A FRET based ratiometric fluorescent probe for detection of sulfite in food

Ratiometric fluorescent platform for on-site monitoring of sodium pyrosulfite in preserved fruits

The rapid detection of pyrosulfites in food chemistry is crucial to food safety and health. Here, a coumarin-type ratiometric fluorescent probe was developed based on the Michael addition reaction to detect sodium pyrosulfite (Na2S2O5). The probe exhibited high selectivity and fast response (t1/2 = 6 s) to Na2S2O5 and a low detection limit (26 nM). Because of its excellent ratiometric response performance, the probe was successfully applied to measure the amount of Na2S2O5 in preserved fruits. Colour information analysis and formula calculations were performed to quickly determine the sodium pyrosulfite amount in an actual sample by using a smartphone. Therefore, the intelligent strategy of combining the sensing process and smartphone provides a convenient and efficient method for the fast monitoring of sodium metabisulfite in actual food.

Visible-light Excited and Highly Photostable Organic Fluorophore with dual-state Emission

Organic fluorophores with dual-state emission (DSE) are rare or difficult to observe because most of them display either aggregation-induced emission (AIE) or aggregation-caused quenching (ACQ). Amazing works have been accomplished, yet most of the DSE compounds were excited by UV light which limits their wide application in bioimaging. In this work, we achieved a visible-light excited DSE fluorophore and realized its imaging in SKOV-3 cells and zebrafish. The naphtho[2',3':4,5]imidazo[1,2-a]pyridine (NIP) core ensures its emission in dilute solution. Meanwhile, the twisted phenyl ring blocks fluorescence quenching induced by the π-π stacking and leads to the emission of the solid. The fluorescence intensity is steady even after 6 h of continuous intense sunlight. More importantly, photostability of NIP in cells is much better than commercial dye (mitochondrial green).

Reversible colorimetric and NIR fluorescent probe for sensing SO2/H2O2 in living cells and food samples

Preservative sulfur dioxide (SO₂) and bleach hydrogen peroxide (H₂O₂) were widely used in the food industry, at the same time, they were also a redox pair in biological systems. Therefore, the reversible sensing SO₂/H₂O₂ was of great significance in food safety and biology. In this paper, a colorimetric and NIR fluorescent dual channels response probe (DCA-Bba) for SO₂/H₂O₂ based on chromene-barbiturate was developed. DCA-Bba exhibited a rapid and sensitive recognition of SO₂, and the adduct DCA-Bba-HSO₃^- could detect H₂O₂ in PBS (with 10 % DMSO, v/v, pH 7.4) solution. The reversible response of DCA-Bba was implemented by HSO₃^- involved 1,4-addition and H₂O₂ induced elimination reaction. DCA-Bba showed a strong red fluorescence based on the intramolecular charge transfer (ICT) process, after the recognition of SO₂, the fluorescence of the adduct was quenched based on the photoinduced electron transfer (PET) process. And importantly, DCA-Bba had been applied for imaging SO₂/H₂O₂ redox cycles in living cells, as well as could detect the levels of SO₂ in white sugar, biscuit, Chinese liquor and red wine samples.

An Imidazo[1,5-a]pyridine Benzopyrylium-Based NIR Fluorescent Probe with Ultra-Large Stokes Shifts for Monitoring SO2

A mitochondria-targeted NIR probe based on the FRET mechanism was developed. It shows ultra-large Stokes shifts (460 nm) and emission shifts (285 nm). Furthermore, we also realized the imaging of SO₂ in living SKOV-3 cells, zebrafish and living mice which may be useful for understanding the biological roles of SO₂ in mitochondria and in vivo.

Simple and Commercially Available 6-chloroimidazo[1,2-a]pyridine-2-carboxylic Acid-based Fluorescent Probe for Monitoring pH Changes

Commercially available compounds that can be directly used as fluorescent probes will greatly promote the development of fluorescent imaging. Based on our previously work related to nitrogen bridgehead heterocycles, herein, a commercially available compound, 6-chloroimidazo[1,2-a]pyridine-2-carboxylic acid, has been detected for monitoring pH value (3.0-7.0). The probe proves to have high selectivity and sensitivity, brilliant reversibility, and extremely short response time. The real-time imaging of pH changes in yeast was also conducted.

Progress in the Development of Imidazopyridine-Based Fluorescent Probes for Diverse Applications

Different classes of Imidazopyridine i.e., Imidazo[1,2-a]pyridine, Imidazo[1,5-a] pyridine, Imidazo[4,5-b]pyridine, have shown versatile applications in various fields. In this review, we have concisely presented the usefulness of the fluorescent property of imidazopyridine in different fields such as imaging tools, optoelectronics, metal ion detection, etc. Fluorescence mechanisms such as excited state intramolecular proton transfer, photoinduced electron transfer, fluorescence resonance energy transfer, intramolecular charge transfer, etc. are incorporated in the designed fluorophore to make it for fluorescent applications. It has been widely employed for metal ion detection, where selective metal ion detection is possible with triazole-attached imidazopyridine, β-carboline imidazopyridine hybrid, quinoline conjugated imidazopyridine, and many more. Also, other popular applications involve organic light emitting diodes and cell imaging. This review shed a light on recent development in this area especially focusing on the optical properties of the molecules with their usage which would be helpful in designing application-based new imidazopyridine derivatives.

Small-Molecule Fluorescent Probe for Detection of Sulfite

Sulfite is widely used as an antioxidant additive and preservative in food and beverages. Abnormal levels of sulfite in the body is related to a variety of diseases. There are strict rules for sulfite intake. Therefore, to monitor the sulfite level in physiological and pathological events, there is in urgent need to develop a rapid, accurate, sensitive, and non-invasive approach, which can also be of great significance for the improvement of the corresponding clinical diagnosis. With the development of fluorescent probes, many advantages of fluorescent probes for sulfite detection, such as real time imaging, simple operation, economy, fast response, non-invasive, and so on, have been gradually highlighted. In this review, we enumerated almost all the sulfite fluorescent probes over nearly a decade and summarized their respective characteristics, in order to provide a unified platform for their standardized evaluation. Meanwhile, we tried to systematically review the research progress of sulfite small-molecule fluorescent probes. Logically, we focused on the structures, reaction mechanisms, and applications of sulfite fluorescent probes. We hope that this review will be helpful for the investigators who are interested in sulfite-associated biological procedures.

All-in-one: Accurate quantification, on-site detection, and bioimaging of sulfite using a colorimetric and ratiometric fluorescent probe in vitro and in vivo

SO₂ and its derivatives (SO₃^2-/HSO₃^-) are used widely in food, beverages, and pharmaceutical production. However, they could induce multiple diseases in respiratory, nervous, and cardiovascular systems. Although several fluorescent probes have been developed for detecting SO₃^2-/HSO₃^-, reports on rapid fluorescent probes for the on-site detection of SO₂ derivatives are scarce. Herein, a colorimetric and ratiometric fluorescent probe 1 based on the intramolecular charge transfer (ICT) was reported. Probe 1 resulted in a 122 nm blue-shift in fluorescent emission and decrement of absorbance at 500 nm upon the addition of sulfite. Therefore, probe 1 could quantify SO₃^2-/HSO₃^- using both UV-Vis and fluorescent methods (LOD: UV-Vis method 34 nM; fluorescent method 51 nM). Importantly, probe 1 was used for a rapid (60 s) and convenient (1 step, on-site) measurement of the SO₂ derivatives in real samples (LOD: 0.47 µM) using smartphone based on the colorimetric method. The SO₃^2-/HSO₃^--sensing mechanism was confirmed as the Michael addition reaction. Furthermore, the probe was used for the real-time monitoring of SO₃^2-/HSO₃^- in A549 cells and zebrafish. In summary, an all-in-one fluorescent probe was successfully developed for the accurate quantification, on-site detection, and bioimaging of SO₃^2-/HSO₃^-.

A turn-on fluorescent probe based on indolizine for the detection of sulfite

Numerous SO32−/HSO3− fluorescent probes have been reported based on various mechanisms.

Simple inorganic base promoted polycyclic construction using mucohalic acid as a C3 synthon: synthesis and AIE probe application of benzo[4,5]imidazo[1,2-a]pyridines

Using mucohalic acid as C3 synthon via a transition metal-free multicomponent reaction, an eco-friendly protocol to synthesize C1-functionalized benzo[4,5]imidazo[1,2-a]pyridines which can be applied as fluorescence probe for picric acid is described.

Solid-state and solution characterizations of [(TMPA)Cu(II)(SO3)] and [(TMPA)Cu(II)(S2O3)] complexes: Application to sulfite and thiosulfate fast detection

Sulfite (SO₃^2-) and thiosulfate (S₂O₃^2-) ions are used as food preservative and antichlor agent respectively. To detect low levels of such anions we used Cu(II) complex of the Tris-Methyl Pyridine Amine (TMPA) ligand, denoted L. Formation of [LCu(SO₃)] (1) and [LCu(S₂O₃)] (2) in solution were monitored using UV-Vis, EPR and cyclic voltammetry, while the solid-state X-ray structures of both complexes were solved. In addition, we also evaluated the pH range in which the complexes are stable, and the anions binding affinity values for the [LCu(solvent)]²⁺ (3) parent complex. As a matter of illustration, we determined the sulfite content in a commercial crystal sugar.

Simple inorganic base promoted C-N and C-C formation: synthesis of benzo[4,5]imidazo[1,2-a]pyridines as functional AIEgens used for detecting picric acid

Metal-free catalyzed intermolecular tandem Michael addition/cyclization has been developed for the synthesis of benzo[4,5]imidazo[1,2-a]pyridines from α-bromocinnamaldehyde and 2-substituted benzimidazoles. The reaction promoted by a simple inorganic base displays moderate to good yields and good functional group tolerance. The optical properties of some typical products have been investigated. We found that, due to the presence of the benzene ring at the C1-position of benzo[4,5]imidazo[1,2-a]pyridines which restricts intramolecular motion, as a new type of aggregation-induced emission (AIE) luminogen (AIEgen), they show very good solid-state fluorescence with quantum yields up to 88.80%. Importantly, the AIE performance of compound 3b can be useful to detect the nitroaromatic explosive picric acid (PA) with a detection limit and quenching constant of 42.5 nM and 7.27 × 10⁴ M^-M, respectively.

Access to 6-hydroxy indolizines and related imidazo[1,5-a]pyridines through the SN2 substitution/condensation/tautomerization cascade process

A simple and efficient cascade reaction was developed for the construction of hydroxy substituted indolizines from pyrrole-2-carbaldehydes and commercially available 4-halogenated acetoacetic esters.

Tetranitrile-anthracene as a probe for fluorescence detection of viscosity in fluid drinks via aggregation-induced emission

An AIE-based fluorescent probe was developed for monitoring the viscosity change during the spoilage process of fluid drinks.

Aptamer Induced Multicolored AuNCs-WS2 "Turn on" FRET Nano Platform for Dual-Color Simultaneous Detection of AflatoxinB1 and Zearalenone

Mycotoxins posit serious threats to human and animal health, and numerous efforts have been performed to detect the multiple toxins by a single diagnostic approach. To best of our knowledge, for the first time, we synthesized an aptamer induced "turn on" fluorescence resonance energy transfer (FRET) biosensor using dual-color gold nanoclusters (AuNCs), l-proline, and BSA synthesized AuNCs (Lp-AuNCs and BSA-AuNCs), with WS₂ nanosheet for simultaneous recognition of aflatoxinB₁ (AFB₁) and zearalenone (ZEN) by single excitation. Here, AFB₁ aptamer stabilized blue-emitting AuNCs (AFB₁-apt-Lp-AuNCs) (at 442 nm) and ZEN aptamer functionalized with red-colored AuNCs (ZEN-apt-BSA-AuNCs) (at 650 nm) were employed as an energy donor and WS₂ nanosheet as a fluorescence quencher. With the addition of AFB₁ and ZEN, the change in fluorescence intensity (F.I) was recorded at 442 and 650 nm and can be used for simultaneous recognition with a detection limit of 0.34 pg mL^-1 (R² = 0.9931) and 0.53 pg mL^-1 (R² = 0.9934), respectively. Most importantly, the semiquantitative determination of AFB₁ and ZEN can also be realized through photovisualization. The current approach paves a new way to develop sensitive, selective, and convenient metal nanocluster-based fluorescent "switch-on" probes with potential applications in multipurpose biosensing.

An ESIPT-Based Ratiometric Fluorescent Probe for Highly Sensitive and Rapid Detection of Sulfite in Living Cells

The novel ratiometric fluorescent probe HPQRB with an ESIPT effect based on Michael addition for highly sensitive and fast detection of sulfite in living HepG2 cells is reported. HPQRB can be easily synthesized by a two-step condensation reaction. HPQRB has a large emission shift (Δλ=116 nm), which is beneficial for fluorescence imaging research, and its sulfite-responsive site is based on a rhodamine-like structure with the emission peak at 566 nm, which decreases with increasing sulfite concentration. and its HPQ structure always has an ESIPT effect throughout the reaction process, keeping the emission peak at 450 nm as a self-reference. In particular, HPQRB has high selectivity for sulfite and responds quickly (within 30 s) with a low detection limit (44 nM). Furthermore, HPQRB has been successfully used for fluorescence imaging of sulfite in HepG2 cells, demonstrating the superior ability to detect sulfite under physiological conditions.

A pyrazolo[1,5-a]pyridine-based ratiometric fluorescent probe for sensing Cu2+ in cell

Ratiometric fluorescent probes based on FRET mechanism have attracted great attention due to their large pseudo-Stokes shifts and built-in correction for environmental effects. However, most donors failed to meet the requirement that the emission of the donor must overlap well with the absorption of the acceptor. Therefore, searching for new fluorophore to construct FRET system is in great need. In this paper, a new fluorescent dye pyrazolo[1,5-a]pyridine was synthesized and used as a donor in the FRET system for ratiometric sensing of Cu²⁺. The probe is based on FRET and PET mechanism. It shows high selectivity and sensitivity toward Cu²⁺ (detection limit 30 nM). Furthermore, it was successfully used to detect Cu²⁺ in Glioma cells.

Fluorescent sensor for water based on photo-induced electron transfer and Förster resonance energy transfer: anthracene-(aminomethyl)phenylboronic acid ester-BODIPY structure

An anthracene-(aminomethyl)phenylboronic acid ester-BODIPY (DJ-1) was designed and developed as a fluorescent sensor based on photo-induced electron transfer (PET) and Förster resonance energy transfer (FRET) for the detection of a trace amount of water in solvents, where the anthracene skeleton and BODIPY skeleton are the donor fluorophore and the acceptor fluorophore in the FRET process, respectively. It was found that the addition of water to organic solvents containing DJ-1 causes both the suppression of PET in the anthracene-(aminomethyl)phenylboronic acid ester as the PET-type fluorescent sensor skeleton and the energy transfer from the anthracene skeleton to the BODIPY skeleton through a FRET process, thus resulting in the enhancement of the fluorescence band originating from the BODIPY skeleton. This work demonstrates that the PET/FRET-based fluorescent dye composed of the donor fluorophore possessing PET characteristics and the acceptor fluorophore in the FRET process can act as a fluorescent sensor with a large SS for the detection of a trace amount of water in solvents.

An anthracene-(aminomethyl)phenylboronic acid ester-BODIPY (DJ-1) structure was developed as a fluorescent sensor based on photo-induced electron transfer (PET) and Förster resonance energy transfer (FRET) for detection of water in solvents.

A ratiometric fluorescent probe for detection of exogenous mitochondrial SO2 based on a FRET mechanism

A novel imidazo[1,5-a]pyridine-hemicyanine based ratiometric fluorescent probe for detection of mitochondrial SO₂ was designed and synthesized. The probe is based on a fluorescence resonance energy transfer (FRET) mechanism. It exhibits high selectivity and sensitivity towards SO₃²⁻ with a fast response time (3 min) and detection limit of 0.13 μM. Further, it showed low cytotoxicity and was successfully applied to image exogenous mitochondrial SO₂ in cells.

A novel imidazo[1,5-a]pyridine-hemicyanine based ratiometric fluorescent probe for detection of mitochondrial SO₂ was designed and synthesized.

A highly sensitive and selective fluorescent probe for quantitative detection of Al3+ in food, water, and living cells

Three novel β-pinene-based fluorescent probes 2a–2c were designed and synthesized for the selective detection of Al³⁺. Probe 2a showed higher fluorescence intensity toward Al³⁺ than the other two compounds. Probe 2a determined the concentration of Al³⁺ with a rapid response time (45 s), wide pH range (pH = 1–9), excellent sensitivity (LOD = 8.1 × 10⁻⁸ M) and good selectivity. The recognition mechanism of probe 2a toward Al³⁺ was confirmed by ¹H NMR, HRMS and DFT analysis. Probe 2a was successfully used as a signal tool to quantitatively detect Al³⁺ in food samples and environmental water samples. Furthermore, probe 2a was successfully utilized to label intracellular Al³⁺, indicating its promising applications in living cells.

Probe 2a exhibiting high sensitivity, good selectivity, wide pH range, lower detection limit, and rapid detection for Al³⁺, probe 2a was applied for the successful detection of Al³⁺ in water samples, food samples and HeLa cells.