Sensor molecules

Rousseau, R., Dietrich, G., Kruckeberg, S., Lutzenkirchen, K., Marx, D., Schweikhard, L. and Walther, C. (1998) Probing cluster structures with sensor molecules methanol adsorbed onto gold clusters. Chemical Physics Letters, 295, 41-46. 

Molecular Dynamics of a Sensor Molecule in Various Hosts (Example Ferrocene (FC))... 

By using NFS, information on both rotational and translational dynamics can be extracted. In many cases, it would be favorable to obtain separate information about either rotational or translational mobility of the sensor molecule. In this respect, two other nuclear scattering techniques using synchrotron radiation are of advantage. Synchrotron radiation-based perturbed angular correlations (SRPAC) yields direct and quantitative evidence for rotational dynamics (see Sect. 9.8). NIS monitors the relative influence of intra- and inter-molecular forces via the vibrational density of states (DOS) which can be influenced by the onset of molecular rotation (see Sect. 9.9.5). 

Boson Peak, a Signature of Delocalized Collective Motions in Glasses (Example FC as Sensor Molecule)... 

FC as sensor molecule has been used to investigate the low-energy mobility, i.e., the nature of the Boson peak and of the trawi-Boson dynamics, of toluene, ethylbenzene, DBF and glycerol glasses [102]. The spectator nucleus Fe is at the center of mass of the sensor molecule FC. In this way, rotations are disregarded and one selects pure translational motions. Thus, the low-energy part of the measured NIS spectra represents the DOS, g(E), of translational motions of the glass matrix (below about 15 meV in Fig. 9.39a). 

Fig. 9.39 (a) Density of states (DOS), g E), obtained from NIS at 22 K on ferrocene as sensor molecule in toluene glass, (b) Reduced DOS, g(E)/E, for various glasses. Arrows indicate the energy of the Boson peak. (Taken from [102])... 

Here Fmin is the fluorescence intensity without binding and /,max is the intensity when the sensor molecules are totally occupied. Kd is the dissociation constant. The differences in intensities in the numerator and denominator allow compensating for the background signal, and the obtained ratio can be calibrated in target concentration. But since F, Fmin and Fmax are expressed in relative units, they have to be determined in the same test and in exactly the same experimental conditions. This requires proper calibration, which is difficult and often not possible. 

In intensity sensing, the most efficient and commonly used method of intrinsic referencing is the introduction of a reference dye into a sensor molecule (or into support layer, the same nanoparticle, etc.) so that it can be excited together with the reporter dye and provide the reference signal [1], The reference dye should conform to stringent requirements ... 

Takakusa, H., Kikuchi, K., Urano, Y., Sakamoto, S., Yamaguchi, K. and Nagano, T. (2002). Design and synthesis of an enzyme-cleavable sensor molecule for phosphodiesterase activity based on fluorescence resonance energy transfer. J. Am. Chem. Soc. 124, 1653-1657. 

Applications exploiting porous silica to encapsulate sensor molecules, enzymes and many other compounds are developing rapidly. Nowadays, sol-gel technology is being used in various fields of modem technology, as for example the basis for optodes, integrated systems, fiber optics, lasers, and new materials. 

Li YQ, Bricks JL, Resch-Genger U et al (2006) Bifunctional charge transfer operated fluorescent probes with acceptor and donor receptors. 2. Bifunctional cation coordination behavior of biphenyl-type sensor molecules incorporating 2, 2 6, 2"-terpyridine acceptors. J Phys Chem A 110 10972-10984... 

Norrild and coworkersd showed that this structure is only valid as an initial complex formed under completely non-aqueous conditions. In the presence of water, a rapid rearrangement from the a-D-glucopyranose form to the a-D-glucofuranose occurs. In the latter form, all five free hydroxy groups of glucose are covalently bound to the sensor molecule (Figure B.10.4.2). 

Luminescent Sensor Molecules Based on Coordinated Metals A Review of Recent Developments, Coord. Chem. Rev. 

In spite of the numerous studies on practical applications of luminescence sensors, the understanding of the fundamental primary processes and underlying photophysics is still in its infancy. Only when details of the types of sites occupied by the sensor molecules, the local environment, and the quenching processes are understood and correlated with a variety of sensor molecules and supports can new materials and supports be rationally designed. 

Figure 9.1. Radiative and nonradiative paths to the ground state of excited sensor molecules m. All possible mechanisms by which an excited molecule may return to the ground state are sorted in two sets. One set, characterized by the rate constant kr, is referred as the radiative path. The second set, characterized by the rate function knr, is referred to in this chapter as the nonradiative path.

The sensor molecule probability of remaining in the excited state pm as a function... 

A luminescent material in which all excited molecules return to the ground state with the same probability is defined here as a homogeneous sensor. In this case, the number of excited sensor molecules in the excited state, as a function of time is given by a single exponential. 

From the practical point of view, the radiative decay rate kr may be assumed to be independent of the external parameters surrounding the excited sensor molecule. Its value is determined by the intrinsic inability of the molecule to remain in the excited state. The radiative decay rate kr is a function of the unperturbed electronic configuration of the molecule. In summary, for a given luminescent molecule, its unperturbed fluorescent or phosphorescent decay rate (or lifetime) may be regarded to be only a function of the nature of the molecule. 

In contrast, the nonradiative decay rate k r may be viewed to be determined by the localized environment of the luminescent molecule. The localized environment perturbs the natural electronic configuration of the sensor molecule increasing the probability of its decay. The functional form of knr is determined by the nature of the interaction between the excited sensor and its surrounding perturbation. For example, the knr may be proportional to the concentration, partial pressure, or value of a [Parameter] of interest ... 

Sensors are usually attached chemically or physically to other materials here referred as the carrier, like polymers, antibodies, and optical fibers in order to facilitate the sensing process. These carriers generally affect the luminescent characteristics of the sensor molecules. The modification of the luminescent characteristics of the sensor is caused by the creation of more than one microphase or microenvironment for the sensor. Each molecule in its particular microenvironment may return to the ground state following a different set of processes or mechanisms. Alternatively, the nonra-diative decay rate of each microphase may be different for each sensor molecule. Depending on the characteristics of the carrier and the sensor, the number of microphases may be one, two, three, or an infinite number. 

Consider, for example, a sensor composed of n phases each with a different nonradiative decay rate. The fraction of the excited sensor molecules Pi in phase ith may decay with an overall decay rate kr + k ri. In this case, the average probability of the sensor molecules of remaining in the excited state is given by... 

The time constant or lifetime t of the sensor luminescence is determined by the value of [Parameter]. For sensor-carrier preparations with a uniform composition in which all sensor molecules return to the ground state with the same probability we have ... 

Going back to Figure 9.1, the probability of finding a sensor molecule pm in the excited state is modified in the presence of steady-state excitation e(t) ... 

Frequency domain measurements require the use of periodic excitation sources. The luminescent molecules respond to the periodic excitation exhibiting the same frequency of modulation. This luminescence exhibits a phase delay and a demodulation with respect to the excitation due to the inability of the sensor molecule to respond to the higher frequencies of the excitation. This inability of the sensor molecules roughly begins at modulation frequencies /modulation of the same order of magnitude or faster than the decay rate... 

Figure 6.6. Various switching factors in 148 and 149 sensor molecules.

Some analytes, such as riboflavin (vitamin B2)16 and polycyclic aromatic compounds (an important class of carcinogens), are naturally fluorescent and can be analyzed directly. Most compounds are not luminescent. However, coupling to a fluorescent moiety provides a route to sensitive analyses. Fluorescein is a strongly fluorescent compound that can be coupled to many molecules for analytical purposes. Fluorescent labeling of fingerprints is a powerful tool in forensic analysis.17 Sensor molecules whose luminescence responds selectively to a variety of simple cations and anions are available.18 Ca2+ can be measured from the fluorescence of a complex it forms with a derivative of fluorescein called calcein. 

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