Water molecule modulation, detection

The formation of these ternary luminescent lanthanide complexes was the result of displacement of the two labile metal-bound water molecules, which was necessary because the energy transfer process between the antenna and the Ln(III) metal centre is distance-dependent. This ternary complex formation was confirmed by analysis of the emission lifetimes in the presence of DMABA and showed the water molecules were displaced by a change in the hydration state q from 2 to 0, with binding constants of log fCa = 5.0. The Eu(III) complexes were not modulated in either water or buffered solutions at pH 7.4. Lifetime analysis of these complexes showed that the metal-bound water molecules had not been displaced and that the ternary complex was not formed. Of greater significance, both Tb -27 and Tb -28 could selectively detect salicylic acid while aspirin was not detected in buffered solutions at pH 7.4, using the principle as discussed for DMABA where excitation of the binding antenna resulted in a luminescent emission upon coordination of salicylic acid to the complex. 

The relaxivity of a responsive probe should be selectively influenced by the physiological parameter to be detected. The relaxivity is related to the microscopic properties of the contrast agent. The most important parameters that can be tailored by the chemist are the hydration number, the exchange rate of the water molecules with the surrounding water (bulk), and the motional dynamics of the molecules. All these three parameters can be modulated through supramolecular interactions. The literature on responsive probes is quite extensive here we present few examples to illustrate the importance of supramolecular interactions in the field of responsive probes. 

We have developed a novel ultrasensitive detection method, thermal lens microscopy (TLM), for nonfluorescent species [13]. TLM is photothermal spectroscopy under an optical microscope. Our thermal lens microscope (TLM) has a dual-beam configuration excitation and probe beams [13]. The wavelength of the excitation beam is selected to coincide with an absorption band of the target molecule and that of the probe beam is chosen to be where the sample solution (both solvent and solute) has no absorption. For example, in determination of methyl red dye in water, cyclohexane, and n-octanol, a 514-nm emission line of an argon-ion laser and a 633-nm emission line of a helium-neon laser were used as excitation and probe beams, respectively [21], Figure 4 shows the configuration and principle of TLM [13]. The excitation beam was modulated at 1 kHz by an optical chopper. After the beam diameters were expanded, the excitation and probe beams were made coaxial by a dichroic mirror just before they were introduced into an objective lens whose magnification and numerical aper-... 

Raman spectroscopy (RS) is a well known technique to detect the vibrational characteristics of molecules in various media and is therefore extensively used in physics chemistry and biologyGenerally this technique is easily implemented, and does not require sample preparation. In addition RS has the advantage that it can be applied in water solutions, in contrast to IR absorption. In a classical picture RS results from the inelastic interaction between a molecular system and the electromagnetic field of a laser source." The electronic polarizability is modulated by the vibration mode associated with the motion of the molecule, at a frequency (Raman shift) which is the difference (Stokes scattering) or the sum (anti-Stokes scattering) between the laser and the molecular frequencies. The induced dipole moment can be written as ... 

EMIRS has been successfully applied to many systems. Briefly it can be mentioned the study of adsorbates at the electrode surface [10], the detection of adsorbed reaction intermediates for the oxidation of small organic molecules [12], and the determination of the water structure in the double layer [13]. However, the potential modulation in EMIRS is its drawback, since it prevents the study of irreversible processes as the system must return to the same conditions each time the potential is changed. Other important limitations of EMIRS are related to both the electrical and chemical relaxation effects caused by the potential modulation at 12 Hz. The electrical relaxation is due to the high ohmic drop of the electrolyte confined in the thin solution layer required for the in situ measurements. The chemical relaxation is due to ion migration induced by the change in solution composition caused by the electrode potential change. These aspects have been discussed in detail in the following text [14-16] (see Sect. 3.4.2.3). 

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