Stern-Volmer Approach Research Articles

Truxene-to-Fluorenone Energy Transfer in a Robust Mesoporous Zn-MOF.

A new metal-organic framework (MOF; [Zn4O(hett)4/3(fluo)1/2(bdc)1/2]n; TFT-MOF) constructed on chromophoric ligands 5,5',10,10',15,15'-hexaethyltruxene-2,7,12-triacetate (hett), 9-fluorenone-2,7-dicarboxylate (fluo), terephthalate (bdc), and the Zn4O node has been prepared and identified by powder X-ray diffraction. This luminescent MOF exhibits large mesoporous pores of 2.7 nm based on computer modeling using density functional theory (DFT) calculations. The steady-state and time-resolved fluorescence spectra and photophysical parameters of TFT-MOF have been investigated and compared with those of the free ligands and their basic chromophores. All in all, TFT-MOF exhibits particularly efficient singlet-singlet energy-transfer processes described as 1(hett)* → (fluo) and 1(bdc)* → (fluo), leading to fluorescence arising for the fluo lumophore operating only through Förster resonance energy transfer (FRET) with an efficiency of transfer of up to >95%. This experimental conclusion was corroborated by DFT and time-dependent DFT (TDDFT). For the 1(hett)* → (fluo) process, the approximated overall rate constant of energy transfer was evaluated to be at most 2.04 × 1010 s-1 (using a Stern-Volmer approach of solution data and the relationship between distance and concentration). This process was analyzed using the Förster theory, where two intrapore energy transfer paths of center-to-center distances of 13 and 25 Å have been identified. TFT-MOF photosensitizes the formation of singlet oxygen (1O2 (1Σg)) as detected by its phosphorescence signal at 1275 nm.

Lophine-cinnamoyl hybrids: Synthesis, photophysical properties and a spectroscopic and molecular dynamics approach towards interactions with bovine serum albumin

Lophine and cinnamic acid derivatives have gained considerable attention due their chemical, biological and fluorescence emission properties. Herein we reported the synthesis of a new family of lophine-cinnamoyl hybrids connected by a methylene chain with two to nine carbon atoms. The hybrids show absorption in the ultraviolet and emission in the blue-violet region independent of the length of the alkyl chain spacer. Non-covalent interactions of the hybrids to bovine serum albumin (BSA) were investigated by UV–Vis absorption and fluorescence spectroscopy. Overall, the hybrids display moderate affinities for BSA (binding constants in the range 10–14 × 104 M−1) and can be effectively employed as protein probes using BSA as a model. The theoretical calculations showed preferential interactions of the hybrids with the binding pocket located near to Trp213 and the number of binding sites equal to 1 are in agreement with the experimental data obtained through the Stern-Volmer approach.

Interaction of cationic chlorin photosensitizers with non-ionic surfactant Tween 80

The interaction between cationic chlorophyll photosensitizers (PSs) for antimicrobial photodynamic therapy and non-ionic surfactant Tween 80 as a potential carrier system was studied using absorption and fluorescence spectroscopy. The results, analyzed in terms of our approach and the Stern–Volmer approach, indicate that all three PSs form stable molecular complexes with Tween 80, the PS molecules being located at the periphery of surfactant micelles next to hydrophilic head groups. The higher the charge of the macrocyclic cation, the stronger the interaction of PS with the surfactant.

In Situ-Grown Cdot-Wrapped Boehmite Nanoparticles for Cr(VI) Sensing in Wastewater and a Theoretical Probe for Chromium-Induced Carcinogen Detection.

In modern society, massive industrialization escalates environmental degradation by liberating various contaminants into the environment. Hexavalent chromium is a heavy metal that is being discharged from tannery and other industries, resulting in various carcinogenic diseases. This study reports a carbon dot (cdot)-based fluorometric probe for detecting hexavalent chromium in water. This is the very first time that cdots are tailored over the boehmite nanoparticle's surface using an in situ approach. Validation of formation of the nanocomposite has been discussed in detail employing the Rietveld refinement-based X-ray crystallography method. Vibrational spectroscopy and electron microscopy of the sample authenticate the nucleation process and the growth mechanism. The Stern-Volmer approach and time-resolved fluorescence measurements justify the sensitivity of the sensor (∼58 nM), and selectivity is analyzed by exposing the material to different ionic environments. Density functional theory (DFT) is applied herein to analyze the origin of fluorescence and the sensing mechanism of the probe, which shows that photoinduced electron transfer is responsible for the turn-off-based sensing of Cr(VI). The molecular docking simulation is carried out to ensure the binding of cdots to the binding pocket of the glutathione enzyme, which is responsible for treating reactive oxygen species-mediated DNA damage due to elements such as hexavalent chromium. Time-dependent density functional calculations show that the fluorometric probe is capable of detecting Cr(VI) in living cells making it an early stage chromium-mediated carcinogen detector.

Study of energy transfer mechanism from CdS quantum dots to Rhodamine 101 in reverse micelle medium

In this study, fluorescence resonance energy transfer (FRET) between CdS quantum dots (QDs) and Rhodamine 101 was investigated in reverse micelles. CdS QDs as donor were synthesized by using reverse micelle method. The particle sizes of CdS QDs were found as 1.18 ± 0.05 for QD1 and 2.11 ± 0.25 nm for QD2 with TEM measurements. It was determined that the fluorescence intensity of the QDs quenched as the concentration of Rh101 increased. The effect of the concentration of dye on the quenching of the fluorescence intensity of QDs was evaluated by the Stern-Volmer approach and KSV and kq values were calculated. It was observed that the quenching was not diffusion controlled. The FRET parameters were also calculated by using fluorescence spectroscopy. The distances between donor-acceptor (r) were also calculated as 4.74 nm for QD1 and 2.26 nm for QD2. The steady-state transfer efficiencies were calculated as 0.24 and 0.53 for QD1 and QD2, respectively. The time-resolved transfer efficiencies were also determined as 0.11 for QD1 and 0.48 for QD2. The results demonstrated that the novel donor-acceptor pairs may play an important role in the applications of many research areas such as FRET-based nanosensors and light harvesting devices.

(Invited) Spectroscopic Investigations of Stable Nitroxyl Radical-Containing Polymers

Stable nitroxide radical bearing organic polymer materials are attracting much attention for their application as next generation energy storage materials. A greater understanding of the inherent charge transfer mechanisms in such systems will ultimately be paramount to further advancements in the understanding of both intrafilm and interfacial ion- and electron-transfer reactions. This work is focused on advancing the fundamental understanding of these dynamic charge transfer properties by exploiting the fact that these species are efficient fluorescence quenchers. We systematically incorporated fluorescent perylene dyes into solutions containing the 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) radical and controlled their interaction by binding the TEMPO moiety into macromolecules with varying morphologies (e.g., chain length, density of radical pendant groups). In the case of the model compound, 4-oxo-TEMPO, quenching of the perylene excited state was found to be dominated by a dynamic (collisional) process, with a contribution from an apparent static process that is described by an ∼2 nm quenching sphere of action. When we incorporated the TEMPO unit into a macromolecule, the quenching behavior was altered significantly. The results can be described by using two models: (A) a collisional quenching process that becomes less efficient, presumably due to a reduction in the diffusion constant of the quenching entity, with a quenching sphere of action similar to 4-oxo-TEMPO or (B) a collisional quenching process that becomes more efficient as the radius of interaction grows larger with increasing oligomer length. This is the first study that definitively illustrates that fluorophore quenching by a polymer system cannot be explained using merely a classical Stern-Volmer approach but rather necessitates a more complex model. We extend this to a spectroelectrochemical study of their redox reactions in solution and cast as thin films on conducting substrates, and utilize these measurements to probe electron transfer to and/or from the nitroxyl radical-containing polymers.

Quenching of the perylene fluorophore by stable nitroxide radical-containing macromolecules.

Stable nitroxide radical bearing organic polymer materials are attracting much attention for their application as next generation energy storage materials. A greater understanding of the inherent charge transfer mechanisms in such systems will ultimately be paramount to further advancements in the understanding of both intrafilm and interfacial ion- and electron-transfer reactions. This work is focused on advancing the fundamental understanding of these dynamic charge transfer properties by exploiting the fact that these species are efficient fluorescence quenchers. We systematically incorporated fluorescent perylene dyes into solutions containing the 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) radical and controlled their interaction by binding the TEMPO moiety into macromolecules with varying morphologies (e.g., chain length, density of radical pendant groups). In the case of the model compound, 4-oxo-TEMPO, quenching of the perylene excited state was found to be dominated by a dynamic (collisional) process, with a contribution from an apparent static process that is described by an ∼2 nm quenching sphere of action. When we incorporated the TEMPO unit into a macromolecule, the quenching behavior was altered significantly. The results can be described by using two models: (A) a collisional quenching process that becomes less efficient, presumably due to a reduction in the diffusion constant of the quenching entity, with a quenching sphere of action similar to 4-oxo-TEMPO or (B) a collisional quenching process that becomes more efficient as the radius of interaction grows larger with increasing oligomer length. This is the first study that definitively illustrates that fluorophore quenching by a polymer system cannot be explained using merely a classical Stern-Volmer approach but rather necessitates a more complex model.

Riboflavin Photosensitized Oxidation of Myoglobin

The reaction of the fresh meat pigment oxymyoglobin, MbFe(II)O₂, and its oxidized form metmyoglobin, MbFe(III), with triplet-state riboflavin involves the pigment protein, which is oxidatively cleaved or dimerized as shown by SDS-PAGE and Western blotting. The overall rate constant for oxidation of MbFe(II)O₂ by ³Rib is (3.0 ± 0.5) × 10⁹ L·mol⁻¹·s⁻¹ and (3.1 ± 0.4) × 10⁹ L·mol⁻¹·s⁻¹ for MbFe(III) in phosphate buffer of pH 7.4 at 25 °C as determined by laser flash photolysis. The high rates are rationalized by ground state hydrophobic interactions as detected as static quenching of fluorescence from singlet-excited state riboflavin by myoglobins using time-resolved fluorescence spectroscopy and a Stern-Volmer approach. Binding of riboflavin to MbFe(III) has K(a) = (1.2 ± 0.2) × 10⁴ mol·L⁻¹ with ΔH° = -112 ± 22 kJ·mol⁻¹ and ΔS° = -296 ± 75 J·mol⁻¹·K⁻¹. For meat, riboflavin is concluded to be a photosensitizer for protein oxidation but not for discoloration.

Signal drift of oxygen optical sensors. Part II: “Smart” drift correction algorithm and its experimental check with a light intensity detection based sensor

The drift nature associated to oxygen optical sensors was used to mathematically setup a “smart” iterative algorithm allowing the accurate determination of oxygen percentage present in gas mixtures even with the light intensity detection based sensor usually considered unsuitable for that measurement owing to intrinsic weakness of the detection system. Oxidative, photochemical and thermal degradations, acting on both luminophore and polymeric membrane were considered as drift sources. The only requirement for the algorithm correct functioning was the reading of the signal coming from a reference oxygen mixture (air). It allowed the accurate quantification of the nominal %O2 concentration in all 9 days experiment demonstrating to be drift free. On the other hand, the “classical” Stern–Volmer approach, in the same experimental conditions, was useless as it always produced a large positive systematic error increasing with time and with %O2. The proposed algorithm gave sensor accuracy as good as the most expensive phase shift based commercial sensors even in the chosen test conditions producing, on purpose, heavy membrane degradation. Even in those drastic conditions, the mean accuracy was Δ%O2=0.11, %RSD always lower than 5% and close to 1% for higher %O2.

The Quenching Effect of Flavonoids on 4-Methylumbelliferone, a Potential Pitfall in Fluorimetric Neuraminidase Inhibition Assays

Many assays aimed to test the inhibitory effects of synthetic molecules, and naturally occurring products on the neuraminidase activity exploit the hydrolysis of 2'-O-(4-methylumbelliferyl)-N-acetylneuraminic acid (4-MUNANA). The amount of the released product, 4-methylumbelliferone (4-MU), is then measured fluorimetrically. The authors attempted an analysis of the inhibitory properties of 35 naturally occurring flavonoids on neuraminidase N3, where only 29 of them were sufficiently soluble in the assay medium. During the analysis, the authors noticed a strong quenching effect due to the test compounds on the fluorescence of 4-MU. The quenching constants for the flavonoids were determined according to the Stern-Volmer approach. The extent of fluorescence reduction due to quenching and the magnitude of the fluorescence reduction measured in the inhibition assays were comparable: for 11 of 29 compounds, the two values were found to be coincident within the experimental uncertainty. These data were statistically analyzed for correlation by calculating the pertinent Pearson correlation coefficient. Inhibition and quenching were found to be positively correlated (r = 0.71, p(uncorr) = 1.5 × 10(-5)), and the correlation was maintained for the whole set of tested compounds. Altogether, the collected data imply that all of the tested flavonoids could produce false-positive results in the neuraminidase inhibition assay using 4-MUNANA as a substrate.

Time-resolved fluorescence spectroscopy of two-photon laser-excited8p,9p,5f, and6flevels in neutral xenon

Laser-induced fluorescence of two-photon excited 8p, 9p, J=0, 2 and 5f, 6f, J=2 levels in neutral xenon has been investigated in the pressure regime between 4 and 120 Pa. Radiative lifetimes and collisional deactivation rates have been deduced especially for the 5f[3/2](2), 5f[5/2](2), and 8p[1/2](0) levels using the Stern-Volmer approach. The spontaneous lifetimes for 5f[3/2](2), 5f[5/2](2), and 8p[1/2](0) levels are 94, 78, and 207 ns, respectively. These lifetimes have been calculated also by applying extended multiconfiguration Dirac-Fock wave functions and are found in agreement with experiment within 10-25 %.

New Fluorescence Parameters for the Determination of QARedox State and Excitation Energy Fluxes

A number of useful photosynthetic parameters are commonly derived from saturation pulse-induced fluorescence analysis. We show, that qP, an estimate of the fraction of open centers, is based on a pure 'puddle' antenna model, where each Photosystem (PS) II center possesses its own independent antenna system. This parameter is incompatible with more realistic models of the photosynthetic unit, where reaction centers are connected by shared antenna, that is, the so-called 'lake' or 'connected units' models. We thus introduce a new parameter, qL, based on a Stern-Volmer approach using a lake model, which estimates the fraction of open PS II centers. We suggest that qL should be a useful parameter for terrestrial plants consistent with a high connectivity of PS II units, whereas some marine species with distinct antenna architecture, may require the use of more complex parameters based on intermediate models of the photosynthetic unit. Another useful parameter calculated from fluorescence analysis is ΦII, the yield of PS II. In contrast to qL, we show that the ΦII parameter can be derived from either a pure 'lake' or pure 'puddle' model, and is thus likely to be a robust parameter. The energy absorbed by PS II is divided between the fraction used in photochemistry, ΦII, and that lost non-photochemically. We introduce two additional parameters that can be used to estimate the flux of excitation energy into competing non-photochemical pathways, the yield induced by downregulatory processes, ΦNPQ, and the yield for other energy losses, ΦNO.

A Simple Device to Demonstrate the Principles of Fluorometry

A simple device to demonstrate the principles of fluorimetry was constructed using low-cost components that are readily obtained. These components were a mini ultraviolet fluorescent lamp, a plastic cell holder, borosilicate glass tubes, and a photoresistor (LDR). The LDR resistance was measured by a multimeter interfaced to a computer for data acquisition and processing. A calibration curve was constructed to relate the fluorescence of quinine bisulfate (QBS) solutions with the LDR resistance. QBS fluorescence quenching by NaCl was studied by the Stern-Volmer approach. The apparatus was designed to permit the direct observation of the following features of the fluorescence phenomenon: (i) the wavelength shift between excitation and emission light, (ii) the increase of emission intensity with increasing fluorophore concentration, and (iii) the inner filter effect for highly concentrated QBS solutions.

A fluorescence study of thianthrene complexes

A fluorometric method is used to study complexes between thianthrene and various solvents including CCl 4, CHCl 3, acetone and dichloroethane. Dichloromethane and dichloroethane are used as solvents and systems are analyzed at 20 and 30°C. Complex formation constants ( K f ) determined in dichloroethane are corrected for the solvent-thianthrene complex and are seen to be similar to those determined in dichloromethane. The Stern-Volmer equation is shown to be an incorrect choice in the analysis despite its yielding a linear plot. The correct method yields a different value for K f than does the Stern-Volmer approach. Enthalpies of complex formation are also determined.