The surface modification of aqueous silver colloidal particles with the amino acid cysteine and the cross-linking of the colloidal particles in solution is described. Capping of the silver particles with cysteine is accomplished by a thiolate bond between the amino acid and the nanoparticle surface. The silver colloidal particles are stabilized electrostatically by ionizing the carboxylic acid groups of cysteine. Aging of the cysteine-capped colloidal solution leads to aggregation of the particles via hydrogen bond formation between amino acid molecules located on neighboring silver particles. The aggregation is reversible upon heating the solution above 60°C. The rate of cross-linking of the silver particles via hydrogen bond formation may be accelerated by screening the repulsive electrostatic interactions between the particles using salt. The process of aggregation and heat-induced dispersion of the particles has been studied by UV-vis spectroscopy, laser light scattering, and transmission electron microscopy measurements.

A comparative study of antimicrobial activity is done using three different electrospun nanofibers namely-CA, PAN, and PVC used as control and with various amounts of AgNO(3) being treated with UV-irradiation leading to the enhancement of silver nanoparticles. DMF is used as the common solvent which helps to undergo spontaneous slow reduction at room temperature to form silver nanoparticles followed by UV-irradiation using a 400 W source. The time required for the formation of silver nanoparticles is short and they are more or less well dispersed with few such aggregates. The presence of silver nanoparticles is confirmed using various characterization techniques. The antimicrobial activity is studied using nanofibers with fabricated functionality.

The biotin−avidin reaction is well studied and is often used as a prototypical interaction in the development of immunoassays. In this paper, this reaction is studied on the surface of colloidal silver and gold particles as a first step in the development of a sol-based assay. More specifically, silver and gold colloidal particles were biotinylated by self-assembly of a biotin disulfide molecule, and the reaction of the surface-modified colloidal particles with avidin molecules was followed using optical absorption spectroscopy. The specific interaction of avidin, a tetrameric protein, with biotin leads to cross-linking of the colloidal particles (“flocculation”) and a consequent growth of a long wavelength absorption peak. The degree of flocculation was quantified using a semiempirical flocculation parameter, and the dependence of this parameter on the extent of biotinylation of the colloidal particle surface as well as the concentration of avidin in solution was studied to determine the optimum working conditions of the sol. The silver sol required electrostatic stabilization of the biotin-capped particles through the simultaneous incorporation of a charged bifunctional molecule, 4-carboxythiophenol, in the capping monolayer while the gold sol was stable with biotin capping. Both biotinylated silver and gold sols showed a visible color change on addition of avidin. However, changes in the optical absorption spectra were more marked for the silver sol.