Fluorescence quenching by metals

De Schryver et al. [44] applied this model to analyse the pyrene fluorescence quenching by metal ions in SDS micelles. The situation described by the inequality (40) was observed for nickel, copper and lead ions. The values were determined from the slope of the linear plot of S2/SS vs. [M] [see Eq. (43)]. For europium and chromium ions, both interfacial exchange processes in micelle-micelle and micelle-bulk solution are very slow as compared with pyrene fluorescence decay. Here, the kinetics fits well to Case 2 discussed above. For silver and thallium ions, the rates of the fluorescence... 

Pyrene fluorescence quenching by metal ions in SDS micelles was studied at both constant and pulse illumination by Ziemiecki and Cherry [50, 51]. In order to analyse the kinetic data they applied the same kinetic model as De Schryver et al. The values of kf. obtained at steady-state excitation and at pulse excitation coincide. Thus, the pyrene fluorescence quenching by metal ions in SDS is an example of pure dynamic quenching within the micelles. The data of De Schryver et al. [44] and of Ziemiecki and Cherry [50, 51] are compiled in Table 1. 

Fluorescence quenching by I of a methylated analogue of trans-4,4 -DPE leads to the radical cation [515], Exciplexes were found for StNPs [290] and several other arylstyrenes [516]. Radical anions and cations were observed for styrylanthracenes and styrylphenanthrenes [436,517]. Energy and electron transfer from excited metal complexes to olefins has been reported, from Ru(bpy) + to a methylated 4-StP [518] or from Cr(bpy)] +... 

One further feature of SERS merits attention at this point. This is the widely recognized fact that strongly fluorescent molecules, whose Raman spectra are obscured by their much stronger and broader fluorescence emission, commonly have their strong fluorescence quenched by adsorption at a SERS-active metal surface. The fluorescence quenching mechanism is presumably closely related to that for the CT effect shown in Fig. 2. 

In two more recent papers on single molecule fluorescence near thin metallic layers, Enderlein reminds us that the quenching by metal nanoparticles shortens the lifetime of the excited state and by doing this, increases the number of excitation cycles that the molecule can survive before it is photobleached. This is an extremely important point, especially when dealing with single molecules, as fluorescence quantum yields are not nearly as important as the number of photons that are emitted before photobleaching occurs. 

Other limitations found in fluorescent derivatives include quenching by oxygen, metals, or other species, chemical or photochemical instability, and biodegradability. 

Fig. 3a, b. Schematic representation of (a) conventional fluorescent sensor and (b) fluorescent sensor with signal amplification. Open rhombi indicate coordination sites and black rhombi indicate metal ions. The curved arrows represent quenching processes. In the case of a den-drimer, the absorbed photon excites a single fluorophore component, which is quenched by the metal ion regardless of its position... 

It has been demonstrated that dendrimers can be used also as fluorescent sensors for metal ions. Poly(propylene amine) dendrimers functionalized with dansyl units at the periphery like 34 can coordinate metal ions by the aliphatic amine units contained in the interior of the dendrimer [80]. The advantage of a dendrimer for this kind of application is related to the fact that a single analyte can interact with a great number of fluorescent units, which results in signal amplification. For example, when a Co ion enters dendrimer 34, the fluorescence of all the 32 dansyl units is quenched with a 32-fold increase in sensitivity with respect to a normal dansyl sensor. This concept is illustrated in Fig. 3. 

Chen YG, Zhao D, He ZK, Ai XP (2007) Fluorescence quenching of water-soluble conjugated polymer by metal cations and its application in sensor. Spectrochim Acta A Mol Biomol Spectrosc 66 448 -52... 

Fig. 4 (a) Changes in the absorption spectra of Mcy-I upon titration by Hg2+ (b) absorption spectra changes of Mcy-1 upon addition of different cations (10 equiv) (c) fluorescence responses of Mcy-1 to various metal cations (d) bars represent the percentage of fluorescence quenched (reprint from ref. [53], Copyright 2008 American Chemical Society)... 

Yet the majority of cation sensors are mediator-based sensing. One type of optical sensor is based on the fluorescence quenching of fluor-ophore Rhodamine 6G by transition metals such as Co(II), Cr(III),... 

Masuhara H, Shioyama H, Saito T et al (1984) Fluorescence quenching mechanism of aromatic hydrocarbons by closed-shell heavy metal ions in aqueous and organic solutions. J Phys Chem 88 5868-5873... 

Calixarene containing a dioxotetraaza unit, PET-18, is responsive to transition metal ions like Zn2+ and Ni2+. Interaction of Zn2+ with the amino groups induces a fluorescence enhancement according to the PET principle. In contrast, some fluorescence quenching is observed in the case of Ni2+. PET from the fluorophore to the metal ion is a reasonable explanation but energy transfer by electron exchange (Dexter mechanism) cannot be excluded. 

Sensors based on the fluorescence quenching ofrhodamine 6G in resins by iodide ions(43) and in Nafion polymer by metal ions in solution 44,45) have been demonstrated. However, complex fluorescence decay mechanisms often hinder interpretation in lifetime-based sensing and much progress is still to be made in this area before the true potential of lifetime-based sensing becomes a reality. For example, rhodamine 6G in... 

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