Published: April 20, 2011 r2011 American Chemical Society 6026 dx.doi.org/10.1021/la200166r | Langmuir 2011, 27, 6026–6030 ARTICLE pubs.acs.org/Langmuir Effect of Gold Oxide in Measurements of Colloidal Force Rico F. Tabor, †,^ Anthony J. Morfa, ‡,§ Franz Grieser, §,^ Derek Y. C. Chan, ||,^,# and Raymond R. Dagastine* ,†,^ † Department of Chemical and Biomolecular Engineering, University of Melbourne, Parkville 3010, Australia ‡ Bio21 Institute, University of Melbourne, Parkville 3010, Australia § School of Chemistry, University of Melbourne, Parkville 3010, Australia ) Department of Applied Maths and Statistics, University of Melbourne, Parkville 3010, Australia ^ Particulate Fluids Processing Centre, University of Melbourne, Parkville 3010, Australia # Faculty of Life and Social Sciences, Swinburne University of Technology, Hawthorne, VIC 3122, Australia ’ INTRODUCTION Gold is an attractive material due to its high level of chemical inertness (nobility), high surface energy, and the fact that it can be generated as nanoparticles or easily deposited in thin layers (of a few nanometers or less). These properties have made it an important material in many areas of scientific research, particu- larly in surface and colloidal force measurements. It is often em- ployed as a coating on silica or mica substrates to give a well- characterized, high-energy surface 1À4 that can be used as a support for self-assembled monolayers. 5,6 However, its reported properties still appear to vary significantly between studies, as the gold surface can be readily oxidized, 7 changing its chemical nature significantly. Additionally, because it is such a high-energy surface, it is very prone to contamination, which can leave doubt as to the purity of surfaces used in measurements. 2,8 There has been a great deal of controversy about the wetting behavior of gold, with reports claiming either highly hydrophilic or hydrophobic “pure” gold surfaces. 7,9À11 The greater body of evidence suggested that the surface of pure gold is in fact hydro- phobic but that it can be readily rendered hydrophilic by oxidation, 7 with trace water itself appearing to play a key role. 10 More recently, Stacchiola et al. demonstrated that water forms a locked bilayer on pure Au(111) surfaces, which is itself hydro- phobic, as it has no exposed hydrogen-bonding sites, 12 support- ing this theory. It has been shown that many of the standard laboratory cleaning procedures for generating highly pure sur- faces for force measurements, particularly for use in the surface force apparatus (SFA) and atomic force microscope (AFM), tend to generate oxide on gold. 7,13,14 Preparations such as cleaning in piranha solution (concentrated H 2 O 2 and H 2 SO 4 ), UV/ozone cleaning, and water or oxygen plasma cleaning all generate highly hydrophilic surfaces, suggesting the presence of gold oxide. Gold can also be readily oxidized by electrochemical methods, 15,16 that are of particular interest due to the use of gold in electrodes and electro- chemical self-assembled monolayer (SAM) production. There appears to be some discrepancy between force mea- surements involving gold surfaces, particularly when fitting the electrical double-layer behavior to understand surface charging in aqueous solutions. Wang and Yoon found the potential of gold in their AFM measurements to be around À56 mV in water (pH unspecified). 8 Giesbers et al. found that the zeta potentials and double-layer potentials of gold varied significantly with pH, exhibiting an isoelectric point at around pH 5 and a value of ≈À20 mV at pH 6À8. 17 The data of Barten et al. support these observations. 18 It is important to note that in all of these cases the possibility of oxide being present on the gold surface is men- tioned, but their effects have not been quantified explicitly. In this work, we use measurements of surface forces in order to better understand and explain the behavior of gold, particularly in the context of surface oxidation. Apparent ambiguities in Received: January 14, 2011 Revised: April 4, 2011 ABSTRACT: Atomic force microscopy, contact-angle, and spectroscopic ellipso- metry measurements were employed to investigate the presence and properties of gold oxide on the surface of gold metal. It was found that, in agreement with available literature, unoxidized gold surfaces were hydrophobic, whereas oxidation rendered the surface highly hydrophilic. The oxide could be removed with ethanol or base but appeared to be stable over long periods in water or salt solutions between pH 3 and 7. After oxidation, the oxide layer thickness, determined using ellipsometry, was consistent with an approximate monolayer of AuÀO bonds at the gold surface. The presence of gold oxide was found to alter significantly the electrical double-layer characteristics of the gold surface below pH 6 and may explain the apparent inconsistencies in observed force behavior where gold is employed as well as aiding in design of future microfluidic systems which incorporate gold as a coating.