Amphiphilic cyclodextrins as capping agents for gold colloids: A spectroscopic investigation with perspectives in photothermal therapy

Amphiphilic cyclodextrins (CDs) modified in the upper rim with thiohexyl groups and in the lower rim with oligoethylene amino (SC6NH2) or oligoethylene hydroxyl groups (SC6OH) can bind gold colloids, yielding Au/CD particles with an average hydrodynamic radius (RH) of 2 and 25 nm in water solution. The systems were investigated by UV-vis, quasi-elastic light scattering, and FTIR-ATR techniques. The concentration of amphiphiles was kept above the concentration of gold colloids to afford complete covering. In the case of SC6NH2, basic conditions (Et3N, pH 11) yield promptly the decoration of Au, which can be stabilized by linkage of CD amino and/or thioether groups. The critical aggregation concentration of SC6NH2 was measured (4microM) by surface tension measurements, pointing out that about 50% of CDs are present in nonaggregated form. Whereas Au/SC6NH2 colloids were stable in size and morphology for at least one month, the size of the Au/SC6OH system increases remarkably, forming nanoaggregates of 20 and 80 nm in two hours. Under physiological conditions, the gold/amino amphiphiles system can internalize in HeLa cells, as shown by extinction spectra registered on the immobilized cells. The gold delivered by cyclodextrins can induce photothermal damage upon irradiation, doubling the cell mortality with respect to uncovered gold colloids. These findings can open useful perspectives to the application of these self-assembled systems in cancer photothermal therapy.