Self-assembly of cysteine-functionalized silver nanoparticles at solid/liquid interfaces

Cysteine-functionalized silver nanoparticles (AgNPs) of an average size of 20 +/- 5 nm, applied in the studies, were synthesized in a chemical reduction method. Dynamic light scattering (DLS) measurements showed that the nanoparticles were stable for ionic strength lower than 3x10(-2) M and at pH 4.0 and 9.0. The electrophoretic mobility measurements enabled to determine that the nanoparticles were positively charged with the zeta potential equal to + 51 mV at pH 4.0 and ionic strength 10(-2) M. Under the same ionic strength and at pH 9.0, the zeta potential of AgNPs was negative attaining value of -52 mV. The self-assembly of the nanoparticles on bare and polyelectrolyte-modified surfaces of silicon was investigated using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The influence of ionic strength and pH on the coverage and structure of the monolayers was studied in detail. It was found that the diffusion-controlled deposition of the nanoparticles on bare silicon surface, under acidic conditions, allows to obtain silver monolayers of coverage up to 0.25. The monolayers of coverage up to 0.26 were formed also under alkaline conditions on poly(diallyldimethylammonium chloride) (PDDA)-modified surfaces. The density and structure of the monolayers were tuned by ionic strength. The experimental results were interpreted in terms of the random sequential model (RSA) which allowed to confirmed that the electrostatic interactions play a dominant role in the self-assembly of the nanoparticles at solid/liquid interfaces. The characteristics of optical properties of the monolayers revealed that the monolayers deposited under acidic conditions reduce reflection properties of silicon wafers. The decrease in specular reflectance was explained in terms of the plasmonic properties of the nanoparticles. It was shown that the changes in the reflectance strongly depend on the coverage of the AgNP monolayers and on the presence of supporting layer of PDDA. It was conclude that stable cysteine-capped AgNPs can be used for the construction of antireflection materials and in the biology as mimics of such proteins as human serum albumin (HSA).