Engineered protein-iron and/or gold-protein-iron nanocomposites in aqueous solutions upon UVA light: Photo-induced electron transfer possibilities and limitations

Abstract

Occurrence/suppression of photo-induced electron transfer (PI-ET) process in solutions of nanocomposites represents a very hot topic in several photo-related research domains. In this work, this phenomenon is investigated in aqueous solutions using iron-containing mono-/bi-metallic nanocomposites that have been engineered through protein-templated syntheses, with the aim to potentially create new biocompatible imaging contrast agents for medical diagnostics. Two types of iron-containing nanocomposites, mono-metallic (protein-Fe) and bi-metallic (Au-protein-Fe), have been synthesized and characterized with the aid of several experimental techniques, such as high-resolution transmission electron microscopy (HR-TEM), steady-state fluorescence, electron paramagnetic resonance (EPR) and light-induced EPR (LEPR). Theoretical model of the N-terminal region of the bovine serum albumin protein interacting with Fe cations and simulations of the EPR spectral features upon UVA light irradiation complemented the material’s analysis. Two iron forms within the mono-/bi-metallic nanocomposites have been detected experimentally: (i)complexed iron cations (giving EPR signal at g = 4.29) and (ii)superparamagnetic iron oxide nanoparticles (SPIONs; expressing EPR resonances at g// = 2.21 and g⊥ = 2.09). Upon UVA-light irradiation (325 nm), PI-ET between the two iron forms have been observed in the protein-Fe nanocomposite; however, this electronic communication is suppressed in the Au-protein-Fe system. The presence of luminescent Au nanoclusters and dissimilar sizes of SPIONs in bi-metallic nanocomposites (around 5 nm vs. approx. 2 nm in the monometallic protein-Fe system) represent the two possible reasons underneath suppression of the PI-ET process in the former. Moreover, OH radicals were detected in aqueous solutions in both iron-containing nanocomposites (Fe and Au/Fe systems) when irradiated at 325 nm for 5 min at r.t. The reported evidence of PI-ET in iron-containing nanocomposite aqueous solutions can thus have a large impact on their potential medical and/or environmental applications.