Understanding the Effect of Amino Acid Conjugation on Optoelectronic Properties of Graphene Quantum Dots: Insights From DFT and TD-DFT Analysis.

Graphene quantum dots (GQDs) have attracted immense attention from both academia and industry as an advanced nanoscale carbon material having widespread applications. Attributes like unique optical and electronic properties, nontoxicity, solubility, and biocompatibility make them desirable candidates for drug delivery and cell-targeted bioimaging. Suitability of GQDs for these coveted applications can be improved through conjugation with amino acid systems. Influence of amino acid conjugation on the optoelectronic features of GQDs therefore needs to be analyzed clearly. To this end, quantum chemistry investigations within the framework of DFT and TD-DFT calculations have been carried out in the present work. The amino acids have been classified and scrutinized based on their side chain polarity, aromatic, and aliphatic contents. Our calculation interestingly reveals that amino acid-containing nonpolar side chain along with heterocyclic aromatic component favors a redshift in the near infrared (NIR) region of the absorption spectrum. This fosters the suitability of GQDs for biomedical applications. Our work also illustrates that tuning the polarity of the side chains normally tends to produce a blueshifted spectrum. Thus, amino acid conjugation of GQDs offers "plenty of room at the bottom" for us to explore further and provide rational guidance using quantum chemistry techniques.