Carbon quantum dot-based fluorometric nitrite assay by exploiting the oxidation of iron(II) to iron(III)

A Dual-mode Ratiometric Fluorometric and Colorimetric Platform Based on Nitrogen-doped Carbon Dots and o-phenylenediamine for the Detection of Nitrite

In this study, a dual-mode ratiometric fluorometric and colorimetric platform for the determination of nitrite in pickles was proposed by exquisitely employing the fact that non-fluorescent o-Phenylenediamine (OPD) was oxidized by nitrite under acidic conditions to form fluorescent 2,3-diaminophenazine (DAP) (Em = 575), which meanwhile quench the fluorescent nitrogen-doped carbon dots (N-CDs) at 455 nm, the ratio of fluorescence intensity of DAP to N-CDs (F575/F455) changed with the increase of nitrite accompanied by visible color changes. Thus, nitrite can be quantitatively detected within a wide linear range (10-500 µM) with a low detection limit of 0.45 µM due to the high quantum yield of 39.7% of N-CDs. In addition, the colour of the N-CDs/OPD system changed from transparent to yellow when the nitrite was introduced, enabling colorimetric and on-site visual detection. The detection limit of the colorimetric method was 3.03 µM with a linear range of 10-500 µM. The proposed ratiometric fluorometric method has pleasant selectivity and good immunity to interference.

Carbon quantum dots for fluorescent detection of nitrite: A review

NO₂^- is commonly found in foods and the environment, and excessive intake of NO₂^- poses serious hazards to human health. Thus, rapid and accurate assay of NO₂^- is of considerable significance. Traditional instrumental approaches for detection of NO₂^- faced with limitations of expensive instruments and complicated operations. Current gold standards for sensing NO₂^- are Griess assay and 2,3-diaminonaphthalene assay, which suffer from slow detection kinetics and bad water solubility. The newly emerged carbon quantum dots (CQDs) exhibit integrated merits including easy fabrication, low-cost, high quantum yield, excellent photostability, tunable emission behavior, good water solubility and low toxicity, which make CQDs be widely applied to fluorescent assay of NO₂^-. In this review, synthetic strategies of CQDs are briefly presented. Advances of CQDs for fluorescent detection of NO₂^- are systematically highlighted. Lastly, the challenges and perspectives in the field are discussed.

A dual-mode nanoprobe based on silicon nanoparticles and Fe(II)-phenanthroline for the colorimetric and fluorescence determination of nitrite

A fluorometric and colorimetric dual-modal nanoprobe (denoted as Fe²⁺-Phen/SiNPs) has been developed for selective and sensitive determination of nitrite (NO₂^-). The mechanism is based on fluorescence quenching between silicon nanoparticles (SiNPs) and Fe(II)-phenanthroline complex (Fe²⁺-Phen) via inner filter effect and redox. With the addition of increasing NO₂^-, Fe²⁺ is oxidized to Fe³⁺, recovering the fluorescence of SiNPs. Meanwhile, the color of the system gradually changes from orange-red to colorless, which enables colorimetric measurement. The NO₂^- concentration shows a wide linear relationship with fluorescence intensity from 0.1 to 1.0 mM (R² = 0.9955) with a detection limit of 2.4 μM in the fluorometric method (excitation wavelength: 380 nm). By contrast, the linear range of the colorimetric method ranges from 0.01 to 0.35 mM (R² = 0.9953) with a limit of detection of 6.8 μM (proposed selective absorbance: 510 nm). The probe has been successfully applied to nitrite determination in water, salted vegetables, and hams demonstrating broad application prospects for the determination of nitrite in complicated matrices.

Simple method for visual detection of nitrite using fluorescence and colorimetry by poly (tannic acid) nanoparticles

The nitration reaction of nitrite and phenolic substances was first used to identify and detect NO₂^- by taking fluorescent poly (tannic acid) nanoparticles (FPTA NPs) as sensing platform. With the low cost, good biodegradable and convenient water-soluble FPTA NPs, a fluorescent and colorimetric dual modes detecting assay was realized. In fluorescent mode, the linear detection range of NO₂^- was 0-36 μM, the LOD was as low as 3.03 nM, and the response time was 90 s. In colorimetric mode, the linear detection range of NO₂^- was 0-46 μM, and the LOD was as low as 27 nM. Besides, a smartphone with FPTA NPs@ agarose hydrogel formed a portable detection platform to test the fluorescent and visible color changes of FPTA NPs for NO₂^- sensing as well as for accurate visualization and quantitative detection of NO₂^- in actual water and food samples.

A fluorescent carbon dots synthesized at room temperature for automatic determination of nitrite in Sichuan pickles

In this paper, highly fluorescent carbon dots were synthesized from sodium ascorbate and polyethyleneimine at room temperature (R-CDs). The proposed green synthesis method was energy-saving, environmentally friendly and easy online. R-CDs exhibit an optimal emission peak of 490 nm under excitation at 380 nm with a quantum yield of 32 %. R-CDs morphology, composition, and properties were characterized using TEM, FTIR, XPS, UV-vis and fluorescence spectroscopy. The study revealed that nitrite quenched the fluorescence of R-CDs under acidic conditions. Subsequently, this discovered reaction of R-CDs and nitrite was combined with flow-injection technology, and a simple, precise and automatic fluorescence strategy for nitrite determination was accomplished. The response to nitrite was linear in 5-300 μg·L^-1 concentration range and the limit of detection was 2.85 μg·L^-1 (3.3 S/k). This method was applied to nitrite determination in Sichuan pickles during the pickling process and results were consistent with the standard method, demonstrating its feasibility in practical applications.

Intrinsic Dual-Emitting Carbon Quantum-Dot-Based Selective Ratiometric Fluorescent Mercaptopurine Detection

Mercaptopurine (6-MP), an immunosuppressive drug, has been widely prescribed for treating leukemia and autoimmune diseases. The level of the 6-MP drug in body fluids is of great interest due to the severe health problems related to its overdose. This study used a facile microwave preparation route to synthesize carbon quantum dots (CQDs) using glutathione and formamide as carbon sources. The obtained monodispersed quantum dots showed dual fluorescence emission with a sensitive affinity toward the 6-MP drug. The sensor's response was optimized by tuning the temperature, pH, and volume ratio of the probe. The prepared ratiometric fluorescence method showed accurate measurements for determining mercaptopurine in aqueous solutions in the concentration range of 1.4-7.6 mg L^-1 with the limit of detection of 1.3 mg L^-1. The sensor's performance was assessed in complex solutions, human urine, and human plasma sample and recovery values in the range of 88-127% were obtained. The reliable dual fluorometric sensor showed promising results for 6-MP determination and potential application for the determination of other chemical and biochemical species.

Fluorine and nitrogen co-doped near-infrared carbon dots for fluorescence "on-off-on" determination of nitrite

A fluorescence "on-off-on" strategy was established for the determination of nitrite in aqueous solution based on fluorine and nitrogen co-doped near-infrared carbon dots (NIR-CDs). NIR-CDs were prepared via one-step hydrothermal method by using N-(4-aminophenyl)-acetamide and 4,5-difluorobenzene-1,2-diamine as precursors. The photoluminescence quantum yield of NIR-CDs reaches to 17.4%, and the optimal emission peak of NIR-CDs is 675 nm under excitation of 530 nm. The Stokes shift of NIR-CDs (145 nm) is higher than that of some CDs with longer emission wavelengths. The red bathophenanthroline disulfonic acid (BPS)-Fe²⁺ complex can quench the fluorescence of NIR-CDs via inner filter effect and static quench modes. Nitrite can oxidize Fe²⁺ to produce Fe³⁺ in acidic environment, resulting in not only the formation of colorless and unstable BPS-Fe³⁺ complex but also the fluorescence recovery of NIR-CDs. This fluorescence "on-off-on" phenomenon also comes with the color variation of the mixture, resulting in both the fluorescence and the visual determination of nitrite. Under optimal conditions, this assay exhibits a good linear range from 1 to 50 μM and a low detection limit of 0.056 μM for nitrite determination. The method showed good applicability for nitrite determination in soil extract, human urine, and water samples with acceptable results. A convenient fluorescence "on-off-on" strategy for nitrite detection based on fluorine and nitrogen co-doped near-infrared carbon dots (NIR-CDs) and bathophenanthroline disulfonic acid (BPS)-Fe²⁺ complex was innovatively established. This probe showed a low detection limit of 0.056 μM for nitrite in authentic samples, which offered a new sight for fluorescent and visual detection of nitrite in environmental protection and human health areas.

A fluorescence aptasensor based on carbon quantum dots and magnetic Fe3O4 nanoparticles for highly sensitive detection of 17β-estradiol

Trace amounts of 17β-estradiol (E2) in food and the environment poses a threat to human health, which has created the demand for sensitive analytical methods to detect E2. In this study, a novel fluorescent aptasensor was developed for sensitive detection of E2 based on double-chain hybridization between carbon quantum dots-labelled with E2 aptamer (CQDs-aptamer) and Fe₃O₄ nanoparticles modified by complementary DNA (Fe₃O₄-cDNA). Under the optimal conditions, the aptasensor displayed a good linear range of 10^-11-10^-6 M for E2 with the coefficient of determination (R²) of 0.996, and a low detection limit of 3.48 × 10^-12 M was obtained. Besides, the aptasensor showed high selectivity and good reproducibility for E2 detection, which was successfully applied to the sensitive detection of E2 in milk as compared with tap water and lake water with satisfactory recoveries from 85.21% to 114.80%, suggesting the great significance of this aptasensor for detecting food contaminants in the food industry.

Cyanine dye-assembled composite upconversion nanoparticles for the sensing and cell imaging of nitrite based on a single particle imaging method

An upconversion luminescence total internal reflection single particle imaging method was developed for the sensing and cell imaging of nitrite.

Simple and Sensitive Multi-components Detection Using Synthetic Nitrogen-doped Carbon Dots Based on Soluble Starch

Although carbon dots (CDs) as fluorescent sensors have been widely exploited, multi-component detection using CDs without tedious surface modification is always a challenging task. Here, two kinds of nitrogen-doped CDs (NCD-m and NCD-o) based on soluble starch (SS) as carbon source were prepared through one-pot hydrothermal process using m-phenylenediamine and o-phenylenediamine as nitrogenous dopant respectively. Through fluorescence "on-off" mechanism of CDs, NCD-m and NCD-o could be used as a fluorescence sensor for detection of Fe 3+ and Ag + with LOD of 0.25 and 0.51 μM, respectively. Additionally, NCD-m could be used for indirect detection of ascorbic acid (AA) with LOD of 5.02 μM. Moreover, fluorescence intensity of NCD-m also exhibited the sensitivity to pH change from 2 to 13. More importantly, Both NCD-m and NCD-o had potential application for analysis of complicated real samples such as tap water, Vitamin C tablets and orange juice. Ultimately, the small size of NCD-m could contribute to reinforcing intracellular endocytosis, which allowed them to be used for bacteria imaging. Obviously, these easily obtainable nitrogen-doped CDs were able to be used for multi-components detection. Strategy for synthesis of nitrogen-doped carbon dots (NCDs) and a schematic for fabrication of as-prepared NCDs for detection of Fe 3+, Ag + and ascorbic acid (AA).

Surface engineering of carbon selenide nanofilms on carbon cloth: An advanced and ultrasensitive self-supporting binder-free electrode for nitrite sensing

Large uptakes of nitrite have been proven to be detrimental to human health, therefore, the development of high-performance nitrite sensors is highly emergent. Herein, a carbon selenide nanofilms modified carbon fiber cloth (CSe₂ NF/CC) electrode was obtained via in-situ synthesis to detect nitrite. The electrode integrates the collective merits of macroporous CC and pleated carbon selenide nanofilms, possessing a low overpotential of 0.83 V, a high electrochemical active surface area (EASA) of 5.39 cm², great electrical conductivity, and fast charge transport as well as ion diffusion. The proposed electrode achieved a low limit of detection of 0.04 μmol L^-1 (S/N = 3), a high sensitivity of 2048.56 μA mmol L^-1 cm^-2, excellent selectivity, and long-term stability. Additionally, the CSe₂ NF/CC was successfully used for nitrite detection in different food samples such as pickled vegetables and sausage samples.

A novel colorimetric and fluorometric probe for biothiols based on MnO2 NFs-Rhodamine B system

Herein, a novel bimodal ratiometric probe for sensitive and selective detection of biothiols (including glutathione (GSH), cysteine (Cys) and homocysteine (Hcys)) was constructed, which was based on the redox reaction between manganese dioxide nanoflakes (MnO₂ NFs) and rhodamine (RhB) and biothiols. When MnO₂ NFs was added into RhB solution, RhB was oxidized to a series of derivatives, accompanying with the colorimetric color changing from purple to light pink and fluorescence changing from red to green. In the presence of GSH, Cys or Hcys, they could reduce MnO₂ NFs to Mn²⁺, thereby preventing the following oxidization of RhB and the corresponding color and fluorescence changes. The absorption intensity ratio and fluorescence intensity ratio showed good linear relationships with the concentrations of biothiols. The colorimetric detection limits for GSH, Cys and Hcys were 0.057 μM, 0.140 μM and 0.087 μM, respectively. And the fluorescence detection limits were 0.177 μM, 0.282 μM and 0.161 μM. More importantly, this probe was successfully applied to monitor the concentration of GSH/Cys/Hcys in human serum samples, with satisfactory recovery. Thus, this MnO₂ NFs-RhB platform can potentially be a candidate for the detection of biothiols.

Selective detection of Fe3+ ions based on fluorescence MXene quantum dots via a mechanism integrating electron transfer and inner filter effect

Fluorescence quantum dots (QDs) are promising functional nanomaterials in chemical biology and environmental applications, where an analyte-induced responsive system is beneficial for detecting numerous life-related molecules and pollutants. Here, fluorescent Ti3C2 MXene quantum dots (MQDs) with the size of 1.75 nm were synthesized by a simple method of hydrofluoric acid etching and dimethyl sulfoxide exfoliation to form nanosheets followed by a one-step ultrasound method. The as-synthesized MQDs showed excitation-dependent behaviour along with a fluorescence quantum yield value of 7.7%. In addition, the fluorescence of the MQDs can be significantly suppressed by Fe3+. The mechanism for the fluorescence quenching of the MQDs was systematically investigated, which was attributed to the oxidation-reduction reaction between the MQDs and Fe3+ and the inner filter effect (IFE), different from the reported Förster resonant energy transfer (FRET) mechanism for MXene nanosheets. Based on this trait, a fluorescence method for Fe3+ detection based on MQDs was demonstrated with high sensitivity and selectivity, and the limit of detection was 310 nM. The proposed method was successfully used for the sensitive detection of Fe3+ in serum and sea water. This work will not only help to understand the selectivity mechanisms of MQDs as fluorescent probes for metal ions, but also provide a smart sensing platform in biological and environmental detection.

Orange-emitting N-doped carbon dots as fluorescent and colorimetric dual-mode probes for nitrite detection and cellular imaging

Novel orange-emitting N-doped carbon dots (N-CDs) were prepared as fluorescent and colorimetric dual-mode probes for sensing nitrite (NO₂⁻).

Amphiphilic fluorescent carbon nanodots as a selective nanoprobe for nitrite and tetracycline both in aqueous and organic solutions

The dispersibility of carbon dots in organic and/or aqueous solvents plays a critical role in various application fields.

A dual-modal fluorometric and colorimetric nanoprobe based on graphitic carbon nitrite quantum dots and Fe (II)-bathophenanthroline complex for detection of nitrite in sausage and water

A fluorometric and colorimetric dual-mode sensing platform based on graphitic carbon nitrite quantum dots (g-CNQDs) and Fe (II)-bathophenanthroline complex (BPS-Fe²⁺) was designed to the sensitive detection of nitrite (NO₂^-) in sausage and water. In this system, the fluorescence of g-CNQDs was quenched by BPS-Fe²⁺ complex due to the inner filter effect (IFE). When NO₂^- was present, Fe²⁺ was oxidized by nitrite to form BPS-Fe³⁺ complex with BPS, leading to the recovery of the fluorescence from g-CNQDs. Therefore, we constructed a "turn-off-on" fluorescence probe for detection of NO₂^-. Moreover, with the increase of NO₂^- concentration, the color of the solution changed from red to colorless, so the UV-vis measurements and on-site visual detection were realized. The method is capable of detecting NO₂^- in the concentration range of 2.32-34.8 μM with good selectivity and high sensitivity. In addition, the method has the potential to determine NO₂^- in water samples and sausage samples.

Visual and fluorescent detection of mercury ions using a dual-emission ratiometric fluorescence nanomixture of carbon dots cooperating with gold nanoclusters

Mercury (II) ions (Hg²⁺), as one of the most toxic heavy metals, can cause irreversible damage to human health even at very low concentration due to its high toxicity and bioaccumulation. Herein, a facile ratiometric fluorescence nanomixture based on carbon dots‑gold nanoclusters (CDs-Au NCs) was constructed for quantitative detection of Hg²⁺. Lysine functionalized carbon dots (CDs) were prepared by one-pot hydrothermal method, while gold nanoclusters (Au NCs) were synthesized via using chicken egg white (CEW) as reducer and stabilizer. The novel nanomixture exhibited two strong emission peaks at 450 nm and 665 nm under 390 nm excitation, and showed pink fluorescence under UV light. Interestingly, the fluorescence of the CDs-Au NCs nanomixture was selectively response to Hg²⁺. The fluorescence of Au NCs at 665 nm was decreased when Hg²⁺ was presented in the solution, while the fluorescence of CDs at 450 nm stayed constant. The fluorescence color changed from pink to blue obviously with increasing the concentration of Hg²⁺, which indicated that CDs-Au NCs could be used for visual detection Hg²⁺ by the naked eye. Under optimal conditions, this ratiometric fluorescent sensor could detect Hg²⁺ accurately and possess a great sensitivity with a detection limit of 63 nM. In addition, this method was applied to detect Hg²⁺ in real water samples with great recoveries, suggesting its potential in practical application with simplicity, environmentally friendly and low cost.

Hydrothermal synthesis of green fluorescent nitrogen doped carbon dots for the detection of nitrite and multicolor cellular imaging

A fluorescent probe for the determination of nitrite (NO₂^-) was fabricated by using green fluorescent nitrogen doped carbon dots (NCDs). The NCDs were synthesized via a one-pot hydrothermal carbonization of citric acid in the presence of p-phenylenediamine as the nitrogen source. The N content of the NCDs was high to 17.09% and consisted of a variety of functional groups on the NCDs surface, including sp²-hybridized CN, porphyrin C-N-C and amino N in N-(C) ₃ or H-N-(C) ₂ et al. N atoms were also doped within the framework of the NCDs. The almost monodisperse NCDs (average particle diameter = 3.67 nm) exhibited green photoluminescence (PL) with excitation/emission maxima of 360/505 nm. The PL of the NCDs was dependent on both excitation wavelength and solution pH. The NCDs showed a strong PL quenching response to NO₂^- under acidic conditions (pH = 2.5). The PL intensity of the NCDs was inversely proportional to the concentration of NO₂^- between 0.02 and 40 μM (R² = 0.992), with a detection limit of 21.2 nM. The practical use of the nanoprobe for NO₂^- determination in food samples was also demonstrated, successfully. NCD-nitroso compounds formed because of reaction between the abundant amide groups on the surface of NCDs with the NO₂^-, which caused an inner filter effect and static PL quenching. Importantly, the NCDs had low cellular toxicity and were successfully used as a multicolor cellular imaging agent for Hepg2 cells.

Fluorometric determination of nitrite through its catalytic effect on the oxidation of iodide and subsequent etching of gold nanoclusters by free iodine

A method for sensitive detection of nitrite is presented. It is found that the red fluorescence of gold nanoclusters (with excitation/emission maxima at 365/635 nm) is quenched by traces of iodine via etching. Free iodide is formed by oxidation of iodide by bromate anion under the catalytic effect of nitrite. This catalytic process provides a sensitive means for nitrite detection. Under the optimal conditions, fluorescence linearly dropos in the 10 nM to 0.8 μM nitrite concentration range. The limit of detection is 1.1 nM. This is a few orders of magnitude lower than the maximum concentration allowed by authorities. Graphical abstract Schematic representation of a method for detection of nitrite via a redox reaction. Iodine was produced in the reaction and subsequently quenched the fluorescence from gold nanoclusters by etching their metallic cores, and a sensitive assay for nitrite down to 1.1 nM was developed.