Surface chemistry and photophysical properties of a diacetylene-peptide derivative capped quantum dots Langmuir monolayer

A diacetylene derivative, 10,12-pentacosadiynoic acid (PDA), was conjugated to a small peptide chain Cysteine-Cysteine-Glycine (CCG) through the solid-phase peptide synthesis. The (CdSe)ZnS core-shell quantum dots (QDs) capped with trioctylphosphine ligands were modified through a surface ligand reaction to prepare the PDA-CCG QDs conjugate. Both systems, PDA-CCG and PDA-CCG QDs, were investigated as Langmuir monolayer at the air-water interface through the surface pressure-area (π-A) isotherms, compression-decompression cycles, stability measurements, and in situ UV-vis and fluorescence spectroscopy. Two different π-A isotherms were observed for the systems investigated showing the importance of the peptide moiety in PDA-CCG to form a Langmuir monolayer up to a surface pressure of 50 mN m^-1 compared with 15 mN m^-1 for the PDA component alone. The compression-decompression cycles and stability measurements for both systems suggest the formation of a stable Langmuir monolayer over 1 h time period. Although the in situ UV-vis spectroscopy of PDAA-CCG and PDA-CCG QDs does not show an absorption spectrum, we observed by in situ fluorescence spectroscopy the photoluminescence (PL) of the PDA-CCG QDs at 560 nm, with an intensity of the PL increasing linearly with the increase of the surface pressure. Irradiating the PDA-CCG QDs Langmuir monolayer at 254 nm, we observe the photopolymerization with two distinct bands at 575 (blue band) and 630 nm (red band) of the polymer.