Water on Au sputtered films

摘要

Transient changes in the contact angle, Δθ ～ 10°, of water on gold (Au) reveal reversible wetting of near hydrophobic Au films. The recovery time is temperature dependent. Surface flatness is investigated using AFM and profilometery. Au is increasingly used as a platform for many bio and chemical reactions because it is regarded as inert, so-called noble, with low biological and chemical activity. Consistent with such inactivity, it is the only metal which does not form an oxide on heating in air or oxygen at least up to 900 ℃. Such properties are critical to the production of reliable, reproducible and repeatable interfaces capable of both disposable and multiple uses. Many commercial products now incorporate Au nanoparticles because such low activity inhibits bacterial and fungal growth - for example, Au nanoparticle films are thought to act as ideal barriers between skin and dental implants. As a high energy coating it is used for the self-assembly of monolayers and is suitable for catalyzing, or enhancing catalysis of, various important reactions. The ability to form monolayer or nanoscale, films from various deposition techniques or from nanoparticle self-assembly also makes it attractive for biosensing applications. Yet, despite this extensive use and application, the surface properties of Au are not well understood and questions remain about the implicit inert activity of Au. Crucial to all of these applications is the wetting nature of the gold surface which continues to raise ambiguity in the literature. Most literature suggests the pure Au surface is hydrophobic, consistent with thermodynamics of gold oxidation, and this property is even used to reduce the wetting of other noble metals such as platinum by adding gold. Yet despite its high thermodynamic oxidation state, gold is easily turned hydrophilic by chemical means through the formation of AuO when anodized in the presence of an acid. Many standard laboratory cleaning procedures for generating highly pure or clean surfaces, such as those required for accurate force measurements, also oxidise Au. Preparations such as cleaning in piranha solution, UV/Ozone cleaning, and water or oxygen plasma cleaning all generate highly hydrophilic surfaces (with a contact angle as low as θ ～ 7°) suggesting the presence of AuO. Much of the reported hydrophilic and wetting observations are therefore attributed to the presence of gold oxidation or in unclean environments due to surface contamination. Interestingly, the subject of the measured contact angle, θ, of water on thin Au films within the literature also reflects a degree of uncertainty. It is nearly always measured to be hydrophilic or wettable θ ＜ 90°) and rarely hydro-phobic. Eariy and systematic work typically reports θ ～ 66°, consistent with eariy theories, whereas many (though not all) recent works report θ ～ 76° about 10° higher. Variations around these are also reported and may be attributed to the type and quality of gold surface.