Interactions of graphene oxide with luminescent biofunctionalized semiconductor nanoparticles: simultaneous monitoring in a protein–semiconductor coupled system

Abstract

We have demonstrated the physicochemical aspects of the interactions of free graphene oxide (GO) with bovine serum albumin (BSA) encapsulated ZnSe NPs as a representative protein–semiconductor coupled system. The well-resolved emissions of tryptophan and ZnSe NPs in the chosen biofunctional nanomaterial enables to follow interactions of GO with protein and semiconductor components simultaneously. The long average emission lifetime of semiconductor nanoparticles in BSA–ZnSe NPs changed significantly on interactions with GO from 131.5 to 108.6 ns, while there was little change from 5.24–5.08 ns for protein component. Influence of solvent polarity on steady-state emissions provide evidence of non-electrostatic interactions of BSA and charge transfer from ZnSe NPs towards GO sheet. Circular dichroism spectral measurements suggest change in protein secondary structure and iodide quenching studies provide a quantitative estimate of decrease in accessibility of tryptophan residues (f_a) towards polar environment (f_a changes from 42% to 17%) on interactions of GO with BSA–ZnSe NPs. These results are consistent with the observed changes in UV-vis absorption and zeta potential, which also indicate hydrophobic association of GO with BSA–ZnSe NPs. Further, electron transfer process is evident from Raman peak shift and the observed changes in I_D/I_G ratio, which indicate strong interactive nature of BSA–ZnSe NPs towards GO. We also justified the thermodynamic feasibility of electron transfer process and calculated the rate of electron transfer from semiconductor component in BSA–ZnSe NPs to the GO surface to be 2.06 × 10⁹ s⁻¹. Thus, the present study provides useful information for future fabrication of multifunctional single platform combining the graphene, semiconductor and protein molecules.