Enhancing Double-Beam Laser Tweezers Raman Spectroscopy (LTRS) for the Photochemical Study of Individual Airborne Microdroplets.

Jovanny Gómez Castaño,Jean Verwaerde,Luc Boussekey,Yeny Tobón,Myriam Moreau

doi:10.3390/molecules24183325

Abstract

A new device and methodology for vertically coupling confocal Raman microscopy with optical tweezers for the in situ physico- and photochemical studies of individual microdroplets (Ø ≤ 10 µm) levitated in air is presented. The coupling expands the spectrum of studies performed with individual particles using laser tweezers Raman spectroscopy (LTRS) to photochemical processes and spatially resolved Raman microspectroscopy on airborne aerosols. This is the first study to demonstrate photochemical studies and Raman mapping on optically levitated droplets. By using this configuration, photochemical reactions in aerosols of atmospheric interest can be studied on a laboratory scale under realistic conditions of gas-phase composition and relative humidity. Likewise, the distribution of photoproducts within the drop can also be observed with this setup. The applicability of the coupling system was tested by studying the photochemical behavior of microdroplets (5 µm < Ø < 8 µm) containing an aqueous solution of sodium nitrate levitated in air and exposed to narrowed UV radiation (254 ± 25 nm). Photolysis of the levitated NaNO3 microdroplets presented photochemical kinetic differences in comparison with larger NaNO3 droplets (40 µm < Ø < 80 µm), previously photolyzed using acoustic traps, and heterogeneity in the distribution of the photoproducts within the drop.

Highlights

Optical tweezers (OT) are a state-of-the-art technique currently used in several research disciplines such as environmental, basic, pharmaceutical, and materials and engineering sciences for the study and manipulation of a wide variety of nano- to microscale samples captured by the forces exerted by lasers [1].The particles studied using this technique range from biological systems such as microorganisms, cells, and organelles to physical materials, atmospheric, and medical aerosols, combustion and industrial emissions as well as chemical compounds and solutions
The laser tweezers Raman spectroscopy (LTRS) coupling system was tested by conducting a Raman study of an aqueous microdroplet of NaNO3 levitated in air and exposed to UV light (254 ± 25 nm)
An alternative vertical dual-laser coupling for laser tweezers Raman spectroscopy (LTRS) was presented

Summary

Introduction

Optical tweezers (OT) are a state-of-the-art technique currently used in several research disciplines such as environmental, basic, pharmaceutical, and materials and engineering sciences for the study and manipulation of a wide variety of nano- to microscale samples captured by the forces exerted by lasers [1]. The particles studied using this technique range from biological systems such as microorganisms, cells, and organelles to physical materials, atmospheric, and medical aerosols, combustion and industrial emissions as well as chemical compounds and solutions. OT have been employed for the trapping of micrometric samples in solution, and for the levitation of condensed phase particles in a gas or in a vacuum. By using OT, analyses such as: (a) phase behavior, hygroscopic growth, morphology, vapor pressure, and the kinetics of surface mass transference [2,3]; (b) whispering gallery mode (WGM) resonances [4] and other optical properties [5]; (c) heterogeneous oxidation reactions [6];

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