Nature of Interactions of Amino Acids with Bare Magnetite Nanoparticles

Owing to their chemical and magnetic properties, magnetite nanoparticles are an interesting adsorbing material for biomolecules. The understanding of the interactions of simple biomolecules with inorganic nanoparticles is an important approach for research on the bio-nano interface and can constitute the fundamentals to manifold applications in biotechnology, medicine and catalysis. The aim of the work presented here is to compare the interaction of seven different amino acids (l-alanine, l-cysteine, l-glutamic acid, glycine, l-histidine, l-lysine, and l-serine) with magnetite nanoparticles in a colloidal system at pH 6. We investigate the influence of the side chain on the adsorption at a magnetite-water interface with incubation experiments. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and simultaneous thermal analysis (STA) reveal deeper insights into the interactions of amino acids with magnetite nanoparticles. The amino acids that contain polar side chains adsorb on the nanoparticles to a high degree. Cysteine demonstrates the highest adsorption capacity and the formation of cystine is observed. ATR-FTIR spectroscopy results indicate a strong influence of the carboxyl group and side chains on the binding mechanism of amino acids at the iron oxide surface. Our investigation offers novel knowledge into adsorption behavior at the bio-nano interface.