Measurement Of Nitrite Research Articles

Reduction with zinc — Impact on the determination of nitrite and nitrate ions using microfluidic paper-based analytical devices

We used a microfluidic paper-based analytical device (μPAD) to investigate the influence that zinc reduction exerts on the determination of nitrite and nitrate ions in natural water samples. The μPAD consists of layered channels for the reduction of nitrate to nitrite with zinc powder and the subsequent detection of nitrite with Griess reagent. The amount of zinc, number of layers, and reaction time for the reduction were optimized to obtain an intense signal for nitrate. Initially, the sensitivity to nitrate corresponded to 55% that of nitrite, which implied an incomplete reduction. We found, however, that zinc decreased the sensitivity to nitrite in both the μPAD and spectrophotometry. The sensitivity to nitrite was decreased by 48% in spectrophotometry and 68% in the μPAD following the reaction with zinc. One of the reasons for the decreased sensitivity is attributed to the production of ammonia, as we elucidated that both nitrite and nitrate produced ammonia via the reaction with zinc. The results suggest that the total concentration of nitrite and nitrate must be corrected by constructing a calibration curve for nitrite with zinc, in addition to developing curves for nitrate with zinc and for nitrite without zinc. Using these calibration curves, the absorbance at different concentration ratios of nitrite and nitrate ions could be reproduced via calculation using the calibration curves with zinc for nitrite and nitrate. Eventually, the developed μPAD was applied to the determination of nitrite and nitrate ions in natural water samples, and the results were compared with those using a conventional spectrophotometric method. The results of the μPAD are in good agreement with those of conventional spectrophotometry, which suggests that the μPAD is reliable for the measurement of nitrite and nitrate ions in natural water samples.

Embroidered textile-based electrochemical sensors towards urinary nitrite measurement

Wearable biochemical sensors enable the real-time measurement of health parameters in non-invasive manner. In this work, we have developed polyaniline (PANI)/COOH-functionalized multiwalled carbon nanotubes (MWCNTs)/sodium copper chlorophyllin (CuCP) coated conductive thread as a novel wearable platform for monitoring nitrite (NO2–) levels. The linear calibration plots for NO2– were obtained over the concentration range of 0.05 µM–10 mM with detection limit of 0.35 nM. The sensor platform was also tested for measuring NO2– levels in artificial urine samples.

A violet light-emitting diode-based gas-phase molecular absorption device for measurement of nitrate and nitrite in environmental water

A simple and sensitive device for the detection of nitrite and nitrate in environmental waters was developed based on visible light gas-phase molecular absorption spectrometry. By integrating a detection cell (DC), semiconductor refrigeration temperature-controlling system (SRTCY), and nitrite reactor into a sequential injection analysis system, trace levels of nitrite and nitrate in complex matrices were successfully measured. A low energy-consuming light-emitting diode (violet, 400–405 nm) was coupled with a visible light-to-voltage converter (TSL257) to measure the gas-phase molecular absorption. To reduce the interference of water vapor, an SRTCY was used to condense the water vapor on-line before the gas-phase analyte entered the DC. The DC was radiatively heated by the SRTCY to avoid water vapor condensation in the light path. As a result, the obtained baseline noise reduced 3.75 times than that of without SRTCY. Under the optimized conditions, the device achieved limits of detection (3σ/k) of 0.055 and 0.36 mmol/L (0.77 and 5.04 mg N/L) for nitrite and nitrate, respectively, and the linear calibration ranges were 0.1–15 mmol/L (R2 = 0.9946) and 1–10 mmol/L (R2 = 0.9995), respectively. Precisions of 5.2 % and 9.0 % were achieved for ten successive determinations of 0.3 mmol/L nitrite and 1.0 mmol/L nitrate, and the analytical times for nitrite and nitrate determination were 5 and 13 min, respectively. This method was validated against standard methods and recovery tests, and it was applied to the measurement of nitrite and nitrate in environmental waters. Moreover, a device was designed to enable the field measurement of nitrite and nitrate in complex matrices.

Online Measurement of Sodium Nitrite Based on Near-Infrared Spectroscopy

In this study, a method was developed for the rapid online measurement of sodium nitrite solutions using near-infrared spectroscopy. A series of standard solutions of sodium nitrite at different concentrations were prepared, and the samples were measured in cuvettes and flow cells. Following the preprocessing of raw spectra and band selection, partial least squares were used to establish a prediction model, and the coefficient of determination (R2) of the validation set and the root mean square error of prediction (RMSEP) of the model were 0.9989 and 0.0338. The results demonstrate that the established model can meet the demands of online measurement and perform the rapid, nondestructive detection of sodium nitrite solutions, which provides some basis for the automated formulation of feedstock in spent fuel reprocessing.

Polysaccharides derived from Ganoderma lucidum applied to the extraction chitosan and development of electrochemical sensor based on extracted chitosan and graphitic carbon nitride modified electrode for determination nitrite in food samples

The current study focused on extracting chitosan from polysaccharides generated from Ganoderma lucidum and applying it to the construction of electrochemically modified electrodes based on graphitic carbon nitride (g-C3N4) for the measurement of nitrite in water and food samples. Ganoderma Lucidum polysaccharides were isolated and characterized. The chitosan was then isolated enzymatically from the polysaccharides using a chemical technique. The isolated chitosan and g-C3N4 were used to modify a GCE (chitosan/g-C3N4/GCE). The structural investigations employing FT-IR, SEM, and XRD show that chitosan successfully interacted with g-C3N4 in the chitosan/g-C3N4 sample. Electrochemical evaluation using cycle voltammetry (CV), differential pulse voltammetry (DPV), and amperometry revealed that the chitosan/g-C3N4/GCE electrode has good sensitivity and selectivity for determining nitrite. The use of chitosan and g-C3N4 in electrode surface modification improved nitrite electro-oxidation performance by allowing for more effective adsorption and electron transport processes. DPV measurement showed that the sensor had a sensitivity of 0.12103 μA/μM, a linear range of 40 to 2000 μM, and a limit of detection of 21 nM, and amperometry measurement showed the sensor's sensitivity of 0.15487 μA/μM, linear range of 20 to 4230 μM, and an LOD of 10 nM. These findings indicated a more sensitive and wide range of sensor performance using amperometry with respect to the DPV technique. The method's validity and accuracy in determining nitrite in prepared genuine samples (water, milk, apple juice, orange juice, Aloe vera, and tomato juice) were investigated. RSD values (3.19 % to 4.75 %) and recovery values (80.00 % to 98.33 %) were found to be satisfactory. The data also revealed that there were no substantial matrix interferences, demonstrating the accuracy and applicability of the proposed approach for detecting nitrite in food samples. Therefore, the proposed sensor design has several advantages that make it a promising candidate for various applications. Its reusability, stability, wide linear range, accuracy, and low cost make it an attractive option for many industries.

Advancements in Preprocessing and Analysis of Nitrite and Nitrate since 2010 in Biological Samples: A Review.

As a substance present in organisms, nitrite is a metabolite of nitric oxide and can also be ingested. Nitrate is the metabolite of nitrite. Therefore, it is necessary to measure it quickly, easily and accurately to evaluate the health status of humans. Although there have been several reviews on analytical methods for non-biological samples, there have been no reviews focused on both sample preparation and analytical methods for biological samples. First, rapid and accurate nitrite measurement has significant effects on human health. Second, the detection of nitrite in biological samples is problematic due to its very low concentration and matrix interferences. Therefore, the pretreatment plus measuring methods for nitrite and nitrate obtained from biological samples since 2010 are summarized in the present review, and their prospects for the future are proposed. The treatment methods include liquid-liquid microextraction, various derivatization reactions, liquid-liquid extraction, protein precipitation, solid phase extraction, and cloud point extraction. Analytical methods include spectroscopic methods, paper-based analytical devices, ion chromatography, liquid chromatography, gas chromatography-mass spectrometry, electrochemical methods, liquid chromatography-mass spectrometry and capillary electrophoresis. Derivatization reagents with rapid quantitative reactions and advanced extraction methods with high enrichment efficiency are also included. Nitrate and nitrate should be determined at the same time by the same analytical method. In addition, much exploration has been performed on formulating fast testing through microfluidic technology. In this review, the newest developments in nitrite and nitrate processing are a focus in addition to novel techniques employed in such analyses.

Ensuring the validity of measurement results of nitrite and nitrate in meat products

Nitrate and nitrite measurement is a fundamental part of quality control in the meat products industries. The guarantee of technical validity of the results of these measurements can be achieved through the adoption of appropriate metrological tools such as method validation. In this study, the measurement of the mass fractions of nitrite and nitrate ions was performed by ion chromatography with UV detection. The method was linear (r > 0.99) over the working range. Limits of quantification values for measuring nitrite and nitrate in meat product were 8.0 mg kg−1 and 52.4 mg kg−1, respectively. The mean values of recovery at all evaluated mass fraction levels were within the range of 88 % to 106 %. All the validation parameters resulted as satisfactory, and the method was considered adequate for the intended use.

Parts per billion of nitrite in microcrystalline cellulose by ion chromatography mass spectrometry with isotope labeled internal standard

Regulatory authorities like the U.S. Food and Drug Administration (FDA) have set strict specification levels for N-nitrosamines in finished drug products. Nitrite is a potential precursor for the formation of probable carcinogenic N-nitrosamines when secondary or tertiary amines are also present in the active pharmaceutical ingredient (API) synthesis or drug formulation process. An accurate and sensitive determination of nitrite will be useful when a drug product manufacturer chooses to investigate the reaction kinetics between nitrite and amines or to select appropriate excipients for its drug formulation. Pharmaceutical excipient manufacturers may also need an accurate nitrite measurement to investigate the nitrite content in their excipients. This study details the development and validation of an ion chromatography mass spectrometry (IC-MS) method for trace nitrite determination in microcrystalline cellulose materials, one of the important pharmaceutical excipients used in many drug formulations. MS operated under selected ion monitoring mode was used to solve the commonly encountered interference issue with conductivity detection, and nitrite isotope internal standard was employed to address the ion suppression issue with MS detection. The installation of an after-column “jumper” to flush water with an auxiliary pump through the MS when it is not used for data collection avoided sensitivity loss due to trace salt accumulation in the ion source. Validation of the optimized method was satisfactory, with linearity of nitrite in the concentration range of 0.02–7.50 ppm (µg/g) having a regression coefficient of > 0.999, precision of RSD < 9.5% at 0.03 ppm and RSD < 3.4% at 0.4 ppm and recovery of 92.0–103.0%. The limit of detection and limit of quantitation were 0.005 and 0.016 ppm, respectively.

Adding Sustainability in Analytical Chemistry Education through Monitoring Aquarium Water Quality

This paper introduces a captivating topic for upper-level analytical chemistry capstone projects, focusing on aquarium water analysis. This provides a more comprehensive understanding of the role of analytical chemistry towards sustainability and its environmental, economic, societal and education dimensions. Regarding the crucial role of maintaining optimal aquarium water quality for the welfare of aquatic life, students are tasked with envisioning and executing the measurement of key parameters, including pH, ammonium, nitrite, and nitrate contents. This hands-on experience not only engages students in real-world applications, but also allows them to delve into essential analytical chemistry principles. They carefully select measurement methods, considering factors such as instrument availability, ease of use, precision and sensitivity requirements, sample size, and matrix effects. Besides fostering the acquisition of technical and soft skills, one notable aspect of this type of project is the exceptionally high student satisfaction. Furthermore, the project’s outcomes have proven to be significant predictors of learning achievements. Additionally, it lays the foundation for exploring potential designs of aquaponics systems and fosters interdisciplinary projects, expanding the practical applications in the field of chemistry education. Overall, these projects exemplify enriching and engaging educational experiences that empower students with valuable skills and knowledge while encouraging them to explore novel avenues in analytical chemistry.

Bifunctional manganese-doped silicon quantum dot-responsive smartphone-integrated paper sensor for visual multicolor/multifluorescence dual-mode detection of nitrite

Considering great threats of nitrite in food toward public health, it is significant to develop portable, dependable, and cost-effective methods to detect the target. In comparison with traditional colorimetric or fluorometric strategies relying on the intensity variation at a single wavelength, multimodal measurement with notable color changes is a more attractive paradigm because of its stronger resistance against external conditions, expanded application scenario, improved reliability, and naked-eye distinguishable readouts. However, it remains a challenge on how to attain this goal. Here we proposed a smartphone-integrated paper sensor based on manganese-doped silicon quantum dots (Mn-SiQDs) for the visual dual-mode multicolor/multifluorescence determination of nitrite. The Mn-SiQDs featured both photoluminescence and oxidase-mimetic catalytic activity, and could stimulate the colorless 3,3′,5,5′-tetramethylbenzidine (TMB) oxidation to a blue species TMB+. When nitrite existed, the generated TMB+ selectively reacted with the analyte to produce yellow diazotized TMB+. Meanwhile, the photoluminescence of Mn-SiQDs could be suppressed by the diazotized TMB+ via inner filter effect. As a result, with a facile electrostatic assembly of Mn-SiQDs and [Ru(bpy)3]2+, the formed complex Mn-SiQDs/[Ru(bpy)3]2+ led to the variations of both color and photoluminescence upon different levels of nitrite. By combining the principle with paper strips and smartphone sensing, the fabricated sensor offered naked-eye visible multicolor/multifluorescence dual-mode response toward nitrite, exhibiting excellent specificity, performance stability, and practicability in food analysis. Our work provides an effective tool to achieve the multicolor/multifluorescence bimodal measurement of nitrite, and it will inspire future effort on designing versatile materials for advanced sensing applications.

Electrochemical measurements of the levels of nitric oxide metabolites in the blood serum

Background: Sepsis is a serious clinical condition caused by a dysregulated immune response to infection resulting in multiple organ failure. In the pathogenesis of sepsis, especially that of septic shock, great importance is given to the endothelial marker of vascular regulation, nitric oxide (NO). In septic shock, dysregulation of the vascular tone plays a key role in the development of hypotension. Therefore, the control of the level of nitric oxide and its stable metabolites in critically ill patients is a very important task. Aim: the aim of this study was to evaluate the potential of the electrochemical nitrite detection in the patients blood serum. Methods: The levels of nitric oxide stable metabolites in the blood serum of healthy individuals (n=20) and septic patients (n=25) were studied by the electrochemical method using a composite electrode and by the spectrophotometric method using the Griess reagent. Results: The data in the groups of healthy people and patients with sepsis differ significantly (p 0.00001) both when measured using electrochemical and spectrophotometric methods. The median value of the current response in healthy people was 0.41 A (0.33; 0.55), and the total content of nitric oxide metabolites (NOx) was 26.8 mol/L (20.8; 31.0), while in patients with sepsis, these values were 0.79 A (0.61; 1.28) and 38.89 mol/L (29.64; 57.45), respectively. A negative correlation was found between the data obtained for practically healthy persons (r=-0.696, p=0.0007). Conclusion: The obtained results allow us to conclude that the nitrite measurement in the blood serum by amperometry using a composite electrode is promising as a diagnostic technique.

Analysis Of Water Quality Conditions In Pearl Oyster (Pinctada maxima) Hatcheries in Karangasem Bali

Indonesia has a high potential for pearl oyster farming due to its diverse marine habitats, warm waters, and abundant supply of seaweed, which is the preferred food for pearl oysters. Indonesia is home to several species of pearl oysters, including the Pinctada maxima, which are prized for their large size and the quality of their pearls. Water quality plays a crucial role in the cultivation of Pinctada maxima. Factors such as temperature, pH, salinity, dissolved oxygen levels, and the presence of pollutants and pathogens can all impact the health and survival of Pinctada maxima. In addition to feed quality and seed availability, good water quality management is the key to the success of this biota seedling. This research was carried out by measuring parameters including temperature, salinity, pH, and dissolved oxygen levels in the waters temporally in the morning and evening every day of cultivation. Variables of ammonia, nitrite, total suspended solids, total bacteria, and vibrio bacteria were carried out once a week. The results of temperature measurements range from 28.2-290C. Measurement of total suspended solids 0 mg/l. pH measurements ranged from 8.1-8.3. Dissolved oxygen measurements ranged from 4.16-4.27 mg/l. Salinity measurements obtained results of 35 ppt. Ammonia measurements range from 0-1.55 mg/l. Nitrite measurements range from 0.012-0.026. Measurement of total bacteria obtained &lt;1 X 101 cfu/ml from all samples. Measurement of total vibrio yielded &lt;1 X 101 cfu/ml from all samples. Based on the research that has been done, the results show that water quality is still in the optimal range for pearl oyster (Pinctada maxima) hatchery activities.

Nitrite-negative results in urinary tract infection by Enterobacterales: does the nitrite dipstick test have low sensitivity?

Urinary tract infection (UTI) is one of the most common bacterial infections among humans. Urine culture is the gold standard diagnostic method for UTI; however, the dipstick test for nitrite is a widely used method signalling the presence of urinary nitrate-reducing bacteria. Unlike the gold standard, the dipstick test is easy to perform, while it is also less time-consuming and less expensive, and produces a result in a few minutes. This study investigates the sensitivity of the dipstick test for nitrite compared with the Griess test in urine samples from UTI caused by Enterobacterales species. We used the Griess test, which is the gold standard in nitrite measurement, to determine the sensitivity of the nitrite dipstick test. Semiquantitative urine culture was performed using standard procedures, and Enterobacterales identification was performed by manual conventional biochemical tests. In the first sample selection, 3 % (8/267) of urine samples suspected of UTI, analysed from March to April 2016, were nitrite-negative by dipstick test but positive for Enterobacterales in the urine culture. In the second sample selection, 5 % (2/44) of urine samples from October to December 2022 were also nitrite-negative but showed urine Enterobacterales isolation. All nitrite-negative dipstick results were consistent with the Griess test. Escherichia coli was the most prevalent bacterium, followed by Klebsiella pneumoniae, independent of sample selection. The dipstick test is a safe alternative for investigating nitrite in urine samples. We believe that the cause of nitrite-negative results is a lack of dietary nitrate, dilution of urine and exogenous interference (e.g. ascorbic acid). These findings support the idea that standard urine culture is necessary to rule out UTI.

The effect of hyperglycemia on the activation of peritoneal macrophages of albino rats.

Hyperglycemia is one of the main damaging factors of diabetes mellitus (DM). The severity of this disease is most clearly manifested under conditions of the inflammatory process. In this work, we have studied the activation features of rat peritoneal macrophages (MPs) under conditions of high glucose concentration in vitro. Comparison of the independent and combined effects of streptozotocin-induced DM and hyperglycemia on proliferation and accumulation of nitrites in the MPs culture medium revealed similarity of their effects. Elevated glucose levels and, to a lesser extent, DM decreased basal proliferation and NO production by MPs in vitro. The use of the protein kinase C (PKC) activator, phorbol ester (PMA), abolished the proinflammatory effect of thrombin on PMs. This suggests the involvement of PKC in the effects of the protease. At the same time, the effect of thrombin on the level of nitrites in the culture medium demonstrates a pronounced dose-dependence, which was not recognized during evaluation of proliferation. Proinflammatory activation of MPs is potentiated by hyperglycemia, one of the main pathological factors of diabetes. Despite the fact that high concentrations of glucose have a significant effect on proliferation and NO production, no statistically significant differences were found between the responses of MPs obtained from healthy animals and from animals with streptozotocin-induced DM. This ratio was observed for all parameters studied in the work, during analysis of cell proliferation and measurement of nitrites in the culture medium. Thus, the results obtained indicate the leading role of elevated glucose levels in the regulation of MPs activation, which is comparable to the effect of DM and even "masks" it.

AT1R blocker prevents mental stress induced retrograde blood flow in overweight/obese men.

The main goal was to determine the impact of mental stress (MS) on blood flow regulation in overweight/obese men. Fourteen overweight/obese men (27 ± 7 years; 29.8 ± 2.6kg/m2 ) participated in two randomized experimental sessions with oral administration of the AT1R blocker Olmesartan (40 mg; AT1RB) or placebo (PL). After 2 h, a 5-min acute MS session (Stroop Color Word Test) was administered. Blood flow was assessed at baseline and during the first 3 min of MS by vascular ultrasound in the brachial artery. Blood was collected before (baseline) and during mental stress (MS) for measurement of nitrite (chemiluminescence) and endothelin-1 (ELISA kit). The AT1R blocker was able to reverse the MS responses observed in the placebo session for retrograde flow (p < 0.01), retrograde SR (p < 0.01) and oscillatory shear index (p=0.01). Regarding vasoactive substances, no differences were observed in ET-1 (p > 0.05) responses to MS between experimental sessions. However, for nitrite responses, the administration of the AT1R blocker was able to increase circulating levels of NO (p=0.03) Blockade of AT1R appears to prevent the decrease in endothelial function by reducing low shear stress and maintaining the vasoactive substances balance after MS in overweight/obese men.

Effects of media length on biofilms and nitrification in moving bed biofilm reactors

Biofilms grown on free-floating plastic media are increasingly being used to cultivate biofilms in integrated fixed film activated sludge (IFAS) and moving bed bioreactor (MBBR) systems for wastewater treatment with the common goal of increasing nitrogen removal. Fundamental principles of fluid dynamics dictate that the length of internal media channels affects fluid velocities and shear forces across biofilm surfaces, which in turn should affect rates of mass transfer and biofilm growth and activity, but little is known about media length effects on water quality and biofilm characteristics. It was hypothesized that length affects biofilm thickness, microbial populations and their activities, and system performance. Nitrification rates and biofilm characteristics were monitored in parallel continuous flow, bench-scale MBBRs systems with media length as a controlled variable. Longer media produced biofilms with approximately twice the thickness and twice the mass per unit area than did media with one-third their length. Based on calculated head losses, the combined effects of length and constriction of internal channels led to an estimated 77% reduction in fluid velocity through the longer media relative to the shorter media. Longer media demonstrated more rapid development of nitrite oxidizing bacteria (NOB) activity than the shorter media over much of the study, as indicated by measurements of nitrite and nitrate, but AOB activity was similar in the two media. Both biomass and NOB activity were concentrated toward media ends, while ammonia oxidizing bacteria (AOB) activity was uniformly distributed across the media, based on testing of sectioned media. 16s rRNA amplicon sequencing indicated the presence of several putative heterotrophic nitrifying families, particularly Xanthomonadaceae, Comamonadadeae and Microbacteriaceae, as well as the autotrophic Bradyrhizobiacea (which includes the NOB Nitrobacter) were common on both media throughout the study. The short media enriched for Nitrosomonadaceae, which includes the AOB genus Nitrosomonas, while minimal autotrophic AOBs were found in the long media biofilm. These results provide insights to the design of media for improved performance, particularly with respect to nitrite versus nitrate production, which may be useful to improve nitrification and for energy saving processes for nitrogen removal such as deammonification. The research also provides fundamental insights regarding the effects of media geometry on biofilm structure and function, which advances our understanding of environmental factors affecting biofilm development.

Polysulfides derived from the hydrogen sulfide and persulfide donor P* inhibit IL-1β-mediated inducible nitric oxide synthase signaling in ATDC5 cells: are CCAAT/enhancer-binding proteins β and δ involved in the anti-inflammatory effects of hydrogen sulfide and polysulfides?

Hydrogen sulfide (H2S) emerged as an essential signaling molecule exerting beneficial effects in various cardiovascular, neurodegenerative, or musculoskeletal diseases with an inflammatory component, such as osteoarthritis. These protective effects were initially attributed to protein S-sulfhydration, a posttranslational modification of reactive cysteine residues. However, recent studies suggest that polysulfides and not H2S are responsible for S-sulfhydration. To distinguish between H2S and polysulfide-mediated effects in this study, we used the slow-releasing H2S and persulfide donor P*, which can be decomposed into polysulfides. The effects of P* on IL-1β-induced inducible nitric oxide synthase (iNOS), a pro-inflammatory mediator in osteoarthritis, were determined by nitrite measurement, qPCR, and Western blotting in the murine chondrocyte-like cell line ATDC5. Decomposed P* significantly reduced IL-1β-induced iNOS signaling via polysulfides, independently of H2S. In line with this, the fast-releasing H2S donor NaHS was ineffective. In RAW 264.7 macrophages, similar results were obtained. P*-derived polysulfides further diminished IL-1β-induced CCAAT/enhancer-binding protein (C/EBP) β and δ expression in ATDC5 cells, which might play a critical role in P*-mediated iNOS decline. In conclusion, our data support the view that polysulfides are essential signaling molecules as well as potential mediators of H2S signaling. Moreover, we propose that C/EBPβ/δ might be a novel target involved in H2S and polysulfide-mediated anti-inflammatory signaling.

Bifunctional Mn-Doped N-Rich Carbon Dots with Tunable Photoluminescence and Oxidase-Mimetic Activity Enabling Bimodal Ratiometric Colorimetric/Fluorometric Detection of Nitrite.

Multimodal detection is a promising paradigm because of its advantages of expanding usage scenarios and improving reliability. However, it is very challenging to design reasonable strategies to achieve the multimodal sensing of targets. Herein, we developed an unprecedented bimodal ratiometric colorimetric/fluorometric method by exploring a novel bifunctional artificial oxidase mimic, Mn-doped N-rich carbon dots (Mn-CDs), to achieve the high-performance determination of nitrite in complicated matrices. The Mn-CDs exhibited both tunable photoluminescence and high oxidase-like activity, effectively catalyzing the colorless 3,3',5,5'-tetramethylbenzidine (TMB) oxidation to generate blue TMB+. When nitrite was introduced, the TMB+ species generated would specifically react with nitrite to produce diazotized TMB+, resulting in a color change from blue to green and finally to yellow. Simultaneously, the fluorescence of Mn-CDs was quenched by the diazotized TMB+ product via the inner filter effect. Hence, the existence of nitrite could lead to the simultaneous variations of visual color and photoluminescence, providing the principal basis for the bimodal ratiometric colorimetric/fluorometric quantification of the target. With the method, excellent sensitivity, selectivity, reliability, and practicability for nitrite detection were verified. Our work proposes a new bimodal strategy for nitrite measurement using bifunctional CDs-based enzyme mimics, which will inspire future effort on the exploration of promising multifunctional nanozymes and their advanced applications in biochemical sensing.

Unchanged nitrate and nitrite isotope fractionation during heterotrophic and Fe(II)-mixotrophic denitrification suggest a non-enzymatic link between denitrification and Fe(II) oxidation

Natural-abundance measurements of nitrate and nitrite (NOx) isotope ratios (δ15N and δ18O) can be a valuable tool to study the biogeochemical fate of NOx species in the environment. A prerequisite for using NOx isotopes in this regard is an understanding of the mechanistic details of isotope fractionation (15ε, 18ε) associated with the biotic and abiotic NOx transformation processes involved (e.g., denitrification). However, possible impacts on isotope fractionation resulting from changing growth conditions during denitrification, different carbon substrates, or simply the presence of compounds that may be involved in NOx reduction as co-substrates [e.g., Fe(II)] remain uncertain. Here we investigated whether the type of organic substrate, i.e., short-chained organic acids, and the presence/absence of Fe(II) (mixotrophic vs. heterotrophic growth conditions) affect N and O isotope fractionation dynamics during nitrate (NO3–) and nitrite (NO2–) reduction in laboratory experiments with three strains of putative nitrate-dependent Fe(II)-oxidizing bacteria and one canonical denitrifier. Our results revealed that 15ε and 18ε values obtained for heterotrophic (15ε-NO3–: 17.6 ± 2.8‰, 18ε-NO3–:18.1 ± 2.5‰; 15ε-NO2–: 14.4 ± 3.2‰) vs. mixotrophic (15ε-NO3–: 20.2 ± 1.4‰, 18ε-NO3–: 19.5 ± 1.5‰; 15ε-NO2–: 16.1 ± 1.4‰) growth conditions are very similar and fall within the range previously reported for classical heterotrophic denitrification. Moreover, availability of different short-chain organic acids (succinate vs. acetate), while slightly affecting the NOx reduction dynamics, did not produce distinct differences in N and O isotope effects. N isotope fractionation in abiotic controls, although exhibiting fluctuating results, even expressed transient inverse isotope dynamics (15ε-NO2–: –12.4 ± 1.3 ‰). These findings imply that neither the mechanisms ordaining cellular uptake of short-chain organic acids nor the presence of Fe(II) seem to systematically impact the overall N and O isotope effect during NOx reduction. The similar isotope effects detected during mixotrophic and heterotrophic NOx reduction, as well as the results obtained from the abiotic controls, may not only imply that the enzymatic control of NOx reduction in putative NDFeOx bacteria is decoupled from Fe(II) oxidation, but also that Fe(II) oxidation is indirectly driven by biologically (i.e., via organic compounds) or abiotically (catalysis via reactive surfaces) mediated processes co-occurring during heterotrophic denitrification.

A nitrite sensor based on bimetallic zeolitic imidazole framework derived Co/porous carbon nanorods

Measurement of nitrite is needed for the control of nitrite pollution and to decrease the health risks and protect environment. It is attractive to design novel electrocatalysts for the efficient electrochemical quantification of nitrite. Herein, a sequence of Co, N-doped-porous carbon rods (CoN-PCRs) were designed and prepared through the calcination of rod-shaped mixed-metal zeolitic imidazolate framework precursors (ZIF-L) with different Co/Zn ratios. It was observed that the highest surface area and best response were obtained with the CoN-PCRs-0.6. Furthermore, its morphology and structure were characterized by several techniques. Additionally, the electrochemical sensing performances of CoN-PCRs-0.6 to nitrite were studied. Benefiting from the structural merits, the constructed sensor showed a low detection limit of 0.14 μM and achieved two wide dynamic ranges 0.2–4000 μM and 4000–100000 μM. The sensor also possessed good specificity, reproducibility, and stability toward nitrite quantification. Ultimately, the rapid and accurate measurement of nitrite in tap water was successfully realized.