Ultrafast electron transfer at the interface of gold nanoparticles and methylene blue molecular adsorbates

Due to their unique property of possessing localized surface plasmon resonance (LSPR), metal nanoparticles (MNPs) have drastically impacted many applications. For instance, local field enhancement through LSPRs and plasmonic hot electron transfer are known to enhance the efficiency of MNP-based photoreactions. Here, we report on the ultrafast electron transfer from gold nanoparticles (Au-NPs) to methylene blue (MB) molecular adsorbate using femtosecond pump-probe and steady-state absorption and emission spectroscopy techniques. Although the energy band alignment of the interface allows both dipole-dipole Förster resonance energy transfer (FRET) and charge transfer, because the MB emission intensity at the Au-NPs/MB nanocomposite decreased by a factor of ∼3.6, the FRET process was ruled out. Selective excitation of LSPRs at the Au-NPs/MB nanocomposite sample in pump-probe experiments led to the formation of the MB ground-state depletion and a positive induced absorption at wavelengths shorter than ∼500 nm, which was attributed to the shoulder of the MB^- anion absorption. Furthermore, despite the fact that the concentration of Au-NPs in the nanocomposite sample is the same as that in the Au-NPs solution, the initial intensity of the LSPR depletion signal was about six times weaker than that in the Au-NPs sample. These observations suggest that electron transfer from excited Au-NPs to MB adsorbates took place on a time-scale that is shorter than the ∼50 fs experimental temporal resolution.