Photon-Counting Phase-Modulation Fluorometer

We propose a phase-modulation fluorometer that is applicable to a very weak fluorescence intensity level. In order to counter the single-photon event situation, we have introduced a combination of a time-to-amplitude converter and a pulse height analyzer (PHA) to the fluorometer, the combination of which is usually used in the single-photon correlation method to measure fluorescence decay waveforms with pulsed excitation. In the proposed fluorometer, a sinusoidal response waveform that is shifted in phase over the reference one is obtained statistically as a histogram in the PHA memory, and then the fluorescence lifetime can be calculated by the same procedure as the conventional analog phase-modulation method. The excitation light source used was a current-modulated ultraviolet light-emitting diode, whose center wavelength was 370 nm and whose spectral bandwidth was 10 nm. Fluorescence lifetimes of 17.5 ns and 5.7 ns obtained for 10 ppb quinine sulfate in 0.1 N H₂SO₄ and for 10 ppb rhodamine 6G in ethanol, respectively, agreed well with those reported in the literature. In passing, we report that the fluorescence lifetime of benzo[a]pyrene in dichloromethane is 14.6 ns, which is known as one of the carcinogenic, environmental-pollution materials.This paper was originally presented at the 2001 International Conference (2nd Joint OSJ-SPIE Conference) on Optical Engineering for Sensing and Nanotechnology, ICOSN 2001 which was held June 6#x2013;8, 2001 at the Pacifico-Yokohama Conference Center, Yokohama, Japan.     

Particular features of manifestation of energy migration toward impurity in nanoparticles of metal complexes

We measure the fluorescence quantum yields (q _fl) of complexes of Al, Sc, Y, In, Lu, and Gd with dibenzoylmethane in aqueous and isopropanol solutions at different concentration ratios of ions and dike-tone. We reveal that, for the examined solutions, qfl of complexes varies more than by two orders of magnitude under the influence of the heavy atom. It is found that a considerable decrease in q _fl and τ_fl of ligands of complexes of listed ions caused by the influence of heavy atoms weakly affects the intensity of sensitized fluorescence of Nile red and rhodamine 6G molecules introduced into nanoparticles from these complexes in aqueous solutions. The revealed result is explained by the comparability of the singlet exciton free path length and the dimension of nanoparticles under study. We show that a lower fluorescence intensity of heavy metal complexes makes it possible to decrease its contribution in the range of the cofluorescence maximum of rhodamine 6G and to monitor the occurrence of the dye in the aqueous solution down to the concentration of 0.05 nM. We show that, in nanoparticles from Eu complexes, further fluorescence quenching of dibenzoylmethane is observed, as well as the appearance of cofluorescence of rhodamine 6G, the intensity of which is comparable with its intensity in nanoparticles of other complexes. The appearance of this cofluorescence cannot be explained by the existence of S-S energy migration.     

On estimating probability of excited-state intramolecular proton transfer

Higher singlet states can play an important role in various intramolecular processes. Recent investigations of the time-resolved (with a picosecond resolution) spectra of the dual fluorescence of 3-hydroxyflavone molecules excited in the region of the S ₁ and S ₂ absorption bands by pulses with a duration of ∼44 ps have directly shown the occurrence of the proton transfer from the carboxyl to the carbonyl group of the molecule upon excitation into the second singlet absorption band. The reaction times estimated from the emission characteristics are comparable with the electronic level lifetime (several picoseconds), as a result of which the direct measurements are rather difficult. The proton transfer through the S ₂ state is also recorded in the steady-state fluorescence excitation spectra. In this study, it is shown how the reaction rate can be estimated from these data.     

The mutual influence of two different dyes on their sensitized fluorescence (cofluorescence) in nanoparticles from complexes

We have studied the fluorescence sensitization and quenching for pairs of different dyes simultaneously incorporated into nanoparticles from complexes M(diketone)₃phen, where M(III) is La(III), Lu(III), or Sc(III); diketone is p-phenylbenzoyltrifluoroacetone (PhBTA) or naphthoyltrifluoroacetone (NTA); and phen is 1,10-phenanthroline. We have shown that, upon formation of nanoparticles in the solution in the presence of two dyes the concentrations of which are either comparable with or lower than the concentration of nanoparticles (<20 nM), the intensities of the sensitized fluorescence of dyes in nanoparticles in binary solutions and in solutions of either of the dyes coincide. We have found that the intensity of sensitized fluorescence of small (<20 nM) concentrations of rhodamine 6G (R6G) or Nile blue (NB) increases by an order of magnitude upon simultaneous introduction into nanoparticles of 1 μM of coumarin 30 (C30), while the intensity of fluorescence of C30 sensitized by complexes decreases by an order of magnitude. The same effect is observed as 1 μM of R6G are introduced into nanoparticles with NB ([NB] ≤ 20 nM). The increase in the fluorescence of dye molecules upon their incorporation from the solution into nanoparticles from complexes is noticeably lower than that expected from the proposed ratio of concentrations of complexes and dyes in nanoparticles. Analysis of the obtained data indicates that the introduction of large concentrations of C30 or R6G dyes into nanoparticles makes it possible to prevent large energy losses due to impurities or upon transition to a triplet state that arises during the migration of the excitation energy over S ₁ levels of complexes. Energy accumulated by these dyes is efficiently transferred to another dye that is present in the solution at lower concentrations and that has a lower-lying S ₁ level, which makes it possible to increase its fluorescence by an order of magnitude upon its incorporation into nanoparticles.     

Specific Features of XeCl*-Laser Radiation Absorption by Concentrated Solutions of Organic Dyes

The transmission of focused laser radiation with power densities in the range (1–250) MW/cm² through solutions of laser dyes (substituted paraterphenyls and rhodamine 6G) is investigated. The dependence of the transmission on the exciting radiation power density is measured, and numerical modeling is used to estimate the lifetime of the fluorescent state of molecules and the influence of stimulated emission on the lifetime of the S ₁ state. The effect of scattering on the transmission of highly absorbing media is demonstrated under nanosecond excitation.     

Sn-state lifetime determination of dyes

The S _n -state lifetime is determined from two-step excited S _n -S₀fluorescence yield measurements with picosecond light pulses. The theoretical analysis includes single pulse and double pulse consecutive excitation and takes into account the anisotropy of excitation and emission. Experimental results of the single pulse two-step excitation technique are presented for the S₄-state lifetimes of the three mode-locking dyes 5, 9740 and 9860 for Nd-glass lasers.     

Columinescence of dye molecules in nanostructures of metal ion complexes

The mechanism of columinescence (fluorescence sensitization) of dyes incorporated in nanostructures of metal complexes is studied. It is shown for the first time that the columinescence of dyes is due to the transfer of excitation energy from ligands and metal ions of complexes that form nanostructures. It is proven that the dye columinescence of rhodamine 6G (R6G) molecules incorporated into nanostructures of Al(DBM)₃phen, Al(DBM) _n (OH)_{6 ? 2n} , and Eu(DBM)₃phen (DBM is dibenzoylmethane) nanostructures is completely determined by the singlet excitation energy migration from ligands to R6G molecules. It is shown that, at small concentrations of R6G, the R6G columinescence intensity is lower in nanostructures of metal complexes with a high probability of S-T conversion and that this difference disappears at large concentrations of R6G. In the case of Nile blue (whose S ₁ level lies below the ⁵ D ₀ level of Eu(III)) incorporated in nanostructures of Eu(DBM)₃phen complexes, as well as in nanostructures of Al(DBM)₃phen and Gd(DBM)₃phen complexes with admixture of Eu complexes, we observed the S-S energy transfer from DBM to NB in addition to the delayed sensitized fluorescence of NB previously observed in nanostructures of Eu complexes, which was caused by the energy transfer from the ⁵ D ₀ level of Eu(III) to NB. At dye concentrations below 100 nM, the efficiency of NB sensitization due to the migration of singlet excitation energy from DBM is lower than in the case of the energy transfer from Eu(III) ions, while, at large concentrations of the dye, the S-S energy transfer successfully competes with the sensitization of NB by Eu(III) ions. The use of dye columinescence makes it possible to easily determine dye concentrations of 2–100 nM in solutions with standard spectrofluorimeters.     

Fermi-type resonance in quasi-linear fluorescence excitation spectra of triazatetrabenzoporphine

The effect of deuteration of the central NH groups on the quasi-linear fluorescence and fluorescence excitation (with selective monitoring) spectra for triazatetrabenzoporphine, a close analog of phthalocyanine, has been investigated at 77K in n-nonane. Vibrational analysis of the spectra was carried out. The normal mode frequencies were determined for the electronic states S₀ (from fluorescence spectra) and S₁ (for fluorescence excitation spectra). It has been established that N-deuteration lowers the frequency of a vibration involving inplane NH bending down to ∼990 cm⁻¹ and leads to resonant vibrational-electronic (vibronic) interaction of Fermi-type resonance between the zero level of the S₂ state and the vibronic level of the S₁ state upon excitation of this mode. Thereby the possibility of the “vibronic analog of Fermi resonance” (a term coined by G. Herzberg) occurring in a simple (two-component) variant of phthalocyanine-type molecules has been shown. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 6, pp. 796–803, November–December, 2008.     

Excitation relaxation and structure of TPPS4 J-aggregates

The energy relaxation kinetics and the structure of the J-aggregates of water-soluble porphyrin 5,10,15,20-tetrasulphonatophenyl porphine (TPPS₄) were investigated in aqueous medium by means of time-resolved fluorescence spectroscopy and confocal laser-scanning fluorescence microscopy. The excitation of the J-aggregates, at excitation intensities higher than ∼10¹⁵ photons/cm² per pulse, results in a remarkable decrease of the fluorescence quantum yield and in the appearance of an additional, non-exponential energy relaxation channel with a decay constant that depends on the excitation intensity. This relaxation mechanism was attributed to the exciton single-singlet annihilation. The exciton lifetime in the absence of the annihilation was calculated to be ∼150 ps. Using exciton annihilation theory, the exciton migration within the J-aggregates could be characterized by determining the exciton diffusion constant (1.8±0.9)  10⁻³ cm²/s and the hopping time (1.2±0.6) ps. Using the experimental data, the size of the J-aggregate could be evaluated and was seen to yield at least 20 TPPS₄ molecules per aggregate. It was shown by means of confocal fluorescence laser scanning microscopy that TPPS₄ does self-associate in polyvinyl alcohol (PVA) at acidic pH forming molecular macro-assemblies on a scale of ∼1 μm in PVA matrices.     

Mode-selective internal conversion of perylene

We observed fluorescence excitation spectra and dispersed fluorescence spectra for single vibronic level excitation of jet-cooled perylene-h ₁₂ and perylene-d ₁₂, and carefully examined the vibrational structures of the S₀ ¹ A _g and S₁ ¹ B _2u states. We performed vibronic assignments on the basis of the results of ab initio calculation, and found that the vibrational energies in the S₁ state are very similar to those in the S₀ state, indicating that the potential energy curves are not changed much upon electronic excitation. We conclude that the small structural change is the main cause of its slow radiationless transition and high fluorescence quantum yield at the zero-vibrational level in the S₁ state. It has been already reported that the lifetime of perylene is remarkably short at specific vibrational levels in the S₁ state. Here, we show that the mode-selective nonradiative process is internal conversion (IC) to the S₀ state, and the ν₁₆(a _g ) in-plane ring deforming vibration is the promoting (doorway) mode in the S₁ state which enhances vibronic coupling with the high-vibrational level (b _2u ) of the S₀ state.     

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.     

Spectral and luminescent properties of jet-cooled dinaphthofuran

The fluorescence and fluorescence excitation spectra of jet-cooled dinaphtho[2,1-b:1′,2′-d]furan (dinaphthofuran) molecules, as well as their complexation with inert gases Ar, Kr, and Xe, are studied. The indicatrices of the degree of polarization of fluorescence of dinaphthofuran molecules upon excitation of the electronic transitions S ₀?S ₁ and S ₀?S ₂ are calculated as functions of the intramolecular orientation of the transition dipole moments. The fluorescence polarization spectrum is measured under excitation within the rotational contour of the line of the purely electronic transition v ₀ ⁰ = 29 294 cm^?1. In contrast to complex planar molecules, the S ₀?S ₂ fluorescence excitation spectrum of dinaphthofuran is found to be continuous, with the Q branch of the rotational contour being absent. The fluorescence excitation spectra of van der Waals complexes of dinaphthofuran with inert gases exhibit multiplet lines, which is associated with the helical structure of the molecules studied.     

Fabrication of silver-coated silicon nanowire arrays for surface-enhanced Raman scattering by galvanic displacement processes

Silver-coated silicon nanowire (SiNW) arrays were prepared utilizing galvanic displacement processes consisting of three steps: galvanic displacement deposition of silver particles using a HF-AgNO₃ or NH₄F-AgNO₃ aqueous solution; formation of SiNW arrays by a silver-assisted chemical etching process conducted in the HF-H₂O₂ aqueous solution; deposition of silver particles on the SiNW arrays from the NH₄F-AgNO₃ aqueous solution. The effects of the morphology of pre-deposited silver particles and deposition solution on the formation of silver-coated SiNW arrays were discussed. Surface-enhanced Raman scattering (SERS) performances have been studied using Rhodamine 6G (R6G) probe molecules on the silver-coated SiNW substrates.     

Nd3+螯合物的含氢有机溶液光谱性能研究

研究了Nd(TTA)₃螯合物溶于二甲基甲酰胺溶剂的光谱性质，溶液中所有氢未置换为氘.测量了这种溶液体系的吸收谱、荧光谱和荧光寿命.在898和1058 nm波长处观察到明显的Nd³⁺荧光特征峰.用Judd-Ofelt理论对吸收谱进行分析计算，得到了三个强度参数Ω_t(t=2,4,6)分别为Ω₂=4.9×10^-20 cm², Ω₄=5.1×10^-20 cm²和Ω₆=2.5×10^-20 cm².利用强度参数计算了⁴F_3/2能级与⁴I_9/2和⁴I_11/2之间的跃迁强度S_ed、自发辐射系数A_ed以及荧光分支比β等，估算了⁴F_3/2能级的辐射跃迁寿命τ_r＝682 μs.实测1058和898 nm波长处荧光寿命τ大约为460和505 μs，因此荧光量子效率分别高达0.67和0.74.荧光量子效率高表明Nd³⁺在这种溶液中无辐射跃迁比较弱；强度参数Ω₂比较大，表明Nd³⁺在溶液中具有不对称配位场环境，不对称的配位场环境可大大促进Nd³⁺吸收激发能量.光谱质量因子Ω₄/Ω₆>1，使得898 nm的辐射强于1058 nm的辐射. 关键词： Nd 有机溶液 光谱性能 Judd-Ofelt理论     

A Streak Camera System for Time Resolved Fluorimetry

Presently, there are several techniques for measurement of fluorescence lifetimes of organic molecules. These techniques, reviewed by Ware,¹ can be divided into two basic groups, those based upon pulsed sources and those based on modulated sources and phase shift measurement. In the pulsed methods, repetitive, short pulse width, intense excitation pulses excite the fluorophor and the fluorescence decay is measured; the source temporal response must be deconvoluced from the fluorescence decay in order to evaluate the fluorescence signal and lifetime. Typical sources of excitation include nanosecond flashlamps and more recently nanosecond N₂-laser (with or without a dye laser) and mode-locked lasers with picosecond pulse widths^2-6. The modulated source phase shift methods,¹ involve sinusoidal excitation of the fluorophor and measurement of the phase shift between the modulated excitation source and the modulated flourescence. Because modulation frequencies are limited to approximately 20 MHz, fluorescence lifetimes are limited to ～.1 ns and above. In addition, in the phase shift methods, only “one point” lifetimes are obtained, i.e., the entire fluorescence decay curve is not obtained.     

Polarized fluorescence and absorption spectroscopy of 1,32-dihydroxy-dotriacontane-bis-rhodamine 101 ester. A new and lipid bilayer-spanning probe

We report on the properties of 1,32-dihydroxy-dotriacontane-bis-rhodamine 101 ester (Rh101C₃₂Rh101) in lipid bilayers of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and in liquid solvents. The results are compared with those of rhodamine 101 octadecanyl ester (Rh101C₁₈). Both molecules are solubilized in the lipid bilayer and the Rh101 moieties are anchored in the lipid-water interface, so that the electronic transition dipole moments (S ₀ S ₁) are oriented preferentially in the plane of the bilayer. At low concentrations of the dyes in lipid bilayers of DOPC, the fluorescence relaxation is single exponential with a lifetime of =4.9±0.2 ns. The relative fluorescence quantum yield of _C32/_C18 0.95 in DOPC vesicles. These results strongly suggest that only a small fraction of the Rh101C₃₂Rh101 molecules are quenched, by, for example, intra- or intermolecular dimers in the ground state at mole fractions of less than 0.1% in the lipid bilayers. For Rh101C₃₂Rh101 in lipid vesicles, the steady-state and time-resolved fluorescence anisotropies are compatible with efficient intramolecular electronic energy transfer. It is concluded that nearly every Rh101C₃₂Rh101 molecule is spanning across the lipid bilayer of DOPC.     

Ru原子荧光及其寿命的测量

本文设计了一个荧光探测系统,采用了电热原子化激光共振激发的方法,可方便地对难熔金属的原子荧光寿命进行测量,从而确定原子能级的寿命,实验中测得Ru原子[4d⁶5s(⁶D)5p]⁷P₄⁰能级的寿命为70.4ns(±5％)。     

Excited-state inter- and intramolecular proton transfer in methyl 3-hydroxy-2-quinoxalinate: effects of solvent and acid or base concentrations

Absorption, fluorescence excitation and fluorescence spectroscopy, combined with time-dependent spectroscopy and semi-empirical (AM1) and density functional theory using Gaussian 98 program calculations have been used to study the effects of solvent and acid or base concentration on the spectral characteristics of methyl 3-hydroxy-2-quinoxalinate (M3HQ). M3HQ is present as enol in less polar solvents and as keto in polar media. In non-polar solvents, large Stokes shifted fluorescence band is assigned to the phototautomer, formed by the excited-state intramolecular proton transfer, whereas fluorescence is only observed from keto in the polar solvents. In aqueous and polar solvents the monocation (MC5/MC6) is formed by protonating the carbonyl oxygen atom in the ground (S₀) and the first excited singlet states (S₁). Dication is formed by protonating one of N- atom of MC5/MC6. Monoanion is formed by deprotonating the phenolic proton of enol in the basic solution. pK_a values for different prototropic equilibriums were determined in S₀ and S₁ states and discussed.     

A UV–Visible–NIR fluorescence lifetime imaging microscope for laser-based biological sensing with picosecond resolution

This article describes the design and characterization of a wide-field, time-domain fluorescence lifetime imaging microscopy (FLIM) system developed for picosecond time-resolved biological imaging. The system consists of a nitrogen-pumped dye laser for UV–visible–NIR excitation (337.1–960 nm), an epi-illuminated microscope with UV compatible optics, and a time-gated intensified CCD camera with an adjustable gate width (200 ps-10^-3 s) for temporally resolved, single-photon detection of fluorescence decays with 9.6-bit intensity resolution and 1.4-μm spatial resolution. Intensity measurements used for fluorescence decay calculations are reproducible to within 2%, achieved by synchronizing the ICCD gate delay to the excitation laser pulse via a constant fraction optical discriminator and picosecond delay card. A self-consistent FLIM system response model is presented, allowing for fluorescence lifetimes (0.6 ns) significantly smaller than the FLIM system response (1.14 ns) to be determined to 3% of independently determined values. The FLIM system was able to discriminate fluorescence lifetime differences of at least 50 ps. The spectral tunability and large temporal dynamic range of the system are demonstrated by imaging in living human cells: UV-excited endogenous fluorescence from metabolic cofactors (lifetime ∼1.4 ns); and 460-nm excited fluorescence from an exogenous oxygen-quenched ruthenium dye (lifetime ∼400 ns). Received: 23 February 2003 / Published online: 22 May 2003 RID="*" ID="*"Corresponding author. Fax: +1-734/9361-905, E-mail: mycek@umich.edu     

Measurements of theP 3/2-S 1/2-intervals in then=4,n=5 andn=6 states of ionized helium

TheP _3/2-S _1/2-intervals in then=4,n=5 andn=6 states of ionized helium have been measured by a radio frequency method, which permits to determine the disturbing electric fields in the interaction region and to correct their influences. The experimental results for theP _3/2-S _1/2 intervals in then=4,n=5 andn=6 states were (20,180.6±0.8) MHz, (10,332.9±1.4) MHz and (5,979.1±1.2) MHz respectively. From these intervals, the following indirect values for theS _1/2-P _1/2-Lambshifts can be deduced: (1,768.5±0.8) MHz in then=4 state, (905.0±1.4) MHz in then=5 state and (524.3±1.2) MHz in then=6 state. The results agree with the theoretical predictions. The static electric fields in the interaction region, ranging from 2 to 6 V/cm, increased with increasing electron excitation current, but were independent of the helium pressure within the range of 10 to 26 mTorr. All uncertainties are expressed as 68% confidence values.