Fabrication of surface energy/chemical gradients using self-assembled monolayer surfaces

Direct laser patterning of surface energy gradients for alkanethiols on gold has been demonstrated. A homogeneous 1-hexadecanethiol self-assembled monolayer (SAM) on gold (supported by a glass substrate) was selectively desorbed using a focused laser beam. By continually varying the incident laser intensity along a straight line scan, a gradient in desorption was produced. This desorption gradient was then backfilled with the second SAM (16-mercaptohexadecanoic acid), to produce a wettability gradient. The gradient in wettability was characterized by condensation imaging. Secondary ion mass spectroscopy was also used to show variation of the second SAM population from maximum to zero along the length, representative of the chemical gradient. The hexadecanethiol desorption was found to be the most sensitive in a laser intensity range of 29.15-6.5 kW/cm2. By considering the functional behavior of the governing equations, the theoretical trend for desorption as a function of laser intensity (represented by the out-of-focus distance) was determined. It was found to conform to the experimental data. The proposed method is fast, simple, noncontact, and flexible in terms of producing different types of gradients.