Ratiometric fluorescent sensors

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. 

Macrocyclic 25 has been constructed with pseudocryptand 26 which can be discerned within Tsien s ratiometric fluorescent sensors as the Na receptor unit. Conformational changes by 26 upon ion binding allows strong off-on PET sensing action due to the ion-induced increase of its oxidation potential. X-ray structural evidence for the Na -induced conformational changes in 26 shows decreased conjugation with a 40° twist about the C-N bond of the 2-anisidine unit. X-ray evidence is also available for similar conformational changes in the non-macro-cyclic Ca " receptor 23. A related analysis has been made for a Ag" -phenyl-azacrown ether interaction which involves a soft-soft component. ... 

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. 

The racemic bicyclic isoxazolidine 626a, bearing a pyrenyl moiety, was shown to be an effective ratiometric fluorescent sensor selective for Ag(i) <2003JA2884>. The selective complexation of compounds 626a-e and 627 toward Fe(lll), Cu(ll), and Ru(lll) was also analyzed <2005TL173>. 

Yuan, L. Lin, W. Xie, Y Chen, B. Song, J. Development of a ratiometric fluorescent sensor for ratiometric imaging of endogenously produced nitric oxide in macrophage cells. Chem. Commm. 2011, 47, 9372-9374. 

Chang CJ, Javorski J, Nolan EM, Shaeng M, Lippard SJ (2004) A tautomeric zinc sensor for ratiometric fluorescence imaging application to nitric oxide-release of intracellular zinc. Proc Natl Acad Sci USA 101 1129-1134... 

Li H, Xu J, Yan H (2009) Ratiometric fluorescent determination of cysteine based on organic nanoparticles of naphthalene-thiourea-thiadiazole-linked molecule. Sensor Actuat B-Chem... 

Haidekker MA, Brady TP, Lichlyter D, Theodorakis EA (2006) A ratiometric fluorescent viscosity sensor. J Am Chem Soc 128 398-399... 

The chromophore environment can affect the spectral position of the absorption and emission bands, the absorption and emission intensity (eM, r), and the fluorescence lifetime as well as the emission anisotropy, e.g., in the case of rigid matrices or hydrogen bonding. Changes in temperature typically result only in small spectral shifts, yet in considerable changes in the fluorescence quantum yield and lifetime. This sensitivity can be favorably exploited for the design of fluorescent sensors and probes [24, 51], though it can unfortunately also hamper quantification from simple measurements of fluorescence intensity [116], The latter can be, e.g., circumvented by ratiometric measurements [24, 115],... 

Fig. 14 Ratiometric fluorescence images (right RLD, left PDR) of sensor spots in a microwell plate exposed to different hydrogen peroxide concentrations A1 0, A2 0.5, A3 1, A4 5, 51 7.5, 52 10, 53 25, 54 50, Cl 75, C2 100, C3 250, C4 500 ppm H202 (top). Calibration plot (R(j - R)/Ro versus the concentration of hydrogen peroxide in MOPS buffer (pH 6.9). Ro is the initial ratiometric fluorescence intensity of the sensor membrane, and R the intensity in presence of increasing concentrations of H202. Squares PDR image, triangles RLD image (bottom)...

Figure 11.16 Multishell silica nanoparticle for ratiometric fluorescence sensing of Pb2+ ions, (a) Schematic working scheme of the sensor binding of the Pb2+ ions to the thiol groups present on the surface causes the quenching of the emission of the dansylamide dyes (labeled e when unperturbed and q when quenched) in the outer shell but not of the methoxynaphthalene dyes in the core (labeled r). (b) TEM micrograph of the particles (inset) and ratiometric behavior of the fluorescence emission at different Pb2+ concentrations.83 (Reprinted with permission from M. Arduini et al., Langmuir, 2007, 23, 8632—8636. Copyright 2007 American Chemical Society.)...

Figure 25. Two small molecule fluorescent sensors have been reported recently that exhibit an increase in fluorescence emission in the presence of lead (a) A-methyl-9-anthrylthiohydroxamic acid (448) and (b) benz(c,d)indole-l,7-diaza-15,crown-5 (Bl-crown) (447) the BI crown is also ratiometric. In addition, Bradshaw and co-workers recently reported a series of bis(hydroxyquino-line) derivatives of diazatrithia-15-crown-5 [e.g., (c) 3-hydroxy-8,14-bis(8-hydroxyquinolin-2-ylmethyl)-8,14-diaza-l,5,ll-trithiacyclohexadecane] that exhibit shifted fluorescence emission maxima upon binding of Pb + (449).

Paek K, Chung S, Cho C-H, Kim BJ (2011) Fluorescent and pH-responsive diblock copolymer-coated core-shell CdSe/ZnS particles for a color-displaying, ratiometric pH sensor. Chem Commun 47 10272... 

Generally, there exists serious interference between Cd and Zn in most reported sensors due to their similar properties. Interestingly, as a ratiomet-ric fluorescent sensor, probe 13 can distinguish Cd from Zn. Coordination of Cd with 13 induces blue-shifts in absorption and huorescence bands accompanied by a simultaneous increase in fluorescence intensity. As well demonstrated, probe 13 was successfully exploited for imaging intracellular Cd in living cells with both normal huorescence turn-on and ratiometric modes (Scheme 7.15). 

A ratiometric sensor 4 for Cd " and Zn " based on calix[4]arene bearing 1,2,3-triazole-linked pyrene units has been reported (Fig. 28.2) [30]. Probe 4 showed strong excimer and weak monomer emissions, due to face-to-face jt-interactions. Addition of Cd " or Zn produced a ratiometric fluorescence change, with a decrease in excimer emission and an increase in monomer enussion. This ratiometric behavior is due to the complexation of Cd " and Zn " by the triazole nitrogen atoms, which causes a change in the conformation of the pyrene arms. No other metal ions produced such ratiometric response, confirming the high selectivity of probe 4. 

Brown, J. Q. McShane, M. J. Core-referenced ratiometric fluorescent potassium ion sensors using self-assembled ultrathin films on europium nanoparticles. IEEE Sens. J. 2005, 5, 1197-1205. 

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