The Interaction Between Alanine Capped Gold Nanoparticle and Structurally Similar Coumarin Dyes: Interplay Between Proton Transfer and Restriction of Molecular Twisting.

We report a bioinspired alanine-mediated synthesis of alanine-capped gold nanoparticles (ALA-AuNPs), in which alanine serves simultaneously as the reducing and capping agent and characterized it using multiple techniques. The ALA-AuNPs exhibit a surface plasmon resonance peak around 585 nm, a strong positive surface charge, and an average diameter of ∼11.2 ± 2.2 nm. Their interactions with three structurally similar coumarin dyes (CU-343, DCC, and DCCE) were investigated using steady-state, time-resolved spectroscopy, and theoretical calculations. Ultraviolet-visible (UV-vis) studies revealed that CU-343 and DCC undergo protonation upon interaction with ALA-AuNPs, while DCCE remained in its neutral form. Fluorescence and time-resolved fluorescence emission spectral studies showed quenching and reduced lifetime for CU-343 due to energy transfer. Whereas, DCC and DCCE exhibited fluorescence enhancement and increased lifetimes due to inhibition of Twisted Intramolecular Charge Transfer (TICT) state formation. Theoretical results confirmed intermolecular proton transfer (IPT) from the - NH3+ groups of ALA-AuNPs to the deprotonated sites of CU-343 and DCC. These results demonstrate that ALA-AuNPs selectively modulate the photophysical properties of similar dyes, suggesting their potential use in fluorescence-based sensing, nanophotonics, and bioimaging applications.