A thiol with an unusual adsorption- desorption behavior: 6- mercaptopurine on Au(111).

E. PENSA; A. A. RUBERT; G. BENTEZ; P. CARRO; C. VERICAT; R. C. SALVAREZZA

S. C. Bariloche, Argentina

Otro; EULASUR SUMMER SCHOOL AND WORKSHOP. Properties and Applications of nanomaterials.; 2010

The formation of self-assembled monolayers (SAMs) of thiols on metals has been widely studied, not only for its basic interest, but also for their technological and biological applications. In particular, SAMs of thiopurines and thiopirimidines are of interest because they can help to understand certain highly organized biological structures, such as DNA. Moreover, 6-mercaptopurine (6MP) was among the first agents approved by the FDA for the treatment of cancer, particularly leukemia [1]. In this work, we have been studied and compared both 6MP and Hexanethiol (HT) SAMs on Au with (111) preferential orientation by cyclic voltammetry, scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), before and after electrochemical desorption. Also we have been make DFT calculations of 6MP SAMs on Au(111). Both Voltammetric and XPS experiments of SAMs HT and 6MP indicate the presence of a thiolate bond, similar to those already reported [2,3] However, STM images in air of 6MP SAMs on Au(111) reveal islands of height equal or less to that of one atomic gold layer [4]. In many cases the islands are aligned along preferential directions. There is a total absence of the gold vacancies usually found in SAMs of alkanethiols on Au (111), as such HT SAMs on Au(111). High resolution images obtained in the regions not covered by the aligned islands show a rectangular lattice with typical distances 0.75 ± 0.05 nm and 0.6 ± 0.1 nm, consistent with a 2x3√3, and similar to what was found for adenine on gold [5]. Finally, DFT calculations indicate strong chemisorption via a S-Au bond and additional binding to the surface via a N-Au bond. From DFT data, the positive charge on the Au topmost surface atoms is markedly smaller than that found for Au atoms in alkanethiolate SAMs. The adsorption of 6MP originates Au atom removal from step edges but no vacancy island formation at (111) terraces. Then .we propose that the absence of the Au-S interface reconstruction results from the lack of significant repulsive forces acting at the Au surface atoms.[6] _________________________________________ [1] Peter Karran and Natalie Attard, Nature Reviews Cancer, 2008, 8, 24-36. [2] Rafael Madueño, Teresa Pineda, Jose M. Sevilla, and Manuel Blázquez, Langmuir, 2002, 18, 3903- 3909. [3] Boland, T.; Ratner, B. D., Langmuir, 1994, 10, 3845–3852. [4] Thomas Boland and Buddy D. Ratner, Langmuir, 1994, 10, 3845-3852. [5] Tao, N. J.; DeRose, J. A.; Lindsay, S. M., J. Phys. Chem. 1993, 97, 910-919. [6] E. Pensa, P. Carro, A. A. Rubert, G. Benítez, C. Vericat, R. C. Salvarezza, Langmuir, 2010, 26, 17068-17074.