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Establishing a Spectral Database for Airborne Particles

In an article published in the journal Molecules, researchers developed a novel optical-trapping Raman spectroscopy (OT-RS) technique for identifying, characterizing, and detecting single aerosol particles in their native atmospheric state. 

Study: A Collection of Molecular Fingerprints of Single Aerosol Particles in Air for Potential Identification and Detection Using Optical Trapping-Raman Spectroscopy. Image Credit: SvedOliver/Shutterstock.com

The authors gathered optical-trapping Raman spectroscopy (OT-RS)-measured single particle Raman spectra and constructed an optical-trapping Raman spectroscopy (OT-RS) molecular fingerprint library. This molecular fingerprint library was a reference for future aerosol particle characterization, detection, and identification. Aerosol particles from eight distinct categories, including carbons, biological warfare agent (BWA) surrogates, bioaerosols such as pollens, spores, fungus, vitamins, and dust were gathered for their optical-trapping Raman spectroscopy (OT-RS) molecular fingerprints.

Six of the eight categories' spectral molecular fingerprints for the aerosol particles had already been published, and the remaining two were more recent studies. The paper's findings also discussed the difficulties, restrictions, and benefits of employing single particle optical trapping-Raman spectroscopy (OT-RS) for identifying, characterizing, and detecting aerosol particles.

Challenges in Characterization of Single Particles in the Native Atmospheric State

Aerosol particles range in size from 0.01 mm to 100 mm. They are microscopic solids or liquids made of carbon black, mineral dust, metal flakes, sea spray, soot, or biological organisms such as fragments and microbes. Human health, biological security, and climate are all significantly impacted by aerosols.

The study of aerosols entails collaboration across physics, engineering, biochemistry, and chemistry to solve light scattering, aerosol instrumentation, bio-variability, surface chemistry, formation, and loss. One challenge is the characterization, identification, and detection of single particles in their native atmospheric state during sampling.

Several approaches have been employed to identify, characterize, and detect single particles, including optical and scanning electron microscopy, mass spectrometry and X-ray spectrometry, laser-induced fluorescence (LIF), Raman spectroscopy (RS), and polymerase chain reaction.

Here the researchers presented a set of optical trapping-Raman spectroscopy (OT-RS) single particle Raman spectra. The goal of this paper was to launch the formulation of an open-end data reference for the optical trapping-Raman spectroscopy (OT-RS)-studied Raman spectral molecular fingerprints of single particles. Therefore, the single particle Raman spectra of eight distinct categories, including amino acids, B vitamins, BWA, spores, (v) pollens, fungus, interplanetary dust, and terrestrial dust, were collected together with some previously unpublished information.

The single particle spectral database would be a helpful resource for interpreting the Raman spectra of aerosol particles in their native atmospheric state. Such a database would prove tremendously beneficial for the potential characterization, identification, and detection of single particles in the air using optical trapping-Raman spectroscopy (OT-RS). It also briefly addressed what the optical trapping-Raman spectroscopy (OT-RS) technique could do to identify, characterize, and detect aerosol particles in their native atmospheric state.

Understanding the Basic Investigative Setup

A continuous wave Gaussian beam with 532 nm approximate TEM00 mode was employed in the optical trapping-Raman spectroscopy (OT-RS) system's experimental setup for trapping target aerosol particles and activating the Raman spectrometer. The laser's power was adjusted between 1500 and 50 mW to trap a wide range of single aerosol particles. 

Two types of optical forces, radiation pressure force (RPF) and photophoretic force (PPF), were primarily used to capture aerosol particles.

The PPF was produced by photopheresis because of the variations in the thermal accommodation coefficient of the surface and the particle's surface temperature, which were the primary factors in trapping absorbing particles. A highly focused laser beam created the RPF, which consisted of gradient and scattering forces, to capture non-absorbing particles.

The single particle Raman spectral molecular fingerprints produced by the optical trapping-Raman spectroscopy (OT-RS) technique helped to identify amino acid chemical compositions.

Three different types of amino acid particles' characteristic Raman spectral molecular fingerprints were used in the experiment, which included L-glutamic, glycine, and L-threonine. All three spectral molecular fingerprints were collected within the first minute of a particle being successfully trapped.

Vitamins B5, B7, and B12 single-particle Raman spectral molecular fingerprints were generated. The testing conditions were identical to those mentioned for amino acids. Like the amino acids, the three spectra were collected within the first minute of a particle being successfully trapped.

Based on references to the Raman spectra recorded using bulk materials, over 100 chemical functional groups from single particles in the air or their native atmospheric state were assigned. The authors encouraged future work to include molecular fingerprints from individual particles in this open-end collection.

Future of OT-RS for Single Particle Characterization, Identification, and Detection

The Raman spectra of single particles from eight distinct chemical classes, including BWA, amino acid and B-vitamin, fungi, pollens, spores, terrestrial dust, and interplanetary dust, were presented in this work as a novel collection of optical trapping-Raman spectroscopy (OT-RS) Raman spectra. This work was a beginning step toward creating a spectrum database for molecular fingerprints of single particles. 

A comprehensive data bank involved gathering optical trapping-Raman spectroscopy (OT-RS) from continuing additions of other molecular fingerprints of single particles in the air. This open-end molecular fingerprint database would be a valuable resource for future characterization, identification, and detection of single particles utilizing the optical trapping-Raman spectroscopy (OT-RS) approach. 

The findings revealed that the optical trapping-Raman spectroscopy (OT-RS) approach helped recognize single particles because it used spectrum properties to show a particle's chemical make-up and molecular structure. Both the averaged information of bulk materials and the detailed information of individual single particles could be revealed by single particle Raman spectroscopy. It reduced environmental contact, surface alteration, and contamination of the particle.

The optical trapping-Raman spectroscopy (OT-RS) technique could be used to identify and detect single particles. However, several obstacles were overcome, and more advancements in this area were anticipated. 

Reference

Alali, H., Ai, Y., Pan, Y.L., Videen, G., Wang, C. (2022). A Collection of Molecular Fingerprints of Single Aerosol Particles in Air for Potential Identification and Detection Using Optical Trapping-Raman Spectroscopy. Molecules, 27(18), 5966.
https://www.mdpi.com/1420-3049/27/18/5966/htm

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Pritam Roy

Written by

Pritam Roy

Pritam Roy is a science writer based in Guwahati, India. He has his B. E in Electrical Engineering from Assam Engineering College, Guwahati, and his M. Tech in Electrical & Electronics Engineering from IIT Guwahati, with a specialization in RF & Photonics. Pritam’s master's research project was based on wireless power transfer (WPT) over the far field. The research project included simulations and fabrications of RF rectifiers for transferring power wirelessly.

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