Construction of Cyan Blue Fluorescence Probe based on Nitrogen-Doped Carbon Dots for Detecting Nitrite Ion in Ham Sausage.

Nitrite ion is one of the materials widely used in human life, and the accurate, sensitive and stable detection of nitrite ions is of great significance to people's healthy life. In this study, nitrogen-doped fluorescent carbon dots (N-CDs) for detecting nitrite salt solutions were prepared using citric acid monohydrate and Chrysoidin as precursors through a one-pot hydrothermal method. Under the condition of pH = 3, a noticeable quenching phenomenon occurred in the carbon dot solution with the increase in nitrite ion concentration. This quenching effect might be attributed to the diazonium effect. N-CDs have been successfully used as fluorescence probes for NO2- detection. NO2- can effectively quench the fluorescence intensity of N-CDs, providing a linear response to fluorescence quenching efficiency with respect to NO2- concentration within the range of 0-10µM and 10-30µM, and a detection limit of 52nM, showing high sensitivity. In addition, the probe was applied to the determination of NO2- in ham sausage samples with a detection limit of 0.67µM and recoveries in the range of 99.5-102.3%, the fluorescent probe showed satisfactory reliability.

[Validation Study on Developed Methods for Anions and Bromic Acid in Various Mineral Waters by Ion Chromatography].

Five kinds of anions namely fluoride, chlorate, chlorite, nitrate and nitrite ions, and bromic acid were determined in various mineral waters (MWs), and the methods were validated. MWs are varying in the degree of hardness and contents of carbonate. When the five anions were measured based on the official method of tap water, the peak shape of fluoride ion in MWs with high degree of hardness was different from the standard solution, making it difficult to determine. The same phenomenon was also observed when bromic acid was measured. In order to achieve accurate determination, five-fold dilution with ultrapure water was carried out on the samples. With the additional step, the abnormal peak of both analytes was improved, and no difference in the retention times between standard and sample solutions was observed. The validation tests were performed using the developed methods with the additional diluting step, and the results of all target substances met the criteria of the guideline on analytical method validation for MW in Japan. Our results suggested that the methods we developed could be useful for the accurate determination of the anions and bromic acid in various MWs on the market.

Highly sensitive colorimetric determination of nitrite based on the selective etching of concave gold nanocubes.

Concave gold nanocubes are viable optical nanoprobes for the determination of nitrite ions. Herein, a novel approach was developed, based on the measurement of localized surface plasmon resonance absorption. The addition of nitrite ions selectively induced the etching of concave gold nanocubes, abrading the sharp vertices to spherical corners, which resulted in blue-shifted absorption accompanied by a color change from sapphire blue to light violet. The mechanism of selective etching of concave gold nanocube tips was elucidated by using X-ray photoelectron spectroscopy and atom probe tomography. The optimized detection of NO2- via the concave gold nanocube-based probe occurred at pH 3.0 and in 20 mM NaCl concentration at 40 °C. The absorption ratios (A550 nm/A640 nm) were proportional to the NO2- concentrations in the range 0.0-30 µM, with a detection limit of 38 nM (limit of quantitation of 0.12 µM and precision of 2.7%) in tap water. The highly selective and sensitive colorimetric assay has been successfully applied to monitor the nitrite ion concentrations in spiked tap water, pond water, commercial ham, and sausage samples.

A hydrophobic deep eutectic solvent-based vortex-assisted dispersive liquid-liquid microextraction combined with HPLC for the determination of nitrite in water and biological samples.

In recent years, hydrophobic deep eutectic solvents as new generation of green solvents have attracted wide attention in liquid microextraction technique. In this article, four hydrophobic deep eutectic solvents composed of trioctylmethylammonium chloride and oleic acid were designed and prepared firstly. Combined with high-performance liquid chromatography, these deep eutectic solvents were used as an extraction solvent in vortex-assisted dispersive liquid-liquid microextraction for the selective enrichment and indirect determination of trace nitrite from real water and biological samples. This method is based on the diazotization-coupling reaction of nitrite with p-nitroaniline and diphenylamine in acidic water, and then the nitrite is quantified indirectly by measuring the obtained azo compounds. Some factors influencing the extraction efficiency, including the reaction and extraction conditions, were investigated. Under the optimized conditions, the method has a linear range of 1-300 µg/L with a correlation coefficient of 0.9924, limit of detection of 0.2 µg/L, limit of quantitation of 1 µg/L, intraday and interday relative standard deviations of 4.0 and 6.0%. This method was successfully applied in determination of nitrite from three environmental water and two biological samples with the recovery in the range of 90.5-115.2%. In addition, these results were well agreement with those obtained by the conventional Griess method.

Adsorption Capability of Fe-HT3.0 for Nitrite and Nitrate Ions in a Binary Solution System.

In this study, the adsorption capability of Fe-HT3.0 for nitrite and nitrate ions in a binary solution system was evaluated. It was found that the amount of nitrite and nitrate ions adsorbed in a single solution (1.19 and 1.27 mmol/g, respectively) was higher than that in a binary solution (0.36 and 0.90 mmol/g, respectively). Equilibrium adsorption was attained within 6-24 h. The adsorption data were fitted to a pseudo-second-order model (correlation coefficient: 0.999), and indicated that the adsorption of both nitrite and nitrate ions is controlled by chemical sorption. Additionally, the binding energies before and after the adsorption of nitrite and nitrate ions in the binary solution system were measured. After adsorption, new nitrogen peaks (approx. 399 and 403 eV) were detected. The results of this study show the potential of Fe-HT3.0 for the removal of nitrite and nitrate ions from aqueous solution systems.

Adsorption of Nitrite and Nitrate Ions from an Aqueous Solution by Fe-Mg-Type Hydrotalcites at Different Molar Ratios.

In this study, we prepared Fe-Mg-type hydrotalcites (Fe-HT3.0 and Fe-HT5.0) with different molar ratios and evaluated their adsorption capability for nitrite and nitrate ions from aqueous solution. Fe-HT is a typical hydrotalcite-like layered double hydroxide. Adsorption isotherms, as well as the effects of contact time and pH were investigated, and it was found that Fe-HT can adsorb larger amounts of nitrite and nitrate ions than Al-HT (normal-type hydrotalcite). Adsorption isotherm data were fitted to both Freundlich (correlation coefficient: 0.970-1.000) and Langmuir (correlation coefficient: 0.974-0.999) equations. Elemental analysis and binding energy of Fe-HT surface before and after adsorption indicated that the adsorption mechanism was related to the interaction between the adsorbent surface and anions. In addition, the ion exchange process is related to the adsorption mechanism. The adsorption amount increased with increasing temperature (7-25°C). The experimental data fit the pseudo-second-order model better than the pseudo-first-order model. The effect of pH on adsorption was not significant, which suggested that Fe-HT could be used over a wide pH range (4-12). These results indicate that Fe-HT is a good adsorbent for the removal of nitrite and nitrate ions from aqueous solution.

Photodegradation of 2,4-D induced by NO2(-) in aqueous solutions: the role of NO2.

To elucidate the effect of nitrite ion (NO2(-)) on the photodegradation of organic pollutants, a 300 W mercury lamp and Pyrex tubes restricting the transmission of wavelengths below 290 nm were used to simulate sunlight, and the photodegradation processes of 2,4-dichlorophenoxyacetic acid (2,4-D) with different concentrations of NO2(-) in freshwater and seawater were studied. The effect of reactive oxygen species (ROS) on the photolysis of 2,4-D was also demonstrated using electron paramagnetic resonance (EPR). The results indicated that the 2,4-D photolysis reaction followed the first-order kinetics in freshwater and seawater under different concentrations of NO2(-). Meanwhile, the photochemical reaction rate of 2,4-D increased with increasing concentration of NO2(-). When the concentration of NO2(-) was lower than 23 mg/L, the photodegradation rate of 2,4-D in seawater was higher than that in freshwater. However, when the concentration of NO2(-) was reached 230 mg/L, 2,4-D degradation slowed down in seawater. It was important to note that EPR spectra showed NO2 radical was generated in the NO2(-) solution under simulated sunlight irradiation, indicating that 2,4-D photodegradation could be induced by NO2. These results show the key role of NO2(-) in photochemistry and are helpful for better understanding of the phototransformation of environmental contaminants in natural aquatic systems.

Structural evidence of the conversion of nitric oxide (NO) to nitrite ion (NO2-) by lactoperoxidase (LPO): Structure of the complex of LPO with NO2- at 1.89 Å resolution.

Lactoperoxidase (LPO) is a heme containing mammalian enzyme which uses hydrogen peroxide (H2O2) to catalyze the conversion of substrates into oxidized products. LPO is found in body fluids and tissues such as milk, saliva, tears, mucosa and other body secretions. The previous structural studies have shown that LPO converts substrates, thiocyanate (SCN-) and iodide (I-) ions into oxidized products, hypothiocyanite (OSCN-) and hypoiodite (IO-) ions respectively. We report here a new structure of the complex of LPO with an oxidized product, nitrite (NO2-). This product was generated from NO using the two step reaction of LPO by adding hydrogen peroxide (H2O2) in the solution of LPO in 0.1 M phosphate buffer at pH 6.8 as the first step. In the second step, NO gas was added to the above mixture. This was crystallized using 20% (w/v) PEG-3350 and 0.2 M ammonium iodide at pH 6.8. The structure determination showed the presence of NO2- ion in the distal heme cavity of the substrate binding site of LPO. The structure also showed that the propionate group which is linked to pyrrole ring D of the heme moiety was disordered. Similarly, the side chain of Asp108, which is covalently linked to heme moiety, was also split into two components. As a result of these changes, the conformation of the side chain of Arg255 was altered allowing it to form new interactions with the disordered carboxylic group of propionate moiety. These structural changes are indicative of an intermediate state in the catalytic reaction pathway of LPO.

Gold Nanomaterial-Based Microfluidic Paper Analytical Device for Simultaneous Quantification of Gram-Negative Bacteria and Nitrite Ions in Water Samples.

This study presents a rapid microfluidic paper-based analytical device (µPAD) capable of simultaneously monitoring Gram-negative bacteria and nitrite ions (NO2-) for water quality monitoring. We utilize gold nanoparticles (AuNPs) functionalized with polymyxin molecules (AuNPs@polymyxin) to cause color change due to aggregation for the detection of Gram-negative bacteria, and antiaggregation in the presence of o-phenylenediamine (OPD) for NO2- detection. In this study, Escherichia coli (E. coli) serves as the model of a Gram-negative bacterium. Using the developed µPADs, the color changes resulting from aggregation and antiaggregation reactions are measured using a smartphone application. The linear detection ranges from 5.0 × 102 to 5.0 × 105 CFU/mL (R2 = 0.9961) for E. coli and 0.20 to 2.0 µmol/L (R2 = 0.995) for NO2-. The detection limits were determined as 2.0 × 102 CFU/mL for E. coli and 0.18 µmol/L for NO2-. Notably, the newly developed assay exhibited high selectivity with no interference from Gram-positive bacteria. Additionally, we obtained acceptable recovery for monitoring E. coli and NO2- in drinking water samples with no significant difference between our method and a commercial assay by t test validation. The sensor was also employed for assessing the quality of the pond and environmental water source. Notably, this approach can also be applied to human urine samples with satisfactory accuracy. Furthermore, the assay's stability is extended due to its reliance on AuNPs rather than reagents like antibodies and enzymes, reducing costs and ensuring long-term viability. Our cost-effective µPADs therefore provide a real-time analysis of both contaminants, making them suitable for assessing water quality in resource-limited settings.

Designing an intriguingly fluorescent N, B-doped carbon dots based fluorescent probe for selective detection of NO2- ions.

Low-cost nitrogen and boron-doped carbon nanodots (CPAP-CDs) with a high quantum yield (64.07%) were synthesized through a facile hydrothermal treatment. The obtained CPAP-CDs exhibited wide absorption, strong fluorescence, and pH-dependent behavior. The high fluorescence of CPAP-CDs was quenching in the presence of the nitrite ion in a concentration-dependent manner. The detection limit was as low as 6.6 nM with a wide linear detection range of 2 µM - 1 mM. Diazotization between the NO2- ion and CPAP-CDs resulted in the aggregation of CPAP-CDs and aggregation-induced emission quenching. The as-designed method was tested further with different water samples, such as tap, drinking, and seawater.

Oxygen vacancies enhanced cooperative electrocatalytic reduction of carbon dioxide and nitrite ions to urea.

The electrochemical reduction of carbon dioxide and nitrite ions into value-added chemicals represents one of the most promising approaches to relieve the greenhouse gases, while a critical challenge is to search for a highly effective catalyst with low energy input and high conversion selectivity. In this work, we demonstrated low-valence Cu doped, oxygen vacancy-rich anatase TiO2 (Cu-TiO2) nanotubes as a synergetic catalyst for electrochemical co-reduction of both CO2 and NO2-. The incorporation of Cu dopants in anatase TiO2 facilitated to form abundant oxygen vacancies and bi-Ti3+ defect sites, which allowed for efficient nitrite adsorption and activation. The low-valence Cu dopants also served as effective catalytic centers to reduce CO2 into CO* adsorbate. The close proximity of CO* and NH2* intermediates was beneficial for the subsequent cooperative tandem reaction to form urea via the CN coupling. This oxygen vacancy-rich Cu-TiO2 electrocatalyst enabled excellent urea production rate (20.8 µmol⋅h-1) and corresponding Faradaic efficiency (43.1%) at a low overpotential of -0.4 V versus reversible hydrogen electrode, substantially superior than those of undoped TiO2, thus suggesting an exciting approach for cooperative CO2 and nitrogen fixation.

Sensitive and selective nitrite assay based on fluorescent gold nanoclusters and Fe2+/Fe3+ redox reaction.

In recent years, gold nanoclusters (AuNCs) have received considerable attention as optical transducers in chemo/biosensors. Herein, a facile and efficient assay for NO2- has been successfully developed based on the fluorescence quenching of AuNCs co-modified by bovine serum albumin and 3-mercaptopropionic acid (BSA/MPA-AuNCs). In the presence of NO2- under acidic conditions, Fe2+ can be readily oxidized and transformed to Fe3+, which can significantly suppress the fluorescence of BSA/MPA-AuNCs via non-radiative electron-transfer mechanism. The linear range and detection limit for this system were found to be 5-30 µM (r = 0.9975) and 0.7 µM, respectively. Other common anions and cations showed only very minor interference with the NO2- detection. Furthermore, the effectiveness of the proposed sensing strategy was validated by the demonstration of good performance in the determination of the amount of NO2- in ham samples, rendering it a powerful tool for the assessment of food security and water quality.

Blue emitting nitrogen-doped carbon dots as a fluorescent probe for nitrite ion sensing and cell-imaging.

This study examined the efficiency of pH-dependent, fluorescent carbon dots for the sensing of hazardous anions in aqueous media and cell imaging. The nitrite anion, an important water-soluble element for environmental and biological systems, requires continuous monitoring because a high concentration can affect the systems severely. The as-synthesized carbon dots efficiently detected the nitrite anion in aqueous solution through a fluorescent 'Turn Off' phenomenon. The quenching mechanism was investigated through proper microscopic and spectroscopic studies. The limit of detection and linear detection range were 7.9 nM and 2.3µM-7.7 mM, respectively. The sensitivity was tested with different water samples. In a parallel experiment, the as-synthesized carbon dots were used as a cell-imaging probe for HeLa cells, highlighting their potential in different biological studies.

Real-time monitoring of water quality of stream water using sulfur-oxidizing bacteria as bio-indicator.

In aquatic ecosystems, real-time water-quality (WQ) biomonitoring has become the most effective technology for monitoring toxic events by using living organisms as a biosensor. In this study, an online WQ monitoring system using sulfur oxidizing bacteria (SOB) was tested to monitor WQ changes in real-time in natural stream water. The WQ monitoring system consisted of three SOB reactors (one continuous and two semi-continuous mode reactors). The SOB system did not detect any toxicity in relatively-unpolluted, natural stream water when operated for more than six months. When diluted swine wastewater (50:1) was added to the influent of the reactors, the system detected toxic conditions in both the continuous and semi-continuous operational modes, showing 90% inhibition of SOB activity within 1 h of operation. The addition of 30 mg/L NO2--N or 2 mg/L of Cr6+ to the influents of SOB reactors resulted in the complete inhibition of the SOB activity within 1-2 h. The results demonstrated the successful application of an SOB bioassay as an online toxicity monitoring system for detecting pollutants from stream or river waters.

Sensitive naked eye detection and quantification assay for nitrite by a fluorescence probe in various water resources.

An uncontrolled increase of nitrite concentration in groundwater, rivers and lakes is a growing threat to public health and environment. It is important to monitor the nitrite levels in water and clinical diagnosis. Herein, we developed a switch-off fluorescence probe (PyI) for the sensitive detection of nitrite ions in the aqueous media. This probe selectively recognizes nitrite ions through a distinct visual color change from colorless to pink with a detection limit of 0.1 µM. This method has been successfully applied to the determination of nitrites in tap water, lake water and Yellow River water with recoveries in the range of 94.8%-105.4%.

Synthesis, characterization and nitrite ion sensing performance of reclaimable composite samples through a core-shell structure.

The following paper reported and discussed a nitrite ion optical sensing platform based on a core-shell structure, using superamagnetic nanoparticles as the core, a silica molecular sieve MCM-41 as the shell and two rhodamine derivatives as probe, respectively. This superamagnetic core made this sensing platform reclaimable after finishing nitrite ion sensing procedure. This sensing platform was carefully characterized by means of electron microscopy images, porous structure analysis, magnetic response, IR spectra and thermal stability analysis. Detailed analysis suggested that the emission of these composite samples was quenchable by nitrite ion, showing emission turn off effect. A static sensing mechanism based on an additive reaction between chemosensors and nitrite ion was proposed. These composite samples followed Demas quenching equation against different nitrite ion concentrations. Limit of detection value was obtained as low as 0.4µM. It was found that, after being quenched by nitrite ion, these composite samples could be reclaimed and recovered by sulphamic acid, confirming their recyclability.

Nitrite ion mitigates the formation of N-nitrosodimethylamine (NDMA) during chloramination of ranitidine.

Ranitidine (RNT) has been an important tertiary amine precursor of N-nitrosodimethylamine (NDMA) in chlorine-based water treatment, due to reaction with monochloramine (NH2Cl) with exceptionally high molar yields up to 90%. This study examined the effects of nitrite ions (NO2-) on the kinetics of NDMA formation during the chloramination of RNT under variable concentrations of dissolved oxygen (DO, 0.7-7.5mg/L), RNT (5-30µM), NH2Cl (5-20mM), NO2- or NO3- (0-2mM) and pH (5.6-8.6). In the absence of the NO2-, the ultimate molar yield of NDMA after 6h of reaction was primarily influenced by [DO] and pH, while marginally affected by initial [RNT] and [NH2Cl]. A kinetic model, prepared in accordance with the reaction sequence of NDMA formation, suggested that the rate determining step was accelerated with increasing [NH2Cl]0, [DO], and pH. A Kinetic study together with ultra-performance liquid chromatography-quadrupole-time of flight mass spectrometer (UPLC-Q-TOF MS) and gas chromatography (GC)/TOF MS analyses in parallel demonstrated that the nitrite ion inhibited the nucleophilic substitution of the terminal amine on NH2Cl, and reduced the pseudo-steady state concentration of N-peroxyl radicals, significantly decreasing the ultimate yields of NDMA.

Trace analysis of nitrite ions in environmental samples by using in-situ synthesized Zein biopolymeric nanoparticles as the novel green solid phase extractor.

For the first time, a novel green method using Zein biopolymeric nanoparticles as a green dispersive solid-phase extractor is reported for the separation and preconcentration of trace amount of nitrite (NO2-) ions in ppb levels. The Zein protein is a biodegradable hydrophobic plant protein that is obtained from corn and is composed of a number of hydrophobic amino acids. Zein bionanoparticles were synthesized in an anti-solvent process and used as a new biosorbent in the extraction technique. In the proposed technique, by using a standard method at first, a mixture of 1-naphthylamine and sulphanilic acid as selective regents was added to the samples, and in the presence of the nitrite ion, a red azo product was formed. After that, the ethanolic Zein solution (equal to 15mg) was injected rapidly into the sample, based on the anti-solvent process. Zein bionanoparticles (BNPs) were produced, the adsorbed colour product was separated by centrifugation, and finally samples were analysed with the spectrophotometric method. The influence of different variables such as pH, buffer and amount of buffer, amount of adsorbent and effect of time on extraction were investigated and Zein BNPs were characterized by TEM, SEM, and FT-IR techniques. The main advantages of Zein as a new solid-phase extractor are that this biopolymer is non-toxic, stable, widely available, biodegradable, very hydrophobic, and can be fabricated easily. Under optimal experimental conditions, the linear correlation coefficient (r2) was found to be 0.9972 at the concentration range of 5.0-1000ngmL-1. The limit of detection was 2.3ngmL-1 (0.05µM). This method was applied successfully for the analysis of sea and river waters as well as industrial wastewater samples. Finally, this method follows the US EPA (US Environmental Protection Agency) and WHO (World Health Organization) international standards for nitrite analysis. In addition, it has several advantages to warrant its applicability in the near future in separation science as a green biosorbent in both dispersive and normal solid-phase extraction.

Engineered Artificial Nanochannels for Nitrite Ion Harmless Conversion.

Inspired by the delicate functions of living organisms to transport or transform nitrite ions (NO2-), a bioinspired smart nanochannel that can realize harmless conversion of NO2- into N2 is developed by immobilizing a NO2--responsive functional molecule, p-phenylenediamine, onto a single conical polyethylene terephthalate nanochannel. Subsequently, the aromatic primary amine groups could be triggered to transform into a phenyldiazonium molecule based on the acid-activated NO2--binding process. The nanochannel exhibits specific selectivity and highly ultratrace recognition of NO2-. Fascinatingly, the transformed phenyldiazonium molecules could be triggered to generate phenol groups and release N2 by ultraviolet light activation, achieving NO2- harmless conversion. This system could provide inspiration to construct artificial nanofluidic devices for ion-sensing and nitrogen cycle fields.

Nitrite sensing composite systems based on a core-shell emissive-superamagnetic structure: Construction, characterization and sensing behavior.

Two recyclable nitrite sensing composite samples were designed and constructed through a core-shell structure, with Fe3O4 nanoparticles as core, silica molecular sieve MCM-41 as shell and two rhodamine derivatives as chemosensors, respectively. These samples and their structure were identified with their electron microscopy images, N2 adsorption/desorption isotherms, magnetic response, IR spectra and thermogravimetric analysis. Their nitrite sensing behavior was discussed based on emission intensity quenching, their limit of detection was found as low as 1.2µM. Further analysis suggested a static sensing mechanism between nitrite and chemosensors through an additive reaction between NO+ and chemosensors. After finishing their nitrite sensing, these composite samples and their emission could be recycled and recovered by sulphamic acid.