The impact of lipid head-groups in GUVs on electron transfer by surface-adsorbed fluorescent gold nanoclusters

Abstract

Glutathione-coated gold nanoclusters (GSH-AuNCs) have immense medical importance. Being photoinduced electron transfer (PET) facilitators, these nanomaterials are useful as photosenstizers and applied in photocatalysis, radiotherapy, tomography imaging, etc. GSH-AuNCs interact with the lipid bilayer of the cellular membrane and take part in drug delivery and functioning. Since PET is a key player here, hence, understanding the phenomenon in detail is extremely necessary. Herein, we have synthesized GSH-protected orange emitting AuNCs and analyzed their interactions with giant unilamellar vesicles (GUVs) made from lipids, namely, DMTAP and DPPC, with positive and zwitterionic head groups, respectively. The photophysical changes in the process were monitored by steady state and time resolved fluorescence spectroscopy to interpret the possible binding of GSH-AuNCs with lipid vesicles. The AuNCs are hydrophilic and hence adsorb over the surface of GUVs. A well-known electron scavenger, methyl viologen (MV²⁺) was added externally to the system to trigger PET with the GSH-AuNCs at neutral pH. The dynamics were analyzed by the method of fluorescence quenching due to PET. It is found that attachment of GSH-AuNCs with lipid vesicles having differently charged head groups is very essential in deciding the extent of PET since DMTAP GUVs promote PET to a greater extent compared to DPPC GUVs. The experimental conclusions were supported by theoretical studies. The kinetic and thermodynamic parameters of the PET process were also calculated.