Using spectroscopic ellipsometry for quick prediction of number density of nanoparticles bound to non-transparent solid surfaces

We report on the use of spectroscopic ellipsometry (SE) in predicting number density of nanoparticles bound to the surfaces decorated with either organic monolayers or surface-grafted polymers. Two systems are considered that comprise citrate-stabilized gold nanoparticles adsorbed on: (1) 3-aminopropyltriethoxysilane (APTES) self-assembled monolayer (SAM), and (2) surface-tethered polyacrylamide (PAAm). Number density of gold nanoparticles on the surface is varied systematically by gradually increasing either the concentration of APTES molecules in the SAM or molecular weight of grafted PAAm. The adsorption of gold nanoparticles on APTES gradient surfaces is monitored via atomic force microscopy (AFM), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and SE. The partition of gold nanoparticles on PAAm gradient assemblies is characterized by AFM, ultraviolet-visible (UV-vis) spectroscopy, and SE. By correlating the results obtained from the various techniques on nanoparticle coatings, we derive an empirical linear relationship between the number density of nanoparticles on surfaces and cos (Δ) parameter measured in SE. Excellent agreement between nanoparticle number density determined experimentally from AFM scans and that predicted by SE proves the potential of SE as a quick, predictive technique to estimate number density of nanoparticles bound to solid, non-transparent substrates.