Off-on Fluorescence Research Articles

Benzo-Fused BOPAM Fluorophores: Synthesis, Post-functionalization, Photophysical Properties and Acid sensing Applications.

A novel category of asymmetric boron chromophores with the attachment of two BF2 moieties denoted as BOPAM has been successfully synthesized via a one-pot three-step reaction starting from N-phenylbenzothioamide. This synthetic route results in the production of [a] and [b]benzo-fused BOPAMs along with post-functionalization of the [a]benzo-fused BOPAMs. The photophysical properties of these compounds have been systematically investigated through steady-state absorption and fluorescence emission measurements in solvents at both ambient and cryogenic temperatures, as well as in the solid state. Computational methods have been employed to elucidate the emissive characteristics of the benzo-fused BOPAMs, revealing distinctive photophysical attributes, including solvent-dependent fluorescence intensity. Remarkably, certain BOPAM derivatives exhibit noteworthy photophysical phenomena, such as the induction of off-on fluorescence emission under specific solvent conditions and the manifestation of intermolecular charge transfer states in solid-state matrices. Through post-functionalization strategies involving the introduction of electron-donating groups onto the [a]benzo-fused BOPAM scaffold, an intramolecular charge transfer (ICT) pathway is activated, leading to substantial fluorescence quenching via non-radiative decay processes. Notably, one [a]benzo-fused BOPAM variant exhibits a pronounced fluorescence enhancement upon exposure to acidic conditions, thereby underscoring its potential utility in pH-sensing applications.

CeO2 loaded on L-tryptophan functionalized graphitic carbon nitride colorimetric-fluorescent dual-channel detection

In this study, a nanocomposite with CeO2 loaded on L-tryptophan functionalized graphitic carbon nitride (CeO2/L-g-C3N4) was developed by a facile two-step method. Firstly, L-tryptophan functionalized graphitic carbon nitride (L-g-C3N4) was achieved via π-π stacking interactions through a physical ultrasonic treatment process, which had better dispersibility in water than Bulk-g-C3N4. Then the final product of CeO2/L-g-C3N4 was obtained by a hydrothermal co-precipitation technique with cerium nitrate as precursor. The characterization of morphology indicated that CeO2 nanoparticles evenly distributed on the surface of CeO2/L-g-C3N4, which might be attributed to L-tryptophan offering more bonding sites (–COOH) for the deposition of nanoparticles. The material exhibited excellent oxidase-mimicking activity, which could catalyze the colorless 3,3′,5,5′-tetramethylbenzidine (TMB) into the blue-colored oxidized TMB product (oxTMB), and a new hydroquinone (HQ) colorimetric sensing strategy was proposed. And theoretical simulations indicate that the increased enzymatic activity is mainly attributable to the reduced binding energy of absorbed intermediates. Additionally, due to the material’s exceptional fluorescence properties, combined with the inner filter effect and redox reactions, a unique on–off-on fluorescence sensing mechanism had been successfully developed for the detection of Cr(VI) and SO32−.

Synthesis of nitrogen-doped carbon quantum dots (N-CQDs) using microwaveassisted hydrothermal method for tyramine determination

In this study, we develop a rapid, efficient, low-cost and environmentally friendly method to synthesize nitrogen-doped carbon quantum dots (N-CQDs). With simple input materials as citric acid and urea, N-CQDs were sucessfully synthesized in only 5 minutes using a domestic microwave oven. Characterisation results using TEM, UV–VIS, IR and fluorescence methods have demonstrated the successful doping of N into CQDs. The obtained N-CQDs material has a particle size of less than 10 nm, and the fluorescence quantum efficiency (36.6%), which is significantly higher than that of undoped CQDs (17.2%). The N-CQDs material also exhibits an on-off fluorescence effect in the presence of Au nanoparticles (AuNPs) and in the presence of tyramine, a biological amine commonly found in food products such as cheese, fish sauce, soy sauce, kimchi, etc. The fluorescence recovery of N-CQDs/AuNPs is linearly proportional to the tyramine concentration in the range from 0.02 ppm to 1 ppm, showing the possibility of using this material to detect and quantify tyramine in food samples.

A highly specific red-emitting fluorescent probe for monitoring of Cu2+ in living cells and zebrafish.

A novel seminaphthorhodafluor-based fluorescent probe was designed and prepared. It exhibited an outstanding off-on fluorescence response (about 45-fold enhancement) to Cu2+ because the structure changed from a spirocyclic form to a ring-open state. It showed satisfactory detection performances for Cu2+ with fast response speed, high selectivity and anti-interference, and a low detection limit (22.9nM) due to its long emission wavelength and off-on fluorescence response mode. This probe was also satisfactorily applied for the monitoring of Cu2+ in living cells and zebrafish.

Developing an off-on fluorescence sensor based on red copper nanoclusters wrapped by sulfhydryl and polymer double ligands for sensitive detection of N-acetyl-L-cysteine

N-acetyl-L-cysteine (NAC) as a class of thiols is commonly used in the treatment of lung diseases, detoxification and prevention of liver damage. In this paper, 4-mercaptobenzoic acid (4-MBA) coated and polyvinylpyrrolidone (PVP) attached copper nanoclusters (4-MBA@PVP-CuNCs) were successfully synthesized using a simple one-pot method with an absolute quantum yield of 10.98 %, and its synthetic conditions (like effects of single/double ligands and temperature) were studied intensively. Then Hg2+ could quench the fluorescence of the 4-MBA@PVP-CuNCs and its fluorescence was restored with the addition of NAC. Based on the above principles, an off–on switching system was established to detect NAC. That is, the 4-MBA@PVP-CuNCs-Hg probe was prepared by adding Hg2+ to switch off the fluorescence of the CuNCs by static quenching, and then NAC was added to switch on the fluorescence of the probe based on the chelation of NAC and Hg2+. Moreover, the effects of metal ion types and mercury ion doses for the probe construction were also further discussed. The method showed excellent linearity in the range of 0.05–1.25 µM and low detection limit of 16 nM. Meanwhile, good recoveries in real urine, tablets and pellets were observed, which proved the reliability of the method and provided a convenient, fast and sensitive method for NAC detection.

A Mitochondria-Targeting Fluorescent Probe for the Dual Sensing of Hypochlorite and Viscosity without Signal Crosstalk in Living Cells and Zebrafish.

Hypochlorite (ClO-) and viscosity both affect the physiological state of mitochondria, and their abnormal levels are closely related to many common diseases. Therefore, it is vitally important to develop mitochondria-targeting fluorescent probes for the dual sensing of ClO- and viscosity. Herein, we have explored a new fluorescent probe, XTAP-Bn, which responds sensitively to ClO- and viscosity with off-on fluorescence changes at 558 and 765 nm, respectively. Because the emission wavelength gap is more than 200 nm, XTAP-Bn can effectively eliminate the signal crosstalk during the simultaneous detection of ClO- and viscosity. In addition, XTAP-Bn has several advantages, including high selectivity, rapid response, good water solubility, low cytotoxicity, and excellent mitochondrial-targeting ability. More importantly, probe XTAP-Bn is successfully employed to monitor the dynamic change in ClO- and viscosity levels in the mitochondria of living cells and zebrafish. This study not only provides a reliable tool for identifying mitochondrial dysfunction but also offers a potential approach for the early diagnosis of mitochondrial-related diseases.

Real-Time Detection of Multiple Intracellular MicroRNAs using an Ultrasound-Propelled Nanomotor-Based Dynamic Fluorescent Probe.

Multiple intracellular microRNA (miRNA) detection is essential for disease diagnosis and management. Nonetheless, the real-time detection of multiple intracellular miRNAs has remained challenging. Herein, we have developed an ultrasound (US)-powered nanomotor-based dynamic fluorescent probe for the real-time OFF-ON fluorescent determination of multiple intracellular miRNAs. The new probe relies on the utilization of multicolored quantum dot (QD)-labeled single-stranded DNA (ssDNA)/graphene oxide (GO)-coated US-powered gold nanowire (AuNW) nanomotors. The fluorescence of QDs is quenched due to π-π interactions with the GO. Upon binding to target miRNAs, the QDs-ssDNA is now distant from the AuNWs, resulting in effective OFF-ON QD fluorescence switching. Compared with conventional passive probes, the dynamic fluorescent probe enhances probe-target interactions by using the US-propelled nanomotor, resulting in exceptionally efficient and prompt hybridization. Simultaneous quantitative analysis of miR-10b and miR-21 in vitro can be achieved within 15 min with high sensitivity and specificity. Additionally, multicolor QDs provide strong signal intensity and multiplexed detection, enabling one-step real-time discrimination between cancer cells (A549) and normal cells (L02). The obtained results are in good agreement with those from qRT-PCR. This dynamic fluorescent probe based on a nanomotor and QDs enables rapid "on the move" specific detection of multiple intracellular miRNAs in intact cells, facilitating real-time monitoring of diverse intracellular miRNA expression, and it could pave the way for novel applications of nanomotors in biodetection.

An off-on fluorescence method for acid phosphatase assay based on the inner filter effect of MnO2 nanosheets on vitamin B2

Fluorescence analysis has attracted much attention due to its rapidity and sensitivity. The present work describes a novel fluorescence detection method for acid phosphatase (ACP) on the basis of inner-filter effect (IFE), where MnO2 nanosheets (MnO2 NSs) and vitamin B2 (VB2) are served as absorbers and fluorophores, respectively. In the absence of ACP, the absorption band of MnO2 NSs overlaps well with the excitation band of VB2, resulting in effective IFE and inhibition of VB2 fluorescence. In the presence of ACP, 2-phospho-L-ascorbic acid trisodium salt (AAP) is hydrolyzed to generate ascorbic acid (AA), which efficiently trigger the reduction of MnO2 NSs into Mn2+ ions, causing the weakening of the MnO2 NSs absorption band and the recovery of VB2 fluorescence. Further investigation indicates that the fluorescence recovery degree of VB2 increases with the increase of ACP concentration. Under selected experimental conditions, the proposed method can achieve sensitive detection of ACP in the ranges of 0.5–4.0 mU/mL and 4.0–15 mU/mL along with a limit of detection (LOD) as low as 0.14 mU/mL. Finally, this method was successfully applied for the detection of ACP in human serum samples with satisfactory recoveries in the range of 95.0 %-108 %.

A Metal-Free Triazacoronene-Based Bimodal VOC Sensor.

Developing suitable sensors for selective and sensitive detection of volatile organic compounds (VOCs) is crucial for monitoring indoor and outdoor air quality. VOCs are very harmful to our health upon inhalation or contact. Bimodal sensor materials with more than one transduction capability (optical and electrical) offer the ability to extract complementary information from the individual analyte, thus improving detection accuracy and performance. The privilege of manipulating the optoelectronic properties of the polycyclic aromatic hydrocarbon-based semiconducting materials offers rapid signal transduction in multimodal sensing applications. A thiophene-functionalized triazacoronene (TTAC) donor-acceptor-donor (D-A-D) type sensor is reported here for VOC sensing. The single-crystal X-ray structure analysis of the TTAC revealed that a distinctive supramolecular polymer architecture was formed because of cooperative π-π and intermolecular D-A interactions and exhibited rapid signal transduction upon exposure to specific VOCs. The TTAC-embedded green luminescent paper-based test strip exhibited an on-off fluorescence response upon nitrobenzene vapor exposure for 120 s. The selective and rapid response is due to the fast photoinduced electron transfer, as is evident from the time-resolved excited-state dynamics and density functional theory studies. The thick-film-based prototype chemiresistive sensor detects harmful VOCs in a custom-made gas sensing system including benzene, toluene, and nitrobenzene. The TTAC sensor rapidly responds (200 s) at relatively low temperatures (180 οC) compared to other reported metal-oxide-based sensors.

Integration of sustainable chemistry frameworks with a single-pot, on-off fluorescence approach for enabling the facile quantification of mosapride; Application to pharmaceutical tablets and content uniformity

The twelve green chemistry concepts were considered for designing two new feasible, sensitive and selective spectroscopic approaches for the estimation of mosapride (MSP) in pure powder and tablet dosages. These approaches were derived from the reaction of the drug with erythrosine B in buffered acidic medium. The reaction yielded a binary complex, and its absorbance can be measured at its λmax (556 nm) while the incorporation of erythrosine B in the complexation reaction caused quantitative quenching of its native fluorescence that could be measured at 554 nm after excitation at 529 nm. The reaction parameters were carefully optimized for the two methods then the calibration curves were plotted. The absorbance and fluorescence quenching were directly proportional to MSP concentration in the ranges of 1.0–12.0 and 0.15–2.0 μg/mL, respectively. Limits of detection and quantitation were estimated giving values of 0.30 and 0.92 μg/mL for spectrophotometric method and 0.047 and 0.143 μg/mL for spectrofluorimetric method. In accordance with the International Council for Harmonization (ICH) guiding rules, the new methods were assessed proving their high accuracy, precision, and robustness. The methods were adopted for the determination of the cited drug in pharmaceutical tablets. Moreover, the uniformity of tablet contents was successfully tested according to USP utilizing the suggested approaches. Furthermore, the greenness of the suggested procedure was confirmed using three different approaches.

Construction of an off–on fluorescence sensing platform for FRET-based detection of coumarin in cosmetic samples

An off–on nanoprobe was constructed by attaching Rh6G to QDs via a linker. The UV-Vis absorption spectrum of Rh6G overlaps with the fluorescence emission spectrum of the QDs, causing FRET-based coumarin-sensing with high sensitivity and selectivity.

A facile on-off fluorescence approach for fluvoxamine determination in pharmaceutical tablets; application to content uniformity testing.

A green-complied spectrofluorimetric approach for quantification of the antidepressant, fluvoxamine, has been established. The method that has been suggested relies on the development of an association complex between fluvoxamine and erythrosine B in an acetate buffer solution. After being excited at 530 nm, the quenching in erythrosine B's native fluorescence caused by complex formation with fluvoxamine was detected at a wavelength of 552 nm. The values of fluorescence quenching at the most optimal reaction conditions were rectilinear at the concentration range of 0.2-2.0 μg mL-1, with a good correlation coefficient (r = 0.9998). The detection limit for the method was 0.03 μg mL-1 while the quantitation limit was 0.09 μg mL-1. The suggested approach has been validated according to the ICH. The established approach was effectively used to determine the drug under study in its dosage form with an average percent recovery of 98.92 ± 0.87 (n = 5), with no effect caused by the existing excipients. The proposed approach was also successfully used for the content uniformity test.

A buffering fluorogenic probe for real-time lysosomal pH monitoring

Real-time monitoring of lysosomal pH is critical for understanding lysosome-related biological processes and diseases, but few lysosome probes can achieve this response effectively. Therefore, it is essential to develop pH-sensitive probes with better performance. In this study, we designed and synthesized a lysosome-targeting “buffering fluorogenic probe” (Lyso-BFP) by introducing a salicylamide group to naphthalimide for buffering strategy and a hydroxyethyl piperazine unit to target lysosome as well as specific pH response. Lyso-BFP demonstrated excellent photostability, pH specificity, and responsiveness to lysosomal acidification in living cells, thanks to its well-suit pKa. Density functional theory calculation revealed that Lyso-BFP’s excellent pH sensing performance resulted from the inhibition of the photo-induced electron transfer (PET) process from hydroxyethyl piperazine to the naphthalimide moiety after the piperazine ring’s protonation at low pH. Remarkably, Lyso-BFP allowed for wash-free imaging and long-term real-time monitoring of lysosome pH changes based on its off-on fluorescence behavior and buffer strategy. With its superior pH selectivity and real-time pH monitoring capability, Lyso-BFP holds great potential in the diagnosis of lysosome-related disease, and the design concept of this fluorogenic probe provides a new perspective for real-time tracking of cell organelles.

Simple dihydropyridine-based colorimetric chemosensors for heavy metal ion detection, biological evaluation, molecular docking, and ADMET profiling

In this study, two novel chemosensors containing dihydropyridine fragment namely; (2E, 2Eʹ)-1,1ʹ-(2,6-dimethyl-1,4-dihydropyridine-3,5-diyl)bis(3-(4-(dimethylamino)phenyl)prop-2-en-1-one) (1), (2E,2E',4E,4E')-1,1ʹ -(2,6-dimethyl-1,4-dihydropyridine-3,5-diyl)bis(5-(4-(dimethylamino)phenyl)penta-2,4-dien-1-one) (2) have been synthesized and characterized. The solvatochromic behavior was explored in different solvents of various polarities. The visual detection, as well as UV–Vis and fluorescence measurements were carried out to explore the colorimetric and optical sensing properties of the investigated chemosensors towards various metal ions such as Al3+, Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Mg2+, Hg2+ and Zn2+. The chemosensors 1 and 2 have strong detecting abilities, with excellent sensitivity and selectivity for Cu2+ and Fe3+, respectively, over the other metal ions. The chemosensors were totally reversible upon addition of EDTA to the formed complexes and displayed a turn on–off-on fluorescence response based on an effect of chelation-quenching fluorescence. The antioxidant activities of the investigated chemosensors were assessed. They were examined in-silico for their capacity to block the Akt signaling pathway, which is involved in cancer proliferation with interpreting their pharmacokinetics aspects. Furthermore, in-vitro antitumor evaluation against a panel of cancer cell lines for the investigated chemosensors has been examined. Conclusively, chemosensor 1 was more effective at scavenging free radicals and as an anticancer agent and could be exploited as a therapeutic candidate for cancer therapy than chemosensor 2 due to its potential inhibition of Akt protein.

Multi-channel sensing of trivalent metal ions using a simple ferrocenyl Schiff base probe with AIE property

A simple ferrocene-based Schiff base (probe L) exhibiting aggregation-induced emission (AIE) effect has been synthesized and investigated. The probe L showed selective detection for trivalent metal ions (Al3+, Cr3+and Fe3+) over a range of monovalent and divalent metal ions in methanol via multi-channel of detection using colorimetry, fluorescence and electrochemistry. Upon binding with trivalent metal ions, color change from faint yellow to pink for Al3+, Cr3+and Fe3+ was observed by the naked-eye, which was consistent with the change of UV–Vis absorption spectra. In addition, probe L showed a dramatic off-on fluorescence emission along with a large Stocks-shift of about 33 nm in the emission wavelength. The fluorescence enhancement with a blue shift may be attributed to the combined effect of rotation inhibition and an extended intramolecular charge transfer (ICT) mechanism. Moreover, electrochemical detection demonstrated that the oxidation potential difference between probe L and probe l-metal complexes was 0.028 V (Al3+), 0.044 V (Cr3+) and 0.015 V (Fe3+), respectively, indicating that trivalent metal ions induced charge transfer from the ligand to the metal (LMCT) and reduced the electron density of ferrocene moiety. Therefore, Al3+, Cr3+and Fe3+ can be selectively detected using multi-channel of detection techniques. Finally, 1H NMR titration study confirmed that two molecules of probe L bind with one metal ion through the nitrogen atom and oxygen atom of the probe L.

Biogenic Carbon Dot-Embedded Chitosan Hydrogels as a Two-Stage Fluorescence OFF–ON Probe for Sequential and Ratiometric Detection of Fe(III) and Glutathione

Biogenic Carbon Dot-Embedded Chitosan Hydrogels as a Two-Stage Fluorescence OFF–ON Probe for Sequential and Ratiometric Detection of Fe(III) and Glutathione

Ligand-Directed H2 S Probe and Scavenger for Specific Tumor Imaging.

An expanding body of evidence suggests that specifically targeting hydrogen sulfide (H2 S) might potentially benefit both tumor diagnosis and treatment, but there is still a lack of cancer-targeted molecular tools for in vivo applications. Here, we report the first ligand-directed H2 S-specific near-infrared fluorescent sensor PSMA-Cy7-NBD and scavenger PSMA-Py-NBD that target the prostate-specific membrane antigen (PSMA). PSMA-Cy7-NBD displays a 53-fold off-on fluorescence response to H2 S at 803 nm with high specificity. PSMA-Py-NBD can scavenge H2 S fast (k2 =30.8 M-1 s-1 at 25 °C) without interference from biothiols. Both tools are highly water-soluble and can be transported selectively into PSMA-expressing prostate cancer cells. Endogenous H2 S levels in murine 22Rv1 tumor models can be imaged and downregulated by intravenous injection of PSMA-Cy7-NBD and PSMA-Py-NBD, respectively. These tools could potentially help to investigate H2 S cancer biology and with related therapies.

Photofunctional Scope of Fluorescent Dithienylethene Conjugates with Aza-Heteroaromatic Cations.

A series of dithienylethene (DTE) photoswitches with aza-heteroaromatic cationic moieties is synthesized. The switches are characterized regarding their photochemical and photophysical properties in acetonitrile and in water. The efficiency of the switching and the photostationary state composition depend on the degree of π-conjugation of the heteroaromatic systems. Thus, DTEs with acridinium-derived moieties have very low quantum yields for the ring-closing process, which is in contrast to switches with pyridinium and quinolinium moieties. All switches emit fluorescence in their open forms. The involved electronic transitions are traced back to an integrative picture including the DTE core and the cationic arms. The emission can be fine-tuned by the π-conjugation of the heteroaromatic cations, reaching the red spectral region for DTEs with acridinium moieties. On ring-closing of the DTEs the fluorescence is not observable anymore. Theoretical calculations point to rather low-lying energy levels of the highly conjugated ring-closed DTEs, which would originate near-infrared emission (> 1200nm). The latter is predicted to be very weak due to the concurrent non-radiative deactivation, according to the energy-gap law. In essence, an ON-OFF fluorescence switching as the result of the electrocyclic ring-closing reaction is observed.

Structures, photophysical properties, and optical sensing of zinc– and copper–salicyaldimine complexes

Metal–salicyaldimine complexes [M(sal-3-py)2] (1, M = Zn; 2, M = Cu), where Hsal-3-py = 3-(salicylideneimino)pyridine, have been synthesized. Nuclear magnetic resonance (NMR) spectroscopy and mass spectroscopy (MS) indicated that 1 and 2 both have a ML2-type discrete molecular structure in the solution and amorphous phases. Single-crystal X-ray diffraction analysis and X-ray powder diffraction (XRPD), however, proven that 1 and 2 both possess an extended two-dimensional sql layer net in the crystalline phase. The electronic absorption spectra of 1 and 2 in MeOH exhibited intense intraligand charge transfer (ILCT) transitions in the UV region (200–340 nm) and metal–ligand charge transfer (CT) transition in the visible region (380–400 nm). The emission spectra of 1 displayed a cyan fluorescence band at 480 nm in solid-state and a green fluorescence band at 502 nm in MeOH solution. Optical detection of 1 in MeOH toward Cu2+, Al3+, and Cr3+ ions showed remarkable changes in electronic absorption spectra and ON–OFF–ON fluorescence color change from green to blue light emission. The sensing processes involves two stages, i.e., direct central metal displacement for Cu2+ detection and analyte-induced decomplexation for Al3+/Cr3+ detection, and subsequently further recognition through the 3-pyridyl functions of the salicyaldimine ligands. Further, 1 displayed remarkable fluorescence color change from green light in MeOH to cyan light in H2O, being potential candidate as H2O probe in methanol.

Engineering the BASHY Dye Platform toward Architectures with Responsive Fluorescence.

A set of nine boronic-acid-derived salicylidenehydrazone (BASHY) complexes has been synthesized in good to very good chemical yields in a versatile three-component reaction. In an extension to previous reports on this dye platform, the focus was put on the electronic modification of the "vertical" positions of the salicylidenehydrazone backbone. This enabled the observation of fluorescence quenching by photoinduced electron transfer (PeT), which can be reverted by the addition of acid in organic solvent (OFF-ON fluorescence switching). The resulting emission is observed in the green-to-orange spectral region (maxima at 520-590 nm). In contrast, under physiological pH conditions in water, the PeT process is inherently decativated, thereby enabling the observation of fluorescence in the red-to-NIR region (maxima at 650-680 nm) with appreciable quantum yields and lifetimes. The latter characteristic supported the application of the dyes in fluorescence lifetime imaging (FLIM) of live A549 cells.