Probing Plasmon-Driven Ultrafast Charge Dynamics in Methylene Blue-Gold Nanostructures with Femtosecond Transient Absorption Spectroscopy

Understanding the interplay between plasmonic nanostructures and molecular systems is critical for advancing photocatalysis and optoelectronic applications. In this study, we systematically investigate the ultrafast charge transfer dynamics between methylene blue (MB) and gold nanostructures of varying morphologies-nanospheres (GNS), silica-coated nanorods (GNRSi), and nanobipyramids (GNBP)-using femtosecond transient absorption spectroscopy (fs-TAS), supported by steady-state absorption, photoluminescence (PL), and surface-enhanced Raman spectroscopy (SERS). By controlling the spectral overlap and physical contact between MB and the plasmonic substrates, we effectively disentangle the dominant interaction mechanisms, ranging from hot electron transfer (HET) in systems with direct contact to plasmon-induced resonance energy transfer (PIRET) in spatially separated systems. The GNS-MB system, characterized by direct contact, exhibits clear signatures of HET, evidenced by PL quenching, rapid bleach recovery, and shortened electron–phonon coupling times. Conversely, the GNRSi-MB system, where a silica shell impedes direct contact, shows minimal spectral and kinetic perturbation, strongly suggesting that PIRET dominates the plasmon-dye coupling. Crucially, GNBP-MB mixtures exhibit the strongest overall plasmon–dye interaction, owing to their sharp-tip geometry and strong local field confinement, which enables both HET and PIRET processes to contribute synergistically. Dual-wavelength excitation (343 and 515 nm) further highlights this mechanistic distinction, with 343 nm excitation promoting clearer hot electron signatures. SERS measurements demonstrate enhanced charge-transfer-induced chemical enhancement, achieving detection limits as low as 10–12 M with GNBP substrates. These results establish nanoparticle morphology, surface accessibility, and spectral alignment as critical parameters for tailoring plasmon–molecule interactions in advanced photonic and catalytic systems.