Molecular Recognition by Calix[4]arene-Modified Gold Nanoparticles in Aqueous Solution

摘要

The concept of exploiting metal clusters for nanotechnology applications dates back to the development of the original Au55 cluster by Schmid et al. in 1981.[1] Over the past decade, so-called monolayer protected clusters (MPCs) of gold and, to a lesser extent, silver have been studied extensively owing to their extreme stability and the plethora of tunable properties that are controlled by the particle size and by the ligand chemistry.[2]-[8] The stability is usually achieved by the use of thiolate ligands, which form a protective shell around the particles to which they are attached by the strong AuS interaction. Typical sizes of MPCs range from 1 to approximately 40 nm depending on the preparation method used. Since most preparations lead to materials that are insoluble in water a comparatively small number of studies have been carried out in aqueous systems.[9]-[18] In particular, in view of future bioanalytical applications it is, however, desirable to develop new MPC systems that are not only stable and soluble in water, but also capable of molecular recognition in aqueous systems. Very stable, yet chemically versatile water-soluble MPCs are obtained when a sulfanylalkyl oligo(ethylene glycol) is used as a stabilizing ligand.[16] Herein we report the preparation and chemical properties of 14-nm gold MPCs, which are stabilized in this way and, in addition, carry in their ligand shell calix[4]arene moieties, which confer their specific molecular recognition properties to the particles. Calixarenes are host molecules which can bind cationic guests strongly and with high specificity depending on the size of the calixarene cavity.[19], [20] Some of us have recently shown that, in non-aqueous systems, the incorporation of calixarenes in 1.5-4-nm MPCs can enhance their affinity to guest molecules.[20] By themselves, calixarenes are completely insoluble in water, which significantly limits their potential applications as molecular recognition units. The sulfanylalkyl oligo(ethylene glycol) ligands in the system described herein act therefore not only as stabilizers for the MPCs but also as solubilizers for the attached calix[4]arene units. It is thus now possible to demonstrate molecular recognition by an unmodified calixarene cavity in water. The particles prepared have been characterized by transmission electron microscopy (TEM) and by UV/Vis, and NMR spectroscopy. Specific recognition of immobilized cationic pyridinium moieties by the calixarene-modified MPCs in aqueous solution has been demonstrated by a simple color test and by atomic force microscopy (AFM).