Prof. Paulo Saldiva Profile

Prof. Paulo Saldiva

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Proceedings Article | 20 June 2024 Presentation + Paper

Two-photon excitation as a selective tool to characterize black carbon nanoparticles in the environment

Maria Luiza Ferreira Vicente, Sebastião Pratavieira, Mariana Matera Veras, Paulo Saldiva, Hajjoul Houssam, Francisco Eduardo Gontijo Guimarães

Proceedings Volume 12999, 1299918 (2024) https://doi.org/10.1117/12.3022330

KEYWORDS: Nanoparticles, Carbon, Atmospheric particles, Absorption, Particles, Fluorescence, Spectroscopy, Molecular aggregates, Tunable filters, Ocean optics

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Black carbon (BC) pollution particles result from incomplete fossil fuel combustion and are formed by clusters of primary nanoparticles (NPs) with diameters around 50 nm. The impact of BC particles on the environment and climate rise depends on accurately determining optical properties, such as absorption and scattering. The shape and the internal molecular structure of primary NPs influence their optical behavior. In this study, we characterized NPs of genuine soot from direct air filtration, determining their optical properties in water droplets through two-photon microscopy. They are monodispersed in aqueous media, presenting 2P optical features associated with stable molecular aggregates of polycyclic aromatic hydrocarbons (PAHs). The single-photon (1P) spectroscopic features of stable suspensions of pollution NPs in aqueous solution (pH=13) are represented by an optical depth (OD) spectrum relative to the average scattering and absorption in the 300 - 900 nm wavelength range, proportional to λ^-n. The broad 1P absorption spectrum is fitted with three Gaussians. The first, between 500 and 700 nm, is associated with optical transitions between π states in J-type molecular arrangements. The second, between 350 and 450 nm, relates to H-type aggregates. The third could be attributed to optical transitions below 350 nm in weakly perturbed molecular aggregates. Two-photon absorption (2PA) measurements were conducted in the transmission mode of the two-photon microscopy with a tunable laser between 700 and 1050 nm in the infrared. Unlike 1P transitions, these 2P transitions are highly selective to the nature of the aggregates, confined to the spectral region associated with H-aggregated states.

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