title: Surface gradient material: from superhydrophobicity to superhydrophilicity.
abstract: We have developed a simple method to fabricate a gradient from superhydrophobicity to superhydrophilicity. It is based on the controlled self-assembled monolayer consisting of a thiol molecule on a gold surface and the amplifying effect of the wetting property on a rough surface. Using a relatively dilute HS(CH2)11CH3 solution (0.05 mmol/L), we found that the density of molecules on the surface can be controlled by varying the immersion time. Slowly adding the dilute solution to the container holding the rough gold substrate will lead to a density gradient along the surface. After the complementary adsorption of HS(CH2)10CH2OH, the surface exhibits a gradient from superhydrophobicity to superhydrophilicity. The slope of the gradient can be conveniently tuned by varying the speed of addition. Cassie-Baxter and Wenzel equations are employed to explain this special property based on the rough structure and the molecular composition gradient that have been determined by XPS. This kind of material would provide a larger oriented driving force for many important biological and physical processes and might have potential applications in water droplet movement, oriented axonal specification of neurons, protein adhesion, and so on.
authors.affiliation: Key Lab of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, People's Republic of China.
authors.forename: Xi
authors.initials: X
authors.lastname: Yu
authors.forename: Zhiqiang
authors.initials: Z
authors.lastname: Wang
authors.forename: Yugui
authors.initials: Y
authors.lastname: Jiang
authors.forename: Xi
authors.initials: X
authors.lastname: Zhang
mesh_terms: D006046:Gold / Q000737:chemistry*; D057927:Hydrophobic and Hydrophilic Interactions; D008422:Materials Testing / Q000379:methods*; D008855:Microscopy, Electron, Scanning; D013438:Sulfhydryl Compounds / Q000737:chemistry*; D013499:Surface Properties; D017155:Wettability
chemical_list: D013438:Sulfhydryl Compounds; D006046:Gold