A method for on-line background subtraction in frequency domain fluorometry

Time-resolved fluorescence determinations on biochemical samples are often complicated by contributions from background. In the time-correlated single-photon counting method, background subtraction is a routine procedure. A limitation of frequency domain fluorometry, however, arises from the difficulty of performing this operation. This limitation has become increasingly significant as frequency domain methods are being applied to evermore complex biological systems using the higher-frequency capabilities of modern instrumentation. We have devised a method for such a correction in the frequency domain, regardless of the complexity of the background decay, based on measurement of the background phasor and subsequent subtraction from the sample phasor. This method is applicable to both lifetime and dynamic polarization measurements, and it can be readily implemented on commercially available frequency domain fluorometers. Decay curves may be accurately recovered from samples containing background contributions ranging from less than 5% to greater than 90% of the total signal intensity.     

Ionic liquid promoted preparation of 4H-thiopyran and pyrimidine nucleoside-thiopyran hybrids through one-pot multi-component reaction of thioamide

One-pot multi-component reactions of aldehydes, cyanothioacetamide and malononitrile promoted by ionic liquid proved to be an efficient way for the synthesis of thiopyran derivatives. Without any added catalyst, both aromatic and aliphatic aldehydes participated in this reaction smoothly. As an application of this method, a pyrimidine nucleoside-thiopyran chimera with potential biological activities was obtained in high yield from 5-formyl-2’-deoxyuridine. In addition, the ionic liquid used can be easily recovered and effectively reused for at least 5 times. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Frequency-Domain Fluorescence Lifetime Measurements via Frequency Segmentation and Recombination as Applied to Pyrene with Dissolved Humic Materials

In this study, the association behavior of pyrene with different dissolved humic materials (DHM) was investigated utilizing the recently developed segmented frequency-domain fluorescence lifetime method. The humic materials involved in this study consisted of three commercially available International Humic Substances Society standards (Suwannee River fulvic acid reference, SRFAR, Leonardite humic acid standard, LHAS, and Florida peat humic acid standard, FPHAS), the peat derived Amherst humic acid (AHA), and a chemically bleached Amherst humic acid (BAHA). It was found that the three commercial humic materials displayed three lifetime components, while both Amherst samples displayed only two lifetime components. In addition, it was found that the chemical bleaching procedure preferentially removed red wavelength emitting fluorophores from AHA. In regards to pyrene association with the DHM, different behavior was found for all commercially available humics, while AHA and BAHA, which displayed strikingly similar behavior in terms of fluorescence lifetimes. It was also found that there was an enhancement of pyrene’s measured lifetime (combined with a decrease in pyrene emission) in the presence of FPHAS. The implications of this long lifetime are discussed in terms of (1) quenching mechanism and (2) use of the fluorescence quenching method used to determine the binding of compounds to DHM. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Acridones and Quinacridones: Novel Fluorophores for Fluorescence Lifetime Studies

Two new families of fluorescent probe, acridones and quinacridones, whose fluorescence lifetime can be altered to produce a range of lifetimes from 3 ns to 25 ns are described. Both families of fluorophore have fluorescence lifetimes which are unaffected by pH in the range of 5 to 9 and show a marked resistance to photobleaching. The probes have been modified to allow them to be attached to biomolecules and the labelling of a neuropeptide (substance P) is described. The labelled peptides have the same fluorescence lifetime as the free fluorophore. Quinacridone, with an emission around 550 nm offers a long fluorescence lifetime, photostable alternative to fluorescein.     

A new approach to determine radiative recombination lifetime in quantum well solar cells

In this paper, the authors present and extend on an existing model, which has been developed to determine the radiative recombination lifetimes in quantum well solar cells. Given the fact that recombination reduces cell performance, the main future use of this new model is to aid in optimisation of cell designs and increase cell efficiency. In this work the authors introduce a coefficient defined as the delta factor which is based on material parameters into the existing model. The introduction of this factor into the existing model has shown an improvement in radiative recombination lifetime determination of approximately 11% when comparison is made with the previous model. This has lead to an overall average improvement in lifetime determination of approximately 9% when comparisons are made between the new model and experimental data. This is a significant improvement in lifetime determination, which will benefit cell designers.     

Estimates of uncertainties in analysis of positron lifetime spectra for metals

The effects of uncertainties and errors in various constraints used in the analysis of multi-component life-time spectra of positrons annihilating in metals containing defects have been investigated in detail using computer simulated decay spectra and subsequent analysis. It is found that the errors in the fitted values of the main components lifetimes and intensities introduced from incorrect values of the instrumental resolution function and of the source-surface components can easily exceed the statistical uncertainties. The effect of an incorrect resolution function may be reduced by excluding the peak regions of the spectra from the analysis. The influence of using incorrect source-surface components in the analysis may on the other hand be reduced by including the peak regions of the spectra. A main conclusion of the work is that extreme caution should be exercised to avoid introducing large errors through the constraints used in the analysis of experimental lifetime data. Supported in part by the National Research Council of Canada     

眼底视网膜色素上皮层细胞脂褐素及氧化黑色素自体荧光寿命成像研究

利用一种基于时间相关单光子计数器的双光子激发荧光寿命显微成像技术,对猪眼底视网膜色素上皮层细胞内的脂褐素和氧化黑色素颗粒的空间分布及其荧光寿命特性进行了研究,尤其对于这些色素颗粒在光致氧化环境中的荧光寿命差异进行了分析.结果表明,利用荧光寿命测量能有效区分视网膜色素上皮层细胞中的多组分荧光团,利用荧光寿命的衰减参数可分辨正常及异常的荧光现象.该方法有望发展成为一种用于眼科临床诊断及病理学研究的高灵敏度的工具,对眼底细胞随年龄增长的衰老机理的研究具有重要的意义. 关键词： 双光子激发荧光 荧光寿命成像 视网膜色素上皮层     

眼底视网膜色素上皮层细胞脂褐素及氧化黑色素自体荧光寿命成像研究

利用一种基于时间相关单光子计数器的双光子激发荧光寿命显微成像技术，对猪眼底视网膜色素上皮层细胞内的脂褐素和氧化黑色素颗粒的空间分布及其荧光寿命特性进行了研究，尤其对于这些色素颗粒在光致氧化环境中的荧光寿命差异进行了分析.结果表明，利用荧光寿命测量能有效区分视网膜色素上皮层细胞中的多组分荧光团，利用荧光寿命的衰减参数可分辨正常及异常的荧光现象.该方法有望发展成为一种用于眼科临床诊断及病理学研究的高灵敏度的工具，对眼底细胞随年龄增长的衰老机理的研究具有重要的意义.     

本征GaAs中电子自旋极化对电子复合动力学的影响研究

采用时间分辨圆偏振光和线偏振光抽运-探测光谱,研究了9.6 K温度下本征GaAs中自旋极化电子与非极化电子的复合动力学及其随光子能量演化.发现自旋极化对电子复合动力学具有显著影响.仅在导带底附近测量时,两种方法测试到的复合寿命一致,而在高过超能量电子态测量时,两种方法测试到的复合寿命不一致.指出时间分辨法拉第光谱中,用于反演求解电子自旋相干寿命的电子复合寿命应该使用圆偏振光抽运-探测获得的复合寿命,而不是线偏振光抽运-探测获得的寿命.理论计算与实验结果吻合较好. 关键词： 圆偏振光抽运-探测光谱 自旋量子拍 自旋极化 GaAs     

锑化铟中载流子的复合过程

用定态光电导及光磁电的方法测量了p型n型InSb在85—290°K之间的电子及空穴的寿命。在室温附近,所有样品的截流子寿命都趋于同一值,在290°K时为3×10^-8秒。从寿命的绝对值及温度依赖关系,以及掺杂对寿命的影响,可以肯定在室温附近起主要作用的复合过程是带间碰撞复合过程。在200°K以下,p型样品中的电子寿命与空穴寿命有很大差别,表明有陷阱作用。用位于价带之上0.05eV的复合中心及位于导带之下0.11eV的电子陷阱能完满地解释200°K以下的寿命与温度的依赖关系。     

InAs单层和亚单层结构中的自旋动力学研究

利用偏振时间分辨光谱和时间分辨Kerr旋转谱，研究了GaAs中的InAs单层和亚单层的电子自旋动力学.实验发现，在非共振激发条件下，厚度为1/3单层的InAs亚单层中电子自旋弛豫寿命长达3.4ns，而1个单层厚的InAs层的电子自旋寿命只有0.48ns；而在共振激发条件下，亚单层结构中的电子自旋寿命大大减少，只有70ps，单层InAs中电子自旋寿命没有显著变化.分析表明，低温下InAs单层和亚单层结构中，Bir-Aronov-Pikus(BAP)自旋弛豫机理占主导地位.通过改变材料结构特性和激发条件来改变电子空穴的空间相关性，从而达到控制自旋弛豫的目的. 关键词： InAs亚单层 自旋弛豫 BAP机理     

Band-to-band recombination in GaxIn1−xSb

The carrier lifetimes in Ga_xIn_1−xSb for radiative and Auger recombination are calculated for the temperature range 77–300 K and the composition range 0 ⩽ x ⩽ 1. The possible band-to-band Auger recombination mechanisms in direct-gap semiconductors are investigated. The Auger rates are calculated, including Boltzmann statistics and nonparabolic bands, using the Kane-band model. In the low temperature range, for lightly doped material, the carrier lifetime is determined by radiative recombination. At higher temperatures the CHCC process is dominant in n-type Ga_xIn_1−xSb but in p-type material the CHLH process is dominant. The influence of CHSH processes on the carrier lifetime is appreciable in p-type GaSb. The calculations are compared with experimental data reported by other authors.     

HIRFL-CSR实验环中束流损失机制及寿命研究

分析了在储存环中回旋的离子束与残余气体分子、 内靶和冷却电子束相互作用时的损失机制及相应的束流寿命, 针对兰州重离子加速器冷却储存环实验环内靶模式, 计算了50—500 MeV/u 12C6+, 36Ar18+, 132Xe54+和 238U92+等束流在各种损失机制影响下所对应的束流寿命和总的束流寿命。 结果表明: 影响束流寿命的主要因素是与内靶分子（原子）之间的电荷交换及与冷却电子束之间的辐射复合; 对于重离子束 132Xe54+和 238U92+, 与冷却电子束之间的辐射复合是影响其储存寿命的主要因素。The loss mechanism and lifetime of ion beams in collisions with residual gas, internal target and electrons in e cooler in heavy ion cooler storage rings were studied. The partial beam lifetimes resulting from various loss mechanisms and the total beam lifetimes of 50—500 MeV/u12C6+, 36Ar18+, 132Xe54+ and 238U92+ stored in the experimental ring of the Cooler Storage Ring at the Heavy Ion Research Facility in Lanzhou (HIRFL CSR) were calculated. The calculations indicate that the charge exchange process between ion beams and the internal target, as well as the radiative recombination process with the electrons in e cooler restrict the beam lifetime considerably. For heavy ion beams such as 132Xe54+ and 238U92+, the radiative recombination is the dominant loss mechanism     

Frequency Domain Fluorescence Lifetime Study of Crude Petroleum Oils

Frequency domain (FD) fluorescence lifetime data was collected for a series of 20 crude petroleum oils using a 405 nm excitation source and over a spectral range of ~426 to ~650 nm. Average fluorescence lifetimes were calculated using three different models: discrete multi-exponential, Gaussian distribution, and Lorentzian distribution. Fitting the data to extract accurate average lifetimes using the various models proved easier and less time consuming for the FD data than with Time Correlated Single Photon Counting (TCSPC) methods however the analysis of confidence intervals to the computed average lifetimes proved cumbersome for both methods. The uncertainty in the average lifetime was generally larger for the discrete lifetime multi-exponential model when compared to the distribution-based models. For the lifetime distributions, the data from the light crude oils with long lifetimes generally fit to a single decay term. Heavier oils with shorter lifetimes required multiple decay terms. The actual value for the average lifetime is more dependant on the specific fitting model employed than the data acquisition method used. Correlations between average fluorescence lifetimes and physical and chemical parameters of the crude oils were made with a view to developing a quantitative model for predicting the gross chemical composition of crude oils. It was found that there was no significant benefit gained by using FD over TCSPC other than more rapid data analysis in the FD case. For the FD data the Gaussian distribution model for fluorescence lifetime gave the best correlations with chemical composition allowing a qualitative correlation to some bulk oil parameters.

频控变色场致发光器件及机理研究

吸收是发光的前提条件,吸收变了发光的其他特性也会跟着改变。发光寿命长短又会反过来影响吸收的多少,这不仅存在于光致发光中,而且是一个普遍规律。作者从电场引起的场致发光中发现,在同一种材料中掺入两种发光寿命长短相差悬殊的发光中心Cu,Mn,利用激发电源频率的变化明显地显示出器件发光光谱的变化,在一种材料中实现了变色。实验结果证明提高激发频率可以提高短寿命发光中心的激发效率,通过改变激发频率的大小,就可以在掺有两种发光寿命大小不一的材料中实现了发光颜色的变化,这个规律只与发光中心寿命有关,而与激发方式及猝灭原因无关。     

A Ca optical frequency standard: Frequency stabilization by means of nonlinear Ramsey resonances

An optical frequency standard based on frequency stabilization to Ramsey fringes observed in a Ca atomic beam is described. The important operation parameters influencing the uncertainty of such a standard are studied experimentally. Frequency shifts due to phase errors of separated traveling wave excitation can be strongly reduced by means of laser beam reversal. It is expected that the present fractional uncertainty of approximately 2.3×10^–12 to realize the true line center can be reduced below the 10^–14 level by applying atomic beam cooling and improved phase alignment.     

Type-II superlattice detectors for free space optics applications and higher operating temperature conditions

The utmost limit performance of interband cascade detectors optimized for the longwave range of infrared radiation is investigated in this work. Currently, materials from the III–V group are characterized by short carrier lifetimes limited by Shockley-Read-Hall generation and recombination processes. The maximum carrier lifetime values reported at 77 K for the type-II superlattices InAs/GaSb and InAs/InAsSb in a longwave range correspond to ～200 and ～400 ns. We estimated theoretical detectivity of interband cascade detectors assuming above carrier lifetimes and a value of ～1–50 μs reported for a well-known HgCdTe material. It has been shown that for room temperature the limit value of detctivity is of ～3–4×10¹⁰ cmHz^1/2/W for the optimized detector operating at the wavelength range ～10 μm could be reached.     

Lifetime determination of materials that exhibit a stretched exponential luminescent decay

We report frequency- and time-domain luminescent lifetime measurements for materials that exhibit a broad spectrum of lifetimes, specifically, those for which the observed decay dynamics can be described by a stretched exponential function. The spectrum of lifetimes of such materials can be characterized by an average lifetime, and in principle this average can be extracted from either time- or frequency-domain measurements. In practice, this requires some care, because the extent to which the various states are excited depends on how long their lifetime is relative to the excitation pulse width or period. For the complex luminescent materials ZnS:Cu,Al and CdS quantum dots we compare the average lifetime obtained from frequency-domain data and from time-domain data under both pulsed laser excitation and steady-state, dc excitation. The agreement between the average measured lifetimes is good, but not perfect, showing that quoted average lifetimes for complex materials can be dependent on the measurement technique. Finally, a spectrum of lifetimes is given that gives rise to stretched exponential relaxation and this spectrum is used to compute frequency-domain data for a stretched exponential material.     

Hyperpolarized long-lived states in solution NMR: three-spin case study in low field

Recent work has shown that singlet states in two-spin systems can possess lifetimes exceeding the T(1) relaxation time, provided that the system is kept under conditions that minimize the effects of the chemical shift Hamiltonian (for instance under low magnetic field or RF irradiation). Similar observations have been made in hyperpolarized states of multi-spin systems prepared via parahydrogen-induced polarization (PHIP). However, lifetime prolongation mechanisms in multi-spin systems are still under investigation. Here we present experimental observations of a long-lived state in a three-spin system prepared by PHIP and stored at low field. The observed lifetime of the long-lived state is 144s, about twice as long as the longest T(1) measured in the system at high field. The results are analyzed using a recently proposed theory of lifetime prolongation in multi-spin systems in low field. It is shown that quantum mechanical selection rules governing intramolecular dipolar relaxation in low field account for the enhanced lifetime and spectral features of this state.     

Simulations of measurements of fluorescence lifetimes using noise-modulated light

At the Pattern Recognition group at the Delft University of Technology, we are working on new ways to measure fluorescence lifetimes. There are two well-known ways to measure lifetimes; the phase method and the pulse method. In the phase method fluorescent material is stimulated by sinusoidally modulated light. The emitted fluorescent light will have the same modulation frequency, but there will be a phase shift between the excitation and the emission light. Measuring this phase shift will, after some simple calculation, give the lifetime of the fluorescent material. The second method is the pulse method. Short pulses of light are used to excite the material. The emitted light is detected, and from these measurements the decay curve of fluorescent light is determined. In our research we want to use a new method that may allow us to measure a mixture of lifetimes. We want to use excitation light that is modulated by a white noise signal. We are currently building an experimental setup for these measurements. We have been working on numerical and electrical simulations to investigate the properties of noise signals. Some results of these simulations are presented in this paper.