Evanescent waves induced fluorescence

Unambiguous determination of the conditions under which slippage occurs requires a technique able to measure the velocity of the fluid in the immediate vicinity of the solid wall over a thickness comparable to the size of a polymer chain, i.e. a few tens of nanometers. Classical laser Doppler velocimetry does not meet this requirement even if it allows for the determination of velocity profiles which clearly reveal a non-zero velocity within typically a few 10 pm from the wall. We have developed a new optical technique. Near Field Velocimetry (N.F.V.) [14], which combines Evanescent Wave Induced Fluorescence (E.WF.) [27] and Fringe Pattern Fluorescence Recovery After Photobleaching (F.P.F.R.A.P.) [28]. The former technique gives the spatial resolution normal to the solid wall, while the latter one enables the determination of the local velocity of the fluid. A major constraint of the technique is that it needs polymer molecules labelled with an easily photobleachable fluorescent probe. 

Negative adsorption is in most cases very small compared to positive adsorption and therefore not easily detected directly. A sophisticated optical technique (evanescent-wave-induced fluorescence, EWIF) has been used to prove the reduction in segment concentration close to a non-adsorbing surface Depletion also has an effect on the flux of polymer solutions through pores since the viscosity of the liquid near the non-adsorbing surface is lower than that of the polymer solution, the flux is then higher than would be expected on the basis of the bulk viscosity. Negative adsorption at a liquid-air interface leads to a measurable increase In surface tenslon ... 

Time resolved evanescent wave induced fluorescence spectroscopy is a powerful method for the investigation of dye molecules at interfaces. This technique has been used on studies on the popular photosensidzer aluminium phthalocyanine tetra/sulphonate absorbed at fused silica/methanol interfaces . 2nd harmonic detection of sinusoidally modulated two photon excited fluorescence can also be used to obtain luminescence spectra ". ... 

Aptamer-based protein sensors have been developed since 1998. Thus, 0.7 amol of thrombin in 140-pL sample (0.5 pM concentration) was detected in a single-aptamer sensor through binding to a fluorescently labeled DNA aptamer by evanescent-wave-induced fluorescence anisotropy in less than 10 minutes [45]. Performance of DNA... 

The development of antibody-based biosensors presents more difficulties than enzyme-based biosensors as the antigen-antibody interactions are not readily reversible because of the high values of the affinity constants. Another limitation is that the physicochemical changes resulting from the immunochemical reaction are often insufficient to provide detection limits comparable with those of conventional analysis. As a consequence, indirect systems have been developed that rely on the use of enzyme-or fluorescent-tagged reagents. Both competitive and sandwich formats are used. Evanescent wave-induced fluorescence is frequently chosen to avoid possible interferences from the bulk media. For... 

The structure of adsorbed chains at the air-liquid interface is similar to that at the solid-liquid interface. Adsorption at air-liquid interface has been studied by ellipsometry (54), X-ray and neutron reflectivity (55,56), surface tension measurements (57), X-ray evanescent wave-induced fluorescence (58), and Langmuir trough techniques (55). Neutron reflectivity measurements indicate that in the... 

The flow velocity of a fluid at submicrometric distances from a solid wall is investigated using a novel optical technique based on fluorescence recovery after photobleaching and evanescent wave induced fluorescence. For a polymer melt, three different friction regimes, all characterized by slip at the wall, are identified and modeled at the molecular level. For a simple fluid (alkane), slip at the wall is put into evidence and factors influencing the extend of slip are identified. 

Evanescent wave induced fluorescence (EWIF) and small angle neutron scattering (SANS) experiments have been performed at the polymer solution-solid interface, in the case of adsorption. The interfacial layer is characterized by its total surface excess T and by the monomer concentration profile (2) in the direction normal to the interface. 

The first of these techniques is the EWIF method, an acronym for Evanescent Wave Induced Fluorescence. The power of EWIF has been first demonstrated in the case of depletion and the main results have been presented in several publications.However, it is only very recently that the method has been applied to adsorbed layers. 

Lastly in this section we note that Kent et al. (1992) have used x-ray evanescent (XEWIF) wave-induced fluorescence to study the adsorption of poly(dimethyl siloxane) at the air/liquid interface, the liquids being bromohep-tane (a good solvent) and bromocyclohexane (a theta solvent at 29 °C). The signal is rather too noisy to allow a definitive and quantitative comparison with theory. In the good solvent the surface region was inferred to have a pol5nner... 

Since TIRF produces an evanescent wave of typically 80 nm depth and several tens of microns width, detection of TIRF-induced fluorescence requires a camera-based (imaging) detector. Hence, implementing TIRF on scanning FLIM systems or multiphoton FLIM systems is generally not possible. To combine it with FLIM, a nanosecond-gated or high-frequency-modulated imaging detector is required in addition to a pulsed or modulated laser source. In this chapter, the implementation with of TIRF into a frequency-domain wide-field FLIM system is described. 

Optical fibers without cladding can be used to monitor the adsorption of chemical species such as proteins on an optical Hber core [87]. Fluorescence is induced by the evanescent wave field of the light propagating in the core. For example, the intrinsic fluorescence of the amino acid tryptophan in immunoglobulin (IgG) and the fluorescence of fluorescein-labelled IgG were detected and used to calculate the amount of protein adsorbed on hydrophilic glass and quartz surfaces. Fluorescent, nonadsorbing dextran was used as a calibration molecule which approximates the diffusion properties of the protein. Remote spectroscopic sensing of the adsorption of Rhodamine B-labelled IgG at the tip of a 600-pm fiber optic has also been described [88, 89]. 

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