Preparation and Colloidal Stability of Different Binary Mixed Self-Assembled Monolayer Functionalized Au Nanocolloids
Aşkın Gizem Çitak*, Özge Çalışkan, Fuad Bayramov, Deniz Yılmaz, Ramazan KIZIL
Istanbul Technical University, College of Chemical and Metallurgical Engineering, Chemical Eng. Dept. Maslak, 34469 Istanbul-Turkey. Contacts: firstname.lastname@example.org and email@example.com
Au nanoparticles with diameters below 20 nm are one of the most versatile nanomomaterials in nanotechnology, since they can easily be incorporated into the systems of photovoltaics, catalysis and nanodevice fabrication. The unique optical and electronic properties of these nanoparticles can be tailored by changing their size, shape or aggregation state and most importantly the surface chemistry via kovalent bonding of organic or polymeric molecules. Chemisorption of alkanethiols on gold surface has long been exploited to form self-assembled monolayers (SAM) on the surface of nanocolloids. Chemical functionalization of Au nanocolloids with alkanethiols significantly changes the rates of flocculation of the dispersion depending on the pH, chain length and terminal functional group of the alkanethiol to be attached covalently to the surface. Thus, colloidal stability of Au nanocolloids depends highly on the chemical nature of capping agents and the pH of dispersion.
Typically, a single type alkanethiol dissolved in alcohol is chemisorbed by colloidal Au in aqueous phase to form stable SAM-capped colloidal dispersion. The most extensively used alkanethiols for SAM formation are 16-mercaptohexadecaonic acid (MHDA) and 11-mercaptoundecanoic acid (MuA), mainly due to their good association characteristics in water. Formation of SAM on Au surfaces from mixed alkanethiol molecules instead of a single one is expected to give an additional chemical functionality to Au nanocolloids, which may offer advantages of precluding steric effects and enabling the particles possess multifunctional reactive terminal groups. However, a potential risk of preparing mixed-SAM on nano-sized colloidal gold is the loss of colloidal stability or fast agglomeration due to the loss of surface charge.
This poster presentation covers investigation of the colloidal stability of Au nanocolloids functionalized both with a single and binary alkanethiols of different chain lengths and function end groups. For this purpose we used three different MHDA, MuA, mercaptohexanol (MHOL) alkanethiols for making single and binary mixed-SAM capped Au nanocolloids prepared by citrate reduction. Alkanethiols were dissolved in ethanol and added dropwise to the aqueous colloidal dispersion of 13 nm Au nanoparticles. Before the chemisorption of alkanethiols, Au nanoparticles were treated with tween solution for physical adsorption of the surfactant molecules onto the metallic surface which are expected to be replaced by alkanethiols by means of chemical interaction. As MuA and MHDA form very stable SAMs on the surface Au nanoparticles when they are used individually, MHOL did not form any SAM on the particles by itself. The colloidal stability of single SAM derivatized Au nanoparticles by MHDA and MuA was satisfactory enough. Mixed-SAM of binary alkanethiols of different functional end groups (MuA+MHOL and MHDA+MHOL) were prepared mixed with an equimolar mixture of two different alkanethiols dissolved in ethanol. UV-vis spectrum suggested that colloidal stability still remains almost the same when binary-mixed SAMs are prepared and interestingly an improved stability was detected for MuA+MHOL mixed-SAM functionalized Au nanocolloids. The amounts of flocculated nanoparticles were quantified by integrating the spectrum curve from 600 to 800 nm using composite Simpson method. We noticed a considerable red shift in the surface plasmon peak of Au nanoparticles when they are functionalized with mixed-SAMs. The pH effect on these binary SAM systems was also investigated to determine the most stable pH range for such colloidal systems. The poster will summarize findings on the collodial stability as a function of pH and the chemical nature of alkanethiols and provide a discussion of the advantages of using binary mixed-SAMs for functionalization of Au nanocolloids.