Potential of protoporphyrin IX and metal derivatives for single molecule fluorescence studies

Metalloporphyrins are cofactors of a variety of proteins, and are often used as spectroscopic probes of the active site. Many high resolution techniques, such as single molecule spectroscopy, are based on fluorescence contrast and require the replacement of the native metalloporphyrin by a fluorescent analog. We have investigated the potential of several fluorescent analogs of heme, namely free-base protoporphyrin IX and its metal derivatives containing Zn, Sn, and Mg, for single molecule fluorescence studies by determining their room-temperature molecular absorption cross sections and fluorescence quantum yields. According to these data, free-base protoporphyrin IX and its Zn derivative, which have the highest fluorescence quantum yields, are the most suitable heme analogs for single molecule fluorescence studies.     

Identification of molecules through the fluorescence enhancement by a metal tip

A fluorescence enhancement phenomenon, which is realized as a result of a sharp increase in the radiative decay rate of a quantum dipole emitter (QDE) is investigated theoretically in the vicinity of a conical metal tip. The QDE relaxation process is considered as a self-stimulated transition from an excited state into the ground state due to the feedback field formation from the tip. The dynamics of the system shows a stepped relaxation behavior that differs significantly from the conventional exponential decay. This effect can be observed in a small region of the resonance frequency, which is defined by an angle of conical tip. The increase of fluorescence when approaching of molecule to the metal tip on the surface enables one to determine its location.     

Raman scattering and fluorescence emission in a single nanoaperture: Optimizing the local intensity enhancement

We investigate the potential of a single subwavelength aperture milled in an aluminium film to enhance the local electromagnetic field. We compare the Raman scattering of unadsorbed chlorobenzene molecules and the fluorescence emission of Cyanine-5 dyes, having the same excitation and collection setup for both experiments. For the optimal nanoaperture diameter, we report a clear enhancement factor of about 5 of the Raman scattering intensity per unit volume. Since Raman scattering probes the molecular vibrational levels and avoids the resonant pumping of a real excited state, the observed Raman enhancement is disconnected from the effects of the molecular energy levels alteration previously reported for fluorescent dyes. The observations are similar for both Raman and fluorescence experiments, and stand in good agreement with numerical electromagnetic computations of the excitation intensity inside the nanoaperture.     

Excitation and emission of metal electrons in atom-surface collisions

Electron-hole pair excitation and ionization probabilities are calculated for atomic collisions with metal surfaces at high incident energies. The method adopted is based on a Sudden Collision Approximation, and a realistic model is employed for the bound and continuum electronic states involved. The parameters used in the calculations are for Ar, He, H atoms impinging on a Li surface at 300 eV. The main results are: (1) Only single electron-hole pair excitations are important; multiple pair contributions are small. (2) The transitions are dominated by the behavior of the electronic wavefunctions in the tunneling region and may serve as a probe of this regime. (3) The excitation efficiency is in the order H ? Ar ? He, the effectiveness of hydrogen being due to its stronger, longer-range coupling. (4) The maximum excitation probabilities are for electrons ejected with relatively low excess energies. (5) Total transition probabilities are about 0.5 per collision for H, and about 0.1 for Ar, indicating that these are important, easily detectable processes. Experiments in this field should provide important information on electronic wavefunctions at the metal-gas interface, and on gas-metal interactions at high energies.     

金纳米颗粒光散射提高InAs单量子点荧光提取效率

采用光学方法确定InAs/GaAs单量子点在样品外延面上的位置坐标, 利用AlAs牺牲层把含有量子点的GaAs层剥离并放置在含有金纳米颗粒或平整金膜上, 研究量子点周围环境不同对量子点自发辐射寿命及发光提取效率的影响. 实验结果显示, 剥离前后量子点发光寿命的变化小于13%, 含有金纳米颗粒的量子点发光强度是剥离前的7倍, 含有金属薄膜的量子点发光强度是剥离前的2倍. 分析表明在金纳米颗粒膜上的量子点荧光强度的增加主要来自于金纳米颗粒对量子点荧光的散射效应, 从而提高量子点发光的提取效率.     

Modification of single molecule fluorescence using external fields

Controlling and manipulating the fluorescence of single fluorophores is of great interest in recent years for its potential uses in improving the performance of molecular photonics and molecular electronics, such as in organic light-emitting devices, single photon sources, organic field-effect transistors, and probes or sensors based on single molecules. This review shows how the fluorescence emission of single organic molecules can be modified using local electromagnetic fields of metallic nanostructures and electric-field-induced electron transfer. Electric-field-induced fluorescence modulation, hysteresis, and the achievement of fluorescence switch are discussed in detail.     

Influencing the angular emission of a single molecule

We present the first experimental proof for the influence of a nearby nanosized metal object on the angular photon emission by a single molecule. Using a novel angular sensitive detection scheme, we directly quantify the redirection of angular emission for different molecular dipole orientations as an object is scanned laterally over the molecule at different heights. An excellent agreement between experiments and 2D-numerical simulations is found for molecules oriented perpendicular to the sample, whereas, for parallel orientations, the observed behavior contradicts the calculated behavior.     

Excitation of multiple giant dipole resonances: from spherical to deformed nuclei

The effect of deformation on the excitation of multiple giant dipole resonances is studied. Analytical expressions are derived in the framework of the interacting boson model for the energies and E1 properties of giant dipole resonances in spherical and deformed nuclei, and a numerical treatment of transitional nuclei is proposed. Coulomb-excitation cross sections are calculated in ²³⁸U and in the samarium isotopes.     

Field enhancement of gold optical nanoantennas mounted on a dielectric waveguide

In this paper, we investigate the localized field enhancement of optical nanoantennas consisting of different types of coupled gold nanoparticles and fed by a silicon nitrite slab waveguide. The optical nanoantenna is mounted on the surface of the waveguide, and the incident blue-violet light is guided in the slab waveguide and then reflected to the nanoatenna at the end of the waveguide. We also study the performance of the slab waveguide with different optimized parameters and also the field enhancement achieved by different shapes of nanoantennas.     

Sub-poissonian statistics of fluorescence photons of a single molecule

The method previously used by the author for the measurement and calculation of the distribution function w _N(T)of fluorescence photons from a single two-level atom that is continuously excited by laser light and has a unity fluorescence quantum yield is generalized to the case of a single molecule whose fluorescence quantum yield is smaller than unity and to the case of a three-level molecule whose fluorescence is blinking. The functions w _N(T) calculated for these two cases demonstrate a sub-Poissonian distribution of fluorescence photons.     

Enhancement of electroluminescence through a two-dimensional corrugated metal film by grating-induced surface-plasmon cross coupling

We report enhancement of electroluminescence through opaque-topped metal cathodes in organic light-emitting devices (OLEDs), which are periodically corrugated in two dimensions. The enhancement is caused by grating-induced cross coupling and outcoupling of surface plasmons. The emission intensity observed at normal direction is enhanced by a factor of 4 compared with that of OLEDs with one-dimensionally corrugated metal cathodes and is higher than that observed from the transparent substrate side in uncorrugated OLEDs. We attribute the enhancement to the fact that the surface plasmons propagating in all lateral directions are coupled out into free space by the two-dimensional corrugated structure of the OLEDs. In addition, the emission from the transparent substrate side is also much enhanced owing to the efficient coupling out of the waveguide modes trapped in the emissive layer.     

Modification of single molecule fluorescence by a scanning probe

We examine the optical near-field interaction between different types of scanning tips and single oriented fluorescent molecules. We demonstrate the influence of a tip on the excitation intensity as well as on the integrated fluorescence signal, the excited state lifetime, and the angular emission of single molecules. By using a standard model describing the radiation of an oscillating dipole close to a nanosphere or a flat interface, we interpret our observations and describe some central criteria for obtaining fluorescence enhancement or quenching. PACS 33.80.-b; 07.79.Fc; 78.90.+t     

单分子荧光检测在生命科学中的应用

文章对单分子荧光检测在分子马达、离子通道、信号分子、蛋白折叠、蛋白构象变化动力学、酶活性反应、细胞过程实时观察等生命科学领域中的应用进行了介绍.这些研究结果表明,单分子荧光检测在研究生物大分子的活动规律与机制方面不但有着无法替代的优越性,而且有着广阔的发展空间.     

Tunable multiple plasmon resonances and local field enhancement of nanocrescent/nanoring structure

According to the plasmon hybridization theory, the plasmon resonance characteristics of the gold nanocrescent/nanoring(NCNR) structure are systematically investigated by the finite element method. It is found that the extinction spectra of NCNR structure exhibit multiple plasmon resonance peaks, which could be attributed to the result of the plasmon couplings between the multipolar plasmon modes of nanocrescent and the dipolar, quadrupolar, hexapolar, octupolar,decapolar plasmon modes of nanoring. By changing the geometric parameters, the intense and separate multiple plasmon resonance peaks are obtained and can be tuned in a wide wavelength range. It is further found that the plasmon coupling induces giant multipole electric field enhancements around the tips of the nanocrescent. The tunable and intense multiple plasmon resonances of NCNR structure may provide effective applications in multiplex biological sensing.     

First-principles study on current through a single π conjugate molecule for analysis of carrier injection through an organic/metal interface

We have studied current flow through a single π conjugate molecule weakly adsorbed to, and sandwiched between, two electrodes, focusing specifically on carrier injection through an organic/metal interface. This is the first calculation to investigate the effects of the orientation of the molecule and the electrode material on current using a first-principles method: in the past, most calculations of current were based on the assumption of covalent bonding of a molecule to electrodes. We modeled two systems in which a naphthalene molecule is sandwiched between gold (Au) or aluminum (Al) electrodes. First, in both systems, the current through the molecule depends on the orientation of the molecule. This indicates that electrons mainly transfer through the π channel, which is the overlap between the molecular π orbital and the electrode orbital. Next, the current in the Au-naphthalene-Au system is higher than that in the Al-naphthalene-Al system. This shows that Au is more suitable as an electrode material than Al. Therefore, the orientation of the molecule at an organic/metal interface and the materials comprising the electrodes play a key role in carrier injection through the interface.     

Experimental assessment of fluorescence microscopy signal enhancement by stimulated emission

The quantity of photons generated during fluorescence microscopy is principally determined by the quantum yield of the fluorescence dyes and the optical power of the excitation beam. However, even though low quantum yields can produce poor images, it is challenging to tune this parameter, while increasing the power of the excitation beam often results in photodamage. Here, we propose the use of stimulated emission (SE) as a means of enhancing both the signal intensity and signal-to-noise ratio during confocal fluorescence microscopy. This work experimentally confirmed that both these factors can be enhanced by SE radiation, through generating a greater number of photons than are associated with the standard fluorescence signal. We also propose the concept of stimulated emission enhancing fluorescence (SEEF) microscopy, which employs both the SE and fluorescence signals, and demonstrate that the intensity of an SEEF signal is greater than those of the individual SE and fluorescence signals.     

Theoretical simulation of the fluorescence spectrum of a single molecule

The possibility of computer simulation of the fluorescence spectrum of an actual system of single molecules, whose initial isomeric structure is photochemically transformed as a result of a radiationless resonance transition, is demonstrated.Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 71, No. 6, pp. 740–744, November–December, 2004.This revised version was published online in April 2005 with a corrected cover date.     

Room-temperature fluctuations in the fluorescence of a single polymer molecule

A theoretical model of light absorption and emission by a polymer molecule has been developed using recent experimental data on the room-temperature fluctuations in the fluorescence intensity of single molecules of a PPV-PPyV copolymer containing several tens of chromophores. An analysis of the experimental data based on the proposed model shows evidence of a change in the conformation of a polymer molecule in the triplet state. By applying the theory to the PPV-PPyV copolymer, it is possible to determine the rate constants of the conformation variation, the rates of the transition from the singlet to the triplet state, and the lifetime of the triplet state of the molecule. The theory also predicts some new effects which can be verified by experiment.     

Field emission through atoms chemisorbed on a metal surface

An exact general expression for the field emission current is derived under two alternative forms in the frame of a new formalism of tunnelling developed in preceding articles. This formalism is applied to study qualitatively the TED (total energy distribution) in the presence of a chemisorbed atom. It is shown in particular that the overlap between the atomic electronic state and the continuum of states of the vacuum induces, in the high-field regime, a non-negligible variation of the position and width of the resonance peak with the electric field.