Excitation spectra, luminescent probes

The most important applications of luminescence probing in microemulsions involve the deactivation dynamics or excitation energy transfer properties of the excited states. With a brief flash of light a population of excited species is created in the sample, and the subsequent deactivation is observed over time. The decay of the excited probe, and the fluorescence spectrum, may depend on the interactions with the environment, which reveal useful information. In time-resolved luminescence quenching (TRLQ), however, it is the interaction of the probe with another added component, a quencher, that is studied. This method is dealt with here. For micellar systems, several publications have already discussed it in both experimental and theoretical detail [1-6]. 

Aromatic hydrocarbons such as pyrene have also been employed as a luminescence probe of polarity and microviscosity in a variety of organized assemblies (109). Pyrene is a good excimer-forming probe due to the long lifetime of fluorescence and formation of excited-state dimers (excimers) at low concentration. Figure 9 shows an example pyrene luminescence spectrum. The ratio of excimer to monomer fluorescence intensity is often utilized as a measure of pyrene mobility and proximity. The vibronic fine structure of the pyrene monomer is sensitive... 

Luminescence decay curves are also often used to verify that samples do not contain impurities. The absence of impurities can be established if the luminescence decay curve is exponential and if the spectrum does not change with time after pulsed excitation. However, in some cases, the luminescence decay curve can be nonexponential even if all of the luminescing solutes are chemically identical. This occurs for molecules with luminescence lifetimes that depend upon the local environment. In an amorphous matrix, there is a variation in solute luminescence lifetimes. Therefore, the luminescence decay curve can be used as a measure of the interaction of the solute with the solvent and as a probe of the micro-environment. Nag-Chaudhuri and Augenstein (10) used this technique in their studies of the phosphorescence of amino acids and proteins, and we have used it to study the effects of polymer matrices on the phosphorescence of aromatic hydrocarbons (ll). 

Fig. 1.19. Quenching of the coherent vibrational oscillations of MDMO-PPV upon photoinduced charge transfer. The AT/T dynamics for pure MDMO-PPV (continuous line) and for MDMO-PPV/PCBM (1 3 wt. ratio) (dashed line), excited by a sub-10-fs pulse, was recorded at the probe wavelength of 610 nm. The inset shows the Fourier transform of the oscillatory component of the MDMO-PPV signal, the nonresonant Raman spectrum of MDMO-PPV (excitation 1064 nm) and the resonant Raman spectrum of an MDMO-PPV/PCBM sample (excitation 457 nm). For the resonant Raman spectrum of MDMO-PPV, it was necessary to quench the strong background luminescence by adding PCBM...

Fig.11.2 Optical spectra of various optical probes, (a) Photo-luminescence spectra of Q-dots (from the work by Michalet et al. [2]), (b) scattering spectra of gold nanoparticles with different aspect ratio (from the work by Jain et al. [3]), and (c) fluorescence spectrum of FITC (black line) and SERS spectrum (red line) of benzene thiol with 514.5 nm photo-excitation (from the work by Jun et al. [4]). Raman band has much narrower bandwidth than the others...

The first sensor of that kind was presented by Sun et al. [48] who reported on a pH sensor based on the upconversion luminescence of NaYF4 Yb,Er nanorods (see inset Fig. 6b) that were embedded in a matrix of hydrogel along with the longwave absorbing pH probe (bromothymol blue BTB) that causes a pH dependent inner filter effect (Fig. 6a). The emission spectrum of the nanorods at NIR excitation and the pH dependent absorption spectrum of BTB for three pH values are shown in Fig. 6b. 

---