Amphiphilic metalloaggregates: Catalysis, transport, and sensing

Surfactant aggregates and in particular micelles can be regarded as nanosized molecular containers which can capture and concentrate in their small volume lipophilic and ionic species. Exploiting the self-assembling nature of such aggregates is therefore possible to easily construct supramolecular assemblies in which several and potentially different functionalities may perform highly specific functions such as catalysis, transport, and sensing. In this review, we will report the achievements obtained with metallomicelles, which are surfactant aggregates able to concentrate at the aggregate-water interface a high number of metal ions. The most common strategy for the formation of metallomicelles is the use of lipophilic ligands able to bind the desired metal ion. The lipophilic complex thus formed can be micellized together with common surfactants forming a co-micellar aggregate or, depending on its structure, can form a homo-aggregate. The peculiar features of metallomicelles can be exploited in the metallocatalyzed hydrolysis of several relevant substrates such as esters of carboxylic acids, amino acids and phosphoric acids with million-fold observed acceleration, at neutral pH. A detailed kinetic analysis has shown that the relevant parameters for the hydrolytic activity are (a) the proximity of the reactants in the aggregate; (b) the apparent lowering of the pK(d) of the nucleophilic function (metal ion coordinated water molecule or other nucleophiles). Moreover, playing with the structure of the ligand other and more interesting features can be obtained. Important examples are the enantioselective cleavage of amino acid esters, the transport of ionic Substrate across membranes and the controlled release of encapsulated molecules from liposomes. A completely different application of micellized ligands is sensing. In this case, the aggregate behaves as a template favoring the proximity between a fluorescent dye and the ligand so that complexation of the metal ion results in quenching of the fluorescence emission of the dye. Following this strategy self-assembled chemosensors for transition metal ions have been prepared. (C) 2009 Elsevier B.V. All rights reserved.