DOI: 10.1002/cphc.200800292 Micelles as Containers for Self-Assembled Nanodevices: A Fluorescent Sensor for Lipophilicity Giuseppe Chirico, [b] Maddalena Collini, [b] Laura D’Alfonso, [b] Franck Denat, [c] Yuri A. Diaz- Fernandez, [a] Luca Pasotti, [a] Yoann Rousselin, [c] Nicolas Sok, [c] and Piersandro Pallavicini* [a] 1. Introduction The possibility offered by using water as the solvent and con- fining separate hydrophobic species in the small volume of the same micelle has been exploited since the 1970s. In particular, hydrophobic fluorophores and quenchers have been co-micel- lized and the observation of either steady-state or dynamic fluorescence quenching due to intramicellar interactions [1,2] has been used to calculate the aggregation number of the micelle. Moreover, kinetic parameters, such as the rate constants of processes involving the fluorophore/quencher systems inside a micelle, have also been elucidated by means of fluorescence quenching. [3] The dimensions and the shape of micelles have been studied for more than 30 years, and it is well established that in the case of many traditional surfactants, a micelle is an approximately spherical object with a diameter shorter than 10 nm [4] and, accordingly, a volume smaller than 500 nm 3 . However, the use on purpose of micelles as nanocontainers for building self-assembled multicomponent molecular devices is still a largely unexploited area, even if they offer some unique advantages: 1) when two or more hydrophobic mole- cules are confined in the same micelle, they “feel each other” as if they were more concentrated, that is, inside micelles the local concentration of the contained species is huge (even if their bulk concentration is very low); 2) mobility is allowed inside micelles, where the viscosity is comparable to that of an organic solvent droplet; [5] and 3) inside micelles solvation is dramatically lower than in bulk water. [6] The most useful conse- quence is that dynamic interactions are promoted among mol- ecules included in the same micelle, even if they are not linked by a covalent bond, and the micelle and its molecular content may behave as a self-assembled, multicomponent nanosized device. As in classical molecular multicomponent devices, a new overall function may be developed that is different from the mere sum of the properties brought by its molecular com- ponents. With this approach, we and other authors have used intrami- cellar energy and electron transfer between a fluorophore and the M 2 + complex of a lipophilized ligand to obtain fluorescent micellar sensors for cations such as Cu 2 + , [7] Ni 2 + , [7c] and Hg 2 + . [8] A sensor for the inositol triphosphate anion, [9] an off-on-off window-shaped fluorescent sensor for pH, [10] an AND molecular logic gate based on H + and Na + inputs [11] and an indole–qui- nolizine-based sensor for Cu 2 +[12] have also been obtained by using micelles as containers in which interactions among mi- cellized species are promoted. In addition, we have also inves- tigated the role of the micellar shape and of the surfactant type in the response efficiency of self-assembled pyrene-based fluorescent sensors for the Cu 2 + cation. [13] A further step forward may be made with this kind of system if the micelle is not used as a mere container but par- ticipates as a further molecular component, and its peculiar properties are exploited. In particular, it should be remem- bered that molecular species dissolved in a micellar solution partition between bulk water and the micelles, with the ratio of included versus bulk molecules being proportional to the lipophilicity of the molecules. [6,14] Accordingly, a micelle is in- Potentiometric titrations, fluorescence versus pH titrations, dy- namic light scattering and fluorescence polarization anisotropy studies demonstrate that inside the nanodimensioned Triton X-100 micelles, 1-pyrenecarboxylic acid, PCOO  , forms an apical complex with the Zn 2 + cation encircled by a lipophilic cyclen ligand and hugely increasing its fluorescence. The ability of the Zn 2 + -cyclen-PCOO  complex plus its micellar container to act as a fluorescent sensor to evaluate the lipophilicity of molecular spe- cies is demonstrated on the fatty acid series CH 3 ACHTUNGTRENNUNG(CH 2 ) x COOH ACHTUNGTRENNUNG(x=0–16). At pH 7.4 a decrease in fluorescence is observed on the addition of fatty acids that is directly related to their chain length, that is, to their tendency to enter the micellar containers, where they dislocate PCOO  from the Zn 2 + centre. The independ- ent determination of fatty acid pK a values in the presence of Triton X-100 micelles confirms that our fluorescent micellar device is capable of sensing their lipophilicity. [a] Dr. Y.A. Diaz-Fernandez, Dr. L. Pasotti, Prof. P. Pallavicini Dipartimento di Chimica Generale Università di Pavia, viale Taramelli 12, 27100 Pavia (Italy) Fax:(+ 39)0382528544 E-mail:piersandro.pallavicini@unipv.it [b] Prof. G. Chirico, Prof. M. Collini, Prof. L. D’Alfonso Dipartimento di Fisica G. Occhialini Università Milano Bicocca, Piazza della Scienza 3, 20126 Milano (Italy) [c] Prof. F. Denat, Dr. Y. Rousselin, Dr. N. Sok Institut de Chimie MolØculaire ICMUB UniversitØ de Bourgogne, 9 avenue Alain Savary, 21078 Dijon (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.200800292. ChemPhysChem 2008,9,1729–1737 # 2008 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 1729