Physisorption of amino acids on TiO 2 : Changes in the nonlinear optical properties

Abstract

Calculations at HF, DFT (CAM-B3LYP) level were carried out with the 6-31+G(d,p) basis set for estimate the polarizability (α), the first hyperpolarizability (β) and the second hyperpolarizability (γ) for aromatic amino acids: tryptophan (Trp), tyrosine (Tyr) and phenylalanine (Phe), before and after of the physisorption on an anatase nanoparticle model. The results show that, the values for < α >, β _total and < γ > increases as different indole, phenol, or benzene moieties were substituted, according to the sequences Phe < Tyr < Trp. Therefore, these properties enhance when the electronic delocalization is increased due to the substitution of aromatic systems. Moreover, the increase in the value of the polarizability, were accompanied by a decrease in the adsorption energy for the amino acid-anatase interaction, suggesting an increment in the stability for the complex linking by a weak attractive dispersion force to be induced between them. In relation to the adsorption energy, the negative E _ads indicated that the amino acid adsorption is exothermic for the structures studied and thus the adsorption system is energetically stable. Indole, phenol, and benzene substitution on amino acid structure had a low impact on the E _ads values, and the order for physisorption stability was Tyr > Phe > Trp. For β _total , the values showed a much higher sensitivity to the physisorption effect. We propose that amino acid studied can be used as target system in the study of interaction amino acid-anatase with nonlinear optical microscopy techniques.