Fluorescence quenching of 3,7-diamino-2,8-dimethyl-5-phenyl phenazinium chloride by AgCl and Ag nanoparticles

Quenching of fluorescence of the dye 3,7-diamino-2,8-dimethyl-5-phenyl Phenazinium Chloride (Safranine T) has been investigated by AgCl nanoparticles in the W/O microemulsion medium at different [H₂O]/[AOT] ratios (ω) and with Ag nanoparticles and Ag⁺ in aqueous medium. A simple straightforward method has been introduced to prepare AgCl nanoparticles in well-characterized, monodispersed biomimicking nanocavities formed by sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in heptane. Experimental results reveal that the size of the AgCl nanoparticles increases with increase in hydration. The results of the quenching experiment were analysed in the light of Stern Volmer equation. Quenching of fluorescence of the dye has been found to decrease with decrease in the size of the nanoparticles of AgCl and the variation of Stern Volmer quenching constants (K_SV) with particle size is different for two different size regimes.     

Determining the mechanism of interaction between molecules of porphyrin and fullerene and gold nanoparticles,based on luminescence spectroscopy data

Gold nanoparticles are synthesized via laser ablation of a gold target in a liquid. The constants that characterize the efficiency of porphyrins and fullerenes bonding with gold nanoparticles are determined using a modified Stern–Volmer equation. The results from luminescence quenching measurements are presented. It is found that the efficiency of bonding depends on whether there are functional groups in the molecular fragments. Porphyrin containing para-bromphenyl groups at the meso positions of the porphyrin core has the highest affinity for the surfaces of gold nanoparticles.     

Photoinduced relaxation processes in self-assembling complexes from CdSe/ZnS water-soluble nanocrystals and cationic porphyrins

Particular features and quenching mechanisms of exciton luminescence of water-soluble nanocomposites that are formed as a result of the interaction of surface charged semiconductor quantum dots (QDs) CdSe/ZnS (d _CdSe = 2.8 nm) and cationic porphyrins (H₂TMPyrP⁴⁺ and ZnTMPyrP⁴⁺) have been studied theoretically and experimentally. It has been found that, in CdSe/ZnS??Porphyrin conjugates, there occurs long-range inductive resonance electronic excitation energy transfer from surface modified (with thioglycolic or mercaptoundecanoic acid) QDs to porphyrins, which is accompanied by quenching of the exciton luminescence of QDs and an increase in the fluorescence intensity of porphyrin. It has been shown that, when mercaptoundecanoic acid is used as a QD shell, the QD luminescence quenching efficiency by porphyrins follows the F?rster-Galanin theory and depends on the overlap integral between the CdSe/ZnS luminescence band and the absorption spectra of free-base porphyrin H₂TMPyrP⁴⁺ and its metal complex ZnTMPyrP⁴⁺. It has been revealed that, as the QDs ? Zn-porphyrin intercenter distance decreases from 39.1 (mercaptoundecanoic acid) to 30.1), a considerable QD luminescence quenching is observed; however, the energy transfer efficiency substantially decreases, from 55% in the former case to 23% in the latter one. Based on the spectral-luminescent data and quantum-chemical calculations, it has been found that the chemical change of H₂TMPyrP⁴⁺ in the structure of the complex with CdSe/ZnS QDs passivated by thioglycolic or mercaptoundecanoic acid is caused by the formation of a metal complex ZnTMPyrP⁴⁺. Based on calculations of the redox-potentials, it has been concluded that the low luminescence quantum yield of CdSe/ZnS QDs passivated by residues of mercaptocarboxylic acids S^?(CH₂) _n COO^? and its dependence on the number of CH₂ groups are related to the possibility of photoinduced electron transfer from the HOMO of passivating molecules to QDs (QD* ? S^?(CH₂)nCOO^? hole transfer). It has been shown that the quenching of the exciton luminescence of QDs in heterogeneous structures CdSe/ZnS(thioglycolic acid)??ZnTMPyrP⁴⁺, which is complementary to the energy transfer, can be caused by the photoinduced electron transfer that involves the participation of the LUMO of the ZnTMPyrP⁴⁺ molecule (QD* ? ZnTMPyrP⁴⁺).     

Quenching of liquid scintillator fluorescence by chloroalkanes and chloroalkenes

The fluorescence quenching of 2,5-diphenyloxazole (PPO) by a series of chloroalkanes and chloroalkenes including carbon tetrachloride, chloroform, dichloroethane, tetrachloroethane, dichloroethylene, trichloroethylene and tetrachloroethylene was studied in toluene as solvent at room temperature. CCl₄ was found to be the most efficient quencher in the series. The quenching was found to be appreciable and a positive deviation from linearity was observed in the Stern–Volmer (SV) plots for all the quenchers in the concentration range studied. From the studies of effect of temperature, solvent viscosity and excitation wavelength dependence for the PPO–CCl₄ system, it was inferred that non-linearity is due to the presence of a minor static quenching component in an overall dynamic quenching. The static (K_S) and the dynamic (K_D) quenching constants were calculated from the modified SV equation using quadratic least square fits. Fluorescence quenching experiments with CCl₄ were done for four other scintillators (POPOP, α-NPO, BBO and PBBO). The mechanism of quenching was established to be via charge-transfer, with the direction of transfer being from the scintillators to the chloroalkanes and chloroalkenes.     

Fluorescence Quenching Studies of 1,3‐Diphenyl Benzene

Fluorescence quenching of 1,3‐diphenyl benzene (m‐terphenyl) by carbon tetrachloride (CCl₄) at steady state in different solvents, namely n‐hexane, n‐heptane, cyclohexane, toluene, benzene acetonitrile, 1,4‐dioxane, and with a transient method in benzene has been done at room temperature to understand the role of quenching mechanisms. The Stern–Volmer plot was found to be linear for all the solvents studied. The probability of quenching per encounter p was determined in all the solvents and was found to be less than unity. Further, from the studies of rate parameters and lifetime measurements in benzene at different temperatures (30–60°C), it was shown that the phenomenon of quenching is generally governed by the well‐known Stern–Volmer (S‐V) plot. The activation energy E _a (E′_a) of quenching was determined using literature values of activation energy of diffusion E _d, and it was found to be greater than E _d, which confirms the fact that the quenching mechanism is not solely due to material diffusion but there is also contribution from activation energy.     

Gas Phase Oxygen Quenching Studies of Ketone Tracers for Laser-Induced Fluorescence Applications in Nitrogen Bath Gas

In this paper we report the quantitative oxygen quenching effect on laser-induced fluorescence of acetone, methyl ethyl ketone, and 3-pentanone at low pressures (～700 torr) with oxygen partial pressures up to 450 torr. Nitrogen was used as a bath gas in which these molecular tracers were added in different quantities according to their vapor pressure at room temperature. These tracers were excited by using a frequency-quadrupled, Q-switched, Nd:YAG laser (266 nm). Stern–Volmer plots were found to be linear for all the tracers, suggesting that quenching is collisional in nature. Stern–Volmer coefficients (k_sv) and quenching rate constants (k_q) were calculated from Stern–Volmer plots. The effects of oxygen on the laser-induced fluorescence of acetone, methyl ethyl ketone, and 3-pentanone were compared with each other. Further, the Smoluchowski theory was used to calculate the quenching parameters and compared with the experimental results.     

Investigations on structural and optical properties of Zn1−xGdxS nanoparticles

Zn_1−xGd_xS (x = 0.00, 0.02 and 0.04) nanoparticles were synthesized by facile chemical co-precipitation method using PVP as a surfactant. ZnS nanoparticles could be doped with Gd ions during synthesis without altering the XRD patterns of ZnS. Also, the pattern of the powders showed cubic zincblende structure. The particle size obtained from the XRD studies lies in the range 3-5 nm, whereas from TEM analysis it is 4 nm for x = 0.02 sample. The UV-Vis absorption spectra revealed that Zn_1−xGd_xS nanoparticles exhibit strong confinement effect as the blue shift in the absorption spectra with that of the undoped ZnS. The photoluminescence spectra showed enhanced luminescence intensity and the entry of Gd into host lattice.     

Spectroscopic studies on the interaction between ZnSe nanoparticles with bovine serum albumin

The interaction between ZnSe nanoparticles (NPs) and bovine serum albumin (BSA) was studied by UV–vis, fluorescence spectroscopic techniques. The results showed that the fluorescence of BSA was strongly quenched by ZnSe NPs and the quenching mechanism was discussed to be a static quenching procedure, which was proved by quenching constant (K_q). The recorded UV–vis data and the fluorescence data quenching by the ZnSe NPs showed that the interaction between them leads to the formation of ZnSe–BSA complex. Based on the synchronous fluorescence spectra, it was established that the conformational change of BSA was induced by the interaction of ZnSe with the tyrosine micro-region of the BSA molecules. Furthermore, the temperature effects on the structural and spectroscopic properties of individual ZnSe NPs and protein and their bioconjugates (ZnSe–BSA) were also researched. It was found that, compared to the monotonic decrease of the individual ZnSe NPs fluorescence intensity, the temperature dependence of the ZnSe–BSA emission had a much more complex behavior, which was highly sensitive to the conformational changes of the protein.     

Interactions between imazethapyr and bovine serum albumin: Spectrofluorimetric study

The interaction between imazethapyr (IMA) and bovine serum albumin (BSA) was investigated by fluorescence spectroscopy. The Stern–Volmer quenching constant (K_SV) at three temperatures was evaluated in order to determine the quenching mechanism. The dependence of fluorescence quenching on viscosity was also evaluated for this purpose. The results showed that IMA quenches the fluorescence intensity of BSA through a static quenching process. The values of the binding constant for the formed BSA–IMA complex and the number of binding sites were found to be 1.51×10⁵ M^?1 and 0.77, respectively, at room temperature. Based on the calculated thermodynamic parameters, the forces that dominate the binding process are hydrogen bonds and van der Waals forces, and the binding process is spontaneous and exothermic. The quenching of protein fluorescence by iodide ion was used to probe the accessibility of tryptophan residues in BSA and the change in accessibility induced by the presence of IMA. According to the obtained results, the BSA–IMA complex is formed in the site where the Trp-134 is located, causing it to become less exposed to the solvent.     

Study of the Interaction of Fangchinoline with Human Serum Albumin by Fluorescence and UV Spectroscopic Method

The interaction of fangchinoline with human serum albumin (HSA) was studied by use of fluorescence quenching spectra, synchronous fluorescence spectra, and ultraviolet spectra. It was shown that fangchinoline has a strong ability to quench the fluorescence of HSA. The Stern‐Volmer curves based on the quenching of the fluorescence of HSA by fangchinoline indicated that the quenching mechanism of fangchinoline on HSA was static quenching and non‐radiation energy transfer. Based on the Förster theory of non‐radiation energy transfer, the binding distances (r) and the binding constants (K _A) between fangchinoline and HSA were found. The thermodynamic parameters obtained revealed that the interaction between fangchinoline and HSA was mainly driven by hydrophobic force. The conformational changes of HSA were investigated by use of synchronous fluorescence. The result indicates that an ionic electrostatic interaction between fangchinoline and HSA could not be excluded.     

Investigation of three flavonoids binding to bovine serum albumin using molecular fluorescence technique

The three flavonoids including naringenin, hesperetin and apigenin binding to bovine serum albumin (BSA) at pH 7.4 was studied by fluorescence quenching, synchronous fluorescence and UV–vis absorption spectroscopic techniques. The results obtained revealed that naringenin, hesperetin and apigenin strongly quenched the intrinsic fluorescence of BSA. The Stern–Volmer curves suggested that these quenching processes were all static quenching processes. At 291 K, the value and the order of the binding constant were K_{A (naringenin)}=4.08×10⁴<K_{A (hesperetin)}=5.40×10⁴≈K_{A (apigenin)}=5.32×10⁴ L mol^?1. The main binding force between the flavonoid and BSA was hydrophobic and electrostatic force. According to the Förster theory of non-radiation energy transfer, the binding distances (r₀) were obtained as 3.36, 3.47 and 3.30 nm for naringenin–BSA, hesperetin–BSA and apigenin–BSA, respectively. The effect of some common ions such as Fe³⁺, Cu²⁺, Mg²⁺, Mn²⁺, Zn²⁺ and Ca²⁺ on the binding was also studied in detail. The competition binding was also performed. The apparent binding constant (K′_A) obtained suggested that one flavonoid had an obvious effect on the binding of another flavonoid to protein when they coexisted in BSA solution.     

On the nature of optically active centres in selfactivated luminophors of the CaWO4 type

The evidence in favour of the assumption that impurities of V group elements creates defects of the MeO′₄ type (centres of luminescence), Me?₃ and V_?, or WO₃ (capture centres) which are responsible for recombination afterglow different from the steady-state luminescence spectral distribution (Me = As, Sb, Nb, Ta; V₀ - oxygen vacancy) are presented. These same impurities, and phosphorous, lead to quenching of luminescence observed as afterglow with excitation quanta greater than 6.3 eV. This corresponds to the valued of the energy gap. Experimental data together with results of thermodynamic analysis lead to the conclusion that the luminescence of CaWO₄ and of other undoped oxygen containing compounds of transition elements luminophors is caused by direct self-activation connected with ability of these elements to convert spontaneously into a lower valency state and to form variable phases (non-stoichiometric compounds). It is proposed that in case of CaWO₄ centres of luminescence are formed by W⁵⁺.     

Investigation on trap distribution and photoelectronic effect due to UV,IR and visible light excitation in self-activated zns crystals

Photoelectronic effects in IR sensitive ZnS crystals are found for IR, UV and visible light excitations. The transient rise characteristic of UV excited photoluminescence (UPL) saturates faster than UV produced photoconductivity (UPC). The UPC shows a typical S-shaped rise curve for any 365 run excitation irradiance and temperature. Simultaneously measured transient behaviors of IR induced photoconductivity (INP) and IR stimulated luminescence (STL) have a strong IR excitation intensity dependence for λ_IR = 2.5?4.6 microm. A unique phenomenon, quick rise followed by quick decay during the initial 20 msec, is found in INP before reaching final maximum but is not observed in STL. During UV steady irradiation, additional photoconductivity and luminescence are quickly induced by an abrupt IR excitation of 2.5 microm. Then, the photoconductivity reaches a new steady state level below UPC steady state. However, the luminescence sets the same UPL steady state level. This means that photoconductivity exhibits an IR optical quenching but not stimulation, while neither optical quenching nor stimulation is found in luminescence. It is also possible to quench the UPC response with visible light excitation at 570 nm. These observations support the previously reported discussion that IR absorbing impurity centers and shallow traps, including recombination (or luminescence) centers must be involved in IR stimulable ZnS crystals. They also indicate the presence of deep trap centers for E_T ≈ 2.18 eV, which have a strong role in slow UPC rise.     

双核铼配合物复合电纺纤维的制备和氧传感性能的研究

合成了亲脂性双核铼配合物fac-[{Re(CO)₃(d19 phen)}₂(4,4′ bipyridylacetylene)](OTf_⁾2^,将此配合物均匀地分散到聚苯乙烯(PS)和聚甲基丙烯酸甲酯(PMMA)中,通过电纺方法,得到复合材料纤维,并系统地研究了它们的氧传感性能。分别用线性Stern Volmer模型,Demas双格位模型以及Lehrer模型对所得到的Stern Volmer曲线进行了拟合,实验结果表明,这种复合纤维氧传感材料显示了短的响应时间,可逆的氧传感信号,而且由于含有亲脂性的长链烷烃配体的存在,使得铼配合物在聚合物中分散均匀,从而得到线性关系较好的Stern Volmer曲线。     

Energy transfer from the singlet levels of diketones and dyes to lanthanide ions in nanoparticles consisting of their diketonate complexes

The energy transfer from the S ₁ levels of p-phenylbenzoyltrifluoroacetone (PhBTA) and dyes to different Ln³⁺ ions is studied in nanoparticles (NPs) composed of complexes of this diketone with Ln³⁺ and 1,10-phenanthroline (phen) and doped with dye molecules. The quenching rate constants in the NPs consisting from complexes of Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Ho³⁺, Er³⁺, and Tm³⁺ are determined from the data on the quenching of sensitized (cofluorescence) and ordinary fluorescence of coumarin 30 (C30) and rhodamine 6G (R6G). The quenching rate constants vary from ≤5 × 10¹¹ to 10¹³ s^?1 for the fluorescence quenching of PhBTA by different Ln³⁺ ions, while the quenching of dye fluorescence occurs at rates of the order of 10⁹ s^?1. In the case of complexes with the Pr³⁺ ions, the fluorescence quenching of PhBTA in NPs composed of its complexes is accompanied by sensitized luminescence of Pr³⁺. The quenching observed is due to a nonradiative energy transfer from the S ₁ states of ligands and dyes to these ions. It is shown that in NPs composed of complexes with Eu³⁺, Yb³⁺, and Sm³⁺ the cofluorescence of C30 is quenched via the electron-transfer mechanism. The study of quenching of cofluorescence and fluorescence of dyes in NPs composed of mixed complexes of La³⁺ and Nd³⁺ (Ho³⁺) shows that the observed quenching of fluorescence and cofluorescence is governed mainly by the quenching of the S ₁ state of dyes when the Nd³⁺ (Ho³⁺) content does not exceed 5–10% and by the quenching of the S ₁ state of a ligand when the Nd³⁺ (Ho³⁺) content exceeds 50%. It is assumed that the high rate constant of energy transfer from the S ₁ level of ligands to ions Pr³⁺, Nd³⁺, Ho³⁺, Er³⁺, and Tm³⁺ in NPs composed of beta-diketonate complexes is caused by exchange interactions.     

Luminescence of nanocrystalline ZnS:Cu

Temperature dependent luminescence and luminescence lifetime measurements are reported for nanocrystalline ZnS:Cu²⁺ particles. Based on the variation of the emission wavelength as a function of particle size (between 3.1 and 7.4 nm) and the low quenching temperature (T_q=135 K), the green emission band is assigned to recombination of an electron in a shallow trap and Cu²⁺. The reduction in lifetime of the green emission (from 20 μs at 4 K to 0.5 μs at 300 K) follows the temperature quenching of the emission. In addition to the green luminescence, a red emission band, previously only reported for bulk ZnS:Cu²⁺, is observed. The red emission is assigned to recombination of a deeply trapped electron and Cu²⁺. The lifetime of the red emission is longer (about 40 μs at 4 K) and the quenching temperature is higher.     

Electrical behavior of amorphous indium–gallium–zinc oxide thin film transistors by embedding Au nanoparticles in the channel layer

We reported the effects on the electrical behavior of amorphous indium–gallium–zinc oxide (a-IGZO) thin film transistors (TFTs) after introducing various positions and sizes of Au nanoparticles (NPs) in the channel layer. These TFTs showed an off-current increase and threshold voltage (V_th) shift compared to conventional a-IGZO TFTs. The effects of Au NPs are explained to form the carrier conduction path which causes the current leakage in the channel layer, and act as either electron injection sites or trap sites. Therefore, this study demonstrates that the optimized control of size and position of Au NPs in the channel layer is crucial for its application in the electrical stability improvement and V_th control of a-IGZO TFTs.     

Solvent effect on the fluorescence quenching of biologically active carboxamide by aniline and carbon tetrachloride in different solvents using S–V plots

The fluorescence quenching studies of carboxamide namely (E)-N-(3-Chlorophenyl)-2-(3,4,5-trimethoxybenzylideneamino)-4,5,6,7 tetrahydrobenzo[b]thiophene-3-carboxamide [ENCTTTC] by aniline and carbon tetrachloride in six different solvents namely toluene, cyclohexane, n-hexane, n-heptane, n-decane and n-pentane have been carried out at room temperature with a view to understand the quenching mechanisms. The Stern–Volmer (S–V) plots have been found to be non-linear with a positive deviation for all the solvents studied. In order to interpret these results we have invoked the ground state complex formation and sphere of action static quenching models. Using these models various quenching rate parameters have been determined. The magnitudes of these parameters suggest that sphere of action static quenching model agrees well with the experimental results. Hence the positive deviation is attributed to the static and dynamic quenching. Further, with the use of Finite Sink approximation model, it was possible to check these bimolecular reactions as diffusion-limited and to estimate independently distance parameter R′ and mutual diffusion coefficient D. Finally an effort has been made to correlate the values of R′ and D with the values of the encounter distance R and the mutual coefficient D determined using the Edward's empirical relation and Stokes Einstein relation.     

Spectral characteristics and fluorescence quenching of N,N′-bis(4-pyridyl)-3,4:9,10-perylenebis(dicarboximide) (BPPD)

The photophysical properties such as electronic absorption, molar absorptivity, emission spectra, fluorescence quantum yield and fluorescence lifetime of N,N′-bis(4-pyridyl)-3,4:9,10-perylene bis(dicarboximide) (BPPD) have been measured in different solvents. Both electronic absorption and fluorescence spectra are not sensitive to medium polarity, while the fluorescence quantum yield (?_f) is solvent dependent. The ground state geometry has been computed by using density functional theory (DFT), the transition from HOMO to LUMO from perylene core with maximum absorption at 512 nm and HOMO–LUMO energy difference equal 2.53 eV. BPPD dye undergoes molecular aggregation to dimmer or higher aggregates in dimethyl sulfoxide (DMSO). Crystalline solids of BPPD gives excimer-like emission at 676 nm. The fluorescence quenching of BPPD is also studied using hydrated ferric oxide nanoparticle (FeOOH), and the Stern–Volmer rate constants (K_sv) were calculated as 8×10⁶ and 9.2×10⁶ M^?1 in ethanol and ethylene glycol, respectively.     

Towards an identification of chemically different flavin radicals by means of theirg-tensor

Theg-tensors of two chemically different flavin mononucleotide (FMN) radicals, one of which is covalently bound via N(5) of its 7,8-dimethyl isoalloxazine moiety, and the other one non-covalently bound to mutant LOV domains of the blue-light receptor phototropin, LOV1 C57M and LOV2 C450A, respectively, have been determined by very high microwave frequency and high magnetic field electron paramagnetic resonance (EPR) performed at 360 GHz and 12.8 T. Due to the high spectral resolution of the frozen-solution continuous-wave EPR spectra, the anisotropy of theg-tensors could be fully resolved. By least-squares fittings of spectral simulations to expermental data, the principal values ofg have been established:g _X=2.00554(5),g _Y=2.00391(5), andg _Z=2.00247(7) for the N(5)-alkyl-chain-linked FMN radical in LOV1 C57M-675, andg _X=2.00427(5),g _Y=2.00360(5), andg _Z=2.00220(7) for the noncovalently bound FMN radical in LOV2 C450A-605. By a comparison of these values to the ones from the flavin adenine dinucleotide radicals in two photolyases, the radical in LOV2 C450A-605 could be clearly identified as a neutral FMN radical, FMNH. In contrast, LOV1 C57M-675 exhibits significantly shifted principal components ofg, the differences being caused by spin-orbit coupling of the nearby sulfur from the reactive methionine residue, and the modified chemical structure due to the covalent attachment at N(5) of the radical to the apoprotein. The results clearly show the potential of using theg-tensor as probe of the global electronic and chemical structure of protein-bound flavin radicals.