ATR-FTIR Spectroscopy and Its Relevance to Probe the Molecular-Level Interactions Between Amino Acids and Metal-Oxide Nanoparticles at Solid/Aqueous Interface

Abstract

Amino acids play an important rolein the stabilization process of nanoparticles in aqueous solution. The nano–bio combination received considerable attention in various nanoscale applications such as chemical and biological sensing, imaging, biotechnology, medicines, etc. Considering the importance of the nano–bio mimicking system, in the present study we have focused on the structural behavior and the interaction of three amino acids, namely, L-Leucine, L-Cysteine, and L-Serine in the presence of metal-oxide nanoparticles and its impact on bulk water structure. We have employed attenuated total reflectance Fourier-transform infrared (ATR-FTIR) vibrational spectroscopy to probe the structural signatures of the molecular system in the aqueous solution. From the IR spectral features, it is found that the vibrational signatures of the individual amino acids are very sensitive to the number of molecules present in the aqueous bulk solution. However, there is no change in water structure observed with the variation of the amino acid concentrations. Surprisingly, the combination of oxide nanoparticles and the amino acids has a significant impact on the OH-stretching and bending regions of the bulk water for the case of L-Leucine and L-Cysteine. In presence of oxide nanoparticles, it is observedthat there is a significant enhancement in the IR absorption intensity with the appearance of new spectral features of amino acids which was not noticed for the case of amino acids in the pristine aqueous environment. However, there is no enhancement in the intensity observed for the case of L-Serine except the spectral features in the fingerprint region.