Nitrogen-doped Carbon Quantum Dots Research Articles

Iron ion detection and life cycle assessment of cutting fluid based on photoluminescent nitrogen-doped carbon quantum dots

Cutting fluid (CF) ensures machining accuracy and improves tool life in machining, which is of great significance for its impurity content detection and life cycle evaluation. In this paper, citric acid and arginine were used as carbon and nitrogen sources, photoluminescent (PL) nitrogen-doped carbon quantum dots (N-CDs) with a particle size ranging from 8 to 14 nm and well dispersion in water-based CF were synthesized by a one-pot hydrothermal reaction method. The N-CDs have the best signal under 360 nm ultraviolet light excitation, which well avoids the autofluorescence of the CF. The N-CDs dispersion have good selectivity and linearity in the Fe3+ concentration range of 2 to 80 μM, and the detection limit is 1.2 μM. The N-CDs were used to detect the concentration of Fe3+ in CF with different working hours, and the results were consistent with the results of coupled plasma emission spectroscopy. This study provides an intelligent sensing platform for the highly sensitive and selective detection of Fe3+ in the CF environment, and provides a promising strategy for the evaluation of the CF lifetime.

Nitrogen-doped carbon quantum dot as electron acceptor anchored on graphitic carbon nitride nanosheet for improving rhodamine B degradation

Carbon quantum dots (CQDs) hold significant promise for wastewater treatment because of their remarkable attributes, including robust stability, outstanding biocompatibility, minimal toxicity, superior water dispersibility, cost-effective production, and exceptional photo-stability.In this study, CQDs were synthesized using microplasma and tested for the degradation of rhodamine B (RhB) (5 mg/L) under different conditions, with a Solar simulator and UVA illumination. Specifically, graphitic carbon nitride nanosheets (NCN) in conjugation with carbon quantum dots (NCN@CQD) were prepared with different amounts of CQD. The synthesized CQD, NCN, and NCN@CQD underwent characterization using various compositional and structural analytical techniques. The results show that nitrogen species were effectively incorporated into the CQDs framework and CQDs are anchored on NCN forming NCN@CQD successfully. Subsequently, all samples were then investigated in degrading Rhodamin B, RhB, to assess the photocatalytic activity. The results reveal that the presence of CQDs substantially improved the photocatalytic efficiency of NCN. Specifically, the CQDs anchored on NCN exhibited a substantial enhancement in the photoactivity for RhB decomposition under a solar light simulator, surpassing that of pure NCN. The amount of CQD in the NCN@CQD sample exerted a strong influence on the photocatalytic activity. All NCN@CQD samples exhibited remarkable enhancements in both adsorption and photocatalytic performance compared to pure CQD and NCN samples. In an optimization study, a best NCN@CQ photocatalyst was identified, showing a 40 % enhancement of photocatalytic efficiency compared to the use of NCN alone. A visible light photocatalytic activity of 91 % can be attained within 150 min, exhibiting a reaction rate constant of 0.0135 min−1 toward RhB decomposition, which was double as for the individual NCN (0.0067 min−1). The boosted photocatalytic activity resulting from the synergistic effect between NCNs and CQDs can be attributed to the efficient function of CQD as an electron acceptor. The proposed mechanism of photocatalytic degradation of RhB is illustrated in detail.

Bifunctional effects of nitrogen-doped carbon quantum dots on CoS2/mesoporous carbon composites for high-performance lithium-ion batteries

Cobalt disulfide (CoS2) stands as a promising candidate for anode materials in lithium-ion batteries due to its high theoretical capacity, but it faces challenges associated with the shuttle effect of lithium polysulfide during cycling. To address these issues, zeolitic imidazolate framework (ZIF)-derived composites have been extensively explored because of distinct advantages such as the formation of nano-sized particles, heteroatom doping, and highly porous structures. However, ZIF-derived carbon supports primarily consist of ultra-micropores that can impede lithium-ion diffusion. Herein, we aimed to enhance cycling stability by introducing a nitrogen-doped carbon quantum dot (NCQD) solution derived from N-methyl-2-pyrrolidone into cobalt-based ZIF-67 to modify the porosity and dope heteroatoms of CoS2 nanoparticle-embedded heteroatom-doped carbon composites (CoS2/NSC). The mildly acidic NCQD solution resulted in the partial etching of the ZIF-67 structure, along with the deposition of NCQDs as a nitrogen source. Notably, the pore sizes could be adjusted by varying the concentration of the NCQD solution, while retaining the nitrogen functional groups during carbonization. The electrode using CoS2/NSC with the 2.8 mL NCQD pre-treatment exhibited enhanced C-rate capability with the capacity of 392 mAh/g at 2.0 A/g. Moreover, the cycling stability was improved, with a capacity retention of 77 % after 100 cycles.

PHOTOELECTRIC CHARACTERIZATION OF DYE SENSITIZED SOLAR CELL (DSSCS) BASED ON NITROGEN DOPED CARBON QUANTUM DOT (CQDT) AND NATURAL PIGMENT EXTRACTED FROM ROSELLE HYBISCUS SABDARIFA

Citric acid, Ammonia and distilled water were used to synthesized Nitrogen doped carbon quantum dot (CQDT) and natural pigment was extracted from Roselle Hybiscus Sabdarifa flower. These sensitizers were used to fabricate monolithic dye sensitized solar cells, using Doctor Blade technique. Three samples of DSSCs were fabricated with CQDT, Hybiscus Sabdarifa dye and Hybiscus Sabdarifa +CQDT. These fabricated cells were characterized and the optical properties of the cells fabricated shows that the maximum absorbance of CQDT, hybiscus sabdarifa and hybiscus sabdarifa + CQDT was 1.75 %, 2.38 % and 2.17 % respectively. Moreso, the bandgap energy of the CQDT is 2.25 eV while hybiscus sabdarifa and hybiscus sabdarifa + CQDT is 1.88 eV and 1.92 eV respectively. The X-ray diffraction spectrum results reveals that the synthesized cells were polycrystalline in nature. The SEM images further exhibit a nanograins, that is spread throughout the substrate surface which show a stacked like agglomeration due to glass substrate used for the synthesis. The average crystallite size obtained was about 1 to 9 nm from the XRD data. The photoelectrical properties of the CQDT, hybiscus sabdarifa and hybiscus sabdarifa + CQDT revealed that the conversion efficiency was 4.17 %, 2.63 % and 1.17% respectively. This shows that the dye adhere well with the CQDT material thereby creating space for photovoltaic activities.

Fluorescence 'Turn-on' Probe for Chromium Reduction, Adsorption and Detection Based on Cellulosic Nitrogen-Doped Carbon Quantum Dots Hydrogels.

This paper proposes a new, highly effective fluorescence test for Cr(VI) detection. This method utilizes a hydrogel composed of hydroxyethyl cellulose (HEC), nitrogen-doped carbon quantum dots (N-CQDs), and poly(co-acrylamido-2-methyl-1-propane sulfonic acid) (AMPS). The N-CQDs were successfully synthesized using a simple microwave method, and then conjugated with HEC and AMPS. The higher adsorption (99.41%) and higher reduction rate in H1 likely stems from both the presence of N-CQDs (absent in HB) and their increased free functional groups (compared to H2/H3, where N-CQDs block them). This facilitates the release (desorption) of Cr(VI) from the hydrogels, making it more available for reduction to the less toxic Cr(III). The fluorescent brightness of the HEC-N-CQDs-g-poly(AMPS) hydrogel increases gradually when Cr(VI) is added in amounts ranging from 15 to 120 mg/L. The fluorescent enhancement of the HEC-N-CQDs-g-poly(AMPS) hydrogel appeared to exhibit a good linear relationship with the 15-120 mg of the Cr(VI) concentration, with a detection limit of 0.0053 mg/L, which is lower than the standard value published by WHO. Our study found that the HEC-N-CQDs-g-poly(AMPS) hydrogel served effectively as a fluorescent probe for Cr(VI) detection in aqueous solutions, demonstrating high sensitivity.

Green one-step synthesis of N-doped carbon quantum dots for fluorescent detection of lemon yellow in soft drinks

A new fluorescent sensor for the determination of lemon yellow was developed based on nitrogen-doped carbon quantum dots (N-CQDs), which were prepared via a hydrothermal method with dried pomelo peel and L-tyrosine. The N-CQDs exhibited the blue fluorescence with a quantum yield of 28 %. The sensing principle of N-CQDs was quenched by lemon yellow via static quenching. The potential interfering substances showed no influence on the detection of lemon yellow. The limit of detection was 0.023 mg/L and lower than that of national standard. Furthermore, the synthesized N-CQDs have been successfully applied to the measurement of lemon yellow in real samples. Hence, the N-CQDs would be a promising sensor in food analysis.

Preparation and fluorescence detection of nitrogen-doped carbon quantum dots/cotton fiber composites

From the perspective of environmental protection and sustainable development, fluorescent probes have been prepared using inexpensive raw materials for fluorescence detection in wastewater treatment. Nitrogen-doped carbon quantum dots (N-CQDs)/cotton fiber composites were prepared by grafting N-CQDs onto the oxidized cotton fibers using a silane coupling agents. Through a series of characterizations using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and X-ray energy dispersive spectrometry (EDS), it was found that N-CQDs were loaded onto cotton fibers. In the N-CQDs/cotton fiber composite, the fluorescence rate of carbon quantum dots is higher, which is 17.2 %, and the detection limit is 1.21 μM. The optimum conditions for fluorescence detection of azo dye Chrome black T(EBT) with pH 8 and concentration 10 μM were studied. This method for the detection of EBT has advantages such as high sensitivity, simplicity, speed, reusability, and a high potential for application in wastewater detection.

Nitrogen-doped carbon quantum dots as dual mode fluorescence sensors for the determination of food colorant quinoline yellow

Quinoline yellow (QY), as a food coloring agent, will consume a large number of detoxifying substances in the body after being ingested by the human body, interfering with the normal metabolic functions of the human body, and may cause allergies, diarrhea and other symptoms, as well as a certain degree of carcinogenicity, posing a great threat to human health. As a result, it is critical to develop a fast, sensitive, and effective approach to determining quinoline yellow in food. In this study, carbon dots (N-CQDs) with high fluorescence quantum yield were prepared and used to determine the QY content using the dual mode of internal filtering effect and fluorescence emission shift detection. Both methods showed good linearity in the range of QY concentration of 0.3–3.2 μM, and the detection limits were classified as 2.6 nM and 0.18 μM. In addition, in order to achieve visual detection of QY, fluorescent test strips were constructed using the carbon dots and non-fluorescent qualitative filter paper to make the detection of QY more convenient. This probe presents a novel way for detecting quinoline yellow in food analysis.

Fluorescence Detection of Pb2+ in Environmental Water Using Biomass Carbon Quantum Dots Modified with Acetamide-Glycolic Acid Deep Eutectic Solvent.

In this study, a novel green fluorescent probe material, nitrogen-doped carbon quantum dots (N-CQDs), was prepared by a one-step hydrothermal synthesis method using walnut green skin as a carbon source and acetamide-glycolic acid deep eutectic solvent (AGADES) as a modifier. By covalent coupling, the amide chromophore in AGADES is designed to cover the surface of walnut green skin carbon quantum dots (W-CQDs), forming a fluorescence energy resonance effect and improving the fluorescence performance of the carbon quantum dots. The prepared N-CQDs have a uniform particle size distribution, and the fluorescence quantum efficiency has increased from 12.5% to 32.5%. Within the concentration range of 0.01~1000 μmol/L of Pb2+, the linear detection limit is 1.55 nmol/L, which can meet the trace detection of Pb2+ in the water environment, and the recycling rate reaches 97%. This method has been successfully applied to the fluorescence detection and reuse of Pb2+ in actual water bodies, providing new ideas and methods for the detection of heavy metal ions in environmental water.

Morinda coreia fruits derived green-emissive nitrogen-doped carbon quantum dots: Selective and sensitive detection of ferric ions from water

Carbon quantum dot containing nitrogen has been prepared by utilizing a green biomass resource, Morinda coreia fruits, by a single step hydrothermal procedure. The spherical carbon dots are analysed by HR-TEM, XPS, FT-IR, UV–visible and photoluminescence studies. With a mean diameter of 1.99 nm, they possessed functional groups like carboxyl and amide on their surface and emit light of wavelength of 490 nm under 390 nm excitation. The fluorescent Morinda coreia carbon dots (MCCDs), whose quantum yield is found to be 17.1 %, are very sensitive to the presence of iron (III) ions with a low detection limit of 1.32 μM. Successful fluorometric determination of Fe3+ ions from tap water and borewell water were also achieved with good recovery rates. Interactions between surface groups on MCCDs and Fe3+ ions lead to the formation of a non-fluorescent complex, and a static quenching mechanism is proposed.

Engineering Nitrogen-Doped Carbon Quantum Dots: Tailoring Optical and Chemical Properties through Selection of Nitrogen Precursors.

The process of N-doping is frequently employed to enhance the properties of carbon quantum dots. However, the precise requirements for nitrogen precursors in producing high-quality N-doped carbon quantum dots (NCQDs) remain undefined. This research systematically examines the influence of various nitrogen dopants on the morphology, optical features, and band structure of NCQDs. The dots are synthesized using an efficient, eco- friendly, and rapid continuous hydrothermal flow technique. This method offers unparalleled control over synthesis and doping, while also eliminating convention-related issues. Citric acid is used as the carbon source, and urea, trizma base, beta-alanine, L-arginine, and EDTA are used as nitrogen sources. Notably, urea and trizma produced NCQDs with excitation-independent fluorescence, high quantum yields (up to 40%), and uniform dots with narrow particle size distributions. Density functional theory (DFT) and time-dependent DFT modelling established that defects and substituents within the graphitic structure have a more significant impact on the NCQDs' electronic structure than nitrogen-containing functional groups. Importantly, for the first time, this work demonstrates that the conventional approach of modelling single-layer structures is insufficient, but two layers suffice for replicating experimental data. This study, therefore, provides essential guidance on the selection of nitrogen precursors for NCQD customization for diverse applications.

DFT, real sample and smartphone studies of fluorescent probe, N-doped carbon quantum dots for sensitive and selective detection of bilirubin

In this investigation, nitrogen-doped carbon quantum dots (NCQDs) were successfully synthesized, showcasing robust fluorescence characteristics and photostability. These NCQDs demonstrated biocompatibility and were utilized as effective sensing agents for bilirubin (BR) detection. Their remarkable Stern-Volmer constant, attributed to exceptional molar absorptivity and photoluminescence quantum yield, rendered them highly proficient in BR sensing. Various analytical techniques such as X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) were employed to characterize the synthesized NCQDs, providing comprehensive insights into their structural, elemental, chemical composition, particle size, morphology, and optical properties. For sensing purposes, UV–Vis, steady-state, and time-resolved fluorescence spectroscopic techniques were utilized. The average particle size of the NCQDs was determined to be approximately 4.5 ± 0.3 nm. Notably, the Stern-Volmer constant (Ksv) for BR sensing from these quantum dots surpassed previously reported values in the literature. A linear relationship between emission intensity ratio and BR concentration within a range of 1.35–14.20 μM was observed. Accurate quantification of BR was achieved with a high Stern-Volmer constant (Ksv) S11 of approximately 7.48 × 105 M-1 and a limit of detection (LOD) of 4.86 μM. Leveraging the NCQDs as fluorescent probes, a highly sensitive and selective BR detection method based on the Förster resonance energy transfer (FRET) mechanism was established, with emission occurring at 360 nm. Density functional theory (DFT) studies supported FRET as the primary mechanism for fluorescence quenching, indicating electron transfer from NCQDs to BR, as evidenced by the energies of the lowest unoccupied molecular orbitals (LUMOs) of both NCQDs and BR. Real sample analyses of BR levels in human serum yielded a recovery range of 94.8 % to 104.21 %, validating the practical applicability of the method. Furthermore, smartphone-based BR detection using NCQDs achieved an LOD of 11.74 µM.

Excellent Charge Separation of NCQDs/ZnS Nanocomposites for the Promotion of Photocatalytic H2 Evolution.

Carbon Quantum dots (CQDs) are widely studied because of their good optical and electronic characteristics and because they can easily generate photocarriers. Nitrogen-doped CQDs (NCQDs) may exhibit improved hydrophilic, optical, and electron-transfer properties, which are conducive to photocatalytic hydrogen evolution. In this paper, NCQD-modified ZnS catalysts were successfully prepared. Under the irradiation of the full spectrum, the H2 evolution rate of the optimal catalyst 0.25 wt % NCQDs/ZnS achieves 5.70 mmol g-1 h-1, which is 11.88, 43.84, and 5.14 times the values of ZnS (0.48 mmol g-1 h-1), NCQDs (0.13 mmol g-1 h-1), and CQDs/ZnS (1.11 mmol g-1 h-1), respectively. Furthermore, it shows good stability, indicating that the modification of NCQDs prevents the photocorrosion and oxidation of ZnS. The enhanced performance is due to NCQD loading, which promotes the separation of photogenerated carriers, optimizes the structures, and increases the specific surface area. This work highlights the fact that NCQD-modified ZnS may afford a new strategy to synthesize ZnS-based photocatalysts with enhanced H2 production performance.

One-pot hydrothermal method of green-synthesized nitrogen-doped carbon quantum dots for ultra-sensitive dual detection of tannic acid and Hg2+ ions

One-pot hydrothermal method of green-synthesized nitrogen-doped carbon quantum dots for ultra-sensitive dual detection of tannic acid and Hg2+ ions

N‐Doped Carbon‐Quantum‐Dot‐Integrated Colorimetric System for Visible Photoprinting and Oxygen Sensing

AbstractThe design of light‐responsive color switching systems (LCSs) is essential to the implementation of rewritable paper and oxygen detection. Previous LCSs rely on metal‐based semiconductor photocatalysts which are expensive and pose metal‐leaking risk. To solve the issue, for the first time, nitrogen‐doped carbon quantum dots (N‐CQDs) are reported as metal‐free photocatalyst for visible photoprinting and oxygen sensing. On N‐CQD surface, the decorated amino and hydroxyl groups act as hole scavengers and thus realize visible‐light‐responsive single and multiple color switching, after mixing with redox‐sensitive dyes. Combined with polymeric substrates, LCS rewritable papers can be fabricated and exhibit rapid decoloration performances under 420 nm light irradiation, due to the reduction of dye by photogenerated electrons from N‐CQDs. After the decoloration, efficient color reversion can be realized by the oxidization of dye in ambient air (O2). By taking advantage of the controlled recoloration kinetics, an oxygen sensor can be attained for visual quantitative analysis of atmospheric conditions with color variations. These results assist the future development of nonmetal color switching systems for superior functional design of photoprintable papers and colorimetric oxygen sensors.

Internal electric field promoted NCDs/BiOBr/AgBr Z-scheme heterojunction with rich oxygen vacancies for efficient photocatalytic degradation of tetracycline and reduction of Cr (VI)

It is important to prepare efficient photocatalysts for the degradation of antibiotics and heavy metals. In this study, NCDs/BiOBr/AgBr Z-scheme heterojunction photocatalysts with excellent performance were constructed. NCDs/BiOBr/AgBr form a Z-type heterojunction system with pyrrole nitrogen-doped carbon quantum dots (NCDs) as charge transfer channels. The internal electric field formed in NCDs/BiOBr/AgBr greatly facilitated electron transfer. The more easily broken Bi-O bonds were utilized in the synthesis to introduce rich oxygen vacancies (OVs) into NCDs/BiOBr/AgBr to promote molecular oxygen activation. Meanwhile, silver nanorods were reduced in the synthesis to enhance the photodegradation performance. The photocatalytic degradation of tetracycline (TC) and reduction of Cr (VI) were enhanced by about 4 and 6 times. The factors affecting the removal effects of TC and Cr (VI) were analyzed. The catalysts were also tested to have good stability and potential for application in real wastewater. The reactive oxygen species (ROS), which play a dominant role in the photodegradation of TC and Cr (VI) by NCDs/BiOBr/AgBr, were tested, and a possible photocatalytic mechanism was proposed.

PH response mechanism of bifunctional fluorescent carbon quantum dots and application in cancer detection and bioself-targeting imaging

The development of fluorescent probes suitable for lysosome visualisation as well as real-time tracking is crucial because lysosomes play a crucial role in numerous metabolic activities and diseases. In this paper, nitrogen-doped carbon quantum dots (NCQDs) enriched with a large number of hydroxyl, amino, and carboxyl groups were prepared by a simple solvothermal method using tartaric acid and 4-aminosalicylic acid as raw materials. The fluorescence intensity of NCQDs decreased with increasing pH (1–13) due to protonation and deprotonation as well as hydrolysis of surface groups in alkaline environment and detachment of chromophores. This enabled the NCQDs to achieve label-free and rapid visualisation of pH in cells. The disparity in pH levels between cancer cells and normal cells resulted in varying fluorescence brightness of NCQDs observed in these two cells, which enabled the differentiation of cancer cells from normal cells. Most importantly, when NCQDs entered the cell, lysosomes could provide protons for NCQDs, and the protonation of NCQDs' surface groups led to stronger fluorescence after endocytosis of NCQDs in lysosomes. Thus, lysosomal targeted imaging could be achieved without the need to introduce lysosomal-targeted ligands, and NCQDs had a higher biocompatibility and stability compared to commonly used commercial dyes, which made it the ideal choice of lysosomal-targeted fluorescent probes.

Synthesis of green fluorescent, energy efficient nitrogen doped carbon quantum dots

The study of a simple and facile microwave heating technique is introduced for the synthesis of nitrogen-doped carbon quantum dots (N-CQDs) with the most abundant and cheap chemicals- Citric acid (carbon source & precursor) and Urea (nitrogen source & reducing agent). TEM and DLS results indicated that such prepared N-CQDs have an average particle size of ∼0.8 nm and high Quantum Yields of ∼24.69%. UV-Vis and PL Spectroscopic results indicated that these N-CQDs have bright green color fluorescence under the excitation of 405 nm wavelength and are highly dispersible in water and provide a stable optically transparent yellowish solution in ambient light. FTIR results confirmed the Nitrogen Doping over these CQDs as it consists of N-H vibrational structural bonds. XRD results display a diffraction peak at 27°, which corresponds to the (002) plane of a graphitic structure revealing an amorphous carbon phase.TEM results also demonstrated that these materials have good spherical morphology. Their high fluorescence/ photoluminescence, small size, chemical stability, high quantum yields, broad excitation range, and porous nature make them better candidates in the field of optoelectronics such as Solar cells and Light emitting diodes (LEDs), biosensing/imaging, and energy storage such as Li-Ion Batteries and supercapacitors.

Synergetic modification on buried and upper surfaces of perovskites with nitrogen-doped carbon quantum dots for efficient and stable solar cells

Tin oxide (SnO2), which is used as an electron transport layer in perovskite solar cells (PSCs), however, has nonnegligible defects. The surface of SnO2 films contains hydroxyl groups (–OH), which increases film defects and affects the quality of the upper perovskite. The energy level mismatch is also found in SnO2, which leads to severe nonradiative recombination and unsatisfactory photovoltaic performance. Meanwhile, excessive PbI2 residues on the perovskite surface serve as catalytic decomposition sites, significantly undermining device stability. In this study, nitrogen-doped carbon quantum dots (N-CQDs) were synthesized by using human hair as a raw material and were employed as an additive for interface between SnO2 and perovskites to improve interfacial charge transport. In addition, these N-CQDs were also used as a surface modifier on top of the perovskite layer to reduce residual PbI2. These results significantly increase the power conversion efficiency (PCE) of the related devices from 19.22% to 21.49%. The CQDs-modified device also shows significantly enhanced operational stability that when exposed to 30% humidity, the PCE of the optimized device retains 86% of its initial value after 1200 h. This study emphasizes the great potential of N-CQDs as a multifunctional interfacial modifier for fabrication of efficient and stable PSCs.

A Simple Method for Synthesizing Nitrogen-Doped Carbon Quantum Dots for Fluorescent "Turn off" Mercury (II) Ion Sensing.

Here, straightforward and environmentally friendly fluorescent nitrogen doped carbon quantum dots (N-CQDs) with a high blue fluorescence emission at 455nm are used for ultrasensitive Hg2+ ion detection. Folic acid and urea are used as carbon sources in the carbonization process. Two broad absorption bands at around 280 and 370nm from UV-Vis spectrum and characteristic absorption peaks from infrared spectrum confirms the successful synthesis of the N-CQDs. Energy dispersive X-Ray analysis confirmed the elemental composition of the N-CQDs. Transmission electron microscopy showed the homogeneous globular morphology of the N-CQDs with an average particle size of 65nm. Zeta potential measurement established the stability and surface charge of N-CQDs. Dynamic light scattering measurement showed the average size of N-CQDs. With the addition of Hg2+ ion to N-CQDs, the blue fluorescence emission is quenched. Moreover, the N-CQDs can be applied to real water sample such as pond water, river water, and tap water. The detection limit is approximately calculated to be 12 nM and linear range is 0-30 parts per billion.