Glutathione-functionalized graphene quantum dots as selective fluorescent probes for phosphate-containing metabolites

Tumor diagnosis using carbon-based quantum dots: Detection based on the hallmarks of cancer

Carbon-based quantum dots (CQDs) have been shown to have promising application value in tumor diagnosis. Their use, however, is severely hindered by the complicated nature of the nanostructures in the CQDs. Furthermore, it seems impossible to formulate the mechanisms involved using the inadequate theoretical frameworks that are currently available for CQDs. In this review, we re-consider the structure-property relationships of CQDs and summarize the current state of development of CQDs-based tumor diagnosis based on biological theories that are fully developed. The advantages and deficiencies of recent research on CQDs-based tumor diagnosis are thus explained in terms of the manifestation of nine essential changes in cell physiology. This review makes significant progress in addressing related problems encountered with other nanomaterials.

Green Synthesis of Sulfur- and Nitrogen-Doped Carbon Quantum Dots for Determination of L-DOPA Using Fluorescence Spectroscopy and a Smartphone-Based Fluorimeter

Sulfur- and nitrogen-doped carbon quantum dots (S,N-CQDs) were synthesized using feijoa leaves as a green precursor via a novel route. Spectroscopic and microscopic methods such as X-ray photoelectron spectroscopy, fluorescence spectroscopy, and high-resolution transmission electron microscopy were used to analyze the synthesized materials. The blue emissive S,N-CQDs were applied for qualitative and quantitative determination of levodopa (L-DOPA) in aqueous environmental and real samples. Human blood serum and urine were used as real samples with good recovery of 98.4-104.6 and 97.3-104.3%, respectively. A smartphone-based fluorimeter device was employed as a novel and user-friendly self-product device for pictorial determination of L-DOPA. Bacterial cellulose nanopaper (BC) was used as a substrate for S,N-CQDs to make an optical nanopaper-based sensor for L-DOPA determination. The S,N-CQDs demonstrated good selectivity and sensitivity. The interaction of L-DOPA with the functional groups of the S,N-CQDs via the photo-induced electron transfer (PET) mechanism quenched the fluorescence of S,N-CQDs. The PET process was studied using fluorescence lifetime decay, which confirmed the dynamic quenching of S,N-CQD fluorescence. The limit of detection (LOD) of S,N-CQDs in aqueous solution and the nanopaper-based sensor was 0.45 μM in the concentration range of 1-50 μM and 31.05 μM in the concentration range of 1-250 μM, respectively.

An overview on the development of different optical sensing platforms for adenosine triphosphate (ATP) recognition

Adenosine triphosphate (ATP), one of the biological anions, plays a crucial role in several biological processes including energy transduction, cellular respiration, enzyme catalysis and signaling. ATP is a bioactive phosphate molecule, recognized as an important extracellular signaling agent. Apart from serving as a universal energy currency for various cellular events, ATP is also considered a factor responsible for numerous physiological activities. It regulates cellular metabolism by breaking phosphoanhydride bonds. Several diseases have been reported widely based on the levels and behavior of ATP. The variation of ATP concentration usually causes a foreseeable impact on mitochondrial physiological function. Mitochondrial dysfunction is responsible for the occurrence of many severe diseases such as angiocardiopathy, malignant tumors and Parkinson's disease. Therefore, there is high demand for developing a sensitive, fast-responsive, nontoxic and versatile detection platform for the detection of ATP. To this end, considerable efforts have been employed by several research groups throughout the world to develop specific and sensitive detection platforms to recognize ATP. Although a repertoire of optical chemosensors (both colorimetric and fluorescent) for ATP has been developed, many of them are not arrayed appropriately. Therefore, in this present review, we focused on the design and sensing strategy of some chemosensors including metal-free, metal-based, sequential sensors, aptamer-based sensors, nanoparticle-based sensors etc. for ATP recognition via diverse binding mechanisms.

Glutathione-modified graphene quantum dots as fluorescent probes for detecting organophosphorus pesticide residues in Radix Angelica Sinensis

A novel fluorescent sensor was developed in this study based on glutathione-functionalized graphene quantum dots (GQDs@GSH) to detect organophosphorus pesticide residues in Radix Angelica Sinensis. GQDs@GSH was synthesized by a one-step pyrolysis method with a fluorescence quantum yield as high as 33.9% and its structure was characterized by transmission electron microscopy and X-ray photoelectron spectroscopy. GQDs@GSH exhibited excellent fluorescence property showing strong blue fluorescence under UV irradiation. The fluorescence of GQDs@GSH could be quenched by Fe³⁺ by electron transfer and the quenched fluorescence could be recovered due to the strong chelating and reducing ability of phytic acid (PA). Under the catalyzation of acetylcholinesterase (AChE) and choline oxidase (ChOx), acetylcholine (ACh) could be decomposed to H₂O₂, which could further oxidize Fe²⁺ to Fe³⁺ thus quenching the fluorescence of GQDs@GSH once again. Coumaphos, a kind of organophosphorus pesticide, could inhibit AChE activity, thus making the quenched fluorescence turn on again. Several parameters influencing the fluorescence response such as Fe³⁺, PA, ACh and coumaphos concentration, pH value and reaction time were optimized. Based on such a fluorescence "off-on-off-on" ngkmechanism, GQDs@GSH was successfully applied to the detection of coumaphos in Radix Angelica Sinensis. A good linear relationship between the fluorescence intensity and coumaphos concentration was obtained in the range of 0.1-10.0 μmol·L^-1. By a standard addition method, the recoveries were measured to be 101.44-117.90% with RSDs lower than 1.98%. The biosensor system is simple, sensitive and accurate. It has a good application prospect in the detection of organophosphorus pesticide residues in traditional Chinese medicine and agricultural products, and also expanded the application scope for glutathione as a highly selective biological molecule.

Lighting up Micro-/Nanorobots with Fluorescence

Micro-/nanorobots (MNRs) can be autonomously propelled on demand in complex biological environments and thus may bring revolutionary changes to biomedicines. Fluorescence has been widely used in real-time imaging, chemo-/biosensing, and photo-(chemo-) therapy. The integration of MNRs with fluorescence generates fluorescent MNRs with unique advantages of optical trackability, on-the-fly environmental sensitivity, and targeting chemo-/photon-induced cytotoxicity. This review provides an up-to-date overview of fluorescent MNRs. After the highlighted elucidation about MNRs of various propulsion mechanisms and the introductory information on fluorescence with emphasis on the fluorescent mechanisms and materials, we systematically illustrate the design and preparation strategies to integrate MNRs with fluorescent substances and their biomedical applications in imaging-guided drug delivery, intelligent on-the-fly sensing and photo-(chemo-) therapy. In the end, we summarize the main challenges and provide an outlook on the future directions of fluorescent MNRs. This work is expected to attract and inspire researchers from different communities to advance the creation and practical application of fluorescent MNRs on a broad horizon.

Semiconductor quantum dots for photodynamic therapy: Recent advances

Photodynamic therapy is a promising cancer treatment that induces apoptosis as a result of the interactions between light and a photosensitizing drug. Lately, the emergence of biocompatible nanoparticles has revolutionized the prospects of photodynamic therapy (PDT) in clinical trials. Consequently, a lot of research is now being focused on developing non-toxic, biocompatible nanoparticle-based photosensitizers for effective cancer treatments using PDT. In this regard, semiconducting quantum dots have shown encouraging results. Quantum dots are artificial semiconducting nanocrystals with distinct chemical and physical properties. Their optical properties can be fine-tuned by varying their size, which usually ranges from 1 to 10 nm. They present many advantages over conventional photosensitizers, mainly their emission properties can be manipulated within the near IR region as opposed to the visible region by the former. Consequently, low intensity light can be used to penetrate deeper tissues owing to low scattering in the near IR region. Recently, successful reports on imaging and PDT of cancer using carbon (carbon, graphene based) and metallic (Cd based) based quantum dots are promising. This review aims to summarize the development and the status quo of quantum dots for cancer treatment.

Graphene quantum dots: A review on the effect of synthesis parameters and theranostic applications

The rising demand for early-stage diagnosis of diseases such as cancer, diabetes, neurodegenerative can be met with the development of materials offering high sensitivity and specificity. Graphene quantum dots (GQDs) have been investigated extensively for theranostic applications owing to their superior photostability and high aqueous dispersibility. These are attractive for a range of biomedical applications as their physicochemical and optoelectronic properties can be tuned precisely. However, many aspects of these properties remain to be explored. In the present review, we have discussed the effect of synthetic parameters upon their physicochemical characteristics relevant to bioimaging. We have highlighted the effect of particle properties upon sensing of biological molecules through 'turn-on' and 'turn-off' fluorescence and generation of electrochemical signals. After describing the effect of surface chemistry and solution pH on optical properties, an inclusive view on application of GQDs in drug delivery and radiation therapy has been given. Finally, a brief overview on their application in gene therapy has also been included.

Synthesis, Applications, and Prospects of Graphene Quantum Dots: A Comprehensive Review

Graphene quantum dot (GQD) is one of the youngest superstars of the carbon family. Since its emergence in 2008, GQD has attracted a great deal of attention due to its unique optoelectrical properties. Non-zero bandgap, the ability to accommodate functional groups and dopants, excellent dispersibility, highly tunable properties, and biocompatibility are among the most important characteristics of GQDs. To date, GQDs have displayed significant momentum in numerous fields such as energy devices, catalysis, sensing, photodynamic and photothermal therapy, drug delivery, and bioimaging. As this field is rapidly evolving, there is a strong need to identify the emerging challenges of GQDs in recent advances, mainly because some novel applications and numerous innovations on the ease of synthesis of GQDs are not systematically reviewed in earlier studies. This feature article provides a comparative and balanced discussion of recent advances in synthesis, properties, and applications of GQDs. Besides, current challenges and future prospects of these emerging carbon-based nanomaterials are also highlighted. The outlook provided in this review points out that the future of GQD research is boundless, particularly if upcoming studies focus on the ease of purification and eco-friendly synthesis along with improving the photoluminescence quantum yield and production yield of GQDs.

One-Pot Synthesis of Bright Blue Luminescent N-Doped GQDs: Optical Properties and Cell Imaging

High fluorescent graphene quantum dots (GQDs) are promising in bioimaging and optoelectronics. In this paper, bright blue fluorescent N-doped GQDs were synthesized using a ultrasonic-assisted hydrothermal method. The morphology, structure, surface chemistry, optical properties, and stability subject to photo-bleaching, temperature, pH and preservation period for the N-GQDs were investigated in detail using various microscopy and spectroscopy techniques. The results showed that the N-GQDs possessed an average size of 2.65 nm, 3.57% N doping, and up to 54% quantum yield (QY). The photoluminescence (PL) spectra of the N-GQDs are excitation dependent when excited in the range of 300-370 nm and excitation independent in the range of 380-500 nm for the core and surface states emission. The N-GQDs showed excellent photo-bleaching resistance and superior photo-stability. At room temperature and in the pH range of 3-8, the fluorescence of the N-GQDs was almost invariable. The N-GQDs can be stably preserved for at least 40 days. The average decay lifetime of the N-GQDs was 2.653 ns, and the radiative and nonradiative decay rate constants were calculated to be 2.04 × 108 s-1 and 1.73 × 108 s-1, respectively. The PL mechanism was qualitatively explained. The N-GQDs was used for cell imaging, and it showed good results, implying great potential applications for bioimaging or biomarking.

Photo-sonodynamic combination activity of cationic morpholino-phthalocyanines conjugated to nitrogen and nitrogen-sulfur doped graphene quantum dots against MCF-7 breast cancer cell line in vitro

In this work, we explore the reactive oxygen species (ROS) generation abilities of cationic morpholino-substituted-phthalocyanine (Pc) conjugated to nitrogen (NGQDs) and nitrogen-sulfur (NSGQDs) doped-graphene quantum dots upon irradiation with light for photodynamic therapy (PDT), ultrasound for sonodynamic therapy (SDT) and the combination of both in photo-sonodynamic therapy (PSDT). The in vitro cytotoxicity studies were conducted using the Michigan Cancer Foundation-7 breast cancer cell lines (MCF-7 cells). For PDT treatments, only the ¹O₂ was detected for all the sensitizers, whereas both the ¹O₂ and ^•OH radicals were evident after SDT and PSDT treatments. An increase in the ¹O₂ generation was observed for the conjugates compared to the GQDs and the Pc alone. However, the ^•OH radicals were reduced in the conjugates compared to the GQDs and the Pc alone. The NGQDs generally showed better ROS generation efficacy compared to the NSGQDs, alone and in the conjugates. The combination therapy also shows improved efficacy compared to the monotherapies for the Pcs and Pc-GQDs conjugates.

Smartphone-based ratiometric fluorescent definable system for phosphate by merged metal-organic frameworks

Phosphate plays an important role in a wide range of chemical and biological processes, so the development of a new phosphate optical sensor with high sensitivity, specificity and visual recognition function has important practical significance. Herein, a ratiometric fluorescent (RF) probe and a smartphone-integrated colorimetric test paper sensing platform for assay phosphate was fabricated using hybrid fluorescent UiO-66-NH₂ and Eu³⁺@MOF-808 metal-organic frameworks. After continuous addition of phosphate, the blue fluorescence emission of UiO-66-NH₂ and the red emission of Eu³⁺@MOF-808 were regularly enhanced and quenched respectively, and the fluorescence response of the detection platform to phosphate exhibited a clear color change process (red → pink → blue). More importantly, the probe solution and test paper of the integrated smartphone are converted to digital values through RGB channels and successfully used to visualize semi-quantitative recognition of phosphate. In addition, an RF probe and a smartphone integrated fluorescent test paper were developed separately to devise logic gate devices for detecting phosphate. The multifunctional ratio sensing platform integrated by the smartphone furnishes a new strategy and broad prospects for the intelligent online identification of important targets in biological samples and environmental samples.

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.

Ultra-sensitive detection of ATP in serum and lysates based on nitrogen-doped carbon dots

In this work, novel types of nitrogen-doped carbon dots (N-CDs) were prepared from citric acid and glycine (GLY) as precursors through a simple pyrolysis method. The GLY-CDs showed strong fluorescence with a fluorescence quantum yield as high as 33.34% and good water solubility. The fluorescence of GLY-CDs could be selectively quenched by iron(III) ion (Fe³⁺ ) resulting in the non-fluorescent complex. Due to the high affinity of Fe³⁺ to adenosine-5'-triphosphate (ATP), the fluorescence of the GLY-CDs in GLY-CDs-Fe³⁺ could be recovered by ATP. Thereby, quantitatively fluorescent turn-on detection of ATP could be achieved. The fluorescence recovery ratio was linearly proportional to the concentration of ATP with a detection limit as low as 15.0 nM, indicating the CDs have high sensitivity. The GLY-CDs were successfully employed in the detection of ATP in serum and cell lysates.

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.

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.

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

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

Embedding dual fluoroprobe in metal-organic frameworks for continuous visual recognition of Pb2+ and PO43- via fluorescence 'turn-off-on' response: Agar test paper and fingerprint

A novel dual-emissive ratiometric fluorescence (RF) probe CDs/QDs@ZIF-8 has been successfully constructed by employing a simple and effective strategy for in situ encapsulating carbon dots (CDs) and thioglycolic acid-modified CdTe quantum dots (QDs) into porous metal-organic frameworks (MOFs) "cage". The dual responsive colorimetric fluorescence probe was developed for the ultra-high selectivity and sensitivity continuous detection of Pb²⁺ (turn OFF) and PO₄^3- (turn ON) in biological samples. Blue CDs acts as a stable internal standard emission, the emission color of CDs/QDs@ZIF-8 changes from red to blue with introducing Pb²⁺, fluorescence of probe is quenched because of the binding of Pb²⁺ ions to thioglycolic acid on the surface of probe and e- transfer from the photoexcited QDs to Pb²⁺ ions, color can be recovered after the adding PO₄^3- to CDs/QDs@ZIF-8-Pb²⁺ system, which could take away Pb²⁺ ions from the surface of CDs/QDs@ZIF-8. More importantly, fabricated agar test papers was also successfully applied in visual detection of Pb²⁺ and PO₄³- in real samples, which can implement without instrument-specific calibration.

Self-Enhanced Carbonized Polymer Dots for Selective Visualization of Lysosomes and Real-Time Apoptosis Monitoring

Protons are highly related to cell viability during physiological and pathological processes. Developing new probes to monitor the pH variation could be extremely helpful to understand the viability of cells and the cell death study. Carbonized polymer dots (CPDs) are superior biocompatible and have been widely applied in bioimaging field. Herein, a new type of extreme-pH suitable CPDs was prepared from citric acid and o-phenylenediamine (CA/oPD-CPDs). Due to the co-existence of hydrophilic and hydrophobic groups, CA/oPD-CPDs tend to aggregate in neutral condition with a dramatic decrease of fluorescence, but disperse well in both acidic and alkaline conditions with brighter emission. This specialty enables them to selectively illuminate lysosomes in cells. Moreover, CA/oPD-CPDs in the cytoplasm could serve as a sustained probe to record intracellular pH variation during apoptosis. Furthermore, CA/oPD-CPDs present a continuous fluorescence increase upon 2-h laser irradiation in living cells, underscoring this imaging system for long-term biological recording.

Developing an Analytical Method Based on Graphene Quantum Dots for Quantification of Deferiprone in Plasma

In the world of nanotechnology, graphene quantum dots (GQDs) have been considerably employed in numerous optical sensing and bioanalytical applications. Herein, a simple and cost-efficient methodology was developed to the quantification of deferiprone in plasma samples by utilizing the selective interaction of the GQDs and drug in the presence of Fe³⁺ ions. GQDs were synthesized by a bottom-up technique as an advantageous fluorescent probe. Increasing levels of deferiprone ranging from 5 to 50 mg.L^-1, leads to significant fluorescence quenching of GQDs. In addition, the calibration curve was revealed a linear response in this range with a sensitivity of 5 mg.L^-1. The method validation was carried out according to the FDA guidelines to confirm the accuracy, precision, stability and selectivity of the developed method. The results show that this green and low-cost fluorescent probe could be used for the analysis of deferiprone.

Biomass-derived nitrogen doped graphene quantum dots with color-tunable emission for sensing, fluorescence ink and multicolor cell imaging

In this paper, a simple, economical, and green strategy is developed for producing nitrogen doped graphene quantum dots (N-GQDs) with multicolor light emission by hydrothermal treatment of Passiflora edulia Sims. The synthesized N-GQDs exhibit ideal ionic stability, hydrophilicity and anti-photobcleaching properties, and the quantum yield reaches up to about 29%. Because of with the fluorescence quenching effect, the achieved N-GQDs allow to detect Ag⁺ in a linear range of 10 nM-160 μM, and the limit of detection is calculated to be 1.2 nM according to the S/N of 3. Noteworthy, N-GQDs with blue, green and yellow light emissions are demonstrated via regulating the reaction time and temperature, implying a promising fluorescence adjustability. Furthermore, the N-GQDs-based fluorescent probe exhibits low cytotoxicity and favorable biocompatibility. Depending on the superior properties, our N-GQDs are applied in fluorescent ink and multicolor cell imaging. Eventually, the developed sensor is highly selective and accurate for Ag⁺ analysis in real water, which demonstrates the promising practical use in environmental determination and/or biomedical engineering.

Predicting the Time of Entry of Nanoparticles in Lipid Membranes

The number of engineered nanoparticles for applications in the biomedical arena has grown tremendously over the last years due to advances in the science of synthesis and characterization. For most applications, the crucial step is the transport through a physiological cellular membrane. However, the behavior of nanoparticles in a biological matrix is a very complex problem that depends not only on the type of nanoparticle but also on its size, shape, phase, surface charge, chemical composition, and agglomeration state. In this paper, we introduce a streamlined theoretical model that predicts the average time of entry of nanoparticles in lipid membranes, using a combination of molecular dynamics simulations and statistical approaches. The model identifies four parameters that separate the contributions of nanoparticle characteristics (i.e., size, shape, solubility) from the membrane properties (density distribution). This factorization allows the inclusion of data obtained from both experimental and computational sources, as well as a rapid estimation of large sets of permutations in membranes. The robustness of the model is supported by experimental data carried out in lipid vesicles encapsulating graphene quantum dots as nanoparticles. Given the high level of interest across multiple areas of study in modulating intracellular targets, and the need to understand and improve the applications of nanoparticles and to assess their effect on human health (i.e., cytotoxicity, bioavailability), this work contributes to the understanding and prediction of interactions between nanoparticles and lipid membranes.

Graphene Quantum Dots for Optical Bioimaging

Graphene quantum dots (GQDs) have shown great potential in bioimaging applications due to their excellent biocompatibility, low cytotoxicity, feasibility for surface functionalization, physiological stability, and tunable fluorescence properties. This Review first introduces the intriguing optical properties of GQDs that are suitable for biological imaging, and is followed by the GQDs' synthetic strategies. The emergent and latest development methods for tuning GQDs' optical properties are further described in detail. The recent advanced applications of GQDs in vitro, particularly in cell imaging, targeted imaging, and theranostic nanoplatform fabrication, are included. The applications of GQDs for in vivo bioimaging are also covered. Finally, the Review is concluded with the challenges and prospectives that face this nascent yet exciting field.

A persistent luminescence-based label-free probe for the ultrasensitive detection of hemoglobin in human serum

Hemoglobin (Hb) plays an important role in oxygen carriage for mammals, which is also a typical biomarker for certain diseases. Although numerous methods had been developed for the detection of Hb in red blood cells, analytical technology for the monitoring of low-abundance Hb in serum or plasma is still a challenge. Herein, persistent luminescence nanoparticles (PLNPs) with strong near-infrared (NIR) emission character behaving as a label-free probe for the highly sensitive and selective detection of Hb were developed. Further studies revealed that the sensing mechanism should be attributed to the Hb-induced dynamic quenching process. Moreover, the nanoprobe showed high selectivity to Hb against the common existing substances in human serum and a linear response to Hb ranging from 1 to 50 nM with an extremely high limit of detection (LOD) of 0.13 nM. Finally, applicability of the proposed probe for the detection of Hb in human serum samples was validated.

A fluorometric displacement assay for adenosine triphosphate using layered cobalt(II) double hydroxide nanosheets

A turn-on fluorometric method is described for the determination of adenosine-5'-triphosphate (ATP). It is based on the displacement of a dye-labeled oligonucleotide from a cobalt(II) based layered double hydroxide (LDH). Due to the electrostatic and ligand exchange interaction, the FAM-labeled DNA is readily adsorbed on the LDH. This leads to complete and fast quenching of the green fluorescence of the label. However, on addition of ATP, the DNA is detached from the LDH because of the stronger affinity of ATP for LDH. This results in the restoration of the green fluorescence. The effect was used to design a sensitive assay that has a linear response in the 0.5-100 μM ATP concentration range and a 0.23 μM lower detection limit. It was applied to the determination of ATP in spiked serum samples. Graphical abstract Schematic presentation of a fluorometric ATP assay based on the displacement of a dye-labeled oligonucleotide from a layered double hydroxide (LDH).

Cytotoxicity and autophagy induction by graphene quantum dots with different functional groups

Graphene quantum dots (GQDs) possess great potential in various applications due to their superior physicochemical properties and wide array of available surface modifications. However, the toxicity of GQDs has not been systematically assessed, thus hindered their further development; especially, the risk of surface modifications of GQDs is largely unknown. In this study, we employed a lung carcinoma A549 cells as the model to investigate the cytotoxicity and autophagy induction of three types GQDs, including cGQDs (COOH-GQDs), hGQDs (OH-GQDs), and aGQDs (NH₂-GQDs). The results showed hGQDs was the most toxic, as significant cell death was induced at the concentration of 100 μg/mL, determining by WST-1 assay as well as Annexin-V-FITC/PI apoptosis analysis, whereas cGQDs and aGQDs were non-cytotoxic within the measured concentration. Autophagy detection was performed by TEM examination, LC3 fluorescence tracking, and Western-blot. Both aGQDs and hGQDs induced cellular autophagy to various degrees except for cGQDs. Further analysis on autophagy pathways indicated all GQDs significantly activated p-p38MAPK; p-ERK1/2 was inhibited by aGQDs and hGQDs but activated by cGQDs. p-JNK was inhibited by aGQDs and cGQDs, while activated by hGQDs. Simultaneously, Akt was activated by hGQDs but inhibited by aGQDs. Inhibition of autophagy by 3-MA significantly increased the cytotoxicity of GQDs, suggesting that autophagy played a protective role against the toxicity of GQDs. In conclusion, cGQDs showed excellent biocompatibility and may be considered for biological applications. Autophagy induction may be included in the health risk assessment of GQDs as it reflects the stress status which may eventually lead to diseases.

Kanamycin detection at graphene quantum dot-decorated gold nanoparticles in organized medium after solid-phase extraction using an aminoglycoside imprinted polymer

This is a description of the indirect determination of kanamycin sulfate though the photoluminescence enhancement of an aqueous dispersion of amino-functionalized graphene quantum dots (amino-GQDs) coupled with gold nanoparticles (AuNPs) in a cationic surfactant-rich medium. Specifically, cetyltrimethylammonium bromide (CTAB) was used as the cationic surfactant in our work. Previously, solid phase extraction with a cartridge packed with aminoglycoside-selective imprinted polymer ensured selectivity in kanamycin determination in yellow-fever vaccine and veterinary pharmaceutical samples. The proposed method has trace analysis capability and it is simple to perform as it does not involve the use of toxic reagents employed for chemical derivatization of aminoglycoside antibiotics.

Fluorescent probes guided by a new practical performance regulation strategy to monitor glutathione in living systems

A practical regulation strategy for the design of glutathione specific probes and their application in living systems.

Glutathione (GSH) plays an important role in the body's biochemical defense system, and the detection of GSH in a physiological system is an important tool for understanding redox homeostasis. Protection–deprotection strategies have proven to be the most reliable, among existing detection methods. However, the understanding of how various electronic and steric effects influence a probe's ability to recognize a substrate is still lacking. In this study, we have analyzed various substituent effects on a GSH probe template via theoretical calculations and constructed the performance regulation and control strategy for this kind of probe. We then developed a series of guided probes using eighteen different acrylic ester derivatives to mask the fluorescence of fluorescein. The optical performance differences between the guided probes strongly supported the applicability of our proposed guiding strategy. Moreover, the positively guided probes are excellent for imaging GSH distribution in living cells and mice.

Preparation of Highly Catalytic N-Doped Carbon Dots and Their Application in SERS Sulfate Sensing

Carbon dots (CD) have excellent stability and fluorescence activity, and have been widely used in fluorescence methods. However, there are no reports about using CD as catalysts to amplify SERS signals to detect trace sulfate. Thus, preparing CD catalysts and their application in SERS sulfate-sensing are significant. In this article, highly catalytic N-doped carbon dots (CD_N) were prepared by a hydrothermal procedure. CD_N exhibited strong catalysis of the gold nanoparticle (AuNP) reaction between HAuCl₄ and H₂O₂. Vitoria blue 4R (VB4R) has a strong SERS peak at 1614 cm⁻¹ in the formed AuNP sol substrate. When Ba²⁺ ions were added, they were adsorbed on a CD_N surface to inhibit the CD_N catalytic activity that caused the SERS peak decreasing. Upon addition of analyte SO₄²⁻, a reaction with Ba²⁺ produced stable BaSO₄ precipitate and CD_N, and its catalysis recovered to cause SERS intensity increasing linearly. Thus, an SERS method was developed for the detection of 0.02–1.7 μmol/L SO₄²⁻, with a detection limit of 0.007 μmol/L.

A novel fluorescence "turn off-on" nanosensor for sensitivity detection acid phosphatase and inhibitor based on glutathione-functionalized graphene quantum dots

In this paper, we developed a label-free and sensitive fluorescence sensor for acid phosphatase (ACP) and its inhibitor parathion-methyl (PM) detection based on glutathione-functionalized graphene quantum dots (GQDs@GSH). Upon addition of MnO₂ nanosheets, the fluorescence of GQDs@GSH could be efficiently quenched via a fluorescence resonance energy transfer. ACP could easily catalyze the hydrolysis of L-Ascorbic acid-2-phosphate (AAP) to ascorbic acid (AA), which could reduce MnO₂ nanosheets to Mn²⁺ in acidic environment, leading to dramatically increase of the fluorescence intensity of GQDs@GSH. Quantitative detection of ACP in a broad range from 0.1 to 9 mU mL^-1 with a detection limit of 0.027 mU mL^-1 could be achieved. The feasibility of the proposed sensor in real samples analysis was also studied and satisfactory results were obtained. Furthermore, the fluorescence assay strategy could also be used for the detection of parathion-methyl (PM) as ACP inhibitor.

Luminescence phenomena of carbon dots derived from citric acid and urea - a molecular insight

In this report, we present the results of our investigations into the elucidation of the chemical structure of moieties responsible for the blue and green luminescence of CDs derived from the microwave-assisted pyrolysis of citric acid in the presence of urea. The molecular fluorophore that forms during the synthesis of green fluorescing CDs is 4-hydroxy-1H-pyrrolo[3,4-c]pyridine-1,3,6(2H,5H)-trione (HPPT).

A novel fluorimetric sensing strategy for highly sensitive detection of phytic acid and hydrogen peroxide

In this paper, we developed a sensitive sensor for phytic acid (PA) and hydrogen peroxide (H₂O₂) detection based on glutathione-functionalized graphene quantum dots (GQDs@GSH). The fluorescence of GQDs@GSH was found to be effectively quenched by Fe³⁺ ions via photo-induced electron transfer (PET) process. Upon the addition of PA to GQDs@GSH/Fe³⁺ system, the fluorescence of GQDs@GSH was significantly restored due to the strong chelating and reducing ability of PA, Fe³⁺ ions could be reduced to Fe²⁺ ions by PA and formed PA/Fe²⁺ complex. Therefore, the "off-on" fluorescence method was constructed to detect PA by using GQDs@GSH/Fe³⁺ as a fluorescent probe. Furthermore, the method can be used for the detection of H₂O₂. H₂O₂ can destroy the chelate structure of PA/Fe²⁺, release Fe²⁺ ions and oxidize Fe²⁺ ions to produce Fe³⁺ ions, leading to the fluorescence quenching of GQDs@GSH again. Under optimal conditions, the fluorescence sensing platform showed good linear relationship between the relative fluorescence intensity I/I₀ and the concentration of PA and H₂O₂ in the range of 0.05-3 μmol L^-1 and 0.5-10 μmol L^-1, respectively. The detection limits of PA and H₂O₂ were 14 nmol L^-1 and 0.134 μmol L^-1, respectively. Furthermore, the fluorescence assay method was also applied in real sample analysis and satisfactory results were obtained.

Ratiometric fluorescent detection of chromium(VI) in real samples based on dual emissive carbon dots

As we know, hexavalent chromium (Cr(VI)) was usually used as an additive to improve the color fastness during the printing and dyeing process, and thus posing tremendous threat to our health and living quality. In this work, the dual emissive carbon dots (DECDs) were synthesized through hydrothermal treatment of m-aminophenol and oxalic acid. The obtained DECDs not only exhibited dual emission fluorescence peaks (430 nm, 510 nm) under the single excitation wavelength of 380 nm, but also possessed good water solubility and excellent fluorescence stability. A ratiometric fluorescent method for the determination of Cr(VI) was developed using the DECDs as a probe. Under the optimal conditions, a linear range was obtained from 2 to 300 μM with a limit of detection of 0.4 μM. Furthermore, the proposed ratiometric fluorescent method was applied to the analysis of Cr(VI) in textile, steel, industrial wastewater and chromium residue samples with satisfactory recoveries (88.4-106.8%).

Optical sensing at the nanobiointerface of metal ion-optically-active nanocrystals

Optically-active nanocrystals (such as quantum dots and plasmonic noble metal nanoparticles) have received great attention due to their size-tunable optical properties. The indicator displacement assay (IDA) with optically-active nanocrystals has become a common practice for optical sensor development, since no sophisticated surface functionalization of nanoparticles is required. Among the IDA-based optical sensors, the use of metal ions as receptors seems to be attractive. Therefore, in this review, the research progress of optical sensing at the nanobiointerface of metal ion-optically-active nanocrystals has been summarized. In particular, metal ion-mediated selective recognition has been summarized here based on the classical Hard-Soft-Acid-Base (HSAB) principle, which has been seldom mentioned before. Most of the references were therefore categorized according to their located place based on the HSAB theory. Besides, several metal ion modulation strategies that were not related to the HSAB theory (e.g., redox modulation) were also included. Finally, due to the cross-talk of metal ions in selective recognition, we have also summarized sensor array development based on multiple metal ion receptors in IDA sensing with optically-active nanocrystals. Several interesting applications of the IDA sensing with metal ions as receptors and optically-active nanocrystals as indicators are presented, with specific emphasis on the design principles and photophysical mechanisms of these probes.

Facile and Highly Effective Synthesis of Controllable Lattice Sulfur-Doped Graphene Quantum Dots via Hydrothermal Treatment of Durian

Recently, the biomass "bottom-up" approach for the synthesis of graphene quantum dots (GQDs) has attracted broad interest because of the outstanding features, including low-cost, rapid, and environmentally friendly nature. However, the low crystalline quality of products, substitutional doping with heteroatoms in lattice, and ambiguous reaction mechanism strongly challenge the further development of this technique. Herein, we proposed a facile and effective strategy to prepare controllable sulfur (S) doping in GQDs, occurring in a lattice substitution manner, by hydrothermal treatment of durian with platinum catalyst. S atoms in GQDs are demonstrated to exist in the thiophene structure, resulting in good optical and chemical stabilities, as well as ultrahigh quantum yield. Detailed mechanism of the hydrothermal reaction progress was investigated. High-efficiency reforming cyclization provided by platinum was evidenced by the coexistence of diversified sp²-fused heterocyclic compounds and thiophene derivatives. Moreover, we also demonstrated that saccharides in durian with small molecular weight (<1000 Da) is the main carbon source for the forming GQDs. Because of the desulfurizing process, controllable photoluminescence properties could be achieved in the as-prepared GQDs via tuning doping concentrations.

L-tyrosine methyl ester-stabilized carbon dots as fluorescent probes for the assays of biothiols

Over the past few decades, assays of biothiols had attracted much attention due to the essential role they played in human physiology, especially using the fluorescent analysis. In most cases, competitive mechanism was often employed, where the metal ions were often introduced as the quenchers and thiols competed with metal ions due to the high binding affinity and strong thiophilicity for 'signal-on' assays. To develop a metal ions-free approach for the assays of thiols, here, L-tyrosine methyl ester capped carbon dots (Tyr-CDs) were employed and prepared as the fluorescent probes. The as-prepared Tyr-CDs displayed narrow size distribution and distinct blue fluorescence with high quantum yield (12.9%) compared with the unmodified CDs. Moreover, Tyr-CDs exhibited higher quenching efficiency due to the efficient energy transfer between Tyr-CDs and the quinone products in the presence of tyrosinase. When the targeted biothiols was present, the catalytic reaction of the tyrosinase to the formation of quinone was inhibited and the fluorescence signal was recovered in a biothiols-concentration-dependent manner, which provided the basis for the analysis of biothiols. The practical application of the present system was demonstrated by testing the biothiols in human plasma samples and good recovery was obtained, indicating that the sensing platform we proposed hold great promise in the accurate detection of biothiols in complex biosystems.

Functional Carbon Quantum Dots: A Versatile Platform for Chemosensing and Biosensing

Carbon quantum dot has emerged as a new promising fluorescent nanomaterial due to its excellent optical properties, outstanding biocompatibility and accessible fabrication methods, and has shown huge application perspective in a variety of areas, especially in chemosensing and biosensing applications. In this personal account, we give a brief overview of carbon quantum dots from its origin and preparation methods, present some advance on fluorescence origin of carbon quantum dots, and focus on development of chemosensors and biosensors based on functional carbon quantum dots. Comprehensive advances on functional carbon quantum dots as a versatile platform for sensing from our group are included and summarized as well as some typical examples from the other groups. The biosensing applications of functional carbon quantum dots are highlighted from selective assays of enzyme activity to fluorescent identification of cancer cells and bacteria.