Structural Characterization of Microcontact Printed Arrays of Hexa(ethylene glycol)-Terminated Alkanethiols on Gold

Abstract

This paper reports on the structural characteristics of microcontact printed oligo(ethylene glycol)-terminated alkanethiol layers, HS(CH2)15CONH-(CH2CH2O)6-H (hereafter EG6), on gold. Microwetting, contact angle goniometry, imaging null ellipsometry, and infrared reflection-absorption spectroscopy (IRAS) are used to characterize the printed EG6 layers, and the quality of these layers in terms of layer thickness and the crystallinity of the alkyl and ethylene glycol portions is compared with data obtained from analogous layers prepared by solution self-assembly. The outcome of the printing process is critically dependent on the experimental parameters used to prepare the patterns. It is found that high quality layers, consisting of densely packed all-trans alkyl chains terminated with relatively helical hexa(ethylene glycol) tails, are formed by inking the poly(dimethylsiloxane) (PDMS) stamp with a 1 mM EG6 solution and contacting it with gold for 15 min. The homogeneity of printed layers is not as good as the homogeneity of those prepared from solution under similar conditions, most likely because of simultaneous transfer of low molecular weight residues from the PDMS stamp. These residues, however, can be easily removed upon ultrasonication in ethanol without affecting the quality of the printed layer. Further on, the microscopic square-shaped bare gold patterns formed after microcontact printing (muCP) are subsequently filled with 16-hexadecanoic acid (hereafter THA) or HS(CH2)15CONH-(CH2CH2O)6-COOH (hereafter EG6COOH) to provide a microarray platform for further covalent attachment of biomolecules. Well-defined structures in terms of wettability contrast, sharpness, and height differences between the printed and back-filled areas are confirmed by imaging null ellipsometry and microscopic wetting.

Structural and kinetic properties of laterally stabilized, oligo(ethylene glycol)-containing alkylthiolates on gold: A modular approach

“…48,58,59 Furthermore, the OEG-terminated hydrogellike domains, when functionalized with specific anchoring groups, can be used to properly immobilize functionally active proteins 7,8,60 Also, the hydrophobic barriers are of high importance in the fabrication of protein arrays via piezodispensing. 32 Our study suggests that the EG 6 C 16 SAMs could be utilized as such hydrophobic barriers in the fabrication of protein and lipid membrane arrays. The superior thermal stability of the SAM could be advantageous in various micro-and nanofabrication processes of biochips, e.g., UV photolithography, laser ablation, etc.…”

“…43 Interestingly, a strong peak that normally does not appear in spectra of alkanethiolate SAMs is found at 2907 cm −1 . In previous studies by Yamamoto et al, this peak was assigned to "softening" of methylene CH vibrations upon direct interaction with a metal surface 32,44,45 A comparison of the relative peak intensities in the CH region, Fig. 4͑a͒, with the above cited spectroscopic investigations of long chain alkane films on gold suggests that the amount of the material adsorbed during the first minutes corresponds to a fraction of a monolayer of EG 6 C 16 molecules with disordered alkyl chains in a flat-on arrangement.…”

“…Microcontact printing process was carried out as described in detail by Zhou, Valiokas, and Liedberg, 32 except that the PDMS stamp was used without oxygen plasma treatment. The PDMS stamps were inked with the thiols ͑equimo-lar amount of EG 1 and EG 4 ͒ at a total concentration of 100 M in ethanol ͑99.5%, Kemetyl, Haninge, Sweden͒ for 5 min followed by drying in N 2 .…”

“…We have previously shown that ordered hydrophilic micropatterns can be obtained via microcontact printing OEG-terminated thiols with hydrophilic ͑treated with oxygen plasma͒ PDMS stamps. 32 In this proof of principle experiment, however, hydrophobic ͑nontreated͒ stamps were used. Backfilling with EG 6 C 16 for several hours, after EG n printing, revealed a sharp water condensation pattern on the gold surface, Fig.…”

Addressable adsorption of lipid vesicles and subsequent protein interaction studies

“…19 The poly͑dimethylsiloxane͒ stamp was produced as described elsewhere. 34 The pattern consists of 100-m-wide protruding frames separating a 12ϫ 12 array of 300-mwide recessed circles. Prior to its use, the stamp was thoroughly rinsed in ethanol and dried in a stream of N 2 .…”