Luminescent sensors/probes

Fig. 2 Luminescent sensors/probes. (Weir this art in color at www.dekker.com.)...

Until relatively recently, most work focused on organic luminophores as sensor-probe materials. However, luminescent transition metal complexes, especially those with platinum metals (Ru(II), Os(II), Re(I), Rh(III), and Ir(III)) have shown considerable promise and are receiving increasing attention. More recently Pt(II) complex have shown promising results.(4) Many of these materials have highly desirable features ... 

An important class of luminescence sensors are those based on the decrease of luminescence intensity and lifetime of the probes as function of analyte concentration. Assume that the probe intensity decays by a single exponential with an unquenched lifetime tq. If quenching occurs only by a dynamic (collisional) mechanism, then the ratio to/t is equal to Fq/F and is described by the classic Stern-Volmer equation... 

Aggregation as the cause of luminescence was also probed in fluid solutions, since a dilute colorless solution was not emissive, but a concentrated (2 x 10-2 M) solution was weakly emissive a result that is consistent with the aggregation of these units in solution. Undoubtedly, this complex has potential for practical applications as a luminescent sensor for the detection of volatile organic compounds (VOCs). 

Fig. 6 Schematic representation of a time-resolved measurement of pC>2. Oxygen quenches the luminescence of the sensor probe and decreases its decay time r [33]...

The HT luminescence characterization described above can also be applied to other materials, such as catalysts and sensor/probe molecules. 

Several papers reported on the spatial determination of photoactive molecule in organized assemblies (76-79), and Pallavicini and coworkers reviewed on the use of luminescence as probe in self-assembly of multicomponent fluorescent sensors (80). Also, luminescence quenching studies on [Rufbpyls] in sodiiun dodecyl sulfate (SDS) micelles and hemimicelles by using a variety of quenchers were reported by Tiuro and coworkers (81) and then reviewed by De Schryver and coworkers (82). 

It is common to use the term "probe to describe a small dye molecule added passively to a system for study. By contrast, a label is attached covalently to some component of the system. Fluorescent probe experiments are easy to carry out, but the onus rests with the experimenter to prove where in the system the probe is located. Labelling experiments are more demanding because of the need to synthesize the labelled component. Data interpretation is often easier because the dilemma of dye location is less severe. We like the phrase luminescent sensor to describe the general case where a dye is used either as a label or as a probe. 

There is a substantial literature indicating that fluorescence and phosphorescence behavior of many dyes are sensitive to various relaxation processes of the polymer matrices in which they are dissolved (17,18). Large scale motions of a probe couple to chain motions responsible for the glass transition and provide a measure of Tg. Smaller scale rotations can sense matrix chain motions responsible for more localized (3 or y) relaxation processes. It is also well known that diffusion of gasses in glassy polymers is also very sensitive to these relaxations. If this diffusion leads to luminescence quenching, one expects the emission intensity or decay time of a luminescent sensor to be sensitive to these relaxation processes. 

It is clear from the many examples discussed in this chapter that lanthanide lununescent sensors, probes and imaging agents have established themselves as a major player within the field of luminescent sensing. And we can only foresee that the role of these rare-earth ions will continue to grow in the years to come, and that the field of lanthanide luminescence sensing has a very bright and varied future. 

The commercialization of inexpensive robust LED and laser diode sources down to the uv region (370 nm) and cheaper fast electronics has boosted the application of luminescence lifetime-based sensors, using both the pump-and-probe and phase-sensitive techniques. The latter has found wider application in marketed optosensors since cheaper and more simple acquisition and data processing electronics are required due to the limited bandwidth of the sinusoidal tone(s) used for the luminophore excitation. Advantages of luminescence lifetime sensing also include the linearity of the Stem-Volmer plot, regardless the static or dynamic nature of the quenching mechanism (equation 10) ... 

Even if few systems are proposed for inorganic compounds (with regard to the number of potential pollutants), instruments or sensors for parameters used for treatment process control are available UV systems for residual chlorine in deodorization, electrochemical sensors for dissolved oxygen (with nowadays a luminescent dissolved-oxygen probe utilizing a sensor coated with a luminescent material) and a colorimetric technique for residual ozone. 

Abstract Silver clusters, composed of only a few silver atoms, have remarkable optical properties based on electronic transitions between quantized energy levels. They have large absorption coefficients and fluorescence quantum yields, in common with conventional fluorescent markers. But importantly, silver clusters have an attractive set of features, including subnanometer size, nontoxicity and photostability, which makes them competitive as fluorescent markers compared with organic dye molecules and semiconductor quantum dots. In this chapter, we review the synthesis and properties of fluorescent silver clusters, and their application as bio-labels and molecular sensors. Silver clusters may have a bright future as luminescent probes for labeling and sensing applications. 

In contrast to the 02 generator, the polymer used in the O2 sensor must be highly gas permeable but solvent impenetrable. Solvent penetration will alter the probe properties and make calibration dependent on environment. Furthermore, good solvents for the probe will leach the probe and destroy the sensor. Again, it is important that the support dissolve the probe well and not greatly quench the luminescence. 

The fluorescence and phosphorescence of luminescent materials are modulated by the characteristics of the environment to which these materials are exposed. Consequently, luminescent materials can be used as sensors (referred also as transducers or probes) to measure and monitor parameters of importance in medicine, industry and the environment. Temperature, oxygen, carbon dioxide, pH, voltage, and ions are examples of parameters that affect the luminescence of many materials. These transducers need to be excited by light. The manner in which the excited sensor returns to the ground state establishes the transducing characteristics of the luminescent material. It is determined by the concentration or value of the external parameter. A practical and unified approach to characterize the luminescence of all sensors is presented in this chapter. This approach introduces two general mechanisms referred as the radiative and the nonradiative paths. The radiative path, in the general approach, is determined by the molecular nature of the sensor. The nonradiative path is determined by the sensor environment, e.g., value or concentration of the external parameter. The nonradiative decay rate, associated with the nonradiative path, increases... 

The use of fluorescent dyes in biological probes and sensors is covered in some detail in Chapter 3 (section 3.5.6). Because there are marked solvatochromic effects on the luminescent spectra of many fluorophores, this phenomenon is utilised to tune their performance and application in biological and other systems. 

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