Fluorescent Photoinduced Electron Transfer Research Articles

Dual-Functional Manganese-Doped ZnO-MOF Hybrid Material with Enhanced Hydrolytic Stability: A Fluorescent Photoinduced Electron Transfer Sensor for the Ultraselective Detection of Acetic Acid and Chromium (VI).

In recent years, the synthesis of metal-organic framework (MOF)─nanocomposites has received wide attention from the scientific fraternity due to the presence of a tunable hierarchical architecture and invasive versatility in applications. The present work focuses on the solvothermal synthesis of a novel hybrid MOF-nanocomposite through the impregnation of Mn-doped ZnO nanoparticles onto the matrix of a pioneer metal-organic framework that is composed of zinc metal connected with terephthalic acid linkers (MOF-5). The hierarchical arrangements of the prepared material were further assessed by Fourier-transform infrared spectroscopy (FTIR), powder X-ray diffraction (PXRD), high-resolution transmission electron microscopy (HR-TEM), UV-visible, photoluminescence (PL), and dynamic light scattering (DLS) measurements. The porosity analysis via nitrogen sorption measurements at 77 K showed that the material is porous with hierarchical micro-, wide micro-, and mesopores. The SAED pattern confirms the polycrystallinity of the material, which is in good agreement with the data obtained from PXRD analysis. Effective integration of Mn-doped ZnO onto the MOF structure was confirmed by XPS analysis, and the study further identified the oxidation state of the elements present. The synthesized analyte is an efficient fluorescent chemosensor for the detection of acetic acid, which can find further potential applications in intracellular imaging. Interestingly, the same compound also selectively detects the presence of Cr(VI) ions, thereby acting as a dual sensor, which finds applications in the sensing and removal of environmental contaminants. The material showed a sharp and intense emission at 569 nm at an excitation wavelength of 320 nm, and it exhibits high quenching efficiencies of 99.87 and 71.43% toward the sensing of μM level concentration of acetic acid and Cr2O72-, respectively. The highly efficient fluorescent sensing of pollutants, even at a shorter linear range, discarded the possibility of sensing the pollutants at higher concentration ranges. The Ksv value for the detection of acetic acid and Cr(VI) is found to be 3.7017 × 106 and 11.0324 × 106 M-1, respectively, which further confirms the higher sensing ability of the synthesized fluorophore. The mechanistic studies and density functional theory calculations of Mn-doped ZnO@MOF-5 reveal that photoinduced electron transfer plays a significant role in the turn-off response toward acetic acid and Cr2O72- ions. In the case of acetic acid, in addition to photoinduced electron transfer, hydrogen bonding interactions may also lead to fluorescence quenching. To the best of our knowledge, no precedent work has been reported for the sensing of acetic acid in the solution state. All other fluorescent sensing reports put forward the sensing and adsorption of acetic acid in the gaseous state, which makes this material a pioneer among others for the detection of acetic acid in the solution phase.

Energy band gap modulation and photoinduced electron transfer fluorescence sensing properties of D–A conjugated polymers containing benzotriazole

AbstractTwo new conjugated polymers containing benzotriazole were synthesized via Suzuki coupling reaction, and the polymer with lower energy band gap was created by designing the structure of the acceptor unit by introducing thiophene unit at both ends of the benzotriazole. Surprisingly, the fluorescence emission of D–A conjugated polymer with modified unit shifted from blue to yellow light, and the polymer had a higher fluorescence quantum efficiency. In addition, thiophen benzimidazole was introduced as the side chain unit for the study of photoinduced electron transfer (PET) mechanism. The addition of Fe3+ and Cu2+ caused significant quenching of fluorescence of polymers, and the polymer with high energy band gap exhibited higher fluorescence quenching ability. Moreover, when exploring the fluorescence sensing of [P‐Fe3+] complex in the presence of anion, it was observed that the PO43− competed with [P‐Fe3+] complex for Fe3+, interrupting the fluorescence sensing mechanism eventually leading to the recovery of fluorescence. The design approach utilized in this survey for D–A conjugated polymer paves the way for a novel mean of functional control of fluorescence sensor based on PET fluorescence sensing.

A Tool, an App and a Field: Fluorescent PET Sensors, Blood Electrolyte Analysis and Molecular Logic as Products of Supramolecular Photoscience from Northern Ireland and Sri Lanka.

The general tool of fluorescent PET (photoinduced electron transfer) sensors/switches - a molecular design principle with engineering features - is outlined, with the aid of frontier orbital energy diagrams. Fluorophores such as anthracene, 1,3-diaryl-Δ2 -pyrazolines and 4-amino-1,8-naphthalimides are employed within this system, alongside receptors such as amines, carboxylates, crown ethers and amino acids. This tool appealed to a multinational corporation for building a medical analyzer for electrolytes such as Na+ , K+ , Ca2+ and gases like CO2 , which became a commercially successful application. Finally, the tool was a springboard for chemistry to cross into computer science. The field of molecular logic can elucidate how molecules inside us handle information. Molecular examples of the simplest logic gates such as YES, NOT, OR, AND are described. A case of a human-level computation - visual edge detection - is also included.

Crossing the divide: Experiences of taking fluorescent PET (photoinduced electron transfer) sensing/switching systems from solution to solid

The author's journey from the initial discoveries of fluorescent PET (photoinduced electron transfer) sensors and logic gates to their applications in solid-based devices is outlined. Functional molecules are immobilized on particles, films, fibres, beads and sheets for a range of purposes in analysis and identification, as well as the emulation of animal and computer behaviour.

Benzothiazole Pyrazoline: Acid-Switchable Absorption and Fluorescence of Photoinduced Electron Transfer (PET).

Acid-responsive fluorescent compounds were prepared by introducing an ortho-hydroxyphenyl to pyrazoline with a benzothiazole backbone. These compounds demonstrated normal fluorescence photoinduced electron transfer (PET) under neutral conditions but the addition of trifluoroacetic acid showed an arctic blue fluorescence, we verified that a protonation process of nitrogen in the thiazole ring which weakened the ability of thiazole to donate electrons to the pyrazoline and changed the photoinduced electron transfer led to photoinduced electron transfer (PET), which was the mechanism of the fluorescence quenching phenomenon under strongly acidic conditions. The photophysical properties of Benzothiazole pyrazoline exhibited blue emission at 421nm in aqueous DMSO. The blue shift in the emission was switched by acid in DMSO, showing the compound's distinct fluorescence peak at 554nm. To investigate solvatochromism, eight different solvents were used. The red-shift emission observed in enhancing the polarity of solvents and emission in DMSO suggested the conformation of the molecule which led to the intramolecular charge transfer by color and emission changes. Furthermore, the probe was also applied using the High-performance liquid chromatography (HPLC) with a UV detector to determine the trifluoroacetic acid in water samples. Interestingly, the method was found to be linear over the range of 10.0µg L-1 to 250.0µg L-1 (0.999). Under the optimum condition, the separation of trifluoroacetic acid was achieved in 20min with the LOD of 1.3µg L-1 and LOQ of 5.1µg L-1. This proposed method also showed satisfactory results when applied for the analysis of trifluoroacetic acid in a water sample.

Two-colour single-molecule photoinduced electron transfer fluorescence imaging microscopy of chaperone dynamics

Many proteins are molecular machines, whose function is dependent on multiple conformational changes that are initiated and tightly controlled through biochemical stimuli. Their mechanistic understanding calls for spectroscopy that can probe simultaneously such structural coordinates. Here we present two-colour fluorescence microscopy in combination with photoinduced electron transfer (PET) probes as a method that simultaneously detects two structural coordinates in single protein molecules, one colour per coordinate. This contrasts with the commonly applied resonance energy transfer (FRET) technique that requires two colours per coordinate. We demonstrate the technique by directly and simultaneously observing three critical structural changes within the Hsp90 molecular chaperone machinery. Our results reveal synchronicity of conformational motions at remote sites during ATPase-driven closure of the Hsp90 molecular clamp, providing evidence for a cooperativity mechanism in the chaperone’s catalytic cycle. Single-molecule PET fluorescence microscopy opens up avenues in the multi-dimensional exploration of protein dynamics and allosteric mechanisms.

Development of silica molecularly imprinted polymer on carbon dots as a fluorescence probe for selective and sensitive determination of cetirizine in saliva and urine

A fluorescence probe based on carbon dots (CDs) coated with silica molecularly imprinted polymer (MIPs) was synthesized for selective and sensitive determination of cetirizine (CTZ). Green source carbon dots were firstly derived from orange peels through a microwave method, and had the merits of eco-friendly and low toxicity. Then a thin silica film was formed on the surface of CDs by reverse microemulsion technique, and molecularly imprinted polymer coated on silica–carbon dots. In this scene, CTZ, 3-aminopropyltriethoxysilane (APTES) and tetraethoxysilane (TEOS) were employed as a template, a functional monomer and cross linker, respectively. The obtained CDs-MIPs can selectively bind CTZ through the specific interaction between recognition sites and template, and obey photoinduced electron transfer fluorescence quenching mechanism. Fluorescence dropped linearly in the range of 0.5–500 ng mL−1, under the optimal conditions, with a detection limit of 0.41 ng mL−1. Furthermore, the proposed method was successfully intended for the determination of trace CTZ in human saliva and urine samples without the interference of other molecules and ions. And recoveries ranged from 95.8% to 99.8% with relative standard deviation less than 3.0%.

Fluorescent Molecular Logic Gates Driven by Temperature and by Protons in Solution and on Solid.

Temperature-driven fluorescent NOT logic is demonstrated by exploiting predissociation in a 1,3,5-trisubstituted Δ2 -pyrazoline on its own and when grafted onto silica microparticles. Related Δ2 -pyrazolines become proton-driven YES and NOT logic gates on the basis of fluorescent photoinduced electron transfer (PET) switches. Additional PASS 1 and YES+PASS 1 logic gates on silica are also demonstrated within the same family. Beside these small-molecule systems, a polymeric molecular thermometer based on a benzofurazan-derivatized N-isopropylacrylamide copolymer is attached to silica to produce temperature-driven fluorescent YES logic.

Structure, interdomain dynamics, and pH-dependent autoactivation of pro-rhodesain, the main lysosomal cysteine protease from African trypanosomes

Rhodesain is the lysosomal cathepsin L-like cysteine protease of Trypanosoma brucei rhodesiense, the causative agent of Human African Trypanosomiasis. The enzyme is essential for the proliferation and pathogenicity of the parasite as well as its ability to overcome the blood–brain barrier of the host. Lysosomal cathepsins are expressed as zymogens with an inactivating prodomain that is cleaved under acidic conditions. A structure of the uncleaved maturation intermediate from a trypanosomal cathepsin L-like protease is currently not available. We thus established the heterologous expression of T. brucei rhodesiense pro-rhodesain in Escherichia coli and determined its crystal structure. The trypanosomal prodomain differs from nonparasitic pro-cathepsins by a unique, extended α-helix that blocks the active site and whose side-chain interactions resemble those of the antiprotozoal inhibitor K11777. Interdomain dynamics between pro- and core protease domain as observed by photoinduced electron transfer fluorescence correlation spectroscopy increase at low pH, where pro-rhodesain also undergoes autocleavage. Using the crystal structure, molecular dynamics simulations, and mutagenesis, we identify a conserved interdomain salt bridge that prevents premature intramolecular cleavage at higher pH values and may thus present a control switch for the observed pH sensitivity of proenzyme cleavage in (trypanosomal) CathL-like proteases.

Rational Design of High-Performance Donor-Linker-Acceptor Hybrids Using a Schiff Base for Enabling Photoinduced Electron Transfer.

Donor-linker-acceptor (D-L-A)-based photoinduced electron transfer (PET) has been frequently used for the construction of versatile fluorescent chemo/biosensors. However, sophisticated and tedious processes are generally required for the synthesis of these probes, which leads to poor design flexibility. In this work, by exploiting a Schiff base as a linker unit, a covalently bound D-L-A system was established and subsequently utilized for the development of a PET sensor. Cysteamine (Cys) and N-acetyl-l-cysteine (NAC) costabilized gold nanoclusters (Cys/NAC-AuNCs) were synthesized and adopted as an electron acceptor, and pyridoxal phosphate (PLP) was selected as an electron donor. PLP can form a Schiff base (an aldimine) with the primary amino group of Cys/NAC-AuNC through its aldehyde group and thereby suppresses the fluorescence of Cys/NAC-AuNC. The Rehm-Weller formula results and a HOMO-LUMO orbital study revealed that a reductive PET mechanism is responsible for the observed fluorescence quenching. Since the pyridoxal (PL) produced by the acid phosphatase (ACP)-catalyzed cleavage of PLP has a weak interaction with Cys/NAC-AuNC, a novel turn-on fluorescent method for selective detection of ACP was successfully realized. To the best of our knowledge, this is the first example of the development of a covalently bound D-L-A system for fluorescent PET sensing of enzyme activity based on AuNC nanoprobes using a Schiff base.

4-Amino-1,8-naphthalimide based fluorescent photoinduced electron transfer (PET) pH sensors as liposomal cellular imaging agents: The effect of substituent patterns on PET directional quenching

Four new fluorescent sensors (1–4) based on the 4-amino-1,8-naphthalimide fluorophores (Naps) have been synthesized based on the classical fluorophorespacer-receptor model. These four compounds all gave rise to emission bands centred at ca. 535 nm, which were found to be highly pH dependent, the emission being ‘switched on’ in acidic media, while being quenched due to PET from the amino moieties to the excited state of the Nap at more alkaline pH. The luminescent pH dependence for these probes was found to be highly dependent on the substitution on the imide site, as well as the polyamine chain attached to the position 4-amino moiety. In the case of sensor 2 the presence of the 4-amino-aniline dominated the pH dependent quenching. Nevertheless, at higher pH, PET quenching was also found to occur from the polyamine site. Hence, 2 is better described as a receptor1-spacer1-fluorophore-spacer2-receptor2 system, where the dominant PET process is due to (normally less favourable) ‘directional’ PET quenching from the 4-amino-aniline unit to the Nap site. Similar trends and pH fluorescence dependences were also seen for 3 and 4. These compounds were also tested for their imaging potential and toxicity against HeLa cells (using DRAQ5 as nuclear stain which does now show pH dependent changes in acidic and neutral pH) and the results demonstrated that these compounds have reduced cellular viability at moderately high concentrations (with IC50 values between ca. 8–30 µmol·L−1), but were found to be suitable for intracellular pH determination at 1 µmol ·L−1concentrations, where no real toxicity was observed. This allowed us to employ these as lysosomal probes at sub-toxic concentrations, where the Nap based emission was found to be pH depended, mirroring that seen in aqueous solution for 1–4, with the main fluorescence changes occurring within acidic to neutral pH.

Fabrication of carbon dots@restricted access molecularly imprinted polymers for selective detection of metronidazole in serum

A custom-tailored design was proposed for the fabrication of carbon dots coupled with restricted access materials and molecularly imprinted polymers (CDs@RAM-MIPs) to detect metronidazole (MNZ). Biomass carbon dots (CDs) were derived from longan peels assisted with high pressure microwave, and had the merits of eco-friendly, excellent photostability and low toxicity. In this work, glycidyl methacrylate was used as a co-polymeric monomer to increase hydroxyl groups on the surface of synthetic materials, which eliminated the interference of biological macromolecules. The specific binding cavities of CDs@RAM-MIPs were formed after removing the template molecule (MNZ). The obtained CDs@RAM-MIPs can selectively capture MNZ through the specific interaction between recognition sites and MNZ, and obey photoinduced electron transfer fluorescence quenching mechanism. The highly sensitive and selective fluorescent sensor based CDs@RAM-MIPs had a wide linear range (50–1200 ng mL−1) and a low detection limit (17.4 ng mL−1) for MNZ. It has been utilized to detect MNZ in serum with recoveries of 93.5%–102.7%, and the relative standards (RSDs) were 1.9%–3.6%, respectively. This work provides a thoughtful strategy for preparation and application of CDs@RAM-MIPs, which presages its great potential for detecting trace compounds in real samples.

Molecular memory with downstream logic processing exemplified by switchable and self-indicating guest capture and release

Molecular-logic based computation (MLBC) has grown by accumulating many examples of combinational logic gates and a few sequential variants. In spite of many inspirations being available in biology, there are virtually no examples of MLBC in chemistry where sequential and combinational operations are integrated. Here we report a simple alcohol-ketone redox interconversion which switches a macrocycle between a large or small cavity, with erect aromatic walls which create a deep hydrophobic space or with collapsed walls respectively. Small aromatic guests can be captured or released in an all or none manner upon chemical command. During capture, the fluorescence of the alcohol macrocycle is quenched via fluorescent photoinduced electron transfer switching, meaning that its occupancy state is self-indicated. This represents a chemically-driven RS Flip-Flop, one of whose outputs is fed into an INHIBIT gate. Processing of outputs from memory stores is seen in the injection of packaged neurotransmitters into synaptic clefts for onward neural signalling. Overall, capture-release phenomena from discrete supermolecules now have a Boolean basis.

Physical Insights into Molecular Sensors, Molecular Logic Gates, and Photosensitizers in Photodynamic Therapy

In this article, the importance of charge/electron transfer in two rapidly growing areas of science is highlighted. In the field of molecular sensors, it plays a considerable role on the detection of molecular systems to serve as fluorescence sensors, switches, and molecular logic gates (MLG) replacing the semiconductor electronics, while in the field of photodynamic therapy, it acts competitive. On these scientific fields, a lot of research has been conducted in the last decades to find out potential candidates. In the field of fluorescent sensors, switches, and molecular logic gates, the fluorescent photo-induced electron transfer switching principle is responsible for the quenching of fluorescence. The manipulation of the quenching can lead to the design of an ideal candidate for complicated molecular logic operation. In the field of photodynamic therapy (PDT), the intersystem crossings occurring between excited singlet and triplet states are the key for an ideal photosensitizer (PS) candidate. The triplets must present relatively long lifetimes, and they must lie near or above the energy which is needed for the excitation of molecular oxygen. It this case, charge/electron phenomena can act competitive, and they are not desirable. However, there are a few complexes which are good PSs of singlet oxygen despite the charge transfer (CT) nature of their lowest excited state.

Tuning the Photoinduced Electron Transfer in a Zr-MOF: Toward Solid-State Fluorescent Molecular Switch and Turn-On Sensor.

The immobilization of fluorescent photoinduced electron transfer (PET) switches/sensors into solid state, which usually cannot maintain their identical properties in solution, has remained a big challenge. Herein, a water-stable anthracene and maleimide appended zirconium-based-metal-organic framework (Zr-MOF; UiO-68-An/Ma) is reported. Unlike the regular intramolecular "fluorophore-spacer-receptor" format, the separated immobilization of fluorescent (anthracene) and acceptor (maleimide) groups into the framework of a multivariate MOF can also favor a pseudo-intramolecular fluorescent PET process, resulting in UiO-68-An/Ma with very weak fluorescence. Interestingly, after Diels-Alder reaction or thiol-ene reaction of maleimide groups, the pseudo-intramolecular fluorescent PET process in UiO-68-An/Ma fails and the solid-state fluorescence of the crystals is recovered. In addition, UiO-68-An/Ma shows an interesting application as solid-state fluorescent turn-on sensor for biothiols, with the naked eye response at a low concentration of 50 µmol L-1 within 5 min. This study represents a general strategy to enable the efficient tuning of fluorescent PET switches/sensors in solid state, and considering the fluorescence of the PET-based MOFs can be restored after addition of analyte/target species, this research will definitely inspire to construct stimuli-responsive fluorescent MOFs for interesting applications (e.g., logic gate) in future.

Dual colorimetric and fluorometric monitoring of Bi3+ ions in water using supermicroporous Zr-MOFs chemosensors

We fabricated supermicroporous, switchable, colorimetric and fluorometric chemosensors on a Zr-metal–organic framework platform. The chemosensors could be applied in the selective and sensitive colorimetric and fluorometric detection of ultra-trace concentrations of heavy metals, such as Bi3+ ions, in water sources. We fabricated supermicroporous fluorescent chemosensors (SFCs) through direct dressing of rhodamine ethylene–diamine salicylaldehyde (RES), a water-insoluble organic probe, onto Zr-MOFs. The decorated SFCs with uniform super-microchannel pores, long-range intergrowing crystal structure, and active hook surface sheaths act as sensitive and selective chemosensor carriers. The selectivity of the colorimetric and fluorometric sensing assay for Bi3+ ions in a heterogeneous mixture with multiple cations and anions depends on the structure of the RES, the pH of the system, the composition of the competitive ion system, and the process of Bi-to-RES binding, which occurs through a mechanism that involves fluorescent photoinduced electron transfer. Under optimal working conditions, the SFCs showed a wide range of detection for Bi3+ ions, the low detection limit of approximately 10–9 mol/L, and fast response within seconds during binding with the Bi3+ ion target. The long-term stability of SFCs with RES surface functionality enables the practical and multiple reuse/cycles of Bi3+ ion detection.

Lighting-up protein-ligand interactions with fluorescent PET (photoinduced electron transfer) sensor designs.

The fluorescence of sensors 1 and 6, which are designed in terms of photoinduced electron transfer (PET), is switched 'on' by factors of 15 and 6 with avidin in a rather selective manner. Sensors 3 and 7, simpler derivatives of 1, are switched 'on' by factors of 16 and 7.5 with bovine serum albumin, though at 30-fold higher concentrations.

A selective fluorescent and colorimetric probe for cyanide based on dual-site controlled intramolecular charge transfer–photoinduced electron transfer–Fluorescence resonance energy transfer

A novel dual-site controlled intramolecular charge transfer–photoinduced electron transfer–Fluorescence resonance energy transfer (ICT–PET–FRET) probe 6b and its reference ICT–FRET probe 6a were designed and synthesized. The probes show high selective fluorescent and colorimetric sensing for cyanide ions. The NH and CH groups connected by the π-extension framework in probe 6b were simultaneously employed as the two sensing sites and controlled the fluorescence of naphthalimide unites by PET, ICT followed by the corresponding FRET processes between coumarin and naphthalimide fluorophores.

Dual Site-Controlled and Lysosome-Targeted Intramolecular Charge Transfer-Photoinduced Electron Transfer-Fluorescence Resonance Energy Transfer Fluorescent Probe for Monitoring pH Changes in Living Cells.

Acidic pH is a critical physiological factor for controlling the activities and functions of lysosome. Herein, we report a novel dual site-controlled and lysosome-targeted intramolecular charge transfer-photoinduced electron transfer-Fluorescence resonance energy transfer (ICT-PET-FRET) fluorescent probe (CN-pH), which was essentially the combination of a turn-on pH probe (CN-1) and a turn-off pH probe (CN-2) by a nonconjugated linker. Coumarin and naphthalimide fluorophores were selected as donor and acceptor to construct the FRET platform. Hydroxyl group and morpholine were simultaneously employed as the two pH sensing sites and controlled the fluorescence of coumarin and naphthalimide units by ICT and PET, respectively. The sensing mechanism of CN-pH to pH was essentially an integration of ICT, PET, and FRET processes. Meanwhile, the morpholine also can serve as a lysosome-targeted group. By combining the two data analysis approaches of the ratios of the two emission intensities (R) and the reverse ratio R' (R' = 1/R), the fluorescent ratio of CN-pH can show proportional relationship to pH values in a very broad range from pH 4.0 to 8.0 with high sensitivity. The probe has been successfully applied for the fluorescence imaging of the lysosomal pH values, as well as ratiometrically visualizing chloroquine-stimulated changes of intracellular pH in living cells. These features demonstrate that the probe can afford practical application in biological systems.

Water-soluble amino(ethanesulfonate) and [bis(ethanesulfonate)] anthracenes as fluorescent photoinduced electron transfer (PET) pH indicators and Fe3+chemosensors

Two novel anthracene-based chemosensors appended with amino(ethanesulfonate) groups function as fluorescent PETturn-onprobes for protons andturn-offprobes for ferric ions in water.