A FRET chemsensor based on graphene quantum dots for detecting and intracellular imaging of Hg²⁺

Graphene quantum dots for biosensing and bioimaging

Graphene Quantum Dots (GQDs) are low dimensional carbon based materials with interesting physical, chemical and biological properties that enable their applications in numerous fields. GQDs possess unique electronic structures that impart special functional attributes such as tunable optical/electrical properties in addition to heteroatom-doping and more importantly a propensity for surface functionalization for applications in biosensing and bioimaging. Herein, we review the recent advancements in the top-down and bottom-up approaches for the synthesis of GQDs. Following this, we present a detailed review of the various surface properties of GQDs and their applications in bioimaging and biosensing. GQDs have been used for fluorescence imaging for visualizing tumours and monitoring the therapeutic responses in addition to magnetic resonance imaging applications. Similarly, the photoluminescence based biosensing applications of GQDs for the detection of hydrogen peroxide, micro RNA, DNA, horse radish peroxidase, heavy metal ions, negatively charged ions, cardiac troponin, etc. are discussed in this review. Finally, we conclude the review with a discussion on future prospects.

Carbon dots/Ruthenium(III) nanocomposites for FRET fluorescence detection and removal of mercury (II) via assembling into nanofibers

The determination and removal of mercury(II) (Hg2+) are essential for human health and environmental ecosystems. Herein, an ingenious carbon dots (CDs)-based Förster resonance energy transfer (FRET) system (N, S-CDs/Ru) was fabricated employing CDs and Ru3+ units as energy-transfer doner/acceptor pairs for visual detection and efficient removal of Hg2+. The treatment of Hg2+ induced a remarkable linear enhancement of the ratiometric fluorescence (F613 nm/F478 nm) with a detection limit (LOD) of 95 nM, along with continuous fluorescence color variations from blue to red. Given that the fluorescence color recognition and processing realized the real-time and rapid quantitation of Hg2+ by paper-based smartphone sensing platform. The mechanistic study revealed that the N/S/O-rich surface of the system enabled the Hg2+-triggered self-assembly from dots to nanofibers, combing with the active FRET process. Also, the efficient removal of Hg2+ with a removal efficiency of ∼98 % and an adsorption capacity of ∼372 mg/g was obtained. Furthermore, it was found that N, S-CDs/Ru loaded commercialized SiO2 or SBA-15 could facilitate the removal of Hg2+ with a removal efficiency over 99 % and an adsorption capacity up to ∼562 mg/g. This study provides a potential strategy for environmental monitoring and remediation.

A Novel Turn-On Fluorescent Sensor Based on Sulfur Quantum Dots and MnO2 Nanosheet Architectures for Detection of Hydrazine

In this paper, the SQDs@MnO2 NS as the probe was applied to construct a novel “turn-on” fluorescent sensor for sensitive and selective detection of hydrazine (N2H4). Sulfur quantum dots (SQDs) and MnO2 nanosheets (MnO2 NS) were simply mixed, through the process of adsorption to prepare the architectures of SQDs@MnO2 NS. The fluorescent emissions of SQDs@MnO2 NS play a key role to indicate the state of the sensor. According to the inner filter effect (IFE) mechanism, the state of the sensor at the “off” position, or low emission, under the presence of MnO2 NS, is which the ultraviolet and visible spectrum overlaps with the fluorescence emission spectrum of SQDs. Under the optimal conditions, the emission was gradually recovered with the addition of the N2H4, since the N2H4 as a strong reductant could make the MnO2 NS converted into Mn2+, the state of the sensor at the “on”. Meanwhile, the fluorescent sensor possesses good selectivity and high sensitivity, and the detection concentration of N2H4 with a wide range from 0.1 µM to 10 mM with a detection limit of 0.072 µM. Furthermore, actual samples were successful in detecting certain implications, indicating that the fluorescent sensor possesses the potential application ability to monitor the N2H4 in the water.

Metal ion sensing with graphene quantum dots: detection of harmful contaminants and biorelevant species

Graphene quantum dots (GQDs) are attractive materials for use as highly selective and sensitive chemical sensors, owing to their simple preparation and affordability. GQDs have been successfully deployed as sensors for toxic metal ions, which is a significant issue due to the ever-increasing environmental contamination from agricultural and industrial activities. Despite the success of GQDs in this area, the mechanisms which underpin GQD-metal ion specificity are rarely explored. This lack of information can result in difficulties when attempting to replicate published procedures and can limit the judicious design of new highly selective GQD sensors. Furthermore, there is a dearth of GQD examples which selectively detect biologically relevant alkali and alkaline earth metals. This review will present the current state of GQDs as metal ion sensors for harmful contaminants, highlighting and discussing the discrepancies that exist in the proposed mechanisms regarding metal ion selectivity. The emerging field of GQD sensors for biorelevant metal ion species will also be reviewed, with a perspective to the future of this highly versatile material.

Fluorescence Microscopy-An Outline of Hardware, Biological Handling, and Fluorophore Considerations

Fluorescence microscopy has become a critical tool for researchers to understand biological processes at the cellular level. Micrographs from fixed and live-cell imaging procedures feature in a plethora of scientific articles for the field of cell biology, but the complexities of fluorescence microscopy as an imaging tool can sometimes be overlooked or misunderstood. This review seeks to cover the three fundamental considerations when designing fluorescence microscopy experiments: (1) hardware availability; (2) amenability of biological models to fluorescence microscopy; and (3) suitability of imaging agents for intended applications. This review will help equip the reader to make judicious decisions when designing fluorescence microscopy experiments that deliver high-resolution and informative images for cell biology.

Function-Oriented Graphene Quantum Dots Probe for Single Cell in situ Sorting of Active Microorganisms in Environmental Samples

Functional microorganisms play a vital role in removing environmental pollutants because of their diverse metabolic capability. Herein, a function-oriented fluorescence resonance energy transfer (FRET)-based graphene quantum dots (GQDs-M) probe was developed for the specific identification and accurate sorting of azo-degrading functional bacteria in the original location of environmental samples for large-scale culturing. First, nitrogen-doped GQDs (GQDs-N) were synthesized using a bottom-up strategy. Then, a GQDs-M probe was synthesized based on bonding FRET-based GQDs-N to an azo dye, methyl red, and the quenched fluorescence was recovered upon cleavage of the azo bond. Bioimaging confirmed the specific recognition capability of GQDs-M upon incubation with the target bacteria or environmental samples. It is suggested that the estimation of environmental functional microbial populations based on bioimaging will be a new method for rapid preliminary assessment of environmental pollution levels. In combination with a visual single-cell sorter, the target bacteria in the environmental samples could be intuitively screened at the single-cell level in 17 bacterial strains, including the positive control Shewanella decolorationis S12, and were isolated from environmental samples. All of these showed an azo degradation function, indicating the high accuracy of the single-cell sorting strategy using the GQDs-M. Furthermore, among the bacteria isolated, two strains of Bacillus pacificus and Bacillus wiedmannii showed double and triple degradation efficiency for methyl red compared to the positive control (strain S12). This strategy will have good application prospects for finding new species or high-activity species of specific functional bacteria.

Cytotoxicity and Bioimaging Study for NHDF and HeLa Cell Lines by Using Graphene Quantum Pins

Herein, we report the synthesis of an interesting graphene quantum material called "graphene quantum pins (GQPs)". Morphological analysis revealed the interesting pin shape (width: ~10 nm, length: 50-100 nm) and spectral analysis elucidated the surface functional groups, structural features, energy levels, and photoluminescence properties (blue emission under 365 nm). The difference between the GQPs and graphene quantum dos (GQDs) isolated from the same reaction mixture as regards to their morphological, structural, and photoluminescence properties are also discussed along with the suggestion of a growth mechanism. Cytotoxicity and cellular responses including changes in biophysical and biomechanical properties were evaluated for possible biomedical applications of GQPs. The studies demonstrated the biocompatibility of GQPs even at a high concentration of 512 μg/mL. Our results suggest GQPs can be used as a potential bio-imaging agent with desired photoluminescence property and low cytotoxicity.

Facile and label-free fluorescence sensing of β-galactosidase activity by graphene quantum dots

β-Galactosidase (β-Gal), as a glycoside hydrolase, is closely associated with cell senescence and primary ovarian cancer. However, there is still lack of facile and rapid sensing approach to monitor the β-Gal activity. In this work, a label-free and convenient sensing strategy to detect β-Gal activity has been proposed based on fluorescent graphene quantum dots (GQDs). In the presence of β-Gal, 4-nitrophenyl-β-D-galactopyranoside (NPGal) can be hydrolyzed into 4-nitrophenol (4-NP), which serves as a good quencher to quench the fluorescence of GQDs. The quenching mechanism is proven to be inner filter effect (IFE). Due to the specificity of the enzymatic reaction, this sensing method displays excellent selectivity and high sensitivity. A broad dynamic range from 20 to 200 U L^-1 and a detection limit of 4.4 U L^-1 for the β-Gal assay are achieved. Compared with the previously reported methods, this sensing strategy only needs one fluorescent nanomaterial without any modification and avoids time-consuming handling steps. Therefore, the sensing strategy based on fluorescent GQDs offers great potential for the recognition of disease-correlated enzyme activity.

Comparison of two rhodamine-based polystyrene solid-phase fluorescent sensors for mercury(II) determination

Two novel rhodamine-based polystyrene solid-phase fluorescent sensors PS-AC-I and PS-AC-II with different coordination atoms (O or S) are synthesized and shown to be able to detect Hg(II) ions. They are characterized by Fourier-transform infrared spectroscopy and by scanning electron microscopy analysis. Their fluorescent properties, including response time, pH effects, fluorescence titrations, metal ion competition and recycling, are investigated and compared. Sensor PS-AC-II displayed higher selectivity and sensitivity to Hg(II), with a lower detection limit of 0.032 µM, which was 15 times better than PS-AC-I. A detection mechanism involving the Hg(II) chelation-induced ring-opening of the rhodamine spirolactam is proposed with the aid of theoretical calculations.

One minute synthesis of green fluorescent copper nanocluster: The preparation of smartphone aided paper-based kit for on-site monitoring of nanomolar level mercury and sulfide ions in environmental samples

We wish to report, a minute synthesis of green fluorescent copper nanocluster by simple sonication. 1-Thio-β-d-glucose was used as a capping ligand to synthesis copper nanocluster (TG-CuNCs). The TG-CuNCs exhibit the emission maximum at 430 nm. The synthesized TG-CuNCs was well characterized by UV-vis, fluorescent, XRD, HR-TEM and FT-IR techniques. After the addition of Hg²⁺ or S^2- into TG-CuNCs, the fluorescence was quenched. Based on the quenching of fluorescence, we have calculated the detection limit 1.7 nM and 1.02 nM for Hg²⁺ and S^2-, respectively. Finally, we have applied TG-CuNCs for the detection of Hg²⁺ and S^2- in tap, river, pond water. Importantly, the smartphone aided paper-based kit was developed for on-site monitoring of Hg²⁺ and S^2- ions. To the best of our knowledge, this is the first report for the one-minute synthesis of TG-CuNCs and the preparation of smartphone aided paper-based kit for on-site monitoring of Hg²⁺ and S^2- ions. Further, it is anticipated that this synthesis of TG-CuNCs and smartphone aided paper-based kit for Hg²⁺ and S^2- will be useful materials in the filled with the biosensor, material science and nanotechnology.

Development of Graphene Quantum Dots-Based Optical Sensor for Toxic Metal Ion Detection

About 71% of the Earth's surface is covered with water. Human beings, animals, and plants need water in order to survive. Therefore, it is one of the most important substances that exist on Earth. However, most of the water resources nowadays are insufficiently clean, since they are contaminated with toxic metal ions due to the improper disposal of pollutants into water through industrial and agricultural activities. These toxic metal ions need to be detected as fast as possible so that the situation will not become more critical and cause more harm in the future. Since then, numerous sensing methods have been proposed, including chemical and optical sensors that aim to detect these toxic metal ions. All of the researchers compete with each other to build sensors with the lowest limit of detection and high sensitivity and selectivity. Graphene quantum dots (GQDs) have emerged as a highly potential sensing material to incorporate with the developed sensors due to the advantages of GQDs. Several recent studies showed that GQDs, functionalized GQDs, and their composites were able to enhance the optical detection of metal ions. The aim of this paper is to review the existing, latest, and updated studies on optical sensing applications of GQDs-based materials toward toxic metal ions and future developments of an excellent GQDs-based SPR sensor as an alternative toxic metal ion sensor.

Interband Absorption in Few-Layer Graphene Quantum Dots: Effect of Heavy Metals

Monolayer, bilayer, and trilayer graphene quantum dots (GQDs) with different binding abilities to elemental heavy metals (HMs: Cd, Hg, and Pb) were designed, and their electronic and optical properties were investigated theoretically to understand deeply the optical response under heavy metal exposure. To gain insight into the nature of interband absorption, we performed density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations for thickness-varying GQDs. We found that the interband absorption in GQDs can be efficiently tuned by controlling the thickness of GQDs to attain the desirable coloration of the interacting complex. We also show that the strength of the interaction between GQDs and Cd, Hg, and Pb is strongly dependent on the number of sp²-bonded layers. The results suggest that the thickness of GQDs plays an important role in governing the hybridization between locally-excited (LE) and charge-transfer (CT) states of the GQDs. Based on the partial density-of-states (DOS) analysis and in-depth knowledge of excited states, the mechanisms underlying the interband absorption are discussed. This study suggests that GQDs would show an improved sensing performance in the selective colorimetric detection of lead by the thickness control.

Azocalix[4]arene with three distal ethyl ester residues as a highly selective chromogenic sensor for Ca2+ ions

Three azocalix[4]arenes with distal ethyl ester residues, 5-phenylazo-25,26,27-tris[(ethoxycarbonyl)methoxy]-28-hydroxycalix[4]arene (2), 5-(o-methylphenyl)azo-25,26,27-tris[(ethoxycarbonyl)methoxy]-28-hydroxycalix[4]arene (3), 5-(p-Methylphenyl)azo-25,26,27-tris[(ethoxycarbonyl)methoxy]-28-hydroxycalix[4]arene (4), were synthesized and their binding properties with metal ions were investigated by ultraviolet (UV)/visible spectroscopy. The chromogenic behavior of these compounds upon metal ion complexation indicates a specific selectivity toward Ca2+ ion in the presence of other cations tested. The stoichiometry of 3 to Ca2+ ion in the complex is 1:1 and the stability constant of the complex is 1.28×104m−1.

Sensitive immunoassay of von Willebrand factor based on fluorescence resonance energy transfer between graphene quantum dots and Ag@Au nanoparticles

Graphene quantum dots (GQDs) and core-shell Ag@Au nanoparticles (Ag@Au NPs) were synthetized and they were characterized by transmission electron microscope and X-ray photoelectron spectra, respectively. Von Willebrand factor antibody (vWF Ab) was bound on Ag@Au NPs to construct Ag@Au-Ab nanocomposites (Ag@Au-Ab NCs). The fluorescence of GQDs could be effectively quenched by the prepared nanocomposites owing to fluorescence resonance energy transfer (FRET). The immunoreaction between vWF and Ag@Au-Ab NCs resulted in the declined FRET efficiency and a degree of fluorescence recovery of GQDs. The fluorescence intensity change was found to be proportional to the logarithm of the vWF concentration in the range of 0.1 pg mL^-1-10 ng mL^-1 with a detection limit of 30 fg mL^-1. The proposed fluorescence sensor was employed to investigate the relationship between the release of vWF and the oxidation-injury degree of vascular endothelial cells. The experimental results indicate that the vWF content in the growth medium was enhanced and the cell injury was intensified when the contact time of the cells with H₂O₂ was increased.

Tuning the optical properties of graphene quantum dots for biosensing and bioimaging

This review highlights new insights into the various strategies used to tune the optical features of graphene quantum dots, and their use as attractive and powerful probes for bio-sensing/imaging.

Rhodamine-based ratiometric fluorescent probes based on excitation energy transfer mechanisms: construction and applications in ratiometric sensing

Rhodamine is a convenient platform for the construction of “OFF–ON” ratiometric excitation energy transfer fluorescent probes.

Quantum dots in nanomedicine: recent trends, advances and unresolved issues

The review addresses the current state of progress in the use of ultra-small nanoparticles from the category of quantum dots (QDs), which presently embraces a widening range of nanomaterials of different nature, including "classical" semiconductor groups III-V and II-VI nanocrystals, along with more recently emerged carbon, silicon, gold and other types of nanoparticles falling into this class of nanomaterials due to their similar physical characteristics such as small size and associated quantum confinement effects. A diverse range of QDs applications in nanomedicine has been extensively summarised previously in numerous publications. Therefore, this review is not intended to provide an all-embracing survey of the well documented QDs uses, but is rather focused on the most recent emerging developments, concepts and outstanding unresolved problematic and sometimes controversial issues. Over 125 publications are overviewed and discussed here in the context of major nanomedicine domains, i.e. medical imaging, diagnostics, therapeutic applications and combination of them in multifunctional theranostic systems.

Highly sensitive and selective detection of mercury ions based on up-conversion FRET from NaYF4:Yb3+/Er3+ nanophosphors to CdTe quantum dots

The detection of Hg²⁺ has attracted considerable attention because of the serious health and environmental problems caused by it.