Stabilization of Langmuir and Langmuir Blodgett cetyltrimethylammonium bromide monolayers by gluing with polystyrene sulfonate

Thesis Type: Postgraduate

Institution Of The Thesis: Orta Doğu Teknik Üniversitesi, Faculty of Arts and Sciences, Department of Chemistry, Turkey

Approval Date: 2015




Organic thin films have wide range application areas such as biosensors, permeation selective barriers, molecular electronics, optical devices and coatings. The properties of such films can be tuned by their composition, structure and uniformity. Stability of obtained films has also an important factor in the fabrication and the transfer of the films. For this aim, Langmuir is an elegant technique for both the production and deposition of monolayers onto a solid support. Herein, some optimization studies such as subphase temperature, pH, compression rate and transfer rate were performed with stearic acid (SA) and octadecane-1-thiol (ODT) which are the standard materials of Langmuir system. Concentration and subphase temperature were also optimized for cetyltrimethylammonium bromide (CTAB) which was used to prepare and organize the gold nanorods (GNRs) in a monolayer structure. Lately, it was reported that some water soluble materials can also form Langmuir monolayers but to produce a Langmuir compatible complex, an interaction with these water soluble materials with a water insoluble material is needed. However, CTAB was used by itself without any complex forming reagent and stable monolayers were obtained. Polymers, particularly water soluble ones, are used as a glue to improve the cohesiveness of LB films. This process is known as “gluing” where LB assemblies were cross-linked noncovalently through ionic and hydrophobic association with a water-soluble polymers. The effect of gluing on stability of CTAB monolayers was investigated. Poly(4- styrenesulfonate) (PSS) was used as a gluing agent. The addition of polymers into subphase led to an interaction between surfactant and polyelectrolyte and an increase in monolayer stability. In order to compare the effect of PSS on stability of the obtained monolayers, hysteresis loops (repeatedly compression-expansion cycles) were performed in the presence and absence of PSS in the subphase and it was concluded that more stabilized CTAB monolayers can be obtained by gluing. Moreover, glued CTAB multilayers can be prepared by Langmuir Blodgett technique. GNRs were synthesized by seed mediated growth approach. The aspect ratio of the rods were approximately 2.5. The characterization of GNRs was performed by UV spectrometry. Two characteristic rod plasmon absorptions were observed at 520 and 675 nm. The morphology of the GNRs was characterized by Scanning Electron Microscopy (SEM). After the deposition of negatively charged PSS glued CTAB monolayers, they were used for assembling the cationic gold nanorods on the surface of the substrate. Finally, the performance of the obtained structures as Surface Enhanced Raman Scattering (SERS) substrates were investigated utilizing brilliant cresyl blue (BCB) dye as a model compound.