Highly selective and sensitive fluorescent probe for the detection of nitrite

Rapid, portable and visualizing nitrite detection enabled by a rationally designed meso-aminoindole substituted pyronine-based fluorescent probe

Nitrite (NO2-) widely exists in our daily diet, and its excessive consumption can lead to detrimental effects on the human central nervous system and an elevated risk of cancer. The fluorescence probe method for the determination of nitrite has developed rapidly due to its simplicity, rapidity and sensitivity. Despite establishing various nitrite sensing platforms to ensure the safety of foods and drinking water, the simultaneous achievement of rapid, specific, affordable, visualizing, and on-site nitrite detection remains challenging. Here, we designed a novel fluorescent probe by using Rhodamine 800 as the fluorescent skeleton and 5-aminoindole as the specific reaction group to solve this problem. The probe shows a maximal fluorescence emission at 602 nm, thereby avoiding background emission interference when applied to food samples. Moreover, this unique probe exhibited excellent sensing capabilities for detecting nitrite. These included: a rapid response time within 3 min, a noticeable color change that the naked eye can observe, a low detection limit of 13.8 nM, and a remarkable selectivity and specificity to nitrite. Besides that, the probe can detect nitrite quantitatively in barreled drinking water, ham sausage, and pickles samples, with good recoveries ranging from 89.0 % to 105.8 %. More importantly, based on the probe fixation and signal processing technology, a portable and smart sensing platform was fabricated and made convenient and rapid analysis the content of NO2- in real samples possible. The results obtained in this work provide a new strategy for the design of high-performance nitrite probes and feasible technology for portable, rapid and visual detection of nitrite, and this probe holds the potential as a practical tool for alleviating concern regarding nitrite levels.

An acid-free sensing strategy for detecting nitrite using dihydroquinoline-8-carboxylate as a probe

Nitrite (NO2-) has been identified as a typical pollutant harmful to the human body and heavily assayed in the fields of food safety and water quality control. The mainstream sensing strategies for detecting NO2- depend on Griess reaction or its improved methods which employ Griess reaction to initiate further inter-or intramolecular interaction to generate readout signals. However, a significant drawback of these methods is the use of strongly acidic media. In this study, we designed and synthesized a new NO2--specific fluorescent probe (ethyl 3-cyano-2-hydroxy-5-imino-8-(3-methoxy-3-oxopropyl)-4-(pyridin-2-yl)-5,8-dihydroquinoline-8-carboxylate, DHQC). DHQC exhibited strong green fluorescence in an acetonitrile-PBS (10 mM) mixed system (pH 7.0). In the neutral medium and at room temperature, the fluorescence of DHQC changed from green to blue with the addition of NO2-. The preliminary mechanistic investigation reveals that NO2- can induce the decarboxylation of the probe DHQC. Based on this finding, a high sensitive and selective method for NO2--detection was established, which showed good linearity in a range of 5∼50 μM with a limit detection of 3.5 nM (3σ). Given the unique properties of DHQC, a DHQC-loaded hydrogel bead device was further developed and employed for rapid monitoring of NO2-, exhibiting the advantages of simple preparation, high sensitivity, and fast response compared with traditional sensing reagents. In addition, DHQC was also used as a fluorescent probe for cell-imaging in live cells, exhibiting good cell permeability and biocompatibility. This study proposes a potential strategy for constructing smart fluorimetric probes used for NO2- detection.

A naphthalimide-based fluorescent probe for rapid detection of nitrite and its application in food quality monitoring

Nitrite (NO₂^-) is a widely used food additive and long-term aging of cooked leftovers may also contribute to the formation of NO₂^-, excessive consumption of NO₂^- is harmful to human health. Developing an effective sensing strategy for on-site monitoring of NO₂^- has attracted considerable attention. Herein, a novel colorimetric and fluorometric probe ND-1 based on photoinduced electron transfer effect (PET) was designed for highly selective and sensitive detection of nitrite (NO₂^-) in foods. The probe ND-1 was strategically constructed by employing naphthalimide as the fluorophore and o-phenylendiamine as the specific recognition site for NO₂^-. The triazole derivative ND-1-NO₂^- could be produced exclusively by reacting with NO₂^-, leading to a visible colorimetric change from yellow to colorless accompanied by a significantly enhanced fluorescence intensity at 440 nm. The probe ND-1 exhibited promising sensing performances towards NO₂^- including high selectivity, rapid response time (within 7 min), low detection limit (47.15 nM) and wide quantitative detection range (0-35 μM). In addition, probe ND-1 was capable of quantitative detecting of NO₂^- in real food samples (including pickled vegetables and cured meat products) with satisfactory recovery rates (97.61%-103.08%). Moreover, the paper device loaded by probe ND-1 could be utilized for visual monitoring of NO₂^- levels variation of stir-fried greens. This study provided a feasible method for the accurate, traceable and rapid on-site monitoring NO₂^- in foods.

Aryl-Phenanthro[9,10-d]imidazole: A Versatile Scaffold for the Design of Optical-Based Sensors

Fluorescent and colorimetric sensors are important tools for investigating the chemical compositions of different matrices, including foods, environmental samples, and water. The high sensitivity, low interference, and low detection limits of these sensors have inspired scientists to investigate this class of sensing molecules for ion and molecule detection. Several examples of fluorescent and colorimetric sensors have been described in the literature; this Review focuses particularly on phenanthro[9,10-d]imidazoles. Different strategies have been developed for obtaining phenanthro[9,10-d]imidazoles, which enable modification of their optical properties upon interaction with specific analytes. These sensing responses usually involve changes in the fluorescence intensity and/or color arising from processes like photoinduced electron transfer, intramolecular charge transfer, intramolecular proton transfer in the excited state, and Förster resonance energy transfer. In this Review, we categorized these sensors into two different groups: those bearing formyl groups and their derivatives and those based on other molecular groups. The different optical responses of phenanthro[9,10-d]imidazole-based sensors upon interaction with specific analytes are discussed.

Fast visual monitoring of the freshness of beef using a smart fluorescent sensor

Spoiled meat contains many pathogenic bacteria; hence, the intake of spoiled food can lead to various illnesses. To screen the freshness of food, in this study, we devised a ratiometric fluorescence sensor dicyanovinyl coumarin (CMDC) for the determination of cadaverine, an important biomarker for the spoilage of meat. CMDC underwent aza-Michael addition with cadaverine, exhibiting high sensitivity, fast response (50 s), and distinct fluorescence color transition. Test strips fabricated using CMDC showed a noticeable color change from red to green when exposed to cadaverine vapor. The test strips were successfully used to visually monitor the spoilage of beef based on the fluorescence color change. Furthermore, the as-developed test strip coupled with a smartphone provides a simple tool for consumers and suppliers to obtain information about meat quality.

A boron dipyrromethene (BODIPY) based probe for selective passive sampling of atmospheric nitrous acid (HONO) indoors

People spend up to 90% of their time indoors, and yet our understanding of indoor air quality and the chemical processes driving it are poorly understood, despite levels of key pollutants typically being higher indoors compared to outdoors. Nitrous acid (HONO) is a species that drives these indoor chemical processes, with potentially detrimental health effects. In this work, a BODIPY-based probe was synthesized with the aim of developing the first selective passive sampler for atmospheric HONO. Our probe and its products are easily detected by UV-Vis spectroscopy with molar extinct coefficients of 37 863 and 33 787 M^-1 cm^-1, respectively, and a detection limit of 14.8 ng mL^-1. When protonated, the probe fluoresces with a quantum yield of 33%, which is turned off upon reaction. The synthesized BODIPY probe was characterized using NMR and UV-Vis spectroscopy. Products were characterized by UV-Vis and ultra high-resolution mass spectrometry. The reaction kinetics of the probe with nitrite was studied using UV-Vis spectroscopy, which had a pseudo-first-order rate of k = 7.7 × 10^-4 s^-1. The rapid reaction makes this probe suitable for targeted ambient sampling of HONO. This was investigated through a proof-of-concept experiment with gaseous HONO produced by a custom high-purity calibration source delivering the sample to the BODIPY probe in an acidic aqueous solution in clean air and a real indoor air matrix. The probe showed quantitative uptake of HONO in both cases to form the same products observed from reaction with nitrite, with no indication of interferences from ambient NO or NO₂. The chemical and physical characteristics of the probe therefore make it ideal for use in passive samplers for selective sampling of HONO from the atmosphere.

Quantitative image analysis method for detection of nitrite with cyanine dye-NaYF4:Yb,Tm@NaYF4 upconversion nanoparticles composite luminescent probe

In this work, a quantitative image analysis method based on cyanine dye-upconversion nanoparticles composite luminescent nanoprobe for the detection of nitrite was developed. The nanoprobe was constructed by combining the NaYF₄:Yb,Tm@NaYF₄ upconversion nanoparticles (UCNPs) and the new cyanine dye IR-790. The upconversion nanoparticles transferred energy to IR-790, resulting in the luminescence quenching, while the luminescence of UCNPs was recovered after adding NO₂^-. The increase in photons was related to the concentration of NO₂^-. Under the optimal experimental conditions, the detection range was 0.20-140 μM and the limit of detection was 0.030 μM. The measurement for NO₂^- can be completed in 29 min. The method has the characteristics of fast response (~0.1 s), low sample consumption (10 μL) and powerful data support (550 frame time series images). Furthermore, the quantitative image analysis method was successfully applied for the analysis of nitrite in environmental water and food samples.

A novel ratiometric and colorimetric probe for rapid and ultrasensitive detection of nitrite in water based on an Acenaphtho[1,2-d] imidazole derivative

The concentration of nitrite (NO₂^-) ions above allowable limits in water resources and food stuffs is considered hazardous and has been proven to be of great threat to the environment and public health. In this work, an acenaphtho [1,2-d] imidazole derivative (1) as a ratiometric colorimetric probe is developed. UV-Vis experiments demonstrate that the probe 1 shows excellent selectivity toward NO₂^- in the presence of other potential interfering species, a rapid response (20 s) and a low detection limit (100 nM) by a distinct visual color change with a bathochromic shift of 120 nm from colorless to intense yellow. Besides, this probe is further used for the quantification of nitrite ions in environmental water resources such as tap water, underground water, and surface water samples. The high recoveries (96-99% with relative standard deviations (RSD) of <2.0%) make the probe 1 a promising candidate for practical applications in daily life in the detection of nitrite ions.

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.

A facile dual-function fluorescent probe for detection of phosgene and nitrite and its applications in portable chemosensor analysis and food analysis

Due to the potential threats of phosgene and nitrite to public health and safety, in this work, we first proposed the application of a facile dual-function fluorescent probe 2-(1H-Benzimidazol-2-yl)Aniline (BMA) for the detection of phosgene and nitrite in different solvent environments. BMA had fast response (1 min), high selectivity and sensitivity (the limit of detection was 1.27 nM) to phosgene in CH₃CN solution (containing 10% DMSO), which manifested as a ratiometric fluorescent mode from 416 nm to 480 nm. The response of BMA to nitrite in HCl solution (pH = 1, containing 10% CH₃CN) was also highly selective and sensitive (the limit of detection was 60.63 nM), which shown as a turn-off fluorescent mode at 485 nm. In addition, two portable chemosensors (BMA-loaded TLC plates and test strips) had also been successfully manufactured for the detection of phosgene in the gas phase and nitrite in solution, which displayed good responses. Most importantly, BMA had also been successfully used for detection of nitrite in food samples, and a good recovery (88.5%-107.2%) was obtained by adding standard sodium nitrite.

Ultrasensitive, Specific, and Rapid Fluorescence Turn-On Nitrite Sensor Enabled by Precisely Modulated Fluorophore Binding

The precise regulation of fluorophore binding sites in an organic probe is of great significance toward the design of fluorescent sensing materials with specific functions. In this study, a probe with specific fluorescence properties and nitrite detection ability is designed by precisely modulating benzothiazole binding sites. Only the fluorophore bond at the ortho-position of the aniline moiety can specifically recognize nitrite, which ensures that the reaction products displays a robust green emission. The unique 2-(2-amino-4-carboxyphenyl) benzothiazole (ortho-BT) shows superior nitrite detection performance, including a low detection limit (2.2 fg), rapid detection time (<5 s), and excellent specificity even in the presence of >40 types of strong redox active, colored substances, nitro compounds, and metal ions. Moreover, the probe is highly applicable for the rapid on-site and semiquantitative measurement of nitrite. The proposed probe design strategy is expected to start a new frontier for the exploration of probe design methodology.

Fluorescent Chromophores Containing the Nitro Group: Relatively Unexplored Emissive Properties

Apart from numerous applications, for example in azo dye precursors, explosives, and industrial processes, the nitro group (-NO₂ ) appears on countless molecules in photochemical research owing to its unique characteristics such as a strong electron-withdrawing ability and facile conversion to the reduced substituent. Although it is well known as a fluorescence quencher, fluorescent chromophores that contain the nitro group have also emerged, with 3-nitrophenothiazine being recently reported to have 100 % emission quantum yield in nonpolar solvents. The diverse characters of nitro-containing chromophores motivated us to systematically review those chromophores with nitro substituents, their associated photophysical properties, and applications. In this Review, we succinctly elaborate the advance of the fluorescent nitro chromophores in fields of intramolecular charge transfer, fluorescent probes and nonlinear properties. Special attention is paid to the rationalization of the associated emission spectroscopy, so that the readers can gain insights into the structure-photophysics relationship and hence gain insights for the strategic design of nitro chromophores.

A novel near-infrared fluorescent sensor for zero background nitrite detection via the "covalent-assembly" principle

Different chemical states of nitrogen are present in many freshwater and marine ecosystems, and nitrite ions are one of the most toxic water-soluble nitrogen species. Developing an effective and convenient sensing method to constantly detect the concentration of nitrite has become a wide concern. Here, a novel near-infrared fluorescent probe (AAC) was designed and synthesized via the "covalent assembly" principle, showing excellent selectivity and high sensitivity for nitrite. A new nitrite-quantitative method was established with the help of AAC, and the detection limit of nitrite using the new method was as low as 6.7 nM. AAC was successfully applied for the quantitative detection of nitrite in real-world environmental and food samples (including river water and Chinese sauerkraut), and the detection results were essentially identical to the results obtained from the traditional Griess assay. Moreover, AAC was successfully applied for tracking nitrite in Escherichia coli by fluorescence imaging. Since nitrite can have devastating effects, the method established with AAC allowed us to "see" effectively about the water quality, food quality, etc.

Intrinsically ESIPT-exhibiting and enhanced emission in polymer nanoparticles as signaling for sensing nitrite

A straightforward approach to the fabrication of intrinsically excited-state intramolecular proton transfer (ESIPT)-fluorescent polymer nanoparticles (e-PNPs) was developed. The e-PNPs were obtained by self-assembly of the homopolymers derived from 4-aminosalicylic acid in aqueous solution. By incorporating ESIPT modules into polymer nanoparticles, the ESIPT reaction can be endowed with moderate hydrophobic micro-environment by nanoparticle scaffolds, eliciting enhanced ESIPT emission. The newly developed e-PNPs exhibit strong tautomeric fluorescence(e-FL), good photostability, low-toxicity and favourable biocompatibility in aqueous solution. Upon the addition of NO₂^-, the e-FL can be significantly quenched owing to the reaction of NO₂^- with the amide groups on e-PNPs. From this basis, the fluorescence detection of NO₂^- was implemented, which showed a linear relationship between 0 nM and 110 nM with a detection limit of 2.3 nM. Furthermore, e-PNPs were used as nanoprobes to monitor the NO₂^- levels in HeLa cells by fluorescence imaging, demonstrating the ability of discrimination from different concentrations of NO₂^-. The proposed method can be applied to a wide range of other ESIPT modules to integrate into polymer nanoparticles and offer highly sensitive nanosensing platform for bioanalysis and molecular imaging.

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.

A colorimetric fluorescent probe for rapid and specific detection of nitrite

The method of fluorescent probes has been an important technique for detection of nitrite (NO₂ ^- ). As an important inorganic salt, excessive nitrite would threaten humans and the environment. In this paper, a colorimetric fluorescent probe P-N (1,2-diaminoanthraquinone) with rapid response and high selectivity, which could detect NO₂ ^- by visual colour changes and fluorescence spectroscopy is presented. The probe P-N solution (pH 1) changed from pink to colourless with the addition of NO₂ ^- and fluorescence intensity at 639 nm clearly decreased. Good linear exists between fluorescence intensities and NO₂ ^- concentrations for the range 0-16 μM, and the detection limit was 54 nM (based on a 3σ/slope). Moreover, probe P-N could also detect NO₂ ^- in real water samples, and results were all satisfactory. Probe P-N shows great practical application value for detecting NO₂ ^- in the environment.

Sulfur and phosphorus co-doped graphene quantum dots for fluorescent monitoring of nitrite in pickles

Doping graphene quantum dots (GQDs) with heteroatoms can change their band gap and electronic density, thus enhancing their fluorescence quantum yield (QY). In this work, we for the first time reported a nontoxic, rapid, and one-pot hydrothermal method to synthesize sulfur and phosphorus co-doped GQDs (S, P-GQDs). Citric acid was functioned as a carbon source, whereas sodium phytate and anhydrous sodium sulfate are used as the P and S sources, respectively, in this bottom-up synthesis. The resulting S, P-GQDs exhibit high heteroatomic doping ratios of 9.66 at.% for S and 3.34 at.% for P, and higher QY than those obtained from monoatomic doped GQDs. Additionally, the as-prepared S, P-GQDs exhibit excitation-dependent behavior, pH sensitivity between 8.0 and 13.0, high tolerance of ionic strength. More importantly, the as-synthesized S, P-GQDs show a sensitive and selective behavior for sensing nitrite (NO₂^-) in the concentration range of 0.7-9 μmol/L, and the detection limit was as low as 0.3 μmol/L. Additionally, the S, P-GQDs was successfully used in detecting NO₂^- in pickled foods, showing their promise for potential applications in realistic analysis.

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.

Aggregation-induced emission fluorescent probe for monitoring endogenous alkaline phosphatase in living cells

Alkaline phosphatase (ALP) is a non-specific phosphate monoesterase and often regarded as an important biomarker of hypothyroidism and hepatobiliary diseases in medical diagnosis. In-situ detection of endogenous ALP and exploration of the distribution of ALP in cells are of great importance for the diagnosis of diseases associated with ALP. In this work, we designed and synthesized an aggregation-induced emission (AIE) fluorescent probe, (E)-2-(((9H-fluoren-9-ylidene) hydrazono)methyl)phenyl dihydrogen phosphate (FAS-P), that can respond to ALP with a remarkable large Stokes shift (>200 nm) based on excited state intramolecular proton transfer (ESIPT) mechanism. The probe FAS-P has high selectivity and sensitivity to the detection of ALP. And there is a linear relationship between the fluorescence intensity of FAS-P and ALP activity in the range of 1-100 U L^-1, the limit of detection (LOD) is as low as 0.6 U L^-1. More importantly, we successfully applied FAS-P to detect ALP in living cells and the monitoring of ALP in real time.

A colorimetric and fluorescent lighting-up sensor based on ICT coupled with PET for rapid, specific and sensitive detection of nitrite in food

A colorimetric and fluorogenic sensor exhibiting rapid, specific and sensitive detection of potentially toxic nitrite in food is described.

pH-regulated synthesis of CuOx/ERGO nanohybrids with tunable electrocatalytic oxidation activity towards nitrite sensing

CuO_x/ERGO nanohybrids with diverse morphologies prepared by pH-regulated synthesis display tunable electrocatalytic ability towards nitrite sensing.

Janus PEGylated gold nanoparticles: a robust colorimetric probe for sensing nitrite ions in complex samples

We presented a Janus PEGylated AuNP probe where PEGs and recognition ligands (e.g., 4-aminobenzenethiol, 4-ABT) were asymmetrically functionalized on an AuNP. With this design, the probes showed high colloidal stability, signal robustness, specificity, and sufficient sensitivity in the determination of NO₂^- in various complex samples.

Analysis of Nitrite and Nitrate in Foods: Overview of Chemical, Regulatory and Analytical Aspects

In this chapter, several factors that should be considered for selecting and developing suitable analytical methods for determining nitrite/nitrate are presented. Nitrite and nitrate occurrence and suitability are a controversial issue. Nitrite is an approved additive considered a foremost curing ingredient for the preservation of meat products. Nitrate is a natural constituent of the human diet that, however, raises fears for its suggested potential harmfulness related to carcinogenesis and environmental contamination. Chemical, regulatory, and analytical aspects are discussed in the light of the need to obtain reliable data of nitrite and nitrate for law enforcement purposes, exposure estimates, and investigation of their physiological role in the human body. In addition, current metrological aspects to ensure the "fitness for purpose" of the selected method are suggested and discussed.