Amphiphilic cyclodextrins as capping agents for gold nanoparticles. A spectroscopic investigation with perspectives in biomedical applications

The increasing interest towards nanosystems for diagnostics, biosensing and therapeutics results in a consistent field of research focused on multifunctional colloidal carriers. Amphiphilic cyclodextrins (CDs) rnodified in the upper rim with thiohexyl groups and in the lower rim with olìgo-ethylene amino (SC6NH2) and oligo-ethylene hydroxyl groups (SC60H), respectively, can bind gold colloids (Au) yielding the particles of Au/CD in water solution with average hydrodynamic radius (RH) of 2 and 25 nm.These systems were investigateci by UV-Vis, Quasi Elastic Light Scattering (QELS) and X-ray Photoelectron Spectroscopy (XPS). Thc concentration of amphiphiles was kcpt above the concentration of gold colloids to ensure compiete covcring of Au nanoparticles. In the case of SC6NH2 basic conditions (Et3N, pH 11) yield the decoration of Au which are stabilized from linkage of CD. The critical aggregation concentration of SC6NH2( around 4?M) was determined by surface tension measurements pointing out that about 50% of CDs are present in non-aggregated form. Whereas Au/SC6NH2 colloids were stable in size and morphology for at least 1 month, the size of Au/SC60H system increases remarkably forming nanoaggregates of 20 and 80 nm in 2 hours. XPS investigation of Au 4f line of Au/SC6NH2, anchored on solid substrate, revealed the presence of two oxidation states; Au0 (84.0 eV) and Au+ (85.2 eV). The presence of Au+ species testifies that gold nanoparticles are linked chemically to amphiphilic CDs. The position of S 2p3/2 peak, rcgistered at 163.7 eV is typical for a sulphur in unbonded thiol. Moreover, N 1s spectrum was composed of two components; the first one assigned to the amine group (400.2 eV) and the second - to a metal coordinated amine group (402.4 eV). This result indicates that the gold nanoparticles interact with amphiphilic CDs forming Au - N band. Under physiological conditions, the gold/amino-amphiphiles system can internalize in HeLa cells, as it was shown by the extinction spectra registered far the immobilized cells. Gold delivered by CD can induce photothermal damage upon the irradiation duplicating the cellular mortality with respect to uncovercd gold colloids. These findings can open useful prospects far the application of the systems, based on CD/noble metals, systems in the biosensing and/or in cancer photothermal therapy.