Hydroxyl Radical Formation Is Linked to Fluence Threshold and Mass Conversion during Laser Fragmentation of Microparticles in Water

Laser fragmentation in liquids (LFL) is a laser-based nanoparticle (NP) synthesis approach that combines high productivity with the ability to produce NPs without organic surfactants and with high surface charge densities, which is highly relevant for applications in sensing, biomedicine, and catalysis. The surface functionality of these NPs is determined by their chemical and structural properties, which may stem from the transient formation of reactive oxygen species, in particular hydroxyl radicals (•OH), during synthesis. However, the formation of •OH and the underlying chemical and physical processes during LFL are not yet fully understood. Here, the fluence-dependent formation of •OH during picosecond-pulsed LFL of gold microparticles (Au MP) in aqueous solution was investigated using coumarin as a selective •OH probe molecule. The NPs generated by microparticle laser fragmentation in liquids (MP-LFL) were characterized by analytical disc centrifugation, and •OH formation was monitored via in situ reaction with coumarin and fluorescence spectroscopy of the resulting 7-hydroxycoumarin at various fluences. MP-LFL produced monodisperse NPs in the ultrasmall size regime (∼4 nm), and a detectable fragmentation threshold fluence of 0.53 J·cm–2 was determined. The •OH yield scaled with the extent of mass conversion from MPs to NPs, suggesting that efficient fragmentation promotes plasma-solvent interactions responsible for oxidative radical formation. A mechanistic picture is proposed in which higher fluences increase fragmentation rates, electron temperatures, and plasma density, thereby promoting •OH formation. These findings provide new insight into the interplay between laser parameters, plasma chemistry, and NP generation and establish a basis for tailoring NP surface chemistry through controlled laser processing conditions.