 nanosheets, we have developed a switch-on sensitive fluorescent method for GSH via the recovery of the MnO2 nanosheet-quenched fluorescence of lucigenin. A good linearity range of 1.0-150.0 μM with a low limit of detection (S/N = 3) of 180.0 nM was achieved, revealing the higher sensitivity for GSH determination in comparison with the previously reported MnO2 nanosheet-based turn-on fluorescent methods. The developed fluorescent nano-platform exhibits excellent selectivity with successful application for GSH detection in human serum plasma, indicating its good practicability for GSH sensing in biological and clinical applications.

Red pitaya peels-based carbon dots for real-time fluorometric and colorimetric assay of Au3+, cellular imaging, and antioxidant activity

The synthesis of fascinating multifunctional carbon dots (CDs) attracted immense attention. Here, a facile solvothermal treatment of red pitaya peels in acetic acid produced CDs (designated as ACDs, excitation/emission wavelengths at 357/432 nm). ACDs with high sp²-hybridized carbon and carboxylic group contents can rapidly and selectively reduce Au³⁺ to Au⁰, and stabilize produced Au nanoparticles (AuNPs). The synergetic effect of electron transfer from ACDs to Au³⁺ and inner filter effect (IFE) from ACDs to AuNPs quenches the fluorescence within 30 s. Simultaneously, the resulting AuNPs have a purple color with a maximum absorption at 545 nm for visual detection. Therefore, for the first time, we reported a fluorometric and colorimetric dual-mode sensing system for real-time, highly sensitive and selective detection of Au³⁺. The fluorescence quenching ratio and absorbance change linearly with the increase of Au³⁺ concentration in the range of 0.3-8.0 μM and 3.3-60.0 μM with limits of detection (LODs) at 0.072 μM and 2.2 μM, respectively. The assay was applied for Au³⁺ determination in spiked real water samples with recoveries from 95.5 to 105.0%, and relative standard deviation (RSD) of less than 6.5%. Furthermore, ACDs with good photostability, low cytotoxicity, and excellent biocompatibility were successfully applied for intracellular Au³⁺ sensing and imaging. In addition, ACDs exhibited an extraordinarily high antioxidant activity, with an IC₅₀ value for DPPH radical scavenging (0.70 μg mL^-1) much lower than that of ascorbic acid (4.34 μg mL^-1). The proposed strategy demonstrates the outstanding properties of ACDs in chemical and biomedical analysis. Graphical abstract.

A Sensitive FRET Biosensor Based on Carbon Dots-Modified Nanoporous Membrane for 8-hydroxy-2'-Deoxyguanosine (8-OHdG) Detection with Au@ZIF-8 Nanoparticles as Signal Quenchers

A sensitive fluorescence resonance energy transfer (FRET) biosensor is proposed to detect 8-hydroxy-2′-deoxyguanosine (8-OHdG), which is a typical DNA oxidation damage product excreted in human urine. The FRET biosensor was based on carbon dots (CDs)-modified nanoporous alumina membrane with CDs as fluorescence donors. Gold nanoparticles were encapsulated in zeolitic imidazolate framework-8 to form Au@ZIF-8 nanoparticles as signal quenchers. CDs and Au@ZIF-8 nanoparticles were biofunctionalized by 8-OHdG antibody. The capture of 8-OHdG on the membrane substrates can bring Au@ZIF-8 nanoparticles closely to CDs. With 350 nm excitation, the fluorescence of CDs was quenched by Au@ZIF-8 nanoparticles and FRET effect occurred. The quenching efficiency was analyzed. The limit of detection (LOD) was 0.31 nM. Interference experiments of the FRET biosensor showed good specificity for 8-OHdG detection. The biosensor could detect urinary 8-OHdG sensitively and selectively with simple sample pretreatment processes. It shows applicability for detecting biomarkers of DNA damage in urine or other biological fluids.

Fluorescent probe based on N-doped carbon dots for the detection of intracellular pH and glutathione

Carbon dots (CDs) as fluorescent probes have been widely exploited to detect biomarkers, however, tedious surface modification of CDs is generally required to achieve a relatively good detection ability. Here, we synthesized N-doped carbon dots (N-CDs) from triethylenetetramine (TETA) and m-phenylenediamine (m-PD) using a one-step hydrothermal method. When the pH increases from 3 to 11, the fluorescence intensity of the N-CDs gradually decreases. Furthermore, it displays a linear response to the physiological pH range of 5-8. Au³⁺ is reduced by amino groups on the surface of N-CDs to generate gold nanoparticles (AuNPs), causing fluorescence quenching of the N-CDs. If glutathione (GSH) is then added, the fluorescence of the N-CDs is recovered. The fluorescence intensity of the N-CDs is linearly correlated with the GSH concentration in the range of 50-400 μM with a limit of detection (LOD) of 7.83 μM. The fluorescence probe was used to distinguish cancer cells from normal cells using pH and to evaluate intracellular GSH. This work expands the application of CDs in multicomponent detection and provides a facile fluorescent probe for the detection of intracellular pH and GSH.

The synthesis and modification of highly fluorescent carbon quantum dots for reversible detection of water-soluble phosphonate-1-hydroxyethane-1,1-diphosphonic acid by fluorescence spectroscopy

Photoluminescent (PL) carbon quantum dots (CQDs) were prepared successfully using a facile and green procedure. They exhibited striking blue fluorescence and excellent optical properties, with a quantum yield as high as 61.44%. Due to the fluorescence quenching effect and the stronger complexing ability of the phosphoric acid group of 1-hydroxyethane-1,1-diphosphonic acid (HEDP) to Fe³⁺ , CQDs doped with Fe³⁺ were adequately constructed as an efficient and sensitive fluorescent probe for HEDP-specific sensing. The proposed fluorescent probe had a sensitive and rapid response in the range 5-70 μM. Furthermore, quantitative molecular surface (QMS) analysis based on the Multiwfn program was applied to explore the complexation mode of HEDP and metal ions. The distribution of electrostatic potential (ESP), average local ionization energy (ALIE), the minimum value points and the position of the lone pair electrons on the surface of molecular van der Waals were further determined. More strikingly, this experiment achieved the quantitative detection of water-soluble phosphonate-HEDP, for the first time using fluorescence spectrometry. It has been proved to be an effective and intuitive sensing method for the detection of HEDP in real samples.

Insight into the photophysics of strong dual emission (blue & green) producing graphene quantum dot clusters and their application towards selective and sensitive detection of trace level Fe3+ and Cr6+ ions

Graphene-nanostructured systems, such as graphene quantum dots (GQDs), are well known for their interesting light-emitting characteristics and are being applied to a variety of luminescence-based applications. The emission properties of GQDs are complex. Therefore, understanding the science of the photophysics of coupled quantum systems (like quantum clusters) is still challenging. In this regard, we have successfully prepared two different types of GQD clusters, and explored their photophysical properties in detail. By co-relating the structure and photophysics, it was possible to understand the emission behavior of the cluster in detail. This gave new insight into understanding the clustering effect on the emission behaviour. The results clearly indicated that although GQDs are well connected, the local discontinuity in the structure prohibits the dynamics of photoexcited charge carriers going from one domain to another. Therefore, an excitation-sensitive dual emission was possible. Emission yield values of about 18% each were recorded at the blue and green emission wavelengths at a particular excitation energy. This meant that the choice of emission color was decided by the excitation energy. Through systematic analysis, it was found that both intrinsic and extrinsic effects contributed to the blue emission, whereas only the intrinsic effect contributed to the green emission. These excitation-sensitive dual emissive GQD clusters were then used to sense Fe³⁺ and Cr⁶⁺ ions in the nanomolar range. While the Cr⁶⁺ ions were able to quench both blue and green emissions, the Fe³⁺ ions quenched blue emission only. The insensitivity of the Fe³⁺ ions in the quenching of the green emission was also understood through quantum chemical calculations.

Schematic representation for the origin of blue and green emissions, and the resultant PL emission spectra from the GQD interconnected cluster-type sample.

Ecofriendly Synthesis of Fluorescent Nitrogen-Doped Carbon Dots from Coccinia grandis and its Efficient Catalytic Application in the Reduction of Methyl Orange

Facile and fast hydrothermal process for the synthesis of nitrogen doped carbon dots (N-CDs) from Coccinia grandis (C. grandis) extract is discussed here. The morphology of prepared N-CDs was characterized by high-resolution transmission electron microscopy (HR-TEM), energy dispersive X-ray spectroscopy (EDS), and selected area electron diffraction (SAED) method. The optical properties of the prepared N-CDs were revealed by Ultraviolet-Visible (UV-Vis) and photoluminescence spectroscopy. X-ray diffraction (XRD) and Raman spectroscopic techniques were employed to examine the crystallinity and graphitization of prepared N-CDs. The nitrogen doping was confirmed by Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The prepared nitrogen doped carbon dots released blue fluorescence at 405 nm beneath the excitation of 310 nm. The prepared N-CDs influenced the catalytic performance of NaBH₄ in the reduction of methyl orange. The rate constant for the reduction of organic dye (methyl orange) by NaBH₄ in the presence of the prepared green catalyst was also determined.

Novel Amino-pillar[5]arene as a Fluorescent Probe for Highly Selective Detection of Au3+ Ions

A novel fluorescent probe, amino-pillar[5]arene (APA), was prepared via a green, effective, and convenient synthetic method, which was characterized by nuclear magnetic resonance (NMR), infrared (IR), and high-resolution mass spectrometry. The fluorescence sensing behavior of the APA probe toward 22 metal ions in aqueous solutions were studied by fluorescence spectroscopy. The results showed that APA could be used as a selective fluorescent probe for the specificity detection of Au3+ ions. Moreover, the detection characteristics were investigated by fluorescence spectral titration, pH effect, fluorescence competitive experiments, Job's plot analysis, 1H NMR, and IR. The results indicated that detection of Au3+ ions by the APA probe could be achieved in the range of pH 1-13.5 and that other coexisting metal ions did not cause any marked interference. The titration analysis results indicated that the fluorescence intensity decreased as the concentration of Au3+ ions increased, with an excellent correlation (R 2 = 0.9942). The detection limit was as low as 7.59 × 10-8 mol·L-1, and the binding ratio of the APA probe with Au3+ ions was 2:1. Therefore, the APA probe has potential applications for detecting Au3+ ions in the environment and in living organisms.

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.

Nitrogen-Doped Graphene Oxide Dots-Based "Turn-OFF" H2O2, Au(III), and "Turn-OFF-ON" Hg(II) Sensors as Logic Gates and Molecular Keypad Locks

Fluorescent nitrogen-doped graphene oxide dots (NGODs) have been demonstrated as an on-off nanosensor for the detection of Hg²⁺, Au³⁺, and H₂O₂. As compared to l-cystine, where the luminescence signal recovery results from the detachment of Hg²⁺ from the NGODs, signal recovery through l-ascorbic acid (turn-off-on model) has been attributed to the reduction of Hg²⁺ to Hg⁰. The sustainable recovery of the photoluminescence signal is demonstrated using common citrus fruits containing vitamin C (l-AA), suggesting a promising practical usage of this sensing system. Additionally, the sensitivity of NGOD- and AA-originated signal recovery from the Hg(II)-NGODs mixture has been successfully tested in Hg²⁺ ion-spiked tap water from three different places. Mimic devices were executed and verified on the basis of characteristic spectral changes, and the possible utility of this system in electronic security and memory element devices has also been demonstrated. Considering an easy synthesis process and excellent performance of NGODs, this investigation opens up new opportunities for preparing high-quality fluorescent NGODs to meet the requirements of many applications.

Future Perspectives and Review on Organic Carbon Dots in Electronic Applications

Over the span of the past decade, carbon dots (CDs) synthesized from renewable organic resources (organic CDs) have gathered a considerable amount of attention for their photoluminescent properties. This review will focus on organic CDs synthesized using clean chemistry and conventional synthetic chemistry from organic sources and their fluorescence mechanisms, such as quantum confinement effect and surface/edge defects, before outlining their performance in electronic applications, including organic photovoltaic devices, organic light-emitting devices, biosensors, supercapacitors, and batteries. The various organic resources and methods of organic CDs synthesis are briefly covered. Many challenges remain before the adoption of CDs can become widespread; their characterization, structure, functionality, and exact photoluminescent mechanism all require additional research. This review aims to summarize the current research outcomes and highlight the area where further research is needed to fully use these materials.

Rapid and selective colorimetric sensing of Au3+ions based on galvanic displacement of silver nanoparticles

Au³⁺decolorizes AgNPs through a galvanic displacement reaction.

Nitrogen-doped carbon quantum dots as a fluorescent probe to detect copper ions, glutathione, and intracellular pH

A facile one-step hydrothermal method was developed to synthesize nitrogen-doped carbon quantum dots (N-CQDs) by utilizing hexamethylenetetramine as the carbon and nitrogen source. The quantum yield (QY) of 21.7% was under the excitation wavelength of 420 nm with maximum emission at 508 nm. This N-CQD fluorescent probe has been successfully applied to selectively determine the concentration of copper ion (Cu²⁺) with a linear range of 0.1-40 μM and a limit of detection of 0.09 μM. In addition, the fluorescence of N-CQDs could be effectively quenched by Cu²⁺ and specifically recovered by glutathione (GSH), which render the N-CQDs as a premium fluorescent probe for GSH detection. This fluorescence "turn-on" protocol was applied to determine GSH with a linear range of 0.1-30 μM as well as a detection limit of 0.05 μM. For pH detection, there is good linearity in the pH range of 2.87-7.24. Furthermore, N-CQD is a promising and convenient fluorescent pH, Cu²⁺, and glutathione sensor with brilliant biocompatibility and low cytotoxicity in environmental monitoring and bioimaging applications.

Superwettable Microwell Arrays Constructed by Photocatalysis of Silver-Doped-ZnO Nanorods for Ultrasensitive and High-Throughput Electroanalysis of Glutathione in Hela Cells

Superwettable microwell arrays were constructed for the first time by the synergic photocatalysis of silver-doped-ZnO (Ag-ZnO) nanorods patterned on indium tin oxide (ITO) electrodes for electroanalysis of glutathione (GSH) in hela cells through the signal output of AgCl electrochemistry. The newly prepared Ag-ZnO nanorods with high photocatalysis were dispersed into an octadecyltrichlorosilane (OTS) matrix to be deposited onto ITO substrates, yielding superhydrophobic Ag-ZnO-OTS coatings. Superhydrophilic microwells were further created by the Ag-ZnO photocatalysis under UV irradiation to produce Ag-ZnO microwell arrays featuring the superwettability profile. The resulting Ag-ZnO microwell-modified ITO electrodes were employed further for electroanalysis of GSH through solid-state AgCl electrochemistry, in which the specific Ag-GSH interactions would trigger a rational decrease in the sharp AgCl peak currents at the potential approaching zero. Moreover, benefitting from the superwettability feature, the microwells on the ITO electrodes could facilitate the condensing enrichment of GSH analytes from the sample droplets, achieving improved analysis sensitivity. The as-developed electroanalysis strategy was subsequently demonstrated for the detection of GSH in hela cell supernatant with levels down to about 27.30 pM. Additionally, this synergic photocatalysis-based preparation route can be tailored for the large-scale fabrication of various array platforms with the superwettability feature for high-throughput and sensitive biological analysis.

Visual Assay of Glutathione in Vegetables and Fruits Using Quantum Dot Ratiometric Hybrid Probes

Future food safety monitoring with simple, fast, and visual methods has become increasingly important. Accordingly, this work was designed to construct a new-style dual-emission ratiometric fluorescent probe (CdSe@SiO₂@CdTe) for visual assay of glutathione (GSH) with a "turn on" strategy. After adding Hg²⁺, the red fluorescence of the outer CdTe quantum dots (QDs) was quenched through both electron transfer and ion-binding processes. Upon the addition of GSH, the red fluorescence occurred again owing to the strong affinity between GSH and Hg²⁺, whereas the inner green fluorescence of CdSe QDs was unchanged, leading to a clearly recognizable fluorescence color change (from green to orange-red). In the concentration range from 0.1 to 10 μM, the relative fluorescence intensity ratios ( I₆₁₉/ I₅₃₅) showed an excellent linear correlation with the concentration of GSH, and the detection limit was as low as 42 nM under optimal conditions. Meanwhile, the ratiometric hybrid probes were successfully applied for direct visual sensing GSH in real vegetable and fruit 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.

Hydrothermal conversion of Magnolia liliiflora into nitrogen-doped carbon dots as an effective turn-off fluorescence sensing, multi-colour cell imaging and fluorescent ink

The present work illustrates the potential uses of nitrogen-doped multi-fluorescent carbon dots (N-CDs) for Fe³⁺ sensing, cellular multi-colour imaging, and fluorescent ink. N-CDs were synthesized using Magnolia liliiflora flower by the simple hydrothermal method. The resulted N-CDs was found to be nearly spherical in shape with the size of about 4 ± 1 nm and showed competitive quantum yield around 11%. The synthesized N-CDs with uniform size distribution and high content of nitrogen and oxygen-bearing functional groups exhibit excellent dispersibility in aqueous media. The N-CDs were able to detect a high concentration of Fe³⁺ ions (1-1000 μM) with a limit of detection is about 1.2 μM by forming N-CDs-Fe³⁺ complex due to the functional groups such as nitrogen, carbonyl and carboxyl on the surface of N-CDs. Thus they could be used to remove pollutants from industrial wastewater. The electronic charge on the surface of the N-CDs and N-CDs-Fe³⁺ complex (zeta potential) is around -36 and 18 mV, respectively. In addition, these N-CDs show excitation-dependent fluorescence that was utilized for multi-colour in vitro cellular imaging in rat liver cells (Clone 9 hepatocytes). The N-CDs are rapidly uptake in the cell cytoplasm and showed high cytocompatibility on cellular morphology. Moreover, as the N-CDs possess strong fluorescence and anti-coagulation they could be utilized in fluorescent ink pens.