Preparation, characterization, resistance to protein adsorption, and specific avidin–biotin binding of poly(amidoamine) dendrimers functionalized with oligo(ethylene glycol) on gold

Protein-resistant films derived from the fifth-generation poly(amidoamine) dendrimers (PAMAM G5) functionalized with oligo(ethylene glycol) (OEG) derivatives consisting of various ethylene glycol units (EGn, n=3n=3, 4, and 6) were prepared on the self-assembled monolayers (SAMs) of 11-mercaptoundecanoic acid (MUA) on gold substrates. The resulting films were characterized by ellipsometry, contact angle goniometry, and X-ray photoelectron spectroscopy (XPS). About 35% of the peripheral amines of the dendrimers were reacted with N−hydroxysuccinimide-terminated EGn derivatives (NHS-EGn). The dendrimer films showed improved stability over octadecanethiolate SAMs on gold in hot solvents, attributed to the formation of multiple amide bonds per PAMAM unit with underlying NHS-activated MUA monolayer. The EGn-attached PAMAM surfaces with n=3n=3 reduced the adsorption of fibrinogen to ∼20% monolayer, whereas 2–3% for n=4n=4 or 6. The dendrimer films with various densities of EGn molecules on PAMAM surfaces were prepared by immersion of the NHS-terminated MUA-functionalized gold substrates in ethanolic solutions containing PAMAM and NHS-EGn of various mole ratios. The density (r) of the EGn molecules on the PAMAM surfaces is consistent with the mole ratio (r′r′) of NHS-EGn/free amine of PAMAM in solutions. The resistance to protein adsorption of the resulting surfaces is correlated with the surface density and the length of the EG chains. At their respective r, the EGn-modified dendrimer films resisted ∼95% adsorption of fibrinogen on gold surfaces. Finally, the specific binding of avidin to the ∼5% and ∼40% biotinylated EG3 dendrimers (surface density of biotin with respect to the total number of terminal amino groups on PAMAM G5) gave rise to about 50% and 100% surface coverage by avidin, respectively.

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