Tip-Specific Functionalization of Gold Nanorods for Plasmonic Biosensing: Effect of Linker Chain Length

Pedro M. R. Paulo, Peter Zijlstra, Michel Orrit, Emilio Garcia-Fernandez, Tamara C. S. Pace, Ana S. Viana, and Silvia M.B. Costa

Langmuir, Just Accepted Manuscript

DOI: 10.1021/acs.langmuir.7b00422

Gold nanorods are promising platforms for label-free biosensing. We have functionalized gold nanorods with biotin thiol linkers of increasing chain length and evaluated their ability for molecular detection of streptavidin. We have found an unexpected effect for the increase of linker length, as it resulted in a substantial improvement of the plasmon response at surface saturation. The plasmon peak shift increased from 5 nm to 14 nm, i.e. more than twice the response, between the short and long biotin linker. This effect is only observed for a site selective tip functionalization, whereas for a full biotin coating there is no improvement observed with the linker length. The improved plasmon response for tip functionalization is attributed to a low biotin coverage, but directed to the most sensitive regions, which combined with a longer chain linker reduces steric hindrance for streptavidin binding on the rod’s surface. The model sensors were further characterized by measuring their dose-response curves and binding kinetic assays. Simulations of discrete dipole approximation give theoretical plasmon shifts that compare well with the experimental ones for the long linker, but not with those of the short linker, thus suggesting that sterical hindrance affects the latter. Our results highlight the importance of specifically functionalizing the plasmonic hot-spots in nanoparticle sensors with the adequate density of receptors in order to maximize their response.