Energy-Level Alignment, Ionization, and Stability of Bio-Amino Acids at Amino Acid/Si Junctions

Abstract

The electronic structures of 20 bio-amino acids and amino acid/Si junctions are studied using ab initio calculations. It is shown that the amino acids can be classified into two groups depending on where the highest occupied molecular orbital (HOMO) state is localized. This classification is possible owing to the molecular structural geometry and the constituent atoms in the residue part of amino acids. Moreover, we found that, owing to the hybridization of electronic states between amino acids and Si substrate, the optical transition from the HOMO state of the amino acid to the conduction band states of Si becomes possible at the amino acid/Si interface. This result indicates the possibility of the optical ionization of amino acid by producing amino acid/semiconductor junctions. The present results provide basic data not only for the microscopic understanding of protein electronic structures but also for the electronic design of new protein functions.

Double‐Sided Graphene‐Enhanced Raman Scattering and Fluorescence Quenching in Hybrid Biological Structures

“…[ 29,46 ] The HOMO and LUMO values for the biocompatible nitrocellulose molecules on top of graphene as well as for the phenylalanine and tyrosine amino acids in the kidney tissue below graphene (plus other amino acids) were obtained from literature data. [ 49–51 ] These HOMO/LUMO levels and their energy differences relative to E F of p‐type graphene (Δ E HF /Δ E LF ) are listed in Table S2, Supporting Information. The smaller Δ E HF of ≈0.09, ≈−0.34, and ≈−0.29 for top and bottom molecules in our case indicate a favorable energy‐level alignment of HOMO with p‐doped graphene, which fits very well with the Raman findings.…”

“…[ 41 ] The latter leads to specific HOMO and LUMO energy levels for amino acids that can be used as orientation values to tune the E F position of graphene through external p‐ or n‐type doping for efficient charge transfer in prospective studies. [ 29,51 ] This perspective is presented in Figure 3g with the corresponding data and abbreviations being summarized in Table S3, Supporting Information. It shows that by varying the carrier concentration in graphene, its Fermi level and thus work function can in principle be shifted such that an alignment either to the HOMO or LUMO energies of amino acids is achievable (green and light orange open circles, respectively).…”

“…[ 20,22,32 ] Nevertheless, more research is needed to account for the hybridization of the electronic states in every hybrid biological system that can modify the HOMO and LUMO energies to some extent relative to isolated amino acids. [ 51 ]…”

Optical Ionization of a Phenylalanine on C(111) surfaces

“…The excitation rate of Phe on Si(111) is 10 -3 times smaller than that of substrate [11]. A condition to improve the efficiency is the existence of mid-gap states.…”