Monolayer gold oxide

Long chain alkylmercaptans and disulfides readily self-assemble on gold surfaces to form compact organized monolayers in which the sulfur is chemisorbed to the gold and the hydrocarbon tail is extended away from the surface (1.-5). The mercaptan monolayers strongly inhibit gold oxidation in dilute sulfuric acid and also block diffusion of aqueous ions (e.g. Fe 2 3, Fe(CN) 63, ... 

C.M. Vitus, A.J. Davenport, In situ scanning tunneling microscopy studies of the formation and reduction of a gold oxide monolayer on Au(lll). J. Electrochem. Soc. 141, 1291-1298, 1994. 

Finally, hydroquinone was connected with its C2 via a pentaethylene glycol Hnker to long alkyl chains with terminal thiol functions. The latter self-assembled on a gold surface to give monolayers. Upon oxidation the hydroquinone was oxidized to quinone and could perform a Diels-Alder reaction with cyclopentadienyle-thanol-conjugated carbohydrate analogs. This set-up is used in chip-based carbohydrate arrays for the evaluation of protein binding and modification [268]. 

Figure 42. Cyclic voltammograms for the (111) face of gold. Sweep rate = 20mVs 10 mM HCIO4 + 1 mAf Pbp2 solutions deareated with nitrogen and stirred, for different ranges of the potential scan. Although not shown on the figure, the formation of a monolayer of oxidized compounds was included in each cycle.

That the signal arises from the monolayer of copper on the gold surface can be demonstrated by adjusting the potential to a value (-I- 0.50 V) where the monolayer is oxidized to Cu and dissolved (stripped) into the thin layer of electrolyte. The spectrum of the edge region taken under these conditions is compared to that of the UPD layer in Figs. 15C and D. There are two noticeable differences. First, the edge position for the Cu ions in... 

Davenport et al. [14] have shown that a worm-like structure of gold oxide appears on a crystalline gold surface oxidized electrochemically. By comparing in-situ STM images with electrochemical data it was evident that monolayers of gold oxide were formed at anodic potentials and that a slow reduction of the gold oxide at cathodic potentials recovered the bare surface. The gold surface, however, is not oxidized solely by electrochemical methods. 

The C-C bond formed through this process is very stable compared to say a gold-thiol bond. It can resist temperatures up to 700 K without being lost from the surface [42] and is stable in a wide potential window. A further advantage over other common surface modification techniques (e.g., self-assembly of silane monolayers on oxide) is that the process is quite fast, deposition times are usually on the order of 10 s. In contrast to the enhanced stability, formation of these layers on electrode surfaces is less controlled than for the alkanethiol system. In addition, not all the radicals generated participate in the formation of C-C bonds. It varies based on the surface condition and the substituent in the para position of... 

Monolayers can be transferred onto many different substrates. Most LB depositions have been perfonned onto hydrophilic substrates, where monolayers are transferred when pulling tire substrate out from tire subphase. Transparent hydrophilic substrates such as glass [18,19] or quartz [20] allow spectra to be recorded in transmission mode. Examples of otlier hydrophilic substrates are aluminium [21, 22, 23 and 24], cliromium [9, 25] or tin [26], all in their oxidized state. The substrate most often used today is silicon wafer. Gold does not establish an oxide layer and is tlierefore used chiefly for reflection studies. Also used are silver [27], gallium arsenide [27, 28] or cadmium telluride wafer [28] following special treatment. 

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