Monitoring the covalent binding of quantum dots to functionalized gold surfaces by surface plasmon resonance spectroscopy

Mercapto-propionic acid-capped cadmium selenide nanoparticles were covalently bound to the terminal amino groups of 11-amino-1-undecanethiol self-assembled monolayers (SAM) on the surface of gold electrodes. The nanoparticle binding was followed by surface plasmon resonance spectroscopy (SPR), and the minimum angle shift was related to surface coverage using atomic force microscopy (AFM) and scanning electron microscopy (SEM). An S-shaped binding isotherm was obtained; in all cases binding saturated at submonolayer coverage. Submonolayer formation was attributed to an exponential fall in the association affinity constant, ka, as a function of surface coverage. Effective medium theory was used to extract dielectric constants (λ = 632.8 nm) of ε = 3.4 for the MPA-capped CdSe nanoparticles and ε = 1.76 for the surrounding aqueous media.