2NANO2 Kinetics of Au colloid Monolayer Self-Assembly
The aim of this experiment was to investigate the kinetic factors associated with the construction of 2-Dimensional arrays of metal nanoparticles on derivitised glass surfaces using UV-Vis spectroscopy. After establishing the time-dependence of surface assembly, the equilibrium constant for adsorption of gold (Au) particles onto glass surfaces was obtained by fitting a suitable isotherm to the UV-Vis data as a function of concentration of particles.
Self-assembled monolayer (SAM) is a modification method that is widely used to improve the stability and functionality of nanoscale and bulk materials. The technique is revolutionizing how interfaces and surfaces are modified for applications in biotechnology, molecular electronics, biosensors and chemical sensors (Schreiber, 2000). There has been a dramatic improvement of understanding of the process of self-organization of adsorbed molecular monolayers on surfaces due to recent applications of different in situ techniques. The self-organization process involves a number of steps — starting from transport of bulk solution and surface adsorption to continuing with the 2-dimensional organization on a substrate. The properties of atoms found on the surface of many important materials are the ones that determine the properties of the material.
The inorganic SAM surface chemistry controls the properties of materials such as semiconductor devices, bio-implants, heterogeneous catalysts and many more materials. The knowledge in the area of surface chemistry and morphology has been greatly advanced by the studies that have been done in the areas of mechanical properties and stability of inorganic monolayers. Functionally significant surface features exist on the micron scale of molecules, atoms, or larger particulate or supramolecular assemblies, which chemists are determined to create tailor-made materials with improved surface properties for improved or new applications (Kühnle, 2009). The knowledge of surface chemistry provide chemists with the opportunity to create tailored surface features by integrating various organic functional groups in the adsorbate molecules.
A novel method utilized in controlling surface features is by self-assembly of monodispersed particle layer. Uniform particles can impart a reproducible nanostructure. Organic SAMs have been utilized in sensors, sub-micrometer lithographic patterning schemes and molecular electronics. However, the problem with these SAMs is lack of desirable features such as thermal stability, oxidation when exposed to open air, and desorption in organic solvents – factors which have limited their practical applications (Cao & Wang, 2011). The utilization of nanoparticle-based devices, more specifically those based on self-assembly monolayers, will ultimately improve the performance of these devices.
In this lab experiment, kinetics of Au nanoparticle self-assembly was investigated by use of bi-functional organosilane-coated glass microscope slides. When clean glass slides are exposed to a solution of 1% 3-aminopropyltrimethoxysilane (APTMS), a reaction occurs between the substrate hydroxyl groups and the organosilane, forming a siloxane bond. After rinsing and exposing salinized surfaces to Au colloid, self-assembled monolayer were formed at the surface of the metal (see figure 1).
Figure 1: A schematic diagram showing the process of formation of a monodispersed layer of Au nanoparticles.
The negatively-charged colloid particles of Au bind to the coated surfaces due to electrostatic interactions that occur between these particles and organosilane molecules. Optical spectra is measured by the presence of intense red-wine color of Au nanoparticles as the surface forms on the transparent substrate.
Cao, G. & Wang, Y., 2011. Nanostructures and Nanomaterials: Synthesis, Properties, and Applications. s.l.:World Scientific.
Kühnle, A., 2009. Self-assembly of organic molecules at metal surfaces. Current Opinion in Colloid and Interface Science, Volume 14, p. 157–168.
Schreiber, F., 2000. Structure and growth of self-assembling Monolayers. Progress in Surface Science, Volume 65, pp. 151- 256.