Interactions between organized, surface-confined monolayers and vapor-phase probe molecules .12. Two new methods for surface-immobilization and functionalization of chemically sensitive dendrimer surfaces

In this report we demonstrate two new methods for covalently linking dendrimers to surfaces. In the first method (method 1) a poly(iminopropane-1,3-diyl) dendrimer with 64 terminal-amine groups is first attached to a mixed mercaptoundecanoic acid (MUA)/mercaptopentane (MP) self-assembled monolayer (SAM), and then the unreacted terminal-amine groups of the dendrimer are converted to amide-linked functional groups by condensation with acid chlorides. The second method (method 2) involves bulk-phase coupling of suitable functional groups with the primary-amine-terminated dendrimer followed by reaction of the few unfunctionalized primary amines with the MUA component of the SAM to yield amide linkages. Five different dendrimer terminal groups are considered: primary amine, benzamide, 4-(trifluoromethyl)-benzamide, butanamide, and triphenylacetamide. Fourier transform IR external reflection spectroscopy, ellipsometry, variable takeoff angle X-ray photoelectron spectroscopy, and surface acoustic wave device-based gravimetry reveal that these two approaches result in very different types of dendrimer monolayers. When the dendrimers are prepared by method 2, their surface concentration is lower than when the functionalization is done after attachment. However, the density of surface functionalities on each dendrimer is higher when dendrimer modification is performed prior to surface attachment. When the benzamido-terminated dendrimer surfaces are dosed with a variety of volatile organic compounds (VOCs), we find that the surface prepared by method 2 is more sensitive and that there is enhanced selectivity for the VOCs having pi electrons. This result is interpreted in terms of pi-stacking interactions with the aromatic groups on the dendrimer surfaces.