Highly Selective and Sensitive Detection of Nitrite Ion by an Unusual Nitration of a Fluorescent Benzimidazole

Exploration of a new approach for detection of nitrite with hydroxyl radical fluorescence probe in aqueous solutions

Nitrite (NO2-) has been widely recognized by the international community as an important substance affecting water quality safety and human health, and the detection of NO2- has always been a hot topic for researchers. Fluorescent probe method is an emerging and ideal way for detecting NO2-. Due to the high dependence of the reported reactive NO2- fluorescent probes on strong acidic systems, using the idea of photochemistry, a fluorescence analysis method for detecting NO2- was proposed in this work to change the necessity of strong acidic solutions in probe detection process. A 365 nm UV-LED lamp was used to irradiate NO2- in aqueous solution to convert it into hydroxyl radicals (HO·), and capture the photodegradation product of NO2- using coumarin-3-carboxylic acid as probe 3-CCA that can react with HO· to generate only one type of strong fluorescent substance. This probe has excellent photostability, selectivity, and anti-interference ability, and can realize the quantitative detection of NO2- (0-15 μM) in pure aqueous solution with pH of 7.4. In addition, its application in actual water samples is also satisfactory, with a recovery rate of (85.91 %-107.30 %). Importantly, we hope that this photolysis strategy can open up the novel thinking to develop suitable fluorescent probes for the analysis and detection of some hardly detected analytes.

Indopolycarbocyanine dyes as perspective analytical reagents for spectrophotometric determination of nitrite by radical nitration

A spectrophotometric method for the quantitative determination of nitrite was developed, based on the radical nitration of indopolycarbocyanine dyes in the presence of 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO). The rate of the reaction of the studied dyes with nitrite increases with the lengthening of the polymethine chain and the presence of hydrophilic sulfo groups in the side chain of the dye. TEMPO acts as a co-reagent, significantly accelerating the reaction rate and increasing the sensitivity of nitrite determination. The proposed reaction mechanism is supported by spectrophotometric and HPLC/MS studies. For Ind2 (tetramethine indocarbocyanine cationic dye), the limit of detection for nitrite is 0.50 µM within a linearity range of 1-13 µM. The developed method is sensitive, with a LOD 130 times lower than the maximum contaminant level (MCL) of nitrite in drinking water (65 μM), as specified by the WHO. The method is of low-toxicity and good selectivity, as the determination of nitrite is not significantly affected by the main components of water. The method was successfully applied for the analysis of nitrite in natural and bottled water.

A Portable Integrated Electrochemical Sensing System for On-Site Nitrite Detection in Food

Ensuring an appropriate nitrite level in food is essential to keep the body healthy. However, it still remains a huge challenge to offer a portable and low-cost on-site food nitrite analysis without any expensive equipment. Herein, a portable integrated electrochemical sensing system (IESS) is developed to achieve rapid on-site nitrite detection in food, which is composed of a low-cost disposable microfluidic electrochemical patch for few-shot nitrite detection, and a reusable smartphone-assisted electronic device based on self-designed circuit board for signal processing and wireless transmission. The electrochemical patch based on MXene-Ti3C2Tx/multiwalled carbon nanotubes-cyanocobalamin (MXene/MWCNTs-VB12)-modified working electrode achieves high sensitivity of 10.533 µA mm-1 and low nitrite detection limit of 4.22 µm owing to strong electron transfer ability of hybrid MXene/MWCNTs conductive matrix and high nitrite selectivity of VB12 bionic enzyme-based ion-selective layer. Moreover, the portable IESS can rapidly collect pending testing samples through a microfluidic electrochemical patch within 1.0 s to conduct immediate nitrite analysis, and then wirelessly transmit data from a signal-processing electronic device to a smartphone via Bluetooth module. Consequently, this proposed portable IESS demonstrates rapid on-site nitrite analysis and wireless data transmission within one palm-sized electronic device, which would pave a new avenue in food safety and personal bespoke therapy.

Timescale correlation of shallow trap states increases electrochemiluminescence efficiency in carbon nitrides

Highly efficient interconversion of different types of energy plays a crucial role in both science and technology. Among them, electrochemiluminescence, an emission of light excited by electrochemical reactions, has drawn attention as a powerful tool for bioassays. Nonetheless, the large differences in timescale among diverse charge-transfer pathways from picoseconds to seconds significantly limit the electrochemiluminescence efficiency and hamper their broad applications. Here, we report a timescale coordination strategy to improve the electrochemiluminescence efficiency of carbon nitrides by engineering shallow electron trap states via Au-N bond functionalization. Quantitative electrochemiluminescence kinetics measurements and theoretic calculations jointly disclose that Au-N bonds endow shallow electron trap states, which coordinate the timescale of the fast electron transfer in the bulk emitter and the slow redox reaction of co-reagent at diffusion layers. The shallow electron trap states ultimately accelerate the rate and kinetics of emissive electron-hole recombination, setting a new cathodic electrochemiluminescence efficiency record of carbon nitrides, and empowering a visual electrochemiluminescence sensor for nitrite ion, a typical environmental contaminant, with superior detection range and limit.

Mechanically robust, self-reporting and healable polyurethane elastomers by incorporating symmetric/asymmetric chain extenders

Self-healing elastomers usually show poor mechanical properties and environmental stability, and they cannot self-report mechanical/chemical damage. Herein, an innovative design strategy is reported that combines symmetric/asymmetric chain extenders to create large yet disordered hard domains within polyurethane (PU) elastomers, enabling the integration of mechanical robustness and self-reporting and self-healing capabilities to overcome both mechanical and chemical damage. Specifically, large yet disordered hard domains were created by governing the molar contents of asymmetric fluorescent 2-(4-aminophenyl)-5-aminobenzimidazole (PABZ) and symmetric 4-aminophenyl disulfide (APDS). Such a structural feature led to a small free-volume fraction, prominent strain-induced crystallization (SIC), and high energy of dissipation, enabling the PU elastomer to display outstanding mechanical strength (60.7 MPa) and toughness (177.9 MJ m-3). Meanwhile, the loose stacking of disordered hard domains imposed small restriction on network chains and imparted the network with high relaxation dynamics, leading to high healing efficiency (97.8%). More importantly, the fluorescence intensity was stimulus-responsive and thus the PU elastomer could self-report mechanical/chemical damage and healing processes. The PU elastomer also showed potential application prospects in information encoding and encryption. Furthermore, selecting polydimethylsiloxane as one of the soft segments could effectively endow the PU elastomer with intrinsic hydrophobicity. Therefore, this work provides valuable guidance for designing multi-functional materials with anti-counterfeiting, self-reporting, and healing properties as well as high mechanical properties and hydrophobicity.

Preferential Binding of a Red Emissive Julolidine Derivative to a Promoter G-Quadruplex

The therapeutic potential of G-quadruplexes has increased significantly with the growing understanding of their functional roles in pathogens apart from human diseases such as cancer. Here, we report the synthesis of three julolidine-based molecules and their binding to nucleic acids. Among the synthesized molecules, compound 1 exhibited red emissive fluorescence with a distinct preference for Pu22 G-quadruplex. The binding of compound 1 to Pu22 G-quadruplex, initially identified through a fluorescence-based screening, was further confirmed by UV-vis, fluorescence spectroscopy, and circular dichroism-based experiments. Thermal denaturation of compound 1 in the presence of Pu22 G-quadruplex revealed a concentration-dependent stabilization (~10.0 °C at 1 : 3 stoichiometry). Fluorescence-based experiments revealed 1 : 1 stoichiometry of the interaction and an association constant (Ka ) of 5.67×106  M-1 . CD experiments displayed that the parallel conformation of the G-quadruplex was retained on compound 1's binding and signs of higher order binding/complex formation were observed at high compound 1 to DNA ratio. Molecular docking studies revealed the dominance of stacking and van der Waals interactions in the molecular recognition which was aided by some close-distance interactions involving the quinolinium nitrogen atom.

Micelle-Assisted Confined Coordination Spaces for Benzimidazole: Enhanced Electrochemiluminescence for Nitrite Determination

Selective and sensitive detection of nitrite has important medical and biological implications. In the present work, to obtain an enhanced electrochemiluminescence (ECL) determination of nitrite, a novel nano-ECL emitter CoBIM/cetyltrimethylammonium bromide (CTAB) was prepared via a micelle-assisted, energy-saving, and ecofriendly method based on benzimidazole (BIM) and CTAB. Unlike conventional micelle assistance, the deprotonated BIM (BIM-) preferential placement was in the palisade layer of cationic CTAB-based micelles. Enriching the original CTAB micelle with BIM- disrupted its stability and resulted in the formation of considerably smaller BIM/CTAB-based micelles, providing a confined coordination environment for BIM- and Co2+. As a result, the growth of CoBIM/CTAB was also limited. Owing to the unusual nitration reaction between BIM and nitrite, the prepared CoBIM/CTAB was successfully applied as a novel ECL probe for the detection of nitrite with a wide linear range of 1-1500 μM and a low detection limit of 0.67 μM. This work also provides a promising ECL platform for ultrasensitive monitoring of nitrite and it was applied with sausages and pickled vegetables.

Ratiometric colorimetric detection of nitrite using CoMnO3 nanofibers as an oxidase-like enzyme to induce diazotization reaction

Nitrite is a typical food additive and preservative used in the food industry, which has attracted considerable attention due to its severe adverse effects on human health. Herein, a sensitive and highly selective ratiometric colorimetric sensing platform for the detection of nitrite was created based on a polymetallic oxide nanozyme, CoMnO3 nanofibers (CMO) catalysis integrated with the particular diazotization reaction. The nanozyme has superior oxidase-like activity (Km was 0.105 mM and Vmax was 63.7 × 10-8 M S-1) and could catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to oxidized TMB (oxTMB), as CMO could achieve the conversion of oxygen in the solution to superoxide anion (O2˙-). In addition, it is interesting to note that oxTMB can be diazotized in the presence of nitrite under acidic conditions, causing a shift in the ratio of nitrite concentration to the absorbance peaks at 450 and 652 nm (A450/A652). The ratio of A450/A652 exhibited a positive linear relationship with the concentration of nitrite within the concentration range of 0.2-200 μM, with a detection limit of 0.094 μM. Simultaneously, this method was also successful in quantifying the nitrite produced by brined and pickled foods and the dynamic tracking of the nitrite levels in various types of dishes. The analysis method not only offers dual-signal ratio sensing with high sensitivity but also holds the benefit of outstanding selectivity for the use of the particular reaction, which has a wide range of application prospects in food safety management.

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

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