Enhanced Nonlinear Optical and Time-Resolved Properties of Magic Number Iron (Fe) Nanoclusters Prepared by Egg-White Synthesis

Protein-templated iron (EW:Fe) and gold (EW:Au) nanoclusters (NCs) were synthesized using chicken egg white as a biomolecular template and characterized with high-resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDS). The synthesis yielded small EW:Fe and EW:Au nanoclusters with average diameters of 2.1 and 2.6 nm, respectively. One- and two-photon excited fluorescence measurements showed visible emission centered near 440 nm for both nanoclusters, with additional emission bands at 650 nm for EW:Au and 850 nm for EW:Fe. The fluorescence quantum yields were determined to be 0.23% for EW:Fe and 0.16% for EW:Au. Both systems displayed strong nonlinear optical responses as shown by their two-photon absorption cross sections. Time-resolved fluorescence measurements revealed multicomponent excited-state decay dynamics with characteristic lifetimes on the tens to hundreds of picoseconds time scale. The observed photophysical behavior is interpreted within a superatomic molecular orbital framework, providing new insight into the excited-state dynamics and nonlinear optical properties of novel, protein-templated Fe nanoclusters.