Dynamic viscosity mapping of the oxidation of squalene aerosol particles

Abstract

Organic aerosols (OA) play important roles in multiple atmospheric processes, including climate change, and can impact on human health. The physico-chemical properties of OA are important for all these processes and can evolve through reactions with various atmospheric components, including oxidants. The dynamic nature of these reactions makes it challenging to obtain a true representation of their composition and surface chemistry. Here we investigate the microscopic viscosity of model OA composed of squalene, undergoing chemical aging. We employ Fluorescent Lifetime Imaging Microscopy (FLIM) in conjunction with viscosity sensitive probes termed molecular rotors, in order to image changes in microviscosity in real time during oxidation with ozone and hydroxyl radicals, which are two key oxidising species in the troposphere. We also recorded Raman spectra of levitated particles to follow the reactivity during particle ozonolysis . The levitation of droplets was achieved via optical trapping that enabled simultaneous levitation and measurement via FLIM or Raman spectroscopy and allowed the true aerosol phase to be probed. Our data revealed a very significant increase in viscosity of levitated squalene droplets upon ozonolysis, following their transformation from liquid to solid phase that was not observable when the oxidation was carried out on coverslip mounted droplets. FLIM imaging with sub-micron spatial resolution also revealed spatial heterogeneity in viscosity distribution of oxidised droplets. Overall, a combination of molecular rotors, FLIM and optical trapping is able to provide powerful insights into OA chemistry and the microscopic structure that enables the dynamic monitoring of microscopic viscosity in aerosol particles in its true phase.

Fluorescent Molecular Rotors for Viscosity Sensors

“…[87] More recently,t he dynamics of oxidized squalenea erosolw as monitored in the presence of two oxidizings pecies, ozone and hydroxyl radicals. [88] In this study, BODIPY-C 10 was used as av iscositys ensor and mixed in the aerosol to monitor the real-time microviscosity changes of the organic aerosol duringo xidation. Squalene is a3 0-carbon organic oil naturally occurring in animalsa nd plants.…”

Amorphous phase state diagrams and viscosity of ternary aqueous organic/organic and inorganic/organic mixtures

“…As a result, the majority of the approaches available for measuring aerosol viscosity are indirect, such as the poke-and-flow technique 21 and fluorescence lifetime imaging in which viscosity is inferred from the fluorescence lifetime of a molecular rotor. [22][23][24][25] Despite these experimental challenges, it is now apparent that the addition of functional groups and increasing molecular weight are generally consistent with increasing viscosity. 4,26,27 Numerous studies have now linked increasing viscosity with progressive oxidation using OH radicals or ozonolysis.…”

Enhancing the Viscosity‐Sensitive Range of a BODIPY Molecular Rotor by Two Orders of Magnitude

“…Fortunately,r elativelyf ast and convenient measurements of viscosity can be achieved with an emergingc lass of fluorescent compounds-molecular rotors. [1,2] Previously,t hey have been used for viscosity sensing in polymers, [3,4] polymersomes, [5] aerosols, [6][7][8] model lipid bilayers, [9,10] protein aggregates, [11] and live cells. [12][13][14][15] Within cells, viscosity measurements have been performed in mitochondria, [16,17] lysosomes, [18] an endoplasmic reticulum, [19] anda plasma membrane.…”