Gold-thiol monolayers preparation

IV. PREPARATION OF GOLD-THIOL MONOLAYERS A. Experimental Procedures... 

Other electroactive groups incorporated in gold-thiol monolayers include tetrathiaful-valene365 and fullerene393. Miller and coworkers prepared thiol-derivatized oligoimides with naphthalene diimide electroactive groups. Electron transfer to each electroactive function was observed394,395. 

Surface-enhanced Raman spectroscopy (SERS) provides a means of obtaining the Raman spectmm of a monolayer. Although this method requires careful preparation of a roughened surface (necessary for intensity enhancement), and the absorbance may vary from sample to sample, it is very sensitive to the functionalities located close to the metal surface. The enhancement factor depends inter alia on the distance between the functional group and the metal surface on one hand and the surface coverage on the other. A typical distance at which the enhancement factor decreases to half of its initial value in a well-packed gold-thiol monolayer is about 3.5-7. This technique provides useful... 

Whitesides and coworkers developed a procedure to estimate the density of defects in gold-thiol monolayers. The monolayer surface is exposed to the gold-etching solution, the pits obtained are transferred into pits on the Si surface through further chemical treatment and the number of pits per unit area counted. Long-chain alkanethiolate monolayers prepared by the standard technique show pit densities of ca 100 pits mm . The number of pits sharply increase if the gold surface is stored for a long time in air prior to monolayer deposition . 

Other useful methods for the quantitative destmction of thiolate monolayers on the gold surface include metastable atom °° or kiloelectronvolt ion bombardment and electron beam damage techniques . These methods are successfully used to prepare patterned monolayers. Quantitative removal of the thiol by an electron beam requires an e-beam dose of 10-100 mCcm . Recently, X-rays were shown to damage gold-thiol monolayers . ... 

Electroactive monolayers of thiol-derivatized hydroquinones on the surface of platinum electrodes were prepared and studied by Hubbard and coworkers even before Nuzzo and Allara reported formation of the first gold-thiol monolayers . The redox potentials of quinone- or hydroquinone-containing monolayers vary with pH with a slope of 60 mV pH indicating a 2e -I- 2H+ reaction . This observation was used in an elegant study by Hickman and coworkers. They codeposited hydroquinone and ferrocene-containing thiols on the surface of a gold electrode. The half-wave potential of the latter does not depend on the pH value of the solution, and therefore can be used as a reference. The whole assembly acts thus as a pH-sensitive sensor which does not require any separate reference electrodes . 

Bertilsson L and Liedberg B 1993 Infrared study of thiol monolayers assemblies on gold—preparation, oharaoterization, and funotionalization of mixed monolayers Langmuir 3 141-9... 

The use of tetraoctylammonium salt as phase transfer reagent has been introduced by Brust [199] for the preparation of gold colloids in the size domain of 1-3 nm. This one-step method consists of a two-phase reduction coupled with ion extraction and self-assembly using mono-layers of alkane thiols. The two-phase redox reaction controls the growth of the metallic nuclei via the simultaneous attachment of self-assembled thiol monolayers on the growing clusters. The overall reaction is summarized in Equation (5). 

Underpotentially deposited ultra-thin layers of silver or copper on gold substrates are also suitable for monolayer assembly. Monolayers prepared on such surfaces are different from those that form on the parent bulk metal surfaces and show enhanced stability against thermal desorption or exchange with thiol-containing solutions91 92. 

The order and orientation in monolayers prepared by /iCP was studied and compared with the ordinary gold-thiol films. The former layers differ from the latter mainly by the much shorter contact time used in their preparation, but they were shown to be, under certain conditions, indistinguishable from their planar counterparts519,520. 

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