Single molecule spectroscopy studies of local acidity along bifunctional acid−base gradients are reported. Gradients are prepared by directional vapor phase diffusion and subsequent reaction of 3-aminopropyl-trimethoxysilane with a uniform silica film. Gradient formation is confirmed by spectroscopic ellipsometry and by static water contact angle measurements. X-ray photoelectron spectroscopy is used to characterize the nitrogen content and degree of nitrogen protonation along the gradient. Nile Red is employed as the probe dye in single molecule spectroscopy studies of these gradients. While Nile Red is wellknown for its solvent sensitivity, it is used here, for the first time, to sense the acid/base properties of the film in two-color wide-field fluorescence imaging experiments. The data reveal broad bimodal distributions of Nile Red emission spectra that vary along the gradient direction. The single molecule results are consistent with solution phase ensemble acid/base studies of the dye. The former reveal a gradual transition from a surface dominated by basic aminosilane sites at the high-amine end of the gradient to one dominated by acidic silanol sites at the low-amine end. The sub-diffraction-limited spatial resolution afforded by superlocalization of the single molecules reveals spatial correlations in the acid/base properties of the gradient over ∼200 nm distances. These studies provide data relevant to the use of aminosilane-modified silica in bifunctional, cooperative chemical catalysis.
Graphene oxide serves as a precursor to various technologies, which include batteries, biosensors, solar cells, and supercapacitors. Gold nanoparticles exhibit excellent electrochemical and photophysical properties, allowing for electronic absorption and the ability to absorb light energy at the plasmonic wavelength. Palladium nanoparticles are highly sensitive and functional in room temperature, making it an ideal metal for catalytic applications. We report the synthesis of functional graphene oxide from graphite flakes followed by the insertion of gold and palladium nanoparticles through an oleylamine ligand. In this report, the fermi level of graphene oxide (GOx), gold-graphene oxide (Au-GOx), and palladium-graphene oxide (Pd-GOx) was shown to be effectively controlled. Additionally, each system showed complete solubility in ethanol and in the case of Au-GOx, enhanced solubility was seen in tetrahydrofuran as well.
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