Fluorescence from water-soluble hydrophobically

Fluorescence and Light Scattering from Water-Soluble Hydrophobically Associating Polymers... 

Dendritic hosts can be used in aqueous solution to encapsulate water-soluble fluorescent probes. Changes in the photophysical properties of these encapsulated probes are useful to understand the properties of the microenvironment created by the dendritic interior. For example, adamantyl-terminated poly(pro-pylene amine) dendrimers from the first to the fifth generation (36 represents the third generation) can be dissolved in water at pH<7 in the presence of -cyclodextrin because of encapsulation of the hydrophobic adamantyl residue inside the /1-cyclodextrin cavity and the presence of protonated tertiary amine units inside the dendrimer [72]. Under these experimental conditions, 8-anifi-... 

In most cases, the amphiphilic polymers do not exhibit intrinsic fluorescence and therefore a dye needs to be encapsulated, or the vesicle membrane has to be stained. The first method requires encapsulation of a water-soluble fluorescent dye during vesicle formation followed by a subsequent exclusion of the dye from the extracellular space (e.g., by size exclusion chromatography, dialysis, ultrafiltration, or centrifugation). To stain the membrane either a fluorophore is covalently linked in a certain percentage to the membrane forming molecules, or a lipophilic probe is aggregated in the hydrophobic part of the membrane [116,146,175,176],... 

From a supramolecular perspective, cyclodextrins offer a preformed cavity of defined size within a water-soluble macrocycle. The interaction between guest molecules, or substituents to the cyclodextrin itself, leads to some useful results. Hydrophobic fluorescent substituents will be self-included by the macrocycle in aqueous solution until they are displaced by guests with even greater affinities [6]. The act of displacement will generate a fluorescent response that lends itself to sensor applications. It is also possible to use the hydrophobic core to induce rotaxane formation. This approach has been used by many groups, for example, Liu has recently used it to prepare water-soluble gold-polypseudorotaxanes that capture fullerenes such as C60 [7]. 

The introduction of QDs into aqueous media is usually accompanied by drastic decreases in the luminescence yields of the QDs. This effect presumably originates from the reaction of surface states with water, a process that yields surface traps for the conduction-band electrons [63]. As biorecognition events or biocat-alytic transformations require aqueous environments for their reaction medium, it is imperative to preserve the luminescence properties of QDs in aqueous systems. Methods to stabilize the fluorescence properties of semiconductor QDs in aqueous media (Figure 6.2) have included surface passivation with protective layers, such as proteins [64, 65], as well as the coating of QDs with protective silicon oxide films [66, 67] or polymer films [43, 68, 69). Alternatively, they can be coated with amphiphilic polymers, which have both a hydrophobic side chain that interacts with the organic capping layer of the QDs and a hydrophilic component, such as a poly(ethylene glycol) (PEG) backbone, for water solubility [70, 71). Such water-soluble QDs may retain up to 55% of their quantum yields upon transfer to an aqueous medium. 

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