doi:10.1016/j.jcis.2006.05.012
Copyright © 2006 Elsevier Inc. All rights reserved.
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Substrate effects in poly(ethylene glycol) self-assembled monolayers on granular and flame-annealed gold
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Jonas Rundqvista, Jan H. Hohb and David B. Havilanda, 
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aNanostructure Physics, AlbaNova University Center, Royal Institute of Technology, Roslagsvägen 30 B, SE-106 91 Stockholm, Sweden
bDepartment of Physiology, Johns Hopkins School of Medicine, 725 North Wolfe Street, Baltimore, MD 21205, USA
Received 14 March 2006;
accepted 3 May 2006.
Available online 6 May 2006.
Abstract
Poly(ethylene glycol) (PEG) self-assembled monolayers (SAMs) are surface coatings that efficiently prevent nonspecific adhesion of biomolecules to surfaces. Here, we report on SAM formation of the PEG thiol CH3O(CH2CH2O)17NHCO(CH2)2SH (PEG17) on three types of Au films: thermally evaporated granular Au and two types of Au films from hydrogen flame annealing of granular Au, Au(111), and Au silicide. The different Au surfaces clearly affects the morphology and mechanical properties of the PEG17 SAM, which is shown by AFM topographs and force distance curves. The two types of SAMs found on flame-annealed Au were denoted “soft” and “hard” due to their difference in stiffness and resistance to scratching by the AFM probe. With the aim of nanometer scale patterning of the PEG17, the SAMs were exposed by low energy (1 kV) electron beam lithography (EBL). Two distinctly different types of behaviour were observed on the different types of SAM; the soft PEG17 SAM was destroyed in a self-developing process while material deposition was dominant for the hard PEG17 SAM.
Keywords: Self-assembled monolayer; SAM; Poly(ethylene glycol); PEG; Surface effects; Flame-annealing; Atomic force microscopy; AFM; Electron beam lithography; EBL; Silicide
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Fig. 1. Au surfaces before and after flame annealing. (a) AFM topography image of a thermally evaporated granular Au film. After hydrogen flame annealing the Au film is reconstructed resulting in flat areas several microns wide. The flame-annealed Au is locally different with areas displaying the sixfold symmetry of Au(111) (b), as well as rounded features and holes with no crystalline character visible (c). The surfaces in (a)–(c) were imaged dry by contact mode AFM and the scan sizes are 750 nm. (d)–(f) Incomplete SAMs of PEG17 on granular and flame-annealed Au after 5 h incubation time. (d) PEG17 SAM attachment at the borders between the Au grains on non-flame-annealed Au. (e) On flame-annealed Au, PEG17 SAM growth is characterized by island growth and growth along the crystal axis' of the Au(111). (f) A random distribution and a morphology dominated by nodules are observed on the flame-annealed Au not displaying the sixfold symmetry. The scans in (d)–(f) were captured by contact mode AFM in PBS.
Fig. 2. Images and force distance plots of the two types of PEG17 SAM on flame-annealed Au. (a) The PEG17 SAM on the Au(111) consists of nodules with vacancies in between. (b) The PEG17 SAM on the Au not displaying the sixfold symmetry has fewer and smaller nodules then the soft PEG17. After 20 force curves, the soft PEG17 has been destroyed by the tip–sample contact (c) but the hard PEG17 SAM has not (d). (e) The force versus tip–sample separation for soft (solid points) and hard (open points) PEG17 SAM. Images and force plots were captured by contact mode AFM in PBS.
Fig. 3. (a) EBL exposures on soft and hard PEG17 SAMs. Lines with 40-nm nominal width exposed with electron doses of 160, 320, 640, and 1280 μCcm−2, from left to right, on (a) soft PEG17 SAM and (b) hard PEG17 SAM. Ablation of the SAM is dominant in the exposed areas for the soft PEG17 SAM (a) and material deposition is dominant for the hard PEG17 SAM (b). Images were captured by contact mode AFM in PBS.
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