Time-gated measurements

Attractive for the use of QDs are their long lifetimes (typically 5 ns to hundreds of nanoseconds), compared to organic dyes, that are typically insensitive to the presence of oxygen. In conjunction with time-gated measurements, this provides the basis for enhanced sensitivity [69]. This property can be also favorable for time-resolved applications of FRET. The complicated size-, surface-, and wavelength-dependent, bi- or multi-exponential QD decay behavior (Fig. 2) can complicate... 

In addition, the luminescence lifetime of quantum dots is approximately one order of magnitude longer than that of their organic counterparts.30 Thus, the autofluorescence of biological samples can effectively be filtered, for example, with time-gated measurements, in the case of these inorganic luminophores. Moreover, QDs are also particularly interesting for multiplexed analysis because of two of their inherent... 

TR-FRET measurements present interesting alternatives to the FRET quench format to minimize compound interference. Time-gated measurements allow the exclusion of compound auto-fluorescence from the detection. The technology is very popular due to its sensitivity, ease of use, and simplicity of assay development. The high costs for the reagents are the major disadvantage of... 

Pal R., A. Beeby, D. Parker. Analysis of citrate in low-volume seminal fluid samples using a time-gated measurement of europium luminescence, J. Pharm. Biomed. Anal., 56, 352-358 (2011). 

Fig. 2. Three time gates set to measure backseattered pulse from cylinder interface echo (gate 1), echo from discontinuity (gate 2) and back echo (gate 3).

The Evaluation system is a Windows based open system through DLL, Dynamic Link Library, which provides great flexibility in evaluation and presentation of data. It also makes it possible to customise evaluation and images for special applications The time gates can be set after testing and there is a 256 colour range for amplitude. The software include FFT -facilities which enables measurements on used probes for parameters such as center frequenzy and bandwidth. 

The PE data was obtained by repeating the scanning of the object, now measuring the received echo at Transducer 1. For every position, (x, y), an A-scan was obtained from which we extracted the back wall echo by means of a time gate. This back wall echo is denoted s(x, y). Note that s x, y) is a time signal that can be written s(f, x, y) where t is the time index. One example of such a back wall echo is shown in Figure 2. 

In TG methods, the fluorescence emission is detected in two or more time-gates each delayed by a different time relative to the excitation pulse (see Fig. 3.3). In the case of a detection scheme equipped with two time-gates, the ratio of the signals acquired in the two time-gates is a measure of the fluorescence lifetime. For a decay that exhibits only a single exponent, the fluorescence lifetime is given by ... 

In contrast, time domain instruments attempt to directly measure the decay characteristics of a fluorophore of interest by excitation with ultrashort light pulses and monitoring the decay using either TCSPC [5] or a time gated image intensifier [8],... 

Spin-lattice relaxation times were measured by the fast inversion-recovery method (24) with subsequent data analysis by a non-linear three parameter least squares fitting routine. (25) Nuclear Overhauser enhancement factors were measured using a gated decoupling technique with the period between the end of the data acquisition and the next 90° pulse equal to eibout four times the value. Most of the data used a delay of eibout ten times the Ti value. (26)... 

Fig. 9 Schematic representation of the time-domain DLR (f-DLR) scheme. The shortlived indicator fluorescence and the long-lived phosphorescence of the inert reference beads are simultaneously excited and measured in two time gates. The first (Aex) is in the excitation period where the light source is on and the signal obtained is composed of short-lived fluorescence and long-lived luminescence. The second gate (Aem) is opened in the emission period where the intensity is exclusively composed of the reference luminescence [18]...

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