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

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.

Optical tweezers and manipulators. Modern concepts and future prospect

“…Applications of optical manipulation systems practically always require monitoring the trapping dynamics and recording the position of the object and the forces acting on it [287]. Along with classical wide-field optical microscopy, depending on the character of the trapped objects and the objectives of the experiment, technologies of three-dimensional imaging are used in combination with optical traps, e.g., confocal [295] and two-photon microscopy [296], digital holography [24, 297±299], Raman microspectroscopy [120,170,300], and fluorescence light sheet microscopy [301,302].…”

“…The endoscopic approach based on using optical fibers, for instance, fiber fluorescence microscopy, allows substantial simplification of the trapped particle tracing procedure [311]. At the same time, label-free fluorescence microscopy, for example, Raman spectroscopy [300], is becoming the most requested solution for the noninvasive monitoring of living objects.…”

“…An advantage of this symbiosis consists in the specific fluorescence quenching in an optically trapped particle as compared to the case as if it was located on a substrate. This principle was successfully exploited in studies of fast chemical processes in atmospheric aerosols [170,171,300]. In Ref.…”

Micro‐droplet Trapping and Manipulation: Understanding Aerosol Better for a Healthier Environment

“…Tobon et al. combined AOT and Raman microspectroscopy to study the photolysis of aqueous NaNO 3 droplets and the spatial distributions of photoproducts within the droplet can be determined [50a] . Seng et al.…”

Unraveling the three‐dimensional morphology and dynamics of the optically evolving polystyrene nanoparticle assembly using dual‐objective lens microscopy

“…[ 37 ] Furthermore, the imaging objective has been further used to record complementary spectroscopic information such as fluorescence or Raman spectra. [ 38 ]…”