Time-resolved sensing

Table 1.1. Characteristics of Intensity, Intensity-Ratio, and Time-Resolved Sensing Methods...

Allcock P, Snow PA (2001) Time-resolved sensing of organic vapors in low modulating porous silicon dielectric mirrors. J Appl Phys 90(10) 5052-5057 Amato G, Boarino L, Bellotti F (2004) On the apparently anomalous response of porous silicon to nitrogen dioxide. Appl Phys Lett 85 4409 411... 

Allcock P, Snow PA (2001) Time-resolved sensing of organic vapors in low modulating porous silicon dielectric mirrors. J Appl Phys 90 5052... 

Keywords Anisotropy Intensity sensing Time-resolved fluorimetry Wavelength ratiometry... 

There are several future directions that NIR brain sensing and imaging research can take. In instrumentation, advances in time-resolve spectroscopic equipment may yield less expensive equipment and thus a more prolific use. This will allow for approximation of time of flight parameter providing a possible avenue for inferring path length. Theoretically, there is a need for better theoretical modeling to eliminate crosstalk noise. Possible improvements have already been introduced by Boas, et. al [8]. However, more human subject studies need to be conducted to... 

To illustrate the use of time-resolved data for sensing, assume that each form (free and bound) has a unique decay time, rF and rb- The intensity decay of a mixture is a double-exponential... 

The fluorescence lifetime can be measured by time-resolved methods after excitation of the fluorophore with a light pulse of brief duration. The lifetime is then measured as the elapsed time for the fluorescence emission intensity to decay to 1/e of the initial intensity. Commonly used fluorophores have lifetimes of a few nanoseconds, whereas the longer-lived chelates of europium(III) and terbium(III) have lifetimes of about 10-1000 /tsec (Table 14.1). Chapter 10 (this volume) describes the advantages of phase-modulation fluorometers for sensing applications, as a method to measure the fluorescence lifetime. Phase-modulation immunoassays have been reported (see Section 14.5.4.3.), and they are in fact based on lifetime changes. 

Volume 4 is intended to summarize the principles required for these biomedical applications of time-resolved fluorescence spectroscopy. For this reason, many of the chapters describe the development of red/NIR probes and the mechanisms by which analytes interact with the probes and produce spectral changes. Other chapters describe the unique opportunities of red/NIR fluorescence and the types of instruments suitable for such measurements. Also included is a description of the principles of chemical sensing based on lifetimes, and an overview of the ever-important topic of immunoassays. 

LIBS presently is mainly used for quantitative and semi-quantitative analyses. Much less attention has been paid to the potential of LIBS for qualitative analysis, which may be effectively used in radiometric sorting of minerals. In this sense we do not need to determine a detailed chemical composition of the minerals, but rather the aim is to find the specific lines, which enable us to identify the corresponding mineral. LIBS mineral sorting is in its early stages, but the prospects are very good. We developed the corresponding database of time-resolved LIBS under 1,064, 532, 355 and 266 nm laser excitations for the spectral range from 200 to 900 nm. 

The Applications of Laser-induced Time-resolved Spectroscopic Techniques chapter starts with a short description of laser-induced spectroscopies, which may be used in combination with laser-induced luminescence, namely Breakdown, Raman and Second Harmonic Generation. The chapter contains several examples of the application of laser-based spectroscopies in remote sensing and radiometric sorting of minerals. The proljlem of minerals as geomaterials for radioactive waste storage is also considered. 

The last subject where time-resolved X-ray diffraction techniques proved their exceptional potential concerns chemical physics of gold nanoparticles in water. Belonging jointly to X-ray and nanoparticle physics, this subject is in a certain sense on the borderline of the present chapter [82-84]. Nevertheless, the possibilities offered by time-resolved X-ray techniques in this domain are fascinating. The heart of the problem is as follows. A suspension of gold... 

Subsequently, fluorescent MIPs for cGMP were fabricated [46 18, 66, 67]. For that, 1,3-diphenyl-6-vinyl-1 //-pyrazolo 3.4-/ quinoline (PAQ) was introduced as the fluorescent indicator to interact with cGMP in a thin-layer fluorescent MIP chemosensor. Both steady-state (Fig. 1) and time-resolved fluorescence spectroscopy were used as two independent analytical techniques for investigation of the chemosensor properties in the presence of cGMP. Steady-state fluorescence spectroscopy is a common technique applied to MIP sensing. Nevertheless, the use of time-resolved fluorescence spectroscopy combined with microscopy was a new approach to MIP sensing. 

Lin Z (2004) Time-resolved fluorescence-based europium-derived probes for peroxidase bioassays, citrate cycle imaging and chirality sensing. PhD thesis, University of Regensburg... 

When deciding to study the dynamics of electronic excitation energy transfer in molecular systems by conventional spectroscopic techniques (in contrast to those based on non-linear properties such as photon echo spectroscopy) one has the choice between time-resolved fluorescence and transient absorption. This choice is not inconsequential because the two techniques do not necessarily monitor the same populations. Fluorescence is a very sensitive technique, in the sense that single photons can be detected. In contrast to transient absorption, it monitors solely excited state populations this is the reason for our choice. But, when dealing with DNA components whose quantum yield is as low as 10-4, [3,30] such experiments are far from trivial. 

Most of the time-resolved emission spectroscopy setups are home made in the sense that they are built from individual devices (laser, detection system,. ..) hence they are not of a plug and press type, so that their exact characteristics may vary from one installation to the other. Some of these differences have no impact on the overall capabilities of the system but some have a drastic influence on the way the collected data are processed and analysed. This aspect will be detailed in the next section, while this section deals with a general description of the apparatus. The most basic type of apparatus will be described, with no reference to sophisticated techniques such as Time Correlated Single Photon Counting or Circularly Polarized Luminescence devices. 

Figure 6 shows an outline of a PAS instrument designed for fast time-resolved measurements. The excitation light is a laser pulse of some 20 ns duration, at a wavelength which falls within the absorption spectrum of the sample (e.g. 337 nm with a nitrogen laser). Total absorption of this pulse then deposits an energy E in the sample and this will decay in the course of time into heat which will give rise to the pressure sensed by the detector usual microphones have slow response times, so that piezo-electric devices are used to improve the instrument s time resolution [43]. 

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