Fabrication Of Photo-responsive Nanostructures Of Azobenzene Thiols Via Dip-pen Nanolithography

Nanostructures containing azobenzene functional group were fabricated on Au via dip-pen nanolithography for the first time using synthesised novel azobenzene thiols as the "ink". The DPN-generated nanostructures containing thioled azobenzene show reversible and repeatable photo-responsive properties. The detailed contents are described as follows:A novel series of photo-responsive azobenzene functional monomers containing both hydroxyl and mercaptoalkoxyl groups,4-hydroxyl-4’-(n-mercaptoalkoxyl) azobenzene (HOAZOOCnSH, n= 3,4,5,6,8,12), were synthesized by reductive coupling reaction, alkyl substitution and thiosulfate reaction with 4-nitrophenol as starting material. The experimental conditions were optimized, and the chemical structure of the monomer was confirmed by 1HNMR,13CNMR, Elemental analysis and Mass spectrum. These functional monomers have good and reversible photoresponsive properties as characterized by UV-Vis. The influence of molecular chain length on the photoisomerization rate was discussed.The novel azobenzene thoils were successfully deposited on Au via DPN, and random nanostructures such as dot/line array, square, letters were fabricated. The height of HOAZOOCnSH (n= 3,4,5,6,8,12) in AFM topograpy is almost consistent to the theoretical chain length calculated by Chem 3D molecular simulation, indicating that the azobenzene thiols stand on Au substrate in an almost vertical, slightly tilted way.Take HOAZOOC12SH as an example, the effect of temperature, relative humility, scan rate and contact force on the DPN transfer rate and feature size of nanostructures were discussed in detail. It is found that the transfer rate of azobenzene thiols increases as temperature increases, and relative humility decreases. The amount of transport molecules increases with long exposure time and slow scanning speed. Larger nanodots require higher contact force and shorter alkane chain. AFM was used to visualize in situ the change in height for the DPN-generated square upon alternate irradiation with UV and visible light, and an averaged change in height of 0.2-0.4 nm was observed, which is consistent with the molecular chain different between cis and trans azobenzene calculated by Chem 3D simulation, demonstrating that DPN-generated azobenzene-based nanostructures possess reversible and repeatable photo-responsive properties.