Surface modification of metals using dendritic molecules

VERÓNICA BRUNETTI; JULIETA PAEZ; PABLO FROIMOWICZ; MIRIAM C. STRUMIA; ANA M. BARUZZI

Mar del Plata, Bs. As.

Simposio; IV Latinamerican Symposium on Scanning Probe Microscopy; 2007

Materials & Polymer Systems (Poster)

 

Surface modification of metals using dendritic molecules

 

Verónica Brunetti^a, Julieta I. Paez^b, Pablo Froimowicz^b, Miriam C.Strumia^b and Ana M. Baruzzi^a.

 

^aInstituto de Investigaciones en Fisicoquímica de Córdoba (INFIQC – CONICET), Departamento de Fisicoquímica, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Pabellón Argentina, Ala 1, Piso 2, Ciudad Universitaria, Cordoba CP 5016, Argentina.

^bDepartamento de Química Orgánica, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Medina Allende y Haya de la Torre, Ciudad Universitaria (5000) Córdoba, Argentina

e-mail: brunetti @fcq.unc.edu.ar

  

 

The adsorption and reactivity at the electrode/electrolyte interface is a subject of general interest in electrochemistry surface science. Surface electrochemistry is the part of electrochemistry concerned with the details of what goes on at the surface, in particular how molecules adsorb there and what reactions they undergo. In most of our work the electrode material is a metal. Where possible, we try to use single-crystal electrodes, to control the chemistry as precisely as possible, and to learn the effect that the structure of the electrode has on reactivity. The objectives range from very fundamental work, such as finding the details of adsorption, to more practical objectives, such as making new materials with interesting properties. In particular, we are searching about the stability, ordering and packing of self-assembled monolayers (SAMs) of organic molecules on gold for the development of an electrochemical sensor. We are now focusing on dendritic molecules (dendrons) as building blocks for the modification and functionalization of the metal electrode.

Dendritic macromolecules contain a lot of functional groups that can be efficiently modified to control the properties of the resulting polymers. In particular, dendrimers have a wellcontrollable structure and size with perfect branching. Since the properties of polymers are related directly to structure and size, it is attractive to make and study well-defined macromolecules. Surface immobilization of dendrimers presents an exciting opportunity for creating a wide variety of functionalized polymeric architectures suitable for the immobilization and delivery of biomolecules [1]. However, it has been well documented within the literature that upon immobilization onto a solid surface the spherical dendrimers become distorted and take on a flattened disc shape [2]. We are developing a new approach to generate a highly functionalized surface that avoids this problem using functional dendrons immobilized onto different substrates.

In the present work, we have employed Scanning Tunnelling Microscopy and electrochemical methods to explore the immobilization of two kinds of dendrons onto gold and Highly Oriented Pyrolytic Graphite (HOPG). First, we used aliphatic dendrons (molecule 1) having an amine group as tethered group and tert-butyl as perypheric-groups. Second, we studied aromatic dendrons (molecule 2) having a carboxylic acid group at the focal point and     -NO₂ as perypheric-groups. This molecule can be adsorbed at through two adsorption centers, either the aromatic rings or the carboxylic group.

 

 

                        Molecule 1                                                        Molecule 2

 

The arrangement of adsorbed molecules on a surface depends on both intermolecular interactions and molecule-surface recognition. STM Images from Molecule 1 immobilized onto HOPG displayed a stripe-like ordered structure. The width of stripes is about 4 nm. It is presumed that the interaction of hydrophobic peripheries contribute to the long-range order. On the other hand, no spontaneous formation of 2D assembly from Molecule 1 can be observed onto gold electrodes by STM or electrochemical methods. In this case, we need to immobilize the dendron by a covalent reaction with a thiol bounded to the surface (eg. Mercaptopropionic acid) and the formation of amide linkage with the amine, activating the surface with a “zero length” cross-linker such as 1, ethyl-3-(3-dimethylaminopropil)carbodiimide.

Molecule 2 exhibits spontaneous assembly onto Au(111) electrodes probably because a higher interaction between the dendron and the substrate due to the presence of the aromatic rings. In contrast to normal alkanethiols forming highly molecular structures on metal surfaces, the self-assembled layers of molecule 2 form patterned surfaces with nanometer-sized features (7nm) and in long-range order. Moreover, by an increase of the bias voltage of STM, we can observe that some defects are induced, showing triangular regions of vacancies. In addition, modified electrodes exhibit an important change in the electron transfer rate of redox probes (such Ru(NH₃)₆^2+/3+) and a blocking behaviour of the underpotential deposition of copper reaction, confirming the immobilization of the dendron.

 

 

References:

[1] Godinez et al., Langmuir , 2005, 21, 3013-3021.

[2] A. W. Bosman, H. M. Janssen and E. W. Meijer, Chem. Rev. 1999, 99, 1665-1688.   

 

Acknowledgments: Financial support from CONICET, ANPCyT and SECyT(UNC) are gratefully acknowledged. J.I. Paez thanks the fellowship granted by CONICET.