Hartmut Schroeder

[37] Monitoring specific protein-binding reactions with chemiluminescence

Publisher Summary This chapter discusses the monitoring of specific protein-binding reactions with chemiluminescence. Aminophthalhydrazides, aminonaphthylhydrazides, and their ligand conjugates can be measured at picomolar concentrations by chemiluminescence. Competitive protein-binding assays monitored by chemiluminescence can measure some ligands at concentrations present in biological materials. Light production by the biotin-isoluminol conjugate is enhanced about 10-fold when the biotin moiety is bound to avidin. This enhancement provided a means for monitoring competitive-binding assays for biotin without separation of the protein-bound and free forms. Among the oxidation systems tested, the H 2 O 2 -microperoxidase and the H 2 O 2 -hematin systems provide the most sensitive assays for the chemiluminescent compounds. The sensitivity of competitive-binding assays is determined in part by the lower limit for detection of the label. The lower limits for detection of these compounds using several oxidation systems are presented in the chapter along with their use in protein-binding assays.

From random to controlled small-scale filamentation in water.

Simple apertures such as slits and meshes inserted in the beam path of a powerful Ti:Sapphire laser pulse are suitable to produce stable 1-D and 2-D arrays of filaments in liquids. The thus imposed intensity gradients and diffraction patterns can overcome the inherent beam irregularities which naturally give rise to random small-scale multiple filamentations. This method is visualized by means of two photon fluorescence imaging.

Laser-induced breakdown spectroscopy for detection of heavy metals in environmental samples

The application of LIBS technology as a sensor for heavy metals in solid environmental samples has been studied. This specific application introduces some new problems in the LIBS analysis. Some of them are related to the particular distribution of contaminants in the grained samples. Other problems are related to mechanical properties of the samples and to general matrix effects, like the water and organic fibers content of the sample. An attempt has been made to optimize the experimental set-up for the various involved parameters. The understanding of these factors has enabled the adjustment of the technique to the substrates of interest. The special importance of the grain size and of the laser-induced aerosol production is pointed out. Calibration plots for the analysis of heavy metals in diverse sand and soil samples have been carried out. The detection limits are shown to be usually below the recent regulation restricted concentrations.

White-light generation using spatially-structured beams of femtosecond radiation

We studied white-light generation in water using spatially- structured beams of femtosecond radiation. By changing the transverse spatial phase of an initial Gaussian beam with a 1D spatial light modulator to that of an Hermite-Gaussian (HGn,m) mode, we were able to generate beams exhibiting phase discontinuities and steeper intensity gradients. When the spatial phase of an initial Gaussian beam (showing no significant white-light generation) was changed to that of a HG01, or HG11 mode, significant amounts of white-light were produced. Because self-focusing is known to play an important role in white-light generation, the self-focusing lengths of the resulting transverse intensity profiles were used to qualitatively explain this production. Distributions of the laser intensity for beams having step-wise spatial phase variations were modeled using the Fresnel-Kirchhoff integral in the Fresnel approximation and found to be in good agreement with experiment.

Some fundamental concepts of femtosecond laser filamentation

Powerful femtosecond laser pulses propagate in an apparent form of filamentation in all transparent optical media. This universal nonlinear phenomenon is currently an interesting topic of research at the forefront of applied physics and attracts more and more people to enter this field. This paper attempts to clarify some of the fundamental physics behind filamentation. The basic concepts include the slice-by-slice self-focusing, intensity clamping, white light laser generation and background energy reservoir as well as multiple filamentation competition. Some important potential applications are also discussed.

Efficient broadband emission from condensed media irradiated by low-intensity, unfocused, ultrashort laser light

We report here measurements of the efficiencies of broadband emission in different optical media using an unfocused, ultrashort (~40 fs) laser beam. Two different measurements have been carried out by placing a wire mesh in the path of the incident laser radiation. The wire mesh introduces a periodic intensity distribution in the x-y plane and also in the direction of the laser beam propagation. We measure both on-axis and offaxis components of the broadband emission and also observe modulation in broadband generation as the distance between the mesh and the sample is varied. The experimentally measured locations of broadband emission maxima are in agreement with simulations based on Fresnel diffraction integrals. The off-axis emission efficiencies lie in the range of 16-87%.

Signal processing algorithm for simultaneous multielement analysis by laser-produced plasma spectroscopy

We developed a signal processing algorithm to analyze the signals obtained by an OMA system for laser-produced plasmas. This signal processing program is applied for multi- component analysis of trace elements in particulate materials (e.g. soils and industrial wastes) and is designed to overcome signal fluctuations due to instability of the plasma characteristics and due to some of the matrix effects. The program involves a constrained normalization algorithm, an automatic peak assignment, a functional fit of all peaks of interest and their surroundings, and a principal components regression calibration model. These algorithms, together with experimental optimizations, are shown to solve most of the problems present in laser plasma analysis of particulate material and to produce detection limits in the ppm range.

Real-time detection of hazardous elements in sand and soils

A new method for real-time analysis of hazardous elements in sand and soils has been investigated. The method is based on laser plasma generation and time-resolved spectroscopy. Almost no sample preparation is needed and simultaneous multielemental analysis is possible. The data collection step takes only a few seconds, and the computer analysis and evaluation is carried out within minutes. The method has been tested for analysis of heavy metals in a variety of soil matrices. Good calibration plots have been obtained, with detection limits of ca. 10 ppm. An expert system has been developed for data analysis. The multivariate calibration model has been based on principle component regression technique in order to compensate for possible spectral interferences and some of the matrix effects.

Scattering of electronically excited sulphur dioxide from graphite

So2 molecules in a He-seeded molecular beam were electronically excited with a XeC1 excimer laser and scattered from the basal plane of graphite in an UHV chamber. Their characteristic scattering features were measured by counting those molecules which radiate after scattering. The survival probability of SO2 scattered from graphite is 2.5 * lO-. This implies that the probability of detecting excited molecules which have collided twice on the surface is already negligible. In this favourable case, the dynamics of scattering can be described with a hard cube model. The parallel component of the momentum is conserved, whereas the perpendicular component in our case is reduced to almost half of its initial value.

Formation of nickel silicides by excimer laser CVD of Ni(CO)4

Nickel silicides have been grown on single crystal silicon sub- strates. A XeCl excimer laser was used for all process steps: substrate cleaning, nickel deposition , silicide formation and annealing. The nickel films were grown by photodecoxnposition of Ni(CO)4 adsorbate layers with an excess of CO to prevent homogeneous nucleation and hence the formation of dust. The samples were analysed by X-ray fluorescence, SIMS and RBS. The results indicate that epitaxial silicide layers with a thickness of 50 ma can be obtained after careful choice of laser fluence and Ni film thickness. In an alternative approach we used a molecular beam of Ni(CO)4, part of which is laser excited prior to impinging on the substrate. This allows the combination of CVD and conventional MBE techniques. In this experiment we also investigate the interaction of photofragments with substrate surfaces and other processes responsible for material deposition.