HPLC-DAD Determination of Nitrite and Nitrate in Human Saliva Utilizing a Phosphatidylcholine Column

Intrinsic oxidase-mimicking activity of nitrite upon visible light illumination and its colorimetric detection in saliva

Small molecules with enzyme-like properties have recently attracted considerable attention. Herein, we discovered that nitrite possesses intrinsic oxidase-mimicking activity upon visible light, catalyzing the oxidation of the typical chromogenic substrate in the absence of H2O2. Notably, nitrite exhibited a markedly high value of Kcat, approximately 4, 7, and 4000-fold greater than that of Acr+-Mes, Eosin Y, and Diacetyl, respectively. Comprehensive investigation elucidated that O2•⁻ and •OH are the primary reactive oxygen species (ROS) responsible for the oxidation of 3,3',5,5'-tetramethylbenzidine dihydrochloride hydrate (TMB). Leveraging the linear correlation between the absorbance of oxidized TMB (oxTMB) at 652 nm and nitrite concentration, a simple colorimetric approach for nitrite detection was successfully established in the range of 1-75 μM with a detection limit of 0.14 μM. Moreover, the proposed strategy could be applied to determine the nitrite concentration in saliva, exhibiting a great prospect for clinical diagnosis. This work contributes novel insights into the exploration of small-molecule enzyme mimics.

Ag-CeO2 Based on Electrochemical Sensor for High-Efficient On-Site Detection of Nitrite in Aquaculture Water and Beverages

Nitrite is one of the most common nitrogenous compounds, which is not only an important indicator of aquaculture water but also widely used as a food additive. Its potential toxicity poses a huge threat to aquatic products and human health. Therefore, it is important to develop a convenient and rapid sensor for the high-efficient onsite detection of nitrite. In this work, a novel electrochemical sensor was developed for the qualitative and quantitative analysis of nitrite. The developed nitrite electrochemical detection system is easily applied in onsite detection. The electrochemical working electrode was constructed based on the combination of Ag-CeO2 and conductive carbon paste (CPE) with excellent electrocatalysis activity and rapid electron transfer ability. By the application of the developed system and under the optimal conditions, the linear range was from 40.0 μM to 500.0 μM, and the detection limit was reduced to 4.3 μM. The recovery was between 92.1% and 108.1%, and the relative standard deviations (RSDs) were 0.49%~9.31%. The sensor exhibited superior reproducibility, high stability sensitivity, and anti-interference ability, confirming its effectiveness for nitrite analysis. Finally, the developed electrochemical sensor was successfully applied to detect nitrite in beverages and aquaculture water samples, indicating that this approach has great potential in onsite food testing and environmental monitoring.

Rapid colorimetric assay based on the oxidation of 2,2-azino-bis(3-ethylbenzothiazoline)-6-sulfonic acid-diammonium salt for nitrite detection in meat products

This work developed a rapid colorimetric method for nitrite detection in meat products. The detection was based on the reaction of nitrite with 60 mM HCl to produce radicals which further oxidized ABTS (50 µM) to form a water-soluble blue-green product (ABTS•+). The absorbance was measured at a maximum absorption wavelength of 412.5 nm. Parameters such as concentration of HCl, concentration of ABTS and reaction time were evaluated. The absorbance was linearly proportional to the concentration of nitrite (0.1-20 µM) with the limit of detection of 0.34 µM. The proposed method was a time-saving assay since it required only 2 min to complete one measurement. There was no effect of the interference produced by other ions. The assay was robust with 2.5%RSD (n = 50). In meat product samples, high accuracy was observed with the recoveries between 100 ± 2.2% and 105 ± 3.7%. The amount of nitrite in meat products detected by the ABTS method was found in the range of 5.41 - 7.62 mg/kg. The conventional Griess method was applied to determine nitrite in the same meat products. There was no statistically significant difference between the two methods (P = 0.05).

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.

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.

Bio-Doped Microbial Nanosilica as Optosensing Biomaterial for Visual Quantitation of Nitrite in Cured Meats

A microbial optosensor for nitrite was constructed based on biomimetic silica nanoparticles, which were doped with R5, a polypeptide component of silaffin, as a robust biosilica immobilization matrix entrapped with Raoultella planticola and NAD(P)H cofactor during the in vitro biosilicification process of silica nanoparticles. Ruthenium(II)(bipy)2(phenanthroline-benzoylthiourea), the chromophoric pH probe, was physically adsorbed on the resulting biogenic nanosilica. Optical quantitation of the nitrite concentration was performed via reflectance transduction of the bio-doped microbial nanosilica at a maximum reflectance of 608 nm, due to the deprotonation of phen-BT ligands in the ruthenium complex, while the intracellular enzyme expression system catalyzed the enzymatic reduction of nitrite. Reflectance enhancement of the microbial optosensor was linearly proportional to the nitrite concentration from 1−100 mg L−1, with a 0.25 mg L−1 limit of detection and a rapid response time of 4 min. The proposed microbial optosensor showed good stability of >2 weeks, great repeatability for 5 repetitive assays (relative standard deviation, (RSD) = 0.2−1.4%), high reproducibility (RSD = 2.5%), and a negligible response to common interferents found in processed meats, such as NO3−, NH4+, K+, Ca2+, and Mg2+ ions, was observed. The microbial biosensor demonstrated an excellent capacity to provide an accurate estimation of nitrite in several cured meat samples via validation using a standard UV-vis spectrophotometric Griess assay.

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.

Saliva nitrite is higher in male children with autism spectrum disorder and positively correlated with serum nitrate

OBJECTIVES

Nitric oxide (NO) plays a vital role in neurological development. As an easily accessible and non-invasive fluid, saliva hasn't been evaluated for nitrite among children with autism spectrum disorder (ASD). This study aims to quantify saliva nitrite and explore its relation with serum NO.

METHODS

Saliva sampling and pretreatment methods were optimized, followed by NO measurement via chemiluminescence for 126 ASD children and 129 normally developing children (ND).

RESULTS

In the ASD group, saliva nitrite was significantly higher than that in the ND, with concentrations of 4.97 ± 3.77 μM and 2.66 ± 2.07 μM (p < 0.0001), respectively. Positive correlation was observed between saliva NO2- and serum NO3- in ASD children, which didn't exist in the ND group. Male children in the ASD group had significantly higher NO than that in boys of the ND group, without significant difference between girls in both groups. Correlation was not found between saliva or serum NO and severity of these ASD children.

DISCUSSION

It is reported for the first time that saliva nitrite was positively correlated with serum nitrate in ASD children, with significantly higher NO only in autistic boys. Non-invasive saliva might serve as a predictor of health status of ASD children.

Direct and specific analysis of nitrite and nitrate in biological and non-biological samples by capillary ion analysis for the rapid identification of fatal intoxications with sodium nitrite

In recent years, a significant increase of reports about suicidal cases due to intentional sodium nitrite intake has been described. In the forensic pathology context, the strategy to approach intoxication cases by sodium nitrite, without any preliminary information or hint, is not straightforward. Indeed, in a number of cases the lack of crime scene data and/or specific pathological signs makes difficult the identification of nitrite poisoning. Moreover, the analytical determination of nitrite in blood is challenging, due to its rapid oxidization to nitrate by hemoglobin. Although several methods have been proposed for the clinical analysis of nitrate and/or nitrite in biological samples, none of these is specifically focused on the determination of these ions in cadaveric samples. Consequently, the diagnosis of nitrite fatal intoxication is still based on methemoglobin analysis. The present paper reports the optimization and validation of an analytical method of capillary ion analysis (CIA) with UV detection, for the determination of nitrite and nitrate in biological fluids and its application to two authentic cases of death by nitrite intake. The analyses were carried out in a bare fused-silica capillary (75 µm inner diameter) using 100 mM sodium tetraborate (pH 9.24) as background electrolyte and applying a voltage of - 15 kV between the capillary ends. The detection was obtained by direct UV absorption recorded at 214 nm wavelength. Bromide was used as the internal standard. Linearity was established in the range of 0.25-5 mmol/L). Reproducibility (intraday and day-to-day) was characterized by relative standard deviations (RSDs) 14.7% for peak areas. The method was applied to the determination of nitrite and nitrate in two real forensic cases, where high concentrations of nitrate were found in cadaveric blood samples (6.5 and 4.4 mmol/L, respectively). Nitrite was found only in trace amounts, due to the instability of this ion in cadaveric blood where it is oxidized to nitrate. The present method represents a new tool for the direct and rapid determination of nitrite and nitrate in cases of forensic interest, and thus offers a diagnostic tool more sensitive and precise than the need methemoglobin analysis.

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 (A_{550 nm}/A_{640 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.

Applying Membrane Distillation for the Recovery of Nitrate from Saline Water Using PVDF Membranes Modified as Superhydrophobic Membranes

In this study, a flat sheet direct contact membrane distillation (DCMD) module was designed to eliminate nitrate from water. A polyvinylidene fluoride (PVDF) membrane was used in a DCMD process at an ambient pressure and at a temperature lower than the boiling point of water. The electrical conductivity of the feed containing nitrate increased, while the electrical conductivity of the permeate remained constant during the entire process. The results indicated that the nitrate ions failed to pass through the membrane and their concentration in the feed increased as pure water passed through the membrane. Consequently, the membrane was modified using TiO₂ nanoparticles to make a hierarchical surface with multi-layer roughness on the micro/nanoscales. Furthermore, 1H,1H,2H,2H-Perfluorododecyltrichlorosilane (FTCS) was added to the modified surface to change its hydrophobic properties into superhydrophobic properties and to improve its performance. The results for both membranes were compared and reported on a pilot scale using MATLAB. In the experimental scale (a membrane surface area of 0.0014 m², temperature of 77 °C, nitrate concentration of 0.9 g/Kg, and flow rate of 0.0032 Kg/s), the flux was 2.3 Kgm⁻²h⁻¹. The simulation results of MATLAB using these data showed that for the removal of nitrate (with a concentration of 35 g/Kg) from the intake feed with a flow rate of 1 Kg/s and flux of 0.96 Kgm⁻²h⁻¹, a membrane surface area of 0.5 m² was needed.

A green carbon dots-based fluorescent sensor for selective and visual detection of nitrite triggered by the nitrite–thiol reaction

Selective and visual detection of nitrite is realized through the inner-filter effect triggered by the nitrite–thiol reaction based on green carbon dots.

Levels of the Thiocyanate in the Saliva of Tobacco Smokers in Comparison to e-Cigarette Smokers and Nonsmokers Measured by HPLC on a Phosphatidylcholine Column

The aim of the study was to estimate the thiocyanate levels in saliva of cigarette smokers in comparison to e-cigarette smokers and nonsmokers. To improve our understanding of the influence of smoking on the oral level of thiocyanate, we conducted an assessment of human saliva, in 24 individuals (eight tobacco smokers, eight e-cigarette smokers, and eight nonsmokers). High-Performance Liquid Chromatography with ultraviolet detection (HPLC-UV) using a unique phosphatidylcholine column was applied in this assay. Thiocyanate ion was detected directly by its absorbance at 210 nm. The method presents a new application of the IAM (Immobilized Artificial Membrane) column for quantification of inorganic anions. The whole process meets the criteria of green chemistry because it was carried out without the use of organic solvents. For compensating matrix effects, an eight-point standard addition protocol was used to quantify the thiocyanate level in saliva samples. The calibration graphs were linear in the range of 5-100 mg L-1 with a correlation coefficient higher than 0.99. The thiocyanate concentrations in the saliva of tobacco smokers, e-cigarette smokers, and nonsmokers were found in the range of 121.25-187.54 mg L-1, 121.24-244.11 mg L-1, 33.03-79.49 mg L-1, respectively. The present study indicates an obvious statistically significant elevation in salivary thiocyanate level in tobacco smokers in comparison to nonsmokers. The phosphatidylcholine-based stationary phase proved to be suitable for the detection and quantification of the thiocyanate ion. The salivary thiocyanate levels in e-cigarette smokers were not significantly different in comparison to tobacco smokers but higher if compared to nonsmokers. The criterion for statistical significance was p < 0.05.