(28i) Mixed-SAM of Aliphatic Thiols On Gold Nanocolloids: Collodial Stability and Reactivity

Gold nanoparticles with diameters below 20 nm are extensively employed to numerous fields in nanotechnology, spanning from fabrication of nanodevices to photovoltaics and catalysis. The unique optical and electronic properties, and chemical functionality of Au nanoparticles can be tailored by changing the size, shape or aggregation state and most importantly surface chemistry. Chemisorption of alkanethiols on gold surface has long been exploited to form self-assembled monolayes (SAM) on the nanocolloid surface. Chemical functionalization of Au nanocolloids significantly changes the rates of flocculation of the gold dispersion depending on the pH, the chain length and terminal functional group of the alkanethiol.

For the formation of SAM on colloidal gold, in general, a single type of alkanethiol previously dissolved in alcohol is chemisorbed in aqueous gold colloid dispersion. The most widely used molecules for SAM formation are 16-mercaptohexadecaonic acid (MHDA) and 11-mercaptoundecanoic acid (MUDA) mainly due to good association characteristics in water. It is inevitable that formation of SAM from mixed molecules will provide an additional functionality to gold colloids with apparent 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 loss of surface charge.

 This study investigates formation of bifunctional colloidal gold dispersions with a good colloidal stability and pH dependent agglomeration behavior. Citrate reduced Au nanoparticles in aqueous solutions (particle size of nearly 13 nm) was mixed with an equimolar mixture of two different alkanethiols dissolved in ethanol. Before the chemisorption of alkanethiols, Au nanoparticles were treated with tween solution for physical adsorption of the surfactant molecules onto the metallic surface. The first mixed-SAM on Au nanoparticles was prepared by chemisorption of 0.5 mM MHDA and decanthiol (DT) via Au-S covalent link with exposed carboxylic acid and alkyl ends. The second mixed-SAM on gold was prepared using a mixture 0.5 mM MHDA and butyl 3-mercaptopropionate (BMP). A UV-Vis spectrometer was used to monitor capping of the surface of colloidal gold with mixed-SAM. A red shift in the spectrum of bare Au nanoparticles indicates the surface binding of mixed alkanethiols as a consequence of size extension. The colloidal stability was tested for a pH range of 6.0 to 9.0 by 0.5 increments. While the best colloidal stability was observed at pH 7.5 for MHDA-BMP mixed SAM, a different pH response was recorded for the other type of mixed-SAM. The talk will cover the discussion of agglomeration kinetics of multifunctionalized gold colloids and chemical binding of amine containing biotin molecules with the functional groups on the colloids.