Bifunctional, Chemically Patterned Flat Stamps for Microcontact Printing of Polar Inks Xuexin Duan, Veera B. Sadhu, Andra ´s Perl, Ma ´ria Pe ´ter, David N. Reinhoudt, and Jurriaan Huskens* Laboratories of Molecular Nanofabrication and Supramolecular Chemistry & Technology, MESA+ Institute for Nanotechnology, UniVersity of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands ReceiVed September 26, 2007. In Final Form: December 17, 2007 Different methods to create chemically patterned, flat PDMS stamps with two different chemical functionalities were compared. The best method for making such stamps, functionalized with 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFDTS) and 3-(aminopropyl)triethoxysilane (APTS), appeared to be full functionalization of a freshly oxidized flat PDMS stamp with either adsorbate, followed by renewed oxidation through a mask and attachment of the other adsorbate. These stamps were used to transfer polar inks (a thioether-functionalized dendrimer and a fluorescent dye) by microcontact printing. The PFDTS monolayer was used as a barrier against ink transfer, while the APTS SAM areas functioned as an ink reservoir for polar inks. The printing results confirmed the excellent transfer of hydrophilic inks with these stamps to gold and glass substrates, even from aqueous solutions. Attachment of a fluorescent dye on the amino-functionalized regions shows the possibility of the further modification of the chemically patterned stamps for tailoring of the stamps’ properties. Introduction In recent years, soft lithography has become a widespread technique to chemically pattern various substrates. 1 As such, microcontact printing (μCP) 2,3 has triggered enormous interest because of its processing advantages in comparison to conven- tional lithographic techniques. In a typical μCP approach, an elastomeric stamp with a relief structure is brought into intimate contact with a substrate to transfer ink molecules from the stamp to the substrate. In the contact areas, the ink forms a self-assembled monolayer (SAM), which can then be used, for instance, as a resist against etching. The lateral dimensions of the SAMs formed depend on the dimensions of the relief features on the stamp. It is possible to form relief features with lateral dimensions as small as 50 nm, but the replication of such structures by μCP remains a challenge. 4-10 The main limiting factors of downsizing the printed patterns are (i) the low mechanical stability of the elastomeric stamp, which is prone to collapse and deforma- tion, 4,11,12 and (ii) the absorption of ink in the stamp, promoting ink diffusion and overload of the surface. 13-16 Several solutions have been proposed to overcome these problems, such as developing new stamp materials 17-20 and composite stamps 11,21,22 to avoid stamp collapse during printing, inking with ink pads, 23 and utilizing heavyweight inks or catalytic microcontact printing to overcome ink diffusion. 8,24,25 Currently, the most common stamp material for μCP is poly- (dimethylsiloxane) (PDMS). PDMS is a very soft, chemically cross-linked rubber that easily establishes conformal contact with a substrate and exhibits excellent printing characteristics with apolar inks, such as alkanethiols. However, the elastomeric character of PDMS is also the origin of some of the most serious technical problems. Deformation of the soft polymer stamps due to nonuniform pressures applied during printing, such as pairing, buckling, or roof collapse of structures, renders them unsuitable for high-resolution μCP. In principle, a flat stamp can solve many or all stamp stability issues. Delamarche et al. 26 have shown that flat PDMS stamps can be patterned by a combination of surface oxidation in an oxygen plasma using a mask and subsequent stabilization of the * Corresponding author. E-mail: j. huskens@utwente.nl. (1) Xia, Y. N.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 551-575. (2) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002-2004. (3) Xia, Y. N.; Whitesides, G. M. 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