Molecular Gradients of .omega.-Substituted Alkanethiols on Gold: Preparation and Characterization

Abstract

A general methodology capable of generating a molecular, chemical concentration gradient on a solid substrate in one surface dimension is presented. Although the method can be extended to more than one surface dimension, and to a variety of molecule/substrate systems, we have chosen to demonstrate it by utilizing thiols on gold as the model system. The gradients are prepared by cross-diffusing ω-substituted alkanethiols, HS(CH 2 ) n X (X = OH, CO 2 CH 3 , COOH, and CD 3 , n = 10-19), with or without fully deuterated chains, and simple n-alkanethiols, HS(CH 2 ) m Y (Y = CH 3 , m = 9-17), toward each other from opposite ends of a polysaccharide matrix deposited on top of a gold substrate. The so-prepared gradients are characterized by a number of techniques, including ellipsometry, contact angle measurements, and infrared reflection absorption spectroscopy (IRAS). The observed ellipsometric thicknesses and the limiting contact angles of the gradient assemblies agree very well with the values obtained from the corresponding single component and mixed self-assembled monolayers. The infrared data furthermore suggest that the entire gradient is composed ofwell-organized and densely packed all-trans hydrocarbon chains for thiols with chain length >10. The lateral dimensions of the gradient regions vary between 4 and 8 mm for the combination diffusion pairs employed in this study. The upper and lower values account for diffusion pairs with long and short hydrocarbon chains, respectively. It must be stressed, however, that the infrared and ellipsometric techniques only can provide information about the molecular organization on the macroscopic scale because of the relatively large spot size (1 mm) used in the experiments. Scanning probe microscopy must be employed to further investigate the mechanism of gradient formation, the molecular concentration profile, and mixing behavior on the microscopic (molecular) scale. We believe, however, that the large number of chemical and structural combinations attainable with the present approach can be used to address a broad range of surface physics/chemistry problems in material science and generate novel and unique model organic architectures. We discuss briefly the potential applications ofthese novel gradients in areas of relevance to molecular recognition and biosensing.