Sulfur-doped graphene quantum dots as a novel fluorescent probe for highly selective and sensitive detection of Fe(3+)

Nucleobase chemosensor based on carbon nanodots

A facile and sensitive fluorescence protocol for nucleobase detection was developed based on carbon nanodot (CD) chemosensors. The novel fluorescent CDs were prepared using four kinds of nucleobases (including adenine, guanine, thymine and cytosine) as separate carbon sources via simple hydrothermal strategy. The quantum yield of adenine CDs (A-CDs), guanine CDs (G-CDs), thymine CDs (T-CDs) and cytosine CDs (C-CDs) was checked as 15.1%, 28.3%, 10.6% and 11.7%, respectively. Four CDs can recognize their complementary nucleobases based on the principle of complementary base pairing. Their fluorescence was linearly quenched with the increase of nucleobase concentrations under optimal conditions. Combining the calibration curve, quantitative assay of nucleobase in solution can be realized. For example, A-CDs could determine thymine in the concentration range of 2-20mM with a detection limit of ca. 0.053mM, and the linear equation is fitting as (I₀-I) / I = 0.01961 × C_T(mM) + 0.01756 (R² = 0.994). Thymine can induce the fluorescence lifetime of A-CDs decreasing from 5.58 to 3.34ns, indicating a dynamic quenching mechanism. The novel nucleobase sensors were also evaluated in specific solution environment. A-CDs showed a relatively minor relative standard deviation (< 4.0%) in fetal calf serum solution, indicating a high accuracy and credibility of the sensing system. In view of the excellent sensitivity, preferable biocompatibility as well as simple constructing method, the sensing platform derived from the nucleobase-based CDs present great potential in biological sensing applications.

pH-Responsive fluorescent graphene quantum dots for fluorescence-guided cancer surgery and diagnosis

Cancer remains a major cause of morbidity and mortality around the world. Improved cancer treatment requires enhancement of cancer diagnosis and detection. To achieve this goal, here we report a novel imaging probe, pH-responsive fluorescent graphene quantum dots (pRF-GQDs). pRF-GQDs were prepared by electrolysis of graphite rods in sodium p-toluenesulfonate acetonitrile solution. The resulting pRF-GQDs, which have minimal toxicity, display a sharp fluorescence transition between green and blue at pH 6.8, a pH matching the acidic extracellular microenvironment in solid tumors. We found that this unique fluorescence switch property allows tumors to be distinguished from normal tissues. In addition to fluorescence, pRF-GQDs also exhibit upconversion photoluminescence (UCPL). We demonstrate that the combination of UCPL and fluorescence switch enables detection of solid tumors of different origin at an early developmental stage. Therefore, pRF-GQDs have great potential to be used as a universal probe for fluorescence-guided cancer surgery and cancer diagnosis.

Red Emission B, N, S-co-Doped Carbon Dots for Colorimetric and Fluorescent Dual Mode Detection of Fe3+ Ions in Complex Biological Fluids and Living Cells

Colorimetric and fluorescent dual mode detection methods have gained much attention in recent years; however, it is still desirable to develop new colorimetric and fluorescent dual mode nanosensors with more simple preparation procedures, low cost, and excellent biocompatibility. Herein, a colorimetric and fluorescent nanosensor based on B, N, S-co-doped carbon dots (BNS-CDs) was synthesized by one-step hydrothermal treatment of 2,5-diaminobenzenesulfonic acid and 4-aminophenylboronic acid hydrochloride. Using this nanosensor, a highly sensitive assay of Fe³⁺ in the range of 0.3-546 μM with a detection limit of 90 nM was provided by quenching the red emission fluorescence. It is more attractive that Fe³⁺ can also be visualized by this nanosensor via evident color changes of the solution (from red to blue) under sunlight without the aid of an ultraviolet (UV) lamp. Furthermore, the designed nanosensor can be applied for efficient detection of intracellular Fe³⁺ with excellent biocompatibility and cellular imaging capability, and it holds great promise in biomedical applications.

Surface passivation of carbon dots with ethylene glycol and their high-sensitivity to Fe3+

Surface passivation of carbon dots with ethylene glycol and their application in the field of Fe³⁺ detection.

Bright Multicolor Bandgap Fluorescent Carbon Quantum Dots for Electroluminescent Light-Emitting Diodes

Multicolor bandgap fluorescent carbon quantum dots (MCBF-CQDs) from blue to red with quantum yield up to 75% are synthesized using a solvothermal method. For the first time, monochrome electroluminescent light-emitting diodes (LEDs) with MCBF-CQDs directly as an active emission layer are fabricated. The maximum luminance of blue LEDs reaches 136 cd m-2 , which is the best performance for CQD-based monochrome electroluminescent LEDs.

Controlled synthesis of soluble conjugated polymeric nanoparticles for fluorescence detection

Highly fluorescent soluble conjugated polymeric nanoparticles (SCPNs) were synthesized in confined nanoreactors and used for fluorescence sensing of glucose and Fe ion.

Supramolecular interactions via hydrogen bonding contributing to citric-acid derived carbon dots with high quantum yield and sensitive photoluminescence

Fluorophores coupled with supramolecules lead to “dot” topologies in citric-acid derived carbon dots under the effect of hydrogen bonding.

Thioacetamide-derived nitrogen and sulfur co-doped carbon nanoparticles used for label-free detection of copper(ii) ions and bioimaging applications

Thioacetamide-derived nitrogen and sulfur co-doped carbon nanoparticles as photoluminescent probes for the efficient detection of Cu²⁺ ions.

A core–shell metal–organic-framework (MOF)-based smart nanocomposite for efficient NIR/H2O2-responsive photodynamic therapy against hypoxic tumor cells

In this work, core–shell MOF-based smart nanocomposite UCNPs/MB@ZIF-8@catalase has been constructed for bio-imaging and efficient NIR/H₂O₂-responsive photodynamic therapy against hypoxic tumor cells.

Facile and scalable preparation of highly luminescent N,S co-doped graphene quantum dots and their application for parallel detection of multiple metal ions

New and bright N,S co-doped GQDs are synthesized and serve as fluorescence probes for parallel and selective detection of Fe³⁺, Cu²⁺ and Ag⁺ ions in biological and environmental samples.

Highly Sensitive and Selective Detection of Nanomolar Ferric Ions Using Dopamine Functionalized Graphene Quantum Dots

The good stability, low cytotoxicity, and excellent photoluminescence property of graphene quantum dots (GQDs) make them an emerging class of promising materials in various application fields ranging from sensor to drug delivery. In the present work, the dopamine-functionalized GQDs (DA-GQDs) with stably bright blue fluorescence were successfully synthesized for low level Fe(3+) ions detection. The as-synthesized GQDs are uniform in size with narrow-distributed particle size of 4.5 ± 0.6 nm and high quantum yield of 10.2%. The amide linkage of GQDs with dopamine, confirmed by using XPS and FTIR spectra, results in the specific interaction between Fe(3+) and catechol moiety of dopamine at the interfaces for highly sensitive and selective detection of Fe(3+). A linear range of 20 nM to 2 μM with a detection limit of 7.6 nM is obtained for Fe(3+) detection by DA-GQDs. The selectivity of DA-GQDs sensing probe is significantly excellent in the presence of other interfering metal ions. In addition, the reaction mechanism for Fe(3+) detection based on the complexation and oxidation of dopamine has been proposed and validated. Results obtained in this study clearly demonstrate the superiority of surface functionalized GQDs to Fe(3+) detection, which can pave an avenue for the development of high performance and robust sensing probes for detection of metal ions and other organic metabolites in environmental and biomedical applications.

Fluorescent nanoprobes for sensing and imaging of metal ions: recent advances and future perspectives

Recent advances in nanoscale science and technology have generated nanomaterials with unique optical properties. Over the past decade, numerous fluorescent nanoprobes have been developed for highly sensitive and selective sensing and imaging of metal ions, both in vitro and in vivo. In this review, we provide an overview of the recent development of the design and optical properties of the different classes of fluorescent nanoprobes based on noble metal nanomaterials, upconversion nanoparticles, semiconductor quantum dots, and carbon-based nanomaterials. We further detail their application in the detection and quantification of metal ions for environmental monitoring, food safety, medical diagnostics, as well as their use in biomedical imaging in living cells and animals.

Nitrogen and Phosphorus Co-Doped Carbon Nanodots as a Novel Fluorescent Probe for Highly Sensitive Detection of Fe(3+) in Human Serum and Living Cells

Chemical doping with heteroatoms can effectively modulate physicochemical and photochemical properties of carbon dots (CDs). However, the development of multi heteroatoms codoped carbon nanodots is still in its early stage. In this work, a facile hydrothermal synthesis strategy was applied to synthesize multi heteroatoms (nitrogen and phosphorus) codoped carbon nanodots (N,P-CDs) using glucose as carbon source, and ammonia, phosphoric acid as dopant, respectively. Compared with CDs, the multi heteroatoms doped CDs resulted in dramatic improvement in the electronic characteristics and surface chemical activities. Therefore, the N,P-CDs prepared as described above exhibited a strong blue emission and a sensitive response to Fe(3+). The N,P-CDs based fluorescent sensor was then applied to sensitively determine Fe(3+) with a detection limit of 1.8 nM. Notably, the prepared N,P-CDs possessed negligible cytotoxicity, excellent biocompatibility, and high photostability. It was also applied for label-free detection of Fe(3+) in complex biological samples and the fluorescence imaging of intracellular Fe(3+), which indicated its potential applications in clinical diagnosis and other biologically related study.

Green light-emitting polyepinephrine-based fluorescent organic dots and its application in intracellular metal ions sensing

In this paper, we present a class of bio-dots, polyepinephrine (PEP)-based fluorescent organic dots (PEP-FODs) for selective and sensitive detection of Fe(2+), Fe(3+), and Cu(2+). The PEP-FODs were derived from epinephrine via self-polymerization at relatively low temperature down to 60°C with low cytotoxicity and relative long lifetime (7.24ns). The surface morphology and optical properties of the synthesized PEP-FODs were characterized. We found that the diameters of PEP-FODs were mainly distributed in the narrow range of 2-4nm with an average diameter of 2.9nm. An optimal emission peak located at 490nm was observed when the green light-emitting PEP-FODs were excited at 400nm. It is discovered that Fe(2+), Fe(3+), and Cu(2+)can strongly quench the fluorescence of PEP-FODs through the nonradiative electron-transfer. The detection limit of 0.16, 0.67, and 0.15μM was obtained for Fe(2+), Fe(3+), and Cu(2+), respectively. The independent sensing platform of Fe(2+), Fe(3+), and Cu(2+)could be established by using NaF as a complexing agent and by regulating the reaction time between NaF and metal ions. Cell viability studies reveal that the as-prepared PEP-FODs possess good solubility and biocompatibility, making it as excellent imaging nanoprobes for intracellular Fe(2+), Fe(3+), and Cu(2+)sensing. The developed PEP-FODs might hold great promise to broaden applications in nanotechnology and bioanalysis.

Design of Multiple Logic Gates Based on Chemically Triggered Fluorescence Switching of Functionalized Polyethylenimine

In this study, two new functionalized polyethylenimine (PEI), PEIR and PEIQ, have been synthesized by covalently conjugating rhodamine 6G (R6G) or 8-chloroacetyl-aminoquinoline (CAAQ) and have been investigated for their sensing capabilities toward metal ions and anions basing on fluorescence on-off and off-on mechanisms. When triggered by protons, metal ions, or anions, functionalized PEIs can behave as a fluorescence switch, leading to a multiaddressable system. Inspired by these results, functionalized PEI-based logic systems capable of performing elementary logic operations (YES, NOT, NOR, and INHIBIT) and integrative logic operations (OR + INHIBIT) have been constructed by observing the change in the fluorescence with varying the chemical inputs such as protons, metal ions, and anions. Due to its characteristics, such as high sensitivity and fast response, developing functionalized PEI as a new material to perform logic operations may pave a new avenue to construct the next generation of molecular devices with better applicability for biomedical research.

2-Dimensional graphene as a route for emergence of additional dimension nanomaterials

Dimension has a different and impactful significance in the field of innovation, research and technologies. Starting from one-dimension, now, we all are moving towards 3-D visuals and try to do the things in this dimension. However, we still have some very innovative and widely applicable nanomaterials, which have tremendous potential in the form of 2-D only i.e. graphene. In this review, we have tried to incorporate the reported pathways used so far for modification of 2-D graphene sheets to make is three-dimensional. The modified graphene been applied in many fields like supercapacitors, sensors, catalysis, energy storage devices and many more. In addition, we have also incorporated the conversion of 2-D graphene to their various other dimensions like zero-, one- or three-dimensional nanostructures.

Fluorescent probes for "off-on" highly sensitive detection of Hg²⁺ and L-cysteine based on nitrogen-doped carbon dots

Fluorescent nitrogen-doped carbon dots (NCDs) were synthesized by a facile, and low-cost one-step hydrothermal strategy using citric acid as carbon source and ammonia solution as nitrogen source for the first time. The obtained NCDs show stable blue fluorescence with a high quantum yield of 35.4%, along with the fluorescence lifetime of ca. 6.75 ns. Most importantly, Hg(2+) can completely quench the fluorescence of NCDs as a result of the formation of a non-fluorescent stable NCDs-Hg(2+) complex. Static fluorescence quenching towards Hg(2+) is proved by the Stern-Volmer equation, ultraviolet-visible absorption spectra, temperature dependent quenching and fluorescence lifetime measurements. Subsequently, the fluorescence of the NCDs-Hg(2+) system is completely recovered with the addition L-cysteine (L-Cys) owing to the dissociation of NCDs-Hg(2+) complex to form a more stable Hg(2+)-L-Cys complex by Hg(2+)-S bonding. Therefore, such NCDs can be used as an effective fluorescent "turn-off" probe for rapid, rather highly selective and sensitive detection of Hg(2+), with a limit of detection (LOD) as low as 1.48 nM and a linear detection range of 0-10 μM. Interestingly, NCDs-Hg(2+) system can be conveniently employed as a fluorescent "turn-on" sensor for highly selective and sensitive detection of L-Cys with a low LOD of 0.79 nM and a wide linear detection range of 0-50 μM. Further, the sensitivity of NCDs to Hg(2+) is preserved in tap water with a LOD of 1.65 nM and a linear detection range of 0-10 μM.

Chemically doped fluorescent carbon and graphene quantum dots for bioimaging, sensor, catalytic and photoelectronic applications

Doping fluorescent carbon dots (DFCDs) with heteroatoms have recently become of great interest compared to traditional fluorescent materials because it provides a feasible and new way to tune the intrinsic properties of carbon quantum dots (CQDs) and graphene quantum dots (GQDs) to achieve new applications for them in different fields. Since the first report on nitrogen (N) doped GQDs in 2012, more effort is being focused on exploring different procedures for making new types of DFCDs with different heteroatoms. This mini review will summarize recent research progress on DFCDs. It first reviews various doping categories achieved up to now, looking back on the synthesis method and comparing the differences in synthesis approaches between the DFCDs and the undoped ones. Then it focuses on the advances on how the doping affects the optical properties, especially DFCDs doped with N, which have been investigated the most. Finally, different applications of DFCDs involving bio-imaging, sensing, catalysis and photoelectronic devices will be discussed. This review will give new insights into how to use different synthetic methods for tuning the structure of DFCDs, understanding the correlation between the doping and properties, and achieving new applications.

Highly luminescent N-doped carbon quantum dots from lemon juice with porphyrin-like structures surrounded by graphitic network for sensing applications

Green synthetic approach to synthesizing highly luminescent N-doped carbon quantum dots with porphyrin like moiety for sensitive and selective detection of Fe³⁺.

One-pot synthesis of sulfur-doped graphene quantum dots as a novel fluorescent probe for highly selective and sensitive detection of lead(ii)

A facile strategy was developed for the one-step synthesis of S-GQDs with a monolayer-graphene crystal structure. The change of surface chemistry by S-doping resulted in selective and sensitive detection of Pb²⁺.

Green, simple and large scale synthesis of N-doped graphene quantum dots with uniform edge groups by electrochemical bottom-up synthesis

An electrochemical bottom-up synthesis of N-doped GQDs (N-GQDs) with large amount of well-defined edge groups was developed for the first time. The exclusive edge group of the obtained N-GQDs is –NH₂ results in the excitation wavelength independence behavior.

Facilely synthesized N-doped carbon quantum dots with high fluorescent yield for sensing Fe3+

Schematic illustration of the preparation process for the NCQDs and their application for Fe³⁺ ions detection.

A reversible rhodamine 6G-based fluorescence turn-on probe for Fe3+ in water and its application in living cell imaging

A reversible fluorescent probe L based on rhodamine 6G was synthesized for the optical detection of Fe³⁺ in water.

Graphene quantum dots: recent progress in preparation and fluorescence sensing applications

This paper reviews recent activities in the preparation and fluorescence sensing applications of graphene quantum dots.

Sensitive determination of tannic acid using blue luminescent graphene quantum dots as fluorophore

Colorimetric and fluorimetric determination of tannic acid using blue luminescent graphene quantum dots (GQDs).

Effects of elemental doping on the photoluminescence properties of graphene quantum dots

B/N//P/S atoms were doped into graphene quantum dots to tailor their surface functional groups and structural defects with the aim of improving their photoluminescence properties.

Facile synthesis of multi-responsive functional graphene quantum dots for sensing metal cations

Aminated and phosphorylated graphene quantum dots (PiNGQDs) have been synthesized and used as fluorescent probes for sensing metal cations.

Development of a biosensing system for tacrine based on nitrogen-doped graphene quantum dots and acetylcholinesterase

The graphene quantum dot-based sensor is sensitive to reaction products resulting from acetylcholinesterase activity and it allows for the determination of tacrine acting as an enzyme inhibitor.

Microwave assisted one-pot synthesis of graphene quantum dots as highly sensitive fluorescent probes for detection of iron ions and pH value

Recently, carbon nanomaterials have received considerable attention as fluorescent probes owing to their low toxicity, water solubility and stable photochemical properties. However, the development of graphene quantum dots (GQDs) is still on its early stage. In this work, GQDs were successfully synthesized by one-step microwave assisted pyrolysis of aspartic acid (Asp) and NH4HCO3 mixture. The as-prepared GQDs exhibited strongly blue fluorescence with high quantum yield up to 14%. Strong fluorescence quenching effect of Fe(3+) on GQDs can be used for its high selectivity detection among of general metal ions. The probe exhibited a wide linear response concentration range (0-50 μM) to Fe(3+) and the limit of detection (LOD) was calculated to be 0.26 μM. In addition, GQDs are also sensitive to the pH value in the range from 2 to 12 indicating a great potential as optical pH sensors. More importantly, the GQDs possess lower cellular toxicity and high photostability and can be directly used as fluorescent probes for cell imaging.

Rhodamine-Functionalized Graphene Quantum Dots for Detection of Fe(3+) in Cancer Stem Cells

A turn-on orange-red fluorescent nanosensor based on rhodamine B derivative-functionalized graphene quantum dots (RBD-GQDs) has been successfully synthesized for Fe(3+) detection with high sensitivity and selectivity. By connecting with GQDs, the water solubility, sensitivity, photostability, and biocompatibility of RBD are drastically improved. The most distinctive feature of the RBD-GQDs, which sets them apart from other previously reported fluorophores or GQDs, is that they with the detection limits as low as 0.02 μM are demonstrated as a Fe(3+) turn-on fluorescent nanosensor in cancer stem cells. Fe(3+) binding to such GQDs (RBD-GQDs-Fe(3+)) with orange-red fluorescence of 43% quantum yield were demonstrated to be the biomarkers for cancer stem cell imaging.

Europium-decorated graphene quantum dots as a fluorescent probe for label-free, rapid and sensitive detection of Cu(2+) and L-cysteine

In this work, europium-decorated graphene quantum dots (Eu-GQDs) were prepared by treating three-dimensional Eu-decorated graphene (3D Eu-graphene) via a strong acid treatment. Various characterizations revealed that Eu atoms were successfully complexed with the oxygen functional groups on the surface of graphene quantum dots (GQDs) with the atomic ratio of 2.54%. Compared with Eu free GQDs, the introduction of Eu atoms enhanced the electron density and improved the surface chemical activities of Eu-GQDs. Therefore, the obtained Eu-GQDs were used as a novel "off-on" fluorescent probe for the label-free determination of Cu(2+) and l-cysteine (L-Cys) with high sensitivity and selectivity. The fluorescence intensity of Eu-GQDs was quenched in the presence of Cu(2+) owing to the coordination reaction between Cu(2+) and carboxyl groups on the surface of the Eu-GQDs. The fluorescence intensity of Eu-GQDs recovered with the subsequent addition of L-Cys because of the strong affinity of Cu(2+) to L-Cys via the Cu-S bond. The experimental results showed that the fluorescence variation of the proposed approach had a good linear relationship in the range of 0.1-10 μM for Cu(2+) and 0.5-50 μM for L-Cys with corresponding detection limits of 0.056 μM for Cu(2+) and 0.31 μM for L-Cys. The current approach also displayed a special response to Cu(2+) and L-Cys over the other co-existing metal ions and amino acids, and the results obtained from buffer-diluted serum samples suggested its applicability in biological samples.

Graphene-based nanoprobes for molecular diagnostics

In recent years, graphene has received widespread attention owing to its extraordinary electrical, chemical, optical, mechanical and structural properties. Lately, considerable interest has been focused on exploring the potential applications of graphene in life sciences, particularly in disease-related molecular diagnostics. In particular, the coupling of functional molecules with graphene as a nanoprobe offers an excellent platform to realize the detection of biomarkers, such as nucleic acids, proteins and other bioactive molecules, with high performance. This article reviews emerging graphene-based nanoprobes in electrical, optical and other assay methods and their application in various strategies of molecular diagnostics. In particular, this review focuses on the construction of graphene-based nanoprobes and their special advantages for the detection of various bioactive molecules. Properties of graphene-based materials and their functionalization are also comprehensively discussed in view of the development of nanoprobes. Finally, future challenges and perspectives of graphene-based nanoprobes are discussed.

Green Synthesis of Bifunctional Fluorescent Carbon Dots from Garlic for Cellular Imaging and Free Radical Scavenging

Nitrogen and sulfur codoped carbon dots (CDs) were prepared from garlic by a hydrothermal method. The as-prepared CDs possess good water dispersibility, strong blue fluorescence emission with a fluorescent quantum yield of 17.5%, and excellent photo and pH stabilities. It is also demonstrated that the fluorescence of CDs are resistant to the interference of metal ions, biomolecules, and high ionic strength environments. Combining with low cytotoxicity properties, CDs could be used as an excellent fluorescent probe for cellular multicolor imaging. Moreover, the CDs were also demonstrated to exhibit favorable radical scavenging activity.