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VISUALIZACIN Y CARACTERIZACIN DENANOESTRUCTURADOS DE COLESTEROL SOBRE AU (111)

POR STMAristeo Segura* y Nikola Batina*

Facultad de Ciencias Qumicas. Universidad Autnoma Benito Jurez de

Oaxaca, Av. Univ. S/N, Col. 5 Seores. C.P. 68120. Oaxaca.ass.cquimicas@uabjo.mx

*Laboratorio de Nanotecnologa e Ingeniera Molecular, Depto. de Qumica, CBI,UAM-Iztapalapa. Av. San Rafael Atlixco No. 186, Col. Vicentina, Del. Iztapalapa,

C.P. 09340. Mxico, D.F. bani@xanum.uam.mx

Posgrado en QumicaResumen

El colesterol es una molcula de gran importancia en la vida ascomo en la medicina, biologa y bioqumica, debido al papel que juega enel cuerpo humano y la relacin que tiene con diversas enfermedades. En

el pasado, el colesterol molecular y sus agregados en forma de LDL yHDL, fueron estudiados ampliamente por diversas tcnicas; no obstanteexiste una carencia de informacin sobre el comportamiento molecular delcolesterol y el mecanismo de interaccin con substratos slidos, quepodran ser un paso determinante para el diseo de biosensores en lofuturo y de sistemas nanoelectromecnicos (NEMS). En este estudio, lasmonocapas del colesterol molecular fueron son preparadas sobre elsubstrato de Au(111) y en diversas soluciones: Etilenglicol, agua ymetanol, en diferentes concentraciones y visualizadas con el microscopioelectrnico de barrido de efecto tnel (STM) ex-situ. La caracterizacinestuvo enfocada a la topografa y a las propiedades a nivel nanometrico

(molecular).Las imgenes de STM revelaron informacin muy importanteacerca de la forma en que se agregan las molculas de colesterol sobresubstratos slidos. Estas presentan monocapas con defectos en forma deorificios, con un espesor determinado de las pelculas as como laestructura que tienen estas molculas a nivel molecular en la de unageometra en forma de cajas, en la que podemos identificar la posicin yorientacin de las molculas de colesterol sobre el substrato slido.

De acuerdo con los resultados y auxiliados de un modelo simplese puede explicar el mecanismo de la formacin de monocapas delcolesterol en diferentes soluciones. Los resultados demuestranclaramente que la estructura y las caractersticas de las pelculas

moleculares sobre el Au(111) dependen del tipo de solvente y de laconcentracin del colesterol en la solucin.

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Self-assembled Film of Cholesterol Molecules on the Au(111):An STM StudyAristeo Segura *,** and Nikola Batina **

*Facultad de Ciencias Qumicas, UABJO, Av. Univ. S/N, Col. 5 Seores, C.P. 68120, Oaxaca, Mxico, ass.cquimicas@uabjo.mx**Laboratorio de Nanotecnologa e Ingeniera Molecular, Depto. de Qumica, UAM-I, Av. San Rafael Atlixco No. 186, Col.

Vicentina, Del. Iztapalapa, C.P. 09340, Mxico D.F., Mxico, bani@xanum.uam.mx

Self-assembled films of cholesterol were prepared on Au(111) surfaceand characterized by STM at nanometric level. In dependence of theconcentration, two type of films were observed: one in the form ofmolecular monolayer and second with molecules organized in thesquare boxes, as a building blocks of the self-assembly layer. Botharrangements were characterized in great details which offers newinsides into determination of molecular position, conformation andmechanism of the film growth. Accommodation of the cholesterol intothe self-assembly layers was explained by help of molecular modelsdeveloped in this study. The obtained finding are interesting from thefundamental as well from the technological point of view, in terms ofbetter understanding of the process related to the functionalization ofthe metallic surface, and future design of the nano-scale devices (i.e.bio-medical sensors).

BSTRACT

METHODOLOGYThe cholesterol samples (Aldrich, 99+%) for STM analysis were

prepared from 2x10-4 M and 8x10-12 M solution of cholesterol inmethanol (Aldrich, 99.93%). Molecular adlayer were formed directly onthe gold substrate with (111) orientation (Berliner Glass Co) [5], usingspin-coating technique, at 37 C. Molecular films were visualized byScanning Tunneling Microscopy (STM), Nanoscope IIIa andNanoscope IV, Veeco, USA, at air (ex-situ conditions) and at the roomtemperature (20 C). All glassware was cleaned in chrome-sulfuricacid and rinsed with Milli-Q water.

CONCLUSIONThe cholesterol molecular films prepared from different solutions on theAu (111) substrate were characterized by STM at atomic and molecularlevels. High resolution STM images revealed two type of molecularfilms: monolayer and self-assembly boxes of the cholesterol dimmers,which were characterized in great details. With help of the molecularmodeling, structure of the observed films were reconstructed. It offerednew information over the molecular orientation in the adsorbed layer,mode of molecular attachment to the substrate and molecular

conformation in the adsorbed film. The structure of the molecularadsorbate depends on the molecular concentration. The obtainedresults are interesting from several points of view. First of all, webelieve, it is the first report of the successful visualization of thecholesterol film at the molecular level, with clear identification of thesingle cholesterol molecule. Second, formation and characterization ofthe cholesterol supramolecular assemblies in form of boxes andmonolayers at the Au(111) substrate is very promising issue from thepoint of the surface functionalization and design of the nano-scalesensores.

ACKNOWLEDGMENTSThis research work was realized with the support of InstitutoMexicano del Petrleo (IMP) Mexico, Proyect FIES-98-100-I andProfessors Improvement Program (PROMEP-SEP) Mexico.

REFERENCES1. H. Pollard, A.M. Scanu and E.W. Taylor, Biochemistry, 64, 304-310, 1969.2. M. Cranney, A.J. Mayne, A. Laikhtman, G. Comtet and G. Dujardin, Surface

Science, 593,139-146, 2005.3. V. Arima, F. Matino, J. Thompson, R. Cingolani, R. Rinaldi, R.I.R. Blyth, Surface

Science, 580,63-70, 2005.4. B. Lu, H.J. Zhang, Y.S. Tao, H. Huang, H.Y. Li, S.N. Bao, P. He and Q. Chen,

Applied Physics Letters, 86, 061915, 2005.5. D. M. Kolb, A. S. Dakuri and N. Batina, Nanoscale Probes of the Solid /Liquid

Interface, NATO ASI Seris C, E 288, 1995.

6. R.S. Abendan and J.A. Swift, Langmuir,18,4847-4853, 2002.7. M.S. Brown and J.L. Goldstein, Nobel Lecture, 284-324, 1985.8. S. Huey Sheng, L. G. Orad, and C. E. Nordman, Acta Cryst. B, 37,1538-1543, 1981.9. B. M. Craven., Nature, 260 ,727-729, 1976.

RESULTS

X-range : 52.1 nm

Z-range : 1.0 nm

Y-range

:52.1nm

X-range : 52.1 nm

Z-range : 1.0 nm

Y-range

:52.1nm

1.7 nm

1.7 nm

X-range: 17.3 nm

Y-range:17.3n

m

Z-range: 0.6 nm

X-range: 17.3 nm

Y-range:17.3n

m

Z-range: 0.6 nm

Fig. 2. STM image of the cholesterol molecular film on the Au(111) substrate,prepared from 2x10-4 M methanol solution.

Fig. 2. Cholesterol self-assemblies in form of square boxes visualized over the Au(111) substrate, afterfunctionalization with 8x10-12 M cholesterol/methanol solution.

Fig. 1. Cholesterol structure (a) and dimensions (b) calculated

using the PC Spartan plus molecular model.

X-range: 17.0 nm

Z-range: 0.5 nm

Y-range:17.0nm

X-range: 17.0 nm

Z-range: 0.5 nm

Y-range:17.0nm

X-range: 2.69 nm

Z-range: 0.4 nm

Y-range:2.69

nm

a b

2007 Foro acadmico de la DCBI de la UAM- Iztapalapa, Mexico City,

* ** *

0.288 nm

0.406 nm

Distancenm

Z-Roughness

* * *

*