Photochemical & Photobiological Sciences PAPER Cite this: Photochem. Photobiol. Sci., 2014, 13, 509 Received 9th September 2013, Accepted 1st November 2013 DOI: 10.1039/c3pp50314c www.rsc.org/pps Effect of micellar and sol–gel media on the spectral and kinetic properties of tetracycline and its complexes with Mg 2+ † Alessio Cesaretti, Benedetta Carlotti, Catia Clementi, Raimondo Germani and Fausto Elisei* The spectroscopic and photophysical properties of the broad-spectrum antibiotic tetracycline (TC) and its Mg 2+ complexes were studied in organized media attained by means of three iso-structural quaternary ammonium surfactants able to self-assemble in water at low c.m.c. values, thus giving spherical micelles and sol–gel media upon increasing the concentration. Specific protonated forms of TC and its complexes were introduced in these micro-heterogeneous environments and then investigated through steady-state (both in absorption and emission) and pulsed (up to femtosecond resolution) spectroscopic techniques. Free TC showed minor spectral and kinetic variations while complexes remained unchanged in the pres- ence of spherical micelles, meaning that TC is likely to be placed at the interface between the micelle and the bulk aqueous solution, without altering its bioactivity. Ultrafast transient absorption spectroscopy proved to be a powerful tool to gain deep insight into the distribution of the investigated species between the heterogeneous structure of sol–gel media. In fact, according to the polarity and net charge of free TC and its complexes, these species can be mostly found in the hydrophobic (intertwined worm-like micelles) or in the hydrophilic domains (basically aqueous pools) that the sol–gel is made up of. In the first case, the properties are dramatically altered (highly enhanced fluorescence and lengthened lifetime of the first singlet excited state up to the nanosecond time scale), leading to the improved traceability of the drug. Introduction Confined environments, such as micellar and sol–gel media, have gained increasing interest in the last few years thanks to their ability to interact with a variety of molecules, thus provid- ing them with higher viscosity and lower polarity than the bulk aqueous solution. 1 This kind of organized media results in an environment capable of altering the photophysical be- haviour of entrapped molecules, affecting both their spectral properties and their deactivation dynamics. 2–6 These features earned micelles and sol–gels the title of drug delivery systems: they can be loaded with various compounds, poorly water- soluble drugs as well as inorganic ions, thus carrying them through the body and up to the desired sites. 7–9 Both micelles and sol–gels develop starting from surfactant solutions as precursors. It is well known that when the surfac- tant concentration rises above a critical value, namely the criti- cal micellar concentration (c.m.c.), aggregates are formed. 10 At first, micelles are like spheres, but as the concentration of sur- factants continues to rise, their shape can turn from spherical to worm-like. It has been found that micellar sphere-to-rod growth can occur when the Coulombic repulsion between the charged hydrophilic heads is reduced by the presence of a sufficient amount of an additional salt providing definite counterions. 11–15 The worm-like aggregates can then interact through inter- molecular forces to give an integrated network (sol–gel) whose rheology resembles that of solids rather than that of liquids. 16–20 Such media are able to immobilize drugs and hence can act as carriers for pharmacological purposes. More- over, these viscoelastic systems can be employed not only as drug delivery vehicles, but also as a wide range of agents: detergents, cosmetics and household products, just to mention a few. 21 Therefore, many organic chemistry groups have focused their attention on the synthesis of specific surfactants capable † Electronic supplementary information (ESI) available: Additional graphs, which are numbered consecutively starting with Fig. S1 as referred to in the text, and additional tables (Tables S1 and S2). See DOI: 10.1039/c3pp50314c Chemistry Department, and Centro di Eccellenza sui Materiali Innovativi Nanostrutturati (CEMIN), University of Perugia, via Elce di Sotto 8, 06123 Perugia, Italy. E-mail: fausto.elisei@unipg.it; Fax: +39-075-5855598; Tel: +39-075-5855588 This journal is © The Royal Society of Chemistry and Owner Societies 2014 Photochem. Photobiol. Sci. , 2014, 13, 509–520 | 509