Self‐association,tautomerism and E–Z isomerization of isatin–phenylsemicarbazones – spectral study and theoretical calculations

The self‐association and tautomerism of (E)‐isatin‐3‐4‐phenyl(semicarbazone) Ia and (E)‐N‐methylisatin‐3‐4‐phenyl(semicarbazone) IIa were investigated in solvents of various polarity. In weakly interacting non‐polar solvents, such as CHCl₃ and benzene, phenylsemicarbazone concentrations above 1×10^?5 mol dm^?3 result in the formation of dimers or higher aggregates of E‐isomers Ia and IIa . This aggregate formation prevents room temperature E–Z isomerization of Ia and IIa to more stable Z‐isomers. In contrast to the situation in non‐polar solvents, E–Z isomerization from the monomeric form of phenylsemicarbazone Ia and IIa E‐isomers occurs in highly interactive polar solvents including MeOH and DMF only at temperatures above 70 °C. Moreover, decrease in phenylsemicarbazone concentration below 1×10^?4 mol dm^?3 in these highly solute–solvent interacting systems leads to aggregate dissociation, and a new hydrazonol tautomeric form with a high degree of conjugation predominates in these solutions. Theoretical calculations confirm obtained experimental results. Copyright © 2013 John Wiley & Sons, Ltd.     

Dynamic quenching of the multiband fluorescence of 3-hydroxyflavone

The properties of emission, absorption, and dual fluorescence excitation of 3-hydroxyflavone in acetonitrile are studied under the conditions of dynamic quenching by potassium iodide with concentrations up to 4 × 10^?2 M. The normal and tautomeric forms undergo quenching, which is more efficient for the tautomeric form. An interesting circumstance is that the absorption in the S ₀ → S ₁ and S ₀ → S ₂ singlet bands of the solution increases with increasing quencher concentration in the whole region of concentrations used, the steepest rise being recorded in the concentration region from 0 to 5 × 10^?3 M. The intensities and quantum yields of the two fluorescence bands show rather complicated nonlinear dependences on the quencher concentration. The long-wavelength fluorescence band, which belongs to the tautomeric form of 3-hydroxyflavone, is quenched considerably stronger. The experimental results reveal the kinetic character of the excited-state proton transfer in molecules of 3-hydroxyflavone in acetonitrile.     

Synthesis of Pyridine-Based 1,3,4-Oxadiazole Derivative as Fluorescence Turn-On Sensor for High Selectivity of Ag+

An oxadiazole derivative(OXD) containing symmetrical pyridine-2-formamidophenyl-binded moiety was synthesised as fluorescence turn-on sensor OA1. Its ultraviolet–visible(UV–vis) and fluorescent spectra(FS) gave prominent fluorescence enhancement only for monovalent silver ion(Ag⁺) in HEPES buffer solution (10 mM, pH?=?7.0, DMF-H₂O, 9:1, v/v), which indicated the photo-induced electron transfer(PET) occurred from the donor of pyridine-2-formamidophenyl group to oxadiazole fluorophore. The present study demonstrated that OA1 was a viable candidate as fluorescent receptor for a new Ag⁺ sensor. And the results of fluorescent spectral titration showed this sensor formed 1:1 complex with Ag⁺.     

Prototropy and π‐electron delocalization for purine and its radical ions – DFT studies

Complete tautomeric equilibria and π‐electron delocalization were studied at the B3LYP/6‐311+G** level for neutral purine ( P ) and its charged radicals ( P ^+? and P ^??). All possible nine tautomers (four NH and five CH forms) and all possible 36 tautomeric equilibria (six NⁱH → N^kH, twenty NH → CH, and ten CⁱH → C^kH conversions) were considered. The greatest variations of the tautomeric equilibrium constants (as pK_T) were observed for the NH → CH conversions when proceeding from neutral to reduced purine ( P + e → P ^??). These variations completely change the tautomeric preferences. One‐electron oxidation ( P ? e → P ^+?) has considerably smaller effect on the pK_T values and does not change the tautomeric preferences. π‐Electron delocalization depends on the position of the moving proton and on the type of the electron transfer. For individual tautomers, some linear relations between the relative stabilities and the HOMA (harmonic oscillator model of aromaticity) indices occur for neutral and oxidized purine. For reduced purine, a scatter plot is found. Copyright © 2010 John Wiley & Sons, Ltd.     

Theoretical study of keto–enol tautomerism by quantum mechanical calculations (the QM/MC/FEP method)

In this study, the tautomeric equilibrium between the keto and enol forms has been studied for five typical ketones and aldehydes: i‐butanal, acetaldehyde, acetone, acetylacetone, and dimedone. The level of theory used in the gas‐phase calculation was Becke, three‐parameter, Lee–Yang–Parr/6‐311G(d,p)//Becke, three‐parameter, Lee–Yang–Parr/6‐31G(d). The free energies of solvation were included in the calculation by using the free‐energy perturbation method based on Monte Carlo simulation, that is, the quantum mechanical/Monte Carlo/free‐energy perturbation method. Three different models, incorporating no‐water, one‐water, and two‐waters, were adopted. The results showed that in the gas phase the addition of water molecules to the reaction mechanism caused the activation barriers (ΔG^?_gas) to decrease by half relative to the water‐free mechanism, but there was no effect on the relative difference in free energy, ΔG_gas. The solvation effects (ΔG_sol), based on quantum mechanical/Monte Carlo/free‐energy perturbation calculations, were added to those of the gas‐phase results of the one‐water and two‐waters models. The two‐waters model produced values that were very consistent with the experimental data for all of the tautomers. The differences in the relative Gibbs free energy (ΔG_rxn) were less than 1.0 kcal mol^–1. In summary, the inclusion of solvent molecules in gas‐phase calculations plays a very important role in producing results consistent with experimental data. Copyright © 2012 John Wiley & Sons, Ltd.     

Novel Pyrazoline-Based Selective Fluorescent Sensor for Hg2+

This paper presents the preparation of a pyrazoline compound and the properties of its UV–Vis absorption and fluorescence emission. Moreover, this compound can be used to determine Hg²⁺ ion with selectivity and sensitivity in the EtOH:H₂O?=?9:1 (v/v) solution. This sensor forms a 1:1 complex with Hg²⁺ and shows a fluorescent enhancement with good tolerance of other metal ions. This sensor is very sensitive with fluorometric detection limit of 3.85?×?10^?10 M. In addition, the fluorescent probe has practical application in cells imaging.     

A highly Selective Fluorescent Sensor for Monitoring Cu<Superscript>2+</Superscript> Ion: Synthesis,Characterization and Photophysical Properties

A new fluorescent sensor, 4-allylamine-N-(N-salicylidene)-1,8-naphthalimide (1), anchoring a naphthalimide moiety as fluorophore and a Schiff base group as receptor, was synthesized and characterized. The photophysical properties of sensor 1 were conducted in organic solvents of different polarities. Our study revealed that, depending on the solvent polarity, the fluorescence quantum yields varied from 0.59 to 0.89. The fluorescent activity of the sensor was monitored and the sensor was consequently applied for the detection of Cu²⁺ with high selectivity over various metal ions by fluorescence quenching in Tris-HCl (pH = 7.2) buffer/DMF (1:1, v/v) solution. From the binding stoichiometry, it was indicated that a 1:1 complex was formed between Cu²⁺ and the sensor 1. The fluorescence intensity was linear with Cu²⁺ in the concentration range 0.5–5 μM. Moreso, the detection limit was calculated to be 0.32 μM, which is sufficiently low for good sensitivity of Cu²⁺ ion. The binding mode was due to the intramolecular charge transfer (ICT) and the coordination of Cu²⁺ with C = N and hydroxyl oxygen groups of the sensor 1. The sensor proved effective for Cu²⁺ monitoring in real water samples with recovery rates of 95–112.6 % obtained.     

Dithiane Based Boronic Acid as a Carbohydrate Sensor in an Aqueous Solution at pH 7.5: Theoretical and Experimental Approach

Carbohydrate sensing in an aqueous solution remains a very challenging area of interest. Using the idea of covalent reversible interaction between boronic acids and the diol groups in carbohydrates enable us to design a carbohydrate sensor 1-thianthrenylboronic acid (1T), which has high selectivity towards fructose. To elucidate the sensing and binding properties of 1T with sugars, we have incorporated theoretical (DFT and TD-DFT) and spectroscopic techniques. For an optimized geometry, the complete vibrational assignments were done with FT-IR and FT-Raman spectra. Physiochemical parameters were obtained by implementing frontier molecular orbital (FMO) analysis. Further, excited state properties were determined by performing TD-DFT calculations in solvent and these properties were in good agreement with the experiment. The steady state fluorescence measurements with varying concentration of sugars, revealed that the fluorescence intensity of boronic acid is enhanced by studied sugars due to the structural modification. We also noticed remarkable changes in fluorescence lifetimes and quantum yield after adding sugars. The article also reports influence of pH on boronic acid’s fluorescence intensity with and without sugars. The fluorescence of boronic acid increases with the increase in pH. These changes are due to acid–base equilibrium of boronic acid and led us to estimate the pKa value of 7.6. All the theoretical and experimental evidences suggested that 1T can be used as a possible fluorescent sensor for fructose. In addition, 1T showed very good affinity for Cu²⁺ ion with K_a?=?150?×?10² M^?1, which suggests that 1T can also be used as a chemosensor for Cu²⁺ ions.

     

Study of the Interaction Between Apis mellifera Venom and Micro-Heterogeneous Systems

The bee venom, used in treatment of inflammatory and articular diseases, is a complex mixture of peptides and enzymes and the presence of tryptophan allows the investigation by fluorescence techniques. Steady state and time-resolved fluorescence spectroscopy were used to study the interaction between bee venom extracted from Apis mellifera and three micro heterogeneous systems: sodium dodecylsulphate (SDS) micelles, sodium dodecylsulphate-poly(ethylene oxide) (SDS-PEO) aggregates, and the polymeric micelles LUTROL^® F127, formed by poly(ethylene oxide)-poly(propylene oxide)- poly(ethylene oxide). Fluorescence parameters in buffer solution were typical of peptides containing tryptophan exposed to the aqueous medium, and they gradually changed upon the addition of surfactant and polymeric micelles, demonstrating the interaction of the peptides with the micro heterogeneous systems. Quenching experiments were carried out using the N-alkylpyridinium ions (ethyl, hexyl, and dodecyl) as quenchers. In buffer solution the quenching has low efficiency and is independent of the alkyl chain length of the quencher. In the presence of the micro heterogeneous systems the extent of static and dynamic quenching enhanced, showing that both fluorophore and quenchers reside in the microvolume of the aggregates. The more hydrophobic quencher (dodecyl pyridinium ion) provides higher values for K _SV and dynamic quenching constants, and SDS-PEO aggregates are most efficient to promote interaction between peptides and alkyl pyridinium ions. The results proved that bee venon interacts with drug delivery micelles of the copolymer LUTROL^® F127.     

Analysis of Anomalous Swelling by a Rubber Elasticity Model for Entangled Networks

A complete equation of the swelling activity parameter (S) as a function of swelling deformation (λ_s ) is derived by using a non-Gaussian elastic network model, including a tube concept and the Flory–Rehner model, and by following McKenna's criterion that takes into account the disparity between the Flory–Huggins interaction parameters for cross-linked and uncross-linked polymers. However, only a part of tube at the network chain size scale is extended to that for a large-scale structure according to the “gel tensile blob” model for equilibrium swollen networks. This approach is basically best for the “ideal regular network + simple structured good-solvent” binary systems due to its model character. As a result, it reproduces well the literature data of S versus λ_s ² with a maximum/inflection measured for such actual systems.     

Raman fingerprint of the interaction of K+ with the COO− group of zwitterionic alanine

The interaction of K⁺ with the zwitterionic form of alanine (ZAla) is investigated using Raman spectroscopy and density functional theory calculations. The Raman spectra of an aqueous solution of Ala and its mixture with KOH at different molar concentrations [ZAla + xKOH, x = 1–5 M] have been recorded in the spectral region 400–1800 cm⁻¹. The wavenumber position of the band at ~529 cm⁻¹ shows a red shift of 14 cm⁻¹, while the Raman band at ~634 cm⁻¹ shows a blue shift of 10 cm⁻¹ with the increasing x from 1 to 5 M. The intensity ratio I₆₃₄/I₅₂₉ is increased with increasing x, and it could be because of the increase in concentration of the [ZAla + K⁺] complex in the solution. The new Raman band appeared at ~1079 cm⁻¹ in the Raman spectra of [ZAla + xKOH, x = 1–5] complex. To determine the most probable site for the interaction of K⁺ with ZAla, the structures of ZAla and the [ZAla + K⁺] were optimized at B3LYP/6‐311++G(d,p) level of theory. The electrostatic potential calculation carried out for ZAla reveals that the maximum density of electron is lying over COO⁻, and therefore, COO⁻ would be the most probable site for the interaction of K⁺ with ZAla. The theoretically calculated Raman spectra of ZAla, [ZAla + K⁺] and the [ZAla⁻ + K⁺] are in good agreement with experimentally observed Raman spectra. Thus, the Raman bands at ~529, 634, and 1079 cm⁻¹ may be used as the Raman fingerprint for the interaction of K⁺ with COO⁻ of the ZAla and ZAla⁻. Copyright © 2015 John Wiley & Sons, Ltd.     

UV‐induced transformations of matrix‐isolated 1,3,4‐thiadiazole‐2‐thiones

Monomers of 5‐mercapto‐1,3,4‐thiadiazole‐2‐thione (bismuthiol) were studied using an experimental matrix‐isolation technique as well as by carrying out theoretical quantum chemical calculations. The calculations, performed using the quadratic configuration interaction method with single and double excitations (QCISD)/6‐31++G(d,p)//DFT(B3LYP)/6‐311++G(2d,p), predict that the thione–thiol tautomer of bismuthiol should be significantly (by more than 19 kJ mol^?1) more stable than other tautomeric forms. Accordingly, only the signatures of the thione–thiol tautomer were observed in the FT‐IR spectrum of bismuthiol, recorded directly after deposition of an Ar matrix. UV (λ > 320 nm) irradiation induced the conversion of the thione–thiol tautomer into the dithiol form. Analogous investigations were carried out for two related compounds: 5‐methyl‐1,3,4‐thiadiazole‐2‐thione and 5‐methylthio‐1,3,4‐thiadiazole‐2‐thione. For these two species, only the thione tautomeric forms were observed after deposition of Ar matrices. These tautomers were predicted (by QCISD calculations) to be more stable (by at least 19 kJ mol^?1) than other tautomeric forms. Upon UV irradiation, the most stable thione forms of these compounds were transformed into the corresponding thiol tautomers. Direct observation of the thione → thiol phototautomeric processes provides a clear proof that intramolecular proton transfer reaction can occur in molecules, such as bismuthiol, in spite of the increased NH···S distance, in comparison to other phototautomerizing species studied so far. All the isomers of the studied compounds (substrates and products of the photoreactions) were identified by comparison of their IR spectra with the spectra calculated at the DFT(B3LYP)/6‐311++G(2d,p) level of theory for possible isomeric structures. Copyright © 2009 John Wiley & Sons, Ltd.     

Improvement of thermal and spectroscopic behavior of Er<Superscript>3+</Superscript>/Ce<Superscript>3+</Superscript> co-doped tellurite glass for lasing materials

Tellurite glasses (TeO₂–ZnO–Nb₂O₅) mono-doped Er³⁺ and co-doped Er³⁺/Ce³⁺ have been prepared using the melt-quenching technique. To evaluate the effect of Ce³⁺ on the structural, thermal stability of glass hosts and fluorescence properties of Er³⁺, X-ray diffraction patterns, Ftir spectra, differential scanning calorimeter curves, absorption spectra, fluorescence emission spectra, fluorescence lifetimes, up-conversion emission spectra of glass samples were measured and investigated. Using Judd–Ofelt theory, we calculated intensity parameters (Ω₂, Ω₄ and Ω₆), spontaneous emission probabilities, the radiative lifetime, luminescence branching factors and the quantum yield of luminescence for ⁴I_13/2 → ⁴I_15/2 transition. The co-doping with Ce³⁺ was effective on the suppression of up-conversion emission of Er³⁺ owing to the phonon-assisted energy transfer: Er³⁺:⁴I_11/2 + Ce³⁺:²F_5/2 → Er³⁺:⁴I_13/2 + Ce³⁺:²F_7/2 which contributed the effective enhancement of 1.53 µm fluorescence emission. The change in optical properties with the addition of Ce³⁺ ions have been discussed and compared with other glasses. Using the Mc Cumber method for the ⁴I_13/2 → ⁴I_15/2 transition, absorption cross-section, calculated emission cross-section, and gain cross-section values support that TZNEr1Ce1 glass is a potential material for developing broad-band and high-gain erbium-doped fiber amplifiers applied for 1.53 µm.     

A Novel Quinazolinone Derivative as Fluorescence Quenching Off-On Sensor for High Selectivity of Fe3+

A novel quinazolinone compound containing quinazoline-fused moiety has been synthesized as fluorescence Off-On sensor QQ. The probe exhibited highly selective and sensitive recognition toward trivalent ferric ion (Fe³⁺) over other metal ions in HEPES buffer solution (10 mM, pH?=?7.0, DMF-H₂O, 9:1, v/v). The significant quenching in the fluorescence spectral could be served as a selective fluorescence Off-On sensor. The titration study indicated the formation of 1:1 complex between QQ and Fe³⁺.     

Steady‐State and Time‐Resolved Emission Studies of 6‐Methoxy Quinoline

Abstract

Steady‐state absorption, fluorescence excitation, and emission spectra of 6‐methoxy quinoline (6‐MQ) were measured at room temperature in cyclohexane, dioxane, ethanol, acetonitrile, water, and water–dioxane solvents. Absorption spectra of cyclohexane, n‐hexane, and isopentane solutions show resolved vibronic structure at room temperature. However, the excitation spectrum of cyclohexane solution is structureless and is found to be emission wavelength dependent, indicating the formation of at least two distinct species in the ground state. Similar behavior was observed in dioxane and water–dioxane solutions. For all other solutions, the fluorescence excitation spectrum of 6‐MQ was found to be the same for different emissions. Emission of 6‐MQ in all solvents consisted of two bands with their maxima around 355 nm (I) and 430 nm (II), the actual positions and the relative intensities being dependent on the solvent used. The bands I and II were respectively attributed to normal and protonated/H‐bonded species of either ¹L_a or ¹L_b states or mixed (¹L_a/¹L_b) state of ππ* character. Fluorescence decay of this dye in all solvents monitored over each emission maximum showed biexponential behavior, and the analysis yielded two different lifetime components for each emission band. The short and long fluorescence decay components were respectively in the range of 0.30–3.00 ns and 18–20 ns. The observed emission characteristics coupled with the nature of the fluorescence polarization spectra and two different decay components for each emission suggest the existence of two different conformers having two different excited electronic states.     

Kinetics and thermodynamics of reversible disproportionation–comproportionation in redox triad oxoammonium cations – nitroxyl radicals – hydroxylamines

Kinetics and equilibrium of the acid‐catalyzed disproportionation of cyclic nitroxyl radicals R₂NO^? to oxoammonium cations R₂NO⁺ and hydroxylamines R₂NOH is defined by redox and acid–base properties of these compounds. In a recent work (J. Phys. Org. Chem. 2014, 27, 114‐120), we showed that the kinetic stability of R₂NO^? in acidic media depends on the basicity of the nitroxyl group. Here, we examined the kinetics of the reverse comproportionation reaction of R₂NO⁺ and R₂NOH to R₂NO^? and found that increasing in –I‐effects of substituents greatly reduces the overall equilibrium constant of the reaction K₄. This occurs because of both the increase of acidity constants of hydroxyammonium cations K_3H+ and the difference between the reduction potentials of oxoammonium cations E_R2NO+/R2NO? and nitroxyl radicals E_R2NO?/R2NOH. pH dependences of reduction potentials of nitroxyl radicals to hydroxylamines E_1/3Σ and bond dissociation energies D(O–H) for hydroxylamines R₂NOH in water were determined. For a wide variety of piperidine‐ and pyrrolidine‐1‐oxyls values of pK_3H+ and E_R2NO+/R2NO? correlate with each other, as well as with the equilibrium constants K₄ and the inductive substituent constants σ_I. The correlations obtained allow prediction of the acid–base and redox characteristics of redox triads R₂NO^?–R₂NO⁺–R₂NOH. Copyright © 2014 John Wiley & Sons, Ltd.     

Fluorescence of N,N-di(2-carboxyethyl)-<Emphasis Type="Italic">p</Emphasis>-anisidine in solution and crystalline states

The fluorescence properties of N,N-di(2-carboxyethyl)-p-anisidine (I) in solvents of various nature and in the crystalline state have been studied at room temperature (273 K) and at the boiling point of liquid nitrogen (77 K). Fluorescence in aqueous solutions of I with protonated (λ _ex /λ _fl ^max = 225/290 nm) and unprotonated (λ _ex /λ _fl ^max = 270/380 nm) amino nitrogen has been detected. On going from aqueous solutions to nonaqueous, the fluorescence band of unprotonated I experiences a blue shift and its intensity rises. The fluorescence intensity of the band in aprotic polar solvents is higher than that in protic solvents. A linear dependence of the fluorescence intensity of deprotonated I on Cu(II) concentration (ranging from 1.0 to 5.0 mg/dm³) in aqueous solution has been found. The fluorescence intensity of I in aqueous solutions at 77 K and pH 1–6 has been shown to increase in the presence of Zn(II) (1–170 mg/dm³) and Cd(II) (2–330 mg/dm³) although a similar dependence is not observed at 293 K.     

Radiative lifetimes of 7d, 8d <Superscript>1</Superscript>D<Subscript>2</Subscript> excited states of Hg I

Radiative lifetimes of 7d, 8d ¹ D ₂ excited states of Hg I are measured using pulsed two-photon excitation from the ground [Xe]5d ¹⁰6s ^{2 1} S ₀ mercury state, detecting the decay of the laser-induced fluorescence. The results are compared with theoretical values, obtained by means of a Hartree-Fock single configuration method, taking into account electron configuration interaction. The radiative lifetime value dependence on the effective principal quantum number for the nd ¹ D ₂ series is analyzed and compared with the quantum defect dependence. Received 25 February 2000 and Received in final form 26 July 2000     

Conformational analysis of 4,4‐dimethyl‐1‐(trifluoromethylsulfonyl)‐1,4‐azasilinane and 2,2,6,6‐tetramethyl‐4‐(trifluoromethylsulfonyl)‐1,4,2,6‐oxazadisilinane

4,4‐Dimethyl‐1‐(trifluoromethylsulfonyl)‐1,4‐azasilinane 1 and 2,2,6,6‐tetramethyl‐4‐(trifluoromethylsulfonyl)‐1,4,2,6‐oxazadisilinane 2 were studied by variable temperature dynamic ¹H, ¹³C, ¹⁹F NMR spectroscopy and theoretical calculations at the DFT (density functional theory) and MP2 (Møller‐Plesset 2) levels of theory. Both kinetic (barriers to ring inversion) and thermodynamic data (frozen conformational equilibria) could be obtained for the two compounds. The computations revealed two minima on the potential energy surface for molecules 1 and 2 corresponding to the rotamers with the CF₃SO₂ group directed ‘inward’ and ‘outward’ the ring, the latter being 0.2–0.4 kcal/mol (for 1 ) and 1.1 kcal/mol (for 2 ) more stable than the former. The vibrational calculations at the DFT and MP2 levels of theory give the values of the free energy difference ΔG^o for the ‘inward’ ‘outward’ equilibrium consistent with those determined from the experimentally measured ratio of the rotamers. The structure of crystalline compound 2 was ascertained by X‐ray diffraction analysis. Copyright © 2011 John Wiley & Sons, Ltd.     

A Fiber-Optic Evanescent Wave O<Subscript>2</Subscript> Sensor Based on Ru(II)-Doped Fluorinated ORMOSILs

A novel fiber-optic evanescent wave sensor (FOEWS) for O₂ detection based on [Ru(bpy)₃]²⁺-doped hybrid fluorinated ORMOSILs (organically modified silicates) has been developed. The sensing element was fabricated by dip-coating the optical fiber with [Ru(bpy)₃]²⁺-doped hybrid fluorinated ORMOSILs composed of n-propyltrimethoxysilane (n-propyl-TriMOS) and 3, 3, 3-trifluoropropyltrimethoxysilane (TFP–TriMOS). Fluorophores of [Ru(bpy)₃]²⁺ were excited by the evanescent wave field produced on the fiber core surface and the emission fluorescence was quenched by O₂. Spectroscopic properties have been characterized by FTIR and UV–VIS absorption measurements. By using the presented hybrid fluorinated ORMOSILs, which enhances the coating surface hydrophobicity, the quenching response is increased. The sensitivity of the sensor is 7.5, which is quantified in terms of the ratio I _N2/I _O2 (I _N2 and I _O2 represent the fluorescence intensities in pure N₂ and pure O₂ environments, respectively). The limit of detection (L.O.D.) is 0.01% (3σ) and the response time is about 1 s. Meanwhile, the proposed FOEWS has the advantages of easy fabrication, low cost, fast response and suitable sensitivity for oxygen monitoring using a cheap blue LED as light source and coupling a miniature PMT detector directly to the optical fiber probe.