AVS2009 Session BI+NS-MoM: Nanoparticles and Self Assembly

In this contribution we present and discuss bio- and chemo-functionalization of a novel system based on plasmonic gallium (Ga) nanoparticles (NPs) self-assembled on a large variety of semiconductor substrates including Si, SiC, GaN and GaAs. The advantages of this system for a plasmonic metal/semiconductor sensing platform are presented and compared to the standard gold (Au) nanoparticles system.

The Ga and Au NPs are directly deposited on semiconductors by evaporation and/or plasma sputtering.

For both systems Ga (NPs)/semiconductor and Au(NPs)/semiconductor we discuss semiconductor surface treatments affecting the interface chemistry and the dynamics of interface phenomena playing a role in tailoring the surface plasmon resonance.

Those systems have also been functionalized by bio-molecules, e.g., antigens and antibodies for biosensing and by porphyrins for gas sensing to nitric oxide (NO).

The peculiarity of our study is the exploitation of Plasmonic spectroscopic ellipsometry (HORIBA UVISEL, Jobin Yvon) for the optical monitoring of the plasmon resonance tuning in real time during the nanoparticles deposition on semiconductor surfaces and for the functionalization of metal nanoparticles by anchoring biomolecules to the semiconductor nanostructures.

For these plasmon-enhanced semiconductor nanoscale sensors, the degree of coverage by the porphyrin and/or protein, the aggregation, the type of binding and orientation of biomolecules on a semiconductor nanostructures surface is important for the sensing activity. Therefore, plasmonic ellipsometry is used to investigate the interplay among the nanostructures size and shape, the functionalizing solution concentration and dipping time, the mechanism of anchoring of the functionalizing molecules.

Data are corroborated by atomic force and electrical force microscopies.

This work is supported by the 7FP European Project NanoCharM_Multifunctional NanoMaterial Characterization exploiting ellipsometry and polarimetry

It is know that specific molecules can spontaneously arrange on various surfaces forming two-dimensional poly-crystalline mono-molecular layers called self-assembled monolayers (SAMs). These organic coatings are used to impart targeted optical, electronic and biological properties to surfaces. Very often SAMs composed of more than one type of molecule (mixed-SAMs) are used to simultaneously impart multiple properties. Scanning tunneling microscopy (STM) studies have shown that, in mixed SAMs, molecules phase-separate in domains of random shape and size.

We will show that when mixed SAMs are formed on surfaces with a radius of curvature smaller than 20 nm they spontaneously phase-separate in highly ordered phases of unprecedented size. The reason for this supramolecular phenomenon is purely topological and can be rationalized through the “hairy ball theorem”. In the specific case of mixed SAMs formed on the surface of gold nanoparticles, the molecular ligands separate into 5 Å wide phases of alternating composition that encircle or spiral around the particle metallic core. This new family of nano-structured nano-materials shows new properties solely due to this novel and unique morphology. For example, we will show that the cell uptake of these particles strongly depends on the particle’s composition and the ligand shell morphology.

Gold has been a fascinating element that drew the interest of people since early origins of human history. However, it is just over the last two decades that it gained a resurgence of interest, in conjunction with the advent and development of nanotechnology. Gold nanoparticles have a wide variety of bio-applications such as labeling, sensing, and targeted drug delivery. A directed assembly of gold nanoparticles in the form of nanowires or networks is extensively used as nanodevices. Nanoparticles come in a variety of shapes; spheres, rods and cubes being the frequently observed ones. Surface functionalization facilitates tailoring of the gold surface for various applications. The objective here is to investigate the surface modification of gold nanorods by chemisorption of alkanethiols.

An important property of gold nanorods is their aspect ratio, which is defined as the width-height ratio. The lateral surface and edge surface mostly comprise of [100[ and [111[ surfaces respectively. By appropriate choice of capping agents, the anisotropic surfaces can be tailored for different applications. One of the most-successful methods for synthesis of gold nanorods relies on the use of cationic surfactant CetylTrimethyl Ammonium Bromide (CTAB). CTAB provides stability, size and shape control. The displacement of CTAB allows fuctionalization with other molecules. In this work, the displacement of CTAB is carried out with alkanethiols of varying chain length, viz. 16-mercaptohexadecanoic acid, 11- mercaptoundecanoic acid, 3-mercapto propionic acid. An indirect investigation method is used to analyze the behavior of gold nanorods. Planar surfaces with the same crystalline structures as gold nanorods (111 and 110) are selected for the experiment. Surface modification of planar surfaces is conducted in two steps: formation of CTAB bilayers by immersing planar substrates followed by displacement of CTAB bilayers with alkanethiols.