DNA adsorption and cationic bilayer deposition on self-assembled monolayers

A cationic bilayer adsorbed to a self-assembled monolayer (SAM) of alkylthi ols could be a useful substrate for DNA immobilization and patterning. Ther efore, we studied the layer formation of a self-assembled system consisting of a base layer of a negatively charged SAM chemisorbed on gold, a middle layer of an electrostatically adsorbed cationic bilayer, and a top layer of double-stranded DNA that electrostatically adsorbs to the cationic bilayer . The formation of DNA, lipid, and alkylthiol layers was monitored by surfa ce plasmon spectroscopy. Cationic lipids readily formed layers with thickne ss between 32 and 33 A on self-assembled alkylthiols possessing terminal ca rboxylic acid groups within 24 h and at pH > 2. Fluorescence bleaching expe riments indicated that these layers were homogeneous and relatively immobil e. For comparison, we found that cationic lipids do not form layers on alky lthiols possessing a terminal alcohol group, while zwitterionic lipids form ed bilayers on these surfaces and on the carboxylated surfaces with a thick ness of approximately 38-44 Angstrom. The use of self-assembled alkylthiols with diethylene glycol groups prohibited the formation of both cationic an d zwitterionic lipid layers and also prevented DNA adsorption. Finally, DNA was adsorbed to cationic lipid bilayers which were electrostatically attac hed to negatively charged SAMs. The results indicate that DNA forms a layer of 8 Angstrom calculated thickness, which is consistent with a monolayer p ossessing average interhelical distances of 50 Angstrom, in agreement with other studies using different techniques. Hence this surface is useful for immobilizing DNA. No differences were observed in kinetics of deposition or the thickness of the DNA monolayer when different cationic lipids were use d.