A simple, azulene-based colorimetric probe for the detection of nitrite in water

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.

Tailoring liquid crystalline self-assembly and de Vries behavior of azulenes via lateral and core substitution

The azulene moiety is a highly attractive building block in optoelectronic applications due to its unique properties. For high-performing devices, the molecular orientation is crucial and can be controlled through liquid-crystalline self-assembly. Recent work showed that liquid crystalline derivatives bearing the 2-phenyl-azulene-1-nitrile core formed broad de Vries-type SmA and SmC phases. For exact understanding of the structure-property relationship, a series of 2-(hetero)aryl-azulenes has been synthesized varying the chain linkage, the lateral substituent, and the aromatic ring. Small changes of the molecular structure determined whether the orthogonal SmA phase or the tilted SmC phase is predominant. Implementation of alkyne chains instead of alkoxy chains resulted in the reduction of phase transition temperatures and formation of mesophases at room temperature. Furthermore, de Vries-like behavior was investigated and reduction values between R = 0.35 and 0.74 were measured which supported the hypothesis that in this system de Vries-like behavior is caused by steric repulsion of the lateral substituent. The control of the phase geometry by the molecular structure might be used for improved molecular orientation in optoelectronic materials.

Highly sensitive and specific detection of silver ions using a dual-color fluorescence co-localization strategy

Ag⁺ ions are widely used in various fields of human life due to their unique properties and they threaten the environment and human health. The traditional methods for Ag⁺ detection commonly suffer from disadvantages including limited sensitivity, expensive equipment and complicated operating steps. Herein, we developed a highly specific dual-color fluorescence co-localization (DFC) strategy based on the C-Ag⁺-C structure for Ag⁺ detection. In this strategy, Ag⁺ ions can be captured to form C-Ag⁺-C base pairs, and these ions enable single-stranded DNAs to form double strands. The DFC strategy can exclude nonspecific interaction sites and greatly improve the sensitivity and specificity. By DFC of the QDs and Cy5 linked to the DNA strands, highly sensitive Ag⁺ detection was achieved in the concentration range from 0.14 pM to 200 nM, with a limit of detection (LOD) of 0.14 pM. Moreover, this method has been applied for the detection of Ag⁺ ions in real environmental samples with satisfactory recoveries. We believe that the DFC strategy is promising for Ag⁺ detection.

The Eschenmoser's Salt as a Formylation Agent for the Synthesis of Indolizinecarbaldehydes and Their Use for Colorimetric Nitrite Detection

C-H bond formylation is the most immediate way to incorporate the versatile formyl group into (hetero)aromatics. However, the type of reagents and severe conditions involved in the classical formylation methods often curtail their application, especially in the presence of other functional groups. Herein, we present the Eschenmoser's salt, a commercially available (dimethylamino)methylating chemical, as a useful reagent for the C-H formylation of indolizines and other compounds. The method is straightforward and mild, furnishing indolizinecarbaldehydes in modest-to-good yields with exclusive and remote regioselectivity. Furthermore, these compounds can be easily transformed into push-pull dyes and are highly selective in the colorimetric detection of nitrite, a substance extensively employed as preservative in the food industry, the concentration of which is crucial to control to prevent harmful effects in living organisms. The assay is simple, allowing the naked-eye detection of nitrite in solution or on a cotton swab for a wide range of concentrations.

Azulene-based fluorescent chemosensor for adenosine diphosphate

AzuFluor® 435-DPA-Zn, an azulene fluorophore bearing two zinc(II)-dipicolylamine receptor motifs, exhibits fluorescence enhancement in the presence of adenosine diphosphate. Selectivity for ADP over ATP, AMP and PPi results from appropriate positioning of the receptor motifs, since an isomeric sensor cannot discriminate between ADP and ATP.

Ratiometric Colorimetric Detection of Nitrite Realized by Stringing Nanozyme Catalysis and Diazotization Together

Due to the great threat posed by excessive nitrite in food and drinking water to human health, it calls for developing reliable, convenient, and low-cost methods for nitrite detection. Herein, we string nanozyme catalysis and diazotization together and develop a ratiometric colorimetric approach for sensing nitrite in food. First, hollow MnFeO (a mixture of Mn and Fe oxides with different oxidation states) derived from a Mn-Fe Prussian blue analogue is explored as an oxidase mimic with high efficiency in catalyzing the colorless 3,3′,5,5′-tetramethylbenzidine (TMB) oxidation to blue TMBox, presenting a notable signal at 652 nm. Then, nitrite is able to trigger the diazotization of the product TMBox, not only decreasing the signal at 652 nm but also producing a new signal at 445 nm. Thus, the analyte-induced reverse changes of the two signals enable us to establish a ratiometric colorimetric assay for nitrite analysis. According to the above strategy, facile determination of nitrite in the range of 3.3–133.3 μM with good specificity was realized, providing a detection limit down to 0.2 μM. Compared with conventional single-signal analysis, our dual-signal ratiometric colorimetric mode was demonstrated to offer higher sensitivity, a lower detection limit, and better anti-interference ability against external detection environments. Practical applications of the approach in examining nitrite in food matrices were also verified.

C4-aldehyde of guaiazulene: synthesis and derivatisation

Guaiazulene is an alkyl-substituted azulene available from natural sources and is a much lower cost starting material for the synthesis of azulene derivatives than azulene itself. Here we report an approach for the selective functionalisation of guaiazulene which takes advantage of the acidity of the protons on the guaiazulene C4 methyl group. The aldehyde produced by this approach constitutes a building block for the construction of azulenes substituted on the seven-membered ring. Derivatives of this aldehyde synthesised by alkenylation, reduction and condensation are reported, and the halochromic properties of a subset of these derivatives have been studied.

An amplified fluorescent biosensor for Ag+ detection through the hybridization chain reactions

Ag is widely distributed in nature and it is used in almost all areas of human life. However, due to the widespread use of Ag materials, Ag⁺ pollution seriously threatens the human health and environment. The traditional detection methods for Ag⁺ suffer from disadvantages including high operational cost, complicated operating unit and instrument, and high requirements for professionals. Thus, in this study, a new type of Ag⁺ detection biosensor based on the hybridization signal amplification was designed to overcome these problems. Combining cytosine-Ag⁺-cytosine mismatch structure with the hybridization chain reaction, this biosensor converted the conventional detection signal into the nucleic acid amplification signal, which realized efficient, rapid, sensitive, and specific detection of Ag⁺. The limit-of-detection of this sensor reached 0.69 pM, which is much less than the maximum concentration (0.1 mg L^-1, 927 nM) suggested for drinking water by the World Health Organization, and the maximum concentration (0.05 mg L^-1, 464 nM) suggested by the United States Environmental Protection Agency. This method provides a promising new platform for detecting Ag⁺ concentrations at ultralow levels.

Azulene-A Bright Core for Sensing and Imaging

Azulene is a hydrocarbon isomer of naphthalene known for its unusual colour and fluorescence properties. Through the harnessing of these properties, the literature has been enriched with a series of chemical sensors and dosimeters with distinct colorimetric and fluorescence responses. This review focuses specifically on the latter of these phenomena. The review is subdivided into two sections. Section one discusses turn-on fluorescent sensors employing azulene, for which the literature is dominated by examples of the unusual phenomenon of azulene protonation-dependent fluorescence. Section two focuses on fluorescent azulenes that have been used in the context of biological sensing and imaging. To aid the reader, the azulene skeleton is highlighted in blue in each compound.

Azulene Functionalization by Iron-Mediated Addition to a Cyclohexadiene Scaffold

The functionalization of azulenes via reaction with cationic η5-iron carbonyl diene complexes under mild reaction conditions is demonstrated. A range of azulenes, including derivatives of naturally occurring guaiazulene, were investigated in reactions with three electrophilic iron complexes of varying electronic properties, affording the desired coupling products in 43-98% yield. The products were examined with UV-vis/fluorescence spectroscopy and showed interesting halochromic properties. Decomplexation and further derivatization of the products provide access to several different classes of 1-substituted azulenes, including a conjugated ketone and a fused tetracycle.

Colorimetric detection of Hg2+ with an azulene-containing chemodosimeter via dithioacetal hydrolysis

Azulene is a bicyclic aromatic chromophore that absorbs in the visible region. Its absorption maximum undergoes a hypsochromic shift if a conjugated electron-withdrawing group is introduced at the C1 position. This fact can be exploited in the design of a colorimetric chemodosimeter that functions by the transformation of a dithioacetal to the corresponding aldehyde upon exposure to Hg2+ ions. This chemodosimeter exhibits good chemoselectivity over other metal cations, and responds with an unambiguous colour change clearly visible to the naked eye. Its synthesis is concise and its ease of use makes it appropriate in resource-constrained environments, for example in determing mercury content of drinking water sources in the developing world.

A Colorimetric Chemosensor Based on a Nozoe Azulene That Detects Fluoride in Aqueous/Alcoholic Media

Colorimetry is an advantageous method for detecting fluoride in drinking water in a resource-limited context, e. g., in parts of the developing world where excess fluoride intake leads to harmful health effects. Here we report a selective colorimetric chemosensor for fluoride that employs an azulene as the reporter motif and a pinacolborane as the receptor motif. The chemosensor, NAz-6-Bpin, is prepared using the Nozoe azulene synthesis, which allows for its rapid and low-cost synthesis. The chemosensor gives a visually observable response to fluoride both in pure organic solvent and also in water/alcohol binary solvent mixtures.

Azulene-Derived Fluorescent Probe for Bioimaging: Detection of Reactive Oxygen and Nitrogen Species by Two-Photon Microscopy

Two-photon fluorescence microscopy has become an indispensable technique for cellular imaging. Whereas most two-photon fluorescent probes rely on well-known fluorophores, here we report a new fluorophore for bioimaging, namely azulene. A chemodosimeter, comprising a boronate ester receptor motif conjugated to an appropriately substituted azulene, is shown to be an effective two-photon fluorescent probe for reactive oxygen species, showing good cell penetration, high selectivity for peroxynitrite, no cytotoxicity, and excellent photostability.