Sensors wavelength-ratiometric

Badugu R, Lakowicz JR, Geddes CD. A wavelength-ratiometric fluoride-sensitive probe based on the quinolinium nucleus and boronic acid moiety. Sensors and Actuators B 2005, 104, 103-110. 

A spectral shift due to interconversion between monomer and excimer/exciplex emission can be exploited to develop temperature sensors for the macroscopic environment. For example, a thermochromic sensor film was developed by immobilising perylene in a polystyrene film with iV-aUyl-iV-methylaniline (NA) [41]. In solution and in the absence of NA, perylene emits blue fluorescence — 475 nm). In thin films and in the presence of NA, an additional broad red emission band is observed (Aem = 551 nm), which is attributed to the perylene-NA exciplex. The fluorescence spectrum is temperature dependent on heating between 25 and 85 °C the relative intensity of the blue monomer emission increases at the expense of the exciplex emission band, indicating that at higher temperatures the monomer-exciplex equilibrium is shifted in favour of the monomer. A wavelength ratiometric approach based on the relative intensities of the two emission peaks as a function of temperature was used to calibrate the sensor film [41]. 

Another feature is that the absence of spectral change precludes ratiometric measurements. However, dual-wavelength Cl sensors have been constructed. For instance, in compound A-6 (Figure 10.30), 6-methoxyquinolinium (MQ) as the Cl -sensitive fluorophore (blue fluorescence) is linked to 6-aminoquinolinium (AQ) as the Cl--insensitive fluorophore (green fluorescence), the spacer being either rigid or flexible. 

System 22 is an earlier example which incorporates Tsien s selective calcium receptor 23. ° System 23 has also been employed for the construction of ratiometric fluorescent sensors involving wavelength shifts. System 22 and other related PET sensors provide some of the most visually dramatic fluorescence off-on switching induced by biologically relevant levels of calcium ions in addition to their consistent predictability of most sensor parameters. 

In order to obtain a ready-for-use sensor array, the probe was immobilized in a block copolymer matrix (polyacrylonitrile-co-polyacrylamide Hypan), which is completely penetrated by water if exposed to it [102], Prior to immobilization, the sensor membrane was cast onto an optically transparent ethyleneglycol-terephthalate polyester support (Mylar). The resulting sensor foil was glued on a black 96-microwell format matrix. The sensor arrays were analyzed by means of time-resolved RLI and PDI methods (see Sect. 2.1) with an optical set up as illustrated in Fig. 6 at an excitation wavelength of 405 nm. The ratiometric images resulted in similar calibration plots for both methods (Fig. 14). The limit of detection and the dynamic range of this sensor foil are comparable to those observed with [Eu(Tc)] in solution [103]. 

This receptor shows a remarkable selectivity for Mg2+ over Ca2+ under physiological conditions and has found applications in 19F NMR probes and ratiometric fluorescent sensors based on wavelength shifts.[62] In high concentrations, however, both Ca2+ and Mg2+ can be bound. The similarity of fluorescence enhancements with both ions is the result of essentially identical conformational changes produced upon complexation. Each ion-bound state effectively decouples the amine substituent from the oxybenzene unit, so that PET is similarly suppressed. This means that the charge density difference between the two cations is of secondary importance in these conformationally switchable systems. 

A disadvantage of the use of fluorescein in sensors is that fluorescein is difficult to enqrloy as a wavelength-ratiomet-ric probe. This is because the absotption and emission intensity are tow for 4S0-mn excitation (Figure 19.32). The pH probe S-hydroxypyrene-l,3,6,8-trisulfonate (HPTS) displays more favorable properties as a wave-tongth-ratiometric probe. In HPTS (Figure 19.34), the sulfonate groups provide solubility in water, and the hydroxyl group provides sensitivity to pH. 

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