Zeta Potential and Colloidal Stability of Mixed Self-Assembled Monolayer Functionalized Au Nanocolloids
Özge Çalışkan* , Aşkın Gizem Çitak, 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
One of the facile routes of improving optical, electronic and chemical functionality of colloidal Au nanoparticles is attachment of organic layers as self-assembled monolayers (SAM) on the surface. Chemisorption of alkanethiols on gold surface has long been exploited to form SAM on the surface of nanocolloids without disrupting the colloidal stability. 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.
It is well known that a single type alkanethiol dissolved in alcohol forms SAM on Au surfaces with good colloidal stability, but the system is restricted with the chemical functionality or reactivity of only one molecule. 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 when functionalized with two different type alkanethiols, namely; mercaptohexanol (MHOL) and mercaptohexadecanoic acid (MHDA). This binary mixed-SAM comprises both carboxylic acid and alcohol end groups. For formation of mixed-SAM, 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. An equimolar mixture of MHOL and MHDA is then added step by step to the colloidal system to prevent aggregation. This step took about 15 minutes to complete functionalization of a 20 ml colloid dispersion. Following the alkanethiol addition step, the colloidal system was kept mixing around 3 hours to complete simultaneous chemisorption of MHDA and MHOL. The colloidal stability of these mixed-SAM derivatized Au nanoparticles was monitored using a UV-vis spectrophotometer. The extent of flocculation was calculated from the UV-vis spectrum by computing the area under curve between 600 and 800 nm using the composite Simpson’s method. It was found that there was only slight decrease in the colloidal stability after an initial change due to alkanethiol addition. Zeta potentials of the colloids derivatized with SAM of MHDA and mixed-SAM coated colloids were measured using a zetasizer from pH 11 to 2 and the isoelectric points (IEP) for both cases were determined. It was found that there is a significant difference in the zeta potential of MHOL+MHDA mixed-SAM functionalized Au nanocolloids and colloids coated only with MHDA. The IEP for mixed-SAM was 2.5, whereas it was 3.2 when colloids comprise only carboxylic group of MHDA. We have shown preparation of a stable mixed-SAM derivatized with both alcohol and carboxylic acid end groups. This bifunctional SAM layer can be used for different sort of chemical interactions of the colloidal system for bioanalytical purposes or molecular biology applications.