New Approaches for Measuring Accelerated Chemical Reactions in Single Aerosol Particles
Lead Research Organisation:
University of Bristol
Department Name: Chemistry
Abstract
Aerosols are liquid or solid particles suspended in a gas such as air. They are used in advanced manufacturing, freeze-drying, and inhaled medications, are implicated in the spread of respiratory diseases, and are pervasive in our natural world. Indeed, the role of aerosols in atmospheric processes represents one of the largest uncertainties in predicting climate change. Environmental aerosols also degrade air quality and are strongly linked to respiratory and cardiovascular illnesses. Yet, carefully designed aerosols are often utilised to deliver inhaled drugs to the lungs to treat disease, and to manufacture new materials.
Recent studies have reported that chemical reactions can speed up by factors of up to a million in aerosols compared to reactions in beaker-scale solutions currently used to synthesise a range of chemicals, including pharmaceuticals such as benzopyridines (used in anaesthetics and vasodilators). The potential generality of reaction enhancement upon aerosolization may remove the need for expensive catalysts in many industrial synthetic processes for the manufacture of pharmaceuticals, agrochemicals, and other fine chemicals. Accelerated reactions have also been harnessed in analytical techniques, providing rapid degradation of bulky proteins for fast characterisations of monoclonal antibodies. Moreover, accelerated aerosol reactions may greatly affect the chemistry of atmospheric aerosols, with tremendous impacts on climate, air quality and human health. For example, the enhanced in-aerosol chemistry forming sunlight-absorbing organic molecules in air influences the warming of the Earth's atmosphere, and hence changes to the climate.
Despite the potential socioeconomic impacts of reaction acceleration via aerosolization, the generality and underlying mechanisms of these enhanced chemistries are unknown. Electrospray ionisation mass spectrometry is the dominant measurement tool used to study accelerated aerosol reactions. High levels of electrical charge are deliberately imparted to generated aerosol droplets to enable analysis in a mass spectrometer. While electrical charge is known to drive chemistry in microdroplets, the impact of charge and the associated high electric field strengths on accelerated in-aerosol reactions is not understood or quantified. Moreover, such approaches probe evolving properties for an aerosol ensemble comprising tens of thousands of particles per cubic centimetre and therefore provide highly averaged properties. Yet an ensemble will exhibit a considerable level of particle-to-particle variability in chemical composition, mixing state and size. Consequently, measurements on aerosol samples containing hundreds or thousands of microdroplets prevent clear links being made between aerosol properties and reaction rate enhancements. Indeed, there are a range of physical phenomena that are unique to aerosols that could be driving enhanced chemistries, but new measurement approaches are needed to understand the causes.
Advances in optical spectroscopy on single aerosols levitated in an optical trap allow the measurement of the physical and chemical properties of aerosol droplets with superior accuracy and sensitivity, on unlimited timescales, and under highly controlled conditions. In this project, we will build and benchmark a new integrated instrument combining three proven technologies to detect the formation of the products of in-aerosol reactions, using optical spectroscopy approaches in combination with laser based optical trapping of single particles. Through a comprehensive series of laboratory experiments, we will quantify reaction rates for an exemplar class of reaction that forms light absorbing products, and connect these rates to in-droplet reactant concentrations (including supersaturated solutions unique to aerosols), particle acidity, and the role of heterogeneous processes at the aerosol particle surface.
Recent studies have reported that chemical reactions can speed up by factors of up to a million in aerosols compared to reactions in beaker-scale solutions currently used to synthesise a range of chemicals, including pharmaceuticals such as benzopyridines (used in anaesthetics and vasodilators). The potential generality of reaction enhancement upon aerosolization may remove the need for expensive catalysts in many industrial synthetic processes for the manufacture of pharmaceuticals, agrochemicals, and other fine chemicals. Accelerated reactions have also been harnessed in analytical techniques, providing rapid degradation of bulky proteins for fast characterisations of monoclonal antibodies. Moreover, accelerated aerosol reactions may greatly affect the chemistry of atmospheric aerosols, with tremendous impacts on climate, air quality and human health. For example, the enhanced in-aerosol chemistry forming sunlight-absorbing organic molecules in air influences the warming of the Earth's atmosphere, and hence changes to the climate.
Despite the potential socioeconomic impacts of reaction acceleration via aerosolization, the generality and underlying mechanisms of these enhanced chemistries are unknown. Electrospray ionisation mass spectrometry is the dominant measurement tool used to study accelerated aerosol reactions. High levels of electrical charge are deliberately imparted to generated aerosol droplets to enable analysis in a mass spectrometer. While electrical charge is known to drive chemistry in microdroplets, the impact of charge and the associated high electric field strengths on accelerated in-aerosol reactions is not understood or quantified. Moreover, such approaches probe evolving properties for an aerosol ensemble comprising tens of thousands of particles per cubic centimetre and therefore provide highly averaged properties. Yet an ensemble will exhibit a considerable level of particle-to-particle variability in chemical composition, mixing state and size. Consequently, measurements on aerosol samples containing hundreds or thousands of microdroplets prevent clear links being made between aerosol properties and reaction rate enhancements. Indeed, there are a range of physical phenomena that are unique to aerosols that could be driving enhanced chemistries, but new measurement approaches are needed to understand the causes.
Advances in optical spectroscopy on single aerosols levitated in an optical trap allow the measurement of the physical and chemical properties of aerosol droplets with superior accuracy and sensitivity, on unlimited timescales, and under highly controlled conditions. In this project, we will build and benchmark a new integrated instrument combining three proven technologies to detect the formation of the products of in-aerosol reactions, using optical spectroscopy approaches in combination with laser based optical trapping of single particles. Through a comprehensive series of laboratory experiments, we will quantify reaction rates for an exemplar class of reaction that forms light absorbing products, and connect these rates to in-droplet reactant concentrations (including supersaturated solutions unique to aerosols), particle acidity, and the role of heterogeneous processes at the aerosol particle surface.
