Off-on Fluorescence Research Articles

Green synthesized carbon quantum dots from Prosopis juliflora leaves as a dual off-on fluorescence probe for sensing mercury (II) and chemet drug

In this article, a facile and green dual fluorescence sensor was developed for mercury and chemet as an anti-poisoning drug for heavy metal ions. The method is based on “off-on” fluorescence of carbon quantum dots (CQDs) prepared from Prosopis juliflora leaves as a green and low cost source. The synthesis of CQDs was easily performed with a rapid one pot process and the prepared CQDs exhibited a blue luminescence with a 5% quantum yield. The off fluorescent is observed in the presence of different concentrations of mercury (II) that is specifically recovered with the addition of chemet. The gradually recovered fluorescence is due to the high affinity of chelating property of succimer drug towards mercury (II) with a good restoration. Under the optimized condition, two linear calibrations were validated in the range of 5–500 ng mL−1 and 2.5–22.5 ng mL−1 for Hg (II) and chemet, respectively. The limit of detection (LOD) based on 3Sb was 1.26 ng mL−1 (or 6.3 nM) for Hg (II) and 1.4 ng mL−1 for chemet. The relative standard deviation of the fluorescent probe for 50 ng mL−1 of Hg (II) and 10 ng mL−1 of chemet was 4.5% and 1.3% (n = 10), respectively. This methodology was applied to successful measurements of Hg (II) and chemet in the water and human serum samples, respectively.

An easy-to-synthesize multi-photoresponse smart sensor for rapidly detecting Zn2+ and quantifying Fe3+ based on the enol/keto binding mode

A readily available salicylaldazine-modified fluorene Schiff base (EASA-F) exhibits fast fluorescent OFF–ON response to Zn2+ and OFF–ON–OFF behavior to Fe3+ synchronously accompanied the diverse absorption-ratiometric and colorimetric changes.

Pd nanoparticle-decorated graphitic C3N4 nanosheets with bifunctional peroxidase mimicking and ON-OFF fluorescence enable naked-eye and fluorescent dual-readout sensing of glucose.

In this work, we report a novel naked-eye and fluorescent dual-readout glucose sensing platform based on the bifunctional peroxidase mimicking and ON-OFF fluorescence of Pd nanoparticle-decorated graphitic C3N4 nanosheets (PdNPs/g-C3N4). The PdNPs/g-C3N4 hybrid not only exhibited a strong peroxidase-like activity to catalyze colorless o-phenylenediamine (OPD) into its yellow product OPDox in the presence of H2O2, but it also acted as a turn-off fluorescent sensor for the assembly of the generated OPDox on g-C3N4. When coupled with glucose oxidase (GOx), the platform could provide both visual and fluorescent responses for glucose specifically. Under optimal conditions, the platform was able to detect the target at concentrations as low as 50 μM and 0.4 μM in the linear ranges of 50-2000 μM and 1-1000 μM with the naked eye and a fluorometer, respectively. The developed platform integrates the equipment-free merit of naked-eye observation with the excellent performance of fluorometric analysis well, providing a new and powerful tool for both rough assessment and accurate detection of glucose in diabetes management and other applications.

Nitrogen-doped carbon dots as fluorescence ON-OFF-ON sensor for parallel detection of copper(ii) and mercury(ii) ions in solutions as well as in filter paper-based microfluidic device.

Due to improper garbage disposal and rapid industrialization, concentrations of different metal ions are rising to toxic levels in natural water sources. Development of novel, selective and sensitive sensors for different metal ions is in high demand for rapid detection and remediation. Herein, we report nitrogen-doped carbon dots (NCDs) with high blue fluorescence, synthesized by a new one-step pyrolytic method using urea and ethylenediaminetetraacetic (EDTA) acid as precursors. The NCDs were used for parallel detection of Hg2+ and Cu2+ ions in aqueous medium through a fluorescence ON–OFF–ON process. The minimum detection limit for Hg2+ and Cu2+ were 6.2 nM and 2.304 nM, respectively, in aqueous medium, which is close to or below the allowed levels of Hg2+ and Cu2+ ions, i.e., 6 ppb and 2 ppm, respectively, in drinking water as per World Health Organisation (WHO). Hg2+ and Cu2+ ions were discriminated with vitamin C (ascorbic acid) and trisodium citrate by a fluorescence turn on process. A filter paper based microfluidic device loaded with NCDs, vitamin C and trisodium citrate was developed using candle wax channels on a filter paper as a proof of principle, projecting NCDs as a promising material for parallel detection of multiple metal ions. The device demonstrated herein is capable of detecting Hg2+ and Cu2+ ions up to 0.1 μM. This simple, low cost, disposable paper-based device will be very useful for rapid onsite analysis.

Reactive oxygen species-triggered off-on fluorescence donor for imaging hydrogen sulfide delivery in living cells.

Hydrogen sulfide (H2S), an important gasotransmitter, can mediate a variety of pathophysiological processes, and H2S-based donors have been intensively explored for the therapy of cardiovascular injury, nerve damage and intestinal disorders. However, most of the H2S donors are not capable of simultaneously real-time tracking intracellular H2S delivery, which limits their biological application for elucidating the specific function of H2S. Herein we develop the first reactive oxygen species (ROS)-triggered off-on fluorescence H2S donor (NAB) by incorporating ROS-responsive arylboronate into a fluorophore through thiocarbamate. The donor NAB can release carbonyl sulfide (COS) and the fluorophore with a fluorescence off-on response via a ROS-triggered self-immolative reaction, and then COS is quickly converted to H2S by the ubiquitous carbonic anhydrase. This dual function makes NAB suitable for not only in situ and real-time monitoring of the intracellular H2S release but also rescuing RAW264.7 cells from the hazardous oxidative environment under the stimulation of phorbol-12-myristate-13-acetate, revealing the possible potential of NAB as a therapeutic prodrug with the fluorescence imaging capacity.

Selective visualization of live-cell mitochondrial thiophenols and their induced oxidative stress process by a rationally designed rhodol-based fluorescent probe

Mitochondria as cellular powerhouses are the preferential targets affected by thiophenols, an important class of highly toxic environmental pollutants, and are linked to the production of pathogenic reactive oxygen species (ROS) induced by trace thiophenol residues. For real-time and accurate sensing, it is critically important to develop highly sensitive fluorescent probes for the specific detection of mitochondrial thiophenols. Herein, we report the first mitochondria-targeted fluorescent probe (ROAL) to image thiophenols in living cells. The development of ROAL was based on a novel red-emitting rhodol derivative (ROAP). ROAL proved to be highly selective to thiophenols among various analytes including aliphatic thiols, and renders an ultrasensitive off-on fluorescence response to thiophenols with a remarkable detection limit (8.1 nM). The probe efficiently stains mitochondria with a high Pearson’s co-localization coefficient (0.95) using Mito Tracker Green FM as reference, thereby ensuring the specific detection of mitochondrial thiophenols in living HepG2 and HeLa cells. In particular, using this probe we for the first time proved that endogenous reactive oxygen species have the capacity to eliminate thiophenols in living cells, suggesting that thiophenols might induce cellular oxidative stress.

Epoxy-based polymer incorporating 1-naphthylamine and sebacic acid moieties: A selective fluorescent sensor for ferric ions

A new epoxy-based polymer sensor 1 incorporating 1-naphthylamine and sebacic acid moieties was synthesised. The sensor shows ON-OFF type fluorescence selectivity for Fe3+ over other coexistent metal ions such as Cu2+, Zn2+, Co2+, Ni2+, Cd2+, Hg2+, Mg2+, Sr2+, Pb2+ and Ca2+ ions.

Effective GQD/AuNPs nanosensors for selectively bifunctional detection of lysine and cysteine under different photophysical properties

This work presented the development of the optical nanosensor based on AuNPs and GQDs which perform the efficiency dual optical sensor. Importantly, the AuNPs with GQDs was prepared by one-pot chemical reaction by using GQDs as an excellent bifunction of the reducing agent and stabilizer to obtain the disaggregated AuNPs/GQDs in red color property. Apart from its photophysical sensing ability, this hybrid material demonstrated the dual sensing of cysteine and lysine regarding to colorimetric and fluorometric techniques, respectively, in PBS solution pH 7.4. The presence of Cys in the AuNPs/GQDs solution rapidly lead to the color change from red to blue with concomitant of the red shift of the absorption band at 525 to 645 nm undergone the aggregation process. On the other hand, AuNPs/GQDs nanosensor in the presence of Lys performs the off-on fluorescence at 450 nm without color change in PBS solution pH 7.4. Interestingly, no interference of any amino acids towards Cys and Lys detection highlighted a high efficacy in selective sensing aspect. For sensing application, the detection limit of Cys and Lys was 5.88 μM and 16.14 μM, respectively, with an acceptable % recovery of 101–106 in real samples.

Excited-State Intramolecular Proton Transfer-Based Multifunctional Solid-State Emitter: A Fluorescent Platform with "Write-Erase-Write" Function.

The excited-state intramolecular proton transfer (ESIPT)-based molecular probes have drawn significant attention owing to their environment-sensitive fluorescence properties, large Stokes shift, and emerged as building blocks for the development of molecular sensors and switches. However, most of the ESIPT-based fluorophores exhibit weak emission in the solid state limiting the scope of real-time applications. Addressing such issues, herein, we presented a C3 symmetric-like molecular architecture employing a simple one-step Schiff base condensation between triaminoguanidinium chloride and 3,5-di- tert-butyl-2-hydroxybenzaldehyde (TGHB). The temperature-dependent fluorescence studies including at 77 K indicated the strong emission from the keto tautomer compared to that of the enol tautomer. The facile ESIPT in TGHB in the solid-state led to a remarkable enhancement of fluorescence quantum yield of 1600 times compared to that of the solution (λem = 545 nm) by restricting the intramolecular rotation and subsequently suppressing the nonradiative deactivation. The excited-state processes were further elucidated through time-resolved fluorescence measurements. TGHB exhibited turn on-off fluorescence upon exposure to acid/base vapor in the form of a powder as well as a transparent, free-standing thin film. A rewritable and erasable fluorescent platform was demonstrated using TGHB as molecular ink, which offers a potential testbed for performing "write-erase-write" cycles multiple times. In addition, TGHB, possessing multiple binding sites (O and N donors) involving the central core of the triaminoguanidinium cation displayed selective turn-on fluorescence with Zn2+. The structure-property relationship revealed in the present study provides insight into the development of novel cost-effective multifunctional materials, which are promising for stimuli-responsive molecular switches.

Dual-Site and Dual-Excitation Fluorescent Probe That Can Be Tuned for Discriminative Detection of Cysteine, Homocystein, and Thiophenols.

Thiols play a vital role in both the physiological process and organic synthesis field, including aliphatic thiols (e.g., Cys, Hcy, and GSH) and thiophenols. As a result of the similarities of thiols in terms of molecular structure and chemical properties, it is difficult for conventional fluorescent probes to distinguish them, which hinders the progress of biological and pathological research. Keeping this in mind, a dual-site and dual-excitation fluorescent probe (YY) was designed to distinguish among Cys, Hcy, and thiophenols by three different reaction paths. When excited at 470 nm, YY only exhibits a fluorescence OFF-ON response toward thiophenols. However, when excited at 453 nm, YY not only displays a fluorescence OFF-ON response toward Hcy and thiophenols (λem = 499 and 561 nm) but also presents a two-stage fluorescence response toward Cys, which possesses a fluorescence OFF-ON response in the first stage (λem = 501 nm) and then a fluorescence ON-OFF response in the second stage (λem = 556 nm). This specific fluorescence response indicates that YY has ability to overcome the above-mentioned challenge to achieve discriminative detection of Cys, Hcy, and thiophenols qualitatively, which promotes the study of thiols in the fields of physiology and pathology. Furthermore, cell-imaging studies show that YY can be applied to the imaging of exogenous Cys, Hcy, and thiophenols through two different emission channels.

Recent Advances in the Development of Optical Imaging Probes for γ-Glutamyltranspeptidase.

γ-Glutamyltranspeptidase (GGT) is a cell-membrane-bound protease that participates in cellular glutathione and cysteine homeostasis, which are closely related to many physiological and pathological processes. The accurate measurement of GGT activity is useful for the early diagnosis of diseases. In the past few years, many efforts have been made to build optical imaging probes for the detection of GGT activity both in vitro and in vivo. In this Minireview, recent advances in the development of various optical imaging probes for GGT, including activatable fluorescence probes, ratiometric fluorescence probes, and activatable bioluminescence probes, are summarized. This review starts from the instruction of the GGT enzyme and its biological functions, followed by a discussion of activatable fluorescence probes that show off-on fluorescence in response to GGT. GGT-activatable two-photon fluorescence imaging probes with improved imaging depth and spatial resolution are also discussed. Ratiometric fluorescence probes capable of accurately reporting on GGT levels through a self-calibration mechanism are discussed, followed by describing GGT-activatable bioluminescence probes that can offer a high signal-to-background ratio to detect GGT in living mice. Finally, current challenges and further perspectives for the development of molecular imaging probes for GGT are addressed.

Synthesis and Evaluation of a Schiff-Based Fluorescent Chemosensors for the Selective and Sensitive Detection of Cu2+ in Aqueous Media with Fluorescence Off-On Responses.

Copper being an essential nutrient; also pose a risk for human health in excessive amount. A simple and convenient method for the detection of trace amount of copper was employed using an optical probe R1 based on Schiff base. The probe was synthesized by Schiff base condensation of benzyl amine and 2-hydroxy-1-napthaldehyde and characterized by single X-ray diffraction, 1H NMR and FTIR. By screening its fluorescence response in a mixture of DMSO and H2O (20:80, v/v) R1 displayed a pronounced enhancement in fluorescence only upon treatment with copper. Other examined metal ions such as alkali, alkaline and transition had no influence. Within a wide pH range 5-12 R1 could selectively detect copper by interrupting ICT mechanism that results in CHEF. From Job's plot analysis a 2:1 binding stoichiometry was revealed. The fluorescence response was linear in the range 1-10 × 10-9M with detection limit 30 × 10-9M. Association constant was determined as 1 × 1011M-2 by Benesi-Hilderbrand plot. As a fast responsive probe it possesses good reproducibility and was employed for detection of copper in different water samples.

Enhanced photon collection enables four dimensional fluorescence nanoscopy of living systems

The theoretically unlimited spatial resolution of fluorescence nanoscopy often comes at the expense of time, contrast and increased dose of energy for recording. Here, we developed MoNaLISA, for Molecular Nanoscale Live Imaging with Sectioning Ability, a nanoscope capable of imaging structures at a scale of 45–65 nm within the entire cell volume at low light intensities (W-kW cm−2). Our approach, based on reversibly switchable fluorescent proteins, features three distinctly modulated illumination patterns crafted and combined to gain fluorescence ON–OFF switching cycles and image contrast. By maximizing the detected photon flux, MoNaLISA enables prolonged (40–50 frames) and large (50 × 50 µm2) recordings at 0.3–1.3 Hz with enhanced optical sectioning ability. We demonstrate the general use of our approach by 4D imaging of organelles and fine structures in epithelial human cells, colonies of mouse embryonic stem cells, brain cells, and organotypic tissues.

Near-infrared off-on fluorescence probe activated by NTR for in vivo hypoxia imaging

Selective and efficient detection and imaging of nitroreductase (NTR) overexpressed in hypoxic tissues is of great importance for better understanding its biological functions. The effective optical probes equipped for NTR detection in vivo is still lacking, so we developed an innovative near infrared (NIR) fluorescent on-off probe (Cy-NO2) composed of fluorescence reporting unit (an aminocyaine dye) and recognition unit (p-nitrobenzylcarbamate group). The response and mechanism of the NTR activated reduction of Cy-NO2 were evaluated in vitro through kinetic optical studies, mass spectra analysis and docking calculations. The results demonstrated that Cy-NO2 could rapidly recognize NTR with high selectivity and sensitivity. The efficient NTR detection ability of Cy7-NO2 was further validated by fluorescence imaging of hypoxic cells (A549, PC-12 and HUVEC cell). Furthermore, the in vivo hypoxia imaging by Cy-NO2 were evaluated on tumor-bearing mouse, cerebral ischemia (CIS) and deep vein thrombosis models. The results indicated a rapid and distinct enhancement of its NIR fluorescence highly appropriate for in vivomonitoring of NTR in all of the three animal models, which revealed aspiring clinical value of this NIR fluorescent hypoxia probe.

Synthesis of 1,8-naphthalimide-based probes with fluorescent switch triggered by flufenamic acid.

In this work, we report the synthesis of novel fluorescent molecules, based on 1,8-naphthalimide thio- and amino-derivatives, designed to display an OFF-ON and ON-OFF photoelectron transfer fluorescence switch upon interaction with a proton-donor drug. We have functionalized the new probes to allow easy formation of a covalent link to polymer matrices, for future applications as drug delivery sensors. We have investigated the fluorescent switch of the new probes using flufenamic acid (FA, pKa 3.65), a water insoluble, non-steroidal anti-inflammatory drug, as a model drug and proton source. The rapid interaction of the new probes with FA resulted in fluorescence enhancement or decrease and a large Stokes shift, all of which allowed the detection of the drug in the nanomolar range. The new 1,8-naphthalimide fluorescent dyes reported in this work are interesting tools for the detection and quantification of acidic drugs and biomolecules.

Dual-Analyte Fluorescent Sensor Based on [5]Helicene Derivative with Super Large Stokes Shift for the Selective Determinations of Cu2+ or Zn2+ in Buffer Solutions and Its Application in a Living Cell.

A new fluorescent sensor, M201-DPA, based on [5]helicene derivative was utilized as dual-analyte sensor for determination of Cu2+ or Zn2+ in different media and different emission wavelengths. The sensor could provide selective and bifunctional determination of Cu2+ in HEPES buffer containing Triton-X100 and Zn2+ in Tris buffer/methanol without interference from each other and other ions. In HEPES buffer, M201-DPA demonstrated the selective ON-OFF fluorescence quenching at 524 nm toward Cu2+. On the other hand, in Tris buffer/methanol, M201-DPA showed the selective OFF-ON fluorescence enhancement upon the addition of Zn2+, which was specified by the hypsochromic shift at 448 nm. Additionally, M201-DPA showed extremely large Stokes shifts up to ∼150 nm. By controlling the concentration of Zn2+ and Cu2+ in a living cell, the imaging of a HepG2 cellular system was performed, in which the fluorescence of M201-DPA in the blue channel was decreased upon addition of Cu2+ and was enhanced in UV channel upon addition of Zn2+. The detection limits of M201-DPA for Cu2+ and Zn2+ in buffer solutions were 5.6 and 3.8 ppb, respectively. Importantly, the Cu2+ and Zn2+ detection limits of the developed sensors were significantly lower than permitted Cu2+ and Zn2+ concentrations in drinking water as established by the U.S. EPA and WHO.

A unique off-on near-infrared cyanine-based probe for imaging of endogenous alkaline phosphatase activity in cells and in vivo

Alkaline phosphatase (ALP) is a class of enzymes that are widely found in various tissues of the human body. It plays important roles in regulating diverse cellular functions. The aberrant levels of serum ALP are implicated in diseases. The development of sensitive and accurate detection tool for evaluating the level changes of ALP in living organisms will be very helpful in the fields of biochemistry, cytology and clinical medicine. Herein, we develop a near-infrared probe (QcyP) for the specific detection of different ALP levels both in different cell lines and in tumor-bearing mice models. The probe is composed of two moieties: a unique heptamethine cyanine fluorophore and a phosphate monoester. ALP can trigger an off-on fluorescence switch via an enzyme-catalyzed cleavage of the phosphate group. The fluorescence regulation mechanism is based on the rearrangement of the conjugated π-electron system. The probe exhibits high selectivity and good sensitivity towards ALP. The results demonstrate that the probe QcyP not only can be applied to discriminate the levels of ALP in different cell lines, but also can distinguish the ALP level changes between hepatic tumor mice model and normal mice.

Design and Synthesis of a BODIPY-Tetrazole Based "Off-On" in-Cell Fluorescence Reporter of Hydrogen Peroxide.

BODIPY-linked bithiophene-tetrazoles were designed and synthesized for bioorthogonal photoclick reactions in vitro and in vivo. The reactivity of these tetrazoles toward dimethyl fumarate was found to depend on the BODIPY attachment site, with the meta-linked BODIPY-tetrazole being the most reactive. The resulting pyrazoline cycloadduct showed drastically reduced BODIPY fluorescence. However, BODIPY fluorescence recovered after treatment with hydrogen peroxide. This turn-on effect was attributed to conversion from the pyrazoline to a pyrazole. Finally, we showed that this unique BODIPY-tetrazole off-on fluorescence probe can be used to detect hydrogen peroxide inside HeLa cells.

Sensitive and Selective Fluorescent Probe for Selenol in Living Cells Designed via a p Ka Shift Strategy.

Selenocysteine (Sec) is a primary kind of reactive selenium species in cells, and its vital roles in physiological processes have been featured. Thus, the development of highly sensitive and selective methods for the sensing of Sec is of great significance. This work reports a turn-on fluorescent probe for selenol based on the unique fluorescence OFF-ON switching between the Schiff base (SB) and its complementary protonated Schiff base (PSB) form of merocyanine dyes. The probe consists of a merocyanine Schiff base fluorophore and a 2,4-dinitrobenzenesulfonamide moiety that reacts especially with selenol. The fluorescence turn-on response of MC-Sec is realized via the selective removal of the strongly electron withdrawing 2,4-dinitrobenzenesulfonyl group by Sec, leading to a shift in the p Ka of the imine nitrogen of the probe from 6.40 to 9.04 and thus significantly increasing the population of the fluorescent PSB form of the dye at physiological pH. MC-Sec shows good selectivity and sensitivity for Sec and has been applied in the imaging of exogenous and endogenous selenol in living cells by confocal fluorescence microscopy. The proposed mechanism should be useful for developing future probes directed to other target molecules by employing this simple but effective p Ka shift strategy.

Dihydroindeno[1,2-b]pyrroles: new Al3+ selective off-on chemosensors for bio-imaging in living HepG2 cells.

In this study, a new molecular organic probe has been designed and synthesized by using recyclable, inexpensive and non-toxic polyethylene glycol (PEG-400) as a promoting reaction medium in water under environmentally benevolent conditions. The probe has been explored as a potential chemosensor to detect Al3+ ions using a HEPES buffer (pH = 7.4) solution. Investigations of the fluorescence behaviour of this sensor in DMSO/H2O (2 : 8, v/v) solution displayed a dramatic switch-on response only in the presence of Al3+, while other metal ions, like Li+, Na+, K+, Ag+, Ca2+, Mg2+, Mn2+, Ba2+, Cu2+, Ni2+, Co2+, Fe2+, Zn2+, Cd2+, Hg2+, Pb2+, Sr2+, Fe3+ or Cr3+, have almost no influence on the fluorescence behaviour. Various common anions, such as ClO4-, Cl-, or NO3- in the form of Al3+ salts [e.g. Al(ClO4)3, AlCl3 or Al(NO3)3], had no influence on the fluorescence behaviour of the sensors. The detection limit for Al3+ is in the order of 10-6 M in DMSO/H2O (2 : 8, v/v) HEPES buffer (pH = 7.4) solution. Notably, this is the first report of a dihydroindeno[1,2-b]pyrrole moiety acting as a sensor for the selective detection of Al3+ ions through an off-on fluorescence response. The potential of the probe was also confirmed by employing it for fluorescence bio-imaging with Al3+ on HepG2 cells.