Stability and exchange studies of alkanethiol monolayers on gold-nanoparticle-coated silica microspheres

Self-assembled monolayers (SAMs) of carboxylate- and amine-terminated alkan ethiols were formed on gold-nanoparticle-coated microspheres. The stability and place-exchange reactions of fluorescently labeled derivatives of these monolayers were studied as a function of time and storage conditions. Chan ges in the fluorescence intensity of the derivatized SAMs were monitored us ing fluorescence microscopy. Gold-nanoparticle-coated microspheres were pre pared by first derivatizing silica microspheres with a thiol-containing sil ane followed by self-assembly of the gold nanoparticles onto the microspher e surfaces. Nanoparticle assembly was performed by mixing thiol-activated s ilica with a citrate-stabilized solution of gold nanoparticles in ultrapure water. The mean diameter of the gold particles was 14.5 +/- 0.9 nm as dete rmined by transmission electron microscopy (TEM). The mean diameter of the gold nanoparticles, after assembly onto the microsphere surfaces, was essen tially unchanged (14.0 +/- 2.8 nm). The relative surface coverage of the si lica microspheres with gold was found to be dependent on the concentration of gold nanoparticles in solution and on the incubation time. Field-emissio n scanning electron microscopy (FE-SEM) was used to obtain high-resolution images of the microspheres before and after the gold coating procedure. As the surface coverage increased, the measured surface roughness of the silic a microspheres also increased. Tapping mode atomic force microscopy (AFM) w as used to measure the surface roughness of individual microspheres. The su rface roughness of the microspheres and the chemical composition of the SAM were found to correlate with differences in SAM stability. SAMs on microsp heres with relatively higher surface roughness were less stable and were mo re susceptible to place-exchange reactions. Carboxylate-terminated monolaye rs were found to be more stable than amine-terminated SAMs. A novel method for determining the relative amounts of exchange of fluorescently labeled a lkanethiols between different monolayer-coated microspheres is reported. In this method, the transfer of fluorescently labeled alkanethiols composing SAMs on one set of gold-coated microspheres to nonlabeled SAMs on another s et of gold-coated microspheres is monitored as a function of time using flu orescence microscopy. A collision-dependent mechanism was found to influenc e the rate and amount of exchange of alkanethiol between microspheres. The stability and exchange properties of SAMs on gold-nanoparticle-coated micro spheres are presented in this paper.