Aggregation-induced-enhancement fluorescence

Keywords Aggregation-induced enhanced emission (AIEE) Fluorescent organic nanoparticles Ion-association method Organic dye nanoparticles Reprecipitation method Restriction of intramolecular rotation Size-dependent optical properties... 

Fig. 5.16 Selected fluorophores 44 with aggregation-induced enhanced emission. soi, fluorescence quantum yield in dioxane < agg, fluorescence quantum yield in dioxane-water (20 80).

A self-assembled chitosan polymeric nanoparticle conjugated with fluorescent stilbene derivatives that exhibit aggregation-induced enhanced emission (AIEE) in vivo was reported by Lim et al. The AIEE phenomenon occnrs in a special class of dyes that have lower flnorescence in solution, but in a concentrated environment (snch as when encapsnlated in a nanoparticle or vesicle), the dyes show a marked enhancement in emission intensity, in contrast to common organic dyes. Dipolar tricyanostilbene derivatives... 

Fig. 16 Sensors used in enantioselective aggregation induced enhancement (a) Phenylglycinol-derived Schiff base 52 (b) proposed mechanism of oligomeric interaction of 52 with mandelic acid enantiomers (c) chiral derivatives of 2-phenyl-3-(p-ammophenyl)acrylonitrile 53,54 showing an AIEE effect in the presence of bases and acids respectively (d) fluorescence response as a function of enantiomeric excess of dibenzoyl tartrate observed using the amine 54 (adapted from [113]. Copyright (2010) with permission from Elsevier) (e) AIEE sensor 55, based on BINOL...

In addition to the field enhancement, the increases of the radiative decay rate of the molecule also lead to the fluorescence enhancement. This happens when molecules are S -20nm away from metal nanoparticies aggregated on surfaces [19-21]. Lakowicz and coworkers have characterized this phenomena by using silver island films deposited on the internal surface of two quartz plates which sandwich a bulk fluorophore solution [20]. The fluorophores are physically placed close to silver islands so that there are a range of distances between the fluorophore and metal. The fluorescence enhancement is accompanied by decreased lifetimes and increased photostability. This phenomenon shows that the silver island increases the radiative decay rate of the fluorophore and therefore induces the fluorescence enhancement. 

The other key point is that the recognition event should then produce differences in the fluorescence properties of the sensor molecule in the presence of the two enantiomers of the analyte. Several mechanisms can be used to generate the fluorescence response (Figs. 2 and 11) by changing the electronic properties and the conformation of the sensor. Aggregation induced emission enhancement (AIEE) [49] was recently shown to be able to generate an enantioselective fluorescence response. Special optical techniques such as circularly polarized luminescence (CPL) are now available to detect enantioselective interactions occurring in the excited state or in the second coordination-sphere interactions [49]. 

NTPs), such as ATP, UTP, CTP, TTP, and GTP, over mono- and diphosphates (NMPs and NDPs, respectively), with enhancement of the fluorescence emission. In addition, it was postulated that, in the presence of probe 23b, NTPs induce aggregation, leading to an increase in fluorescence emission called aggregation-induced emission. 

Kimura, Shirai and coworkers used two chiral dimeric porphyrins 95 and 96 to investigate their self-assembling behavior [162,163]. While incorporation into fibers made of the alkylamide derivatives of (fl,fl)-DACH, 95 formed stable well-resolved fibrous assemblies as visualized by transmission electron microscopy, the fluorescence of which was not quenched by external electron acceptors [162]. However, the induced CD was not detected indicating an inability of 95 to form chirally orientated aggregates under the applied conditions. In contrast, 96 was able to produce optically active inter molecular self-assemblies with an enhanced chiroptical response through the //-oxo bridging in an alkali solution, while intramolecular //-oxo dimer formation was excluded on the basis of steric reasons [163]. 

Similarities between CD and absorbance methods are also found between CD and fluorescence and CD and circularly polarized luminescence (CPL). Three prerequisites are needed to produce FDCD and CPL activities. Intense emission signals normally associated with fluorescence are attractive because limits of detection are lowered considerably. FDCD finds more uses as a chromatographic detection device. A CD signal is usually induced by some kind of molecular complexation reaction. Association can be with a simple molecule or with an aggregate of molecules, such as chiral micelles, which are known to be fluorescence enhancers. In cases of color induction combined with fluorescence induction, FDCD can lead to even higher levels of selectivity among analytes that have been derivatized by the same color reagent. 

Primary screening of amyloid sensing was carried out with insulin amyloid fibrils generated from fresh insulin, which were induced to aggregate at low pH conditions to form a cross- 3-sheet secondary structure. Thirteen compounds from the 320 compounds in the library were selected based on the observed increase or enhancement in their fluorescence intensities upon addition of insulin amyloid fibril. These 13 selected compounds were further tested in synthetic A 340 and A 342 aggregates. The two A(3 variants, A 340 and A (142, which differ by truncation at the carboxyl... 

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