Fluorescence Color Change Research Articles

Synthesis of fluorescent, ortho-azonaphthol-containing conjugated polymer for ratiometric fluoride ion sensing

A conjugated polymer containing the ortho-azonaphthol (o-AN) group was synthesized via the Suzuki coupling reaction, exhibiting an orange color and red fluorescence in its solution. Because the o-AN group contained an OH group adjacent to the azo group, it had an intramolecular H bond, providing backbone planarity. The H bond could be altered by exposure to F ions, showing an intense color change. The o-AN unit in the main chain of the polymer played an important role in F-ion detection and the resultant ratiometric color change. The o-AN monomer was not fluorescent in solution, but the o-AN-containing polymer did fluoresce in its solution, enabling ratiometric fluorescence color change from red to blue. The deprotonation of the OH group in the polymer, abstracted by F ions, enabled chromic and fluorogenic detection with suitable limits of detection of 0.96 and 2.55 μM, respectively, for drinking water.

A ratiometric fluorescent probe with an extremely large emission shift for detecting ClO− and its application in test strips and cell imaging

Hypochlorite (ClO−) is one of the important reactive oxygen species (ROS), which playing an irreplaceable influence towards various physiological activities, it is significant to establish an analytical tool for identifying and monitoring of ClO−. Here, a ratiometric fluorescent probe MYQ with an extremely large emission shift containing 1,8-naphthalimide and benzopyran fluorophores was designed for the detection of ClO−. The probe MYQ presented red fluorescence emission at 655 nm, however, in the presence of ClO−, it exhibited strong blue fluorescence at 462 nm with fluorescent color change from red to blue. Meanwhile, MYQ showed excellent selectivity and selectivity toward ClO− over other common anions in the PBS (60% CH3OH, pH = 7.4) system. The limit of detection for ClO− was calculated to be 25.3 nΜ. The presumed mechanism for detecting ClO− was verified by mass spectrometry and liquid chromatography. Furthermore, the potential practicability of MYQ was demonstrated by strip test and cell imaging.

Dye-based dual-emission Eu-MOF synthesized by Post-modification for the sensitive ratio fluorescence visualization sensing of ClO-

As we all know, excessive hypochlorite will be transformed into highly toxic substances, while insufficient hypochlorite can not completely kill bacteria and viruses in water. Therefore, it is desirable to develop a new analytical method to detect ClO- in environmental water. Here, a novel and simple fluorescence sensor was constructed for monitoring ClO- by an effective strategy. An Acriflavine@lanthanide metal-organic framework (Acr@Eu(BTEC)) was designed by covalently integrating amino-rich dye (Acr) and carboxyl-rich Eu(BTEC) via post-synthesis method. The created fluorescence sensor has two emission centers originating from Acr and Eu(BTEC), respectively. In the presence of ClO-, the strong green fluorescence derived from Acr was significantly quenched, while the invariant red emission from Eu3+ acted as the reference signal. Thus, Acr@Eu(BTEC) with two emissions was developed as a ratiometric fluorescence sensor for highly sensitive and selective detection of ClO-. The limit of detection (LOD) was as low as 10.75 nM. Moreover, visual detection of ClO- by the naked eyes is feasible with obvious fluorescent color changes from green to orange and then red. This method shows excellent performance in practical application, which suggests that it has great potential in water quality monitoring.

Multi-stimuli responsive fluorescence switching behaviours of AIE polymers for acid-base vapour sensing and highly sensitive ferric ion detection

Novel aggregation-induced emission (AIE) polymers, i.e., P1-SP and P2-SP, consisting of AIEgenic tetraphenylethylene (TPE) and photochromic spiropyran (SP) units are reported. Both P1-SP and P2-SP exhibit excellent AIE behaviours of TPE moieties in near-aqueous solutions THF/H2O (95% water) and solid states with strong cyan and blue emissions at 494 and 477 nm, respectively. Upon acidification, the close forms of P1-SP and P2-SP can be opened and switched to open P1-MCH+ and P2-MCH+ structures of protonated merocyanine (MCH+), resulting in fluorescence quenching of TPE units through corresponding intramolecular charge transfer (ICT) and Förster resonance energy transfer (FRET) processes. The fluorescence recovery of TPE can be monitored by the neutralization of P1-MCH+ and P2-MCH+ with base solutions under ambient/visible light attributed to the re-close P1-SP and P2-SP structures, which led to respective turn-off ICT and FRET phenomena. Distinct fluorescence quenching behaviours of P1-SP and P2-SP can be observed under various stimuli, including acid conditions and ferric ion interactions, which are also confirmed by theoretical studies. Moreover, P1-SP and P2-SP can be utilized as fluorescence quenching sensors for nanomolar level detections of ferric ion (LOD = 166 and 110 nM for corresponding P1-SP and P2-SP) with excellent selectivities in near-aqueous solutions and paper test strips. Additionally, the interesting visible and fluorescence colour changes of coated cellulosic papers, blended PMMA films and powder forms based on P1-SP and P2-SP under multi-external stimuli (acid-base vapour and visible light/heating processes) are also explored in this report.

Visualizing voltage in mitochondria via a unique polarity-responsive fluorescent probe

Mitochondrial membrane potential (ΔΨm) plays key roles in important life processes such as aerobic respiration, and is also an important parameter for assessing mitochondrial health and cell status. Therefore, detecting ΔΨm is crucial for revealing important life processes and detecting cellular activity. However, the currently reported fluorescent probes for ΔΨm have inherent defects, and their signals are facilely affected by factors such as staining concentration and pH fluctuations. To overcome these shortcomings, in this work, we proposed a new strategy to construct probes for ΔΨm via subcellular migration and polarity-responsive mechanisms, and constructed a two-color fluorescent probe for detection of ΔΨm. The probe targeted mitochondria in living cells and emitted blue and red fluorescence due to the lower polarity of the inner mitochondrial membrane. After the ΔΨm decreased, the probe migrated into the RNA, giving decreased blue emission and enhanced red fluorescence due to the higher polarity of the RNA restriction of intermolecular rotation. In this manner, the probe was capable of detecting ΔΨm by two fluorescence signal changes in fluorescence color and staining position. Using this probe, the reversible change of ΔΨm was successfully observed, and the process of cellular oxidative damage and apoptosis was visualized. The probe can serve as an important molecular tool to promote the development of ΔΨm, cell damage and apoptosis and related fields.

A Versatile Tetraphenylethene Derivative Bearing Excitation Wavelength Dependent Emission, Multistate Mechanochromism, Reversible Photochromism, and Circularly Polarized Luminescence and Its Applications in Multimodal Anticounterfeiting

A Versatile Tetraphenylethene Derivative Bearing Excitation Wavelength Dependent Emission, Multistate Mechanochromism, Reversible Photochromism, and Circularly Polarized Luminescence and Its Applications in Multimodal Anticounterfeiting

An imidazole-derived polarity sensitive probe for lipid droplet target and in vivo tumor imaging

Accurate differentiation of tumor cells from normal cells is a fundamental and critical challenge for the early diagnosis of cancer. The intrinsic low-polarity and increased numbers of lipid droplets can be utilized as potential biomarkers for tumor cells. Therefore, we synthesized and screened the polar fluorescent probe (PI-CHO), which exhibiting notable fluorescence color change and spectral shift in solvents of different polarities. PI-CHO has a high sensitivity to polarity changes in the linear range of ET(30) = 37.4–63.1, with a fluorescence enhancement of more than 474-folds. Intracellular imaging experiments showed that PI-CHO has low cytotoxicity, excellent lipid droplet-targeted performance and discernibility between tumor and normal cells. Ex vivo imaging of the tumors and major organs demonstrated the great reliability and sensitive responsiveness of PI-CHO towards tumors. Moreover, the proposed probe was successfully applied to detect tumors in living mice, exhibiting the potential to become a powerful tool in early cancer diagnosis.

Ratiometric fluorescent detection of sulfide ions in Radix Codonopsis and living cells based on PVP-supported gold/copper nanoclusters with tunable dual emission

Herein, a novel fluorometric-sensor with dual-emission system was constructed on the basis of polyvinylpyrrolidone (PVP) and 2-mercaptobenzothiazole (MBT) co-functionalized gold/copper nanoclusters (PVP/MBT-Au@CuNCs) by a facile and eco-friendly one-pot approach. The sensor exhibited ratiometric fluorescence emission (F590 nm/F422 nm) for visual and selective detection of S2− with a sensitive detection limit of 11.9 nM. Besides, fluorescence quenched sensing of S2− was chalked up by a quickly selectivity monitoring time of 30 s, owing to the strongly binding of Cu2S and Au2S by hard-soft-acid-base theory and the destruction of the aggregated structure of PVP/MBT-Au@CuNCs. Furthermore, the platform also provided the portable analysis for visual detection of S2− by capturing the change in fluorescence color with a single dual-emissive ratiometric paper strip. It is worth mentioning that the fluorescent gold-copper nanoclusters showed excellent application activities in the selective detection of S2− in Radix Codonopsis or Tremella samples and recognition of S2− in HeLa cells or macrophages by confocal microscopy fluorescent imaging. Overall, the sensing system paved a new avenue for effectively developing a convenient ratiometric fluorescent sensor platform for evaluating the safety of food with S2− pollution in environment and biological system.

A smartphone readout device for portable and sensitive estimation of Hg2+ via coumarin-modified paper

The development of cost-effective sensing system for Hg2+ quantitation is highly demanded for environmental monitoring. In this work, we reported an on-site, rapid and portable smartphone-based Hg2+ sensing system integrating coumarin-modified paper as a probe and smartphone readout device as a processing platform. For fluorescent sensing of Hg2+, the modified paper was fabricated by chemically grafting a coumarin-based probe N-((7-hydroxy-coumarin-3-yl)carbamothioyl)acetamide (HCCA) onto paper, and its physicochemical properties and sensing performance were characterized by elemental analysis, FTIR, XPS, SEM, and fluorescence spectral analysis. The modified paper displayed high selectivity and sensitivity for Hg2+, accompanied with a distinct fluorescence color change. For portable detection of the fluorescent sensors, a smartphone readout device was designed, consisting of a removable 3D-printed hardware device and an application. Remarkably, the smartphone readout device in cooperation with the modified paper is highly selective and sensitive to Hg2+ with the limit of detection (LOD) of 0.46 ppb, which avoids the requirement of sophisticated equipment and complex operation. The cost-effective sensing system based on smartphone shows a great potential for monitoring of Hg2+ in environmental water.

Dual-ligand lanthanide metal–organic framework for ratiometric fluorescence detection of the anthrax biomarker dipicolinic acid

Dipicolinic acid (DPA) is a unique biomarker of Bacillus anthracis. Development of a simple, fast, sensitive and timely DPA detection method is of great importance and interest for preventing mass disease outbreaks and treatment of anthrax. In this work, a novel lanthanide-doped fluorescence probe was constructed by coordination of Eu3+ with bifunctional UiO-66-(COOH)2-NH2 MOFs materials for efficient monitoring DPA. UiO-66-(COOH)2-NH2 MOFs were prepared using Zr4+ as a metal node, 1,2,4,5-benzenetetracarboxylic acid (H4BTC) and 2-aminoterephthalic acid (NH2-BDC) as bridging ligand through a simple one-pot synthesis method. By virtue their abundant carboxyl groups, UiO-66-(COOH)2-NH2 can readily grasp Eu3+ to form UiO-66-(COOH)2-NH2/Eu with coordinated water molecules at Eu sites. Upon interaction with DPA molecules, the coordinated H2O molecules were replaced by DPA molecules which transfer energy to Eu3+ in UiO-66-(COOH)2-NH2/Eu and sensitize Eu3+ luminescence. Meanwhile, DPA has a characteristic absorption band at 270 nm, which overlapped with the excitation spectrum of NH2-BDC, allowing the fluorescence of UiO-66-(COOH)2-NH2/Eu at 453 nm to be greatly quenched by DPA through inner filter effect (IFE). Therefore, the rationally designed UiO-66-(COOH)2-NH2/Eu complex not only exhibits strong hydrophilicity and high dispersion, but also serves as ratiometric fluorescence sensing platform for monitoring DPA concentration. This sensing platform showed a satisfactory linear relationship from 0.2 μM to 40 μM with a limit of detection of 25.0 nM and a noticeable fluorescence color change from blue to red, holding a great promise in practical applications.

Lactosylation leads to a water-soluble fluorescent probe for detection of S2− in water

Detection of sulfide anion (S2−), which is a common environmental pollutant in water and a threat to human health through drinking or food, holds great significance. However, most of existing S2− probes are insensitive in aqueous solution due to poor water-solubility. Here, a water-soluble small molecular fluorescent probe L-CC was first developed via introducing lactose to coumarin derivative (L-C) and then coordinating with copper ions (Cu2+). Water solubility endowed by lactose moiety makes L-CC ultrasensitive toward S2− in water. The fluorescence of L-CC was recovered quickly within 10 s in aqueous solution by displacement of Cu2+ after adding S2− with an extremely low detection limit for S2− measured to be 5.20 nM, which make L-CC suitable for testing S2− spiked in some real water samples. What’s more, fluorescent test strips produced by filter papers embedded with L-CC were capable of visual and quantitative analysis of S2− in water with obvious fluorescent color changes. Thus, this work provides a fluorescent probe with good water solubility endowed by lactosylation as well as its derived fluorescent test strip for simple and sensitive detection of S2− in water, which shows good potential in on-site water quality monitoring.

Synthesis and photophysical properties of 2-aryl-4-(morpholin-4-yl)quinazoline chromophores: The effect of π-linker moiety

A few-stages synthetic approach to novel 2-aryl-4-(morpholin-4-yl)quinazolines has been proposed. The photophysical properties of target compounds have been investigated in two solvents and compared with those for 2-thienyl-4-(morpholin-4-yl)quinazoline counterparts. The influence of nature, arrangement, and the length of π-linker moiety on the photophysical characteristics of dyes have been analyzed. In general, the replacement of thienylene ring with phenylene one results in blue-shifted absorption and emission maxima, whereas the compound with shortened π-system exhibits slight changes in both absorption and emission in toluene solution compared to the corresponding bi-phenylene counterpart. The compounds emit in solutions with quantum yield up to 75% in toluene and up to 46% in acetonitrile. Fluorescence colour and intensity change upon gradual addition of acid to the solution of the chromophore in toluene. It was found that (5-(4-diphenylaminophenyl)thiophen-2-yl)-4-(morpholin-4-yl)quinazoline is capable of generating white luminescence. Additionally, emission behaviour of quinazolines in the MeCN/water mixtures with different water content has been studied. The changes in position and intensity of the emission band were noted for all the compounds upon addition of water to the solution of quinazoline in MeCN; some chromophores show enhanced emission at 70–80 vol% of water fraction. Finally, 4-morpholinyl chromophores exhibit emission in the solid state. The biphenylene quinazolines emit deep-blue light in powder, while 2-(4-diphenylamino)phenylquinazoline and 2-(5-aminoaryl)thiophen-2-yl counterparts possess green or blue-green fluorescence.

Hue Recognition Competitive Fluorescent Lateral Flow Immunoassay for Aflatoxin M1 Detection with Improved Visual and Quantitative Performance.

Immunological detection of small molecules in a point-of-care (POC) format is of great significance yet remains challenging for accurate visual discrimination and quantitative analysis. Here, we report a novel hue recognition competitive fluorescent lateral flow immunoassay (HCLFIA) strip that allows both visual and quantitative detection of aflatoxin M1 (AFM1). The HCLFIA strip works on the basis of the ratiometric change of emission, arising from the overlap of fluorescence signals of two nanocomposites tagged with probe antibodies and coated antigens. A visually discernible orange-red-to-green fluorescence color change allows the naked eye semiquantitative readout of AFM1 around the threshold concentration (0.05 ng mL-1), yielding a visible detection limit of 0.02 ng mL-1. Moreover, using a custom smartphone-based device and color chart analysis, ultrasensitive quantitative detection of AFM1 can be achieved with a low limit of detection at 0.0012 ng mL-1, which is considerably better than those of the previously reported colorimetric and fluorescent strips. The accuracy performed in spiked milk samples ranged from 97.91 to 113.12% with a coefficient of variation below 7.8%, showing good consistency with the results from isotope dilution liquid chromatography-tandem mass spectrometry. Thanks to the unique hue recognition scheme, the HCLFIA strip holds great potential for POC detection of small molecules.

Ratiometric fluorescence sensor for point-of-care testing of bilirubin based on tetraphenylethylene functionalized polymer nanoaggregate and rhodamine B

Monitoring of free bilirubin in a point-of-care fashion is urgently needed in particular for newborns and liver dysfunction patients. Herein, a novel hybrid ratiometric fluorescence sensor was designed and fabricated for the rapid, sensitive and selective detection of free bilirubin in water and urine. The sensor was composed of Rhodamine B and polymer nanoaggregate of tetraphenylethylene functionalized hyperbranched polyamidoamine, which exhibited dual emission peaked at 577 nm and 465 nm. The fluorescence peak at 465 nm could be sufficiently suppressed by Förster resonance energy transfer between the polymer nanoaggregate and bilirubin, while the peak at 577 nm arising from Rhodamine B was insensitive to bilirubin, which resulted in a visually discernible blue to orange fluorescence color change. The concentrations of bilirubin could be evaluated by the fluorescence intensity ratios and the detection limit was as low as 25 nM. A portable hydrogel-based point-of-care testing device was constructed by integrating the recognition sensor in hydrogel and employed for bilirubin test. This hydrogel-based assay is convenient and affordable, providing a promising tool for at-home bilirubin detection and bilirubin-related disease diagnosis.

Nitrogen-doped carbon dots coupled with morin-Al3+: Cleverly design an integrated sensing platform for ratiometric optical dual-mode and smartphone-assisted visual detection of fluoride ion

Ratiometric fluorescence sensor has high selectivity and good sensitivity; however, its development is limited by intricate design, tedious synthesis, etc. Herein, a facile and effective ratiometric fluorescence sensing platform for fluoride ion (F-) detection was developed by simply combining nitrogen-doped carbon dots (N-CDs) and morin-Al3+ based on inner filter effect (IFE). The competitive binding of F- to Al3+ obviously decreased morin-Al3+ fluorescence and increased N-CDs fluorescence, attributing to the inhibition of IFE between N-CDs and morin-Al3+. The as-constructed ratiometric fluorescence sensing platform can be used for F- detection with a wide linear range (0.5–150 μM) and a low detection limit (55.8 nM). Interestingly, with the introduction of F- into the N-CDs/morin-Al3+ sensing platform, a distinguishable change in fluorescence color from green to blue enabled the N-CDs/morin-Al3+ system to be used as a smartphone-assisted visual sensing platform for F- detection with a detection limit of 2.09 μM. This platform was successfully applied for the onsite monitoring of F- in various water samples with satisfying results. These findings provide a novel guidance for the facile construction of a ratiometric optical dual-mode and smartphone-assisted sensing platform based on CDs, revealing the broad application prospect of CDs in environmental monitoring field.

Ultrafast Sulfur Mustard Simulant Gas Fluorescent Chemosensors Based on Triazole AIEE Material with High Selectivity and Sensitivity at Room Temperature.

Herein, a novel blue aggregation-induced enhanced emission (AIEE) material 4-N-(naphthalen-l-yl)-3,5-bis(4-N-phenyl-1-naphthylamine)phenyl-4H-1,2,4-triazole (NDTAZ) is developed and used as a fluorescent chemosensor for sulfur mustard (SM) simulant 2-chloroethyl ethyl sulfide (2-CEES) vapor. The NDTAZ chemosensor is designed by introducing an electron-donating N-phenyl-1-naphthylamine group at 3 and 5 position of 4H-1,2,4-triazole (TAZ) to enhance the nucleophilicity of the TAZ group, and a naphthalene ring is connected to 4 position of the TAZ group to construct an AIEE molecule. The NDTAZ films show extraordinary stability and then are further used as reliable and portable fluorescent chemosensors. Upon exposure to 2-CEES vapor, the NDTAZ chemosensor exhibits an instantaneous fluorescence response (not more than 1 s). What should be noted is that this fluorescent chemosensor realizes the visualized detection of fluorescent color change from blue to green at "room temperature", which is rarely reported. The limit of detection is estimated to be 0.55 ppm, which is below the AEGL-1 (0.6 ppm for 1 min) safety ceiling level to SM exposure. Moreover, the NDTAZ chemosensor shows high selectivity toward 2-CEES vapor over closely related substances, including alkylating agents, aryl halide compounds, sulphur-containing compounds, and nerve agent mimics. More impressively, the NDTAZ chemosensor demonstrates good recyclability by water treatment. Also, the sensing mechanism is adequately proved by using multiple experimental methods and theoretical calculation. In addition, the NDTAZ-based facile filter paper-constructed test strips are fabricated for real-time and on-spot detection of leaked 2-CEES gas specifically. Therefore, this fluorescent chemosensor with excellent sensing performance greatly advances the practical detection of SM species at room temperature.

Green fluorescent carbon dots from chitosan as selective and sensitive “off-on” probes for nitrite and “on-off-on” probes for enrofloxacin detection

In this work, green emission fluorescence carbon dots (G-CDs) were prepared using chitosan and p-phenylenediamine as precursors. Furthermore, the fluorescence of the prepared G-CDs was quenched by Nitrite (NO 2 - ) with excellent selectivity and sensitivity due to internal filtration effect (IFE) Therefore, G-CDs was applied to monitor NO 2 - as an “on-off” fluorescence probe and obtained a wide linear range (1–300 μM) with limit of detection (LOD) 1.00 μM. Another hand, the fluorescence of G-CDs was also quenched by Au 3+ due to fluorescence resonance energy transfer (FRET) effect caused by the in-situ formed Au nanoparticles (AuNPs). Interestingly, the addition of enrofloxacin (Enr) to G-CDs/AuNPs system led to the aggregation of AuNPs with partly lager size G-CDs, and the quenched fluorescence of the G-CDs/AuNPs system was restored, accompanying with visible fluorescence color change from green to blue. The “on-off-on” detection method through the constructed G-CDs/AuNPs system to Enr obtained good effects, such as high selectivity, sensitivity, visibility, relatively wide linear range (1–50 mg/L) and low LOD (0.04 mg/L). Additionally, the prepared G-CDs could be used for the detection of NO 2 - and Enr in actual water samples, proving it is an excellent candidate for environmental detection of NO 2 - and Enr. • Synthesis of nitrogen doped green carbon dots (G-CDs) with chitosan and p-Phenylenediamine. • Fabrication of an “off-on” fluorescent probe for NO2- and “on-off-on” fluorescent probe for enrofloxacin based on G-CDs. • Investigation of the detection mechanism of G-CDs by NO2- and enrofloxacin. • G-CDs displayed high specificity, sensitivity and anti-interference ability for the detection of NO2- and enrofloxacin.

Uracil-Appended Fluorescent Sensor for Cu2+ and Hg2+ Ions: Real-Life Utilities Including Recognition of Vitamin B2 (Riboflavin) in Milk Products and Invisible Ink Applications.

A simple uracil-appended fluorescent sensor (1) has been developed by one pot reaction and characterized by using common spectroscopic methods such as UV-vis, Fluorescence, HRMS and FT-IR analyses. Upon addition of various metal ions to the CH3CN solution of sensor 1, the fluorescence was quenched in the presence of Cu2+ / Hg2+ ions. The limit of detection for Cu2+ and Hg2+ was calculated to be 3.31 and 0.316 µM, respectively. Further, the sensor was applied for real-life applications in the determination of Vitamin B2 (riboflavin) and its presence in milk products. With the incorporation of different sources of vitamin-B to acetonitrile solution of it, there was discernible fluorescence enhancement only in the presence of vitamin B2. Also, it has been successfully applied for the detection of Vitamin B2 (riboflavin) in milk and curd. Moreover, based on the fluorescent color changes, the sensor was utilized for invisible ink applications.

A luminescent covalent organic framework with recognition traps for nitro-pesticides detection, pH sensing and metal ions identification

Special recognition sites in luminescent covalent organic frameworks (LCOFs) are extremely important to improve the selectivity of detection. Herein, we report on a novel LCOF named TFBD-Py-COF with three-in-one recognition traps composed of hydrogen bonding, reversible protonation of N and O–H⋯NC chelating unit. TFBD-Py-COF is synthesized via 3,3′,5,5′-tetraformyl-4,4′biphenyldiol (TFBD) and 4,4′,4″,4‴-(pyrene-1,3,6,8-tetrayl) tetra aniline (PyTTA), which has a two-dimensional dual rhombic pore structure based on AA stacking with the pore diameters of 0.88 and 1.38 nm. The hydroxyl group of TFBD-Py-COF can form hydrogen bonds with imines and induce the trans conformation of the mesoimine, as evidenced by the single crystal structure of the model compound. TFBD-Py-COF exhibits good chemical and thermal stability with a high BET surface area of 750 m2 g−1. TFBD-Py-COF shows visible fluorescent color from blue to green when dispersed in different organic solvents. TFBD-Py-COF has the fluorescence chemo-sensing ability for nitro-pesticides based on electron transfer-induced fluorescence quenching mechanism, allowing a high sensitivity and low detection limit of 4.0 × 10−2 ppm for p-nitroaniline. In addition, TFBD-Py-COF shows reversible acidochromic behaviors and concomitant distinct fluorescent color change due to the protonation of the imine N. Furthermore, the fluorescence of TFBD-Py-COF is selectively quenched by Fe3+ with a Ksv of 1.1 × 104 and a detection limit of 0.2 μM, owing to the coordination of Fe3+ with the O–H⋯NC chelating unit. This study demonstrates great potential of LCOFs for multifunctional detection.

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.