Nanoscale defolding influence of polypeptides in the charge-transfer process through an organic–inorganic nanohybrid system

A biologically important polypeptide [with an alternate sequence of alanine (ALA) and 2-aminobutyric acid (AiB)] is used as a linker molecule to investigate the charge-transfer phenomenon between CdSe nanoparticle (NP) (diameter ∼6-7 nm) assemblies and gold (Au) substrates. The (ALA-AiB)n polypeptides, with varying chain lengths n = 5, 8, 11, were attached to the surface to form self-assembled monolayers (SAMs) through a thiol group located either at the N-terminal or C-terminal of the sequence. Temperature dependent photoluminescence (PL) spectra showed anomalous behavior in the quenching regime of CdSe NPs in the 237 K to 290 K region. In principle, the fluorescence intensity of any fluorophore decreases with a gradual increase in temperature, due to dominating non-radiative relaxation over radiative relaxation. PL spectral intensity follows this general trend from 77 K to 237 K for all chain lengths. For chain length n > 5 (n = 5 showed a kink, but the extent of the kink is negligible in comparison with n > 5) polypeptide-based monolayers, there is a sudden increase in fluorescence intensity above 237 K. This sudden increase is probed using molecular dynamics simulations which reveals that this unprecedented behavior arises due to interchain polypeptide interactions. An insertion of an alkyl chain with an almost similar length of peptide along with polypeptide (in a 3 : 1 ratio, in terms of concentration) diminishes the interchain polypeptide hydrogen-bonding interactions and manifests the normal trend of PL spectra.